JPH0454929B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides that a part of the emitted light of a light emitting element used in an optical communication device is reflected at the end face of an optical fiber, and a part of the reflected light is The present invention relates to an improvement in an optical isolator configured to prevent feedback from being fed back to the light emitting element.

In an optical communication system, when an optical signal emitted from a light emitting element of an optical communication device is propagated through an optical fiber, a part of the optical signal is reflected at the end face of the optical fiber. There is a demand for an optical isolator that prevents light from entering the light emitting element.

[Conventional technology]

FIG. 2 is a block diagram showing the basic configuration of a conventional optical isolator. In the figure, 1 is an optical isolator, 1-1 and 1-2 are birefringent polarizing prisms made of, for example, rutile, 2 is a Faraday rotator made of a magneto-optical material, such as YIG, 3 is incident light, and 4 is outgoing light. , 5 is a magnet, and 6 is a central axis.

Note that the same numbers refer to the same members throughout the drawings.
In Fig. 2, birefringent polarizing prisms (hereinafter referred to as polarizing prisms) 1-1 and 1-2 have the same shape, but polarizing prism 1-1 is different from polarizing prism 1-2. The crystal axis of the polarizing prism 1-1 is arranged at an angle of 45 degrees, and the crystal axis of the polarizing prism 1-1 is arranged perpendicular to the plane of the paper (indicated by the X-axis direction in the figure).

Further, the entrance and exit surfaces of each of the polarizing prism 1-1, the Faraday rotator 2, and the polarizing prism 1-2 have a certain inclination with respect to the optical path, so that the incident light is not reflected in the incident direction.

Further, the incident light 3 enters the polarizing prism 1-1 of the optical isolator 1, passes through the Faraday rotator 2, and is output from the polarizing prism 1-2, but the output light 4 and the incident light 3 are arranged in parallel. It is composed of

The optical isolator 1 configured as described above has irreversible characteristics, and the light in the opposite direction is transmitted to the polarizing prism 1 by using the polarizing prisms 1-1, 1-2, the birefringence characteristics, and the characteristics of the Faraday rotator 2. -1, the light in the opposite direction that entered the polarizing prism 1-2, for example, the reflected light, is transmitted in a direction 3 different from the incident light 3.
-1 or 3-2, so that the incident light 3 has a function of preventing reflected light from entering.

In the figure, the dotted line indicates an example of the distribution of the magnetic field H of the Faraday rotator 2. A magnetic field H is generated from the magnet 5 of the optical isolator 1 in the direction of the optical central axis 6 of the Faraday rotator 2.
is applied, and the magnetic field in the vicinity of the optical central axis 6
_H1 has become uniform, and the magnetic field H2 at the periphery is H1
It has become uneven compared to That is, the magnetic field H1 and the magnetic field H2 are not equal.

FIG. 3 is an explanatory diagram showing the operating principle of a conventional optical isolator. In the figure, Figure a shows the case where light is transmitted in the forward direction, and Figure b shows the case where light is transmitted in the reverse direction.

In the figure, 7-1 and 7-2 are crystal axes of polarizing prisms, 8 is a light emitting source, such as a laser diode (hereinafter referred to as LD), 8-1 is light polarized perpendicular to 7-1, 8-2 is the light polarized parallel to 7-1, 9 is the optical path of LD 8, 10 is the optical fiber, 10-1 is the end face of the optical fiber, ()
indicates the optical path of extraordinary light, and () indicates the optical path of normal light.

In FIG. 3a, the light from the light emitting source LD8 enters the polarizing prism 1-1 through the optical path 9. In FIG. The above incident light has a polarization plane perpendicular to the crystal axis 7-1 of the polarizing prism 1-1 as normal light o, and a light with a polarization plane parallel to the crystal axis 7-1 as extraordinary light e.
becomes.

Of this incident light, light with a plane of polarization perpendicular to the crystal axis 7-1 (the crystal axis perpendicular to the plane of the paper) (light parallel to the plane of the paper in the figure) is refracted with the normal light refractive index No. 1, and The light having a polarization plane parallel to the crystal axis 7-1 (light perpendicular to the plane of the paper in the figure) is refracted by the extraordinary refractive index Ne1.

The normal light o formed as above is the optical path ()
The extraordinary light e travels through the system of optical path (), and the extraordinary light e travels through the system of optical path ().

The above normal light o and extraordinary light e enter the Faraday rotator 2 and are refracted, and the plane of polarization of each light is rotated by 45 degrees by the magnetic field H, and the polarizing prism 1-2
It is incident on and refracted.

The crystal axis of polarizing prism 1-2 is polarizing prism 1
-1 clockwise as seen from the direction of movement with respect to the crystal axis
It is installed at a 45-degree angle, and the incident light is emitted as is and refracted.

That is, the extraordinary light e is refracted by the extraordinary light refractive index Ne2, and the normal light o is refracted by the normal light refractive index No2, and each of them is incident on the end face 10-1 of the optical fiber 10.

In this case, the abnormal light e entering the optical fiber 10
The polarization plane of the normal light o and the polarization plane of the normal light o are orthogonal to each other as shown, and both are rotated by 45 degrees clockwise compared to the polarization plane of the light emitted from the LD 8.

Further, the incident light of the polarizing prism 1-1 and the output light of the polarizing prism 1-2 are parallel to each other.

A part of the light in the forward direction is reflected by the end face 10-1 of the optical fiber 10, and the light passes through the optical path 11 in the reverse direction as shown in FIG. travels along the optical path ()', and the extraordinary light e at 1-2 travels through the optical path ()'.

In Fig. 3b, the reflected light is reflected from the polarizing prism 1-
2, and among this reflected light, the polarizing prism 1-
The normal light component o for the polarizing prism 1-2 is refracted at the normal light refractive index No.2, and the extraordinary light component e for the polarizing prism 1-2 is refracted at the extraordinary light refractive index No.2, and each of them is incident on the Faraday rotator 2 and refracted. .

At the Faraday rotator 2, the plane of polarization is rotated by 45 degrees by the magnetic field H, similar to the forward direction light described above, but this rotation is 90 degrees compared to each of the incident lights on the optical path 9 (on the LD 8 side). become.

Due to this 90 degree rotation, the normal light o becomes the abnormal light e
Then, the extraordinary light e changes into the normal light o, and each of them enters the polarizing prism 1-1.

In the polarizing prism 1-1, the normal light o is refracted by the normal light refractive index No1 and outputted to the optical path 11-2, and the extraordinary light e is refracted by the extraordinary light refractive index Ne1 and outputted to the optical path 11-1.

In this case, the combination of refractive indices of the system in optical path ()' is No2, Ne1, and the combination of refractive indices in optical path ()' is Ne2, No1, both of which are combinations of refractive indices with different values. Therefore, the emitted light is not parallel to the reflected light (on the optical fiber side), and in rutile, the extraordinary light refractive index Ne1 is larger than the normal light refractive index No1 (Ne1>No1), so the optical path 11-
1 and 11-2 spread as shown in the figure, and the reflected light is LD
8 will not be returned.

Therefore, the optical isolator 1 can be operated with the configuration shown in FIG. 3 above.

The above optical isolator 1 is a Faraday rotator 2
When the distribution of the magnetic field H is assumed to be uniform, the actual distribution of the magnetic field H is non-uniform, so the transmitted light becomes elliptically polarized, and as explained in Figs. 4 and 5 below, The elliptically polarized light component is fed back to LD8, and the LD
The S/N of the optical signal of No. 8 is lowered.

FIG. 4 is a diagram showing the optical path of the elliptically polarized component of reflected light.

In the figure, 12-1 is the optical path of the normal light component o to prism 1-1 of the elliptically polarized light that has passed through the Faraday rotator on optical path ()', and 12-2 is the optical path of normal light component o that has passed through the Faraday rotator on optical path ()'. The optical path of the extraordinary light component e of the elliptically polarized light with respect to the prism 1-1 is shown.

FIG. 5a shows polarized light 13 that ideally should be normal light o for the prism 1-1 but becomes elliptic when it passes through the Faraday rotator.
13-1 is a diagram showing the normal light o component, 13-2 is a diagram showing the extraordinary light e component, and diagram b ideally shows the prism 1-1.
14-1 shows the e-component of the extraordinary light, and 14-2 the o-component of the normal light. be.

The long axis component and short axis component of the above elliptically polarized light are orthogonal to each other, and the ratio of the short axis component to the long axis component is called the extinction ratio, which represents the quality of the optical signal (degree of elliptically polarized light). There is.

In Fig. 4, the optical path of reflected light ()′,
If the angle formed between ()' and the magnetic fields H1 and H2 is large, the reflected light will pass through a non-uniform part of the magnetic field, so the reflected light will become elliptically polarized as shown in FIG. 1.

The long axis component (ordinary light o component) 13- of the elliptically polarized light 13 shown in FIG. 5a that has entered the polarizing prism 1-1
1 is refracted at the normal light refractive index No. 1, and the optical path 11-
2, and the short axis component (extraordinary light e component) whose polarization plane differs by 90 degrees from this is the extraordinary light refractive index.
It is refracted at Ne1 and output to the optical path 12-2.

Further, the long axis component (extraordinary light e component) 14-1 of the elliptically polarized light 14 shown in FIG.
Short axis component with different plane of polarization (normal light o component) 14
-2 is refracted at normal light refractive index Ne1, and optical path 12
-1.

The combination of refractive indexes of the above optical path ()', 12-1 is No2, No1, and the optical path ()', 1
The refractive index combination of 2-2 is Ne2, Ne1,
Since both have the same refractive index, the optical paths 12-1 and 12-2 are parallel to the optical path 11 of the reflected light in FIG. 13-2, 14- which is the short axis component
The light of 2 is returned to LD8.

As explained above, linearly polarized light incident from opposite directions causes the Faraday rotator 2 to have a non-uniform magnetic field (H1≠
H2), the light becomes elliptically polarized, and the short axis direction components 13-2, 14-
2 is fed back to LD8 and reduces the S/N of the optical signal of LD8.

[Problem that the invention seeks to solve]

As explained above, in conventional optical isolators, linearly polarized light incident from the opposite direction is elliptically polarized by the Faraday rotator, of which the short axis direction component is LD
There is a problem in that the signal-to-noise ratio of the optical signal that is fed back to the LD and output from the LD is reduced.

[Means for Solving the Problems] The above problem is solved by adjusting the optical path in the Faraday rotator so that the part of the uniform magnetic field near the optical central axis is approximately parallel to the direction of the uniform magnetic field. The optical isolator of the present invention selects the taper angle of the birefringent polarizing prism and the second birefringent polarizing prism, and the incident angle from the light emitting element to the first birefringent polarizing prism. It is resolved accordingly.

[Effect]

According to the present invention, the optical isolator is configured such that the optical path of the optical signal passing through the Faraday rotator is made to have a uniform magnetic field near the optical center axis of the Faraday rotator, so that the optical path is substantially parallel to the direction of this magnetic field. Therefore, the linearly polarized light does not become elliptically polarized, thereby eliminating the incidence of reflected light on the light emitting element and preventing a decrease in the S/N of the optical signal.

〔Example〕

The optical isolator of the present invention will be described below with reference to the drawings.

Figure 1a shows the configuration of an optical isolator according to an embodiment of the present invention, in which the Faraday rotator and polarizing prism are made of the same materials as in the conventional example, and the crystal axes of the polarizing prisms are also different from each other by 45 degrees. The magnetic field H is applied to the Faraday rotator using the same method as in the conventional example.

Further, the entrance/exit surfaces of each of the polarizing prism 16-1, the Faraday rotator 15, and the polarizing prism 16-2 are configured to have a certain inclination with respect to the optical path so that the incident light is not reflected in the incident direction.

In the figure, 1' is an optical isolator, 15 is a Faraday rotator, 16-1 and 16-2 are polarizing prisms, i is the entrance and exit angle, and α1 is the polarizing prism 1.
The taper angles of 6-1 and 16-2 are shown.

In FIG. 1a, light in the opposite direction to the incident light 3 and the outgoing light 4 of the optical isolator 1 is transmitted near the optical central axis 6 of the Faraday rotator 15 in a direction substantially parallel to the optical central axis 6. Polarizing prism 16-
1, 16-2, the input and output angles i, and the taper angle α1 are selected.

FIGS. 1b and 1c are explanatory diagrams showing the operating principle of an optical isolator according to an embodiment of the present invention based on the configuration of the optical isolator 1 shown in FIG. 1a.

Figure b shows the case where light is transmitted in the forward direction, and Figure c shows the case where the light is transmitted in the reverse direction.

In the figure, reference numeral 17 indicates an optical path, b indicates an optical path of normal light o to the forward polarizing prism 16-1, and b indicates an optical path of extraordinary light e to the forward polarizing prism 16-1.

In FIG. 1b, the light from LD8 as described above passes through the optical path 17 and passes through the polarizing prism 16--with the taper angle α1.
1, and among this incident light, the polarized component perpendicular to the crystal axis 7-1 is refracted at the normal light refractive index No. 1 and becomes the normal light o, and the polarized component parallel to the crystal axis 7-1 is The extraordinary light is refracted at the refractive index Ne1 and becomes extraordinary light e.

The above-mentioned normal light o travels through the optical path B, and the extraordinary light e travels through the optical path A. Each light enters the Faraday rotator 15 and passes through the vicinity of the optical central axis 6.

The polarization planes of the normal light o and the extraordinary light e are rotated by 45 degrees by the uniform magnetic field H1 as explained in FIG. 2, the normal light o is refracted by the normal light refractive index No2, the extraordinary light e is refracted by the extraordinary light refractive index Ne2, and the respective lights are emitted to optical paths E and H.

The emitted light beams E and F of the optical paths A and B become parallel to the light of the optical path 17 and enter the optical fiber 10.

A part of the above incident light is transmitted to the end face 1 of the optical fiber 10.
A portion of the reflected light travels along optical paths C and D in the opposite direction to optical paths A and B in FIG. 1B.

In FIG. 1c, the above reflected light passes through the optical path 18 and enters the polarizing prism 16-2 at an incident angle i, and among the incident lights, the normal light o to the prism 16-2 has a normal light refractive index. The extraordinary light e directed to the prism 16-2 is refracted by the extraordinary light refractive index Ne2 and proceeds along the optical path C. Each light beam enters the Faraday rotator 15 and is refracted.

The light incident on the Faraday rotator 15 passes through a portion of the uniform magnetic field H1 near the optical central axis 6.

Due to this uniform magnetic field H1, normal light o and abnormal light e
The plane of polarization of is rotated by 45 degrees, but it is not elliptically polarized.

Due to this 45 degree rotation, each plane of polarization is rotated by 90 degrees compared to the incident light, and the normal light o to the polarizing prism 16-2 is
-1 to abnormal e, and the polarizing prism 16-
The extraordinary light e for the polarizing prism 16-1 changes into the normal light o for the polarizing prism 16-1, and enters the polarizing prism 16-1.

In the polarizing prism 16-1, the extraordinary light e is refracted by the extraordinary light refractive index Ne1 and emitted to the optical path H, and the normal light o is refracted by the normal light refractive index No1 and emitted to the optical path H. In FIG. 3b, by the same method as explained, the optical paths T and H are expanded as shown, and the reflected lights O and E are not incident on the LD 8.

Therefore, the optical isolator 1 can be operated with the configuration shown in FIG. 1 above.

Next, the measured data of the row isolators is shown.

When the incident angle i = 50 degrees, the taper angle = 30 degrees, and the internal angles A = 110 degrees and B = 70 degrees of the Faraday rotator 15 (Fig. 1 b, c), with respect to the central axis 6 of the Faraday rotator 15. The normal light o travels at an angle of about 6.7 degrees, and the extraordinary light e travels at an angle of about 3 degrees. Under these conditions, the extinction ratio of the two polarized lights after passing through the Faraday rotator is 35 dB.
As described above, it is possible to prevent the S/N of the LD 8 from decreasing due to reflected light, and the function of the optical isolator can be sufficiently performed.

〔Effect of the invention〕

As explained above, according to the present invention, the incident angle from the light emitting element to the polarizing prism and the taper angle are selected so that the light is not elliptically polarized by the Faraday rotator, so that the reflection at the end face of the optical fiber is This has the effect of preventing the light from being fed back to the LD and preventing the S/N of the optical signal from decreasing.

[Brief explanation of the drawing]

FIG. 1a is a diagram showing the configuration of an optical isolator according to an embodiment of the present invention, FIG. 1b is a diagram showing a forward optical path of the optical isolator of the present invention, and FIG. Figure 2 is a diagram showing the configuration of a conventional optical isolator, Figure 3 is a diagram showing the optical path of the direction.
The figures show the operating principle of a conventional optical isolator, Figure 3a shows the optical path in the forward direction of the optical isolator, Figure 3b shows the optical path in the reverse direction of the optical isolator, and Figure 4 shows FIG. 5, which is a diagram showing the optical path of reflected light, is an explanatory diagram of elliptically polarized light. In the figure, 1, 1' are optical isolators, 1-1, 1
-2, 16-1, 16-2 are polarizing prisms, 2,
15 is a Faraday rotator, 3 is incident light, 4 is output light, 5 is magnet, 6 is optical central axis, 7-1, 7-
2 is the crystal axis, 8 is LD, 8-1 is light polarized perpendicular to 7-1, 8-2 is light polarized parallel to 7-1, 9 and 17 are LD8 optical path,
10 is an optical fiber, 10-1 is an end face, 11 is an optical path of reflected light, 11-1 is an outgoing extraordinary light e, and 11-
2 is the emitted light of the normal light o, 12-1 and 12-2 are the emitted light of the short-axis direction component of the elliptically polarized light, 13 is the elliptically polarized light whose long axis is perpendicular to the crystal axis of the prism, 13-1 , 14-1 is the long axis component, 13-2,
14-2 is a short axis component, 14 is elliptically polarized light having a long axis in the direction of the crystal axis of the prism, and 18 is an optical path of reflected light.

Claims (1)

[Scope of Claims] 1. A first birefringent polarizing prism into which light emitted from a light emitting element is incident, a Faraday rotator, and a second birefringent polarizing prism and a second birefringent polarizing prism whose crystal axes are inclined at 45 degrees with respect to each other. birefringent polarizing prisms are arranged in sequence, and a magnetic field is applied so as to be parallel to the optical central axis of the Faraday rotator, so that the incident light of the first birefringent polarizing prism and the second birefringent polarizing prism are In the optical isolator, the light emitted from the birefringent polarizing prism is parallel to the light emitted from the polarizing prism, and the optical path in the Faraday rotator aligns the uniform magnetic field part near the optical central axis with the direction of the uniform magnetic field. The taper angles of the first birefringent polarizing prism and the second birefringent polarizing prism and the angle of incidence of the light emitting element on the first birefringent polarizing prism are parallel to each other. An optical isolator characterized by being selectively configured.