Phase-generated carrier combined with the Hilbert transform for phase demodulation in frequency-scanning interferometry

Abstract

Frequency-scanning interferometry (FSI) with an external-cavity diode laser (ECDL) is an effective technology for the absolute distance measurement of a target. The measurement accuracy of the FSI system is directly determined based on the phase extraction accuracy of an interference signal. However, because the optical frequency scanning of the ECDL exhibits nonlinearity, a large phase extraction error is introduced into the phase calculation result, which decreases the measurement accuracy. In this study, a composite algorithm for implementing phase-accurate demodulation is developed by combining the phase-generated carrier (PGC) technology and the Hilbert transform (HT) error compensation principle. In PGC technology, the demodulated phase contains the nonlinear error and quadrature phase deviation resulting from an amplitude ratio that is not equal to 1, and non-strict orthogonality of the two quadrature components. To compensate for these two errors, HT is utilized. The amplitude and phase of one of the two quadrature components are compensated by using the analytic signals on the complex plane, which ensures that the two quadrature components have the equal amplitudes and strict orthogonality. The effectiveness and stability of this composite algorithm are verified by a series of simulations and experiments. The experimental results indicate that the proposed method is capable of maintaining the single-cycle relative error of phase extraction within the 0.3% limit, thereby improving the measurement accuracy of the FSI system.