Carrier-Suppressed Single Sideband Signal for FMCW LiDAR Using a Si Photonic-Crystal Optical Modulators

We have studied frequency-modulated continuous-wave (FMCW) light detection and ranging which uses externally modulated light instead of a wavelength-chirped laser diode and a complicated clock sampling circuit. In this method, the FMCW beat signal is accompanied by a 3 dB average loss in principle when pseudo frequency-modulated light simply generated by an intensity modulation with frequency-swept signals is used. In this article, we first fabricated a Si in-phase/quadrature-phase modulator equipped with compact photonic-crystal phase shifters and generated carrier-suppressed single sideband modulation signals. Furthermore, we obtained almost ideal frequency modulation, with a side-mode suppression ratio of 22.3 dB, by temporally controlling the frequency and phase of the modulation signals. When the FMCW beat signal was produced with this modulated signal in a fiber delay interferometer, its intensity was increased by 5.3 dB, as compared with that using intensity modulation. This improvement corresponds to an 80% increase in the limit distance of FMCW ranging. We also generated similar signals using a single modulator with a bypass waveguide, which simplified modulation. The side-mode suppression ratio for the frequency-swept modulation was limited to 9.8 dB, while comparably large increase in the beat intensity was confirmed.