Abstract

In an unbalanced interferometer, the phase noise level is positively related to the optical path length difference (OPD) and the wavelength instability of the light source. The effective current solution for reducing phase noise is to use an auxiliary reference interferometer to measure the phase noise signal. However, this method requires that all interferometers have the same OPD, otherwise the phase modulation depth of different interferometers with the same light source will be different and the noise reduction effect will be severely reduced. It is difficult to provide interferometers with precisely the same OPD in practical. In this paper, an improved phase-generated carrier (PGC) demodulation technique for phase noise cancellation that does not require strictly equal OPDs of interferometers is proposed. Combining an auxiliary reference interferometer scheme with an ellipse fitting algorithm (EFA) can eliminate the effect of different phase modulation depths due to different OPDs, and phase modulation depth drift on demodulation results, significantly reducing harmonic distortion. The additional phase modulation signal ensures the correct calculation of small signals by the EFA, while the OPDs of the interferometers are evaluated in real time to achieve constant phase noise cancellation capability under different OPD. Experimental results show that the proposed technique achieves a highly stable phase demodulation result with a maximum phase noise reduction of about 9 dB and a minimum THD of −74.59 dB in interferometers with non-strict equal OPD.

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