Hyperfine optical vector analysis of ultranarrowband optical bandpass filters based on coherent two-tone sweeping and fixed low-frequency detection

A hyperfine optical vector analyzer (OVNA) is proposed and experimentally demonstrated to characterize ultra-narrowband optical bandpass filters (OBPFs). In this scheme, a coherent two-tone optical signal (TTOS) with a small frequency interval is generated to act as a probe light. Through finely sweeping the TTOS across the passband of the OBPF via electro-optic modulation, the magnitude and phase responses of the OBPF under test can be measured with a high signal-to-noise ratio based on fixed low-frequency detection. The frequency response measurement is immune to external disturbance. In the experiment, the magnitude and group delay responses of a fiber-based Fabry-Perot tunable filter (FFP-TF) and a fiber-based Fabry-Perot interferometer (FFP-I) with 3-dB bandwidths of 1.5 GHz and 60 MHz, respectively, are successfully measured. In addition, the measurement uncertainty is theoretically and experimentally analyzed. This method paves a way to characterize ultra-narrowband OBPFs with high out-of-band rejection ratios.