Liquid crystal based polarized low coherence interferometer for optical demodulation in sensors

Abstract

The resolution of the measurement detection and sensitivity of a polarized low coherence interferometer (PLCI) can be pre-engineered by optimizing the key parameters of the birefringent wedge, which is rarely reported. In this work, we introduce a liquid crystal (LC) wedge in the PLCI and use it to demodulate Fabry–Perot (FP) cavity length. The birefringence property of the nematic LC is used to convert the optical path difference (OPD) of the sensor into a spatial distribution. This results in the production of localized interference fringe patterns. The formation of PLCI fringes and the related shift of the interferogram with a variation in the displacement of the FP displacement sensor is explained with reference to the OPD matching between an LC wedge and the FP cavity. The displacement value is demodulated from the obtained fringe pattern by tracking the centroid position of the fringe envelope and also considering the birefringence dispersion. An additional simulation study shows that the spatial position of the interferogram signal coupled with the dispersion coefficient is almost identical to the experimental data. The demodulated results from both the simulation and experimental investigations are found to be consistent with each other and closely agree with the actual cavity length. Further, the possibility to enhance the sensing resolution is examined by modulating the interferogram fringes using an electric field. Compared to birefringent crystals, the LC wedge presented here is found to be advantageous for high precision and tunability of the measurement range, which is useful for robust fiber optic sensing applications.

Extended centroid position method in liquid-crystal-based interrogation system for electric-field-modulated Fabry-Perot sensing

“…The schematic structure of the PLCI system is shown in figure 1. A broadband white light source (MCWHF2, Thorlabs) with a Gaussian spectrum over a wavelength range of 400 to 800 nm is used to illuminate the fiber optic F-P sensor through a three-port wideband circulator (WMC3L1F, Thorlabs) [30]. The output light from the sensor port containing the information about cavity length propagates into the PLCI demodulator through another port of the coupler.…”

“…The resultant PLCI signal of the interference fringe pattern is a modulated version of the Gaussian spectrum (broadband white light source) with the multiple beam interference intensity of the sensing interferometer. Therefore, the light intensity formed near the thickness t of the LC wedge and the specific cavity length L of the F-P sensor can be expressed as [7,11,30]:…”

“…This modified PLCI signal introduces the degree of asymmetry following the value of β and hence the usual peak position detection method fails to obtain the accurate position of the zero OPD. In our previous work [30], we considered the centroid position of the envelope and demonstrated the detection of the actual value of cavity length up to a certain accuracy limit. Therefore, it often produces a dissimilarity between the real value and the obtained value of cavity length due to several effects of the fringe pattern, such as distortion of the interferogram width, amplitude and period.…”

“…Considering the maximum or peak value of this envelope as the peak position (t 0 ) of the interferogram, the estimated cavity length L 0 = ∆n 0 .t 0 /2 at this point is 19.9810 µm which does not correspond to the actual value of the cavity length. Now for the initial compensation of the asymmetrical nature of the interferogram or the envelope, a centroid position (t c ) of the envelope is determined by using the following relation [27,30,33],…”

“…Still, there is a limitation in the measurement range due to the fixed value of optical birefringence, as well as the dimension of such birefringent wedges, which effectively needs further modification. In our earlier work [30], we demonstrated the use of birefringent liquid crystal (LC) instead of the crystal wedge, which has the advantage of birefringence tunability by applying a small electric field. Not only does it provide the tunability feature of the operation limit, but it also enhances the resolution of detection.…”