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The influence of the frequency modulation induced laser amplitude modulation on the calibration of the 3×3 coupler parameters and its suppression

The photoelectric signal detection technology based on the 3×3 coupler is widely used in various interferometric sensing systems. In practice, the three-way asymmetry of the 3×3 coupler seriously affects the demodulation of the interferometric optical fiber sensing system. Therefore, the accurate and stable calibration of the demodulation parameters in the 3×3 demodulation, such as the DC quantity, the AC quantity, and the phase difference between the couplers, is of vital importance to the 3×3 demodulation. In the process of studying the ellipse fitting calibration parameters based on optical frequency modulation, it is found that the laser's associated amplitude modulation fluctuation has a greater impact on the demodulation. Based on this, this paper conducts an in-depth study on the impact of the associated amplitude modulation through simulation and experiments and propose a method to eliminate the associated amplitude modulation, that is, the three-channel interference signal is proportionally divided by the laser output signal to eliminate the laser associated amplitude modulation fluctuation caused by optical frequency modulation. The experimental results show that this method can eliminate the associated amplitude modulation well. The influence of amplitude modulation improves the calibration accuracy of 3×3 demodulation parameters, realizes the stable coefficients calibration of the interferometric optical fiber sensing system, and greatly improves the stability and reliability of the system.