RU2567116C1 - Fibre-optic sensitive element of sensor of electric current and magnetic field - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: sensitive element of the sensor is made of a piece of a fibre-optic waveguide with a spiral structure of built-in linear birefringence (spun-fibre) laid spirally: in the initial part of the sensor with reduction of curvature radius, in the middle part - with constant radius, and in the final part - with increase of radius.

EFFECT: increasing contrast of an interferometer and a possibility of designing sensors with temperature compensation of sensitivity, miniature sensors, high-sensitivity sensors with multiturn outlines, and sensors operating under conditions of strong vibrations.

3 cl, 2 dwg

Description

The invention relates to fiber optics, in particular to fiber optic current and magnetic field sensors.

Known fiber-optic current sensors (VCTD) and magnetic fields operate on the principle of the Faraday effect. The current flowing in the wire induces a magnetic field, which, due to the Faraday effect, rotates the plane of polarization of linearly polarized radiation propagating in the fiber sensitive element. The rotation of the plane of polarization of radiation, proportional to the current in the wire, is measured by an interferometer, which includes a sensitive element.

Known fiber sensitive elements of the current sensor are made in the form of a circumferentially laid segment of a quartz fiber waveguide with internal birefringence (DLF) having a spiral structure [1-3]. Such fiber optic fibers (spun fibers) [4-6] are obtained by hood during rotation of the workpiece with a strong built-in linear linear DLP. Spun fibers due to the integrated DLP are preferred for current sensors, since their magneto-optical sensitivity is relatively weakly dependent on the bending radius. This is especially true for spun fibers of a microstructural structure [7–8], which have a higher DLP.

A disadvantage of the known fiber sensing elements is the reduction in contrast (visibility) of the current sensor interferometer due to bending due to winding. The lower the stronger, the smaller the bending radius. This disadvantage is most pronounced in small-sized sensitive elements, as well as in sensitive elements, where a certain bending radius plays an important role.

Closest to the proposed device is a device [9]. This device uses spun fiber winding around a circle at a certain radius, which reduces the temperature error of the current sensor. The fiber bending radius required for this is such a value at which the contrast of the interferometer is noticeably reduced.

As a result, factors arise that affect the threshold sensitivity of the sensor. So, when using spun fibers with this bending radius, the amplitude of the light waves participating in the formation of the interference signal, or, in other words, the contrast of the interferometer, is significantly reduced. This is due to the fact that the ellipticity of the polarization states (PS) of the radiation in a bent spun fiber decreases [6]. When light waves with reduced ellipticity are reflected from the mirror, the contrast of the interferometer is lost [8]. As a result, the threshold sensitivity of the sensor deteriorates, i.e. the signal to noise ratio decreases. This decreases the dynamic range of the sensor.

An object of the invention is the creation of a fiber sensitive element of an electric current and magnetic field sensor with increased contrast of an interferometer using a strongly bent spun fiber, which is necessary to create sensors with temperature compensation of sensitivity, miniature sensors (with a sensitive element of the order of 5 mm or less), highly sensitive sensors with multi-turn circuits, sensors operating in conditions of strong vibrations.

The technical result of the invention is to increase the dynamic range of the fiber optic sensor.

где L_b - длина биений встроенного линейного двулучепреломления, L_tw - длина шага спиральной структуры волокна; максимальные радиусы кривизны спиралей, по которым уложены волокна начальной и конечной частей волоконного контура, R_нач и R_кон удовлетворяют условию $${\text{R}}_{\text{нач}}\text{, }{R}_{кон}>r\sqrt{\frac{20L_b}{\mathrm{22,792}}\frac{n^3}{\lambda }},$$

где r - радиус кварцевой оболочки волокна, L_b - длина биений встроенного линейного двулучепреломления волокна, λ - рабочая длина волны излучения, n - среднее значение показателя преломления волокна.The specified technical result is achieved by the fact that in the sensitive element of the fiber-optic sensor of electric current or magnetic field, including a fiber circuit made of a segment of an optical fiber with a spiral structure of the axes of the built-in linear birefringence, the fiber circuit consists of three parts, while in the middle part of the fiber contour, the fiber is laid in a circle, in the initial part - in a spiral with decreasing radius of curvature, and in the final part - in a spiral with increasing radius curvature. Moreover, the best technical result is achieved when the lengths of the fibers of the initial and final parts L _beg and L _{con of the} fiber circuit satisfy the condition $${\text{L}}_{\text{beg}}\text{,}{L}_{\mathrm{to}\mathrm{about}n}>6\frac{L_{tw}{L}_b}{\sqrt{{\mathrm{<figure-callout\; id="2"\; label="L\; t\; w"\; filenames="00000015.png"\; state="\{\{state\}\}">L\; </figure-callout>}}_{tw}^{}+\mathrm{four}{L}_b^2-2L_b}},$$

where L _b is the beat length of the integrated linear birefringence, L _tw is the step length of the spiral structure of the fiber; the maximum radii of curvature of the spirals along which the fibers of the initial and final parts of the fiber circuit are laid, R _beg and R _con satisfy the condition $${\text{R}}_{\text{beg}}\text{,}{R}_{\mathrm{to}\mathrm{about}n}>r\sqrt{\frac{\mathrm{twenty}{L}_b}{\mathrm{22,792}}\frac{{\mathrm{<figure-callout\; id="3"\; label="n"\; filenames="00000015.png"\; state="\{\{state\}\}">n</figure-callout>}}^3}{\lambda }},$$

where r is the radius of the quartz fiber sheath, L _b is the beat length of the integrated linear birefringence of the fiber, λ is the working radiation wavelength, n is the average value of the refractive index of the fiber.

A way to increase the degree of polarization of radiation in a spun fiber and, as a consequence, the contrast of the interference pattern of the HFTC is based on the properties of the spun fiber to transform the state of polarization of the propagating radiation in combination with a slow change in the radius of bending of the fiber compared to the speed of these transformations.

Consider the propagation of monochromatic radiation through a curved spun fiber. The conversion of the complex amplitude of the electric field of a light wave into a spun fiber can be described using the apparatus of Jones matrices [6, 10]. In the coordinate system of circular polarizations, the evolution equation has the form:

where E _r , E _l are the complex amplitudes of the right- and left-circularly polarized waves, N _c is the differential Jones matrix of the spun fiber with bending, which has the form:

here γ is the slew rate of the phase delay between waves with circular orthogonal polarizations due to the Faraday effect, β = k _y -k _x = 2π / L _b is the slew rate of the phase delay between waves with orthogonal linear polarizations, determined by the built-in linear DLP with the beat length _{L b, ξ = 2π / L} tw - spatial frequency axes of rotation of embedded linear birefringence with the pitch length of the spiral structure _{L tw, δ = 2π / L} ind - rate of rise of phase delay between the waves with orthogonal linear polarizations, defined with birefringence bending length th beat L _ind, z - the coordinate along the fiber axis.

In an elliptical coordinate system of polarization states, the azimuth of which in the laboratory coordinate system at each point of the fiber coincides with the azimuth of the DLS axes, and the ellipticity of the basis states E _u and E _v is equal to the eigenvalue of a given fiber [10], the Jones differential matrix can be decomposed into two terms:

The first term is the Jones differential matrix of the direct spun fiber:

.Where $$\Omega =\sqrt{{\mathrm{<figure-callout\; id="2"\; label="\beta "\; filenames="00000015.png"\; state="\{\{state\}\}">\beta </figure-callout>}}^2+\mathrm{four}{\xi }^2}$$

.

The second term can be considered as a perturbation introduced by a bend:

where ψ = arctan (L _tw / (2L _b )).

Solving the evolution equation by the iteration method, it can be shown that the solutions have the following form:

where f (z) is a monotonically increasing function of the distance z, W is the integral of the product of the coefficient δ and the rapidly oscillating function with a frequency of 2ξ, a _i and b _i are the functions of W, and a _i (0) = b _i (0) = 0 .

Let the coefficient δ at the beginning and end of the fiber be sufficiently small compared to the coefficient β, which means that these spun fiber segments have maximum sensitivity. This condition can be determined by the inequality δ / β = L _b / L _ind <20 [6] or, using the expression for the beat length of a bending DLP [6], by the inequality for the radius of curvature of the fiber:

In the middle of the fiber, the coefficient δ is equal to some predetermined value δ _k corresponding to the required radius of the fiber winding. Let the transition from the initial value to δ _to and then back to the final value proceed quite smoothly and have a length of at least 5-6 periods 2π / (Ω-2ξ), which, expressing Ω and ξ in terms of the beat lengths of the integrated DLP and the step length of the spiral structure is determined by the inequality

.

.

It can be shown that the integral W of the product of such a function δ (z) and a rapidly oscillating function along the entire fiber length is negligible, and with it all the functions a _i (L) ≈b _i (L) ≈0. Thus, with this configuration of the spun fiber, after the passage of light radiation through the fiber, the polarization modes are not mixed. This means maintaining a contrast close to contrast for a contour with a large winding radius.

In FIG. 1 shows a spun fiber winding circuit of the sensing element, where 1 is the initial part of the fiber circuit with decreasing radius of curvature of the winding, 2 is the middle part, 3 is the end part of the fiber circuit with increasing radius of curvature of the winding.

For the experiment, a spun fiber with an elliptical core was selected, the spiral structure pitch of which L _tw = 3 mm, the beat length of the integrated linear DLP L _b = 9.45 mm.

A sensitive element of the traditional VODT circuit [6] with a λ / 4 plate was made from a section of the investigated fiber 19 m long. Measurement of the contrast of the VODT interference pattern was performed with two configurations of the sensitive fiber. In the first configuration, the sensitive fiber was completely wound on a quartz tube with a diameter of 14 mm. In the second configuration of the sensing element, the first two meters of fiber were wound with a smooth change in the diameter of the winding from 200 mm to 14 mm, and the last two meters of fiber were wound with a smooth change in the diameter of the winding, respectively, from 14 mm to 200 mm. The tested sensitive elements were welded to the reflective interferometer circuit of the laboratory model of VODT.

The contrast measurement results are illustrated in FIG. 2, where the oscillograms of the output signal U _pd (t) combined with the known (dashed line) and proposed (solid line) sensitive elements are shown in one figure. The signals were taken from the output of the photodiode and were periodic functions of time at the doubled working frequency of the modulation of the relative phase used in the TDCF for recording the information signal. The amplitude of the relative phase modulation of the light waves was π radians. Under these conditions, the amplitude of these signals from maximum to minimum determines the contrast of the interferometer, which is calculated by the well-known formula:

K = (U _{pd max} - U _{pd min} ) / (U _{pd max} + U _{pd min} )

According to contrast measurements, for the first configuration of the WFTC sensitive element corresponding to the prototype circuit, the value K ₁ = 0.36 was obtained, and for the second configuration corresponding to the proposed scheme, K ₂ = 0.79. From this it can be seen that the contrast in the proposed circuit increased 2.2 times, which ensures the achievement of a technical result, namely, it increases the dynamic range of the current sensor.

Literature

1. Frosio G, Dancliker R. "Reciprocal reflection interferometer for a fiber-optic Faraday current sensor", Applied Optics, vol. 33, no. 25, 6111-6122 (1994).

2. Blake J.N. "Fiber optic current sensor." U.S. Patent. No. 6,188,811, dated 13.02.2001.

3. Boev A.I., Gubin V.P., Morshnev S.K., Przhiyalkovsky Y.V., Ryabko M.V., Sazonov A.I., Starostin N.I., Chamorovsky Y.K. "Fiber optic current sensor." RF patent No. 2437106 dated 12/29/09 IPC: G01R 15/24.

4. Laming R.I., Payne D.N., J. Lightwave Technology, 1, 2084 (1989).

5. Polynkin P., Blake J., J. of Lightwave Technol., 23, 3815 (2005).

6. Gubin V.P., Isaev V.A., Morshnev S.K., et al. Quantum. Electron., 36, No. 3. 287-291 (2006).

7. Chamorovsky Yu.K., Starostin N.I., Ryabko M.V. et al., Opt. Comm., 282, 4618 (2009).

8. Gubin V.P., Morshnev S.K., Starostin N.I. et al. Quantum Electronics, 41, 815 (2011).

9. Morshnev S.K., Chamorovsky Yu.K. RF patent No. 2437107 dated 05/19/2010.

10. Przhiyalkovsky Y.V., Starostin N.I., Morshnev S.K., Gubin V.P. Quantum Electronics, 43 (2), 167 (2013).

11. Rashleigh S.C. Origins and control of polarization effect in single-mode fibers // J. Lightwave Tech. 1983. LT-1. No. 2. P. 312-331.

12. Gubin V.P., Morshnev S.K., Starostin N.I. et al. Radio engineering and electronics, 53, 971 (2008).

Claims (3)

1. The sensing element of the fiber-optic sensor of electric current or magnetic field, including a fiber circuit made of a segment of an optical fiber with a spiral structure of built-in linear birefringence, characterized in that the fiber circuit consists of three parts, while in the middle part of the fiber circuit fiber laid in a circle, in the initial part - in a spiral with a decrease in the radius of curvature, and in the final part - in a spiral with an increase in the radius of curvature. 2. The sensing element according to claim 1, characterized in that the lengths of the fibers of the initial and final parts L _beg and L _{con of the} fiber circuit satisfy the condition:

where L _b is the beat length of the integrated linear birefringence, L _tw is the step length of the spiral structure of the fiber. 3. The sensitive element according to claim 1, characterized in that the maximum radii of curvature of the spirals along which the fibers of the initial and final parts of the fiber circuit are laid, R _beg and R _con satisfy the condition:

where r is the radius of the quartz fiber sheath, L _b is the beat length of the integrated linear birefringence of the fiber, λ is the working radiation wavelength, n is the average value of the refractive index of the fiber.

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