All-in-One 112-Gb/s DP-QPSK Optical Receiver Front-End Module Using Hybrid Integration of Silica-Based Planar Lightwave Circuit and Photodiode Arrays

We propose a new hybrid integrated optical front-end module for a 100-Gb/s-class coherent receiver using silica-based planar lightwave circuit (PLC) technology. We have developed an integrated structure composed of multichannel microcollimator optics between PLC waveguides and photodiodes (PDs), and compact hermetically sealed optical-to-electrical converters with a PD array and a transimpedance amplifier array to maintain reliability. We used the technology to fabricate an all-in-one optical front-end module for a dual polarization quadrature phase-shift-keying (DP-QPSK) coherent receiver, and confirmed that the module operated successfully for 112-Gb/s DP-QPSK signal detection.

I. Introduction

Along-haul optical communication network system with a wavelength channel rate of over 100 Gb/s is being developed using technology that combines digital signal processor (DSP)-based coherent optical detection with multi-level modulation. Dual polarization quadrature phase shift keying (DP-QPSK) is fast becoming a standard modulation format for 100-Gb/s-class coherent optical communication. The optical front-end in a DP-QPSK coherent receiver has a complicated configuration that incorporates two polarization beam splitters (PBSs), two 90° optical hybrid mixers (90°OHs), eight photodiodes (PDs) incorporating four balanced photodetectors, and four transimpedance amplifiers (TIAs) with differential outputs. In such an optical front-end, the skew and the responsivity imbalance between the PD channels at each balanced photodetector degrade the common-mode rejection ratio (CMRR), which affects the coherent receiver performance. To eliminate the skew and minimize the responsivity imbalance, the optical front-end module must be highly integrated. Various integrated optical front-end modules have been proposed using, for example, monolithic semiconductor integration or a free space optical component assembly [1], [2]. However, for practical use, these approaches require further development in terms of performance, reliability and cost.