Electromagnetic interferences are directly proportional to the change in current or voltage as a function of the time and the serial inductance of the circuit. PCBs always generate EMI, so a number of considerations for minimizing them should be taken into account.

Design of Advanced Digital Systems Based on High-Speed Optical Links 349

Clock

Rt

D1

Rt

D2

Clock

D2

D3

In high-speed designs other considerations are taken into account, depending on the

For example, at data rates of 622 Mbps and higher, the skin effect is extremely important. This could cause signal attenuations over long distances. The result is a low pass filter with attenuation, which increases with frequency. For this reason, the traces should be wider.

On the other hand, connectors and vias cause impedance discontinuities. In order to reduce them, specific connectors with shielding features with many ground connections must be

High-performance cables are also very important to have a good bandwidth and for

When working at 2.5 Gbps and beyond, the problems become substantially more difficult to eliminate. There are many copper and dielectric losses. It is needed to pay attention to every consideration for PCB and layout design. A backplane thickness of less than 0.200 inches gives the best result. Vias used for interconnection between layers create line discontinuities. These routed layers should be as few as possible and thus limiting the number of vias, so the vias are shorter, minimizing line discontinuities. Buried vias can be used to reduce this

The material used in boards is very important. FR4 dielectric commonly used has significant losses above 2 Gbps. For this reason, low-loss dielectric PCB material can be used such as

Digital system design has therefore moved deep into the Multi-GHz range, with signal rise and fall-times of the order of 100ps or less. It is a well-known fact that Signal Integrity (SI) simulations become necessary when system designs break the 50-100MHz barrier, and are virtually mandatory in the GHz range. SI simulations are used to ensure the quality and accurate timing of electrical signals. The benefits of SI analysis early in the design cycle are well established, as it allows the identification and resolution of SI problems like overshoot,

ringing, crosstalk, delay mismatches, etc. before the first prototype is built.

Clock D1

Rt

Rt

Rt

D1 D2

**3.6 Other considerations**

minimizing attenuation losses.

problem in thicker boards.

Rogers 4350, GETEK or ARLON.

**4. Signal integrity studies**

Fig. 14. Types of clock signal routing.

amount of information needed to be sent.

Stub

Rt

used.

One of the main sources of EMI is the current loop. The other is common mode problems. The differential mode is the mode where the signals travel forming a path and a return in opposite direction. When signals travel in the same direction, both signal and return, is called common mode. This occurs because the ground is not a perfect driver and there is an undesired associated inductance. This effect is illustrated in Figure 13.

Fig. 13. Common and differential mode.

The main considerations in order to reduce this effect are the following:

## **3.5 Clock transmission line**

We have mentioned differential lines but we have not introduced single-ended lines connecting a source with load or receiver. They are used in point-to-point routing, signal clock routing, low-speed lines and non-critical I/O lines. Signal clock routing is the most remarkable point in single-ended lines. The following considerations are given to improve signal integrity in clock signals:

The clock signals can be routed in several ways. If a daisy chain routing is used, undesired stubs or short traces can appear, so degrading the signal quality and producing reflexions. When considering a star routing, the clock signal arrives to all devices at the same time, so lines must have the same length. Each load must be identical for minimizing signal integrity problems. To design traces with the same length, serpentine techniques for time adjusting are used. Several types of clock signal routing are illustrated in Figure 14.

Fig. 14. Types of clock signal routing.

## **3.6 Other considerations**

348 Optical Fiber Communications and Devices

One of the main sources of EMI is the current loop. The other is common mode problems. The differential mode is the mode where the signals travel forming a path and a return in opposite direction. When signals travel in the same direction, both signal and return, is called common mode. This occurs because the ground is not a perfect driver and there is an

undesired associated inductance. This effect is illustrated in Figure 13.

The main considerations in order to reduce this effect are the following:

Ensure that exists a good capacitive coupling between planes.

Do not use multiple signal layers for clock signals.

Use terminations to minimize reflexions.

Keep a solid reference and continuous plane for each line. Trace the critical lines as

We have mentioned differential lines but we have not introduced single-ended lines connecting a source with load or receiver. They are used in point-to-point routing, signal clock routing, low-speed lines and non-critical I/O lines. Signal clock routing is the most remarkable point in single-ended lines. The following considerations are given to improve

Keep lines as straight as possible. Use rounded shapes instead of sharp angled ones.

 Do not use vias in clock lines. They change the impedance and they cause reflexions. Place a ground plane next to the outer layer to minimize the noise. If you use inner

The clock signals can be routed in several ways. If a daisy chain routing is used, undesired stubs or short traces can appear, so degrading the signal quality and producing reflexions. When considering a star routing, the clock signal arrives to all devices at the same time, so lines must have the same length. Each load must be identical for minimizing signal integrity problems. To design traces with the same length, serpentine techniques for time adjusting

layers for routing a clock trace, form a sandwich with both layers.

are used. Several types of clock signal routing are illustrated in Figure 14.

Fig. 13. Common and differential mode.

striplines.

Reduce presence of stubs.

**3.5 Clock transmission line**

signal integrity in clock signals:

Use point-to-point traces.

In high-speed designs other considerations are taken into account, depending on the amount of information needed to be sent.

For example, at data rates of 622 Mbps and higher, the skin effect is extremely important. This could cause signal attenuations over long distances. The result is a low pass filter with attenuation, which increases with frequency. For this reason, the traces should be wider.

On the other hand, connectors and vias cause impedance discontinuities. In order to reduce them, specific connectors with shielding features with many ground connections must be used.

High-performance cables are also very important to have a good bandwidth and for minimizing attenuation losses.

When working at 2.5 Gbps and beyond, the problems become substantially more difficult to eliminate. There are many copper and dielectric losses. It is needed to pay attention to every consideration for PCB and layout design. A backplane thickness of less than 0.200 inches gives the best result. Vias used for interconnection between layers create line discontinuities. These routed layers should be as few as possible and thus limiting the number of vias, so the vias are shorter, minimizing line discontinuities. Buried vias can be used to reduce this problem in thicker boards.

The material used in boards is very important. FR4 dielectric commonly used has significant losses above 2 Gbps. For this reason, low-loss dielectric PCB material can be used such as Rogers 4350, GETEK or ARLON.

## **4. Signal integrity studies**

Digital system design has therefore moved deep into the Multi-GHz range, with signal rise and fall-times of the order of 100ps or less. It is a well-known fact that Signal Integrity (SI) simulations become necessary when system designs break the 50-100MHz barrier, and are virtually mandatory in the GHz range. SI simulations are used to ensure the quality and accurate timing of electrical signals. The benefits of SI analysis early in the design cycle are well established, as it allows the identification and resolution of SI problems like overshoot, ringing, crosstalk, delay mismatches, etc. before the first prototype is built.

Design of Advanced Digital Systems Based on High-Speed Optical Links 351

The topology is extracted according to the manufacturer's datasheet optical connector.

An ideal study without transmission line is performed. The simulation for this case is:

Fig. 16. Unrouted differential lines.

Fig. 17. Extracted topology.

Fig. 18. Waveforms for differential lines.

## **4.1 Pre-layout studies**

The pre-layout simulations are required at the earliest stages of the PCB design. In this stage the designer evaluates several topologies and selects the one that fulfils all the specifications such as size, component number and performance. In addition, the results of these simulations help to set crucial parameters for the transmission structure, such as trace width, trace spacing, maximum trace length, and critical component placement. It is important to understand these simulations are intended for selecting the components and topologies as well as for fine tuning of the signal path. The results analysis is used to set the rules that will be incorporated into the layout.

As an example, and taking into account the design considerations described in the previous section, a real design is going to be studied. The main elements in this design are:

Fig. 15. Elements of Advanced Digital Systems based on High-Speed Optical Links in PCB.

The optical connector has a differential output (LVPECL), routed to the entrance of Transmitter/Receiver Chip Set. These transmission lines are the most critical of the study because is a point-to-point serial data transmission with a very high bit rate.

Another point to be taken into account is the connection between data acquired in the Transmitter/Receiver Chip Set and the FPGA. In this case, the importance of the study is not based in data frequency. We must avoid the data bus crosstalk.

## **4.1.1 Differential lines**

This signal has a critical jitter and the topology, geometry, length, and termination impedance must be carefully studied. In this case, it is observed the differential line without being routed.

Fig. 16. Unrouted differential lines.

350 Optical Fiber Communications and Devices

The pre-layout simulations are required at the earliest stages of the PCB design. In this stage the designer evaluates several topologies and selects the one that fulfils all the specifications such as size, component number and performance. In addition, the results of these simulations help to set crucial parameters for the transmission structure, such as trace width, trace spacing, maximum trace length, and critical component placement. It is important to understand these simulations are intended for selecting the components and topologies as well as for fine tuning of the signal path. The results analysis is used to set the

As an example, and taking into account the design considerations described in the previous

Fig. 15. Elements of Advanced Digital Systems based on High-Speed Optical Links in PCB.

The optical connector has a differential output (LVPECL), routed to the entrance of Transmitter/Receiver Chip Set. These transmission lines are the most critical of the study

Another point to be taken into account is the connection between data acquired in the Transmitter/Receiver Chip Set and the FPGA. In this case, the importance of the study is not

This signal has a critical jitter and the topology, geometry, length, and termination impedance must be carefully studied. In this case, it is observed the differential line without

because is a point-to-point serial data transmission with a very high bit rate.

based in data frequency. We must avoid the data bus crosstalk.

section, a real design is going to be studied. The main elements in this design are:

**4.1 Pre-layout studies**

rules that will be incorporated into the layout.

Transmitter/Receiver Chip Set (2)

FPGA (3)

**4.1.1 Differential lines**

being routed.

SFP Optical Connector with two receiver channels (1)

The topology is extracted according to the manufacturer's datasheet optical connector.

Fig. 17. Extracted topology.

An ideal study without transmission line is performed. The simulation for this case is:

Fig. 18. Waveforms for differential lines.

Design of Advanced Digital Systems Based on High-Speed Optical Links 353

The waveforms 800, 5000, 9000 y 12000 mils in black, green, red and blue provide a delay to the length, which increases to approximately 2.14 ns in the worst case. This shows a mismatched line. Also the waveforms will degrade depending on the length of the track due to the increasing influence of the reflections, since an overshoot and ringing growing.

The worst case above (12000 mils) but with a termination is now studied. The topology is:

Fig. 21. Topology of single-ended line with active termination.

Fig. 22. Waveform of single-ended line with active termination.

The results are:

In black and green color RX+ and RX- signals are observed and in blue color the resulting differential signal is presented. The voltage values obtained are the same that the manufacturer recommendations.

The next step, seeing the topology is correct, it would add a transmission line and observe the maximum length that could have the routing. In the next section another types of line will be studied.

## **4.1.2 Parallel data bus**

In this case, the connection between the transmitter/receiver chip set and the FPGA is studied. Its speed of transmission is not comparable with that of the differential line. Even so, the maximum length must be checked and a termination may be needed.

First, a study of a single line to detect maximun length and optimal termination has to be performed. The study was performed for a bus of 32 lines at 80 MHz.

Fig. 19. Topology of single-ended line.

The waveforms for each length (800, 5000, 9000 y 12000 mils) are as follows:

Fig. 20. Waveform for single-ended line.

The waveforms 800, 5000, 9000 y 12000 mils in black, green, red and blue provide a delay to the length, which increases to approximately 2.14 ns in the worst case. This shows a mismatched line. Also the waveforms will degrade depending on the length of the track due to the increasing influence of the reflections, since an overshoot and ringing growing.

The worst case above (12000 mils) but with a termination is now studied. The topology is:

Fig. 21. Topology of single-ended line with active termination.

The results are:

352 Optical Fiber Communications and Devices

In black and green color RX+ and RX- signals are observed and in blue color the resulting differential signal is presented. The voltage values obtained are the same that the

The next step, seeing the topology is correct, it would add a transmission line and observe the maximum length that could have the routing. In the next section another types of line

In this case, the connection between the transmitter/receiver chip set and the FPGA is studied. Its speed of transmission is not comparable with that of the differential line. Even

First, a study of a single line to detect maximun length and optimal termination has to be

so, the maximum length must be checked and a termination may be needed.

The waveforms for each length (800, 5000, 9000 y 12000 mils) are as follows:

performed. The study was performed for a bus of 32 lines at 80 MHz.

manufacturer recommendations.

Fig. 19. Topology of single-ended line.

Fig. 20. Waveform for single-ended line.

will be studied.

**4.1.2 Parallel data bus**

Fig. 22. Waveform of single-ended line with active termination.

Design of Advanced Digital Systems Based on High-Speed Optical Links 355

Where there are three bus lines, taking the top and bottom as aggressors and the middle line

In blue and green color, we have waveforms of the FPGA inputs, that correspond to the agressor signal. The red one is the receiver input that flowing by the victim trace, and the black one is the output driver of the victim trace. In this case a crosstalk about 2149mV is obtained and the simulation does not pass the test (the victim signal is in an undefined logic

In the previous section is commented that the solution is adapting the transmission lines using terminations. If an active termination is added to the lines and then the simulation is

as victim.

The results for high steady state are:

Fig. 25. Waveforms of three parallel lines.

made in each line, the result is as follows.

Fig. 26. Topology of three parallel lines with active termination.

state, below 3V).

Which greatly improves the previous case:

Fig. 23. Waveform of single-ended line without termination.

Working with a data bus is essential to examine the crosstalk between the lines. The topology studied is as follows:

Fig. 24. Topology of three parallel lines.

Where there are three bus lines, taking the top and bottom as aggressors and the middle line as victim.

The results for high steady state are:

354 Optical Fiber Communications and Devices

Working with a data bus is essential to examine the crosstalk between the lines. The

Which greatly improves the previous case:

Fig. 23. Waveform of single-ended line without termination.

topology studied is as follows:

Fig. 24. Topology of three parallel lines.

Fig. 25. Waveforms of three parallel lines.

In blue and green color, we have waveforms of the FPGA inputs, that correspond to the agressor signal. The red one is the receiver input that flowing by the victim trace, and the black one is the output driver of the victim trace. In this case a crosstalk about 2149mV is obtained and the simulation does not pass the test (the victim signal is in an undefined logic state, below 3V).

In the previous section is commented that the solution is adapting the transmission lines using terminations. If an active termination is added to the lines and then the simulation is made in each line, the result is as follows.

Fig. 26. Topology of three parallel lines with active termination.

Design of Advanced Digital Systems Based on High-Speed Optical Links 357

In this case, crosstalk level keeps the victim signal within the permissible limits either in/for high level (figure 30) or low level (figure 31). The values are 1365 mV for high level and 1514

Fig. 30. Waveforms of three parallel lines with serial termination (high level).

Fig. 31. Waveforms of three parallel lines with serial termination (low level).

Fig. 29. Topology of three parallel lines with serial termination.

mV for low level.

Fig. 27. Waveforms of three parallel lines with active termination (high level).

It can be observed a clear improvement in the crosstalk level, keeping the victim signal inside the permitted high level range. However, at low level it is shown a crosstalk of 1421 that causes the victim signal introduces in the forbidden region for low level, and this can cause errors.

Fig. 28. Waveforms of three parallel lines with active termination (low level).

Then, the initial solution of placing an active termination is not suitable. As theory says, we should test a serial termination and check what happen.

Fig. 27. Waveforms of three parallel lines with active termination (high level).

Fig. 28. Waveforms of three parallel lines with active termination (low level).

should test a serial termination and check what happen.

Then, the initial solution of placing an active termination is not suitable. As theory says, we

cause errors.

It can be observed a clear improvement in the crosstalk level, keeping the victim signal inside the permitted high level range. However, at low level it is shown a crosstalk of 1421 that causes the victim signal introduces in the forbidden region for low level, and this can

Fig. 29. Topology of three parallel lines with serial termination.

In this case, crosstalk level keeps the victim signal within the permissible limits either in/for high level (figure 30) or low level (figure 31). The values are 1365 mV for high level and 1514 mV for low level.

Fig. 30. Waveforms of three parallel lines with serial termination (high level).

Fig. 31. Waveforms of three parallel lines with serial termination (low level).

Design of Advanced Digital Systems Based on High-Speed Optical Links 359

In this case, as in the above, we extract the real topology. We studied a serial termination in

The results are as expected, we see that the data sent between the transmitter/receiver chip set and the FPGA function properly at the selected frequency and the distance routed.

The wide range of applications of optical fibers has been continuously supported by their friendly integration with classic electronics. Being the optical transceiver the key element in such hybrid systems, their design is not straightforward, and need to take into account a

In this chapter the main handicaps when developing electronics specifically for high-speed fiber optic communications have been highlighted. Some of these issues fully lay in the field

**4.2.2 Parallel data bus**

Fig. 34. Real topology of parallel data bus.

Fig. 35. Real waveforms of parallel data bus.

**5. Conclusions**

good number of particularities.

bus line.

## **4.2 Post-layout studies**

When the layout is complete, a post-layout simulation is performed on the critical sections of the board to ensure there are no major signal integrity problems. Based on the results of the post-layout simulation, any changes required are incorporated in the layout and the layout is released to the fabrication house for board manufacturing. The post-layout simulation process requires the layout of all active layers on the board as well as the physical properties of the dielectric and metal layers. The post-layout simulations use the physical properties supplied by the fabrication house that may differ from those published.

## **4.2.1 Differential lines**

Taking the pre-layout studies conducted on the differential lines and after to route them with the defined conditions, we extract the real topology that will be sent to the manufacturer for verification.

Fig. 32. Real topology of differential lines.

We can see that actual results match those obtained in the ideal simulation (Figure 18):

Fig. 33. Real waveforms of differential lines.

## **4.2.2 Parallel data bus**

358 Optical Fiber Communications and Devices

When the layout is complete, a post-layout simulation is performed on the critical sections of the board to ensure there are no major signal integrity problems. Based on the results of the post-layout simulation, any changes required are incorporated in the layout and the layout is released to the fabrication house for board manufacturing. The post-layout simulation process requires the layout of all active layers on the board as well as the physical properties of the dielectric and metal layers. The post-layout simulations use the physical properties supplied by the fabrication house that may differ from those published.

Taking the pre-layout studies conducted on the differential lines and after to route them with the defined conditions, we extract the real topology that will be sent to the

We can see that actual results match those obtained in the ideal simulation (Figure 18):

**4.2 Post-layout studies**

**4.2.1 Differential lines**

manufacturer for verification.

Fig. 32. Real topology of differential lines.

Fig. 33. Real waveforms of differential lines.

In this case, as in the above, we extract the real topology. We studied a serial termination in bus line.

Fig. 34. Real topology of parallel data bus.

The results are as expected, we see that the data sent between the transmitter/receiver chip set and the FPGA function properly at the selected frequency and the distance routed.

Fig. 35. Real waveforms of parallel data bus.

## **5. Conclusions**

The wide range of applications of optical fibers has been continuously supported by their friendly integration with classic electronics. Being the optical transceiver the key element in such hybrid systems, their design is not straightforward, and need to take into account a good number of particularities.

In this chapter the main handicaps when developing electronics specifically for high-speed fiber optic communications have been highlighted. Some of these issues fully lay in the field

**17**

*1,2,4Malaysia 3Indonesia*

**Fiber Optic Temperature Sensors**

*1Photonic Research Center, University of Malaya, Kuala Lumpur*

*2Department of Electrical Engineering, University of Malaya, Kuala Lumpur*

*3Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya 4Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), Shah Alam*

S. W. Harun1,2, M. Yasin1,3, H. A. Rahman1,2,4, H. Arof2 and H. Ahmad1

The need for temperature measurement exists in many applications such as in automated consumer products, automated production plants and high performance processors. Recent works have mainly focused on temperature sensors that satisfy user requirements for specific applications, and the main considerations are performance, dimension and reliability. In fact, traditional low-cost solutions, such as thermocouples and resistance temperature detectors (RTDs), do not always yield satisfactory performance, e.g., when the fluid temperature has to be measured in hostile environments, in the presence of electromagnetic, chemical, and mechanical disturbances. Since signals from the thermoelectric sensors are normally mixed with intrinsic noise and extrinsic interferences, they may contain intolerable errors if not properly filtered. Therefore, this type of sensors is inept for gauging temperature in microfluidic or nano-sized devices, in extreme marine environments, and underground geological sites where long distance measurement with precision is required. For such applications, fiber optical sensors offer a better alternative since the optical signal does not suffer from interference by electromagnetic fields and can be transmitted over extremely long distances without any significant loss [Yasin et al., 2010; Li et al., 2010; Ahmad et al., 2009; Lim et al., 2009; Ahmad et al., 2009; You et al., 2005; Xu et al., 2005]. Furthermore, they are relatively small in size, and compatible with other optical

To date, various types of fiber optic temperature sensors have been reported in the literatures and they are mostly based on fiber interferometric [Choi et al., 2008] and fiber Bragg grating (FBG) [Han et al., 2004]. However, the first type of sensors are rather expensive to produce and complicated to implement on-site [Golnabi, 2000]. Fiber Bragg gratings are very efficient at temperature sensing and are easy to implement; however, they always need additional techniques to discriminate the Bragg shifts by temperature and by strain/compression and they also require expensive phase-masks. In this chapter, a temperature sensor is demonstrated based on four different techniques; intensity modulated

**1. Introduction**

fiber devices.

of electronic engineering. Other aspects, and due to the high frequency involved in such systems, need some additional analysis. In this sense, the electromagnetic theory, as used for wave propagation in dielectric materials, has to be applied to the electric transmission lines present in some cases.

Being experiments the common analysis tools in hybrid opto-electronic systems, some simulation tools can be sometimes used for fixing specific problems like cross-talking. These numerical tools are of particular interest for proper routing design in PCBs.

After all, these systems have demonstrated their applicability in a huge number of scenarios including communications, medicine, nuclear research, etc.

## **6. References**

## **Fiber Optic Temperature Sensors**

S. W. Harun1,2, M. Yasin1,3, H. A. Rahman1,2,4, H. Arof2 and H. Ahmad1

*1Photonic Research Center, University of Malaya, Kuala Lumpur 2Department of Electrical Engineering, University of Malaya, Kuala Lumpur 3Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya 4Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), Shah Alam 1,2,4Malaysia 3Indonesia*

## **1. Introduction**

360 Optical Fiber Communications and Devices

of electronic engineering. Other aspects, and due to the high frequency involved in such systems, need some additional analysis. In this sense, the electromagnetic theory, as used for wave propagation in dielectric materials, has to be applied to the electric transmission lines

Being experiments the common analysis tools in hybrid opto-electronic systems, some simulation tools can be sometimes used for fixing specific problems like cross-talking. These

After all, these systems have demonstrated their applicability in a huge number of scenarios

[1] Douglas Brooks, "Signal Integrity Issues and Printed Circuit Board Design", Prentice

[2] Stephen C. Thierauf, "High-Speed Circuit Board Signal Integrity", Artech House Inc.,

[3] Mark. I. Montrose, "Printed Circuit Board Design Techniques for EMC", Wiley-

[6] Sackinger, E., "Broadband circuits for optical fiber communication", Wiley, 2005. ISBN:

[7] Cox, C.H., "Analog optical links: Theory and practice", Cambridge University Press,

[8] Muller, P., Leblebici, Y., "CMOS Multichannel Single-Chip Receivers for Multi-Gigabit

[9] ATLAS Trigger and DAQ steering group, "Trigger and Daq Interfaces with FE systems:

[10] Dowell, M. Pearce, "ATLAS front-end read-out link requirements", ATLAS internal

[11] J. Torres, "Estudio, diseño e implementación del módulo de preprocesado de datos del

[12] J. Torres et al, "Signal integrity studies at optical multiplexer board for tilecal system",

[14] Jim Lipman, "Models make the difference in high-speed pc-board design", Electronic

[13] Lynne Green, "Signal Integrity", IEEE Circuits and Devices, November 1999.

Optical Data Communications (Analog circuits and signal processing)", Springer,

Sistema Read Out Driver para el calorímetro TileCal del experimento ATLAS/LHC del CERN", Tesis Doctoral, Servicio de Publicaciones de la Universidad de

[4] Lattice Semi, "High-Speed PCB Design Considerations", Technical Note TB1033, 2011.

[5] Altera, "High-Speed Board Layout Guidelines", Application Note 224, 2009.

Requirement document. Version 2.0", DAQ-NO-103, 1998.

numerical tools are of particular interest for proper routing design in PCBs.

including communications, medicine, nuclear research, etc.

Hall PTR, 2003. ISBN: 0-131-41884-X

Interscience-IEEE, 2000, ISBN: 0-7803-5376-5

2004, ISBN: 1-58053-131-8

2004. ISBN: 0-521-62163-1

2010. ISBN: 978-90-481-7473-7

note, ATLAS-ELEC-1, July 1998

2007 IOP Publishing Ltd and SISSA.

Design News, 15th April 1999.

Valencia, Junio 2005.

9780471712336

present in some cases.

**6. References**

The need for temperature measurement exists in many applications such as in automated consumer products, automated production plants and high performance processors. Recent works have mainly focused on temperature sensors that satisfy user requirements for specific applications, and the main considerations are performance, dimension and reliability. In fact, traditional low-cost solutions, such as thermocouples and resistance temperature detectors (RTDs), do not always yield satisfactory performance, e.g., when the fluid temperature has to be measured in hostile environments, in the presence of electromagnetic, chemical, and mechanical disturbances. Since signals from the thermoelectric sensors are normally mixed with intrinsic noise and extrinsic interferences, they may contain intolerable errors if not properly filtered. Therefore, this type of sensors is inept for gauging temperature in microfluidic or nano-sized devices, in extreme marine environments, and underground geological sites where long distance measurement with precision is required. For such applications, fiber optical sensors offer a better alternative since the optical signal does not suffer from interference by electromagnetic fields and can be transmitted over extremely long distances without any significant loss [Yasin et al., 2010; Li et al., 2010; Ahmad et al., 2009; Lim et al., 2009; Ahmad et al., 2009; You et al., 2005; Xu et al., 2005]. Furthermore, they are relatively small in size, and compatible with other optical fiber devices.

To date, various types of fiber optic temperature sensors have been reported in the literatures and they are mostly based on fiber interferometric [Choi et al., 2008] and fiber Bragg grating (FBG) [Han et al., 2004]. However, the first type of sensors are rather expensive to produce and complicated to implement on-site [Golnabi, 2000]. Fiber Bragg gratings are very efficient at temperature sensing and are easy to implement; however, they always need additional techniques to discriminate the Bragg shifts by temperature and by strain/compression and they also require expensive phase-masks. In this chapter, a temperature sensor is demonstrated based on four different techniques; intensity modulated

Fiber Optic Temperature Sensors 363

4

3

50:50 Fiber coupler

Fig. 1. Experimental setup for the proposed temperature sensor using a POF-based coupler thermocouple has a resolution of 10C and a temperature range of -500C to 13000C. The heat to the aluminum rod is controlled by varying the heat intensity produced by the hotplate

(Display)

Fig. 2 show the efficiency of the FODS as a function of displacement obtained both experimentally and theoretically without the temperature effect. The characteristic of the proposed sensor can be compared with the case of coupling two similar collinear fibers which are separated at the end-faces and with both axis aligned as discussed in [Yang et al., 2011]. Given that the the distance between the parallel end-faces is called z, a and NA is the radius and numerical aperture of the fiber respectively, the efficiency η for small values of

The fiber receives the maximum light when the gap between the tip of fiber probe and the reflected surface is zero, and thus the measured intensity of the reflected light is maxima as shown in the figure. However, the measured intensity of the reflected light decreases almost linearly as the distance or gap increases especially for close distance target. Theoretically, the distance and the reflected power vary according to the inverse square law and the ratio between the reflected power and the transmitted power is given by

�(��)� �������(��) � ���� � ��� � (1)

Hotplate

Aluminum rod

Fiber probe

ranging from room temperature (250C) to 900C.

Silicon detector

Chopper

Lock-in

����

� �

amplifier Computer

2

1

2

z/a is [Van Etten, 1991]

[Kulkarini et al., 2006]

for z/a <<1

A He-Ne Laser

Modulator driver

fiber optic displacement sensor (FODS), lifetime measurements, microfiber loop resonator (MLR) and stimulated brillouin scattering. The first sensor is based on a rugged, low cost and very efficient FODS utilizing a plastic optical fiber (POF)-based coupler as a probe and a linear thermal expansion of aluminum. The second temperature sensor, which is based on fluorescence decay time in Erbium-doped silica fiber has the advantage of incorporating a time based encoding system, which is less sensitive to system losses such as those associated with optical cables and connectors. The MLR is formed by coiling a microfiber, which was obtained by heating and stretching a piece of standard silica single-mode fiber (SMF). The MLR is embedded in a low refractive index material for use in temperature measurement. The MLR-based temperature sensor has a low loss splicing with a standard SMF. Lastly, a temperature sensor is demonstrated using an SBS effect, which requires measurement of frequency shift. In the proposed sensor, a Brillouin pump is injected into one end of a ring cavity resonator, in which a sensing fiber is located, and then the frequency shift between the BP and the Brillouin fiber laser (BFL) output is measured using a heterodyne method.

## **2. FODS based temperature sensing**

POFs have widespread uses in the transmission and processing of optical signals for optical fiber communication system compatible with the Internet. POFs also have potential applications in WDM systems, power splitters and couplers, amplifiers, sensors, scramblers, integrated optical devices, frequency up-conversion, and etc. [Yasin et al., 2009; Yang et al., 2011]. Recently, an intensity modulated FODSs have been demonstrated to be efficient for many applications including sensor. They are relatively inexpensive, easy to fabricate and suitable for deployment in harsh environments. In this section, a low cost temperature sensor is demonstrated using POF-based coupler as a probe based on a linear thermal expansion of aluminum. The temperature sensor is schematically shown in Fig. 1. The sensor is essentially a FODS with a 3 dB multimode fiber coupler as a probe. A 594 nm He-Ne beam is launched into port 1 of the coupler. Light travels to port 3 and is scattered when it exits the fiber end. It is then reflected by the top surface of an aluminium rod with dimensions of 0.5 cm diameter and 7 cm length. The port 3 probe is held in position about 1 mm perpendicular to the top surface of the aluminium rod so that the reflected light can be easily launched back into the same port. The collected light is sent to port 2 by the 3dB coupler and measured by a silicon photo-detector. The detector converts the light into electrical signal, which is then processed by the lock-in amplifier and finally displayed and stored onto the computer.

In the calibration stage, the static operating range of the probe is identified and this process requires the probe to be mounted on a translational stage, which is rigidly attached to a vibration free table. Firstly, the output from port 2 at zero point is measured, where the aluminum rod and the probe are in close contact. Then the aluminum rod is moved away from the probe in 50 µm steps and at each position, under vibrationless condition, the output voltage is recorded. A graph of displacement (gap) against output voltage is drawn and a linear range on the graph is identified. A position at the center of the linear range is chosen and the gap between the probe and the top surface of the aluminum rod is fixed at the chosen displacement point. Then an experiment is carried out where the aluminum is fixed onto a hotplate for heating purpose. A thermocouple placed at the upper region of the aluminum rod is used to display and monitor the temperature of the aluminum rod. The

fiber optic displacement sensor (FODS), lifetime measurements, microfiber loop resonator (MLR) and stimulated brillouin scattering. The first sensor is based on a rugged, low cost and very efficient FODS utilizing a plastic optical fiber (POF)-based coupler as a probe and a linear thermal expansion of aluminum. The second temperature sensor, which is based on fluorescence decay time in Erbium-doped silica fiber has the advantage of incorporating a time based encoding system, which is less sensitive to system losses such as those associated with optical cables and connectors. The MLR is formed by coiling a microfiber, which was obtained by heating and stretching a piece of standard silica single-mode fiber (SMF). The MLR is embedded in a low refractive index material for use in temperature measurement. The MLR-based temperature sensor has a low loss splicing with a standard SMF. Lastly, a temperature sensor is demonstrated using an SBS effect, which requires measurement of frequency shift. In the proposed sensor, a Brillouin pump is injected into one end of a ring cavity resonator, in which a sensing fiber is located, and then the frequency shift between the BP and the Brillouin fiber laser (BFL) output is measured using a heterodyne method.

POFs have widespread uses in the transmission and processing of optical signals for optical fiber communication system compatible with the Internet. POFs also have potential applications in WDM systems, power splitters and couplers, amplifiers, sensors, scramblers, integrated optical devices, frequency up-conversion, and etc. [Yasin et al., 2009; Yang et al., 2011]. Recently, an intensity modulated FODSs have been demonstrated to be efficient for many applications including sensor. They are relatively inexpensive, easy to fabricate and suitable for deployment in harsh environments. In this section, a low cost temperature sensor is demonstrated using POF-based coupler as a probe based on a linear thermal expansion of aluminum. The temperature sensor is schematically shown in Fig. 1. The sensor is essentially a FODS with a 3 dB multimode fiber coupler as a probe. A 594 nm He-Ne beam is launched into port 1 of the coupler. Light travels to port 3 and is scattered when it exits the fiber end. It is then reflected by the top surface of an aluminium rod with dimensions of 0.5 cm diameter and 7 cm length. The port 3 probe is held in position about 1 mm perpendicular to the top surface of the aluminium rod so that the reflected light can be easily launched back into the same port. The collected light is sent to port 2 by the 3dB coupler and measured by a silicon photo-detector. The detector converts the light into electrical signal, which is then processed by the lock-in amplifier and finally displayed and

In the calibration stage, the static operating range of the probe is identified and this process requires the probe to be mounted on a translational stage, which is rigidly attached to a vibration free table. Firstly, the output from port 2 at zero point is measured, where the aluminum rod and the probe are in close contact. Then the aluminum rod is moved away from the probe in 50 µm steps and at each position, under vibrationless condition, the output voltage is recorded. A graph of displacement (gap) against output voltage is drawn and a linear range on the graph is identified. A position at the center of the linear range is chosen and the gap between the probe and the top surface of the aluminum rod is fixed at the chosen displacement point. Then an experiment is carried out where the aluminum is fixed onto a hotplate for heating purpose. A thermocouple placed at the upper region of the aluminum rod is used to display and monitor the temperature of the aluminum rod. The

**2. FODS based temperature sensing**

stored onto the computer.

Fig. 1. Experimental setup for the proposed temperature sensor using a POF-based coupler

thermocouple has a resolution of 10C and a temperature range of -500C to 13000C. The heat to the aluminum rod is controlled by varying the heat intensity produced by the hotplate ranging from room temperature (250C) to 900C.

Fig. 2 show the efficiency of the FODS as a function of displacement obtained both experimentally and theoretically without the temperature effect. The characteristic of the proposed sensor can be compared with the case of coupling two similar collinear fibers which are separated at the end-faces and with both axis aligned as discussed in [Yang et al., 2011]. Given that the the distance between the parallel end-faces is called z, a and NA is the radius and numerical aperture of the fiber respectively, the efficiency η for small values of z/a is [Van Etten, 1991]

$$\eta = 1 - \frac{z}{a} \frac{2}{\pi (NA)^2} \left[ \arcsin(NA) - NA \sqrt{1 - NA^2} \right] \tag{1}$$

for z/a <<1

The fiber receives the maximum light when the gap between the tip of fiber probe and the reflected surface is zero, and thus the measured intensity of the reflected light is maxima as shown in the figure. However, the measured intensity of the reflected light decreases almost linearly as the distance or gap increases especially for close distance target. Theoretically, the distance and the reflected power vary according to the inverse square law and the ratio between the reflected power and the transmitted power is given by [Kulkarini et al., 2006]

Fiber Optic Temperature Sensors 365

1.20 mV. The output signal starts to significantly increase with increasing rod temperature at 420C, which henceforward forms a linear function until the rod temperature reaches 900C. It is observed that the maximum difference between the two runs is about 0.04 mV, which is small compared to the full range of 1.20 mV. The temperature sensor is observed to be very stable with the measurement error of less than 0.8% which is obtained at 900C (see inset of Fig. 3). The output voltage are recorded for 200 seconds and the standard deviation obtained at 250C, 600C and 900C are 0.5%, 0.5% and 0.8% respectively. The performance of the temperature sensor is summarized in Table 2 where the sensitivity of the linear function for the first run is 0.0044 mV/0C with 98% linearity whereas the sensitivity of the linear

Fig. 3. Linear relationship between the output voltage and aluminum rod temperature for 250C to 900C. Inset shows the standard deviation values for output voltage versus rod

Sensitivity 0.0044 mV/oC 0.0041 mV/oC Linear Range 42 - 90 oC 42 - 90 oC Linearity More than 98% More than 96% Standard Deviation ± 0.01 mV ± 0.01 mV Resolution 2.4 oC 2.4 oC

Table 2. The performance of the temperature sensor using FODS.

**Parameter Value (Run No. 1) Value (Run No. 2)**

The temperature sensor has a higher sensitivity (0.0044 mV/0C) compared to the displacement sensor (0.0005 mV/μm) even though the initial gap between the aluminum rod and fiber tip of port 3 is fixed at 1 mm, which is within the linear range of the displacement response without temperature effect. This is because as temperature increase,

function for the second run is 0.0041 mV/0C with 96% linearity.

temperature at 250C, 600C and 900C.

$$\frac{P\_r}{P\_t} = \frac{d^2}{(2\pi \tan \theta)^2} \tag{2}$$

where Pr, Pt, d, x, and θ are the reflected power, transmitted power, core diameter, axial displacement and fiber's acceptance angle, respectively. The characteristic of the displacement curve is summarized in Table 1 where the sensitivity is obtained at 0.0005 mV/μm and the slope shows a good linearity of more than 99% within the displacement range of 1400 µm. The displacement sensor is observed to be very stable with the measurement error of less than 0.8%.

Fig. 2. Efficiency as a function of displacement obtained both experimentally and theoretically.

Table 1. The Performance of fiber optic displacement sensor using the aluminum rod.

Fig. 3 show the linear function of the output signal against the aluminum rod temperature for two different runs. The two different runs were taken because repeatibility of results is a crucial factor in the operation of any sensor system. In the experiment, the gap between the aluminum rod and fiber tip of port 3 is fixed at 1 mm, which is within the linear range of the displacement response without the temperature effect. The temperature of the aluminum rod is then varied from 250C to 900C, resulting in an output signal ranging from 0.95 mV to

��� ��� ��� (2)

<sup>=</sup> ��

where Pr, Pt, d, x, and θ are the reflected power, transmitted power, core diameter, axial displacement and fiber's acceptance angle, respectively. The characteristic of the displacement curve is summarized in Table 1 where the sensitivity is obtained at 0.0005 mV/μm and the slope shows a good linearity of more than 99% within the displacement range of 1400 µm. The displacement sensor is observed to be very stable with the

�� ��

Fig. 2. Efficiency as a function of displacement obtained both experimentally and

Sensitivity 0.0005 mV/µm Linear Range 0 – 1400 µm Linearity More than 99% Standard Deviation 0.01 mV (0.8%) Resolution 19 µm

**Parameter Value**

0 2 4 6 8 10 12

experiment

theory

**Displacement (z/a)**

Table 1. The Performance of fiber optic displacement sensor using the aluminum rod.

Fig. 3 show the linear function of the output signal against the aluminum rod temperature for two different runs. The two different runs were taken because repeatibility of results is a crucial factor in the operation of any sensor system. In the experiment, the gap between the aluminum rod and fiber tip of port 3 is fixed at 1 mm, which is within the linear range of the displacement response without the temperature effect. The temperature of the aluminum rod is then varied from 250C to 900C, resulting in an output signal ranging from 0.95 mV to

measurement error of less than 0.8%.

0

0.2

0.4

**Efficiency,**

**η**

**(V/Vmax)**

0.6

0.8

1

theoretically.

1.20 mV. The output signal starts to significantly increase with increasing rod temperature at 420C, which henceforward forms a linear function until the rod temperature reaches 900C. It is observed that the maximum difference between the two runs is about 0.04 mV, which is small compared to the full range of 1.20 mV. The temperature sensor is observed to be very stable with the measurement error of less than 0.8% which is obtained at 900C (see inset of Fig. 3). The output voltage are recorded for 200 seconds and the standard deviation obtained at 250C, 600C and 900C are 0.5%, 0.5% and 0.8% respectively. The performance of the temperature sensor is summarized in Table 2 where the sensitivity of the linear function for the first run is 0.0044 mV/0C with 98% linearity whereas the sensitivity of the linear function for the second run is 0.0041 mV/0C with 96% linearity.

Fig. 3. Linear relationship between the output voltage and aluminum rod temperature for 250C to 900C. Inset shows the standard deviation values for output voltage versus rod temperature at 250C, 600C and 900C.

Table 2. The performance of the temperature sensor using FODS.

The temperature sensor has a higher sensitivity (0.0044 mV/0C) compared to the displacement sensor (0.0005 mV/μm) even though the initial gap between the aluminum rod and fiber tip of port 3 is fixed at 1 mm, which is within the linear range of the displacement response without temperature effect. This is because as temperature increase,

Fiber Optic Temperature Sensors 367

WDM

Heater

EDF

Digital Oscilloscope

Fig. 4. Experiment setup of fiber optic high temperature sensor.

Fast Photo-diode Detector (Ge)

980nm

**Output Power (dBm)**

Pulsed laser Diode 980nm

Modulator

Fig. 5. Output spectrum of the generated ASE with 980nm pump power of 31mW.

approximately 2 mW and the results for temperature measurements of 85oC and 130oC are shown in Figs. 6(a) and (b), respectively. Fitting the exponential curve produces lifetimes of 4.85 and 4.75 ms for temperatures of 85oC and 130oC accordingly. The fluorescence lifetime data taken over the range of 25oC to 160oC are presented in Fig. 7. The graph shows the existence of an inverse linear relationship between the Erbium lifetime and the temperature. The fluorescence lifetime data shown in Fig. 7 are mean values of consecutive measurements at corresponding stabilized temperature. The sensitivity of the sensor is obtained at 0.009 ms/oC with a linearity more than 94%. The lifetime reduction is attributed to the quenching of the Erbium luminescence, which results in less efficient excitation. The quenching is mainly due to a decrease in the absorption coefficient of Erbium ion as the temperature is increased.

1480 1500 1520 1540 1560 1580

**Wavelength (nm)**

the reflectivity of aluminum will also increase [Guo et al., 2003], hence sensitivity increases when increasing temperature is applied to the aluminum rod. The possible sources of error in the sensor operation can be due to light source fluctuation, stray light and possible mechanical vibrations. To reduce these effects a well-regulated power supply is used for the 594 nm He-Ne laser and this minimizes the fluctuation of source intensity. The sensor fixture is also designed so that the stray light cannot interfere with the source light and room light does not have any effect on the output voltage. To reduce the mechanical vibrations, the experimental set-up is arranged on a vibration free table

### **3. Fiber optic temperature sensor based on lifetime measurement**

Fluorescence-based sensors are widely used for measuring various parameters due to its relatively independent of ambient conditions. This approach is widely used for temperature sensor, which is normally based on the detection of fluorescence lifetimes in various rareearth-doped silica fibers [Zhang et al., 2009; Lopez et al., 2004; Seat et al., 2002; Baek et al., 2006]. In principle, fluorescence is induced by pump power at a certain wavelength that is suitable for the doped ions, and only a straightforward detection procedure is needed. An additional advantage of such rare-earth-doped fiber sensors is that they are compatible with a wide range of existing fiber-optic multiplexing schemes that can simultaneously detect multiple physical parameters. The underlying principle behind the ability of the rare-earth doped materials to be used as temperature sensors [McSherry et al., 2005] is their properties of emission and absorption that are dependent on the temperature. This behavior is due to the homogeneous broadening of the line width and the changing population of the energy levels with temperature. In the earlier work, a remote temperature sensor has been proposed using fluorescence intensity-ratio technique [Castrellon-Uribe, 2005]. In this section, a temperature sensor is proposed based on fluorescence decay time in Erbiumdoped fiber (EDF).

Fig. 4 shows the schematic diagram of the proposed sensor set-up. The 980nm laser pump beam is launched into a piece of 90 cm long Erbium-doped fiber (EDF) via a wavelength division multiplexing (WDM) coupler. The EDF is placed in a vacuum oven which allows us to vary the fiber temperature within 25 to 200oC interval. The fluorescence signal from the forward pumped EDF is detected by a Ge photo-detector and processed with a digital oscilloscope. The 980 nm pump beam is chopped so as to generate a square-wave modulated signal with pulse width of 2.2ms, peak power of approximately 124mW and frequency of 45Hz. When the erbium-doped fiber is pumped with the photon energy of 980 nm, the 4I11/2 erbium level is excited and the 4I13/2 metastable level is quasi-instantaneously populated due to the non-radiative transition. The population inversion between 4I13/2 and 4I11/2 level is responsible for the emission of fluorescence at around 1550 nm. When the EDF is pumped at a fixed rate, the fluorescence variation including a lifetime change can reflect corresponding temperature. The temperature dependent fluorescence lifetimes of the spontaneous emission of the EDF is investigated and studied in this work.Fig. 5 shows the output spectrum of the amplified spontaneous emission (ASE) of the forward pumped EDF when the continuous wave pump power is fixed at 31 mW. The ASE spectrum peaks at 1529 nm with the average power of around -58 dBm. The temperature sensing mechanism in this work is based on the temperature dependence of the Erbium fluorescent lifetime decay. The Erbium fluorescence lifetime is measured using a modulated pump laser with power of

the reflectivity of aluminum will also increase [Guo et al., 2003], hence sensitivity increases when increasing temperature is applied to the aluminum rod. The possible sources of error in the sensor operation can be due to light source fluctuation, stray light and possible mechanical vibrations. To reduce these effects a well-regulated power supply is used for the 594 nm He-Ne laser and this minimizes the fluctuation of source intensity. The sensor fixture is also designed so that the stray light cannot interfere with the source light and room light does not have any effect on the output voltage. To reduce the mechanical vibrations,

Fluorescence-based sensors are widely used for measuring various parameters due to its relatively independent of ambient conditions. This approach is widely used for temperature sensor, which is normally based on the detection of fluorescence lifetimes in various rareearth-doped silica fibers [Zhang et al., 2009; Lopez et al., 2004; Seat et al., 2002; Baek et al., 2006]. In principle, fluorescence is induced by pump power at a certain wavelength that is suitable for the doped ions, and only a straightforward detection procedure is needed. An additional advantage of such rare-earth-doped fiber sensors is that they are compatible with a wide range of existing fiber-optic multiplexing schemes that can simultaneously detect multiple physical parameters. The underlying principle behind the ability of the rare-earth doped materials to be used as temperature sensors [McSherry et al., 2005] is their properties of emission and absorption that are dependent on the temperature. This behavior is due to the homogeneous broadening of the line width and the changing population of the energy levels with temperature. In the earlier work, a remote temperature sensor has been proposed using fluorescence intensity-ratio technique [Castrellon-Uribe, 2005]. In this section, a temperature sensor is proposed based on fluorescence decay time in Erbium-

Fig. 4 shows the schematic diagram of the proposed sensor set-up. The 980nm laser pump beam is launched into a piece of 90 cm long Erbium-doped fiber (EDF) via a wavelength division multiplexing (WDM) coupler. The EDF is placed in a vacuum oven which allows us to vary the fiber temperature within 25 to 200oC interval. The fluorescence signal from the forward pumped EDF is detected by a Ge photo-detector and processed with a digital oscilloscope. The 980 nm pump beam is chopped so as to generate a square-wave modulated signal with pulse width of 2.2ms, peak power of approximately 124mW and frequency of 45Hz. When the erbium-doped fiber is pumped with the photon energy of 980 nm, the 4I11/2 erbium level is excited and the 4I13/2 metastable level is quasi-instantaneously populated due to the non-radiative transition. The population inversion between 4I13/2 and 4I11/2 level is responsible for the emission of fluorescence at around 1550 nm. When the EDF is pumped at a fixed rate, the fluorescence variation including a lifetime change can reflect corresponding temperature. The temperature dependent fluorescence lifetimes of the spontaneous emission of the EDF is investigated and studied in this work.Fig. 5 shows the output spectrum of the amplified spontaneous emission (ASE) of the forward pumped EDF when the continuous wave pump power is fixed at 31 mW. The ASE spectrum peaks at 1529 nm with the average power of around -58 dBm. The temperature sensing mechanism in this work is based on the temperature dependence of the Erbium fluorescent lifetime decay. The Erbium fluorescence lifetime is measured using a modulated pump laser with power of

the experimental set-up is arranged on a vibration free table

doped fiber (EDF).

**3. Fiber optic temperature sensor based on lifetime measurement**

Fig. 4. Experiment setup of fiber optic high temperature sensor.

Fig. 5. Output spectrum of the generated ASE with 980nm pump power of 31mW.

approximately 2 mW and the results for temperature measurements of 85oC and 130oC are shown in Figs. 6(a) and (b), respectively. Fitting the exponential curve produces lifetimes of 4.85 and 4.75 ms for temperatures of 85oC and 130oC accordingly. The fluorescence lifetime data taken over the range of 25oC to 160oC are presented in Fig. 7. The graph shows the existence of an inverse linear relationship between the Erbium lifetime and the temperature. The fluorescence lifetime data shown in Fig. 7 are mean values of consecutive measurements at corresponding stabilized temperature. The sensitivity of the sensor is obtained at 0.009 ms/oC with a linearity more than 94%. The lifetime reduction is attributed to the quenching of the Erbium luminescence, which results in less efficient excitation. The quenching is mainly due to a decrease in the absorption coefficient of Erbium ion as the temperature is increased.

Fiber Optic Temperature Sensors 369

opportunities for developing micro-photonic devices such as resonators [Jiang et al., 2006], couplers [Tong et al., 2006], and sensors [Li & Tong, 2006]. As one of the basic functional elements, microfiber resonators, in forms of loops, knots, or coils, have shown high intrinsic optical quality and have exhibited promising applications such as filters and lasers [Jiang et al., 2006]. Microfiber-based sensors are amongst the simplest of such devices, as they do not require expensive and complex fabrication procedures. Recently, strain, high temperature and refractive index sensing using tapered micro-structured optical fiber has been reported [Nguyen et al., 2005; Minkovich et al., 2005; Villatoro et al., 2006]. However, the authors used tapered micro-structured fiber which is expensive and difficult to work with in term of

A microfiber fabrication set-up is shown in Fig. 8. A coating removed standard SMF was held by two fiber holders which were fixed on two translation stages. One stage is fixed and another stage is a motorized stage that can be moved in one dimensional direction and the speed of the motor can be controlled. The fiber ends were connected to an amplified spontaneous emission (ASE) source and an optical spectrum analyzer (OSA). The microfiber was fabricated by heating the fiber to its softening temperature, and then pulling the ends apart to reduce the fiber's diameter down to a around 1-3 mm. A high temperature and stable micro-burner fueled by clean butane gas is used in the tapering. Mounted on the movable stand, the micro-burner can be swung vertically to flame brush the tapered fiber. The flame should be clean and the burning gas flow should be controlled carefully so that the air convection does not break the fiber during the drawing process. During the tapering process, we monitored both the inter-modal interference and the insertion loss of the fiber using the ASE source in conjunction with the OSA. The flame is applied to the fiber in an

getting low loss splicing with standard optical fiber.

optimized angle to make sure the heat distribution is homogeneous.

Fig. 8. Schematic diagram of the microfiber fabrication set-up.

The MLR is fabricated by coiling the microfiber onto itself using two surface attractions, van der Waals force and electrostatic force, which keep the loop stable since the forces overcome the elastic force to make the fiber straight. The fabricated MLR is laid on an earlier prepared

Fig. 6. Fluorescence lifetime decay of EDF at (a) 85 oC and (b) 130 oC from a digital oscilloscope.

Fig. 7. Lifetime of EDF as function of Temperature.

#### **4. Microfiber loop resonator based temperature sensor**

Optical ring resonators have attracted considerable attention owing to their simple structure, compact size, and many applications in communication, optical signal processing and optical devices [Amarnath et al., 2005; Harun et al., 2010]. Planar waveguide micro-ring resonators have been well developed, but suffer from larger connection-losses with fibers and higher cost. Recently, research on low-loss microfibers has opened up new

Parameter: Yo=16.58206; Xo=-43.43441; A1=6.98419; t1=4.75061; R2=0.92964

(a) 85 oC (b) 130 oC

Parameter: Yo=16.56105; Xo=-44.04324; A1=7.72526; t1=4.8562; R2=0.94302

Fig. 7. Lifetime of EDF as function of Temperature.

**4. Microfiber loop resonator based temperature sensor**

Optical ring resonators have attracted considerable attention owing to their simple structure, compact size, and many applications in communication, optical signal processing and optical devices [Amarnath et al., 2005; Harun et al., 2010]. Planar waveguide micro-ring resonators have been well developed, but suffer from larger connection-losses with fibers and higher cost. Recently, research on low-loss microfibers has opened up new

oscilloscope.

Fig. 6. Fluorescence lifetime decay of EDF at (a) 85 oC and (b) 130 oC from a digital

opportunities for developing micro-photonic devices such as resonators [Jiang et al., 2006], couplers [Tong et al., 2006], and sensors [Li & Tong, 2006]. As one of the basic functional elements, microfiber resonators, in forms of loops, knots, or coils, have shown high intrinsic optical quality and have exhibited promising applications such as filters and lasers [Jiang et al., 2006]. Microfiber-based sensors are amongst the simplest of such devices, as they do not require expensive and complex fabrication procedures. Recently, strain, high temperature and refractive index sensing using tapered micro-structured optical fiber has been reported [Nguyen et al., 2005; Minkovich et al., 2005; Villatoro et al., 2006]. However, the authors used tapered micro-structured fiber which is expensive and difficult to work with in term of getting low loss splicing with standard optical fiber.

A microfiber fabrication set-up is shown in Fig. 8. A coating removed standard SMF was held by two fiber holders which were fixed on two translation stages. One stage is fixed and another stage is a motorized stage that can be moved in one dimensional direction and the speed of the motor can be controlled. The fiber ends were connected to an amplified spontaneous emission (ASE) source and an optical spectrum analyzer (OSA). The microfiber was fabricated by heating the fiber to its softening temperature, and then pulling the ends apart to reduce the fiber's diameter down to a around 1-3 mm. A high temperature and stable micro-burner fueled by clean butane gas is used in the tapering. Mounted on the movable stand, the micro-burner can be swung vertically to flame brush the tapered fiber. The flame should be clean and the burning gas flow should be controlled carefully so that the air convection does not break the fiber during the drawing process. During the tapering process, we monitored both the inter-modal interference and the insertion loss of the fiber using the ASE source in conjunction with the OSA. The flame is applied to the fiber in an optimized angle to make sure the heat distribution is homogeneous.

Fig. 8. Schematic diagram of the microfiber fabrication set-up.

The MLR is fabricated by coiling the microfiber onto itself using two surface attractions, van der Waals force and electrostatic force, which keep the loop stable since the forces overcome the elastic force to make the fiber straight. The fabricated MLR is laid on an earlier prepared

Fiber Optic Temperature Sensors 371

A MLR has a comb transmission spectrum similar to that of a Fabry–Perot filter. The resonant wavelength must satisfies *λm* = 2π*Rn*eff/*m*, where *R* is the radius of the ring, *neff* is the effective index of the ring, *m* is the resonant mode number. The resonant frequency

����� <sup>=</sup> �

� =

extinction ratio is about 4.0 dB, and the FSR is 0.08 nm.

Fig. 10. Transmission spectrum of the fabricated MLR

�������

where *c* is the velocity of light in vacuum. The quality factor of a ring resonator can be

�� Δ�����

where Δλ*FWHM* is the 3 dB bandwidth of the resonant peak. The lower the coupling coefficient between the ring and straight waveguide, the higher the *Q* is. If the radius and the coupling coefficient of the ring are chose properly, the desired resonant frequency spacing and bandwidth of the resonant peak can be obtained. The measured comb transmission spectrum of the MLR is shown in Fig. 10, which was obtained by using the ASE source in conjunction with OSA. The resonant response of the MLR is obvious. The

1532 1532.2 1532.4 1532.6 1532.8 1533

**Wavelength (nm)**

(3)

(4)

spacing, i.e. Free Spectral Range (FSR), is given by [Sumetsky et al., 2006];

expressed as

**Transmitted power (dBm)**

glass plate with a thin and flat layer of low refractive index material to address the temporal stability of the device as shown in Fig. 9. The thickness of the low refractive index material is approximately 0.5mm which is thick enough to prevent leakage of optical power from the microfiber to the glass plate. Some uncured resin is also applied on surrounding the MLR before it is sandwiched by another glass plate with the same low refractive index resin layer from the top. It is essentially important to ensure that minimum air bubbles and impurity are trapped around the fiber area between the two plates. This is to prevent refractive index non-uniformity in the surrounding of microfiber that may introduce loss to the system. The uncured resin is solidified by the UV light exposure for 3 ~7 minutes and the optical properties of the MLR are stabilized. The packaged MLR is located in an oven with temperature control to investigate how the comb spectrum changes with the temperature.

(b) Snapshot Fig. 9. Packaging of the MLR (a) schematic diagram (b) snapshot

glass plate with a thin and flat layer of low refractive index material to address the temporal stability of the device as shown in Fig. 9. The thickness of the low refractive index material is approximately 0.5mm which is thick enough to prevent leakage of optical power from the microfiber to the glass plate. Some uncured resin is also applied on surrounding the MLR before it is sandwiched by another glass plate with the same low refractive index resin layer from the top. It is essentially important to ensure that minimum air bubbles and impurity are trapped around the fiber area between the two plates. This is to prevent refractive index non-uniformity in the surrounding of microfiber that may introduce loss to the system. The uncured resin is solidified by the UV light exposure for 3 ~7 minutes and the optical properties of the MLR are stabilized. The packaged MLR is located in an oven with temperature control to investigate how the comb spectrum changes with the temperature.

(a) diagram

(b) Snapshot

Fig. 9. Packaging of the MLR (a) schematic diagram (b) snapshot

A MLR has a comb transmission spectrum similar to that of a Fabry–Perot filter. The resonant wavelength must satisfies *λm* = 2π*Rn*eff/*m*, where *R* is the radius of the ring, *neff* is the effective index of the ring, *m* is the resonant mode number. The resonant frequency spacing, i.e. Free Spectral Range (FSR), is given by [Sumetsky et al., 2006];

$$
\Delta\nu\_{FSR} = \frac{c}{2\pi R n\_{eff}}\tag{3}
$$

where *c* is the velocity of light in vacuum. The quality factor of a ring resonator can be expressed as

$$Q = \frac{\lambda\_m}{\Delta \lambda\_{\text{FWHM}}} \tag{4}$$

where Δλ*FWHM* is the 3 dB bandwidth of the resonant peak. The lower the coupling coefficient between the ring and straight waveguide, the higher the *Q* is. If the radius and the coupling coefficient of the ring are chose properly, the desired resonant frequency spacing and bandwidth of the resonant peak can be obtained. The measured comb transmission spectrum of the MLR is shown in Fig. 10, which was obtained by using the ASE source in conjunction with OSA. The resonant response of the MLR is obvious. The extinction ratio is about 4.0 dB, and the FSR is 0.08 nm.

Fig. 10. Transmission spectrum of the fabricated MLR

Fiber Optic Temperature Sensors 373

extinction ratio against the temperature. As seen, the slope of the extinction ratio reduction against temperature was about 0.043dB/oC. The dependence of the extinction ratio on temperature is due to the change in the material's refractive index, which increase the loss at higher temperature and thus reduces the extinction ratio. In the proposed sensor, the optical path length is not much changed with the temperature and therefore, the comb spacing is

Brillouin scattering is an effect caused by the nonlinearity of a medium, whereby an incident pump photon can be converted into a scattered photon of slightly lower energy, usually propagating in the backward direction, and a phonon. At high optical pump powers, the Stimulated Brillouin scattering (SBS) is one of the most important nonlinear effects in fibers and has a significant influence on the operation of optical transmission systems as well as fiber sensors. Brillouin-based optical fiber sensors have been widely reported in recent years owing to their possibilities to perform distributed strain and temperature sensing along an optical fiber [Bao et al., 1993; Nikles et al., 1996; Culverhouse et al., 2008; Kee et al., 2002; Rathod et al., 1994]. Several techniques have been proposed for effective sensing using either spontaneous Brillouin scattering (SpBS) or stimulated Brillouin scattering (SBS) effects. Brillouin reflectometers are the most widely exploited in Brillouin-based optical sensing due to their simplicity. They are implemented by injecting a light beam into one of the sensing fibers' ends and are not affected by nonlocal effect, thus appearing more suitable for long range sensing applications. However, these type of sensors suffer from a small amplitude of

The configuration of the proposed sensor is shown in Fig. 13. It consists of a Brillouin pump (BP), an Erbium-doped fiber amplifier (EDFA), a ring cavity Brillouin gain block and a frequency shift measurement setup. The gain block consists of a piece of sensing fiber and a 3 dB coupler, which the Brillouin gain oscillates in an anti-clockwise direction to generate a BFL. The sensing fiber is located in an oven with temperature control and is shown in the dashed square in Fig. 13. An external cavity tunable laser source (TLS) with a line-width of approximately 20MHz is used as the BP. The BP light operating at 1555 nm is split into reference and probe signals, by using a 10dB coupler. The probe signal from 10% part is optically amplified by an EDFA to provide sufficient power for this study. This amplified signal is launched into a piece of sensing fiber through an optical circulator, which is also used to couple into the photodiode the backward propagating light from the ring cavity, including a BFL operating at wavelength downshifted by 0.08 nm from the BP wavelength. The output Brillouin signal is combined with the reference or BP signal by a 3 dB coupler for Brillouin shift measurement using a heterodyne technique. The combined optical signal is converted into an electrical signal by a phodiode which is connected to an RF spectrum

To compare the performance achieved by various sensing fibers, a standard single-mode fiber (SMF), non-zero dispersion shifted fiber (NZ-DSF) and photonic crystal fiber (PCF) are alternatively employed as a gain medium in the BFL. The lengths of SMF, NZ-DSF and PCF are fixed at 25 km, 10 km and 100 m, respectively. The NZ-DSF has a core diameter of 4.2 μm and cut-off wavelength of 1150nm. The PCF has a triangular core with average diameter of 2.1 ± 0.3 μm and cladding diameter of 128 ± 5 μm. The average air hole diameter of the

**5. Stimulated Brillouin scattering based temperature sensor**

the signal due to the use of spontaneous Brillouin scattering.

unchanged as shown in Fig. 11.

analyzer.

The temperature response of the packaged MLR is then investigated. Fig. 11 shows the transmission spectrum of another packaged MLR at various temperatures. As shown in this figure, the spacing of the transmission comb spectrum is unchanged with the temperature. However, we observed a linear dependence of the extinction ratio of the MLR on temperature; higher temperature MLRs had a smaller extinction ratio. Fig. 12 shows an

Fig. 11. Transmission spectra of the fabricated MLR obtained at various temperatures

Fig. 12. Extinction ratio of the MLR as a function of the temperature

The temperature response of the packaged MLR is then investigated. Fig. 11 shows the transmission spectrum of another packaged MLR at various temperatures. As shown in this figure, the spacing of the transmission comb spectrum is unchanged with the temperature. However, we observed a linear dependence of the extinction ratio of the MLR on temperature; higher temperature MLRs had a smaller extinction ratio. Fig. 12 shows an

Fig. 11. Transmission spectra of the fabricated MLR obtained at various temperatures

Fig. 12. Extinction ratio of the MLR as a function of the temperature

extinction ratio against the temperature. As seen, the slope of the extinction ratio reduction against temperature was about 0.043dB/oC. The dependence of the extinction ratio on temperature is due to the change in the material's refractive index, which increase the loss at higher temperature and thus reduces the extinction ratio. In the proposed sensor, the optical path length is not much changed with the temperature and therefore, the comb spacing is unchanged as shown in Fig. 11.

## **5. Stimulated Brillouin scattering based temperature sensor**

Brillouin scattering is an effect caused by the nonlinearity of a medium, whereby an incident pump photon can be converted into a scattered photon of slightly lower energy, usually propagating in the backward direction, and a phonon. At high optical pump powers, the Stimulated Brillouin scattering (SBS) is one of the most important nonlinear effects in fibers and has a significant influence on the operation of optical transmission systems as well as fiber sensors. Brillouin-based optical fiber sensors have been widely reported in recent years owing to their possibilities to perform distributed strain and temperature sensing along an optical fiber [Bao et al., 1993; Nikles et al., 1996; Culverhouse et al., 2008; Kee et al., 2002; Rathod et al., 1994]. Several techniques have been proposed for effective sensing using either spontaneous Brillouin scattering (SpBS) or stimulated Brillouin scattering (SBS) effects. Brillouin reflectometers are the most widely exploited in Brillouin-based optical sensing due to their simplicity. They are implemented by injecting a light beam into one of the sensing fibers' ends and are not affected by nonlocal effect, thus appearing more suitable for long range sensing applications. However, these type of sensors suffer from a small amplitude of the signal due to the use of spontaneous Brillouin scattering.

The configuration of the proposed sensor is shown in Fig. 13. It consists of a Brillouin pump (BP), an Erbium-doped fiber amplifier (EDFA), a ring cavity Brillouin gain block and a frequency shift measurement setup. The gain block consists of a piece of sensing fiber and a 3 dB coupler, which the Brillouin gain oscillates in an anti-clockwise direction to generate a BFL. The sensing fiber is located in an oven with temperature control and is shown in the dashed square in Fig. 13. An external cavity tunable laser source (TLS) with a line-width of approximately 20MHz is used as the BP. The BP light operating at 1555 nm is split into reference and probe signals, by using a 10dB coupler. The probe signal from 10% part is optically amplified by an EDFA to provide sufficient power for this study. This amplified signal is launched into a piece of sensing fiber through an optical circulator, which is also used to couple into the photodiode the backward propagating light from the ring cavity, including a BFL operating at wavelength downshifted by 0.08 nm from the BP wavelength. The output Brillouin signal is combined with the reference or BP signal by a 3 dB coupler for Brillouin shift measurement using a heterodyne technique. The combined optical signal is converted into an electrical signal by a phodiode which is connected to an RF spectrum analyzer.

To compare the performance achieved by various sensing fibers, a standard single-mode fiber (SMF), non-zero dispersion shifted fiber (NZ-DSF) and photonic crystal fiber (PCF) are alternatively employed as a gain medium in the BFL. The lengths of SMF, NZ-DSF and PCF are fixed at 25 km, 10 km and 100 m, respectively. The NZ-DSF has a core diameter of 4.2 μm and cut-off wavelength of 1150nm. The PCF has a triangular core with average diameter of 2.1 ± 0.3 μm and cladding diameter of 128 ± 5 μm. The average air hole diameter of the

Fiber Optic Temperature Sensors 375

Fig. 14. Stimulated Brillouin scattering monitored by OSA in minimum and maximum used

υυ

where Δ*vB* is the full-width at half maximum (FWHM). The Brillouin gain spectrum peaks at the Brillouin frequency shift *vB* (or the frequency difference between the two beams), and

0 2

where *p12* is the longitudinal elasto-optic coefficient, *ρ0* is the density, *λp* is the pump wavelength and *c* is the vacuum velocity of light [Heiman et al., 1979; Agrawal, 1989]. The Brillouin linewidth for a spontaneous Brillouin scattering in a standard SMF is reported to be approximately 30 MHz, which can be calculated using Eq. (7). In this work, the Brillouin linewidth is observed to be around 15 MHz as shown in Fig. 13 due to the proposed sensor, which is based on SBS effect. The Brillouin linewidth is also expected to slightly increases

The temperature–dependence of the Brillouin gain shift is also investigated for various types of sensing fibers and the result is shown in Fig. 16. The beat frequency is obtained at approximately 10.85, 10.75 and 9.80 GHz with the sensing fibers of SMF, NZ-DSF and PCF, respectively. It is also observed that the central frequency linearly increases with the temperature over a wide temperature range. The slope coefficients of 0.77, 0.56 and 1.45 MHz/oC are obtained for BFLs using SMF, NZ-DSF and PCF, respectively. The error bars

λ ρ

<sup>2</sup> ( ) *<sup>B</sup>*

υ

*n p g g c V*

*B*

the peak value is given by the Brillouin gain coefficient

with the temperature increase.

*g g*

υ

2

<sup>Δ</sup> <sup>=</sup> − +Δ (6)

= = <sup>Δ</sup> (7)

 υ

7 2 12

*p a B*

υ

0

π

0 2 2 ( 2) ( ) ( ) ( 2) *B*

υ

*B B*

temperature.

Fig. 13. Experimental setup for the spontaneous Brillouin scattering measurements.

fiber is 0.8 μm with 1.5 μm pitch. The PCF is made from pure silica with 17.4 wt% of Gedoped core region. The SMF and NZ-DSF have core diameters of 9 and 4.2 μm, respectively. The sensing fiber is incorporated in a ring BFL configuration to reduce the required BP power for the operation. The Brillouin threshold is relatively lower for the SBS.

Fig. 14 shows the backward propagating Brillouin spectrum obtained from the ring cavity BFL, which is measured after the optical circulator by an OSA with resolution of 0.015 nm. In this experiment, the sensing fiber is NZ-DSF and the temperature is varied from 25oC to 80oC. The Brillouin Stokes laser is generated from interactions between the optical mode and acoustic modes in the fiber core. Thermally excited acoustic waves (acoustic phonons) produce a periodic modulation of the refractive index. Brillouin scattering occurs when light is diffracted backward on this moving grating, giving rise to frequency shifted Stokes and anti-Stokes components. This process can be stimulated when the interferences of the laser light and the Stokes wave reinforce the acoustic wave through electrostriction. Since the scattered light undergoes a Doppler frequency shift, the Brillouin shift depends on the acoustic velocity and is given by [Alahbabi et al., 2005]

$$
\omega\_B = \frac{2nV\_a}{\lambda} \tag{5}
$$

where �� is the acoustic velocity within the fiber, *n* is the refractive index and λ is the vacuum wavelength of the incident lightwave. As shown in Fig. 14, the Brillluin Stokes is slightly shifted to a longer wavelength as the temperature is increased from 25oC to 80oC. This is attributed to the refractive index of the sensing medium, which increases with the temperature. Fig. 14 shows the Brillouin shift for the BFL with NZ-DSF, measured by the RF spectrum analyzer at two different temperatures; 25oC and 80oC. As shown in the figure, the Brillouin shift peak power shifted toward a higher frequency with increasing temperature. For a sensing fiber of NZ-DSF, at room temperature of 25oC, a value for �� is experimentally obtained at 10.7 GHz at 1550 nm wavelength region as shown in Fig. 15.

The strong attenuation of sound waves in silica determines the shape of the Brillouin gain spectrum. Actually, the exponential decay of the acoustic waves results in a gain presenting a Lorenzian spectral profile [Xiao-qiang et al., 2009].

**Stokes**

**1**

**PM**

Fig. 13. Experimental setup for the spontaneous Brillouin scattering measurements.

 **50%**

power for the operation. The Brillouin threshold is relatively lower for the SBS.

�� <sup>=</sup> ����

where �� is the acoustic velocity within the fiber, *n* is the refractive index and λ is the vacuum wavelength of the incident lightwave. As shown in Fig. 14, the Brillluin Stokes is slightly shifted to a longer wavelength as the temperature is increased from 25oC to 80oC. This is attributed to the refractive index of the sensing medium, which increases with the temperature. Fig. 14 shows the Brillouin shift for the BFL with NZ-DSF, measured by the RF spectrum analyzer at two different temperatures; 25oC and 80oC. As shown in the figure, the Brillouin shift peak power shifted toward a higher frequency with increasing temperature. For a sensing fiber of NZ-DSF, at room temperature of 25oC, a value for �� is experimentally obtained at 10.7 GHz at 1550 nm wavelength region as shown in Fig. 15.

The strong attenuation of sound waves in silica determines the shape of the Brillouin gain spectrum. Actually, the exponential decay of the acoustic waves results in a gain presenting

� (5)

**Photodiode 20GHz**

 **FUT**

 **50%**

 **RF spectrum analyzer**

**Oven**

acoustic velocity and is given by [Alahbabi et al., 2005]

 **10%**

 **TLS EDFA**

 **ISO**

**90%**

a Lorenzian spectral profile [Xiao-qiang et al., 2009].

fiber is 0.8 μm with 1.5 μm pitch. The PCF is made from pure silica with 17.4 wt% of Gedoped core region. The SMF and NZ-DSF have core diameters of 9 and 4.2 μm, respectively. The sensing fiber is incorporated in a ring BFL configuration to reduce the required BP

**100% PM**

Fig. 14 shows the backward propagating Brillouin spectrum obtained from the ring cavity BFL, which is measured after the optical circulator by an OSA with resolution of 0.015 nm. In this experiment, the sensing fiber is NZ-DSF and the temperature is varied from 25oC to 80oC. The Brillouin Stokes laser is generated from interactions between the optical mode and acoustic modes in the fiber core. Thermally excited acoustic waves (acoustic phonons) produce a periodic modulation of the refractive index. Brillouin scattering occurs when light is diffracted backward on this moving grating, giving rise to frequency shifted Stokes and anti-Stokes components. This process can be stimulated when the interferences of the laser light and the Stokes wave reinforce the acoustic wave through electrostriction. Since the scattered light undergoes a Doppler frequency shift, the Brillouin shift depends on the

Fig. 14. Stimulated Brillouin scattering monitored by OSA in minimum and maximum used temperature.

$$g\_B(\nu) = g\_0 \frac{\left(\Delta \nu\_{\rm B}/2\right)^2}{\left(\nu - \nu\_{\rm B}\right)^2 + \left(\Delta \nu\_{\rm B}/2\right)^2} \tag{6}$$

where Δ*vB* is the full-width at half maximum (FWHM). The Brillouin gain spectrum peaks at the Brillouin frequency shift *vB* (or the frequency difference between the two beams), and the peak value is given by the Brillouin gain coefficient

$$\mathbf{g}\_{\mathcal{B}}(\boldsymbol{\nu}) = \mathbf{g}\_0 = \frac{2\pi n^7 p\_{12}^2}{c\lambda\_p^2 \rho\_0 V\_a \Delta \boldsymbol{\nu}\_{\mathcal{B}}} \tag{7}$$

where *p12* is the longitudinal elasto-optic coefficient, *ρ0* is the density, *λp* is the pump wavelength and *c* is the vacuum velocity of light [Heiman et al., 1979; Agrawal, 1989]. The Brillouin linewidth for a spontaneous Brillouin scattering in a standard SMF is reported to be approximately 30 MHz, which can be calculated using Eq. (7). In this work, the Brillouin linewidth is observed to be around 15 MHz as shown in Fig. 13 due to the proposed sensor, which is based on SBS effect. The Brillouin linewidth is also expected to slightly increases with the temperature increase.

The temperature–dependence of the Brillouin gain shift is also investigated for various types of sensing fibers and the result is shown in Fig. 16. The beat frequency is obtained at approximately 10.85, 10.75 and 9.80 GHz with the sensing fibers of SMF, NZ-DSF and PCF, respectively. It is also observed that the central frequency linearly increases with the temperature over a wide temperature range. The slope coefficients of 0.77, 0.56 and 1.45 MHz/oC are obtained for BFLs using SMF, NZ-DSF and PCF, respectively. The error bars

Fiber Optic Temperature Sensors 377

less than that of the other silica fibers. The reason is due to the Germania concentration in the fiber core such that the inhomogeneous distribution within the core of the fiber is responsible for multi peaks in Brillouin gain spectrum. The PCF used is composed of a small-scale solid silica core comprising a Ge-doped center region with multiple air holes arranged in a hexagonal lattice around the core which act as cladding. With the combination of a large refractive index contrast between the silica core and the air-filled microstructure, the PCF can confine the mode tightly in the core that results in a low effective mode area and thereby produce a large nonlinear coefficient. This characteristic avoids the difficulty in distributed fiber sensors that use long SMF and NZ-DSF which contributes to uncertainty in the sensing results. Although the threshold is reduced by the increased length necessary to provide the SBS, it creates other problems like reduced coefficient for sensing temperature.

Four different approaches for temperature sensing have been demonstrated; intensity modulated FODS, lifetime measurements, MLR-based and SBS-based sensors. The first sensor uses a fiber-optic displacement sensor based on a multimode POF coupler to measure temperature ranging from 420C to 900C. The displacement curve has a sensitivity of 0.0005 mV/μm and a linearity of more than 99% within a measurement range between 0 to 1400 μm. By placing the aluminum rod within the linear range, the measured output signal is observed to be a linear function of the aluminum rod temperature with a sensitivity of 0.0044 mV/oC for temperature detection with a linearity of more than 98% and a resolution of 2.4 oC. The second sensor is based on a lifetime measurement of 90 cm long of EDF, which is diode-pumped by a modulated 980 nm laser. The sensitivity of the sensor is obtained at 0.009 ms/oC with a linearity of more than 94%. This temperature sensor is shown to be effective and useful for its high resolution and precision. The third sensor is based on MLR, which is fabricated from a microfiber, which is derived using a flame heating method. The MLR is constructed by coiling the microfiber to form a loop resonator device. The device is embedded into low refractive index polymer for robustness. The spacing of the transmission comb spectrum of the MLR is observed to be unchanged with the temperature. However, the extinction ratio of the spectrum is observed to be linearly decreasing with the temperature. The slope of the extinction ratio reduction against temperature was about 0.043dB/oC. The dependence of the extinction ratio on temperature is due to the change in the material's refractive index. The temperature-dependence of the Brillouin frequency shift is demonstrated in various types of fiber in the last approach. The beat frequency between the SBS and pump signals is measured using the heterodyne method. The central Brillouin beat frequency increases linearly with increasing temperature with temperature coefficients of 0.77, 0.56 and 1.45MHz/0C. The phenomena are demonstrated for 25 km long SMF, 10 km

Alahbabi M N, Cho Y T, Newson T P. 150-km-range distributed temperature sensor based

optical fiber. Optics Letters, 1993, 18(18): 1561–1563.

on coherent detection of spontaneous Brillouin backscatter and in-line Raman amplification. Journal of the Optical Society of America: B, 2005, 22(6): 1321–1324. Bao X, Webb D J, Jackson D A. 32-km distributed temperature sensor using Brillouin loss in

**6. Conclucions**

long NZ-DSF and 100 m PCF, respectively.

**7. References**

Fig. 15. The typical SBS beat frequency spectra at the minimum and maximum temperatures using the NZ-DSF as the sensing fiber.

Fig. 16. Brillouin frequency shift as functions of temperature.

are ±3 MHz, corresponding to the BFS fluctuations when the temperature is fixed. All fiber samples are only coated with a thin acrylate micro jacket, so that the coating influence is expected to be kept below 0.1 MHz/ oC. This experiment indicates that the PCF with small core is preferable for this application. As also shown in Fig. 16, the Brillouin shift for PCF is

 Fig. 15. The typical SBS beat frequency spectra at the minimum and maximum temperatures

are ±3 MHz, corresponding to the BFS fluctuations when the temperature is fixed. All fiber samples are only coated with a thin acrylate micro jacket, so that the coating influence is expected to be kept below 0.1 MHz/ oC. This experiment indicates that the PCF with small core is preferable for this application. As also shown in Fig. 16, the Brillouin shift for PCF is

using the NZ-DSF as the sensing fiber.

Fig. 16. Brillouin frequency shift as functions of temperature.

less than that of the other silica fibers. The reason is due to the Germania concentration in

the fiber core such that the inhomogeneous distribution within the core of the fiber is responsible for multi peaks in Brillouin gain spectrum. The PCF used is composed of a small-scale solid silica core comprising a Ge-doped center region with multiple air holes arranged in a hexagonal lattice around the core which act as cladding. With the combination of a large refractive index contrast between the silica core and the air-filled microstructure, the PCF can confine the mode tightly in the core that results in a low effective mode area and thereby produce a large nonlinear coefficient. This characteristic avoids the difficulty in distributed fiber sensors that use long SMF and NZ-DSF which contributes to uncertainty in the sensing results. Although the threshold is reduced by the increased length necessary to provide the SBS, it creates other problems like reduced coefficient for sensing temperature.

## **6. Conclucions**

Four different approaches for temperature sensing have been demonstrated; intensity modulated FODS, lifetime measurements, MLR-based and SBS-based sensors. The first sensor uses a fiber-optic displacement sensor based on a multimode POF coupler to measure temperature ranging from 420C to 900C. The displacement curve has a sensitivity of 0.0005 mV/μm and a linearity of more than 99% within a measurement range between 0 to 1400 μm. By placing the aluminum rod within the linear range, the measured output signal is observed to be a linear function of the aluminum rod temperature with a sensitivity of 0.0044 mV/oC for temperature detection with a linearity of more than 98% and a resolution of 2.4 oC. The second sensor is based on a lifetime measurement of 90 cm long of EDF, which is diode-pumped by a modulated 980 nm laser. The sensitivity of the sensor is obtained at 0.009 ms/oC with a linearity of more than 94%. This temperature sensor is shown to be effective and useful for its high resolution and precision. The third sensor is based on MLR, which is fabricated from a microfiber, which is derived using a flame heating method. The MLR is constructed by coiling the microfiber to form a loop resonator device. The device is embedded into low refractive index polymer for robustness. The spacing of the transmission comb spectrum of the MLR is observed to be unchanged with the temperature. However, the extinction ratio of the spectrum is observed to be linearly decreasing with the temperature. The slope of the extinction ratio reduction against temperature was about 0.043dB/oC. The dependence of the extinction ratio on temperature is due to the change in the material's refractive index. The temperature-dependence of the Brillouin frequency shift is demonstrated in various types of fiber in the last approach. The beat frequency between the SBS and pump signals is measured using the heterodyne method. The central Brillouin beat frequency increases linearly with increasing temperature with temperature coefficients of 0.77, 0.56 and 1.45MHz/0C. The phenomena are demonstrated for 25 km long SMF, 10 km long NZ-DSF and 100 m PCF, respectively.

## **7. References**

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Bao X, Webb D J, Jackson D A. 32-km distributed temperature sensor using Brillouin loss in optical fiber. Optics Letters, 1993, 18(18): 1561–1563.

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## *Edited by Moh. Yasin, Sulaiman W. Harun and Hamzah Arof*

This book is a collection of works dealing with the important technologies and mathematical concepts behind today's optical fiber communications and devices. It features 17 selected topics such as architecture and topologies of optical networks, secure optical communication, PONs, LANs, and WANs and thus provides an overall view of current research trends and technology on these topics. The book compiles worldwide contributions from many prominent universities and research centers, bringing together leading academics and scientists in the field of photonics and optical communications. This compendium is an invaluable reference edited by three scientists with a wide knowledge of the field and the community. Researchers and practitioners working in photonics and optical communications will find this book a valuable resource.

Optical Fiber Communications and Devices

Optical Fiber

Communications and Devices

*Edited by Moh. Yasin,*

*Sulaiman W. Harun and Hamzah Arof*

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