Increasing dynamic range of a fibre Bragg grating edge-filtering interrogator with a proportional control loop

We present a fibre Bragg grating (FBG) interrogator that uses a microcontroller board and a tunable optical filter in a proportional control loop to increase dynamic range and achieve high strain sensitivity. It is an edge-filtering interrogator with added proportional control loop that locks the operating wavelength to the mid-reflection point on the FBG spectrum. The interrogator separates low-frequency (LF) components of strain and measures them with extended dynamic range, while at the same time measuring high-frequency (HF) strain without loss in strain sensitivity. In this paper, we describe the implementation of the interrogator and analyse the characteristics of individual components, such as the speed and voltage resolution of the microcontroller and the tunable optical filter. We measure the performance of the proportional control loop at frequencies up to 1?kHz and characterize the system using control theory. We illustrate the limitation of the conventional interrogator to measure strains greater than 40 ?? and demonstrate successful application of the proposed interrogator for simultaneous measurement of 450 ?? LF strain at 50?Hz superimposed with 32?kHz HF strain.