Polarization mode coupling effect distributed optical fiber temperature sensor

Polarization mode coupling effect distributed optical fiber temperature sensor

The mode coupling effect type distributed optical fiber temperature sensor utilizes the coupling effect sensing between two orthogonal polarization modes generated by polarization-maintaining optical fiber when it is subjected to external pressure.

The system block diagram is shown in Figure 3. The quasi-monochromatic light emitted by the SLD is injected along one polarization direction of the polarization-maintaining fiber. Assume that the light signal emitted by the light source excites the HE11 mode in the polarization-maintaining fiber, and the input optical power spectrum function is Gaussian distribution. According to the partial coherent light theory, the optical power expression related to the sensing information on the detector is obtained as follows:

Figure 3 System block diagram of coupled-mode distributed temperature sensor

In the experiment, the change of external temperature can be converted into the change of pressure on the optical fiber. The specific method is to place the optical fiber and a metal cylindrical rod with a large expansion coefficient side by side in a quartz tube. When the temperature rises, the metal rod expands and squeezes the optical fiber, thereby generating a lateral pressure on the optical fiber. The optical fiber produces a coupling effect at this point. By calibrating the system for temperature, the relationship between temperature and coupling coefficient can be obtained. In addition, as shown in Figure 3, the movable arm can be moved to maximize the coherent light intensity on the detector. At this time, the group delay 0τ=, and the light intensity P0 is measured. Substituting 0τ=, the coherent light intensity P0 into P(τ), the coupling coefficient can be obtained. The temperature can be known by checking the relationship between the coupling coefficient and temperature. At the same time, the value when the coherent light intensity reaches the maximum and the relational delay expression can be used to determine the position of the temperature point LΔ2/LL=Δ, where B is the birefringence of the optical fiber.