JP2611352B2 - Chromatic dispersion compensation method for optical transmission line - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] The present invention relates to a method for compensating chromatic dispersion characteristics of an optical transmission line. both polarization of the short wavelength side edge wavelength lambda _S and the long wavelength side edge wavelength lambda _L signal, and a rotator for optical rotation so as to be perpendicular, between exit end and the incident end, the long-wavelength side edge wavelength lambda _L has two optical paths of the optical path and the short-wavelength side edge wavelength lambda _S, each of the wavelength dispersion compensation device that is more configuration as the optical path system assembly where the time difference to pass through the optical path is set to a desired, or the above rotator Is disposed such that the plane of polarization of the long wavelength side end wavelength λ _L coincides with a plane including the optical path and the optical axis, and the required transmission time of light of each wavelength is set to a desired birefringence. One of the crystalline crystal and the chromatic dispersion compensator composed of The longer wavelength side end wavelength λ _L
And said short propagation time required difference of the light transmission path of the wavelength-side edge wavelength lambda _S, in the wavelength dispersion compensator, a structure to cancel.

[Industrial applications]

The present invention relates to a method for compensating for chromatic dispersion characteristics of an optical transmission line.

[Conventional technology]

A semiconductor laser light source used in an optical communication system has an emission spectrum width.

Therefore, the longer wavelength end wavelength λ _L and the shorter wavelength end wavelength λ _S
A semiconductor laser having an emission spectrum width of
The optical pulse signal is not composed of a single wavelength, but is composed of a wavelength band from the long wavelength side end wavelength λ _L to the short wavelength side end wavelength λ _S.

Therefore, when a rectangular optical pulse signal is propagated through an optical transmission line,
Due to the dispersion characteristics, for example, a long wavelength (λ _L ) component advances quickly, and a short wavelength (λ _S ) component delays. Therefore, the rectangular optical pulse waveform is broken at the emission end after propagating through the optical transmission line.

Since a long optical transmission line has chromatic dispersion characteristics as described above, when two pulse signals that are close in time are transmitted over a long distance, the tails of the pulse signals overlap on the optical receiver side, and intersymbol interference and the like occur. As a result, the propagation characteristics deteriorate.

In a single mode optical fiber, for example,
It is known that when the wavelength becomes 1.27 μm or more, the material dispersion changes to a negative characteristic (a component having a short wavelength advances quickly).

Therefore, conventionally, as a means for compensating for the chromatic dispersion of an optical transmission line, a single mode optical fiber having a characteristic in which a core diameter and a refractive index are selected is used near a wavelength of 1.27 μm to make material dispersion a negative characteristic. A method of compensating for the chromatic dispersion of an optical transmission line at a specific wavelength, which cancels the structural dispersion of the positive characteristic, has been attempted.

[Problems to be solved by the invention]

However, the above-mentioned conventional approach has a problem that the wavelength and the optical transmission line are limited and are not practical.

It is an object of the present invention to provide a method for compensating for chromatic dispersion of an optical transmission line, which is created in view of such points of the present invention and has no restrictions on wavelengths and optical transmission lines.

[Means for solving the problem]

In order to achieve the above object, as shown in FIG. 1, according to the present invention, the polarization planes of both the short wavelength end wavelength λ _S and the long wavelength end wavelength λ _L of the optical pulse signal are set to be orthogonal. The optical rotator 11 has two optical paths, an optical path having a long wavelength side end wavelength λ _{L and} an optical path having a short wavelength side end wavelength λ _S , between the input end and the output end. An optical path system assembly 20 set as desired and a chromatic dispersion compensator 10 composed of the optical path assembly 20 are provided.

Alternatively, as shown in FIG. 2, an optical rotator 11 for optically rotating the polarization planes of both the short wavelength side end wavelength λ _S and the long wavelength side end wavelength λ _L of the optical pulse signal so as to be orthogonal to each other, The birefringent crystal 30 is disposed so that the polarization plane of the side end wavelength λ _L coincides with a plane including the optical path and the optical axis C, and the required transmission time of light of each wavelength is set as desired. , A chromatic dispersion compensating device 10 is provided.

Such a chromatic dispersion compensator 10 is inserted into one end of the optical transmission line 3 connecting the optical transmitter 1 and the optical receiver 2.

The difference between the required propagation time of the long wavelength side end wavelength λ _L and the short wavelength side end wavelength λ _{S through} the optical transmission line 3 is calculated by the chromatic dispersion compensator 10.
Thus, a configuration for compensating is provided.

[Action]

As described above, the optical rotators 11 are arranged in the chromatic dispersion compensator 10, and the polarization planes of both the short wavelength side end wavelength λ _S and the long wavelength side end wavelength λ _L of the optical pulse signal transmitted from the optical transmitter are set. Are rotated so as to be orthogonal.

Therefore, the polarization planes of the other wavelengths in the wavelength band of the optical pulse signal are inclinedly arranged in the order of the wavelength length between the two orthogonal polarization planes.

The optical pulse signal sequences thus the wavelength order of polarization, in the first invention, by using the optical path system assembly 20, the light in the vicinity of the short-wavelength side edge wavelength lambda _S is passed through a short optical path,
Light near the long wavelength side end wavelength λ _L is transmitted through a long optical path.

The time difference between the time when the short wavelength side end wavelength λ _S passes through the short optical path and the time when the long wavelength side end wavelength λ _L passes through the long optical path is defined as the long wavelength side end wavelength λ _L and the short wavelength side end wavelength λ. _S is set to be equal to the difference in required propagation time of the optical transmission line 3.

In other words, the chromatic dispersion compensator 10 delays and cancels the time that the long wavelength side end wavelength λ _L travels through the optical transmission line 3 quickly.
Therefore, the spread of the optical pulse signal due to chromatic dispersion is suppressed.

On the other hand, in the second invention, a birefringent crystal 30
Are disposed such that the plane of polarization of the long wavelength side end wavelength λ _L coincides with a plane including the optical path C and the optical path C.

That is, the longer wavelength end wavelength λ _L is incident on the birefringent crystal 30 as extraordinary light, and is refracted at the extraordinary light refractive index. Therefore, it is possible to make the time required for the long wavelength side end wavelength λ _L to pass through the birefringent crystal 30 longer than the short wavelength side end wavelength λ _S incident as ordinary light.

By selecting the refractive index and thickness of the birefringent crystal 30, the short-wavelength end wavelength λ _S and the long-wavelength end wavelength λ _L are converted to the birefringent crystal.
The difference in required time for transmission through 30 is set to be equal to the difference in required time for propagation in the optical transmission line 3 between the long wavelength end wavelength λ _L and the short wavelength end wavelength λ _S.

That is, the chromatic dispersion compensator 10 delays and cancels out the time that the longer wavelength end wavelength λ _L advances faster in the optical transmission line 3.
Therefore, the spread of the optical pulse signal due to chromatic dispersion is suppressed.

〔Example〕

Hereinafter, the present invention will be specifically described with reference to the drawings.
The same reference numerals denote the same objects throughout the drawings.

FIG. 1 is a block diagram of an embodiment of the first invention, and FIG.
FIG. 3 is an optical path diagram of an embodiment of the first invention, and FIG. 4 is an optical path diagram of an embodiment of the second invention.

1 and 2, an optical transmitter 1 using a semiconductor laser as a light source transmits an optical pulse signal to an optical receiver 2 via an optical transmission line 3 composed of a long optical fiber.

A chromatic dispersion compensator 10 is inserted between the optical transmitter 1 and the optical transmission line 3.

This chromatic dispersion compensator 10 is, in the first invention,
As shown in FIG. 1 and FIG. 3, it comprises a rotator 11 and an optical path system assembly 20.

The optical rotator 11 is an optical rotator made of cholesteric liquid crystal, and sets a wavelength at which selective reflection that increases the optical rotation angle occurs to a required wavelength.

The optical rotation angle of the optical rotator 11 is set such that the wavelength λ _S is ‥‥ (2m + 1) × (π / 2) and the wavelength λ _L is ‥‥ (2m) × (π / 2).

That is, the polarization planes of both the short wavelength end wavelength λ _S and the long wavelength end wavelength λ _L of the optical pulse signal are rotated so as to be orthogonal to each other. For example, it has a function of optical rotation such that the polarization plane of the short wavelength side end wavelength λ _S is orthogonal to the paper surface and the polarization plane of the long wavelength side end wavelength λ _L is parallel to the paper surface.

The optical path system assembly 20 has details as shown in FIG.
A polarization splitter 21 is arranged to face the output light of the optical rotator 11 at an angle of 45 degrees and is arranged on the optical axis of the transmitted light of the polarization splitter 21 and at an angle of 45 degrees to the incident surface of the optical transmission path 3. The polarization splitters 22 facing each other are arranged.

The polarization splitters 21 and 22 transmit light on the polarization plane orthogonal to the paper surface, that is, light near the end wavelength λ _S on the end wavelength side,
Light on the plane of polarization parallel to the paper surface, that is, light near the long wavelength side end wavelength λ _L , is a polarization separation splitter provided with a polarization separation film that reflects the light.

The mirror 23 is arranged at an angle of 45 degrees with respect to the reflected light of the polarization splitter 21 so that the reflected light of the polarization splitter 21 is incident, and the mirror 24 is mirrored so that the reflected light of the mirror 23 is incident. 23 tilt 45 degrees to the reflected light,
Further, it is disposed so as to face the polarization splitter 22.

Therefore, the light near the long wavelength side end wavelength λ _L reflected by the polarization separation splitter 21 passes through the long optical path of the polarization separation splitter 21-mirror 23-mirror 24-polarization separation splitter 22-optical transmission path 3 to transmit light. Light enters the road 3.

The optical path length of the long optical path is set as desired, and the short-wavelength end wavelength λ of the optical pulse signal passing through the optical path assembly 20 is set.
The time difference between the passing time of the light near _S and the passing time of the light near the long wavelength end wavelength λ _L , that is, the long wavelength end wavelength λ
The delay time of _L is made equal to the difference in the required propagation time of the optical transmission line 3.

That is, since the longer wavelength side end wavelength λ _L is delayed by the chromatic dispersion compensator 10 and canceled out by the time during which the longer wavelength λ _L advances faster in the optical transmission line 3, it is generated by the propagation of the optical pulse signal through the optical transmission line 3. Chromatic dispersion compensator
10 can compensate.

In the second invention, as shown in FIGS. 2 and 4, the chromatic dispersion compensator 10 comprises an optical rotator 11 and a birefringent crystal.
It consists of 30.

The optical rotator 11 has a function to rotate the light so that the polarization plane of the short wavelength side end wavelength λ _S is orthogonal to the paper surface and the polarization plane of the long wavelength side end wavelength λ _L is parallel to the paper surface.

Numeral 30 denotes a birefringent crystal made of, for example, titanium oxide, which is arranged at a position where the light emitted from the optical rotator 11 enters and the emitted light enters the optical transmission path 3.

Further, the birefringent crystal body 30 is arranged in such a posture that the plane of polarization of the long wavelength side end wavelength λ _L coincides with a plane including the optical path and the optical axis C.

That is, of the light emitted from the optical rotator 11, the longer wavelength side end wavelength λ _L
Is incident on the birefringent crystal 30 as extraordinary light, and is refracted at an extraordinary light refractive index. On the other hand, the short wavelength side end wavelength λ _S enters the birefringent crystal 30 as ordinary light, and is refracted at the ordinary light refractive index.

Accordingly, the time required for the long wavelength side end wavelength λ _L to pass through the birefringent crystal 30 is longer than the short wavelength side end wavelength λ _S incident as ordinary light.

Required travel time difference Delta] t to the short wavelength side edge wavelength lambda _S and the long wavelength side edge wavelength lambda _L is transmitted through the birefringent crystal _{30, Δt = (n e -n o} ) × d ÷ c n e ‥‥ the extraordinary refractive The index n _o ‥‥ the ordinary light refractive index d ‥‥ the thickness of the birefringent crystal c ‥‥ the speed of light.

Therefore, by appropriately selecting the refractive index and the thickness of the birefringent crystal 30, the required time difference Δt is set to the long wavelength side end wavelength λ _L
Is set to be equal to the difference between the required propagation time of the optical transmission line 3 and the short wavelength side end wavelength λ _S.

That is, since the longer wavelength side end wavelength λ _L is delayed by the chromatic dispersion compensator 10 and canceled out by the time during which the longer wavelength λ _L advances faster in the optical transmission line 3, it is generated by the propagation of the optical pulse signal through the optical transmission line 3. Chromatic dispersion compensator
10 can compensate.

〔The invention's effect〕

As described above, the present invention relates to a chromatic dispersion compensation method for an optical transmission line, in which a chromatic dispersion compensator is inserted at an end of an optical transmission line that connects an optical transmitter and an optical receiver. It can be applied to any optical transmission line of a fiber or a multi-mode optical fiber, and can suppress the wavelength dispersion of an optical pulse signal without limiting the wavelength of an optical pulse signal to be used. There is an excellent effect on the above.

[Brief description of the drawings]

 FIG. 1 is a block diagram of an embodiment of the first invention, FIG. 2 is a block diagram of an embodiment of the second invention, FIG. 3 is an optical path diagram of the embodiment of the first invention, and FIG. FIG. 4 is an optical path diagram of an embodiment of the second invention. In the figure, 1 is an optical transmitter, 2 is an optical receiver, 3 is an optical transmission line, 10 is a chromatic dispersion compensator, 11 is an optical rotator, 20 is an optical path system assembly, 21, 22 is a polarization splitter, 23, 24 Denotes a mirror, 30 denotes a birefringent crystal, and C denotes an optical axis.

Claims (2)

(57) [Claims] 1. An optical rotator (11) for rotating both polarization planes of a short wavelength side end wavelength λ _S and a long wavelength side end wavelength λ _{L of} an optical pulse signal so as to be orthogonal to each other, and between an input end and an output end. An optical path assembly (20) having two optical paths, an optical path having the long wavelength side end wavelength λ _{L and} an optical path having the short wavelength side end wavelength λ _S , and having a desired time difference for passing through each optical path; Chromatic dispersion compensator (10)
Is inserted into one end of an optical transmission line (3) connecting the optical transmitter (1) and the optical receiver (2), and the longer wavelength side end wavelength λ _L and the shorter wavelength side A chromatic dispersion compensating method for an optical transmission line, characterized in that a difference in required propagation time of an optical transmission line (3) having an end wavelength λ _S is canceled out by the chromatic dispersion compensator (10). 2. An optical rotator (11) for rotating both polarization planes of the short wavelength end wavelength λ _S and the long wavelength end wavelength λ _{L of} an optical pulse signal so as to be orthogonal to each other; The birefringent crystal (30) in which the polarization plane of λ _L is disposed so as to coincide with a plane including the optical path and the optical axis, and the required transmission time of light of each wavelength is set as desired. And chromatic dispersion compensator (10)
Is inserted into one end of an optical transmission line (3) connecting the optical transmitter (1) and the optical receiver (2), and the longer wavelength side end wavelength λ _L and the shorter wavelength side A chromatic dispersion compensating method for an optical transmission line, characterized in that a difference in required propagation time of an optical transmission line (3) having an end wavelength λ _S is canceled out by the chromatic dispersion compensator (10).