Correction of laser sweeping nonlinearities using ultralow-loss on-chip 7 m spiral resonators

Swept laser interferometry is an extremely powerful solution embedded in several recent technologies such as absolute distance measurement, light detection and ranging (LiDAR), optical frequency domain reflectometry, optical coherence tomography, microresonator characterization, and gas spectroscopy. Nonlinearity in the optical frequency sweeping of tunable lasers is a fatal drawback in gaining the expected outcome from these technologies. Here, we introduce an on-chip, millimeter-scale, 7 m spiral resonator that is made of ultralow-loss Si₃N₄ to act as a frequency ruler for correction of the tunable lasers sweeping nonlinearities. The sharp 2 MHz frequency lines of the 8.5 × 10⁷ high-quality factor resonator and the narrow-spaced 25.566 MHz frequency ticks of the 7 m spiral allow unprecedented precision for an on-chip solution to correct the laser sweeping nonlinearity. Accurate measurements of the ruler’s frequency spacing, linewidth, and temperature and wavelength sensitivities of the frequency ticks are performed here to demonstrate the quality of the frequency ruler. In addition, the spiral resonator is implemented in an frequency-modulated continuous-wave LiDAR experiment to demonstrate a potential application of the proposed on-chip frequency ruler.