Polarization properties of interferometrically interrogated fiber Bragg grating and tandem-interferometer strain sensors

Abstract

Abstract-Lead sensitivity in low-coherence interferometric fiber-optic sensors is a well-known problem. It can lead to a severe degradation in the sensor resolution and accuracy through its effect on the fringe visibility and interferometric phase. These sensitivities have been attributed to birefringence in the various components. In the current work, an analysis of the polarization properties of fiber Bragg grating and tandem-interferometer strain sensors, using Stokes calculus and the Poincaré sphere, is presented. The responses of these sensors as a function of the birefringence properties of the various components under different illuminating conditions are derived. The predicted responses demonstrate very good agreement with experimentally measured responses. These models provide a clear insight into the evolution of the polarization states through the sensor networks. Methods to overcome the lead sensitivity are discussed and demonstrated, which yield a differential strain measurement accuracy of 18 nε · rms for a fiber Bragg grating sensor.

Differential phase tracking applied to Bragg gratings in multi-core fibre for high accuracy curvature measurement

“…However in environments where the interferometer and sensor downlead are perturbed the measurement accuracy will be degraded. Birefringence fluctuations in the interferometer and downlead result in increased drift owing to a combination of the degree of polarisation of the BBS, birefringence in the sensing grating and any polarisationdependent loss in the system [7]. These affects can be reduced by reducing the degree of polarisation of the source.…”

Bend and Twist Sensing in a Multiple-Core Optical Fiber

“…Systematic errors in indicated strain reported by the system are typically within a few µε, but extensive measures were explored to systematically reduce these artifacts to below the level of twist-correlated signals. For example, periodic non-linearity in indicated wavelength versus strain caused by CCD pixel non-uniformity was addressed through look-up tables, and handling-induced birefringence effects [7] were suppressed by depolarizers. Because each rosette has slightly different characteristics, a calibration machine and associated procedures were developed to extract the sets of bend and twist coefficients and correction factors.…”

Differential phase tracking applied to Bragg gratings in multicore fiber for high-accuracy curvature measurement

“…The accuracy of the interrogation system is affected by errors in the interferometric phase measurement and polarization induced phase measurement errors, which arise due to a combination of the degree of polarization of the broadband source, birefringence in the interrogating interferometer and birefringence in the downlead and in the FBGs. A more detailed analysis of these effects are reported elsewhere 27 ; however to achieve the lowest possible polarization induced phase errors it is important to minimize the birefringence in the system and illuminate it with unpolarized light. Birefringence in the FBG structures was also characterized by measuring the insertion loss of each grating as a function of polarization using a polarization sensitive optical frequency domain reflectometer, (OVA-CT, Luna Technologies, Blacksburg, VA).…”