The effect of the free carrier effect is more significant in the mid-infrared band when compared with the 1310 band and 1550 band. In this paper, we propose a cascaded Mach-Zehnder interference (MZI) structure to improve the extinction ratio (ER) of the modulator in the mid-infrared band. The cascaded compensation method is to add the next-stage equal-arm MZI device to the two phase shifters of the major MZI. The output light intensity of the two phase shifters can be maintained at the same level by adjusting the output loss of both the equal-arm MZI. With the cascaded compensation method, the simulated ER of the optical modulator is increased from 36 dB to 55 dB under −4 Vbias while the device still maintains a low insertion loss (IL) of 12.5 dB. Through the cascaded compensation method, the modulation depth of the modulator at −2 V, −4 V, −6 V, and −8 V are 58 dB, 53 dB, 57 dB, and 59 dB, respectively. Meanwhile, the dynamic ER is 9.2 dB at a data rate of 40 Gbps, which is 4.5 dB higher than that of the original one.

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The edge-coupler of fiber-to-chip with ultra-low coupling loss is demonstrated on SOI platform. The edge-coupler is consisted of the cantilevered SiO2 waveguide, the amorphous silicon (α-Si) nano taper and the crystal silicon (c-Si) nano tapers. The thin α-Si layer is deposited on the c-Si layer to improve the pattern matching with fiber. The optical input signal from the optical fiber is launched into the suspended SiO2 waveguide, then coupled into the α-Si nano taper at the center of the SiO2 waveguide, and finally coupled into the c-Si nano taper. We characterized the cantilevered edge-coupler using cleaved single-mode optical fiber with a mode field diameter of 10.5 μm. The measured coupling loss is as low as -1.7 dB per facet for TE mode without index matching liquid at 1550 nm. The 1 dB bandwidth is more than 100 nm with 1 dB alignment tolerances of ±2.0 μm in both horizontal and vertical directions. Besides, potential hybrid optical integration could also be allowed with this results in the future.

In this paper, we propose a large wavelength bandwidth Mach-Zehnder modulator based on light intensity equalization structure. The modulator is designed into a symmetric structure to increase the optical operating bandwidth. Besides, two auxiliary MZIs are connected to the respective phase shifters of the main modulator to balance the optical intensity in them. Compared with a conventional Mach-Zehnder modulator, this design has a larger optical operating bandwidth over 56.1 nm. Besides, the maximum extinction ratio of this modulator is 26.2dB, which is 12 dB higher than that of a conventional one. The dynamic extinction ratios at speed of 40Gbps are 4.10 dB in the wavelength of 1530nm and 4.02 dB in the wavelength of 1560 nm respectively. The 3 dB electro-optical bandwidth of this modulator under different voltages are almost consistent with the performance of the conventional one. All of these prove that this structural design can effectively increases the performance of Mach-Zehnder modulators.