RU2539849C2 - Method and apparatus for distributed measurement of birefringence in fibres with polarisation preservation (versions) - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to fibre optics, particularly to distributed fibre-optic sensors which measure parameters of an optical fibre under the effect of external physical fields. The disclosed methods for distributed measurement of birefringence in optical fibres with polarisation preservation, in addition to known methods which employ three signals, employ a fourth signal. Detection and statistical processing of recorded signals is performed using means of processing and storing signals with a zero mean. The fourth signal in the form of a linearly polarised signal can be fed into the optical fibre towards a probing signal and in the polarisation of the probing signal. In another version of the method, the fourth signal is mixed with a scattered signal during detection thereof.

EFFECT: improved spatial resolution, measurement accuracy and/or range (length) of the investigated section.

8 cl, 13 dwg

Description

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.

и частоте Стокса

, сдвинутых относительно друг друга примерно на величину ВРМБ-сдвига Ω₀. Один из сигналов непрерывный, другой - импульсный. По меньшей мере один из сигналов сканируется по частоте, обеспечивая тем самым сканирование разности частот

в окрестности средней частоты ВРМБ-сдвига

. Проходящий через волокно импульсный сигнал взаимодействует в процессе ВРМБ с непрерывным сигналом, вызывая локальные изменения его интенсивности. Далее, датчиком регистрируют изменение интенсивности непрерывного сигнала, прошедшего через волокно, как функцию времени и разности частот

. В результате статистической и математической обработки полученных данных определяют пространственно-частотные характеристики ВРМБ резонанса, а именно пространственное распределение коэффициента ВРМБ усиления (или ширину линии ВРМБ усиления), величину Бриллюэновского сдвига Ω₀ [1, 2], положение пиков ВРМБ резонансов в волокнах с несколькими резонансами [3].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 is that two optical signals at the pump frequency are applied to the optical fiber used as a sensitive element from opposite ends

and Stokes frequency

shifted relative to each other by approximately the size of the SBS shift Ω ₀ . One of the signals is continuous, the other is pulsed. At least one of the signals is scanned in frequency, thereby providing a frequency difference scan

in the vicinity of the average frequency of the SBS shift

. The pulse signal passing through the fiber interacts during the SBS with a continuous signal, causing local changes in its intensity. Next, the sensor records the change in the intensity of the continuous signal transmitted through the fiber, as a function of time and frequency difference

. As a result of statistical and mathematical processing of the obtained data, the spatial-frequency characteristics of the SBS resonance are determined, namely, the 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 SBS resonance peaks in fibers with several resonances [3].

From the patent of the Russian Federation No. 2179374 (published on 02/10/2002; IPC H04B 10/08, G01M 11/02, G01R 31/11), device variants are known for measuring fiber characteristics of an optical cable. In the first embodiment, the device comprises a low-coherent laser emitting continuous wave light with a pump frequency ω _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 to the opposite end with an optical reflector, and 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 continuous wave light with a pump frequency ω _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 by the output to the input of the processor that processes the measurement results, differing in those m, 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 power divider connected to the second output to the output of the passage of the first directional coupler, the output of the main the channel of which is connected to the opposite end of the first additional fiber of the optical cable connected by the proximal end to the output of the main the channel of the second directional power coupler, the lead-through arm of which is connected to the second output of the power divider, and the branch arm is connected to the proximal end of the second additional fiber of the optical cable, the opposite end of which is connected through the optical amplifier to the input of the second power divider, and the branch of the first directional arm the coupler is connected to the 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.

и частоте Стокса

, сдвинутых относительно друг друга на величину ВРМБ сдвига Ω₀, вводятся в волокно с противоположных концов в поляризации одной из главных осей волокна. Их взаимодействие в волокне приводит к образованию динамической решетки звуковой волны, распространяющейся вдоль всей области взаимодействия сигналов. В поляризации другой главной оси со стороны накачки в волокно подается короткий (наносекундный) зондирующий, или пробный, импульс (или сигнал) на частоте

, сдвинутой относительно основной частоты накачки. Отражение этого импульса от звуковой решетки в процессе его распространения по волокну приводит к образованию встречного излучения в виде длинного импульса на частоте

, во временном распределении мощности которого заключена информация об эффективности отражения зондирующего сигнала от динамической решетки на различных участках вдоль волокна.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 pump frequency signals

and Stokes frequency

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 of the 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, a short (nanosecond) probing, or probe, pulse (or signal) at a frequency

shifted relative to the fundamental 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

, in the temporal distribution of power which contains information on the reflection efficiency of the probe signal from the dynamic array in various sections along the fiber.

относительно его резонансной частоты

при четырехволновом ВРМБ взаимодействии, определяемой соотношением:The reflection efficiency at each point of the fiber depends on the position of the optical frequency of the probe signal

relative to its resonant frequency

with four-wave SBS interaction, defined by the ratio:

$${\omega }_{L0}^Y=\frac{n_X}{n_Y}\frac{\left(\mathrm{one}+\frac{v{n}_Y}{c}\right)}{\left(\mathrm{one}+\frac{v{n}_X}{c}\right)}{\omega }_{L0}^X\approx \frac{n_X}{n_Y}{\omega }_{L0}^X,\left(\mathrm{one}\right)$$

и

- оптическая частота основной накачки и резонансная оптическая частота зондирующего сигнала, n_X и n_Y - показатели преломления принципиальных поляризационных мод волокна, с и v - скорости света в вакууме и звука в волокне.Where

and

are the optical frequency of the main pump and the resonant optical frequency of the probe signal, n _X and n _Y are the refractive indices of the fundamental polarization modes of the fiber, and c and v are the speeds of light in vacuum and sound in the fiber.

зондирующего сигнала в данной точке волокна относительно

, по разнице частот

определяется пространственное распределение двулучепреломления в волокне:Thus, registering the position of the resonant frequency

probe signal at a given point of the fiber relative to

, by frequency difference

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].

, сдвинутой относительно стоксовой частоты

, а отражение этого импульса от звуковой решетки в процессе его распространения по волокну приводит к образованию встречного излучения в виде длинного импульса на частоте

, во временном распределении мощности которого заключена информация об эффективности отражения зондирующего сигнала от динамической решетки на различных участках вдоль волокна.Note that achieving good spatial resolution is also possible in the case when a short probe pulse in polarization Y is introduced into the fiber from the side of the Stokes wave at a frequency

shifted relative to the Stokes frequency

, and 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

, in the temporal distribution of power which contains information on the reflection efficiency of the probe signal from the dynamic array 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 No. 102589857 (published July 18, 2012; IPC G01M 11/02), 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.

и обеспечивается согласованное сканирование разности частот

,

и

в окрестности значений соответственно Ω₀ и H₀, где Ω₀ - величина бриллюэновского сдвига,

- величина сдвига частоты накачки при четырехволновом ВРМБ резонансе, а детектирование и статистическая обработка регистрируемых сигналов реализуются средствами обработки и накопления сигналов с нулевым средним.The specified technical result is achieved by the fact that in the proposed method for the distributed measurement of birefringence in optical fibers while maintaining polarization, in addition to the known one, a fourth signal at a frequency

and consistent frequency difference scanning is provided

,

and

in the vicinity of the values of Ω ₀ and H ₀ , respectively, where Ω ₀ is the Brillouin shift value,

- the magnitude of the shift in the pump frequency at 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.

в виде линейно-поляризованного сигнала может вводиться в оптическое волокно навстречу зондирующему импульсу и в поляризации зондирующего импульса, что приводит к увеличению соотношения сигнал/шум.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.

в волокно не вводится, а смешивается с рассеянным сигналом при его детектировании, что является еще одним вариантом реализации способа распределенного измерения двулучепреломления в оптических волокнах с сохранением поляризации.The indicated result can also be achieved when the fourth signal at a frequency

it is not introduced into the fiber, but 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 conservation 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.

,

,

,

в окрестности частот соответственно

,

,

,

и оснащен средствами, обеспечивающими стабилизацию и независимую перестройку в необходимых пределах разности частот

,

и

, контроль формы импульса, интенсивности и/или фазы излучения на этих частотах, а упомянутый процессор обеспечивает статистическую обработку регистрируемых сигналов известными методами обработки и накопления сигналов с нулевым средним.The mentioned optical generator generates narrow-band optical radiation simultaneously at four different frequencies

,

,

,

in the vicinity of frequencies, respectively

,

,

,

and equipped with means providing stabilization and independent tuning within the necessary limits of the frequency difference

,

and

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.

Figure 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

.

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.

(трехчастотный метод).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

(three-frequency method).

Figure 11 shows the zoom of the signals presented in figure 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.

,

и стоксовых сигналов

,

вводится в волокно с противоположных концов навстречу друг другу в поляризациях, соответствующих главным поляризационным осям волокна (X и Y). Вблизи условий резонанса, описываемых уравнением (1) (в присутствии всех сигналов на всех четырех частотах), пары волн накачки и Стоксовых волн, относящиеся к разным поляризациям, взаимодействуют друг с другом через единую гиперзвуковую волну, образованную в среде этими двумя парами. Необходимым условием такого резонанса является близость резонансного сдвига частоты

зондирующего импульса относительно первой частоты накачки

, описываемого уравнением (1), и аналогичный сдвиг частоты

второго стоксового сигнала относительно частоты

первого стоксового сигнала (см. фиг.1). Производя детектирование и обработку сигналов, полученных с выхода волокна при согласованной амплитудной модуляции и частотном сканировании задающих оптических полей, измеряются пространственно-частотное распределение резонанса, в частности, распределение по волокну положения резонансного сдвига H₀, ширины резонанса, его профиль. По положению резонанса репродуцируется распределение Δn. Другие характеристики резонанса также могут быть использованы как независимые параметры для измерения внешних воздействий.In one embodiment of the method of the invention, radiation at pump frequencies

,

and Stokes signals

,

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

probe pulse relative to the first pump frequency

described by equation (1), and a similar frequency shift

second Stokes signal relative to frequency

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 resonance shift position H ₀ , the resonance width, and its profile over the fiber. According to the position of the resonance, the distribution Δn is reproduced. 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 interaction point:

- фаза волны на частоте

в месте взаимодействия; i=X, Y; j=L, S.Where

- phase of the wave at the frequency

in the place of interaction; i = X, Y; j = L, S.

For -π / 2 <Ф <π / 2, the interaction of the pairs leads to an increase in the total increment of amplification in each of them; for π / 2 <Ф <3π / 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.

,

и стоксового сигнала

вводится в волокно с противоположных концов навстречу друг другу в поляризациях, соответствующих главным поляризационным осям волокна (X и Y). Четвертый сигнал на частоте

смешивается с рассеянным сигналом при его детектировании, что приводит к увеличению соотношения сигнал/шум.In another embodiment of the method according to the invention, the radiation at pump frequencies

,

and Stokes signal

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). Fourth frequency signal

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 with 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 resonance shift position H ₀ , the resonance width, and its profile over the fiber. According to the position of the resonance, the distribution Δn is reproduced. 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.

с последующей обработкой сигналов на выходе из волокна. Это позволяет повысить соотношение сигнал/шум, а следовательно, повысить пространственное разрешение и точность измерений и увеличить длину тестируемого участка.A significant difference between the two proposed variants of the method of distributed measurement of birefringence in optical fibers while maintaining polarization from those known from the prior art is the use of an additional fourth signal at the Stokes frequency

followed by signal processing at the output of the fiber. 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.

,

,

,

в окрестности резонансных частот четырехволнового ВРМБ взаимодействия - соответственно

,

,

,

, связанных соотношениями (1), и

, где Ω₀ - величина ВРМБ сдвига, а также наличие дополнительных средств, обеспечивающих стабилизацию и независимую перестройку в необходимых пределах разности частот

, $${\Omega }^X={\omega }_L^X-{\omega }_S^X$$

и

известными методами, и контроль известными методами формы импульса, интенсивности и (или) фазы излучения на этих частотах.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 from existing analogues is the presence of a generator (1) of optical radiation that generates narrow-band optical radiation simultaneously at four different frequencies

,

,

,

in the vicinity of the resonant frequencies of the four-wave SBS interaction, respectively

,

,

,

related by (1), and

, 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

, $${\Omega }^X={\omega }_L^X-{\omega }_S^X$$

and

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

и

оптически связаны с волоконным поляризационным комбайнером (2). Поляризационный комбайнер (2) - это известный волоконно-оптический элемент, который обеспечивает объединение линейных поляризаций от двух входных волокон с сохранением поляризации в двух ортогональных поляризациях одного выходного волокна с сохранением поляризации. При этом излучение на частоте

вводится в X-поляризацию выходного волокна поляризационного комбайнера (2), а излучение на частоте

- в Y-поляризацию выходного волокна поляризационного комбайнера (2). Волоконно-оптический поляризационный комбайнер (2) оптически связан с волоконно-оптическим РМ циркулятором (3). РМ циркулятор (3) - это известный волоконно-оптический элемент, который обеспечивает сохранение поляризации и развязку излучения, поступающего с противоположных входов. Излучение на частотах

и

через РМ циркулятор (3) и устройство ввода (4) в волокно в двух независимых линейных поляризациях соответственно X и Y поступает в оптическое волокно (5), выступающее в качестве чувствительного элемента.Two fiber optic generator outputs at frequencies

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, the radiation at the frequency

is introduced into the X-polarization of the output fiber of the polarizing combiner (2), and the radiation at the frequency

- in the Y-polarization of the output fiber of the 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

and

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

и

оптически связаны с волоконным поляризационным комбайнером (6), аналогичным поляризационному комбайнеру (2), который через устройство ввода (7) передает излучение в оптическое волокно (5) с сохранением поляризации, выступающее в качестве чувствительного элемента (верхний рисунок на фиг.2).To implement the first variant of the proposed method, two fiber-optic outputs of the generator (1) at frequencies

and

optically coupled to a fiber polarizing combiner (6), similar to a polarizing combiner (2), which, through an input device (7), transmits radiation to an optical fiber (5) while maintaining polarization, acting as a sensitive element (upper figure in figure 2).

смешивается с рассеянным сигналом при его детектировании, нет необходимости в использовании поляризационного комбайнера (6) (нижний рисунок на фиг.2). В этом случае устройство дополнительно оснащено оптическим волоконным РМ ответвителем (8), который отводит излучение генератора (1) оптического излучения на частоте

и направляет его непосредственно на быстрый фотодетектор (9) для регистрации совместно с сигналом, поступающим с поляризационного делителя (10) в Y-поляризации.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

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 of the optical radiation generator (1) at a frequency

and directs it directly to a fast photodetector (9) for registration together with the signal coming from the polarization divider (10) in Y-polarization.

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

и

, прошедшее оптическое волокно (5) и содержащее информацию о распределении тестируемого параметра H₀ вдоль волокна (полученное через его нелинейное взаимодействие в волокне с излучением на частотах

и

), через устройство ввода (4) и циркулятор (3) подается на поляризационный делитель (10). Поляризационный делитель (10) передает излучение в ортогональных поляризациях в два оптических канала. Оптические сигналы из этих двух каналов преобразуются быстрыми селективными фотодетекторами (9) в электрические сигналы, которые поступают в процессор (11) для накопления, математической обработки и извлечения информации о пространственном распределении упомянутых параметров.Radiation at frequencies

and

passing 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

and

), through the input device (4) and the circulator (3) is fed 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 (11) ensures the 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 photodetectors (9).

Figure 3 presents the diagram of the generator (1) of optical radiation, made on the basis of tunable semiconductor lasers with distributed feedback. 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 outputs 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.

,

и стоксовых сигналов

,

вводится в волокно с противоположных концов навстречу друг другу, ниже приведены результаты моделирования работы заявляемого устройства для реализации данного варианта способа. Расчеты выполнены в приближении, достаточном для анализа датчиков длиной менее 10 км. Уравнения получены из системы уравнений, описывающих динамику бриллюэновского рассеяния [10, 11]:As an example of a specific implementation of the method, in which radiation at pump frequencies

,

and Stokes signals

,

is introduced into the fiber from opposite ends towards each other, the following are the results of modeling the operation of the inventive device for implementing this variant of the method. 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, 11]:

Where

,

,

, $$W_2^{X,Y}=\frac{{\left(\mathrm{one}+\frac{\delta {\Omega }^{X,\mathrm{<figure-callout\; id="2"\; label="Y"\; filenames="00000117.png"\; state="\{\{state\}\}">Y</figure-callout>}2}}{{\Omega }_{00}}\right)}^2}{{\left(\mathrm{one}+\frac{\delta {\Omega }_0^{X,Y}}{{\Omega }_{00}}\right)}^2\left(\left(\mathrm{one}+\frac{\delta {\Omega }^{X,Y}}{{\Omega }_{00}}\right)-i\frac{\gamma }{{\Omega }_{00}}\right)}$$

,

,

,

- комплексные амплитуды соответствующих оптических и акустических полей (H), g - фактор бриллюэновского усиления, ρ^X,Y(t,x) - Ланжевеновские источники акустического шума, α - коэффициент линейного затухания, Ω₀₀ - частота бриллюэновского сдвига в невозмущенном волокне, H₀₀ - частота сдвига H₀ для невозмущенного волокна,

- отстройка частот волн от резонансов

, соответствующих значениям Ω₀₀ и H₀₀, $$\delta {\Omega }_0^{X,Y}={\Omega }_0^{X,Y}-{\Omega }_0$$

- отклонение частоты бриллюэновского сдвига от Ω₀₀, δH₀=H₀-H₀₀ - отклонение величины H₀ от H₀₀,

,

.t and x are the current time and coordinate along the fiber,

are the complex amplitudes of the corresponding optical and acoustic fields (H), g is the Brillouin gain factor, ρ ^{X, Y} (t, x) are Langevin sources of acoustic noise, α is the linear attenuation coefficient, Ω ₀₀ is the Brillouin shift frequency in the undisturbed fiber, H ₀₀ - shear frequency H ₀ for unperturbed fiber,

- detuning wave frequencies from resonances

corresponding to the values of Ω ₀₀ and H ₀₀ , $$\delta {\Omega }_0^{X,Y}={\Omega }_0^{X,Y}-{\mathrm{<figure-callout\; id="0"\; label="\Omega "\; filenames="00000122.png,00000125.png"\; state="\{\{state\}\}">\Omega </figure-callout>}}_0$$

is the deviation of the frequency of the Brillouin shift from Ω ₀₀ , δH ₀ = H ₀ -H ₀₀ is the deviation of the value of H ₀ from H ₀₀ ,

,

.

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 δΔn≈5 · 10 ^-7 .

и стоксовы сигналы на резонансных частотах $${\omega }_{S0}^X$$

и $${\omega }_{S0}^Y$$

представляли собой непрерывное излучение мощностью соответственно 50 мВт, 10 мВт и 50 мВт (фиг.4). Накачка на частоте $${\omega }_{L0}^X$$

- импульс длительностью 1 нс и амплитудой 50 мВт. Фаза импульса накачки менялась от импульса к импульсу равновероятно от 0 до 2π.Example 1. The scheme of the numerical experiment was as follows. Resonant frequency pump $${\omega }_{L0}^Y$$

and Stokes signals at resonant frequencies $${\omega }_{S0}^X$$

and $${\omega }_{S0}^Y$$

consisted of continuous radiation with a capacity of respectively 50 mW, 10 mW and 50 mW (figure 4). Frequency Pumping $${\omega }_{L0}^X$$

- a pulse of 1 ns duration and an amplitude of 50 mW. The phase of the pump pulse varied from pulse to pulse equally likely from 0 to 2π.

(фиг.5). Этот сигнал дифференцировался (фиг.6), нормировался на рассчитанную амплитуду гиперзвуковой волны, и затем по реализациям бралось среднеквадратичное отклонение. Результат этой операции для 10 реализации представлен на фиг.7 и в увеличенном масштабе - на фиг.8. Хорошо видно, что даже такое небольшое усреднение позволяет четко наблюдать сигнал отклонения Δn с разрешением не менее чем в 20 см. Неоднородность в 1,2 м (во второй точке) также хорошо разрешается.The response signal was observed in the time scan of the Stokes radiation at a frequency $${\omega }_{S0}^X$$

(figure 5). This signal was differentiated (Fig.6), normalized to the calculated amplitude of the hypersonic wave, and then the standard deviation was taken as the implementation. The result of this operation for implementation 10 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 Δn with a resolution of at least 20 cm. Inhomogeneity of 1.2 m (at the second point) is also well resolved.

и $${\omega }_{S0}^Y$$

представляли собой непрерывное излучение мощностью 10 мВт каждый (фиг.9). Накачки на частотах $${\omega }_{L0}^X$$

и $${\omega }_L^X$$

- импульсы длительностью 1 нс и амплитудой 250 мВт. Фаза импульса накачки менялась от импульса к импульсу равновероятно от 0 до 2π.Example 2. The scheme of the numerical experiment was as follows. Stokes signals at resonant frequencies $${\omega }_{S0}^X$$

and $${\omega }_{S0}^Y$$

constituted a continuous radiation with a power of 10 mW each (Fig.9). Frequency Pumping $${\omega }_{L0}^X$$

and $${\omega }_L^X$$

- 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.

и $${\omega }_{S0}^Y$$

. Аналогично Примеру 1, эти сигналы дифференцировались, нормировались на рассчитанную амплитуду гиперзвуковой волны, и затем по реализациям бралось среднеквадратичное отклонение. Результат этой операции для 10 реализации представлен на фиг.10 (сверху) и в увеличенном масштабе - на фиг.11. Хорошо видно, что даже такое небольшое усреднение позволяет четко наблюдать сигнал отклонения Δn с разрешением не менее чем в 20 см. Неоднородность в 1,2 м (во второй точке) также хорошо разрешается. При условии, когда непрерывный сигнал накачки на частоте $${\omega }_{S0}^X$$

не подавался, т.е. имело место просто рассеяние импульса на динамической решетке, выявить сигнал на фоне шума не удавалось (фиг.10, снизу).The response signal was observed in time scans of Stokes radiation at frequencies $${\omega }_{S0}^X$$

and $${\omega }_{S0}^Y$$

. 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 implementation 10 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 Δn 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 $${\omega }_{S0}^X$$

not served, 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.

в волокно не вводится, а добавляется к рассеянному излучению при его регистрации. Также как и ранее, сигнал отклика дифференцировался, нормировался на рассчитанную амплитуду гиперзвуковой волны, и затем по реализациям бралось среднеквадратичное отклонение. Результат этой операции для 25 реализации представлен на фиг.12 и в увеличенном масштабе - на фиг.13. Хорошо видно, что даже такое небольшое усреднение позволяет четко наблюдать сигнал отклонения Δn с разрешением не менее чем в 20 см. Неоднородность в 1,2 м (во второй точке) также хорошо разрешается.Example 3. The parameters of the signals at four frequencies are chosen to be the same as in Example 1. The difference is that the radiation at the frequency $${\omega }_{S0}^Y$$

it is not introduced into the fiber, but is added to the scattered radiation during its registration. 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 implementation 25 is shown 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 Δn with a resolution of at least 20 cm. Inhomogeneity 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

1. X. Bao, Q. Yu, and L. Chen, "Simultaneous strain and temperature measurements with polarization-maintaining fibers and their error analysis by use of a distributed Brillouin loss system," Opt. Lett. 29, 1342-1344.

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 No. 2346235 (published on 02/10/2009; IPC G01B 11/16).

5. Y. Dong, L. Chen, and X. Bao, "Truly distributed birefringence measurement of polarization-maintaining fibers based on transient Brillouin grating," Optics Lett. 35, 193-195.

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.

8. Y. Dong, L. Chen, and X. Bao, "High-Spatial-Resolution Time-Domain Simultaneous Strain and Temperature Sensor Using Brillouin Scattering and Birefringence in a Polarization-Maintaining Fiber," Phot. Techn. Lett. 22, 1364-1366.

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

10. B.C. Starunov, I.L. Fabelinsky "Stimulated Mandelstam-Brillouin scattering and stimulated entropy (temperature) scattering of light" UFN 98 441-491 (1969).

11. A.A. Fotiadi et al., "Statistical properties of stimulated Brillouin scattering in singlemode optical fibers above threshold". Opt. Lett., Vol. 27, pp. 83-85 (2002).

Claims (8)

1. The method of distributed measurement of birefringence in optical fibers while maintaining polarization, in which
optical signals are introduced into the optical fiber segment from opposite ends in the polarization of one of the main axes of the fiber with an effective refractive index n _X at frequencies $${\omega }_L^X$$

and $${\omega }_S^X\approx {\omega }_L^X-{\Omega }_0$$

where Ω ₀ - frequency SBS shear,
introduce into the polarization of the other main axis of the fiber with an effective refractive index n _{Y a} probing signal at a frequency $${\omega }_L^Y$$

co-directional with a signal at a frequency $${\omega }_L^X$$

causing modulation of the oncoming 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
determined by the formula n _Y -n _X ≈n _Y H _{0 the} value of birefringence,
characterized in that
in the optical fiber towards the probing signal and in the polarization of the probing signal, a fourth optical signal at a frequency is additionally introduced $${\omega }_S^Y\approx {\omega }_L^Y-{\Omega }_0$$

,
provide consistent frequency difference scanning $${\omega }_L^X-{\omega }_L^Y\equiv H_L$$

and $${\omega }_S^X-{\omega }_S^Y\equiv H_S$$

relative to the value of H ₀ and the difference in frequencies $${\omega }_L^X-{\omega }_S^Y\equiv \Omega$$

with respect 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
optical signals are introduced into the optical fiber segment from opposite ends in the polarization of one of the main axes of the fiber with an effective refractive index n _X at frequencies $${\omega }_L^X$$

and $${\omega }_S^X\approx {\omega }_L^X-{\Omega }_0$$

where Ω ₀ - frequency SBS shear,
introduce into the polarization of the other main axis of the fiber with an effective refractive index n _{Y a} probing signal at a frequency $${\omega }_S^Y$$

co-directional with a signal at a frequency $${\omega }_S^X$$

causing modulation of the oncoming 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
determined by the formula n _Y -n _X ≈n _Y H _{0 the} value of birefringence,
characterized in that
in the optical fiber towards the probing signal and in the polarization of the probing signal, a fourth optical signal at a frequency is additionally introduced $${\omega }_L^X\approx {\omega }_S^X+{\Omega }_0$$

,
provide consistent frequency difference scanning $${\omega }_L^X-{\omega }_L^Y\equiv H_L$$

and $${\omega }_S^X-{\omega }_S^Y\equiv H_S$$

relative to the value of H ₀ and the frequency difference $${\omega }_L^X-{\omega }_S^Y\equiv \Omega$$

with respect to Ω ₀ ,
they process the recorded signals by the method of processing and accumulating signals with a zero mean. 3. 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 in the polarization of one of the main axes of the fiber with an effective refractive index n _X at frequencies $${\omega }_L^X$$

and $${\omega }_S^X\approx {\omega }_L^X-{\Omega }_0$$

where Ω ₀ - frequency SBS shear,
introduce into the polarization of the other main axis of the fiber with an effective refractive index n _{Y a} probing signal at a frequency $${\omega }_L^Y$$

co-directional with a signal at a frequency $${\omega }_L^X$$

causing the generation of oncoming 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
determined by the formula n _Y -n _X ≈n _Y H _{0 the} value of birefringence,
characterized in that
when detecting counterpropagating radiation in polarization Y, a fourth optical signal at a frequency $${\omega }_S^Y\approx {\omega }_L^Y-{\Omega }_0$$

,
provide consistent frequency difference scanning $${\omega }_L^X-{\omega }_L^Y\equiv H_L$$

and $${\omega }_S^X-{\omega }_S^Y\equiv H_S$$

relative to the value of H ₀ and the difference in frequencies $${\omega }_L^X-{\omega }_S^Y\equiv \Omega$$

with respect to Ω ₀ , and
they process the recorded signals by the method of processing and accumulating signals with a zero mean. 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 in the polarization of one of the main axes of the fiber with an effective refractive index n _X at frequencies $${\omega }_L^X$$

and $${\omega }_S^X\approx {\omega }_L^X-{\Omega }_0$$

where Ω ₀ - frequency SBS shear,
introduce into the polarization of the other main axis of the fiber with an effective refractive index n _{Y a} probing signal at a frequency $${\omega }_S^Y$$

co-directional with a signal at a frequency $${\omega }_S^X$$

causing the generation of oncoming 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
determined by the formula n _Y -n _X ≈n _Y H _{0 the} value of birefringence,
characterized in that
when detecting counterpropagating radiation in polarization Y, a fourth optical signal at a frequency $${\omega }_L^X\approx {\omega }_S^Y+{\Omega }_0$$

,
provide consistent frequency difference scanning $${\omega }_L^X-{\omega }_L^Y\equiv H_L$$

and $${\omega }_S^X-{\omega }_S^Y\equiv H_S$$

relative to the value of H ₀ and the difference in frequencies $${\omega }_L^X-{\omega }_S^Y\equiv \Omega$$

with respect to Ω ₀ , and
they process the recorded signals by the method of processing and accumulating signals with a zero mean. 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 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 $${\omega }_L^X$$

, $${\omega }_L^Y$$

, $${\omega }_S^X$$

, $${\omega }_S^Y$$

,
the optical radiation generator is equipped with stabilization and independent frequency difference tuning $${\omega }_L^X-{\omega }_L^Y\equiv H_L$$

, $${\omega }_S^X-{\omega }_S^Y\equiv H_S$$

, $${\omega }_L^X-{\omega }_S^Y\equiv \Omega$$

, 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 PM coupler, to which the signal from the optical radiation generator is directed at a frequency $${\omega }_S^Y$$

and through which the specified signal is combined with the signal from the polarization divider in Y-polarization for joint registration,
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 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 $${\omega }_L^X$$

, $${\omega }_L^Y$$

, $${\omega }_S^X$$

, $${\omega }_S^Y$$

,
the optical radiation generator is equipped with stabilization and independent frequency difference tuning $${\omega }_L^X-{\omega }_L^Y\equiv H_L$$

, $${\omega }_S^X-{\omega }_S^Y\equiv H_S$$

, $${\omega }_L^X-{\omega }_S^Y\equiv \Omega$$

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

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