One-Chip Integrated Dual Polarization Optical Hybrid Using Silica-Based PLC Technology

Fig.3 Digital coherent detection with polarization multiplexing, such as dual polarization quadrature phase-shift keying (DP-QPSK), has been attracting considerable attention for constructing 100 Gbit/s transmission systems. The optical front-end for the digital coherent receiver consists of polarization beam splitters (PBSs), optical hybrids, and four sets of balanced photodetectors (PDs). The integration of these components has been extensively investigated with a view to reducing the receiver size. To realize a compact receiver, we have integrated two PBSs and two optical hybrids using silica-based planar lightwave circuit (PLC) technology. Figure 1 shows the configuration of our dual polarization optical hybrid (DPOH) and the fabricated module. The signal and local oscillator (LO) lights are launched into corresponding PBSs, and separated into the TE and TM modes. Then, the signal and LO components enter the optical hybrid for each polarization state. The output ports are connected to balanced PDs via eight equal length fibers. To realize a compact waveguide-type PBS, we introduced stress release grooves along one waveguide arm of a Mach-Zehnder interferometer and adjust the waveguide birefringence. In the optical hybrid, we adjusted the waveguide arm lengths to achieve a phase difference of 90° between the in-phase and quadrature (I and Q) channels. Figure 2 shows the measured transmission spectra of the fabricated PBS. The polarization extinction ratio was more than 25 dB across the C-band. Figure 3 shows the phase difference between the I and Q channels. The phase difference deviation from 90° was less than 3°. The excess loss without intrinsic loss was less than 2.5 dB, and the loss imbalance between all the outputs was less than 0.2 dB. We confirmed its applicability as a coherent receiver component by measuring the receiver performance with a back-to-back configuration for 111 Gbit/s DP-QPSK signals. The notable advantage is that our DPOH requires no adjustment in practical use unlike other waveguide-type optical hybrids. We believe that silica-waveguide DPOHs will contribute greatly to providing compact and cost-effective optical front-ends for digital coherent receivers. We demonstrated a dual polarization optical hybrid (DPOH) achieved by integrating polarization beam splitters (PBSs) and optical hybrids using silica-based planar lightwave circuit (PLC) technology. The fabricated DPOH exhibited good optical characteristics for the coherent detection of 100 Gbit/s DP-QPSK signals.