Polarization-Discriminated RSOA–EAM for Colorless Transmitter in WDM–PON

Chengliang Zuo,Xun Li

doi:10.3390/app10249049

Abstract

The integrated reflective semiconductor optical amplifier (RSOA) and electro-absorption modulator (EAM) is viewed as an appealing solution to the colorless transmitter on the optical network unit (ONU) side of wavelength-division multiplexed (WDM) passive optical networks (PONs), for its broad modulation bandwidth and high optical gain. However, the conventional RSOA–EAM usually exhibits a poor upstream signal eye-diagram because it can hardly simultaneously saturate the downstream signal and boost the upstream signal as required. By exploiting the polarization-depended RSOA gain, we propose a polarization-discriminated RSOA–EAM to improve the quality of the upstream signal eye-diagram. In this device, the transverse electric polarized downstream signal is saturated by the high gain in the RSOA active region made of compressively strained multiple quantum wells, whereas the upstream signal is linearly amplified after polarization rotation. We find that, as the quality of the upstream signal eye-diagram improves with an increased polarization rotation angle, its power drops, which indicates that there exists an optimized rotation angle to reach a compromise between the upstream signal integrity and power. Simulation results show that the dynamic extinction ratio and output power of the upstream signal can reach 8.3 dB and 11 dBm, respectively, through the proposed device with its rotation angle set at an optimum value (80°), which exceeds the specification (6 dB and 4 dBm) of the upstream transmitter as required by the next-generation PON stage two. The quality of the upstream signal eye-diagram measured in Q-factor is improved by 10 dB compared to the conventional RSOA–EAM design without polarization rotation introduced.

Highlights

With the increasing demand for bandwidth from the subscriber end, it is unlikely that a traditional time-division multiplexed passive optical network (TDM–passive optical networks (PONs)) will satisfy the market’s future requirements [1,2]
Needs to be set in its saturation state) and in amplifying the upstream signal a large Faraday rotation angle is in favor of the downstream signal erasing but offers simultaneously
To be able to find an optimum rotation angle that makes a compromise between the upstream signal integrity and power, we introduce the following dimensionless figure of merit (FOM): FOM = Q2 ×

Summary

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]. 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). 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, 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 monolithically integrated with the SOA/RSOA with the current technology.

Device Structure and Working Principle

Results

Downstream Signal Suppression

Upstream Signal Integrity and Power

Faraday

Conclusions