  Swept laser interferometry is an extremely powerful solution embedded in
several recent technologies such as absolute distance measurement, light
detection and ranging, 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 onchip, millimeter scale, 7 m spiral resonator that is made of
ultralow loss silicon nitride to act as a frequency ruler for correction of the
tunable lasers sweeping nonlinearities. The sharp 2 MHz frequency lines of the
85 M high-quality resonator and the narrow spaced 25.57 MHz frequency ticks of
the 7 m spiral allow unprecedented precise nonlinearity correction on an
integrated photonics platform. Accurate measurements of the rulers 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 FMCW LiDAR
experiment to demonstrate a potential application of the proposed onchip
frequency ruler.
