Electroabsorptive modulators (EAM) are known to be used for modulating RF signals. There are many known benefits associated with optical modulation of RF signals including very small size, higher operating frequencies, immunity to electromagnetic interference, and relatively wide signal bandwidth.
An EAM is typically a semiconductor waveguide device in which the absorption region (also known as the absorption layer) also serves as an optical waveguiding layer. As is known in the art, an electric field applied across the waveguide causes a change in the optical absorption of the device, which in turn causes the intensity of the light passing through the waveguide to be modulated. By applying an RF signal to the device, the intensity modulation of the input light signal will vary in accordance with the variation of the RF signals.
However, in order to maximize the performance of an optical system, it is necessary to increase the optical power delivered to the detector in the system. One method of maximizing system performance is to minimize optical insertion losses in the system. In particular, it is desirable to minimize optical insertion losses, also known as coupling losses, between a laser transmitter and the waveguide portion of the EAM. Another method of maximizing system performance is to increase the output of the laser transmitter to offset any coupling losses. However, EAMs suffer from a charge screening effect that minimizes the modulation efficiency of EAMs at high optical insertion power. As a result, optical power levels at the input of an EAM must be limited to values below the saturation level of the optical element at high optical insertion power.
It is also known to design EAMs using optical couplers, for example peripheral coupled waveguides, to minimize electroabsorption saturation by controlling an amount of optical power absorbed in the waveguide from the laser transmitter through an input facet. It is further known to reduce an optical confinement factor in an electroabsorption region of the EAM to prevent saturation of the EAM. In one method, the electroabsorption region of the EAM having a reduced optical confinement factor is placed in optical communication with a laser transmitter to limit the optical power absorbed by the EAM. However, if the optical confinement factor is constant along the waveguide portion of the EAM, an input region of the waveguide absorbs the most optical power and tends to saturate first at high optical input powers, thereby reducing the modulation efficiency. At very high optical input power, the input region of the waveguide of the EAM may undergo catastrophic failure due to heat produced by sustained generation of high photocurrent.
A peripheral coupled waveguide EAM is disclosed in U.S. Pat. No. 7,167,605, incorporated herein by reference in its entirety, wherein an electroabsorption material is placed within an evanescent tail of an optical wave guided within an optical waveguide. A modulation voltage is applied to the electroabsorption material within the evanescent tail of the optical wave to modulate the optical wave. U.S. Pat. No. 7,167,605 further discloses optimizing an optical confinement factor to maximize performance of the EAM.
An optical waveguide taper has been shown to improve the performance of a single modulator by changing the confinement factor of the waveguide as a function of position along the length of the waveguide. For example, combining a reduced optical confinement factor at an input region of the waveguide with an increasing optical mode confinement factor in the center of the waveguide can improve the optical saturation power of the EAM. However, the optical confinement factor must be reduced at the exit of the waveguide portion of the EAM if the EAM is to couple to an external photonic component, for example on another EAM chip.
Certain applications, such as a photonics-based satellite communications front end for example, require two or more modulators to perform needed frequency translation functionality. However, the process of coupling optical signals from a laser chip to a modulator chip and then to a second modulator chip produces excess coupling losses that minimize the effectiveness of the system. It is therefore desirable to develop an EAM device and method to optimize the coupling and performance of multiple EAMs independently of the optical input.