Objective: The demodulation algorithm based on the 3×3 coupler is one of the most widely used interferometric demodulation algorithms. It relies on the physical characteristics of the coupler to achieve symmetrical output signal, and is independent on external loading wave modulation and feedback control to achieve signals with high sensitivity and large dynamic range. Its cost and noise have additional advantages. The homodyne symmetric (NPS) demodulation method based on the 3×3 coupler was put forward for the symmetry condition of the 3×3 coupler to give full play to its advantages. However, the NPS method based on the 3×3 coupler is affected by the deviation of the splitter ratio of the coupler and difference in the light intensity coefficient of the photodetector. The three-channel light intensity direct current (DC) component coefficient and alternating current (AC) component coefficient produces real-time deviation. This leads to the residual DC component in the operation of eliminating the DC component for the original NPS algorithm, which destroys the differential cross-multiplication operation. Additionally, the uneven coefficient of the AC component causes the automatic gain control (AGC) operations failure to eliminate the influence on the optical signal from fluctuations, such as the light source. Eventually, the demodulated phase becomes inaccurate, which affects the sensitivity of the hydrophone. However, existing methods for compensating amplitude deviation cannot satisfy long-term stable balance and real-time correction. To solve the influence of real-time deviation of the AC component coefficients of the three output signals in the NPS system on the demodulation, a method of real-time correction of NPS is applied to the demodulation of distributed feedback (DFB) fiber laser hydrophone, which makes it easy to realize independent and stable demodulation on unmanned platforms. Methods: Wrap the single arm of the interferometer in the NPS demodulation system on a piezoelectric ceramic (PZT), and apply a sinusoidal signal with an amplitude greater than π rad to initiate full amplitude signals of the three outputs. Obtain the maximum and minimum values of the three output signals in the current PZT vibration period, calculate the eigenvalues of the DC component coefficients of each path, and obtain the average value of current eigenvalue and the sum of the DC component coefficients of all previous PZT vibration periods as the current period DC component coefficient; calculate the characteristic value of the AC component coefficient of each channel, and obtain the average value of current characteristic value and the sum of the AC component coefficients of all previous PZT vibration periods as the AC component coefficient of the current period. The three output signals of each period minus the respective periods. The DC and AC components are divided by their respective AC component coefficients to obtain the normalized three-channel AC component signals in real-time and NPS subsequent operations are performed. Finally, the measured phase signal is obtained, and demodulation is completed. Results and Discussions: According to the experiment, comparing the demodulation effect of real-time correction NPS and the unmodified demodulation effect, it is found that the real-time correction demodulation can effectively compensate for the deviation of DC and AC component coefficients, and its demodulation can truly reflect the measured physical quantity, as shown in Figs. 7(a), 7(b), 8(a), and 8(b). When the experimental system works continuously for 15 h, the overall demodulation effect is stable, as shown in Fig. 9. In the underwater vibrating liquid column test experiment, the algorithm can effectively restore the vibration signal even though noise is introduced from an external source and other factors. Conclusions: A signal demodulation method for real-time correction of NPS for fiber laser hydrophone is proposed. This method is used to test 200 Hz4 kHz vibration signals, and the system runs continuously for 15 h. The demodulation results can restore the vibration signal. Theoretical and experimental results show that this method can effectively compensate for the deviation of DC and AC component coefficients, improve the demodulation accuracy of DFB fiber laser hydrophone, and decrease the hardware costs. The calculation amount is not large, and it is expected to realize real-time high-speed miniaturization demodulation. © 2021, Chinese Lasers Press. All right reserved.
