The following description relates to systems and techniques for reducing the polarization mode dependency of an optical device. The techniques may be used on in conjunction with optical devices such as those used in optical communications systems.
Different optical devices, including optical amplifiers, optical filters and combined optical, amplifying filters may be used in optical communication systems. These devices may be integrated in, for example, components such as switches, routers, multiplexers and other components for optical-signal-processing.
Optical signals that are employed in communication systems may be polarized and may include more than one polarization mode. Two polarization modes are known as transverse-electrical (TE) and transverse-magnetic (TM). The optical signals may propagate through the communication system on optical fibers and may have an arbitrary polarization state. The TM component may be thought of as propagating perpendicular to an axis of the optical waveguide and the TE polarization mode may be thought of as propagating parallel to the axis of the optical waveguide.
The performance of some optical components, including optical amplifiers and optical waveguide coupled components used in an optical communication system, may depend upon the polarization state of an incident optical signal received by the component. The optical devices may be polarization-dependent, meaning that the device has a different influence on the different modes of an incident signal.
Incident signals with different polarization states may be affected by an optical device in different ways. In an optical amplifier, for example, the modal gain may be polarization mode dependent. The gain of the transverse-electrical and transverse-mechanical waves may be different. This variation in the response of an optical device to the different polarization states is known as polarization mode dispersion (PMD). Polarization mode dispersion is an inherent property of optical media. It can be caused by the difference in the propagation velocities of light in the orthogonal principal polarization states of the transmission medium. The net effect is that if an optical pulse contains both polarization components, then the different polarization components will travel at different speeds and arrive at different times, smearing the resultant optical signal. One result is that the gain may differ for TE-polarized and TM-polarized waves. The difference in gain between the differently polarized waves may result in an amplification of the TM wave that is different from the TE wave. Thus, the output optical signal from the amplifier may include TM and TE polarized waves that are in a different proportion than the input optical signal. The output signal of a polarization mode dependent device may have a different polarization state than the incident received signal.