High-resolution and wideband optical vector analysis using optical FMCW and fixed low-frequency detection

Abstract

We propose a wideband optical vector analyzer (OVA) with high frequency resolution using optical frequency-modulated continuous-wave (FMCW) and fixed low-frequency detection. In the proposed OVA, an optical FMCW is divided into two portions. One portion propagates through an acousto-optic modulator (AOM) to introduce a fixed frequency shift, and the other undergoes a variable optical delay line (VODL) to match the delay of the two branches. Then, the two optical FMCWs with a fixed frequency interval are combined as the probe signal. After passing through an optical device under test (DUT), the probe signal carrying the response of DUT is transmitted into a low-speed photodetector to perform frequency downconversion. Hence, an intermediate photocurrent (IF) with a fixed frequency is generated and sampled by a low-speed analog-to-digital converter (ADC). Finally, the frequency response of the DUT can be obtained by utilizing a short-time Fourier transform (STFT) algorithm. The proposed OVA is low-cost with a simple structure and has a wide measurement range. Besides, by applying the optical FMCW with a large chirp rate, the proposed FMCW-based OVA can realize a high measurement speed and a high frequency resolution. An experiment is performed in which the chirp rate of the optical FMCW is 1000 nm/s. As a result, the measurement speed achieves 2 ns/point and the frequency resolution is 0.22 MHz within the measurement range of 17 nm.