JP2006309123A - Faraday rotator mirror and method for fabricating the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small Faraday rotator mirror exhibiting excellent fabrication workability, reliability, and coupling efficiency. <P>SOLUTION: In the Faraday rotator mirror 1, a single mode fiber 2; a graded index fiber 3; a Faraday rotator 5; and a total reflection mirror 6 are successively arranged, and optical adhesives 8a and 8b are interposed at least either between the graded index fiber 3 and the Faraday rotator 5 or between the Faraday rotator 5 and the total reflection mirror 6. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a Faraday rotating mirror of an optical passive component applied to stabilize the operation of an optical fiber sensor system, an optical amplification system, and the like, and a manufacturing method thereof.

  The optical fiber sensor has a system path mainly composed of an optical fiber and has a detection element in one of the optical paths of the optical fiber. Here, the detection element is a component that receives an optical characteristic change according to the amount detected from the detection target. In an optical fiber sensor, when a single mode fiber is used as a detection element to detect external forces such as vibration, pressure, temperature, electric field, magnetic field, and sound wave, for example, the change in optical path length of the single mode fiber due to these external forces is a fiber interferometer. Detected by.

  However, in such an optical fiber sensor, there arises a problem that fluctuation of the output interference fringe and disappearance of the signal occur due to an accidental change in the polarization state of light due to birefringence in the optical fiber.

  In order to solve such a problem, Non-Patent Document 1 proposes to use a Faraday rotating mirror as a part of the fiber interferometer. This Faraday rotating mirror is an optical component that has a function of suppressing a fluctuation in polarization state caused by birefringence in an optical fiber and maintaining an arbitrary input polarization state.

  FIG. 8 is a cross-sectional view showing a configuration of a conventional Faraday rotating mirror 21. The Faraday rotation mirror 21 includes an optical fiber 22, a coupling lens 23, a Faraday rotator 25, a total reflection mirror 26, and a magnet 27.

  The optical fiber 22 is a single mode fiber. The coupling lens 23 is a member for efficiently coupling light reflected by a total reflection mirror 26 described later to the optical fiber 22, and is disposed so as to face one end of the optical fiber 22. The Faraday rotator 25 is a member having a function of rotating a polarization direction of incident light by a predetermined angle by applying a predetermined magnetic field, and is made of, for example, a bismuth-substituted garnet crystal. The thickness of the Faraday rotator 25 is set so that, for example, the polarization direction of incident light rotates 45 °. The total reflection mirror 26 is a member for reflecting the light emitted from the optical fiber 22, and is disposed so as to face one end of the optical fiber 22 through the coupling lens 23 and the Faraday rotator 25. Yes. The magnet 27 is a member for applying a predetermined magnetic field (for example, the saturation magnetic field strength of the bismuth-substituted garnet crystal or more) to the Faraday rotator 25.

  FIG. 9 is a diagram for explaining the polarization state of light in the Faraday rotation mirror 21 as viewed from the direction of the optical fiber 22. Hereinafter, the operating principle of the Faraday rotating mirror 21 will be described with reference to FIG. For convenience, the light emitted from the optical fiber 22 is referred to as incident light, the light reflected by the total reflection mirror 26 is referred to as reflected light, the traveling direction of the incident light is referred to as the forward direction, and the traveling direction of the reflected light is referred to as the reverse direction. . Further, although the incident polarization direction is a linear polarization, this description is not limited to this, and the present invention can be applied to any polarization direction.

  First, incident light (FIG. 9A) emitted from the optical fiber 22 is transmitted through the Faraday rotator 25, and its polarization direction is rotated 45 ° clockwise as viewed from the forward direction (FIG. 9B). . Thereafter, the reflected light (FIG. 9C) reflected by the total reflection mirror 26 enters the Faraday rotator 25 from the opposite direction again. The reflected light transmitted through the Faraday rotator 25 in the reverse direction is further rotated 45 ° clockwise as viewed in the forward direction (FIG. 9D), and enters the optical fiber 22. As a result, the reflected light of the Faraday rotating mirror 21 has a polarization direction orthogonal to the incident light, and receives birefringence that is just opposite to that received by the incident light. The polarization state is stabilized in a state orthogonal thereto.

  The Faraday rotating mirror 21 as shown in FIGS. 8 and 9 is applied to an optical fiber amplification system in addition to an optical fiber sensor system. In an optical fiber amplification system, a single mode fiber (length: several tens to several hundreds of meters) doped with erbium is generally used, so that the polarization state changes due to birefringence in the optical fiber. There is a problem of polarization mode dispersion that causes signal waveform degradation in a long-distance optical fiber communication system. However, since these are compensated by using the Faraday rotating mirror 21, a more stable output can be obtained.

  Further, in Patent Document 1, in order to reduce the size, a core expansion fiber (having the same outer diameter as the optical fiber) is used instead of the coupling lens. Furthermore, in order to reduce the number of processes and simplify the assembly, a total reflection mirror film is formed on one end face of the Faraday rotator, or the core expansion fiber and the Faraday rotator are brought into close contact via an optical adhesive. Proposals have been made.

  FIG. 10 is a cross-sectional view illustrating a configuration of the Faraday rotation mirror 31 of Patent Document 1. The Faraday rotation mirror 31 includes a core expansion fiber 33, a Faraday rotator 35, a total reflection mirror film 36, a cylindrical magnet 37, and an optical adhesive 38.

  The core expansion fiber 33 includes a core 33a and a clad 33b. The core expansion fiber 33 is produced by locally heating a general single mode fiber. When such heating is performed, the core 33a expands because Ge doped into the core 33a is thermally diffused. The Faraday rotator 35 is a member having the same configuration as the Faraday rotator 25 described above. The total reflection mirror film 36 is made of a multilayer dielectric and is directly formed on one end surface of the Faraday rotator 35. The total reflection mirror film 36 has a small light loss and a high reflectance (for example, 99% or more). The cylindrical magnet 37 is a member having the same function as the magnet 27 described above.

  In general, the divergence angle of the radiation beam from the optical fiber decreases as the core diameter increases and approaches the parallel light. Therefore, when the divergence angle is large, it becomes difficult for the reflected light to be coupled to the core expansion fiber 33. In Patent Document 1, a decrease in coupling efficiency, that is, an increase in insertion loss is suppressed by increasing the core diameter by 3 to 4 times. On the other hand, as the distance from the end face of the core expansion fiber 33 to the total reflection mirror film 36 becomes longer, the radiation beam diverges and the coupling efficiency decreases, so the Faraday rotator 35 is mounted in close contact with the core expansion fiber 33. . In Patent Document 1, the thickness of the optical adhesive 38 is extremely small and is set to 10 μm or less.

  Furthermore, Patent Document 2 proposes that the return light of unnecessary reflected light is suppressed by using a core expansion fiber and a trapezoidal Faraday rotator.

  FIG. 10 is a sectional view showing the Faraday rotation mirror 41 of Patent Document 2. The Faraday rotation mirror 41 includes a core expansion fiber 43, a Faraday rotator 45, a total reflection mirror film 46, a cylindrical magnet 47, and an optical adhesive 48.

The core expansion fiber 43 is different from the above-described core expansion fiber 33 only in that one end surface (the surface facing the Faraday rotator 45) is inclined with respect to the vertical axis of the optical axis. The Faraday rotator 45 is configured in a shape (trapezoidal shape in FIG. 10) such that the other end surface is perpendicular to the optical axis while being connected to the core expansion fiber 43 at one end surface. The total reflection mirror film 46, the cylindrical magnet 47, and the optical adhesive 48 are members having the same configuration as the total reflection mirror film 36, the cylindrical magnet 37, and the optical adhesive 38 described above.
Electronics Letter 14th March 1991 vol27 No6 Japanese Patent No. 3548283 Japanese Patent No. 3602891 JP 2002-14253 A

  However, as shown in FIG. 7, the Faraday rotating mirror 21 using the coupling lens 23 has the following problems.

  In the detection unit of the fiber interferometer and the optical fiber amplification system, the Faraday rotation mirror 21 is connected to an optical fiber, an optical fiber coupler, or the like, and is mounted in a housing. The outer diameter of the optical fiber is 0.25 mm, and the optical fiber coupler Since the Faraday rotating mirror 21 is as large as 5 mm with respect to the outer diameter of 2 to 3 mm, it causes an increase in the size of the apparatus. The Faraday rotation mirror 21 has a size of about φ5 mm, in addition to the diameter of the coupling lens 23 being about 2 mm, and a metal fitting for joining the coupling lens 23 to other components. Etc., it becomes as large as φ5 mm.

  Further, in the Faraday rotation mirror 21, it is necessary to optically adjust the optical fiber 22, the coupling lens 23, and the total reflection mirror 26 with high accuracy. Therefore, in the Faraday rotation mirror 21, since the number of processes is increased and the assembly is complicated, it takes time to manufacture.

  On the other hand, the Faraday rotating mirror 31 proposes a reduction in the number of steps and simplification of assembly while adopting a core expansion fiber 33 having an outer diameter of φ0.125 mm. However, the Faraday rotating mirror 31 requires heating at a high temperature (for example, 1200 ° C.) for a long time (for example, 12 hours) in order to expand the core diameter of the single mode fiber by 3 to 4 times when the core expansion fiber 33 is manufactured. Therefore, workability is poor, and shortening of production time cannot be improved sufficiently. Further, in the Faraday rotating mirror 31, reliability may be deteriorated due to long-time heat treatment at a high temperature. Further, the Faraday rotating mirror 31 has a problem that unnecessary reflected light (returned light) generated at the end face of the Faraday rotator 35 is almost coupled to the core expansion fiber 33.

  On the other hand, in the Faraday rotation mirror 41, it has been proposed to suppress unnecessary reflected light by employing a trapezoidal Faraday rotator 45. However, in the Faraday rotating mirror 41, as in the Faraday rotating mirror 31, problems such as workability, manufacturing time, and reliability cannot be sufficiently solved.

  A total reflection mirror film 46 is directly deposited on the Faraday rotator 45 in order to reduce insertion loss. Generally, the divergence angle of the radiation beam from the core expansion fiber 43 decreases as the core diameter increases, and tends to approach parallel light. Therefore, the smaller the core diameter (the greater the divergence angle), the more difficult the reflected light is coupled to the core expansion fiber 43. Therefore, the Faraday rotating mirror 41 suppresses a decrease in coupling efficiency, that is, an increase in insertion loss, by expanding the core diameter by 3 to 4 times. Further, since the radiation beam diverges as the distance from the end face of the core expansion fiber 43 to the total reflection mirror film 46 increases, the Faraday rotator mirror 41 directly forms the total reflection mirror film 46 on the Faraday rotator 45. The increase in insertion loss is suppressed.

  11A and 11B are diagrams showing an optical coupling system disclosed in Patent Document 3. FIG. 11A is an enlarged cross-sectional view of a main part thereof, FIG. 11B is an enlarged cross-sectional view of a main part of the graded index fiber 53, and FIG. FIG. 6 is an image diagram of light rays that pass through an index fiber 53. The optical coupling system shown in FIG. 11 includes an optical fiber 52, a graded index fiber 53, and a coreless fiber 54. By replacing a part of the coreless fiber 54 with an optical functional element, an optical passive component of a transmission optical system is provided. can do. The graded index fiber 53 includes a core 53a and a clad 53b, and the core 53a has a maximum refractive index distribution at the center and a refractive index distribution in which the refractive index decreases stepwise (or continuously) toward the outer periphery. The light beam transmitted through the graded index fiber 53 is periodically bent and propagated. The graded index fiber 53 functions as a lens that can cope with parallel light or focused light by cutting to an appropriate length.

  Such a basic structure of the graded index fiber 53 is used for a multimode fiber used for optical communication, but it can be produced in large quantities by producing a base material having a large outer diameter and extending the base material. This is possible, and is characterized by extremely high productivity as compared with the core expansion fibers 33 and 43.

  However, since the lens performance of the graded index fiber 53 varies depending on the length as described above, it is necessary to adjust the fiber length with high accuracy. The graded index fiber 53 is cut by cleavage like a general optical fiber, and can be easily cut by, for example, a fiber cutter. Therefore, the graded index fiber 53 is excellent in workability. For example, length variation of several tens of μm occurs. Therefore, when the Faraday rotator is connected in close contact with the end face of the fiber as shown in FIG. 10A, the coupling distance does not match due to the length variation of the graded index fiber 53, so that sufficient coupling cannot be obtained. There is a problem.

  The present invention has been conceived under such circumstances, and an object of the present invention is to provide a Faraday rotating mirror that is small in size and excellent in manufacturing workability, reliability, and coupling efficiency.

  The Faraday rotation mirror according to the first aspect of the present invention is a single mode fiber, a graded index fiber, a Faraday rotator, and a total reflection mirror, which are sequentially disposed, and the graded index fiber, the Faraday rotator, And an optical adhesive is interposed between at least one of the Faraday rotator and the total reflection mirror.

  The Faraday rotation mirror according to the first aspect further includes a coreless fiber interposed between at least one of the graded index fiber and the Faraday rotator and between the Faraday rotator and the total reflection mirror. May be.

  In the Faraday rotating mirror according to the first aspect, a surface of the coreless fiber facing the Faraday rotator is inclined with respect to a plane perpendicular to the optical axis of the graded index fiber, and the Faraday rotator is connected to the coreless fiber. It is preferable that the total reflection mirror is in close contact with the facing surface and is arranged perpendicular to the optical axis of the graded index fiber.

  In the Faraday rotating mirror according to the first aspect, the thickness of the optical adhesive is preferably 10 μm or more.

  In the Faraday rotating mirror according to the first aspect, the total reflection mirror is preferably formed directly on the Faraday rotator.

  In the Faraday rotating mirror according to the first aspect, the length of the graded index fiber is preferably 0.31 to 0.5 times one period of light propagating through the graded index fiber.

  In the Faraday rotating mirror according to the first aspect, the single mode fiber and the graded index fiber are preferably disposed in the through hole of the ferrule having a through hole.

  In the Faraday rotating mirror according to the first aspect, it is preferable that the outer shape of the total reflection mirror is positioned within the range of the outer shape of the Faraday rotator when viewed from the optical axis direction of the graded index fiber.

  In the Faraday rotator mirror according to the first aspect, when viewed from the optical axis direction of the graded index fiber, the Faraday rotator and the total reflection mirror are substantially square, the outline of the Faraday rotator and the total reflection It is preferable that the diagonal of the mirror is substantially parallel.

  According to a second aspect of the present invention, there is provided a method for manufacturing a Faraday rotator mirror, in which a single mode fiber, a graded index fiber, a Faraday rotator, and a total reflection mirror are sequentially arranged, and the graded index fiber and the Faraday mirror are arranged. A step of interposing an optical adhesive between the rotor and at least one of the Faraday rotator and the total reflection mirror, a step of adjusting the thickness of the optical adhesive, and an optical adhesive And a step of curing.

  In the Faraday rotation mirror according to the first aspect of the present invention, an optical adhesive is interposed between the graded index fiber and the Faraday rotator and between the Faraday rotator and the total reflection mirror. Yes. Therefore, in this Faraday rotating mirror, for example, by adjusting the thickness of the optical adhesive, the characteristics of the Faraday rotating mirror (insertion loss, etc.) due to processing variations in the length of the graded index fiber and the thickness of the Faraday rotator Can be prevented. That is, this Faraday rotating mirror is suitable for downsizing compared to, for example, a coupling lens for efficiently coupling light reflected by a total reflection mirror to an optical fiber, and compared with, for example, a core expansion fiber. Although it is suitable for improving manufacturing workability and reliability, it is possible to apply a graded index fiber in which deterioration of characteristics (such as insertion loss) due to processing variations is a concern. Therefore, the Faraday rotating mirror is small in size, excellent in workability and reliability, and excellent in terms of reducing insertion loss.

  As an example, a Faraday rotating mirror according to the present invention will be described below.

  FIG. 1 schematically shows the configuration of a Faraday rotating mirror 1 according to the first embodiment of the present invention. The Faraday rotation mirror 1 includes a single mode fiber 2, a graded index fiber (GIF) 3, a coreless fiber 4, a Faraday rotator 5, a total reflection mirror 6, a magnet 7, and optical adhesives 8a and 8b.

  The optical fiber 2 guides light so that light incident from one end thereof is emitted from the other end. Examples of the material constituting the optical fiber 2 include quartz glass, multicomponent glass, and plastic.

  The graded index fiber 3 is an optical fiber having a predetermined refractive index distribution that maximizes the center of the core portion of the optical fiber 2. As shown in FIG. 11C, the light propagating through the graded index fiber 3 meanders with a periodicity with respect to the length of the graded index fiber 3 (meandering period) and transmits. This meandering period is constant regardless of the wavelength of normal light. FIG. 11C shows a state where the length of the graded index fiber 3 corresponds to one cycle of the sine curve, and this length is defined as one pitch. Note that the length of the graded index fiber 3 is within a range in which the exit angle of the graded index fiber 3 is narrowed (pitch is in the range of 0.25 to 0.5) so that the graded index fiber 3 can appropriately perform the lens function. Is set. When the pitch is 0.25, the optimum coupling distance is theoretically infinite, and when the pitch is 0.5, the optimum coupling distance is theoretically zero.

  FIG. 2 shows the relationship between the length (GIF length) of the graded index fiber 3 and its optimum coupling distance. As best shown in FIG. 2, as the length of the graded index fiber 3 increases, the optimum coupling distance decreases. When the difference between the optimum coupling distance and the thickness of the Faraday rotator 5 is small (for example, 300 μm or less), the coreless fiber 4 may be omitted. Here, the optimum coupling distance means the distance between the point where the light at the exit end of the single mode fiber 2 forms an image with the graded index fiber 3 and the end face of the graded index fiber 3. When the total reflection mirror 6 is disposed at the image formation point, the reflected light forms an image at the exit end of the single mode fiber 2, so that the coupling with the maximum reflectance can be obtained. Note that the optimum coupling distance in the present embodiment is substantially equal to the sum of the length of the coreless fiber 4, the thickness of the Faraday rotator 5, and the thicknesses of the optical adhesives 8a and 8b. However, the thickness of the Faraday rotator 5 is not a physical thickness but a refractive index equivalent to that of the single mode fiber 2, the graded index fiber 3, the coreless fiber 4, and the optical adhesives 8a and 8b (for example, 1.46). ) To match the optical length. For example, when the Faraday rotator 5 is a bismuth-substituted garnet, the refractive index is 2.34, and the optical length is a value obtained by multiplying the physical length by 1.46 / 2.34.

  FIG. 3 shows the relationship between the pitch that defines the length of the graded index fiber 3 and the return loss. As clearly shown in FIG. 3, when the pitch exceeds 0.31, the return loss becomes less than −30 dB. Here, the reflection attenuation amount is the ratio of the reflection amount of reflection points other than the total reflection mirror 6 to the total reflection amount of the Faraday rotation mirror 1. Reflection points other than the total reflection mirror 6 are generated at the interfaces of the constituent members shown in FIG. In particular, the return loss amount increases at the interface of the Faraday rotator 5 having a large refractive index difference. Of these reflection points other than the total reflection mirror 6, the reflected light that does not pass through the Faraday rotator 5 is not a light whose polarization state rotates as described with reference to FIG. Further, “−30 dB” is a numerical value given by “10 × log (1/1000)” and indicates a reflection amount (0.1% noise) of 1/1000 with respect to the total reflection amount. . Therefore, “less than return loss −30 dB” means that the noise component is less than 0.1%.

  The coreless fiber 4 is a member for adjusting the optical coupling distance, and has a homogeneous configuration substantially having no refractive index distribution.

  The Faraday rotator 5 is a member having a function of rotating the polarization direction of incident light by a predetermined angle, for example, by applying a predetermined magnetic field, and is composed of a bismuth-substituted garnet crystal or the like. The thickness of the Faraday rotator 5 is set so that, for example, the polarization direction of incident light rotates by 45 °. Specifically, although it differs depending on the wavelength of incident light, in the case of 1550 nm, it is 350 to 500 μm (physical thickness). The Faraday rotator 5 is preferably provided with an antireflection film (not shown) for preventing reflection of light on the surface thereof. As this antireflection film, for example, an AR coat is used, and as the formation position thereof, for example, when the total reflection mirror 6 is formed on one main surface of the Faraday rotator 5, the other main surface is used. By applying such an antireflection film to the surface of the Faraday rotator 5, unnecessary reflection (reflected return light) of the Faraday rotator mirror 1 can be reduced.

Here, an example of a method for manufacturing the Faraday rotator 5 will be described. First, polishing processing is performed on a large (for example, 10 mm square or more) flat plate. Next, an antireflection film is applied to a predetermined portion of the polished surface as necessary. Next, the flat plate provided with the antireflection film is cut into a predetermined size (for example, 1 mm square or less) by dicing cutting or the like. As described above, 100 or more Faraday rotators 5 are taken out from one large flat plate. Since such a manufacturing method eliminates the need for individual processing, the total reflection mirror 6 is excellent in manufacturing workability and mass productivity. The total reflection mirror 6 is a member for substantially totally reflecting light. The main surface of the substrate and the coreless fiber) and the main surface of the Faraday rotator 5 are formed. As the total reflection mirror 6, for example, a dielectric multilayer film itself, a multilayer dielectric in which a dielectric multilayer film is deposited on a substrate made of glass or the like, a metal having a high reflectance (such as aluminum) deposited on the substrate, In addition, an aluminum plate and the like can be mentioned, but a dielectric multilayer film or a multilayer dielectric is preferable from the viewpoint of high reflectivity (for example, 99% or more) and low light loss.

  The magnet 7 is a member for applying a predetermined magnetic field (a magnetic field parallel to the optical axis 10) to the Faraday rotator 5, and is a cylindrical shape in this embodiment. Note that when the Faraday rotator 5 that does not require application of a magnetic field is employed, the magnet 7 may be omitted.

  The optical adhesives 8 a and 8 b are members that have a function of adjusting the separation distance between the graded index fiber 3 and the total reflection mirror 6. Examples of the optical adhesives 8a and 8b include epoxy-based and acrylic-based translucent resins, and those having a refractive index equivalent to that of the coreless fiber 4 (for example, 1.45 to 1.5) are particularly suitable. Further, as the optical adhesives 8a and 8b, those having curability with respect to visible light, ultraviolet light and heat are preferable from the viewpoint of workability. The adjustment with the optical adhesives 8a and 8b is performed by changing the thickness of at least one of the optical adhesive 8a and the optical adhesive 8b. Specifically, first, the Faraday rotator 5 and the total reflection mirror 6 are brought into close contact with each other (for example, the distance between the two is 10 μm or less), and the optical adhesive 8b is cured, whereby the Faraday rotator 5 and The total reflection mirror 6 is integrated. Next, the thickness of the optical adhesive 8a interposed in a non-cured state (flowable state) with a predetermined thickness (for example, 30 μm or more) while measuring insertion loss to confirm the bonding state Is appropriately changed. Insertion loss is measured by using a light source (trade name: Laser Source 81553SM, manufactured by Agilent Technology Co., Ltd.) and a Faraday rotating mirror through an optical circulator (model number: PICA-1550-S, Applied Photoelectric Laboratory Co., Ltd.). 1 and the reflected light is again passed through the optical circulator (model number: PICA-1550-S, Applied Photoelectric Laboratory Co., Ltd.) with a light receiver (trade name: Optical Head 81521B, manufactured by Agilent Technologies). It was done by receiving. Next, the optical adhesive 8a changed to a desired thickness (for example, a thickness determined according to the optimum coupling distance) is cured by ultraviolet rays, heat, or the like. The above adjustment can be performed as described above. In addition, after the said adjustment, the Faraday rotation mirror 1 can be obtained by arrange | positioning the magnet 7 in a predetermined position as needed.

  In the Faraday rotating mirror 1 according to the present embodiment, optical adhesives 8 a and 8 b are interposed between the graded index fiber 3 and the Faraday rotator 5 and between the Faraday rotator 5 and the total reflection mirror 6. ing. Therefore, in the Faraday rotating mirror 1, the characteristics of the Faraday rotating mirror 1 due to processing variations in the length of the graded index fiber 3 and the thickness of the Faraday rotator 5 are adjusted by adjusting the thicknesses of the optical adhesives 8a and 8b. Deterioration of (insertion loss, etc.) can be suppressed. That is, in the Faraday rotation mirror 1, the outer diameter is smaller than that of a coupling lens for efficiently coupling the light reflected by the total reflection mirror 6 to the optical fiber. Although it is suitable for improving the reliability, it is possible to apply the graded index fiber 3 in which the deterioration of characteristics (such as insertion loss) due to processing variations is concerned. Therefore, the Faraday rotating mirror 1 is small in size and excellent in manufacturing workability (mass productivity) and reliability, and is also excellent in reducing insertion loss.

  In the Faraday rotating mirror 1, without using the trapezoidal Faraday rotator 45 which is difficult to process as shown in FIG. 11, the flat Faraday rotator 5 which is simple to process is used to adjust the pitch to reduce reflection. The amount can be reduced.

  In the Faraday rotating mirror 1, the thickness of the optical adhesive 8b is 10 μm or more. The optimum coupling distance is substantially equal to the sum of the length of the coreless fiber 4, the optical thickness of the Faraday rotator 5, and the thickness of the optical adhesives 8 a and 8 b, but the coreless fiber 4 and the Faraday rotator 5 are In addition, since a variation of several tens of μm occurs in the manufacture, there is a case where a significant effect cannot be obtained by adjusting the unit of several μm. Accordingly, the Faraday rotating mirror 1 is suitable for performing significant variation adjustment.

  By adopting the coreless fiber 4, the Faraday rotating mirror 1 does not unnecessarily increase the thickness of the optical adhesive 8 a. Since the optical adhesive 8a usually has a curing shrinkage rate of several percent, if the thickness of the optical adhesive 8a increases unnecessarily, the movement of the Faraday rotator 5 in the optical axis direction during the curing increases accordingly. In some cases, according to the present configuration, such movement can be suppressed.

  FIG. 4 schematically shows the configuration of the Faraday rotation mirror 11 according to the second embodiment of the present invention. The Faraday rotator mirror 11 has a surface facing the Faraday rotator 5 in the coreless fiber 4 as an inclined surface inclined at a predetermined processing angle with respect to the optical axis 10, and the Faraday rotator is provided on the inclined surface via an optical adhesive 8a. The total reflection mirror 6 is disposed so that the main surface is perpendicular to the optical axis 10, and the processing variation of the graded index fiber 3 and the like is adjusted by the optical adhesive 8b. Thus, it is different from the Faraday rotating mirror 1. Other configurations of the Faraday rotating mirror 11 are the same as those described above with respect to the Faraday rotating mirror 1.

  FIG. 5 shows the relationship between the processing angle of the inclined surface and the return loss. As clearly shown in FIG. 5, the return loss decreases as the processing angle increases. Therefore, the Faraday rotating mirror 1 ′ can obtain a higher attenuation effect, and thus is superior in terms of reflection attenuation.

  FIG. 6 schematically shows the configuration of the Faraday rotation mirror 12 according to the third embodiment of the present invention. The Faraday rotating mirror 21 is different from the Faraday rotating mirror 11 in that the single mode fiber 2, the graded index fiber 3 and the coreless fiber 4 are held by the ferrule 13.

  In the Faraday rotating mirror 12, for example, even if the diameter of each of the fibers 2 to 4 (for example, 125 μm) is smaller than the physical thickness (for example, 350 to 500 μm) of the Faraday rotator 5, the mounting surface of the Faraday rotator 5 in the ferrule 13. Is adjusted to an optimum size (determined according to the physical thickness of the Faraday rotator 5), and can be made structurally more stable. Further, in the Faraday rotating mirror 12, when the inclined surface is formed on the coreless fiber 4, it is possible to polish the entire ferrule 13, so that only the coreless fiber 4 is polished or cut by cleavage. Compared to processing variations, it can be reduced.

  Although specific embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the invention.

  In the Faraday rotating mirrors 1, 11, and 12, an optical adhesive 8 a or an optical adhesive is attached to either the graded index fiber 3 and the Faraday rotator 5 or between the Faraday rotator 5 and the total reflection mirror 6. What is necessary is just to have the agent 8b. Even in such a configuration, the same effect can be obtained.

  In the Faraday rotation mirrors 1, 11, and 12, a coreless fiber may be interposed between the Faraday rotator 5 and the total reflection mirror 6. Even in such a configuration, the same effect can be obtained.

  FIG. 7 shows the positional relationship and the size relationship between the Faraday rotator 5 and the total reflection mirror 6. In the Faraday rotating mirrors 1, 11, and 12, the size of the total reflection mirror 6 is preferably set to be equal to or smaller than the size of the Faraday rotator 5 (for example, the outer dimension). According to such a configuration, the amount of the optical adhesives 8 a and 8 b that wrap around the side surface of the Faraday rotator 5 can be suppressed.

  In the Faraday rotation mirrors 1, 11, and 12, the total reflection mirror 6 is preferably bonded so as to be positioned at the center of the Faraday rotator 5 in a plan view as shown in FIG. 7. According to such a configuration, since the amount of the optical adhesive 8b attached to the side surface of the total reflection mirror 6 becomes substantially equal, the coupling performance can be further improved.

  In the Faraday rotation mirrors 1, 11, and 12, as shown in FIGS. 7C and 7D, the total reflection mirror 6 may be rotated by 45 ° about the optical axis with respect to the Faraday rotator 5. . According to such a configuration, in addition to being suitable for equalizing the adhesion amount of the optical adhesive 8b adhering to the side surface of the total reflection mirror 6, the position of the total reflection mirror 6 is determined by the surface tension of the optical adhesive 8b. Since the deviation is corrected spontaneously, it is excellent in mass production stability.

  The Faraday rotating mirror 12 may employ a capillary instead of the ferrule 13. Even in such a configuration, the same effects as in the case of employing the ferrule 13 are obtained.

  Next, examples and comparative examples of the present invention will be described.

[Example 1]
<Preparation of Faraday Rotating Mirror> As the Faraday rotating mirror according to Example 1 of the present invention, the Faraday rotating mirror 1 shown in FIG. 1 was prepared. A specific manufacturing method will be described below. First, a single mode fiber 2 for 1550 nm, a graded index fiber 3 having a relative refractive index difference of 1.5% and a thickness of about 540 nm (0.35 pitch), a fiber length of about 40 μm, and a pure fiber with no additive The coreless fiber 4 made of quartz was fusion-bonded while aligning the outer shape to produce a fusion-bonded body made of the fibers 2 to 4. Next, when the wavelength of the incident light is 1550 nm, the physical thickness is set to about 380 μm (optical length: 240 μm) so that the polarization direction of the incident light rotates by 45 °, and on one main surface The other main surface of the Faraday rotator 5 obtained by cutting a 10 mm square Faraday rotator mother board having an AR coating formed as an antireflection film with a predetermined thickness into a 0.8 mm square has a reflectance of 99. The Faraday rotator 5 with a total reflection mirror 6 was produced by closely connecting the total reflection mirror 6 made of a multilayer dielectric adjusted to be% or more via an acrylic ultraviolet curable optical adhesive. Next, the Faraday rotator 5 with the total reflection mirror 6 was bonded to the end face of the coreless fiber 4 in the fusion spliced body via an acrylic ultraviolet curing optical adhesive. Next, before curing the acrylic ultraviolet curable optical adhesive, the insertion loss (ratio of incident light and reflected light of the single mode fiber 2) is adjusted to 0.7 dB or less, and then the ultraviolet light is applied. The acrylic UV curable optical adhesive was cured by irradiation. Five Faraday rotating mirrors in this example were produced.

<Measurement of insertion loss> Using a light source with a wavelength of 1550 nm (trade name: Laser Source 81553SM, manufactured by Agilent Technologies, Inc.), the output light is an optical circulator (model number: PICA-1550-S, Applied Photoelectric Laboratory) Is incident on the Faraday rotating mirror 1 through the second port, the reflected light is incident on the second port of the optical circulator again, and the third port is connected to the light receiver ( (Product name: Optical Head 81521B, manufactured by Agilent Technologies, Inc.). The insertion loss is measured by connecting the light emitted from the second port of the optical circulator directly to the photoreceiver and then measuring the amount of reflected light using the photoreceiver by connecting the second port to the Faraday rotating mirror 1. Derived as a ratio of outgoing light and reflected light at the second port of the optical circulator. The true insertion loss is a value obtained by subtracting the insertion loss of the second port and the third port of the circulator measured in advance from the insertion loss measured by the light receiver.

<Measurement of Return Loss> It was performed using a return loss measuring device (trade name: Precision Reflectometer 8504B, manufactured by Agilent Technologies, Inc.) capable of measuring the amount of reflected light at a predetermined position in the optical axis direction. . A light source having a wavelength of 1550 nm (trade name: Laser Source 81553SM, manufactured by Agilent Technologies) was selected, and measurement was performed by connecting the single mode fiber 2 in the Faraday rotating mirror 1 to the output port of the measuring instrument. Since the amount of other reflected light is very small compared to the amount of light reflected by the total reflection mirror 6, the ratio of the amount of light reflected at each reflection point with respect to the amount of light of the total reflection mirror 6 with the amount of light reflected by the total reflection mirror 6 as the total amount of reflected light. Is the return loss.

[Example 2]
<Preparation of Faraday Rotating Mirror> As the Faraday rotating mirror according to Example 2 of the present invention, the Faraday rotating mirror 11 shown in FIG. 4 was prepared. A specific manufacturing method will be described below. First, a single mode fiber 2 for 1550 nm, a graded index fiber 3 having a relative refractive index difference of 1.5% and a thickness of about 540 nm (0.35 pitch), a fiber length of about 40 μm, and an end face setting processing angle Was 4 ° ± 1 °, and the coreless fiber 4 made of pure quartz with no additive was fusion-bonded while aligning the outer shape to produce a fusion-bonded body composed of the fibers 2 to 4. Next, when the wavelength of the incident light is 1550 nm, the polarization direction of the incident light is rotated by 45 ° on the end face of the coreless fiber 4 in the fusion spliced body via an acrylic ultraviolet curable optical adhesive. A 10 mm square Faraday rotator mother board having a physical thickness of about 380 μm (optical length: 240 μm) and an AR coating formed on one main surface as an antireflection film with a predetermined thickness is set to 0.0.mu.m. The Faraday rotator 5 obtained by cutting to 8 mm square was tightly connected. Next, a total reflection mirror 6 made of a multilayer dielectric adjusted to have a reflectance of 99% or more is bonded to the other main surface of the Faraday rotator 5 via an acrylic ultraviolet curable optical adhesive. did. Next, before curing the acrylic ultraviolet curable optical adhesive, the insertion loss (ratio of incident light and reflected light of the single mode fiber 2) is adjusted to 0.7 dB or less, and then the ultraviolet light is applied. The acrylic UV curable optical adhesive was cured by irradiation. In this manner, five Faraday rotating mirrors in this example were produced.

<Measurement of Return Loss> The return loss was measured by the same measurement method as in Example 1, and the results are shown in Table 1.

[Example 3]
<Preparation of Faraday Rotating Mirror> As the Faraday rotating mirror according to Example 3 of the present invention, the Faraday rotating mirror 12 shown in FIG. 6 was prepared. A specific manufacturing method will be described below. First, a single mode fiber 2 for 1550 nm, a graded index fiber 3 having a relative refractive index difference of 1.5% and a thickness of about 540 nm (0.35 pitch), a fiber length of about 40 μm, and a pure fiber with no additive The coreless fiber 4 made of quartz was fusion-bonded while aligning the outer shape to produce a fusion-bonded body made of the fibers 2 to 4. Next, after inserting the ferrule 13 (outer diameter: 2.5 mm, inner diameter: 0.125 mm (tolerance 0.001 mm)) using an epoxy thermosetting adhesive, the end face is set to a set inclination angle. Polishing was performed at 4.5 ° ± 0.5 °. Next, when the wavelength of the incident light is 1550 nm on the end surface on the coreless fiber 4 side of the ferrule 13 into which the fusion spliced body has been inserted, the incident light is polarized when the wavelength of the incident light is 1550 nm. A physical thickness of about 380 μm (optical length: 240 μm) is set so that the wave direction rotates by 45 °, and an AR coating is formed on one main surface as an antireflection film with a predetermined thickness. A Faraday rotator 5 obtained by cutting a Faraday rotator mother plate into a 0.8 mm square was closely connected. Next, a total reflection mirror 6 made of a multilayer dielectric adjusted to have a reflectance of 99% or more is bonded to the other main surface of the Faraday rotator 5 via an acrylic ultraviolet curable optical adhesive. did. Next, before the optical adhesive is cured, the insertion loss (ratio of incident light and reflected light of the single mode fiber 2) is adjusted to 0.7 dB or less, and then acrylic is irradiated by irradiating ultraviolet rays. The UV curable optical adhesive was cured. In this manner, five Faraday rotating mirrors in this example were produced.

<Measurement of Inclination Angle> A laser pointer with a wavelength of 650 nm (trade name: Sashi-40, manufactured by KOKUYO Co., Ltd.) was irradiated on the inclined surface, and the inclination angle was measured based on the reflection position. A laser pointer is irradiated from a position about 1.5 m away from the Faraday rotating mirror 1 on the extension line of the optical axis of the single mode fiber 2, and a concentric light receiving plate is placed at a position about 1.4 m away from the Faraday rotating mirror 1. The displacement x (m) from the optical axis was measured. This measured value was applied to the equation “x / 1.4 = tan θ” to derive the tilt angle.

<Measurement of Return Loss> The return loss was measured by the same measurement method as in Example 1, and the results are shown in Table 1.

[Comparative Example 1]
<Preparation of Faraday Rotating Mirror> The Faraday rotator 5 with the total reflection mirror 6 is closely connected to the end face of the coreless fiber 4 in the fusion spliced body by optical adhesive without adjusting the position with the optical adhesive. In the same manner as in Example 1, a Faraday rotating mirror of this comparative example was produced.

<Measurement of Insertion Loss> Insertion loss was measured by the same measurement method as in Example 1, and the results are shown in Table 1.

<Measurement of Return Loss> The return loss was measured by the same measurement method as in Example 1, and the results are shown in Table 1.

<Evaluation> In the Faraday rotating mirror of Comparative Example 1, the insertion loss is 1.3 dB at the maximum and the return loss is -29 dB at the maximum, so that sufficient characteristics (such as insertion loss) cannot be obtained. In contrast, in the Faraday rotating mirrors of Examples 1 to 3, since the insertion loss is 0.7 dB or less and the return loss is less than −30 dB, sufficient characteristics (such as insertion loss) are obtained. I was able to. In the Faraday rotating mirror of Example 2, the return loss was less than −40 dB, and more excellent characteristics could be obtained. Furthermore, in the Faraday rotating mirror of Example 3, the variation in the tilt angle could be kept within ± 0.5 °, and the return loss was less than −50 dB, and more excellent characteristics could be obtained. Therefore, according to the Faraday rotating mirrors according to the first to third embodiments, even when a graded index fiber that is small in size and has excellent workability (mass productivity) and reliability is employed, there is no increase in insertion loss. It was confirmed that it could be.

It is sectional drawing which represents typically the principal part structure of the Faraday rotary mirror which concerns on the 1st Embodiment of this invention. It is a graph which shows the relationship between the length (GIF length) of a graded index fiber, and its optimal coupling distance. It is a graph which shows the relationship between the pitch which prescribes | regulates the length of a graded index fiber, and a return loss. It is sectional drawing which represents typically the principal part structure of the Faraday rotary mirror which concerns on the 2nd Embodiment of this invention. It is a graph which shows the relationship between a processing angle and a return loss amount. It is sectional drawing which represents typically the principal part structure of the Faraday rotary mirror which concerns on the 3rd Embodiment of this invention. The arrangement | positioning relationship between the Faraday rotator and the total reflection mirror in the Faraday rotation mirror which concerns on this invention is represented, (a) is a side view of a 1st state, (b) is a top view of a 1st state. (C) is a side view of a 2nd state, (d) is a top view of a 2nd state. It is sectional drawing which represents typically the principal part structure of the conventional Faraday rotation mirror. It is a schematic diagram explaining the polarization state of the light in the Faraday rotation mirror shown in FIG. The Faraday rotation mirror of patent document 1 is represented, (a) is sectional drawing which represents the principal part structure typically, (b) is sectional drawing which represents typically only a core expansion fiber. It is sectional drawing which represents typically the principal part structure of the Faraday rotation mirror of patent document 2. FIG. The optical coupling system of Patent Document 3 is schematically represented, (a) is a cross-sectional view illustrating the main structure thereof, (b) is a diagram illustrating a refractive index distribution of a graded index fiber, and (c) is It is a figure showing the meandering period of a graded index fiber.

Explanation of symbols

1, 11, 12, 21, 31, 41 Faraday rotating mirror 2, 22, 52 Single mode fiber 3, 53 Graded index fiber 3a, 33a Core 3b, 33b Clad 4, 54 Coreless fiber 5, 25, 35, 45 Faraday Rotor 6, 26, 36, 46 Total reflection mirror 7, 27, 37, 47 Magnet 8a, 8b, 38, 48 Optical adhesive 10 Optical axis 13 Ferrule

Claims (10)

Single mode fiber, graded index fiber, Faraday rotator and total reflection mirror are arranged in order,
An Faraday rotating mirror characterized by interposing an optical adhesive between at least one of the graded index fiber and the Faraday rotator and between the Faraday rotator and the total reflection mirror. The Faraday rotation mirror according to claim 1, further comprising a coreless fiber interposed between at least one of the graded index fiber and the Faraday rotator and between the Faraday rotator and the total reflection mirror. The surface of the coreless fiber facing the Faraday rotator is inclined with respect to a plane perpendicular to the optical axis of the graded index fiber,
The Faraday rotator is in close contact with the facing surface of the coreless fiber;
The Faraday rotation mirror according to claim 2, wherein the total reflection mirror is arranged perpendicular to the optical axis of the graded index fiber. The Faraday rotating mirror according to claim 1, wherein the optical adhesive has a thickness of 10 μm or more. The Faraday rotation mirror according to any one of claims 1 to 4, wherein the total reflection mirror is directly formed on the Faraday rotator. 6. The Faraday rotation mirror according to claim 1, wherein a length of the graded index fiber is 0.31 to 0.5 times one period of light propagating through the graded index fiber. The Faraday rotation mirror according to any one of claims 1 to 6, wherein the single mode fiber and the graded index fiber are arranged in the through hole of a ferrule having a through hole. 8. The Faraday rotating mirror according to claim 1, wherein an outer shape of the total reflection mirror is located within a range of an outer shape of the Faraday rotator when viewed from an optical axis direction of the graded index fiber. . The Faraday rotator and the total reflection mirror are substantially square when viewed from the optical axis direction of the graded index fiber, and an outline of the Faraday rotator and a diagonal of the total reflection mirror are substantially parallel. Item 9. The Faraday rotating mirror according to Item 8. A single mode fiber, a graded index fiber, a Faraday rotator, and a total reflection mirror are sequentially arranged, between the graded index fiber and the Faraday rotator, and between the Faraday rotator and the total reflection mirror. And interposing an optical adhesive in at least one of
Adjusting the thickness of the optical adhesive;
And a step of curing the optical adhesive.

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