Fig. 2: Frequency-locked microwave, optical, and electron spectra.
From: Unifying frequency metrology across microwave, optical, and free-electron domains
a Photograph of the photonic chip mounted on a custom holder. Inset: optical micrograph of the microresonator and schematic depiction of the electron beam position. b Microwave spectrum recorded with an ESA. The beatnote around 250 MHz is from the repetition rate frep of the optical frequency comb, and the other beatnotes are generated by beating the CW laser with the frequency comb teeth. Inset: microwave spectrum when the OPLL is activated; the smoothed beatnote shows a 3-dB bandwidth of 598 kHz. c Optical spectrum recorded with an OSA. The narrow peak corresponds to the CW laser and the lower plateau is from the bandpass-filtered frequency comb. Individual comb lines are not resolved by the OSA. d Electron spectrum recorded with the electron spectrometer. The raw spectrum (gray) is first smoothed (black) with a Gaussian pre-filter, and then deconvolved to generate a comb-like spectrum with well-separated peaks (green). The electron spectrum without optical modulation (light green shaded area) is illustrated for comparison. Inset: zoomed-in view of the central part of the spectra.
