Fig. 1: Phase-locked breather comb generation and characterization.
From: Frequency microcomb stabilization via dual-microwave control
a Schematic of the current f–2f nonlinear interferometry to measure fceo. By comparing the higher-frequency comb segment and the second-harmonic of the lower-frequency segment, a beat note at fceo with sufficient signal-to-noise-ratio (SNR) is generated on a photodetector (PD). This often requires comb spectral broadening in a highly nonlinear fiber and/or a broad octave-level comb span. b The unique generation mechanism of frequency microcombs provides an alternative full-stabilization route that does not require external nonlinear processes. Modulation instability and four-wave mixing then generates the initial comb lines with Δ spacing, and subsequently secondary lines with frep spacing. Often, Δ is not an integer multiple of frep. The frequency microcomb therefore has an offset frequency ξ innately. As elaborated later, ξ resembles fceo in directly gauging the optical frequency instability. c Example frequency microcomb spectrum showing subcombs around primary lines yet to fully merge. d Primary comb lines are formed with a spacing (Δ) of 1.1 nm and then generate overlapping subcombs. e Electrical spectrum measures two distinct beat notes of frep = 17.9 GHz and ξ = 523.35 MHz, confirming the presence of a unique ξ (f) and (g) Free-running change in frep and ξ at different spectral regions are measured to be the same within error bars of ≈2 kHz and ≈200 kHz, respectively. At wavelengths where the beat notes have SNR higher than 10 dB (100 kHz RBW), 10 measurements are taken to determine the mean values of the comb spacing and offset frequencies. Measurement error bar is the peak-to-peak deviation from 10 measurements.
