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8LD-O-14
LASER DIAGNOSTICS AND SPECTROSCOPY
Fiber-optical Faraday current sensor with enhanced SNR
Y. Przhiyalkovskiy1'2. N. Starostin1'2, S. Morshnev1'2, A. Sazonov1'2
1 - SPC Profotech, Skolkovo innovation center area, Bolshoi bulvar 42, bld. 1,, Moscow, 121205, Russia 2 - Kotelnikov Institute of Radio Engineering and Electronics (Fryazino Branch) of the Russian Academy of Sciences,
Vvedensky Sq. 1, Fryazino, Moscow region, 141190 Russia email address: yankus.p@gmail.com
At present, fiber optical Faraday current sensors are widely recognized in the power industry due to their high accuracy, easy installation, and immunity to electromagnetic interference [1]. However, given the fast reaction of the Faraday effect, the optical method of current measuring is also of interest for measuring low-amplitude current pulses, for example, in particle accelerators. To achieve the required sensor characteristics in this application, one thus should maximize the signal-to-noise ratio (SNR) and the sensitivity of the sensing coil.
In our work we design and investigate a fiber-optical current sensor with enhanced SNR for measuring current pulses (Fig. 1) [2]. The sensor is based on a reflective interferometer with the sensing spun fiber and has an additional reference optical channel for suppressing excess noise which is caused by beating of the spectral components of light [3]. The attained suppression of excess noise in the proposed sensor amounts to 15.3 dB.
To increase the sensor sensitivity, the number of fiber turns in a sensing coil should be maximized. While keeping the fiber length constant to ensure high time resolution of the sensor, this can be achieved by reducing the winding radius down to several millimeters. However the experiment shows that such strong bending of the fiber leads to a decrease in the interference visibility of the sensor and degradation of the SNR [2]. Mode coupling which occurs in the wound spun fiber makes the polarization state of light highly elliptical. So when light hits the mirror at the end of the fiber, each incident elliptically polarized mode excites both backward propagating polarization modes. If the winding radius is small, the incoherent components emerged in this way have significant amplitudes, which eventually results in a disturbance of the optical spectrum and, therefore, in low efficiency of noise suppression. Thus, for the radius of the sensing coil of 5 mm used in the experiment, the suppression coefficient reduces to 6 dB.
The theory shows, that if the winding radius change smoothly, polarization modes of the bent spun fiber adapt to the new radius with minimal coupling [4]. So to avoid the above degradation of excess noise suppression, we apply the modified configuration of the fiber winding in which the beginning and end 1-m segments of the spun fiber are wound spirally with change of the winding radius, respectively, from a large value to small and vice versa. Hence, the almost circular polarization of light is restored when it hits the mirror which prevents the emergence of incoherent waves there. As a result, the optical spectrum remains undisturbed and the excess noise suppression increases to 14.8 dB.
The work was carried out within the framework of the state task of the Kotelnikov Institute Radio Engineering and Electronics of RAS.
Fig. 1
[1] Bohnert, K., et al. "Optical fiber sensors for the electric power industry." Optics and Lasers in Engineering 43.3-5 (2005): 511526.
[2] Gubin, V. P., et al. "A broadband Faraday fiber-optic current sensor with excess noise compensation." Results in Physics 18 (2020): 103286.
[3] Morkel, P. R., R. I. Laming, and D. N. Payne. "Noise characteristics of high-power doped-fibre superluminescent sources." Electronics Letters 26.2 (1990): 96-98.
[4] Przhiyalkovskiy, Yan V., et al. "Polarization Dynamics of Light Propagating in Bent Spun Birefringent Fiber." Journal of Lightwave Technology 38.24 (2020): 6879-6885.
