FBG interrogator based on two-stage digital PI controller for high-frequency signals

A fiber Bragg grating (FBG) interrogator with temperature compensation using digital proportional-integral (PI) controllers is proposed. This interrogator uses a two-stage controller design along with the edge-filter technique. The first stage is used to define the initial wavelength of the laser source, while the second stage locks the wavelength in order to follow the reflected optical power. We demonstrated the working principle of the interrogator using a climatic chamber, varying the temperature over an FBG from 25 to $60 ^{\circ}\mathrm{C}$. The reflected power remained constant, with a relative error of approximately 1%. We also applied a 50 kHz acoustic pulse to cause a vibration over the FBG, demonstrating that while the system compensates slow temperature variations, it can respond to fast signals, such as acoustic waves.