Investigation of a Silicon Photonic Optical Serial-to-Parallel Converter With Mach–Zehnder Delay Interferometers and Integrated Ge-PDs

A silicon photonic optical serial-to-parallel converter (OSPC) with phase operation, which consists of Mach-Zehnder delay interferometers (MZDIs) with 1to 8-bit delay lengths, was fabricated for 8-bit processing at 40 Gb/s. The OSPC performs a 1-bit extraction from the packet in each MZDI with a π phase bias by interfering a target label bit with a π/2-phase-shifted preamble setting at the top of the packet. An improvement in the number of processed bits is verified by introducing an output-side coupler with unequal splitting ratio, which compensates the power unbalance in the MZDI caused by a larger propagation loss in the delay line. We set a splitting ratio of 32:68 as the optimized value, for example, for the 20-bit-delay MZDI, assuming a propagation loss of 2.6 dB/cm. The Si waveguide was 0.50-μm-wide and 0.21-μm-thick, and it was buried in 2-μm-thick lower and upper SiO₂ cladding layers. The delay lengths were designed as an integer multiple of the unit length where light propagates in a 1-bit duration, and the unit length was 1.83-mm-long for 40Gb/s data. Thus, the delay length ranged from 1.83 to 14.64 mm for 1to 8-bit delay MZDIs. Because of the small bending radius of 10-μm, the total device size could be downsized, and it was set to 2 mm × 1.8 mm. First, we evaluated the transmissivity spectrum of the MZDIs, and free spectral ranges of 40 GHz divided by one to eight were obtained, which were less than 5% deviation from the designed values. The difference of the transmissivity between the upper and the lower output port of the MZDI affects the output signal intensity, and its improvement was investigated by changing the splitting ratio of the output side splitter with various lengths of the multimode interferometers. The control of the transmissivity difference could be confirmed. Next, we investigated a Ge-photodetector (PD) integrated OSPC. We successfully obtained the preliminary result- for the OSPC operation in the case of 1-bit delay OSPC with integrated Ge-PDs. The suppression ratios, defined as the ratios of the extracted bit amplitude to the maximum amplitude of the canceled bits, of 11 dB was obtained. The performance could be improved by the optimized splitting ratio of the output-side MMI, the suppression of the input signal chirp, the reduction of insertion loss, and the improvement of the PD sensitivity.