Compact polarization transformation-enabled on-chip delay line for frequency-swept laser nonlinearity calibration

An on-chip optical delay line with low-loss and compact waveguide configurations is a crucial component for functional photonic integrated circuits, such as optical coherence tomography, optical gyroscopes, and frequency-modulated continuous-wave (FMCW) LiDAR systems. In this paper, we proposed a low-loss and compact on-chip delay line structure integrated with a polarization transformation-enabled 2-pass architecture. A total delay of 10.46 ns is achieved using a 0.8-m-long silicon nitride waveguide spiral. The losses of the delay line operating in the TE and TM modes are 0.083 dB/cm and 0.213 dB/cm, respectively. Further, the nonlinearity calibration of the frequency-swept laser is conducted with the assistance of the designed delay line. It is indicated that the calibration method is validated in an FMCW LiDAR system, and a significant enhancement in the signal-to-noise ratio is achieved. This work offers valuable insights into developing highly integrated on-chip FMCW LiDAR systems with an improved linearity of frequency-swept output light.