JPH0820623B2 - Optical isolator - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polarization-independent optical isolator.

(Prior Art) Conventionally, various optical isolators having no polarization dependence have been proposed, such as Japanese Patent Publication Nos. 58-28561 and 60-49297.

Therefore, a conventional example will be described with reference to FIG. The optical isolator has the same thickness of 2 as shown in Fig. 2 (a).
Birefringent crystal plates 9, 10, the magneto-optical material 11 having a Faraday rotation angle of 45 degrees, and the optical axis and the thickness were set so that the polarization direction of the incident light and the polarization direction of the emitted light differ by 45 degrees. It is composed of an optical rotatory crystal plate 12 and is arranged between two optical fibers 13 and 14 facing each other.

The incident light is split into ordinary light and extraordinary light by the first birefringent crystal plate 9 on the optical fiber 13 side, and then the polarization direction of the light passing through the magneto-optical material 11 is counterclockwise when viewed from the optical fiber 14 side. 45 ° in the direction (counterclockwise), further rotated by 45 ° by the optical rotatory crystal plate 12, enters the second birefringent crystal plate 10, and ordinary light and extraordinary light are recombined, and the optical fiber 14 Incident on.

When the backward light emitted from the optical fiber 14 passes through the second birefringent crystal plate 10 and the optical rotatory crystal plate 12, its polarization direction is the same as that of the forward light, but When the light enters the optical material 11, it is rotated by 45 degrees counterclockwise as viewed from the side of the optical fiber 14 due to the Faraday effect, so that the polarization direction differs from that of the light in the forward direction by 90 degrees. The light that has entered the refraction crystal plate 9 is not combined as it is divided into ordinary light and extraordinary light, and is emitted to the optical fiber 13 side at a position different from the incident axis in the forward direction.

Forward points P1 and P from optical fiber 13 to optical fiber 14
The change of the polarization direction in 2, P3, P4 and P5 will be described with reference to FIG. At the time of incidence of (a), the vertical polarization component F1 is shown by a dotted line, the horizontal polarization component F2 is shown by a solid line, the centers C1 and C2 of the polarization components are dots, and the central axis O where the incident light travels is a white circle. It shows with.

In the incident light, the polarization component F1 in the vertical polarization direction goes straight as ordinary light in the first birefringent crystal plate 9 and the horizontal polarization component F2 moves in parallel as extraordinary light and exits. The center C2 of the component F2 moves and the center C1 of the polarization component F1 moves.
And the position is different.

The polarization component of the light emitted from the first birefringent crystal plate 9 is rotated 45 degrees counterclockwise when viewed from the optical fiber 14 side by the magnetization when passing through the magneto-optical material 11. , The polarization component F1 and the polarization component F2 are
Tilt 45 degrees.

Further, as shown by a point P4, the polarization components of these lights are incident on the optical rotatory crystal plate 12 and are rotated by 45 degrees, so when compared with the polarization direction at the time of incidence on the first birefringent crystal plate 9, It means that it is rotated 90 degrees.

The polarization components of these lights are the second birefringent crystal plate.
When incident on 10, the polarization component F1 moves in parallel and the polarization component F2 goes straight, so as shown by the point P5, the center C of the polarization components F1, F2
1, C2 will match.

Further, as shown in FIG. 2C, the polarization components F1a and F2a of the backward light travel to the point P3 in the same manner as the polarization components of the forward light as shown by the points P5, P4 and P3. Then, as shown by the point P2, the polarization components F1a and F2a are changed to 45 by the magnetization of the magneto-optical material 11.
After being rotated by a degree, at the point P1, the polarization components P1a and P2a are located such that their centers C1a and C2a are apart from the incident axis O.

At points P2, P3, and P4, the center C1a of the polarization component F1a is located on the central axis O, so that the polarization component of the light in the opposite direction has the first birefringence on the central axis O of the forward incident light. It will pass until it enters the crystal plate 9. Therefore, the performance of the isolator having this configuration largely depends on the performance of the birefringent crystal plate 9 and the assembly accuracy.

(Problems to be Solved by the Invention) In the optical isolator of the above example, the two birefringent crystals are used as a means for enhancing the function of preventing the light incident from the opposite direction from returning to the optical fiber on the light source side. It is easily conceivable to combine the plate, the magneto-optical material, and the optical rotatory crystal plate as one unit, and to combine a plurality of units. However, even if the number of units is simply increased and the performance is improved by the increased amount, the number of parts is also increased accordingly, resulting in higher cost and further inconvenience in that the size cannot be reduced. Also, since the return light (light in the backward direction) passes on the central axis of the forward incident light to the first birefringent crystal plate,
Performance tends to deteriorate.

An object of the present invention is to provide an optical isolator having a high performance with a small number of parts.

(Means for Solving the Problem) The optical isolator according to the present invention comprises four plate-shaped birefringent crystals (first birefringent crystal, second birefringent crystal, which are provided between two optical waveguides, A third birefringent crystal and a fourth birefringent crystal), and two magneto-optical materials whose lengths are determined so that the polarization directions of the incident light and the outgoing light differ by about 45 degrees due to magnetization. is there.

The second and third birefringent crystals are respectively arranged between the first birefringent crystal and the fourth birefringent crystal, and the first and second birefringent crystals are arranged between the first and second birefringent crystals. The first magneto-optical material is placed between the third birefringent crystal and the fourth birefringent crystal, and the second magneto-optical material is placed between them.

The thickness of the first birefringent crystal and the thickness of the fourth birefringent crystal are equal. The thickness of each of the second birefringent crystal and the third birefringent crystal is √2 times the thickness of the first birefringent crystal (or the fourth birefringent crystal).

The plane of polarization of the second birefringent crystal (perpendicular to the transmission end face,
And the plane including the crystal optical axis) is (45 + 90m) ° with respect to the plane of polarization of the first birefringent crystal (where m is an integer of 0 or more).
The polarization plane of the third birefringent crystal is orthogonal to the polarization plane of the second birefringent crystal, and the polarization plane of the fourth birefringent crystal is rotated with respect to the polarization plane of the third birefringent crystal. (45 + 90n) degrees (where n is an integer of 0 or more).

(Embodiment) An embodiment of the present invention will be described below with reference to FIG.

The optical isolator according to the present invention is configured by combining four plate-shaped birefringent crystals 1, 2, 3, 4 and two magneto-optical materials 5, 6. These constituent members 1 to 6 are 2
It is arranged between the optical fibers 7 and 8 which are two optical waveguides. The arrangement order is from the first birefringent crystal 1 on the optical fiber 7 side to the first magneto-optical material 5, the second birefringent crystal 2, and the third birefringent crystal 2 in the light traveling direction (downward direction in the figure). The refraction crystal 3, the second magneto-optical material 6 and the fourth birefringence crystal 4 are in this order. The birefringent crystals 1 to 4 use rutile, the first and fourth birefringent crystals 1 and 4 have a plate thickness t, and the second and third birefringent crystals 2 and 3 are used.
Is the plate thickness √2t.

The polarization planes of the second and third birefringent crystals 2 and 3 (the planes perpendicular to the transmission end face and including the crystal optical axis) are orthogonal to each other, and the polarization planes of the first birefringent crystal 1 are different from each other. 45 each
It is rotated once. The polarization plane of the fourth birefringent crystal 4 is rotated by 45 degrees with respect to the polarization planes of the second and third birefringent crystals 2 and 3, and the polarization plane of the first birefringent crystal 1 is , 18
It is rotated 0 degrees.

Garnet is used as the first and second magneto-optical materials 5 and 6, and the length is determined so that the polarization directions of the incident light and the emitted light are different by about 45 degrees.

 Next, the operation of the optical isolator will be described.

The light from the light source is first incident on the first birefringent crystal 1 from the optical fiber 7, where it is split into ordinary light and extraordinary light, and then passes through the first magneto-optical material 5 to change the polarization direction. It rotates 45 degrees clockwise (clockwise) when viewed from the side of the optical fiber 8 and is translated by the second and third birefringent crystals 2 and 3, and then enters the second magneto-optical material 6 and is polarized. The direction is further rotated by 45 degrees and the light is emitted, and in the fourth birefringent crystal 4 on the optical fiber 8 side, the branched lights are combined again and enter the optical fiber 8.

The returning (reverse direction) light emitted from the optical fiber 8 passes through the fourth birefringent crystal 4, and the ordinary light and the extraordinary light branched by the fourth birefringent crystal enter the second magneto-optical material 6. The first birefringent crystal 1 is rotated further 45 degrees by the first magneto-optical material 5 after being translated by the first and second birefringent crystals 3 and 2 and translated in parallel. The ordinary light and the extraordinary light travel toward the optical fiber 7 at positions far away from the central axis of the forward incident light.

Here, the change in the polarization direction at each point P1, P2, P3, P4, P5, P6, P7 of the light traveling from the optical fiber 7 to the optical fiber 8 is first described.
The change in the polarization direction of the light in the opposite direction at each of the above points will be described with reference to FIG.

It is to be noted that the parts corresponding to those in FIGS. 2A and 2B have the same reference numerals.

In the incident light of FIG. 1 (a), in the first birefringent crystal 1, the vertical polarization component F1 at the time of incidence goes straight as ordinary light, and the horizontal polarization component F2 moves in parallel as extraordinary light, so at the point P2. The center C1 of the polarization component F1 and the center C2 of the polarization component F2 have different positions.

As shown by the point P3, the polarization components F1 and F2 of the light emitted from the first birefringent crystal 1 are rotated in the polarization direction by 45 degrees by the magnetization in the first magneto-optical material 5.

As shown by the point P4, the light is incident on the second birefringent crystal 2.
The component F1 goes straight, and as shown by a point P5, the polarization component F2 incident on the third birefringent crystal 3 goes straight and the polarization component F1 moves in parallel.

In the second magneto-optical material 6 as shown at point P6, the polarization components F1 and F2 have their polarization directions further rotated by 45 degrees by the Faraday effect, and as shown at point P7,
The centers C1 and C2 of the polarized light component F1 and the polarized light component F2 incident on the birefringent crystal 4 of are coincident with each other.

The change in the polarization direction of the light emitted from the optical fiber 8 in the opposite direction is as follows.

As shown by points P7 and P6, the light in the opposite direction is branched by passing through the fourth birefringent crystal 4, and the center C1a of the polarization component F1a moves.

As shown at point P5, when the polarization components F1a and F2a are incident on the second magneto-optical material 6, they are rotated by 45 degrees, so that their polarization directions are the polarization directions of the polarization components F1 and F2 at the forward point P5. It will differ from the direction by 90 degrees.

As shown by the point P4, the polarization component F1a incident on the third birefringent crystal 3 goes straight and the polarization component F2a horizontally moves, so that the center C2a of the polarization component F2a moves.

As shown by the point P3, the polarized light component F2a that has entered the second birefringent crystal 2 advances straight, and the polarized light component F1a moves in parallel and then exits, so that the center C1a of the polarized light component F1a moves.

As shown by the point P2, the polarization components F1a and F2a have their polarization directions rotated by 45 degrees in the first magneto-optical material 5,
As shown by the point P1, in the first birefringent crystal 1, the polarization component F1
a goes straight and the polarization component F2a moves in parallel, so the polarization component
The center C2a of F2a moves.

The relationship between the central axis O and the polarization components F1a and F2a of the light in the opposite direction will be described. From the point P7 to the point P1, the polarization components F1a and F2
Neither center C1a nor center C2a of a is located on the central axis O. At the point P1, the distances between the centers C1a and C2a of the polarization components F1a and F2a and the central axis O of the forward incident light are far apart from each other as compared with the other points P7 to P2.

(Effects of the Invention) As described above, according to the present invention, the first, second, third and fourth birefringent crystals having a flat plate shape are combined with the first and second magneto-optical materials. As a result, it is possible to provide an optical isolator that has a high performance and can be downsized with a small number of parts. Further, since the return light does not pass any polarization component on the central axis of the forward incident light, the performance is stable.

[Brief description of drawings]

FIG. 1 is an explanatory view according to the present invention, as viewed from the optical fiber 8 side, and FIG. 2 is an explanatory view showing a conventional example, as viewed from the optical fiber 14 side. 1 ... 1st birefringent crystal, 2 ... 2nd birefringent crystal, 3 ... 3rd birefringent crystal, 4 ... 4th birefringent crystal, 5 ... 1st magneto-optical material, 6 ... Second magneto-optical material, 7,8 ... Optical waveguide (optical fiber), F1, F1a, F2, F2a ... Polarization component, O ... Central axis.

Claims (1)

[Claims] 1. A plate-shaped first, second, third and fourth birefringent crystal provided between two optical waveguides, and polarization directions of incident light and outgoing light are approximately 45 degrees due to magnetization. A first and a second magneto-optical material having different lengths, and a second and a third birefringent crystal between the first birefringent crystal and the fourth birefringent crystal. And the first magneto-optical material is placed between the first birefringent crystal and the second birefringent crystal, and the first magneto-optical material is placed between the third birefringent crystal and the fourth birefringent crystal. The second magneto-optical material is arranged respectively, and the thickness of the first birefringent crystal is equal to the thickness of the fourth birefringent crystal, and the second birefringent crystal and the third birefringent crystal have the same thickness. Each thickness is √2 times each thickness of the first birefringent crystal and the fourth birefringent crystal, and the polarization plane of the second birefringent crystal (transmission The plane perpendicular to the plane and containing the crystal optical axis is rotated by (45 + 90m) ° (where m is an integer of 0 or more) with respect to the plane of polarization of the first birefringent crystal, and the third birefringent crystal Is orthogonal to the plane of polarization of the second birefringent crystal, and the plane of polarization of the fourth birefringent crystal is (45 + 90n) ° (where n Is an integer of 0 or more) is rotated. An optical isolator.