Fiber optic interferometric sensors (FOIS) are characterized by numerous advantages, including high sensitivity, large dynamic range, compact size, lightweight construction, and immunity to electromagnetic interference. However, in practical applications, the continuously fluctuating external environment can significantly disrupt the FOIS array, particularly affecting the optical transmission path. This disruption introduces additional noise, which negatively impacts detection capabilities. The phase generated carrier (PGC) algorithm exhibits superior suppression of common mode noise generated by optical paths, owing to its compatibility with nearly-balanced interferometers, thereby surpassing conventional heterodyne techniques. Furthermore, it utilizes a singular optical pulse for both transmission and reception, which provides advantages over the 3×3 method in terms of time-division multiplexing and the reduction of required hardware channels. Nonetheless, the PGC algorithm must accurately calculate the phase delay and modulation depth of the received interference signal to mitigate nonlinear errors in the demodulation results. This calculation process is intricate and time-consuming, particularly in large-scale array scenarios. To address this challenge while maintaining the advantages of PGC in optical configurations and overcoming the complexities associated with parameter calculations, this paper proposes an improved demodulation technique for FOIS.

The methodology utilizes a periodic linear frequency-modulated (LFM) continuous wave to drive a singular acousto-optic modulator (AOM). This configuration produces a frequency-modulated optical pulse that experiences self-mixing interference within a nearly-balanced Michelson interferometer. Consequently, this procedure results in the generation of a difference carrier component, facilitating the heterodyne demodulation of the signal, designated as AOM-LFM heterodyne (ALH). The ALH is fully compatible with the conventional PGC algorithm in terms of optical configuration. It also supports the transmission and reception of individual optical pulse, demonstrates robustness against environmental disturbances, and achieves a time slot utilization rate approaching 100%. Furthermore, the ALH demonstrates reliability without necessitating calculations for carrier phase delay and modulation depth, significantly reducing the computational demands. To address the discrepancy in diffraction efficiency of the AOM resulting from different frequencies, an amplitude pre-distortion (APD) compensation module is incorporated. Additionally, to address the carrier frequency deviation arising from differences in the optical path difference (OPD) of the FOIS, the implementation of orthogonal signal normalization (OSN) is recommended. This paper provides a detailed discussion on the simulation of algorithmic principles, the amplitude pre-distortion of frequency modulation drive signals, the normalization of orthogonal signals, and the analysis of empirical experimental data.

The paper systematically adjusts the phase values and observes the corresponding changes in total harmonic distortion (THD) and signal-to-noise and distortion (SINAD) in the demodulation results. The research findings indicate that as the phase delay varies from 0° to 360°, the actual variations of THD and SINAD are approximately ±3.5 dB, without significant fluctuations observed. This suggests that the ALH algorithm, similar to traditional dual-frequency heterodyne algorithms, demonstrates insensitivity to changes in the initial phase.

Subsequent to the employ of the APD module, the inconsistency in amplitude among the time-division pulse lights within the array is significantly reduced. The introduced OSN module proficiently tackles the issue of orthogonal signal distortion caused by variations in optical path difference, thereby alleviating the nonlinearity present in the demodulation outcomes. The experimental results demonstrate that at a frequency of 1 kHz, the demodulation noise can be minimized to -108 dB/$\sqrt[]{\mathrm{H}\mathrm{z}}$, while the maximum dynamic range reaches 24.4 dB. Furthermore, during an uninterrupted operational period of 8 h, the amplitude variation of the array demodulation results remains below 0.4 dB, with no spurious line spectrum detected within the specified frequency band. All performance metrics satisfy the criteria for practical applications.

The ALH algorithm directly drives the AOM using a periodic linear frequency-modulated sine pulse signal, achieving dual modulation of both the frequency and pulse of the input light. The generated interrogation optical pulse enters the nearly balanced interferometric sensor element, where it experiences self-mixing interference, and produces a difference frequency carrier term, thereby enabling low-frequency heterodyne demodulation of external signals. The ALH scheme effectively retains the high duty cycle and robust anti-interference capabilities of the nearly balanced interference optical path, while simplifying the engineering implementation of the algorithm software. Additionally, it offers the advantage of flexible configuration for both the modulation period and the difference frequency carrier frequency, allowing it to better accommodate sensing arrays with varying application requirements. Furthermore, unlike traditional phase modulators, which necessitate high polarization maintenance and vibration resistance in the optical path, AOM devices do not require polarization-maintaining inputs. This characteristic significantly mitigates the integration challenges associated with multi-wavelength laser sources. Simultaneously, the high stability of AOM devices ensures that the entire demodulation results are devoid of spurious line spectra, while maintaining consistent demodulation amplitude and SINAD across array elements.