High-Resolution Resonant Optical Fiber Sensor Interrogation Technique Using a Square-Wave Modulated DFB Laser Diode

We demonstrate a high-resolution, low-cost interrogation technique using a square-wave modulated DFB laser diode. The method does not require knowledge of the frequency chirp function, instead using only three parameters: the laser diode average power, intensity modulation amplitude, and the average frequency. The last of which, we show can be easily measured on an optical spectrum analyzer. We improve on previous methods by recovering the lineshape and its first derivative, and by averaging over the modulation cycle to use the full signal power. Using a tunable Fabry-Pérot, we demonstrate a resolution that increases with bias point and modulation amplitude with a minimum of 17 fm.