Fiber Bragg grating written into the core by a sideways exposure to an ultra violet-laser interference pattern have shown great promise for use as practical strain sensors in large structures. One way to sense the strain of the grating is by using active interrogation whereby the fiber Bragg grating is used as the optical feedback element of a laser cavity, and the lasing wavelength is monitored as the system output. Compared to passive broadband techniques, the fiber Bragg laser sensor provides much stronger optical signals, thus leading to a much improved signal-to- noise ratio. In order to optimize the power output from this sensor, one wishes to model the output from the fiber laser in terms of the Er-doped fiber parameters, the pump characteristics, the cavity mirror reflectivities and losses in the cavity. In this paper we solve the rate and propagation equations for a Fabry Perot cavity to obtain explicit closed form equations for the output power, threshold pump power, as well as for the optimum length. Experiments where we used an electron cyclotron resonance plasma enhanced chemical vapor deposition apparatus to deposit dielectric thin films on one fiber end point in order to change the reflectivity of a cavity mirror, while monitoring the reflectivity in situ, verify the validity of the model.