JP2005196089A - Fiber bragg grating element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high cut off quantity exceeding 40 dB in a broad band exceeding 1nm with a simple structure. <P>SOLUTION: An FBG part 40 filters at a high cut off of a light input exceeding 40 dB in a broad band exceeding 1nm, a photosensitive material having a sensitivity to ultraviolet light is provided in a clad region 42 and a chart Bragg grating which is the same as a core region 41 is formed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fiber Bragg grating element having a function as an optical filter that performs high-bandwidth, high-blocking filtering on optical input.

Conventionally, in an optical communication apparatus, an optical filter using a fiber Bragg grating (FBG) is often used to cut off a desired wavelength band. This FBG utilizes a “photo-induced refractive index change” that increases the refractive index when an optical fiber is irradiated with ultraviolet rays, and a large blocking amount can be obtained by increasing the photo-induced refractive index change. The FBG is obtained by forming a periodic refractive index change on the fiber core, and this periodic refractive index change is formed by a method using a two-beam interference method or a phase mask. Due to this periodic refractive index change, the Bragg center wavelength λ _B is reflected, and as a result, the light in the Bragg center wavelength λ _B region is blocked. The Bragg center wavelength λB is expressed by λ _B = 2nΛ. Here, n is an effective refractive index of the optical fiber, and Λ is a grating pitch that means an interval of periodic refractive index change. Such an FBG is used not only as a WDM communication system but also as a multiplexer / demultiplexer, a line monitoring filter, a temperature sensor, and a strain sensor.

JP 2003-249898 A

  By the way, for example, when trying to cut off the optical input in a wide band exceeding 1 nm, if FBG is made chirped grating, and if it is going to obtain a big cut off amount in this band, a big cut off amount is realized by lengthening the grating length. doing. For example, FIG. 16 shows the result of blocking light in the 10 nm band with a wavelength of 1650 nm by a 7 mm chirped grating. The blocking amount is about 35 dB (in the figure, it is indicated by transmission loss, but hereinafter referred to as blocking amount). ) FIG. 17 shows the result of blocking 10 nm band light of the same wavelength 1650 nm band with a 13 mm chirped grating, and a blocking amount of about 40 dB is obtained.

  However, when the chirped grating length is approximately doubled, the blocking amount should be approximately doubled to obtain a blocking amount of about 80 dB, but the blocking amount shown in FIG. 17 is slightly (about 5 dB). It has only increased the amount of interception. That is, even if the chirped grating length is simply increased, there is a limit in obtaining a large blocking amount.

  On the other hand, in recent optical communication fields, etc., it is necessary to reliably separate the light of the supervisory communication system from the light of the actual communication system. The influence of this supervisory communication system light on the actual communication system is affected. In order to eliminate as much as possible, for example, a cutoff amount of about 60 dB may be required. However, as described above, the FBG that obtains a large blocking amount in a wide band is limited to about 40 dB, and the appearance of an FBG that can obtain a blocking amount exceeding 40 dB in a wide band exceeding 1 nm has been demanded.

  The present invention has been made in view of the above, and an object of the present invention is to provide a fiber Bragg grating element capable of obtaining a high cutoff amount exceeding 40 dB in a wide band exceeding 1 nm with a simple configuration.

  In order to solve the above-mentioned problems and achieve the object, a fiber Bragg grating element according to claim 1 is a fiber Bragg grating element that performs high-interruptible filtering exceeding 40 dB in a wide band exceeding 1 nm for a desired optical input. In this case, a photosensitive material sensitive to ultraviolet rays is attached to the cladding region of the optical fiber, and the same chirped Bragg grating as that of the core region is formed.

  The fiber Bragg grating element according to claim 2 is a fiber Bragg grating element that performs high-blocking filtering exceeding 40 dB in a wide band exceeding 1 nm with respect to optical input, and having a numerical aperture of 0.2 or more. A chirped Bragg grating is formed on the optical fiber.

  According to a third aspect of the present invention, there is provided a fiber Bragg grating element that performs a high-blocking filtering exceeding 40 dB in a wide band exceeding 1 nm with respect to an optical input, wherein the core is disposed outside the core region. A ring-shaped region having a refractive index lower than that of the center of the region is provided, and a chirped Bragg grating is formed at least in the core region.

  The fiber Bragg grating element according to claim 4 is a fiber Bragg grating element that performs high-blocking filtering exceeding 40 dB in a wide band exceeding 1 nm with respect to optical input, and at least a chirped Bragg grating in the core region. And a part or all of the core region in which the chirped Bragg grating is formed is covered with a material outside the cladding region and having a refractive index equal to or higher than the refractive index of the cladding region.

  According to a fifth aspect of the present invention, there is provided the fiber Bragg grating element according to the above invention, wherein a part or all of the chirped Bragg grating is made of a material having a refractive index greater than a refractive index of the cladding region outside the cladding region. It is characterized by covering.

  According to a sixth aspect of the present invention, a connector includes the fiber Bragg grating element according to any one of the first to fifth aspects, and couples an optical fiber.

  In the fiber Bragg grating element according to the present invention, a photosensitive material that is sensitive to ultraviolet rays is attached to the cladding region of the optical fiber to form the same chirped Bragg grating as that of the core region, or the numerical aperture is 0. A chirped Bragg grating is formed in two or more optical fibers, a ring-shaped region having a refractive index lower than that of the cladding region is provided outside the core region, and at least the chirped Bragg grating is formed in the core region. Or, a chirped Bragg grating is formed at least in the core region, and a part of the core region in which the chirped Bragg grating is formed by a material outside the cladding region and having a refractive index equal to or higher than the refractive index of the cladding region Or cover it all, and further Te, even short grating length, with a simple structure, an effect that it is possible to obtain a high cutoff amount exceeding 40dB wideband exceeding 1 nm.

  A fiber Bragg grating element that is the best mode for carrying out the present invention will be described below.

(Embodiment 1)
FIG. 1 is a diagram showing a schematic configuration of an optical branch line monitoring system using a fiber Bragg grating element according to Embodiment 1 of the present invention. In this optical branch line monitoring system, a trunk optical line 22 is connected to the electric transmission device 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 branched into multiple branch lines 1a _{1 to} 1a ₈ by the outdoor optical splitter 3. Each of the branched optical lines 1 a _{1 to 1 a 8 that} have been subjected to optical multi-branching is connected to each ONU (Optical Network Unit) 20 in the user 24. The optical splitter 3 includes an optical line monitoring device (not shown). The 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 the monitoring light having a variable wavelength output from the OTDR (Optical Time Domain Reflectometer) 2 and outputs the monitoring light to the fiber selector (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 FBGs 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 FBGs 21-1 to 21-8 is an FBG having the same configuration and has a characteristic of blocking about 60 dB of monitoring light in a band of about 10 nm.

The control unit 26 periodically emits 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 passes through 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 .

As shown in FIG. 2, the monitoring light having the wavelength λc ₁ is reflected by the FBG 21-1, but the communication light having the wavelength λb is received as it is by the optical receiving unit 31-1, and is optically transmitted by the O / E unit 32-1. / Electrically converted 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 such as the wavelength λc ₁ 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 lights are converted into FBGs 21-1 to 21. It is necessary to shut off reliably with -8. Here, as described above, the FBGs 21-1 to 21-8 have a blocking amount of about 60 dB in the wavelength band of monitoring light of about 10 nm, so that the monitoring light can be reliably blocked. The FBGs 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 21 (21-1 to 21-8) will be described. As shown in FIG. 3, FBG21 are chirped gratings the grating pitch Λ, corresponding to the wavelength λc ₁ ~λc ₈ is changed in the longitudinal direction, as shown in FIG. 4, the Bragg center wavelength of the wavelength λc ₁ ~λc because it is _8, can be reflected in a wide band up to λc ₁ ~λc _8, it is possible to cut off the light in this wavelength band. As a result, the ONU 20 can use the same FBG 21. The FBG 21 may be formed with the wavelength of each monitoring light as the Bragg center wavelength.

  FIG. 5 is a longitudinal sectional view of the FBG portion 40 including the FBG 21, and FIG. 6 illustrates a transverse sectional view and a refractive index profile of the FBG portion 40 including the FBG 21. 5 and 6, the FBG 21 blocks light of wavelengths λc1 to λc8 by a chirped grating formed in the longitudinal direction of the core 41 region, and its length is 13 mm. Here, the outside of the clad 42 of the FBG portion 40 is covered with a high refractive index portion 43 having a refractive index equal to or higher than the refractive index of the clad 42. The high refractive index portion 43 is preferably the entire periphery of the FBG portion 21, but may be a part. The member used for the high refractive index portion 43 only needs to have a refractive index higher than the refractive indexes of the core 41 and the clad 42, and may be, for example, matching oil or an adhesive. The high refractive index portion 43 may have a refractive index higher than the refractive index of the core 41.

  FIG. 7 is a diagram illustrating the cutoff characteristics of the FBG unit 40. In FIG. 7, the matching oil is the high refractive index portion 43, and the FBG portion 40 blocks light input by 60 dB or more in a band of about 10 nm centering on 1650 nm. The transmission loss fluctuates in the vicinity of -70 dB because it is a measurement limit.

  In the first embodiment, the clad 42 corresponding to the core 41 having the FBG 21 is covered with a high refractive index portion 43 having a refractive index equal to or higher than the refractive index of the clad 42 so as to be about 60 dB even in a wide band of about 10 nm. The above blocking characteristics can be obtained. On the other hand, it goes without saying that even with a short grating length, it is possible to obtain a larger cutoff characteristic than in the prior art.

Although FIG. 1 shows a configuration in which multiple branches are made by the optical splitter 3, as shown in FIG. 8, the ONU 20 is directly connected to a single optical line 1a that is not branched downstream from the optical splitter 28, and is monitored. The light may be transmitted and received via the optical coupler 18. In this case, the single wavelength λc ₄ is preferable. This is because the center wavelength shift of the light emitted from the OTDR 2 is large. For this reason, a broadband FBG 20 exceeding 1 nm is required.

(Embodiment 2)
Next, a second embodiment of the present invention will be described. In the first embodiment described above, the FBG 21 that is a chirped grating is formed only on the core 41. However, in the second embodiment, the same chirped grating is applied not only to the core 51 but also to the cladding 52 region. Try to form.

  9 shows a longitudinal sectional view of the FBG portion 50 having the FBG 21a according to the second embodiment of the present invention, and FIG. 10 shows a transverse section and a refractive index profile of the FBG portion 50. As shown in FIG. 9, in the FBG portion 50, the FBG 21 a is formed up to the clad 52 a that is a partial region of the clad 52. The FBG 21a formed in the region of the clad 52a prevents light passing through the clad 52 from coupling the fundamental mode and the higher order mode. It is designed not to leak to the output side. In the formation of the chirped grating in the clad 52a region in the clad 52, approximately the same amount of Ge as that added to the core 51 is added to the clad 52a region. As in the case of forming a chirped grating, the two-beam interference method or a method using a phase mask can be used. In the second embodiment, Ge is added to the clad 52a. However, the present invention is not limited to this, and it is sufficient if a chirped grating can be formed in the clad 52a region. For example, a material sensitive to ultraviolet rays such as phosphorus is clad. What is necessary is just to add to the area | region of 52a. Further, the high refractive index 43 shown in FIG. 5 may be formed outside the cladding 52 as necessary.

  According to the second embodiment, as in the first embodiment, as shown in FIG. 7, the FBG section 50 that can obtain a cutoff characteristic of about 60 dB or more even in a high band of about 10 nm can be realized.

(Embodiment 3)
Next, a third embodiment of the present invention will be described. In the second embodiment, the chirped grating is formed in the cladding 52 region. However, in the third embodiment, the refractive index of the core 61 region is higher than usual.

  11 shows a longitudinal sectional view of the FBG portion 60 having the FBG 21b according to the third embodiment of the present invention, and FIG. 12 shows a transverse section and a refractive index profile of the FBG portion 60. As shown in FIG. 11, the FBG section 60 forms an FBG 21 b that is a chirped grating in the region of the core 61, similarly to the core 41 of the first embodiment. Here, it is preferable that the core 61 has a higher refractive index than that of the clad 62 and has a numerical aperture (NA) of 0.2 or more, for example. Further, the high refractive index 43 shown in FIG. 5 may be formed outside the cladding 52 as necessary.

  FIG. 13 is a diagram showing a cutoff characteristic in a fiber having a numerical aperture of 0.34. As shown in FIG. 13, according to the third embodiment, a cutoff of about 60 dB or more is realized in a high band of about 10 nm centering on 1650 nm.

(Embodiment 4)
Next, a fourth embodiment of the present invention will be described. In the fourth embodiment described above, the refractive index of the core 61 region is higher than the refractive index of the cladding 62 region. However, in the fifth embodiment, the refractive index of the outer region in the core 71 region is changed. It is lower than the region of the clad 72.

  14 shows a longitudinal sectional view of the FBG portion 70 having the FBG 21c according to the fourth embodiment of the present invention, and FIG. 15 shows a transverse section and a refractive index profile of the FBG portion 70. As shown in FIG. 14, the FBG unit 70 forms an FBG 21 c that is a chirped grating in the region of the core 71 as in the core 41 of the first embodiment. Here, as shown in FIG. 15, in the core 71, the refractive index of the outer region in the core 71 region is made lower than the refractive index of the region of the cladding 72, and the profile thereof is W-type (displaced cladding fiber). It is trying to become. Thus, by making the refractive index profile W-shaped, the cladding mode M1 generated in the cladding 72 is prevented from coupling. Furthermore, the cladding mode M1 itself may not exist by providing the high refractive index portion 73 as shown in the first embodiment. In any case, since the cladding mode does not exist, the cutoff characteristic can be remarkably improved.

  According to the fourth embodiment, as in the first embodiment, as shown in FIG. 7, the FBG section 70 that can obtain a cutoff characteristic of about 60 dB or more even in a high band of about 10 nm can be realized.

It is a figure which shows schematic structure of the optical branch line monitoring system using the fiber Bragg grating element concerning Embodiment 1 of this invention. It is a figure which shows the transmission state of the monitoring light and communication light in the ONU side. It is a schematic diagram which shows the structure of a chirped grating. It is a schematic diagram which shows the concept of the broadband filter by a chirped grating. It is a longitudinal cross-sectional view of the fiber Bragg grating element which is Embodiment 1 of this invention. It is a figure which shows the cross section and refractive index profile of the fiber Bragg grating element which are Embodiment 1 of this invention. It is a figure which shows the experimental result which obtained the cutoff characteristic of 60 dB or more in the band of about 10 nm of 1650 nm band with the fiber Bragg grating element which is Embodiment 1 of this invention. It is a block diagram which shows an example of the other system to which the fiber Bragg grating element which is this Embodiment 1 is applied. It is a longitudinal cross-sectional view of the fiber Bragg grating element which is Embodiment 2 of this invention. It is a figure which shows the cross section and refractive index profile of the fiber Bragg grating element which are Embodiment 2 of this invention. It is a longitudinal cross-sectional view of the fiber Bragg grating element which is Embodiment 3 of this invention. It is a figure which shows the cross section and refractive index profile of the fiber Bragg grating element which are Embodiment 3 of this invention. It is a figure which shows the experimental result which obtained the cutoff characteristic of 60 dB or more in the band of about 10 nm of 1650 nm band with the fiber Bragg grating element which is Embodiment 3 of this invention. It is a longitudinal cross-sectional view of the fiber Bragg grating element which is Embodiment 4 of this invention. It is a figure which shows the cross section and refractive index profile of the fiber Bragg grating element which are Embodiment 4 of this invention. It is a figure which shows the interruption | blocking characteristic of the conventional broadband fiber Bragg grating element in case a grating length is 7 mm. It is a figure which shows the interruption | blocking characteristic of the conventional broadband fiber Bragg grating element in case the grating length shown in FIG. 16 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, 211-1 to 21-8, 21a, 21b, 21c FBG
22 Basic Optical Line 24 User 25 Fiber Selector 26 Control Unit 30-1, 30-3 Optical Connector 31-1, 31-3 Optical Receiver 32-1, 32-3 O / E Unit 33-1, 33-3 Reception processing unit 40, 50, 60, 70 FBG section 41, 51, 61, 71 core 42,52,62,72 clad 43,73 high refractive index portions [lambda] a, the wavelength of the wavelength λc ₁ ~λc ₈ monitoring light λb communication light

Claims (6)

A fiber Bragg grating element that performs high cutoff filtering exceeding 40 dB in a wide band exceeding a desired 1 nm for optical input,
A fiber Bragg grating element, wherein a photosensitive material sensitive to ultraviolet rays is attached to a cladding region of an optical fiber, and the same chirped Bragg grating as that of a core region is formed. A fiber Bragg grating element that performs filtering with a high cut-off exceeding 40 dB in a wide band exceeding 1 nm for a desired optical input,
A fiber Bragg grating element, wherein a chirped Bragg grating is formed on an optical fiber having a numerical aperture of 0.2 or more. A fiber Bragg grating element that performs high cutoff filtering exceeding 40 dB in a wide band exceeding a desired 1 nm for optical input,
A fiber Bragg grating element, wherein a ring-shaped region having a refractive index lower than that of a cladding region is provided outside a core region, and a chirped Bragg grating is formed at least in the core region. A fiber Bragg grating element that performs filtering with a high cut-off exceeding 40 dB in a wide band exceeding 1 nm for a desired optical input,
A chirped Bragg grating is formed at least in the core region, and a part or all of the core region in which the chirped Bragg grating is formed by a material having a refractive index equal to or higher than the refractive index of the cladding region outside the cladding region. A fiber Bragg grating element characterized by covering.   The part or the whole of the chirped Bragg grating is covered with a material having a refractive index equal to or higher than the refractive index of the cladding region outside the cladding region. Fiber Bragg grating element.   6. A connector comprising the fiber Bragg grating element according to claim 1 and incorporating an optical fiber.

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