Design of Carrier-Depletion-based Modulators on Commercially Available SOI Substrates of Varying Device-Layer Thickness

We compare modulator designs on thick-film and thin-film silicon-on-insulator (SOI) substrates based on the carrier-depletion effect. This effect exhibits a lower junction capacitance as compared to its injection counterparts, leading to a higher electro-optic bandwidth, although the effective refractive index change is low. In this work, commercially available standard SOI substrates with device layer thicknesses of 5 μm, 3 μm, and 0.22 μm are chosen for designing carrier-depletion-based phase shifters, which are then utilized for the design of Mach-Zehnder modulators (MZMs). These MZMs are tested for various on-off-keying (OOK) modulation bit rates employing non-return-to-zero (NRZ) pseudo-random binary signal (PRBS). For the comparison of given designs, we find that the thick-film MZMs, which can provide higher fabrication tolerance and high power handling capabilities, support maximum modulation speeds in the order of a few Gbps, whereas it can go up to 20 Gbps for thin-film MZMs.