Method for manufacturing a Faraday rotator and an optical isolator

Abstract

To provide a Faraday rotator manufacturing method which allows for stably manufacturing high-quality Faraday rotators, in particular, forming high-quality anti-reflection films on light transmissive surfaces of Faraday rotators.SOLUTION: A Faraday rotator manufacturing method is provided, comprising spraying dry ice fine particles produced from liquefied carbon dioxide onto a light transmissive surface, or the light transmissive surface and side surfaces, of a substantially rectangular flat plate, a precursor of a Faraday rotator, preferably under a setting of a particle diameter within a given range and injection pressure within a given range for cleaning, and forming an anti-reflection film on the light transmissive surface.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing an optical isolator used in optical communications, and more particularly to a method for manufacturing a Faraday rotator used in an optical isolator. [Background technology]

[0002] In laser light sources used in optical processing machines, optical measuring instruments, and optical communications, if the emitted laser light is reflected by the surface of a component installed along the transmission path and part of the reflected light returns to the laser light source and enters it, the laser oscillation becomes unstable. In order to block such reflected light, an optical isolator is used that uses a Faraday rotator that rotates the polarization plane of the reflected light non-reciprocally.

[0003] An optical isolator is primarily composed of a polarizer, an analyzer, a Faraday rotator, and a permanent magnet that magnetizes the Faraday rotator to induce a Faraday rotation angle. It is installed between a light source and an optical transmission component, and transmits only the light traveling toward the optical transmission component and blocks the reflected light returning toward the light source.

[0004] For example, the structure of a polarization-dependent optical isolator is such that a polarizer is provided on the input side and an analyzer is provided on the output side. A Faraday rotator (with an external magnet for applying a magnetic field) is placed between the polarizer and the analyzer, and rotates the polarization axis of the incident light by 45 degrees. The polarizer and the analyzer are also placed so that the relative angle between their transmission polarization axes is 45 degrees. The light incident on the optical isolator passes through the polarizer and the polarization plane is rotated 45 degrees by the Faraday rotator. After that, the polarization plane of the light coincides with that of the analyzer, so the light is transmitted through the analyzer.

[0005] On the other hand, the reflected light enters the analyzer and then the Faraday rotator. The Faraday rotator rotates the polarization plane of the reflected light by 45 degrees, making it perpendicular to the transmission polarization axis of the polarizer, so it cannot pass through the polarizer. Therefore, the reflected light cannot return to the laser light source, and does not adversely affect laser oscillation.

[0006] In optical communication, magnetic materials such as bismuth-substituted rare earth iron garnet single crystals are used as Faraday rotators in optical isolators, and extremely clean cleaning is required. After single crystal growth, circular wafers are cut into dimensions according to the required specifications using a cutting device such as a dicing saw, and generally cut into roughly square chip shapes with sides of 11 to 15 mm square. In the subsequent process, anti-reflective coatings are usually formed on both sides of the chips to make foreign body (particle) inspections more efficient. In order to form a high-quality anti-reflective coating, it is extremely important to completely remove particles adhering to the surface of the chips by cleaning before forming the anti-reflective coating. In the above-mentioned cutting process, a dicing device is usually used, but when cutting with a blade embedded with abrasive grains, powder from the work itself is generated, and in cutting, lapping, and polishing processes, fine powder from the work itself is also generated, and these are attached to the surface of the chips as particles. A method for cleaning these chips is to attach a large number of chips to a cleaning jig and clean them with an alkaline detergent or pure water using an ultrasonic cleaning device.

[0007] However, since the material of the Faraday rotator itself is an insulator, it is prone to adhesion of particles such as cutting powder, polishing powder, debris, and surrounding foreign matter generated by cutting and polishing processes, and the above-mentioned cleaning method cannot completely remove particles that were attached before cleaning from the chip. Furthermore, if an anti-reflection coating is formed with particles still attached to the chip, the anti-reflection coating is applied on top of the attached particles, and if the particles fall off for some reason, there is a major problem that the anti-reflection coating in that part will be chipped.

[0008] In addition, cleaning of Faraday rotators cannot be completed by ultrasonic cleaning equipment, so it is necessary to wipe off particles with a cotton swab or cleaning paper soaked in a solvent, which hinders the improvement of work efficiency.

[0009] To address such problems, Patent Document 1 describes a method for cleaning a Faraday rotator using an ultrasonic cleaning device by applying ultrasonic waves while the rotator is placed in a magnetic field. However, even if cleaning is performed by applying ultrasonic waves in a magnetic field, it is difficult to completely remove particles. This makes it difficult to form a high-quality anti-reflection film, and therefore to manufacture a high-quality Faraday rotator.

[0010] Moreover, Patent Document 2 describes a method in which the side portions of a Faraday rotator processed into a chip shape are covered and protected with a resin layer, and then cleaned with an ultrasonic cleaning device. However, although this method can prevent particles from adhering to the side surfaces, ultrasonic cleaning cannot completely remove particles adhering to the light transmitting surface, making it difficult to form a high-quality anti-reflection coating and, ultimately, to manufacture a high-quality Faraday rotator. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2007-121992 A [Patent Document 2] Patent No. 2849655 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide a method for manufacturing a Faraday rotator that can obtain a high-quality antireflection film when the antireflection film is formed on the light-transmitting surface of a Faraday rotator material. [Means for solving the problem]

[0013] In order to achieve the above object, the present invention provides a method for manufacturing a Faraday rotator capable of forming a high-quality Faraday rotator, particularly a Faraday rotator having a high-quality antireflection film. More specifically, the present invention provides a method for manufacturing a Faraday rotator, comprising a cutting step of cutting an approximately square flat plate (hereinafter referred to as "approximately square flat plate") to be used as a Faraday rotator from a single crystal circular wafer, and a cleaning step of the approximately square flat plate, in which the cleaning step includes spraying dry ice fine particles generated from liquefied carbon dioxide onto the light-transmitting surface, preferably the light-transmitting surface and side surfaces, of the approximately square flat plate, characterized in that, when an antireflection film is formed, dry ice fine particles, preferably having a particle diameter of 5 μm to 100 μm (more preferably 10 μm to 50 μm), generated from liquefied carbon dioxide, are sprayed onto the light-transmitting surface, preferably the light-transmitting surface and side surfaces of the approximately square flat plate (chip) to be used as a Faraday rotator, before the formation of the antireflection film.

[0014] The particle diameter of the dry ice particles is preferably 5 μm or more and 100 μm or less, because by making the size of the dry ice particles 100 μm or less, damage caused by spraying the approximately square flat plate used as the Faraday rotator is reliably eliminated, while particles less than 5 μm in size may result in incomplete particle removal.

[0015] By cleaning the approximately square plate with dry ice particles of this size, it becomes possible to remove particles that were not completely removed by an ultrasonic cleaning device, and a high-quality anti-reflection film can be obtained in the subsequent formation of the anti-reflection film. Thus, the present invention provides an improved method for manufacturing an optical isolator, and in particular, an improved method for manufacturing a Faraday rotator.

[0016] That is, the present invention provides a method for producing a flat Faraday rotator including a cleaning step, characterized in that in one of the cleaning steps, dry ice particles generated from liquefied carbon dioxide are sprayed onto the light transmitting surface of the approximately rectangular flat plate, preferably the light transmitting surface and side surfaces, for cleaning, and the dry ice particles at this time have a particle diameter of 5 μm to 100 μm, for example, as determined by a laser diffraction scattering method. In the laser diffraction scattering method, when a laser beam is applied to a particle group, concentric diffracted and scattered light is generated, and the distribution of the intensity depends on the particle diameter and the amount of the particles present. Therefore, the amount of the particles present is fixed, the intensity of the diffracted and scattered light is measured by a light receiving unit, and the particle diameter is determined from the distribution. In the present invention, the particle diameter of the dry ice particles is a diameter equivalent to 50% of the volume-based cumulative distribution of the dry ice particles determined by the laser diffraction scattering method. In addition, the dry ice particles used in the present invention can be formed by adiabatic expansion of high-pressure liquefied carbon dioxide and lowering the temperature using the Joule-Thomson effect.

[0017] In a preferred embodiment, the particle diameter of the dry ice particles is 10 μm or more and 50 μm or less.The pressure at which the dry ice particles are blown onto the approximately rectangular plate is preferably 0.1 MPa to 5 MPa.

[0018] The present invention also provides a method for manufacturing an optical isolator comprising a polarizer, an analyzer, a Faraday rotator, and a permanent magnet for magnetizing the Faraday rotator to induce a Faraday rotation angle, the Faraday rotator being produced by a process including a cutting step of cutting out an approximately square plate to be used as the Faraday rotator from a single crystal circular wafer, and a cleaning step of the approximately square plate, the cleaning step including spraying dry ice particles, preferably having a particle diameter of 5 μm to 100 μm (more preferably 10 μm to 50 μm) generated from liquefied carbon dioxide, onto the light transmitting surface of the approximately square plate, preferably the light transmitting surface and side surfaces, before forming an anti-reflection film, if any. The Faraday rotator is obtained by applying the above-mentioned method for manufacturing a Faraday rotator of the present invention, and then an optical isolator is manufactured according to a conventional method. Effect of the Invention

[0019] According to the present invention, before depositing an anti-reflection film on the light-transmitting surface of the Faraday rotator, dry ice particles are sprayed onto the light-transmitting surface (and side surface) of the approximately rectangular flat plate, which makes it possible to remove particles adhering to the light-transmitting surface of the Faraday rotator without damaging it, and thus makes it possible to deposit a high-quality anti-reflection film. If particles are found on the anti-reflection film, it is also possible to spray more dry ice particles onto the anti-reflection film. In this case, it is desirable that the diameter of the dry ice particles is a little over 5 μm, and that the dry ice spray pressure described below is a little over 0.1 MPa. [Brief description of the drawings]

[0020] [Figure 1] 1 is a conceptual diagram showing a cleaning step for a substantially square flat plate in the present invention and a cleaning device used in the cleaning step. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention will be described in more detail with reference to the drawings. FIG. 1 is a conceptual diagram showing one embodiment of a cleaning step of an approximately square flat plate 6 in the manufacturing method of a Faraday rotator of the present invention. As can be seen from this figure, the approximately square flat plate 6 is cleaned by spraying dry ice particles onto the light-transmitting surface of the approximately square flat plate 6. As shown in FIG. 1, liquefied carbon dioxide gas in a liquefied carbon dioxide gas container 1 is supplied to a dry ice cleaning device 3 (manufactured by Air Water Corporation) together with nitrogen as a propellant gas in a nitrogen container 2, and is sprayed as dry ice particles from a cleaning nozzle 5 onto the approximately square flat plate 6 through a cleaning nozzle unit 4. In FIG. 1, the light-transmitting surface of the approximately square flat plate 6 is cleaned, but the side of the approximately square flat plate 6 can be cleaned by changing the direction of the nozzle 5 or the arrangement of the approximately square flat plate 6. In the present invention, the side of the approximately square flat plate refers to a surface other than the light-transmitting surface of the approximately square flat plate.

[0022] The pressure at which the dry ice particles are sprayed is preferably 0.1 MPa to 5 MPa. If the pressure is less than 0.1 MPa, the pressure may be insufficient and it may not be possible to remove particles adhering to the light transmitting surface. If the pressure exceeds 5 MPa, the pressure may be too high and may damage the approximately square flat plate used as the Faraday rotator.

[0023] The particle diameter of the dry ice particles measured by the laser diffraction scattering method is preferably in the range of 5 μm to 100 μm, more preferably in the range of 10 μm to 50 μm. If the particle diameter of the dry ice particles is less than 5 μm, the sublimation energy of the dry ice particles may be insufficient due to the sudden temperature change when the particles collide with the light-transmitting surface of the approximately square flat plate used as the Faraday rotator, and the removal effect by scattering the particles attached to the light-transmitting surface may not be obtained. On the other hand, if the particle diameter exceeds 100 μm, the impact when colliding with the light-transmitting surface becomes large, and the approximately square flat plate used as the Faraday rotator may be damaged. In addition, the size of the dry ice particles can be adjusted by selecting the nozzle shape for irradiating the dry ice. EXAMPLES

[0024] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0025] [Example 1] A bismuth-substituted rare earth iron garnet single crystal film was grown on a 3-inch circular Ca, Mg, Zr, etc.-substituted gadolinium-gallium-garnet substrate (SGGG substrate: manufactured by Saint-Gobain) by liquid phase epitaxy. Next, to adjust the thickness of the bismuth-substituted rare earth iron garnet single crystal film, both sides were polished to a thickness of 0.4 mm and the light-transmitting surface was polished to a mirror finish. Next, cutting was performed using a 0.1 mm thick diamond blade (#600) in a peripheral blade dicing device, and 20 chips measuring 11 mm x 11 mm square were obtained from one bismuth-substituted rare earth iron garnet single crystal film. The light transmitting surface of the chip was cleaned by spraying dry ice particles at a pressure of 3 MPa using a dry ice cleaning device (QuickSnow QS-A1321F-R: manufactured by Air Water Corporation) equipped with a spray nozzle with a dry ice particle diameter of 30 μm (laser diffraction scattering method). After that, the chip was set in a vacuum deposition device and an anti-reflective film was formed on the surface of the chip using ion beam assistance. The formed anti-reflective film was observed with an optical microscope to determine the number of missing parts of the anti-reflective film and the number of chips around the chip.

[0026] [Example 2] An anti-reflective film was formed on the surface of the chip in the same manner as in Example 1, except that in addition to the light-transmitting surface of the chip, the side surfaces were also cleaned by spraying dry ice particles onto them, and the number of missing areas in the anti-reflective film and the number of chips in the peripheral area of ​​the chip were determined.

[0027] [Example 3] An anti-reflective film was formed on the surface of the chip in the same manner as in Example 2, except that the chip was cleaned by spraying dry ice particles using a dry ice cleaning device equipped with a spray nozzle with a dry ice particle diameter of 10 μm (laser diffraction scattering method), and the number of missing areas in the anti-reflective film and chips in the periphery of the chip were determined.

[0028] [Example 4] An anti-reflective film was formed on the surface of the chip in the same manner as in Example 2, except that the chip was cleaned by spraying dry ice particles using a dry ice cleaning device equipped with a spray nozzle with a dry ice particle diameter of 90 μm (laser diffraction scattering method), and the number of missing areas in the anti-reflective film and chips in the periphery of the chip were determined.

[0029] [Example 5] An anti-reflective film was formed on the surface of the chip in the same manner as in Example 2, except that the chip was cleaned by spraying dry ice particles using a dry ice cleaning device equipped with a spray nozzle with a dry ice particle diameter of 1 μm (laser diffraction scattering method), and the number of missing areas in the anti-reflective film and chips in the periphery of the chip were determined.

[0030] [Example 6] An anti-reflective film was formed on the surface of the chip in the same manner as in Example 2, except that the chip was cleaned by spraying dry ice particles using a dry ice cleaning device equipped with a spray nozzle with a dry ice particle diameter of 150 μm (laser diffraction scattering method), and the number of missing areas in the anti-reflective film and chips in the periphery of the chip were determined.

[0031] [Example 7] An anti-reflective film was formed on the surface of the chip in the same manner as in Example 2, except that cleaning was performed by spraying dry ice particles at a pressure of 0.08 MPa, and the number of missing areas in the anti-reflective film and chips in the periphery of the chip were counted.

[0032] [Example 8] An anti-reflective film was formed on the surface of the chip in the same manner as in Example 2, except that cleaning was performed by spraying dry ice particles at a pressure of 6 MPa, and the number of missing areas in the anti-reflective film and chips in the periphery of the chip were counted.

[0033] [Comparative Example 1] Instead of the dry ice cleaning device, a cleaning solution of ammonia water, hydrogen peroxide water, and ultrapure water mixed in a ratio of 1:1:18 (volume ratio) was prepared in an ultrasonic cleaning device, and the chip was immersed in the cleaning solution heated to 60°C and ultrasonic cleaning was performed for 20 minutes under conditions of 950 kHz and 1200 W. After that, spin drying with alcohol was performed, and then the chip was set in a vacuum deposition device to form an anti-reflection film on the surface of the chip, as in Example 2. The formed anti-reflection film was observed with an optical microscope, and the number of missing parts of the anti-reflection film and the number of chips around the chip were determined.

[0034] The results of the above Examples 1 to 8 and Comparative Example 1 are shown in Table 1. The numbers in the table for the number of missing anti-reflection film locations and the number of chips around the chip are the average values ​​for five cleaned 11 mm × 11 mm square chips in Examples 1 to 8 and Comparative Example 1.

[0035] [Table 1]

[0036] [Results / Discussion] In Examples 1 to 4, dry ice particles with a particle size of 10 μm to 90 μm were used, and the Faraday rotator was washed before forming the anti-reflection film, which prevented the occurrence of defects due to chipping of the anti-reflection film after formation. In addition, no damage to the Faraday rotator was confirmed during cleaning by spraying dry ice particles.

[0037] On the other hand, in Examples 5 and 6, which used dry ice particles not having the particle size defined as preferred in the present invention, there were a somewhat large number of defects due to chipping of the anti-reflection film after the anti-reflection film was formed, or the number of defects due to chipping around the chip was somewhat large, and the results were inferior to those of the other Examples. Also, in Comparative Example 1, which does not employ the present invention and in which cleaning was performed with an ultrasonic cleaning device using a cleaning liquid instead of dry ice cleaning, the results were extremely inferior to all the Examples.

[0038] Furthermore, in Example 7, where the pressure with which the dry ice particles were blown was insufficient, there was some damage to the anti-reflection film, which was probably due to the particles not being completely removed, while in Example 8, where the blowing pressure was excessive, there was some chipping of the chip. However, these two examples were not so severe as to invalidate the effect of the present invention.

[0039] As described above, it was found that by spraying dry ice microparticles with a particle diameter of 5 μm to 100 μm generated from liquefied carbon dioxide onto the light-transmitting surface (and sides) of the chip to clean it before forming an anti-reflective film on the light-transmitting surface of the chip to be used as a Faraday rotator, it is possible to form a high-quality anti-reflective film with no missing parts after formation.

[0040] The present invention is not limited to the above embodiment. The process before forming the anti-reflection film is described because the formation of the anti-reflection film makes it easier to observe particles under a microscope, but the cleaning and removal effect of adhering particles is effective regardless of whether the anti-reflection film is formed or not. In particular, the removal effect is large when using an insulating material such as in this embodiment.

[0041] The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention. [Explanation of symbols]

[0042] 1...Liquid carbon dioxide gas container 2. Nitrogen container 3. Dry ice cleaning equipment 4…Cleaning nozzle unit 5…Cleaning nozzle 6…Substantially rectangular flat plate

Claims

1. A method for manufacturing a Faraday rotator, comprising: a cutting step of cutting a substantially rectangular flat plate to be used as a Faraday rotator from a single crystal circular wafer; and a cleaning step of the substantially rectangular flat plate, the step of cleaning the substantially rectangular flat plate includes spraying dry ice particles generated from liquefied carbon dioxide gas onto at least the light transmitting surface of the substantially rectangular flat plate; The particle diameter of the dry ice fine particles measured by a laser diffraction scattering method is 5 μm or more and 100 μm or less, and A method for manufacturing a Faraday rotator, characterized in that the pressure at which the dry ice fine particles are blown is 3 MPa or more and 5 MPa or less.

2. 2. The method for manufacturing a Faraday rotator according to claim 1, wherein the dry ice particles are also sprayed onto the side surfaces of the approximately rectangular plate.

3. 3. The method for manufacturing a Faraday rotator according to claim 1, wherein the particle diameter of the dry ice particles measured by a laser diffraction scattering method is 10 μm or more and 50 μm or less.

4. 3. The method for manufacturing a Faraday rotator according to claim 1, further comprising the step of forming an anti-reflection film on the light transmitting surface of the approximately square plate after the step of cleaning the approximately square plate.

5. 5. The method for producing a Faraday rotator according to claim 4, wherein the particle diameter of the dry ice particles measured by a laser diffraction scattering method is 10 μm or more and 50 μm or less.

6. A method for manufacturing an optical isolator including a polarizer, an analyzer, a Faraday rotator, and a permanent magnet for magnetizing the Faraday rotator to induce a Faraday rotation angle, comprising the steps of: the Faraday rotator is produced by a process including a cutting step of cutting a substantially rectangular flat plate to be used as the Faraday rotator from a single crystal circular wafer, and a cleaning step of the substantially rectangular flat plate; the step of cleaning the substantially rectangular flat plate includes spraying dry ice particles generated from liquefied carbon dioxide gas onto at least the light transmitting surface of the substantially rectangular flat plate; The particle diameter of the dry ice fine particles measured by a laser diffraction scattering method is 5 μm or more and 100 μm or less, and A method for manufacturing an optical isolator, characterized in that the pressure at which the dry ice fine particles are blown is 3 MPa or more and 5 MPa or less.

7. 7. The method for manufacturing an optical isolator according to claim 6, wherein the dry ice particles are also sprayed onto the side surfaces of the substantially rectangular plate.

8. 8. The method for producing an optical isolator according to claim 6, wherein the particle diameter of the dry ice particles measured by a laser diffraction scattering method is 10 μm or more and 50 μm or less.

9. 8. The method for manufacturing a Faraday rotator according to claim 6, further comprising the step of forming an anti-reflection film on the light transmitting surface of the approximately square plate after the step of cleaning the approximately square plate.

10. 10. The method for manufacturing a Faraday rotator according to claim 9, wherein the particle diameter of the dry ice particles measured by a laser diffraction scattering method is 10 μm or more and 50 μm or less.

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