High-speed interrogation system of multi-encoding weak FBGs based on FDML wavelength swept laser

A high-speed and high-capacity interrogation system, which uses Fourier domain mode locked (FDML) and multi-encoding weak FBGs, is proposed. As a wavelength-swept laser, the FDML transforms the wavelength domain into the time domain to achieve WDM; furthermore, its ultra-fast scanning facilitates the multiplex of WDM combinations in the form of TDM. The application of FDML enables the time division of these two multiplexing methods using only one scanning. Next, a multi-encoding weak FBG array is deigned; each FBG can be distinguished and interrogated by its unique code at high repetition rate. Theoretically, the array maintains the high-speed interrogation of the WDM while breaking through the bandwidth limitation of the wavelength-swept laser to expand the multiplexing capacity using the TDM. A proof-of-concept system consisting of 35 multi-encoding weak FBGs with 5-wavelength by 7-time codes is built to demonstrate the sensing performance. In the temperature and vibration experiments, an interrogation linearity above 0.9993 and a 100-kHz interrogation speed are achieved.