JP2001053680A - Dispersion compensator - Google Patents

Abstract

(57) [Problem] To compensate for chromatic dispersion of a WDM optical signal over a wide wavelength band using a plurality of fiber gratings, and to improve optical characteristics of the compensated optical signal. A dispersion compensator includes a transmission line, a wavelength demultiplexer, and a plurality of dispersion-compensating fiber gratings DCFG1,..., DCFGm. The incident light 5 incident on the wavelength demultiplexer 12 is demultiplexed into a plurality of optical pulses having different wavelengths λ1,.
2 is output from each output terminal. Each output terminal of the wavelength demultiplexer 12 is sequentially connected to each dispersion compensating fiber grating DC.
FG1,..., DCFGm were connected. Each of the dispersion-compensating fiber gratings DCFG1,..., DCFGm is set so that each of the wavelengths λ1,.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dispersion compensator for compensating chromatic dispersion of an optical signal in optical communication using an optical fiber.

[0002]

2. Description of the Related Art A transmission optical fiber used in an optical communication system has a so-called chromatic dispersion characteristic in which a transmission speed (group speed) varies depending on a wavelength of a signal light (optical pulse) to be transmitted. . The chromatic dispersion accumulated in the transmission optical fiber has an adverse effect such as an increase in the pulse width of the transmitted optical pulse, and thus is a factor in limiting the transmission capacity of optical communication. In particular, for example, when an optical amplifier transmits an optical pulse without being converted (reproduced relay) into an electric signal by a regenerative repeater or the like by an optical amplifier between transmission and reception over a long distance, the entire length of the optical fiber transmission path In this case, the chromatic dispersion accumulated increases, which has a great adverse effect on transmission characteristics. Also, when the transmission speed of the optical pulse is increased, the pulse width of the optical pulse and the interval between adjacent optical pulses are reduced,
Even if the spread of the pulse width due to chromatic dispersion is slight, there is a possibility that adjacent optical pulses will interfere with each other and deteriorate transmission characteristics. For example, the following two methods are known as techniques for compensating for chromatic dispersion accumulated in the transmission line of such an optical fiber.

One method uses a dispersion compensating optical fiber, and an optical fiber (dispersion compensating optical fiber) having a dispersion value of a chromatic dispersion accumulated in a transmission line and a dispersion of the opposite sign is used for a receiver or the like. This is to perform dispersion compensation by inserting the optical fiber in front of the receiver.For example, when the optical fiber of the transmission path has a positive dispersion characteristic, the optical fiber having the negative dispersion characteristic is inserted in front of the receiver and accumulated. Can be compensated for. However, since the length of the optical fiber to be inserted is increased in proportion to the magnitude of the dispersion compensation amount, the insertion loss increases and, for example, when the length of the optical fiber is about several km to several tens km. In this case, there is a problem that the volume at the time of mounting becomes large and bulky. In particular, in the case of wavelength division multiplexing (WDM) communication in which a plurality of optical pulses of different wavelengths are multiplexed and transmitted through one optical fiber, the amount of dispersion of chromatic dispersion differs for each wavelength of optical pulse. A dispersion compensating optical fiber having a different length for each wavelength is required, which causes a problem that the communication system is further enlarged.

[0004] The other method uses a fiber grating (FG). The fiber grating is formed such that the refractive index of the core is periodically changed by ultraviolet irradiation or the like in the longitudinal direction of the optical fiber. , An optical component that reflects light of a specific wavelength. Hereinafter, a method of compensating for chromatic dispersion according to an example of the related art will be described with reference to the accompanying drawings. Here, FIG.
FIG. 12B is a side sectional view of the fiber grating 1 according to an example of the prior art. FIG. 12B shows the refractive index change portions 4 and 4 adjacent to each other in the longitudinal direction (x direction) of the fiber grating 1 shown in FIG. Distance (period) ∧ (x)
12C is a diagram showing the wavelength (reflection wavelength) of each light reflected by the refractive index changing portion 4 in the longitudinal direction (x direction) of the fiber grating 1 shown in FIG. FIG. 13A is a diagram showing the relationship between the time of a light pulse in which chromatic dispersion has occurred and the light intensity.
FIG. 14B is a diagram showing the relationship between the time and the wavelength of an optical pulse in which chromatic dispersion has occurred, and FIG. 14A is a diagram showing the relationship between the time of the optical pulse before entering the fiber grating 1 and the light intensity. FIG. 14B shows a fiber grating 1
FIG. 14C is a diagram showing the relationship between the time and the wavelength of an optical pulse before incidence, and FIG. 14C is a diagram showing a state in which the optical pulse incident on the fiber grating 1 is reflected;
FIG. 14D is a diagram showing the relationship between the time and the wavelength of an optical pulse after entering the fiber grating 1.
(E) is a diagram showing the relationship between the time of the light pulse after entering the fiber grating 1 and the light intensity. FIG. 15 shows a plurality of wavelengths constituting the incident light 5 during reciprocation in the fiber grating 1. FIG. 7 is a diagram illustrating a delay time that occurs;
FIG. 16A is a side sectional view of the DCFG 6 configured by connecting a plurality of different fiber gratings in series, and FIG. 16B is a view in the longitudinal direction (x direction) of the DCFG 6 shown in FIG. Adjacent refractive index changing parts 4, 4
It is a figure which shows the distance (x) between.

As shown in FIG. 12A, a fiber grating 1 according to an example of the prior art is made of, for example, a single mode optical fiber, and includes a core 2, a clad 3 having a smaller refractive index than the core 2, and a plurality of refractive indices. Rate change part 4,
, 4 are provided. The refractive index changing section 4
For example, a predetermined portion of the core 2 is irradiated with ultraviolet rays or the like, and is formed so as to have a larger refractive index than the core 2.
The core 2 has a predetermined length in the longitudinal direction (x direction). The plurality of refractive index changing parts 4,...
As shown in (b), one end 2 of the core 2 in the longitudinal direction is provided.
A, the refractive index changing portions 4 and 4 adjacent to the other end 2B
The distance ∧ (x) between them is formed so as to gradually increase. Here, as shown in FIG. 13A, the light pulse α before being incident on the transmission line is a rectangular pulse, and is composed of light having a wavelength that has an appropriate spread with respect to the center wavelength λ0.
When the optical pulse α propagates along the transmission path of the optical fiber, the time width of the optical pulse β expands like the optical pulse β shown in FIG. This is because, due to the wavelength dispersion of the optical fiber, the speed at which the light pulse α propagates through the optical fiber varies depending on the wavelength constituting the optical fiber α, and the time required to exit the optical fiber varies depending on the wavelength.
As shown in the figure, with the center value of the propagation time in the transmission path being τ0, in the light pulse β that has spread so as to have a time width of τ0 ± Δτ, the wavelength of light gradually changes with time. . Note that the propagation time is τ0 at the wavelength of the center wavelength λ0.
And

Next, a description will be given of a method of compensating for the dispersion of the optical pulse β which has spread due to chromatic dispersion by the fiber grating 1 shown in FIG. FIG.
As shown in (a) and (b), when an optical pulse β that has spread due to chromatic dispersion is incident on the fiber grating 1, the position where the light pulse β is reflected in the fiber grating 1 differs depending on the wavelength of the optical pulse β. Therefore, the time until the light is emitted from the inside of the fiber grating 1 is different. Therefore, as shown in FIG. 14C, the longer the wavelength of the propagation time in the transmission line, the more the wavelength is reflected at the incident end side in the fiber grating 1, that is, the time until the light exits from the fiber grating 1. , The light pulse β that has spread over time is
As shown in (d) and (e), the light pulse α having no spread
It can be returned to the same state as. For example, assuming that the distance between the refractive index changing portions 4 and 4 at both ends of the core 2 of the fiber grating 1 is L, the refractive index of the core 2 is n, and the speed of light is c, the wavelength λm (λ1 <
.. <Λm), the round trip time of the light pulse reciprocating in the core 2 has a delay time of 2 nL / c compared to the round trip time of the light pulse of wavelength λ1 reciprocating in the core 2. Therefore,
In the upstream transmission line (not shown), if chromatic dispersion occurs in the incident light 5 such that the transmission speed increases as the wavelength of the optical pulse increases, the incident light 5 is incident on the fiber grating 1. Accordingly, the chromatic dispersion of the incident light 5 is compensated by the delay time generated in the fiber grating 1.

[0007] A fiber grating used for the purpose of compensating for chromatic dispersion, like the above-described fiber grating 1, is particularly called a dispersion compensating fiber grating (DCFG). Generally, DCFG has an advantage that its length is about several cm to 1 m and its volume is smaller than that of dispersion compensating fiber. Here, DCF
G length L (cm) and chromatic dispersion D (ps /
nm) and the operating bandwidth BW (nm) have the relationship of the following equation (1).

[0008]

(Equation 1)

The length L of the DCFG is limited by the length of the phase mask used when fabricating the DCFG,
About m is the maximum length. This 10cm long DC
When an attempt is made to compensate for chromatic dispersion of, for example, 1000 ps / nm in the FG, the operating bandwidth BW is 1 from Equation (1).
nm. In the case of dispersion compensation when transmission is performed at a single wavelength, the wavelength bandwidth is, for example, at most about 0.2 nm even when the transmission rate is 10 Gb / s, so that dispersion compensation can be performed with an operation bandwidth BW of 1 nm. It is. However,
In WDM communication, the wavelength interval of multiplexed signals is about 0.4 to 1.6 nm, and several to several tens of waves are multiplexed, so that the signal wavelength bandwidth of the entire incident light 5 is tens to several tens. 10 nm, which is much larger than the operating bandwidth BW of DCFG, and it is impossible to perform dispersion compensation at all signal wavelengths. Therefore, as shown in FIG. 16A, there is a method of sequentially connecting a plurality of fiber gratings FG1,..., FGm manufactured by a plurality of different phase masks in series to manufacture a pseudo long DCFG6. Are known. In this case, as shown in FIG.
, A plurality of fiber gratings FG1,.
With respect to FGm, the distance ∧ (x) between the adjacent refractive index changing portions 4 and 4 changes in proportion to the position x in the longitudinal direction of the DCFG 6.

[0010]

In the DCFG 6 having the above structure, the distance ∧ (x) between the adjacent refractive index changing portions 4 and 4 is, for example, about 0.5 μm.
As shown in an enlarged side sectional view of the connecting portion 7 of the fiber gratings FG1 and FG2 shown in FIG. 7A, the vicinity of the connecting portion 7 of the adjacent fiber gratings FG1 and FG2 is changed between the adjacent refractive index changing portions 4 and 4. Distance ∧ (x)
It is difficult to form the fiber grating so that it changes smoothly and becomes continuous, and the fiber grating F shown in FIG.
As shown in an enlarged side sectional view of the connecting portion 7 of G1 and FG2, the distance ∧ (x) becomes discontinuous.
There is a problem that the change in the reflection wavelength of the light pulse reflected at the position x in the longitudinal direction of the FG 6 becomes discontinuous.
As a method of eliminating the discontinuity of the reflection wavelength,
A method (trimming) of changing the refractive index by irradiating the connection portion 7 with, for example, ultraviolet light or the like is known, but there is a problem that it is very difficult to obtain continuity.

Next, as described above, a problem in optical characteristics in a case where a change in the distance ∧ (x) between adjacent refractive index changing portions 4 and 4 has a discontinuity will be described with reference to the accompanying drawings. I will explain it. FIG. 18 shows three different fiber gratings FG1, FG2,
It is a figure which shows the characteristic of FG3, (a) and (b)
(C) and (d) of the fiber grating FG1
(E) and (f) of the fiber grating FG2
Is for each of the fiber gratings FG3,
FIG. 19 is a diagram showing the relationship between the reflection wavelength and the reflectance, and the relationship between the reflection wavelength and the group delay time. FIG. 19 shows the fiber gratings FG1, FG2, and FG3 shown in FIGS.
Is a diagram showing characteristics when the distance ∧ (x) between adjacent refractive index changing portions 4 and 4 is connected so as to be smoothly continuous, and (a) shows a relationship between a reflection wavelength and a reflectance. (B) is a diagram showing the relationship between the reflection wavelength and the group delay time, respectively, and FIG. 20 is a diagram showing the fiber gratings FG1, FG2, and FG3 shown in FIGS.
Is a diagram showing characteristics in a case where the distance ∧ (x) between adjacent refractive index changing units 4 and 4 is discontinuously connected, and (a) shows a relationship between a reflection wavelength and a reflectance. b)
FIG. 3 is a diagram illustrating a relationship between a reflection wavelength and a group delay time.

As shown in FIGS. 18 (a) to 18 (f), the center wavelengths of the reflection wavelengths of the fiber gratings FG1, FG2, FG3 are set to wavelengths λ1, λ2, λ3, respectively, In each wavelength band, the group delay time changes linearly in proportion to the reflection wavelength. These fiber gratings FG1,
When FG2 and FG3 are connected in series, ideally, FIG.
9 (a) and 9 (b), the wavelength λ1 to the wavelength λ3
Shows a smooth reflectivity and a group delay time that changes linearly with respect to the reflection wavelength. In the connection part 7 for each of the fiber gratings FG1, FG2, and FG3, the adjacent refractive index change parts 4, 4 If there is a discontinuity in the change in the distance ∧ (x) between the wavelength bands, as shown in FIGS. 20A and 20B, the reflectance decreases and fluctuates between the wavelength bands, and further, the group delay time increases. Fine fluctuation (ripple) occurs. If the chromatic dispersion of the incident light 5 is compensated for in the region where the ripple exists, the waveform of the optical pulse constituting the incident light 5 is deteriorated due to deformation, for example, and there is a problem that it is not practical. . Even if it is set to compensate for chromatic dispersion only in the wavelength region where there is no ripple, if the ambient temperature or the like fluctuates, the position of the ripple moves, and the ripple moves to the wavelength region used for chromatic dispersion compensation. May get in.

A plurality of fiber gratings FG
When one DCFG 6 is manufactured by connecting 1,..., FGm, there is a problem that the yield is reduced. For example,
If the yield at the time of producing one fiber grating is 90%, for example, if ten fiber gratings are produced by the same process, (0.9) ¹⁰ = 35
% Yield is reduced. In addition, it is necessary to remove the coating of the optical fiber over the length L of each of the fiber gratings FG1,..., FGm, and it is difficult to secure the mechanical strength of the optical fiber after the coating is removed. There's a problem.

Since the wavelength used in the WDM communication is a fixed specific wavelength, as shown in FIG. 21, individual DCFGs 1 and 2 whose operation band is limited to the vicinity of each signal wavelength are used.
, DCFGm are manufactured and connected in series by fusion splicing to form a dispersion compensating element. In this method, as shown in FIG. Since there is a portion where the rate becomes zero, the above-described phenomenon such as ripple does not occur. Further, DCFG1, ...,
Since DCFGm is individually manufactured and non-defective products are selected and used, the overall production yield is equal to the individual production yield. However, light having a wavelength at which dispersion compensation is performed by, for example, DCFGm at a position away from the incident end of the incident light 5 is
.., DCFG by the time DCFGm is reached.
The group delay time characteristic is adversely affected by the influence of m-1. Also, since the operating bandwidth BW is narrow, the signal wavelength may deviate from the operating bandwidth BW when the temperature of the dispersion compensating element fluctuates.

The present invention has been made in view of the above circumstances, and takes advantage of the fact that the wavelength of a signal in WDM communication is a specific determined wavelength, and uses a plurality of fiber gratings to cover a wide wavelength band. It is an object of the present invention to provide a dispersion compensator capable of improving the optical characteristics of an optical signal after compensation when compensating for chromatic dispersion of a WDM optical signal over a range of:

[0016]

In order to solve the above-mentioned problems and to achieve the above object, a dispersion compensator according to the present invention according to a first aspect of the present invention divides incident light into a plurality of lights having different wavelengths. A first wavelength demultiplexer that emits light, and a plurality of fiber gratings on which the light is incident, wherein the light is incident on the fiber grating that differs for each wavelength, and a reflection wavelength of the fiber grating. Is characterized by being made equal to the wavelength of the incident light. In the dispersion compensator having the above configuration, a plurality of fiber gratings are connected in parallel to the first wavelength demultiplexer, and each fiber grating is independent. For example, a plurality of fiber gratings are connected in series. As in the case of the joints between fiber gratings in the case where the fiber gratings overlap, there is an overlap region in the wavelength band of the reflection wavelength of each fiber grating, etc. The chromatic dispersion can be compensated without showing continuity or the like and without deteriorating the optical characteristics of the incident light.

Further, even if the reflection wavelength of the fiber grating shifts due to a change in ambient temperature or the like,
For example, like the joint between fiber gratings,
Since there is no wavelength region where a large ripple exists, chromatic dispersion can be compensated over a wide temperature range. In addition, different types of fiber gratings can be connected to each wavelength split by the wavelength splitter, and the amount of compensation dispersion can be set according to the wavelength, so that flexible compensation is performed for each wavelength. be able to. Furthermore, since each fiber grating is independent, for example, if a fiber grating failure occurs, only the defective fiber grating needs to be replaced, and it does not affect other normal fiber gratings. The production yield of the container can be improved.

Further, the dispersion compensator according to the present invention includes a circulator to which an input fiber, an output fiber, and the first wavelength demultiplexer are connected,
The incident light is incident on the first wavelength demultiplexer via the incident fiber of the circulator, and reflected light reflected by the fiber grating and emitted from the first wavelength demultiplexer is The light is output to the output fiber via the circulator. In the dispersion compensator having the above configuration, since the circulator is provided, for example, in the middle of the transmission path,
The chromatic dispersion can be compensated at an appropriate position in the communication system, and a flexible communication system can be constructed.

Further, in the dispersion compensator according to the present invention, a second wavelength demultiplexer is connected to the output fiber, and the reflected light is split and output for each wavelength. It is characterized by that. In the dispersion compensator having the above configuration, for example, at the receiving end or the like of the communication system, it is possible to separate and extract the optical signal after the chromatic dispersion is compensated for each wavelength.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a dispersion compensator according to an embodiment of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a configuration diagram showing a dispersion compensator 10 according to an embodiment of the present invention, FIG. 2 is a configuration diagram showing a wavelength demultiplexer 12 shown in FIG. 1, and FIG. 3 is a wavelength demultiplexer shown in FIG. FIG. 4 is a view showing the spectrum of the transmission loss with respect to the wavelength of the device 12, FIG. 4 is a side sectional view of each of the DCFG1,..., DCFGm shown in FIG.
6A is a diagram showing the relationship between the reflection wavelength and the reflectance, FIG. 6B is a diagram showing the relationship between the reflection wavelength and the group delay time, and FIG. 6 is the dispersion compensation shown in FIG. 7A and 7B are diagrams illustrating characteristics of the device 10, wherein FIG. 9A illustrates a relationship between a reflection wavelength and a reflectance, and FIG. 8B illustrates a relationship between a reflection wavelength and a group delay time. Note that the same reference numerals are assigned to the same parts as those in the above-described conventional technology, and the description will be simplified or omitted. The dispersion compensator 10 according to the present embodiment includes a transmission line 11 composed of, for example, a single mode optical fiber, a wavelength demultiplexer 12, and a plurality of dispersion compensating fiber gratings DCFG1,.
And DCFGm. The incident light 5 incident on the wavelength demultiplexer 12 from the transmission line 11 is composed of, for example, a plurality of optical pulses having different wavelengths λ1,..., Λm. It is assumed that chromatic dispersion has occurred such that the transmission speed increases as the length increases.

The wavelength demultiplexer 12 is, for example, an arrayed waveguide grating (AWG).
It is composed of In the wavelength demultiplexer 12, as shown in FIG. 2, a plurality of substantially U-shaped arrayed waveguides 26,... In these array waveguides 26,..., 26, the waveguide difference (optical path length difference) between adjacent array waveguides 26 is set to ΔL, and both ends of the array waveguides 26,. Slab waveguides 27a and 27b are provided, which interfere with optical pulses guided through the array waveguides 26,..., 26, respectively.
One slab waveguide 27a has one waveguide 2
., 29 are provided in the other slab waveguide 27a. Note that a silicon substrate or the like is used as the substrate 25.
The AWG is manufactured by, for example, providing a quartz thin film layer on a silicon substrate, and doping the quartz thin film layer with germanium or the like according to a predetermined waveguide pattern.

Here, for example, the incident light 1 consisting of a plurality of light pulses of different wavelengths λ1,.
When 3 is incident, each optical pulse is distributed to the arrayed waveguides 26,..., 26 in the slab waveguide 27a. Each optical pulse is transmitted by these array waveguides 26,.
6, a waveguide difference is generated, and the light is distributed to the arrayed waveguides 29,..., 29 via the slab waveguide 27b and output. That is, assuming that the output ends of the array waveguides 29,..., 29 are CH1,..., CHm in order, as shown in FIG. . Conversely, array waveguides 29,.
(CH1,..., CHm), wavelengths λ1,.
When each light pulse of λm is incident, the slab waveguide 27b
Are distributed to each array waveguide 26,. Then, each light pulse is applied to the array waveguide 26,.
When the light passes through, the optical pulses interfere with each other in the slab waveguide 27a and are combined to form an output wave 30, which is output to the transmission line 11 through the waveguide 28.

Each array waveguide 29 of the wavelength demultiplexer 12,
, 29 are connected to the dispersion-compensating fiber gratings DCFG1,.
m, and each of the dispersion-compensating fiber gratings DCFGk (k = 1,..., m) is made of, for example, a single mode optical fiber as shown in FIG.
, A cladding 3 having a smaller refractive index than the core 2, and a plurality of refractive index changing portions 4. The refractive index changing portion 4 is formed, for example, by irradiating a predetermined portion of the core 2 with ultraviolet rays or the like so as to have a higher refractive index than the core 2, and has a predetermined length in the longitudinal direction of the core 2. I have. The plurality of refractive index changing portions 4,..., 4 extend from one end 2A in the longitudinal direction of the core 2 to the other end 2B,
It is formed so that the distance between adjacent refractive index changing portions 4 and 4 becomes gradually longer. Each output end CH1,..., CHm of the wavelength demultiplexer 12 has, for example, one end 2 of each dispersion-compensating fiber grating DCFGk (k = 1,.
A is connected.

Each dispersion compensating fiber grating DCF
Gk (k = 1,..., M) is, as shown in FIG.
The reflection wavelength is formed such that the center wavelength is a wavelength λk (k = 1,..., M) and has a wavelength band of a predetermined width, and this wavelength bandwidth is equal to adjacent wavelength bands, that is, two dispersion compensating fibers. Grating DCFGj-1, D
For CFGj (j = 2,..., M), for example, it is set such that overlapping wavelength regions occur. FIG.
As shown in (b), each dispersion-compensating fiber grating DCFGk (k = 1,..., M) is formed so as to have a group delay time that changes linearly in proportion to the reflection wavelength.

Next, the operation of the dispersion compensator 10 according to this embodiment will be described with reference to the accompanying drawings. First, in the transmission line 11, a plurality of different wavelengths λ
In the incident light 5 composed of light pulses of 1,..., Λm, for example, chromatic dispersion occurs such that the longer the wavelength of the light pulse, the higher the transmission speed. Next, when the incident light 5 enters the wavelength demultiplexer 12, it is split into optical pulses of wavelengths λ1,..., Λm, and is sequentially incident on the dispersion-compensating fiber gratings DCFG1,. Each dispersion compensating fiber grating DCFGk (k = 1,..., M)
Then, the wavelength λk (k =
The light pulses (1,..., M) are reflected by the predetermined refractive index changing section 4 and returned to the one end 2A of the core 2 again. At this time, as the wavelength of the optical pulse reciprocating in the core 2 increases, the reciprocation time increases, and as shown in FIGS. 6A and 6B, each wavelength λk (k = 1,... Compensation for chromatic dispersion is performed with the wavelength center.

The above-described dispersion compensator 1 according to the present embodiment.
0, a plurality of fiber gratings DCFGk
(K = 1,..., M) are connected in parallel to the wavelength demultiplexer 12, and each fiber grating DCFGk
Since (k = 1,..., M) are independent of each other, for example, a plurality of fiber gratings DCFGk (k = 1,
, M) are connected in series, the fiber gratings DCFGj−1, DCFGj (j = 2,.
m) As in the case of a joint between two fiber gratings, the overlap region exists in the wavelength band of the reflection wavelength of each of the fiber gratings DCFGk (k = 1,..., m). This does not occur, and the chromatic dispersion can be compensated without deteriorating the optical characteristics of the incident light 5.

Even if the reflection wavelength of the fiber grating DCFGk (k = 1,..., M) shifts due to a change in ambient temperature or the like, for example, the fiber gratings DCFGj−1, DCFGj (j = 2,
.., M), since there is no portion where a large ripple exists, such as a joint portion between), chromatic dispersion can be compensated over a wide temperature range. Further, for example, different types of fiber gratings can be connected to the optical pulses for each wavelength demultiplexed by the wavelength demultiplexer 12, and even if compensation of chromatic dispersion in a wide wavelength band is performed, Flexible compensation can be performed for each wavelength. Further, since each fiber grating DCFGk (k = 1,..., M) is independent, for example, the fiber grating DCFG
When a defect of k (k = 1,..., m) occurs, only the defective one needs to be replaced, which may affect other normal fiber gratings DCFGk (k = 1,..., m). Since there is no dispersion compensator 10, the production yield of the dispersion compensator 10 can be improved.

In the present embodiment, the wavelength demultiplexer 12 is composed of an arrayed optical waveguide grating (AWG), but is not limited to this. For example, a Mach-Zehnder interference filter using a fiber coupler, a dielectric A filter using a multilayer film or the like may be used. Further, in the present embodiment, it is assumed that chromatic dispersion occurs in the transmission line 11 such that the transmission speed increases as the wavelength of each optical pulse increases, but the present invention is not limited to this. Wavelength dispersion that may increase the speed may occur. In this case, the other ends 2B of the dispersion-compensating fiber gratings DCFGk (k = 1,..., M) may be connected to the output ends CH1,.

Further, in the present embodiment, the output light 30 output from the wavelength demultiplexer 12 after being subjected to dispersion compensation is returned to the transmission line 11, but the present invention is not limited to this. Compensator 40 according to a first modification of the present embodiment shown
The optical circulator 41 may be provided between the transmission line 11 and the wavelength demultiplexer 12, as shown in the configuration diagram of FIG. Here, the optical circulator 41 outputs the incident light 5 from the transmission line 11 to the wavelength demultiplexer 12 and outputs the output light 30 from the wavelength demultiplexer 12 to the optical transmission line 42. In this case, as shown in the configuration diagram of the communication system shown in FIG. 8, the dispersion compensator 40 includes, for example, a transmitter 45 and a receiver 4
6, a transmission line 47, and an optical amplifier 48 such as an EDFA. Also, as shown in the configuration diagram of the dispersion compensator 50 according to the second modification of the present embodiment shown in FIG.
An optical circulator 41 may be provided between the optical circulator 41 and the wavelength demultiplexer 12, and a wavelength demultiplexer 51 may be provided at an output end of the optical transmission line 42 connected to the optical circulator 41. In this case, the dispersion compensator 50 is connected to, for example, an output end of an optical communication system such as an optical pulse receiving terminal or the like, and receives each wavelength λ.
Each light pulse of 1,..., Λm can be extracted.

Next, an example of the chromatic dispersion compensation performed using the dispersion compensator 50 according to the second modification of the present embodiment will be described. In the following, an example in which chromatic dispersion is compensated for the incident light 5 using the dispersion compensator 50 will be described as an example, and a long DCF according to an example of the above-described prior art will be described.
The case where the wavelength dispersion is compensated for the incident light 5 using G6 is taken as a comparative example. In the comparative example, the center wavelengths of the reflection wavelengths are λ1 = 1546.19 nm and λ2 = 154, respectively.
6.75 nm, the wavelength bandwidth is 0.71 nm each,
0.75 nm, chromatic dispersion +670 ps · km each
DCFG6 was constructed by connecting fiber gratings FG1 and FG2 each having a length of 5 cm / nm and L = 5 cm in series. In the embodiment, the center wavelength of the reflection wavelength is λ
1 = 1545.40 nm, λ2 = 1546.32 nm,
λ3 = 1547.17 nm, λ4 = 1547.94 n
m and wavelength bandwidths are 0.95 nm and 1.01 n, respectively.
The dispersion compensator 50 is constructed by connecting the dispersion compensating fiber gratings DCFG1,..., DCFG4 having m, 0.92 nm, 0.95 nm and chromatic dispersion of 900 ps · km / nm to the wavelength demultiplexer 12. FIGS. 10A and 11A show changes in reflectance with respect to the reflection wavelength, and FIGS. 10B and 11B show changes in group delay velocity with respect to the reflection wavelength for the comparative example and the example, respectively. b).

In the comparative example shown in FIGS. 10A and 10B, the group delay time and the reflectivity greatly fluctuate near the connection between the two fiber gratings FG1 and FG2. In the examples shown in FIGS. 11A and 11B, it can be confirmed that the chromatic dispersion is compensated without any problem at the wavelengths λ1, λ2, λ3, and λ4.

[0032]

As described above, according to the dispersion compensator of the first aspect of the present invention, only a plurality of fiber gratings having different reflection wavelengths are connected to the wavelength demultiplexer. Is not connected, so that there is no ripple or the like in the change of the group delay time with respect to the reflected wavelength unlike the joint between fiber gratings, and a wide wavelength band without deteriorating the optical characteristics of incident light. Can compensate for chromatic dispersion. Further, according to the dispersion compensator of the present invention, chromatic dispersion can be compensated at an appropriate position in the communication system, for example, in the middle of a transmission line, and a flexible communication system is constructed. can do. Further, according to the dispersion compensator of the present invention, for example, at the receiving end of the communication system, it is possible to separate and extract the optical signal after the chromatic dispersion is compensated for each wavelength. .

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating a dispersion compensator according to an embodiment of the present invention.

FIG. 2 is a configuration diagram showing the wavelength demultiplexer shown in FIG.

FIG. 3 is a diagram illustrating a transmission loss spectrum with respect to a wavelength of the wavelength demultiplexer illustrated in FIG. 2;

4 is a side sectional view of each DCFG1,..., DCFGm shown in FIG.

5A and 5B are diagrams showing characteristics of DCFG1,..., DCFGm shown in FIG. 1, wherein FIG. 5A shows a relationship between a reflection wavelength and a reflectance, and FIG. 5B shows a relationship between a reflection wavelength and a group delay time. FIG.

6A and 6B are diagrams illustrating characteristics of the dispersion compensator illustrated in FIG. 1, wherein FIG. 6A illustrates a relationship between a reflection wavelength and a reflectance, and FIG. 6B illustrates a relationship between a reflection wavelength and a group delay time. is there.

FIG. 7 is a configuration diagram of a dispersion compensator according to a first modification of the present embodiment shown in FIG. 1;

8 is a configuration diagram of a communication system including the dispersion compensator illustrated in FIG.

FIG. 9 is a configuration diagram of a dispersion compensator according to a second modification of the embodiment shown in FIG. 1;

10A and 10B are diagrams showing the characteristics of the DCFG according to an example of the related art shown in FIG. 14A, where FIG. 10A shows the relationship between the reflection wavelength and the reflectance, and FIG. 10B shows the relationship between the reflection wavelength and the group delay time. It is a figure each showing a relationship.

11A and 11B are diagrams illustrating characteristics of the dispersion compensator illustrated in FIG. 9, where FIG. 11A illustrates a relationship between a reflection wavelength and a reflectance;
(B) is a figure which shows the relationship between a reflection wavelength and group delay time, respectively.

12 (a) is a side sectional view of a fiber grating according to an example of the prior art, and FIG. 12 (b) is a refraction adjacent to the fiber grating shown in FIG. 12 (a) in the longitudinal direction (x direction). Distance between rate change parts (period)
FIG. 12C is a diagram showing ∧ (x), and FIG.
Wavelength (reflection wavelength) of light reflected by the refractive index changing portion in the longitudinal direction (x direction) of the fiber grating shown in FIG.
FIG.

FIG. 13A is a diagram showing the relationship between the time of a light pulse in which chromatic dispersion has occurred and the light intensity, and FIG.
FIG. 3 is a diagram showing the relationship between the time and wavelength of an optical pulse in which chromatic dispersion has occurred.

FIG. 14 (a) is a diagram showing the relationship between the light pulse time and the light intensity before entering the fiber grating, and FIG. 14 (b) is the time and wavelength of the light pulse before entering the fiber grating. FIG. 14 is a diagram showing the relationship with FIG.
FIG. 14C is a diagram illustrating a state in which the light pulse incident on the fiber grating is reflected, and FIG.
FIG. 14 is a diagram showing the relationship between the time and the wavelength of the light pulse after entering the fiber grating, and FIG. 14E is a diagram showing the relationship between the time of the light pulse and the light intensity after entering the fiber grating.

FIG. 15 is a diagram illustrating a delay time generated at the time of reciprocation in the fiber grating for a plurality of wavelengths constituting incident light.

FIG. 16 (a) is a side sectional view of a DCFG configured by connecting a plurality of different fiber gratings in series, and FIG. 16 (b) is a DCFG shown in FIG. 16 (a).
FIG. 9 is a diagram illustrating a distance ∧ (x) between adjacent refractive index changing portions in a longitudinal direction (x direction) of the FG.

17 is an enlarged side sectional view showing a connection portion of the fiber gratings FG1 and FG2, and FIG. 17 (a) shows a case where a distance ∧ (x) between adjacent refractive index change portions changes smoothly; (B) is a case where the distance ∧ (x) between adjacent refractive index changing portions is discontinuous.

FIG. 18 is a diagram showing a configuration 3 of the DCFG shown in FIG.
Different fiber gratings FG1, FG2, F
It is a figure which shows the characteristic of G3, (a) and (b) respectively with respect to the fiber grating FG1, (c) and (d) with respect to the fiber grating FG2, (e) and (f) with respect to the fiber grating FG3. FIG. 4 is a diagram showing the relationship between the reflection wavelength and the reflectance, and the relationship between the reflection wavelength and the group delay time.

FIG. 19 is a cross-sectional view of the fiber gratings FG1, FG2, and FG3 shown in FIGS. 18A to 18F when the distance ∧ (x) between adjacent refractive index changing portions changes smoothly.
FIG. 16A is a diagram showing the characteristics of the DCFG shown in FIG. 16A, where FIG. 16A shows the relationship between the reflection wavelength and the reflectance;
(B) is a figure which shows the relationship between a reflection wavelength and group delay time, respectively.

FIG. 20 shows a case where the fiber gratings FG1, FG2, and FG3 shown in FIGS. 18A to 18F are discontinuously connected at a distance ∧ (x) between adjacent refractive index changing portions. It is a figure which shows about the characteristic of DCFG shown to (a), (a) is about the relationship between a reflection wavelength and a reflectance,
(B) is a figure which shows the relationship between a reflection wavelength and group delay time, respectively.

FIG. 21 shows a plurality of DCFGs 1 according to an example of the prior art;
FIG. 3 is a diagram showing a dispersion compensating element to which DCFGm is connected.

FIG. 22 is a view showing reflection characteristics with respect to wavelength of the dispersion compensating element shown in FIG. 21; It is.

[Explanation of symbols]

 Reference Signs List 5 incident light 10, 40, 50 dispersion compensator 11 transmission line (incident fiber) 12 wavelength demultiplexer (first wavelength demultiplexer) 30 reflected light 41 circulator 42 optical transmission line (output fiber) 51 wavelength demultiplexer (Second wavelength demultiplexer) DCFG1, ..., DCFGm Fiber grating

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Ryozo Yamauchi 1440, Mutsuzaki, Sakura-shi, Chiba F-term in Fujikura Sakura Works (reference) 2H047 KA02 KA03 KA12 LA02 LA19 LA26 QA04 RA00 TA13 5K002 BA05 BA21 CA01 FA02

Claims (3)

[Claims] A first wavelength splitter that splits incident light into a plurality of light beams having different wavelengths and emits the light; and a plurality of fiber gratings into which the light is incident. The dispersion compensator is incident on the fiber grating different for each wavelength, and a reflection wavelength of the fiber grating is made equal to a wavelength of the incident light. 2. The dispersion compensator includes a circulator to which an input fiber, an output fiber, and the first wavelength demultiplexer are connected, and the incident light passes through the input fiber of the circulator. And the reflected light reflected by the fiber grating and emitted from the first wavelength demultiplexer is output to the output fiber via the circulator. The dispersion compensator according to claim 1, wherein: 3. The dispersion according to claim 2, wherein a second wavelength demultiplexer is connected to the output fiber, and the reflected light is output after being demultiplexed for each wavelength. Compensator.