RU2552399C1 - Distributed fiber optical high sensitivity temperature sensor - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to devices of measurement of temperature distribution where optical fiber is used as a sensitive element, namely it is a sensitive element of the distributed temperature sensor which uses the method based on the phenomenon of stimulated Brillouin scattering (SBS) occurring in optical fiber. The fiber-optical sensor for temperature distribution monitoring systems on the basis of registration of parameters of stimulated Brillouin scattering contains, at least, one optical module consisting of the tube comprising, at least, one, freely laid with it, quartz optical fiber. The fiber has a mechanically linked shell, and the coefficient of longitudinal linear thermal expansion of optical fiber in the shell is minimum forty times exceeds the coefficient of linear thermal expansion of quartz glass.

EFFECT: increase of sensitivity of high sensitivity temperature sensor.

6 cl, 1 dwg

Description

The invention relates to temperature distribution measuring devices in which an optical fiber is used as a sensing element, namely, it is a sensitive element of a distributed temperature sensor in which a method based on the Mandelstamm-Brillouin stimulated scattering phenomenon (SBS) arising in an optical fiber is used.

Currently, instruments that use a method for measuring the distribution of deformation and / or temperature of an optical fiber along its axis (tension or compression), based on the phenomenon of stimulated Mandelstamm-Brillouin scattering, are manufactured and are commercially available. An example of such a device is the Ditest STA-R Brillouin analyzer manufactured by Omnisens SA, Switzerland [URL: http://omnisens.ch/ditest/3521-ditest-sta-r.php, accessed 05/08/2013].

A device is known - a distributed sensor for measuring deformation and / or temperature (see RF Patent No. 2346235, publ. July 27, 2008), which uses a method based on the phenomenon of stimulated Mandelstamm-Brillouin scattering that occurs in an optical fiber. In this solution, the optical fiber is used as a sensing element for detecting deformation and / or temperature in the environment where the optical fiber is located. The known device consists of a pumping light source, a sensitive optical fiber, an optical coupler, a sounding radiation source and a detector. A sensitive optical fiber is connected at one end to a pump light source, and from a second end, using an optical coupler, to a sounding light source and a detector. The known device measures the distribution of the SBS spectra along a sensitive optical fiber with a given spatial resolution. The Brillouin frequency shift ν _V is one of the SBS parameters measured by the known device. The Brillouin frequency shift ν _V (in MHz) in an optical fiber is determined by the following equation:

where n is the refractive index of the optical fiber (dimensionless quantity), ν _a is the speed of sound in the optical fiber (in m / s), λ is the wavelength of light radiation (in microns). Since the speed of sound depends on the strain and temperature of the optical fiber, the strain and / or temperature can be measured by measuring the Brillouin frequency shift.

It is known that when using sensor cables with free laying of a standard single-mode optical fiber with a diameter of 125 μm, an increase in temperature by 1 ° C causes an increase in the Brillouin frequency shift by about 1 MHz, while an extension of the optical fiber by 1% causes an increase in the Brillouin frequency shift by a value of about 500 MHz [F. Ravet, F. Briffod, M. Nikles, Extended distance fiber optic monitoring for pipeline Leak and ground movement detection. 2008 International Pipeline Conference, Calgary, paper No. IPC2008-64521, pp. 689-697, published 2008.]. Moreover, the contribution to the temperature dependence of the frequency shift due to thermal expansion of the primary coating of the optical fiber, which usually has a diameter of 250 μm, is insignificant [M. Nikles, L. Thevenaz, R.A. Rober, “Brillouin gain spectrum characterization in single-mode optical fibers”, Journal of Lightwave Technology, vol. 15, no.10, pp. 1842-1851, published 1997],

Known fiber optic sensor cable [Japanese Patent JP 2009103496 (A), publ. 05/14/2009], designed to measure the distribution of deformation, which contains at least one strain-sensitive optical fiber, a tube with at least one optical fiber used for temperature compensation (temperature measurement), freely placed in it.

The closest analogue, the prototype is the well-known fiber-optic sensor (see RF patent for utility model No. 122773, published December 10, 2012) for monitoring the distribution of strain and temperature based on the registration of parameters of the fine structure of the scattered radiation, in particular, which allows you to register the distribution deformation by means of a tight, without slipping, connection of the optical fiber with the reinforcing coating and the outer shell and, at the same time, register temperature changes through parallel free placement of stacked unit in a polymer optical fiber.

However, the design of this cable does not take measures to increase the sensitivity when measuring the temperature distribution of the SBS analyzer, namely, to convert the temperature deformation of the buffer coating of the fiber into deformation of the sensitive optical fiber so that it is uniform along the length of the cable, and in a unique way. Also, no measures have been taken with the known cable to ensure mechanical coupling of the protective buffer coating with the optical fiber and the free laying of the sensitive module (fiber in the buffer coating) in the cable.

The technical result is aimed at increasing the sensitivity of the temperature sensor increased sensitivity.

The technical result is achieved due to the fact that in the known fiber-optic sensor for monitoring temperature distribution based on the registration of stimulated scattering parameters Mandelstamm-Brillouin, containing at least one optical module consisting of a tube comprising at least one freely laid with it, a quartz optical fiber, according to the claimed invention, the fiber has a sheath mechanically associated with it, and the coefficient of longitudinal linear thermal expansion is optical th fiber in the sheath is not less than forty times the coefficient of linear thermal expansion of quartz glass.

Optical fibers may contain an additional tight coating of carbon or metal.

The optical module may be twisted.

The sensor may have a reinforcing coating and an outer protective polymer shell.

The free space in the optical modules can be filled with a hydrophobic filler.

The tube of the optical module can be made of polybutylene terephthalate or steel.

The figure shows in cross section a fiber optic temperature sensor of increased sensitivity. The sensor contains a quartz optical fiber 1, in a sealed coating 2, a sheath 3 mechanically connected with the optical fiber, located in the optical module, formed by a tube 5 made of polybutylene terephthalate or steel made of a hydrophobic filler 4. The optical module is in a twist formed around the core 6 and containing the filling corps 7. A reinforcing coating 8 and an outer protective sheath 9 are applied over the twist with the optical module.

The essence of the invention is to increase the temperature sensitivity of the optical fiber of the sensor due to deformation.

In order to increase the sensitivity, the sensor optical fiber is coated with an additional layer of material, the coefficient of thermal linear expansion of which is much higher than that of quartz glass, and the force created by the additional coating due to thermal expansion / contraction leads to such a deformation of the sensitive optical fiber that an additional change in the Brillouin shift the frequency in it is no less than a change in the Brillouin frequency shift due to the temperature dependence. That is, when the temperature is increased by 1 ° C, the optical fiber should be deformed (lengthened) by 0.002% or more. As a result, the temperature of the claimed sensor cable by 1 ° C causes an increase in the Brillouin frequency shift by an order of 2 MHz or more.

To ensure that the Brillouin frequency shift only depends on temperature, the design of the claimed sensor provides free laying of the sensitive element (optical fiber with an additional coating) and, therefore, its protection against deformation caused by external forces acting on the sensor cable.

As a result, the accuracy of temperature measurement improves by 2 or more times.

The change in the Brillouin frequency shift Δν (in MHz) in the optical fiber with a change in its temperature by ΔT and its longitudinal relative tensile / compression strain ε (in%) is given by the following equation:

Δν = Δν ₀ + С _T ΔТ + C _ε ε,

where Δν ₀ is the constant (in MHz), and C _T and C _ε are the sensitivity of the optical fiber to temperature changes and to the longitudinal relative tensile / compression deformation, respectively, which, as mentioned above, are approximately equal to _{T T} ≈ 1 MHz / ° C and C _ε ≈500 MHz /%.

Let us find the longitudinal relative tensile / compression strain of the optical fiber caused by the thermal expansion of the sheath mechanically connected with it with a cross-sectional area S _o (in mm2), linear thermal expansion coefficient α _o (in% / ° C) and Young's modulus E _o ( in MPa). Optical fiber deformation is calculated using Newton’s third law and Hooke’s law.

,

,

where S is the cross-sectional area of the optical fiber (in mm2), E is the Young's modulus of the optical fiber (in MPa), and, as insignificant, the contribution of the thermal expansion of the optical fiber, which consists of fused silica with a linear thermal expansion coefficient of about 0.5 × 10 ^-4 % / ° C, which is significantly less than typical shell materials, for example, for a shell made of polyvinyl chloride - ~ 70 × 10 ^-4 % / ° C, of polyamide ~ 70 × 10 ^-4 % / ° C, from polyester (60-125) × 10 ^-4 % / ° C, from polyethylene (130-200) × 10 ^-4 % / ° C, from polypropylene ~ 200 × 10 ^-4 % / ° C.

The contribution to the change in the Brillouin frequency shift Δν (in MHz) in an optical fiber when its temperature is changed by ΔТ due to thermal expansion of the sheath and in the absence of other causes of longitudinal relative deformation is given by the following equation6

.

.

Thus, we obtain that the absolute sensitivity K of the optical fiber in a mechanically connected shell as a temperature sensor, which is equal to the ratio of the change in the output signal, i.e., the change in the Brillouin frequency shift, to the absolute change in the measured temperature is

K = C _T + C _D ,

- дополнительный вклад в чувствительность за счет теплового расширения оболочки, механически связанной с оптическим волокном. Из данного выражения видно, что при выполнении условияWhere $$C_D=C_{\epsilon }{\alpha }_o\frac{E_o{S}_o}{ES+E_o{S}_o}$$

- an additional contribution to sensitivity due to thermal expansion of the shell mechanically coupled to the optical fiber. From this expression it is clear that when the condition

C _D ≥C _T

the presence of the sheath significantly increases the sensitivity of the optical fiber to temperature when measuring the frequency shift of the SBS. This condition means that the thermal expansion of the sheath causes a longitudinal strain / compression strain of the fiber greater than or equal to the sensitivity of the fiber to temperature divided by the sensitivity to said strain. In the case of using a standard single-mode optical fiber with the above sensitivities, a temperature change of 1 ° C should result in thermal deformation of the optical fiber caused by thermal expansion by at least 0.002%. Thus, the coefficient of longitudinal linear thermal expansion of the optical fiber in the cladding should be no less than forty times the coefficient of linear thermal expansion of silica glass. Using the above relations, we write this condition in the following form:

where K _o = E _o S _o and K = ES are the stiffness (in N) during longitudinal tensile / compression deformation for the sheath and optical fiber, respectively. That is, the product of the coefficient of thermal linear expansion of the sheath and the sensitivity of the optical fiber to the relative tensile / compression strain, divided by the sensitivity of the optical fiber to the temperature change, is not less than one greater than the ratio of the stiffness to the longitudinal tensile / compression of the optical fiber to the same stiffness of the sheath.

Since the properties of polymers are highly dependent on temperature, in order to ensure the material contribution of the shell to the increase in sensitivity over the entire range of measured temperatures, it is necessary that the above ratio also be satisfied over the entire range of measured temperatures.

To ensure uniformity of fiber sensitivity, which simplifies the conversion of the measured frequency shift to temperature, it is necessary to ensure uniform cross-sectional area of the sheath associated with the fiber.

In order that thermal expansion does not cause bending, but leads only to longitudinal tensile / compression deformation, it is necessary that the axis of the optical fiber be in the geometric center of the cross section of the mechanical sheath associated with it (center of gravity). For example, if the shell is cylindrical, then the axes of the optical fiber and the shell must match.

To protect the optical fiber in the sheath from external mechanical influences and to ensure its free laying, it can be placed in a tube. The tube can be made of a polymer, for example, polybutylene terephthalate, using technology widely used in the production of optical cable [URL: http://www.ruscable.ru/info/cable/sev_13.html, accessed 05/08/2013]. The tube can also be made of metal, for example, stainless steel, using the technology common in the production of optical cable built-in to the ground wire technology [URL: http://sarko.ru/new/opticheskiy-grozotros/kabel-okgt-ts.html, date appeal 05/08/2013].

To prevent the formation of irregularities in the laying of the optical fiber in the tube and to prevent longitudinal propagation that reduces the mechanical characteristics of the optical fiber of water, it can be filled with gel. As a gel, an intra-modular hydrophobic filler, widely used in the production of an optical communication cable using known technology, can be used.

To increase the range of longitudinal deformation of the optical fiber, in which the optical fiber is freely laid in a shell in the tube, the tube can be twisted, for example, around a central element or around a central element, thereby forming a coil. In addition, twisting allows you to increase the range of permissible external influences (temperature, power), which provides free laying of the optical fiber. Twisting can be done using technology widely used in the production of optical communication cable.

To increase the allowable range of longitudinal deformation of the optical fiber, which ensures its reliable operation, the optical fibers may contain an additional tight coating of carbon or metal, which are commercially available.

To protect against external influences, the sensor may have a reinforcing coating and an outer polymer sheath, widely used in the construction of optical communication cables.

In the case when a standard quartz optical fiber (that is, consisting mainly of quartz glass) with a diameter of 0.125 mm was used with a Young's modulus E = 70,000 MPa and coefficients characterizing the sensitivity of the optical fiber C _T = 1 MHz / ° C and C _ε = 500 MHz /%, and the sheath is made of polyamide with Young's modulus E _o = 1000 MPa and linear thermal expansion coefficient α _o = 70 × 10 ^-4 % / ° C, we find that the presence of the sheath significantly increases the sensitivity of the optical fiber to temperature when measuring the frequency shift SBS provided that when shell exceeds tkost

K _o ≥344 N,

which corresponds to a cladding area of ≥0.344 sq. mm, and in the case of a cylindrical cladding around a centrally located optical fiber. This corresponds to an outer shell diameter of ≥0.674 mm. Moreover, the deformation of such an optical fiber due to thermal expansion of its shell will be:

ε = 2.8 × 10 ^-3 ΔТ.

Given that, most standard optical fibers allow tensile during operation up to 0.2% without risk of damage [URL: http://www.specialtyphotonics.com/about/white_papers/Fiber%20Strength-rev21%282%29.pdf, accessed 05/08/2013], it is possible to use such a casing in the temperature range 71 ° C, in which the casing will cause deformation of the optical fiber of not more than 0.2% and, therefore, will not lead to the risk of damage. To increase the range of operating temperatures, it is possible to use special fibers with a sealed coating, for example of metal or carbon, which allow greater stretching during operation without the risk of damage [URL: http://www.specialtyphotonics.com/about/white_papers/Aqueous%20White% 20Paper.pdf, accessed 05/08/2013].

Claims (6)

1. Fiber-optic sensor for temperature distribution monitoring systems based on registration of Mandelstamm-Brillouin stimulated scattering parameters, comprising at least one optical module, consisting of a tube, including at least one quartz optical module, freely laid with it fiber, characterized in that the fiber has a sheath mechanically associated with it, and the coefficient of longitudinal linear thermal expansion of the optical fiber in the sheath is not less than forty times the linear coefficient th thermal expansion of quartz glass. 2. The sensor according to claim 1, characterized in that the optical fibers contain an additional tight coating of carbon or metal. 3. The sensor according to claim 1, characterized in that the optical module is in a twist. 4. The sensor according to claim 1, characterized in that it has a reinforcing coating and an outer protective polymer shell. 5. The sensor according to claim 1, characterized in that the free space in the optical modules is filled with a hydrophobic filler. 6. The sensor according to claim 1, characterized in that the tube of the optical module is made of polybutylene terephthalate or steel.