JP4345955B2 - Reflective variable optical attenuator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
In the present invention, the light input from one end side travels through the optical element group and reaches the mirror means on the other end side, and the reflected light from the mirror means travels backward through the optical element group and is output from the one end side. The present invention relates to a reflection-type variable optical attenuator in which the amount of light is controlled by a variable Faraday rotator while reciprocating an element group.
[0002]
[Prior art]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-161076
[0003]
In an optical communication system or an optical measurement system, a variable optical attenuator for controlling the amount of transmitted light is required. A variable optical attenuator usually has a configuration in which a polarizer and an analyzer are installed on the input side and output side of a variable Faraday rotator, and the amount of light transmitted through the analyzer is controlled by varying the magnetic field applied to the Faraday element. It has come to be.
[0004]
As a variable optical attenuator for optical communication, it is important to have polarization independence, high reliability, and good matching with an optical fiber. To meet such requirements, for example, it is required by an optical element group in which various optical elements such as a birefringent element that controls the optical path according to the polarization plane and a variable Faraday rotator that controls the rotation angle of the polarization plane are combined and arranged. There is a structure which makes it an optical function part.
[0005]
In the past, a transmission type in which light is input from one end side of the optical element group, travels through the optical element group, and is output from the other end side has been common. However, in recent years, from the viewpoint of miniaturization of the optical device, the arrayed optical element group and the mirror unit are combined, and light input from one end side travels through the optical element group and reaches the mirror on the other end side. A reflection type has been developed in which light reflected by a mirror travels backward through an optical element group and is output from one end side, and the amount of light is controlled while the light reciprocates through the optical element group. For example, in Patent Document 1, a two-core ferrule having an input fiber and an output fiber, a lens, a birefringent plate, a Faraday element (magneto-optic crystal), and a mirror (reflector) are arranged, and a magnetic field is formed on the Faraday element. An optical device having a configuration in which the Faraday rotation angle is changed by applying the voltage is disclosed.
[0006]
In these optical devices, the amount of light is controlled by controlling the energization current to the electromagnet of the variable Faraday rotator. Outgoing light emitted from the ferrule end face to the Faraday element (magneto-optic crystal) of the variable Faraday rotator is collimated by the lens, and the return light reflected by the mirror is condensed on the ferrule end face by the lens. Yes. In addition, the electromagnet used for light quantity control usually has a structure in which a coil is wound around a C-shaped yoke, and the Faraday element inserted in the optical path is arranged so as to be sandwiched between the gaps, and the leakage magnetic field Thus, a variable magnetic field in a necessary direction can be applied to the Faraday element.
[0007]
[Problems to be solved by the invention]
Such a reflection type optical device can provide the necessary function by allowing the reflected light to pass through the optical element group again, so that the number of parts can be reduced and the length in the optical axis direction can be greatly reduced. Have. However, in the prior art as described above, since the light beam is collimated when passing through the Faraday element, it is necessary to set the effective diameter of the Faraday element large. Along with this, the size in the direction (width direction) perpendicular to the optical axis of the C-shaped yoke around which the coil is wound increases, so that the entire optical device is not sufficiently reduced in size (smaller diameter). . In particular, when a plurality of optical attenuators are juxtaposed, there is a drawback that the enlargement becomes remarkable.
[0008]
An object of the present invention is to provide a reflective variable optical attenuator that can shorten not only the dimension in the optical axis direction but also the dimension in the direction orthogonal to the optical axis, and can be greatly reduced in size as a whole.
[0009]
[Means for Solving the Problems]
The present invention includes an optical element group having at least a birefringent element and a Faraday element, mirror means, and variable magnetic field applying means for applying an external magnetic field to the Faraday element, and a variable Faraday rotator by the Faraday element and variable magnetic field applying means. The light input from one end side travels through the optical element group and reaches the mirror means on the other end side, and the reflected light from the mirror means travels backward through the optical element group and is output from the one end side. This is a variable optical attenuator in which the amount of light is controlled according to the Faraday rotation angle while reciprocating the element group. In the reflective variable attenuator according to the present invention, the optical element group further includes first lens means for collimating the input light and second lens means for condensing the collimated light, and the second lens means. The mirror means is located in the vicinity of the focal point, the Faraday element is installed in the optical path immediately before the mirror means, and the Faraday element is reciprocated with the collimated light narrowed down by the second lens means. There are features.
[0010]
The optical element group includes, for example, a two-core fiber ferrule, a first lens unit having a lens inserted independently in both optical paths, and a birefringent element that controls the optical path according to the polarization state of transmitted light. The second lens means comprising a lens inserted in common in both optical paths and the Faraday element are arranged in that order so that the fiber end face of the two-core fiber ferrule is located near the focal point of the first lens means There is a configuration. In this case, a structure in which the birefringent element, the second lens unit, the variable Faraday rotator, and the mirror unit are positioned and bonded on the same substrate is preferable. When a plurality of reflective variable optical attenuators are arranged side by side, all of them can be mounted on the same substrate in a state of being close to each other.
[0011]
The other optical element group includes, for example, a first optical fiber including a two-core fiber ferrule, a birefringent element that controls an optical path according to the polarization state of transmitted light, and a lens that is inserted in both optical paths in common. The lens means, the prism means for correcting the optical path with respect to the optical axis, the second lens means comprising a lens inserted in common in both optical paths, and the Faraday element in that order and the first There is also an arrangement in which the fiber end face of the two-core fiber ferrule is positioned near the focal point of the lens means.
[0012]
In the present invention, the variable Faraday rotator includes, for example, a rod-like or plate-like permanent magnet provided behind the mirror means, and an electromagnet in which a coil is wound around a C-shaped yoke, and the C-shaped yoke has a Faraday element in the gap. Is constructed such that a saturation magnetic field is applied in a direction perpendicular to the surface of the Faraday element by a permanent magnet, and a variable magnetic field is applied in a direction parallel to the surface of the Faraday element by an electromagnet.
[0013]
The mirror means may be a single mirror, or may be a mirror film formed on the back surface of the Faraday element, or a mirror film formed on one end face (a Faraday element facing surface) of the permanent magnet. When the mirror film is used in this way, the number of parts is reduced, and the distance between the Faraday element and the permanent magnet can be minimized, so that the magnetic efficiency is improved and the permanent magnet can be further reduced in size.
[0014]
As the birefringent element, for example, a rutile single crystal or lithium niobate is used. As the Faraday element, for example, a magnetic garnet single crystal, particularly a bismuth-substituted rare earth iron garnet single crystal film grown by an LPE (liquid phase epitaxial) method is suitable. Both surfaces are polished, and an AR (non-reflective) coat is applied to the light transmitting surface.
[0015]
【Example】
FIG. 1 is an explanatory view showing an embodiment of a reflective variable optical attenuator according to the present invention, and shows the arrangement structure of components and the polarization state of light at the position of each optical element. This variable optical attenuator includes a two-core fiber ferrule 10, first lens means 12 comprising two lenses 12a and 12b inserted independently in both optical paths, and an optical path according to the polarization state of the transmitted light. The birefringent element 14 for controlling the lens, the lens (second lens means) 16 inserted in common in both optical paths, the Faraday element 18 and the mirror 20 are arranged in this order along the optical axis. It is. A permanent magnet 22 and an electromagnet 24 are installed in the vicinity of the Faraday element 18 to form a variable Faraday rotator 26.
[0016]
In order to make the explanation easy to understand, the coordinate direction in which the arrangement direction (optical axis) of the optical element group is the z axis, and the two directions orthogonal to the z axis are the x axis (horizontal axis) and the y axis (vertical axis), respectively. Set. For convenience, the x direction is the right direction (right side in the direction in which the forward light travels), and the y direction is upward (upward in the direction in which the forward light travels).
[0017]
An input fiber 30 and an output fiber 32 are connected to one end side of the two-core fiber ferrule 10. The input fiber 30 is provided on the right side of the middle stage, and the output fiber 32 is provided on the left side of the middle stage. That is, the input fiber 30 and the output fiber 32 have the same position in the y direction, but are shifted in the x direction.
[0018]
The first lens means 12 is composed of lenses 12a and 12b inserted into both optical paths, respectively, and both lenses 12a and 12b are plano-convex lenses made of, for example, a homogeneous transparent material. The distance between the fiber end face of the two-core fiber ferrule 10 and both the lenses 12a and 12b is set so as to substantially match the focal length of the lens. Accordingly, light emitted from the fiber end face becomes collimated light by one lens 12a, and conversely incident collimated light is condensed on the fiber end face by the other lens 12b.
[0019]
The birefringent element 14 is a rectangular parallelepiped made of a birefringent crystal such as rutile or lithium niobate, and its optical axis is set in the yz plane and tilted from the z axis. Since the ordinary light travels straight and the extraordinary light is refracted, this birefringent element 14 separates light of the same optical path whose polarization direction is orthogonal to light of different optical paths up and down, and the light of different optical paths up and down is the same. It performs the function of combining with the light in the optical path.
[0020]
The lens (second lens means) 16 is a single plano-convex lens made of, for example, a homogeneous transparent material. A mirror 20 is positioned in the vicinity of the focal point of the lens 18, and the Faraday element 18 is installed in the optical path immediately before the mirror 20. Accordingly, the collimated light from the lens 12a passes through the birefringent element 14, is narrowed down by the lens 16, passes through the Faraday element 18, and is incident and reflected so as to be focused by the mirror 20. Then, it passes through the Faraday element 18 between the narrow beams and returns to the collimated light by the lens 12b.
[0021]
A rod-like permanent magnet 22 is located behind the mirror 20. The permanent magnet 22 is magnetized in the longitudinal direction, and a saturation magnetic field is applied in a direction perpendicular to the surface of the Faraday element 18 by the permanent magnet. Further, a C-type yoke 34 is provided so as to surround the permanent magnet 22 and the Faraday element 18 is inserted into the gap, and the electromagnet 24 is formed by winding a coil 36 around the C-type yoke 34. The electromagnet 24 applies a variable magnetic field in a direction parallel to the surface of the Faraday element 18. In the variable Faraday rotator 26, the variable magnetic field generated by the electromagnet 24 and the fixed magnetic field generated by the permanent magnet 22 are combined, and these combined magnetic fields are applied to the Faraday element 18, so that the Faraday rotation angle of incident light is 0 to 45 degrees. It can be freely changed within the range.
[0022]
Next, the operation of this variable optical attenuator will be described. The light input from the input fiber 30 to the middle right optical path becomes collimated light by the lens (first lens means) 12a, the ordinary light travels straight through the middle right optical path by the birefringent element 14, and the extraordinary light is in the + y direction (upward). Refract and travel along the upper right optical path. The separated light is focused by the lens (second lens means) 16, passes through the Faraday element 18, and reaches the mirror 20.
[0023]
(45 degree Faraday rotation: Decay rate ≒ 0%)
In the forward path, the separated light is narrowed by the condensing action of the lens (second lens means) 16, and the polarization plane of the light is rotated by 45 degrees at the variable Faraday rotator 26 (when passing through the Faraday element 18). , Is incident on one point of the mirror 20 and reflected. Next, on the return path, the reflected light is switched by the variable Faraday rotator 26 while the optical path is switched vertically and horizontally (when passing through the Faraday element 18), and the polarization plane of each light is further 45 degrees (therefore, the forward path). And a total of 90 degrees on the return path). Collimated light passes through the lens (second lens means) 16, ordinary light travels straight through the birefringent element 14, and extraordinary light is refracted in the −y direction (downward) to synthesize light. Since all the combined light passes through the left optical path in the middle stage, it is collected by the lens (first lens means) 12 b and coupled to the output fiber 32. Therefore, the input light from the input fiber 30 is output to the output fiber without being attenuated.
[0024]
(At 0 degree Faraday rotation: Decay rate ≒ 100%)
In the forward path, the separated light is narrowed by the condensing action of the lens 16, and the polarization plane of the variable Faraday rotator 26 does not rotate, but is incident on one point of the mirror 20 and reflected. In the return path, the reflected light is switched in the up / down / left / right direction, and the polarization plane of the variable Faraday rotator 26 does not rotate while being narrowed down, and becomes collimated light by the lens 16. In the birefringent element 14, the ordinary light in the upper optical path travels straight, and the extraordinary light in the middle optical path is refracted in the -y direction (downward), so that the light is further separated so as to pass through the upper left optical path and the lower left optical path. Therefore, it does not pass through the lens 12b and is not coupled to the output fiber 32 at all.
[0025]
(When rotating 22.5 degrees Faraday: Decay rate ≒ 50%)
When the Faraday rotation angle is between 45 degrees and 0 degrees, it is as follows. As a typical example, an intermediate case of 22.5 degrees will be described. In the forward path, the separated light is narrowed by the condensing action of the lens 16, and the polarization plane of the light is rotated 22.5 degrees by the variable Faraday rotator 26, and is incident on one point of the mirror 20 and reflected. On the return path, the reflected light is switched between top, bottom, left, and right, and the polarization plane of the light is further rotated by 22.5 degrees in the variable Faraday rotator 26 in the narrowed state (therefore, a total of 45 degrees is rotated on the forward path and the return path). The collimated light is generated by the lens 16. The light in the upper optical path travels straight through the birefringent element 14 and the extraordinary light is refracted in the -y direction (downward). Even in the middle stage optical path, the ordinary light travels straight through the birefringent element 14 and the extraordinary light is refracted in the −y direction (downward). Therefore, the extraordinary light in the upper optical path and the ordinary light in the middle optical path are combined, and the remaining light remains separated. That is, only the combined light in the middle left optical path is collected by the lens 12 b and coupled to the output fiber 32, and the remaining light (ordinary light in the upper optical path and abnormal light in the lower optical path) is not coupled to the output fiber 32. In this way, the input light from the input fiber 30 is coupled to the output fiber 32 according to the Faraday rotation angle, and as a result, the output to the output fiber is attenuated.
[0026]
In this way, by freely varying the Faraday rotation angle of the variable Faraday rotator 16 in the range of 0 to 45 degrees, the light coupling ratio to the output fiber 32 can be obtained according to the controlled Faraday rotation angle. The output light quantity can be controlled.
[0027]
In the reflection type variable light attenuator according to the present invention, the collimated light is narrowed by the second lens means (lens 16) and reciprocates through the Faraday element 18 in the narrowed state. Therefore, the effective diameter of the Faraday element 18 is extremely small. It's okay. Accordingly, the dimensions (vertical and horizontal dimensions) of the Faraday element itself may be very small. For example, when lenses having the same focal length are used, the beam size at the mirror position is equivalent to the fiber diameter, and is about φ10 μm in the case of a single mode fiber. Therefore, the Faraday element 18 has a side of about 0.1 mm and can secure a sufficient optical effective area. As shown in the figure, since the variable magnetic field generated by the electromagnet 24 is applied in a direction parallel to the surface of the Faraday element 18, the length of one side can be reduced, so that the length perpendicular to the optical axis of the C-shaped yoke 34 can be reduced. In addition, since the gap is reduced, a sufficient magnetic field can be applied with a small electromagnet, and the synergistic effect of these can significantly reduce the size of the electromagnet. For example, in the above numerical example, the width of the C-shaped yoke 34 can be reduced to 5 mm or less.
[0028]
In the reflection type optical device, since light is reflected by a mirror, the light does not reach behind the mirror, and even if a non-optical component is arranged behind the mirror, the optical path is not hindered. In the present invention, by utilizing this, a rod-shaped permanent magnet is installed behind the mirror, and further, the C-shaped yoke of the electromagnet is provided so as to surround the mirror and the permanent magnet, thereby effectively using the internal space. I am trying. As a result, the variable optical attenuator can be shortened and reduced in diameter in the optical axis direction.
[0029]
An example of the structure of the mirror means and the variable Faraday rotator is shown in FIG. 2A is the same as that shown in the embodiment of FIG. 1, and has a structure in which a mirror 20 as a separate member is installed between the Faraday element 18 and the rod-shaped permanent magnet 22. For example, the Faraday element 18, the mirror 20, the permanent magnet 22, and the C-shaped yoke 34 are formed on the substrate 40 by positioning and bonding. In order to avoid the winding portion of the coil 36, the lateral length of the substrate 40 may be shortened as shown in the figure, or a relief hole (or a relief recess) may be provided. This structure is the simplest.
[0030]
FIG. 2B shows an example in which a mirror film 44 is formed on the back surface of the Faraday element 42. After the Faraday element 42 is polished on both sides, an AR coating is applied to one surface and a mirror film 44 is formed on the other surface. This structure is relatively easy to work and optimal. FIG. 2C shows an example in which a mirror film 48 is formed on one end face of a rod-like permanent magnet 46. After polishing one end face of the permanent magnet 46, a process for forming the mirror film 48 is performed. Such a configuration is also possible depending on the material of the permanent magnet. In both the examples of B and C in FIG. 2, the distance between the end face of the permanent magnet and the Faraday element can be minimized, so that the length in the optical axis direction can be shortened and the magnetic efficiency is good. In these cases as well, as in FIG. 2A, it is easy to handle by assembling on the substrate.
[0031]
More preferably, as shown in FIG. 3, the birefringent element 14, the lens (second lens means) 16, the Faraday element 18, the mirror 20, the permanent magnet 22, and the C-shaped yoke 34 are positioned on the single substrate 60. To be bonded. Thus, the optical attenuator can be easily manufactured only by aligning with the two-core fiber ferrule 10 to which the lenses (first lens means) 12a and 12b are attached. It is also effective to assemble a plurality of birefringent elements, lenses, Faraday elements, mirrors, permanent magnets, and C-shaped yokes on the same substrate. In the present invention, since the width dimension of the C-shaped yoke can be reduced, high-density mounting is possible. It goes without saying that the configuration of B or C in FIG. 2 can be similarly mounted on the substrate.
[0032]
FIG. 4 is an explanatory view showing another embodiment of the reflection type variable optical attenuator according to the present invention, showing the arrangement structure of components and the polarization state at the position of each optical element. Here, in order to make the explanation easy to understand, the coordinate direction in which the arrangement direction (optical axis) of the optical element group is the z axis, and the two directions orthogonal to the z axis are the x axis (horizontal axis) and the y axis (vertical axis), respectively. Set. For convenience, the x direction is the right direction and the y direction is the upward direction.
[0033]
This variable optical attenuator includes a two-core fiber ferrule 70, a birefringent element 74 that controls the optical path in accordance with the polarization state of transmitted light, and a lens (first lens means) that is inserted in both optical paths in common. 72, prism means 75 for optical path correction for controlling the inclination of the optical path with respect to the optical axis, a lens (second lens means) 76 inserted in common in both optical paths, a Faraday element 78, and a mirror 80. The arrangement is in that order. A permanent magnet 82 and an electromagnet 84 are installed in the vicinity of the Faraday element 78 to form a variable Faraday rotator 86.
[0034]
Here, the birefringent element 72 is a rectangular parallelepiped made of a birefringent crystal such as rutile or lithium niobate, and its optical axis is set in the yz plane and tilted from the z axis. This birefringent element 72 also has a function of separating light of the same optical path whose polarization directions are orthogonal to each other into light of different optical paths up and down, and combining light of different optical paths up and down with light of the same optical path. .
[0035]
The first lens means is composed of a lens 72 inserted in common in both optical paths, for example, a plano-convex lens made of a homogeneous transparent material. The distance between the fiber end face of the two-core fiber ferrule 70 and the lens is set so as to substantially match the lens focal length. Therefore, the outgoing light from the fiber end face passes through the birefringent element 74 and becomes collimated light at the lens 72. On the contrary, the collimated light is condensed on the fiber end face by the lens 72. However, since the optical path does not pass through the center of the lens, the optical path on the side facing the birefringent element 74 of the lens is parallel to the optical axis due to the refractive action of the lens, but the optical path on the side facing the prism means 75 is the optical axis. Leaning against.
[0036]
The prism means 75 for correcting the optical path is, for example, a single prism having a pyramid shape (pyramid shape) or a pyramid-shaped portion, and the optical path inclined with respect to the optical axis is parallel to the optical axis and conversely the optical axis. It functions to tilt the optical path parallel to the optical axis with respect to the optical axis. The prism means 75 may be configured by combining four wedge-shaped prisms.
[0037]
The second lens means is, for example, a plano-convex lens made of a homogeneous transparent material. A mirror 80 is positioned in the vicinity of the focal point of the lens, and the Faraday element 78 of the variable Faraday rotator 86 is installed in the optical path immediately before the mirror 80. Accordingly, the collimated light that has passed through the birefringent element 74 is narrowed down by the lens (second lens means) 76, transmitted through the Faraday element 78, reflected by the mirror 80, and transmitted through the Faraday element 78 again with a narrow beam. The lens 76 returns to the collimated light.
[0038]
A rod-shaped permanent magnet 82 is positioned behind the mirror 80, and a C-shaped yoke is provided so as to surround the permanent magnet 82 between the opposing end faces of the Faraday element 78, and a coil is wound around the C-shaped yoke. Thus, the electromagnet 84 is obtained. These Faraday element 78, permanent magnet 82, and electromagnet 84 constitute a variable Faraday rotator 86. A saturation magnetic field is applied in a direction perpendicular to the surface of the Faraday element 78 by the permanent magnet 82, and a variable magnetic field is applied in a direction parallel to the surface of the Faraday element 78 by the electromagnet 84. That is, by applying these combined magnetic fields to the Faraday element 78, the Faraday rotation angle of the incident light can be freely varied in the range of 0 to 45 degrees.
[0039]
The operation of this variable optical attenuator will be described. Light input from the input fiber 90 to the middle right optical path is birefringent element 72, and ordinary light travels straight through the middle right optical path, and extraordinary light is refracted in the + y direction (upward) and travels along the upper right optical path. The separated light becomes collimated light by the lens 72 but is inclined with respect to the optical axis. The light inclined with respect to the optical axis becomes parallel to the optical axis by the prism means 75 for optical path correction. Therefore, the light in the middle right optical path passes through the upper left optical path, and the light in the upper right optical path passes through the middle left. These lights are collected by the lens 76, pass through the Faraday element 78, and reach the mirror 80.
[0040]
(45 degree Faraday rotation: Decay rate ≒ 0%)
In the forward path, the separated light is narrowed down by the condensing action of the lens 76 and transmitted through the Faraday element 78. At that time, the polarization plane of the light is rotated 45 degrees by the variable Faraday rotator 86, and is incident on one point of the mirror 80. And reflect. In the return path, the reflected light is switched by the variable Faraday rotator 86 while the optical path is switched vertically and horizontally, and the polarization plane of the light is further rotated 45 degrees (therefore, a total of 90 degrees is rotated in the forward path and the return path). The light is collimated by the lens 76, is refracted by the prism means 75 for correcting the optical path, passes through the optical path inclined with respect to the optical axis, and is condensed by the lens 72 by switching the optical path vertically and horizontally. In the birefringent element 74, the ordinary light travels straight, the extraordinary light is refracted in the -y direction (downward), and the light is synthesized. All the combined light passes through the middle left optical path and is coupled to the output fiber 92. Therefore, the input light from the input fiber 90 is output to the output fiber 92 without being attenuated.
[0041]
(At 0 degree Faraday rotation: Decay rate ≒ 100%)
In the forward path, the separated light is focused by the condensing action of the lens 76 and transmitted through the Faraday element 78. At that time, the polarization plane of the variable Faraday rotator 86 does not rotate, but is incident on one point of the mirror 80 and reflected. In the return path, the reflected light is switched in the up / down / left / right direction, and the polarization plane of the variable Faraday rotator 86 is not rotated and is collimated by the lens 76 while being narrowed. The light path is refracted by the prism means 75 for correcting the optical path, and the optical path is inclined with respect to the optical axis. In the birefringent element 74, the ordinary light in the upper stage optical path travels straight, and the extraordinary light in the middle stage optical path is refracted in the -y direction (downward), so that the light is further separated so as to pass through the upper stage left optical path and the lower stage left optical path. Thus, the output fiber 92 is not coupled at all.
[0042]
(When rotating 22.5 degrees Faraday: Decay rate ≒ 50%)
When the Faraday rotation angle is between 45 degrees and 0 degrees, it is as follows. As a typical example, an intermediate case of 22.5 degrees will be described. In the forward path, the separated light is narrowed by the condensing action of the lens 76, and the polarization plane of the light is rotated 22.5 degrees by the variable Faraday rotator 86, and is incident on one point of the mirror 80 and reflected. On the return path, the reflected light is switched between top, bottom, left, and right, and the polarization plane of the light is further rotated by 22.5 degrees with the variable Faraday rotator 86 in the narrowed state (thus, a total of 45 degrees is rotated on the forward path and the return path). The lens 76 generates collimated light. The light path is refracted by the prism means 75 for correcting the optical path, and the optical path is inclined with respect to the optical axis. The light in the upper stage optical path travels straight through the birefringent element 74 and the extraordinary light is refracted in the −y direction (downward). Even in the middle stage optical path, the ordinary light travels straight through the birefringent element 12, and the extraordinary light is refracted in the -y direction (downward). That is, the extraordinary light in the upper stage optical path and the ordinary light in the middle stage optical path are combined, and the remaining light remains separated. Therefore, only the combined light in the middle left optical path is coupled to the output fiber 92, and the remaining light (ordinary light in the upper optical path and abnormal light in the lower optical path) is not coupled to the output fiber. In this way, a part of the input light from the input fiber 90 is coupled to the output fiber 92 according to the Faraday rotation angle, and as a result, the output to the output fiber is attenuated.
[0043]
In this way, by freely varying the Faraday rotation angle of the variable Faraday rotator in the range of 0 to 45 degrees, the light coupling ratio to the output fiber can be obtained according to the controlled Faraday rotation angle, and the output The amount of light can be controlled.
[0044]
【The invention's effect】
As described above, in the reflection-type variable optical attenuator according to the present invention, the collimated light is narrowed down by the second lens means and is configured to pass back and forth through the Faraday element in a narrowed state. As long as it has a very small effective diameter. Therefore, the dimensions (vertical and horizontal dimensions) of the Faraday element itself can be made extremely small, the length in the direction perpendicular to the optical axis of the electromagnet that applies a variable magnetic field to the Faraday element can be reduced, and the gap is reduced, so that the small electromagnet A sufficient magnetic field can be applied, and the synergistic effect of these can significantly reduce the size of the electromagnet.
[0045]
Further, since the light is reflected by the mirror, even if a non-optical component is arranged behind the mirror, it does not hinder the optical path. By utilizing this fact, a bar-shaped permanent magnet is installed behind the mirror, and further, a C-shaped yoke of an electromagnet is provided so as to surround the mirror and the permanent magnet, so that space can be saved. As a result, shortening and diameter reduction in the optical axis direction can be realized. Further, when a plurality of variable optical attenuators are arranged in parallel, the effect of reducing the size of the whole becomes even more remarkable.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing an embodiment of a reflective variable optical attenuator according to the present invention.
FIG. 2 is an explanatory diagram showing an example of mirror means.
FIG. 3 is an explanatory diagram showing an example of an assembly structure.
FIG. 4 is an explanatory view showing another embodiment of the reflective variable optical attenuator according to the present invention.
[Explanation of symbols]
10 2-core fiber ferrule
12 First lens means
14 Birefringent elements
16 lens (second lens means)
18 Faraday element
20 mirror
22 Permanent magnet
24 electromagnet
26 Variable Faraday rotator
30 Input fiber
32 output fiber

Claims (5)

An optical element group having at least a birefringent element and a Faraday element; mirror means; and variable magnetic field applying means for applying an external magnetic field to the Faraday element. A variable Faraday rotator is configured by the Faraday element and the variable magnetic field applying means. progressing forward path of the light optical element group inputted from the side reaching the other end side of the mirror means, and reversing the backward light optical element group reflected Ri by the said mirror unit, light reciprocates optical element group and light intensity according to the Faraday rotation angle is controlled during the, in the variable optical attenuator that be output from the output position shifted in the normal direction of the plane including the optical axis of the birefringent element to the input position of the one end ,
The optical element group further includes first lens means for collimating the input light and second lens means for condensing the collimated light. The mirror means is located near the focal point of the second lens means, and the Faraday A reflection type variable optical attenuator, wherein an element is installed in an optical path immediately before the mirror means, and the Faraday element is reciprocated in a state where collimated light is narrowed down by the second lens means. The optical element group includes a two-core fiber ferrule that is held in a state in which the input fiber and the output fiber are shifted in a direction perpendicular to the optical axis, and both optical paths of the input light that exits from the input fiber and the output light that travels to the output fiber. First lens means having a lens inserted independently, and a rectangular parallelepiped, the optical axis being in a plane perpendicular to the direction of deviation between the input fiber and the output fiber and parallel to the optical axis, and transmitting light A birefringent element that does not change the optical path according to the polarization state of the optical fiber or controls the optical path to be shifted in the normal direction of the surface including both the input fiber and the output fiber, and a lens that is inserted in both optical paths in common a second lens unit consisting of, a Faraday element, the fiber end face of the two-core fiber ferrule near the focal point of the array to and the first lens unit toward the other end from one end side in that order Reflection type variable optical attenuator according to claim 1 wherein as to location.   3. The reflection type variable optical attenuator according to claim 2, wherein the birefringent element, the second lens means, the variable Faraday rotator, and the mirror means are positioned and bonded on the same substrate. The optical element group includes a two-core fiber ferrule that is held in a state where the input fiber and the output fiber are shifted in a direction perpendicular to the optical axis, and a rectangular parallelepiped, in which the optical axis is a shift direction between the input fiber and the output fiber. In the plane perpendicular to the optical axis and parallel to the optical axis, the optical path is not changed according to the polarization state of the transmitted light, or the optical path is shifted in the normal direction of the plane including both the input fiber and the output fiber. The birefringent element to be controlled, the first lens means comprising a lens inserted in common in both optical paths, and the optical path inclined with respect to the optical axis is parallel to the optical axis, and conversely the optical path parallel to the optical axis The optical path correction prism means for controlling the inclination of the optical path with respect to the optical axis , the second lens means comprising a lens inserted in common in both optical paths, and the Faraday element, one end in that order Reflection type variable optical attenuator according to claim 1, wherein the fiber end face of the two-core fiber ferrule near the focal point of the array to and the first lens unit toward the Luo other end was positioned.   The variable Faraday rotator includes a rod-like or plate-like permanent magnet provided behind the mirror means, and an electromagnet in which a coil is wound around a C-shaped yoke. The C-shaped yoke has a Faraday element inserted into the gap. 5. The device according to claim 1, wherein a saturation magnetic field is applied in a direction perpendicular to the surface of the Faraday element by a permanent magnet, and a variable magnetic field is applied in a direction parallel to the surface of the Faraday element by an electromagnet. The reflective variable optical attenuator as described.

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