Distance measurement error sources in Fourier-based frequency sweeping interferometry using a gas cell for laser frequency sweep speed determination

Abstract

Fourier-based frequency sweeping interferometry (FSI) is an interferometric technique, which offers absolute distance measurement based on the detection of interference beat frequencies of reflected signals. Through the multiple beat frequencies detected, Fourier-based FSI performs robust and simultaneous measurements of absolute distances to multiple targets. This measurement technique is less sensitive to the variations in reflected optical signal intensity because frequency peaks in the Fourier spectrum are easily retrievable for weak interference signals without any significant degradation of the measurement accuracy. Moreover, the tolerance for variations in the reflected light intensity makes Fourier-based FSI useful for distance measurements using various reflector types, even those with low reflectance materials. The drawback of Fourier FSI method is the need to use complex and time-consuming computations, consisting of fast Fourier transform, laser sweep speed estimation and data resampling, to correct the non-uniform sweep speed of the laser. The complexity of the computations and the quality of the measurement results are also strongly dependent on the setup of the FSI components. In addition, the detected beat frequencies are highly impacted by the target retroreflector movements or vibrations, which introduce Doppler shifts to the detected frequencies that causes drift effects. This paper describes the distance measurement error sources of Fourier-based FSI interferometer components and assesses their impact on the measurement uncertainty. All analyses are based on a CERN FSI interferometer setup, operating in Cband optical sweep frequency range, with a sweep speed of 2000 nm/s, equipped with a hydrogen cyanide gas cell for frequency tracking purposes.