JP2006208710A - Optical isolator element, its manufacturing method, and fiber with optical isolator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem in which back-scattering light from an optical isolator element 1 becomes return light to an optical fiber or to a laser to cause instability of laser output. <P>SOLUTION: In the optical isolator element 1 in which a polarizer with metallic particles diffused and a Faraday rotator are adhered to each other, the metallic particles of the polarizer are less on the non-joint face side than on the joint face side to the Faraday rotator. Consequently, production of unnecessary scattering light is reduced, and the problem of lowered isolation and the return light to the optical fiber and to the laser are solved. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical isolator element used for removing return light generated when light emitted from a light source is introduced into various optical elements and optical fibers, a manufacturing method thereof, and a fiber with an optical isolator.

  In an optical communication module or the like, light emitted from a light source such as a laser light source is incident on various optical elements or optical fibers, but a part of the incident light is reflected on the end surfaces or inside of the various optical elements or optical fibers. It is scattered. A part of the reflected or scattered light tries to return to the light source as return light, and an optical isolator is used to prevent the return light.

  Conventionally, this type of optical isolator is configured by installing a flat Faraday rotator between two polarizers and storing these three components in a cylindrical magnet via a component holder. It was. Normally, the Faraday rotator is adjusted to a thickness that rotates the polarization plane of light having a predetermined wavelength in the saturation magnetic field by 45 °, and the two polarizers rotate so that their transmission polarization directions are shifted by 45 ° rotation direction. Coordinated and configured.

  The optical isolator having such a configuration requires a Faraday rotator and two polarizers as separate parts and a holder for each element, which increases the number of parts and the number of assembling steps. The adjustment work is complicated and incurs high costs. It was difficult to reduce the size.

  For this reason, an optical isolator has been proposed in which an optical isolator element having a configuration in which flat polarizers are bonded and integrated on both sides of a flat Faraday rotator is arranged in the center of a cylindrical magnet.

  A conventional miniaturized optical isolator element 15 shown in FIG. 4 is shown, and its configuration will be described below (Patent Document 1).

  The optical isolator element 15 is composed of an optical isolator element 20 in which a Faraday rotator 16 and polarizers 17 and 18 are bonded with an optical adhesive 19 having a good light transmittance and a controlled refractive index, and a cylindrical magnet 21. Here, the polarizers 17 and 18 have a function of absorbing a polarization component in one direction of transmitted light and transmitting a polarization component orthogonal to the polarization component, and the Faraday rotator 16 has a saturation magnetic field strength. 1 has a function of rotating the polarization plane of light of a predetermined wavelength by about 45 degrees.

  The two polarizers 17 and 18 are arranged so that their transmission polarization directions are deviated by about 45 degrees. As the polarizer, a glass polarizer in which metal particles are dispersed in a glass substrate is used.

  This is a glass having polarization characteristics by arranging metal particles stretched to a length of about submicron in one direction in the glass itself. Light having a polarization plane perpendicular to the extending direction of the metal particles is transmitted, and light having a parallel polarization plane is absorbed. Since the metal particles in the glass are precipitated by reducing the metal halide, the metal halide is reduced to metal particles and exhibits polarization characteristics at a thickness of about 50 μm on both surfaces of the glass.

  FIG. 5 is a view showing a method of manufacturing the conventional optical isolator element 20.

  First, as shown in FIGS. 5A and 5B, a 10 mm square polarizer substrate 22, a Faraday rotator substrate 23, and a polarizer substrate 24 are bonded and integrated. Here, the transmission polarization direction of the polarizer substrate 22 is set to a direction parallel to one side, and the transmission polarization direction of the polarizer substrate 24 is set to a direction of 45 degrees on one side. Each optical substrate is fixed by adhering the polarizer substrate 22, the Faraday rotator 23, and the polarizer substrate 24 so that one side of each is parallel.

  Further, as described above, an optically transparent resin is used as an adhesive for bonding and integrating the optical elements, and generally an epoxy or acrylic organic adhesive is used.

  Here, when high isolation is required for the optical isolator element 20, the rotational deviation between the polarizer substrate 22 and the polarizer substrate 24 is precisely 45-α degrees with respect to the polarization rotation angle 45 + α degrees of the Faraday rotator. It is necessary to adjust to. Specifically, light is incident from the opposite direction (from the side of the polarizer substrate 24), and the polarizer substrate 22 and the polarizer substrate 24 are rotationally adjusted so that the transmitted light is minimized.

  Next, as shown in FIGS. 5C and 5D, the optical isolator element substrate 25 is processed into a large number of small chip-like optical isolator elements 20 by dicing or the like.

  When manufacturing this optical isolator element 20, a method is used in which large polarizer substrates and Faraday rotator substrates are alternately stacked, and after the completion of bonding, a large number of optical isolator elements 20 are obtained. As a result, workability and production volume can be increased, and the number of parts can be further reduced.

In addition, as the optical adhesive, generally, an epoxy or acrylic organic adhesive is used.
JP-A-4-338916

  However, in the conventional optical isolator and the fiber with the optical isolator, reflection different from the reflection (Fresnel reflection) from the junction interface or the surface of each optical element may occur. This phenomenon becomes more prominent as the isolator is arranged at the light condensing position.

  As the conventional polarizers 17 and 18, an absorption polarizer having a function of absorbing a unidirectional polarization component of incident light is used. In the absorption polarizer, ellipsoidal metal particles are dispersed in glass. Made of polarizing glass having a different structure. This polarizing glass is a glass having polarization characteristics by arranging long stretched metal particles in one direction in the glass itself, light having a polarization plane perpendicular to the stretch direction of the metal particles is transmitted, Light with a parallel polarization plane is absorbed. This polarization characteristic is caused by the light-plasma resonance of the metal particles, and the resonance state changes depending on the major axis and minor axis directions of the elliptical metal particles. The metal particles have a minor axis length of about 100 nm and a major axis length of about 500 nm. This metal particle absorbs light having a plane of polarization parallel to the long axis and simultaneously generates scattered light.

  Scattered light usually has no traveling direction, so it does not return directly to the semiconductor laser or transmission line as reflected return light. However, when the scattered reflection point is at the near end of the optical fiber, In this case, there is a problem that part of the scattered light becomes return light.

  Specifically, when the optical isolator element 20 is used as an isolator with an optical fiber, a polarizer is disposed at the near end of the fiber and at the condensing position, so that the isolation is lowered, that is, the optical fiber side or the laser side is returned. There is a problem that it becomes light and causes instability of the laser output.

  Further, the conventional optical isolator has a problem that the optical isolator is thick, and the isolator cannot be inserted between the lens and the optical fiber, resulting in a large apparatus.

  The present invention has been made in view of the above problems, and in an optical isolator element in which a polarizer in which metal particles are dispersed and a Faraday rotator are bonded to each other, the metal particles of the polarizer are Faraday rotators. The non-joint surface side is less than the joint surface side.

  Furthermore, a dielectric multilayer film is formed on a non-joint surface of the polarizer with the Faraday rotator, and the arithmetic average surface roughness (Ra) of the dielectric multilayer film is 0.2 μm or less.

A first step of bonding and integrating the polarizer substrate and the Faraday rotator substrate into a composite substrate;
A second step of polishing both surfaces of the composite substrate bonded and integrated;
And a third step of producing a large number of optical isolator elements by cutting the composite substrate.

  Further, the second step is characterized in that both surfaces of the composite substrate are simultaneously lapped.

  The optical isolator element is bonded to the end face of the optical fiber.

  According to the configuration of the present invention, in the optical isolator element in which the Faraday rotator and the polarizer are bonded via an adhesive, the non-bonding surface side of the polarizer with the Faraday rotator has metal particles as compared to the bonding surface side. By reducing the number, generation of unnecessary scattered light is reduced, and the problems of lowering isolation and returning light to the optical fiber and laser can be solved.

  Further, the thickness for the optical isolator is reduced, and the apparatus can be miniaturized.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a perspective view showing an embodiment of an optical isolator element of the present invention.

  As shown in the figure, the optical isolator element 1 of the present invention is bonded to the polarizer 3, the Faraday rotator 2, and the polarizer 4 in this order via an adhesive, and the Faraday rotator of the polarizer 3 and the polarizer 4. 2 has a region in which elliptical metal particles are dispersed, and metal particles are dispersed on the non-joint surface side of the polarizer 3 and the polarizer 4 with the Faraday rotator. The area does not exist.

  The transmission polarization direction of the polarizer 3 is set to be approximately 45 degrees with respect to the transmission polarization direction of the polarizer 4, and the Faraday rotation angle of the Faraday rotator 2 is set to approximately 45 degrees. The best insertion loss characteristic and isolation characteristic can be obtained by applying a magnetic field in the H direction to the optical isolator element 1.

  As the polarizers 3 and 4, an absorption polarizer having a function of absorbing a polarization component in one direction of incident light is used, and the absorption polarizer is a polarization having a structure in which ellipsoidal metal particles are dispersed in glass. Made of glass. This polarizing glass is a glass having polarization characteristics by arranging long stretched metal particles in one direction in the glass itself, light having a polarization plane perpendicular to the stretch direction of the metal particles is transmitted, Light with a parallel polarization plane is absorbed. This polarization characteristic is because the metal fine particles embedded in the dielectric exhibit light-plasma resonance.

  It has been found that this metal particle causes light scattering as well as polarization characteristics. This scattered light is scattered about 1% of the optical power in the polarization direction (transmission polarization) of the ellipsoidal metal particles, and in the polarization direction (absorption polarization) in the long axis direction. The light power of about 6% is scattered. Unlike the Fresnel reflection, the scattered light does not have a traveling direction, so the amount of light that returns to the semiconductor laser or transmission line as reflected return light is small, but the optical fiber whose scattering reflection point is the semiconductor laser or transmission line. When the light is near the end of the light source or near the light condensing position, a part of the scattered light becomes light returning to the semiconductor laser or the transmission path. In particular, the scattered light in the polarizer on the incident side is generated in the vicinity of the semiconductor laser, so that the influence is particularly great. Assuming that 6% of all scattered light power is incident on an optical fiber or a semiconductor laser, the reflected return light is 12 dB, which is far below the isolation 30 dB of a normal isolator, and causes unstable operation and errors in the laser or transmission path. The problem occurs.

  Accordingly, the polarizers 3 and 4 are formed such that the metal particles have fewer metal particles on the non-bonding surface side with the Faraday rotator 2 on the light incident side than the metal particles on the bonding surface side. It is particularly preferable that the non-bonding surface is not present. Such a manufacturing method will be described later.

  The Faraday rotator 2 is, for example, a bismuth-substituted garnet crystal, and the thickness thereof is set so that the polarization plane of incident light having a predetermined wavelength rotates 45 degrees. In order to rotate the plane of polarization, it is necessary to apply a sufficient magnetic field H in the direction of the optical axis L of the incident light, and generally a permanent magnet (not shown) is often used. Further, if the Faraday rotator 2 has a very large magnetic hysteresis and a magnetic force retaining material that retains its own magnetic force even without a magnet, such as (TbBi) 3 (FeAlGa) 5O12, a magnet that applies a magnetic field is not required. Become.

  As described above, the polarization characteristics of the polarizers 3 and 4 are such that the elliptical metal particles embedded in the dielectric exhibit light-plasma resonance, and therefore the transmittance varies depending on the orthogonal polarization of the incident light. In a conventional product in which metal particles are dispersed on both sides of a polarizer, the transmission power ratio (called extinction ratio) of orthogonally polarized waves is as large as 50 dB. As in the present invention, the extinction ratio is slightly smaller in the case of a polarizer in which one side has more metal particles than the other side, or there is a region in which metal particles are dispersed only on one side, and particularly only on one side. It was confirmed that the extinction ratio of the polarizer in which the region where the metal particles are dispersed is about 47 dB.

  This extinction ratio is a sufficient value for the characteristics of the optical isolator isolation 40 dB generally required.

  As described above, the metal particles inside the polarizer cause backscattered light. Since this backscattered light has no directivity, the more it is generated at a point near the light source or optical fiber, the more it becomes reflected return light and adversely affects the system. Therefore, a metal particle dispersion region is placed near the light source or optical fiber. It is not preferable to do so. The present invention pays attention to this point, and reduces or eliminates the metal particle dispersion region closer to the light source or the optical fiber by means such as polishing, thereby realizing a reduction in reflected return light.

  Here, when the metal particle dispersion region is removed by polishing, not the metal particles but the metal halide particles are dispersed on the surface of the polarizer on the non-bonding surface side with the Faraday rotator after polishing.

  In the state where one side of the polarizer substrates 22 and 24 is masked, a metal halide dispersed on the other side is reduced to a metal, and after passing through a step of depositing silver particles from the silver halide, a polarizer substrate is used, and a metal particle dispersion region May be present only on one side of the polarizer, and there is also a method of halogenating the non-joint surface side with the Faraday rotator as a process for preventing scattered light by the metal particles.

  In any case, in the present invention, the metal halide particles are present on the non-joint surface side of the polarizer with the Faraday rotator, but are scattered in comparison with the metal particles because they transmit light in the optical communication band. No light is generated. Therefore, no reflected return light is generated, the system is not adversely affected, and a stable laser output can be obtained.

  Further, it is desirable that a non-junction surface of the polarizer with the Faraday rotator is provided with an antireflection film made of a dielectric multilayer film. At that time, if the arithmetic average surface roughness (Ra) measured based on JIS B0601 “surface roughness of the contour curve” is larger than 0.2 μm, scattered light on the surface is generated, and the effect of the present invention is diminished. This is because the arithmetic average surface roughness (Ra) is 0.2 μm, which is not negligible with respect to the wavelength of 1.31 μm of optical communication, and it is confirmed by experiment that a scattering loss of 0.1 dB occurs. This is converted into reflected return light of a maximum of 17 dB.

  FIG. 2 is a cross-sectional view of a fiber with an optical isolator in which the optical isolator element 1 of the present invention is bonded to the end face of the optical fiber.

  An optical isolator-equipped fiber 10 according to the present invention is configured by bonding an optical isolator element 1 to an end face of an optical fiber 6 and a ferrule 8 to which the optical fiber 6 is fixed, and disposing a magnet 7 therearound.

  The light emitted from the laser diode 31 is condensed on the optical fiber 6 by the lens 30 and transmitted. Here, since metal particles that cause backscattered light are not disposed at the near end portion of the optical fiber 6, an improvement in isolation characteristics is realized as compared with a conventional fiber with an optical isolator.

  Next, the manufacturing method of the optical isolator element of this invention is demonstrated.

  First, like the prior art, the present invention uses a polarizer substrate 22, a Faraday rotator substrate 23, and a polarizer substrate 24 of about 10 mm square. Polarizer substrates 22 and 24 are processes in which a metal halide such as AgCl, AgI, or AgBr is melted, stretched, and polished on both sides, and then the metal halide is reduced to metal and silver particles are precipitated from silver halide. Is used as the polarizer substrate, and here the metal particle dispersion regions exist on both sides of the polarizer.

  As shown in FIGS. 5A and 5B, the polarizer substrate and the Faraday rotator substrate are bonded after rotation adjustment, and then both surfaces of the integrated optical isolator element substrate are polished by double-sided lapping or the like. The both surfaces of the composite substrate are simultaneously polished with an apparatus to remove the metal particle dispersion region. Here, the metal particle dispersion region is generally about 50 μm. For example, when a polarizer substrate having a thickness of 200 μm is used, the polishing amount is preferably 50 μm or more and 100 μm or less. Here, in polishing of 100 μm or more, there is a possibility that the metal particle dispersion region on the Faraday rotator substrate side becomes close and only the same effect as that before polishing can be obtained.

  From here on, as in the prior art, as shown in FIGS. 5C and 5D, the optical isolator element substrate 25 is processed into a large number of small chip-like optical isolator elements 20 by dicing or the like.

  When this optical isolator element 20 is manufactured, a large polarizer substrate and a Faraday rotator substrate are alternately laminated, and both surfaces are polished together after completion of bonding, and this is cut to obtain a large number of optical isolator elements. By using the method of obtaining 20, the workability and the production amount are high, and the polishing process is increased as compared with the process of the conventional product, but since it is a large substrate and batch processing is possible, The increase in man-hours is very small.

  In the manufacturing method of the present invention, an example in which an optical isolator element substrate is manufactured by bonding a polarizer substrate to both surfaces of a Faraday rotator substrate has been shown. However, the present invention is not limited thereto, and a large polarizer substrate and a large Faraday substrate are provided. The same effect can be obtained even if the step of bonding the rotor substrate and polishing only the surface of the polarizer substrate is performed.

  FIG. 3 is a diagram showing the measurement result of the return loss.

  FIG. 3A shows the measurement result of the fiber A with an optical isolator using a conventional optical isolator, and FIG. 3B shows the measurement result of the fiber B with an optical isolator of the present invention.

  In the measurement, light was incident from the optical fiber side of the fiber with an optical isolator, and the reflected light reflected back to the optical fiber was measured. Here, in order to know the reflection point of the light accurately, a precision reflectometer was used as a measuring instrument.

  As shown in FIG. 3A, the reflection measurement of the conventional fiber A with an optical isolator is generated from the near end portion of the end face of the optical fiber, and the reflection position covers a relatively wide range. The peak value of the return loss was about 24 dB at the first peak C point and about 43 dB at the second peak D point. Here, both the reflection at the point C and the point D is due to back scattered light from the metal particle dispersion region inside the polarizer, the point C is the metal particle dispersion region on the optical fiber side, and the point D is on the Faraday rotator side. It is expected to be a metal particle dispersion region.

  On the other hand, the reflection measurement of the fiber B with an optical isolator according to the present invention occurs at a point slightly away from the end face of the optical fiber as shown in FIG. The return loss peak E point is about 47 dB, which is a position corresponding to the return loss peak point D of the conventional product. That is, in the present invention, since the metal particle dispersion region is not formed on the optical fiber side, the reflection at the point C in FIG. 3A does not occur, and the return loss is greatly reduced. .

  As described above, according to the configuration of the present invention, in the optical isolator element in which the Faraday rotator and the polarizer are integrated through the adhesive, the non-joint surface side of the polarizer with the Faraday rotator is a metal particle. Is not dispersed, the generation of unnecessary scattered light is reduced, and the problems of lowering isolation and returning light to the optical fiber and laser can be solved.

  The present invention is particularly effective for the fiber with an optical isolator shown in the second embodiment. In addition to solving the above problems, the thickness of the optical isolator element is reduced, and the apparatus can be downsized.

  As an example of the present invention, a fiber A with an optical isolator using the conventional optical isolator shown in FIG. 4 and a fiber B with an optical isolator of the present invention shown in FIG. The configuration of the optical fiber portion was exactly the same for both A and B. Each component and configuration will be described below.

  The polarizer substrate uses a polar core (product name) manufactured by Corning, the size is 10 mm square and the thickness is 0.2 mm, and the polarizer on the incident side transmits the polarization direction parallel to the reference side. The exit side polarizer used was set so as to transmit the polarization direction of 45 degrees with respect to the reference side. The Faraday rotator substrate was made of bismuth-substituted garnet, had a size of 10 mm square, a thickness of 0.25 mm, and a polarization rotation angle of 45 degrees in the saturation magnetic field strength. Both are elements that operate with respect to light having a wavelength of 1.31 μm, and an antireflection film of an adhesive (n = 1.5) is applied to both surfaces of the Faraday rotator substrate.

  Two polarizer substrates and one Faraday rotator substrate were rotationally adjusted, and the three substrates were bonded and fixed with a translucent adhesive to produce an optical isolator element substrate. The optical isolator element B used for the fiber B with an optical isolator was polished on both sides. The both surfaces were polished by about 0.08 mm, and the total thickness of the optical isolator element B was 0.49 mm. Further, the optical isolator element A used for the fiber A with the optical isolator was not subjected to double-side polishing, and the total thickness was 0.65 mm.

  Thereafter, both optical isolator element substrates A and B were processed into a 0.6 mm square by a dicing apparatus to produce optical isolator elements A and B and 169 chips each.

  Both ferrules A and B with optical isolators were made of zirconia ceramics. After inserting and fixing an optical fiber SMF-28 manufactured by Corning into the ferrule, the end face was polished so as to have an inclination of 6 degrees. The conventional optical isolator element A was bonded to the polished end face of the fiber A with an optical isolator, whereas the optical isolator element B of the present invention was bonded to the fiber B with an optical isolator.

  The magnet was a donut-shaped magnet made of samarium cobalt, and was adhered to a metal holder to which a zirconia ferrule was fixed in order to arrange it around the optical isolator element.

  Of the two types of optically isolator-equipped fibers thus produced, 10 reflection loss measurements were made by sampling and a total of 20 fibers were measured. The measurement was performed using a precision reflectometer manufactured by Agilent Technologies, and the return loss has polarization characteristics. Therefore, the return loss was measured by rotating the polarization of the measurement light, and the worst return loss was used. .

  Regarding the fiber A with an optical isolator, the average value of the 10 reflection attenuations is as low as 23 dB, and it is expected that the cause is backscattered light of metal particles existing in the vicinity of the end face of the optical fiber. On the other hand, in the fiber B with an optical isolator, the average value of the 10 reflection losses was 45 dB, and the improvement effect of the present invention was sufficiently confirmed.

It is a perspective view which shows embodiment of the optical isolator element of this invention. It is sectional drawing which shows the fiber with an optical isolator of this invention. (A) which shows the measurement result of a return loss is a graph of a comparative example, (b) is a graph of an Example. It is a perspective view which shows the structure of the conventional optical isolator miniaturized. It is a figure which shows the manufacturing method of the normal optical isolator element reduced in size, (a) is before bonding, (b) is after bonding, (c) is a dicing process, (d) The perspective appearance which shows after a dicing process FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 20: Optical isolator element 10: Fiber with optical isolator 15: Optical isolator 2, 16: Faraday rotator 3, 4, 17, 18: Polarizer 5: Metal particle dispersion area | region 6: Optical fiber 7: Magnet 8: Ferrule 9: Metal holder 22, 24: Polarizer substrate 23: Faraday rotator substrate 25: Optical isolator element substrate 30: Lens 31: Laser diode

Claims (5)

In an optical isolator element in which a polarizer in which metal particles are dispersed and a Faraday rotator are bonded to each other, the number of metal particles in the polarizer is smaller on the non-joint surface side than on the joint surface side with the Faraday rotator. A characteristic optical isolator element. The dielectric multilayer film is formed on the non-bonding surface side of the polarizer with the Faraday rotator, and the arithmetic average surface roughness (Ra) of the dielectric multilayer film is 0.2 μm or less. Item 4. The optical isolator element according to Item 1. A first step of bonding and integrating a polarizer substrate and a Faraday rotator substrate into a composite substrate;
A second step of polishing both surfaces of the composite substrate;
And a third step of producing a plurality of optical isolator elements by cutting the composite substrate. 4. The method of manufacturing an optical isolator element according to claim 3, wherein the second step is a step of simultaneously lapping both surfaces of the composite substrate. A fiber with an optical isolator, comprising the optical isolator element according to claim 1 or 2 bonded to an end face of the optical fiber.

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