JPH02160213A - Plane of polarization rotating device - Google Patents

Abstract

PURPOSE:To rotate a plane of polarization stably at an optional angle by holding a Faraday rotator whose one surface crossing an optical axis is formed slantingly to the other surface rotatably around an axis which is not aligned with the optical axis in a specific magnetic field. CONSTITUTION:The Faraday rotator 4 is so constituted that one surface crossing the optical axis 1 is formed slantingly to the other surface 3, and held rotatably around the axis 5 which is not aligned with the optical axis 1 in the specific magnetic field. The angle theta of rotation of the plane of polarization is expressed by an equation I, where (t) is the thickness of the part of the Faraday rotator 4 that the optical axis 1 penetrates and H is an applied magnetic field in the direction of the optical axis. The Faraday rotator 4 is placed in the specific magnetic field, so the H in the equation I is constant and the angle of rotation of the plane of polarization per unit thickness of the Faraday rotator at the optical axis part is constant. Then, the Faraday rotator 4 is rotated around the axis 5 to cause the thickness of the Faraday rotator 4 at the optical axis part to vary, so the angle of rotation of the plane of polarization can be set optionally corresponding to the variation in the thickness.

Description

[Detailed Description of the Invention] Overview Regarding a polarization plane rotation device that rotates the polarization plane of light, the present invention aims to provide a polarization plane rotation device that can stably rotate the polarization plane at an arbitrary angle. A Faraday rotator, in which one surface that intersects with the other surface is formed obliquely with respect to the other surface, is held rotatably about an axis that does not coincide with the optical axis in a predetermined magnetic field.

INDUSTRIAL APPLICATION FIELD The present invention relates to a polarization plane rotation device for rotating the polarization plane of light.

When constructing a system such as an optical communication system, there may be a request to rotate the plane of polarization of signal light, for example. Specifically, it is necessary to rotate the plane of polarization of light in the following cases.

(a) When inserting an optical film such as a coupler film or an optical multiplexing/demultiplexing film that has strong polarization dependence into an optical circuit, the plane of polarization of the light incident on the film must be set at an arbitrary angle in order to obtain the desired film characteristics. It is necessary to adjust the rotation.

(b) When configuring the receiving optical system in a coherent optical transmission system, the plane of polarization of the received light or local light can be rotated and adjusted at any angle to prevent a decrease in the interference efficiency between the received light and the local light. There is a need to.

(1) When configuring an optical isolator, exactly 45
It is necessary to rotate the plane of polarization by 100°, and stable rotation of the plane of polarization is required.

BACKGROUND ART Conventionally, as a method for rotating the plane of polarization of light, there is a method in which a predetermined magnetic field is applied to a Faraday rotator such as YIG, and the plane of polarization of light is rotated by the optical rotation characteristics of the Faraday rotator. In addition, the propagation mode HEz in single mode optical fiber
There is a method in which the plane of polarization of light is rotated by twisting, bending, or applying other external pressure to the fiber by utilizing mode coupling between -, HF, and 117.

Problems to be Solved by the Invention In the conventional method using a Faraday rotator, a relatively stable polarization plane rotation angle can be obtained by applying a saturation magnetic field to the Faraday rotator. There was a problem in that the rotation angle could not be adjusted arbitrarily. Considering the fact that the polarization plane rotation angle in a Faraday rotator depends on the strength of the applied magnetic field and the thickness of the Faraday rotator in the optical axis direction, by changing the applied magnetic field,
Adjusting the rotation angle of the polarization plane can be easily proposed, but when adjusting the rotation angle of the polarization plane using this method, the reproducibility of the rotation angle of the polarization plane is not good due to the presence of magnetic hysteresis, and it is difficult to stabilize the rotation angle of the polarization plane. It is difficult to perform polarization plane rotation.

On the other hand, the conventional method using a single mode optical fiber has the problem that it is difficult to arbitrarily adjust the polarization plane rotation angle, and the polarization plane rotation angle is not stable. Furthermore, as a result of mode coupling in a single-mode optical fiber, not only does the plane of polarization rotate, but also the linearly polarized light becomes elliptically polarized, so methods using single-mode optical fibers are not suitable for systems where extinction ratio is an issue. It is.

The present invention was created in view of the above circumstances, and an object of the present invention is to provide a polarization plane rotation device that can stably rotate the polarization plane at an arbitrary angle.

Means to solve problems FIG. 1 is a principle diagram showing the basic configuration of the present invention.

4 is a Faraday rotator, and one surface 2 intersecting the optical axis 1 is formed obliquely with respect to the other surface 3.

The Faraday rotator 4 is held rotatably about an axis 5 that does not coincide with the optical axis 1 in a predetermined magnetic field.

In addition, in FIG. 1, the light of Faraday rotator 4 @1
Although the plane 3 of the Faraday rotator 4 is shown to be perpendicular to the optical axis 1, the present invention is not limited to this configuration. Both surfaces 2 and 3 intersecting with the optical axis 1 may not be perpendicular to the optical axis 1.

Further, although the optical axis 1 and the axis 5 are shown to be parallel in the figure, they are not necessarily required to be parallel.

Function: If the thickness of the part of the Faraday rotator 4 penetrated by the optical axis 1 is t1, and the applied magnetic field in the optical axis direction is H, then the polarization plane rotation angle (optical rotation angle) θ is as follows: θ=F-t-H (F is expressed by a material-specific constant called the Verdet constant. In the configuration of the present invention, since the Faraday rotator 4 is placed in a predetermined magnetic field, H in the above equation is constant. Therefore, the polarization plane rotation angle per unit thickness of the Faraday rotator in the optical axis portion is constant. By rotating the Faraday rotator 4 about the axis 5, the thickness of the Faraday rotator 4 at the optical axis portion changes, so an arbitrary polarization plane rotation angle can be set according to this change in thickness. Can be done. In the present invention, since the Faraday rotator 4 is placed in a predetermined magnetic field, the reproducibility of the polarization plane rotation angle is good without being affected by magnetic hysteresis, etc., and a stable polarization plane rotation angle can be obtained.

Now, when the Faraday rotator 4 is rotated around the axis 5 (Fig. 2), the difference in the thickness of the optical axis portion between the time when it is the smallest and the time when it is the largest is Δ.
Assuming t, this thickness difference Δt is caused by rotating the Faraday rotator 4 by 180 degrees, so the change in thickness of the optical axis portion when the Faraday rotator 4 is rotated by 1 degree is Δt/180. expressed. Therefore, if the polarization plane rotation angle per unit thickness of the Faraday rotator 4 is set as a, then by rotating the Faraday rotator 4 by 1 degree, φ=Δt-a/180...(1) is given. The polarization plane rotation angle φ can be obtained.

As described above, according to the present invention, by arbitrarily setting Δt, that is, by arbitrarily setting the inclination angle formed by the plane 2.3 that intersects the optical axis l in the Faraday rotator 4, A polarization plane rotation device that can perform polarization plane rotation adjustment with precision is provided.

Example Embodiments of the present invention will be described below based on the drawings.

FIG. 3 is a perspective view of a polarization plane rotation device showing an embodiment of the present invention, and FIG. 4 is a sectional view taken along the rV-rV line in FIG. 3. A slit-shaped opening 10a is formed in the substrate 10, and a bearing member 13 is attached to the disk-shaped Faraday rotator 12 so that its outer peripheral portion comes out from the opening tOa of the substrate.
It is rotatably held inside the substrate 10 by. 1
4 is a permanent magnet having a half-cylindrical shape, and this permanent magnet 14
is fixed to the side of the substrate 10 where the Faraday rotator 12 is held in order to apply a predetermined magnetic field to the portion of the Faraday rotator 12 through which the optical axis OA passes.

FIG. 5 is a diagram showing the manufacturing process of the Faraday rotator 12. First, as shown in FIG. 2A, a magneto-optic crystal 16 of an appropriate thickness is formed on a transparent substrate 14 by epitaxial crystal growth of, for example, Bi-substituted YIG.

Next, as shown in FIG. 4B, the magneto-optic crystal 16 is polished to a plane C that is inclined at a predetermined angle with respect to the transparent substrate 14. Then, as shown in FIG. 2C, the shaft member 18 is fixed to the surface of the transparent substrate 14 on which the magneto-optic crystal 16 is formed and the surface on which the magneto-optic crystal 16 is not formed.

According to this embodiment, by rotating the Faraday rotator 12, it is possible to continuously change the thickness of the magneto-optic crystal 16 that contributes to rotation of the plane of polarization in the portion where the optical axis OA passes through. An arbitrary polarization plane rotation angle can be set according to the rotation angle of the Faraday rotator 12. In addition, since a saturation magnetic field or a constant magnetic field J is always applied by the permanent magnet 14 to the portion of the Faraday rotator 12 through which the optical axis OA passes, the polarization is reproducibly reproducible regardless of the direction of rotation of the Faraday rotator 12. Wavefront rotation angle can be set.

FIG. 6 shows a configuration in which tapered birefringent prisms 20 and 22 are arranged on the transmitting light source side and the receiving side of the Faraday rotator 12 in the polarization plane rotation device shown in FIGS. 3 and 4, respectively. A cutaway side view of an optical isolator is shown. This optical isolator is made of rutile (T i 0
This method takes advantage of the fact that when light passes through a birefringent prism 20, 22 made of a birefringent prism 20, 22 made of a birefringent prism (20, 22, etc.), ordinary light and extraordinary light have different refraction angles. The operation of this optical isolator will be briefly explained. When light from a light source (not shown) enters the birefringent prism 20 in the forward direction (from left to right in FIG. 6), the refractive index differs depending on the polarization direction, so the incident light is divided into ordinary light and extraordinary light. The beam is refracted in a different direction and enters the Faraday rotator 12. The polarization plane rotation angle in the Faraday rotator 12 is set to 45°,
The ordinary light and extraordinary light transmitted through the Faraday rotator 12 have their polarization planes rotated by 45 degrees and pass through the birefringent prism 22.
is incident on the The optical axis of the birefringent prism 22 is 45 degrees around the optical axis direction with respect to the optical axis of the birefringent prism 20.
Since the directions are rotated by .degree., the ordinary light and the extraordinary light correspond to the ordinary light and the extraordinary light inside the birefringent prism 22, respectively. For this reason, the birefringent prism 22
The ordinary light and extraordinary light that have passed through the optical fiber are emitted in parallel with each other, and therefore, these parallel lights can be focused by a lens (not shown) and introduced into an optical fiber (optical transmission path). On the other hand, light in the opposite direction (direction from right to left in FIG. 6) enters the birefringent prism 22, is divided into ordinary light and extraordinary light, refracted in different directions, and enters the Faraday rotator 12. The plane of polarization is rotated by 45 degrees and the beam is emitted. The ordinary light of the birefringent prism 22 whose polarization plane has been rotated by 45° becomes polarized light whose polarization plane has been rotated by 90° with respect to the optical axis of the birefringent prism 20, and therefore is refracted as extraordinary light by the birefringent prism 20. receive. The extraordinary light of the birefringent prism 22 whose polarization plane has been rotated by 45 degrees is similarly refracted as ordinary light by the birefringent prism 20. For this reason, the propagation directions of the ordinary light and the extraordinary light after passing through the birefringent prism 20 in the opposite direction are different, and in a lens arrangement with low loss in the forward direction, the light in the reverse direction does not return to the light source.

In an optical isolator that operates in this way, if the polarization plane rotation angle of the Faraday rotator and the inclination angle (45°) between the optical axes of the birefringent crystal do not almost completely match, reflections propagating in the opposite direction will occur. Feedback light removal performance deteriorates. For this reason, conventionally, the polarization plane rotation angle in the Faraday rotator was set to exactly 45° by polishing the magneto-optic crystal with extremely high precision. However, as a magneto-optical crystal, for example, Bi has a large Verdet constant and can be made thinner.
When using substitution type YIG, since the thickness to obtain a polarization plane rotation angle of 45° is approximately 200 μm,
Accurately setting the polarization plane rotation angle is extremely difficult. On the other hand, according to this embodiment, since the rotation angle of the polarization plane in the Faraday rotator 12 can be set arbitrarily, the rotation angle of the polarization plane can be easily adjusted after assembling the optical isolator, and the reflection feedback An optical isolator with good light removal performance can be provided. For example, the formula % formula % (
) By rotating the Faraday rotator 12 by 1°, the polarization plane rotation angle can be changed by 0.05°, making it possible to adjust the polarization plane rotation angle with high precision.

FIG. 7 is a diagram showing an example of use of the polarization plane rotation device shown in FIGS. 3 and 4. When the light from the LD (semiconductor laser) 24 is branched by the branching prism 26 having the branching film 26a, the relative positional relationship between the plane of incidence on the branching film 26a and the polarization plane of the incident light is directly branched. Since it affects the ratio, a polarization plane rotation device 28 is interposed between the LD 24 and the branching prism 26. According to such a configuration, the plane of polarization of the light beam incident on the branching film 26a can be arbitrarily rotated and adjusted by the polarization plane rotation device 28, so that the light from the LD 24 can be branched at a desired branching ratio. .

FIG. 8 shows a case where the polarization plane rotation device is applied to an optical system of a receiving section in a coherent optical transmission system.

The optical coupler 3 transmits the signal light transmitted through the optical transmission line 30.
2, it is combined with the local light from the local light source 34 and sent to the light receiver 3.
6, the polarization state of the signal light is detected by a polarization state detector 38, and the polarization plane rotation device 40 is feedback-controlled so that the polarization plane of the signal light and the polarization plane of the local light coincide. It is. For example, the amplitude of the hetero gain detection signal generated by the square law detection characteristic of the photoreceiver 36 is:
It is maximum when the polarization plane of the signal light and the polarization plane of the local light match, and it is minimum (zero) when the polarization plane of the signal light and the polarization plane of the local light are orthogonal. By controlling the polarization plane so that the polarization plane of the local light coincides with the polarization plane of the local light, a detected signal with the maximum amplitude can always be obtained, and deterioration of receiving sensitivity can be prevented. In this embodiment, since it is necessary to drive the polarization plane rotation device 40 according to the signal from the polarization state detector 38, the Faraday rotator is manually rotated as shown in FIGS. 3 and 4. Instead, it is necessary to rotate the Faraday rotator using an electric signal using a pulse motor or the like.

FIG. 9 is an explanatory diagram of the main parts of a polarization plane rotation device showing another embodiment of the present invention. In this embodiment, two transparent substrates 14 are each fixed to an independently rotatable shaft member 18, and the magneto-optic crystals 42 and 44 formed on each transparent substrate are polished so that their inclination angles are different. There is. For example, Δt in the magneto-optic crystal 42 is 400 μm, Δt in the magneto-optic crystal 44 is 40 μm, and the value of a is 0.2.
25 (deg/μm), by rotating the Faraday rotator on the left side in Fig. 9 by 1°, the
A polarization plane rotation angle of 0.05 can be obtained by rotating the Faraday rotator on the right side of the figure by 1 degree.
It is possible to obtain a polarization plane rotation angle of °. In this way, Δt
By arranging a plurality of Faraday rotators with different values on the optical axis OA, coarse adjustment and fine adjustment can be performed in stages.

As described in detail, the polarization plane rotation device of the present invention has the effect of stably rotating the polarization plane at any angle.

[Brief explanation of the drawing]

FIG. 1 is a diagram of the principle of the present invention, FIG. 2 is an explanatory diagram of the operation of the present invention, FIG. 3 is a perspective view of a polarization plane rotation device showing an embodiment of the present invention, and FIG. 5 is a manufacturing process diagram of a Faraday rotator showing an embodiment of the present invention; FIG. 6 is a cutaway side view of an optical isolator showing an embodiment of the present invention; FIG. 8 is an explanatory diagram of another usage example of the polarization plane rotation device showing an embodiment of the invention. FIG. 9 is another embodiment of the invention. FIG. 2 is an explanatory diagram of main parts of a polarization plane rotation device. 4.12... Faraday rotator, 16.42.44... Magneto-optic crystal, 14... Permanent magnet, 28.40... Polarization plane rotation device.

Claims (1)

[Claims] A Faraday rotator (4) in which one surface (2) intersecting the optical axis (1) is formed obliquely to the other surface (3) is rotated in a predetermined magnetic field with an axis that does not coincide with the optical axis (1). (5) A polarization plane rotation device configured to be rotatably held around a center.