JPH03290611A - Optical isolator device - Google Patents

Abstract

PURPOSE:To reduce the insertion loss to realize high isolation by arranging two pairs of wedged birefringence crystal and 45 deg. Farady rotators on both sides of prismatic birefringence crystal and specifying the direction of inclination of each crystal face and the direction of the crystal axis. CONSTITUTION:First wedged birefringence crystal B1 and prismatic birefringence crystal P are so arranged that directions of inclination of their surfaces facing each other are equal to each other, and second wedged birefringence crystal B2 and prismatic birefringence crystal P are so arranged that directions of inclination of their surfaces facing each other are equal to each other. Directions of optical axes of first and second wedges birefringence crystal B1 and B2 are shifted from that of prismatic birefringence crystal P. In this constitution, the incident light from a fiber F1 is separated to ordinary light (o) and extraordinary light (e) by crystal B1, and they go in parallel in crystal P and are synthesized on the same axis by crystal B2 and are coupled to a fiber F2 without angular deviation with a small loss. Meanwhile, light in the opposite direction is refracted to be separated by crystal B1 and is not coupled to the fiber F1.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to the configuration of an optical isolator device used for optical communications and the like.

(Prior Art) FIG. 2 is a block diagram of a conventional optical isolator device consisting of a wedge-shaped birefringent crystal and a 45-degree Faraday rotator, and (a) in the same figure shows the case of a one-stage configuration. (b) in the diagram is 2
The cases of stage configuration are shown (Special Publication Publication 1986-58).
(See Publication No. 811). In addition, (a) and (
b) The operation of the one-stage optical isolator device is
Figures (a) and (b) are diagrams for explaining the operation of a two-stage optical isolator device.

In FIGS. 2 to 4, Fl and F2 are optical fibers,
Ll, L2 are optical lenses, Bl, B2°B3. B4 is a wedge-shaped birefringent crystal, and R1 and R2 are 45 degree Faraday rotators. Furthermore, (a), (b), (tsu), and (1) in FIGS. 3 and 4.

(E) shows the distribution state of the light ray at the passing point in the plane perpendicular to the optical axis, and CB~CB4 shows the wedge-shaped birefringent crystal B.
The optical axes of 1 to B4 are shown, respectively.

In a conventional optical isolator device, in the case of a one-stage configuration, a 45 degree Faraday rotator R1 is arranged between wedge-shaped birefringent crystals B1 and B2, and a wedge-shaped birefringent crystal B
The optical axis of the wedge-shaped birefringent crystal B1 is rotated by 45 degrees in the same direction as the rotation of polarized light.

Also, in the case of the same two-stage configuration, wedge-shaped birefringent crystal B
A first-stage optical isolator with a 45-degree Faraday rotator R1 arranged between B1 and B2, and a wedge-shaped birefringent crystal B3.
and a second-stage optical isolator in which a 45-degree Faraday rotator R2 is arranged between The inclination direction of the wedge-shaped birefringent crystal is shifted by 90 degrees.

Next, the operation of the above configuration will be explained using FIG. 3 and FIG. 4.

First, the operation of the one-stage optical isolator device will be explained based on FIG.

When the propagation direction of the light beam is from the optical fiber F1 to F2 (hereinafter referred to as the forward direction), as shown in FIG. The light beam incident on the wedge-shaped birefringent crystal B
When passing through ray 1, it is separated into ordinary ray 0 and extraordinary ray e,
refract at different angles.

Next, the polarization directions of both the ordinary ray and the extraordinary ray are rotated by 45 degrees by the 45-degree Faraday rotator R1, and then the rays enter the wedge-shaped birefringent crystal B2. At this time, the optical axis of the wedge-shaped birefringent crystal B2 is rotated by 45 degrees with respect to the optical axis of the wedge-shaped birefringent crystal B1 in the same direction as the rotation of polarized light, so the ordinary rays and extraordinary rays are wedge-shaped birefringent crystals. The refractive crystal B2 also has an ordinary ray and an extraordinary ray. Therefore, the ordinary ray and extraordinary ray that have passed through the wedge-shaped birefringent crystal B2 become two parallel rays.

These two parallel rays are then focused by lens L2 and coupled to optical fiber F2.

Also, if the rays travel in the opposite direction to the forward direction, the third
As shown in (b) of the figure, from the optical fiber F2 to the lens L
The light incident on wedge-shaped birefringent crystal B2 via B2 is
It is separated into ordinary ray 0 and extraordinary ray e, which are refracted at different angles.

Next, the polarization direction of both the ordinary ray and the extraordinary ray is rotated by 45 degrees by the 45-degree Faraday rotator R1, but since the rotation is in the opposite direction to the forward operation described above, the optical direction of the wedge-shaped birefringent crystal B1 is The ordinary and extraordinary rays will be interchanged with respect to the axis.

Therefore, since these two light beams are refracted so as to be further separated by the wedge-shaped birefringent crystal B1, they are not coupled to the optical fiber F1 even though they pass through the lens L1.

Next, the operation of the two-stage optical isolator device will be explained based on FIG.

When a ray of light travels in the forward direction, the ray that enters the wedge-shaped birefringent crystal B1 from the optical fiber F1 via the lens L1, as shown in FIG. .While passing through the 45 degree Faraday rotator R1 and the wedge-shaped birefringent crystal B2, each beam is separated into two beams, and each beam is split into two beams while passing through the second stage optical isolator.
Separated into rays.

The four separated light beams are parallel, and are focused by the lens L2 and coupled to the optical fiber F2.

In addition, if the rays travel in the opposite direction, (b) in Figure 4
As shown in FIG. 2, the light beam is separated into two beams while passing through the second-stage optical isolator in reverse, and each beam is further separated into two beams while passing through the first-stage optical isolator. The four light beams thus obtained are not coupled to the optical fiber F1 because they all have positional and angular deviations.

(Problem to be solved by the invention) However, in the conventional device, when the light beam travels in the forward direction, in the case of a one-stage optical isolator device, the lens L
Since the light ray before entering L2 is separated into two parallel light beams, an angular shift occurs when the light is focused on the optical fiber, as shown in (1) in Figure 3, and the coupling to the optical fiber L2 is Efficiency deteriorates. Therefore, there are disadvantages in that the insertion loss in the forward direction is low and the loss differs depending on the polarization direction.

FIG. 5 shows how parallel light rays are coupled to an optical fiber F using a lens L. As shown in FIG. 5, the angle deviation is θ, and the core radius of the optical fiber is ω. , where the wavelength of light is λ, the coupling efficiency η is expressed by the following equation.

η=exp(-ω.2π2θ2/λ2)...(1)
Here, for example, the angular deviation θ is 3 degrees and the core radius of the optical fiber is ω. When is 5 am and the wavelength λ of light is 1.55 μm, the coupling efficiency η is 76% (-1,
2dB).

Similarly, in the case of a two-stage optical isolator, (
As shown in 1), it is separated into four parallel rays, so
Further, there were drawbacks in that the coupling efficiency deteriorated, and the forward insertion loss and polarization independence deteriorated.

Conversely, in both the one-stage and two-stage configurations, in order to obtain high coupling efficiency, it is possible to make the wedge-shaped birefringent crystal less inclined, but this would require a large angular shift in the opposite direction. Therefore, there was a drawback that it was difficult to obtain high isolation.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical isolator device with low forward insertion loss and polarization dependence, and high isolation.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides an optical isolator device including a wedge-shaped birefringent crystal, a prism-shaped birefringent crystal, and a 45-degree Faraday rotator. Refractive crystal, first 45 degree Faraday rotator,
A prismatic birefringent crystal, a second 45-degree Faraday rotator, and a second wedge-shaped birefringent crystal are arranged in the indicated order, and the first wedge-shaped birefringent crystal and the prismatic birefringent crystal face opposing surfaces. are arranged so that the inclination directions of the second wedge-shaped birefringent crystal and the prismatic birefringent crystal are equal, and the inclination directions of the opposing surfaces of the second wedge-shaped birefringent crystal and the prism-shaped birefringent crystal are the same; The optical axes of the birefringent crystal and the birefringent prism were shifted by 45 degrees.

(Function) According to the present invention, when a forward ray incident on the first wedge-shaped birefringent crystal passes through the first wedge-shaped birefringent crystal, it is separated into an ordinary ray and an extraordinary ray. , refracted at different angles.

Then, the first 45 degree Faraday rotator transforms the ordinary ray,
After the polarization directions of both extraordinary rays are rotated by 45 degrees, they enter the prism-shaped birefringent crystal. At this time, the optical axis of the prism-shaped birefringent crystal is shifted by 45 degrees with respect to the optical axis of the first wedge-shaped birefringent crystal, so the ordinary rays and extraordinary rays are also normal to the prism-shaped birefringent crystal. It has become a ray, an abnormal ray.

Therefore, the ordinary ray and the extraordinary ray become two parallel rays within the prism-shaped birefringent crystal. Since the exit surface of the prism-shaped birefringent crystal is inclined in the opposite direction to the entrance surface with respect to the plane perpendicular to the optical axis, the ordinary ray and the extraordinary ray are refracted in the direction in which they are combined.

These two rays are then rotated by 45 degrees by a second 45 degree Faraday rotator before entering a second wedge-shaped birefringent crystal. At this time, the ordinary ray and the extraordinary ray are the second
They are also ordinary rays and extraordinary rays for wedge-shaped birefringent crystals.

Therefore, the ordinary ray and the extraordinary ray are combined on the same axis.

On the other hand, the light rays incident on the second wedge-shaped birefringent crystal and traveling in the opposite direction are separated into ordinary rays and extraordinary rays, and are refracted at different angles.

Then, a second 45 degree Faraday rotator transforms the ordinary ray,
The polarization direction of both extraordinary rays is rotated by 45 degrees, but since they are rotated in the opposite direction to the forward motion described above, the ordinary and extraordinary rays are swapped with respect to the optical axis of the prism-shaped birefringent crystal. .

Therefore, the two rays are separated by the prismatic birefringent crystal.
Since the beam is refracted so as to further separate, it does not couple to the end face of the optical fiber, etc. from which the beam traveling in the forward direction is emitted.

(Embodiment 1) FIG. 1A is a configuration diagram showing a first embodiment of the optical isolator device according to the present invention, and FIG. 1B is a diagram for explaining the operation of FIG. 1A. 1B is a diagram for explaining the operation for light rays traveling in the forward direction, and FIG. 1B (b) for explaining the operation for light rays traveling in the reverse direction.

In addition, in FIG. 1A and FIG. 1B, the same components as those in FIG. 2 showing the conventional example are represented by the same reference numerals. That is, Fl and F2 are optical fibers, Ll and R2 are optical lenses, B1 is a first wedge-shaped birefringent crystal, B2 is a second wedge-shaped birefringent crystal, R1 is a first 45 degree Faraday rotator, and R2 is a second 45 degree Faraday rotator, and P is a prismatic birefringent crystal. Also, in Figure 1B, (a).

(a), (tsu), (1), and (e) are cross-sectional distribution diagrams of rays at passing points, CB□* c, l CB□ is the first
and the optical axes of the second wedge-shaped birefringent crystals Bl, B2 and the prism-shaped birefringent crystal P, respectively.

The optical isolator device of FIG. 1A includes a first wedge-shaped birefringent crystal B1, a first 45-degree Faraday rotator R1, a prismatic birefringent crystal P, and a second 45-degree Faraday rotator R.
2. Arrange the second wedge-shaped birefringent crystals B2 in the order indicated with respect to forward propagation of the light beam, and The second wedge-shaped birefringent crystal B2 and the prism-shaped birefringent crystal P are arranged so that the inclination directions of the opposing surfaces are equal, and the first and second wedge-shaped birefringent crystals Bl , B2 and the optical axes of the prism-shaped birefringent crystal P are shifted by 45 degrees.

Next, the operation of the above configuration will be explained using FIG. 1B.

First, the operation when a light beam travels in the forward direction will be explained based on (a) of FIG. 1B.

When a forward ray enters the first wedge-shaped birefringent crystal B1 from the optical fiber F1 via the lens L1, or when it passes through the first wedge-shaped birefringent crystal B1, the ordinary ray O and the extraordinary ray e are separated and refracted at different angles.

Next, after the polarization directions of both the ordinary ray and the extraordinary ray are rotated by 45 degrees by the first 45 degree Faraday rotator R1,
The light is incident on a prismatic birefringent crystal P. At this time, the optical axis of the prism-shaped birefringent crystal P is rotated 4 times in the same direction in which the polarized light is rotated with respect to the optical axis of the first wedge-shaped birefringent crystal B1.
Since it is rotated by 5 degrees, the ordinary rays and extraordinary rays also become ordinary rays and extraordinary rays for the prism-shaped birefringent crystal P.

Therefore, the ordinary ray and the extraordinary ray become two parallel rays within the prism-shaped birefringent crystal P. Since the exit surface of the prism-shaped birefringent crystal P is inclined in the opposite direction to the entrance surface with respect to the plane perpendicular to the optical axis, the ordinary ray and the extraordinary ray are refracted in the direction in which they are combined.

These two rays are then transferred to a second 45 degree Faraday rotator R
2, and then enters the second wedge-shaped birefringent crystal B2. At this time, the optical axis of the second wedge-shaped birefringent crystal B2 is the same as that of the first wedge-shaped birefringent crystal B1.
Since it is rotated by 45 degrees in the same direction as the rotation direction of polarized light with respect to the optical axis, the ordinary rays and extraordinary rays are also ordinary rays and extraordinary rays for the second wedge-shaped birefringent crystal B2.

Therefore, the ordinary ray and the extraordinary ray are combined on the same axis and coupled to the optical fiber F2 by the optical lens L2.

In this way, in the forward direction operation in the above configuration, no angular shift occurs due to separation of the light beams, so there is no polarization dependence of the coupling efficiency, and the coupling efficiency to the optical fiber is high.

Next, the operation when the light beam travels in the opposite direction will be explained based on (b) of FIG. 1B.

The light ray that enters the second wedge-shaped birefringent crystal B2 from the optical fiber F2 via the lens L2 is separated into an ordinary ray 0 and an extraordinary ray e, which are refracted at different angles.

Next, the polarization direction of both the ordinary ray and the extraordinary ray is rotated by 45 degrees by the second 45-degree Faraday rotator R2.
Because it rotates in the opposite direction to the forward motion described above,
The ordinary ray and the extraordinary ray are interchanged with respect to the optical axis of the prism-shaped birefringent crystal P.

Therefore, the two light beams are refracted to be further separated by the prismatic birefringent crystal P, so that the optical fiber F1
is not combined with

Furthermore, the leakage light caused by the imperfection of the second 45-degree Faraday rotator R2 is also transmitted to the first 45-degree Faraday rotator R2.
1 and the first wedge-shaped birefringent crystal B1 causes an angular shift, so that it is not coupled to the optical fiber F1.

As explained above, according to this embodiment, since no angular shift occurs due to separation of light rays in the forward direction, it is possible to make the inclinations of the wedge-shaped birefringent crystal and the prism-shaped birefringent crystal considerably larger than conventional ones. It is. Therefore, the angular shift in the opposite direction becomes large, and an optical isolator device with higher isolation and better polarization independence than before can be realized.

Furthermore, whereas the conventional device uses four wedge-shaped birefringent crystals in addition to two 45-degree Faraday rotators, this embodiment uses two first and second wedge-shaped birefringent crystals. Since only the refractive crystals Bl and B2 and one prism-shaped birefringent crystal P are required, there is an advantage that the number of constituent elements can be reduced.

(Example 2) FIG. 6 is a configuration diagram showing a second example of the optical isolator device according to the present invention.

In this second embodiment, as shown in FIG. The angle of inclination of the entrance and exit surfaces of the Faraday rotators R1 and R2 is set to the first and second wedge-shaped birefringent crystals Bl with respect to a plane perpendicular to the optical axis.

B2 and the prismatic birefringent crystal P are tilted in the same direction.

The operation of the optical isolator device having such a configuration is as follows:
This is the same as the first embodiment.

However, in the case of the second embodiment, between the first and second wedge-shaped birefringent crystals Bl and B2 and the first and second 45-degree Faraday rotators R1 and R2, and between the prismatic birefringent crystal P and the first and second 45 degree Faraday rotators R
Since the space between 1 and R2 can be removed, in addition to the effect of the first embodiment, there is an advantage that a more compact optical isolator device can be manufactured.

(Effects of the Invention) As explained above, according to the present invention, the forward direction insertion loss is low, there is no polarization dependence, and there is a high It is possible to provide an optical isolator device having rations.

Further, it has the advantage that it requires fewer constituent elements than conventional devices, is easy to manufacture, and can realize miniaturization and cost reduction.

[Brief explanation of drawings]

FIG. 1A is a configuration diagram showing a first embodiment of the optical isolator device according to the present invention, FIG. 1B is a diagram for explaining the operation of FIG. 1A, and FIG. 2 is a configuration diagram of a conventional optical isolator device. , FIG. 3 is a diagram for explaining the operation in FIG. 2 (a), FIG. 4 is a diagram for explaining the operation in FIG. 2 (b),
FIG. 5 is a diagram showing how a parallel beam is coupled to an optical fiber by a lens, and FIG. 6 is a configuration diagram showing a second embodiment of the optical isolator device according to the present invention. In the figure, Fl, F2... optical fiber, Ll, R2...
Optical lens, B1...first wedge-shaped birefringent crystal, B
2...Second wedge-shaped birefringent crystal, R1...First 45-degree Faraday rotator, R2...Second 45-degree Faraday rotator, P...Prismatic birefringent crystal.

Claims (1)

[Claims] An optical isolator device comprising a wedge-shaped birefringent crystal, a prismatic birefringent crystal, and a 45-degree Faraday rotator, comprising: a first wedge-shaped birefringent crystal, a first 45-degree Faraday rotator, a prismatic-shaped birefringent crystal; The birefringent crystal, the second 45-degree Faraday rotator, and the second wedge-shaped birefringent crystal are arranged in the indicated order, and the opposing surfaces of the first wedge-shaped birefringent crystal and the prismatic birefringent crystal are tilted. The second wedge-shaped birefringent crystal and the prism-shaped birefringent crystal are arranged so that the directions of the opposing surfaces are equal to each other, and the first and second wedge-shaped birefringent crystals are An optical isolator device characterized in that the directions of the optical axes of a refractive crystal and a prism-shaped birefringent crystal are shifted by 45 degrees.