Method of manufacturing faraday rotator and 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 supercooled droplets and frozen particles ejected from a Laval nozzle equipped with a compressed air supply port and a water supply port onto a light transmissive surface, or the light transmissive surface and side surfaces, of a flat plate, a precursor of a Faraday rotator, 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 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] Laser light sources used in optical processing machines, optical measuring instruments, and optical communications can emit laser light that is reflected by the surface of a component along the transmission path, causing laser oscillation to become unstable if some of the reflected light returns to the laser light source.To block this reflected light, an optical isolator is used that uses a Faraday rotator, which rotates the polarization plane of the reflected light non-reciprocally.

[0003] An optical isolator is mainly 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 light traveling toward the optical transmission component, while blocking reflected light returning toward the light source.

[0004] For example, a polarization-dependent optical isolator has a polarizer on the input side and an analyzer on the output side. A Faraday rotator is placed between the polarizer and the analyzer to rotate the polarization plane of the incident light by 45 degrees. The polarizer and the analyzer are also placed so that their polarization directions differ by 45 degrees from each other.

[0005] Light incident on the optical isolator passes through a polarizer and its polarization plane is rotated by 45 degrees by the Faraday rotator. This rotation causes the polarization plane of the light to match the polarization plane of the analyzer, allowing the light to pass through the analyzer.

[0006] The reflected light enters the analyzer from the opposite direction, and then enters the Faraday rotator. The Faraday rotator rotates the polarization plane of the reflected light by 45 degrees, making it perpendicular to the polarization plane 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.

[0007] In optical communications, magnetic materials such as bismuth-substituted rare-earth iron garnet single crystals are used as Faraday rotators in optical isolators, and they require extremely clean cleaning. In the manufacture of Faraday rotators, circular wafers obtained after single crystal growth are cut into chip shapes with dimensions determined according to the required specifications, typically 11 to 15 mm square, using a cutting device such as a dicing saw. In the subsequent processing step, an anti-reflection coating is then formed on both sides of the chip. To ensure high-quality anti-reflection coating, it is extremely important to thoroughly remove particles adhering to the chip surface by cleaning before depositing the anti-reflection coating.

[0008] The cutting process typically uses a dicing machine, but when cutting with a blade embedded with abrasive grains, dust from the workpiece itself is generated, which adheres to the surface of the chip as particles. Furthermore, when cutting, lapping, or polishing, fine powder from the workpiece itself is generated, which also adheres to the surface of the chip as particles. The chips are cleaned by attaching a large number of chips to a cleaning jig and cleaning them with alkaline detergent or pure water using an ultrasonic cleaning device.

[0009] To address the above-mentioned problems, Patent Document 1 describes a method of cleaning a Faraday rotator with an ultrasonic cleaner by applying ultrasonic waves while placing the rotator in a magnetic field with a magnetic flux density of 500 G (gauss) or more.

[0010] Furthermore, Patent Document 2 describes a method in which the side surfaces of a Faraday rotator processed into a chip shape are covered with a resin layer for protection, and then cleaned with an ultrasonic cleaner. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-121992 [Patent Document 2] Patent No. 2849655 Summary of the Invention [Problem to be solved by the invention]

[0012] However, since the Faraday rotator itself is an insulator, particles such as cutting powder and polishing powder from the workpiece itself generated during cutting and polishing processes tend to adhere to the Faraday rotator, and the cleaning method described above was unable to completely remove the particles that had adhered before cleaning. Furthermore, if an anti-reflection coating is deposited while particles remain attached to the chip, there is a major problem in that the particles may fall off for some reason, causing the anti-reflection coating to break off, since the anti-reflection coating is applied on top of the particles that have adhered.

[0013] Faraday rotators cannot be cleaned by ultrasonic cleaning equipment, and require the use of cotton swabs or cleaning paper soaked in solvents to wipe off particles, which also reduces work efficiency.

[0014] To address this problem, the aforementioned Patent Document 1 proposes a method of cleaning a Faraday rotator using an ultrasonic cleaner by applying ultrasonic waves while the rotator is placed in a magnetic field. However, even if ultrasonic waves are applied in such a magnetic field, it is difficult to completely remove particles, and it is difficult to deposit a high-quality anti-reflection film.

[0015] Furthermore, the aforementioned Patent Document 2 proposes a method of covering and protecting the side surfaces of a Faraday rotator processed into a chip shape with a resin layer and then cleaning the surface with an ultrasonic cleaner. Although this method can prevent particles from adhering to the side surfaces, cleaning by ultrasonic cleaning cannot completely remove particles adhering to the light-transmitting surface, making it difficult to deposit a high-quality anti-reflection film.

[0016] Another known method is the dry ice jet method, in which dry ice particles generated from carbon dioxide gas are sprayed onto the area to be cleaned to remove any particles that have adhered to it. However, when particle removal using this method was attempted, it was found that while it was able to remove micron-sized particles, it was unable to completely remove submicron-sized particles (particularly the cutting chips generated during the cutting process). Furthermore, because dry ice particles easily sublimate into gas, it was necessary to take care in the usage environment, such as by installing a local exhaust system, to prevent oxygen deficiency due to the carbon dioxide generated from the dry ice.

[0017] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a Faraday rotator and an optical isolator manufacturing method that can form a high-quality antireflection film by vapor deposition when an antireflection film is vapor-deposited on the Faraday rotator. [Means for solving the problem]

[0018] As a result of extensive research into achieving the above object, the present inventors have found that cleaning a flat plate, which is a precursor of a Faraday rotator, with supercooled droplets and frozen particles ejected from a de Laval nozzle makes it possible to remove particles, particularly minute particles of submicron size, which cannot be completely removed by an ultrasonic cleaner or dry ice jet, and that a high-quality antireflection coating can be obtained by the subsequent vapor deposition of an antireflection coating, which has led to the present invention.

[0019] Therefore, the present invention provides the following Faraday rotator and optical isolator manufacturing method. [1] A method for manufacturing a Faraday rotator, comprising: a cutting step of cutting a flat plate to be used as a Faraday rotator from a single crystal wafer; and a cleaning step of the flat plate, a cleaning method for a Faraday rotator, characterized in that in the step of cleaning the flat plate, supercooled droplets and frozen particles ejected from a Laval nozzle equipped with a supply port for compressed air and water are sprayed onto at least the light transmitting surface of the flat plate. [2] The method for manufacturing a Faraday rotator according to [1] above, wherein the supercooled droplets and frozen particles ejected from the Laval nozzle are also sprayed onto the side surface of the flat plate. [3] The method for manufacturing a Faraday rotator according to [1] or [2] above, wherein the amount of water supplied to the Laval nozzle is 1 ml / min to 20 ml / min. [4] A method for manufacturing an optical isolator including a polarizer, an analyzer, a Faraday rotator, and a permanent magnet that magnetizes the Faraday rotator to induce a Faraday rotation angle, comprising the steps of: the Faraday rotator is formed by a process including a cutting step of cutting a flat plate to be used as the Faraday rotator from a single crystal wafer and a cleaning step of the flat plate, A method for manufacturing an optical isolator, characterized in that the step of cleaning the flat plate includes spraying supercooled droplets and frozen particles ejected from a Laval nozzle equipped with compressed air and water supply ports onto at least the light-transmitting surface of the flat plate. [5] The method for manufacturing an optical isolator according to [4], wherein the supercooled droplets and frozen particles ejected from the Laval nozzle are also sprayed onto the side surface of the flat plate. [6] The method for manufacturing an optical isolator according to [4] or [5] above, wherein the amount of water supplied to the Laval nozzle is 1 ml / min to 20 ml / min. [Effects of the Invention]

[0020] According to the present invention, before depositing an antireflection film on the light-transmitting surface of a Faraday rotator, supercooled droplets and frozen particles ejected from a Laval nozzle are sprayed onto the light-transmitting surface (preferably also onto the side surface) of the flat plate, thereby making it possible to remove minute particles adhering to the light-transmitting surface of the Faraday rotator without damaging it, and thus making it possible to deposit and form a high-quality antireflection film. [Brief explanation of the drawings]

[0021] [Figure 1]FIG. 1 is a conceptual diagram showing a cleaning step of a flat plate used as a Faraday rotator and a Laval nozzle used in the cleaning step in one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be specifically described below with reference to the drawings.

[0023] FIG. 1 is a conceptual diagram illustrating a cleaning process for a substantially rectangular flat plate 4 used as a Faraday rotator according to one embodiment of the present invention, and a Laval nozzle 3 used in the cleaning process. The Laval nozzle 3, which includes a compressed air supply port 1 and a water supply port 2, sprays supercooled droplets (supercooled water droplets) and frozen particles (ice particles) onto the light-transmitting surface of the substantially rectangular flat plate 4 for cleaning. Compressed air and water are supplied to the Laval nozzle 3, a two-fluid nozzle, through the compressed air supply port 1 and the water supply port 2, respectively. The supplied compressed air undergoes adiabatic expansion in a divergent region within the nozzle, lowering its temperature. Meanwhile, the supplied water is atomized into minute water droplets, which are accelerated and cooled by the flow of low-temperature compressed air, forming minute supercooled droplets (average droplet size: 5-10 μm) and ice particles. Commercially available Laval nozzles include those manufactured by Ricks Corporation. The flow path of compressed air inside the pipe of the Laval nozzle 3 is made up of a convergent portion, a throat portion, and a divergent portion (not shown), and forms a Laval nozzle shape.

[0024] 1, the light-transmitting surface of the substantially square flat plate 4 is cleaned, but by changing the direction of the Laval nozzle 3 or the position of the substantially square flat plate 4, the side surface of the substantially square flat plate 4 can also be cleaned. By spraying the side surface of the flat plate as well, the effect of removing minute particles adhering to the flat plate can be further enhanced. In the present invention, the side surface of the flat plate refers to the surface other than the light-transmitting surface of the flat plate.

[0025] In the present invention, as described above, water is accelerated by compressed air through a Laval nozzle 3 to generate minute supercooled droplets and ice particles, which are then sprayed from the nozzle outlet (spray port) (average velocity at the nozzle outlet: typically 400 to 450 m / s). The supercooled droplets sprayed from the Laval nozzle outlet freeze upon impact with the workpiece (chip) and turn into sherbet. At this time, they form ice blocks, enveloping particles adhering to the workpiece surface. Subsequently, ice particles and air collide with the ice blocks containing the particles, blowing them away and removing the particles. Cleaning the workpiece with supercooled droplets and ice particles sprayed from the Laval nozzle in this way enables the removal of particles, particularly submicron-sized particles (such as cutting dust generated during the cutting process), which is incomplete with ultrasonic cleaners or dry ice jets. This allows for the subsequent deposition of a high-quality anti-reflective coating. Furthermore, because no carbon dioxide is used, the process reduces environmental impact and running costs.

[0026] The amount of water supplied to the Laval nozzle 3 is preferably 1 ml / min to 20 ml / min, and more preferably 5 ml / min to 15 ml / min. If the amount is less than 1 ml / min, the particle capture efficiency decreases when the supercooled droplets freeze upon impact with the Faraday rotator, and there is a risk that particles adhering to the light-transmitting surface will not be sufficiently removed. Furthermore, if the amount of water exceeds 20 ml / min, the amount of water is too large and the generation of frozen particles may be insufficient.

[0027] The pressure of the compressed air supplied to the Laval nozzle 3 is preferably 0.2 MPa to 1.0 MPa, and more preferably 0.4 MPa to 0.6 MPa. If the pressure is less than 0.2 MPa, the particle capture efficiency decreases when the supercooled droplets freeze upon impact with the Faraday rotator, and there is a risk that particles adhering to the light-transmitting surface will not be sufficiently removed. Furthermore, if the pressure exceeds 1.0 MPa, the pressure will be too strong and there is a risk that the generation of frozen particles will be insufficient.

[0028] The flow rate of the compressed air supplied to the Laval nozzle 3 is preferably 200 NL / min to 900 NL / min, and more preferably 400 NL / min to 600 NL / min. If the flow rate is less than 200 NL / min, the particle capture efficiency decreases when the supercooled droplets freeze upon impact with the Faraday rotator, and there is a risk that particles adhering to the light-transmitting surface will not be sufficiently removed. Furthermore, if the flow rate exceeds 900 NL / min, the air flow rate will be too high and the generation of frozen particles may be insufficient.

[0029] The temperature of the compressed air and water supplied to the Laval nozzle 3 is preferably in the range of 20° C. to 30° C. If it is outside this range, there is a risk that particles will not be sufficiently removed.

[0030] In the method for manufacturing a Faraday rotator according to the first embodiment of the present invention, the Faraday rotator to be manufactured is not particularly limited. For example, Y3Fe5O 12 (YIG), Y3Al5O 12 (YAG), Tb3Ga5O 12 The Faraday rotator is made of a bismuth-substituted rare earth iron garnet single crystal such as bismuth-substituted rare earth iron garnet (TGG). In many cases, an anti-reflection film is applied to both light-transmitting surfaces of the Faraday rotator, and the manufacturing method of the present invention is preferably applied to such cases.

[0031] A second aspect of the present invention is a method for manufacturing an optical isolator including a polarizer, an analyzer, a Faraday rotator, and a permanent magnet that magnetizes the Faraday rotator to induce a Faraday rotation angle, wherein the Faraday rotator is manufactured by the manufacturing method according to the first aspect of the present invention. In the method for manufacturing the optical isolator, a Faraday rotator is obtained by applying the method for manufacturing a Faraday rotator of the present invention described above, and then an optical isolator is manufactured according to a conventional method. [Example]

[0032] EXAMPLES The present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0033] [Example 1] A bismuth-substituted rare earth iron garnet single crystal film was grown by liquid phase epitaxy on a 3-inch circular gadolinium-gallium garnet substrate (SGGG substrate: manufactured by Saint-Gobain) substituted with Ca, Mg, Zr, etc. Then, to adjust the thickness of the bismuth-substituted rare earth iron garnet single crystal film, the substrate was polished on both sides to a thickness of 0.4 mm and to a mirror finish on both sides. Next, the bismuth-substituted rare earth iron garnet single crystal film was cut using a 0.1 mm thick diamond blade (#600) in an outer blade dicing device, and 20 chips measuring 11 mm x 11 mm were obtained from one piece of the bismuth-substituted rare earth iron garnet single crystal film. Next, the light-transmitting surface and side surfaces of the chip were cleaned by spraying supercooled droplets and frozen particles formed from supplied compressed air (0.6 MPa, 500 NL / min) and water (distilled water) using a cleaning device (manufactured by RIX) equipped with a Laval nozzle (the supply temperature of the compressed air and water was 25°C). At this time, water was supplied to the Laval nozzle at 5 ml / min. The chip was then placed in a vacuum deposition device, and an anti-reflective film was deposited on the chip surface. The deposited anti-reflective film was observed under an optical microscope to determine any missing areas in the anti-reflective film and the number of chips in the peripheral area.

[0034] [Example 2] The chip was cleaned and an anti-reflection film was deposited in the same manner as in Example 1, except that water was supplied to the Laval nozzle at a rate of 15 ml / min. The deposited anti-reflection film was observed under an optical microscope to determine the number of missing areas and peripheral chips in the anti-reflection film.

[0035] [Example 3] The chip was cleaned and an antireflection film was deposited in the same manner as in Example 1, except that water was supplied to the Laval nozzle at a rate of 1 ml / min. The deposited antireflection film was observed under an optical microscope to determine the number of missing areas and peripheral chips in the antireflection film.

[0036] [Example 4] The chip was cleaned and an anti-reflection film was deposited in the same manner as in Example 1, except that water was supplied to the Laval nozzle at a rate of 20 ml / min. The deposited anti-reflection film was observed under an optical microscope to determine the number of missing areas and peripheral chips in the anti-reflection film.

[0037] [Example 5] The chip was cleaned and an antireflection film was deposited in the same manner as in Example 1, except that water was supplied to the Laval nozzle at a rate of 25 ml / min. The deposited antireflection film was observed under an optical microscope to determine the number of missing areas and peripheral chips in the antireflection film.

[0038] [Comparative Example 1] Instead of a cleaning device equipped with a Laval nozzle, a cleaning solution containing a 1:1:18 mixture of ammonia water, hydrogen peroxide, and ultrapure water was prepared in an ultrasonic cleaning device. The chip obtained in the same manner as in Example 1 was immersed in the cleaning solution heated to 60°C and ultrasonically cleaned for 20 minutes at 950 kHz and 1200 W. After spin drying with alcohol, the chip was placed in a vacuum deposition device and an anti-reflection film was deposited on the chip surface, as in Example 1. The deposited anti-reflection film was observed with an optical microscope to determine the number of missing areas and peripheral chips in the anti-reflection film.

[0039] The results of Examples 1 to 5 and Comparative Example 1 are shown in Table 1. The numbers in the table are the average values ​​of the five chips obtained by cutting in Examples 1 to 5 and Comparative Example 1.

[0040] [Table 1]

[0041] (Results / Discussion) In Examples 1 to 5, by using a Laval nozzle and cleaning the Faraday rotator before depositing the antireflection coating, it was possible to minimize the occurrence of defects due to missing parts of the antireflection coating after deposition. In addition, no damage to the Faraday rotator due to the cleaning was confirmed. On the other hand, in Comparative Example 1, which was cleaned with an ultrasonic cleaner using a chemical solution, the results were inferior to those of Examples 1 to 5. From these findings, it is believed that the present invention can remove even minute submicron-sized particles that are generated during cutting processes, etc., which have been difficult to remove with conventional cleaning methods. As described above, it has been found that, before depositing an antireflection film on the light-transmitting surface and side surface of a chip to be used as a Faraday rotator, the light-transmitting surface and side surface of the chip are cleaned by spraying supercooled droplets and ice particles ejected from a Laval nozzle onto them, and then the antireflection film is deposited, thereby preventing any missing parts of the antireflection film after deposition and enabling the deposition of a high-quality antireflection film.

[0042] The present invention is not limited to the above-described embodiments. 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 that exhibits similar effects is included within the technical scope of the present invention. [Explanation of symbols]

[0043] 1 Compressed air supply port 2 Water supply port 3. Laval nozzle 4 Almost square plate

Claims

1. A method for manufacturing a Faraday rotator, comprising: a cutting step of cutting a flat plate to be used as a Faraday rotator from a single crystal wafer; and a cleaning step of the flat plate, a cleaning method for a Faraday rotator, characterized in that in the step of cleaning the flat plate, supercooled droplets and frozen particles ejected from a Laval nozzle equipped with a supply port for compressed air and water are sprayed onto at least the light transmitting surface of the flat plate.

2. 2. The method for manufacturing a Faraday rotator according to claim 1, wherein the supercooled droplets and frozen particles ejected from the Laval nozzle are also sprayed onto a side surface of the flat plate.

3. 3. The method for manufacturing a Faraday rotator according to claim 1, wherein the amount of water supplied to the Laval nozzle is 1 ml / min to 20 ml / min.

4. 1. A method for manufacturing an optical isolator including a polarizer, an analyzer, a Faraday rotator, and a permanent magnet that magnetizes the Faraday rotator to induce a Faraday rotation angle, comprising: the Faraday rotator is formed by a process including a cutting step of cutting a flat plate to be used as the Faraday rotator from a single crystal wafer and a cleaning step of the flat plate, A method for manufacturing an optical isolator, characterized in that the step of cleaning the flat plate includes spraying supercooled droplets and frozen particles ejected from a Laval nozzle equipped with compressed air and water supply ports onto at least the light-transmitting surface of the flat plate.

5. 5. The method for manufacturing an optical isolator according to claim 4, wherein the supercooled droplets and frozen particles ejected from the Laval nozzle are also sprayed onto the side surface of the flat plate.

6. 6. The method for manufacturing an optical isolator according to claim 4, wherein the amount of water supplied to the Laval nozzle is 1 ml / min to 20 ml / min.

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