Performance of a fiber optic ring depolarizer in fiber sensing applications

摘要

Numerous fiber optic measurement systems making use of sensors such as Fabry-Perot or fiber Bragg gratings incorporate superluminescent or other edge emitting light sources. These sources often have a high degree of polarization. The combination with birefringence in fibers results in measurement errors. A possibility to overcome these errors is to depolarize the light source. Low coherence lengths make passive means of depolarization suitable. A common solution is the fiber Lyot depolarizer, which works especially well with very low coherence lengths. For coherence lengths corresponding to for example the reflection spectrum of narrow band fiber Bragg grating, long fiber lengths are required. A second way is offered by fiber ring depolarizers, where the coherence length is of minor concern. To estimate the performance of the fiber ring depolarizer in a practical measurement system, we employ both concepts. The measurement system is a CCD based spectrometric interrogation unit, with a superluminescent diode as light source. The source itself is well polarized. We observe the effect of birefringence in a transversally loaded fiber Bragg grating array consisting of eight sensors, when the polarization on the path to the sensor is rotated. The improvement in polarization dependency when using the two depolarization methods is compared.