RU2730887C1 - Fiber-optic device for detecting vibration effects with phase recovery with reduced effect of instabilities of the recording interferometer - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to fiber-optic sensor systems based on phase-sensitive reflectometry with phase recovery used in systems of monitoring extended objects. Disclosed fiber-optic device for recording vibration effects with phase recovery with reduced influence of instabilities of the recording interferometer includes: a highly stable narrow-band radiation source, from which through splitter 1×2 greater part of radiation falls into controlled line, and smaller serves for direct registration of parameters of radiation source and receiving part of device. After one of the outputs of splitter 1×2 there arranged in series are an optical signal amplifier (booster), an acoustooptical modulator controlled by a driver, optical circulator, controlled line of optical fiber, weak signal amplifier (preamplifier), filter for required wavelength. At the filter outlet there is a circulator, in the forward direction of which first splitter 3×3 is located, non-equable Mach-Zehnder interferometer with optical path difference, second splitter 3×3, first three radiation receivers. Splitter 1×2 second output is connected to the circulator in the receiving part in front of the receivers so that radiation from the highly stable narrow-band radiation source, passing in the reverse direction, second splitter 3×3, non-equable Mach-Zehnder interferometer with optical path difference, first splitter 3×3, falls on the second three radiation receivers. Outputs of all receivers are connected to analogue-to-digital converters of computing device.

EFFECT: reduced error of recovery of the phase detected from the monitored signal line.

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Description

Technology area

The invention relates to fiber-optic sensor systems based on phase-sensitive reflectometry with phase recovery, used in monitoring systems for extended objects, and can be used to monitor the condition and integrity, as well as determine the presence of objects or actions along the monitored line, including pipelines, perimeters, in oil well logging systems based on the phase-sensitive reflectometry method.

State of the art

The phase-sensitive reflectometry method allows recording a scattering signal (reflectogram) randomly distributed along the length of the sensor, which remains unchanged over time, provided that the frequency of the radiation source is stable and the sensitive optical fiber is not affected, and changes in the presence of thermal or mechanical influences. Electronic processing of the recorded signal today is mainly developed to isolate mechanical influences, the frequency range of which lies in the range from 10 Hz to 10 kHz, which is in good agreement with the range of acoustic signals of various nature that cause vibration of the sensor fiber. This vibration occurs when acoustic waves propagate in the medium from the vibration source (walking person, running engine, vehicle movement) to the laid sensor cable. Acoustic waves vibrate the cable at their own frequency, which stands out against the background of a random signal during subsequent processing.

The basic device and method of phase-sensitive reflectometry have been described in US patent US5194847 (IPC G01H 9/00; G01L 1/24; G01L 11/02; G08B 13/12; G08B 13/186; (IPC 1-7): G08B 13/10 ; G08B 13/18, publ. 1993-03-16). The method of preparation and use of coherent reflectometry includes the following main stages: - placement of a sensitive optical cable-sensor along the controlled object; - supply of pulses of coherent optical radiation to a line of a certain length located along an extended object; - reception of backscattered signals and separation of a signal that reveals the fact of external action by disturbances in the indicated backscatter signals.

The basic method corresponds to the basic schemes of devices for the implementation of the method, as well as many derived schemes of phase-sensitive reflectometry devices.

When coherent radiation passes through an optical fiber, part of it, due to the presence of a large number of randomly located inhomogeneities with a characteristic diameter of the order of a fraction of a wavelength, is scattered back. This backscattered signal in the OTDR circuit is random along the fiber and is constant in the absence of external influences.

Known invention for US patent US10162245 B2 (IPC G02F 1/335) publ. 25.12.2018 In the patent, a distributed acoustic sensor system based on an optical hybrid phase demodulator with a delay can be made in various configurations. These configurations can include an optical hybrid with 2 to 4 outputs connected to an appropriate number of Faraday mirrors and photodetectors. One or more outputs leading to the Faraday mirrors can contain optical delay lines.

The disadvantage of this invention is the influence of the instability of the wavelength of the radiation source, as well as possible temperature fluctuations, on the measurement results. The influence of these instabilities will lead to an increase in the measurement error of the change in the recorded phase.

Known invention of the Russian Federation RU 2530244 "Distributed coherent reflectometric system with phase demodulation (options)" (IPC G01D 5/26 (2006.01) publ. 10.10.2014). Variants of the present invention provide for the presence of a Mach-Zehnder interferometer in the receiving part of the system, in one of the arms of which a delay line is located, and the second is connected to a phase modulator.

The disadvantage of this invention is the influence of temperature instability, as well as instability of the wavelength of the laser source, leading to an increase in the measurement error of the change in the recorded phase.

Known device according to patent CN 105806465 A (IPC G01H 9/004, publ. 03.11.2016), selected as the closest analogue (prototype). In it, the registration of back-reflected radiation is divided in the receiving part by a 1 × 2 splitter, enters the Mach-Zehnder interferometer containing a delay line, and then is connected by a 2 × 3 splitter, each of the three outputs of which is connected to the radiation receiver.

The disadvantage of this invention is the presence of instability of the wavelength of the laser source, as well as temperature instability of the receiving part, leading to an increase in the measured phase error.

Disclosure of invention

The objective of the invention is to reduce the influence of errors caused by the instability of the wavelength of the radiation source, as well as temperature and vibration oscillations of the fiber interferometer in the receiving unit, which leads to a decrease in the signal measurement error.

часть излучения попадает в усилитель оптического сигнала (бустер) 2, после чего модулируется управляемым драйвером акустооптическим модулятором 3, далее через оптический циркулятор 4-1 полученные импульсы следуют в контролируемую линию оптического волокна 5, где происходит рассеяние излучение на неоднородностях волокна во всех направлениях, в том числе и обратно в сторону оптического циркулятора 4-1. Обратнорассеянное излучение, прошедшее через оптический циркулятор 4-1, поступает на усилитель слабого сигнала (предусилитель) 6, после которого расположен фильтр на требуемую длину волны 7. Далее через циркулятор 4-2 и разветвитель 3×3 8, в прямом ходе которого излучение попадает в неравноплечий интерферометр Маха-Цендера с оптической разностью хода 9, на выходе которого стоит разветвитель 3×3 10. От каждого из выходов разветвителя 3×3 10 излучение идет на приемники 11, причем от двух выходов идет напрямую, а от третьего через циркулятор 4-3, далее сигналы на приемниках 11 оцифровываются АЦП 12-1. Меньшая часть излучения от высокостабильного узкополосного источника излучения 1 через разветвитель 1×2 14 в обратном ходе циркулятора 4-3, разветвителя 3×3 10, неравноплечего интерферометра Маха-Цендера с оптической разностью хода 9, разветвителя 3×3 8 попадает на приемники 15, причем от двух выходов разветвителя 3×3 8 идет напрямую, а от третьего через циркулятор 4-2, далее сигналы на приемниках 15 оцифровываются АЦП 12-2. Оцифрованный сигнал от АЦП 12-1 и 12-2 обрабатывается вычислительным устройством 13.The technical result is achieved due to the fact that the fiber-optic device for recording vibration effects with phase recovery with a decrease in the influence of instabilities of the recording interferometer includes a highly stable narrow-band radiation source 1, from which through a splitter 1 × 2 14

part of the radiation enters the optical signal amplifier (booster) 2, after which it is modulated by the acousto-optic modulator 3 controlled by the driver, then through the optical circulator 4-1 the received pulses follow the controlled line of the optical fiber 5, where the radiation is scattered on the fiber inhomogeneities in all directions, in including back towards the optical circulator 4-1. The backscattered radiation, passed through the optical circulator 4-1, enters the weak signal amplifier (preamplifier) 6, after which there is a filter for the required wavelength 7. Then, through the circulator 4-2 and the 3 × 3 splitter 8, in the forward path of which the radiation enters into an unequal-arm Mach-Zehnder interferometer with an optical path difference of 9, at the output of which there is a 3 × 3 10 splitter. From each of the outputs of the 3 × 3 10 splitter, radiation goes to the receivers 11, and from two outputs it goes directly, and from the third through the circulator 4 -3, then the signals at the receivers 11 are digitized by the ADC 12-1. A smaller part of the radiation from a highly stable narrow-band radiation source 1 through a 1 × 2 splitter 14 in the return path of a circulator 4-3, a 3 × 3 splitter 10, an unequal Mach-Zehnder interferometer with an optical path difference of 9, a 3 × 3 splitter 8 falls on the receivers 15, moreover, from the two outputs of the splitter 3 × 3 8 goes directly, and from the third through the circulator 4-2, then the signals at the receivers 15 are digitized by the ADC 12-2. The digitized signal from the ADCs 12-1 and 12-2 is processed by the computing device 13.

A decrease in the error in phase recovery is achieved due to the fact that the backscattered signal from the monitored line of the optical fiber 5, successively passed the circulator 4-1, amplified by a weak signal amplifier (preamplifier) 6, filtered by a filter for the required wavelength 7, and then passed through a 3 × 3 splitter 8, a non-equal-arm Mach-Zehnder interferometer with an optical path difference 9, a 3 × 3 splitter 10, is recorded at the receivers 11 and compared with the signal at the receivers 15, recorded from a highly stable narrow-band radiation source 1, which has successively passed through the 1 × 2 splitter 14 in the reverse stroke circulator 4-3, splitter 3 × 3 10, unequal Mach-Zehnder interferometer with optical path difference 9, splitter 3 × 3 8, and from two outputs of the splitter 3 × 3 8 goes directly, and from the third through circulator 4-2. This comparison of signals will allow separating the useful signal received from the monitored line of the optical fiber 5 from the internal instabilities of the system, such as the instability of the laser wavelength and temperature and vibration fluctuations of the receiving part, since all these components are present on the receivers 11, and the receivers 15 register only the components responsible for instabilities.

List of figures

FIG. 1 shows a diagram of the proposed device.

FIG. 2 shows the graphs of the signal and its components.

Implementation of the invention

FIG. 1 shows a diagram of the proposed device. The device contains a highly stable narrowband radiation source 1, an optical signal amplifier (booster) 2 controlled by an acousto-optic modulator driver 3, optical circulators 4-1, 4-2 and 4-3, a controlled optical fiber line 5, a weak signal amplifier (preamplifier) 6, filter for the required wavelength 7, and 3 × 3 splitters 8 and 10, unequal-arm Mach-Zehnder interferometer with optical path difference 9, receivers 11 and 15, ADC 12-1 and 12-2, computing device 13, through a splitter 1 × 2 fourteen.

Continuous radiation from a highly stable narrow-band radiation source 1 is divided by a 1 × 2 splitter 14. The 1 × 2 splitter 14 serves to separate the radiation so that a larger fraction falls into the monitored line, and a smaller one directly into the receiving part of the device. Most of the radiation after the splitter 1 × 2 14 is amplified by an optical signal amplifier (booster) 2, after which continuous radiation is formed into probing pulses by a driver-controlled acousto-optic modulator 3 and is directed through an optical circulator 4-1 into the controlled line of an optical fiber 5. Backscattered by fiber inhomogeneities the radiation from the probing pulse in the return path of the optical circulator 4-1 is amplified by the weak signal amplifier (preamplifier) 6, and then enters the filter at the required wavelength 7. At the output of the filter at the required wavelength 7 there is an optical circulator 4-2, in the forward path directing backscattered radiation to one of the outputs of the 3 × 3 splitter 8, which, being separated, passes the unequal-arm Mach-Zehnder interferometer with an optical path difference of 9, and then, passing the 3 × 3 10 splitter, the radiation goes to the receivers 11, and from two outputs it goes directly , and from the third through the circulator 4-3, then the signals at the reception mniks 11 are digitized by ADC 12-1. A smaller fraction of the radiation after the 1 × 2 splitter 14 is directed to the receiving part of the device. In the return stroke of the circulator 4-3, standing between one of the receivers 11 and one of the outputs of the 3 × 3 10 splitter, a smaller fraction of the radiation passes the 3 × 3 splitter 10, the unequal-arm Mach-Zehnder interferometer with an optical path difference of 9, the 3 × 3 splitter 8 , after which it is registered on the receivers 15, and from the two outputs of the 3 × 3 splitter 8 goes directly, and from the third through the circulator 4-2, then the signals on the receivers 15 are digitized by the ADC 12-2. The digitized signal from the ADCs 12-1 and 12-2 is processed by the computing device 13.

The backscattered signal from the monitored optical fiber line 5, successively passed by the circulator 4-1, amplified by a weak signal amplifier (preamplifier) 6, filtered by a filter for the required wavelength 7, after which the transmitted 3 × 3 splitter 8, unequal-arm Mach-Zehnder interferometer with an optical difference path 9, a 3 × 3 splitter 10 and recorded at the receivers 11 is compared with the signal at the receivers 15 recorded from a highly stable narrow-band radiation source 1, which successively passed through the 1 × 2 splitter 14 in the reverse stroke of the circulator 4-3, the 3 × 3 splitter 10, unequal Mach-Zehnder interferometer with optical path difference 9, splitter 3 × 3 8, and from two outputs of the splitter 3 × 3 8 goes directly, and from the third through the circulator 4-2. This comparison of signals makes it possible to separate the useful signal received from the monitored line of the optical fiber 5 from the internal instabilities of the system, such as instability of the laser wavelength and temperature and vibration fluctuations of the receiving part, since all these components are present on the receivers 11, and the receivers 15 register only the components responsible for instabilities.

Analytically, the implementation of this operation can be described as follows. The signal of a fiber-optic device for recording vibration effects with phase recovery with a decrease in the influence of instabilities of the recording interferometer from an area equal to the spatial resolution of the device is formed as the interference of all backscattered waves at the scattering centers in accordance with the expression I (s, t) = I _11const (s, t) ⋅ (1 + cos (Δϕ)). In this case, the unequal-arm Mach-Zehnder interferometer with an optical path difference 9, located between the 3 × 3 splitters 8 and 10 in the receiving part, forms on three receivers 11 signals with a phase shift caused by various shifts during the transition of light waves from one fiber to another in the splitter 3 × 3 10:

I _11.1 (s, t) = I _11const (s, t) ⋅ (1 + cos (Δϕ _p - 2π / 3))

I _11.2 (s, t) = I _11const (s, t) ⋅ (1 + cos (Δϕ _p ))

I _11.3 (s, t) = I _11const (s, t) ⋅ (1 + cos (Δϕ _p + 2π / 3))

Based on these three graphs, you can restore the phase based on the expression:

removing jumps through the value of π, which is a well-known and well-proven algorithm. In this expression, the signal phase is influenced by three main factors:

where λ (t) is the wavelength of the laser source at time t,

s is the sensitivity of the controlled line of optical fiber 5 to the impact, which depends on the type of soil, cable design and other parameters,

P (t) - the magnitude of the external influence on the monitored line of optical fiber 5 - this is the signal, the receipt of which is the purpose of the device,

- колебания разности плеч неравноплечего интерферометра Маха-Цендера

- fluctuations in the difference between the arms of an unequal-armed Mach-Zehnder interferometer

with an optical path difference of 9 from temperature changes and vibrations in the device, which usually cannot be completely eliminated.

In the same unequal-arm Mach-Zehnder interferometer with an optical path difference of 9, in the reverse stroke, the interference of the laser source directly occurs, the result of which is recorded on the receivers 15. The signals on them can be described by the expressions:

I _15.1 (s, t) = I _15const (s, t) ⋅ (1 + cos (Δϕ _l - 2π / 3))

I _15.2 (s, t) = I _15const (s, t) ⋅ (1 + cos (Δϕ _l ))

I _15.3 (s, t) = I _15const (s, t) ⋅ (1 + cos (Δϕ _l + 2π / 3))

Based on these three graphs, you can restore the phase based on the expression:

removing jumps over the value i, which is a well-known and well-proven algorithm.

In this expression, the signal phase is influenced by two main factors:

where λ (t) is the wavelength of a highly stable narrowband radiation source 1 at time t,

- колебания разности плеч неравноплечего интерферометра Маха-Цендера с оптической разностью хода 9 от изменений температуры и вибраций в приборе.

- fluctuations in the difference between the arms of an unequal Mach-Zehnder interferometer with an optical stroke difference of 9 from changes in temperature and vibrations in the device.

FIG. 2 shows the graphs of the signal and its components.

и λ(t) а в другом (выражение 2) - только шумовые составляющие.Thus, we get two signals, one of which (expression 1) contains both useful information P (t) and noise, in this case, the components

and λ (t) and in the other (expression 2) - only noise components.

Therefore, subtracting the second signal from the first will compensate for the instability introduced by an unequal-arm Mach-Zehnder interferometer with an optical path difference of 9:

In this expression, the measured phase value is proportional to the applied value, and the oscillations of the laser wavelength are a quantity that introduces at least an order of magnitude less noise than the compensated oscillations of the difference between the interferometer arms.

As a result, the proposed technical solution solves the problem of reducing the error in phase recovery, which leads to an increase in the accuracy of recording a signal from an external influence and, as a consequence, to an increase in the detectability and quality of the classification of the effect.

Claims (1)

A fiber-optic device for recording vibration effects with phase restoration with a decrease in the influence of instabilities of the recording interferometer includes: a highly stable narrow-band radiation source 1, characterized by the presence of a splitter 1 × 2 14,

part of the radiation after which it enters the monitored line, and the smaller part serves for direct registration of the parameters of the radiation source and the receiving part of the device; after one of the outputs of the splitter 1 × 2 14, an optical signal amplifier (booster) 2, a driver-controlled acousto-optic modulator 3, an optical circulator 4-1, a controlled optical fiber line 5, a weak signal amplifier (preamplifier) 6, a filter for the required wavelength are located in series 7, at the filter outlet there is a circulator 4-2, in the direct course of which the first 3 × 3 splitter 8, the unequal-arm Mach-Zehnder interferometer with an optical path difference 9, the second 3 × 3 splitter 10, the first three radiation receivers 11 are located; the second output of the splitter 1 × 2 14 is connected to the circulator 4-3 in the receiving part in front of the receivers 11 so that the radiation from the highly stable narrow-band radiation source 1, passing in the reverse direction the second splitter 3 × 3 10, the unequal-arm Mach-Zehnder interferometer with the optical path difference 9, the first splitter 3 × 3 8 falls on the second three radiation receivers 15; the outputs of all receivers are connected to analog-to-digital converters 12-1 and 12-2 of the computing device 13.

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