A novel integrable optical analog-to-digital converter (OADC) scheme based on phase-shifted optical quantization has been proposed for years, the quantization module of which is mainly composed of a 1×2 multi-mode interference (MMI) coupler, a phase modulator and a 4×4MMI. Although the manufacture technology of 1×2 MMI and phase modulator has been very mature, the 4×4MMI on a silicon-on-insulator (SOI) substrate with superior performance has not been realized. Since the 4×4MMI is crucial for the realization of the OADC, we designed and fabricated a 4×4 MMI coupler on SOI substrate. The width and length of the multi-mode section are 10.2 um and 182.6 um respectively. Measurement results show that the imbalance of the four output ports at 1550-nm is around 1.9 dB, while the insertion losses are 5.6 dB.
Based on side-port multimode interference coupler, a novel design of 1.31/1.55-μm wavelength multiplexer/demutiplexer on SOI platform with conventional channel waveguides is proposed and analyzed by using wide-angle beam propagation method. With a 25.9μm long ultra-short MMI section, nearly an order of magnitude shorter than that of the previously reported 1.31/1.55-μm wavelength MMI splitters on SOI, simulation results exhibit contrasts of 28dB and 25dB at wavelength 1.31 and 1.55 μm, respectively, and the insertion losses are both below 0.55dB. Meanwhile, the analysis shows that the proposed structure has larger fabrication tolerances than restricted MMI based structures and the present design methodology also applies to split other wavelengths and in different material platforms, such as InP, GaAs and PLC guides, etc.
A novel 4 × 4 multimode interference couplers based phase-shifted photonic quantization scheme using multiwavelength mode locked pulse lasers as sampling source for all-optical analog-to-digital converter is proposed. Numerical analysis indicates that 8-bit quantization resolution operating at 40 GHz bandwidth could be achieved with an incident average optical power of 1.932 mW to each photodiode. The whole scheme can be integrated on a InP-based chip.
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