As the DFB fiber laser hydrophone of sensitive element, the DFB fiber laser has a very important influence on the sensing characteristics and so on laser bending under the condition of dynamic characteristics, it is necessary to study the DFB fiber laser hydrophone practical research, solve the problem of anti-interference, and guide the design of the DFB fiber laser hydrophone probe has very important significance.
Considering the fiber is usually composed of fused silica control, material is hard, and the stiffness of the elastic restoring force is mainly composed of itself, when bending beam characteristics; In the application and laser usually need a fixed on both ends. So based on the theory of beam on both ends fixed on the dynamic characteristics of the DFB fiber laser are analyzed.
2 the DFB fiber laser hydrophone pressure sensitivity
DFB fiber laser hydrophone using the DFB fiber laser as the sensitive element. Place the DFB fiber laser hydrophone in sound field, sound pressure will cause the strain of the laser cavity, so that the laser output wavelength lambda FL change. Will be detected the change, can measure the level of motivation of laser cavity, thus the pressure signal demodulation. The output wavelength is by the resonant cavity phase shift of Bragg grating wavelength, namely the lambda FL = lambda. B. The following optical equation for Bragg reflection wavelength.
Among them, the lambda B for the Bragg wavelength, Λ is grating period, neff is an effective refractive index of fiber core, the average value of 1.456.
In the case of uniform laser stretching, output wavelength and the relationship between the axial strain satisfy:
Among them, the delta lambda FL exit for the DFB fiber laser wavelength drift capacity, the strain sensitivity coefficient k for relative wavelength drift (under uniform axial stress when the size of 0.784), for optical fiber epsilon z uniform tensile axial strain.
Using non-equilibrium interferometer system to the DFB fiber laser hydrophone demodulation, wavelength drift information after entering non-equilibrium interferometer is converted to phase shift information, its relationship to meet:
One for wavelength drift caused by the phase drift, d for non equilibrium path length