**4. Adaptation of mode fields**

As depicted in chapter 3, comparison of the optical fields of a butt ended standard single‐ mode fiber (SMF)and of edge emitting laser diodes shows a great mismatch. This mismatch is the reason for the very low coupling efficiency of approx. 15% for a butt ended fiber

This low efficiency can be overcome by a better adoption of the two optical mode fields with lenses. A coupling efficiency of more than 90% has been shown. Disadvantages occur at the handling of the parts because there are several parts including one ore two lenses, the fiber and the chip, which must be handled for optical alignment. The consequence is a rather cost‐ ly of opto-electronic packaging.

**Figure 6.** left side: Sketch of a melted fiber taper in front of an OEIC, right side: photograph.

nearly perfect coupling between the two wave-guides is possible. Due to limitations in costs, several lens configurations have been proposed for the optical coupling of laser diodes to single mode fibers. As shown in figure 5, simple ball lenses can be used to adapt the two mode fields. Coupling efficiencies of up to 30% are possible. A better approach is the use of graded index lenses or Selfoc-lenses. For a focusing lens, so-called half-pitch devices are used. Whereas quarter-pitch lenses are used to form parallel beams. These can reach effi‐ ciencies of 70%. Another approach is to form a lens at the end of the fiber, which is called

As depicted in chapter 3, comparison of the optical fields of a butt ended standard single‐ mode fiber (SMF)and of edge emitting laser diodes shows a great mismatch. This mismatch is the reason for the very low coupling efficiency of approx. 15% for a butt ended fiber

This low efficiency can be overcome by a better adoption of the two optical mode fields with lenses. A coupling efficiency of more than 90% has been shown. Disadvantages occur at the

fiber taper. With this device, efficiencies up to 90% have been achieved.

424 Optoelectronics - Advanced Materials and Devices

**Figure 5.** Mode field adaptation by several micro-optic solutions.

**4. Adaptation of mode fields**

**Figure 7.** X and Y-axis: 2µm tolerance for 0.5 dB additional coupling loss.

As an example for integrated lens design, lenses can be made at the end of the fiber by melting the glass fiber and pulling it. This kind of fiber end is called fiber taper and works like a lens with typical diameters from 20 μm to 50 μm. In figure 6 you can see at the left side the chip facet and its wave guide and at the right side the melted fiber taper in front of the wave-guide. The fiber is additionally fixed into a metal cannula. With these tapered fibers a coupling efficiency of more than 50% can be realized. Unfortunately, a high precision fixation of better than 0.5 μm is necessary to mount the tapered fiber in front of the OEIC without additional losses. Therefore, the mechanical resolution of the coupling mechanism must be better than this value. The fixing procedure after coupling should not introduce additional displacements and must be stable enough to fix the cou‐ pling mechanism, which is important for a good long-term stability. The short working distance of 10 μm between fiber taper and laser which can be seen in the photograph is also dangerous for the life of the laser diode if it comes into contact with the with the fi‐ ber end. But there is only one low-priced device on the market, which makes this device very comfortable for use in small and very reasonably priced modules.

periment in a fixed relative position. The legs support the tabletop: Air suspension

Opto-Electronic Packaging http://dx.doi.org/10.5772/51626 427

In order to align a fiber optically to another component, one has to move either the fiber, the component or both. Three linear and three angular motions are necessary to describe fully the motion and position of a solid body in space. The figure below identifies the six degrees of freedom using the common Cartesian frame of reference. When each of these degrees of freedom is singularly constrained by a hardware device, the device is labelled kinematic.

The device used to move a component in the linear x, y, or z direction is a translation stage. The device used to move a component in the angular θx, θy, or θz direction is a rotation stage.

Actuators are used to move the component on a translation stage to its desired position. There are three basic types of actuators used with precision stages: manual drives, stepper motor drives, and piezoelectric transducers. Manual drives and stepper-motor drives can move components over long distances, constrained only by the size of the manual drive or, in the case of stepper motors, the length of the lead screw. Piezoelectric transducers can move components over very short distances with nanometre precision. The range and reso‐

lution of the various drives and stage technologies is shown in figure 10 below.

**Figure 10.** Translation ranges of available micro mechanical stages and screw systems.

mechanisms reduce practically all vibrations by two orders of magnitude.

**Figure 9.** Influence of vibrations for micro positioning.

The tolerances for lateral and longitudinal fixing of the fiber taper in front of the opto-elec‐ tronic circuit or OEIC are shown in figure 7 and figure 8. Both graphs show the distance in micrometers at the x-axis and a relative intensity of the coupling efficiency between the ta‐ pered fiber and the OEIC. You can see in figure 7 that within 2 micrometers the intensity will not be lower than 0.5 dB of the maximum intensity. For the longitudinal direction the tolerance is much greater: in this case 8μm, which can be seen in figure 8.

**Figure 8.** Z-axis with higher tolerance of 8µm.

Please remember: it is easier by a factor of four to perform the optical coupling in longitudi‐ nal direction in comparison to the lateral direction.