Takashi Inoue , Hiroshi Kawashima , Noritaka Matsubara , Kazutaka Nara
A silica-based planar lightwave circuit (PLC) that is required for a receiver frontend used in digital coherent telecommunication system is developed by integrating a polarization-beam splitter (PBS) and a 90-degree hybrid mixer. At first, by configuring a PBS for signal with a double-pass Mach-Zehnder interferometer, a PBS-integrated coherent mixer chip is developed and a polarization extinction ratio (PER) higher than 30dB is achieved. In the next stage, downsizing of a PLC chip is studied. For this purpose, the relative refractive index difference Δ is improved to 1.8%, and a circuit layout of folded waveguide is employed. As a result, an ultra-small PBS integrated mixer chip with dimension of 12mm×12mm is realized.
Using the developed chips in digital coherent receivers, experiments to demodulate dualpolarization quadrature phase-shift keying (DP-QPSK) signals at 40 Gbit/s and 100 Gbit/s are conducted, and excellent performances were confirmed in both cases.