Study on the processing of the heterodyne interference signal for phononic crystal detections

To satisfy the demands for the resonance frequency detection of the phononic crystal, that is to get the form of vibration of the phononic crystals (PCs), which include the amplitude and the phase information of the vibration continually, we use the optical heterodyne interference technique to detect the vibration so as to get the photocurrent which contains the information we need to process. The phononic crystal was excited by the piezoelectric excitation. As the photocurrent is weak, we use the auto-gain-control (AGC) circuit to enlarge it, and then we use the phase-locked-loop (PLL) frequency-mixing method to demodulate the signal which reflects the vibration amplitude and the phase. The AGC circuit used in this system has overcome the difficulties of the large-scale variation of the photocurrent caused by the incline of the test-bed and other factors, thus we can realize the accurate and continuous detection of the weak vibration in time. Its anti-jamming ability, quick response, and high signal to noise ratio (SNR), make this method more important to the on-line and continuous detection of the phononic crystal.