Fiber-Bragg-grating cavity sensor based on a self-seeded Fabry-Perot laser diode

We propose a low-cost, general-purpose fiber Bragg grating (FBG) cavity sensing technique, where optical picosecond pulses generated from a self-seeded Fabry-Perot laser diode are used to interrogate a fiber cavity formed with two chirped FBGs and the optical loss in the cavity is determined from the peak power ratio of the first two pulses reflected from the cavity. This technique does not require intensity referencing and the sensor output is insensitive to drifts in the center wavelengths of the FBGs. We demonstrate this technique experimentally with a long-period fiber grating (LPFG) placed in the fiber cavity as a refractive-index sensor. The results from the cavity-loss measurement are consistent with the wavelength-shift measurement of the LPFG in response to a change in the surrounding index. This technique is applicable to general intensity-based sensors and has the potential to be further developed for remote sensing of a wide range of physical or chemical parameters. It also provides a new way of converting the wavelength shift of an LPFG sensor into an intensity-based signal for easy processing.