Effect of laser phase-induced intensity noise on multiplexed fiber-optic sensor system using optical loop with frequency shifter

We have proposed a multiplexed fiber-optic sensor system using an optical loop with a frequency shifter. The measured output power spectrum of the system has shown that the multiplexed signals superimpose upon a noise pedestal which is like a series of hill peaks. In this paper, the output power spectrum is theoretically analyzed from the output intensity autocorrelation function. It displays that the noise pedestal originates from the laser phase-induced intensity noise. The noise level depends on the coherence time of the laser source. The positions of peaks are decided by the working frequency of the frequency shifter in the optical loop. The sensitivity of the system are related to the bandwidth B, the coherence time T-C, the sensor number n to be multiplexed, the loop loss alpha, and the fiber coupler parameters. Properly choosing these parameters is beneficial to improve the sensitivity of system.