Wideband and High-Resolution Measurement of Magnitude-Frequency Response for Optical Filters Based on Fixed-Low-Frequency Heterodyne Detection

A high-resolution electrical method is proposed for magnitude-frequency response measurement of optical filters based on fixed-low-frequency heterodyne detection. The method consists of an acousto-optic frequency shifter and two phase modulators located in a Mach-Zehnder interferometer architecture. The modulation sidebands in the test arm pass through the optical filter under test and heterodyne with the frequency-shifted modulation sidebands in the reference arm, which allows extracting the magnitude-frequency response of the optical filter from two fixed-frequency heterodyne electrical components in the low-frequency region. In the demonstration, the magnitude-frequency response of a phase-shifted fiber Bragg grating (FBG) is experimentally extracted from low-frequency heterodyne signals fixed at 69.5 and 70.5 MHz, respectively, with hyperfine resolution up to 50 kHz and wide frequency range of 140 GHz. The proposed electrical method enables wideband magnitude-frequency response measurement for both band-pass and band-stop filters with low-frequency detection. Moreover, it features not only the immunity to modulation nonlinearity but the bidirection sweeping with doubled measurement range as well. Besides, the method only needs to receive and analyze low-frequency signals at two fixed frequencies, which largely speeds up the swept frequency measurement.