Suppression of the spectral overlap in interferometric optical vector analyzers by introducing a common mode branch

Abstract

In this paper, a novel optical vector analyzer (OVA) is proposed and experimentally demonstrated by us based on coherent optical frequency modulated continuous-wave (FMCW) interferometry. The optical vector analysis of optical devices under test (ODUTs) maintains high accuracy despite strong sweep nonlinearity with our method. Basically, due to the linear frequency sweep of optical FMCW, the beat photocurrent consisting of a baseband frequency (BF) signal and an intermediate frequency (IF) signal is generated. Usually we extract the IF signal by filtering and then Hilbert transform to get the frequency response. However, when the sweep nonlinearity is strong, the spectral overlap will occur, and simple filtering will cause severe distortion. We innovatively introduced a common mode branch whose signal is the same as the BF signal in the beat photocurrent. Due to the introduction of the common mode signal, the non-aliased IF signal can be extracted without filtering, which solves the measurement distortion caused by spectrum aliasing and filtering. What’s more, an optical tunable bandpass filter (OTBPF) is added to the correction path to locate the optical frequency for the first time. The phase-shift-fiber grating (PS-FBG) in 1550nm band is measured, and the effectiveness of this method is verified. We eliminate the distortion brought about by the spectral aliasing and filtering with the introduction of the common mode signal path and the measurement accuracy is greatly improved. Frequency response resolution accuracy of this method reaches 33MHz which is mainly limited by the presentation difference.