JP2005115192A - Temperature compensation of fiber grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a structure of a fiber grating with temperature compensation in which the wavelength fluctuation of the fiber grating caused by change in temperature is suppressed with high accuracy. <P>SOLUTION: A compressive external force imparting structure formed by a combination of materials having a different thermal expansion coefficient is provided in multistages and combined. Compressive external force of a different amount in a different temperature range is applied to a fiber grating, thereby suppressing the wavelength fluctuation of the fiber grating due to change in temperature. To be concrete, a second stage structure is installed in the rear of the compressive external force imparting structure of the first stage that fixes the fiber grating, and the compressive external force is applied by the first stage structure to the fiber grating in a low temperature range while the compressive external force is applied by the second stage structure to the fiber grating through the adjacent first stage structure in a high temperature range by utilizing the elasticity of adhesive resin for fixing the thermal expansion material in the first stage structure. Thus, different temperature compensations are performed in different temperature ranges, thereby suppressing the wavelength fluctuation over the entire desired temperature range. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a fiber grating having a configuration that suppresses the property that the reflection wavelength of light reflected by a fiber grating formed in a core portion of an optical fiber fluctuates with a change in environmental temperature.

The fiber grating is a filter that gives a periodic refractive index change to the core of the optical fiber, reflects only a specific wavelength of incident light, and passes all other light. The wavelength band spectral width of reflected light is usually extremely narrow. Therefore, by utilizing the feature of having this narrow band characteristic, various devices, apparatuses, and systems that operate by specifying the wavelength with high accuracy are, for example, high Widely applied to various sensors such as density wavelength multiplexing optical communication, fiber laser resonator, temperature and strain.
In these device applications that require high-accuracy wavelength control, naturally, stability against changes in the use environment temperature is strictly required. Although some of them have been incorporated into active devices that control the temperature at a constant level, there is a strong demand for ensuring temperature stability by passive control.

The reflection center wavelength λ ₀ of the fiber grating is determined as λ ₀ = 2n _eff × Λ by the effective refractive index n _eff of the core portion and the refractive index modulation period Λ (grating period). The reflection center wavelength is usually shifted by about 1 nm, that is, about 10 pm / ° C. between −20 ° C. and 80 ° C. mainly due to the positive temperature dependence of the refractive index n _eff . In order to suppress the wavelength fluctuation due to the temperature change, the temperature compensation is performed by controlling the grating period Λ with an external force against the refractive index change. As means for controlling the grating period Λ, (1) a system in which materials having different coefficients of thermal expansion are combined and an external force of compression is applied to the fiber grating in response to a positive temperature change (different thermal expansion coefficient material combination system), (2 ) There are a method in which FBG is directly attached to a negative expansion material to give negative expansion (negative expansion material method), and (3) a hybrid method in which the above (1) and (2) are combined. By these methods, the wavelength temperature change of the fiber grating is suppressed to a wavelength shift amount of approximately 30 to 70 pm, that is, 0.3 to 0.7 pm / ° C. between −20 ° C. and 80 ° C. (for example, non- (See Patent Document 1, Non-Patent Document 2, and Non-Patent Document 3.) This wavelength shift amount is about 3 to 7% in the case without temperature compensation.
Satoshi Inoue, “Fabric grating fabrication and device application”, Optics, vol.32, no.5, pp.317-319, May 2003. GW Yoffe, PA Krug, F. Ouellette, and DA Thorncraft, `` Passive temperature-compensating package for optical fiber gratings '', APPLIED OPTICS, vol.34, no.30, pp.6859-6861, Oct. 1995. Satoshi Inoue, Toru Iwashima, Kazuaki Sakai, Tatsuya Ito, Motoki Tsunoi, Tadashi Enomoto, Hiroo Kanamori, “Development of Fiber Grating for WDM Transmission”, SEI Technical Review, No. 152, pp.30-35, Mar. 1998 .

However, in order to fully develop the characteristics of the fiber grating that has a narrow band characteristic, the shift amount of the center wavelength with respect to changes in the environmental temperature should be further suppressed, and it can be applied to various devices, apparatuses, systems, etc. User demands are very strong. Our goal is to provide a temperature-compensated fiber grating that is much smaller than the wavelength shift achieved so far, preferably to a wavelength shift of less than 30 pm between -20 ° C and 80 ° C. .

Our experimental data on the temperature dependence of fiber gratings under no load is shown in FIG. The reflection center wavelength is 1548.54 nm at a room temperature of 20 ° C., and the amount of variation is represented in the figure based on this value. At first glance, it appears to change linearly with respect to temperature, but it is slightly off the straight line at the middle part of the temperature. That is, it has not only a linear function of temperature but also a dependency of a quadratic function. The actual value data # 1 in FIG. 3 is fitted to a linear function of temperature, and a difference between values calculated from the actual value data and the linear function fitting line is shown in a curve # 2 in FIG.

The materials used in the conventional methods, for example, the above-mentioned (1) different material combination method, (2) negative expansion material method, and (3) hybrid method, are, for example, aluminum, SUS, quartz, etc. It is constant with respect to the temperature in the temperature range from 80 ° C to 80 ° C. This means that the linear function of the temperature change can be compensated. Therefore, even if optimized, the curve # 2 in FIG. 4 is the theoretical limit of the wavelength shift suppression, and the value Is about 25 pm between −20 ° C. and 80 ° C., and it is impossible in principle to suppress it below that. Curve # 3 in FIG. 4 is a result of our experiment in which temperature compensation was performed by a conventional combination method of different materials, and the wavelength shift amount was −36 pm between −20 ° C. and 80 ° C. Even if we further improve our experiments, the limit is about 25 pm, and it is impossible to suppress it below that limit. Whether temperature compensation is performed using a negative expansion material method or a hybrid method is the same as long as compensation is performed with a goal of linear function fitting.

The method that has been most often employed in the past is the dissimilar material combination method shown in FIG. The material 3 is a low coefficient of thermal expansion (for example, quartz glass), the material 4 is a high coefficient of thermal expansion (for example, aluminum), and both are bonded and fixed with a resin 6. This adhesive fixing is made in pairs on both sides, and so on. The part of the fiber grating 2 formed on the fiber 1 is bonded and fixed to the high thermal expansion coefficient material 4 with resin 5 at two places. When the environmental temperature rises, the thermal expansion of the material 4 in the left-right inward direction is larger than the thermal expansion of the material 3 in the left-right outward direction, so that the fiber grating 2 receives an external force of compression. The temperature dependence of the wavelength is cancelled. Optimal temperature compensation can be obtained by appropriately designing the coefficients of thermal expansion of the two materials, their lengths, and the positions and spacings of the resins 5 and 6. In the negative expansion material method (not shown), the fiber formed with the fiber grating is bonded to a base made of a negative expansion material by using resin at two locations, and an external force of compression is applied to the fiber grating. Temperature compensation is obtained. In this case, it is necessary to select a negative expansion material having an optimum negative expansion coefficient. However, if the thermal expansion coefficient of the prepared negative expansion material deviates from the optimum value, the figure is placed on the base of the negative expansion material. It is possible to perform optimum temperature compensation by a hybrid method to which the different material combination method shown in FIG. 2 is applied. However, as described above, the compensation by these conventional methods is a linear function compensation, and it is impossible to suppress the center wavelength fluctuation amount below the curve # 2 in FIG. In fact, in our experiment using the different material combination method of FIG. 2, the curve # 3 of FIG. 4 is obtained, and the curve # 2 is the fundamental limit even if it is skillfully produced.

This principle limit in the conventional method is to cancel the wavelength shift that does not depend on the temperature as a linear function over the entire temperature range to be combined with a material that thermally expands as a linear function with respect to the temperature. Is due to It is impossible to cancel a quadratic function-dependent phenomenon with respect to the temperature by canceling it with a linear function-dependent phenomenon, and it is possible only after performing a quadratic function-dependent compensation of the opposite sign. If the target temperature region is divided into two, for example, a low temperature region and a high temperature region, and temperature compensation is performed separately, compensation as a quadratic function, that is, compensation approximating a quadratic function becomes possible. The limit in the above-described conventional method can be exceeded. However, it is necessary here that the temperature compensation in one temperature region does not disturb the temperature compensation in the other temperature region. This is the origin of the idea of the present invention.

In order to realize this, first, it is necessary to perform temperature compensation separately in two temperature regions, divided into a low temperature region and a high temperature region. If this is applied to the data in FIG. 3 divided into a low temperature region of −20 ° C. to 20 ° C. and a high temperature region of 20 ° C. to 80 ° C., the result is as shown in FIG. Curve # 4 is for the low temperature region of −20 ° C. to 20 ° C., curve # 5 is for the high temperature region of 20 ° C. to 80 ° C., and the multistage compression external force imparting configuration of the present invention is made in two stages. If applied and temperature compensation is performed separately in each temperature region, the wavelength shift amount in the entire temperature region of −20 ° C. to 80 ° C. is theoretically determined by the larger wavelength shift amount in each temperature region. It becomes a limit. The respective wavelength shift amounts are from curve # 4 to 4 pm at −20 ° C. to 20 ° C., and from curve # 5 to 6 pm at 20 ° C. to 80 ° C. In this case, 6 pm is the theoretical limit of the wavelength shift amount in the entire temperature range. It becomes. The wavelength shift amount in the conventional method that compensates the entire temperature range with a linear function is 25 pm as described above with respect to curve # 2 in FIG. Is improved.
If the target temperature region is divided into more than two, and the multistage compression external force application configuration is adopted, the limit value of the wavelength shift amount can be further reduced.

From the experimental results of Examples and Comparative Examples described later, it has been proved that the temperature compensation by the method of the present invention has a very large effect.

According to the first aspect of the present invention, a compression external force applying structure composed of a combination of materials having different coefficients of thermal expansion is used, and a compression external force is applied to the fiber grating fixed thereto from both ends or one end thereof by changing the environmental temperature. In a fiber grating with temperature compensation that suppresses wavelength fluctuation, if a plurality of compression external force applying structures are provided, it is possible to obtain temperature compensation that exceeds the temperature compensation according to the conventional method. This is because different amounts of compression external force are applied to the fiber grating in different temperature regions. In the above-described embodiment, the effect of the present invention was proved by temperature compensation by a combination of two-stage compression external force imparting configurations. However, if the number of stages is increased to three stages and four stages, the assembly work increases in complexity. Naturally, it is expected that the wavelength shift amount can be further reduced than the experimental result of the above-described embodiment.

According to the invention of claim 2, in the compression external force application structure according to claim 1, the material for fixing the materials having different thermal expansion coefficients in the compression external force application structure may be a resin having elasticity at one place or a plurality of places. For example, since different amounts of compression external force are continuously applied to the fiber grating due to the elasticity of the resin in different temperature regions, it is possible to obtain temperature compensation that exceeds the temperature compensation according to the conventional method. Of course, it is necessary that the resilience of the elasticity of the resin does not change due to repeated increases and decreases in temperature.
Next, embodiments of the present invention will be described.

  In order to express the characteristics of the present invention most clearly, we have added a second-stage compression external force application configuration to the one-stage compression external force application configuration employed in the conventional method, and the rear of the first-stage configuration. Due to the elasticity of the resin used, it was possible to obtain different temperature compensation in the low and high temperature regions. Here, the term “rear” means that the compression external force imparting configuration for directly fixing the fiber grating is the first-stage configuration and is located on the outer side. The experimental results in the case where the compression external force imparting configuration is two stages will be described in Examples, and the comparison with the conventional method will be described in Comparative Examples.

The temperature-compensated fiber grating of the present invention produced by the method of the present invention will be described with reference to FIG. 1, FIG. 3, FIG. 4, FIG. The resin coating portion of the optical fiber was removed over 17 mm, and a fiber grating having a length of 10 mm was produced by a phase mask method using ultraviolet laser light. Before fixing as shown in FIG. 1, the center wavelength when measured with a spectrum analyzer at room temperature of 20 ° C. under no load was 15548.54 nm. FIG. 3 shows the result of measuring this wavelength by setting this in a thermostat and changing the temperature from −20 ° C. to 80 ° C. at intervals of 20 ° C. The center wavelength data at each temperature was read for 30 minutes after being maintained at that temperature. In the figure, the deviation from the wavelength at room temperature of 20 ° C. is plotted. The curve # 2 in FIG. 4 is data obtained by plotting the shift amount from the fitting of the wavelength data at each temperature on the basis of 20 ° C. when the linear function fitting is performed on the curve # 3 in this figure. When the temperature compensation of the conventional method is applied, the limit value for suppressing the wavelength shift amount is the difference between the maximum value and the minimum value in the range of −20 ° C. to 80 ° C. from the curve # 2, and 25 pm in this curve. Become. In contrast, according to the method of the present invention, the data in FIG. 3 is divided into two temperature regions, a low temperature region of −20 ° C. to 20 ° C. and a high temperature region of 20 ° C. to 80 ° C. The result of the next function fitting is curve # 4 and curve # 5 in FIG. The figure plots the amount of deviation from the fitting of wavelength data at each temperature, with 20 ° C. being the boundary between the two temperature regions as a reference. When the temperature compensation of the present invention is applied, the wavelength shift amount suppression limit value is the difference between the maximum value and the minimum value of curve # 4 in the low temperature region of −20 ° C. to 20 ° C. (in this case, 4 pm), It is determined by the larger of the difference between the maximum value and the minimum value of curve # 5 (in this case, 6 pm) in the high temperature range of 20 ° C. to 80 ° C. That is, in this case, 6 pm is the limit value for suppressing the wavelength shift amount, and it is clear that this can be greatly improved with respect to the limit value of 25 pm in the conventional method.

Prepare two pieces of thermal expansion materials 4 and 7 each having a rectangular section of 2 mm × 1 mm and a length of 55 mm made of quartz and a rectangular section of 1.5 mm × 1 mm and a length of 10 mm made of aluminum. Then, it was bonded and fixed as shown in FIG. 1 with 6, 8, and 5 using an ultraviolet curable epoxy resin. The range occupied by the resin is about 2 mm in diameter. The distance between the two aluminum thermal expansion materials 4 on both sides of the fiber grating is 15 mm, and both ends of the optical fiber coating removal are embedded in the ultraviolet curable epoxy resin 5. The portion where the thermal expansion material 4 is bonded and fixed to the substrate 3 with the ultraviolet curable epoxy resin 6 is the first-stage compression external force imparting configuration, and the thermal expansion material 7 is bonded and fixed to the substrate 3 with the ultraviolet curable epoxy resin 8. The portion is the second stage compression external force application structure. The aluminum thermal expansion material 4 bonded and fixed to the quartz substrate 3 with the ultraviolet curable epoxy resin 6 by the first stage compression external force imparting configuration thermally expands with increasing environmental temperature on both sides, and the ultraviolet curable epoxy resin. The fiber grating whose left and right sides are fixed at 5 is subjected to compression external force in the contraction direction, and temperature compensation is possible. Further, in the second-stage compression external force imparting configuration, the left and right aluminum thermal expansion materials 7 fixed to the substrate 3 with the ultraviolet curable epoxy resin 8 are heated with reference to the ultraviolet curable epoxy resin 8 as the temperature rises. The external force which expands and pushes the aluminum thermal expansion material 4 which adjoins and adjoins to the fiber grating side generate | occur | produces. When the ultraviolet curable epoxy resin 6 in the first stage compression external force application structure has elasticity due to this external force, the aluminum thermal expansion material 4 is pushed toward the fiber grating, and the fiber grating is In addition to the external force provided by the first-stage compression external force application structure, the external force is further compressed by the external force provided by the second-stage compression external force application structure, and the external force is different from the external force of the first-stage compression external force application structure. Acts to allow for different amounts of temperature compensation. Here, the thermal expansion amount of the aluminum thermal expansion material 7 having the second-stage compression external force application structure fixed to the substrate 3 by the ultraviolet curable epoxy resin 8 is small in the low temperature region and does not work. As a result, separate temperature compensation is possible in the two temperature regions. Optimization of temperature compensation in the entire temperature range of −20 ° C. to 80 ° C. can be performed by adjusting the adhesive fixing position of the ultraviolet curable epoxy resins 8 and 6 to the left and right. The result of this optimization is shown by curve # 6 in FIG. The difference between the maximum value and the minimum value of the wavelength shift in the entire temperature range of −20 ° C. to 80 ° C. is 8 pm, which can be achieved up to 6 pm, which is the theoretical suppression limit value of the present invention described above. The effect was fully proved.

In order to confirm the elastic restoring force of the resin adhesive used, the configuration shown in FIG. 1 is composed of the substrate 3, the left aluminum thermal expansion material 4, and the left ultraviolet curable resin 6 (not shown). ) Was separately assembled, and an experiment was performed in which a load was repeatedly applied from the left side to the left aluminum thermal expansion material 4 using a side pressure machine. Even when the load by the side pressure machine was applied up to 800 gf and this was repeated many times, no deterioration of the applied load was observed, and it was confirmed that the restoring force of the ultraviolet curable resin could be taken sufficiently.

In this embodiment, as shown in FIG. 1, the second aluminum thermal expansion material 7 is arranged symmetrically in the left-right direction.

Comparative example

An example will be described in which the temperature compensation method of the fiber grating, which has been most often employed in the past, is compared with the temperature compensation method according to the present invention. The configuration of the conventional method is shown in FIG. In the fiber grating, as in the above example, the resin coating portion of the optical fiber was removed over 17 mm, and a fiber grating having a length of 10 mm was produced by a phase mask method using an ultraviolet laser beam. Before fixing as shown in FIG. 2, the center wavelength at room temperature of 20 ° C. measured in a no-load state is very similar to that of the above-described embodiment, and its temperature dependency is also curve # 1 of FIG. 3 of the above-described embodiment. Since the fiber grating was fabricated with high reproducibility, the resulting figure is omitted. The theoretical suppression limit value of the wavelength shift amount in the entire temperature range of −20 ° C. to 80 ° C. is the difference between the maximum value and the minimum value in the temperature range on curve # 2 in FIG. 4, that is, 25 pm.

As shown in FIG. 2, the substrate 3 having a rectangular section of 2 mm × 1 mm and a length of 35 mm made of quartz, the thermal expansion material 4 having a rectangular section of 1.5 mm × 1 mm and a length of 10 mm made of aluminum, and Two pieces of 7 were prepared and bonded and fixed at 6 and 5 with an ultraviolet curable epoxy resin. The range occupied by the resin is about 2 mm in diameter. By this combination, a compression external force application structure was formed. The distance between the aluminum thermal expansion materials 4 on both sides of the fiber grating is 15 mm, and both ends of the optical fiber coating removal are buried in the ultraviolet curable epoxy resin 5. The temperature dependence of the wavelength shift amount of the temperature-compensated fiber grating assembled in this way is shown by curve # 3 in FIG. The amount of wavelength shift suppression in the entire temperature range of −20 ° C. to 80 ° C. was 36 pm. The wavelength compensation amount limit value for temperature compensation in this conventional method is 25 pm as described above, and it can be further improved by adjusting the fixing position of the ultraviolet curable epoxy resin 6, but it is highly skilled. Even if it can be improved, 25 pm is the limit value. On the other hand, in the temperature compensation assembled with the two-stage compression external force imparting configuration of the present invention, when the target temperature range is divided into -20 ° C to 20 ° C and 20 ° C to 80 ° C, it is As stated, the theoretical limit was 6 pm and the experimental result was 8 pm. FIG. 6 compares the experimental results of the wavelength shift amount between the conventional temperature compensation and the temperature compensation according to the method of the present invention. Curve # 3 in FIG. 6 is an experimental result of temperature compensation by the conventional method, and curve # 6 is an experimental result of temperature compensation by the method of the present invention. The wavelength shift amount obtained by the conventional method is 36 pm, the wavelength shift amount obtained by the method of the present invention is 8 pm, and it was proved that the wavelength shift amount can be remarkably suppressed by the temperature compensation by the method of the present invention. .

It is a figure which shows the structure of the temperature compensation of the fiber grating of this invention system. It is a figure which shows the structure of the temperature compensation of the fiber grating by a conventional system. It is a figure which shows the amount of center wavelength fluctuations with respect to environmental temperature in the no-load state of a fiber grating. It is a figure which shows the environmental temperature dependence of the amount of wavelength shifts. Curve # 2 is the temperature dependence of the difference between the fitting value and the actual measurement value at each temperature when curve # 1 in FIG. 3 is fitted to a linear function of temperature. Curve # 3 is the temperature dependency of the wavelength shift amount of the temperature-compensated fiber grating manufactured based on the conventional method. It is a figure which shows the environmental temperature dependence of the amount of wavelength shifts. When the curve # 1 in FIG. 3 is divided into a low temperature region of −20 ° C. to 20 ° C. and a high temperature region of 20 ° C. to 80 ° C. and fitted to a linear function of temperature, the fitting value at each temperature and the actual measurement It is the temperature dependence of the difference in values. Curve # 4 is for the low temperature region and curve # 5 is for the high temperature region. Temperature dependency of the wavelength shift amount of the fiber grating with temperature compensation manufactured based on the conventional method (curve # 3) and temperature dependency of the wavelength shift amount of the fiber grating with temperature compensation manufactured based on the method of the present invention (curve) FIG. 6 is a diagram showing # 6).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber 2 Fiber grating 3 Substrate 4,7 Thermal expansion material 5,6,8 UV curable resin

Claims (2)

Wavelength variation of the fiber grating due to environmental temperature change by applying compression external force from both ends or one end to a fiber grating fixed to the compression external force application structure using a compression external force application structure composed of a combination of materials having different coefficients of thermal expansion A temperature-compensated fiber grating and a method for manufacturing the same, wherein two or more stages of the compression external force applying structure are provided at both ends or one end of the fiber grating. 2. The compression external force applying structure according to claim 1, wherein the material for fixing materials having different coefficients of thermal expansion in the compression external force applying structure is made of a resin having elasticity at one or a plurality of positions. Fiber grating and manufacturing method thereof.

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