JPH0712970Y2 - Optical isolator - Google Patents

Description

[Detailed description of the device] [Industrial applications]

The present invention relates to an optical isolator used for optical fiber communication and optical fiber amplifier.

[Prior art]

Conventionally, an optical isolator including a birefringent crystal and a Faraday element has been known as an optical isolator having a small polarization dependence. That is, usually rutile (TiO ₂ ) is used as a birefringent crystal, and a plurality of rutile plates are arranged,
A Faraday element that rotates the polarization plane by 45 ° is arranged between the rutile plates.

[Problems to be solved by the device]

However, in the optical isolator having such a configuration,
There is a problem that the optical path lengths of the two separated polarization components reaching the emission end are different. There is no problem when the incident light is a parallel light beam, but there is a big problem in the case of focused light or divergent light because the focal position shifts due to the difference in optical path length. In view of the above, the present invention provides an optical isolator which is improved so that the optical path lengths of two separated polarization components to the emission end become equal to each other so that no problems occur in focused light and divergent light. The purpose is to

[Means for Solving the Problems]

To achieve the above object, in an optical isolator according to the present invention, a first birefringent crystal, a first Faraday element for rotating a polarization plane by 45 °, and a 45 ° rotation with respect to a first birefringent crystal. Of the first birefringent crystal A second birefringent crystal having a thickness of 1 and a first birefringent crystal rotated 135 ° with respect to the first birefringent crystal. A third birefringent crystal having a thickness of, a second Faraday element that rotates a plane of polarization by 45 °, and a first birefringent crystal rotated by 180 ° with respect to the first birefringent crystal. A feature is that a fourth birefringent crystal having the same thickness is arranged in order in the optical axis direction.

[Action]

A first-stage isolator is formed by the first and second birefringent crystals and the first Faraday element sandwiched between them.
The third and fourth birefringent crystals and the second Faraday element sandwiched therebetween form a second-stage isolator. The optical paths of the first-stage isolator and the second-stage isolator are configured to be different from each other. Therefore, the optical path lengths of the respective polarization components can be matched.

【Example】

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In FIG. 1, rutile (TiO ₂ ) is used as a birefringent crystal, and four rutile plates 11, 12, 13,
14 and two Faraday elements 21, 22 are a rutile plate 11, a Faraday element 21, a rutile plate 12, a rutile plate 13, and a Faraday element 2.
2. The rutile plates 14 are arranged in series on the optical axis in this order. The C-axis directions of the rutile plates 11, 12, 13, 14 are the directions as shown by the arrows in FIGS. 31, 32, 33, and 34 when viewed from the incident direction of light. That is, rutile board
The C-axis direction of 12 is a direction rotated by 45 ° in the clockwise direction with respect to the C-axis direction of the rutile plate 11, and the C-axis direction of the rutile plate 13 is clockwise with respect to the C-axis direction of the rutile plate 11. 135
The rutile plate 14 is rotated by 180 ° in the clockwise direction with respect to the C axis direction of the rutile plate 11. The thickness of each of the rutile plates 11, 12, 13, 14 is t, t. First, with respect to the first rutile plate 11, the position 41 shown in FIG. 3A is used.
Suppose light is incident on. Of this incident light, the polarization component whose polarization plane is in the vertical direction is refracted as an extraordinary light component, and the polarization component whose polarization plane is in the horizontal direction is directly transmitted as an ordinary light component. Therefore, both polarization components incident on the position 41 are positioned at the exit end face as shown in FIG. 3B.
It is divided into 41 and 42 and emitted. This separation angle becomes the largest when the angle between the C-axis and the plate surface is 42 °, and is about 5.
It will be 8 °. The distance between the positions 41 and 42 is proportional to the thickness t, and the proportional coefficient is the tangent of the above angle. So the distance is t · tan 5.8 °. In the Faraday element 21, the polarization planes of the two polarization components thus separately emitted are rotated by 45 °.
In this case, the positional relationship between the two polarization components does not change,
The polarization planes and positions of the two polarization components incident on the rutile plate 12 are as shown in FIG. 3C. This rutile plate 12 has a C-axis direction as shown by an arrow in FIG. 32, and its thickness is Has become. Therefore, of the polarized light components incident on the rutile plate 12, the polarized light component incident on the position 41 becomes an extraordinary light component and is refracted similarly to the rutile plate 11 described above. As a result of refraction, the component incident on this position 41 is at position 43 as shown in FIG. 3D.
Will be emitted from. Position 43 is to the upper right of position 41
The angle between them is 45 °, and the distance between them is Proportional to. On the other hand, the polarization component incident on the position 42 is an ordinary light component in this case, and is emitted at the position 42 through the rutile plate 12 without being refracted. In addition, rutile board 11, 1
All of 2, 13, and 14 have the same angle formed by the C axis and the plate surface, that is, the separation angle. The two polarization components emitted from this rutile plate 12 are the rutile plate.
Enter 13. The rutile plate 13 has a C-axis direction as shown by an arrow in FIG. Has become. Therefore, of the polarized light components incident on the rutile plate 13, the polarized light component incident on the position 42 becomes an extraordinary light component which is refracted and is emitted from the position 44 as shown in FIG. 3E. Position 44 is at an angle of 45 ° to the lower right with respect to position 42, and the distance between them is Proportional to. On the other hand, the polarized light component incident on the position 43 is an ordinary light component in this case, and is emitted at the position 43 through the rutile plate 13 without being refracted. The two polarization components emitted from the rutile plate 13 pass through the Faraday element 22 and their respective polarization planes are rotated by 45 °,
It is incident on the next rutile plate 14. The position and polarization plane of each polarization component incident on the rutile plate 14 are as shown in FIG. 3F. Since the rutile plate 14 has the C-axis direction as shown by the arrow in FIG. 34, the polarization component incident on the position 43 becomes an extraordinary light component and is refracted and goes downward. This refraction angle is the same as that of the other rutile plates 11, 12, and 13, and therefore, the amount of displacement due to refraction is proportional to the thickness, and the thickness of this rutile plate 14 is t.
Therefore, the positional deviation amount is a distance proportional to t. As a result, the emission positions of the two polarization components coincide with each other at the position 44 (see FIG. 3G). This will be described with reference to FIG. FIG. 4 shows the positions of the incident point and the outgoing point as seen from the incident direction of light. The polarization component refracted upward in the rutile plate 11 first moves from the position 41 to the position 42, and then in the lower right portion 45 of the rutile plate 13.
It bends in the ° direction and moves to position 44. The distance between these positions 41-42 is proportional to the thickness t, and the distance between positions 42-44 is the thickness t. Will be proportional to. On the other hand, the polarized light component that was not refracted by the rutile plate 11 is refracted by the rutile plate 12 in the upper right 45 ° direction, and the amount of positional deviation (distance between positions 41 and 43) is the thickness. Will be proportional to. This polarization component is a rutile plate
It refracts downward at 14, and the amount of positional deviation (distance between positions 43 and 44) is proportional to the thickness t. These proportional coefficients are all the same for tan θ (θ is the separation angle, for example, 5.8 °). Therefore, the above two polarization components are emitted at the same position 44. Further, the total displacement amount of the positions 41-42-44 is equal to the total displacement amount of the positions 41-43-44. That the positional deviation amounts of the respective polarization components in the direction perpendicular to the optical axis are the same means that the lengths of the optical paths through which the respective polarization components pass are also equal. This is because the length in the direction parallel to the optical axis, that is, the thickness of each rutile plate is the same for refracted light (extraordinary light) and non-refracted light (ordinary light). Ie thickness The optical path lengths of the extraordinary light and the ordinary light at the rutile plates 12 and 13 are set to Ne and No, respectively. If Ne ′, No ′, the total optical path length of the polarization components that move to positions 41-42-44 is Ne ′ + No + Ne + No ′, while the total optical path length of the polarization components that move to positions 41-43-44 is No '+ Ne + No + Ne' and both are equal. On the other hand, when the light passes from the left to the right in the forward direction, but from the right to the left in the reverse direction, the emission positions of the two polarization components are separated and different from the forward incidence position. That is, as shown in FIG. 5, a polarized component of light incident in the backward direction at the exit position 44 in the forward direction is shifted to the position 43 by the rutile plate 14, and further shifted to the position 45 by the rutile plate 12, The light is emitted at this position 45. The other polarization components are shifted to the position 46 on the rutile plate 13 and then to the position 47 on the rutile plate 11 and emitted at the position 47.
These emission positions 45 and 47 are different from each other, and are also greatly different from the incidence position 41 in the forward direction. Therefore, the isolation characteristic that there is no loss of light in the forward direction and a large loss in the reverse direction is obtained. Although rutile is used as the birefringent crystal in the above, other birefringent crystals such as sapphire and calcite are also conceivable.

[Effect of device]

According to the optical isolator of the present invention, it is possible to equalize the optical path lengths of the two polarized light components reaching the emission end,
Therefore, in the case of focused light, divergent light, or the like, the focal position does not shift. Moreover, the number of constituent parts can be reduced, the assembly can be simplified, the cost can be reduced, and the size can be easily reduced.

[Brief description of drawings]

FIG. 1 is a schematic side view of an embodiment of the present invention, FIG. 2 is a view showing the C-axis direction of a rutile plate, and FIG. 3 is a diagram showing each polarization component in the forward direction in each part of the same embodiment. FIG. 4 is a schematic diagram showing a polarization plane and a positional relationship, FIG. 4 is a diagram showing a positional relationship of each polarization component in the forward direction and a change order thereof, and FIG. 5 is a positional relationship of each polarization component in the reverse direction and its change. It is a figure which shows an order. 11,12,13,14 …… Rutile plate, 21,22 …… Farade element.

Claims (1)

[Scope of utility model registration request] 1. A first birefringent crystal, a first Faraday element for rotating a plane of polarization by 45 °, and a first birefringent crystal.
Of the first birefringent crystal rotated 45 ° A second birefringent crystal having a thickness of 1 and a first birefringent crystal rotated 135 ° with respect to the first birefringent crystal. A third birefringent crystal having a thickness of, a second Faraday element that rotates a plane of polarization by 45 °, and a first birefringent crystal rotated by 180 ° with respect to the first birefringent crystal. An optical isolator characterized in that a fourth birefringent crystal having the same thickness is arranged in order in the optical axis direction.