JP2000089045A - Manufacture of fiber grating and manufacturing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To expand the control range of wavelength characteristics by enabling a shift control of wavelength characteristics toward the short wavelength side and the impression of a high tension, and to obtain a manufacturing device for a fiber grating in which the wavelength is controlled. SOLUTION: Both side parts in the fiber axis direction, while interposing ultraviolet laser beam-irradiating region of a coated optical fiber 1, in which a covering layer is formed of an ultraviolet ray transmissive resin, between them, are wound around a pair of winding drums 34, 35 so as not to overlap each other, are fixed so as not to be relatively moved. One winding drum 35 is forcibly rotated by a set rotational amt. around a Y axis with a pulse motor, thereby impressing a tension, thus keeping a core in a state of being under a prescribed elongation strain (tension impressing stage). In this state, the grating is written on the core by irradiating the covering layer with an ultraviolet laser beam via a phase mask from above the covering layer (irradiating stage), the motor is rotated in the opposite direction, thereby releasing the elongation strain, the coated optical fiber is contracted to an original state, thereby making the grating pitch narrow (tension releasing stage), and again, screening is carried out by impressing a tension for testing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of writing a diffraction grating (grating) having a refractive index difference in a stripe pattern on a core of an optical fiber, and reflecting light having a wavelength characteristic corresponding to the grating pitch by the grating. Also, the present invention relates to an apparatus and a method for manufacturing a fiber grating used as a filter that transmits light.

[0002]

2. Description of the Related Art In a fiber grating, a refractive index modulation fringe is formed at a predetermined grating pitch (period) in a fiber axis direction with respect to a core of an optical fiber.
Conventionally, a two-beam interference method or a phase mask method has been known as a method for producing a fiber grating by writing a grating composed of such refractive index modulation fringes on the core (for example, Japanese Patent Application Laid-Open No.
JP-A-235808, JP-A-7-140311, and Patent No. 2521708). In such a fiber grating manufacturing method, a silica glass (core) doped with germanium (Ge) is irradiated with a coherent ultraviolet laser beam to generate a photo-induced refractive index change at a corresponding portion, thereby causing the above-described refractive index modulation. Stripes are generated (written).

As such a fiber grating, a short-period grating having a grating pitch of about 1 μm or less (fiber Bragg grating: FBG
Also called. ) And grating pitch of several hundred microns
There is a long period grating of about m.

[0004] The short-period grating has a wavelength characteristic of reflecting or transmitting light of a specific wavelength corresponding to the grating pitch. Here, assuming that the grating pitch is P and the effective refractive index is n, the specific wavelength λB reflected by the grating is expressed by the following equation.

ΛB = 2 · n · P That is, the specific wavelength λB is proportional to the grating pitch P. By controlling the wavelength characteristics described above, applications to applications such as filters, duplexers, dispersion compensators, fiber laser mirrors, EDF gain equalizers, resonators, and temperature sensors are being considered. Therefore, when applying to such various applications, it is important to control the wavelength characteristics of the fiber grating to be set appropriately according to the application.

[0006] Long-period gratings have the property that the forward propagation mode of the core couples to the backward propagation mode of the cladding. Coupling to multiple cladding modes results in multiple specific wavelength peaks. As with the short-period grating, it is important for the long-period grating to appropriately control the wavelength characteristics according to various applications.

As a method of controlling the wavelength characteristics, in the phase mask method, the grating pitch is determined in accordance with the interval (mask pitch) between the phase gratings of the phase mask. By preparing a mask and irradiating the optical fiber with ultraviolet rays via the phase mask, the wavelength characteristics of the fiber grating are controlled to a required one.

Further, as a method of controlling the wavelength characteristics of the fiber grating, after writing the grating by irradiating ultraviolet rays, a tension is applied to the fiber grating on which the grating has been written in the fiber axis direction so that the tensile strain remains. A method of shifting the grating pitch at the time of writing to a longer wavelength side by setting the state is known (Hiroyuki Uno et al., "Bragg wavelength change of fiber grating due to tensile strain", 1998 IEICE General Conference. , C-3-
123, p. 289, March 1998). That is, the shift of the grating pitch on the long wavelength side makes it possible to shift the wavelength reflected by the grating (reflection peak wavelength) to a longer wavelength side.

[0009]

However, in the case of applying the tension after writing the above-mentioned grating, since the strain on the tensile side is given, the grating pitch at the time of writing is shifted to the longer wavelength side as a control of the wavelength characteristic. Although it is possible to shift the grating pitch, there is a decisive inconvenience that the grating pitch at the time of writing cannot be shifted to the shorter wavelength side.

In the method of shifting the wavelength characteristics to the longer wavelength side, it is necessary to fix the fiber grating in a state where the tensile strain remains by means of, for example, adhesive fixing in order to maintain the wavelength shift. Therefore, there is a restriction on the device configuration when applied to the above various applications.

In addition, it is necessary to apply the tension by a means and a method that does not damage the optical fiber.
Means that enable mass production, not as a test facility
It must be done by the method.

In addition, the writing of the grating by the irradiation of ultraviolet rays is to generate a refractive index modulation fringe by causing a light-induced refractive index change in the core of the optical fiber by the irradiation of the ultraviolet rays. Usually, from the viewpoint of ensuring an effective photoinduced refractive index change in the core, the coating layer in the region to be irradiated with ultraviolet light is removed. Therefore, in the conventional method of writing a grating on an optical fiber consisting of a core and a clad without a coating layer and applying a tension after the writing, there is a possibility that the applied optical fiber may break as the applied tension. It is necessary to set the applied tension value (tensile load value) to such an extent as not to occur as an upper limit. Here, in the case of the above-mentioned optical fiber, when the amount of elongation exceeds approximately 1%, the fiber usually breaks, so that tension application is stopped in a low tension region where the amount of elongation is less than 1%. As a result, only a very small shift to the longer wavelength side can be expected. Regarding this point, the tensile load value corresponding to the elongation of 1% in the case of the ordinary optical fiber is approximately 1 kg, and according to the above-mentioned document describing the method of applying the tension after writing the grating, 0.5 kg It is reported that the wavelength shift amount is extremely small at 5.4 nm. In addition, it is shown that the amount of wavelength shift increases substantially in proportion to an increase in the value of the tensile load.

Further, in order to write a plurality of gratings on one optical fiber, it is necessary to remove a coating layer in an intermediate region of the optical fiber. When the coating layer in the intermediate region of the optical fiber is mechanically removed using a stripper or the like, the surface of the optical fiber is damaged, and the tensile strength is reduced to about several hundred grams. Therefore, when a plurality of gratings are written on one optical fiber, the yield is significantly reduced. In addition, the method of removing the coating layer using a chemical does not damage the surface of the optical fiber strand, but since the glass portion is directly exposed to the outside air, the strength is reduced when the coating layer in the middle area of the optical fiber is removed. Causes deterioration. Therefore, a plurality of continuous gratings cannot be formed on one optical fiber.

In the production of ordinary optical fibers, screening tests for mechanical properties are usually performed on the produced optical fibers. However, in fiber gratings, although inspection methods for transmission characteristics such as wavelength characteristics have been almost established, inspection methods for mechanical characteristics such as strength have not yet been established. This is because the conventional fiber grating has no practical mechanical strength and is used in a state where no mechanical strength is applied to the fiber grating (for example, in a packaged state), so that no screening of the mechanical strength of the fiber grating is required. This is because the.

Here, it is conceivable to carry out a screening test on a fiber grating manufactured using a fiber grating manufacturing apparatus equipped with an ultraviolet irradiation system or the like using the above-described ordinary optical fiber screening test apparatus. . However, in this case, the production of the fiber grating and the screening test are performed separately, and the steps and operations are complicated.

The present invention has been made in view of the above circumstances, and an object of the present invention is to fabricate a fiber grating capable of controlling the shift of the wavelength characteristic to a shorter wavelength side by using a tension applying method. It is to provide a method and a manufacturing device. In addition, the application of high tension enables the shift amount of wavelength characteristics to be increased, the control range of wavelength characteristics to be expanded, and the inspection of mechanical characteristics by screening test after the fiber grating is manufactured. It is also an object of the present invention to perform the process in a series of steps by using an apparatus, and to provide an apparatus suitable for mass production as an apparatus for manufacturing a fiber grating using the shift of the wavelength characteristic (wavelength control).

[0017]

In order to achieve the above object, the present invention provides a method for writing a grating in a state where tension is applied and tensile strain is generated, and then the applied tension is reduced. If the tensile strain is elastically restored by releasing, the grating pitch of the grating written in the core becomes narrower by the elastic restoration, whereby the wavelength characteristics can be shifted to the shorter wavelength side, and The focus is on the fact that the state shifted to the short wavelength side can be stably maintained. The shift amount in the present specification refers to the characteristic wavelength (reflection peak wavelength or transmission peak wavelength) of the grating when strain (tension) is applied and the characteristic wavelength of the grating when strain (tension) is not applied (open state). And the difference.

(First Invention) Specifically, a first invention according to a fiber grating manufacturing method is to apply a tension in a fiber axial direction to a writing area of a grating in an optical fiber to be manufactured. A tension applying step of applying a tensile strain in the fiber axis direction by applying a voltage, and a grating having a predetermined grating pitch in the fiber axis direction by irradiating the optical fiber with tension applied in the tension applying step with ultraviolet rays. An irradiation step of writing the core of the optical fiber, and a tension release step of shifting the grating pitch of the grating written on the core to a shorter wavelength side by releasing the application of the tension after the irradiation step. Is a particular matter.

In the case of the first aspect of the present invention, a tensile strain (elongation strain) occurs in the core of the optical fiber in the fiber axis direction by the tension applying step, and the core is extended in the fiber axis direction. Next, an irradiation step is performed in this state, and a grating having a predetermined grating pitch is written to the core in the above-mentioned extended state.
Then, the tension on the optical fiber on which the grating has been written is released by the next tension releasing step, whereby the elongation strain is elastically restored, and the grating pitch of the written grating is narrowed accordingly. Transition. As a result, the wavelength of the light reflected by the grating is shifted (shifted) to the shorter wavelength side as the grating pitch becomes narrower. Therefore, the present method enables the shift control of the wavelength characteristic to the shorter wavelength side, which cannot be realized by the conventional tension applying method in which the tension is applied after writing the grating. Also, in this case, unlike the above-described conventional tension applying method, the grating is written in advance in a tension applied state, and then the tension is released. Therefore, the grating shifted to the short wavelength side does not exist. This means that the fiber grating is formed in a stable state with respect to the core of the optical fiber in the loaded state, and the wavelength characteristics of the fiber grating shifted to the short wavelength side can be obtained stably. In addition, compared to the above-mentioned conventional tension application method, there is no need to individually maintain the optical fiber after the writing of the grating in the tension application state, and it is excellent in handling and mass-produces a fiber grating whose wavelength is controlled by applying tension. Will be easily realized.

Further, according to the present invention, a plurality of gratings having different pitches can be manufactured on one fiber core by irradiating ultraviolet rays while applying tensions having different magnitudes. Since the pitch of the grating is determined by the tension (strain) applied to the fiber core when writing the grating, the pitch (intensity distribution) of the writing light may be the same. For example, when writing a grating using a phase mask method, gratings with different pitches can be manufactured using the same phase mask. Therefore, according to the present invention, a plurality of gratings can be easily manufactured with high productivity.

Further, when a plurality of gratings are written on one fiber core, when applying a tension to a region including the previously formed grating and writing the subsequent grating, the wavelength of the previously formed grating is used. By measuring the characteristics (reflection characteristics and / or transmission characteristics), it is possible to determine the magnitude of the tension actually applied to the fiber core based on the measured wavelength characteristics. That is, the previously formed grating can be used as a strain sensor. Therefore, based on the actual tension obtained in this way, it is possible to accurately determine the tension necessary for the grating to be written. According to this method, for example, a short-period grating in which the difference between the reflection peak wavelengths of adjacent gratings is 1 nm or less can be manufactured with high accuracy and high productivity. Here, the method of forming the grating for measuring the applied tension may be any method. For example, it may be formed by irradiating ultraviolet rays using a predetermined phase mask without applying tension.

In the first aspect of the present invention, the optical fiber used for fabricating the fiber grating may be an optical fiber (core optical fiber) without a coating layer comprising a core and a clad, or a grating writing area. An optical fiber core wire from which the coating layer has been removed may be used, but the present invention is not limited to this, and an optical fiber core wire having a coating layer formed on the above optical fiber strand may be used. In this case, it is preferable that the coating layer is formed of an ultraviolet-transmissive resin in order to transmit the irradiated ultraviolet rays and effectively generate a photo-induced refractive index change in the core due to the ultraviolet irradiation. When the ultraviolet irradiation is performed from above such a coating layer, the tension that can be applied in the tension applying step can be set to a high value, and the higher the value of the applied tension, the more the tension is generated in the core. By increasing the amount of tensile strain, the amount of elastic recovery by the tension releasing step, that is, the amount of contraction of the grating pitch can be increased. As a result, the shift amount of the wavelength characteristic to the short wavelength side also increases, and the range in which the wavelength control can be performed can be expanded.
For example, in the case of an optical fiber, an application of a tension corresponding to an elongation amount of 1% may cause a breakage. In the case of a core wire, the amount of elongation that may cause breakage is generally 6%. Therefore, when performing the tension applying step on the optical fiber core wire, a high tension corresponding to at least 4% or more of the elongation amount is required. It becomes possible to apply.

Further, after applying and releasing the tension for controlling the wavelength, a screening step of performing a screening test on the writing area of the grating by applying a predetermined set tension to the manufactured fiber grating. May be performed subsequently. As a result, the mechanical characteristics of the wavelength-controlled grating writing area, that is, the inspection for the strength and the presence or absence of surface flaws can be performed together with the production of the fiber grating. It becomes suitable as a system.

The above screening step is carried out following the usual method of fabricating a fiber grating in which the grating is written by performing only the irradiation step without performing both the tension applying step and the tension releasing step. You may. As a result, in the production of the fiber grating, the writing of the grating and the screening test on the fiber grating on which the grating is written are performed in a series of steps, and in this case also, it is suitable as a mass production system for fiber gratings. It becomes something.

(Second Invention) A second invention according to a fiber grating manufacturing apparatus for carrying out the fiber grating manufacturing method according to the first invention as described above is directed to an optical fiber for which a fiber grating is manufactured. An ultraviolet irradiation system for irradiating ultraviolet rays so that a grating having a predetermined grating pitch is written on the core of the optical fiber in the fiber axis direction, and an optical fiber in an area irradiated with ultraviolet rays by the ultraviolet irradiation system in the fiber axis direction. The present invention is particularly characterized in that a tension applying mechanism for temporarily applying a tension so as to generate a tensile strain is provided. Here, the above-mentioned `` ultraviolet irradiation system '' includes all devices and mechanisms for writing a grating by irradiating ultraviolet rays, preferably an ultraviolet laser beam, to the optical fiber over an axial range of writing the grating, For example, a laser source, a phase mask,
It includes a laser light irradiation position changing mechanism and the like. Further, as the above-mentioned "tension applying mechanism", the optical fibers at both axial positions sandwiching the ultraviolet irradiation region are gripped, bonded, and fixed in position by means of friction or the like, and one or both of the fixed regions is fixed. What is necessary is just to comprise so that it may be in the state where tension was given to the said ultraviolet irradiation area | region by moving to an axial direction.

In the case of the second aspect of the present invention, the above-described tension applying step is performed by applying a tension in the fiber axial direction to the ultraviolet irradiation region by the above-mentioned tension applying mechanism and maintaining the tension, and the tension is applied. Irradiation step is performed by irradiating ultraviolet rays from an ultraviolet irradiation system in the state where the core is in a state where tensile strain is generated in the fiber axis direction due to the application of the tension due to the writing of the grating by the ultraviolet irradiation. Will be performed.
Then, after writing the grating, the tension release step is performed by releasing the tension application by the tension application mechanism. As a result, the core contracts and restores in the fiber axis direction, and as the core contracts, the written grating also contracts in the fiber axis direction,
The grating pitch will also be narrower than originally written. As a result, it becomes possible to shift the wavelength of the light reflected by the grating to a shorter wavelength side than the grating written under tension.

Here, the above-mentioned "tension applying mechanism"
Specifically, a pair of fixing means for fixing the optical fibers on both sides separated from each other in the fiber axis direction with the region irradiated with ultraviolet rays by the ultraviolet irradiation system, and at least one of the pair of fixing means And a moving means for forcibly moving in the fiber axis direction with respect to the other.

More specifically, for example, as each of the above-mentioned "fixing means", an optical fiber is wound around an axis orthogonal to the fiber axis direction, and based on the frictional resistance between the optical fiber and the optical fiber. To fix the above optical fiber. That is, by winding the optical fiber around the outer peripheral surface of the winding drum, the optical fiber is fixed so as not to relatively move in the axial direction due to frictional resistance generated between the optical fiber and the outer peripheral surface of the winding drum. . In this case, the winding drum on the side moved by the moving means is rotatably supported around the orthogonal axis at the same position in the fiber axial direction, and the moving drum on the moving side is the optical fiber. The "moving means" may be constituted by a motor that forcibly rotates by a set amount of rotation in a state in which is wound. Since the position of the optical fiber is fixed by the frictional resistance as described above, it is possible to fix the optical fiber in a state in which the possibility of damaging the optical fiber itself is surely avoided, and furthermore, the winding drum is connected to the center axis thereof. By forcibly rotating the optical fiber around the optical fiber, it is possible to easily and reliably apply tension to the optical fiber. Note that a cylindrical or columnar shape is adopted as the above-mentioned "winding drum", and a pulse motor or the like whose rotation amount is determined in proportion to the number of input pulses due to ease of tension application control is used as the "motor". It is preferable to employ it.

In particular, when fabricating a short-period fiber grating using the phase mask method, the pitch of the phase mask becomes 1 μm or less, so that it is necessary to accurately hold the fiber with high precision with respect to the phase mask. . As described above, when positioning with high accuracy is required, it is preferable to use a combination of a stepping motor and a worm gear. Further, in order to suppress or prevent the vibration of the fiber, it is preferable to provide a mechanism for pressing both ends of the fiber irradiated with ultraviolet rays. With such a configuration, it is possible to suppress and prevent the transmission of the vibration of the stepping motor to the ultraviolet irradiation region, and to reduce the holding current necessary for holding the stepping motor at a fixed position.

Further, a groove is provided on the outer peripheral surface (surface in contact with the fiber) of the mandrel (for example, 351 in FIG. 5) of the cylindrical or cylindrical winding drum, and the inner surface of the groove and at least one surface of the fiber are provided. By adopting a configuration in which the fibers contact each other, the frictional resistance can be increased, and the fibers can be prevented from overlapping each other.

Further, when the above-described fiber grating manufacturing apparatus is used, even when only the above-described irradiation step and screening step are performed, both writing of the grating and screening test of the fiber grating on which the grating is written are performed. This can be performed using the same manufacturing apparatus. That is, after writing the grating to the unloaded optical fiber by ultraviolet irradiation from the ultraviolet irradiation system, a tension is applied by a tension applying mechanism so that a predetermined elongation strain for a screening test is generated in the fiber axis direction. What should I do?

In the present specification, the terms “tension” and “strain” are used substantially interchangeably. That is, applying a predetermined “tension” corresponds to applying a predetermined “strain”. The “tension applying mechanism” in the present invention may be any mechanism that can apply a substantially predetermined strain in the axial direction of the fiber. In order to apply a predetermined strain to the fiber, a mechanism for applying a tension that can obtain a predetermined amount of strain using the “strain amount” as a direct control parameter may be used, or an elastic characteristic (tension) inherent to the fiber core may be used. A mechanism that obtains a tension necessary to obtain a predetermined amount of distortion based on (strain relationship) and applies the necessary tension using “tension” as a direct control parameter may be used.

[0033]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a fiber grating manufactured by using the fiber grating manufacturing method according to the embodiment of the present invention, and 1 denotes an optical fiber core having a predetermined length as an optical fiber to be manufactured. 2, the core on which the grating 21 is written, 3
Is a cladding of the optical fiber core 1, 4 is a coating layer covering the outer surface of the cladding 3, and 5 is a phase mask for writing the grating 21. As shown in FIG. 2, the optical fiber core 1 is obtained by coating a coating layer 4 on an optical fiber 1 'comprising a core 2 and a clad 3 manufactured by drawing from an optical fiber preform. . Then, ultraviolet laser light as ultraviolet light 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 fringe (periodic in the fiber axis direction). (Grating) is written to produce a fiber grating. 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 light from the outside of 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.

As the core 2, Sn, or Sn and Al in addition to Ge having the same concentration as that of the optical fiber of the normal specification.
Alternatively, it is preferable to use a dopant to which Sn, Al and B dopants are added in order to constantly increase the photoinduced refractive index change. Here, the normal specification optical fiber is an optical fiber core to be connected to the optical fiber core 1, and such an optical fiber core has a relative refractive index difference with respect to its core. Ge amount of about 0.9%
Is produced by doping. The core 2 of the optical fiber core 1 is provided with the same amount of Ge as that of the core of the above-mentioned normal specification optical fiber (an amount having a relative refractive index difference of 0.9%) and a concentration of 10,000 ppm or more. Preferably, Sn having a concentration of 10,000 to 15000 ppm, or Sn having such a concentration and Al having a concentration of 1000 ppm or less may be co-doped. The above-mentioned doping may be performed by various known methods. For example, when the doping is performed by liquid immersion, the above Ge or Sn compound (for Sn, for example, SnCl2.2H2O) is mixed with methyl alcohol,
What is necessary is just to immerse 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 after the step of drawing the optical fiber 1 ′. As a material for forming the coating layer 4, an ultraviolet transmitting resin having a property of transmitting ultraviolet light is used. As the ultraviolet transmission type resin, a specific wavelength band (for example, 240 n
m-270 nm), and it is particularly preferable to transmit the ultraviolet light of the above specific wavelength band without substantially absorbing it, and to transmit the ultraviolet light of a shorter or longer wavelength than the above specific wavelength band. What absorbs and causes a hardening reaction may be used. In other words, although the same resin has different ultraviolet absorption characteristics depending on the wavelength, it is an ultraviolet ray transmitting type in the above specific wavelength band, and is an ultraviolet curing type resin in a wavelength range shorter or longer than the above specific wavelength band. Most preferably, the coating layer 4 is formed. As such a resin, for example, 2 to urethane acrylate or epoxy acrylate is used.
What used may be a compound containing a photoinitiator (photoinitiator) that initiates and accelerates the curing reaction by receiving ultraviolet rays in a wavelength range shorter than 40 nm or a wavelength range longer than 270 nm.

It is preferable to fill the core 2 with hydrogen before irradiating the optical fiber core 1 covered with the coating layer 4 with ultraviolet rays, in particular, in order to increase the photo-induced refractive index change. In the case of filling with hydrogen, the optical fiber core wire 1 may be placed in a sealed container filled with hydrogen and left at room temperature under a pressure of about 20 MPa for about 2 weeks.

Next, a fiber grating manufacturing apparatus for manufacturing a fiber grating by performing shift control of the wavelength characteristic using the above-described optical fiber core 1 will be described with reference to FIG. The writing of the grating 21 by ultraviolet irradiation may be performed by using various well-known methods. FIG. 3 shows an example in which the writing is performed by, for example, a phase mask method.

That is, in the fiber grating manufacturing apparatus, a grid-like phase mask 5 is disposed immediately before the side of the optical fiber core wire 1, and the phase mask 5 is Nd−
The YAG laser source 6 irradiates, for example, a 266 nm coherent ultraviolet laser beam, which is a fourth harmonic (4ω) thereof, in a state of being condensed by the cylindrical lens system 7. Thereby, the ultraviolet laser light is applied to the phase mask 5.
Then, the light passes through the coating layer 4, and 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, so that the grating 21 is written. In FIG. 3, reference numeral 8 denotes a beam expander for enlarging the ultraviolet laser beam into a parallel beam, 9 denotes a slit having a minute width for cutting out a portion where the power of the above-mentioned parallel laser beam is uniform, and 10 denotes the light beam. This is a movable reflection mirror that can be moved in the fiber axis direction of the fiber core wire 1 (see the dashed line arrow). The above-described phase mask 5, Nd-YAG laser source 6, cylindrical lens system 7, beam expander 8, slit 9, and movable reflection mirror 10 constitute an ultraviolet irradiation system. By moving the movable reflection mirror 10, it is possible to irradiate the ultraviolet ray by sweeping. 11 is an optical spectrum analyzer, 12 is an optical isolator, 1
Reference numeral 3 denotes an optical coupler, which is a device for inspecting the wavelength characteristics of the manufactured fiber grating.

On the premise of the above-described apparatus configuration, in this embodiment, a tension applying mechanism 30 for applying a tension in the fiber axial direction to the optical fiber core 1 is added. As shown in FIG. 4 in detail, the tension applying mechanism 30 includes a frame 31 disposed so as to surround an ultraviolet irradiation area of the optical fiber core 1, and a fiber axis of the optical fiber core 1 from the frame 31. A pair of arm members 32, 33 protruding from both sides in the direction, respectively;
The winding drum 34 as a pair of fixing means supported at the tip of
And a winding drum 35 on one side (the right side in FIG. 4) in the fiber axial direction.
And a motor 36 (see FIG. 5) for rotationally driving the motor.

The frame 31 has an opening 311 at at least a side portion (upper portion in FIG. 4) of the optical fiber core 1 through which an ultraviolet laser beam can pass, and holds the pair of arm members 32 and 33. There is no restriction on the shape or the like as long as it can be performed. Each of the arm members 32 and 33 is L
One end is fixed to the frame 31, and the other end is connected to the winding drums 34 and 35. Each of the winding drums 34 and 35 is a mandrel 3 that constitutes a winding drum main body.
41, 351 and a pair of flanges 342, 352 disposed on both sides of each. A winding drum 34 on one side in the fiber axial direction (right side in FIG. 4) is rotatably connected to the arm member 33 around an axis Y arranged in a direction orthogonal to the fiber axial direction, while the other side in the fiber axial direction. The winding drum 34 (on the left side in FIG. 4) is fixed so as not to rotate relative to the arm member 32. In addition, the motor 3
Reference numeral 6 denotes a pulse motor, the output shaft of which 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 for manufacturing a fiber grating using the above-described fiber grating manufacturing apparatus will be described.

To fabricate a fiber grating,
The tension applying step, the irradiation step, the tension releasing step, and the screening step are sequentially performed. That is, as the tension applying step, first, the optical fiber core wires 1 on both sides of the writing area of the grating 21 are doubled or overlapped so as not to overlap with the outer peripheral surfaces of the mandrels 341 and 351 of the winding drums 34 and 35. The optical fiber core wire 1 is set in a state where it is wound around three times (see FIG. 5) and the optical fiber core wire 1 is linearly extended. Thereby, the mandrel 3 of each of the winding drums 34 and 35 is formed.
The optical fiber core 1 is fixed so as not to move relative to the outer peripheral surface of each of the mandrels 341 and 351 in the fiber axial direction due to frictional resistance between the outer peripheral surface of the optical fiber 41 and the outer surface of the optical fiber core 1. . Next, the motor 36 is operated to forcibly rotate the mandrel 351 by the set rotation amount.
This state is maintained. Thereby, the optical fiber core wire 1 between the pair of mandrels 341 and 351 is forcibly extended in the fiber axis direction by a circumferential length corresponding to the forcible rotation amount of the mandrel 351, that is, tension is applied. The core 2 is in a state in which elastic strain (elongation strain) on the tensile side is generated, and the next irradiation step is performed in this state.

In the irradiation step, first, the phase mask 5 is set on the writing area of the grating 21 of the optical fiber core 1 and the phase mask 5 is moved from one end to the other end in the fiber axis direction of the phase mask 5. Ultraviolet laser light from an ultraviolet irradiation system is applied to the optical fiber core 1 through the phase mask 5 over the range. The change of the irradiation position of the ultraviolet laser light in the fiber axial direction range is performed by moving the reflection mirror 10 in the fiber axial direction. Then, the phase grating 5 is applied to the core 2 in a state where the above-described elongation strain is generated by the irradiation of the ultraviolet laser light.
Will be written with a grating pitch corresponding to the lattice pitch of

After the writing of the grating 21 is performed in this irradiation step, a tension releasing step is performed. In the tension releasing step, the motor 36 is rotated reversely by the above-mentioned set amount of rotation, and the optical fiber core 1 is rotated. Is restored to the original state before the application of the tension, and the state becomes a no-load state. As a result, the elongation strain generated in the core 2 is restored to the original state, that is, contracted, and the contraction of the grating causes the grating pitch of the written grating 21 to be narrowed. For this reason, the wavelength characteristic of the grating 21 is shifted to the shorter wavelength side by the narrowing of the grating pitch.

Although the fabrication of the fiber grating itself is completed as described above, in the present embodiment, a screening step is subsequently performed. That is, in this screening step, a constant elongation strain is applied to the fiber grating in the fiber axis direction for a predetermined time by operating the motor 36 of the tension applying mechanism 30, and a screening test for mechanical strength characteristics is performed. Then, the defective fiber grating is eliminated from the product, and the fiber grating without the defect is used as the product. As a result, in the production of the fiber grating, both the formation of the grating 21 whose wavelength characteristics are shift-controlled and the screening of the fiber grating on which such a grating 21 is formed can be performed by a series of steps in the same production apparatus. it can.
Therefore, the present fiber grating manufacturing apparatus and method provide a material suitable for mass production of fiber gratings.

In the shift control of the wavelength characteristics to the shorter wavelength side by the above-described tension applying step, irradiation step and tension releasing step, the optical fiber core 1 on which the coating layer 4 is formed is used as a target for writing the grating 21. Therefore, the tension applied in the tension applying step can be set to a value significantly higher than the case where the optical fiber 1 ′ from which the coating layer 4 is removed is targeted. For this reason, the irradiation step can be performed in a state where a high tension is applied, that is, in a state where a large elongation strain is generated in the core 2, and accordingly, the contraction amount of the core 2 due to the tension release step, that is, the grating pitch is reduced. The degree of narrowing can be increased. Therefore, the shift amount of the wavelength characteristic of the fiber grating to the short wavelength side after the tension releasing step can be made extremely large, and the shift control to the short wavelength side can be performed over an extremely wide wavelength range. .

In the actual wavelength characteristic shift control (wavelength control), 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 the shift control is performed based on this relationship. It is sufficient to set an applied tension corresponding to the shift amount of the wavelength to be performed, and determine the set rotation speed of the motor 36 so that the applied tension is generated in the optical fiber 1.

In the above-described fiber grating manufacturing method, in order to more reliably write the grating 21 by irradiating the ultraviolet laser light from above the coating layer 4, the ultraviolet laser light irradiation is performed as follows. You may.

That is, the irradiation with the ultraviolet laser light is performed so that the irradiation energy density becomes about 1.5 kJ / cm ² . Thereby, when the ultraviolet laser light is irradiated from the outside of the coating layer 4, the coating layer 4 has a thickness of approximately 30 μm.
Even when the core 2 has a considerably thick film thickness as described above, high refractive index modulation is generated on the core 2 through the coating layer 4 so that the Bragg grating 21 with high reflectivity can be written.

In addition, as shown in FIG. 6, the optical fiber 1 to be written 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 Light irradiation is performed. The beam pattern BP is obtained by converging a parallel beam incident on the cylindrical lens system 7 so as to be directed to the focal point F, and the entire optical fiber core 1 is located inside the beam pattern BP with respect to the beam pattern BP. And the optical fiber core 1 is positioned such that the outer peripheral surface of the coating layer 4 of the optical fiber core 1 is inscribed in the outer edge of the beam pattern BP. In addition,
If such a positional relationship is satisfied, the arrangement position of the optical fiber core wire 1 is located in front of the focal point F as shown by a solid line in FIG. It does not matter if it is the side. For example, when the focal length L1 is 100
mm, the optical fiber core wire 1 having an outer diameter of 200 μm may be disposed on the optical axis at a distance L2 of about 2 mm from the focal point F. By positioning the entire optical fiber core 1 inside the beam pattern BP, it becomes possible to irradiate the entire coating layer 4 with an ultraviolet laser beam at a uniform irradiation energy density. 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 wire 1 is disposed at a position closer to the focal point F side. Irradiation to the optical fiber core wire 1 can be performed at a position where the irradiation energy density is the highest within a range where the occurrence of excessive strength deterioration can be prevented, and the time required for writing the grating can be reduced. <Other Embodiments> Note that the present invention is not limited to the above-described embodiments, but includes various other embodiments. That is, in the above-described embodiment, the tension application by the tension application mechanism is performed by using one of the winding drums 35 as the arm member 33.
, And the winding drum 35 is forcibly rotated by a motor 36. However, the present invention is not limited to this, and both the winding drums 34, 35 are not rotated with respect to the arm members 32, 33. The one end 331 of one of the arm members 33 is guided and supported so as to be movable in the fiber axis direction with respect to the frame 31 as shown by a chain line in FIG. The optical fiber core 1 is constructed by configuring the device to be forcibly moved to the right side in FIG. 4 by a combination of a mechanism and a motor or an actuator such as a hydraulic cylinder.
May be applied with tension.

The method for producing a fiber grating of the present invention is suitably applied to the production of both a short-period grating and a long-period grating. The short-period grating has a pitch of about 1 μm or less, and the long-period grating has a pitch of about several hundred μm.

Hereinafter, embodiments of the present invention will be described.

[0054]

(Embodiment 1) In Embodiment 1, a plurality of short-period gratings were manufactured on the same fiber core using the same phase mask.

A silica glass fiber co-doped with Ge and Sn was used as a fiber strand for producing a short-period fiber. The relative refractive index difference (Δ) of this fiber strand is 0.5.
97%, cutoff wavelength (λc) is 1.27 μm, Sn
The concentration was 1,500 ppm.

The surface of the fiber was covered with an ultraviolet-transmissive UV-curable resin having a high transmittance to ultraviolet rays for writing the grating. In this embodiment,
Using an aliphatic urethane acrylate (photopolymerization initiator: 2,4,6, -trimethylbenzoyldiphenylphosphine oxide) having a transmittance of about 10% or more for ultraviolet rays having a wavelength of about 240 nm to about 270 nm, a thickness of about 60 μm
A coated fiber core was obtained by forming a coating layer (monolayer) of m. In order to increase the photo-induced refractive index change of the coated fiber core, the coated fiber core was left in a high-pressure hydrogen gas of about 20 MPa for about two weeks to perform a hydrogen filling treatment.

A grating was written on the fiber strand and the coated fiber core filled with hydrogen by using a phase mask method. For writing the grating, the fiber grating manufacturing apparatus shown in FIGS. 3 and 5 was used. While applying strain (tension) in the axial direction of the fiber core, the fourth harmonic (2) of the Nd-YAG laser is applied.
66 nm: intensity of 10 mW) was swept and irradiated (about 22 mm). The laser irradiation time was adjusted so that the reflection level at the reflection peak wavelength was the same. In the distortion application (tension application), a stepping motor and a worm gear were combined, and the amount of distortion was controlled by the rotation amount (the number of steps) of the stepping motor.

No strain was applied (0 step), strain was 0.21% (300 steps),
A grating was written in each state with a distortion of 0.35% (500 steps). FIG. 7 shows the reflection spectrum of the obtained grating. As can be seen from FIG. 7, shortening of the wavelength by 3.4 nm and 4.4 nm was confirmed at 0.21% strain and 0.35% strain, respectively.
In the case of the fiber strand, the tensile strength is weak and the fiber is broken by about 1% strain, so that the degree of shortening the wavelength by applying the tension is small.

Twenty different strains were applied to the hydrogen-filled coated fiber core, and 20 gratings were produced on one fiber core at intervals of about 5 cm. For the 20 different applied strains, the magnitude of the applied strain was adjusted by controlling the amount of rotation of the stepping motor so that the wavelength difference between the reflection peaks between adjacent gratings was equal. Conditions in which 20 different strains are applied to the fiber core (rotation amount (number of steps) and distortion amount of the stepping motor),
Table 1 shows the reflection peak wavelength (strain applied state and strain released state) of the obtained grating.

[0060]

[Table 1]

FIG. 8 shows the reflection spectrum of a continuously manufactured fiber grating having 20 gratings after the hydrogen removal stabilization treatment. After the stabilization of hydrogen removal, the wavelength was further reduced to about 1.7 nm. From FIG. 8, it can be seen that gratings of 20 waves are formed at an average wavelength interval of about 0.82 nm between adjacent gratings. Further, it was confirmed that the wavelength was shortened by about 17 nm at a distortion amount of 1.4%. in this way,
Since the coated fiber has a high tensile strength, the grating can be written in a state where a strain exceeding the breaking strain (about 1%) of the conventional exposed fiber core (fiber strand) is applied.

FIG. 9 shows the relationship between the applied distortion (calculated from the rotation of the stepping motor) and the reflection peak wavelength in the reflection spectrum. In FIG. 9, x indicates the reflection peak wavelength in the state of applying strain during writing of the grating, and ○ indicates the reflection peak wavelength in the state where the strain is released. As is clear from FIG. 9, in each case, the applied distortion amount and the degree of shortening of the reflection peak have a substantially linear relationship. The relation between the applied strain amount (x) and the reflection peak wavelength (y) in each of the strain applied state and the strain released state is expressed by the following equations (1) and (2).

Strain applied state: y = −2.3177x + 1544.1 (1) Strain released state: y = −12.138x + 1543.3 (2) As shown in FIG. 10, in the strain applied state (during irradiation). The reflection peak wavelength (x) and the reflection peak wavelength (y) in the strain released state have a linear relationship represented by the following equation (3).

Y = 5.2302x−6552.4 (3) Therefore, from the above relational expression, the amount of applied strain necessary to obtain a predetermined reflection peak wavelength in the strain release state (the reflection peak wavelength in the applied strain state) Can be requested. Further, when a plurality of gratings are manufactured on one fiber core, the amount of applied strain actually applied is determined from the amount of shift of the reflection peak wavelength of a specific grating (referred to as a reference grating) formed earlier. It can be obtained as a relative value to the amount of strain applied when the reference grating was manufactured. Therefore, by monitoring the reflection peak wavelength of the reference grating,
The relative values (wavelength intervals) of the reflection peak wavelengths of a plurality of gratings can be controlled more accurately. That is, the reference grating can be used as a strain sensor.

Specifically, in a method of manufacturing a plurality of gratings having different reflection peak wavelengths at a predetermined wavelength interval, the applied strain can be controlled as follows.

A first grating is manufactured while applying a predetermined first strain so as to obtain a predetermined wavelength shift. Of course, it is not necessary to apply a strain when producing the first grating. Thereafter, in the step of applying a second strain for producing the second grating, the amount of applied strain is increased while actually monitoring the reflection peak wavelength of the first grating. When the shift amount of the reflection peak wavelength of the first grating reaches a predetermined wavelength interval, the amount of applied strain is fixed, and the second grating is written. By repeating this operation, a plurality of adjacent gratings can be formed with high accuracy at a constant group length interval. A reference grating used as a strain sensor in a grating forming process after the second grating may be appropriately selected from already formed gratings. To accurately measure the tension applied to the grating during fabrication,
It is preferable to use a grating at a close position as a reference grating.

The grating for monitoring the reflection peak wavelength at the time of writing the grating is preferably in the same applied state as the grating to be written from now on, but the amount of distortion applied to the grating is calculated from the reflection peak wavelength of the grating in the strain released state. Using the relationship between and the actual reflection peak wavelength, the amount of strain to be applied to the grating to be written can be calibrated. Of course, this calibration does not need to be performed during the grating writing process, and a calibration curve may be formed in advance for each fiber.

As described above, according to the present invention, a plurality of gratings having different reflection peak wavelengths can be formed on a single fiber core using a single mask. In addition, by using the previously formed grating as a kind of strain sensor, the amount of strain applied for forming the subsequent grating can be accurately controlled.

The short-wavelength grating of this embodiment is suitably used as a filter for extracting light of a specific wavelength in multiplex optical communication, for example. Note that, in the above-described embodiment, the configuration in which the reflection peak wavelengths of the 20 gratings are all different is shown. However, the present invention is not limited to the above configuration, and the relationship between the reflection peak wavelengths of a plurality of gratings is arbitrary. Good.

(Embodiment 2) In Embodiment 2, a long period grating is manufactured. A coated fiber core subjected to the same hydrogen filling treatment as in Example 1 was used as the fiber core. 3 and FIG.
Was performed in the same manner as in Example 1 using the grating manufacturing apparatus shown in FIG. However, the pitch of the phase mask is 20
0 microns.

Table 2 shows the conditions (the amount of rotation (the number of steps) and the amount of distortion of the stepping motor) under which different strains were applied to the fiber core, and the transmission peak wavelength (strain released state) of the obtained grating. FIG. 11 shows a transmission spectrum of the obtained fiber grating in a strain-relaxed state.

[0072]

[Table 2]

As can be seen from FIG. 11, light due to coupling with the seventh-order Cladd mode in the measurement wavelength range of 1.2 to 1.7 μm is observed, and the respective transmission peak wavelengths shift to shorter wavelengths due to the application of strain. It was confirmed that. From the relationship between the amount of applied distortion and the transmission peak wavelength shown in FIG. 12, it can be seen that the higher the transmitted light, the greater the amount of shift to the shorter wavelength side due to distortion. 1. Used for optical communication
FIG. 13 shows the transmission spectrum of light near 58 μm, and FIG. 14 shows the relationship between the amount of applied strain and the transmission peak wavelength near 1.58 μm. As can be seen from these figures, 1.58
The transmission peak wavelength near μm is shortened to about 6.6 nm at a distortion amount of 0.42%.

As described above, according to the present invention, a plurality of long-period gratings having different transmission peak wavelengths can be manufactured using the same mask. The long-period grating according to the present invention is suitably used as a band-pass filter. Furthermore, by forming a plurality of long-period gratings having different transmission peak wavelengths on one fiber core, bandpass filters having different bandwidths can be manufactured.

[0075]

According to the fiber grating manufacturing method of the present invention, it is possible to control the shift of the wavelength characteristic to the shorter wavelength side, which cannot be realized by the conventional tension applying method in which the tension is applied after writing the grating. become.
Further, the wavelength characteristics of the fiber grating shifted to the short wavelength side can be stably obtained. In addition, compared to the above-mentioned conventional tension applying method, there is no need to individually maintain the optical fiber after the writing of the grating in the tension applied state. It can be mass-produced.

According to the present invention, a plurality of gratings having different pitches can be manufactured on one fiber core by irradiating ultraviolet rays while applying tensions having different magnitudes. For example, when writing a grating using the phase mask method, gratings having different pitches can be manufactured on one fiber core using the same phase mask. Furthermore, if a subsequent grating is written while applying tension to a region including the previously formed grating, the wavelength characteristics (reflection characteristics and / or transmission characteristics) of the previously formed grating can be monitored, and Since the magnitude of the tension applied to the fiber core can be determined at the same time, the tension required for the grating to be written can be accurately determined. Therefore,
A plurality of gratings having a small difference in grating pitch can be manufactured with high accuracy and high productivity.

Further, by using an optical fiber core having a coating layer as an optical fiber used for fabricating a fiber grating, the tension that can be applied is greatly reduced as compared with the case where an optical fiber without the coating layer is used. Accordingly, the shift amount of the wavelength characteristic to the short wavelength side can be increased, and the range in which the wavelength control can be performed can be greatly expanded. In addition, 1
A decrease in yield due to writing a plurality of gratings on one optical fiber core is also suppressed.

Further, by applying a predetermined set tension to the produced fiber grating to perform a screening test for a writing area of the grating, a screening step is continuously performed. Inspection of the mechanical properties of the region, that is, the strength and the presence or absence of surface flaws, etc. can be performed together with the production of the fiber grating, which is suitable as a mass production system of fiber grating with wavelength control It can be.

According to the fiber grating manufacturing method of the present invention, in the manufacturing of the fiber grating, the writing of the grating by irradiating ultraviolet rays and the screening test of the fiber grating on which the grating is written are completed by a series of continuous steps. Thus, the present invention can be suitably used as a mass production system for fiber gratings.

According to the fiber grating manufacturing apparatus of the present invention, a method of manufacturing a fiber grating in which the wavelength characteristics of the above-described tension applying step, irradiation step, and tension releasing step are controlled to be shifted to the shorter wavelength side can be easily and reliably implemented. Thus, a fiber grating with wavelength control as described above can be easily and reliably manufactured.

Further, the screening process of the shift-controlled fiber grating can be easily and reliably performed by the same manufacturing apparatus.

When the fiber grating manufacturing apparatus of the present invention is used, even when only the irradiation step and the screening step are performed, both the writing of the grating and the screening test of the fiber grating on which the grating is written are the same. This can be performed using a manufacturing apparatus.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an optical fiber core wire and a grating to be manufactured according to the present invention.

FIG. 2 is a cross-sectional view of the optical fiber ribbon of FIG. 1;

FIG. 3 is a schematic diagram illustrating a fiber grating manufacturing apparatus according to an embodiment.

FIG. 4 is an enlarged explanatory view of the tension applying mechanism of FIG. 3;

FIG. 5 is an enlarged sectional view taken along line AA of FIG. 4;

FIG. 6 is a diagram showing a positional relationship between an optical fiber core wire and a cylindrical lens system.

FIG. 7 is a reflection spectrum of a plurality of short-period gratings obtained by applying different strains.

FIG. 8 is a reflection spectrum of a fiber grating including a short-period grating of 20 waves.

FIG. 9 is a graph showing a relationship between an applied strain amount at the time of writing a grating and a reflection peak wavelength in a reflection spectrum (in a strain applied state and in a strain released state) of the obtained fiber grating.

FIG. 10 is a graph showing a relationship between a reflection peak wavelength in a strain applied state and a reflection peak wavelength in a strain released state.

FIG. 11 is a transmission spectrum of a plurality of long-period gratings obtained by applying different strains.

FIG. 12 is a graph showing a relationship between an applied strain amount at the time of writing a grating and a transmission peak wavelength in a transmission spectrum (strain released state) of the obtained fiber grating.

FIG. 13 shows spectra around 1.58 μm of a plurality of long-period gratings obtained by applying different strains.

FIG. 14 is a graph showing the relationship between the amount of applied strain and the transmission peak wavelength near 1.58 μm.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber core wire (optical fiber) 2 Core 3 Cladding 4 Coating layer 21 Grating 30 Tension applying mechanism 34 Winding drum (fixing means) 35 Winding drum (moving drum on the side to be moved; fixing means) 36 Motor (moving means) Y axis Axis orthogonal to fiber axis direction

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Katsuaki Kondo 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. Itami Works (72) Inventor Tadahiko Nakai 4-3-1 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries Inside Itami Works (72) Inventor Yasuhide Sudo 4-3 Ikejiri, Itami-shi, Hyogo Prefecture Mitsubishi Cable Industries Co., Ltd. Itami Works

Claims (8)

[Claims] A first step of preparing a fiber core having a core and a cladding; and applying a first tension to a first region of the fiber core to generate a first tensile strain in a fiber axial direction. A tension applying step, and irradiating the fiber core wire in a state where the first tensile strain is generated with ultraviolet rays, thereby forming a first grating having a first pitch in a fiber axis direction in the first region of the fiber core wire. A first irradiation step of writing the core in the first, and a first tension releasing step of shifting the first pitch of the first grating written on the core to a shorter wavelength side by releasing the first tension. Includes a fiber grating manufacturing method. 2. The method according to claim 1, further comprising, before the first irradiation step, a step of forming another grating on the fiber core wire, wherein the first tension applying step includes a step of applying the other tension to the first region including the other grating. The step of applying the first tension, the step of measuring the wavelength characteristic of the other grating, and the step of measuring the wavelength characteristic of the other grating based on the measured wavelength characteristic of the other grating. The fiber grating manufacturing method according to claim 1, further comprising: determining a magnitude; and determining the magnitude of the first tension based on the magnitude of the determined tension. 3. After the first irradiating step, by applying a second tension different from the first tension to a second region including the first region of the fiber core in a fiber axial direction. A second tension applying step for generating a second tensile strain, and irradiating the fiber core wire in a state where the second tensile strain is generated with ultraviolet rays, thereby forming a second grating having a second pitch in a fiber axis direction. A second irradiation step of writing to the core in the second region of the fiber core; shifting the second pitch of the second grating written to the core to a shorter wavelength side by releasing the second tension; A second tension releasing step, wherein the second tension applying step includes: measuring a wavelength characteristic of the first grating; and measuring a measured wave of the first grating. A step of determining the magnitude of the tension applied to the second region based on the characteristic; and a step of determining the magnitude of the second tension based on the determined magnitude of the tension. The second pitch in the second irradiation step is the first pitch.
2. The method of manufacturing a fiber grating according to claim 1, wherein the first pitch in the irradiation step is the same as the first pitch. 4. The fiber grating fabrication according to claim 1, wherein a fiber core further comprising a coating layer made of an ultraviolet transmitting resin covering the clad is used as the fiber core. Method. 5. The fiber according to claim 1, further comprising, after the first tension releasing step, a screening step of performing a screening test on a grating writing area by applying a set tension to the fiber core. Grating fabrication method. 6. An ultraviolet irradiation system for irradiating an ultraviolet ray to a fiber core to be manufactured, so that a grating having a predetermined grating pitch is written on a core of the fiber core in a fiber axis direction. A fiber grating manufacturing apparatus, comprising: a tension applying mechanism for temporarily applying a tension so that a tensile strain in a fiber axial direction is generated in a fiber core in an area irradiated with ultraviolet rays by an irradiation system. 7. The tension applying mechanism comprises: a pair of fixing means for fixing fiber core wires at both positions separated from each other in a fiber axis direction with a region irradiated with ultraviolet light by the ultraviolet irradiation system; The fiber grating manufacturing apparatus according to claim 6, further comprising: moving means for forcibly moving at least one of the fixing means with respect to the other in the fiber axis direction. 8. The fiber core according to claim 1, wherein each of the pair of fixing means winds the fiber core around an axis orthogonal to a fiber axis direction, based on frictional resistance between the fiber core and the fiber core. And a winding drum that is moved by the moving means is rotatably supported around the orthogonal axis at the same position in the fiber axis direction, and the moving means The fiber grating manufacturing apparatus according to claim 7, further comprising a motor that forcibly rotates a winding drum on a side to be rotated by a set rotation amount in a state where the fiber core is wound.