# Demodulation Technique Based on Laser Interference for Weak Photo-Acoustic Signals on Water Surface
## 4 Discussion  
To address spectrum aliasing that occurs during underwater acoustic source detection with laser interference, a demodulation method capable of measuring multiple frequency bands is proposed in this paper. Our method achieves lower frequency detection and tracking of change signals,  
Figure 10: Demodulation result and fitting curve of amplitude modulation signal.  
Figure 9: Standard deviation of amplitude measurement.  
which cannot be realized by previous research. In our approach, the spectrum of interference signals was analyzed, and different solutions were chosen depending on frequencies: for frequencies higher than 2 kHz, a characteristic ratio method was adopted, while for those lower than 2 kHz, a PGC-based improved arc tangent function demodulation method was used. In the former, the central point of a high-frequency band was used as the measurement frequency of the detection signals, and characteristic ratio \(R\) was calculated before the amplitude was obtained in a lookup table. The PGC-based demodulation method enables demodulation with an improved arc tangent function, which gives initial phase and modulation depth of high-frequency carriers. After low-frequency filtering, amplitude and frequency of WSAW can be extracted in the optical dark chamber (SNR about 14 dB). The detection approach adopted in this paper applies to the frequency range between 200 Hz and 10 kHz while providing a frequency measurement accuracy better than 1 Hz. It also features high repeatability for amplitude measurement, with a standard deviation not exceeding 2.5 nm. As demodulated signals are generated through demodulation, time-varying signals may be measured as well. The results of amplitude modulated signal tests show that this approach is able to track signal changes.  
Certain restrictions still exist in the approach adopted in this paper. The lower limit of frequency detection is dependent on filter performance and water surface disturbance waves, due to which the demodulation effect for signals below 200 Hz is not significant. On the other end of the spectrum, the upper limit of frequency detection is related to the high-frequency carriers, and should be limitless in theory. As an underwater speaker was used in our study to excite WSAWs as carriers, the frequency response attribute of the speaker also plays a role. Frequency shift devices will therefore be incorporated in subsequent research, and will hopefully enable carriers with a frequency much higher than that of acoustic waves. Furthermore, the next experiment is to consider the impact of natural light in the natural environment, perhaps considering optical filters and other devices to improve system SNR.  
Conceived the Method and Wrote the Paper, Z.-X.L.; Designed and Performed the Experiments, M.-H.J.; Edited the Manuscript, T.-W.Y. This study was funded by the National Natural Science Foundation of China (61108073); Shanghai Aerospace Science and Technology Innovation Foundation (SAST2015029); Postdoctoral Researchers Settled in Heilongjiang Research Foundation (AUGA4120006016).  
The authors declare no conflict of interest.