RU2082086C1 - Fiber-optical detector - Google Patents

Abstract

FIELD: transducers of small-scale variations in pressure, speed of flow of liquids and gases, level meters. SUBSTANCE: device has reference and information reflecting surfaces in opposite to which ends of reference and information light guides are located. Said light guides are connected to non-symmetry arms of V-shaped fiber-optical dividers which symmetrical arms are connected to light sources and photodetectors which convert optical signals to electric ones. Latter signals are processed by device. Said reference and information planes are designed as two reflecting surfaces which are rigidly attached to monitored object and are oriented in opposite sides in parallel to each other. Ends of reference and information light guides are located at equal distance from reference and information surfaces respectively and are directed to opposite sides. Output electric signal is generated by means of relative analog subtraction of electric signals from reference and information channels. EFFECT: increased functional capabilities. 3 dwg

Description

 The invention relates to the field of measuring equipment, in particular to the field of fiber-optic measuring instruments. The invention can be used to control the movements of material environmental objects.

Недостатками этой схемы являются зависимость результатов измерений от флуктуаций, возникающих в элементах оптического тракта (световодах, разветвителях) под влиянием изменяющихся параметров внешней среды (таких как давление, температура, механические воздействия), зависимость их от отражающих свойств поверхности, а также нелинейная зависимость выходного сигнала от величины перемещения контролируемой поверхности.Known single-channel water of linear displacements of the reflective type, consisting of an emitter, a photodetector, an information fiber connected to the arm of the optical divider, which can be used as a fiber-optic V-shaped divider (hereinafter - the "divider"), having two symmetrical shoulders and one single-ended [1]
During measurements, the end of the information fiber is located opposite the surface whose displacement is measured (information surface). The diverging light flux from the end of the information fiber is radiated in the direction of the information surface and is reflected from it. Part of the reflected light flux P ₁ = P ₀ η _in (x) through the same end returns to the information fiber and, propagating through it in the opposite direction, passes through the divider to the photodetector. The magnitude of the electrical signal at the output of the photodetector is equal to:
_O U = q • k ² • k _c • k _n • α • F ₀ • η (x) (1)
where q is the conversion factor of the light flux into an electrical signal; k ^{2 is} the loss coefficient in the information fiber and the asymmetric arm of the divider (splitter), taking into account the double passage of the light flux in the forward and reverse (when reflected) directions; α is the reflection coefficient of the controlled surface; k _with and K _n - loss coefficients in the symmetrical arms of the divider; h _in (x) the input efficiency of the reflected light flux through the end of the information fiber, defined as the ratio of the light flux coming through the end to the information fiber P ₁ to the amount of light flux emitted from the end face P ₀ :
η (x) = P ₁ / P ₀ , 0 <η (x) ≅ 1 (2)
dimensionless function, depending only on the distance x of the end of the fiber to the controlled surface. The coefficients q, k, α and P ₀ in [2] are independent of the displacement x and determine the constant amplitude of the output signal U _{o, o} in relation (1) so that
U _o = U o _{, 0} • η (x).
If the initial distance x ₀ between the end of the information fiber and the surface under control and the explicit form of the function η (x) are known, then the initial value of the output signal is known, from which the output signal is subsequently measured: U _o (x ₀ ) = U o _{, 0} • η (x ₀ ). Any linear movement of the controlled surface relative to the end by Δx will lead to a change in the output signal by ΔU, which can be represented as an expansion in powers of Δx:

The disadvantages of this scheme are the dependence of the measurement results on fluctuations arising in the elements of the optical path (optical fibers, couplers) under the influence of changing environmental parameters (such as pressure, temperature, mechanical stresses), their dependence on the reflective properties of the surface, and the nonlinear dependence of the output signal from the amount of movement of the controlled surface.

 The influence of changing environmental parameters can be significantly reduced by introducing into the measurement circuit a second (reference) channel [2] that includes, in the general case, a reference light guide, a fixed (reference) surface, a radiator, and a photodetector. The distance between the end of the reference fiber and the supporting surface is fixed during the measurement. The components of the signals in the reference and information channels, due to external influences, are proportional. The influence of external influences on the measurement results can then be eliminated by comparing the values of the reference and information signals.

 With a fixed position of the supporting surface relative to the end of the reference fiber, the efficiency of the input of the reflected beam is constant accurate to fluctuations due to external influences, therefore, the output signal in the reference channel is also constant. Accordingly, the nonlinear dependence of the output signal on the linear displacements of the information surface is also preserved. In addition, in this circuit, the dependence of the output signal on the reflective properties of the surface being monitored is preserved.

 The closest analogue to the invention in technical essence is a fiber optic sensor containing a first reflecting surface, a first and second fiber optic fibers, a first fiber optic splitter, an asymmetric arm of which is coupled to the end of the first fiber, the first emitter and photodetector, optically paired with symmetric shoulders of the fiber-optic divider, modulator, signal processing and change unit [3] An output signal that non-linearly depends on the amount of movement is controlled of the second object, is obtained in this circuit by analog processing of the signals of the first and second fiber optical fibers.

 The disadvantage of this scheme is the effect on the magnitude of the signals of the reflecting properties of various parts of the surface of the controlled object due to their differences in the absence of special processing (grinding, polishing, etc.). In addition, in this circuit, the signals in the first and second optical fibers change in phase, which does not reduce the nonlinearity of the output signal.

 The nonlinear dependence of the output signal on the displacement of the controlled object and the influence on its magnitude of the reflective properties of parts of the surface reduce the accuracy of measurements and reduce the functionality of this sensor.

 The objective of the invention is to increase the accuracy of measurements and expand the functionality of the sensor.

 The specified technical result is achieved by the fact that in the fiber-optic sensor of micro displacement, containing the first reflecting surface, the first and second fiber optic fibers, the first fiber-optic divider, the asymmetric arm of which is coupled to the end of the first fiber, the first emitter and photodetector, optically paired with symmetric the shoulders of a fiber optic divider, a modulator, a signal processing and measurement unit, a differential amplifier, a second fiber optic divider, W are introduced a second emitter and a photodetector optically coupled to the symmetrical arms of the second fiber optic divider, the asymmetric arm of which is optically coupled to the end of the second fiber optic fiber, a reflector mounted with the possibility of hard mounting on the controlled object and made with the first reflective surface on one side and the second the reflecting surface on the side opposite to the first, while the surfaces are identical and parallel, the end face of the second fiber optic fiber Since it is connected to the second reflecting surface, the ends of the optical fibers are located at the same distance from the corresponding reflecting surfaces.

 The differences of the invention are the implementation of the first and second reflective surfaces in the form of a two-sided reflector, the oppositely oriented reflective surfaces of which are subjected to the same treatment in order to equalize their reflective characteristics, as well as the rigid connection between them during the measurement process. These differences result in the fact that when the controlled fiber is moved, the signal values in the first and second fiber fibers move in antiphase, which allows one to linearize the characteristics of the sensor by mutually subtracting them. It should be noted that since the first and second channels are identical, the difference between them is arbitrary and in Fig. 1, which shows the sensor implementation diagram, the position numbering for the first channel is odd (the "odd" channel), and for the second channel is even ( "even" channel).

 In the diagram of figure 1, the following notation: 1 controlled object; 2 reflector; 3.4 ends of the first / 3 / and second / 4 / optical fibers 5 and 6, respectively; 7 and 8 optical connectors (OS) of unbalanced arms of the splitters; 9, 10, 11, 12 symmetrical shoulders of the splitters; 13, 14 photodetectors; 15, 16 emitters; 17 differential amplifier; 18 is a modulator.

 In the even channel, the luminous flux 16 from the emitter 16 through the symmetrical arm 12, the asymmetric arm with the optical connector 8 hits the end 4 of the fiber 6. From the end 4, the diverging light flux propagates in the direction of the surface 2 and is reflected back from it in the direction of the end 4. Part of the reflected luminous flux enters the end 4 and through the light guide 6, the symmetrical arm of the splitter with the optical connector 8 and the second symmetrical arm 10 of the splitter falls on the photodetector 14. Another part of the reflected flux through the the tertiary arm 12 extends back to the emitter and is lost. Similarly for an odd channel.

где q₁₄ и q₁₃ коэффициенты фотоэлектрического преобразования (чувствительности) фотоприемников в четном и нечетном каналах соответственно; k₅, k₆, k₇, k₈, k₉, k₁₀, k₁₁, k₁₂ потери в световодах 5, 6 и плечах 7, 8, 9, 10, 11 и 12 разветвителей; η_ч и η_н эффективности ввода отраженных световых потоков через торцы 4 и 3 соответственно, определяемых как в /2/; P₁₅ и P₁₆ световые потоки на выходе излучателей 15 и 16.The magnitude of the signals at the output of the photodetectors in the even and odd channels are equal, respectively:

where q ₁₄ and q _{13 are} the photoelectric conversion (sensitivity) coefficients of the photodetectors in the even and odd channels, respectively; k ₅ , k ₆ , k ₇ , k ₈ , k ₉ , k ₁₀ , k ₁₁ , k ₁₂ losses in the optical fibers 5, 6 and shoulders 7, 8, 9, 10, 11 and 12 of the splitters; η _h and η _{n the} efficiency of input of reflected light fluxes through the ends 4 and 3, respectively, defined as in / 2 /; P ₁₅ and P ₁₆ light fluxes at the output of emitters 15 and 16.

С учетом того, что фазы изменений сигналов на входе дифференциального усилителя при перемещении контролируемого объекта на величину дельта X противоположны, можно получить их выражения для четного U(X_ч) и нечетного U(X_н) каналов соответственно:

Сигнал на выходе дифференциального усилителя равен разности сигналов на выходах каналов.By analogy with a single-channel sensor / 3 /, it is possible to present relations (5) for each channel in a simplified form:

Taking into account the fact that the phases of changes in the signals at the input of the differential amplifier when the controlled object is moved by delta X are opposite, we can obtain their expressions for even U (X _h ) and odd U (X _n ) channels, respectively:

The signal at the output of the differential amplifier is equal to the difference of the signals at the outputs of the channels.

In a particular case, optical fibers with a stepped profile of a refractive index index of 50 μm in diameter were used; fiber aperture NA 0.23. Therefore, with an error of less than 0.1% n _h (x) = η _n (x) = η (x). Figure 2 shows the dependence η (x) in graphical form, taken from [4] and recounted in relation to the case of pairing the mirror surface with the end of the fiber with the above parameters. To determine the nature of the dependence of the magnitude of the output signal U _res on the linear displacement Δx of the controlled surface, it is necessary to set a certain initial position of the ends of the optical fibers 3 and 4 (Fig. 2), which are the same in the proposed device: x _{n, 0} = x _{h, 0} = x ₀ . Regulating independently the pump currents of the emitters, it is possible to obtain equal signals at the differential inputs of the remote control in such a way that U _{h, 0} = U _{n, 0} = U ₀ . Then, in the initial position, at xx _{0, the} signal at the remote control output in accordance with expressions (6) and (7) is equal to zero, the signals at the remote control output are subtracted.

When the controlled object is moved by an amount Δx output control signal becomes equal to U = U _{0 res} [η '(x ₀₎ Δx • η "' (x ₀₎ (Δx) ^3/3 + ...] and a first approximation, is linear with respect to Δx.

 In practice, however, in relations (6) and (7), to increase accuracy, it is necessary to take into account terms of a higher order. This leads to the fact that the linear dependence of the output signal takes place in a limited range of linear movements.

In FIG. 2 shows the calculated graphs of the normalized value of the output signal U _res / U ₀ [η (x) for a single-channel sensor, curve 1 and Δη for a two-channel, curves 2 5] on the distance between the end of the left fiber (Fig. 2) and the controlled object (xx ₀ + Dx) for various values of x ₀ calculated by numerical interpolation using the results of [4] Curves 2, 3, 4, 5 constructed a prototype (corresponding dependence / 1 /); 2 x ₀ 175 μm; 3 for x ₀ 125 μm, 4 for x ₀ 100 μm, 5 for x ₀ = 175 μm.

The calculations were performed under the assumption of a uniform distribution of radiation at the ends of the optical fibers with a stepped profile of the refractive index / 4 /. From FIG. Figure 2 shows that in the region x ₀ 100 μm the dependence of the output signal almost linearly depends on the displacement of the controlled object in the entire range of permissible displacements (0 200 μm), i.e. The sensitivity of the proposed device is constant in this range.

Figure 3 presents the curves of the dependence of sensitivity on x for various values of x ₀ in the vicinity of x ₀ 100 μm. The figure shows that in the range x ₀ from 110 to 120 μm, the relative change in the sensitivity does not exceed 5%
It should be noted that the maximum values of the normalized output signals in the proposed device is always less in absolute value of the output signal in a single-channel sensor.

 In the proposed device, optical fibers with other parameters can be used depending on the requirements for the device, however, in any case, knowledge of the dependence η (x) is necessary.

Claims (1)

 A fiber-optic micromotion displacement sensor containing a first reflecting surface, a first and second fiber optic fibers, a first fiber optic splitter, an asymmetric arm of which is interfaced with the end of the first fiber optic fiber, a first emitter and a photodetector optically coupled with symmetrical arms of the first fiber-optic splitter, a modulator , a signal processing and measurement unit, characterized in that a differential amplifier, a second fiber optic divider, a second emitter and a receiver, optically coupled to the symmetrical shoulders of the second fiber optic divider, the asymmetric arm of which is coupled to the end of the second fiber waveguide, a reflector mounted with the possibility of hard mounting on the controlled object and made with the first reflecting surface on one side and with the second reflecting surface on the side opposite the first, while the surfaces are identical and parallel, the end face of the second fiber is optically coupled to the second reflect s surface, the ends of optical fibers are arranged at equal distances from the respective reflecting surfaces.

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