Ultra-compact and low-loss silica-based dual polarization optical hybrid for digital coherent receiver with excellent common-mode rejection ratio

An ultra-compact 3 × 12 mm² DPOH on a 1.5%-Δ silica-based PLC employing a novel coupler is reported. The excess loss is <;2 dB and the CMRR is <;-27 dBe over the C- and L-bands.

1. Introduction

The next generation photonic network requires a high-speed optical transport system that combines multi-level modulation formats, coherent detection, and digital signal processing. A dual-polarization quadrature phase shift keying (DP-QPSK) coherent receiver with an excellent common-mode rejection ratio (CMRR) was developed that incorporates a silica-based dual polarization optical hybrid (DPOH), InP/InGaAs photodiodes, and dual-channel InP-based HBT transimpedance amplifiers [1]. As the demand for ultra-small pluggable transceivers increases for short-reach communications, namely, for installation in data centers, it is also becoming important to reduce the size of coherent receivers for long-reach communication. A DPOH based on silica planar lightwave circuit (PLC) technology offers excellent features including low loss, high CMRR, and the integration of polarization beam splitters (PBSs) with a high polarization extinction ratio (PER). However, the chip size was larger than chips made of other waveguide materials, such as InP [2] and silicon [3], because of the lower refractive index difference.