Increased Spurious-Free Dynamic Range in Frequency Sweeping Interferometry by Suppression of Parasitic Amplitude Modulation

We propose an envelope detrending method to correct parasitic amplitude modulation (AM) in frequency sweeping interferometry (FSI). Parasitic AM arises from effects such as device spectral response and residual AM of laser. The periodic component in parasitic AM introduces spurious harmonics, which degrade the spurious-free dynamic range (SFDR) and interfere with information analysis. The proposed method employs a Hilbert transform to obtain a time-varying amplitude fluctuation curve from an auxiliary interferometer. This curve is then employed to detrend the parasitic AM in the main interferometric signal, thus restoring the point spread function and improving the SFDR. An optimal SFDR can be obtained when the optical path differences (OPDs) of the two interferometers are equal, while mismatched OPDs lead to a rapid degradation of the SFDR correction. Experimental results demonstrate the effectiveness of the proposed method and the optimal matching condition. A maximum SFDR of 90.2dB can be achieved when the OPD matching condition is satisfied. By applying the method to an optical frequency domain polarimeter, we realized a measured 81.1dB polarization extinction ratio. We believe that the proposed method can also help to improve the SFDR in other FSI systems.