JP3176180B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called polarization-independent optical isolator which has no polarization dependency and is suitable for use in semiconductor lasers and optical fiber communication using an optical fiber amplifier.

[0002]

2. Description of the Related Art In optical communication and optical measurement systems centered on semiconductor lasers, reliable optical communication such as high-speed and high-density signal transmission and high-precision optical measurement require a laser oscillation malfunction. An optical isolator for removing reflected return light from the optical system to be used is disposed. This optical isolator has a function of passing only a certain polarization plane and blocking a polarization direction orthogonal to the polarization direction,
It is composed of a Faraday rotator that rotates the passing polarized light irreversibly, a permanent magnet that magnetizes the Faraday rotator, and an analyzer that has a passing polarization plane inclined by 45 ° with respect to the polarizer.

Recently, an optical fiber amplifier using an Er-doped fiber has been attracting attention. Even in this case, oscillation occurs in the Er-doped fiber due to reflected light returned from each optical component or a connection point, which increases noise. Cause
An optical isolator for removing reflected return light is required.

In general, when signal light is transmitted through an Er-doped fiber, the polarization plane of the signal light is not preserved. Therefore, a polarization-independent optical isolator that does not depend on the polarization direction of the signal light is used for the optical fiber amplifier.

Here, FIGS. 6 to 8 show a conventional example of a polarization-independent optical isolator. In each of the figures, (a) shows a polarized light propagation state in a forward direction, and (b) shows a polarized light propagation state in a reverse direction. Is represented.

The optical isolator shown in FIG. 6 includes three flat birefringent crystals and one Faraday rotator.

When the signal light passes through the plate-type birefringent crystal 11, the linearly polarized light having the extraordinary light direction is displaced with respect to the plate-type birefringent crystal 11. ), And upon passing through the Faraday rotator 12, the planes of polarization of two orthogonal linearly polarized lights are rotated. Thereafter, when the light passes through the plate-type birefringent crystal 13 and the plate-type birefringent crystal 14, the linearly polarized light having an extraordinary optical direction is displaced with respect to each of the plate-type birefringent crystals. The two linearly polarized light beams are orthogonally combined by the plate-type birefringent crystal 14.

The optical isolator shown in FIG. 7 comprises three flat birefringent crystals and two Faraday rotators.

When the signal light passes through the plate-type birefringent crystal 15, the linearly polarized light having the extraordinary light direction is displaced with respect to the plate-type birefringent crystal 15. ). Then, the polarization planes of the two linearly polarized light beams orthogonal to each other are rotated by the Faraday rotators 16 and 17. Plate-type birefringent crystal 18, plate-type birefringent crystal 19
In contrast, the linearly polarized light having the extraordinary light direction is displaced, and the linearly polarized light separated by the flat birefringent crystal 15 is orthogonally synthesized by the flat birefringent crystal 19.

The optical isolator shown in FIG. 8 includes two wedge-shaped birefringent crystals and one Faraday rotator. The signal light from the optical fiber 21 passes through the lens 22 and is converted by the wedge-shaped birefringent crystal 23 into two orthogonal linearly polarized lights (ordinary light and ordinary light).
The polarization plane of each linearly polarized light is rotated by the Faraday rotator 24. And a wedge-shaped birefringent crystal 2
5, the two linearly polarized lights become parallel lights, and the lens 26
Collimates the parallel light onto the optical fiber 27.

In the opposite direction of the above conventional example, the return light separated into two orthogonal linearly polarized lights by the birefringent crystal polarizer is displaced in a direction away from the optical fiber due to the non-reciprocal action of the Faraday rotator. Does not enter the optical fiber. Therefore, it functions as an optical isolator.

[0012]

Generally, an optical isolator used immediately before a semiconductor laser effectively works only on a certain polarization plane. Therefore the polarization state is temperature, wind, pressure,
If this optical isolator is arranged between ordinary optical fibers that constantly change under environmental conditions such as vibration, there is a drawback that signal light is greatly lost depending on the polarization direction.

In the polarization-independent optical isolator shown in the above-mentioned conventional example, a difference in optical path length occurs between two orthogonal linearly polarized lights in the process of separating and combining signal light into two orthogonal linearly polarized lights. . Degradation of signal light is caused by polarization dispersion caused by the difference in optical path length, and this is a serious problem particularly in long-distance optical communication.

Further, in the above-mentioned conventional polarization-independent optical isolator, two linearly polarized lights orthogonal to each other in the forward direction are separated and combined in the forward direction due to the non-reciprocal action of the Faraday rotator, but in the reverse direction. Return light that becomes two orthogonal linearly polarized lights is largely separated and does not enter the optical fiber, so that it functions as an optical isolator.

However, since the return light is not completely blocked by absorption or the like, it is very difficult to adjust the position of the optical system during the assembly so that the return light does not enter the optical fiber. There is a disadvantage that isolation is reduced due to misalignment of the system.

The present invention has been made in view of the above points, and an object of the present invention is to provide a polarization-independent optical isolator which is compatible with long-distance optical communication and which can easily perform optical adjustment at the time of assembly. is there.

[0017]

In order to solve the above-mentioned problems, the present invention provides a birefringent element for separating incident light into two orthogonal linearly polarized lights and a birefringent element for combining two separated linearly polarized lights. It consists of a rotator that rotates the polarization direction by 45 ° and has a reciprocal action, a Faraday rotator that rotates the polarization direction by 45 ° and has a nonreciprocal action, and a permanent magnet that applies a magnetic field to the Faraday rotator. The optical isolator is characterized in that the optical rotator and the Faraday rotator are arranged in series in the respective optical paths of the separated linearly polarized light in a combination in which the optical path lengths of the two linearly polarized light are equal.

Further, in order to facilitate the adjustment of the optical axis of the optical system at the time of assembly, a polarizer is arranged between the optical rotator and the Faraday rotator, and both polarizers return two linearly polarized lights. Alternatively, it is an optical isolator for removing at least one of them.

[0019]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of an optical isolator according to a first reference example. FIG. 1A shows the state of propagation of polarized light in the forward direction, and FIG. 1B shows the state of propagation of polarized light in the reverse direction. The propagation state of polarized light when passing through each of the constituent elements indicated by a, b, c, d, and e is always the direction of polarization when the optical fiber 6 is viewed from the optical fiber 1.

From FIG. 1A, the signal light in the propagation state a passing through the lens 2 from the optical fiber 1 is separated into two orthogonal linearly polarized lights as in the propagation state b by a polarizer 3 made of a birefringent crystal. . At this time, the two orthogonal linearly polarized lights change the polarization direction parallel to the normal vector of the main section stretched by the vector indicating the optical axis direction O1 of the polarizer 3 and the wave vector of the light incident on the polarizer 3 to the ordinary light direction. The orthogonal direction is defined as the extraordinary light direction (the polarization direction perpendicular to the main section is defined as the ordinary light direction, and the polarization direction perpendicular to the ordinary light direction is defined as the extraordinary light direction).

Of the two orthogonal linearly polarized light beams separated by the polarizer 3, the optical path of the linearly polarized light coinciding with the azimuth of ordinary light is set to 8
Assuming that the optical path of the linearly polarized light that coincides with the extraordinary light direction is 8, the Faraday rotator F1 and the rotator R
1. On the other optical path 8, an optical rotator R2 and a Faraday rotator F2 are arranged in this order.

Here, when viewed from the optical fiber 1 in the forward direction,
The clockwise rotation with respect to the traveling direction of the polarized light is positive. Therefore, the polarization direction of the linearly polarized light passing through the optical path 8 is rotated by + 45 ° by the Faraday rotator F1 and the rotator R1. The polarization direction of the linearly polarized light passing through the optical path 8 is rotated by + 45 ° by the optical rotator R2 and the Faraday rotator F2. The propagation state at this time is as shown in c and d.

Therefore, the optical path 8 that has passed through the optical rotator R1
The polarization direction of the linearly polarized light and the polarization direction of the linearly polarized light in the optical path 8 that has passed through the Faraday rotator F2 are rotated by + 90 ° with respect to the polarization direction when emitted from the polarizer 3.
Since the polarizer 4 is a birefringent crystal having the same optical axis direction O2 as the optical axis direction O1 of the polarizer 3, when entering the polarizer 4, the linearly polarized light in the optical path 8 has an extraordinary optical direction and the optical path 8 has The two orthogonal linearly polarized lights separated from each other are orthogonally combined because they coincide with the direction of the ordinary light, pass through the lens 5, and enter the optical fiber 6. In the above separation and combination, the optical path 8 that the two linearly polarized light traces and the optical path 8 have the same optical path length.

Further, from FIG. 1B, the return light from the optical fiber 6 passes through the lens 5 and is separated into two orthogonal linearly polarized lights by the polarizer 4, and coincides with the ordinary light direction as in the propagation state d. The linearly polarized light passes through the optical path 8, and the linearly polarized light that coincides with the extraordinary light direction passes through the optical path 8.

The polarization direction of the linearly polarized light passing through the optical path 8 is rotated by -45 ° by the reciprocal rotator R1 and +45 degrees by the non-reciprocal Faraday rotator F1.
゜ It is rotated. On the other hand, the polarization direction of the linearly polarized light passing through the optical path 8 is +4 due to the Faraday rotator F2 having a nonreciprocal action.
Rotated by 5 °, -45 by the reciprocal rotator R2
゜ It is rotated. This propagation state is as shown in c and b.

Therefore, the polarization direction of the linearly polarized light in the optical path 8 that has passed through the Faraday rotator F1 and the polarization direction of the linearly polarized light in the optical path 8 that has passed through the optical rotator R2 match the polarization direction when the light has passed through the polarizer 4. . Further, for the polarizer 3,
The direction of the linearly polarized light in the optical path 8 coincides with the extraordinary light direction, and the linearly polarized light in the optical path 8 coincides with the ordinary light direction. Therefore,
The linearly polarized light in the optical path 8 is displaced in the opposite direction to the optical fiber 1 and does not enter the optical fiber 1. Further, since the linearly polarized light in the optical path 8 travels straight without being displaced, it does not enter the optical fiber 1.

FIG. 2 is a schematic configuration diagram showing a second reference example in which the configuration of FIG. 1 is integrated. Thus, there is no problem in the characteristics even when the constituent elements are integrated.

FIG. 3 is a schematic configuration diagram showing a third embodiment of the present invention. The propagation state of polarized light when passing through each of the constituent elements indicated by a, b, c, d, and e is always from the optical fiber 1 to the optical fiber 6.
This is the operation state of polarized light when seeing FIG. From FIG. 3 (a),
The signal light in the propagation state a, which has been emitted from the optical fiber 1 and passed through the lens 2, is incident on the polarizer 3 made of a birefringent crystal and having a crystal optical axis in the direction indicated by O1, as shown in the propagation state b. It is separated into two linearly polarized lights. Here, as viewed from the optical fiber 1 in the forward direction, the optical path of the linearly polarized light that coincides with the ordinary light direction among the two orthogonal linearly polarized lights separated by the polarizer 3 with the right rotation being positive with respect to the traveling direction of the polarized light. 8
Assuming that the optical path of the linearly polarized light coincident with the extraordinary light direction is 8, the optical path 8 includes a Faraday rotator F3 that rotates the polarization direction by + 45 ° and an optical rotator R3 that rotates the polarization direction by + 45 °. The optical rotator R4 for rotating −45 ° and the Faraday rotator F4 for rotating the polarization direction by −45 ° further apply a magnetic field to the permanent magnet M1 and the Faraday rotator F4 for applying a magnetic field to the Faraday rotator F3, and A permanent magnet M2 whose magnetic field is applied in a direction opposite to that of the permanent magnet M1 is arranged.

Now, the signal light from the optical fiber 1 is
Are separated into two orthogonal linearly polarized lights as in the propagation state b, and the separated linearly polarized lights are obtained by rotating the linearly polarized light in the optical path 8 and the linearly polarized light in the optical path 8 in opposite directions as in the propagation states c and d. You. Accordingly, the linearly polarized light in the optical path 8 is displaced with respect to the optical axis O2 of the polarizer 4 in accordance with the extraordinary light direction, and the linearly polarized light in the optical path 8 is straightly moved in accordance with the ordinary light direction and orthogonally combined by the polarizer 4. Are condensed by the lens 5 and enter the optical fiber 6. In the above separation and combination, the optical path 8 that the two linearly polarized light traces and the optical path 8 have the same optical path length.

From FIG. 3B, the return light from the optical fiber 6 is split into two linearly polarized lights orthogonal to each other by the polarizer 4, and the linearly polarized light that coincides with the ordinary light direction with respect to the polarizer 4 has an optical path. 8, the linearly polarized light coincident with the extraordinary light direction
Pass through. The eight linearly polarized lights that have passed through the Faraday rotator F3 are displaced in the opposite direction to the optical fiber 1 because they have an extraordinary optical direction with respect to the polarizer 3, and do not enter the optical fiber 1. Since the eight linearly polarized lights that have passed through the optical rotator 4 have an ordinary light direction with respect to the polarizer 3, they go straight as they are and do not enter the optical fiber 1.

FIG. 4 is a schematic configuration diagram of an optical isolator according to an embodiment of the present invention. FIG. 4A shows the state of propagation of polarized light in the forward direction, and FIG. 4B shows the state of propagation of polarized light in the reverse direction. The propagation state of polarized light when passing through each of the constituent elements indicated by a, b, c, d, and e is the operation state of polarized light when the optical fiber 6 is always viewed from the optical fiber 1.

As shown in FIG. 4A, the signal light in the propagation state a passing through the lens 2 from the optical fiber 1 is converted into two orthogonal linearly polarized lights b by the polarizer 3 made of a birefringent crystal.
Separate as

Assuming that the optical path of ordinary light is 8 and the optical path of extraordinary light is 8 among the two orthogonal linearly polarized lights separated by the polarizer 3, the optical rotator R5, the Faraday rotator F5 and the other are disposed on the optical path 8. Faraday rotator F on the optical path 8
6, the optical rotator R6 is arranged in this order, and between the optical rotator and the Faraday rotator on the optical path of each linearly polarized light, the polarizer 7 is commonly arranged on both optical paths.

The polarizer 7 is a polarizer provided with a dielectric multilayer film that reflects polarized light of unnecessary components and transmits polarized light orthogonal to the polarized light, or a polarizer that absorbs polarized light of unnecessary components and transmits polarized light perpendicular thereto. Although there is a polarizer, in the present embodiment, the polarizer 7 is the latter type of polarizer.

Here, when viewed from the optical fiber 1 in the forward direction,
If the clockwise rotation is positive with respect to the direction of polarization, the light path 8
Is -4 due to the optical rotator R5.
The polarization direction of the linearly polarized light that rotates by 5 ° and passes through the optical path 8 becomes like the propagation state c by rotating by + 45 ° by the Faraday rotator F6, and the polarization direction of the linearly polarized light on both optical paths is It matches the polarization transmission direction. Therefore, there is no loss due to absorption when passing through the polarizer 7.

The polarization direction of the linearly polarized light in the optical path 8 having passed through the polarizer 7 is rotated by -45 ° by the Faraday rotator F5, and the polarization direction of the linearly polarized light in the optical path 8 is rotated by + 45 ° by the optical rotator R6. As shown in the state e, the light is rotated by + 90 ° from the polarization direction when emitted from the polarizer 3.

The polarizer 4 also has the same optical axis direction O2 as the polarizer 3.
When the light enters the polarizer 4, the linearly polarized light in the optical path 8 becomes extraordinary light, and the linearly polarized light in the optical path 8 becomes ordinary light. The light is orthogonally combined as in the state f, passes through the lens 5, and enters the optical fiber 6. In the above separation and combination, the optical path 8 that the two linearly polarized light traces and the optical path 8 have the same optical path length.

As shown in FIG. 4B, the return light from the optical fiber 6 passes through the lens 5 and is separated by the polarizer 4 into two orthogonal linearly polarized ordinary lights and extraordinary lights. Is the optical path 8, and the linearly polarized light that has become the extraordinary light is the optical path 8.
Pass through.

The polarization direction of the linearly polarized light passing through the optical path 8 is rotated by -45 ° by the Faraday rotator F5 having a nonreciprocal action, so that the linearly polarized light coincides with the polarization absorption direction of the polarizer 7, and the linearly polarized light is absorbed.

The polarization direction of the linearly polarized light passing through the optical path 8 is rotated by -45 ° by the optical rotator R6 having a reciprocal action. Since this polarization direction matches the polarization transmission direction of the polarizer 7, the polarization direction is not absorbed by the polarizer 7 and passes through the polarization direction as it is,
The light enters the Faraday rotator F6. Faraday rotator F
6 has a nonreciprocal action, so that the polarization direction is rotated by + 45 °. Therefore, it matches the ordinary light direction of the polarizer 3. Therefore, this linearly polarized light passes through the polarizer 3 without being displaced,
It does not enter the optical fiber 1.

FIG. 5 is a schematic structural view showing a second embodiment of the present invention. The propagation state of polarized light when passing through each of the constituent elements indicated by a, b, c, d, and e is always from the optical fiber 1 to the optical fiber 6.
This is the operation state of polarized light when seeing FIG.

As shown in FIG. 5A, the signal light in the propagation state a, which has exited the optical fiber 1 and passed through the lens 2, is incident on the polarizer 3 made of a birefringent crystal having the crystal optical axis direction O1, and propagates. The light is separated into two linearly polarized lights as shown in a state b. Here, assuming that the optical path of the ordinary azimuth is 8 and the optical path of the extraordinary azimuth is 8, an optical rotator R7 for rotating the polarization direction by −45 ° and a Faraday rotator F7 for rotating the polarization direction by −45 ° are provided in the optical path 8. On the other hand, the polarization direction is +
An optical rotator R8 for rotating by 45 ° and a Faraday rotator F8 for rotating the polarization direction by + 45 ° are arranged. Therefore, as shown in the propagation state c, the linearly polarized light in the optical path 8 is rotated by −45 ° by the optical rotator R7, and conversely, the linearly polarized light in the optical path 8 is rotated by + 45 ° by the optical rotator R8, and the polarization direction is changed to the absorption type. It coincides with the polarized light passing direction of the polarizer 7. In addition, the polarizer 7
The linearly polarized light that has passed through passes through the state of b as shown in the propagation state d of the polarized light. Next, the linearly polarized light in the optical path 8 is rotated by -45 ° by the Faraday rotator F5, and conversely, the linearly polarized light in the optical path 8 is rotated by + 45 °. Become. For this reason, in the polarizer 4, the linearly polarized light in the optical path 8 has an extraordinary optical direction and is refracted, and the optical path 8 has an ordinary optical direction and travels straight. Accordingly, the linearly polarized lights separated into the optical paths 8 and 8 are orthogonally combined by the polarizer 4, condensed by the lens 5, and incident on the optical fiber 6. As can be seen from the above description,
The optical path lengths from separation of two linearly polarized lights to synthesis are equal.

Further, as shown in FIG. 5B, the return light from the optical fiber 6 is split into two orthogonal linearly polarized lights by the polarizer 4, and the linearly polarized light that has become ordinary light becomes abnormal light in the optical path 8. The linearly polarized light passes through the optical path 8, and the Faraday rotator F8 and the Faraday rotator F7 rotate the linearly polarized light of the optical path 8 by -45 ° and the linearly polarized light of the optical path 8 by + 45 °, respectively. 7, the return light is absorbed by the polarizer 7 and completely removed. Thereby, optical adjustment at the time of assembling the optical isolator becomes very easy.

4 and 5 have been described above,
It is natural that the components of the optical isolator are integrated for reasons such as miniaturization.

In both the fourth embodiment shown in FIG. 4 and the fifth embodiment shown in FIG. 5, even when the polarizer 7 is removed, the light in the forward direction is synthesized by the polarizer 4 and the light is returned in the reverse direction. Light is optical path 8
Since the linearly polarized light of the optical path 8 is displaced because of the extraordinary azimuth, the linearly polarized light of the optical path 8 travels straight with the ordinary azimuth, so that FIGS.
Operates as an optical isolator equivalent to the embodiment shown in FIG.

Although the present invention has been described in detail above, the present invention is not limited to the above-described embodiment, and various changes can be made such as the arrangement of the reciprocal element and the non-reciprocal element and the combination of each polarization rotation direction. .

[0047]

As described in detail above, the separation / combination type optical isolator according to the present invention has two signal lights.
Since each optical path length until the light is separated into two linearly polarized lights and combined can be made equal, there is no deterioration of the signal light due to polarization dispersion which is a problem in long-distance optical communication.

Further, in the optical isolator according to the present invention, since the return light is one light beam or completely blocked,
At the time of assembling the optical isolator, it is possible to reduce a decrease in isolation due to misalignment of the optical system, and it is possible to very easily adjust the position of the optical system.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an optical isolator according to a reference example of the present invention.

FIG. 2 is a schematic configuration diagram showing an optical isolator according to a reference example of the present invention.

FIG. 3 is a schematic configuration diagram showing an optical isolator according to a reference example of the present invention.

FIG. 4 is a schematic configuration diagram showing an optical isolator according to a first example of the present invention.

FIG. 5 is a schematic configuration diagram showing an optical isolator according to a second embodiment of the present invention.

FIG. 6 is a schematic diagram showing an optical isolator showing a first conventional technique.

FIG. 7 is a schematic diagram showing an optical isolator showing a second conventional technique.

FIG. 8 is a schematic diagram showing an optical isolator showing a third conventional technique.

[Explanation of symbols]

 1,6 Optical fiber 2,5 Lens 3,4 Polarizer (birefringent crystal) 7 Absorption polarizer F1-F8 Faraday rotator (non-reciprocal element) R1-R8 Optical rotator (reciprocal element) M, M1, M2 Permanent magnet

Claims (1)

(57) [Claims] A first birefringent element that separates incident light into two orthogonal linearly polarized lights; and a second birefringent element that combines two orthogonal linearly polarized lights. A non-reciprocal element and a reciprocal element that rotate the polarization direction by 45 ° are disposed on each of the first optical path and the second optical path through which two linearly polarized light propagates, and are arranged in the order separated by the first birefringent element. Directional light is synthesized by the second birefringent element, and return light separated by the second birefringent element from the reverse direction is incident on the first birefringent element and the forward light is incident. The arrangement order of the non-reciprocal elements and the reciprocal elements and the respective polarization rotation directions are set so that the light is emitted to a position away from the position, and a common polarizer is disposed between the non-reciprocal elements and the reciprocal elements. An optical isolator characterized by: