Signal processing for strain measurement system with fiber Bragg grating

It is well known that the Bragg wavelength of a fiber Bragg grating (FBG) is shifted according to the applied strain. How to encode the wavelength information into a proper corresponding signal is one of the research points. This paper describes a proportional-integrated (PI) control, which enables real-time wavelength interrogation. The system employs 2 pieces of FBGs. One, called sensor FBG, is to accept the induced strain; the other one, called filter FBG, is to detect the Bragg wavelength of the sensor. In another word, if the Bragg wavelength of the filter is made the same to the Bragg wavelength of the sensor, the strain on the sensor can be read from the filter. The PI control theory, which is to control the filter FBG's Bragg wavelength, is described. The relationship between the induced strain on the sensor FBG and the measured strain from the filter FBG is analyzed and discussed.