RU2793155C1 - Method for passive compensation of the temperature dependence of the optical parameters of a fibre bragg grating - Google Patents

Abstract

FIELD: fibre-optic technologies.

SUBSTANCE: invention relates to methods for passive compensation of the temperature dependence of the optical parameters of a fibre Bragg grating (FBG) formed in the core of an optical fibre. The method for passive compensation of temperature dependence of optical parameters of a fibre Bragg grating formed in the core of an optical fibre includes packaging said fibre, bonded with tension to a fastening element into a hollow tube made of a material with thermal expansion coefficient (CTE) lower than the CTE of the element with which the fibre is bonded, and to attach the fibre, two capillaries are used, which are symmetrically placed on both sides of the fibre and fixed, maintaining the value of the pre-calculated distance between the attachment points of the fibre to the attachment element. The resulting structure is placed in the above hollow tube, with which one of the capillaries is fastened and the fibre is tensioned, and with the resulting tension, the second capillary is fixed in the hollow tube. The resulting structure is annealed at a temperature of 110°C for 60 min.

EFFECT: expansion of the temperature range with a decrease in the unevenness of the readings along the wavelength of the Bragg resonance.

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Description

The invention relates to fiber-optic technologies, in particular to a method for passive compensation of the temperature dependence of the optical parameters of a fiber Bragg grating (FBG) formed in the core of an optical fiber.

A known method of passive temperature compensation FBG [Lee S.-M., Gu X. Passive Temperature Compensation Package for Optical Long Period Fiber Gratings // J. Opt. soc. Korea, JOSK. Optical Society of Korea, 1999. Vol. 3, No. 2, pp. 74–79], implemented using an elongated Invar element, to which the fiber is attached with pretension so that the Bragg grating is located between the attachment points. The ends of the Invar component are attached to the fiber with bronze tubes. As the temperature increases, bronze and invar elements expand in accordance with their coefficients of thermal expansion (TEC), compensating for the temperature dependence of the lattice. The unevenness of the readings of the central wavelength of the Bragg resonance in the range [0; 60] °C is 0.018 nm, which corresponds to a temperature sensitivity of the grating of 0.3 pm/°C.

There are two ways of passive thermal compensation FBG [Lo Y.-L., Kuo C.-P. Packaging a fiber Bragg grating without preloading in a simple athermal bimaterial device // Advanced Packaging, IEEE Transactions on. 2002 Vol. 25. P. 50-53]. The first way is that a metal element with a high CTE is placed in a tube made of a material with a low CTE. A fiber with a grating is threaded into the structure and fixed on one side to the outer tube, on the other - to the metal element with the initial tension. In the second method, a metal tube is attached to the fiber at two points, and the fiber is attached to the outer element on both sides of the body. With an increase in temperature in the first method of packaging, the metal element expands towards the lattice, because on the other hand it is limited by an outer tube, and in the second, expansion occurs in both directions. With both methods of thermal compensation, the shift of the Bragg resonance wavelength is 0.1 nm in the range [-40; 80] °C.

A known method of passive thermal compensation FBG [Lo Y.-L., Kuo C.-P. Packaging a Fiber Bragg Grating With Metal Coating for an Athermal Design // J. Lightwave Technol., JLT. IEEE, 2003. Vol. 21, No. 5. P. 1377], which uses a metal coating applied to the fiber using a welding machine. The shift of the Bragg resonance wavelength is 0.05 nm in the range [30; 80] °C, which corresponds to a grating sensitivity of about 0.01–0.001 nm/°C.

2003. Vol. 23, №. s1. P. 283], заключающийся в том, что решетка помещена в трубочку из политетрафторэтилена. Волокно с записанной на нем ВБР закрепляется в тефлоновую трубку с предварительным натяжением к двум концам с помощью адгезива. В трубке проделаны вентиляционные отверстия для поддержания условий окружающей среды внутри трубки. При изменении температуры окружающей среды, тефлоновая трубка расширяется в соответствии с КТР и компенсирует температурную зависимость ВБР, воздействуя механическим напряжением на волокно. Температурная чувствительность атермального корпуса составляет 9,69 пм/°С в диапазоне [20; 50] °С, что соответствует сдвигу длины волны брэгговского резонанса 0,28 нм. A known method of passive thermal compensation FBG [Liu T. et al. The Wavelength Shifting and Temperature Athermalization of Fiber Bragg Grating //

2003 Vol. 23, no. s1. P. 283], which consists in the fact that the grating is placed in a tube of polytetrafluoroethylene. The fiber with the FBG recorded on it is fixed into a Teflon tube with pretension to both ends using an adhesive. Ventilation holes are made in the tube to maintain the environmental conditions inside the tube. When the ambient temperature changes, the Teflon tube expands in accordance with the CTE and compensates for the temperature dependence of the FBG, acting as a mechanical stress on the fiber. The temperature sensitivity of the athermal body is 9.69 pm/°C in the range [20; 50] °C, which corresponds to a shift in the wavelength of the Bragg resonance of 0.28 nm.

A known method of passive thermal compensation FBG [Patent US 6907164 B2, 06/14/2005], based on the pre-tension of the fiber, with the FBG recorded in it, fixed inside the structure at two points, between which there is a sensitive element. The design consists of a hollow structure in which a free and threaded element is placed. On the one hand, the fiber is attached to the threaded part for adjusting the fiber tension, on the other hand, to the free element. The hollow structure, the free element, and the threaded element have thermal expansion coefficients chosen so that the design compensates for the temperature dependence of the Bragg resonance wavelength. The unevenness of the readings of the central wavelength of the Bragg resonance in the range [5; 70] °C is 0.015 nm, which corresponds to a temperature sensitivity of the grating of 1 pm/°C.

The disadvantages of the above methods of passive thermal compensation FBG is a narrow temperature range with a relatively large uneven readings along the wavelength of the Bragg resonance. Also, in the creation of athermal cases, elements are used that require high-precision manufacturing.

A known method of passive thermal compensation FBG, selected as a prototype [Yoffe G.W. et al. Passive temperature-compensating package for optical fiber gratings // Appl. Opt., A.O. Optica Publishing Group, 1995. Vol. 34, No. 30. P. 6859–6861], in which a fiber with a recorded grating is fixed on one side with epoxy glue to a threaded aluminum tube, and on the other hand, to an aluminum element without a thread. The whole structure is placed in a quartz tube, on one side of which there is a nut for a threaded element, on the other - the "wings" of the aluminum component. The fiber is tensioned and secured with a nut. As the temperature increases, the metal components expand according to their CTE, relieving tension, thus compensating for the temperature dependence of the lattice. The thermal expansion of other components in this system can be neglected. The change in the wavelength of the Bragg resonance was less than 0.1 nm in the range [-30; 70] °С.

The disadvantage of the above described method of passive thermal compensation FBG is a narrow temperature range with a relatively large uneven readings along the wavelength of the Bragg resonance. Also, in the creation of athermal cases, elements are used that require high-precision manufacturing.

The invention solves the problem of expanding the temperature range with a decrease in the non-uniformity of readings along the wavelength of the Bragg resonance.

The problem is solved in the following way.

In a method for passive compensation of the temperature dependence of the optical parameters of a fiber Bragg grating formed in the core of an optical fiber by packaging said fiber, bonded with tension to a fastening element into a hollow tube made of a material whose thermal expansion coefficient (CTE) is lower than the thermal expansion coefficient (CTE) of the element , with which the fiber is bonded, to fix the fiber, two capillaries are used, which are symmetrically placed on both sides of the fiber and fixed, maintaining the distance L _b between the points of attachment of the fiber to the attachment element, calculated by the formula:

– упрогооптический коэффициент волокна. Полученную конструкцию размещают в вышеуказанную полую трубку, с которой скрепляют один из капилляров, осуществляют натяжение волокна, при этом F – силу, которую прикладывают для натяжения волокна, определяют из соотношения:de α _a – CTE of capillaries, L _a – length of capillaries α _f – CTE of fiber, L _b – distance between attachment points of fiber to capillaries, α _s – CTE of hollow tube, L _s – length of hollow tube, ξ – thermooptic coefficient of fiber,

is the elastic-optic coefficient of the fiber. The resulting structure is placed in the above hollow tube, with which one of the capillaries is fastened, the fiber is tensioned, while F is the force that is applied to tension the fiber, is determined from the ratio:

With the resulting tension, the second capillary is fixed in a hollow tube, then the resulting structure is annealed at a temperature of 110 °C for 60 minutes.

The essence of the proposed method is explained as follows.

The optical fiber, in which the Bragg grating is written, is attached under a certain tension to two capillaries, high CTE elements, so that the grating is between these elements, which, in turn, are attached to a hollow tube, a low CTE element. As the temperature increases, the compensating elements and the fiber grating will expand according to their CTE. That is, the distance between the attachment points of the fiber to the high CTE element will decrease, resulting in a decrease in fiber tension by exactly the amount necessary to compensate for the shift in the wavelength of the Bragg resonance of the grating caused by the increase in temperature.

The design is symmetrical for identical on both sides of the impact of mechanical stress on the FBG when the temperature changes, this characteristic reduces the unevenness of the readings along the wavelength of the Bragg resonance.

After assembly, the structure is subjected to annealing to relieve stresses both in individual structures of the body elements and in the structure as a whole. The relaxation of materials as a result of annealing improves the stability of the housing and reduces the uneven readings along the wavelength of the Bragg resonance.

The shift in the wavelength of the Bragg resonance of the grating as a result of a change in mechanical stress caused by the thermal expansion of the elements of the athermal body, and temperature, taking into account the elastic-optical and thermo-optical effects, is described as follows:

(1)

(1)

– центральная длина волны брэгговского резонанса,

– изменение температуры окружающей среды,

– КТР алюминиевых капилляров,

– длина алюминиевых капилляров, участвующая в термокомпенсации решетки,

– КТР волокна,

– термооптический коэффициент волокна,

– расстояние между точками крепления волокна к алюминиевым капиллярам,

– КТР кварцевой полой трубки,

– длина кварцевой полой трубки,

– упрогооптический коэффициент волокна, определяемый выражением:Where

is the central wavelength of the Bragg resonance,

– change in ambient temperature,

– CTE of aluminum capillaries,

is the length of the aluminum capillaries involved in the lattice thermal compensation,

– fiber KTP,

is the thermooptic coefficient of the fiber,

is the distance between the attachment points of the fiber to the aluminum capillaries,

– KTR quartz hollow tube,

is the length of the quartz hollow tube,

is the elasto-optic coefficient of the fiber, determined by the expression:

– компоненты упрогооптического тензора,

– коэффициент Пуассона. Для стандартного телекоммуникационного одномодового оптического волокна p₁₁=0.113, p₁₂=0.252, ν=0.16 и n=1.482 [Othonos A. et al. Fibre Bragg Gratings // Springer Series in Optical Sciences. 2006. Vol. 123. P. 189–269]. Первое слагаемое уравнения (1) описывает влияние теплового расширения алюминиевых капилляров на длину волны брэгговского резонанса решетки. Второе слагаемое – сдвиг длины волны брэгговского резонанса в результате температурной зависимости решетки. Третье и четвертое слагаемые – температурное расширение самого волокна и внешней кварцевой полой трубки, соответственно.Where

are the components of the elastic-optical tensor,

- Poisson's ratio. For a standard telecommunications single-mode optical fiber p ₁₁ =0.113, p ₁₂ =0.252, ν=0.16 and n=1.482 [Othonos A. et al. Fiber Bragg Gratings // Springer Series in Optical Sciences. 2006 Vol. 123. P. 189-269]. The first term of equation (1) describes the effect of thermal expansion of aluminum capillaries on the wavelength of the Bragg resonance of the grating. The second term is the shift of the Bragg resonance wavelength as a result of the temperature dependence of the grating. The third and fourth terms are the thermal expansion of the fiber itself and the outer quartz hollow tube, respectively.

The efficiency of the passive temperature compensation method, that is, the unevenness of readings along the Bragg resonance wavelength, also depends on the distance between the attachment points of the fiber to two capillaries, which depends on the dimensions and CTE of the selected structural elements, and is determined by the following expression:

– упрогооптический коэффициент волокна.where α _a is the CTE of capillaries, L _a is the length of capillaries, α _f is the CTE of the fiber, L _b is the distance between the attachment points of the fiber to the capillaries, α _s is the CTE of the hollow tube, L _s is the length of the hollow tube, ξ is the thermooptic coefficient of the fiber,

is the elastic-optic coefficient of the fiber.

The temperature range of operation of the athermal body is directly proportional to the amount of tension with which the fiber is connected to the compensating elements, and is expressed as follows:

– центральная длина волны брэгговского резонанса, ξ – упругооптический коэффициент волокна.where F is the force applied to tension the fiber, ε is the sensitivity of the central wavelength of the Bragg resonance to the relative elongation of the fiber, E is the Young's modulus of the fiber, r is the radius of the fiber, α is the CTE of the fiber,

is the central wavelength of the Bragg resonance, ξ is the elasto-optic coefficient of the fiber.

The essence of the claimed method of passive thermal compensation FBG is illustrated by drawings (Fig. 1, Fig. 2, Fig. 3, Fig. 4).

Accepted designations on the figures:

λ is the wavelength

P - optical power

1 - optical fiber

2 - VBR

3 - capillaries

4 - hollow tube

5 - limiter for adhesive bonding

6 - adhesive connection

On FIG. Figure 1 shows a housing diagram for passive temperature compensation of the FBG. Fiber 1 with a grating 2 written on it and two capillaries 3 symmetrically placed on it on both sides are placed in a hollow tube 4 and fastened to it using a limiter 5 for adhesive bonding with glue 6.

Fiber 1 placed in a hollow tube 4 with a grating 2 written on it and a capillary 3 put on it on one side is fastened to the tube 4.

A force is applied to the opposite end of the fiber 1, and with the resulting fiber tension, the second aluminum capillary 3 is fixed in the hollow tube 4 with glue 6, using special stops 5.

On FIG. 2 shows the dimensions of the elements of the athermal body, where L_s is the length of the hollow tube, L_b is the distance between the attachment points of the fiber to the capillaries, L_a- length capillaries involved in the temperature compensation of the lattice.

On FIG. Figure 3 shows examples of the dependences of the Bragg resonance wavelength on temperature. The red line shows the dependence for a conventional FBG when the temperature changes in the range [-15; 105] °C, the black line shows the dependence for the athermal lattice, measured in the same temperature range. The unevenness of the readings of the Bragg resonance wavelength is 70 pm.

On FIG. Figure 4 shows examples of the reflection spectrum of a grating located in a thermally compensating housing, where the blue line shows the spectrum at a temperature of -15°C, and the red one at a temperature of 105°C.

A specific example of the FBG passive thermal compensation method was implemented as follows. In optical fiber SMF-28 (standard G.657.A2), subjected to preliminary hydrogen treatment and CTE equal to 0.55×10 ^-6 С ^-1 , FBG is recorded by the phase mask method with a Bragg resonance wavelength of 1534.2 nm and a length of 1 mm. On fiber 1 (Fig. 1) with a radius of 62.5 μm with grating 2 recorded on it, two aluminum capillaries 3 are symmetrically placed on both sides and fixed with glue 6 (epoxy glue with higher temperature stability EPO-TEC, operating up to 350 °C), in this case, the length of the aluminum capillaries involved in temperature compensation is 5 mm. The distance between the attachment points of the fiber to the capillaries is 22 mm. Then the design is placed in a quartz hollow tube 4 34 mm long and glued on one side. Next, a weight of 200 g is hung on the opposite end of the fiber, which determines the force applied to tension the fiber, which is about 2 N, and with the resulting fiber tension, the second aluminum capillary is fixed in a quartz hollow tube. Since the glue is distributed unevenly during pouring into the structure, the length of the aluminum capillaries involved in compensating for the shift in the Bragg resonance wavelength in the experimental sample differs from the calculated value due to symmetry breaking. And changing this parameter leads to incorrect operation of the thermally compensating body, since other parameters remain unchanged. Therefore, to maintain symmetry, special limiters 5 are used, fixed in such a way that the epoxy glue is located at the points of the structure calculated earlier.

Then the sample is annealed on a heating plate at a temperature of 110 °C for 60 minutes. The unevenness of the readings of the wavelength of the Bragg resonance in the temperature range [-15; 105] °C is 70 pm.

Thus, the claimed method provides an extension of the temperature range of the case compared to the prototype from [-30; 70] °С to [-15; 105] °С and a decrease in the non-uniformity along the wavelength of the Bragg resonance from 0.1 nm to 70 pm.

Claims (6)

A method for passively compensating for the temperature dependence of the optical parameters of a fiber Bragg grating formed in the core of an optical fiber by encapsulating said fiber, bonded with tension to a fastening element into a hollow tube made of a material whose thermal expansion coefficient (CTE) is lower than the thermal expansion coefficient (CTE) of the element, with which the fiber is bonded, characterized in that, to secure the fiber, two capillaries are used, which are symmetrically placed on both sides of the fiber and fixed, maintaining the distance L _b between the points of attachment of the fiber to the attachment element, calculated by the formula:

where α _a is the CTE of capillaries, L _a is the length of capillaries, α _f is the CTE of the fiber, L _b is the distance between the attachment points of the fiber to the capillaries, α _s is the CTE of the hollow tube, L _s is the length of the hollow tube, ξ is the thermooptic coefficient of the fiber,

is the elasto-optical coefficient of the fiber, and the resulting structure is placed in the above hollow tube, with which one of the capillaries is fastened, the fiber is tensioned, while F is the force that is applied to tension the fiber, is determined from the ratio:

where E is the Young's modulus of the fiber, r is the fiber radius,

is the central wavelength of the Bragg resonance, α _f is the CTE of the fiber, ξ is the elastic-optical coefficient of the fiber,

is the change in the ambient temperature, ε is the sensitivity of the central wavelength of the Bragg resonance to the relative elongation of the fiber, with the resulting tension, the second capillary is fixed in a hollow tube, then the resulting structure is annealed at a temperature of 110 °C for 60 min.

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