JP2005315906A - Optical isolator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical isolator which does not need a special holder or the like for accurately combining a polarizer, a Faraday rotator and an analyzer, is easy in assembling adjustment and can be miniaturized. <P>SOLUTION: The optical isolator has a configuration, in which a parallel plate-like 45°-Faraday rotator 14 is arranged in a cylindrical permanent magnet 16, and polarizers 12a, 12b are arranged by being opposed respectively to two light-incident and emitting surfaces. These polarizers 12a, 12b are structures in which plate-shaped bodies 22a, 22b having a double-layer construction, in which plate-shaped metal and plate-shaped dielectric are closely joined are numerously arranged in parallel, and the two structures are fixed respectively so that directions of their plate surfaces form an angle of 45 degrees mutually in two polarizers. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical isolator used mainly in the field of optical communication, and more particularly to an optical isolator using a polarizer and a Faraday rotator.

  A semiconductor laser used for optical fiber communication is supplied as a module having an optical fiber for optical output, and is often connected to an optical fiber for transmission by providing an optical connector at the end thereof. In this case, it is known that the reflected return light generated in the optical connector portion is reinjected into the semiconductor laser and the operation state of the semiconductor laser becomes unstable. For this reason, an optical isolator is used to remove the return light.

  FIG. 7 shows a configuration of a typical conventional optical isolator. In this optical isolator, a Faraday element 54 is fixed in a magnetic field, and polarizing plates 52a and 52b are disposed with the Faraday element 54 interposed therebetween. By rotating the polarization direction of both polarizing plates 52a and 52b by 45 °, both polarizing plates function as a polarizer and an analyzer, respectively.

  Regarding the optical isolator composed of the above-described components, the relationship and operation of the components will be described. As shown in FIG. 8A, the linearly polarized light 50b that has passed through the polarizer 52a out of the light 50a incident on the polarizer 52a in the direction indicated by arrow A from the semiconductor laser has a polarization direction of 45 ° by the Faraday rotator 54. Rotate. The linearly polarized light 50c rotated by 45 ° passes through the analyzer 52b arranged so as to be 45 ° different from the polarizer 52a, and is emitted.

  8B, the linearly polarized light 60b that has passed through the analyzer 52b out of the light 60a incident from the opposite direction indicated by the arrow B is non-reciprocal due to the non-reciprocity of the Faraday rotation element 54. After passing through, the polarization direction is rotated by 45 °, and is orthogonal to the polarization direction of the polarizer 52a and cannot pass through the polarizer 52a.

  Conventional optical isolators are difficult to miniaturize because a polarizer, a Faraday rotator, and an analyzer are combined as individual components in order to constitute the optical system as described above. Further, when assembling them, the polarization direction of the polarizer and the analyzer had to be precisely adjusted so that they are at an angle of 45 ° to each other, so that the assembly process was complicated and the cost was high.

  Further, when this conventional optical isolator is mounted on, for example, a semiconductor laser module, the polarization direction of the polarizer and the analyzer is difficult to understand because the surface is smooth. For this reason, it has been necessary to adjust the optical axis so that light is actually incident and the output from the optical isolator is maximized. Thus, even when an optical isolator is mounted, there is a problem that adjustment is difficult.

Specific attempts to solve the above problems are disclosed in, for example, Patent Documents 1 to 3.
For example, in the optical isolator disclosed in Patent Document 1, a rectangular Faraday element 64 is inserted and fixed in an opening of a disk-shaped permanent magnet 66 as shown in FIG. 7, and a polarizing plate 62a, Each 62b is fixed using a semi-cylindrical holder 65. The two polarizing plates 62a and 62b are simply inserted into the notch 68 provided in the holder 65, and are attached so that the polarization direction differs by 45 ° depending on the angle difference between the two notches, so that the angle adjustment of the polarizing plate is not required. It has been devised.

Patent Documents 2 and 3 describe a method of directly forming a photonic crystal on the surface of a Faraday rotator. By integrating the Faraday rotator and the polarizer, the number of components can be reduced, and assembly can be facilitated.
JP-A-10-68907 JP 2000-56133 A JP 2003-172901 A

  However, the method of adding a device to the holder described in Patent Document 1 does not improve the point that a polarizer, a Faraday rotator, and an analyzer must be combined as individual components. There is also a problem that a special holder with a cut cut must be prepared.

  In addition, although the method of Patent Document 2 or 3 can reduce the number of parts, a large number of high refractive index materials and low refractive index materials are alternately laminated on the surface of the Faraday rotator in a direction perpendicular to the substrate, so that it takes time for production. There was a problem that the production cost increased. Further, the complexity of the process has not been solved, because the polarization direction of the polarizer and the analyzer must be aligned at an angle of 45 ° with respect to the optical axis.

  The present invention has been made to solve the above-described conventional problems, and an object thereof is to provide an optical isolator that can be downsized and easily adjusted.

The optical isolator according to the present invention has a configuration in which polarizers are arranged opposite to two opposing light incident / exit surfaces of a parallel plate type 45 degree Faraday rotator, and this polarizer has a large number of plate-like bodies parallel to each other. It shall have the structure arranged in this. The two structures are arranged so that the plate-like members constituting them have substantially the same angle with respect to the light incident / exit surface of the Faraday rotation element, and the directions of the plate surfaces form an angle of 45 ° with each other. Fix each one.
By adopting such a configuration, an optical isolator that is small and easy to adjust can be provided.

  The plate-like body has a double layer structure in which a plate-like metal and a plate-like dielectric are in close contact. Since such a plate-like body can be produced by a film forming process, the polarizer can be easily produced on the surface of the Faraday rotation element.

  It is desirable that the plate-like members constituting the polarizer are directly formed on both light incident / exit surfaces of the Faraday rotator. Assembly and adjustment can be simplified by forming the polarizer directly on the surface of the Faraday rotator.

  Also, a means for inserting a parallel flat transparent substrate between the Faraday rotator and one of the polarizers can be employed. By providing a space between the Faraday rotator and one polarizer, the incident position of incident light from the first direction can be shifted from the emission position of light incident on the same optical axis from the second direction. It is possible to avoid the incidence of light incident on the light source of the first incident light from the second direction.

When the parallel plate-like transparent substrate is inserted, a plate-like body is directly formed on the light incident / exit surface opposite to the light incident / exit surface facing the parallel plate-like transparent substrate of the Faraday rotator, It is desirable to directly form a plate-like body on the surface opposite to the surface facing the light incident / exit surface of the Faraday rotator of the transparent substrate.
Assembly and adjustment can be simplified by directly forming the polarizer on the surface of the Faraday rotator and the surface of the parallel flat plate-like transparent substrate.

  Furthermore, it is desirable that the Faraday rotator element having the plate-like body and the parallel flat plate-like transparent substrate are bonded together to be fixed inside the through hole of the permanent magnet provided with the through hole. By integrally forming the components of the optical isolator, the size can be reduced, and assembly and adjustment can be facilitated.

  In addition, the Faraday rotator and the parallel plate-like transparent substrate are fixed to each other, and the Faraday rotator is tilted with respect to the optical axis inside the through hole of the permanent magnet provided with a through hole. It is preferable to fix so. Since light is incident on the Faraday rotator at an angle, mounting the optical isolator is facilitated by fixing it at an angle.

  In the optical isolator of the present invention, since the polarizer, the Faraday rotator, and the analyzer are integrated, the length in the optical axis direction can be shortened, and the optical isolator can be configured in a small size. Further, since the angle adjustment between the polarizer and the analyzer, which has been required so far, is unnecessary, the assembly process of the optical isolator is simplified. Furthermore, since the polarizer and analyzer on the surface have a linear concavo-convex structure, it is easy to specify the polarization direction when mounting the optical isolator.

  FIG. 1 shows an embodiment of an optical isolator according to the present invention. FIG. 1B is a cross-sectional view of the optical isolator of the present invention as seen from the side. In the optical isolator, a first polarizer 12a, a Faraday rotator 14, a transparent substrate 11 attached thereto, and a second polarizer 12b formed on the opposite surface of the transparent substrate 11 are arranged in this order. The broken block is housed in a cylindrical permanent magnet 16. The polarization directions of the first polarizer 12a and the second polarizer 12b are different from each other by 45 °.

  The optical isolator of the present invention is characterized by the polarizer used. As shown in FIG. 2, this polarizer is a structure made up of a plurality of parallel plate-like bodies 22 standing almost vertically on a substrate 25. The plate-like body 22 has a structure in which a plate-like metal 27 and a plate-like dielectric 26 are in close contact with each other. The directionality of the plate surface of the plate-like body 22 (direction indicated by y in the figure) gives polarization selectivity to light transmitted through this structure. As will be described later, in order to easily control the direction of the plate surface of the plate-like body 22, it is desirable to interpose a layer 28 having an uneven structure on the surface of the substrate 25.

  In the optical isolator of the present invention, as shown in FIG. 1B, the plate-like body is directly formed on one light incident / exit surface of the Faraday rotator element 14 to form a polarizer 12a. FIG. 1A is a view as seen from a direction perpendicular to the light incident / exit surface of the Faraday rotator 14, and shows a state in which a plurality of plate-like bodies 22a are arranged in parallel in a certain direction to constitute a polarizer 12a. This is shown schematically.

  FIG. 1C schematically shows a state in which the plate-like body 22b is also formed on the surface of the transparent substrate 11 to constitute the polarizer 12b. Here, the angles of the plate surfaces of the plate-like bodies 22a and 22b with respect to the light incident / exit surface of the Faraday rotator are substantially vertical and equal to each other. Depending on the production conditions of the plate-like body, the plate-like body may be inclined from the vertical, but both plate-like bodies may be used having substantially the same angle. Here, the plate-like body 12b is formed in a direction different from the plate surface direction of the plate-like body 12a by 45 °.

As shown in FIG. 2, when the average interval between the individual plate-like bodies 22 is d, the height of the plate-like bodies is H, and the thickness of the plate-like metal 27 is W, when the wavelength used is λ,
0.07λ <d <0.20λ
0.15λ <H <0.09λ
0.05λ <W <0.18λ
It is preferable to set it as the range.

  The structure constituted by these plate-like bodies reflects the TE component parallel to the plate surface direction and transmits only the perpendicular TM component with respect to the polarization of the incident light. Accordingly, as shown in FIGS. 1A and 1C, when the direction of the plate-like body is changed by changing the direction of 45 °, the light transmitted through one of the polarizers has a polarization direction of 45 ° after that. Only when rotated, the other polarizer can be transmitted.

  The operation as an optical isolator is shown in FIG. As shown in the drawing, the optical isolator of the present invention is set with an angle with respect to the normal optical axis 100 when used.

  FIG. 3A shows a case where the light 10 is incident from the Faraday rotator 14 side, and the polarizer 12a is arranged on the surface so that the direction of the plate-like body is as shown in FIG. As a result, this polarizer 12a passes only TM polarized light out of the light emitted from the semiconductor laser, like the element 52a of FIG. For this reason, the polarization of the semiconductor laser is set to coincide with this direction. This TM polarized wave is rotated by 45 degrees in the direction indicated by 50 c in FIG. 8 by the Faraday rotator 14. As shown in FIG. 1C, the direction of the polarizer 12b is inclined 45 ° with respect to the direction of the polarizer 12a, and the polarizer 12b arranged in this direction is rotated 45 ° by the Faraday rotator 14. Transmits TM polarized light.

  FIG. 3B shows a case where the light 20 is incident from the reverse direction. Since the polarizer 12b on the transparent substrate 11 side is disposed by being rotated by 45 ° around the optical axis with respect to the direction of the polarizer 12a, from the back side, the polarizer 12b is perpendicular to the direction of the plate of the polarizer 12b. Only TM polarization is transmitted. Since the polarization direction of the TM polarization is rotated by 45 ° by the Faraday rotator 14, the TM polarization becomes TE polarization with respect to the polarizer 12a, and is reflected backward.

  At this time, the optical isolator body is tilted with respect to the optical axis 100 as shown in FIG. 3, so that the light 30 reflected backward is reflected at an angle with respect to the optical axis 100. The reflected light 30 is incident on the polarizer 12b as TE polarized light after the polarization plane is further rotated by 45 ° by the Faraday rotator 14. The TE polarized wave is reflected by the polarizer 12b and enters the Faraday rotator 14 again.

  At this time, the reflection direction is parallel to the optical axis 100, but the position of the reflected light 40 is deviated from the extended line in the direction of the incident light 20. The reflected light 40 is further rotated by 45 ° by the Faraday rotator 14 and passes through the polarizer 12a as TM polarized light. Since this position is deviated from the position where the semiconductor laser was placed as shown in Patent Document 2, it does not return to the semiconductor laser. With this structure, an optical isolator in which a polarizer, a Faraday rotator, and an analyzer are integrated can be realized.

  The above is an example in which the entire isolator is tilted. However, as shown in FIG. 4, a block in which the polarizers 12 a and 12 b are integrated with the Faraday rotator 14 and the transparent substrate 11 attached thereto is integrated with the optical axis 100. The same operation can be performed even if it is inclined with respect to the axial direction of the cylindrical permanent magnet 16.

  In the case where it is not necessary to shift the position of light incident from the opposite direction as in the above case, a polarizer may be formed in close contact with both surfaces of the Faraday rotator without providing a transparent substrate. In this case, it is desirable to form a plate-like body directly on both light incident / exit surfaces of the Faraday rotator.

As an example of the present invention, an optical isolator operating at a wavelength of 1550 nm was manufactured.
As shown in FIG. 1, this isolator is formed by integrating a Faraday rotator 14, a polarizer 12a formed on the surface thereof, a transparent substrate 11 made of glass attached to the Faraday rotator 14, and a polarizer 12b formed on the back surface thereof. And a cylindrical permanent magnet 16. This block was prepared by preparing the substrate of the Faraday rotator 14 and the transparent substrate 11, forming the polarizers 12 a and 12 b on each of them, and bonding them together and cutting them into chips.

  First, a Bi-substituted iron garnet substrate (BIG) was prepared as a Faraday rotator element substrate. This substrate has a thickness of 400 μm, a size of 10 mm × 10 mm, and a Faraday rotation angle of 45 ° is obtained with respect to incident light of 1550 nm by applying a magnetic field of 95 KA / m.

  A plate-like body 22 as shown in FIG. 2 was formed on the surface of the Faraday rotator element substrate. The average distance d between the plate-like bodies is 277 nm. This interval corresponds to 0.18λ for light having a wavelength of 1550 nm.

The structure composed of the plate-like body was produced by the following method.
First, a method for manufacturing a substrate for forming a plate-like body will be described. This substrate includes a layer 28 having a linear concavo-convex structure in which a plurality of V-shaped grooves parallel to the surface of the flat substrate 25 are formed. In this embodiment, a tetraethoxysilane (TEOS) sol solution as a sol-gel material is applied to a Faraday rotator substrate by a spin coater, and has a cross-sectional shape as shown in FIG. 5 to form a plurality of parallel V-shaped grooves. The formed mold 88 was pressed.

At this time, the linear direction of the concavo-convex structure was set to be parallel to the side surface of the Faraday rotation element substrate. In this state, heating and drying were performed, and then the mold 88 was released. Further, by heating to 350 ° C., a layer 28 having a linear concavo-convex structure mainly composed of SiO ₂ is formed on the surface of the Faraday rotator element substrate.

Next, as shown in FIG. 2, a plate-like body in which a double layer made of a dielectric and a metal stands in a substantially vertical direction is formed along the linear concavo-convex structure. This double layer was formed using the long-distance sputtering apparatus shown in FIG. An Ag target was attached to the magnetron cathode 201, and an SiO ₂ target was attached to the magnetron cathode 202. A Faraday rotator element substrate having a linear concavo-convex structure formed on the surface at a position 210 shown in the figure was placed so that the concavo-convex structure surface was directed downward. The magnetron cathodes 211 and 212 were installed so that the sputtered particles were incident perpendicular to the linear direction of the linear concavo-convex structure. The magnetron cathodes 211 and 212 were respectively arranged at positions inclined by 80 ° with respect to the normal direction of the substrate.

After the substrate was attached, the pressure inside the sputtering chamber 220 was evacuated to about 1 × 10 ⁻⁴ Pa using a rotary pump and a cryopump. Argon gas was introduced into the target chamber 212, and argon gas mixed with 2% oxygen was introduced into the target chamber 211. At this time, the pressure inside the sputtering chamber was 3 × 10 ⁻² Pa. Next, a negative voltage was applied to the magnetron cathode 201 by a DC power source to cause glow discharge, and further, a high frequency (frequency 13.56 MHz) was applied to the magnetron cathode 202 to generate glow discharge.

Next, the power supplied to the magnetron cathode 201 is adjusted so that the deposition rate of Ag is 10 nm / min on the surface of the substrate, and the magnetron cathode 202 is also set so that the volume rate of the SiO ₂ film is also 10 nm / min. The power supplied to was adjusted.
Subsequently, the shutters 206 and 207 attached to the entire surfaces of the magnetron cathode 201 and the magnetron cathode 202 were simultaneously released to start film formation and left for about 20 minutes. After 20 minutes, the two shutters were closed at the same time to complete the film formation.

Thus, when the cross-sectional structure of the obtained sample was observed with the transmission electron microscope, the structure as shown in FIG. 2 was formed in the substrate surface. Formed in a state where a plate-like dielectric 26 mainly composed of SiO ₂ and a plate-like metal 27 mainly composed of Ag are in contact with each other on a layer 28 having a linear concavo-convex structure formed on the surface of the substrate 25. A plurality of the plate-like bodies 22 are arranged in parallel.

  This plate-like body 22 was in a state of standing substantially perpendicular to the surface of the substrate 25. There were voids between the plate-like bodies. With this structure, a double layer of the plate-like metal 27 and the plate-like dielectric 26 can be realized with a height of H = 200 nm. This corresponds to 0.13λ for a wavelength of 1550 nm. W is 138.5 nm, approximately equal to d / 2, and is 0.09λ for 1550 nm light.

  The plate-like structure reflects TE polarized light, which is a polarization component parallel to the direction of the plate, and transmits TM polarized light perpendicular to the concavo-convex structure. In this example, 95% of TM polarization was transmitted, and the transmitted TE polarization was 0.01% or less. As a result, a reflective polarizer could be formed on the surface of the Faraday rotator substrate.

  Note that the plate-like body is not necessarily formed directly on the surface of the Faraday rotator element substrate. A transparent substrate such as a flat glass substrate may be prepared, a plate-like body may be formed on the surface, and bonded to the Faraday rotator substrate.

As the other polarizer, a plate-like structure was formed on a 10 mm × 10 mm square glass substrate so as to form an angle of 45 ° with respect to one side thereof. The manufacturing method is exactly the same as described above.
The Faraday rotator element substrate and the glass substrate thus obtained were bonded together so that the polarizer formed on one surface thereof was on the outside. Further, a chip for incorporation into the optical isolator was cut out from this substrate. The size of the chip was 1.4 × 1.4 mm. By cutting out parallel to the substrate end surface, the direction of the plate-like structure is automatically formed on the front surface in parallel to the chip end surface, and the back surface is automatically inclined in a direction inclined by 45 °.

  A structure including the Faraday rotator 14, the polarizers 12a and 12b, and the transparent substrate 11 was mounted in a ring-shaped permanent magnet 16 as shown in FIG. A magnetic field of 95 KA / m or more can be applied to the Faraday rotator by the permanent magnet, and the Faraday rotator acts on linearly polarized light incident upon application of the magnetic field, and rotates the polarization direction by 45 °.

  As shown in FIG. 3A, this optical isolator is fixed at an angle of 6 ° with respect to the optical axis, and linearly polarized light having a wavelength of 1550 nm from the Faraday rotator side becomes TM polarized with respect to the direction of the plate. So that it was incident. The light incident as TM polarized light passes through the polarizer 12a, undergoes 45 ° rotation of the polarization plane by the Faraday rotator 14, enters the polarizer 12b as TM polarized light, and can pass through the optical isolator. At this time, 95% of the light passed through the incident light.

  Since the polarizer of the present invention has a linear plate-like structure, the polarization direction of the optical isolator can be known from the diffraction pattern or the like when observed at the wavelength of visible light. Thereby, the angle around the optical axis of the optical isolator could be easily set so that the incident light was TM polarized with respect to the optical isolator.

  Conversely, when light was incident from behind the optical isolator, only the TM polarization component passed through the polarizer 12b. The TM linearly polarized light passes through the Faraday rotator 14 and is reflected by the polarizer 12a as TE polarized light. This light undergoes 45 ° rotation of the polarization plane again by the Faraday rotator 14, and is reflected by the polarizer 12b. The reflected light passes through the Faraday rotator 14 three times, enters the polarizer 12a as a TE wave, and passes through the polarizer 12a.

  At this time, as shown in FIG. 3B, it was confirmed that the emitted light 40 was emitted with a deviation of about 100 μm from the optical axis of the incident light 10. This deviation was sufficient as a distance that had no effect when returned to the semiconductor laser.

After the Faraday rotator substrate on which the polarizer was formed and the transparent substrate were bonded together in the same manner as in Example 1, the chip was cut out obliquely so that the end surface thereof was tilted by 6 ° with respect to the thickness direction of the substrate. As a result, as shown in FIG. 4, a block in which the polarizers 12 a and 12 b, the Faraday rotator 14, and the transparent substrate 11 attached thereto are integrated is inclined by 6 ° with respect to the optical axis, and the permanent magnet 16 Installed inside.
When this optical isolator was installed parallel to the optical axis and operated in the same manner as in Example 1, the same operation was confirmed.

It is a figure which shows the basic composition of the optical isolator of this invention. It is a perspective schematic diagram of the structure which consists of a plate-shaped object used for the polarizer of the optical isolator of this invention. It is a figure explaining operation | movement of the optical isolator of this invention. It is a cross-sectional schematic diagram which shows the other Example of the optical isolator of this invention. It is a cross-sectional schematic diagram of the layer which has the uneven structure produced by the shaping | molding method. It is a schematic diagram of the film-forming apparatus which produces a plate-shaped object. It is a figure which shows the conventional optical isolator. It is explanatory drawing of the operation principle of an optical isolator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Transparent substrate 12a, 12b Polarizer 14 Faraday rotator 16 Permanent magnet 22, 22a, 22b Plate-like body 25 Substrate 26 Plate-like dielectric 27 Plate-like metal 28 Layer having concavo-convex structure 88 Mold 100 Optical axis

Claims (7)

  In an optical isolator in which a polarizer is disposed opposite to two opposing light incident / exit surfaces of a parallel plate 45-degree Faraday rotator, the polarizer has a structure in which a large number of plate-like bodies are arranged in parallel to each other. In the two structures, the angles of the plate surfaces of the plate-like members constituting them are substantially equal to each other with respect to the light incident / exit surface of the Faraday rotation element, and the directions of the plate surfaces are 45 ° to each other. An optical isolator characterized by being fixed so as to form each of them.   2. The optical isolator according to claim 1, wherein the plate-like body has a double layer structure in which a plate-like metal and a plate-like dielectric are in close contact with each other.   The optical isolator according to claim 2, wherein the plate-like body is directly formed on both light incident / exit surfaces of the Faraday rotation element.   3. The optical isolator according to claim 2, wherein a parallel plate-like transparent substrate is inserted between the Faraday rotation element and one of the polarizers.   The plate-like body is directly formed on the light incident / exit surface opposite to the light incident / exit surface facing the parallel flat transparent substrate of the Faraday rotation element, and the light input of the Faraday rotator of the parallel flat transparent substrate is 5. The optical isolator according to claim 4, wherein a plate-like body is also directly formed on the surface opposite to the surface facing the emission surface.   6. The Faraday rotation element having the plate-like body and a parallel flat plate-like transparent substrate are bonded together to be fixed inside the through hole of a permanent magnet provided with a through hole. Optical isolator. The light incident / exit surface of the Faraday rotator element with respect to the optical axis is formed in the through hole of the permanent magnet in which the Faraday rotator element formed with the plate-like body and the parallel plate-like transparent substrate are bonded and integrated to provide a through hole. The optical isolator according to claim 6, wherein the optical isolator is fixed so as to be inclined.

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