The present invention relates to a fiber Bragg grating element that can serves as an optical filter capable of high rejection filtering of an input optical signal over a wide bandwidth.

Conventionally, there are many optical communication devices that make use of optical filters using fiber Bragg gratings (FBG) to cut off light over a desired bandwidth (see patent document 1). This FBG utilizes "laser-induced refractive-index change" in which an optical fiber is irradiated with ultraviolet light thereby to increase the refractive index. As the laser-induced refractive-index change is larger, a higher rejection can be obtained. The FBG is a device that creates a periodic perturbation in the refractive index of a fiber core. This periodic perturbation in the refractive index is formed by two-beam interferometry, phase-mask method or the like. With this periodic perturbation in the refractive index, light is reflected in a wavelength range having a center wavelength &lgr;_B that is called Bragg center wavelength, and is finally rejected in the Bragg center wavelength range. The Bragg center wavelength &lgr;_B is expressed as &lgr;_B= 2n&Lgr;. Here, n is an effective refractive index of an optical fiber and &Lgr; is a grating pitch which means a period of periodic perturbation in the refractive index. Such FBG is used in a WDM communication system as well as a multiplexer/demultiplexer, a line monitoring filter, a temperature sensor and a distortion sensor.

Patent document 1: Japanese Patent Laid-open Publication No.2002-328238

Meanwhile, when an optical signal is to be cut off over a wide bandwidth of about 10 nm, the FBG is formed as chirped grating. In addition, when a higher rejection is desired over this wide bandwidth, the grating length is increased to realize the higher rejection. For example, Fig. 14 shows light is rejected over the bandwidth of about 10 nm of from 1650 to 1659 nm by chirped gratings of 7 mm in grating length, resulting in rejection of about 30 to 35 dB (in Fig. 14, the rejection is expressed as "transmission loss", and this "transmission loss" is referred to as "rejection" below). Besides, Fig. 15 shows light is rejected over the bandwidth of about 10 nm of from 1650 to 1659 nm by chirped gratings of 13 mm in grating length, resulting in rejection of about 35 to 40 dB.

Here, it was expected when the chirped grating length was twice longer, a rejection would be doubled and about 80 dB in the case of Fig, 15. However, the rejection shown in Fig. 15 is little (about 5 dB) larger. That is, there is a limit to obtain a higher rejection simply by elongating the chirped grating.

On the other hand, in the field of optical communications in recent years, there has been a need to separate monitoring light from communication signal light. In order to minimize influences on the communication signal light due to leakage of monitoring light into the communication signal light, it is sometimes desired, for example, to obtain a rejection of about 40 dB stably over a wide bandwidth. For this purpose, there is an increasing demand for FBG capable of stably obtaining a rejection in excess of 40 dB over a wide bandwidth.

The present invention was carried out in view of the foregoing and has an object to provide a fiber Bragg grating element which is simply configured however is capable of obtaining high rejection exceeding 40 dB over a wide bandwidth.

In order to solve the above-mentioned problems and achieve the object, the invention of claim 1 is a fiber Bragg grating element performing high rejection filtering on an input optical signal over a desired bandwidth, the fiber Bragg grating element comprising: a plurality of gratings formed in an optical waveguide having a core and a cladding around the core, the gratings being formed with a grating pitch between adjacent two of the gratings increasing toward a center in a longitudinal direction of the optical waveguide.

Further, the invention of claim 2 is a fiber Bragg grating element performing high rejection filtering on an input optical signal over a desired bandwidth, the fiber Bragg grating element comprising: a plurality of gratings formed in an optical waveguide having a core and a cladding around the core, the optical waveguide having two optical fibers bonded at respective ends to each other, the gratings being formed in the optical fibers with a grating pitch between adjacent two of the gratings increasing toward the respective ends.

Further, the fiber Bragg grating element of claim 3 is characterized in that the cladding of the optical waveguide is doped with a material photosensitive to ultraviolet radiation to have formed in the cladding same gratings as the gratings in the core.

Further, the fiber Bragg grating element of claim 4 is characterized in that the optical waveguide has a numerical aperture equal to or greater than 0.2.

Further, the fiber Bragg grating element of claim 5 is characterized in that the core of the optical waveguide has a peripheral portion of ring-shaped section having a refractive index higher than a refractive index of the core, and the gratings are formed in at least the core.

Further, the fiber Bragg grating element of claim 6 is characterized in that in the optical waveguide, the gratings are formed in at least the core, and a material having a refractive index higher than a refractive index of the cladding is provided on an outer surface of the cladding so as to cover the gratings in the core partially or entirely.

Further, the fiber Bragg grating element of claim 7 is characterized in that the desired bandwidth is equal to or greater than 10 nm.

Further, the fiber Bragg grating element of claim 8 is characterized in that a rejection is equal to or greater than 40 dB over the desired bandwidth.

As the fiber Bragg grating element of this invention is configured to have a pitch between adjacent gratings increasing toward the center in the longitudinal direction of the fiber, it brings about an effect to obtain, with a simple configuration, a high rejection exceeding 40 dB in any wide range stably.

• Fig. 1 is a view schematically illustrating a configuration of an optical branch line monitoring system using a fiber Bragg grating element according an embodiment of the present invention;
• Fig. 2 is a view illustrating monitoring line and communication light which are transmitted to the ONU (Optical Network Unit);
• Fig. 3 is a view schematically illustrating a configuration of FBG in an embodiment 1;
• Fig. 4 is a conceptual view of a wide-band filter using the FBG;
• Fig. 5 is a graph showing an experimental result of rejection by the FBG element of the embodiment 1;
• Fig. 6 is a view schematically illustrating a configuration of an FBG element of an embodiment 2;
• Fig. 7 is a graph showing an experimental result of rejection by the FBG element of the embodiment 2;
• Fig. 8 is a block diagram illustrating another system adopting a fiber Bragg grating element according to the present invention;
• Fig. 9 is a view schematically illustrating a configuration of an FBG element of an embodiment 3;
• Fig. 10 is a view schematically illustrating a configuration of an FBG element of an embodiment 4;
• Fig. 11 is a view schematically illustrating a configuration of an FBG element of an embodiment 5;
• Fig. 12 is a graph showing an experimental result of rejection by the FBG element of the embodiment 5;
• Fig. 13 is a view schematically illustrating a configuration of an FBG element of an embodiment 6;
• Fig. 14 is a graph showing rejection by a conventional wide-band fiber Bragg grating element with a grating length of 7 mm; and
• Fig. 15 is a graph showing rejection by the conventional wide-band fiber Bragg grating element of Fig. 14 of which the grating length is increased to 13 mm.

1a₁ - 1a₈

optical branch line

2

OTDR

3, 28

optical splitter

10

transmitting unit

18

optical coupler

20

ONU

21, 21-1 ... 21-8, 21a, 21b, 21c, 21d

FBG

22

optical trunk line

24

user

25

fiber selector

26

controller

30-1, 30-3

optical connecter

31-1, 31-3

optical receiving portion

32-1, 32-3

O/E portion

33-1, 33-3

reception processing portion

40, 50, 60, 70

FBG element

41, 51, 61, 71

core

42, 52, 62, 72

cladding

43, 73

high refractive-index portion

44, 45

optical fiber

44a, 45a

end

&lgr;a, &lgr;b

wavelength of communication light

&lgr;c₁ - &lgr;c₈

wavelength of monitoring light

Hereinafter, preferred embodiments of a fiber Bragg gating element of the present invention will be described.

Fig. 1 is a view schematically illustrating a configuration of an optical branch line monitoring system using a fiber Bragg grating element according an embodiment 1 of the present invention. This optical branch line monitoring system has a transmitting unit 10 connected to an optical trunk line 22, and the optical trunk line 22 is divided by an optical splitter 28 into a plurality of optical trunk lines 22. Each of these divided optical trunk lines 22 extends outside the system via an optical coupler 18 and further divided into plural optical branch lines la₁ to la₈ by an optical splitter 3. Each of the optical branch lines la₁ to la₈ is connected to an ONU (Optical Network Unit) 20 of a user 24. Here, the optical splitter 3 has an optical line monitoring device (not shown) which receives and outputs only monitoring light of wavelengths &lgr;c₁ to &lgr;c₈ corresponding to the optical branch lines 1a₁ to 1a₈, respectively.

A controller 26 controls output of monitoring light of variable wavelength from an OTDR (Optical Time Domain Reflectometer) 2 to output the light to a fiber selector 25 (hereinafter referred to as "FS") and also controls reception measurements. The OTDR 2 is connected via these controller 26 and FS 25 to the optical couplers 18.

Each ONU 20 is provided with FBG (fiber Bragg gratings) (one of FBGs 21-1 to 21-8) which is assigned a unique wavelength of the monitoring light and reflects monitoring light of the assigned wavelength to prevent the monitoring light from being output to the receiving side. The FBGs 21-1 to 21-8 have the same configuration and property of cutting off monitoring light over a bandwidth of about 10 nm by about 60 dB.

The controller 26 periodically outputs monitoring light of wavelengths &lgr;c₁ through &lgr;c₈, which is sent to the optical trunk lines 22 via the FS 25 and each optical coupler 18. Then, the FS 25 selects an optical trunk line 22 to which the FS 25 outputs the monitoring light. In the optical coupler 18, communication light of wavelength &lgr;b propagating from the transmitting unit 10 and, for example, monitoring light of wavelength &lgr;c₁ are input to the optical splitter 3, and then, the communication light of wavelength &lgr;b is input to the ONUs 20 via the optical branch lines la₁ to la₈ and the monitoring light of wavelength &lgr;c₁ is input to the ONU 20 connected to the optical branch line 1a₁.

As illustrated in Fig. 2, the monitoring light of wavelength &lgr;c₁ is reflected by the FBG 21-1, however, the communication light of wavelength &lgr;b is received as it is by an optical receiving portion 31-1, optical/electrical-converted by an O/E portion 32-1 and input to a reception processing portion 33-1. Likewise, when, monitoring light of wavelength &lgr;c₃ and communication light of wavelength &lgr;b are input to the optical branch line 1a₃, the monitoring light of wavelength &lgr;c₃ is reflected by the FBG 21-3, however, the communication light of wavelength &lgr;b is received as it is by an optical receiving portion 31-3, optical/electrical-converted by an O/E portion 32-3 and input to a reception processing portion 33-3. If the communication light of wavelength &lgr;c₁ or the like is input to the optical receiving portions 31-1 to 31-8, there occurs a communication error with a large influence over communication. Therefore, it is necessary to cut off the monitoring light by use of the FBGs 21-1 to 21-8 with reliability. As described above, the FBGs 21-1 to 21-8 have rejection of about 60 dB of the monitoring light over the wavelength bandwidth of about 10 nm, and therefore, reliable rejection of the monitoring light is allowed. Here, the FBGs 21-1 to 21-8 are fixed by ferrules (not shown) inside the connectors 30-1 to 30-8, respectively.

The following description is made about a configuration of FBG 21 (21-1 to 21-8). As illustrated in Fig. 3, the FBG 21 is chirped gratings having a grating pitch &Lgr; between adjacent gratings varying in the longitudinal direction over the wavelengths of &lgr;c₁ to &lgr;c₈. As illustrated in Fig. 4, the Bragg center wavelengths are the wavelengths &lgr;c₁ to &lgr;c₈, reflection is allowed over the wide bandwidth of wavelengths &lgr;c₁ to &lgr;c₈, and light can be rejected over this bandwidth. Consequently, the ONUs 20 are allowed to use the same FBG 21 (as 21-1 to 21-8). However, the FBG may be configured to be different among 21-1 to 21-8 with respective wavelengths of the monitoring light used as a Bragg center wavelength.

The FBG 21 is configured to have a pitch &Lgr; between adjacent gratings increasing toward the center in the longitudinal direction of the optical fiber. This grating structure is allowed to provide stable reflection of signal light over a wide bandwidth of wavelengths &lgr;c₁ to &lgr;c₈. This reflection is 40 dB or more over the wide bandwidth of &lgr;c₁ to &lgr;c₈.

Fig. 5 is a graph showing rejection by the FBG 21. The FBG 21 rejects input signal light of 40 dB or more over a bandwidth of about 10 nm centered at 1650 nm. In this embodiment 1, as the grating pitch &Lgr; of the FBG 21 is formed increasing toward the center in the longitudinal direction of the optical fiber, it is possible to provide rejection of about 40 dB or more even over the wide wavelength bandwidth of about 10 nm.

Fig. 6 is a view of a modification of Fig. 3, illustrating an FBG element 40 including the FBG 21. In Fig. 6, a cladding 42 of the FBG element 40 has its outer surface covered with a high refractive-index portion 43 that has a refractive index higher than that of the cladding 42. The high refractive-index portion 43 preferably covers the whole FBG element 40, however may cover only a part of the FBG element 40. The high refractive-index portion 43 may use any material that has a refractive index higher than that of the cladding 42, such as matching oil or an adhesive agent. The high refractive-index portion 43 may have the refractive index higher than that of a core 41.

Fig. 7 is a graph showing rejection by the FBG element 40. In Fig. 7, the matching oil is used in the high refractive-index portion 43, and the FBG element 40 shows rejection of 60 dB or more of input light over a bandwidth of about 10 nm having a center wavelength of 1650 nm. Here, the transmission loss fluctuates in the vicinity of -70 dB because of measurement limits.

In this embodiment 1, as the FBG 21 is formed such that the grating pitch &Lgr; of adjacent gratings increases toward the center in the longitudinal direction of the optical fiber in which the FBG is formed, it is possible to provide rejection of about 40 dB or more even over the wide bandwidth of about 10 nm.

Here, in Fig. 1, the line is divided into optical branch lines la₁ to la₈ by the optical splitter 3. However, a configuration illustrated in Fig. 8 may be adopted such that the line is divided into optical branch lines la₁ to la₈ by an optical splitter 28, each of the optical branch lines la₁ to la₈ is then, directly connected to an ONU 20 and monitoring light is input into or output from the line via an optical coupler 18. In this configuration, the monitoring light is preferably of a single wavelength (for example, &lgr;c₄). This is because light emitted by the OTDR 2 shows a large shift of the center wavelength and therefore, an FBG 21 is required to reject light over a wide bandwidth that exceeds 1 nm per Bragg center wavelength.

Next description is made about an embodiment 2. In the above-described embodiment 1, the FBG 21 is formed in a single optical fiber. In other words, the gratings are formed in a single optical fiber such that the grating pitch increases toward the center in the longitudinal direction of the optical fiber. In this embodiment 2, instead of the gratings formed in the single optical fiber, two optical fibers are prepared each having gratings formed with a grating pitch &Lgr; increasing toward its end and the ends of the two optical fibers are fusion-bonded to each other thereby to form an FBG.

Fig. 9 is a vertical cross-sectional view of an FBG 21a of the embodiment 2 of the present invention. The FBG 21a is formed in two optical fibers 44 and 45 each having gratings with a grating pitch &Lgr; increasing toward its end and being fusion-bonded to each other at the respective ends 44a and 45a. This FBG 21a is finally formed, like in the embodiment 1, with the grating pitch &Lgr; increasing toward the center in the longitudinal direction of the connected optical fibers (toward the ends 44a and 45a of the respective optical fibers 44 and 45 in Fig. 9) and almost the same rejection as in the embodiment 1 can be obtained. In other words, the rejection of the FBG 21a in Fig. 9 is identical to that shown in Fig. 5. As the FBG 21a is formed as shown in Fig. 9, it is possible to shorten the UV radiation width (the length of each optical fiber radiated with UV) in forming of the gratings, which enables easy manufacturing and highly accurate forming of the gratings.

In the above-described embodiments 1 and 2, the FBGs 21 and 21a are formed only in the core 41 such that the grating pitch &Lgr; increases toward the center in the longitudinal direction of the optical fiber. Now, in this embodiment 3, the FGB is chirped gratings formed not only in the core but in the cladding.

Fig. 10 is a vertical cross-sectional view of an FBG element 50 having an FBG 21b of the embodiment 3. As illustrated in Fig. 10, the FBG 21b of the FBG element 50 is formed in both the core 51 and a cladding 52a which is a part of the cladding 52. The FBG 21b formed in the cladding 52a prevents light propagating through the cladding 52 from being leaked to the output side (to the right side on Fig. 10). The chirped gratings in this cladding 52a are formed by doping the cladding 52a with almost the same amount of Ge as that of the core 51 and performing two-beam interferometry, phase mask or the like, like in forming the chirped gratings in the core 51. In the embodiment 3, the cladding 52a is doped with Ge, however the dopant is not limited to Ge and any dopant that allows chirped gratings to be formed in the cladding 52a may be used. For example, the cladding 52a may be doped with a material photosensitive to ultraviolet radiation such as phosphorus. Besides, the high refractive-index portion 43 shown in Fig. 6 may be formed on the outer surface of the cladding 52, when necessary.

According to this embodiment 3, it is also possible to realize the FBG element 50 capable of obtaining rejection of about 40 dB and more over the wide bandwidth of about 10 nm.

Next description is made about an embodiment 4. In the embodiment 3, chirped gratings are formed in the cladding 52. Now in this embodiment 4, the core has a higher refractive index than usual.

Fig. 11 is a vertical cross-sectional view of an FBG element 60 having an FBG 21c according to the embodiment 4 of the present invention. As illustrated in Fig. 11, the FBG 21c of the FBG element 60 is chirped gratings formed in the core 61, like in the core 41 of the embodiment 1. Here, the core 61 has a refractive index higher than the refractive index of a cladding 62, and the numerical aperture (NA) is preferably 0.2 or more. In addition, the high refractive-index portion 43 shown in Fig. 6 may be formed on the outer surface of the cladding 62 when necessary.

Fig. 12 shows rejection of the FBG 21c formed in a fiber with a numerical aperture of 0.34. As shown in Fig. 12, it is also possible to realize a rejection of 40 dB or more over a wide bandwidth of about 10 nm centered at 1650 nm.

Next description is made about an embodiment 5. In the above-described embodiment 4, the refractive index of the core 61 is set higher than that of cladding 62. Now in this embodiment 5, the refractive index of a peripheral portion of the core is lower than the refractive index of the cladding.

Fig. 13 is a vertical cross-sectional view of an FBG element 70 having an FBG 21d according to the embodiment 5 of the present invention. As illustrated in Fig. 13, the FBG 21d of the FBG element 70 is chirped gratings formed in the core 71, like in the core 41 of the embodiment 1. In the core 71, the refractive index of a peripheral portion of the core 71 is lower than that of the cladding 72, which provides the fiber with a W-shaped refractive-index profile (displaced clad fiber). This W-shaped refractive-index profile prevents coupling of the cladding mode in the cladding 72. Further, as illustrated in Fig. 13, a high refractive-index portion 73 may be provided to prevent the cladding mode itself from existing. In any case, it is possible to eliminate the influences by the cladding mode, thereby allowing significant improvement of rejection.

Also in this embodiment 5, it is possible to realize the FBG element 70 capable of obtaining a rejection of about 40 dB or more over the wide bandwidth of about 10 nm.

The present invention is applicable to optical communication devices which need an optical filter capable of high rejection filtering of an input optical signal over a wide bandwidth.