Experimental study of the fiber laser output intensity behavior and its application to a water flow sensor

Fiber Bragg gratings (FBGs) and doped fibers have allowed the development of laser systems that can be used as sensors. The typical experimental arrangement is composed of two FBGs (with different Bragg wavelength, λB) as mirrors to form a Fabry-Perot cavity. One of the gratings is the reference (FBG-R) and the other one is the sensing element (FBG-S), which is in contact with the physical parameter to be evaluated. Generally, these systems determine the magnitude of a physical variable with the aid of an optical spectrum analyzer (OSA). However, one interesting aspect of these systems is the transition towards the optimal laser emission when the FBGs spectra overlapping increases. That is, just using the behavior of the laser output intensity as the FBG-S Bragg wavelength (λBS) is approaching to FBG-R one (λBR). This intensity can be measured quite easily with a conventional photodetector. In this work, the behavior of such intensity was experimentally studied and was used to evaluate the water flow-rate in a pipeline. As a result, it was possible to measure the flow-rate in a linear range from 0 to 200 ml/s with a resolution of approximately 8 ml/s.