WO2001001174A1

WO2001001174A1 - Method for making fiber grating, component for optical communication, and temperature sensor

Info

Publication number: WO2001001174A1
Application number: PCT/JP2000/004219
Authority: WO
Inventors: Kazuo Imamura; Takeshi Genji; Norio Naka; Satoshi Uramatsu; Katsuaki Kondo
Original assignee: Mitsubishi Cable Industries, Ltd.
Priority date: 1999-06-29
Filing date: 2000-06-27
Publication date: 2001-01-04

Abstract

A method for making a fiber grating by increasing the photosensitivity of glass and writing a grating. The outer surface of an optical fiber is covered with a cover layer formed of ultraviolet-transmitting resin. Ultraviolet radiation is applied to the core in which the distortion in the axial direction is in the range from +0.8% to +6% to write a grating. As a result, the rate of formation of a grating is improved, and the reflectance of the grating after the tension is released is increased. A method for making a fiber grating preferably used for a temperature sensor having a simple structure and adapted for measuring temperature ranging to cryogenic temperature and a temperature sensor are also disclosed. Before the step of forming the cover layer covering the optical fiber, the material of the resin and the thickness of the cover layer are determined so that the rate of change (temperature coefficient) of the reflection peak wavelength of the grating to be produced with temperature may be a predetermined value.

Description

Description Fiber grating manufacturing method, optical communication components, and temperature sensor

The present invention relates to a method for manufacturing a fiber grating in which the core of an optical fiber has a striped refractive index distribution, an optical communication component having a fiber grating, and a temperature sensor. Background art

A fiber grating is manufactured by forming a periodic refractive index modulation structure on the core of an optical fiber by a two-beam interference method or a phase mask method (Japanese Patent Application Laid-Open No. Hei 6-235808, Japanese Patent Application Laid-Open No. — Refer to Japanese Patent Application Laid-Open No. 140301 / Patent No. 2521078). In such a fiber grating, a coherent ultraviolet laser beam is applied to silica glass (core) doped with germanium (Ge) to cause a photo-induced refractive index change in a corresponding portion, thereby causing a change in the refractive index. We are creating (writing) a grating structure. By changing the period of the refractive index change and the modulation form, applications to filters, demultiplexers, dispersion compensators, fiber laser mirrors, EDF gain equalizers, resonators, temperature sensors, etc. are considered. It has been. Fiber gratings used for these various applications are required to have predetermined transmission characteristics as a required function, and naturally have predetermined mechanical strength characteristics regardless of the application. Is important for practical use.

However, when trying to satisfy the transmission characteristics, the mechanical strength characteristics are sacrificed, and processing to complement them is required.On the other hand, when trying to satisfy the mechanical strength characteristics, the transmission characteristics are sacrificed. It is difficult to achieve both transmission characteristics and mechanical strength characteristics. The reason will be described below.

The optical fiber on which the grating is written is typically a core and a cladding. The outer peripheral surface of the optical fiber consisting of the following is coated with a coating layer of an ultraviolet curable resin that absorbs ultraviolet rays and causes a curing reaction. Even if an interferometry or a phase mask method is used, the mechanical strength characteristic tends to decrease because the coating is usually performed in a state where the coating layer of the portion to be written is removed. For this reason, after the writing of the grating is completed, the portion where the coating layer is removed is re-coated. However, in order to perform the re-coating, a processing technique for re-coating such as re-coating or packaging is required. When the coating layer is removed, the outer surface of the optical fiber (the outer surface of the clad) comes into contact with the outside air, and the optical fiber is deteriorated due to contact with air during the writing operation. Transmission characteristics may be degraded. In addition, the removal of the coating layer at the portion to be written is performed not by mechanical means but by a chemical treatment of dissolving with a chemical, for example, to prevent damage to the optical fiber. Therefore, it is a factor that hinders the efficiency of mass processing of grating writing.

On the other hand, instead of removing the coating layer to write the grating after the formation of the coating layer as described above, it has been attempted to write the grating with a so-called single line using a so-called single pulse before forming the coating layer. In this case, although it has been shown that strength deterioration does not occur as described above (V. Hagemann et al, Mechanical resistance oi draw-tower-Bragg-grating sensors, Electron. Lett. , 34, pp. 211-212, 1998), and the degree of increase in the refractive index due to UV irradiation is low, and the reflectivity of the written grating is correspondingly low.

Also, even after the formation of the coating layer, the effective writing of the grating by irradiating ultraviolet rays from outside the coating layer without removing the coating layer requires the core of the optical fiber to be written. It is conceivable to increase the sensitivity (photosensitivity) to the light-induced refractive index change of the part. As a method of increasing this photosensitivity, that is, a method of causing a relatively large change in the refractive index induced by light, as a core to be written, a normal density (a relative refractive index difference between the core and the clad is, for example, 0.9%). Higher than the density (the relative refractive index difference is, for example, 1.5 to 2.0%) It has been proposed to use a Ge-doped core of about the same concentration) or to use a core doped with normal concentration of Ge and then filled with hydrogen under high pressure (Transactions of the Institute of Electronics, Information and Communication Engineers). Vol. J79.1, No.11, pp. 415, January 1996.

However, when a fiber grating is manufactured using a core doped with a high concentration of Ge, a normal specification optical fiber connected (fused) to the fiber grating has a core doped with a normal concentration of Ge. Therefore, there is a disadvantage that matching between the two cores cannot be achieved, and the connection loss increases due to the difference in the Ge doping concentration.

On the other hand, when fabricating a fiber grating using a core filled with high-pressure hydrogen, bubbles may be generated between the glass part and the coating layer, which may cause a problem that the adhesion of the coating layer to the glass part is reduced. is there. If the degree of adhesion of the coating layer is poor, the mechanical strength of the optical fiber core is reduced, and the optical fiber is not practical. Therefore, there is a need to develop a grating writing method that can reduce the Ge concentration as much as possible or make it possible to relax or omit high-pressure hydrogen filling.

Also, among the fiber gratings, short-period gratings with a grating pitch of about 1 m or less (Fiber plug grating: also called FBG) reflect light of a specific wavelength (peak wavelength) corresponding to the grating pitch. Characteristics. Assuming that the grating pitch is P and the effective refractive index is n, the peak wavelength B of the reflected light from the grating is expressed by the following equation (1).

B = 2nP (1) The peak wavelength of the reflected light from the fiber grating The temperature sensor that measures the temperature by detecting the change in the person B is, for example, a water wave, 0 plus E, No. 2 0 5, 8 1-84 (1996) and Gupta et al., Applied 0 ptics, 35 (2 5), 52 0 2-52 0 5 (1996) ing. When the temperature of the fiber grating changes, the grating pitch P and the refractive index change, and the reflection peak wavelength; IB changes according to the following equation (2).

Δ ΛΒ / λΒ = (+ ξ ) Δ Τ (2) where the flight is a value in the vicinity of room temperature thermal expansion coefficient (quartz fiber 0. 5 5 x 1 0 one ⁶ / Deg) and is, changes with the thermo-optic coefficient (about 8 X 1 0- ⁶ / deg, the temperature and G e concentration representing the temperature change of the refractive index), delta T represents the width of the temperature change, respectively. Since the thermal expansion coefficient of quartz fiber is small, the reflection peak wavelength of the quartz fiber grating changes mainly due to the temperature change of the refractive index.

The conventional temperature sensor (for use near room temperature or for cryogenic temperature) disclosed in the above-mentioned document is used for a substrate with a large coefficient of thermal expansion (for example, aluminum substrate or acrylic substrate) in order to improve the sensitivity of the sensor. The quartz fiber was fixed. As a temperature sensor for high temperature (0 ° C to 800 ° C), a temperature sensor having a structure in which a fiber is not fixed to a substrate is also known.

However, conventional temperature sensors using fiber gratings have the following problems.

If a structure in which the fiber is fixed to the substrate (hereinafter, referred to as “package structure”) is adopted, the sensor becomes relatively large and it becomes difficult to deform (for example, bend) the external shape of the sensor. There is a problem that the place to install is limited. In addition, since the conventional temperature sensors for low temperature and normal temperature described above use uncoated fibers, the mechanical strength at cryogenic temperatures is weak, handling is difficult, and it is difficult to use for a long period of time. there were. In particular, in a sensor having a package structure, the fiber has a large difference in thermal expansion coefficient between the substrate and the fiber. Furthermore, since the rate of change of the reflection peak wavelength of the sensor with respect to temperature changes greatly depending on the temperature, there is a problem that the measurable temperature range is narrow.

The present invention has been made in view of the circumstances described above, and one of the main objects of the present invention is to provide a method of manufacturing a fiber grating in which a grating is written in a state where the photosensitivity of glass is increased. To provide.

Another main object of the present invention is to provide a component for optical communication that can more effectively utilize the performance of the fiber grating manufactured as described above.

Further, another main object of the present invention is to provide a fiber grating which can be suitably used for a temperature sensor capable of measuring a temperature up to a very low temperature with a simple structure without requiring a package structure. —To provide a manufacturing method of a laser and a temperature sensor using such a fiber grating. Disclosure of the invention

A method of manufacturing a fiber grating according to the present invention includes a step of covering an outer peripheral surface of a fiber including a core on which a grating is to be written and a cladding surrounding the core with a coating layer formed of an ultraviolet-transmissive resin; Writing a grating in the core by irradiating the core from outside of the coating layer, wherein the grating is written in the core. The core is irradiated with the ultraviolet ray in a state where a strain of + 0.8% or more and + 6% or less is generated in the direction.

In the step of writing the grating on the core, it is preferable to execute the writing of the grating while applying an axial tension to the core. After executing the writing of the grating, it is preferable to release the axial tension. .

An optical communication component according to the present invention is characterized by comprising: a fiber grating manufactured by any one of the fiber grating manufacturing methods described above; and means for supporting the fiber grating.

It is preferable that the means for supporting the fiber grating supports the fiber grating so that a strain smaller than an axial strain applied to the core is generated in a step of writing the grating on the core.

The method for manufacturing a fiber grating according to the present invention is a method for manufacturing a fiber grating, comprising: a fiber strand having a core and a cladding; and a coating layer covering a surface of the fiber strand. A step of forming a coating layer covering the surface of the fiber strand using a resin material; and a step of changing the reflection peak wavelength of the grating to be manufactured to a temperature change before the step of forming the coating layer. And a coating layer designing step of selecting the resin material and setting the thickness of the coating layer so that the predetermined value is obtained as a predetermined value. In the coating layer designing step, the selection of the resin material and the setting of the thickness of the coating layer are performed based on the elastic modulus and the thermal expansion coefficient of the fiber and the elastic modulus and the thermal expansion coefficient of the resin material. The process may be performed.

The rate of change of the reflection peak wavelength with respect to temperature change may be constant in the range of 196 ° C. to + 170 ° C.

The rate of change of the reflection peak wavelength with respect to temperature change is the change of the reflection peak wavelength of the grating formed on the uncoated fiber strand with respect to the temperature change at −20 ° (: to + 60 ° C.). It may be the same as the rate.

The temperature sensor according to the present invention includes: a fiber element having a core and a clad; a fiber grating having a coating layer covering a surface of the fiber element; a light source that emits light to the fiber element; A temperature sensor that receives the reflected light from the fiber grating and detects a wavelength of the reflected light, wherein a rate of change of the wavelength of the reflected light with respect to a temperature change is from −196 ° C. It is constant within the range of 170 ° C. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram illustrating a principle of manufacturing a fiber grating according to an embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of the optical fiber.

FIG. 3 is a schematic diagram showing a manufacturing apparatus.

FIG. 4 is a graph showing the relationship between the applied tension at the time of writing the grating and the grating fabrication time.

FIG. 5 is a graph showing the relationship between the applied tension at the time of writing the grating and the grating formation speed.

FIG. 6 is a graph showing the relationship between the tension applied to the fiber to be irradiated, the final arrival reflectance during fabrication, and the reflectance when the tension of the fiber is released after fabrication. Figure 7 is a graph showing the relationship between the applied tension and the increase (%) in reflectance due to release of the tension.

FIG. 8 is an enlarged explanatory view of the tension applying mechanism of FIG. FIG. 9 is an enlarged sectional view taken along line AA of FIG.

FIG. 10 is a diagram showing a positional relationship between an optical fiber core and a cylindrical lens system.

FIG. 11 is a graph showing the temperature dependence of the reflection peak wavelength of the fiber grating according to the embodiment of the present invention.

FIG. 12 is a schematic diagram showing a temperature sensor using the fiber grating according to the embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]

An embodiment of a method for manufacturing a fiber grating that writes a grating while increasing the photosensitivity of glass will be described.

First, refer to FIG. FIG. 1 shows an optical fiber core 1 of a predetermined length to be subjected to the grating writing. The optical fiber core 1 is composed of a core 2 on which a grating 21 is written, a cladding 3 formed around the core 2, and a coating layer 4 covering the outer surface of the cladding 3. I have. As shown in FIG. 2, the coating layer 4 was coated on the optical fiber 1 ′ composed of the core 2 and the clad 3 made by drawing from the optical fiber preform. Things. Ultraviolet laser light as ultraviolet rays is irradiated from outside the coating layer 4 through the phase mask 5, so that the core 2 of the optical fiber core 1 has a periodic refractive index modulation in the fiber axis direction. The stripes (gratings) are written to create a fire bug rating. The interval between these many refractive index modulation fringes is the grating pitch. In order to effectively perform writing of the grating by irradiating the ultraviolet laser beam from outside the coating layer 4 as described above, it is preferable to adopt a special configuration as the core 2 and the coating layer 4 as described below. . The core 2 is doped with Ge having a concentration similar to that of the Ge contained in the core of the normal specification optical fiber. Here, the normal specification optical fiber is an optical fiber core connected to the optical fiber core 1. Such an optical fire The core of the core wire is usually doped with Ge so that the relative refractive index difference is about 0.9%.

In the core 2 of the optical fiber core 1 shown in the figure, in addition to Ge, Sn,! ! Preferably, the core 2 is doped with dopants of Sn, Al, and B. For example, in addition to the same amount of Ge as the above-mentioned core of the optical fiber of the normal specification (an amount such that the relative refractive index difference becomes 0.9%), a concentration of 1000 O ppm or more, preferably 10,000 to 15000 ppm Sn or Sn at such a concentration and A 1 at a concentration of 1000 ppm or less may be co-doped. Such de one-flop may be performed by various known methods, for example, when carried out by immersion, the compound of the Sn (For Sn, for example, _{S n C 1 2 · 2 H} 2 0) of methyl alcohol And dipped in the solution.

The coating layer 4 is formed so as to have a thickness of at least about 30 μm by a single coating method following the step of drawing the optical fiber 1 ′ including the core 2 and the clad 3. In the present embodiment, the material of the coating layer 4 has both a property of curing with ultraviolet light of a certain wavelength band (first ultraviolet light) and a property of transmitting ultraviolet light of another wavelength band (second ultraviolet light). Use the resin that was used. Such a resin may be referred to as “ultraviolet-transmissive ultraviolet-curable resin” in the present specification.

The ultraviolet-transmissive ultraviolet-curing resin transmits at least ultraviolet rays of a specific wavelength band (for example, a wavelength band of 240 nm to 270 nm) to be irradiated to the core for writing the grating 21 (preferably, almost all of the ultraviolet rays are absorbed) On the other hand, it absorbs ultraviolet light having a wavelength shorter or longer than the specific wavelength band to cause a curing reaction. In other words, although the same resin has different UV absorption characteristics depending on the wavelength, it is a UV-transmitting type resin in a specific wavelength band, and a UV-curable resin in a shorter or longer wavelength range than the above specific wavelength band. Thus, the coating layer 4 is formed.

In the present embodiment, for example, a photoinitiator (a photo-initiator (a photo-initiator) that initiates and promotes a curing reaction by receiving ultraviolet rays in a wavelength range shorter than 240 nm or a wavelength range longer than 270 nm is applied to urethane acrylate or epoxy acrylate. Initiator Evening) The resin containing is used as “ultraviolet transparent ultraviolet curing resin”.

After coating the outer peripheral surface of the optical fiber with such a resin layer, first, the coating layer is irradiated with first ultraviolet rays to cure the coating layer 4.

In the present embodiment, an ultraviolet curing resin is used. However, when another type of resin is used, the first ultraviolet irradiation step is omitted, and another resin curing step (for example, a curing step using heat) is performed. Will be executed.

It is preferable to fill the core 2 with hydrogen before irradiating the second ultraviolet ray to the optical fiber core 1 covered with the cured coating layer 4 in order to increase the photoinduced refractive index change. . Therefore, in this embodiment, this high-pressure hydrogen filling is performed. Specifically, the optical fiber core 1 is placed in a sealed container filled with hydrogen and left at room temperature under a pressure of about 2 OMPa for about 2 weeks.

After the above-mentioned high-pressure hydrogen filling is performed, the core 2 is written with the grating 21 by irradiating the second ultraviolet ray from outside the optical fiber core wire 1, that is, outside the coating layer 4. .

The writing of the grating 21 may be performed by using various known methods. For example, when the phase mask method is used, a lattice-shaped phase mask 5 is disposed immediately before the optical fiber core 1 as shown in FIG. On the other hand, the Nd-YAG laser source 6 may irradiate, for example, a coherent ultraviolet laser beam having a fourth harmonic (4ω) of 2666 nm, which is condensed by the cylindrical lens system 7. As a result, the ultraviolet laser light passes through the phase mask 5 and the coating layer 4, the refractive index of the portion of the grating pitch corresponding to the grating pitch of the phase mask 5 with respect to the core 2 is increased, and the Bragg grating 21 is written. Will be. In FIG. 3, reference numeral “8” is a beam expander that expands the ultraviolet laser beam into a parallel beam, and reference numeral “9” is a portion where the power of the above-mentioned parallel laser beam is uniform. The slit “10” is a movable reflecting mirror that can be moved in the longitudinal direction of the optical fiber core wire 1 (see the dashed line arrow), and the symbol “1 1” is light. Spectrum analyzer, reference number “1 2” is optical isolator, reference number “1 3” is optical power blur.

In writing the grating according to the present embodiment, the tension applying mechanism 30 shown in FIG. Is used to apply tension in the long axis direction to the optical fiber 1 to be written. The specific method and effect of such tension application will be described later in detail with reference to the drawings.

For example, an Nd—YAG laser source 6 (see Fig. 3) with a maximum average power of 100 mW, a pulse width of 50 ns, and a pulse frequency of 10 Hz can be used as an ultraviolet light source that can be used for grating writing. . Ultraviolet laser light of 266 nm, which is the fourth harmonic of this Nd-YAG laser, is irradiated onto the optical fiber core 1 on the coating layer 4 so that the irradiation energy density becomes, for example, 1.5 kJ / cm ² . In this case, the average power incident on the phase mask 5 is, for example, 10 mW, and the dimension of the ultraviolet light applied to the optical fiber core 1 having an outer diameter of 200 m is about 2 mm (in the direction of the fiber axis). It is 2 mm (in the direction of the fiber).

As the phase mask 5, a mask having a grating pitch of, for example, 1065 nm and a length of 25 mm can be used. Then, by moving the movable mirror 10 smoothly and continuously in the fiber axial direction (longitudinal direction), a 24 mm long grating 25 can be written in the axial direction.

FIG. 4 shows the relationship between the applied tension at the time of writing the grating and the grating fabrication time. FIG. 5 shows the relationship between the applied tension at the time of writing the grating and the grating forming speed. Here, the applied tension is represented by axial strain (%).

As can be seen from Figs. 4 and 5, the fiber grating formation speed increases more rapidly when the tension is 0.8% or more than when no tension is applied, and the grating formation time is shortened. When the tension is 1.0% or more, the fiber grating forming speed increases 2 to 6 times compared to the case where the tension is 0.2% or less. However, even if the tension exceeds 1.0%, the fiber grating formation speed is almost saturated.

From the above, the applied tension is preferably 0.8% or more, and more preferably 1.0% or more. A preferable upper limit of the applied tension is 6%. If the tension exceeds 6%, the fiber may break mechanically.

The reason why the fiber grating formation speed is improved by applying tension It is thought that UV sensitivity was increased by the application of tension, but the detailed mechanism has not been elucidated.

In addition, according to the method of stripping the coating of the fiber before irradiation with ultraviolet light, it is practically impossible to apply a tension of 1.0% or more because the mechanical strength of the fiber is reduced. For this reason, unless a method of forming a coating layer using an ultraviolet-transmitting resin and then irradiating the core with ultraviolet light so as to transmit the coating layer is used, the effect of increasing the ultraviolet sensitivity by applying tension is exerted. It is difficult to do that.

The effect of applying a tension to the increase in the fiber grating forming speed has been described above. Next, the effect of increasing the reflectance will be described.

In Fig. 6, the horizontal axis shows the tension applied to the fiber to be irradiated, the horizontal axis shows the final arrival reflectance during fabrication, and the reflectance when the tension of the fiber is released after fabrication. The applied tension was set in four steps within an elongation ratio of about 1% to 3%. For all tensions, the reflectance after releasing the tension exceeds the ultimate reflectance at the time of fabrication. The fiber used in this experiment was a Sn-doped fiber, which had been subjected to hydrogen treatment for 2 weeks in a hydrogen atmosphere at 3.8 atm (= about 0.38 MPa).

In Fig. 7, the horizontal axis shows the applied tension, and the vertical axis shows the increase (%) in reflectance due to release of the tension. It can be seen that the greater the applied tension during fabrication, the greater the increase in reflectance after releasing the tension. Irradiation on the coating using UV-transmitting resin can apply a maximum of about 6% tension to the fiber during fiber grating production, making it possible to significantly increase the reflectance after releasing the tension. . This, in combination with the above-mentioned sensitivity enhancement, makes it possible to efficiently produce a high-performance grating in a shorter time.

In order for the reflectivity of the fiber grating to increase in the optical communication component, it is necessary to ensure that the member supporting the fiber grating in the component does not apply a large tension to the fiber.

Tension applying mechanism>

An example of the tension applying mechanism 30 for applying a tension in the fiber axis direction to the optical fiber core 1 will be described below. As shown in FIG. 8 in detail, the tension applying mechanism 30 includes a frame 31 disposed so as to surround the ultraviolet irradiation region of the optical fiber core 1, and A pair of arm members 32, 33 protruding from the frame 31 on both sides of the optical fiber core 1 in the fiber axial direction, and a pair of arm members 32, 33 supported at the distal ends of the arm members 32, 33, respectively. It is provided with winding cylinders 34 and 35 as fixing means, and a motor 36 (see FIG. 9) for rotating and driving the winding cylinder 35 on one side in the axial direction of the fiber (the right side in FIG. 8). The frame 31 has an opening 311 through which the ultraviolet laser light can pass at least at a side portion (upper portion in FIG. 8) of the optical fiber core 1, and the pair of arm members 32, 3 There is no restriction on the shape and the like as long as 3 can be maintained. Each of the arm members 32 and 33 is formed in an L shape, and one end is fixed to the frame 31 and the other end is connected to the winding drums 34 and 35. Each of the above winding cylinders 34, 35 is composed of a mandrel 341, 351, which constitutes a winding drum main body, and a pair of flanges 34, 32, 35 2 disposed on both sides thereof. ing. The winding cylinder 35 on one side in the fiber axis direction (the right side in FIG. 8) is rotatably connected to the arm member 33 around an axis Y arranged in a direction orthogonal to the fiber axis direction, while being connected to the fiber axis direction. The other side (left side in FIG. 8) of the winding cylinder 34 is fixed so as not to rotate relative to the arm member 32. The motor 36 is constituted by a pulse motor, and its output shaft is directly connected to the mandrel 351 or connected via a connecting member. The motor 36 receives a control signal from a controller (not shown) and forcibly rotates the mandrel 351 by a set rotation amount.

Next, a method of manufacturing a fiber grating using the above-described fiber grating manufacturing apparatus will be described.

In order to fabricate a fiber grating, a tension applying step, an irradiation step, a tension releasing step, and a screening step are sequentially performed. That is, in the tension applying step, first, the optical fiber core wires 1 on both sides of the writing area of the grating 21 are attached to the outer peripheral surfaces of the mandrels 341, 351 of the winding drums 34, 35 with respect to each other. Wrap it twice or three times (see Fig. 9) so that it does not overlap, and set the optical fiber core wire 1 in a state where it extends in a straight line. As a result, the optical fiber core 1 is moved by the frictional resistance between the outer peripheral surface of the mandrel 341, 351 of each of the winding drums 34, 35 and the outer surface of the optical fiber core 1. The mandrel is fixed so that it does not move relative to the outer peripheral surface of the mandrel 3 4 1, 3 5 1 in the fiber axis direction. Next, activate the mode 3 6 Forcibly rotate the drain 3 5 1 by the set amount of rotation to maintain this state. As a result, the optical fiber core wire 1 between the pair of mandrels 341, 351 is forcibly extended in the fiber axial direction by a circumferential length corresponding to the forcible rotation amount of the mandrel 351. In other words, the tension is applied, and the core 2 is in a state in which elastic strain (elongation strain) on the tensile side is generated. In this state, the next irradiation step is performed.

In the irradiation step, first, the phase mask 5 is set with respect to the writing area of the grating 21 of the optical fiber core 1, and one end to the other end of the phase mask 5 in the fiber axis direction. Ultraviolet laser light from an ultraviolet irradiation system is applied to the optical fiber core 1 through the phase mask 5 over a range up to. The change of the irradiation position of the ultraviolet laser light in the above-mentioned fiber axis direction range is performed by moving the reflection mirror 10 in the fiber axis direction. Then, the grating 21 having a grating pitch corresponding to the grating pitch of the phase grating 5 is written into the core 2 in a state where the above-described elongation distortion occurs due to the irradiation of the ultraviolet laser light. After writing of the grating 21 in this irradiation step, a tension release step is performed. In this tension release step, the motor 36 is rotated in the reverse direction by the above-mentioned set rotation amount, and the optical fiber core is rotated. Line 1 is restored to its original state before the tension was applied, and there is no load. As a result, the elongation strain generated in the core 2 is restored to its original state, that is, contracted, and the grating pitch of the written grating 21 is reduced in accordance with the contraction. Therefore, the wavelength characteristic of the grating 21 is shifted to the shorter wavelength side by the narrower grating pitch. In addition, as described above, the reflectance of the grating is improved as compared to before the release of the tension.

Thus, the fabrication of the fiber grating is completed, but in the present embodiment, the screening step is performed subsequently. In other words, in this screening step, a constant elongation strain is given to the fiber grating in the fiber axis direction for a predetermined time by operating the motor 36 of the tension applying mechanism 30 to screen for mechanical strength characteristics. Perform the test. Then, defective fiber gratings are eliminated from the product, and fiber gratings without defects are used as products. Fiber gratings that have passed the screening test will be combined with members supporting fiber gratings and other components to form optical communication components.

To control the wavelength shift caused by releasing the tension, the relationship between the applied tension and the shift amount of the wavelength characteristic to the shorter wavelength side is determined in advance by a test, and based on this relationship, the relationship between the shift amount of the wavelength to be shifted and the wavelength is controlled. The applied tension is set, and the set rotational speed of the motor 36 is determined so that the applied tension is generated in the optical fiber core 1. In the above fiber grating manufacturing method, in order to make writing of the grating 21 by irradiation of the ultraviolet laser light from above the coating layer 4 more reliable, irradiation of the ultraviolet laser light is performed as follows. You may do so.

That is, the irradiation with the ultraviolet laser light is performed so that the irradiation energy density becomes about 1.5 kJ / cm ² . Thus, when ultraviolet laser light is irradiated from the outside of the coating layer 4, even if the coating layer 4 has a considerably thick film thickness of about 30 / m or more, the ultraviolet light is transmitted through the coating layer 4. As a result, high-refractive-index modulation is generated on the core 2 to enable writing of the high-reflection Bragg grating 21.

In addition, as shown in Fig. 10, the optical fiber to be written ^; the core 1 is positioned at a specific position with respect to the beam pattern BP of the ultraviolet laser light focused by the cylindrical lens system 7, and in this state, the ultraviolet laser Irradiation of light is performed. The beam pattern BP is obtained by converging the parallel beam incident on the cylindrical lens system 7 so as to be directed to the focal point F. In contrast to the beam pattern BP, the entirety of the optical fiber core 1 is the beam pattern BP. The optical fiber 1 is positioned so that the outer peripheral surface of the coating layer 4 of the optical fiber 1 is inscribed in the outer edge of the beam pattern BP. If such a positional relationship is satisfied, it is assumed that the optical fiber core 1 is located in front of the focal point F as shown by a solid line in FIG. 10 as indicated by a dashed line in FIG. It does not matter if it is behind the focus F. For example, when the focal length L1 is 100 mm, the optical fiber core wire 1 with an outer diameter of 200 m is placed on the optical axis at a distance L2 of approximately 2 mm from the focal point F. Just set it. By positioning the entire optical fiber core 1 inside the beam pattern BP, the entire coating layer 4 can be irradiated with ultraviolet laser light at a uniform irradiation energy density. And be able to. Furthermore, it is possible to prevent local damage (deterioration in strength) of the coating layer 4 which is likely to occur when the optical fiber core 1 is disposed closer to the focal point F side, and Irradiation to the optical fiber core wire 1 can be performed at a position where the irradiation energy density is highest within a range where the occurrence of a large strength deterioration can be prevented, and the time required for writing the grating can be reduced. .

In the above embodiment, the application of the tension by the tension applying mechanism is performed by rotatably supporting one of the winding drums 35 with respect to the arm member 33 and forcibly rotating the winding drum 35 with the motor 36. However, the present invention is not limited to this, and both winding cylinders 34 and 35 are fixed to the arm members 32 and 33 so as not to rotate together, and one end 33 of one arm member 33 is fixed. 8 is guided and supported movably in the fiber axis direction with respect to the frame 31 as shown by a dashed line in FIG. 8, and this arm member 33 is combined with a transmission mechanism such as a rack and a binion and a motor, or The tension may be applied to the optical fiber core 1 by configuring the apparatus such that the hydraulic cylinder or the like is forcibly moved to the right in FIG.

The method for manufacturing a fiber grating of the present embodiment is suitably applied to the manufacture of both a short-period grating and a long-period grating. The short-period grating has a pitch of about 1 m or less, and the long-period grating is a grating having a pitch of about several hundred m.

[Second embodiment]

A method of manufacturing a fiber grating that can be suitably used for a temperature sensor and an embodiment of a temperature sensor using such a fiber grating will be described. As a result of a detailed study of the temperature dependence of the reflection peak wavelength of a grating formed on a coated fiber whose surface is covered with a coating layer (hereinafter referred to as a “coated fiber grating”), It has been found that it is possible to manufacture a fiber bag rating that does not require a package structure and has a simple structure and can be suitably used for a temperature sensor capable of measuring a temperature up to an extremely low temperature.

The coating layer of the coated fiber grating uniformly compresses the fiber at low temperatures. The reflection peak wavelength of the grating formed on the fiber strand is Shifts under the influence of compression force. The compressive force of the coating layer is mainly determined by the coefficient of thermal expansion and elastic modulus of the fiber strand, and the elastic modulus, coefficient of thermal expansion and thickness of the coating layer. Therefore, by appropriately selecting the material of the coating layer and forming the coating layer of an appropriate thickness, the rate of change of the reflection peak wavelength of the fiber grating with respect to temperature change (hereinafter simply referred to as the “temperature coefficient of the reflection peak wavelength”) ) May be set to a predetermined value. Further, since the coating layer uniformly compresses the fiber in the low temperature region, non-uniform stress is not applied to the fiber unlike the package structure, so that the coating has stable mechanical strength even at a low temperature. Also, unlike a packaged sensor, it is small and can be bent, so it can be placed in various locations.

In addition, by appropriately setting the material and thickness of the coating layer, the temperature coefficient of the reflection peak wavelength of the grating is kept constant over a wide temperature range (particularly from a temperature above room temperature to a very low temperature). be able to. As described above, when the temperature coefficient of the reflection peak wavelength of the grating is constant over a wide range, a wide temperature range can be easily measured. The change in the temperature coefficient of the reflection peak wavelength of the grating caused by the stress caused by the coating layer offsets the change caused by other factors such as external distortion of the temperature coefficient of the reflection beak wavelength of the fiber grating. A layer can be formed, and a temperature sensor that can easily measure the temperature can be obtained.

As described above, it is preferable to exhibit a temperature dependency expressed by a constant temperature coefficient over a wide temperature range from extremely low temperature to room temperature or higher.However, a constant temperature coefficient is shown in each of a plurality of temperature ranges. , The respective temperature coefficients may be different. That is, if the temperature dependence of the reflection peak wavelength can be approximated by a plurality of continuous straight lines, a temperature sensor that can easily measure the temperature in each temperature range that can be approximated by a straight line can be obtained. It is preferable that the temperature range of each is wide, but it may be set appropriately in consideration of the temperature range of the object to be measured and the required measurement accuracy.

Similarly to the first embodiment, the fiber grating of the present embodiment is formed by using the optical fiber 1 in which the surface of the optical fiber 1 ′ having the core 2 and the clad 3 is covered with the coating layer 4.

As the core 2, similarly to the first embodiment, in addition to Ge having the same concentration as that of the optical fiber of the normal specification, the dope of Sn, or 3] 1 and 81, or Sn, A 1 and B is used. It is preferable to use a material to which the light is added in order to constantly increase the photoinduced refractive index change.

The coating layer 4 is formed by a single coating following the step of drawing the optical fiber 1 '. The material for forming the coating layer 4 and the thickness of the coating layer 4 are selected and the thickness is determined so that the temperature coefficient of the reflection peak wavelength of the grating becomes a predetermined value. By using the coating layer 4 having a large coefficient of thermal expansion, the temperature coefficient of the reflection peak wavelength of the grating can be increased, and conversely, by using the coating layer 4 having a small coefficient of thermal expansion, the reflection peak of the grating can be increased. The temperature coefficient of the wavelength can be reduced. In addition, by controlling the thickness of the coating layer 4, the degree of contribution of the reflection peak wavelength by the coating layer 4 to the temperature coefficient can be changed.

This coating layer design process includes the elastic modulus (Young's modulus E), thermal expansion coefficient (linear thermal expansion coefficient), temperature coefficient of refractive index (thermo-optic coefficient), and material of the coating layer of the optical fiber 1 '. Material selection based on elastic modulus (Young's modulus) and thermal expansion coefficient (linear thermal expansion coefficient), so that the temperature coefficient of the reflection peak wavelength of the grating in the temperature range measured using the fiber grating becomes a predetermined value. And the thickness of the coating layer is determined. Typically, the fiber grating is designed so that the temperature coefficient of the reflection peak wavelength is constant from a temperature above room temperature (for example, 1 10 ° C) to a very low temperature (for example, -196 ° C). . As a material for forming the coating layer 4, it is preferable to use an ultraviolet ray transmitting type ultraviolet curable resin as in the first embodiment.

The writing process of the grating is performed while applying tension (or strain) to the fiber core 1 using the fiber grating manufacturing apparatus shown in FIG. 3 to reduce the shift amount of the reflection peak wavelength of the grating. Can be controlled. It should be noted that the writing itself of the grating 21 by ultraviolet irradiation may be performed by using various well-known methods, and FIG. 3 shows an example in which the writing is performed by, for example, a phase mask method.

Hereinafter, the present embodiment will be described using a specific example.

Fiber 1 as,, and G e and S n using a co-doped quartz glass-based fiber (diameter: 1 2 5 zm, thermal expansion coefficient:. 0 5 5 X 1 0- 6 ( room temperature) Zd eg , Modulus: 73 GPa (room temperature)). The relative refractive index difference (△) of this fiber was 0.97%, the cut-off wavelength (person c) was 1.27〃m, and the Sn concentration was 15,000 ppm. The surface of the fiber strand 1 ′ was coated with a UV-transmissive UV-curable resin having a high transmittance to ultraviolet rays for the writing of a grating to form a coating layer 4. In this specific example, an aliphatic urethane acrylate having a transmittance of about 10% or more for ultraviolet rays having a wavelength of about 240 nm to about 270 nm (photopolymerization initiator: 2,4,6,1-trimethylbenzoyldiphne) By forming a coating layer 4 (single layer: thickness of about 75 μm on both sides) with a thickness of about 37.5 μm using enylphosphine oxide), 200 m). The thermal expansion coefficient of the coating layer 4 is 1 × 10 " ⁴ / deg, and the elastic modulus is 54 OMPa (normal temperature). In order to increase the photoinduced refractive index change of the obtained coated fiber core 1, The coated fiber core 1 was left in a high-pressure hydrogen gas of about 2 OMPa for about 2 weeks, and was filled with hydrogen.Grating was written on the coated fiber core using the phase mask method. The writing of the grating was performed using the fiber grating manufacturing apparatus shown in Fig. 3. No tension was applied to the fiber core 1 (concrete example 1), and the tension was applied in the axial direction of the fiber core 1 ( While applying 3.9 N) (Specific Example 2), a fourth harmonic (266 nm, intensity 10 mW) of the Nd-YAG laser was swept and irradiated (about 22 mm). The laser irradiation time was adjusted so that the levels were the same. The length was 154.4 nm at 25 ° C., and the reflection peak wavelength of Example 2 was 1539.4 nm at 25 ° C. Further, Example 1 and Example 2 produced by the above method The fiber gratings from which the coating layer was removed were referred to as Comparative Examples 1 and 2, respectively.

FIG. 11 shows the results of measuring the temperature dependence of the reflection peak wavelengths of the fiber gratings of Examples 1 and 2 and Comparative Examples 1 and 2.

As is clear from Fig. 11, the reflection peak wavelength of the fiber grating of Example 1 changes almost linearly from about -70 ° C to about 170 ° C, and its slope (temperature coefficient: ΔλΒ / ΔΤ) Was 0.012 nm / ° C. Further, this temperature coefficient was the same as the temperature coefficient of the reflection beak wavelength of Comparative Example 1 at −20 ° C. to + 60 ° C. Furthermore, the temperature dependence from the room temperature of Example 1 (here, 20 ° C) to -196 ° C The property was well approximated by a straight line, and the temperature coefficient was 0.013 nm / ° C. Note that here, the force at room temperature of 20 ° C is not limited to this, but “room temperature” refers to the ambient temperature of the fiber (the temperature of the area other than the temperature measurement target, the temperature of the working environment).

The temperature coefficient of the reflected peak wavelength from -70 ° C to 1-96 ° C (liquid nitrogen temperature) in Specific Examples 1 and 2 can be approximated by a straight line. 0.013 nm / ° C, which is a value close to the temperature coefficient on the high temperature side (0.012 nm / ° C). By increasing the thickness or increasing the elastic modulus of the coating layer, the temperature coefficient on the low temperature side can be increased, so that a single temperature coefficient can be obtained over a wide temperature range (for example, -196 ° C to 10170 ° C). (Here, ΔΐΒ / ΔΤ = 0.012 nm / ° C), and a fiber grating with a temperature dependence of the reflection peak wavelength that can be approximated well can be obtained.

On the other hand, the temperature coefficient (0.005 nm / ° C) of the reflection peak wavelength on the low temperature side (approximately −75 ° C or less) of Comparative Example 1 is the temperature coefficient (0.012 nm / ° C), indicating a different temperature dependence from the high temperature side. A decrease in the temperature coefficient on the low temperature side is also observed in Comparative Example 2. This phenomenon is thought to be due to the fact that the temperature change of the refractive index of the core forming the grating becomes smaller on the low temperature side.

In the gratings of Examples 1 and 2 according to the present embodiment, the reflection at the low temperature side is controlled by controlling the magnitude of the compressive stress applied to the fiber by the coating layer, which increases as the temperature decreases. The temperature coefficient of the peak wavelength is adjusted. As described above, the adjustment of the temperature coefficient on the low temperature side is not performed so as to match the temperature coefficient on the high temperature side, but may be adjusted so as to increase the temperature coefficient depending on the application. A large temperature coefficient means high temperature measurement sensitivity, so if accurate measurement of cryogenic temperatures (for example, less than 10 oC) is required, May be designed so as to increase the temperature coefficient of the coating layer. For example, the temperature coefficient in a temperature range of −100 ° C. or less is preferably larger than 0.01 nm / ° C.

In addition, it is preferable that the temperature dependence of the reflection peak wavelength be expressed by a single temperature coefficient (it can be approximated by a straight line), because the temperature can be easily obtained from the measured wavelength. Even if it cannot be expressed by a single temperature coefficient, for example, if a graph as shown in Fig. 11, that is, a calibration curve is created in advance, and the temperature is determined from the measured reflection peak wavelength of the fiber grating and the calibration curve, Good.

FIG. 12 schematically shows an embodiment of a temperature sensor 50 using a fiber grating according to the present embodiment.

The temperature sensor 50 receives the reflected light from the optical fiber 1 on which the grating 21 is formed, the light source 52 for emitting light to the optical fiber 1, and the grating 21 and detects the wavelength of the reflected light. An optical spectrum analyzer 58 is provided. If necessary, an optical isolator 54 may be provided to select light having a specific wavelength from the light emitted from the light source 52. Further, an optical coupler 56 is provided to couple an optical path for transmitting light from the light source 52 to the grating 21 and an optical path for guiding the light reflected from the grating 21 to the optical spectrum analyzer 58. You may.

The fino 1 on which the gratings 21a and 21b are formed is arranged, for example, in a tank 60 filled with liquefied methane gas (1183 ° C). The gratings 21a and 21b are formed using the above-described tension application method, and have mutually different reflection peak wavelengths. Therefore, by detecting the reflection peak wavelength, it is possible to determine which of the gratings 21a and 21b is the reflected light. Therefore, by using one fiber having a plurality of gratings, it is possible to easily measure temperatures at different positions. Of course, it is not necessary to provide a plurality of gratings.

Further, since the temperature sensor of the present embodiment does not have a package structure for fixing the fiber, it can be easily installed on a curved surface, a narrow place, or the like. In addition, the coated fiber has high mechanical strength and does not break, especially at low temperatures. Therefore, the temperature sensor of the present embodiment is suitably used as a temperature sensor for measuring the temperature of a curved surface at a low temperature like the above-mentioned liquefied natural gas tank. Industrial applicability

According to the present invention, when the grating is written, the core is distorted in the axial direction, thereby increasing the photosensitivity to ultraviolet light. The rate of change of the refractive index due to irradiation increases. Therefore, the time required for the writing operation can be reduced.

If the tension applied to the core at the time of writing is released, the increase in the refractive index is promoted.If the fiber grating is used with a tension smaller than the above applied tension, the reflectance will be higher than the reflectance when the grating is manufactured. High reflectivity can be realized.

Further, according to the present invention, the temperature coefficient of the reflection peak wavelength of the grating formed on the coated fiber can be adjusted to a desired value. Therefore, a package structure is not required, and a simple structure is realized. A fiber grating that can be suitably used for a temperature sensor capable of measuring a temperature down to a low temperature can be manufactured.

Furthermore, by adjusting the temperature coefficient at low temperatures, manufacturing fiber gratings used in temperature sensors that can measure low temperatures with high sensitivity and temperature sensors that can easily measure a wide temperature range from high to extremely low temperatures. Can be.

Claims

The scope of the claims

1. a step of covering an outer peripheral surface of a fiber having a core on which a grating is to be written and a clad surrounding the core with a coating layer formed of an ultraviolet-transmissive resin, and applying ultraviolet light to the core from outside the coating layer. Writing a grating in the core by irradiating;

A method for producing a fiber grating, comprising:

In the step of writing the grating on the core, the core is irradiated with the ultraviolet rays in a state where a distortion of + 0.8% or more and + 6% or less is generated in the axial direction. A method for manufacturing a fiber grating.

2. The method for manufacturing a fiber grating according to claim 1, wherein, in the step of writing the grating on the core, the writing of the grating is performed while applying an axial tension to the core.

3. The method according to claim 2, wherein the axial tension is released after the writing of the grating is performed.

4. A fiber grating manufactured by the fiber grating manufacturing method according to any one of claims 1 to 3,

Means for supporting the fiber grating;

Component for optical communication with.

5. The means for supporting the fiber grating is characterized in that the fiber grating is supported such that in the step of writing the grating into the core, a strain smaller than an axial strain applied to the core is generated. The optical communication component according to claim 4.

6. A method for producing a fiber grating, comprising: a fiber strand having a core and a cladding; and a coating layer covering a surface of the fiber strand,

A step of preparing a fiber strand;

A step of forming a coating layer covering the surface of the fiber strand by using a resin material; and before the step of forming the coating layer, a rate of change of the reflection peak wavelength of the grating to be manufactured with respect to temperature change is predetermined. A coating layer design step of selecting the resin material and setting the thickness of the coating layer so as to be a value; A method for producing fiber gratings, comprising:

7. The coating layer designing step includes selecting the resin material and determining the thickness of the coating layer based on the elastic modulus and thermal expansion coefficient of the fiber strand and the elastic modulus and thermal expansion coefficient of the resin material. 7. The method for manufacturing a fiber grating according to claim 6, which is a step of setting.

8. The method for manufacturing a fiber grating according to claim 6, wherein a rate of change of the reflection peak wavelength with respect to a temperature change is constant in a range of _196 ° C to 110 ° C.

9. The rate of change of the reflection peak wavelength with respect to temperature change is the rate of change of the reflection peak wavelength of the grating formed on the uncoated fiber strand with respect to the temperature change at −20 ° C. to + 60 ° C. 9. The method for producing fiber gratings according to claim 8, which is the same.

10. A fiber grating having a core fiber having a core and a clad, a coating layer covering the surface of the core fiber, a light source for emitting light to the fiber grating, and a reflected light from the fiber grating And a detector for detecting the peak wavelength of the reflected light, wherein the rate of change of the peak wavelength of the reflected light with respect to temperature change is from 1196 ° C to 110 ° C. Temperature sensor that is constant in the range of C.