Simultaneous Sensing of Temperature and Displacement Using a Multimode Fiber Bragg Grating

We present an experimental method for the operation of a multimode fiber Bragg grating as a sensor of temperature and displacement. A Bragg grating in a multimode fiber has multiple reflection peaks. The bending of the multimode fiber induces the redistribution of modes and affects the measured reflection spectrum of a Bragg grating. The bending sensitivity is used for sensing of displacement. In the experiment, we bend a graded-index multimode fiber and observe the reflection and transmission spectra. As the fiber is bent, the number of reflection peaks decreases, and the center reflection wavelength shifts to the longer wavelength, toward the fundamental mode. We also measure the temperature dependence of the wavelength of each reflection peak. The bending and temperature dependence in transmission measurements is almost the same as that in the reflection measurements. Next, we present an experiment on the simultaneous sensing of temperature and displacement. The shift of each transmission or reflection wavelength of the fiber Bragg grating is only a function of temperature. The experimental results demonstrate the possibility of the practical application of this sensing approach.