Phase Encoding for FBG Code-Division Multiplexing Sensing System in the Ultrasonic Frequency Range

This paper presents a phase encoding method for fiber Bragg grating (FBG) code division multiplexing (CDM) sensing systems, based on coherent detection technology and Hadamard sequences. A four-channel CDM sensing system has been constructed based on this method, enabling synchronous monitoring of four ultrasonic vibration signals within the frequency range of 50 kHz to 500 kHz. The experimental results demonstrate that the time-domain signal to noise ratio (SNR) of the demodulated signal typically exceeds 30 dB. The crosstalk suppression that meets the engineering standard (15 dB) reaches over 98%, indicating that the sensing system has good monitoring stability. Furthermore, within the 500 kHz test frequency range, the CDM sensing system can simultaneously detect up to 32 channels and has the potential for integration with other multiplexing techniques, such as wavelength division multiplexing (WDM), to further increase the multiplexing number of FBGs in sensing system. Finally, the multiplexing experiment of special waveform signals was conducted to verify the system's applicability in practical scenarios.