SU1739228A1 - Fibre-optical pressure indicator - Google Patents

Abstract

This invention relates to a measurement technique and is intended to measure pressures. The purpose of the invention is to simplify the design and reduce the temperature error. The interferometric fiber-optic sensor is made on a two-core fiber with two fiber-optic channels. The coherent laser radiation is injected simultaneously into both strands of the fiber, at the output of which an interference field is formed. The fiber is wound on a hollow elastic insulated cylinder so that the plane of location of the veins is perpendicular to its axis. When pressure is applied to the cylinder, the interference fringes shift in proportion to the pressure. 1 il.

Description

The invention relates to fiber optic instrument making, in particular to fiber optic sensors of physical quantities, and can be used in hydroacoustics and other areas of metrology for measuring pressure.

Single-mode two-fiber interferometric sensors are known.

A typical fiber-optic interferometric sensor circuit on two single-mode fibers is a modification of the classical Mach-Zehnder interferometer and includes a radiation source (laser), collimating systems (lenses), beam-splitting elements, micro-lenses, and a phase difference meter. To work according to this scheme, the condition of quadrature is required, which implies the necessity of stabilizing the phase of the wave in the reference fiber (isolating the fiber from external factors). Under laboratory conditions, the stabilization of the phase of the support arm is achieved by thermostating this element, which deprives the fiber-optic interferometric sensors of one of its main advantages — compactness. Other methods of stabilizing the working point of the interferometer are also used that do not lead to compactness. The presence of collimating and light separating elements complicates the operation of such an interferometric sensor (the need to take into account the level of back reflection in the laser resonator from various optical elements of the circuit) and increases its mass compared to the fiber-optic interferometer in the integral design (fiber splitter and combiner). The absence in the circuits of interferometric sensors of the above elements of stabilization of the FZ-beam-splitting and collimating elements leads to a decrease in mass

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sensor, simplicity of design and, as a consequence, suitability for industrial use. The operation of fiber-optic interferometric sensors is influenced by external influences (vibrations, thermal fluctuations) on the switching part of the fibers (fiber sections between the sensing element, laser and phase difference meter), which also leads to the limited practical application of such fiber-optic sensors.

The purpose of the invention is to simplify the design of the sensor and reduce the temperature error.

This goal is achieved due to the fact that in a fiber-optic pressure sensor containing a source of coherent radiation, a fiber sensing element and an optical phase difference meter for optical waves, two fiber-optic information transmission channels are made in the form of a two-core optical fiber, and the sensing element is in the form of an elastic cylinder, wherein the fiber is wound so that the plane of location of the cores is perpendicular to the axis of the cylinder.

The drawing shows a diagram of the sensor.

The sensor contains a source of coherent radiation 1, a micro-lens 2, a twin-core optical fiber 3, a portion of which is wound on a pressure-sensitive element (elastic cylinder) 4, and a meter 5 of the phase difference of optical waves.

The sensor works as follows.

The coherent radiation of source 1 by micro-lens 2 is introduced into double-core fiber 3 into both cores. When light waves propagate through the fiber (in each core), their mutual coherence remains. After the waves exit the fiber in the region of the beam overlap, a stable interference field is formed.

On the screen (the slit plane of the phase difference meter), located at a distance I from the output end, interference bands are observed whose width is determined by the expression x I I d, where d is the distance between the core centers, and I is the wavelength of optical radiation. The observed interference bands are perpendicular to the plane of location of the veins at the fiber end. In this way, the plane of the cores at each end of the fiber is determined. The fiber section with a specific core plane is tensioned around a hollow elastic cylinder so that the core plane is perpendicular to the axis of the cylinder. With such a winding

A pair of fibers per cylinder causes a static phase difference of the waves in the cores due to their different deformations. The pressure, which changes the radius of the elastic cylinder, leads to a change in the static phase difference of the waves, which manifests itself in a shift of the interference fringes proportional to the pressure or a change in the radius Dg. The ratio between D g and P is determined by

by expression

Lg 7 Ґ 0-2g /) + (1 + g;) -

where n, G2 - external and internal radii

cylinder, E, V- Young coefficients and Poisson cylinder material. A useful sensor signal, proportional to pressure, is extracted from the analysis of the observed interference pattern, in particular from

interference fringe measurements. The measurement of the offset of the interference fringes was carried out by measuring the offset of the interference maximum using a moving slit with a photodiode

located on the micrometer table. The change in the phase difference is equal to Dy 2l: x / dx where x is the displacement of the strip. The slit width (10–20 µm) was chosen many times smaller than the width of the interference fringes (1-2 cm). The measurement of the offset of the interference fringes was also carried out using the technical vision system ST-3-1 with a spatial resolution of 20-25 µm with access to a computer. Temperature fluctuations do not affect the operation of the sensor, since both strands of the fiber are at the same temperature and there is no change in the phase difference. Temperature sensitivity of such a sensor

is determined only by the temperature expansion of the fiber winding cylinder.

Example. Implementing a fiber optic pressure sensor on a twin core fiber. The sensor used quartz fiber with an external polymer coating, the radius of each core was 1.3 µm, the distance between the core centers was d 28 µm. The fiber cores were single mode for emitting a He-Ne laser at a wavelength

I am 6328 Ah. Uch astok fiber with a length of L 80 cm was wound on a hollow cylinder of a radius of 9 cm of rubber and fastened on it. Behind the output end of the fiber, a phase difference meter for optical waves was located.

Relative sensitivity of the realized sensor showing the specific change in the phase difference determined by the expression / J, 6 rad The sensitivity measurement was carried out

when winding fiber on a piezoceramic cylinder of the same radius.

A model of an interferometric sensor in fiber design has been implemented.

Thus, compared with the known, a dual core fiber sensor has several advantages.

The use of a two-core fiber leads to a simplification of the design and a reduction in the mass of the sensor, since in the implemented scheme there are no light separating and combining elements, optical collimating systems. The implementation of two optical channels of the interferometer in one fiber, which are in the same temperature conditions, leads to an increase in noise immunity to temperature changes and the absence of the need for thermal stabilization of the reference fiber, which also simplifies the design and reduces the sensor mass,

Claims (1)

Fiber Optic Pressure Transducer, containing a sensitive element, a source of coherent radiation, two fiber optic transmission channels and a phase difference meter for optical waves, in order to simplify and reduce the temperature error , there are two fiber-optic information transmission channels in the form of a twin-core optical fiber, and the sensing element is designed as a cylinder, while the two-core optical fiber is wound on a sensitive element positioning a plane perpendicular to the veins of the cylinder axis.