RU111635U1 - DISTRIBUTED FIBER-OPTICAL TEMPERATURE CONTROL SYSTEM BASED ON FORCED COMBINATION SCATTERING - Google Patents

Abstract

 Distributed fiber-optic temperature control system based on stimulated Raman scattering, including a semiconductor pulsed laser emitter, a device for inputting laser radiation into an optical fiber, photodetectors with interference filters emitting wavelengths of stimulated Raman scattering, a signal amplifier of a photodetector, a system of digitization, processing and indication of distribution temperature along the fiber, characterized in that, in order to reduce power consumption and increase As the range is increased, a mirror reflector is installed at the fiber end face, the system is equipped with a generator with a linearly increasing pulse repetition rate, starting from the frequency inverse of the laser pulse propagation time to the fiber end and back to doubling the frequency, and the photodetector amplifier is equipped with an integrator with a time constant equal to the period of the final frequency, as well as a gain unit with a quadratic increase in gain as the pulse repetition rate increases.

Description

The invention relates to the field of measuring equipment, telemetry and optoelectronics and can be used to control temperature along the optical fiber in the form of a cable laid along a controlled area up to 50 km long. The device can be used in fire alarm systems, technological monitoring of wells up to 5 km deep, in coal mines, elevators and other long-distance objects (up to 50 km).

The methodically presented invention relates to optical time domain reflectometry (OTDR) and apparatuses for performing such operations.

In the conventional OTDR method, a short laser pulse of light at a working wavelength (λ ₀ ) is injected at one end of the optical fiber, and Rayleigh optical radiation is measured, scattered back from each point along the fiber. In a time-distributed reading, an oscillogram of backscattered radiation is used. It measures optical losses and defects in various places of the fiber. In typical OTD reflectometers used to determine the characteristics of an optical fiber or cable, a Rayleigh backscatter signal is used, for example, to determine the location of defects (gaps, malfunctions) in the fiber or to measure the attenuation characteristics of the fiber and the fiber length.

Backscattered signals may result from elastic or inelastic scattering processes. Rayleigh scattering produces random (stochastic) signals from elastic scattering, basically with the same wavelength as the input signal (λ ₀ ). The signals of Brillouin and Raman scattering arise as a result of inelastic scattering processes, each of which produces pairs of spectral bands. Each pair of first-order bands includes one band (Stokes component), centered around a longer wavelength (λ ₊₁ ) than the input signal (λ ₀ ), and another band (anti-Stokes component), centered at a shorter wavelength (λ _-1 ) than the input signal. Thus, a pair of components is located symmetrically near the entered signal wavelength. The spectrum of Brillouin and Raman scattering signals usually contains several successive orders (at wavelengths λ _{± n} , where n = 1,2,3 ...), which are the result of particular scattering processes, and the intensity of the bands decreases with increasing order.

In the patent GB-2140554, G01D 5/26 dated 06/12/92, a method and apparatus is proposed in which pulsed laser light is introduced at one end of an optical fiber and Raman and Stokes signals that are backscattered are extracted and measured. The ratio of the measurements of their powers over time allows us to obtain the temperature distribution along the fiber.

In the following known device EP 0502283, G01K 11/32 from 04/11/94, optical signals at a wavelength of 1.32 μm from a source including a solid-state laser pumped by a diode were used, which were sent along an optical fiber with increased Raman scattering.

Europatent EP 0 636 868 A1, G01K 11/32, G01D 5/26 dated 07/19/94, taken as a prototype, proposes a distributed fiber-optic temperature control system based on stimulated Raman scattering, including a semiconductor pulsed laser emitter, a laser input device into optical fiber, photodetectors with interference filters emitting Stokes and anti-Stokes stimulated Raman wavelengths, photodetector signal amplifiers, a system for digitizing, processing and indicating distribution I temperature along the fiber. Various methods have also been proposed for limiting and converting Stokes components of the first order to higher-order components, as well as additional laser emitters that make it possible to increase the value of the Stokes components and the length of the distributed sensor line to 50 km. It also notes the undesirability of high probe radiation powers due to the appearance of nonlinear effects that impair the measurement accuracy.

The disadvantage of this system is that to increase its range up to 50 km, it is proposed to connect additional radiation sources at the wavelengths corresponding to the Stokes and anti-Stokes components. This complicates the measuring system, increases energy consumption and reduces its reliability. At the same time, the cost of the system increases, which prevents the widespread use of the prototype.

The essence of the invention consists in the use of not only Raman Raman scattering during the passage of a probe pulse propagating along the optical fiber in the forward direction, but also Raman scattering stimulated by a pulse reflected from the end end of the fiber and propagating in the opposite direction.

The technical result consists in increasing the range of the system without additional generators, reducing the power of the probe radiation a , and therefore, the power consumption, weight and cost of the measuring system.

The specified technical result is achieved by the fact that the system uses a mirror reflector at the end end of the fiber line and a special generator with a ramp-up repetition rate of the probe pulses, starting from the frequency of the inverse time of propagation of the laser pulse to the end of the fiber and back to doubling the mentioned frequency. In addition, the photodetector amplifiers are equipped with integrators with a time constant equal to the period of the final frequency, as well as an amplification unit with a quadratic increase in gain as the pulse repetition rate increases.

The proposed device is a distributed fiber-optic temperature control system based on stimulated Raman scattering is illustrated by the structural diagram presented in figure 1.

The following notations are used in the diagram: a pulse generator with a ramp pulse repetition rate 1 connected to a semiconductor laser 2; a device for inputting laser radiation 3 into an optical fiber 4; a reflecting mirror 5 located at the end end of the fiber 4; photodetectors with filters 6 emitting Stokes and anti-Stokes Raman components; preamplifier 7 with integrator 8; variable gain amplifier 9; analog-to-digital converter (ADC) 10; A computer 11 with an indicator device 12 (display).

Elements 1, 5, 8 and 9 of the proposed device constitute a distinctive part from devices in known systems and prototype.

The device operates as follows.

The pulse generator 1 generates short electrical pulses of a duration of 20 nsec with a linearly increasing pulse repetition rate, which are fed to a semiconductor laser 2, where they are converted into laser radiation pulses. These optical pulses enter the radiation input device 3 and, through it, into the optical fiber 4 from 0.1 to 50 km in length. Having reached the end of the fiber, optical pulses are reflected from mirror 5 and returned back through the fiber to photodetectors with filters 6. Interference filters emit Stokes and anti-Stokes Raman components and convert them into electrical signals proportional to their intensity. Next, the signals are amplified in the pre-amplifier 7, fed to the integrator 8, smoothing the pulse signals, and then to the amplifier with a variable gain 9, to align the signals of different distances, the analog-to-digital converter (ADC) 10 and computer 11, where they are mathematically processed and served on the indicator device 12 (display) in the form of a temperature distribution along the fiber. Received serial digital codes received by the computer can be used in the database of the computer control room.

The interaction pattern of light pulses propagating in the forward and reverse directions is illustrated in FIG. 2, where it is shown that the pulse 2 reflected from the mirror 5 and moving in the opposite direction enhances the Raman scattering of the next pulse 1 moving along the optical fiber 4 in the forward direction. The propagation pattern of the probe 1 and reflected 2 pulses along the fiber 4 before the interaction before interaction is shown in figure 2, a . The diagram of the passage of these pulses after passing through the interaction region 3 is shown in figure 2, b .

Due to the change in the frequency of the pulse generator, the meeting point of the direct and reflected pulses will move from the input end of the fiber to its end, “looking through” each section of the fiber in series.

Examples of the invention

Example 1 of the implementation of the proposed system based on quartz-germanate multimode fiber with standard geometric dimensions ⌀125 / 50 μm, with a parabolic refractive index and numerical aperture of 0.25, attenuation of 0.29 dB / km, 5 km long.

Optical pulses with a duration of 20 ns and a wavelength of 1.06 μm were used, with an introduced pulse power between 0.3 W and 1.0 W. The attenuation at the Raman anti-Stokes wavelength of 1.00 μm was 0.95 dB / km and at the Stokes wavelength of 1.12 μm, 0.62 dB / km. The observed reflected signal was attenuated by 3 ... 5 dB, i.e. amounted to 0.25 ... 0.35 watts. The signal of the Stokes component was weaker by 20 dB, and the anti-Stokes component by 30 dB. The level of these signals after filtering was quite high and amounted to 45 and 50 μW, i.e. did not require highly sensitive photodetectors. A deviation of 1% from the value for the linear mode of changing the signal power ratio of the Stokes and anti-Stokes components on a logarithmic scale from the fiber length was observed with a temperature change of 1 ° C.

Thus, the device allowed to record the temperature distribution along the fiber with a length of 5 km with a resolution of 1 ° C and a spatial resolution of 15 m.

Example 2 of the implementation of the invention on a single-mode fiber with standard geometric dimensions ⌀125 / 8.5 μm, with an unbiased dispersion attenuation at a wavelength of 1.55 μm 0.20 dB / km, a numerical aperture of 0.12 50 km long.

Optical pulses with a duration of 20 ns and a wavelength of 1.35 μm were used, with an introduced pulse power between 0.9 W and 1.0 W. The attenuation at the Raman anti-Stokes wavelength of 1.25 μm was 0.49 dB / km and at the Stokes wavelength of 1.45 μm it was 0.42 dB / km. The observed reflected signal at a working wavelength of 1.35 μm was attenuated by 25 ... 26 dB, i.e. amounted to 0.8 ... 1.0 mW. The signal of the Stokes component of Raman scattering was weaker by 20 dB, and the anti-Stokes component by 30 dB. The level of these signals after filtering was, respectively, 45 and 5 μW, i.e. required moderately sensitive photodetectors. A deviation of 1% from the value for the linear mode of changing the signal power ratio of the Stokes and anti-Stokes components on a logarithmic scale from the fiber length was observed with a temperature change of 1 ° C.

Thus, the device allowed to record the temperature distribution along the fiber with a length of 50 km with a resolution of 1 ° C and a spatial resolution of 15 m.

Table 1 shows the comparative characteristics of the known and proposed devices.

According to the data given in table 1, the proposed device has a wider range of operating wavelengths and range, requires less power per pulse, does not require additional emitters. To register the Stokes and anti-Stokes Raman components, highly sensitive receivers are not required.

Table 1. Comparative characteristics of the known and proposed devices No pp Parameters and specifications Known device (prototype) Proposed device one Operating wavelength ranges, microns 1.3 ... 1.55 0.85 ... 1.55 2 The radiation power in a pulse, W 1,5 ... 3,0 0.5 ... 1.0 3 The presence of an additional emitter to enhance Raman scattering there is no four Receiver sensitivity, mkW 0.1 5 5 The length of the optical fiber in the system without additional emitters, km 1 ... 30 1 ... 50 6 Spatial Resolution, m fifteen fifteen 7 Temperature resolution 1.5 ° C 1 ° C 8 Power consumption of equipment, W one hundred fifty

Claims (1)

Distributed fiber optic temperature control system based on stimulated Raman scattering, including a semiconductor pulsed laser emitter, a device for inputting laser radiation into an optical fiber, photodetectors with interference filters emitting wavelengths of stimulated Raman scattering, a signal amplifier of a photodetector, a system for digitizing, processing and indicating distribution temperature along the fiber, characterized in that, in order to reduce power consumption and increase As the range is increased, a mirror reflector is installed at the end end of the fiber, the system is equipped with a generator with a linearly increasing pulse repetition rate, starting from the frequency inverse of the laser pulse propagation time to the end of the fiber and back to doubling the frequency, and the photodetector amplifier is equipped with an integrator with a time constant equal to the period of the final frequency, as well as a gain unit with a quadratic increase in gain as the pulse repetition rate increases.