# Polarization-Discriminated RSOA-EAM for Colorless Transmitter in WDM-PON
## 2 Device Structure and Working Principle  
The proposed device structure is schematically shown in Figure 1a. It is functionally composed of three sections: the SOA, the EAM, and the FR [28; 29], all on III-V compound semiconductors with InP as the common substrate. The active region of the SOA and the EAM both contain MQW structure made by the AlGaInAs materials. The SOA quantum wells are compressively strained to offer a pure TE mode gain, whereas the EAM quantum wells are slightly tensile, strained for polarization independent absorption [30]. Both the SOA and EAM are made of ridge waveguide with a length of 1200 \(\upmu\)m and 100 \(\upmu\)m, respectively. Their electrodes are separated by an isolation trench typically in 10 \(\upmu\)m. The FR is made by Fe-doped InGaAsP/InP material with a bulk core region [29]. The reported Verdet coefficient is up to 33\({}^{\circ}\)/mm/T in the FR structure [28]. g the Verdet coefficient [29] will be able to offer a 90\({}^{\circ}\) round-trip Faraday rotation. Hence, a section length of 1 mm under an external magnetic field of 1.37 T [31; 32] or less (by increasinIt is worth mentioning that the proposed device shown in Figure 1a needs to be fabricated by the butt-joint regrowth technique due to the inhomogeneity of the material system as well as the layer stack structure in each section. To fabricate the ridge waveguide structure for light guiding and confinement, one may need to exploit different etching depth and/or define different ridge width, in order to achieve a smooth coupling from section to section with negligible reflection and to obtain a strong light confinement, particularly in the FR section for efficiently utilizing the Faraday effect [29].  
In this device, the polarization state of the downstream signal (\(\theta_{DS}\)) is chosen as TE, which can be converted from the random polarized light through the polarization diversified circuits [33, 34, 35]. After the downstream signal has its bit stream pattern erased by the saturation amplification in the SOA, it is uploaded with the upstream signal in the EAM. Upon the reflection of the light, the FR rotates its polarization state by an angle (i.e., \(\theta_{FR}\) in the inlet of Figure 1a, defined in a round-trip). In the light's returning path, the upstream signal in a differently polarized state (\(\theta_{US}\)) will experience a different gain from the downstream signal in the SOA. By exploiting such a polarization-dependent gain discrimination, we can simultaneously saturate the downstream signal and linearly amplify the upstream signal. Apparently, a small (close to 0\({}^{\circ}\)) Faraday rotation angle brings in no significant gain discrimination. We will face the contradictory requirement in dealing with the down- and upstream signals in the same SOA, i.e., we can hardly obtain a satisfactory result in erasing the downstream signal (where the SOA needs to be set in its saturation state) and in amplifying the upstream signal (where the SOA needs to be set in its linear amplification region) simultaneously. On the contrary, a large (close to 90\({}^{\circ}\)) Faraday rotation angle is in favor of the downstream signal erasing but offers almost no amplification to the upstream signal, as the SOA provides zero gain to the transverse magnetic (TM) mode. An optimum rotation angle, therefore, must exist in balancing the integrity and power of the upstream signal, as the former depends on the erasing of the downstream signal, whereas the later relies on the linear amplification of the upstream signal. A main task of our followed work is to find this optimum rotation angle with different requirements on the upstream signal.