串行声光调制的非保偏光链路外差算法实现光纤水听器低噪声稳定解调

Low-Noise Stabilization of Demodulation in Fiber-Optic Hydrophones Using Heterodyne Algorithm with Non-Polarization-Maintaining Optical Path and Serial Acousto-Optic Modulation

传统外差算法用于干涉型光纤水听器信号解调时,需要产生具有稳定频差的双脉冲光,常见的解决方案是在马赫-曾德尔干涉仪(MZI)的两臂并行接入不同频率的声光调制器(AOM)。上述方案中,两台独立AOM的运行差异会引入额外噪声,而含有AOM的保偏光链路体积大,难以有效隔振,抗环境干扰极差。基于此,引入迈克耳孙干涉仪(MI)光路先产生同频同偏振的前后光脉冲,再利用串行在输出光路上的后级AOM分时切换调制频率,使双脉冲光之间产生光频差。相比于传统方案,所提出的差频产生光路的频差由同一AOM产生,无分立器件性能差异的影响,同时更小体积的非保偏MI光路易进行抗振设计和封装,因此所提方案具有更强的抗干扰能力。实验结果表明:MI加串行声光调制的外差方案在10 Hz、100 Hz、1000 Hz 3个频点的解调噪声功率谱密度(PSD)分别低至-90dB/√Hz、-100dB/√Hz和-104dB/√Hz;8 h内,10 Hz频点的解调噪声起伏小于2 dB,100 Hz和1000 Hz两个频点的解调噪声起伏均低于1 dB,实现了光纤水听器低噪声高稳定解调,满足实际应用需求。

Objective Realizing stable and low-noise phase demodulation is crucial for the highly sensitive detection of weak external signals in interferometric fiber-optic hydrophones.The heterodyne method is one of the most popular phase recovery algorithms due to its ease of hardware implementation and minimal algorithmic complexity.The common heterodyne algorithm involves interfacing acousto-optic modulators (AOMs) of different frequencies in parallel within the two arms of the Mach?Zehnder interferometer (MZI).There are two drawbacks to the aforementioned scheme.The disparate operation of the two independent AOMs introduces additional noise,while the large volume of the AOM-incorporated polarizationpreserving optical path hinders effective vibration isolation and renders the system highly susceptible to environmental interference.To address these challenges,we propose the Michelson interferometer (MI) to directly generate front and back dual-pulse light with identical frequency and polarization.Subsequently,the paper utilizes the back-stage AOM serialized on the output optical path to switch the modulation frequency in real time,creating an optical frequency difference between front and rear optical pulses.Unlike conventional schemes,the difference frequency generation optical path has the frequency difference produced by the same AOM without being affected by performance differences in discrete devices.Furthermore,a compact non-bias-preserving MI optical path is devised and assembled to withstand vibrations,enhancing the overall system’s resistance to interference.Methods Firstly,we provide an overview of the fundamental principles of the heterodyne demodulation algorithm.It is demonstrated through the process of formula derivation that achieving demodulation results with low noise and high stability requires ensuring the algorithm’s high stability in difference frequency and mitigating environmental interference in the optical path.Subsequently,the principle of AOM explains that the primary determinants influencing the difference frequency stability of the algorithm are frequency and amplitude.With the theoretical foundation,the simulation substantiates the influence of difference frequency stability on demodulation noise,as illustrated in Fig.3.Specifically,frequency and amplitude shifts in the differential frequency lead to a notable degradation of the demodulation noise and subsequent distortion.Analysis of the traditional algorithm’s optical path for difference frequency generation confirms significant inadequacies in both the stability of the difference frequency and its resilience to environmental interference.Therefore,a scheme integrating MI with serial AOMs is proposed,as illustrated in Fig.5.This enhanced scheme fundamentally addresses the challenges inherent in the traditional scheme.Finally,the experimental optical path before and after the improvement is constructed for a comparison test to validate the theoretical and experimental advantages of the enhanced scheme.Results and Discussions By constructing the actual optical experiment as illustrated in Fig.7,the results indicate that utilizing the MI with a serial AOM heterodyne scheme yields the demodulated noise power spectral density (PSD) values as low as -90dB/√Hz、-100dB/√Hz,and -104dB/√Hz respectively at the frequency points of 10,100,and 1000 Hz,as illustrated in Fig.10.Over 8 hour,the demodulated noise exhibits fluctuations of less than 2 dB at the 10 Hz frequency point,and less than 1 dB at both the 100 Hz and 1000 Hz frequency points,as illustrated in Fig.11.For the conventional algorithm relying on MZI with parallel AOM,there are distinct high frequency oscillations present in the time-domain signals and multiple spectral lines of various sizes in the frequency domain spectrum,with the overall noise floor being elevated by nearly 10 dB.Conclusions The MI-based optical path can effectively avoid the incorporation of a large number of polarizationpreserving devices in the difference frequency generation optical path.This scheme greatly reduces the package volume of the optical path structure,simplifies the package scheme,and enhances vibration resistance.Furthermore,the serial use of the AOM ensures that the modulation carriers of the front and back pulse light involved in the interference originate from a singular AOM,significantly reducing additional noise from frequency stability differences between parallel AOMs.The experimental data shows that the proposed difference frequency improved optical path scheme exhibits characteristics such as low noise,absence of conspicuous spurious spectral lines within the bandwidth,and heightened long-term stability in the demodulation of fiber-optic hydrophone signals via the heterodyne algorithm and has substantial engineering applicability.