RU2170439C1 - Microresonator fiber-optical electric current pickup - Google Patents

Abstract

FIELD: fiber-optical converters of physical quantities with use of micromechanical resonators excited by light. SUBSTANCE: collimation of beam interacting with microresonator carrying mirror reflector is realized in fiber-optical pickup with the aid of fiber autocollimator. Change of value of measured electric current in conductor with current is aligned with change o characteristics of microresonator under effect of magnetic field of this conductor with current. In this case change of characteristics of microresonator leads to change of resonance frequency in system " fiber-optical laser- microresonator ". Fiber autocollimator is manufactured in the form of section of single-mode quartz light guide with spherical microlens formed right on butt of this light guide. Longitudinal axis of microresonator coming in the form of film of magnetic material coincides with direction of magnetic forces of magnetic field of conductor with current. EFFECT: remote measurement of electric current, increased stability of measurements. 2 cl, 1 dwg

Description

 The invention relates to fiber-optic converters of physical quantities (temperature, pressure, electromagnetic fields, etc.) using micromechanical resonators (MR) excited by light.

 Structurally, MRs, as a rule, are a microbeam, a micro-console, a micro-membrane, etc., made of silicon or piezoelectric quartz single crystals by anisotropic etching, plasma chemistry of single-crystal materials. An external action deforms the MR substrate and, through a change in internal mechanical stress, changes the resonant frequency of acoustic vibrations excited by light.

 Due to the small amplitude of the MP oscillations (~ 0.1 μm) in fiber-optic sensors (VOD) of physical quantities, both the interferometric method of taking information about the resonant frequency of MR using the Fabry-Perot interferometer and the frequency one are used.

 The closest to the proposed technical solution in terms of technical nature and the achieved result is the VOD of physical quantities with the optical method of exciting MR oscillations and frequency data reading (see RF patent N 2135957, BI N 24 of 08/27/99).

The device contains a fiber optic laser (VOL), an MR made in the form of a microbridge (microbridge on the membrane), a collimator made in the form of a gradient rod lens (GSL) in a quarter of the period forming a Gaussian beam, a single-mode isotropic fiber, a translucent mirror, as which is the fiber-air interface with a reflection coefficient R ₁ = 3.2%, a photodetector, a spectrum analyzer, a semiconductor pump laser at a wavelength of λ _n = 0.98 μm.
In the known technical solution, one end of the fiber optic waveguide is optically coupled to a collimator located between this end and the MR, and the second end is the output and is connected to the spectrum analyzer through a photodetector, while the reflecting surface of the MR forms a two-mirror optical resonator of the optical fiber with the output end of the fiber, and the reflecting the surface of the MR in the initial position is oriented to the optical axis of the collimated beam at a certain given angle θ _and .
The discrete form of the output signal of the VOD, the large length of the transmission channel and the high accuracy of measuring the resonant frequency make this type of VOD promising when used in physical quantity measurement systems.

The disadvantage of this solution is the following. The presence of an autocollimator based on rod lenses in the water design reduces the stability of the collimated beam parameters under the influence of such destabilizing factors as changes in temperature, pressure, acceleration, etc. Instability of the collimated beam parameters worsens physical characteristics of microresonator water such as reliability, accuracy, performance, and reduces the efficiency of interaction with OX MR due instability _and θ - the angle of the initial orientation of the optical and the collimated beam relative to the normal to the reflecting surface of the MR. In addition, in the known technical solution, the design and manufacturing technology of MRs limit the possibility of its use for remote measurement of electric current. Indeed, an MR made of a silicon single crystal by anisotropic etching and plasma chemistry is also provided with an additional metal coating, which acts as the second VOL cavity mirror (the first is the fiber-air interface at the photodetector input). Such MR is not very effective for remote measurement of electric current.

 The problem solved by this invention is the development of microresonator water of physical quantities for remote measurement of electric current based on the application of the magnetospheric effect arising from the interaction of magnetic field of the conductor with current i, which is to be measured. The magnetic field of a conductor with current changes the characteristics of MR, which leads to a change in the resonant frequency in the VOL-MR system in proportion to the magnitude of the current in the conductor.

 The solution to this problem is provided by the fact that in a microresonator fiber-optic electric current sensor, including a fiber-optic laser as an optical radiation source, a microresonator with a mirror reflector, an autocollimator, a photodetector, a spectrum analyzer, a fiber autocollimator made in the form of a single-mode is used as an autocollimator a quartz fiber with a spherical lens formed at the end of the fiber, and the microcavity is made in the form of a film of magnetic material and and in that the magnetic material as used microresonator spin glass, and in that the magnetic material as used microresonator iron garnet single crystal.

 The essence of the claimed technical solution is to develop an electric current water supply, in which the beam interacting with the MR is collimated using a fiber autocollimator, and a change in the measured electric current of the conductor with current is associated with a change in the characteristics of the MR under the influence of the magnetic field of this conductor with current. In this case, a change in the characteristics of the MR leads to a change in the resonant frequency in the VOL-MR system (when Q-switching the two-mirror optical resonator due to photoinduced angular deviations of one of the mirrors of the VOL resonator, which is the MR).

 The proposed fiber autocollimator is made in the form of a section of a single-mode quartz fiber with a spherical microlens formed directly at the end of this fiber.

где d - диаметр коллимированного пучка, формируемого микролинзой на ее выходе;
θ_o - угол расходимости коллимированного пучка;
d_c - диаметр световедущей сердцевины световода;
NA - числовая апертура одномодового световода;
l - длина микролинзы;
n - показатель преломления материала микролинзы.In the Gaussian approximation, the dependence of the parameters of the collimated beam d, θ _o on the geometric dimensions of the microlens and the characteristics of the fiber is described by the expressions:

where d is the diameter of the collimated beam formed by the microlens at its output;
θ _o is the divergence angle of the collimated beam;
d _c is the diameter of the light guide core of the fiber;
NA is the numerical aperture of a single-mode fiber;
l is the length of the microlens;
n is the refractive index of the microlens material.

(2)
которая получена из условия, что торец световода располагается в фокальной плоскости микролинзы, а показатель преломления среды (воздуха), в которой распространяется коллимированный пучок, принят равным 1. Оптимальное значение расстояния между микролинзой и МР определяется экспериментально из условия максимального значения отношения сигнал-шум.The radius of the microlens R is calculated by the formula:

(2)
which is obtained from the condition that the end of the fiber is located in the focal plane of the microlens, and the refractive index of the medium (air) in which the collimated beam propagates is taken to be 1. The optimal distance between the microlens and the MR is determined experimentally from the condition of the maximum signal-to-noise ratio.

 The stability of the collimated beam parameters is ensured, firstly, by the design of an autocollimator, which is a monolithic structure of single-mode material, in which the connection of a quartz fiber with a micro lens of quartz glass is carried out by welding in an electric arc, which allows to obtain high mechanical strength and effective optical coupling of elements, secondly, the weak influence of destabilizing factors (changes in temperature, pressure, electromagnetic fields, etc.) on the indicator refractions and geometric dimensions of a microlens.

Это значит, что в диапазоне температур 0 - 800^oC изменения параметров коллимированного пучка не превышают соответственно 5% и 3%.So, in accordance with formula (1), based on the known values of thermooptical and photoelastic characteristics for quartz glass (fiber), we obtain estimates:

This means that in the temperature range 0 - 800 ^o C the changes in the parameters of the collimated beam do not exceed 5% and 3%, respectively.

Further, we note that with this method of exciting self-oscillations in the VOL-MR system, the main factor determining the efficiency of the VOL and MR interaction is the beam divergence angle θ _o , which determines the interval width Δθ _and = θ ₂ - θ ₁ , namely, the smaller the divergence angle θ _o, the larger the width _and interval of Δθ and vice versa.

In the proposed design of the AK autocollimator, it is possible to vary θ _o values over a wide range, which leads to a significant increase in the width of the zone of existence of stable self-oscillations in the VOL-MR system and, therefore, increases the efficiency of their interaction and improves the technical characteristics of the device: accuracy, reliability, stability.

To illustrate the possibilities of a fiber AK, estimates of the parameters of collimated beams (d, θ _o ) are given below for some typical values of the geometric dimensions of fiber AKs and the characteristics of a single-mode fiber.

We have, for
- the diameter of the microlens, D (microns), 200 ... 300
- the length of the microlens, l (microns), 700 ... 900
- the radius of the microlens, R (microns), 200 ... 300
- parameters of a single-mode fiber (λ = 1.55 μm, NA = 0.15, d _c 6.5 μm)
the following parameters of collimated beams:
- the diameter of the collimated beam formed by the microlens at its output (μm), d = 50 ... 150,
- the divergence angle of the collimated beam (rad) θ _o = 8 • 10 ^-3 ... 2 • 10 ^-2 .

 With regard to the essence of the magnetospheric effect and its use, the following should be noted.

где μ - масса единицы длины МР;
E - модуль Юнга МР;
J - момент инерции поперечного сечения МР.The differential equation of the bending vibrations of the MR, which is under the action of the longitudinal force F _x directed along its length, is generally written in the form:

where μ is the mass of a unit length MP;
E - Young's modulus MR;
J is the moment of inertia of the cross section of the MR.

где

K⁴ = E_x/EJ
здесь ω_к - частота собственных колебаний МР.Representing the bending vibrations y (x, t) in the form Φ (x) e ^iωt , we obtain:

Where

K ⁴ = E _x / EJ
here ω _to - the frequency of natural oscillations of MR.

 The solution of equation (3) with allowance for (4) requires the introduction of appropriate boundary conditions.

где l - длина микроконсоли.Consider, as an example, an MR performed in two versions: in the form of a micro-console and a micro-beam. For a microconsole, the boundary conditions in this case can be represented as

where l is the length of the microconsole.

 When an MR is placed in the form of a micro-console (made of a soft magnetic amorphous alloy), the characteristics of the MR change in the external magnetic field of the conductor with current, which leads to a change in the frequency of the natural oscillations of the MR.

If the magnetic field of a conductor with current magnetizes the console to saturation, then
F _x = S • M _s • H _x
where H _x is the magnetic field strength of the conductor with current i;
F _x is the magnetic force acting on the MR;
S is the cross-sectional area of the console (a • d);
M _s is the saturation magnetization;
a is the width of the console;
d is the thickness of the console.

При H_x ≅ H_A (где H_A - поле анизотропии, параметр ферромагнетика) эффект магнитосилового взаимодействия мал, и в выражении (6) можно пренебречь соответствующим членом в квадратных скобках по сравнению с единицей. Тогда частота собственных колебаний МР будет изменяться, главным образом, за счет изменения модуля упругости E от H_x

Не нарушая общности рассуждений, рассмотрим взаимосвязь частоты собственных колебаний МР в системе ВОЛ-МР с магнитным полем бесконечного прямолинейного проводника с током i, расположенного на расстоянии Z₀ от МР.Solving equations (3) and (5) together, we obtain a transcendental equation, from which it follows that for F _x l ² / EJ≅ 1, the natural frequency of the MP oscillations depends linearly on the value of H _x :

At H _x ≅ H _A (where H _A is the anisotropy field, the ferromagnet parameter), the effect of magneto-force interaction is small, and the corresponding term in square brackets can be neglected in expression (6) compared to unity. Then the frequency of the natural oscillations of the MR will change mainly due to a change in the elastic modulus E from H _x

Without violating the generality of reasoning, we consider the relationship between the frequency of the natural oscillations of the MR in the VOL-MR system with the magnetic field of an infinite rectilinear conductor with current i located at a distance Z ₀ from the MR.

Из выражения (8) следует, что при известном значении аргумента H_x, определяемом из выражения (7), вычисляется ток i, протекающий по проводнику, расположенному от МР на расстоянии r₀. Аналогично по величине H_x дистанционно измеряется ток i проводников иных конфигураций: прямоугольного витка, кругового витка, тороида, соленоида и т.п.Given the principle of superposition of magnetic fields, we have:

From the expression (8) it follows that for a known value of the argument H _x determined from the expression (7), the current i flowing through the conductor located from the MR at a distance r _{0 is} calculated. Similarly, by the value of H _x , the current i of the conductors of other configurations is remotely measured: a rectangular coil, a circular coil, a toroid, a solenoid, etc.

Решая уравнение (3) с учетом (9) в функциях Крылова, находим, что частота f собственных колебаний МР, нагруженного продольной силой F_x, связана с частотой ненагруженного резонатора f₀ следующим образом:
f² = f₀ ²(1+f² = f $\genfrac{}{}{0ex}{}{\text{2}}{\text{0}}$ (1+αF_xl²/EJ) (10)F_xl²/EJ) (10)
Константа α зависит от F_x и геометрических размеров МР (l, d, a). При F_xl²/EJ ≅ 10 значение α практически постоянно и равно α = 0,0246. В этом приближении уравнение (10) можно линеаризовать с точностью 0,5%, что дает следующую зависимость f(F_x):

где ρ - плотность образца МР.When using MR in the form of a beam fixed on both sides, the course of reasoning is similar. In relation to the beam, the boundary conditions are written in the form:

Solving equation (3) taking into account (9) in the Krylov functions, we find that the frequency f of the natural oscillations of the MR loaded by the longitudinal force F _x is related to the frequency of the unloaded resonator f ₀ as follows:
f ² = f ₀ ² (1 + f ² = f $\genfrac{}{}{0ex}{}{\text{2}}{\text{0}}$ (1 + αF _x l ² / EJ) (10) F _x l ² / EJ) (10)
The constant α depends on F _x and the geometric dimensions of the MP (l, d, a). For F _x l ² / EJ ≅ 10, the value of α is almost constant and equal to α = 0.0246. In this approximation, equation (10) can be linearized with an accuracy of 0.5%, which gives the following dependence f (F _x ):

where ρ is the density of the MR sample.

With an increase in the longitudinal force F _{x, the} influence of the elastic properties of the beam on the bending stiffness decreases, and in the limit at F _x ___ → ∞, formula (11) goes over into the formula for the string vibration frequency with α = 0.0197.
The drawing shows a diagram of a microresonator VOD of electric current, where 1 is a fiber optic waveguide activated by erbium, which is pumped at a wavelength of λ _n = 0.98 μm, 2 is a single-mode fiber, 3 is an AK made in the form of a section of a single-mode quartz fiber with a spherical microlens formed directly on the end face of the light guide 4 - the mirror M _1, the optical resonator, which serves as the boundary between the fiber-air, 5 - MP representing a tape (film) of amorphous magnetically soft alloy (e.g., spin-on glass ( Ferrous materials-tone), a single crystal yttrium iron garnet) as a microconsole (mikrobalki) 6 - the angle θ _between the normal to the reflecting surface MR 5 and the optical beam axis formed fiber AK 3, 7 - the mirror M _2, as which used the reflecting surface MP 5, l is the length of the microlens, D is the diameter of the microlens, d is the diameter of the collimated beam, H is the distance between the microlens and MP 5, 8 is the conductor with current, H _x is the magnetic field of the conductor with current 8, 9 is the photodetector, 10 is spectrum analyzer.

 The device operates as follows.

The electric current VOD, calibrated over the range of the measured electric current, is placed relative to the conductor with current 8 at a certain distance Z ₀ so that the longitudinal axis MP 5 and the direction of the magnetic forces F _{x of the} magnetic field of the conductor with current 8 coincide. Regardless of the configuration of the MR (console, beam), the magnetic field of the conductor H _x , acting on the MP 5, leads to a change in the characteristics of the MP 5 and, therefore, its own resonant frequency associated with the measured current. In the VOL 1 - MP 5 system, a self-oscillating mode is established with the oscillation frequency F, which coincides with the resonant frequency of the i-th mode of MP oscillations: f _i = F, where i = 1, 2, ... m. In this case, the self-oscillating regime in the VOL-MR system is carried out by modulating the amplitude of the reflection coefficient of the VOL optical resonator due to photoinduced angular deviations of the mirror, which is the MR.

 Thus, a new principle is proposed for constructing a microresonator VOD of an electric current containing a fiber autocollimator, which ensures high stability of the collimated beam parameters in a wide range of destabilizing factors.

The invention allows to obtain the following positive properties:
- reduction in the mass and dimensions of water;
- improving the reliability, accuracy, stability, speed;
- an increase in the efficiency of interaction between a fiber-optic laser and a microresonator.

Claims (3)

 1. Microresonator fiber-optic electric current sensor, including a fiber-optic laser as an optical radiation source, a microcavity with a mirror reflector, an autocollimator, a photodetector, a spectrum analyzer, characterized in that a fiber optic autocollimator made in the form of a single-mode quartz optical fiber is used as an autocollimator with a spherical lens formed on the end of the fiber, and the microcavity is made in the form of a film of magnetic material, while the longitudinal axis of the micro resonator coincides with the direction of the magnetic forces of the magnetic field of the conductor with the current.  2. Microresonator fiber optic electric current sensor according to claim 1, characterized in that spin glass is used as the magnetic material of the microresonator.  3. Microresonator fiber-optic electric current sensor according to claim 1, characterized in that a single crystal of iron-yttrium garnet is used as the magnetic material of the microresonator.