JPH0854525A - Fiber type dispersion compensating device - Google Patents

Abstract

PURPOSE:To provide a small-sized fiber type dispersion compensating device which hardly generates polarization dispersion. CONSTITUTION:This dispersion compensating device is provided with a dispersion compensating optical fiber coil 6 imparted with a polarization dispersion characteristic by coiling a dispersion compensating optical fiber 3 having a negative dispersion characteristic. The exit end side of the dispersion compensating optical fiber 3 is formed as a looped optical fiber 18 which is rotated about 90 deg. around the optical axis and is fused by a coupler part 11b. The incident end side of the dispersion compensating optical fiber coil 6 is provided with a coupler part 11a to propagate incident light from the incident end 5 in the dispersion compensating optical fiber coil 6. The propagated exit light propagated dividedly to the polarization-component axes of different propagation constants formed orthogonally in the dispersion compensating optical fiber 3 of the coil 6 is returned as the polarized rotating light rotated about 90 deg. around the optical axis by a polarization adjusting means 20 composed to have the coupler part 11b and the looped optical fiber 18 to the dispersion compensating optical fiber coil 6 and is emitted from the exit end 16 via the coupler part 11a and the optical fiber 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber type dispersion compensator for compensating for wavelength dispersion of an optical fiber or a semiconductor laser.

[0002]

2. Description of the Related Art A communication system in which light from a semiconductor laser or the like is modulated with a pulse signal and incident on an optical fiber, and the optical fiber is used as a transmission line has been put into practical use.

By the way, the optical fiber is usually formed of silica glass, and the refractive index of the silica glass becomes smaller as the wavelength of light becomes longer. Therefore, due to the known material dispersion of light, an optical fiber has a higher propagation speed of light as the wavelength of the light wave propagating through the optical fiber becomes faster, and a shorter wavelength of the light wave causes the propagation speed of the light as much as possible. Is known to have a so-called positive dispersion characteristic (wavelength dispersion). For example, currently widely laid 13
The 00nm zero-dispersion optical fiber has a chromatic dispersion per unit length of about 18psec / nm / km at a wavelength near 1550nm and a length of 10nm.
In a zero-dispersion compensation optical fiber of 0 km, the chromatic dispersion reaches 1800 psec / nm.

Therefore, in general, when light of a semiconductor laser having a certain spread with respect to the center wavelength is made incident on an optical fiber having such a dispersion characteristic, the optical fiber is compared with the pulse width of the incident light. The pulse width of the emitted light after propagating through the laser beam becomes wide.

Therefore, in order to be able to identify the independent pulses of the emitted light when the emitted light is received by the light receiving side, the pulse interval of the incident light incident on the optical fiber is sufficiently wide. Therefore, if the pulse interval of the incident light is widened, it becomes difficult to perform high-speed communication, and as it is, the construction of a high-speed, large-capacity communication system using a semiconductor laser and an optical fiber is hindered. Will end up.

Therefore, for example, as shown in FIG. 9, a fiber type dispersion compensator 4 is provided between a semiconductor laser 1 and a propagation optical fiber 15 for propagation, and the fiber type dispersion compensator 4 A communication system (communication system) has been proposed in which the dispersion characteristic of the propagation optical fiber 15 is compensated (dispersion compensation) so that the dispersion value of the entire transmission optical path is almost zero.

The fiber type dispersion compensator 4 uses the structural dispersion of the optical fiber in which the refractive index distribution of the core, which is a portion of the optical fiber for propagating light, has a special distribution structure. The dispersion compensating optical fiber 3 having a so-called negative dispersion characteristic that is slower and shorter is faster. For example, the dispersion compensating optical fiber coil 6 is formed by winding a plurality of times in a coil shape. The negative dispersion value of the optical fiber 3 is equivalent to the positive dispersion value of the propagation optical fiber 15. Further, as described above, the space occupied by the dispersion compensating optical fiber 3 is minimized by winding the dispersion compensating optical fiber 3 in a coil shape.
A small fiber type dispersion compensator 4 is used, and it is housed in the optical transmitter together with the semiconductor laser 1 in combination with an optical amplifier or the like.

According to the communication system as shown in FIG. 9, the fiber type dispersion compensator 4 is used to propagate the optical fiber for propagation.
Since the dispersion characteristic of 15 is compensated, it is possible to prevent the pulse width of the emitted light emitted from the propagation optical fiber 15 from being widened as described above.

[0009]

However, the fiber type dispersion compensator 4 has the dispersion compensating optical fiber coil 6 formed by winding the dispersion compensating optical fiber 3 in the form of a coil having a small diameter. When the long dispersion compensating optical fiber 3 is wound into a coil having a small diameter, stress strain due to bending is applied to the optical fiber 3, whereby the cross-sectional structure of the axially symmetric refractive index of the dispersion compensating optical fiber 3 collapses, A phenomenon called birefringence occurs in which the refractive index distributions of the X-axis and the Y-axis, which are orthogonal to each other in the fiber cross section (transverse cross section), change. The birefringence due to the coil winding is mainly caused by the bending due to the coil winding, and is caused by the change in the refractive index of the axis vertical to the bending direction. This birefringence is partly small, but in a long optical fiber, the total amount of the birefringence is large, so that the X-axis polarization component of the light wave propagating in the optical fiber. And a Y-axis polarization component have different propagation velocities, so-called polarization dispersion occurs.

That is, a slow axis which is a polarization component axis in which the propagation speed of light propagating in the dispersion compensating optical fiber 3 is slow, and a phase advance which is a polarization component axis in which the propagation speed of light is faster than this slow axis. The axes are formed so as to be orthogonal to each other, which causes polarization dispersion.

When polarization dispersion (polarization mode dispersion) occurs in the dispersion compensating optical fiber coil 6 as described above, the pulse width of the laser light incident on the dispersion compensating optical fiber 3 is similar to the material dispersion. In particular, since the above-mentioned polarization dispersion largely changes due to temperature change, temperature management of the communication system is also necessary.
This was a problem because it hindered the construction of high-speed, large-capacity communication systems such as long-distance high-speed communication at 10 Gbps.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a small-sized fiber type dispersion compensator which hardly causes polarization dispersion.

[0013]

In order to achieve the above object, the present invention is constructed as follows. That is, the present invention has a dispersion compensating optical fiber having a negative dispersion characteristic, and the polarization compensating optical fiber is provided with the polarization compensating optical fiber. Due to the wave dispersion, the outgoing light of the dispersion compensating optical fiber, which is propagated by being divided into polarization component axes having different propagation constants and formed in the directions of two axes perpendicular to the cross section of the dispersion compensating optical fiber, is approximately centered on the optical axis. Polarization adjusting means for returning the polarization-rotated light rotated by 90 ° to the exit end of the dispersion compensating optical fiber is provided, and the incident light is made incident on the dispersion compensating optical fiber at the entrance end side of the dispersion compensating optical fiber. At the same time, optical branching means for branching and emitting the return light returning to the incident end side of the dispersion compensating optical fiber via the polarization adjusting means is provided.

Further, the polarization adjusting means has a loop-shaped optical fiber, and rotates the optical fiber about 90 ° about the optical axis to rotate the propagating outgoing light of the dispersion compensating optical fiber. The polarization adjusting means has a loop-shaped optical fiber, and the loop-shaped optical fiber is provided with force applying means for applying at least one of bending force and side pressure. Rotating the propagating outgoing light of the dispersion compensating optical fiber by the applying means is also a characteristic configuration of the present invention.

Further, the polarization adjusting means is a quarter wavelength plate connected to the emission end side of the dispersion compensating optical fiber (for example, tilted at 45 ° with respect to the optical axis of the dispersion compensating optical fiber) and the 1/4 wavelength plate.
It has a reflection end provided on the emission end side of the four-wave plate,
The output light propagating from the dispersion compensating optical fiber is propagated to the reflection end side through the quarter-wave plate, reflected at the reflection end, and again 1
/ 4 wavelength plate is returned to the dispersion compensating optical fiber to rotate the propagating outgoing light of the dispersion compensating optical fiber, and the polarization adjusting means is a Faraday rotator connected to the outgoing end side of the dispersion compensating optical fiber. It has a reflection end provided on the emission end side of the Faraday rotator, propagates the emitted light propagating from the dispersion-compensating optical fiber to the reflection end side via the Faraday rotator, reflects it at the reflection end, and again It is also a characteristic configuration of the present invention that the propagating outgoing light of the dispersion compensating optical fiber is rotated by returning to the dispersion compensating optical fiber via the Faraday rotator.

Further, the dispersion compensating optical fiber is coiled to form a dispersion compensating optical fiber coil, which is also a characteristic configuration of the present invention.

[0017]

In the present invention having the above-mentioned structure, the incident light enters the dispersion compensating optical fiber through the optical branching means, and is split into polarization component axes having different propagation constants formed in the dispersion compensating optical fiber. When the propagation output light is incident on the polarization adjusting means, the polarization adjusting means converts the propagation output light into polarization-rotated light whose polarization plane is rotated by approximately 90 °. It is returned to the emitting end. Then, of the polarization component axes (slow axis and fast axis) formed in the two orthogonal directions of the cross section of the dispersion compensating optical fiber, the light propagates through the slow axis to the emission end side of the dispersion compensating optical fiber. The emitted light returns to the entrance end side of the dispersion compensating optical fiber through the fast axis, and conversely, the light that propagates to the exit end side of the dispersion compensating optical fiber through the fast axis passes through the slow axis. Since it returns to the incident end side of the dispersion compensating optical fiber, the propagation time difference between the two does not occur, and the polarization dispersion of the fiber type dispersion compensator becomes almost zero.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. In the description of the present embodiment, the same reference numerals will be given to the same names as those in the conventional example, and the detailed description thereof will be omitted.
FIG. 1 shows a first fiber type dispersion compensator according to the present invention.
The configuration of the essential parts of this embodiment is shown. In the figure, the dispersion-compensating optical fiber coil 6 is a dispersion-compensating optical fiber 3 having a length of 10 km, which has a negative dispersion characteristic of -90 ps / nm / km in chromatic dispersion per unit length at wavelengths near 1550 nm. Diameter 50mm
The dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6 is formed by winding it around a reel having a diameter of 20 mm and a plurality of times. Dispersion properties are given.

A loop-shaped loop-shaped optical fiber 18 is formed on the emission end side of the dispersion-compensating optical fiber 3, and between the loop-shaped optical fiber 18 and the emission-end side of the dispersion-compensating optical fiber coil 6. The branching ratio is 1: 1, which is formed by fusing the side surface of the optical fiber while making it thin, as shown in FIG.
The fusion-type coupler section 11b of FIG. In addition, as shown in FIG. 1, the loop-shaped optical fiber 18 is indicated by an arrow A in the figure.
As described above, they are fused at the coupler portion 11b in a state of being rotated about 90 ° about the optical axis, and the optical fiber end surface 21b of the coupler portion 11b is obliquely polished. The light is not reflected.

A polarization adjusting means 20 is formed by including the coupler section 11b and the loop-shaped optical fiber 18, and the polarization adjusting means 20 is formed in the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6. The emitted light that propagates while being split into polarization component axes with different propagation constants is approximately 90 ° around the optical axis.
It functions to return the polarization-rotated light rotated by ° to the emission end of the dispersion compensation optical fiber 3 (the emission end of the dispersion compensation optical fiber coil 6).
Is rotated about the optical axis by about 90 °, so that the outgoing light propagating from the dispersion compensating optical fiber 3 is rotated.

On the incident end side of the dispersion compensating optical fiber 3, incident light is made incident on the dispersion compensating optical fiber coil 6 formed by the dispersion compensating optical fiber 3, and the dispersion compensating light is transmitted via the polarization adjusting means 20. A fusion-type coupler section 11a that functions as a light branching unit that branches and returns the return light returning to the incident end side of the fiber 3 is provided, and the coupler section 11a is provided with the dispersion-compensating optical fiber 3 and the emission-side light. It is formed in the same manner as the coupler portion 11b by fusing the fiber 23, and the optical fiber end face 21 of the optical fiber 23 is formed.
Similarly to the optical fiber end face 21b, a is also obliquely polished.

This embodiment is constructed as described above,
Next, the operation will be described. When the incident light is made incident from the incident end 5, the light is incident on the dispersion compensating optical fiber coil 6 through the coupler section 11a, propagates through the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6, and the propagating emitted light is It is incident on the coupler section 11b. In the dispersion compensating optical fiber coil 6, since the slow axis and the fast axis having different propagation constants are formed orthogonal to each other in the cross section of the dispersion compensating optical fiber 3 in the two orthogonal directions, The incident light is propagated while being divided into the linearly polarized wave in the slow axis direction and the linearly polarized wave in the fast axis direction, and the propagating outgoing light propagated separately is incident on the coupler section 11b.

The light incident on the coupler section 11b is
The light intensity is branched into 1: 1 to become right-handed light as indicated by arrow b and left-handed light as indicated by arrow c, which propagate through the loop-shaped optical fiber 18, and the left-and-right-handed light is respectively It returns to the coupler section 11b again and returns from the coupler section 11b to the emission end side of the dispersion compensating optical fiber coil 6. At this time, the propagating light is constituted by the coupler section 11b and the loop-shaped optical fiber 18. The polarized light adjusting means 20 turns the polarized light into a polarized light rotated by about 90 ° about the optical axis, and returns to the dispersed optical fiber coil 6 side as shown by an arrow d in the figure, and the polarized light is rotated. Light propagates through the dispersion compensation optical fiber coil 6.

As a result, this polarization-rotated light is rotated about 90 ° around the optical axis of the outgoing light propagating from the dispersion-compensating optical fiber 3 of the dispersion-compensating optical-fiber coil 6, so that the dispersion-compensating optical-fiber coil 6 is rotated. Of the polarization component axes (slow axis and fast axis) formed at right angles to the cross section of the dispersion compensating optical fiber 3, the dispersion compensating light passes through the slow axis in the direction of arrow a in the figure. The light propagating through the fiber coil 6 travels in the direction of arrow d in the figure through the fast axis of the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6, returns to the incident end side, and vice versa. The light propagated through the dispersion compensating optical fiber coil 6 through the phase axis returns to the incident end side through the slow axis of the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6.

Therefore, for example, through the fast axis of the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6, it reaches from the point P on the incident end side to the point Q on the emitting end side of the dispersion compensating optical fiber 3. When the arrival time of light is t1 and the arrival time from the point P to the point Q through the slow axis is t2, the arrival time from the point P to the point P via the point Q is , "Fast axis → Polarization adjustment means 20 →
In the case of linearly polarized wave propagating along the “slow axis” path, it is approximately t1.
+ T2, in the case of the linearly polarized wave propagating through the path of "slow axis → polarization adjusting means 20 → fast axis", it becomes approximately t2 + t1, and no matter which path is propagated, the point P-Q of the light Round-trip propagation times are equal.

Therefore, the point P-Q is reciprocated to and from the point P.
There is no difference in the propagation time of the light returning to, the polarization dispersion becomes almost zero, the light enters the coupler section 11a, propagates to the optical fiber 23 side through the coupler section 11a, and is emitted from the emission end 16. It

As a result of actually measuring the polarization dispersion value of the fiber type dispersion compensator 4 of this embodiment, the polarization dispersion value is 0.9 ps.
It was less than ec, and the polarization dispersion was close to zero. When the polarization dispersion value was measured by changing the room temperature from -30 ℃ to 80 ℃, the change in polarization dispersion value was within 0.2 ps. Furthermore, the chromatic dispersion of this optical fiber type dispersion compensator 4 is −
It was 1800 psec / nm, and it was confirmed to have a sufficient compensation effect.

According to this embodiment, as described above, the polarization emission means 20 propagates the emitted light propagating in the dispersion compensating optical fiber coil 6 in the direction of the arrow a in the figure about 90 ° about the optical axis. By returning to the dispersion compensating optical fiber 6 side as the polarization-rotated light rotated by °, the light propagating along the slow axis of the dispersion compensating optical fiber 3 returns through the fast axis, and the light propagating along the fast axis is delayed. When the light propagates in the dispersion compensating optical fiber coil 6 in the direction as shown by the arrow a in the figure, the propagation time difference between the slow axis and the fast axis caused by the light is polarization adjusting means. It is possible to cancel out when going in the direction indicated by arrow d via 20 and returning, and as a result, the polarization dispersion of the fiber type dispersion compensator 4 can be made substantially zero. Therefore, when pulsed light having a certain pulse width is incident on the fiber type dispersion compensator 4 from the semiconductor laser 1, the light can be emitted without widening the pulse width.

Since the fiber type dispersion compensator 4 of this embodiment also has a sufficient compensation effect, the fiber type dispersion compensator 4 of this embodiment is as shown in FIG. If it is used by incorporating it into a communication system, the material dispersion of the propagation optical fiber 15 can be sufficiently compensated, and the pulse interval of the incident light incident on the fiber type dispersion compensator 4 from the semiconductor laser 1 can be made sufficiently narrow. Also, when the incident light is made incident on the propagation optical fiber 15 through the fiber type dispersion compensator 4 and propagated, and the light emitted from the propagation optical fiber 15 is received, an independent pulse is identified on the light receiving side. Is possible. Therefore, by using the fiber type dispersion compensator 4 of this embodiment, it is possible to construct a communication system capable of high-speed and large-capacity communication.

In this embodiment, after the incident light is incident on the dispersion compensating optical fiber coil 6, the loop optical fiber is
In order to make the return light returning via 18 enter the dispersion-compensating optical fiber coil 6 and emit it again, the light travels back and forth within the dispersion-compensating optical fiber coil 6; When forming 6,
When the coil diameter is the same as that of the conventional one, the dispersion compensating optical fiber 3 having half the length of the conventional one can be used to form the coil 6 having the same chromatic dispersion compensating effect as the conventional one.
Therefore, the dispersion compensating optical fiber coil 6 can be made smaller by that much, and the fiber type dispersion compensator 4 can be made smaller.

As a comparative example of this embodiment, the same dispersion compensation is performed by using the dispersion compensating optical fiber 3 having a length of 10 km similar to that of this embodiment so as to have the same chromatic dispersion compensation effect as this embodiment. An optical fiber coil 6 is formed, and two dispersion compensating optical fiber coils 6 are connected to form a fiber type dispersion compensator 4. When the polarization dispersion value and the chromatic dispersion are measured, the chromatic dispersion is -1800 psec / The value was the same as that of the above-mentioned example in nm, but the polarization dispersion value was as large as 4.3 ps, which was an obstacle to high-speed communication. The fluctuation of the polarization dispersion value from -30 ℃ to 80 ℃ was also large at 1.6 ps.

FIG. 3 shows the essential structure of a second embodiment of the fiber type dispersion compensator according to the present invention. This embodiment is different from the first embodiment in that the waveguide type Y branch chips 12a and 12b are provided on the incident end side and the emitting end side of the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6, respectively. An optical fiber 22 on the incident side and an optical fiber 23 on the output side are connected to the branch end side of the waveguide type Y branch chip 12a, and a loop-shaped optical fiber is connected to the branch end side of the waveguide type Y branch chip 12b. 18 is connected. In this embodiment, the loop-shaped optical fiber 18 and the waveguide type Y branch chip are used.
12b and the polarization adjusting means 20 are formed, and the waveguide type Y branching chip 12a functions as an optical branching means.

The present embodiment is configured as described above,
This embodiment also operates in the same manner as the first embodiment and can achieve the same effect. As a result of actually measuring the polarization dispersion value and the chromatic dispersion of the fiber type dispersion compensator 4 of the present embodiment, it was confirmed that the values were equivalent to those of the above embodiment.

FIG. 4 shows the configuration of the essential parts of a third embodiment of the fiber type dispersion compensator according to the present invention. This embodiment is different from the first embodiment in that a force applying means 19 for applying a bending force to the loop-shaped optical fiber 18 is used.
That is, the loop-shaped optical fiber 18 is fused by the coupler portion 11b without rotating about the optical axis.

As shown in FIG. 5A, the force applying means 19 is formed by winding a part of the loop-shaped optical fiber 18 around a small reel 24 having a diameter of 20 mmφ. The bending force applied to the loop-shaped optical fiber 18 can be adjusted by adjusting the height and the like. In this embodiment, the force applying means
A bending force is applied to the loop-shaped optical fiber 18 by 19 to cause birefringence in the loop-shaped optical fiber 18, thereby controlling the rotation of the polarization plane of the light and compensating the dispersion of the dispersion compensating optical fiber coil 6. Connect the optical fiber 3 to the arrow a
The propagating outgoing light that propagates like
It can be rotated and returned as shown by arrow d.

In this embodiment as well, the same operation as in the first embodiment can be achieved and the same effect can be obtained. It has been confirmed that the polarization dispersion value of the fiber type dispersion compensator 4 of this embodiment is 0.5 psec and the chromatic dispersion is −1800 psec / nm, and the room temperature is changed from −30 ° C. to 80 ° C. The change of the polarization dispersion value was 0.2 psec or less.

FIG. 6 shows the configuration of the essential parts of a fourth embodiment of the fiber type dispersion compensator of the present invention. This embodiment is different from the first to third embodiments in that the polarization adjusting means 20 is connected to the dispersion compensating optical fiber coil 6 at the emission end side of the dispersion compensating optical fiber 3 4 wavelength plate 2
And the reflecting end 13 provided via the optical fiber 25 on the emitting end side of the quarter wavelength plate 2. Further, in the present embodiment, the dispersion compensating optical fiber coil 6
A circulator 14 is disposed on the incident end side of the dispersion compensating optical fiber 3, and the circulator 14 allows incident light to enter the dispersion compensating optical fiber 3 and polarization adjusting means.
It functions as an optical branching unit that branches the return light returning to the incident end side of the dispersion compensating optical fiber 3 via 20 and emits it.

The quarter-wave plate 2 is arranged with its axis azimuth inclined by 45 ° with respect to the axis azimuth of the fast axis of the dispersion compensating optical fiber 3. To the reflection end 13 side through the quarter-wave plate 2 and
By reflecting at 13 and returning again to the dispersion compensating optical fiber 3 via the quarter wavelength plate 2, the propagating outgoing light of the dispersion compensating optical fiber 3 is rotated by about 90 ° about the optical axis. .

The present embodiment is constructed as described above,
Also in this embodiment, when the incident light is made incident from the incident end 5 as in the first to third embodiments, the light is circulated.
14 enters the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6, and the outgoing light propagating from the dispersion compensating optical fiber 3 propagates to the reflecting end 13 side via the quarter wavelength plate 2 and
It is reflected at 13 and returned again to the emission end side of the dispersion compensating optical fiber 3 via the 1/4 wavelength plate 2. At this time, the propagating emission light of the dispersion compensating optical fiber 3 is reflected by the 1/4 wavelength plate 2 and The polarization adjusting means 20 configured with the end 13 rotates the optical axis about 90 ° about the optical axis and returns the polarization-rotated light.

Therefore, as in the above embodiment, when the dispersion compensating optical fiber 3 propagates as shown by the arrow a, the light propagating along the slow axis of the dispersion compensating optical fiber 3 is When passing through the dispersion compensating optical fiber 3 as indicated by an arrow a in the figure, the light travels through the fast axis when returning as shown by an arrow d in the figure. When the light returns through the dispersion compensating optical fiber 3 as indicated by an arrow d in the figure, the light returns through the slow axis thereof, and the same effect as that of the above embodiment is obtained.

In FIG. 7, the axis direction of an arbitrary reference axis of the quarter wave plate 2 is set to zero, and each quarter wave plate 2 is tilted by 30 ° from the reference axis. The horizontal axis is the axis direction, and the polarization dispersion value (PMD;
ode Dispassion) is shown. As shown in the figure, in the quarter-wave plate 2 of the above embodiment, when the axis azimuth of the reference axis is zero, the polarization dispersion value becomes almost zero when the axis azimuth becomes 120. ing. And 1 /
When the axial azimuth of the four-wave plate 2 is set to this axial azimuth, as described above, the axial azimuth of the quarter-wave plate 2 is inclined by 45 ° with respect to the fast axis of the dispersion compensating optical fiber 3. It is supposed to be.

As described above, when setting the axial direction of the quarter-wave plate 2, the axial direction of the quarter-wave plate 2 is gradually changed with respect to an arbitrary reference axis, and the polarization dispersion value is changed. It is possible to find the place where the value becomes the smallest and set it as the axial direction of the quarter-wave plate 2.

FIG. 8 shows the configuration of the essential parts of a fifth embodiment of the fiber type dispersion compensator of the present invention. This embodiment is different from the fourth embodiment in that the Faraday rotator 8 having a rotation angle of 45 ° is connected to the emission end side of the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6. The reflection end 13 is provided on the emission end side of the Faraday rotator 8, and the polarization adjusting means 20 is configured by including the reflection end 13 and the Faraday rotator 8. In the present embodiment, the dispersion compensating optical fiber coil 6 has the bobbin 9 and the dispersion compensating optical fiber 3.
Is coiled.

In the present embodiment, the outgoing light propagating from the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6 is propagated to the reflecting end 13 side via the Faraday rotator 8 and is reflected by the reflecting end 13 to be reflected by Faraday again. By returning to the dispersion compensating optical fiber 6 via the rotor 8, the dispersion compensating optical fiber 3
The output light propagating from is rotated by about 90 ° about the optical axis, operates in the same manner as in the fourth embodiment, and has the same effect.

The present invention is not limited to the above-mentioned embodiments, and various embodiments can be adopted. For example, in the third embodiment, the loop-shaped optical fiber 18 is provided with the force applying means 19 as shown in FIG. 5A, and the bending force is applied by the force applying means 19. As shown in (b) of the figure, a force applying means 19 for applying a side pressure is provided in the loop-shaped optical fiber 18, and a side pressure is applied by the force applying means 19 so that the dispersion compensating optical fiber coil 6 The emitted light propagating from the dispersion compensating optical fiber 3 may be rotated about 90 ° about the optical axis. Further, by providing the force applying means 19 for applying both the bending force and the side pressure, and applying both the bending force and the side pressure, the propagation output light of the dispersion compensating optical fiber 3 is similarly rotated. You may allow it.

In the first and third embodiments, the coupler parts 11a and 11b are provided on both the incident end side and the output end side of the dispersion compensating optical fiber 3 of the dispersion compensating optical fiber coil 6. However, either one of the coupler parts 11a and 11b may be configured by the waveguide type Y branch chip 12a or 12b. Further, in the third embodiment, instead of the coupler parts 11a and 11b, the waveguide type Y branch chip 12 is used.
The fiber type dispersion compensator 4 may be configured by providing a and 12b.

Further, in the first to third embodiments, instead of the coupler portion 11a and the waveguide type Y branch chip 12a,
It may be configured by the circulator 14.

Further, in the fourth and fifth embodiments, the circulator 14 is provided as the optical branching means,
The circulator 14 may be replaced by a coupler portion functioning as an optical branching means, a waveguide type Y branching chip, or the like.

Further, the length of the dispersion compensating optical fiber 3 forming the polarization dispersion compensating coil 6 and the number of coil turns are not particularly limited, and for example, the propagation optical fiber in the optical communication system shown in FIG. It is appropriately set so as to compensate for 15 wavelength dispersions.

Further, in each of the above embodiments, one dispersion compensating optical fiber coil 6 is formed by the dispersion compensating optical fiber 3, but the fiber type dispersion compensating device of the present invention has two or more dispersion compensating devices. The configuration may include the optical fiber coil 6, and the dispersion compensating optical fiber 3 does not necessarily have to be coiled. However, by making the dispersion-compensating optical fiber 3 into a coil, the space occupied by the dispersion-compensating optical fiber 3 can be made as small as possible, and the fiber-type dispersion compensating device can be made a small device accordingly.

[0051]

According to the present invention, the propagating outgoing light of the dispersion compensating optical fiber is rotated by about 90 ° about the optical axis to form the light in the biaxial direction perpendicular to the cross section of the dispersion compensating optical fiber. Of the polarization component axes (slow axis and fast axis) with different propagation constants, the light propagated through the slow axis to the exit end side of the dispersion compensation optical fiber passes through the fast axis and enters the dispersion compensation optical fiber. The light that has propagated to the exit end side of the dispersion compensation optical fiber through the fast axis can be returned to the entrance end side of the dispersion compensation optical fiber through the fast axis. Therefore, the propagation time difference between the two propagating lights can be made almost zero, the polarization dispersion of the fiber type dispersion compensator can be made almost zero, and the fluctuation of the polarization dispersion value due to the temperature change can be made very small. be able to.

Therefore, for example, if the fiber type dispersion compensating apparatus of the present invention is provided in a communication system for modulating light of a semiconductor laser or the like with a pulse signal and transmitting it to an optical fiber for propagation, the conventional fiber type dispersion compensating device is provided. Unlike the device, the pulse width of the laser light having a certain wavelength spread is not widened by the polarization dispersion of the fiber type dispersion compensator, and the pulse width due to the material dispersion is compensated by compensating the material dispersion of the optical fiber for propagation. It is possible to construct a high-speed, large-capacity communication system because it is possible to prevent the spread of data.

Further, according to the present invention, after the incident light is incident on the dispersion compensating optical fiber, the return light returned through the polarization adjusting means is incident on the dispersion compensating optical fiber again and is emitted. In addition, the light travels back and forth in the dispersion compensating optical fiber. Therefore, when forming a fiber type dispersion compensator having the same chromatic dispersion compensating effect as the conventional one,
The length of the dispersion compensating optical fiber is half that of the conventional one, and the fiber type dispersion compensator can be downsized accordingly.

[Brief description of drawings]

FIG. 1 is a main part configuration diagram showing a first embodiment of a fiber type dispersion compensator according to the present invention.

FIG. 2 is an explanatory view showing a fusion-bonded state of the dispersion compensating optical fiber 3 of the coupler section 11b of the first embodiment.

FIG. 3 is a main part configuration diagram showing a second embodiment of a fiber type dispersion compensator according to the present invention.

FIG. 4 is a main part configuration diagram showing a third embodiment of a fiber type dispersion compensator according to the present invention.

FIG. 5 is an explanatory diagram showing both a main part configuration (a) of the force applying means 19 and another example (b) of the force applying means 19 of the third embodiment.

FIG. 6 is a main part configuration diagram showing a fourth embodiment of a fiber type dispersion compensator according to the present invention.

FIG. 7 is a quarter-wave plate 2 used in the fourth embodiment.
3 is a graph showing the correlation between the axis orientation of the above and the polarization dispersion value of the fiber type dispersion device.

FIG. 8 is a main part configuration diagram showing a fifth embodiment of a fiber type dispersion compensator according to the present invention.

FIG. 9 is an explanatory diagram showing an example of a communication system using a fiber type dispersion compensator.

[Explanation of symbols]

 2 1/4 wavelength plate 3 Dispersion compensating optical fiber 6 Dispersion compensating optical fiber coil 8 Faraday rotator 11a, 11b Coupler part 12a, 12b Waveguide type Y-branch chip 13 Reflecting end 18 Loop optical fiber

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Claims (6)

[Claims] 1. A dispersion compensating optical fiber having a negative dispersion characteristic, wherein the dispersion compensating optical fiber is provided with a polarization dispersion characteristic, and the dispersion compensating optical fiber is provided on the emission end side of the dispersion compensating optical fiber. Polarization rotation light generated by rotating the propagation emission light of the dispersion compensating optical fiber, which propagates by splitting into the polarization component axes with different propagation constants, which are formed in the two orthogonal directions of the cross section of the fiber, about 90 ° about the optical axis. As the polarization adjusting means for returning to the emission end of the dispersion compensating optical fiber is provided, the incident light is made incident on the dispersion compensating optical fiber at the incident end side of the dispersion compensating optical fiber, and the polarization adjusting means is used. A fiber-type dispersion compensator comprising: a light branching unit for branching and returning return light returning to the incident end side of the dispersion compensating optical fiber. 2. The polarization adjusting means has a loop-shaped optical fiber, and the propagation emission light of the dispersion compensating optical fiber is rotated by rotating the optical fiber by about 90 ° about the optical axis. The fiber type dispersion compensator according to claim 1, which is characterized. 3. The polarization adjusting means has a loop-shaped optical fiber, and the loop-shaped optical fiber is provided with force applying means for applying at least one of bending force and side pressure. 2. The fiber type dispersion compensator according to claim 1, wherein the output light propagating from the dispersion compensating optical fiber is rotated by the force applying means. 4. The polarization adjusting means has a quarter wavelength plate connected to the emission end side of the dispersion compensating optical fiber and a reflection end provided on the emission end side of the quarter wavelength plate, Dispersion by propagating outgoing light propagating from the dispersion compensating optical fiber to the reflection end side through the quarter wavelength plate, reflecting at the reflection end, and returning to the dispersion compensating optical fiber through the quarter wavelength plate again. The fiber type dispersion compensator according to claim 1, wherein the propagating outgoing light of the compensating optical fiber is rotated. 5. The polarization compensating means has a Faraday rotator connected to the emission end side of the dispersion compensating optical fiber and a reflecting end provided on the emission end side of the Faraday rotator. Propagation emission light from the dispersion compensation optical fiber is propagated to the reflection end side through the Faraday rotator, reflected at the reflection end, and returned to the dispersion compensation optical fiber through the Faraday rotator again. The fiber type dispersion compensator according to claim 1, which is rotated. 6. The fiber type dispersion compensator according to claim 1, wherein the dispersion compensating optical fiber is coiled to form a dispersion compensating optical fiber coil.