Interrogation of fiber Bragg grating displacement/bending sensors using dual-wavelength pulses generated from a self-seeded Fabry-Perot laser diode

We demonstrate a simple technique for the interrogation of fiber Bragg grating (FBG) displacement/bending sensors using a self-seeded Fabry-Perot laser diode (FP-LD). The sensing element consists of two almost identical FBGs mounted on the surface of a cantilever beam, which results in a reflection spectrum containing two close peaks with a separation depending on the amount of bending of the beam. The sensing element is made to select a particular mode of a gain-switched FP-LD, so that stable short pulses at the two wavelengths defined by the reflection peaks of the FBGs are generated simultaneously when the modulating frequency of the FP-LD is set properly. The wavelength separation of the dual-wavelength pulses is converted into a time difference by transmitting the pulses through a dispersive fiber, so that the wavelength measurement is replaced by the less expensive time measurement. To multiplex similar sensors that are sufficiently far apart spectrally, it is only necessary to change the modulating frequency of the FP-LD. This technique provides an intensity-referencing-free demodulation scheme, an easy way to interrogate a series of sensors, and an effective means to compensate for drifts due to temperature changes.