Fiber-Bragg-grating force sensor based on a wavelength-switched self-seeded Fabry-Perot laser diode

We demonstrate a simple fiber-optic sensor for the measurement of transverse force, where a fiber Bragg grating (FBG) is used as a sensing element as well as a wavelength-selection element for a self-seeded Fabry-Perot laser diode (FP-LD). When a transverse force is applied on the FBG, the birefringence induced in the fiber causes, the FP-LD to emit short pulses at two close wavelengths. The wavelength separation of the pulses is converted into a time difference between the two pulse trains by using a dispersive single-mode fiber, so that wavelength measurement is replaced by the much less expensive time-difference measurement. To extend the measurement range, the laser is made to emit at the two wavelengths alternately by using a polarization-switching technique. The sensor also allows compensation for any isotropic drift on the Bragg wavelength of the FBG.