JPH07245584A - Wavelength dispersion compensating device - Google Patents

Abstract

PURPOSE:To compensate wavelength dispersion in a signal light having plural wavelengths by the use of the device applied to an existing transmission installation by providing an optical circulator, a wavelength dispersion compensating fiber as a wavelength dispersion compensating means and a wavelength selective reflector to the compensating device. CONSTITUTION:The device is provided with an optical circulator having at least three terminals to output a light received by a 1st terminal 1a from a transmission optical fiber 4a from a 2nd terminal 1b and to output the light received by a 2nd terminal 1b to a transmission optical fiber 4b via a 3rd terminal 1c. Furthermore, a wavelength dispersion compensation means whose one terminal connects to the 2nd terminal 1b is provided. The wavelength dispersion compensation means is made up of wavelength dispersion compensation fibers 2a-2e to compensate wavelength dispersion as to a prescribed wavelength and up of wavelength selective reflectors 3a-3d arranged in the path at each prescribed interval and substantially fully reflecting the light of the wavelength whose dispersion is compensated to the 2nd terminal 1b by reciprocating the propagated light through the distance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromatic dispersion compensator for canceling chromatic dispersion in a transmission optical fiber, and more particularly to a chromatic dispersion for compensating chromatic dispersion of each signal light in wavelength division multiplexing transmission using the transmission optical fiber at substantially the same time. It concerns a compensator.

[0002]

2. Description of the Related Art Conventionally, a 1.55 μm band optical transmission using a 1.3 μm band single mode optical fiber (hereinafter referred to as a transmission optical fiber) has been widely performed. When the value is increased or the transmission distance is increased, distortion occurs in the signal light to be transmitted due to wavelength dispersion in the transmission optical fiber, so that such optical transmission has a limit.

In order to solve such a problem, as a first conventional example, for example, EFMurphy, etc. "All-Optical, Fiber"
-Based 1550 nm Dispersion Experiment over 1310nm O
As shown in ptimized Fiber ", OFC'92 Paper PD-14, the polarity is opposite to the chromatic dispersion characteristic of the transmission optical fiber (for example, negative chromatic dispersion characteristic when the transmission optical fiber has a positive chromatic dispersion characteristic). An effective means is to cancel the chromatic dispersion of the transmission optical fiber by connecting in series optical fibers having the chromatic dispersion characteristic (hereinafter, referred to as chromatic dispersion compensating fiber).

FIG. 2 shows a predetermined wavelength band for each of a transmission optical fiber which is a transmission medium for propagating signal light of a plurality of wavelengths and a chromatic dispersion compensating fiber for compensating chromatic dispersion in the transmission optical fiber. 6 is a diagram showing wavelength dispersion characteristics in FIG. In the figure, the horizontal axis represents wavelength (n
m) and the vertical axis represents chromatic dispersion (ps / km / nm) per unit distance (km). Further, the curves indicated by symbols A to C in this figure are as follows. That is, curve A: chromatic dispersion characteristic in a predetermined wavelength band of the transmission optical fiber curve B: chromatic dispersion characteristic in a predetermined wavelength band of the chromatic dispersion compensation fiber curve C: these transmission optical fiber and chromatic dispersion compensation fiber the in construction connected in series, (that the chromatic dispersion to zero in the transmission optical fiber for signal light having a wavelength lambda ₀₎ cancels the wavelength dispersion in the transmission optical fiber for signal light having a wavelength lambda ₀ to as chromatic dispersion compensation Dispersion characteristics of the structure when the length of the optical fiber is adjusted (wavelength band is the same as the above curves A and B) Normally, as can be seen from this figure, the signal to be transmitted in the first conventional example Regarding the wavelength of light The slope of chromatic dispersion in a chromatic dispersion compensating fiber has the same characteristics as chromatic dispersion in a transmission optical fiber (see this figure). Oite, also it has the characteristics of the upward-sloping one of the optical fiber).

Further, in the second conventional example, for example, AVBe
lov, etc. "Single-mode dispersion compensator for
1.31 / 1.55-μm long-haul communication lines ", OFC /
As shown in IOOC'93 Technical Digest, Thj2, p.203, the slope of the chromatic dispersion characteristic of the chromatic dispersion compensating fiber in the predetermined wavelength band (eg, the slope of each curve shown in FIG. 2) is in the predetermined wavelength band. Refraction of the chromatic dispersion compensating fiber so that it is opposite to the chromatic dispersion characteristic of the transmission fiber (for example, when the chromatic dispersion characteristic of the transmission optical fiber has an upward-sloping slope, it becomes a downward-sloping slope). The rate distribution has a special structure. It has been proposed that the transmission optical fiber and the wavelength dispersion compensating fiber be connected in series to cancel the wavelength dispersion of the transmission optical fiber over a wide wavelength range.

[0006]

The conventional chromatic dispersion compensator configured as described above (both the first and second conventional examples are composed of only dispersion compensating fiber) is, for example, the case of the first conventional example. Even if the length of the chromatic dispersion compensating fiber is adjusted so that the chromatic dispersion in the transmission optical fiber for the signal light of a specific wavelength (λ ₀ ) is canceled (becomes 0), If the wavelength of the signal light propagating through the optical fiber deviates from the adjusted wavelength (λ ₀ ), there is a problem that the chromatic dispersion in the transmission optical fiber cannot be canceled completely. When performing wavelength-division multiplex transmission (multiplex communication system using a plurality of signal lights of different wavelengths) using one transmission optical fiber, the transmission optical fiber for each signal light (wavelength is different) to be transmitted There is a problem that it is not possible to cancel all chromatic dispersion.

On the other hand, the second conventional example is for solving the problem of the first conventional example described above.
Since the refractive index distribution of the chromatic dispersion compensating fiber has a special structure, not only this chromatic dispersion compensating fiber is difficult to manufacture, but also the manufactured chromatic dispersion compensating fiber has its transmission loss as described above. There is a problem that the transmission loss becomes larger than the transmission loss of the fiber (first conventional example).

The present invention has been made to solve the above problems and is applicable to existing transmission equipment.
An object of the present invention is to obtain a chromatic dispersion compensator capable of compensating chromatic dispersion in a transmission optical fiber substantially simultaneously for signal lights having a plurality of wavelengths.

[0009]

The chromatic dispersion compensator according to the present invention outputs the light input to the first terminal from the second terminal (input means) and outputs the light input to the second terminal. Third
In order to output from the terminal (output means), an optical circulator having at least three terminals, and a chromatic dispersion compensating means,
One end of the optical circulator is connected to the second terminal, and a chromatic dispersion compensating fiber for compensating the chromatic dispersion of light propagating in the transmission optical fiber (the chromatic dispersion characteristic of the transmission optical fiber and Of the light having a chromatic dispersion characteristic of opposite polarity), and that is disposed at a predetermined distance from the second terminal in the path of the chromatic dispersion compensating fiber and that propagates through the chromatic dispersion compensating fiber. It is characterized in that it is provided with a wavelength selective reflector that substantially totally reflects light having a wavelength capable of chromatic dispersion compensation by reciprocating to the second terminal side.

In this specification, total reflection means a state where the reflectance is 90% or more, and the reflectance is always 100%.
Does not mean.

In particular, the wavelength selective reflector in the chromatic dispersion compensating means may be constructed by forming a diffraction grating in a core region of a predetermined portion of the chromatic dispersion compensating fiber, and separately diffracting light in the core region. An optical fiber in which a grating is formed may be used, and the optical fiber may be connected in series between the wavelength dispersion compensation fibers.

The optical fiber (wavelength dispersion compensating fiber or an optical fiber separately prepared) in which a diffraction grating is formed as the wavelength selective reflector has Ge added to its core region, and A region having a relative refractive index difference of 1% or more between the region and the cladding region is used.

Further, as the wavelength selective reflector,
In addition to the above-mentioned fiber type diffraction grating, a dielectric multilayer filter may be used.

The wavelength selective reflectors have wavelengths λ
D has a wavelength dispersion characteristics of D _T (lambda), and a transmission optical fiber of length L _T, in this wavelength lambda in
_When dispersion compensation is performed with a chromatic dispersion compensating fiber having a chromatic dispersion characteristic of _C (λ), the distance L _C from the second terminal of the optical circulator in the path of the chromatic dispersion compensating fiber is D _T (Λ) · L _T = −2 · D _C (λ) · L _C , respectively.

[0015]

The chromatic dispersion compensator according to the present invention is provided with an optical circulator having at least three terminals, and the optical circulator allows signal light (1 or 2 or more) from the transmission line (optical fiber for transmission) to enter the chromatic dispersion compensator. ), Or retransmitting the signal light from the inside of the chromatic dispersion compensator to the transmission line. Therefore, in the chromatic dispersion compensator, one end of the transmission line (the input end side or the output end side of the signal light in the transmission optical fiber), or the transmission line is composed of a plurality of transmission optical fibers like existing equipment. If there is such a relay site (optional)
Can be installed in

The chromatic dispersion compensator is provided with chromatic dispersion compensating means, one end of which is connected to one terminal of the optical circulator, and the chromatic dispersion compensating means is the chromatic dispersion in the transmission optical fiber which is a transmission line. Dispersion compensation fiber (having a chromatic dispersion characteristic having a polarity opposite to that of the transmission optical fiber) for compensating for the above, and light propagated through the chromatic dispersion compensation fiber (for example, the transmission optical fiber Of one or two or more of the signal lights transmitted by the above), the light having a predetermined wavelength is substantially totally reflected, and the signal lights of other wavelengths are transmitted. Therefore, the signal light incident on the chromatic dispersion compensating fiber reciprocates in the chromatic dispersion compensating fiber located between the optical circulator and the wavelength selective reflector, so that the chromatic dispersion in the transmission optical fiber is compensated. Therefore, the fiber length required for the wavelength dispersion compensator may be half the length, which enables further miniaturization.

That is, for a transmission optical fiber whose dispersion wavelength characteristic is D _T (λ) at a predetermined wavelength λ and whose length is L _T , the wavelength of the chromatic dispersion compensating fiber for compensating the chromatic dispersion. When the dispersion characteristic is D _C (λ) (the sign of the chromatic dispersion characteristic is opposite) at a predetermined wavelength λ, the length L of the chromatic dispersion compensating fiber required to cancel the chromatic dispersion in the transmission optical fiber. _CC
Is determined so as to satisfy D _T (λ) · L _T + D _C (λ) · L _CC = 0.

On the other hand, the chromatic dispersion compensator is actually necessary because the signal light reciprocates between the optical circulator and the wavelength selective reflector in the chromatic dispersion compensating fiber in the chromatic dispersion compensating means. The length L _C of the chromatic dispersion compensating fiber is L _C = L _CC / 2. Therefore, the wavelength-selective reflector is located at a position satisfying D _T (λ) · L _T = −2 · D _C (λ) · L _C from the optical circulator (the connecting surface between the optical circulator and the chromatic dispersion compensating fiber). To a reflection surface of the reflector at a position where L _C (λ) is provided, the fiber length required for wavelength dispersion compensation can be secured.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the chromatic dispersion compensator according to the present invention will be described below with reference to FIG.

FIG. 1 is a diagram showing a configuration of an embodiment of a chromatic dispersion compensator according to the present invention. In the figure, reference numeral 1 denotes an input means from a transmission optical fiber 4a to a first terminal 1a.
The light input to the second terminal 1b is output from the second terminal 1b, and the light input to the second terminal 1b is used as an output unit.
It is an optical circulator having at least three terminals for outputting from c to the optical fiber 4b for transmission. In this embodiment, by connecting the first terminal 1a to the emission end of the first transmission optical fiber 4a and connecting the third terminal 1c to the incidence end of the second transmission optical fiber 4b, Although the case where it is installed in the transmission path is shown, in addition to this embodiment, the first
Connect the terminal 1a of No. 1 to a signal light emitting means (not shown),
The third terminal 1c may be connected to the transmission optical fiber, or the first terminal 1a may be connected to the emission end of the transmission optical fiber and the third terminal 1c may be a light receiving means (not shown). It may be configured to be connected to.

Further, the chromatic dispersion compensator is provided with chromatic dispersion compensating means, one end of which is connected to the second terminal 1b. One end of the chromatic dispersion compensating means is the second terminal of the optical circulator 1. 1b and chromatic dispersion compensating fibers 2a to 2e for compensating chromatic dispersion for a predetermined wavelength, and these chromatic dispersion compensating fibers 2
The chromatic dispersion compensating fibers 2a to 2e are disposed at predetermined distances from the second terminal 1b in the paths a to 2e.
Wavelength selective reflection that substantially totally reflects (reflectance of 90% or more) to the second terminal 1b side the light having a wavelength capable of wavelength dispersion compensation by reciprocating the distance among the light propagating in e. 3a to 3d (indicated by # 1 to #i and #j in the figure).

In particular, the wavelength selective reflectors 3a to 3d.
May be realized by forming a diffraction grating in the core region of the chromatic dispersion compensating fibers 2a to 2e. Further, it may be realized as an optical fiber in which a diffraction grating is formed in the core region, and these optical fibers may be connected in series to each of the wavelength dispersion compensation fibers 2a to 2e.

The chromatic dispersion compensating fibers 2a to
2e or germanium (Ge) is added to the core regions of these fibers to directly form a diffraction grating in the core regions of the optical fibers, and the relative refractive index difference between the core region and the cladding region is 1% or more. To do. G as above
Silica glass in which e is added to the core region is ultraviolet light (UV
(Light), the refractive index of the irradiation area increases according to the irradiation amount of the UV light. Utilizing such an optical effect, the refractive index in the core region is made to interfere with the above UV light to form the interference fringes as shown in C-243 of the 1993 IEICE Spring Conference Proceedings. A diffraction grating is built by changing the corresponding refractive index distribution.

Further, the position where each wavelength selective reflector is arranged differs depending on the wavelength of the signal light to be reflected. For example, as shown in FIG. 1, each of the wavelength selective reflectors 3a to 3d (# 1, ..., #i, #j) outputs a signal light of wavelength λi (i = 1, ..., i, j). In the case of substantially total reflection (reflectance of 90% or more), the wavelength dispersion characteristic of the wavelength λi is D _T (λi), and the length is a wavelength of the transmission optical fiber (4a and / or 4b). The length L _C required for a chromatic dispersion compensating fiber having a wavelength dispersion characteristic of D _C (λi) with respect to λi is obtained by the following mathematical formula 1.

D _T (λi) · L _T = −2 · D _C (λi) · L _C (1) The length L _C required for this chromatic dispersion compensation fiber is
It is substantially the distance from each reflecting surface of each wavelength selective reflector 3a to 3d to the connecting surface between the optical circulator 1 (second terminal 1b) and the chromatic dispersion compensating fiber 2a.

As described above, the wavelength selective reflectors 3a to 3d are provided in the path of the chromatic dispersion compensating fiber at a predetermined distance from the optical circulator 1 according to the signal light of the wavelength to be chromatic dispersion compensated. By arranging, the chromatic dispersion caused by the transmission optical fiber for the signal light of the predetermined wavelength can be compensated by the chromatic dispersion compensator.

Next, the configuration of the chromatic dispersion compensator will be described using specific numerical values.

The optical circulator is a three-terminal type optical circulator, and a polarization independent type is used. As the chromatic dispersion compensating fiber, the optical fiber shown in Table 1 below, in which Ge is added to the core region, is used.

[0029]

[Table 1]

For the signal light of each wavelength (nm),
Chromatic dispersion characteristics (ps / k of the chromatic dispersion compensating fiber)
m / nm) is a value as shown in Table 2 below.

[0031]

[Table 2]

As the wavelength selective reflector, the above-mentioned Proceedings of the 1993 Spring Conference of the Institute of Electronics, Information and Communication Engineers, C-2.
This is realized by directly writing a diffraction grating in the core region of each chromatic dispersion compensating fiber using the holographic grating method shown in 43 (in this case, each chromatic dispersion compensating fiber is Ge is contained in high concentration).

On the other hand, the transmission optical fiber is a 1.3 μm band single mode optical fiber, and its length is 80
It is set to km. The chromatic dispersion in the transmission optical fiber for the signal light of each wavelength is as shown in Table 3 below.

[0034]

[Table 3]

Further, the positions of the diffraction gratings written in the above-mentioned chromatic dispersion compensating fiber are as shown in Table 4 below from the above-mentioned formula 1 according to the wavelength (nm) of the signal light to be reflected. The distance in Table 4 is the distance from the connection surface between the optical circulator and the chromatic dispersion compensating fiber to the reflection surface of the wavelength-selective reflector, whereby the signal light of each wavelength as shown in Table 4 is obtained. In this case, the chromatic dispersion compensating fiber length required to substantially compensate the chromatic dispersion is secured.

[0036]

[Table 4]

[0037]

As described above, according to the present invention, the length of the chromatic dispersion compensating fiber is half that of the conventional fiber (it can be downsized), and the transmission medium is a transmission medium for a plurality of wavelength bands. Since chromatic dispersion in an optical fiber can be almost completely compensated, even if the wavelength of signal light to be transmitted is arbitrarily changed, it can be used without changing the main configuration of the chromatic dispersion compensator. At the same time, there is an effect that it can be easily applied to existing transmission equipment.

Particularly, in the wavelength division multiplex transmission, it is possible to substantially simultaneously and almost completely compensate the chromatic dispersion in the transmission optical fiber as the transmission medium for the signal light of a plurality of wavelengths.

[Brief description of drawings]

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

FIG. 2 is a diagram showing chromatic dispersion characteristics in a predetermined wavelength band for each of the transmission optical fiber and the chromatic dispersion compensation fiber.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical circulator (3 terminals), 2a-2e ... Fiber for wavelength dispersion compensation, 3a-3d ... Wavelength selective reflector, 4
a, 4b ... Optical fiber for transmission.

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Claims (7)

[Claims] 1. At least light for inputting to a first terminal is output from a second terminal as input means, and light for inputting to the second terminal is output as a output terminal from a third terminal. An optical circulator having three terminals, one end of which is connected to a second terminal of the optical circulator, and a chromatic dispersion compensating fiber for compensating for chromatic dispersion of light propagating in a transmission optical fiber, and the wavelength. Light having a wavelength which is disposed at a predetermined distance from the second terminal in the path of the dispersion compensating fiber and which can be chromatic dispersion compensated by reciprocating the distance among the light propagating in the chromatic dispersion compensating fiber. A chromatic dispersion compensator comprising: a chromatic dispersion compensating means that is a wavelength selective reflector that substantially totally reflects the light to the second terminal side. 2. The wavelength selective reflector in the chromatic dispersion compensating means is a diffraction grating formed in a core region of the chromatic dispersion compensating fiber.
Chromatic dispersion compensator. 3. The chromatic dispersion compensating fiber in the chromatic dispersion compensating means is characterized in that Ge is added to its core region and the relative refractive index difference between the core region and the cladding region is 1% or more. The chromatic dispersion compensator according to claim 2. 4. The chromatic dispersion compensator according to claim 1, wherein the wavelength selective reflector in the chromatic dispersion compensating means is an optical fiber having a diffraction grating formed in its core region. 5. The optical fiber, which is the wavelength selective reflector, is characterized in that Ge is added to its core region and the relative refractive index difference between the core region and the cladding region is 1% or more. The chromatic dispersion compensator according to claim 4. 6. The chromatic dispersion compensator according to claim 1, wherein the wavelength selective reflector in the chromatic dispersion compensating means is a dielectric multilayer filter. 7. Each wavelength-selective reflector in the wavelength-dispersion compensating means has a transmission optical fiber having a wavelength dispersion characteristic of D _T (λ) at a wavelength λ and a length of L _T. When dispersion compensation is performed with a wavelength dispersion compensation fiber having a wavelength dispersion characteristic of D _C (λ) at λ, the distance L _C from the second terminal in the optical circulator in the path of the wavelength dispersion compensation fiber. Is disposed at a position satisfying a condition of D _T (λ) · L _T = −2 · D _C (λ) · L _C. 7. The wavelength according to claim 1, wherein Dispersion compensator.