JP4712648B2 - Fiber Bragg grating element and fiber adapter - Google Patents

Description

  The present invention relates to a fiber Bragg grating element and a fiber adapter having a function as an optical filter for filtering a high cutoff amount over a wide band with respect to an incident optical signal.

Conventionally, in an optical communication apparatus, an optical filter using a fiber Bragg grating (FBG) is widely used to cut off a desired wavelength band (see Patent Document 1). Some FBGs are formed using “photo-induced refractive index change” in which the refractive index increases when an optical fiber is irradiated with ultraviolet rays. The FBG is obtained by forming a periodic refractive index change in the fiber core, and this periodic refractive index change is formed by using a two-beam interference method, a method using a phase mask, or the like. Due to this periodic refractive index change, the light having the Bragg center wavelength λ _B is reflected, and as a result, the light in the Bragg center wavelength λ _B region is blocked. Here, the Bragg center wavelength λ _B is represented by λ _B = 2nΛ. Here, n is the effective refractive index of the optical fiber, Λ is the period of refractive index change, and is called the grating pitch. Such an FBG is used not only as a WDM (Wavelength Division Multiplexing) communication system but also as a multiplexer / demultiplexer, a line monitoring filter, a temperature sensor, and a distortion sensor.
JP 2002-328238 A

  Here, when an optical signal is to be blocked over a wide band of about 10 nm in width, the FBG is changed to a so-called chirped FBG. In order to obtain a large cutoff amount in this band, the grating length is increased. For example, for a chirped FBG with a grating length of 7 mm, light with a wavelength band of about 10 nm with a wavelength of about 1650 nm can be cut off by about 30 to 35 dB, and for a chirped FBG with a grating length of 13 mm, about 35 to 40 dB can be cut off. The amount is obtained.

In the recent optical communication field, etc., 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 as much as possible, for example, a stable cutoff amount of about 40 dB in a wide band may be required. In order to meet this requirement, an FBG element (hereinafter referred to as a composite FBG element) in which a plurality of uniform FBGs and chirped FBGs having a uniform grating pitch are arranged has been developed. With this configuration, as shown in FIG. 19 , a cutoff amount exceeding 40 dB can be obtained in a wide band.

  However, in the conventional composite FBG element, a ripple may occur in the wavelength dependency characteristic of the cutoff amount, and if a ripple of a predetermined value or more occurs in the wavelength band to be used or in the vicinity thereof, there is a problem that it cannot be used in this wavelength band. there were. As a result, the production yield of the composite FBG element has been reduced.

  An object of the present invention is to provide a fiber Bragg grating element and a fiber adapter that can avoid or reduce the ripple of the cutoff amount that occurs in the wavelength band to be used.

According to a first aspect of the present invention, there is provided a core, a clad provided outside the core, and provided at least in the core. A grating pitch gradually changes linearly, and the core and the outside of the core are arranged. A plurality of gratings for performing filtering to cut off an optical signal incident on an optical waveguide having the clad provided in a predetermined wavelength band, wherein at least two of the gratings are arranged so as to face each other apart from each other; The wavelength of the optical signal in the vicinity of 1650 nm is configured to be Bragg-reflected. As the distance from the center of the opposed grating decreases, the grating pitch becomes shorter, and the interval between the gratings arranged to face each other is 3 to 5 mm. With multiple chirped fiber Bragg gratings It is a fiber Bragg grating element that.

According to a fifth aspect of the present invention, there is provided a core, a clad provided outside the core and having both or at least one of a depressed region and a co-doped region surrounding the core, and provided in at least the core, and a grating pitch Is a plurality of gratings that perform filtering that cuts off an optical signal incident on an optical waveguide having a predetermined wavelength band that gradually changes linearly and has the core and the cladding provided outside the core , The two gratings are arranged so as to face each other away from each other, and are configured to Bragg-reflect the wavelength of an optical signal in the vicinity of 1650 nm. As the distance from the center of the facing gratings increases, the grating pitch becomes shorter and faces each other. The spacing between the gratings arranged in such a way is 3 A counter chirped FBG fiber having a plurality of chirped fiber Bragg gratings of 5 mm, a ferrule that holds the counter chirped FBG fiber therein, and a housing that stores the ferrule therein This is a fiber adapter.

  According to the present invention, at least a pair of chirped fiber Bragg gratings disposed opposite to each other in the core are separated, thereby avoiding or reducing the amount of cutoff ripple generated in the wavelength band to be used. A fiber Bragg grating element and a fiber adapter can be realized.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(Embodiment 1)
FIG. 1 is a cross-sectional view schematically showing the configuration of the fiber Bragg grating element according to the first aspect of the present invention. In FIG. 1, a fiber Bragg grating element 10 includes an optical waveguide 1 having a core 1a and a clad 1b provided outside the core 1a, and a plurality of chirped fiber Bragg gratings (provided on the core 1a). (Hereinafter referred to as chirped FBG) 2a and 2b, and the chirped FBG2a and the chirped FBG2b are separated from each other and arranged to face each other. The fiber Bragg grating element 10 may be provided with a high refractive index material 3 on the outer periphery of the cladding 1b as shown in FIG.

In FIG. 1, a plurality of lines oriented in a direction orthogonal to the longitudinal direction of the core 1a represent a portion having a high refractive index. The grating pitch is a distance between two adjacent high refractive index portions. The chirped FBGs 2a and 2b function in the same way as when FBGs with different grating pitches are continuously arranged, since the grating pitch gradually changes linearly, and reflect or block light of a predetermined wavelength band. To do. The chirped FBGs 2a and 2b have a grating length la and lb of 4 to 8 mm, for example, and are configured to cause Bragg reflection over ± 5 nm or more of the Bragg center wavelength λ _B of about 1650 nm, for example. The change range of the grating length and the grating pitch differs depending on the application target, and may take other values and ranges. Specifically, the grating lengths la and lb may be 5 to 6 mm.

The core 1a is surrounded by a cladding 1b formed using SiO _2, is formed using, for example, a SiO ₂ of Ge-doped. The core 1a in the optical waveguide having such a configuration is irradiated with UV light using, for example, a two-beam interference method, a phase mask method, or the like to form a refractive index level corresponding to the grating pitch. It should be noted that other treatments that make it easy to cause a light-induced refractive index change, such as holding the optical fiber in a high-pressure hydrogen atmosphere for a predetermined time before irradiation with UV light, may be performed. Since the formation method of chirp FBG is well-known, further description is abbreviate | omitted. As shown in FIG. 2, in the optical signal incident on the fiber Bragg grating element 10, the light components in the bands of chirped FBGs 2a and 2b have different reflection conditions depending on the wavelengths λ _{1 to} λ _n , and the chirped FBGs 2a and 2b. Reflected at different locations inside.

  3 to 11 show the wavelength characteristics of the cutoff amount when the distance d between the chirped FBG2a and the chirped FBG2b (hereinafter referred to as the grating distance) d is continuously changed from 0.3 to 5 mm. FIG. 19 shows the wavelength characteristics of the cutoff amount of a conventional fiber Bragg grating element in which the grating interval d is 0 mm. In the wavelength characteristic of the cutoff amount shown in FIG. 19, 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.

  3 to 11, it can be seen that the ripple decreases substantially monotonously as the grating interval d increases. FIG. 12 shows such a dependency of the ripple on the grating interval. From FIG. 12, when the grating interval d 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 1/4 or less. From the viewpoint of ripple, by setting the grating interval d to 2 mm or more, a practical cutoff amount such as 40 dB can be obtained in a target wavelength band, and by setting it to 4 mm or more, the ripple is 1 to 2 dB. It is preferable that it can be within the range.

(Embodiment 2)
FIG. 13 is a cross-sectional view schematically showing the configuration of the fiber adapter according to the second aspect of the present invention. In FIG. 13A, the fiber adapter 20 includes a counter chirped FBG fiber 21, a ferrule 22, a split sleeve 23, and a housing 24. As shown in FIG. 13A, the opposed chirped FBG fiber 21 includes an optical waveguide having a core 21a and a clad 21b provided outside the core 21a, and a plurality of chirped gratings 21c provided on the core 21a. 21d, and the chirped grating 21c and the chirped grating 21d are arranged on the core 21a side of the clad 21b having a depressed region 21e surrounding the core 21a, with the grating interval d being spaced apart and opposed to each other. . Here, the depressed region is a region on the core 21a side of the clad 21b and has a lower refractive index than the other regions of the clad 21b.

An example of the refractive index distribution in the radial direction in the optical waveguide is shown in FIG. In FIG. 14, the horizontal axis represents the radial distance (μm) from the center of the core 21a, and the vertical axis represents the refractive index difference. In the example shown in FIG. 14, the diameter of the core 21a is about 10 μm, the diameter of the clad 21b is about 125 μm, and the radial width of the depressed region 21e is about 10 to 25 μm. The width in the radial direction of the depressed region 21e is preferably about 15 to 20 μm from the viewpoint of performance and production . The core 21a has a refractive index higher by about 0.005 than the clad 21b, and the depressed region 21e has a refractive index lower by about 0.001 than the clad 21b.

The clad 21b is usually made of SiO _2, but the depressed region 21e is obtained by doping SiO ₂ with F, Cl or the like. By doing so, since the refractive index of the depressed region 21e is lower than the refractive index of the other regions, the mode light propagating in the core 21a and the mode light propagating in the clad 21b can be separated. Coupling between these modes can be avoided or mitigated. Since the chirped gratings 21c and 21d are the same as those described in the second embodiment, description thereof will be omitted.

  The ferrule 22 includes a flange portion 22a, butted portions 22b and 22c attached to both sides of the flange portion 22a, a fiber hole 22d provided in the longitudinal direction at the center in the radial direction, and the butted portion 22b side of the flange portion 22a. And a slit 22f formed in the flange 22e and positioned with respect to the housing 24. The ferrule 22 is formed by using a material such as stainless steel such as SUS and plastics for the flange portion 22a, and a material such as zirconia for the butting portions 22b and 22c. The ferrule 22 is accommodated in the housing 24 with a pressing spring 26 interposed between a protrusion 24b and a flange 22e described later. A counter chirped FBG fiber 21 is attached to the fiber hole 22 d of the ferrule 22.

  The opposed chirped FBG fiber 21 is fixed to the fiber hole 22d using an adhesive. In the opposed chirped FBG fiber 21 fixed to the fiber hole 22d, the portions exposed at the end faces of the butted portions 22b and 22c of the ferrule 22 are mirror-polished using a technique such as micro-wrapping. Here, the facing chirped FBG fiber 21 has a coating made of a synthetic resin such as nylon applied to the surface of the bare fiber. As an adhesive to be used, it is preferable to use an epoxy adhesive having a refractive index equal to or higher than the refractive index of the clad 21b of the opposed chirped FBG fiber 21. When the portion of the opposed chirped FBG fiber 21 from which the coating has been removed is bonded and fixed to the fiber hole 22d, the chirped gratings 21c and 21d are made to have a refractive index greater than that of the cladding 21b. The influence of the cladding mode that does not pass can be reduced, and the wavelength characteristics can be improved.

  The split sleeve 23 is a cylindrical member made of zirconia, phosphor bronze, or the like having a slit 23a. The split sleeve 23 is attached to the abutting portion 22c of the ferrule 22 and is not shown for an optical connector that is fitted to the abutting portion 22c side of the fiber adapter 20. The ferrule is positioned with respect to the ferrule 22. The housing 24 is a cylindrical body made of metal such as SUS that accommodates the ferrule 22 together with the split sleeve 23, and is formed into a male mold on the abutting portion 22b side and a female mold on the abutting portion 22c side. Projections 24a and 24b are formed in the circumferential direction. The ridge 24a is a portion for locking the flange portion 22e of the ferrule 22 accommodated in the housing 24, and the ridge 24b is a portion for locking the push spring 26 interposed between the protrusion 24b and the flange portion 22e.

  The connection nut 25 is detachably screwed to the outer periphery of the optical connector fitted to the abutting portion 22b of the fiber adapter 20, and maintains a connection state with the optical connector, and is formed using SUS or the like. . The fiber adapter 20 is configured as described above, and is used by fitting a female optical connector (not shown) on the abutting portion 22b side and a male optical connector (not shown) on the abutting portion 22c side. . Thereby, the optical fiber of the optical connector fitted to both sides of the fiber adapter 20 via the opposite chirped FBG fiber 21 is optically connected.

  FIG. 15 is a diagram illustrating a grating interval dependency characteristic of the cutoff amount for the fiber adapter 20. From FIG. 15, it can be seen that the cutoff characteristic of the fiber adapter 20 decreases substantially monotonously as the grating interval d increases. From FIG. 12, when the grating interval d 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 1/4 or less. From the viewpoint of ripple, by setting the grating interval d to 1.6 mm or more, for example, a practical cutoff amount of 40 dB or the like can be obtained in a target wavelength band. This is preferable because it can be within a range of 2 dB.

  With this configuration, when used in a single mode, it was possible to obtain a fiber adapter 20 having a cutoff amount higher by about 17 dB than a conventional adapter having an FBG. In the above description, the fiber adapter incorporating the opposed chirped FBG fiber has been described. However, the present invention is similarly applied to a fiber connector incorporating the opposed chirped FBG fiber.

(Embodiment 3)
FIG. 16: is sectional drawing which shows typically the structure of the fiber adapter of the 3rd aspect of this invention. The fiber adapter 30 is obtained by replacing the opposed chirped FBG fiber 21 of the fiber adapter 20 with an opposed chirped FBG fiber 31 having the structure shown in FIG. As shown in FIG. 16A, the opposed chirped FBG fiber 31 is provided over an optical waveguide having a core 31a and a clad 31b provided outside the core 31a, and a region extending over a part of the core 31a and the clad 31b. A plurality of chirped gratings 31c and 31d, the chirped grating 31c and the chirped grating 31d are arranged to face each other while being separated from the grating interval d, and a co-doped region 31e is formed so as to surround the core 31a. The chirped gratings 31c and 31d described above are formed in the co-doped region 31e.

An example of the refractive index distribution in the radial direction in the optical waveguide is shown in FIG. In FIG. 17, the horizontal axis represents the radial distance (μm) from the center of the core 31a, and the vertical axis represents the refractive index difference. In the example shown in FIG. 17, the core 31a has a diameter of about 10 μm, the clad 31b has a diameter of about 125 μm, and the co-doped region 31e has a radial width of about 2 to 10 μm. The radial width of the co-doped region 31e is preferably about 4 to 5 μm from the viewpoint of performance and production .
The core 31a has a refractive index higher by about 0.008 than the clad 31b, and the co-doped region 31e has a refractive index lower by about 0.001 than the clad 31b.

The clad 31b is usually formed using SiO _2, while the core 31a is formed using SiO ₂ doped with, for example, B, Ge or the like that causes a photoinduced refractive index change, and the co-doped region 31e is formed with B, Ge, or the like. It is formed using SiO ₂ doped with F, Cl or the like. B, Ge, etc. in the co-doped region 31e are doped to form the chirped gratings 31c, 31d, and F, Cl, etc., lower the refractive index in the co-doped region 31e below the refractive index in the core 31a. To be doped. For example, the refractive index in the co-doped region 31e may be made lower than the refractive index of other regions in the cladding 31b by doping with F, Cl, or the like. An example of the refractive index distribution in the radial direction of another optical waveguide manufactured in this way is shown in FIG.

  By doing so, since the region where the chirped grating 31c is formed is extended to the outside of the core 31a, the mode light propagating in the clad 31b can be reduced, and the mode light propagating in the core 31a can be reduced. And the coupling between modes of light propagating in the clad 31b can be avoided or alleviated.

  By comprising in this way, the fiber adapter of the interruption | blocking amount higher about 17 dB than the conventional adapter which has FBG was able to be obtained. In the above description, the fiber adapter incorporating the opposed chirped FBG fiber has been described. However, the present invention is similarly applied to a fiber connector incorporating the opposed chirped FBG fiber.

FIG. 1 is a cross-sectional view schematically showing the configuration of the fiber Bragg grating element according to the first aspect of the present invention. FIG. 2 is an explanatory diagram of the function of the fiber Bragg grating element. FIG. 3 is a diagram showing the wavelength characteristics of the cutoff amount when the grating interval is 0.3 mm. FIG. 4 is a diagram showing the wavelength characteristic of the cutoff amount when the grating interval is 1.0 mm. FIG. 5 is a diagram illustrating the wavelength characteristics of the cutoff amount when the grating interval is 1.5 mm. FIG. 6 is a diagram illustrating the wavelength characteristic of the cutoff amount when the grating interval is 2.0 mm. FIG. 7 is a diagram showing the wavelength characteristics of the cutoff amount when the grating interval is 2.5 mm. FIG. 8 is a diagram showing the wavelength characteristics of the cutoff amount when the grating interval is 3.5 mm. FIG. 9 is a diagram illustrating the wavelength characteristics of the cutoff amount when the grating interval is 4.0 mm. FIG. 10 is a diagram illustrating the wavelength characteristics of the cutoff amount when the grating interval is 4.5 mm. FIG. 11 is a diagram illustrating the wavelength characteristics of the cutoff amount when the grating interval is 5.0 mm. FIG. 12 is a diagram showing the grating interval dependence characteristics of ripples. FIG. 13 is a cross-sectional view schematically showing the configuration of the fiber adapter according to the second aspect of the present invention. FIG. 14 is a diagram illustrating an example of a refractive index distribution in the radial direction within the optical waveguide of the fiber adapter illustrated in FIG. 13. FIG. 15 is a diagram illustrating a grating interval dependency characteristic of the cutoff amount for the fiber adapter 20. FIG. 16: is sectional drawing which shows typically the structure of the fiber adapter of the 3rd aspect of this invention. FIG. 17 is a diagram illustrating an example of a refractive index distribution in the radial direction in the optical waveguide of the fiber adapter illustrated in FIG. 16. 18 is a diagram showing another example of the refractive index distribution in the radial direction in the optical waveguide of the fiber adapter shown in FIG. FIG. 19 is a diagram showing the wavelength characteristics of the cutoff amount of a conventional fiber Bragg grating element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical waveguide 1a, 21a, 31a Core 1b, 21b, 31b Clad 2a, 2b, 21c, 21d, 31c, 31d Chirped fiber Bragg grating 3 High refractive index material 10 Fiber Bragg grating element 20, 30 Fiber adapter 21 Opposed chirped FBG fiber 21e Depressed region 22 Ferrule 22a Flange 22b, 22c Butt 22d Fiber hole 22e Saddle 22f Slit 23 Split sleeve 23a Slit 24 Housing 24a, 24b Rib 25 Connection nut 26 Push spring 31e Co-doped region

Claims (2)

The core,
A clad provided outside the core;
Filtering provided in at least the core, wherein the grating pitch gradually changes linearly, and blocks an optical signal incident on an optical waveguide having the core and the cladding provided outside the core in a predetermined wavelength band. The at least two gratings are arranged so as to face each other apart from each other, are configured to Bragg reflect the wavelength of an optical signal in the vicinity of 1650 nm, and are separated from the center of the facing grating And a plurality of chirped fiber Bragg gratings having an interval between the gratings of 3 to 5 mm arranged so as to face each other, the grating pitch being shortened, and a fiber Bragg grating element. A core, a cladding provided outside the core and having at least one of a depressed region and a co-doped region surrounding the core, and at least one in the core, and the grating pitch gradually changes linearly, And a plurality of gratings for performing filtering to cut off an optical signal incident on an optical waveguide having a cladding provided outside the core in a predetermined wavelength band, wherein at least two of the gratings are opposed to each other. The grating pitch is configured to be Bragg-reflected at a wavelength of an optical signal in the vicinity of 1650 nm, and the grating pitch decreases as the distance from the center of the opposing grating increases, and the distance between the gratings arranged to face each other A plurality of chars with a length of 3-5 mm A counter chirped FBG fiber having a preparative fiber Bragg grating,
A ferrule holding the opposed chirped FBG fiber therein;
A fiber adapter comprising: a housing for storing the ferrule therein.