# Polarization-Discriminated RSOA-EAM for Colorless Transmitter in WDM-PON
## 1 Introduction  
With the increasing demand for bandwidth from the subscriber end, it is unlikely that a traditional time-division multiplexed passive optical network (TDM-PON) will satisfy the market's future requirements [1, 2]. As an alternative technology, the wavelength-division multiplexed passive optical network (WDM-PON), which supports a higher bandwidth, better security, and easier upgradability, is promising for future broadband access networks [2, 3, 4]. However, large-scale deployment of the WDM-PON is rare due to the drastically increased cost on specific wavelength source in optical network units (ONUs) [3, 4]. Therefore, the key to reducing the cost of the WDM-PON is to make the ONU colorless, i.e., all ONUs are made identical and interchangeable [5, 6]. To this end, many colorless transmitter solutions have been proposed based on the reflective semiconductor optical amplifier (RSOA) or RSOA integrated with an electro-absorption modulator (RSOA-EAM), such as the spectrum-sliced RSOA [7], the RSOA fiber-cavity laser (FCL) [8, 9, 10, 11], the externally seeded RSOA [12, 13, 14, 15], and thewavelength-reuse RSOA [16; 17; 18; 19]. The RSOA-FCL balances the cost and performance among the first three schemes and is thus viable for short-reach applications up to 10 Gb/s [20], but dispersion-shifted fiber (DSF) is required for long reach in preventing multimode dispersion [21]. The wavelength-reuse scheme is free of this requirement, but its upstream performance is subject to the unsuppressed downstream components [17]. Therefore, in the wavelength-reuse configuration, the requirements imposed on such RSOAs/RSOA-EAMs in their simultaneous dealing with the down- and upstream signals are contradictory: The downstream signal needs to be erased by saturated amplification, which requires the SOA to operate in its nonlinear saturated mode, whereas the upstream signal needs to be boosted without distortion, which requires the SOA to operate at its linear amplification state [22]. Obviously, the conventional RSOA/RSOA-EAM design cannot satisfy this contradictory requirement. The quality of its upstream signal eye-diagrams is therefore largely restricted by the extinction ratio (ER) of the downstream signal [16; 17; 18]. To effectively remove the downstream signal by maximizing the saturation, various structures have been explored, such as the ultralong SOA (UL-SOA) [23], the cascade SOAs/RSOAs [24; 25; 26], and the self-feedback SOA [27]. These schemes, however, need an extra SOA/RSOA to boost the downstream signal power and a circulator to separate the down- and upstream channels, which increases both device complexity and cost, especially when the circulator cannot be monolithtically integrated with the SOA/RSOA with the current technology. Noticing the difference in the propagation direction between the down- and upstream signals, we recently reported a horn-waveguide RSOA-EAM that simultaneously saturates the downstream signal and linearly amplifies the upstream signal by linearly narrowing the ridge width of the SOA section [22]. Although the asymmetric structure has some advantages over conventional ones, the cross-gain saturation (XGS) may jeopardize the device performance since the down- and upstream signals still share the same material gain [22]. In this work, we propose a polarization-discriminated (PD) RSOA-EAM to deal with the down- and upstream signals in different polarization states. Different from the horn-waveguide RSOA-EAM, the PD RSOA-EAM has a uniform ridge width, but it incorporates a Faraday rotator (FR) at the back end of the RSOA-EAM where the SOA active region is made of compressively strained multiple-quantum-well (CS-MQW). As such, the transverse electric (TE) polarized downstream signal is saturated by the substantial TE gain provided by the CS-MQW active region of the SOA, hence is turned into a DC carrier. Upon the reflection with its polarization state rotated, the DC carrier becomes the upstream signal after its remodulation by the EAM.