WO2014171859A1

WO2014171859A1 - Method and device for the distributed measurement of birefringence in polarization-maintaining fibres (variants)

Info

Publication number: WO2014171859A1
Application number: PCT/RU2014/000250
Authority: WO
Inventors: Михаил Владимирович ГРЕКОВ; Антон Сергеевич КУЗИН; Александр Николаевич НАУМОВ; Евгений Викторович ФИГУРА; Андрей Александрович ФОТИАДИ
Original assignee: Общество с ограниченной ответственностью "Уникальные волоконные приборы"
Priority date: 2013-04-17
Filing date: 2014-04-04
Publication date: 2014-10-23
Also published as: RU2539849C2; RU2013117585A

Abstract

The invention relates to fibre optics, and more particularly to distributed fibre-optic sensors in which the parameters of an optical fibre being subjected to the action of external physical fields are measured. In the claimed methods for the distributed measurement of birefringence in polarization-maintaining optical fibres, it is proposed to use a fourth signal as a supplement to the known methods which use three signals. The detection and statistical processing of recorded signals are carried out by means for processing and storing signals with a neutral mean. In a first variant of the method, a fourth signal in the form of a linearly polarized signal can be introduced into an optical fibre counter to a sounding signal and in the polarization of the sounding signal, resulting in an increase in the signal-to-noise ratio. In a second variant of the method, a fourth signal on the frequency is not introduced into a fibre, but rather is mixed with a scattered signal when the latter is detected. The invention furthermore relates to devices for the distributed measurement of birefringence in polarization-maintaining optical fibres, by means of which the claimed methods are realized. The technical result is an improvement across a variety of parameters such as three-dimensional resolution, measurement accuracy and/or the range (length) of the section being tested.

Description

METHOD AND DEVICE OF DISTRIBUTED MEASUREMENT

TWO-REFRACTION IN POLARIZED CONSERVATION FIBERS (OPTIONS) The invention relates to fiber optics, in particular to distributed fiber optic sensors, in which the parameters of an optical fiber under the influence of external physical fields are measured.

Methods for measuring physical fields based on the dynamics of stimulated Mandelstam-Brillouin scattering (SBS) in polarized conservation fibers (PM fibers) are well known and are used in BOTDA-type fiber optic sensors. A feature of PM fibers is that the signal introduced into the fiber along one of the polarization modes retains its polarization during propagation through the fiber. Therefore, during SBS interaction of such signals inside the fiber, there is no mutual depolarization of signals, introducing local uncertainty into the effectiveness of SBS interaction.

In commercial sensors based on SBS (the so-called Brillouin analyzers, or BOTDA), methods for distributed monitoring of physical fields, such as temperature and longitudinal tension, based on measuring SBS resonance parameters, are implemented [1-4]. Their common essence lies in the fact that two optical signals at the pump frequency and Stokes frequency <y, shifted relative to each other by approximately the SBS shift ^' a ₀ , are applied to the optical fiber used as a sensitive element. One of the signals is continuous, the other is pulsed. At least one of the signals is scanned in frequency, thereby providing a scan of the frequency difference ω - <"in the vicinity of the average frequency of the SBS shift

(Ω ₀ ). The pulse signal passing through the fiber interacts in the process

SBS with a continuous signal, causing local changes in its intensity. Further, a change in the intensity of a continuous signal transmitted through the fiber is recorded by the sensor as a function of time and the frequency difference ω - ω. As a result of statistical and mathematical processing of the obtained data, the spatial-frequency characteristics of the SBS resonance are determined, namely, spatial distribution of the SBS gain coefficient (or the SBS gain line width), the Brillouin shift value Ω ₀ [1, 2], the position of the peaks

SBS resonances in fibers with several resonances [3].

From the patent of the Russian Federation JY ₂ 2179374 (published 02.10.2002; IPC Н04В10 / 08, G01M11 / 02, G01R31 / 11), variants of a device for measuring the characteristics of optical fiber fibers are known. In the first embodiment, the device comprises a low-coherent laser emitting continuous wave light with a pump frequency co _L , a power divider, a measuring directional coupler, the output of the main channel of which is connected to the near end of the fiber of the optical cable, connected at the opposite end to the optical reflector, the output of the measuring branch arm directional coupler connected to the input of the photodetector connected by the output to the input of the processor processing the result measurements, characterized in that the output of the low coherent laser is connected to the output of the passage arm of the first directional coupler, the output of the main channel of which is connected to one output of the active medium element with the effect of stimulated Raman scattering, the Stokes displacement frequency in which is equal to the Stokes displacement frequency in the studied fiber, the second the output of this element is connected to the output of the main channel of the second directional coupler, the output of the passage of which is connected to the first input of the power divider a tee connected by the second output to the first input of the optical adder, and the input through an optical amplifier with the output of the branch arm of the first directional coupler, the second input of the optical adder is connected to the output of the controlled optical shutter, the input of which is connected to the output of the branch arm of the second directional coupler, the output of the optical adder connected to the output of the passage arm of the measuring coupler. In the second embodiment, the device comprises a low-coherent laser emitting a continuous wave light with a pump frequency co _L , connected by an output to the input of a power divider, a measuring directional coupler, the output of the main channel of which is connected to the near end of the fiber of the optical cable, the branch of the measuring arm of the measuring directional coupler is connected with the input of the photodetector connected to the input of the processor that processes the measurement results, characterized in that the first output of the power divider is connected to the input of a controlled optical shutter, the output of which is connected to the output of the passage arm of the measuring directional coupler, the opposite end of the fiber under investigation is connected to the first output of the second divider power connected by the second terminal to the terminal of the passage of the first directional coupler, the main channel of which is connected inen with the opposite end of the first additional fiber of the optical cable connected by the proximal end to the output of the main channel of the second directional power coupler, the output of the passage arm of which is connected to the second output of the power divider, and the output of the branch arm with the proximal end of the second additional fiber of the optical cable, the opposite end of which through an optical amplifier connected to the input of the second power divider, and the output of the branch arm of the first directional coupler ene with an absorber. The technical result is to increase the accuracy and expand the functionality by using the scattering of both Brillouin and Raman.

However, the distributed measurement of birefringence in fibers with conservation of polarization described in the aforementioned patent cannot be realized by methods using BOTDA based on SBS interaction of optical signals at two frequencies.

This problem is solved in the well-known SBS method, disclosed in [5] and [6]. This method of distributed measurement of birefringence in fibers while maintaining polarization using a Brillouin dynamic grating is based on the reflection of the probing signal from a dynamic sound grating formed during SBS interaction of two counterpropagating light waves in RM fibers.

The essence of the known method lies in the fact that not two, but three optical linearly polarized signals are introduced into a segment of an optical PM fiber. Two continuous or quasi-continuous signals at a pump frequency ω £ ₀ and a frequency

Stokes from ₀ , shifted relative to each other by the SBS value of the shift Ω ₀ , are introduced into the fiber from opposite ends in the polarization of one of the main axes fiber. Their interaction in the fiber leads to the formation of a dynamic lattice of a sound wave propagating along the entire region of signal interaction. In the polarization of the other main axis, on the pump side, a short (nanosecond) probe, or probe, pulse (or signal) is applied to the fiber at a frequency ω shifted relative to the main pump frequency. The reflection of this pulse from the sound lattice during its propagation through the fiber leads to the formation of counterpropagating radiation in the form of a long pulse at a frequency ω «ω - Ω ₀ , the temporal power distribution of which contains information on the reflection efficiency of the probe signal from the dynamic lattice in various sections along the fiber .

Reflection efficiency at each point of the fiber depends on the position of the optical probe signal frequency ω about its resonant frequency ώ _Ό by four SBS interaction defined by the relation: ^'

where co _L0 and ω _ιο are the optical frequency of the main pump and the resonant optical frequency of the probing signal, n _x and η _γ are the refractive indices of the fundamental polarization modes of the fiber, and c and ν are the speeds of light in vacuum and sound in the fiber.

Thus, registering the position of the resonant frequency ω _{0 of the} probe signal at a given point of the fiber relative to ω £ ₀ , the frequency difference ^ω ι.ο ^{~ ω} Ιο = But the spatial distribution of birefringence in the fiber is determined:

in particular when fiber is used as a sensing element. This method was used to measure the distribution of temperature and pressure along the fiber with a good spatial resolution of the order of ten centimeters [7, 8]. It should be noted that achieving good spatial resolution is also possible when a short probe pulse in polarization Y is introduced into the fiber from the Stokes wave at a frequency ω ₈ ^γ shifted relative to the Stokes frequency ω ₀ , and the reflection of this pulse from the sound lattice during it propagation through the fiber leads to the formation of counterpropagating radiation in the form of a long pulse at a frequency ω «ωζ + Ω ₀ , in the temporal power distribution of which information is provided on the reflection efficiency of the probe la from the dynamic lattice in various sections along the fiber.

The above method of distributed measurement of birefringence in fibers while maintaining polarization using a Brillouin dynamic lattice according to [5], as well as the corresponding device of the same authors disclosed in Chinese patent application Ν ° 102589857 (published July 18, 2012; IPC G01M1 1/02) are selected as the closest analogues of the claimed methods and devices for the distributed measurement of birefringence in fibers while maintaining polarization.

A disadvantage of the known method and device is the low power of the detected scattered signal, due to the low efficiency of scattering of the probe pulse on the dynamic lattice. For this reason, for reliable registration of the reflected wave (i.e., to ensure an acceptable signal-to-noise ratio), the amplitude of the probe pulse intensity should be chosen large enough, usually exceeding hundreds of mW. This greatly limits the applicability of the method for long fibers, in which the thresholds of nonlinear effects amount to several tens of mW.

The objective of the present invention is to reduce the magnitude of the power of the optical signals needed to register the spatial distribution of birefringence in optical fibers while maintaining polarization. The technical result is to improve the combination of such parameters as spatial resolution, measurement accuracy and / or range (length) of the test site. The indicated technical result is achieved by the fact that in the proposed method for distributed measurement of birefringence in optical fibers with preservation of polarization, in addition to the known one, a fourth signal at a frequency αζ is used and a coordinated scanning of the frequency difference ω - ω, and> [- co _s ^Y and - ω neighborhoods of the values of Ω ₀ and H ₀ , respectively, where the Brillouin shift value, H ₍ ω, is the shift in the pump frequency at the four-wave SBS resonance, and the detection and statistical processing of the recorded signals are implemented by means of processing and accumulating signals with a zero mean.

Fourth frequency signal

in the form of a linearly polarized signal, it can be introduced into the optical fiber towards the probe pulse and into the polarization of the probe pulse, which leads to an increase in the signal-to-noise ratio.

This result can also be achieved when the fourth signal at a frequency af _s is not introduced into the fiber, but is mixed with the scattered signal when it is detected, which is another embodiment of the method of distributed measurement of birefringence in optical fibers with preservation of polarization.

The inventive device designed to implement the method of distributed measurement of birefringence in optical fibers while maintaining polarization, consists of an optical generator, two polarizing combiners, a polarizing divider, PM circulator, optical fiber, at least one photodetector, processor. Another variant of the claimed device consists of an optical generator, one polarizing combiner, an optical fiber coupler, a polarizing divider, a PM circulator, an optical fiber, at least one photodetector, and a processor.

The mentioned optical generator generates narrow-band optical radiation simultaneously at four different frequencies ω £, ω [, ω, o? _s in the vicinity of frequencies, respectively, ω ₀ , ω [ ₀ , ω ₀ , ω ₅ ^γ ₀ and is equipped with providing stabilization and independent tuning within the necessary limits of the frequency difference Η = ω £ -ω [, Q. ^x = ω - ω and Ω =

control of the pulse shape, intensity and / or phase of the radiation at these frequencies, and said processor provides statistical processing of the recorded signals by known methods of processing and accumulating signals with a zero mean.

Further, the invention is disclosed in more detail with reference to the accompanying figures.

Figure 1 shows a typical example of the arrangement of the resonant frequencies of interacting optical signals for a wavelength of 1550 nm (above) and a signal input circuit into the fiber while maintaining polarization.

In figure 2. a schematic diagram of the claimed device is given: at the top is the first option, at the bottom is the second option.

Fig. 3 shows a schematic diagram of an optical generator.

Figure 4 shows the parameters of the optical signals used in the calculations for Example 1.

Figure 5 shows a typical waveform detected at a frequency of ω ₀ .

In Fig.6 shows a view of the time derivative of the signal presented in Fig.5.

7 shows a view of the time derivative of the detected signal, averaged over 10 implementations.

Fig. 8 shows the zoom of the signals (i.e., on a larger scale) shown in Fig. 7.

Figure 9 shows the parameters of the optical signals used in the calculations for Example 2.

Figure 10 shows the time derivative of the detected signal, averaged over 10 realizations (above) and a similar signal (below), but obtained in the absence of a signal at a frequency of ω ₀ (three-frequency method).

11 shows the zoom of the signals shown in FIG. 10 (top).

12 is a view of the time derivative of the detected signal according to Example 3 averaged over 25 implementations. In Fig.13 shows the zoom of the signals presented in Fig.12.

The inventive variants of the method are based on the characteristics of the dynamics of stimulated Mandelstam-Brillouin scattering (SBS) in optical fibers with preservation of polarization (PM fibers) and the particular susceptibility of this process to local birefringence variations (including under the influence of measured external influences), determined by detection and mathematical processing of signals received from the output of the fiber during amplitude modulation and frequency scanning of the master optical fields.

In this method, ordinary optical fibers with conservation of polarization (PM fiber) can be used, and PM fibers of a special design and / or PM fibers enclosed in a cable of a special design can be used, in both cases providing selective sensitivity of the induced birefringence to the measured external field.

The method has advantages in systems designed for distributed measurement of two or more types of external influences at once (in combination with other methods), as well as in cases where it is necessary to provide better spatial resolution, measurement accuracy and (or) in comparison with known methods range of the test site. The inventive device options implement the proposed method options.

The technical result is achieved as follows.

In one embodiment of the method according to the invention, radiation at the pump frequencies cof, ω and Stokes signals ω, co _s ^Y is introduced into the fiber from opposite ends towards each other in polarizations corresponding to the main polarization axes of the fiber (X and Y). Near the resonance conditions described by equation (1) (in the presence of all signals at all four frequencies), pairs of pump waves and Stokes waves belonging to different polarizations interact with each other through a single hypersonic wave formed in the medium by these two pairs. A necessary condition for such a resonance is the proximity of the resonant frequency shift ω of the probe pulse relative to the first pump frequency ω described by equation (1), and a similar frequency shift αζ of the second Stokes signal relative to the frequency ω of the first Stokes signal (see figure 1). By detecting and processing the signals received from the fiber at a consistent amplitude modulation and frequency scanning of the master optical fields, the spatial-frequency distribution of the resonance is measured, in particular, the distribution of the position of the resonance shift H ₀ , the width of the resonance, and its profile on the fiber. The resonance position reproduces the distribution of An. Other resonance characteristics can also be used as independent parameters for measuring external influences.

When processing the detected signals, it should be borne in mind that the effectiveness of the interaction of pairs of waves in the fiber depends on the ratio of their phases at the place of interaction:

= Φϊ -Φ-Φΐ + Φΐ, (h) where <p _j ^l is the phase of the wave at frequency j at the point of interaction; g is X, Y; j = L, S.

For π / 2 <Φ <π / 2, the interaction of the pairs leads to an increase in the total increment of amplification in each of them; for l / 2 <Φ <Zlg / 2, to a decrease. In the general case, the SBS increment of the interaction of pairs of optical signals differs from the gain increment outside resonance, where the SBS process in each pair proceeds independently. However, the dependence of the interaction efficiency on the phase relation Ф makes the measurement results dependent on phase noise. Since the phase noise band, even in long fibers, is 10-100 kHz, such noise does not lead to significant signal distortions in one measurement cycle, and when averaging a signal over many cycles, it is necessary to use well-known statistical signal processing schemes with zero mean. In particular, in the example considered below, the mean time deviations of the time derivative of the recorded signal were used as a useful signal.

In another embodiment of the method according to the invention, the radiation at pump frequencies co,

and the Stokes signal co * is introduced into the fiber from opposite ends towards each other in polarizations corresponding to the main polarization axes of the fiber (X and Υ). The fourth signal at the frequency αζ mixes with the scattered signal when it is detected, which leads to an increase in the signal-to-noise ratio.

Similarly to the first option, the signals obtained from the fiber output are detected and processed at the matched amplitude modulation and frequency scanning of the driving optical fields, the spatial-frequency distribution of the resonance is measured, in particular, the distribution of the position of the resonance shift H ₀ , the width of the resonance, and its profile on the fiber. The resonance position reproduces the distribution of An. Other resonance characteristics can also be used as independent parameters for measuring external influences.

With respect to the second embodiment of the method according to the invention, the above remarks also apply regarding the efficiency of interaction of wave pairs in the fiber, depending on the ratio of their phases at the site of interaction.

A significant difference between the two claimed variants of the method of distributed birefringence measurement in optical fibers with polarization being preserved from those known from the prior art is the use of an additional fourth signal at the Stokes frequency αζ with subsequent signal processing at the fiber exit. This allows you to increase the signal-to-noise ratio, and therefore, increase the spatial resolution and accuracy of measurements and increase the length of the test section.

The implementation of the first and second claimed variants of the method of distributed measurement of birefringence in optical fibers while maintaining polarization can be carried out using the devices shown respectively in the upper and lower figures in figure 2. The fundamental difference between the claimed devices and existing analogues is the presence of an optical radiation generator (1) that generates narrow-band optical radiation simultaneously at four different frequencies ω, <У, ω, ω ₅ ^γ in the vicinity of the resonance frequencies of the four-wave SBS interaction - respectively, ω ₀ , a _L0 , ω ₀ , co _S ^Y _Q , related by relations (1), and ω _ϋ - co _S ^Y _Q «ω [ ₀ -co _S ^Y ₀ « Ω ₀ , where Ω ₀ is the SBS value of the shift, as well as the presence of additional means providing stabilization and independent tuning within the necessary limits of the frequency difference H = ω -

, Ω ^χ = ω * - ω and Q ^y = c _L -

by known methods, and control by known methods of the pulse shape, intensity and (or) phase of radiation at these frequencies.

Two fiber-optic outputs of the generator at frequencies ω and

optically coupled to a fiber polarizing combiner (2). A polarizing combiner (2) is a well-known fiber optic element that provides the combination of linear polarizations from two input fibers while maintaining polarization in two orthogonal polarizations of one output fiber while maintaining polarization. In this case, radiation at a frequency ω is introduced into the X-polarization of the output fiber of a polarizing combiner (2), and radiation at a frequency a> into the Υ-polarization of the output fiber of a polarizing combiner (2).

A fiber optic polarizing combiner (2) is optically coupled to a fiber optic PM circulator (3). The PM circulator (3) is a well-known fiber-optic element, which ensures the conservation of polarization and decoupling of radiation coming from opposite inputs. Radiation at frequencies a> and

through RM, the circulator (3) and the input device (4) into the fiber in two independent linear polarizations, respectively, X and Υ enters the optical fiber (5), which acts as a sensitive element.

To implement the first variant of the proposed method, two fiber-optic outputs of the generator (1) at frequencies <i / and co _s ^{Y are} optically coupled to a fiber polarizing combiner (6) similar to a polarizing combiner (2), which transmits radiation through an input device (7) into the optical fiber (5) with the preservation of polarization, acting as a sensitive element (upper figure in figure 2).

In the second variant of the method of distributed measurement of birefringence in optical fibers with preservation of polarization, in which the fourth signal at a frequency of _s is mixed with the scattered signal when it is detected, there is no need to use a polarizing combiner (6) (bottom figure in figure 2). In this case, the device is additionally equipped with an optical fiber PM coupler (8), which removes the radiation from the generator (1) optical radiation at a frequency of ω ₃ ^γ and directs it directly to a fast photodetector (9) for registration together with the signal coming from the polarization divider (10) in Υ-polarization.

The optical fiber used (5) can be any polarization-preserving fiber (for example, of the PANDA type), which is sensitive to changes in physical parameters (temperature, longitudinal tension, surface pressure) determined by recording the value of the resonance shift Н ₀ .

Radiation at frequencies ω and ω ₃ ^γ transmitted through an optical fiber (5) and containing information on the distribution of the tested parameter H ₀ along the fiber (obtained through its nonlinear interaction in the fiber with radiation at frequencies ω and ω [), through an input device (4) and the circulator (3) is supplied to the polarization divider (10). The polarization divider (10) transmits the radiation in orthogonal polarizations to two optical channels. Optical signals from these two channels are converted by fast selective photodetectors (9) into electrical signals that enter the processor (11) to accumulate, mathematically process, and extract information about the spatial distribution of these parameters.

The processor (1 1) provides synchronous operation of all elements of the system: sets the moment of emission, the shape and intensity of the pulses of the optical signals in the optical radiation generator (1), the collection and processing of signals from photo detectors

(9).

Fig. 3 shows a diagram of an optical radiation generator (1) made on the basis of tunable distributed feedback semiconductor lasers. Here, lasers (12–15) are tunable semiconductor lasers of the RIO type [9], in which the electronic tuning and stabilization of laser frequencies (13–15) relative to the master laser (12) are provided. All lasers (12–15) are optically coupled to fiber amplifiers (16–19) and electro-optical modulators (20–23), which provide, by means of a synchronizer (24), time-synchronized generation of pulses of a given shape and intensity, arriving at four fiber-optic output of the generator (1), made on fibers with conservation of polarization.

The claimed device variants are universal, because in addition to implementing the claimed variants of a method based on four-frequency interaction in a fiber, it is also possible to implement the above-mentioned methods of distributed birefringence measurement in optical fibers with preservation of polarization based on two- and three-frequency interaction.

As an example of a specific implementation of the method, in which radiation at the pump frequencies ω *, co _L ^Y and Stokes signals ω, co _s ^Y is introduced into the fiber from opposite ends towards each other, the results of modeling the operation of the inventive device for implementing this variant of the method are given below. The calculations were performed in an approximation sufficient for the analysis of sensors less than 10 km long. The equations are obtained from a system of equations describing the dynamics of Brillouin scattering [10, 1 1]:

3, - ^ ^d > ^E s = ^~ * (Ε £ + Ε „'φ (ί, ^χ ))' + aE, (d, + _Y W _X ^X ) E * = rWfE * E? + p ^x (t, x)

(four)

d _x --b _t ] El = - E [(E _H ^y + E _H ^x <p '(t, x))' + aE _: (d, + _Y W ^Y ) E _H ^Y = γΨΪΕ [Ε; ^γ + p ^Y (t, x)

Where

f and are the current time and coordinate along the fiber, E _n are the complex amplitudes of the corresponding optical and acoustic fields (H), g is the Brillouin amplification factor, p ^{x, r} (g, x) are the Langevin sources of acoustic noise, is the linear attenuation coefficient, Ω ₀₀ is the frequency of the Brillouin shift in the unperturbed fiber, I ₀₀ is the shear frequency of H ₀ for the unperturbed fiber, LS ~ ^ω ϊΊ ~ ^ω 1 _ϋ so ~~ the detuning of the wave frequencies from the resonances ^Y _so corresponding to the values of Ω ₀₀ and Н ₀₀ , δΩζ ' ^γ = Ω. ' ^γ - Ω ₀ is the deviation of the frequency of the Brillouin shift from Ω ₀₀ , δΗ ₀ = Η ₀ -Η ₀₀ is the deviation of the value of H ₀ from H ₀₀ , δΩ ^l = -δ £,

To demonstrate the applicability of the first variant of the method, this system of equations was solved numerically with parameters corresponding to the length of the tested fiber 100 m when pumped at a wavelength of -1550 nm. It was assumed that at 10 different fiber points on segments of 20 cm (at the second point on a segment of 1, 2 m), the birefringence deviation from the nominal value is δΑη "5 · 10 ^{" 7} .

Example 1. The scheme of the numerical experiment was as follows. The pump at the resonant frequency ω [ ₀ and the Stokes signals at the resonant frequencies ω ₀ and _{so so} were continuous radiation with a power of 50 mW, 10 mW, and 50 mW, respectively (Fig. 4). Pumping at a frequency of ω ₀ is a pulse with a duration of 1 ns and an amplitude of 50 mW. The phase of the pump pulse varied from pulse to pulse equally likely from 0 to 2π.

The response signal was observed in the time scan of the Stokes radiation at a frequency of ω ₀ (Fig. 5). This signal was differentiated (Fig. 6), normalized to the calculated amplitude of the hypersonic wave, and then standard deviation. The result of this operation for 10 implementations is shown in Fig. 7 and on an enlarged scale - in Fig. 8. It is clearly seen that even such a small averaging makes it possible to clearly observe the deviation signal An with a resolution of at least 20 cm. Inhomogeneity of 1.2 m (at the second point) is also well resolved.

Example 2. The scheme of the numerical experiment was as follows. The Stokes signals at the resonant frequencies ω ₀ and co _S ^Y ₀ were continuous radiation with a power of 10 mW each (Fig. 9). Pumps at frequencies ω £ ₀ and ω are pulses of 1 ns duration and an amplitude of 250 mW. The phase of the pump pulse varied from pulse to pulse equally likely from 0 to 2π.

The response signal was observed in time scans of Stokes radiation at frequencies ω * ₀ and co _S ^Y _Q. Similarly to Example 1, these signals were differentiated, normalized to the calculated amplitude of the hypersonic wave, and then the standard deviation was taken as the realizations. The result of this operation for 10 implementations is shown in Fig. 10 (above) and on an enlarged scale - in Fig. 11. It is clearly seen that even such a small averaging makes it possible to clearly observe the deviation signal An with a resolution of at least 20 cm. Inhomogeneity of 1.2 m (at the second point) is also well resolved. Provided that a continuous pump signal at a frequency of ω _{0 was} not supplied, i.e. there was simply a scattering of the pulse on the dynamic lattice, it was not possible to detect the signal against the background of noise (Fig. 10, bottom).

To demonstrate the method according to the second embodiment, the numerical experiment scheme was as follows.

Example 3. The parameters of the signals at four frequencies are chosen to be the same as in Example 1. The difference is that radiation at a frequency ω ^ ₀ is not introduced into the fiber, but is added to the scattered radiation when it is detected. As before, the response signal was differentiated, normalized to the calculated amplitude of the hypersonic wave, and then the standard deviation was taken from the realizations. The result of this operation for 25 implementations is presented in Fig. 12 and on an enlarged scale - in Fig. 13. It is clearly seen that even such a small averaging makes it possible to clearly observe the deviation signal An with a resolution of at least 20 cm. Non-uniformity of 1.2 m (at the second point) is also well resolved.

Thus, the proposed method can significantly increase the signal-to-noise ratio compared with the known method of scattering on a dynamic lattice. In turn, this allows one to increase the spatial resolution, i.e. accuracy of measurements, and increase the length of the test section.

Literature

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2. X. Liu and X. Bao, "Brillouin Spectrum in LEAF and Simultaneous Temperature and Strain Measurement," IEEE J.of Lightwave Techn. 30, 1053-1059.

3. L. Zou, X. Bao, S. A. V., and L. Chen, "Dependence of the Brillouin frequency shift on strain and temperature in a photonic crystal fiber," Optics Lett. 29, 1485-1487.

4. RF patent Jfo 2346235 (published February 10, 2009; IPC G01B11 / 16).

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6. K. Y. Song, W. Zou, Z. He, and K. Hotate, "Optical time-domain measurement of

Brillouin dynamic grating spectrum in a polarization-maintaining fiber, "Opt. Lett. 34, 1381-1383.

7. W. Zou, Z. He, and K. Hotate, "Complete discrimination of strain and temperature using Brillouin frequency shift and birefringence in a polarization-maintaining fiber," Optics Express 17, 1248-1255.

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9. http://www.rio-inc.com/

10. B.C. Starunov, I.L. Fabelinsky "Stimulated scattering

Mandelstam-Brillouin and stimulated entropy (temperature) scattering of light "Phys. Usp. Fiz. Nauk 98 441 ^ * 91 (1969). 11. AA Fotiadi et al., "Statistical properties of stimulated Brillouin scattering in singlemode optical fibers above threshold", Opt. Lett., Vol. 27, pp. 83-85 (2002).

Claims

Claim

1. The method of distributed measurement of birefringence in optical fibers while maintaining polarization, in which

injected into the optical fiber segment from opposite ends in the polarization of one of the principal axes of the fiber with an effective refractive index n _x optical signals at frequencies ω * and ω * "ω * - Ω ₀ , where Ω ₀ is the SBS frequency of the shift, they are introduced into polarizations by another main axis of the fiber with an effective refractive index η _{γ the} probing signal at a frequency ω co-directionally with a signal at a frequency co *, causing modulation of the oncoming radiation,

detecting radiation counter to the probing signal, determining the spatial distribution of the frequency shift H ₀ corresponding to the maximum signal response at a given point of the fiber, and determining the birefringence by the formula η _γ -η _χ yes η _γ Η ₀ ,

characterized in that

in the optical fiber towards the probing signal and in the polarization of the probing signal, an additional fourth optical signal is introduced at a frequency a _s * ω [-Ω ₀ , provide consistent scanning of the frequency difference with * -

= H _L and co * - co _s ^Y = H _s relative to the value of H ₀ , and the frequency difference co - co = Ω relative to Ω ₀ ,

they process the recorded signals by the method of processing and accumulating signals with a zero mean.

2. The method of distributed measurement of birefringence in optical fibers with the preservation of polarization, in which

injected into the segment of the optical fiber from opposite ends in the polarization of one of the main axes of the fiber with an effective refractive index n _x optical signals at frequencies ω * and ω * and so * - Ω ₀ , where Ω ₀ is the SBS frequency of the shift, introducing into the polarizations of the other main axis of the fiber with an effective refractive index η _{γ a} probing signal at a frequency ωζ co-directionally with a signal at a frequency Y, causing modulation of the oncoming radiation,

detecting radiation counter to the probing signal, determining the spatial distribution of the frequency shift H ₀ corresponding to the maximum signal response at a given point of the fiber, and determining the birefringence value according to the formula n _Y -n _x "n _Y H _Q ,

characterized in that

in the optical fiber towards the probing signal and in the polarization of the probing signal, an additional fourth optical signal is introduced at a frequency ω {ααζ + Ω ₀ ,

provide a consistent scan of the frequency difference ω - ω = H _L and ^ω Ξ ~ ^ω 1 ^Ξ Hs with respect to H ₀ , and the frequency difference ω - &> / = Ω with respect to Ω ₀ ,

3. The method of distributed measurement of birefringence in optical fibers with the preservation of polarization, in which

is introduced into a segment of the optical fiber at the opposite ends in the polarization of one of the principal axes of the fiber with an effective refractive index n _x optical signals at frequencies u and ω »ω - Ω _0, where Ω ₀ - SMBS offset frequency is input to the polarization of the other main fiber axis an effective refractive index η _{γ the} probe signal at a frequency ω co-directionally with a signal at a frequency ω, causing the generation of counterpropagating radiation,

detect radiation counter to the probing signal,

determine the spatial distribution of the frequency shift H _0> corresponding to the maximum signal response at a given point of the fiber, and determine by the formula η _γ -η _χ "n _at H _{0 the} value of birefringence,

characterized in that when detecting counterpropagating radiation in the polarization Y, the fourth optical signal at the frequency ω ₈ ^γ »ω - Ω _{0 is} additionally mixed with it, and a coordinated scanning of the frequency difference co - co _L ^Y = H _L and ω * - co _s ^Y = H _s with respect to H ₀ , and the frequency differences ω * - ω * = Ω with respect to Ω ₀ , and

4. The method of distributed measurement of birefringence in optical fibers with the preservation of polarization, in which

optical signals are introduced into the optical fiber segment from opposite ends from opposite ends in the polarization of one of the principal axes of the fiber with the refractive index n _{× x} at the frequencies taf and ω ω ^ - Ω ₀ , where Ω ₀ is the SBS frequency of the shift, they are introduced into the polarizations of the other main axis of the fiber with effective refractive index η _{γ the} probe signal at a frequency ω ₅ ^γ co-directionally with a signal at a frequency co *, causing the generation of counterpropagating radiation,

detect radiation counter to the probing signal,

determine the spatial distribution of the frequency shift H ₀ corresponding to the maximum signal response at a given point of the fiber, and determine by the formula η _γ -η _χ κ · η _Υ Η _{0 the} birefringence,

characterized in that

when detecting counterpropagating radiation in the polarization дополнительно, a fourth optical signal at a frequency

+ Ω,, provide a consistent scan of the frequency difference ω * - а> = H _L and ^ω ~ ^ω 1 ~ Hs relative to the value Н ₀ , and the frequency difference ω ^χ - ω * = Ω relative to Ω ₀ , and

5. A device for distributed measurement of birefringence in optical fibers with the preservation of polarization, including an optical radiation generator for generating optical radiation with predetermined characteristics,

two polarizing combiners for combining linear polarizations from two input fibers while maintaining polarization in two orthogonal polarizations of one output fiber while maintaining polarization,

a polarization divider to separate the input radiation into two orthogonal polarizations into two optical channels,

RM circulator for decoupling radiation coming from opposite fiber outputs while maintaining polarization,

an optical fiber that is sensitive to changes in physical parameters determined by recording the magnitude of the resonant shift, at least one photodetector that converts optical radiation into electrical signals, and

a processor for ensuring the synchronous operation of all elements of the system, setting the moment of emission, the shape and intensity of the pulses of the optical signals in the optical radiation generator, and collecting, accumulating and mathematically processing the signals from photodetectors

moreover, the optical radiation generator is configured to generate narrow-band optical radiation simultaneously at four different frequencies in the vicinity of frequencies <y,

,, α,

the optical radiation generator is equipped with stabilization and independent tuning of the frequency difference 6> - ω [= H _L , ω - ω Ξ = H _S , ω £ - ω = Ω, and

the processor provides statistical processing of the recorded signals by the method of processing and accumulating signals with a zero mean.

6. The distributed birefringence measurement device according to claim 5, characterized in that the physical parameters determined by recording the magnitude of the resonance shift are temperature, or longitudinal tension, or surface pressure.

7. A device for distributed measurement of birefringence in optical fibers with the preservation of polarization, including

an optical radiation generator for generating optical radiation with predetermined characteristics, a polarization combiner for combining linear polarizations from two input fibers while maintaining polarization in two orthogonal polarizations of one output fiber while maintaining polarization,

fiber RM coupler, to which the signal from the optical radiation generator at a frequency co _s ^Y is directed, and by means of which the specified signal is combined with the signal from the polarization divider in Y-polarization for their joint registration,

moreover, the optical radiation generator is filled with the possibility of generating narrow-band optical radiation simultaneously at four different frequencies in the vicinity of the frequencies ω *, ω [, ω *, α,

the optical radiation generator is equipped with stabilization and independent tuning of the frequency difference ω * -co _L ^Y = H _L , co * -ωζ = H _s , co * - co * = Ω, and

8. The distributed birefringence measurement device according to claim 7, characterized in that the physical parameters determined by recording the magnitude of the resonance shift are temperature, or longitudinal tension, or surface pressure.