JP2008058694A - Fiber bragg grating element, and method and apparatus of manufacturing fiber bragg grating element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a fiber Bragg grating element, in which a high cut off value exceeding 40dB is available in a wide band range with a simple structure, and to provide a method of manufacturing a fiber Bragg grating element in which the high cut off value is available and further a desired transmission loss is obtained. <P>SOLUTION: The fiber Bragg grating is characterized in providing: a core in which a plurality of gratings are formed of which the gap increases toward the central part from both end parts in the optical axis direction and the gap at the central part is larger than any widest gaps at parts other than the central part; and a clad of which the surface is provided with a shield part equivalent to the gap of the gratings at the central part. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fiber Bragg grating element which is a main optical component used in an optical communication system by irradiating an optical fiber with ultraviolet rays, a method for manufacturing the fiber Bragg grating element, and a manufacturing apparatus therefor.

  In recent years, research and practical application of high-speed and large-capacity optical networks have been advanced, and various optical components are required for the construction of this network. One of the main components of such an optical functional component is a fiber Bragg grating element.

This fiber Bragg grating element is an optical passive component in which periodic refractive index modulation is applied to the core layer of the optical fiber by irradiating the optical fiber with ultraviolet rays. By this refractive index modulation, a function of reflecting only a signal having a desired wavelength among optical signals propagating in the optical fiber can be provided.
Therefore, this fiber Bragg grating element can function as an optical filter such as a wavelength filter or a dispersion compensator, and is a key device for wavelength division multiplexing (WDM) communication.

  As a method for manufacturing this fiber Bragg grating element, there are several methods such as a method using a phase mask and a method using holographic interference. However, in any of the methods, periodic interference fringes are formed in the core layer of the optical fiber having photosensitivity using light interference, and refractive index modulation is applied. When a fiber Bragg grating element is manufactured using a phase mask, generally, refractive index modulation is formed by using interference between two light beams between plus and minus first-order light from the phase mask.

Of the manufacturing methods described above, in consideration of mass production, it is common to employ a method using a phase mask having excellent reproducibility. This phase mask is formed by forming periodic grooves with a pitch in accordance with a predetermined reflection wavelength on the surface of a material that transmits ultraviolet rays such as quartz glass. When the phase mask is irradiated with ultraviolet rays, the ultraviolet rays are diffracted in a specific direction by the grooves formed on the surface.
US Patent No. 5,619,603 JP 2001-183535 A

  By the way, when an optical signal is to be cut off in a wide band of about 10 nm, the fiber Bragg grating element is a chirped grating whose grating interval (hereinafter referred to as pitch) changes in the longitudinal direction. Further, when a large cutoff amount is to be obtained in this band, an attempt is made to realize a large cutoff amount by increasing the grating length, which is the length in the longitudinal direction of the fiber Bragg grating element.

  For example, FIG. 26 shows a result of blocking light of about 10 nm band having a wavelength of 1650 nm band by a chirped grating having a grating length of 7 mm, and a blocking amount of about 30 to 35 dB is obtained. FIG. 27 shows the result of blocking about 10 nm band light having the same wavelength of 1650 nm with a chirped grating having a grating length of 13 mm, and a blocking amount of about 30 to 35 dB is obtained.

  However, when the grating length is approximately doubled, the blocking amount should be approximately doubled to obtain a blocking amount of about 80 dB. However, the blocking amount shown in FIG. 27 is only a blocking amount of about 5 dB. It is only increasing. That is, there is a limit to obtaining a large blocking amount even if the grating length is simply increased.

  On the other hand, in the field of optical communication in recent years, it is necessary to reliably separate the light of the supervisory communication system from the light of the real communication system. In order to eliminate the influence as much as possible, for example, a blocking amount of about 40 dB that is stable in a wide band may be required. For this reason, there has been a demand for the appearance of a fiber Bragg grating element capable of stably obtaining a cutoff amount exceeding 40 dB in a wide band.

  The present invention has been made in view of the above-described contents, and provides a method for manufacturing a fiber Bragg grating element capable of obtaining a high cutoff amount exceeding 40 dB in a wide band with a simple configuration. Furthermore, the present invention provides a method for manufacturing a fiber Bragg grating element that achieves a desired transmission loss in a fiber Bragg grating element capable of obtaining a high cutoff amount.

  The first aspect of the fiber Bragg grating element of the present invention has a plurality of gaps that extend from both ends toward the center in the optical axis direction, and the gap at the center is wider than the widest gap other than the center. A fiber Bragg grating, comprising: a core formed inside the core; and a clad formed outside the core and provided with a shielding portion on the surface corresponding to a gap between the gratings in the center. It is an element.

  A second aspect of the fiber Bragg grating element of the present invention is a fiber Bragg grating element characterized in that the shielding portion is 2 mm or more.

  A third aspect of the fiber Bragg grating element of the present invention is a fiber Bragg grating element characterized in that the interval between the gratings at the center is 4 mm or more.

  A first aspect of a method for manufacturing a fiber Bragg grating element according to the present invention is a method for manufacturing a fiber Bragg grating element by irradiating an optical fiber with ultraviolet light through a phase mask, and the ultraviolet light is transmitted through a shielding plate. A method for manufacturing a fiber Bragg grating element, in which an optical fiber is irradiated through a phase mask.

  A second aspect of the method for manufacturing a fiber Bragg grating element according to the present invention is a method for manufacturing a fiber Bragg grating element by irradiating an optical fiber with ultraviolet rays through a phase mask, and the ultraviolet rays are transmitted through a phase mask. Then, it is a manufacturing method of a fiber Bragg grating element which irradiates an optical fiber via a shielding plate.

  A first aspect of a fiber grating Bragg grating element manufacturing apparatus according to the present invention includes an optical fiber on which a grating is formed, an ultraviolet laser generator that generates ultraviolet rays applied to the optical fiber, and an optical fiber and an ultraviolet laser generator. A detecting means comprising: an irradiating means including a shielding plate and a phase mask installed between; a light source that transmits an optical signal from one end of the optical fiber; and a spectrum analyzer installed on the other end of the optical fiber; Is a manufacturing apparatus of a fiber Bragg grating element.

  A second aspect of the apparatus for manufacturing a fiber Bragg grating element according to the present invention is an apparatus for manufacturing a fiber Bragg grating element, wherein the phase mask is disposed between the ultraviolet laser generator and the shielding plate. .

  According to a third aspect of the apparatus for manufacturing a fiber Bragg grating element of the present invention, the apparatus for manufacturing a fiber Bragg grating element is characterized in that the shielding plate is disposed between the ultraviolet laser generator and the phase mask. .

  According to the present invention, a fiber Bragg grating element capable of avoiding or reducing a cutoff amount of ripple generated in a wavelength band to be used is realized by separating a pair of gratings arranged opposite to each other in the core. it can.

  Furthermore, when manufacturing a fiber Bragg grating element, a fiber Bragg grating element capable of obtaining a desired transmission loss can be manufactured by irradiating an optical fiber with ultraviolet rays through a phase mask after passing through a shielding plate.

  Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. 1 to 25.

(First embodiment)
FIG. 1 is a diagram showing a schematic configuration of an optical branch line monitoring system using a fiber Bragg grating element (hereinafter referred to as an FBG element) according to a first embodiment of the present invention. In this optical branch line monitoring system, a trunk optical line 22 is connected to the transmission apparatus 10, and the trunk optical line 22 is optically branched by an optical splitter 28 and branched into a plurality of trunk optical lines 22. The branched main optical line 22 extends outside the station via the optical coupler 18, and is optically branched into the branched lines 1 a _{1 to 1 a 8} by the outdoor optical splitter 3.

Each of the branched optical lines 1a _{1 to} 1a ₈ subjected to optical multi-branching is connected to each ONU (Optical Network Unit) 20 in the user 24. The optical splitter 3 has an optical line monitoring device (not shown), and this optical monitoring line device associates only the monitoring lights λc _{1 to} λc ₈ with the branched optical lines 1a _{1 to} 1a ₈ respectively. Input and output.

  The control unit 26 performs output control of variable wavelength monitoring light output from an OTDR (Optical Time Domain Reflectometer) 2 and outputs the monitoring light to a fiber selector (hereinafter referred to as FS) 25 and also controls reception measurement. The OTDR 2 is connected to the optical coupler 18 via the control unit 26 and the FS 25.

  Each ONU 20 is assigned a monitoring light wavelength unique to each ONU 20, and FBG elements 21-1 to 21-8 that reflect the monitoring light in the monitoring light wavelength band and block the output are provided in each ONU 20. . Each of the FBG elements 21-1 to 21-8 is an FBG element having the same configuration, and has a characteristic of blocking monitoring light in a band of about 10 nm by about 60 dB.

The control unit 26 periodically emits the monitoring lights of λc ₁ to λc ₈ to be monitored and outputs the monitoring lights to the trunk optical line 22 via the optical coupler 18. At this time, the FS 25 selects the trunk optical line 22 to be output. In the optical coupler 18, the communication light having the wavelength λb propagated from the transmission device 10 is input to the optical splitter 3 together with the monitoring light having the wavelength λc ₁ , for example, and the communication light having the wavelength λb is transmitted via the branched optical lines 1 a _{1 to 1 a 8} . The monitoring light having the wavelength λc ₁ is input to the ONU 20 connected to the branch optical line 1a ₁ .

FIG. 2 is a schematic diagram illustrating the ONU in detail. As shown in FIG. 2, the monitoring light having the wavelength λc ₁ is reflected by the FBG element 21-1, but the communication light having the wavelength λb is received as it is by the optical receiving unit 31-1, and the O / E unit 32-1. Is subjected to optical / electrical conversion and input to the reception processing unit 33-1. Similarly, when the monitoring light having the wavelength λc ₃ and the communication light having the wavelength λb are input to the branch optical line 1a ₃ , the monitoring light having the wavelength λc ₁ is reflected by the FBG 21-3. The communication light of λb is received as it is by the optical receiving unit 31-3, optical / electrically converted by the O / E unit 32-3, and input to the reception processing unit 33-3.

Here, when monitoring light of wavelength λc ₁ or the like is input to the optical receivers 31-1 to 31-8, a communication error or the like occurs, which greatly affects communication, and these monitoring light is used as the FBG element 21-1. It is necessary to block reliably by ~ 21-8. Here, as described above, since the FBG elements 21-1 to 21-8 have a cutoff amount of about 60 dB in the wavelength band of the monitoring light of about 10 nm, the monitoring light can be reliably cut off. The FBG elements 21-1 to 21-8 are fixed by a ferrule (not shown) and provided in the connectors 30-1 to 30-8.

Here, the configuration of the FBG element 21 (21-1 to 21-8) will be described with reference to the drawings. FIG. 3 is a cross-sectional view of the FBG element, and FIG. 4 is a schematic diagram showing a concept of a wideband filter using the FBG element. As shown in FIG. 3, the FBG element 21 is a chirped grating in which the grating pitch Λ corresponding to the wavelengths λc _{1 to} λc ₈ changes in the longitudinal direction. The FBG element 21 has a grating disposed so as to face the central portion O in the longitudinal direction, and is disposed so that the pitch Λ of the grating becomes shorter with distance from the central portion.

Furthermore, as shown in FIG. 4, since the Bragg center wavelength is the wavelengths λc _{1 to} λc ₈ , it can be reflected in a wide band from λc ₁ to λc ₈ , and light in this wavelength band can be blocked. As a result, the ONU 20 can use the same FBG element 21. The FBG 21 may be formed with the wavelength of each monitoring light as the Bragg center wavelength.

  FIG. 5 is a graph showing the cutoff characteristics of the FBG element. The FBG element 21 blocks an input optical signal by 40 dB or more in a band of about 10 nm centering on 1650 nm. In this embodiment, the grating pitch Λ of the FBG element 21 is formed so as to increase toward the center in the length direction of the optical fiber. Obtainable.

  Next, the basic principle of the FBG device manufacturing method of the present invention will be described with reference to the drawings.

  FIG. 6A shows an FBG element in which interference fringes are formed by interference between two light beams between plus and minus primary light. On the other hand, FIG. 6B shows an FBG element in which interference fringes are formed by interference between two light beams of n-order diffracted light such as zero-order light and plus-minus second-order light that was not intended for other ultraviolet irradiation.

  In the drawing, the FBG element is schematically shown. A black line portion indicates a high refractive index, and a white portion indicates a low refractive index. Further, the optical axis direction of the optical fiber is taken as the X axis, and the direction orthogonal thereto is taken as the Y axis.

  The FBG element is a change in the refractive index of the core layer of an optical fiber formed by ultraviolet interference fringes transmitted through a phase mask. In the interference fringes, since the light intensity distribution is directly changed in refractive index, the refractive index change is a periodic function (sin wave).

  Here, the fiber grating formed by plus-minus primary light shown in FIG. 6A has a sin-like refractive index profile in the X-axis direction, but is always constant in the Y-axis direction.

  On the other hand, an FBG element formed by n-order diffracted light such as zero-order light and plus-minus secondary light that was not intended for irradiation is formed with a certain angle in the Y-axis direction. The direction is not constant. (See θ (mn) in FIG. 6B.)

This can be expressed in more detail and functionally as follows.
Originally, since the transmittance of plus / minus primary light is high, the intensity of the interference fringe between the plus / minus primary light and unnecessary light is relatively strong among interference fringes caused by unintended diffracted light. Become.

Here, the angle of the first-order diffracted light reaching the optical fiber from the phase mask is expressed by the following equation.
[Formula 1]
Λ (ph) × (θi) = I × λuv

On the other hand, if diffracted light that is not intended to be transmitted through the phase mask is m-th order and n-th order diffracted light, the two-constraint interference between the m-th order diffracted light and the n-th order diffracted light is shown in FIG. ), The angle θ (mn) is formed with respect to the light propagation direction in the optical fiber.
[Formula 2]
θ (mn) = (θ (m) + θ (n)) / 2
That is, the main phase of unintended interference fringes has an angle θ (mn) with the direction connecting the phase mask and the optical fiber, that is, the Y-axis direction.

  Accordingly, as shown in FIG. 7, while the ultraviolet ray is irradiated through the phase mask and the FBG element is formed, the interval between the phase mask and the optical fiber is modulated (the phase mask and the light are irradiated during the ultraviolet ray irradiation). When the fiber spacing is changed), the phase of the interference fringes by the m-th order n-th order diffracted light whose grating is at an angle of θ (mn) with the Y-axis is shifted.

  On the other hand, in the case of plus / minus primary light, since the equiphase surface is parallel to the Y axis, even if the interval between the phase mask and the optical fiber is changed, the phase of the interference fringes by the plus / minus primary light has no effect. Not receive.

  In the present invention, by using the above principle, interference fringes formed by plus / minus primary light are not affected at all, and interference fringes formed by diffracted light other than plus / minus primary light are not affected. By changing the phase, the refractive index change can be eliminated.

That is, as shown in FIG. 7, during irradiation of ultraviolet rays through the phase mask, the interval between the phase mask and the optical fiber is modulated, that is, by changing the interval with time, plus or minus primary light The phase of interference fringes due to diffracted light other than the above is changed. If the phase is changed so that the interference fringes having different phases just cancel each other, it is possible to substantially eliminate the refractive index change caused by the diffracted light other than the plus / minus primary light.
On the other hand, as described above, the plus / minus primary light is incident in parallel to the Y axis, so that the phase does not change even if the interval between the phase mask and the optical fiber is modulated.

  Therefore, since there is no influence of a change in refractive index due to diffracted light other than plus-minus primary light, an FBG element having excellent reflection crosstalk can be manufactured without deteriorating the return loss.

  Further, the amount of cutoff when the pitch at the center of the grating of the FBG element is changed will be described with reference to the drawings. FIGS. 8 to 16 show the wavelength characteristics of the cutoff amount when the pitch Λ at the center of the grating of the FBG element is continuously changed from 0.3 mm to 5 mm. FIG. 18 shows the wavelength characteristics of the cutoff amount of a conventional fiber Bragg grating element in which the grating pitch Λ is 0 mm. In the wavelength characteristic of the cutoff amount shown in FIG. 18, it can be seen that a ripple of about 20 dB exists around the wavelength of 1657 nm. Here, the above-mentioned ripple is defined as the height of the highest peak in the target wavelength band.

  From FIG. 8 to FIG. 16, it can be seen that the above ripple decreases substantially monotonously as the grating pitch Λ increases. FIG. 17 shows the pitch dependence characteristics of such a ripple grating. From FIG. 17, when the pitch Λ at the center of the grating is 1 mm or more, this ripple is approximately half or less in dB units, and when it is 2 mm or more, this ripple is approximately ¼ or less. From the viewpoint of ripple, by setting the pitch Λ at the center of the grating to 2 mm or more, a practical cutoff amount of, for example, 40 dB can be obtained in the target wavelength band, and by setting it to 4 mm or more, the ripple Is preferably within the range of 1 to 2 dB.

(Second Embodiment)
Next, the transmission loss of the FBG element described above will be described in detail with reference to the drawings. FIG. 19 shows the wavelength characteristics of the transmission loss of the FBG element in which the grating pitch Λ at the center of the FBG element is 0 mm, and FIG. 20 shows the transmission of the FBG element in which the grating pitch Λ at the center of the FBG element is 2 mm. This is the wavelength characteristic of loss. In addition, the zone | band of the irradiated ultraviolet rays is a zone | band of 1520-1580 nm, and No1-No3 described outside a graph shows a diffraction order.

  As shown in FIG. 19, when the grating pitch at the center of the FBG element is 0 mm, the transmission loss is about 0.47 dB, and when the grating pitch at the center of the FBG element is 2 mm, As shown in FIG. 20, the transmission loss is about 0.76 dB. That is, it can be seen that the transmission loss is increased by setting the pitch of the grating at the center of the FBG element to 2 mm.

  Here, when commercializing an ONU in general, it is desirable to suppress the transmission loss after passing through the connector described in the first embodiment to about 1.1 dB. That is, if a transmission loss of about 0.6 dB occurs in the connector, the transmission loss after passing through the FBG element is preferably about 0.5 dB. Therefore, it is desirable to reduce the transmission loss to less than about 0.5 dB by weighting the first embodiment that can stably obtain a blocking amount of about 40 dB.

  Therefore, the inventor used to suppress the transmission loss to less than about 0.5 dB, and when irradiating the optical fiber with ultraviolet rays, the optical fiber was irradiated through the phase mask as shown in FIG. 7, but as shown in FIG. A shielding plate was placed before passing through the phase mask. That is, when irradiating an optical fiber with ultraviolet rays, a desired transmission loss can be obtained by irradiating the optical fiber with ultraviolet rays through a phase mask after passing through a shielding plate, thereby weighting the desired cutoff amount. I found out. Moreover, when irradiating an optical fiber with an ultraviolet-ray, it discovered that the same result could be obtained also by irradiating an optical fiber with an ultraviolet-ray through a shielding board after passing through a phase mask.

  A cross-sectional view of the obtained optical fiber will be described with reference to FIG. FIG. 28 shows an optical fiber as a result of irradiating ultraviolet rays through a shielding plate. As shown in FIG. 28, an optical fiber composed of a core and a clad, and a plurality of gratings 51 are formed inside the core. The grating 51 is formed so that the interval becomes wider toward the center of the core in the longitudinal direction, and the interval between the gratings 51 in the central portion is larger than the interval of the widest grating other than the central portion. . Further, a shielding portion 53 corresponding to the interval between the gratings 51 at the center is formed on the surface of the clad. The shielding portion 53 is a shielding portion that is formed as a result of irradiating ultraviolet rays through a phase mask after passing through a shielding plate.

  A reduction in transmission loss based on the above-described knowledge will be described with reference to the drawings. FIG. 22 shows the wavelength characteristics of the transmission loss of the FBG element in which the grating interval Λ at the center of the FBG element is 2 mm and the ultraviolet ray is irradiated through the shielding plate. In this case as well, the irradiated ultraviolet band is 1520 to 1580 nm.

  Furthermore, the same verification was performed for other bands. FIG. 23 is a graph comparing the case where the above-described shielding plate is used and the case where it is not used where the grating interval Λ at the center of the FBG element is 2 mm at 1200 nm to 1700 nm. FIG. 24 is a table comparing the worst value and the average value of transmission loss in the band (1) 1310 ± 20 nm, (2) 1550 ± 20 nm, and (3) 1670 nm depending on the presence or absence of a shielding plate. As is clear from FIG. 24, the transmission loss is 0.44 dB to 0.73 dB when the shielding plate is not used, whereas the transmission loss when the shielding plate is used is 0.22 dB to 0.00. It can be suppressed to 4 dB.

  Here, a specific embodiment of a manufacturing method for irradiating an optical fiber with ultraviolet rays through a shielding plate and a phase mask according to the present invention will be described below.

  FIG. 25 shows the manufacturing apparatus. The manufacturing apparatus includes an ultraviolet laser generator 35, an optical fiber 37 (on which a fiber grating is formed), and a phase mask 39 and a shielding plate (not shown) installed between the ultraviolet laser generator 35. A light source 43 that transmits an optical signal from one end of the optical fiber 37, and a detection device that includes a spectrum analyzer 45 installed at the other end of the optical fiber 37.

  Further, the optical fiber 37 is guided by two linear guides 47 installed on both sides of the irradiation device in the longitudinal direction of the optical fiber 37 so as not to move except in the longitudinal vertical direction, and is connected to the linear guide 47. The optical fiber 37 is transported in the vertical direction of its length by the actuator 49.

  First, the optical fiber 37 is transported using the actuator 49 so that the portion where the FBG element is formed comes before the irradiation means. Next, adjustment is performed using the moving device 41 so that the interval between the phase mask 39 and the optical fiber 37 becomes a predetermined value, or the optical fiber 37 is moved by the actuator 49. Here, the phase mask 39 is formed with a groove in which a predetermined interference fringe is formed in the optical fiber 37.

Thereafter, irradiation with ultraviolet rays is started by the ultraviolet laser generator 35.
Further, the transmission loss of the optical fiber is always measured by the spectrum analyzer 45, and the modulation can be performed based on this measured value. Furthermore, the ultraviolet irradiation can be terminated when a predetermined loss value in a predetermined wavelength band is reached.
By the above method, an FBG element in which a change in refractive index due to diffracted light other than plus-minus primary light of the present invention is eliminated can be manufactured.

  Furthermore, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention.

  When manufacturing a fiber Bragg grating element, it is possible to manufacture a fiber Bragg grating element that can obtain a desired transmission loss by irradiating an optical fiber with an ultraviolet ray through a phase mask after passing through a shielding plate. High availability.

FIG. 1 is a diagram showing a configuration of an optical branch line monitoring system using a fiber Bragg grating element according to the present invention. FIG. 2 is a diagram illustrating a transmission state of monitoring light and communication light on the ONU side. FIG. 3 is a schematic diagram illustrating a configuration example of the FBG according to the first embodiment. FIG. 4 is a schematic diagram showing the concept of a broadband filter based on FBG. FIG. 5 is a diagram illustrating an experimental result of the cutoff characteristic by the FBG of the first embodiment. 6A is a schematic diagram showing a fiber grating formed by plus / minus primary light, and FIG. 6B is a schematic diagram showing a fiber grating formed by n-order diffracted light other than plus / minus primary light. It is. FIG. 7 is a diagram showing an ultraviolet irradiation method for changing the interval between the phase mask and the optical fiber. FIG. 8 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 0.3 mm. FIG. 9 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 1 mm. FIG. 10 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 1.5 mm. FIG. 11 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 2 mm. FIG. 12 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 2.5 mm. FIG. 13 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 3.5 mm. FIG. 14 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 4 mm. FIG. 15 is a graph showing the wavelength characteristic of the cutoff amount when the grating pitch at the center of the FBG element is 4.5 mm. FIG. 16 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 5 mm. FIG. 17 is a diagram showing the grating pitch dependence characteristics of ripples. FIG. 18 is a graph showing the wavelength characteristics of the cutoff amount when the grating pitch at the center of the FBG element is 0 mm. FIG. 19 shows the wavelength characteristics of the transmission loss of the FBG element in which the grating pitch Λ at the center of the FBG element is 0 mm. FIG. 20 shows the wavelength characteristics of the transmission loss of an FBG element in which the grating pitch Λ at the center of the FBG element is 2 mm. FIG. 21 is a schematic diagram of irradiating an optical fiber with ultraviolet rays through a shielding plate and a phase mask. FIG. 22 shows the wavelength characteristics of the transmission loss of the FBG element in which the grating interval Λ at the center of the FBG element is 2 mm and the ultraviolet ray is irradiated through the shielding plate. FIG. 23 is a graph comparing the case where the above-described shielding plate is used and the case where it is not used where the grating interval Λ at the center of the FBG element is 2 mm at 1200 nm to 1700 nm. FIG. 24 is a table showing the worst value and average value of transmission loss in the presence and absence of a predetermined band and shielding plate. FIG. 25 is a schematic view showing a manufacturing apparatus of the present invention for manufacturing a fiber grating by modulating the interval between the shielding plate and the phase mask and the optical fiber. FIG. 26 is a graph showing the wavelength characteristic of the cutoff amount when the grating length is 7 mm. FIG. 27 is a graph showing the wavelength characteristics of the cutoff amount when the grating length is 13 mm.

Explanation of symbols

1a _{1 to} 1a ₈ branch optical line 2 OTDR
3, 28 Optical splitter 10 Transmission device 18 Optical coupler 20 ONU
21, 21-1 to 21-8, 21 a, 21 b, 21 c, 21 d FBG element 22 trunk optical line 24 user 25 fiber selector 26 control unit 30-1, 30-3 optical connector 31-1, 31-3 optical receiving unit 32-1, 32-3 O / E unit 33-1, 33-3 Reception processing unit 35 Laser generator 37 Optical fiber 39 Phase mask 43 Light source 45 Spectrum analyzer 47 Linear guide 49 Actuator 51 Grating 53 Shielding unit λa, λb Communication Wavelength of light λc _{1 to} λc ₈ Wavelength of monitoring light

Claims (8)

A core in which a plurality of gratings having a width larger than a widest interval other than the central portion is formed inside, with an interval extending from both ends toward the central portion in the optical axis direction,
A clad formed on the outer surface of the core and provided with a shielding portion on the surface corresponding to the spacing of the grating in the central portion;
A fiber Bragg grating element comprising:   The fiber Bragg grating element according to claim 1, wherein the shielding portion is 2 mm or more.   3. The fiber Bragg grating element according to claim 1, wherein an interval between the gratings in the central portion is 4 mm or more. 4.   A method of manufacturing a fiber Bragg grating element by irradiating an optical fiber with ultraviolet rays through a phase mask, the fiber Bragg irradiating the optical fiber with the ultraviolet rays through a phase mask after passing through the shielding plate A method for manufacturing a grating element.   A method of manufacturing a fiber Bragg grating element by irradiating an optical fiber with ultraviolet rays through a phase mask, the fiber Bragg irradiating the optical fiber with the ultraviolet rays through the shielding plate after passing through the phase mask A method for manufacturing a grating element. An optical fiber on which a grating is formed; an ultraviolet laser generator that generates ultraviolet rays applied to the optical fiber; and a shielding plate and a phase mask installed between the optical fiber and the ultraviolet laser generator. Irradiation means;
A light source for sending an optical signal from one end of the optical fiber, and a spectrum analyzer installed at the other end of the optical fiber,
An apparatus for manufacturing a fiber Bragg grating element.   The apparatus for manufacturing a fiber Bragg grating element according to claim 6, wherein the phase mask is disposed between the ultraviolet laser generator and the shielding plate.   The apparatus for manufacturing a fiber Bragg grating element according to claim 6, wherein the shielding plate is disposed between the ultraviolet laser generator and the phase mask.