Fig. 1. Schematic of the optical vector network analyzer.

a The principle of our VSA is based on a chirping CW laser that is sent to and transmits through a device under test (DUT), or interferes with any light under test (LUT). The DUT can be any passive device, and the LUT can be any broadband laser source. The transmission spectrum of the chirping laser through the DUT, and the beatnote signal generated from the interference between the chirping laser and the LUT, are both time-domain traces. Together with a “frequency ruler” for calibration, the chirping laser coherently maps the time-domain trace into the frequency domain. This trace carries the information of the DUT’s loss, phase and dispersion over the chirp bandwidth (blue). For LUT, the chirping laser beats progressively with different frequency components of the optical spectrum, thus, analyzing the beat signal in the RF domain allows extraction of the spectral information (purple). Critical to this frequency-time mapping is precise and accurate calibration of the instantaneous laser frequency during chirping. This requires referring the chirping laser to the frequency ruler (red). b Experimental setup. The chirping laser unit can be a single laser, or multiple lasers that are bandwidth-cascaded. The latter allows extension of the full spectral bandwidth by seamless stitching of individual laser traces into one trace. The chirping laser power is then split into two branches. The upper branch is directed to the frequency-calibration unit (i.e. the “frequency ruler”), which in our case, is a phase-stable fiber cavity of 55.58 MHz FSR. The lower branch is sent to the DUT or the LUT. For the two branches, the photodetector signals are recorded with an oscilloscope and digitally processed offline. PD photodetector, OSC oscilloscope. Note that the figure and caption are reproduced from the original manuscript¹²