WO2004029698A1

WO2004029698A1 - Variable polarization rotation device, and variable optical attenuator using the same

Info

Publication number: WO2004029698A1
Application number: PCT/JP2002/010072
Authority: WO
Inventors: Hiroshi Nagaeda
Original assignee: Fujitsu Limited
Priority date: 2002-09-27
Filing date: 2002-09-27
Publication date: 2004-04-08
Also published as: JPWO2004029698A1

Abstract

A variable polarization rotation device for rotating the polarized light of the input light by utilizing the magneto-optical effect. The intensity of the magnetic field required for magnetic field generating means (42, 43, 44 and 45) to obtain the Faraday rotation by the same magneto-optical effect is considerably reduced by collecting the input light by a first light collecting means (3), and introducing the light onto a magneto-optical crystal (41) installed in the vicinity of a focal position thereof. The considerably reduces the size of the magnetic field generating means (42, 43, 44 and 45) themselves, which in turn considerably reduces the size of the variable polarization rotation device.

Description

Description Variable polarization rotation device and variable optical attenuator using the same

The present invention relates to a variable polarization rotator that rotates (changes) the polarization state of input light using a magneto-optical effect (Faraday effect) and a variable optical attenuator using the same. Background art

An example of a variable polarization rotator using a conventional variable Faraday rotator is shown in FIG. The device shown in FIG. 24 has, for example, an input collimating lens (collimating lens) 102 and a Faraday rotator 10 on the optical axis between the input optical fiber 101 and the output optical fiber 105. 3. An output collimator (collimating lens) 104 is arranged.

As shown in FIG. 24, the Faraday rotator 104 applies a magnetic field to the Faraday element (magneto-optical crystal) 13 1 and the Faraday element 13 1 in a direction parallel to the optical axis. An electromagnet (magnetic field generating means) for applying a magnetic field to the Faraday element 13 1 from a direction crossing the optical axis approximately 90 degrees with the permanent magnets 13 2 and 13 33 It is composed of 3 and 5. The strength of the magnetic field applied by the electromagnets 134 and 135 is adjusted by a current source 106 as a control signal applying means. In addition, since the permanent magnets 13 2 and 13 33 are practically opaque in general, they have a shape that does not hinder the optical path (for example, a hollow structure).

In such a configuration, the magnetization direction of the Faraday element 131, as described in, for example, Japanese Patent Application Laid-Open No. 6-51255 (hereinafter referred to as Patent Literature 1), can It is the direction of the composite magnetic field of the constant magnetic field by 33 and the variable magnetic field by electromagnets 13 4 and 1 35. Here, assuming that a magnetic field sufficient to saturate the magnetization is generated by the constant magnetic field of the permanent magnets 13 2 and 13 3, the applied electromagnetic field by the electromagnets 13 4 and 13 5 must be varied. As a result, the magnetization vector changes so that its magnitude is constant and only the direction changes. Therefore, the component parallel to the optical axis direction changes according to the direction of the synthetic magnetic field, that is, the magnitude of the variable magnetic field of the electromagnets 13 4 and 13 5, and is determined by the magnetization component parallel to the optical axis direction. The Faraday rotation angle changes according to the magnitude of the magnetic field of the electromagnets 13 4 and 13 5.

Patent Document 1 proposes using such a variable Faraday rotator to configure a variable optical attenuator (variable optical attenuator). That is, as shown in FIG. 25, for example, in the configuration described above with reference to FIG. 24, a polarizer 1336 is provided in front of the magneto-optical crystal 13 1 of the Faraday rotator 103, and the magneto-optical crystal 1 An analyzer 13 7 is provided after 31 to form a variable light athens.

Here, the polarizer 1336 and the analyzer 1337 are each formed of a tapered (wedge-shaped) birefringent crystal (for example, rutile or the like), and a straight line having a predetermined polarization plane of the incident light. It selectively transmits polarized light, and the top and bottom of the polarizer 136 face the bottom and top of the analyzer 137, respectively, and the optical axes of both birefringent crystals (perpendicular to the plane of the paper). Are present so that they are perpendicular to each other. In this case, the variable optical antenna shown in FIG. 25 operates as follows. That is, as shown in FIG. 26, the light emitted from the input optical fiber 101 is collimated by the input collimator 102, and then enters the polarizer 1336, where the ordinary light component o And the extraordinary light component e. Here, the polarization directions of the ordinary light component o and the extraordinary light component e are orthogonal to each other. FIG. 26 is a side view showing an optical path when viewed from the X-axis (arrow B) direction in FIG. However, in FIG. 26, the illustration of the permanent magnets 13 2 and 13 3 and the electromagnets 13 4 and 13 5 is omitted.

When the ordinary light component 0 and the extraordinary light component e pass through the Faraday rotator 103, respectively, the polarization direction is rotated according to the magnitude of magnetization in the direction parallel to the optical axis, and the analyzer 13 It is incident on 7. The analyzer 1337 further separates the ordinary light component o into the ordinary light component o o and the extraordinary light component o e, and further separates the extraordinary light e into the ordinary light component e o and the extraordinary light component e e.

Here, the ordinary light component oo and the extraordinary light component ee emitted from the analyzer 13 37 are the refraction history received by the polarizer 13 36 and the analyzer 13 37 and the polarizer 13 36 and the analyzer, respectively. light Considering the shape and arrangement of the child 1337, they are parallel to each other. Therefore, these ordinary light component oo and extraordinary light component ee can be condensed by the collimating lens 104 and coupled to the core of the output optical fiber 105 (shown by a solid line). On the other hand, since the extraordinary light component oe of the ordinary light component o and the ordinary light component eo of the extraordinary light component e spread not parallel to each other, they are coupled to the core of the output optical fiber 105 even through the collimating lens 104. No (shown by dashed line).

Now, the ratio of the total power of the ordinary light component oo and the extraordinary light component ee to the total power of the extraordinary light component oe and the ordinary light component eo depends on the rotation angle of the Faraday rotator 103, and the Faraday rotation angle is constant. In the state, the total power of the ordinary light component oo and the extraordinary light component ee does not depend on the polarization state of the output light of the input optical fiber 101. For example, when the electromagnetic field applied by the electromagnets 13 4 and 13 5 is 0 The Faraday rotation angle is 90 degrees (the magnetization is parallel to the optical axis), and the ordinary light component o emitted from the polarizer 13 36 is emitted almost as it is from the analyzer 13 37 as the ordinary light component oo, and the polarizer 13 The extraordinary light component e emitted from 6 is emitted from the analyzer 13 7 as it is as the extraordinary light component ee, so that most of the emitted light from the input optical fiber 101 is coupled to the output optical fiber 105. Become.

On the other hand, if the electromagnetic field applied by the electromagnets 13 4 and 13 5 is sufficiently large, the Faraday rotation angle approaches 0 degree, and the ordinary light component 0 emitted from the polarizer 13 36 is almost unchanged from the analyzer. The extraordinary light component e emitted from 13 7 as the extraordinary light component ο e, and the extraordinary light component e emitted from the polarizer 13 36 exits as it is as the ordinary light component eo from the analyzer 13 7. Most of the emitted light will not be coupled to the core of the output optical fiber 105.

In this way, the magnetization of the Faraday element 1331 rotates according to the strength of the electromagnetic field applied by the electromagnets 1334 and 135, and the Faraday rotation angle ranges from about 90 degrees to about 0 degrees. And the amount of light coupled to the core of the output optical fiber 105 changes accordingly, so that the device shown in FIG. 25 functions as a variable optical attenuator. 1 3 1 In 2006, a Bi (bismuth) -substituted rare earth iron garnet single crystal film (LPE film) mainly manufactured by the LPE method (liquid phase epitaxy method) was used. The reason is that the LPE film has the advantage of a larger Faraday rotation coefficient than the YIG (yttrium iron garnet) single crystal due to the contribution of Bi.

In addition, as a conventional variable light beam, as shown in FIG. 27, for example, as shown in FIG. 27, a reflecting element 107 is arranged downstream of the Faraday rotator 103 (Faraday element 13 1), There has also been proposed a reflection-type variable light attenuator that transmits twice through a Faraday rotator 103 (Faraday element 131) in a reciprocating manner (for example, Japanese Patent Application Laid-Open No. H10-161706). Hereinafter, it is referred to as Patent Document 2).

In FIG. 27, reference numeral 115 denotes a two-core ferrule fixing the input optical fiber 101 and output optical fiber 105, and 124 denotes input collimator 110 and output collimator 110. The collimator lenses (collimating lenses) 124 and 138 which also serve as the evening light 104 are polarizers and analyzers which also serve as the above-mentioned polarizers 13 and 13 respectively.

Then, in the variable light beam shown in FIG. 27, as described in the paragraphs [0108] and [009] of Patent Document 2, the light beam is transmitted to the Faraday rotator 10 3. Is transmitted twice back and forth, so that the thickness of the Faraday rotator 103 (Faraday element 13 1) and the required magnetic field strength can be halved. The polarizer 1336 and analyzer 13 7 and the input collimator 10 2 and the output collimator 10 4 can be shared by the polarizer / analyzer 13 8 and the collimator 12 4, respectively. Cost reductions are also being made by reducing the number of parts and the number of parts.

However, even if the Faraday rotator 103 (Faraday element 13 1) can be made smaller in such a reflective variable optical attenuator, the electromagnetic coil occupying most of the size of the Faraday rotator 103 Dramatic miniaturization cannot be expected because the (magnetic field generating means) does not become smaller. Although it is possible to reduce the size of the coil wire by making it thinner, it is difficult at present to reduce the size of the electromagnet coil itself due to problems such as heat generation due to increased electrical resistance.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and has been made in view of the above-mentioned circumstances. And a variable optical attenuator using the same.

Other known technical literature information on the optical device using the Faraday rotator is appended below.

(1) JP 2001-1420240 A (related to a variable optical attenuator using a Faraday rotator)

(2) Japanese Patent Laid-Open No. 2000-56187 (related to a laser module using a Faraday rotator)

(3) Japanese Patent Application Laid-Open No. 2002-23104 (related to an optical isolator using a Faraday rotator) Disclosure of the Invention

In order to achieve the above object, a variable polarization rotator of the present invention comprises: a first condensing means for condensing input light; and a magneto-optical device provided near a focal position of the first condensing means. A crystal, magnetic field generating means for applying a magnetic field to the magneto-optical crystal, and control means for controlling the magnetic field generating means to change the magnetic field.

Here, the first condensing means may be constituted by an input line force lens for condensing the input light linearly, or an input point condensing means for condensing the input light pointwise. It may be constituted by a lens. Further, an input collimator for collimating the input light and inputting the collimated light to the first condensing means may be provided at a stage preceding the first condensing means.

Further, the variable optical attenuator using the variable polarization rotator of the present invention collimates the input light and inputs the collimated light to the input line focus lens before the input line focus lens. A variable polarization rotator provided with an input collimator and having a polarizer or a birefringent plate provided between the input focusing lens and the magneto-optical crystal is used.

Further, the variable optical attenuator using the variable polarization rotator of the present invention includes an input for collimating the input light and inputting the collimated light to the input point condensing lens before the input point condensing lens. A collimator is provided, and the input collimator and the input point are collected. A variable polarization rotator provided with a polarizer or a birefringent plate between the lens and the lens is used.

The variable polarization rotator may include an output collimator for collimating light transmitted through the magneto-optical crystal, at a stage subsequent to the magneto-optical crystal. The output collimator may be constituted by an output line force lens that linearly condenses the light emitted from the magneto-optical crystal, or collects the light emitted from the magneto-optical crystal in a point-like manner. It may be constituted by an output point condenser lens that emits light.

Further, the variable optical attenuator using the variable polarization rotator of the present invention employs a variable polarization rotator provided with an analyzer or a birefringent plate between the magneto-optical crystal and the output lens. It is characterized by:

Further, in the variable optical attenuator using the variable polarization rotator of the present invention, the second condensing means for condensing the light emitted from the output point condensing lens at a stage subsequent to the output point condensing lens is provided. And a variable polarization rotator provided with an analyzer or a birefringent plate between the output point condenser lens and the second condenser means.

Further, the variable polarization rotator of the present invention comprises: an input optical fiber for propagating input light; an input collimator for collimating input light from the input optical fiber; and a collimator emitted from the input collimator. Input light lens for condensing light in a linear manner, a magneto-optical crystal installed near the focal position of the input line focus lens, and collimating light transmitted through the magneto-optical crystal An output line focus lens, an output collimating lens for converging collimated light emitted from the output color information lens, an output optical fiber installed near a focal position of the output collimating lens, and the magneto-optical crystal And a control means for controlling the magnetic field generating means to change the magnetic field.

The variable optical attenuator using the variable polarization rotator of the present invention may further include a polarizer or a first birefringent plate provided between the input color information lens and the magneto-optical crystal. A variable polarization rotator in which an analyzer or a second birefringent plate is provided between the crystal and the output color information lens is used. Here, the polarizer or the first birefringent plate and the analyzer or the second birefringent plate, Each of them may be made of a birefringent crystal having a wedge shape.

Further, the variable polarization rotator of the present invention comprises: a first light condensing means for condensing the input light; a magneto-optical crystal installed near a focal position of the first light condensing means; A magnetic field generating means for applying a magnetic field; a control means for controlling the magnetic field generating means to change the magnetic field; and a reflecting element for reflecting light transmitted through the magneto-optical crystal and returning the light to the magneto-optical crystal. It is characterized by having.

Here, the first light condensing means may be constituted by an input color information lens that condenses the input light linearly. In addition, an input collimator for collimating the input light and entering the first light condensing means may be provided in front of the first light condensing means.

Further, an output collimator for collimating the reflected light reflected by the reflective element and passing through the magneto-optical crystal may be provided, and the output collimator may include the reflected light from the magneto-optical crystal. May be constituted by an output line focus lens that condenses the light into a linear shape.

Further, a second condenser means for condensing the light emitted from the output collimator may be provided at a stage subsequent to the output collimator.

Further, the variable polarization rotator of the present invention includes an input optical fiber that propagates the input light, an input collimator that collimates the light emitted from the input optical fiber, and a collimator that emits the collimated light emitted from the input collimator. A first condensing unit that emits light, a magneto-optic crystal installed near a focal position of the first condensing unit, and a reflection that reflects light transmitted through the magneto-optic crystal and returns the light to the magneto-optic crystal. An element, an output collimator for collimating the light reflected by the reflection element and transmitted through the magneto-optical crystal, and a second condensing means for condensing collimated light emitted from the output collimator; An output optical fiber installed near a focal position of the second light condensing means, a magnetic field generating means for applying a magnetic field to the magneto-optical crystal, and a control means for controlling the magnetic field generating means to change the magnetic field With It is characterized in that.

Here, the first condensing means and the output collimator may each be constituted by a line focus lens that condenses incident light linearly, or condenses incident light linearly. Two line focus lenses may be used. Further, the input collimator and the second condensing means may each be constituted by a collimating lens for collimating the incident light, or may be shared by one collimating lens for collimating the incident light.

Further, the variable optical attenuator using the variable polarization rotator of the present invention further comprises a polarizer or a first birefringent plate provided between the first light collecting means and the magneto-optical crystal. A variable polarization rotator provided with an analyzer or a second birefringent plate between the magneto-optical crystal and the output collimator is used.

Here, the first birefringent plate and the second birefringent plate may be shared by a polarizer and an analyzer. Further, each of the first birefringent plate and the second birefringent plate may be formed of a birefringent crystal having a wedge shape. Further, the above-mentioned polarizer and analyzer may be formed of a birefringent crystal having a wedge shape. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic perspective view showing a configuration of a variable polarization rotation device as one embodiment of the present invention.

FIG. 2 is a schematic perspective view showing a configuration focusing on a main part of the variable polarization rotation device shown in FIG.

FIG. 3 is a schematic top view showing the configuration shown in FIG. 2 when viewed from the direction of arrow A, together with the optical path.

FIG. 4 is a schematic side view showing the configuration of FIG. 2 when viewed from the direction of arrow B, together with the optical path.

FIG. 5 is a schematic perspective view showing a modification of the variable polarization rotation device shown in FIG. FIG. 6 is a schematic perspective view showing a configuration of a variable optical attenuator using a variable Faraday rotator as one embodiment of the present invention, together with an optical path.

FIG. 7 is a schematic top view showing the configuration of the variable optical attenuator shown in FIG. 6 when viewed from the direction of arrow A, together with the optical path.

FIG. 8 is a schematic side view showing the configuration of the variable optical attenuator shown in FIG. 6 when viewed from the direction of arrow B, together with the optical path.

FIG. 9 is a schematic perspective view showing a modification of the variable optical attenuator shown in FIG. FIG. 10 is a schematic top view showing the configuration of the variable optical attenuator shown in FIG. 9 when viewed from the direction of arrow A, together with the optical path.

FIG. 11 is a schematic side view showing the configuration of the variable optical attenuator shown in FIG. 9 when viewed from the direction of arrow B, together with the optical path.

FIG. 12 is a diagram showing an optical path focusing on the ordinary light component and the extraordinary light component when viewed from the direction of arrow B in FIG. 9 for explaining the operation of the variable optical attenuator shown in FIG.

FIG. 13 is a schematic perspective view showing the configuration of a reflection-type variable polarization rotator as one embodiment of the present invention, together with an optical path.

FIG. 14 is a schematic view showing a side surface of the reflective element for explaining the operation of the reflective element of the variable polarization rotator shown in FIG.

FIG. 15 is a schematic perspective view showing a modification of the reflection type variable polarization rotation device shown in FIG.

FIG. 16 is a schematic perspective view showing the main components of the reflective variable polarization rotator shown in FIG. 13 together with the optical path.

FIG. 17 is a schematic perspective view showing the configuration of a reflection-type variable optical attenuator as one embodiment of the present invention, together with an optical path.

FIG. 18 is a schematic top view showing the configuration of the reflection type variable optical attenuator shown in FIG. 17 when viewed from the direction of arrow A, together with the optical path.

FIG. 19 is a schematic side view showing the configuration of the reflection type variable optical attenuator shown in FIG. 17 when viewed from the direction of arrow B, together with the optical path.

FIG. 20 is a schematic side view showing the optical paths of the ordinary light component and the extraordinary light component, respectively, for explaining the operation of the reflection type variable optical attenuator shown in FIGS.

FIG. 21 is an enlarged view of a main part of FIG.

FIG. 22 and FIG. 23 are schematic top views showing the arrangement of components to explain the usefulness of using a cylindrical lens in a reflection type variable optical attenuator. FIG. 24 is a schematic perspective view showing the configuration of a conventional variable polarization rotator together with an optical path.

FIG. 25 is a schematic perspective view showing the configuration of a conventional variable optical attenuator together with an optical path. Figure 26 shows the ordinary light component and the extraordinary light component to explain the operation of the variable optical attenuator shown in Figure 25. It is a typical side view which shows the optical path of a minute.

FIG. 27 is a schematic perspective view showing the configuration of a conventional reflection-type variable optical attenuator. BEST MODE FOR CARRYING OUT THE INVENTION

(A) Description of variable polarization rotator

FIG. 1 is a schematic perspective view showing the configuration of a variable polarization rotator as one embodiment of the present invention. The variable polarization rotator shown in FIG. 1 has an input optical fiber 1, an input collimating lens 2, an input cylindrical lens ( The variable Faraday rotator (line focus lens) 3, variable Faraday rotator 4, output cylindrical lens (line focus lens) 5, output collimator lens 6, output optical fiber 7 and variable current source 8. In the following, the Faraday rotator is also composed of a Faraday element (magneto-optical crystal) 41, permanent magnets 42, 43, and electromagnets 44, 45 as magnetic field generating means. I have.

Here, the input optical fiber 1 propagates the input light, and the input collimating lens (input collimator) 2 collimates the light emitted from the input optical fiber 1, The input cylindrical lens (first condensing means) 3 condenses the collimated light emitted from the input collimating lens 2 linearly (only in one direction of the light wavefront). The collimated light is focused in the X-axis direction shown in the figure.

The Faraday rotator 4 rotates the polarization state of light emitted from the input cylindrical lens 3 according to the principle described above. The center of a magneto-optical crystal 41 such as a garnet single crystal is focused on the input cylindrical lens 3. The permanent magnet is placed near the position, and a permanent magnetic field is applied to the magneto-optical crystal 41 by the permanent magnets 42, 42 in parallel with the light beam, and is substantially perpendicular to the light beam by the electromagnets 44, 45. Electromagnetic fields are applied in the focusing direction of the input cylindrical lens 3 (X-axis direction in FIG. 1).

Also in this case, the permanent magnetic field has a magnetic field strength sufficient to saturate the magnetization of the magneto-optical crystal 41, and the applied field strength by the electromagnets 44 and 45 is controlled by the variable current source 8 as a control means from the outside. The current flowing through the coils of the electromagnets 4 4 and 4 5 based on the control signal of It is controlled by regulating the flow. Also, the permanent magnets 42 and 4 3 are practically opaque in general, so each has a shape that does not hinder the optical path (for example, a hollow structure).

As a result, the magnetization of the Faraday rotator 4 (the magneto-optical crystal 41) is saturated by the permanent magnetic field, and the direction of the magnetization vector is changed by the application of the electromagnetic field by the electromagnets 44 and 45, but the magnitude is does not change. As a result, the amount of Faraday rotation is determined by the component of the magnetization vector parallel to the light beam, so that the amount of Faraday rotation can be controlled by applying an electromagnetic field.

Furthermore, the output cylindrical lens (output collimator) 5 can collect or collimate the incident light depending on its position (distance from the Faraday rotator 4). In this case, The light emitted from the Faraday rotator 4 (the light transmitted and diverging through the magneto-optical crystal 41) is arranged so as to be collimated.

The output collimating lens (second focusing means) 6 focuses the light emitted from the output cylindrical lens 5 and couples the light to the output optical fiber 7. It propagates the output light collected from the collimating lens 6. The lens material of the cylindrical lenses 3 and 5 may be a commonly used glass material such as SFS01 and BK7. The focal length f can be set freely, but here, for example, a lens with f = 1.8 mm is used.

With the configuration as described above, in the variable polarization rotator shown in FIG. 1, the light emitted from the input optical fiber 1 is collimated by the input collimating lens 2, and the collimated light is input to the input cylindrical lens 3. After being condensed, the light is incident on the magneto-optical crystal 41 of the Faraday rotator 4, where it undergoes polarization rotation according to the electromagnetic field generated by the electromagnets 44 and 45, and then is collimated by the output cylindrical lens 5. The light is condensed by the output collimating lens 6 and coupled to the output optical fiber 7.

Here, conventionally, since the distance between the electromagnets 44, 45 of the Faraday rotator 4 must be at least as large as the beam diameter of the incident light, for example, the Faraday rotator width ( It is necessary to set the distance between the electromagnets 44 and 45 to about 460 m and the distance between the electromagnets 44 and 45 to about 460 μιη. As schematically shown in FIG. 3 and FIG. 4, since the incident light to the Faraday rotator 4 is condensed linearly in only one direction (in the X-axis direction) by the input cylindrical lens 3, the beam is greatly increased. The diameter becomes smaller, and for example, the Faraday rotator width can be reduced to 35 / xm, and the distance between the electromagnetic stones 44 and 45 can be reduced to about 45Π1, which can be reduced to about 1/10 of the conventional value.

As a result, the magnetic field resistance is reduced, and the applied magnetic field strength can be smaller than before to give the same Faraday rotation angle. For example, the number of coil windings of the electromagnets 44, 45 can be greatly reduced. Thus, the electromagnets 44, 45 (coils), which conventionally occupy much of the volume of the Faraday rotator 4, can be significantly reduced in size. Therefore, the size of the electromagnets 44 and 45 can be significantly reduced, and the size of the Faraday rotator 4 can be significantly reduced.

FIG. 2 is a schematic perspective view showing the configuration focusing on the essential parts (the portion composed of the input cylindrical lens 3 and the Faraday rotator 4) in FIG. 1 together with the optical path, and FIG. 3 is the configuration shown in FIG. Fig. 4 is a schematic top view showing the configuration when viewed from the direction of arrow A together with the optical path, and Fig. 4 is a schematic side view showing the configuration as viewed from the direction of arrow B and the configuration shown in Fig. 2 along with the optical path. It is.

The input cylindrical lens 3 and the output cylindrical lens 5 described above are, for example, as schematically shown in FIG. The same operation and effect as described above can be obtained by replacing with '. However, in this case, since the light condensed by the point condensing lens 3 ′ in the form of a point passes through the Faraday rotator 4 (the magneto-optical crystal 41), the distance between the electromagnets 44 and 45 can be reduced. In addition, the cross-sectional area of the surfaces of the electromagnets 44 and 45 facing the magneto-optical crystal 41 can be reduced, so that the Faraday rotator 4 can be further reduced in size.

(B) Variable optical attenuator

Next, an embodiment of a variable optical attenuator, which is an application of the variable polarization rotation device using the variable Faraday rotator 4 described above, will be described below.

FIG. 6 is a schematic perspective view showing the configuration of a variable optical attenuator using the above-described variable Faraday rotator 4 together with an optical path, and FIG. 7 is a perspective view of the variable optical attenuator shown in FIG. 8 is a schematic side view showing the configuration when the variable optical attenuator shown in FIG. 6 is viewed from the direction of arrow B, together with the optical path. is there. As shown in FIGS. 6, 7 and 8, the variable optical attenuator of the present embodiment also includes an input optical fiber 1, an input collimating lens 2, an input cylindrical lens 3, a Faraday rotator 4, an output cylindrical lens. 5, an output collimating lens 6, an output optical fiber 7 and a variable current source 8. Here, a polarizer (or between the permanent magnet 42 on the input side and the magneto-optical crystal 41) is provided. A first birefringent plate 9 is provided, and an analyzer (or a second birefringent plate) 10 is provided between the magneto-optical crystal 41 and the permanent magnet 43 on the output side. In the following, the same reference numerals as those described above denote the same or similar ones unless otherwise specified.

Here, the polarizer 9 and the analyzer 10 are each formed of a tapered (wedge-shaped) birefringent crystal (for example, rutile or the like) on the YZ plane in FIG. Of the polarizers 9 selectively transmits linearly polarized light having a predetermined polarization plane. The top and bottom of the polarizer 9 face the bottom and top of the analyzer 10, respectively (in FIG. 6, the polarizer 9 has a wedge shape). The analyzer is installed with the long side down, the analyzer 10 is installed with the short side down), and the optic axes of both birefringent crystals (in the plane perpendicular to the plane of the paper). Are present to be perpendicular to each other.

In this example, each of the collimating lenses 2 and 6 and the cylindrical lenses 3 and 5 uses a lens having a focal length f = 4.0 mm. Therefore, in this case, the distance from the input optical fiber 1 (output optical fiber 7) to the input collimating lens 2 (output collimating lens 6) and the center of the cylindrical lenses 3 and 5 to the Faraday rotator 4 (magneto-optical crystal 41) The distance to each position will be about 4mm. In the variable optical attenuator of the present embodiment configured as described above, the light emitted from the input optical fiber 1 is first collimated by the input collimating lens 2, and then input before the magneto-optical crystal 41. The light is condensed by the cylindrical lens 3 (condensed linearly in the X-axis direction in Fig. 6) (especially, see Fig. 7). Thus, also in the case of the present example, the Faraday rotator width and the distance between the electromagnets 44 and 45 can be significantly reduced as compared with the conventional case. Then, the light emitted from the input cylindrical lens 3 is separated into an ordinary light component and an extraordinary light component by the polarizer 9 and the analyzer 10 in the same manner as described above with reference to FIG. The element 4 receives rotation of the polarization plane according to the applied electromagnetic field of the electromagnets 44, 45, whereby the amount of light coupled to the core of the output optical fiber 7 changes according to the applied voltage.

As described above, according to the present embodiment, in the variable optical attenuator that realizes the variable optical attenuation function using the Faraday rotator 4, by providing the cylindrical lens 3 in front of the magneto-optical crystal 41, the incident light Is condensed beforehand by the cylindrical lens 3 before the magneto-optical crystal 41, so that the Faraday rotator width and the distance between the electromagnets 44, 45 can be significantly reduced as compared with the conventional case, and the variable optical attenuator The size can be reduced.

In the configuration described above with reference to FIG. 6, point condensing lenses 3 ′ and 5 ′ can be applied instead of the cylindrical lenses 3 and 5 as shown in FIG. 9, for example. FIG. 10 (schematic top view) shows the configuration of the variable optical attenuator in this case when viewed from the direction of arrow A, and FIG. 11 also shows the configuration when viewed from the direction of arrow B. (Schematic side view) shows the optical path focusing on the ordinary light component and the extraordinary light component when viewed from the direction of arrow B in FIG.

However, in this case, the (input) point condenser lens 3 ′ is disposed after the polarizer 9 (before the magneto-optical crystal 41), and the (output) point condenser lens 5 ′ is connected to the analyzer 10. It is arranged at the front stage (after the magneto-optical crystal). In this case, the polarizer 9 and the analyzer 10 are both installed with the long side of the wedge shaped downward (in the direction of arrow A in FIG. 9).

It should be noted that the arrangement relationship between the polarizer 9 and the analyzer 10 changes when the point condensing lenses 3 'and 5' are applied for the following reasons. That is, if the point condensing lenses 3 ′ and 5 ′ are arranged at the same position as when the cylindrical lenses 3 and 5 are applied, the light transmitted through the polarizer 9 and the analyzer 10 that causes an angle change in the incident light is obtained. The focusing process is performed in all directions, and the point focused beam is hardly attenuated because the position focused on the output optical fiber 7 hardly changes (the angle tolerance is loose) even if the angle changes slightly. It is.

Therefore, the polarizer 9 and the analyzer 10 are not used at the point focused beam but at the collimated light stage. The polarizer 9 is arranged before the point condensing lens 3 ′, and the analyzer 10 is arranged after the point condensing lens 5 ′ so that an angle change occurs in.

With the configuration described above, in the variable optical attenuator shown in FIG. 9, the ordinary light component and the extraordinary light component travel along the optical path schematically shown in FIG. The amount of light coupled to the core of the output optical fiber 7 changes according to the change in the applied electromagnetic field due to 44 and 45. A point condensing lens 3 ′ is provided in front of the Faraday rotator 4 (magneto-optical crystal 41) to collect light incident on the magneto-optical crystal 41 in a point-like manner in advance (see FIGS. Therefore, also in the case of this example, the Faraday rotator width and the distance between the electromagnets 44 and 45 can be greatly reduced as compared with the conventional case, and the variable optical attenuator can be significantly reduced in size.

(C) Description of reflective variable polarization rotator

Next, an embodiment of a reflection-type variable polarization rotator will be described below. FIG. 13 is a schematic perspective view showing the configuration of a reflection-type variable polarization rotator as one embodiment of the present invention, together with an optical path. The variable polarization rotator shown in FIG. An input collimating lens 2, an input cylindrical lens 3, a magneto-optical crystal 41, a variable Faraday rotator 4 including permanent magnets 42, 43 and electromagnets 44, 45, and a magneto-optical crystal 4. It is composed of a reflective element 11 provided after 1 (before the permanent magnets 4 and 3), an output cylindrical lens 5, an output collimating lens 6, an output optical fiber 7, and a variable current source 8. Have been.

Here, also in this case, the input optical fiber 1 propagates the input light, and the input collimating lens (input collimator) 2 collimates the light emitted from the input optical fiber 1. The input cylindrical lens 3 condenses the light emitted from the input collimating lens 2 in a linear form and makes it incident on the variable Faraday rotator 4 (magneto-optical crystal 4 1). It is arranged so that the center of the magneto-optical crystal 41 is located at the focal length.

Further, the Faraday rotator 4 rotates (changes) the polarization state of the light emitted from the input cylindrical lens 3 in accordance with the intensity of the electromagnetic field applied by the electromagnets 44 and 45, as described above. The reflection element 11 is provided on the output side of the magneto-optical crystal 41, and reflects the light transmitted through the magneto-optical crystal 41 to form a magneto-optical element. In this case, the light from the magneto-optical crystal 41 is directed in a direction different from the input optical path (for example, as schematically shown in FIG. 14, the reflection angle Θ and the Z axis in FIG. 13 are used). (Direction of arrow A)].

As shown in FIG. 14, when the reflective element 11 is brought into close contact with the magneto-optical crystal 41, it is realized by forming a reflective film made of a dielectric multilayer film or the like on the output surface of the magneto-optical crystal 41. it can. Of course, air or another optical medium may be interposed between the reflective element 11 and the magneto-optical crystal 41 without being brought into close contact with each other.

Further, an output cylindrical lens (output collimator) 5 collimates the reflected light reflected by the reflection element 11 and transmitted through the magneto-optical crystal 41 again. The output collimating lens 6 is an output cylindrical lens. The light emitted from 5 is condensed, and is arranged such that its focal length is located at the core of the output optical fiber 7. The focal length f of the collimating lenses 2 and 6 is 4 mm, for example, and the focal length f of the cylindrical lenses 3 and 5 is 1.8 mm, for example.

In the reflective variable polarization rotator of the present embodiment configured as described above, the light emitted from the input optical fiber 1 enters the input collimating lens 2 and is collimated by the collimating lens 2. Then, the light enters the input cylindrical lens 3, is condensed linearly in advance in the X-axis direction in FIG. 13 by the input cylindrical lens 3, and is incident on the Faraday rotator 4 (magneto-optical crystal 41).

Then, the light incident on the Faraday rotator 4 has its polarization plane rotated in the magneto-optical crystal 41 according to the intensity of the applied electromagnetic field by the electromagnetic stones 44 and 45, and has transmitted through the magneto-optical crystal 41. Thereafter, the light is reflected by the reflection element 11 and reenters the magneto-optical crystal 41. In the magneto-optical crystal 41, substantially the same amount of Faraday rotation angle as that of the incident light is given to the magneto-optical crystal 41 in the same rotation direction.

Therefore, the thickness of the magneto-optical crystal 41 itself (the X-axis direction in FIG. 13) can be substantially reduced by half compared to the conventional case, and in this case, the light incident on the magneto-optical crystal 41 is cylindrically formed. Since the light is previously focused linearly in the X-axis direction in Fig. 13 by the lens 3, the Faraday rotator width and the distance between the electromagnets 44, 45 are further reduced, and the Faraday rotator 4 The size can be reduced. The reflected light transmitted through the magneto-optical crystal 41 is collimated by the output cylindrical lens 5, then condensed by the output collimating lens 6, and coupled to the core of the output optical fiber 7.

Note that the angle between the incident light from the magneto-optical crystal 41 to the reflective element 11 and the reflected light from the reflective element 11 to the magneto-optical crystal 41 (reflection angle; see Fig. 14) is 0 degree. Yes. In this case, for example, an optical circuit may be used to spatially separate the reflected light from the incident light.

As a means for further reducing the size and the number of parts of the variable polarization rotator, for example, as schematically shown in FIG. 15, the input collimating lens 2 and the output collimating lens 6 described above are combined into one. The collimating lens 26 may be used in common, and the input cylindrical lens 3 and the output cylindrical lens 5 may be used in common by one cylindrical lens 35 (in such a common use, the reflection angle 0 is small (for example, about 5 °). ) Is easier.) Of course, it is also possible to share only one of the lenses. In each case, a two-core ferrule 17 with an input optical fiber 1 and an output optical fiber 7 can be used.

Further, as a principle of the present invention, for example, as schematically shown in FIG. 16, an input cylindrical lens 3, a Faraday rotator 4 (a magneto-optical crystal 41, permanent magnets 42, 43, an electromagnet 44) , 45) and the reflective element 11, a reflection-type variable polarization rotator can be realized (the collimating lenses 2 and 6 may be unnecessary in the configuration of Fig. 13). However, the output cylindrical lens 5 may be unnecessary).

(D) Description of reflective variable optical attenuator

Next, an embodiment of a reflection-type variable optical attenuator to which the above-described reflection-type variable polarization rotation device is applied will be described.

FIG. 17 is a schematic perspective view showing the configuration of a reflection type variable optical attenuator according to an embodiment of the present invention, together with an optical path. FIG. 18 is a diagram showing the variable optical attenuator shown in FIG. FIG. 19 is a schematic side view also showing the configuration when viewed from the direction together with the optical path, and FIG. 19 is a schematic side view also showing the configuration when viewed from the arrow B direction.

As shown in FIGS. 17 to 19, the reflection-type variable optical attenuator of the present embodiment is , 2-core ferrule 17, collimating lens 26, cylindrical lens 35, Faraday rotator 4 (magneto-optical crystal 41, permanent magnets 42, 43, electromagnets 44, 45), polarizer It is provided with a combined analyzer 91, a reflection element 11 and a variable current source 8. In other words, as can be seen from FIGS. 17 to 19, the variable optical attenuator is based on the configuration of the reflective variable polarization rotator described above with reference to FIG. Between the crystal 41 and the polarizer (first birefringent plate) 9 and the analyzer (second birefringent plate) 10, the polarizer / analyzer 91, which also functions as the above-mentioned polarizer (first birefringent plate) 10, was arranged. It is structured.

The polarizer / analyzer 91 also uses a birefringent crystal (for example, rutile) having a tapered (wedge) shape in the YZ plane in FIG.

By adopting such a configuration, in the present apparatus, the light incident on the Faraday rotator 4 (the magneto-optical crystal 41) is linearly condensed by the cylindrical lens 35 in advance, so that the Faraday rotator width and A reflection-type variable optical attenuator can be realized while significantly reducing the distance between the electromagnets 44 and 45 compared to the conventional one.

That is, as shown in FIG. 20, in the case of the optical path viewed from the direction of arrow B in FIG. 17, the input optical fiber 1 is emitted, and the light beam collimated by the collimating lens 26 passes through the cylindrical lens 35. After that, it remains a collimated light, and when this light is incident on the polarizer / analyzer 91, it is polarized and separated into an ordinary light component o and an extraordinary light component e. The two polarized light beams o and e thus polarized are rotated by the Faraday rotator 4 (magneto-optical crystal 41), reflected by the reflecting element 11 and then again by the Faraday rotator 4 (magnetic The light passes through the optical crystal 41) and undergoes polarization rotation.

Here, when the amount of polarization rotation received in the round trip is 90 °, the polarization state when entering the polarizer / analyzer 91 on the return path is as shown in Fig. 21. oe, the extraordinary light component e in the outward path becomes the ordinary light component eo. In this case, since the light ray o e and the light ray e o are substantially parallel, these two light rays can be simultaneously condensed by the collimating lens 26 and coupled to the output optical fiber 7.

On the other hand, when the amount of polarization rotation received in the round trip is 0 °, the polarization state when entering the polarizer / analyzer 91 on the return path is as shown in FIG. 21. The extraordinary light e in the outward path becomes the extraordinary light component ee again. In this case, the ray Since the light beam 00 and the light beam 66 are not parallel to each other, and furthermore, neither the light beams oe and eo are parallel to each other, in the optical system where the light beam oe and the light beam e 0 are coupled to the core of the output optical fiber 7, The light is not coupled to the core of the output optical fiber 7 and is emitted from the clad of the output optical fiber 7.

Therefore, when the amount of polarization rotation received during reciprocation is between 0 and 90 °, the optical path separation at 0 ° rotation and the optical path separation at 90 ° rotation occur according to the amount of rotation. By controlling the degree (controlling the variable current source 8), the coupling rate to the output optical fiber 7 can be controlled.

In the configuration described above, for example, as shown in FIG. 22, the focal length of the collimating lens 26 is f1, the focal length of the cylindrical lens 35 is f2, and the distance between the collimating lens 26 and the cylindrical lens 35 is If the distance is α, then f 1 = f 2 +.

Here, the arrangement of each element is as described above when the cylindrical lens 35 is used as described above, that is, in order from the input side, the two-core ferrule 17, the collimating lens 26, the cylindrical lens 35, and the polarizer In addition to the arrangement of the combined analyzer 91, Faraday rotator 4, and reflecting element 11, variable attenuation can be achieved even if the positions of the cylindrical lens 35 and the polarizer / analyzer 91 are switched, as shown in Fig. 23, for example. Is possible.

However, since f 1 = f 2 + α as described above, even if a lens having the same focal length is used as the cylindrical lens 35, the collimating lens 26 and the cylindrical lens are arranged in the arrangement shown in FIG. Α <α ′ because the polarizer / analyzer 91 must be placed between the polarizer 35 and 35. Therefore, f 1 and f 1 '. Thus, as the focal length of the collimating lens 26 increases, the collimated beam diameter Φ also increases (see φ ′ in Fig. 23), and as the beam diameter φ increases, the beam enters the Faraday rotator 4 (the magneto-optical crystal 41). Numerical Aperture (NA) increases. As a result, the width of the Faraday rotator (the distance between the electromagnets 44 and 45) must be increased, so that it is difficult to reduce the size of the electromagnets 44 and 45, and it is difficult to reduce the size of the Faraday rotator 4. . Therefore, the arrangement shown in FIG. 22 is more advantageous. When a point condenser lens 3 ′ as described above with reference to FIGS. 5 and 9 is used instead of the cylindrical lens 35, only the arrangement shown in FIG. 23 is used to add a variable attenuation function. I can't get it. This is because, when the cylindrical lens 35 is used, the collimating light propagates between the collimating lens 26 and the reflecting element 11 in any of the arrangements shown in FIGS. The angle of the optical path changes depending on the polarizer / analyzer 91, but when the point condensing lens 3 'is used and the arrangement shown in FIG. 22 is adopted, the light passes through the polarizer / analyzer 91 which causes an angle change. Because light is converging in all directions, the condensing position hardly changes (the angle tolerance is loose) even if the angle changes slightly.

However, since the arrangement shown in FIG. 23 is not preferable as described above, when realizing a reflection type variable optical attenuator, it is extremely preferable to adopt the arrangement shown in FIG. 22 using a cylindrical lens 35. This is advantageous.

Also, in the above-described example, the case where the configuration of the above-described reflective variable polarization rotator is basically described with reference to FIG. 15 is described. For example, the configuration shown in FIG. Even if a polarizer / analyzer 91 that also functions as the polarizer 9 and the analyzer 10 described above is placed between the set of, 5 and the magneto-optical crystal 41, the variable optical attenuation is the same as above. It goes without saying that a vessel can be realized.

The present invention is not limited to the above-described embodiment, and can be implemented with various modifications without departing from the spirit of the present invention.

For example, the variable polarization rotator of the present invention can be used to constitute another optical device such as an optical isolator or a laser module. In such a case, the optical device can be significantly reduced in size. Industrial applicability

As described above, according to the present invention, since the light incident on the magneto-optical crystal constituting the Faraday rotator is condensed in advance using a condensing means such as a cylindrical lens, the Faraday rotator width (distance between electromagnets) ) Can be greatly reduced compared to the conventional method, and the size of the electromagnet itself can be reduced. As a result, the size of the Faraday rotator can be significantly reduced. Therefore, a variable polarization rotator using a Faraday rotator, a variable optical attenuator, etc. The size of the scientific equipment can be significantly reduced, and its usefulness is considered to be extremely high.

Claims

The scope of the claims

1. First focusing means (3) for focusing the input light;

A magneto-optical crystal (41) installed near the focal point of the first light-collecting means (3);

Magnetic field generating means (42, 43, 44, 45) for applying a magnetic field to the magneto-optical crystal (41);

A variable polarization rotator comprising control means (8) for controlling said magnetic field generating means (42, 43, 44, 45) to change said magnetic field.

2. The variable polarization rotation according to claim 1, wherein the first light condensing means (3) is constituted by an input line focus lens that condenses the input light linearly. apparatus.

3. The method according to claim 1, wherein the first light condensing means (3) is constituted by an input point condensing lens (3 ') for condensing the input light in a point shape. A variable polarization rotation device as described in the above.

4. An input collimator (2) for collimating the input light and inputting the collimated light to the first light collecting means (3) is provided in front of the first light collecting means (3). The variable polarization rotator according to claim 1, characterized in that:

5. An input collimator (2) for collimating the input light and inputting the collimated light to the input line focus lens (3) is provided in front of the input line focus lens (3). 3. The variable polarization rotation according to claim 2, wherein a polarizer or a birefringent plate (9) is provided between the kalein focus lens (3) and the magneto-optical crystal (41). Variable optical attenuator using device.

6. Before the input point condenser lens (3 '), collimate the input light and An input collimator (2) for inputting collimated light to the point condenser lens (3 ') is provided, and a polarizer is provided between the input collimator (2) and the input point condenser lens (3'). Alternatively, a variable optical attenuator using the variable polarization rotation device according to claim 3, wherein a birefringent plate (9) is provided. '

7. The output collimator (5) for collimating light transmitted through the magneto-optical crystal (41) is provided downstream of the magneto-optical crystal (41). Variable polarization rotation device.

8. The output collimator (5) is constituted by an output line focus lens that condenses light emitted from the magneto-optical crystal (41) into a linear shape. The variable polarization rotator according to Item.

9. The output collimator (5) is constituted by an output point condensing lens (5 ') for condensing the light emitted from the magneto-optical crystal (41) in a point-like manner. 8. The variable polarization rotator according to claim 7.

10. The method according to claim 8, wherein an analyzer or a birefringent plate (10) is provided between the magneto-optical crystal (41) and the output line focus lens (5). Variable optical attenuator using variable polarization rotation device.

11. A second condenser means (6) for condensing light emitted from the output point condenser lens (5 ') is provided at a stage subsequent to the output point condenser lens (5'). 10. The method according to claim 9, wherein an analyzer or a birefringent plate (10) is provided between the point condenser lens (5 ') and the second condenser means (6). A variable optical attenuator using the variable polarization rotation device described.

12. An input optical fiber (1) that propagates the input light,

An input collimator for collimating the input light from the input optical fiber (1) (2) When,

An input focusing lens (3) for linearly condensing collimated light emitted from the input collimator (2);

A magneto-optical crystal (41) installed near the focal point of the input focus lens (3);

An output line focus lens (5) for collimating light transmitted through the magneto-optical crystal (41);

An output collimating lens (6) for collecting collimated light emitted from the output line focus lens (5);

An output optical fiber (7) installed near a focal position of the output collimating lens (6);

13. A polarizer or a first birefringent plate (9) is provided between the input focus lens (3) and the magneto-optical crystal (41). 13. The variable polarization rotator according to claim 12, wherein an analyzer or a second birefringent plate (10) is provided between the output line focus lens (5) and the output line focus lens (5). Variable optical attenuator.

14. It is characterized in that the polarizer or the first birefringent plate (9) and the analyzer or the second birefringent plate (10) are each composed of a birefringent crystal having a wedge shape. 14. The variable optical attenuator according to claim 13.

15. A first focusing means (3) for focusing the input light;

A magneto-optical crystal (41) installed near the focal point of the first light condensing means (3); Magnetic field generating means (42, 43, 44, 45) for applying a magnetic field to the magneto-optical crystal (41);

Control means (8) for controlling the magnetic field generating means (42, 43, 44, 45) to change the magnetic field;

A variable polarization rotator, comprising: a reflection element (11) for reflecting light transmitted through the magneto-optical crystal (41) and returning the reflected light to the magneto-optical crystal (41).

16. The variable device according to claim 15, wherein the first light condensing means (3) is constituted by an input color information lens that condenses the input light linearly. Polarization rotation device.

17. An input collimator (2) for collimating the input light and entering the first condenser means (3) is provided in front of the first condenser means (3). 17. The variable polarization rotator according to claim 15 or claim 16.

18. An output collimator (5) for collimating reflected light reflected by the reflection element (11) and transmitted through the magneto-optical crystal (41) is provided. 18. The variable polarization rotation device according to any one of items 15 to 17.

19. The output collimator (5) is formed by an output color information lens that linearly condenses the reflected light from the magneto-optical crystal (41). 19. The variable polarization rotation device according to item 18.

20. A second condenser means (6) for condensing light emitted from the output collimator (5) is provided at a stage subsequent to the output collimator (5). 20. The variable polarization rotator according to claim 18 or 19.

21. An input optical fiber (1) that propagates input light,

An input collimator for collimating the light emitted from the input optical fiber (1) 2) and

A first focusing means (3) for focusing collimated light emitted from the input collimator (2);

A reflection element (11) for reflecting light transmitted through the magneto-optical crystal (41) and returning the reflected light to the magneto-optical crystal (41);

An output collimator (5) for collimating the light reflected by the reflective element (11) and transmitted through the magneto-optical crystal (41);

A second focusing means (6) for focusing the collimated light emitted from the output collimator (5);

An output optical fiber (7) installed near the focal point of the second condensing means (6); and magnetic field generating means (42, 43, 44, 45) for applying a magnetic field to the magneto-optical crystal (41). ,

A variable polarization rotator, comprising control means (8) for controlling said magnetic field generating means (42, 43, 44, 45) to change said magnetic field.

22. The first light-condensing means (3) and the output collimator (5) are each constituted by a line focus lens that condenses incident light in a linear manner. 21. The variable polarization rotator according to item 21.

23. The first light-collecting means (3) and the output collimator (5) are shared by one line focus lens (35) for linearly condensing incident light. 22. The variable polarization rotation device according to claim 21.

24. The method according to claim 2, wherein said input collimator (2) and said second condenser means (6) are each constituted by a collimating lens for collimating the incident light. 24. The variable polarization rotator according to any one of 1 to 23.

25. The input collimator (2) and the second condenser means (6) are shared by one collimating lens (26) for collimating incident light. 24. The variable polarization rotation device according to any one of items 21 to 23.

26. A polarizer or a first birefringent plate (9) is provided between the first condensing means (3) and the magneto-optical crystal (41), and the magneto-optical crystal (41) and the magneto-optical crystal (41) are provided. The variable according to any one of claims 21 to 25, characterized in that an analyzer or a second birefringent plate (10) is provided between the output collimator and the collimator (5). Variable optical attenuator using a polarization rotation device.

27. The method according to claim 26, wherein the first birefringent plate (9) and the second birefringent plate (10) are shared by a polarizer and analyzer (91). The variable optical attenuator according to the item.

28. The first birefringent plate (9) and the second birefringent plate (10) are each composed of a birefringent crystal having a wedge shape. 27. A variable optical attenuator according to item 26.

29. The variable optical attenuator according to claim 27, wherein said polarizer and analyzer (91) is made of a birefringent crystal having a wedge shape.