Real-Time Frequency Characterization of Chirped Lasers via Finite-Difference Phase Measurements

Abstract

Laser instantaneous frequency (LIF) describes the time-varying frequency of a laser signal, defined as the temporal derivative of the oscillation phase divided by 2π. This concept is crucial in understanding frequency noise, phase noise, and chirped optical signals, where the frequency changes over time. Most current optical frequency measurements rely on frequency-domain analysis, which face limitations when applied to rapidly chirped lasers. Unlike frequency-domain methods, time-domain methods provide a more accurate representation of the time-varying frequency of chirped lasers, which is crucial for real-time applications. The widely used Hilbert transform-based method describes the LIF measurement but has slow real-time responses, a relatively low measurement speed, a not on-chip-compatible structure, and the ambiguity of the chirp sign, which hampers its use, particularly in real-time applications. This paper introduces a novel real-time LIF estimator for chirped lasers, offering higher accuracy, reduced algorithm complexity, and an on-chip integration solution. Experimental demonstrations include applications for FMCW LiDAR adaptive nonlinearity correction and real-time photoelectric control loops.