Since carrier dispersion and carrier absorption exist at the same time when the phase shifter is under phase modulation, decrease in extinction ratio is inevitable for silicon-based MZI modulator based plasma dispersion effect. In this paper, we demonstrate an optical modulator at 1550 nm wavelength band, using a cascaded compensation method. We balance the optical intensity in the two phase shifters of the MZI structure during modulation. With cascaded compensation method, the modulator has an extinction ratio of 51 dB and a dynamic extinction ratio of 10 dB at bitrates of 40 Gb/s.
Compared with the optical modulator based on 1310 nm and 1550 nm wavelength band, the silicon-based modulator at 2 μm band has a higher absorption loss, since the free carrier effect is more significant in the 2 μm band. In this paper, we demonstrate an optical modulator at 2 μm wavelength band, using a doping compensation method. We reduce absorption loss and keep the modulation extinction ratio at a high level through optimizing waveguide width, PN junction offset and compensated area. With doping compensation, the modulator has an absorption loss by PN junction of 2.8 dB/cm at 0 V and an extinction ratio of 14.2 dB at bitrates of 40 Gb/s.
A high efficient cantilever-type mode size converter applied at 800 nm wavelength is proposed and analyzed in this letter. The converter can compress and couple the light spot from a single mode fiber into the Silicon nitride waveguide effectively and smoothly. The core of the entire structure is supported by the SiO2 cantilever beam to make the device suspended, which can effectively prevent the light leaking from the substrate to cause great coupling loss. A Gaussian light source with a diameter of 4.5 μm and wavelength of 800nm is used for the coupling test. The coupling loss of the device with both TE and TM mode are greater than 0.54 dB. The alignment tolerances for 1-dB excess loss are both ± 0.8 μm in horizontal and vertical directions.
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