SU1755382A1 - Fiber-optic sensor - Google Patents

Abstract

Usage: the field of fiber optics, in particular the measurement of small acoustic and electromagnetic disturbances, constant and displaced in time. SUMMARY OF THE INVENTION: A fiber optic sensor comprises a coherent radiation source, a light guide, a coil, a photodetector, and a phase discriminator. The coherent radiation source is made in the form of a two-frequency laser with orthogonal polarization of modes, and the light guide is made in the form of an anisotropic single-mode optical fiber. The sequentially installed half-wave plate and the first micro objective are placed between the laser and the light guide. Between the light guide and the photodetector there are successively installed a second micro lens and a polarizer-analyzer, the polarization plane of which is oriented at an angle of 45 ° amp; relative to the anisotropy axes of the optical fiber, aligned with the polarization planes of the laser modes. 1 silt

Description

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The invention relates to the measurement of small acoustic and electromagnetic disturbances, constant and variable in time.

Devices for measuring small acoustic disturbances are known that contain radiation sources — a laser, an optical fiber, an optical system for forming a light beam, and a photodetector.

The disadvantage of the devices is the uncontrolled attenuation of signals (fading of signals). This is the result of exposure to the optical fiber environment.

Closest to the present invention is a fiber optic acoustic wave sensor comprising a coherent radiation source, an acousto-optic modulator, a dividing mirror, two optical fibers, a mixing mirror, a photodetector, and a phase discriminator. The laser radiation is divided into two beams by a dividing mirror. One beam passes through an acousto-optic modulator and is introduced into the optical fiber. He plays the role of the reference beam. Another bundle is introduced into the fiber, located in the fluid. An acoustic wave propagating in this fluid causes changes in the refractive index or fiber length. The result is a change in the phase of light propagating in the fiber, according to the law of acoustic disturbance; The light beams that pass through the reference and signal fibers are mixed on the mixing mirror and fed to the photo detector. On a photodetector load, a signal is extracted with a GAO frequency determined by an acousto-optic modulator. Acousto-opticVI signal

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the fAO modulator of a modulator is phase modulated according to the law of acoustic disturbance; to which the fiber was subjected. A phase discriminator is used to restore the information (acoustic) signal.

This device has a significant drawback - uncontrolled attenuation of the signal (fading), leading to the breakdown of the sensor. The fading of a signal is determined as follows. Optical fiber (both reference and signal) under actual laying conditions is exposed to environmental influences (heating, bending, compression, vibration, and shock). This leads to uncontrolled changes in birefringence of the fiber, which are variable in time. As a result, the polarization of the light passing through the fiber changes. Thus, the polarization of the beams on the mixing mirror are mismatched. The visibility of the interference pattern and, therefore, the amplitude of the signal with the frequency of the AO is determined by the change in the external conditions in which the reference and signal fibers are located. If the polarizations of the beams on the mixing mirror are orthogonal, then the amplitude of the signal with frequency Tdo is zero. If the polarizations of the beams coincide, the amplitude of the signal is maximum. So, the environmental impact on the fiber path of the sensor leads to a breakdown of its operation, which ultimately leads to a decrease in the accuracy of the sensor.

The purpose of the invention is to improve the accuracy of the sensor by reducing the influence of external environmental influences on the fiber.

The goal is achieved by the fact that, in a device containing a coherent radiation source, an optical fiber, a photodetector and a phase discriminator, the optical radiation source is made in the form of a two-frequency laser with orthogonal polarization modes, and the fiber is in the form of an anisotropic single-mode optical fiber, The azimuths of polarization of the laser modes coincide with the orientation of the anisotropy axes of the fiber. In addition, a polarizer analyzer located between the output end of the fiber and oriented at an angle of 45 ° relative to the fiber anisotropy axes is included in the device.

The use of a dual-frequency laser with orthogonal polarization and anisotropic fiber causes the optical circuit of the sensor to become single-arm. A fiber optic sensor built according to this scheme is resistant to environmental effects on the fiber. This is determined by the use of anisotropic fiber. The polarization of the radiation propagating in such a fiber varies little under fiber disturbances. In addition, and most importantly, both interfering beams are spatially almost perfectly aligned — they propagate under the same conditions (in the same fiber). ). External disturbances change the propagation conditions of one interfering beam (mode with frequency f 1) in the same way as the conditions of propagation of another interfering beam (mode with frequency h) Therefore, even if the polarizations of interfering beams change (with very strong effects) they change consistently. The orthogonality ratio of the polarizations of the interfering beams propagating in the fiber remains constant in all cases. On the polarizer-analyzer in front of the photodetector, the collinear components of the laser mod fields are highlighted, which, after interfering, produce the signal of the difference frequency fi-fa on the photodetector load. Since the ratio of the orthogonality of the polarizations of the interfering modes is preserved, the amplitude signal of the difference frequency f i-fa is also constant. Thus, a significant distinguishing feature is that the optical scheme of the sensor using the described laser and fiber is single-arm.

The drawing shows a structural diagram of a fiber optic sensor.

The sensor contains a laser 1 with the possibility of generating two modes with frequencies f 1 and g and orthogonal polarizations, half-wave plate 2, microobjects 3, single-mode anisotropic optical fiber 4 wound on coil 5, polarizer-analyzer 6, photodetector 7 and phase discriminator eight.

The device works as follows.

The radiation from laser 1, which generates two modes with frequencies f 1 and fa and with orthogonal polarizations, is introduced into a single-mode anisotropic fiber 4 using a half-wave plate 2 and the first micro-lens 3. A Zeeman laser (LGN) can be used as such an optical source 212) or a two-frequency laser with internal mirrors (LGN-208AB). By controlling the orientation of the half-wave plate 2, the azimuths of the polarizations of the fi and h modes of the laser are achieved.

respectively with the direction of the fast and slow axis of the anisotropy of the fiber. The optical fiber 4 is wound on a sensor coil 5. The latter is placed in a liquid in which sound waves propagate. Sound pressure changes the length of the optical fiber. The phase of the optical radiation propagating in the fiber changes accordingly. If the coil 5 is made of a piezoelectric or magnetostrictive material, then placing the coil 5 in an electric or magnetic field, respectively, also leads to a change in the phase of the light in the optical fiber. Having determined the change in the phase of coherent radiation, information is obtained about the magnitude of the effect to which the sensor coil has been subjected.

Let us consider in more detail the process of modulating the phase of light in the sensor. Two waves with orthogonal polarizations propagate in the sensor fiber. The sex of these waves is determined in time as follows:

2 Yash L

Ei Eiosin (+ Ј2 Ј20 sin (2 l f2t +

I

2 7ГП2 L

):. (t)): (2)

where f i, h are the frequencies of the modes generated by the laser:

A is the radiation wavelength of lasers;

ni, n2 are the effective refractive indices of anisotropic fiber along fast and slow fiber axes, respectively;

L is the fiber length.

The light passing through the fiber then passes through the second micro-lens and a polarizer-analyzer, oriented at an angle of 45 ° relative to the direction of one of the anisotropy axes of the fiber. On the load of the photodetector 7, a difference frequency signal is extracted:

f sln (2tt (fi-f2) t + BL), (3)

where B 2 lg (n1-A2) / A is the anisotropic fiber.

The impact of physical fields measured by the sensor is reduced to a change in fiber length. From formula (3), it follows that a change in the length of the optical fiber leads to modulation of the phase of the difference frequency signal. The photodetector signal enters the phase discriminator. The output signal of the phase discriminator is proportional to the measured physical parameter.

The general configuration of the sensor construction, which allows maintaining the mutual orientation of the polarizations of the fields of the interfering beams unchanged, makes it possible to create a sensor that stably operates under various parasitic environmental influences on the fiber.

Claims (1)

Invention Formula A fiber optic sensor containing a coherent radiation source, a light guide, a coil, a photodetector, and a phase discriminator, characterized in that, in order to increase accuracy by reducing the influence of external environmental influences, the coherent radiation source is designed as a two-frequency laser with orthogonal the mode is polarized, and the fiber is in the form of an anisotropic single-mode optical fiber, sequentially installed inserted half-wave plate and first micro lens are placed between the laser and the fiber, and sequentially inserted second micro lens and polarizer-analyzer are placed between the fiber and photo detector; is oriented at an angle of 45 ° with respect to its anisotropy of the optical fiber, combined with the polarization planes of the laser modes.