Acousto-Optic Modulation Induced Noises on Heterodyne-Interrogated Interferometric Fiber-Optic Sensors

Acousto-optic modulator (AOM) is a key component in heterodyne-interrogated interferometric fiber-optic sensors. Here, we analyze in detail the effect of AOM on the noise floor of interferometric fiber-optic sensors with heterodyne detection. Besides the well-known phase noises caused by the AOM driver, the intensity noise induced by the relative diffraction efficiency fluctuation (RDEF) of AOM is considered and for the first time studied in detail. The power spectrum density of the intensity noise is found 0.8-0.9 times of the “switching noise,” which is defined as the sum of RDEF and relative intensity noise of the laser. The switching noise varies with the driving power of AOM and can be reduced by setting the driving power close to its saturation point. Experimentally, we achieved ~5-dB suppression of the switching noise and ~2-dB reduction of the overall noise floor. By working at the optimal driving power, together with reducing the AOM-induced phase noise to -130 dB level at the frequency offset twice of the heterodyne frequency, the overall noise floor of the sensor V can be optimized to about -101 dB, i.e., the order of 10 μrad/ Hz.