JP2000292620A - Temperature compensating optical fiber black grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small temperature compensating optical fiber black grating usable even when a grating part is long by setting a linear expansion coefficient of a large annular ring smaller than a linear expansion coefficient of a small annular ring, fitting the outer periphery of the small annular ring in close contact to the inner periphery of the large annular ring, forming an annular groove in the small annular ring, and fixing a black grating part to the annular groove. SOLUTION: This temperature compensating optical fiber black grating has a small annular ring 1 having an inside diameter of ϕ1 and an outside diameter of ϕ2 and a large annular ring 2 having an inside diameter of ϕ2 and an outside diameter ϕ3. Both the small annular ring 1 and the large annular ring 2 have a ring shape having an annular ring central part hollowed out in a concentrically circular shape. The outer periphery of the small annular ring 1 is fitted in close contact to the inner periphery of the large annular ring 2 to form a double ring structure of the small annular ring 1 and the large annular ring 2. A linear expansion coefficient of a material of the large annular ring 2 is set smaller than a linear expansion coefficient of a material of the small annular ring 1. An annular groove 3 being a concentrically circular shape with the ring is formed on an annular surface of the small annular ring 1. A straight line-shaped groove 4 is arranged so that a part of the groove has a common part with a part of a groove of the annular groove 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a temperature compensated optical fiber Bragg grating.

[0002]

2. Description of the Related Art An optical fiber Bragg grating is:
An optical fiber component having a characteristic of reflecting light of a specific wavelength by creating a periodic refractive index change in the length direction of the optical fiber. The reflection wavelength λ can be written as follows using the grating period Λ and the effective refractive index n _eff . λ = 2n _eff · Λ (1)

Accordingly, the temperature dependence of the reflection wavelength, ∂λ / ∂T
Is (∂λ / ∂T) = 2 ｛(∂n _eff / ∂T) Λ {+ (∂Λ / ∂T) ・ n _eff )｝ (2) Further, assuming that the linear expansion coefficient of the optical fiber is α, ∂
Because written as Λ / ∂T = α · Λ, Equation (2) is, (∂λ / ∂T) = 2 {(∂n eff / ∂T) · Λ + α · Λ · n eff)} ··· (3 ). Here, temperature dependence of refractive index of fused silica 石英 n _eff
/ ΔT is + 9.8 × 10 ⁻⁶ and the linear expansion coefficient α of fused silica is 4.0 × 10 ⁻⁷ , so that the reflection characteristics of the optical fiber Bragg grating having a reflection wavelength of 1550 nm Is approximately 0.01 (nm /
K) has a temperature dependency. Therefore, when the temperature changes by 100 ° C. in the range of −20 ° C. to 80 ° C., for example, the reflection wavelength of the optical fiber grating becomes 1.0.
It will fluctuate by about nm. In wavelength multiplex communication, since it is necessary to simultaneously transmit several types of wavelengths with a wavelength interval of about 0.8 nm, the above-described temperature dependence of the optical fiber Bragg grating characteristics is a significant problem.

In order to reduce the temperature dependence of such an optical fiber Bragg grating, a temperature-compensated optical fiber Bragg grating 2 as shown in FIG.
0 have been proposed (GW Yoffe et al., Opt.
Ial Fiber Communication
Technical Digest W14, pp13
4-135, 1995). This temperature-compensated optical fiber Bragg grating 20 is a combination of two types of members having different linear expansion coefficients. One member is a pedestal 21 having a smaller linear expansion coefficient, and the other member is a pedestal having a smaller linear expansion coefficient. This is a large beam 22. The grating portion 23 of the optical fiber 15 in the optical fiber Bragg grating 20 includes beams 22 at both ends of the pedestal 21.
Between the optical fiber fixing portions 16, 16 while being tensioned. For example, when the temperature around the installed grating portion 23 rises, the beam 22 expands more than the pedestal 21, and is thus fixed between the optical fiber fixing portions 16 on the beam 22. The tension of the grating section 23 is relaxed, and the grating section 23 between the optical fiber fixing sections 16 and 16 contracts. That is, the apparent linear expansion coefficient of the grating unit 23 becomes a negative value, and the value of the second term on the right side of the above equation (3) becomes negative. Therefore, the linear expansion coefficient and the length of the pedestal 21 and the beam 22 are appropriately adjusted so that the second term on the right side of the equation (3) has the same absolute value as the first term on the right side of the equation (3) and takes a negative value. Thereby, the temperature dependency of the optical fiber Bragg grating can be canceled.

[0005] However, when the length of the grating portion 23 is about 10 cm to 1 m or more, in order to perform the above-mentioned conventional temperature compensation especially in an ultra-narrow band fiber Bragg grating or dispersion compensation fiber Bragg grating, The component for performing the temperature compensation is about twice as long as the grating 23. Therefore, in the production of such a part, distortion or the like of the part occurs due to its length, and the production itself is difficult, or even if it can be produced, it is difficult to use because of its size, There is also a problem in terms of accuracy.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a small temperature-compensated optical fiber Bragg grating which can be used even when the grating section is long. And

[0007]

SUMMARY OF THE INVENTION In order to solve the above problems and achieve the object, an optical fiber Bragg grating according to the present invention is configured as follows. That is, the optical fiber Bragg grating of the present invention is composed of a large ring and a small ring, and the linear expansion coefficient of the material forming the large ring is smaller than the linear expansion coefficient of the material forming the small ring. The outer periphery of the small ring is closely fitted to the inner periphery of the ring, an annular groove is formed in the small ring, and the Bragg grating portion of the optical fiber is fixed to the annular groove. Is what you do. In the temperature compensated optical fiber Bragg grating of the present invention, it is preferable that the large ring is made of metal and the small ring is made of resin. Further, in the temperature compensated optical fiber Bragg grating of the present invention, it is preferable that the metal is titanium and the resin is a fluororesin.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.

FIG. 1A is a diagram showing an example of a temperature-compensated optical fiber Bragg grating according to the present invention. In the figure, reference numeral 1 denotes a small ring having an inner diameter of φ ₁ and an outer diameter of φ ₂ , 2
Denotes a large ring having an inner diameter of φ ₂ and an outer diameter of φ ₃ . 1B is a longitudinal sectional view taken along a dotted line AA 'in FIG. 1A, and FIG. 1C is a side view of the temperature-compensated optical fiber Bragg grating shown in FIG. . Each of the small ring 1 and the large ring 2 has a ring shape in which the center of the ring is concentrically cut out. The outer periphery of the small ring 1 is closely fitted to the inner periphery of the large ring 2, and the small ring 1 and the large ring 2 form a double ring structure. The material forming the large ring 2 has a smaller coefficient of linear expansion than the material forming the small ring 1. An annular groove 3 which is concentric with the ring is formed on the annular surface of the small ring 1. A linear groove 4 is further formed in the small ring 1 and the large ring 2, and the linear groove 4 is formed such that a part of the groove has a common part with a part of the groove of the annular groove 3. It is provided on the annular surface of the ring 1 and the large ring 2. Large ring 2
The upper straight groove 4 has a wider groove than the straight groove 4 on the small ring 1, so that the optical fiber can be easily taken out from the annular groove 3. The linear groove 4 is open at both ends to the outside of the large ring 2. The optical fiber Bragg grating is fixed to the small ring 1 and the large ring 2 as follows. First, the grating portion of the optical fiber Bragg grating (not shown in FIG. 1) is formed into an annular shape in accordance with the shape of the annular groove 3 and housed in the annular groove 3. When the length of the grating portion is longer than one circumference of the annular groove 3, the grating portion is wound around the circumference of the annular groove 3 a plurality of times, and the entire grating portion is housed in the annular groove 3. After the both ends of the grating portion of the optical fiber Bragg grating intersect at the common portion of the linear groove 4 and the annular groove 3, the linear grooves 4 are separately formed.
To complete the fitting of the optical fiber Bragg grating. That is, the end of the grating portion on the side where the grating portion is reached when the grating portion accommodated in the annular groove 3 is turned clockwise from the shared portion of the annular groove 3 and the linear groove 4 in FIG. ) Through the upper side of the linear groove 4. On the other hand, FIG.
The end of the grating portion, which is reached when the grating portion accommodated in the counterclockwise direction is reached, is shifted from the shared portion of the annular groove 3 and the linear groove 4 to the lower side of the linear groove 4 in FIG. Through. At this time, in order to achieve temperature compensation, it is necessary to fix the optical fiber so that the optical fiber including the grating section contracts and expands together with the contraction and expansion of the annular groove 3. There are the following two types of optical fiber fixing methods for achieving the above conditions. One is to put the optical fiber including the grating portion into the annular groove 3 while applying tension to the optical fiber so that the optical fiber is in tight contact with the inner wall of the annular groove 3 in a state where tension is applied to the optical fiber. The optical fiber is fixed at. By doing so, when the ring is contracted, the tension is relaxed and the optical fiber itself is contracted, and when the ring is expanded, the optical fiber is expanded together with the expansion of the inner wall of the ring. However, it is necessary to apply an appropriate tension so that the tension applied to the optical fiber in the temperature compensation range does not become zero. Second, there is a method in which an optical fiber including a grating portion is put in the annular groove 3 and then the entire optical fiber is fixed in the annular groove 3. Typically, this can be achieved by filling the annular groove 3 containing the optical fiber with a thermosetting epoxy resin or the like and solidifying the resin. The optical fiber embedded in the resin also contracts and expands as the ring contracts and expands. When this method is used, it is not necessary to fix the optical fiber with the linear groove 4.

The theory of how the temperature compensation is realized in the temperature compensated optical fiber Bragg grating having the above configuration will be described below. Generally, when there is a temperature rise in an object having a double ring structure such as the temperature-compensated optical fiber Bragg grating 10 of the present invention, the small ring has an inner diameter r as shown in FIG.
_a, an outer diameter of r _b, can be regarded as P _a from inside a circle under pressure outside than P _b ring. In this state, the radial displacement u at the radius r is represented by the following equation (4).

[0011]

(Equation 1)

In the equation, E is Young's modulus, and ν is Poisson's ratio. Here, a case similar to the temperature compensated optical fiber Bragg grating of the present invention as shown in FIG. 3 is considered. FIG. 3A is a longitudinal sectional view of each of the small rings 1 and 2 in a state where the small ring 1 and the large ring 2 are not fitted, and shows one half from the center axis of the rings. is there. If there is a temperature rise ΔT in this state, the small ring 1 and the large ring 2 thermally expand as shown in FIG. As described above, in the present invention, the expansion of the small ring 1 is larger than the expansion of the large ring 2, and therefore α ₁ > α ₂ . Therefore, when the small ring 1 is fitted to the large ring 2, the radius becomes R at the fitted portion as shown in FIG. 2). At this time, P _a = 0 [N / m ² ] and P _b =
P [N / m ^2], and by substituting r = r _b [m], a small circular ring 1
The outer peripheral displacement u _b [m] of is given by the following expression (5).

[0013]

(Equation 2)

Where r _a [m] is the inner circumference of the small ring 1 and r
_b [m] is the outer circumference of the small ring 1. Similarly the inner circumference of the displacement u _c [m] of the great circle ring 2 and _{the, P a = P [N /} m 2], P b = 0 [N
/ M ^2], and r = sought by substituting r _c [m], and becomes the following equation (6).

[0015]

(Equation 3)

Where r _c [m] is the inner circumference of the great ring 2 and r
_d [m] is the outer circumference of the large ring 2. Where the absolute value of the displacement |
The sum of u _b | [m] and | u _c | [m] corresponds to the amount of overlap between the small ring 1 and the large ring 2 due to a temperature change. Since the displacement at the radius r when the temperature changes by ΔT [K] in the ring having the expansion coefficient α is αrΔT [m], | u _b | + | u _c | = (α ₁ −α ₂ ) RΔT・ (7) is obtained. Further, since substantially equal to the r _c [m] and _{r b [m], r b} = r c
= R [m], the pressure P [N / m ² ] generated at the fitting portion of the ring due to the temperature change ΔT [K] is obtained from the above equations (5), (6) and (7). The following equation (8) is obtained.

[0017]

(Equation 4)

[0018] substituting this pressure P [N / m ^2] to P _b of formula _{(4), P a = 0} [N / m 2], When r = r _a [m], the ring due to the temperature change ΔT innermost displacement u _a [m] is to become the following equation (9).

[0019]

(Equation 5)

Here, P [N / m ² ] is the value of equation (8). Total displacement u of the innermost circumference of the ring due to temperature change
_T [m] is an expansion α ₁ r _a ΔT due to heat, the sum of the displacement u _a [m] produced by that there are great circle ring 2 by the following expression (10).

[0021]

(Equation 6)

Since the contraction rate in the circumferential direction is expressed by u / r, the apparent linear expansion coefficient α _T at the innermost circumference of the ring is
The following equation (11) is obtained.

[0023]

(Equation 7)

By making the materials and shapes of the small ring 1 and the large ring 2 appropriate, the vicinity of the innermost diameter of the small ring 1 becomes more inward than the temperature before the temperature rise as shown in FIG. The apparent linear expansion coefficient α _T represented by the equation (11) can take a negative value. Since the temperature dependency of the optical fiber Bragg grating is expressed by the above equation (3), the grating shrinks with the temperature rise as described above,
(3) The first term on the right side of the equation may be canceled by the second term. The contraction rate required at this time, that is, the linear expansion coefficient is-
It is 6.7 × 10 ^-6 . Therefore, an annular groove is provided at a position where the apparent linear expansion coefficient αT of the small ring 1 becomes −6.7 × 10 ^−6, and an optical fiber Bragg grating is fixed in this groove to reduce the contraction and expansion of the surroundings. In addition, the optical fiber Bragg grating has a structure in which the optical fiber Bragg grating contracts and expands, thereby achieving temperature compensation.

Here, when the small ring 1 is made of aluminum and the large ring 2 is made of invar, when the thickness in the radial direction of each ring changes, the apparent appearance at the innermost peripheral portion of the small ring 1 is obtained. Consider the change in the coefficient of linear expansion αT. Table 1 shows the linear expansion coefficient, Young's modulus, and Poisson's ratio of Invar and aluminum. The inner circumference of the small ring 1 is 30 mm, and the thickness of the small ring 1 (the difference between the outer radius and the inner radius) is 3c.
FIG. 4 shows a change in the linear expansion coefficient with respect to the thickness (difference between the outer radius and the inner radius) of the large ring 2 when m, 6 cm, and 10 cm are set. FIG. 5 shows changes in the coefficient of linear expansion with respect to the thickness (same as above) of the small ring 1 when the same was set to 10 cm, 12 cm and 15 cm. As is clear from FIG.
If the thickness of the large ring 2 is not set to a certain value or more, the small ring 1
Α = -6.7 × 10 ^-6 regardless of the thickness of
Is not realized. As a result of examination based on the data of FIGS. 4 and 5, the thickness of the large ring 2 made of invar was set to 11 cm.
It can be seen that temperature compensation can be realized by setting the thickness of the small ring 1 made of aluminum to 4 cm, shrink-fitting the small ring 1 into the large ring 2 and fixing the optical fiber to the innermost periphery thereof.

[0026]

[Table 1]

In the above-described example, the annular groove 3 is provided on the annular surface of the small annular ring 1. However, in order to perform temperature compensation in the temperature-compensated optical fiber Bragg grating of the present invention, the location of the annular groove 3 is required. Need not necessarily be an annular surface. An annular groove 3 can be provided on the inner peripheral surface of the small ring 1. However, in this case, the annular groove 3 and the annular surface of the small ring 1 need to be concentric.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The small ring 1 has a linear expansion coefficient α = 8.0 × 10
^The optical fiber Bragg grating shown in FIG. 1 was prepared by using a fluorine resin of ^-5 and titanium having a linear expansion coefficient of α = 8.6 × 10 ^-6 for the large ring 2.
The size and arrangement of the linear groove and the annular groove are as shown in FIG. 1, and the unit in the figure is mm.
The Young's modulus and Poisson's ratio of titanium are shown in Table 1 above. In this embodiment, the inner diameter φ1 of the small ring ₁ is fixed to 50 mm. Regarding the outer diameter φ ₂ of the small ring 1 and the outer diameter φ ₃ of the large ring 2, each of the three values shown in Table 2 below was prepared (ring # 1 to ring # 3). A grating portion having a length of 10 cm is fitted into the annular groove 3, and both ends thereof are crossed by a common portion of the annular groove 3 and the linear groove 4, and passed through to the end of the linear groove 4, and heat is applied to the entire annular groove 3. An optical fiber including a grating portion was fixed by filling a curable epoxy resin and solidifying the resin. As a control, the temperature characteristics of the optical fiber grating without the temperature compensation structure are shown. The case where the temperature compensation structure is not adopted here is as follows.
A state in which external factors other than temperature are eliminated as much as possible.

[0028]

[Table 2]

The temperature dependence of the reflection center wavelength of the optical fiber Bragg grating in each of rings # 1 to # 3 was measured. The measurement was performed between -20 ° C and 80 ° C, and the central reflection wavelength at -20 ° C was set to 0. The measurement results are shown in FIG.

As can be seen from FIG. 6, when temperature compensation is not performed, the center wavelength of reflection greatly changes to the longer wavelength side as the temperature compensation increases.
In, the reflection center wavelength falls within the range of 0.05 to 0.25 nm even when the temperature changes from -20 to 80C. Especially in the ring # 3, the temperature range is -20 to 80 ° C.
As a result, temperature compensation with a displacement of the center wavelength of 0.05 or less was obtained.

[0031]

The temperature-compensated optical fiber Bragg grating of the present invention has its grating section fitted in the annular groove, so that it can be made smaller than conventional linear temperature-compensated optical fiber Bragg gratings. It is. Therefore, the ease of handling is improved. In addition, along with the miniaturization, it is possible to prevent a decrease in accuracy due to distortion of members, which has been a problem when the conventional linear temperature-compensated optical fiber Bragg grating is increased in size, and therefore, the grating portion. Even if the length is long, it is possible to realize more accurate temperature compensation.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure of a temperature compensated optical fiber Bragg grating of the present invention.

FIG. 2 is a diagram showing a state in which an internal pressure and an external pressure are applied to a ring.

FIG. 3 is a cross-sectional view showing a change in the radial direction of the ring when the temperature of the temperature compensated optical fiber Bragg grating of the present invention rises.

FIG. 4 is a graph showing a change in a linear expansion coefficient of an optical fiber Bragg grating when the thickness of a small ring is fixed and the thickness of a large ring is changed.

FIG. 5 is a graph showing a change in a linear expansion coefficient of an optical fiber Bragg grating when the thickness of a small ring is changed while the thickness of a large ring is fixed.

FIG. 6 is a graph showing the reflection center temperature dependence of optical fiber Bragg gratings in rings # 1 to # 3 of the example.

FIG. 7 is a diagram showing a conventional temperature compensated optical fiber Bragg grating.

[Explanation of symbols]

1 ... small ring, 2 ... large ring, 3 ... annular groove, 1
0 ... Optical fiber Bragg grating, 23 ...
・ Grating part

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Akira Wada, Inventor 1440, Misaki, Sakura City, Chiba Prefecture Fujikura Co., Ltd. Sakura Plant F-term (reference) 2H038 BA24 BA25 2H049 AA02 AA43 AA51 AA59 AA68 2H050 AC82

Claims (3)

[Claims] 1. A material comprising a large ring and a small ring, wherein the material forming the large ring has a smaller linear expansion coefficient than that of the material forming the small ring. A temperature compensation optical fiber Bragg, wherein an outer periphery of the small ring is closely fitted, an annular groove is formed in the small ring, and a grating portion of the optical fiber is fixed to the annular groove. Grating. 2. The temperature compensated optical fiber Bragg grating according to claim 1, wherein said large ring is made of metal and said small ring is made of resin. 3. The temperature compensated optical fiber Bragg grating according to claim 2, wherein said metal is titanium and said resin is a fluororesin.