JP2006276507A - Faraday rotation mirror - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Faraday rotation mirror which is small and also which is excellent in work efficiency/mass-productivity/reliability and also which is excellent in coupling efficiency without processing a Faraday rotator intricately as a means for making the unwanted return light of reflected light small. <P>SOLUTION: The Faraday rotation mirror is provided in which at least three types of fibers: a single mode fiber, a graded index fiber; and a core-less fiber, and a Faraday rotator, and a total reflection mirror are arrange sequentially and consecutive three types of fibers are mounted in the through hole of a ferrule. The Faraday rotation mirror is characterized in that the one side end face of the ferrule has at least each of a face vertical and a face inclined to the optical axis, and the principal plane of the flat board-shaped Faraday rotator is mounted on and in contact with the inclined face and at least a part of the principal plane of the total reflection mirror is mounted on the vertical face so as to become vertical to the optical axis. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a Faraday rotating mirror of an optical passive component that is used in an optical fiber sensor system or an optical amplification system and used to stably operate these systems.

  In the optical fiber sensor, the path of the system is mainly composed of an optical fiber, and has a sensing element in one of the optical paths of the optical fiber. The sensing element undergoes some change in optical characteristics depending on the amount to be measured. . For example, when a single mode fiber is used as a sensing element, it can detect external forces such as vibration, pressure, temperature, electric field, magnetic field, and sound wave, and the change in the optical path length of the optical fiber due to these external forces can be detected by optical fiber interference. Detect by meter.

  However, in such an optical fiber sensor, there is a problem that output interference fringes fluctuate and signals disappear due to an accidental change in the polarization state of light due to birefringence in the optical fiber.

  In order to deal with such a problem, Non-Patent Document 1 proposes to use a Faraday rotating mirror as a part of the optical fiber interferometer. This Faraday rotating mirror removes fluctuations in the polarization state caused by birefringence in the optical fiber, and can preserve an arbitrary input polarization state.

  By the way, the Faraday rotating mirror is mounted together with a single mode fiber or an optical fiber coupler which is a detection element of the optical fiber interferometer. The outer diameter of the optical fiber is 0.25 mm (including the protective coating), and the outer diameter of the optical fiber coupler. In contrast to φ2-3 mm, the Faraday rotating mirror is as large as φ5 mm, which increases the size of the apparatus. This is because the diameter of the coupling lens, which is a component of the Faraday rotating mirror, is as large as about 2 mm. If a metal fitting for joining with other components is included, the size becomes such a size.

  In order to solve such a problem, Patent Document 1 proposes miniaturization of a Faraday rotating mirror by using a core expansion fiber instead of a coupling lens.

  FIG. 4A is a schematic cross-sectional view showing the Faraday rotation mirror 11 of Patent Document 1. As shown in FIG.

  The Faraday rotation mirror 11 includes a core expansion fiber 13, a Faraday rotator 15, a total reflection mirror film 16, a magnet 17, and an optical adhesive 18.

  The magnet 17 is a cylindrical magnet that gives a saturation magnetic field parallel to the optical axis to the internal Faraday rotator 15.

  FIG. 4B is a cross-sectional view of the core expansion fiber 13. The outer shape is 0.125 mm (not including the protective coating) as in a general optical fiber. The core expansion fiber 13 includes a core 13a and a clad 13b, and is made by locally heating a general single mode fiber. The core portion is enlarged by thermally diffusing Ge or the like doped in the core 13a.

  The divergence angle of the radiation beam from the core expansion fiber 13 decreases as the core diameter increases and approaches the parallel light. When the core diameter is small and the divergence angle is large, the reflected light is difficult to be coupled to the core expansion fiber 13. Here, the decrease in the coupling efficiency is suppressed by increasing the core diameter by 3 to 4 times.

  However, as the distance from the end face of the core expansion fiber 13 to the total reflection mirror film 16 becomes longer, the radiation beam diverges and the coupling efficiency is lowered. Therefore, the Faraday rotator 15 is brought into close contact with the core expansion fiber 13 and totally reflected. The mirror film 16 is directly formed on the Faraday rotator 15.

  The Faraday rotator 15 is formed of bismuth-substituted garnet or the like, and a magnetic field greater than the saturation magnetic field strength of the garnet is applied by a magnet 17 in a direction parallel to the optical axis direction. The thickness is adjusted to rotate the polarization direction of incident light by 45 °. Further, the bismuth-substituted garnet has a high refractive index of 2.3, and unnecessary reflected light is generated at the end face. Therefore, an antireflection film is applied to prevent the generation. Further, the surface on the side of the core expansion fiber 13 is inclined and processed with respect to the vertical axis of the optical axis so that unnecessary reflected light is not recombined with the core expansion fiber 13. On the other side, a total reflection mirror film 16 made of a multilayer dielectric is directly formed on one side and processed so as to be perpendicular to the optical axis. For this reason, the cross-sectional shape of the Faraday rotator 15 is a trapezoidal shape. The total reflection mirror film 16 has a small loss of light and has a reflectance of 99% or more.

  FIG. 5 is a diagram for explaining the state of polarization of light in the Faraday rotation mirror 11 as viewed from the direction of the core expansion fiber 13. Hereinafter, the operating principle of the Faraday rotating mirror 11 will be described with reference to FIG. For convenience, the light emitted from the core expansion fiber 13 is referred to as incident light, the light reflected by the total reflection mirror film 16 is referred to as reflected light, the incident light direction is referred to as the forward direction, and the reflected light direction 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. 5-a) emitted from the core expansion fiber 13 is transmitted through the Faraday rotator 15, and its polarization direction is rotated 45 ° clockwise as viewed from the forward direction (FIG. 5-b). ). Thereafter, the reflected light reflected by the total reflection mirror film 16 again enters the Faraday rotator 15 from the opposite direction (FIG. 5-c). The reflected light transmitted through the Faraday rotator 15 in the reverse direction is further rotated 45 ° clockwise when the polarization direction is seen from the forward direction, and enters the core expansion fiber 13 (FIG. 5D).

  As a result, the reflected light of the Faraday rotating mirror 11 has a polarization direction orthogonal to the incident light and receives birefringence opposite to that received by the incident light. The polarization state is stabilized in a state orthogonal thereto.

The Faraday rotating mirror 11 as shown in FIGS. 4 and 5 is applied to an optical fiber amplification system as well as an optical fiber sensor system. Since the optical fiber amplification system generally uses several tens to several hundreds meters of a single mode fiber doped with erbium, there is a problem that the polarization state changes due to birefringence in the optical fiber, and further, a long-distance optical fiber communication system. However, there is a problem of polarization mode dispersion that causes signal waveform deterioration. However, by using the Faraday rotating mirror 11, these can be compensated for and a stable output can be obtained. Moreover, since it is mounted together with an optical fiber and a coupler in the same manner as the optical fiber sensor system, it is effective for downsizing the apparatus.
Electronics Letter 14th March 1991 vol27 No6 Japanese Patent No. 3602891

  However, since the core diameter of the single-mode fiber is increased 3 to 4 times for the production of the core expansion fiber 13, it requires heating for several hours to several tens of hours at a temperature of several hundreds of degrees Celsius, resulting in poor workability. Shortening production time has not been improved. In addition, there is a concern that reliability may be lowered by heat treatment for a long time at a high temperature.

  Furthermore, in order to suppress the return light of unnecessary reflected light and not reduce the coupling efficiency, it is necessary to make the Faraday rotator trapezoidal and minimize the distance between the core expansion fiber 13 and the total reflection mirror film 16, In order to make the Faraday rotator into a trapezoidal shape, it is necessary to form a polishing process, a total reflection mirror film or an antireflection film individually after the shape processing, and the workability is poor and is not suitable for mass production.

  The present invention is for solving these problems, a single mode fiber, a graded index fiber, a coreless fiber, a flat Faraday rotator, and a flat total reflection mirror are sequentially arranged, In the Faraday rotating mirror disposed in the through-hole provided in the ferrule with the graded index fiber, the single mode fiber, and the coreless fiber being in contact with each other, the ferrule is on the Faraday rotator side. The one side end surface or the outer peripheral surface of the optical system has an inclined surface that intersects the optical axis and is inclined with respect to a surface orthogonal to the optical axis, and a vertical surface that intersects perpendicularly with respect to the optical axis. The Faraday rotator is brought into close contact with a surface, and at least a part of the total reflection mirror is arranged substantially parallel to the vertical surface. Characterized in that was.

  Furthermore, an angle formed by the inclined surface and the vertical surface is set to 1.5 ° to 8 °.

  Further, a resin or glass having substantially the same refractive index as that of the optical fiber is filled between the Faraday rotator and the total reflection mirror.

  As described above, according to the present invention, a single mode fiber, a graded index fiber, a coreless fiber, a flat Faraday rotator, and a flat total reflection mirror are sequentially disposed, and the graded index fiber and the single In the Faraday rotating mirror arranged in the through hole provided in the ferrule with the mode fibers and the coreless fiber in contact with each other, the ferrule is placed on one end surface or the outer peripheral surface on the Faraday rotator side. An inclined surface that intersects with the optical axis and is inclined with respect to a surface orthogonal to the optical axis, and a vertical surface that intersects perpendicularly with respect to the optical axis, and the Faraday rotator is disposed on the inclined surface. In addition, it is characterized in that at least a part of the total reflection mirror is disposed substantially parallel to the vertical surface after being in close contact with each other. The optical fiber does not need to be heated at a high temperature for a long time without degrading characteristics compared to the conventional technology, and it is excellent in workability, mass productivity, reliability, and unnecessary reflected return light is reduced. As a means for this, a Faraday rotating mirror that does not require complicated processing of the Faraday rotator can be proposed.

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

  Fig.1 (a) is side sectional drawing which shows schematic structure of the Faraday rotation mirror of this invention.

  FIG. 1B is an overhead cross-sectional view showing a schematic configuration of the Faraday rotation mirror of the present invention.

  As shown in the figure, the Faraday rotating mirror 1 includes a single mode fiber 2, a graded index fiber 3, a coreless fiber 4, a Faraday rotator 5, a total reflection mirror 6, a magnet 7, and a ferrule 9.

  The magnet 7 is a cylindrical magnet and gives a saturation magnetic field parallel to the optical axis 10 to the internal Faraday rotator 5.

  As the Faraday rotator 5, a bismuth-substituted garnet crystal or the like is used, and the thickness thereof is adjusted so that the polarization direction of incident light rotates by 45 °. Further, the bismuth-substituted garnet has a high refractive index of 2.3, and unnecessary reflected light is generated at the end face. Therefore, an antireflection film is applied to prevent the generation. The Faraday rotator 5 has a flat plate shape, and can be obtained by polishing a large flat plate, applying an antireflection film, and then cutting it with a dicing cut or the like. The large Faraday rotator 5 is 10 mm square or larger, while the Faraday rotator 5 is 1 mm square or smaller, and 100 or more elements can be obtained in one large size. Compared with the conventional Faraday rotator 15, workability and mass production Excellent in properties.

  The total reflection mirror 6 is made of a multilayer dielectric and is formed on one surface of a glass body. The total reflection mirror 6 made of this multilayer dielectric has a small light loss and a reflectivity of 99% or more.

  FIG. 1C is a cross-sectional view of the graded index fiber 3. The graded index fiber 3 includes a core 3a and a clad 3b. As shown in the figure, the core 3a has a maximum at the center, and a refractive index distribution in which the refractive index decreases stepwise (or continuously) toward the outer periphery. have.

  FIG. 1 (d) is a diagram in which light rays transmitted through the graded index fiber 3 are imaged.

  Since it has a refractive index distribution as shown in FIG. 1C, the light beam transmitted through the graded index fiber 3 is periodically bent and propagated. By cutting to an appropriate length, it functions as a lens that can cope with parallel light and focused light.

  The basic structure of such graded index fiber 3 is used for multimode fibers used in optical communications, but it can be produced in large quantities by producing a base material with a large outer diameter and stretching the base material. Compared with the core expansion fiber 13 shown in Patent Document 1, there is a feature of extremely high productivity.

  The coreless fiber 4 adjusts the optical coupling distance and is a homogeneous optical fiber having no refractive index distribution. By cutting the coreless fiber 4 and the graded index fiber 3 into appropriate lengths and connecting them, a coupling optical system composed of only the fibers can be designed.

  The ferrule 9a has a cylindrical shape, and has a through hole at the center of the cylinder. Generally used for connectors for connecting optical fibers in the optical communication market and made of ceramics such as zirconia and alumina. The through-hole is for fixing the optical fiber, but the clearance between the through-hole of the ferrule for the single mode fiber and the optical fiber is 1 μm or less, and it is highly accurate, and the coupling optical system composed only of the fiber of the present invention Desirable as a system fixture.

  As shown in FIGS. 1A and 1B, the ferrule 9a is processed so that one end face has two planes. As shown in FIG. 1 (b), the plane on the coreless fiber 4 side is processed to be inclined with respect to the optical axis 10, and the Faraday rotator 5 is in close contact with and mounted by an optical adhesive or the like.

  The total reflection mirror 6 is mounted on the end face so as to be perpendicular to the optical axis 10 with an optical adhesive or the like.

  Here, the term “perpendicular” includes all the ranges that can be adjusted to the inclination optimized by the optical adhesive.

  FIG. 3 is a diagram showing the ferrule processing angle with respect to the vertical axis of the through hole of the ferrule 9a and the return loss of the Faraday rotating mirror 11. The reflection attenuation amount is an amount indicating the ratio of the amount recombined to the single mode fiber 2 having the unnecessary reflection amount generated in the Faraday rotator 5 to the amount recombined from the total reflection mirror 6 to the single mode fiber 2 having the reflection amount. .

  As shown in FIG. 3, it can be seen that the return loss increases as the ferrule processing angle increases. This is because the amount of unnecessary reflection does not change, but the efficiency of recombining with the single mode fiber 2 is reduced. Note that the return loss is −27 dB at a processing angle of 0 °, which is the effect of the antireflection film applied to the Faraday rotator 5.

  2A and 2B are overhead sectional views showing another embodiment of the present invention. Although the shape of the ferrule is different, the Faraday rotator 5 is mounted on the inclined surface and the total reflection mirror 6 is mounted perpendicular to the optical axis 10 as in FIG.

  As shown in FIG. 3, when the ferrule processing angle becomes 1.5 ° or more, the return loss becomes −30 dB or less (1/1000 light quantity), which is a very small value. If it is less than 1.5 °, it will be −30 dB or more, which is not a sufficient return loss. When the angle is 8 ° or more, the angle of incidence on the Faraday rotator 5 increases, the amount of light transmitted through the Faraday rotator 5 decreases, and the amount of reflected light begins to decrease, which causes a problem.

  By the way, the total reflection mirror 6 can be maximized by being mounted perpendicularly to the optical axis. However, if the refractive index before and after the Faraday rotator 5 is different, the Faraday rotator 5 changes to the total reflection mirror 6. The optical axis 10 of the traveling light is refracted and slightly inclined with respect to the mounting surface. For example, when the Faraday rotator 5 is tilted by 3 ° and the refractive index of the optical fiber is 1.5, the air layer is 1, and the tilt of the optical axis 10 is 1.5 ° based on Snell's law. For this reason, when the total reflection mirror 6 is mounted, time is required for alignment. If a resin or glass body whose refractive index matches that of the optical fiber is filled between the Faraday rotator 5 and the total reflection mirror 6, such a problem does not occur, and the end face of the ferrule 9 is substantially perpendicular to the optical axis 10. And alignment becomes easy.

  As described above, according to the Faraday rotating mirror 1 shown in the present embodiment, the graded index fiber 3 having a small outer diameter, excellent mass productivity, and high reliability is used for the coupling lens. Even if the flat Faraday rotator 5 with excellent performance is used, it is possible to show excellent return loss, small size, excellent workability, mass productivity and reliability, return loss, coupling efficiency Can propose a Faraday rotating mirror excellent in

  As an example of the present invention, a Faraday rotating mirror 1 shown in FIG. 1 was prototyped. Each component and configuration will be described below.

  The Faraday rotator 5 has a thickness of about 400 μm adjusted so that the polarization direction of incident light is rotated by 45 ° at 1550 nm, and an antireflection film is applied to the end face to prevent unnecessary reflection. The total reflection mirror 6 was formed by forming a multilayer dielectric on a glass body, and having a reflectivity of 99% or more, both of which were obtained by cutting a large 10 mm square into 0.8 mm square.

  The magnet 7 was a cylindrical magnet having an outer diameter of 2.5 mmφ and applied a saturated magnetic field parallel to the optical axis 10 to the internal Faraday rotator 5.

  The single mode fiber 2 was a single mode fiber for 1550 nm. The graded index fiber 3 having a relative refractive index difference of 1.5% and a thickness of about 0.5 mm was used. The coreless fiber 4 has the same refractive index as that of the quartz base material, and the fiber length is about 0.4 mm.

  As the ferrule 9a, a cylindrical zirconia made of no side surface was used.

  These fibers were connected by fusion using an outer diameter alignment. The fusion spliced fiber was mounted on the ferrule 9a with an adhesive. Thereafter, the ferrule was processed with a dicing saw as shown in FIG. The processing angle was 5 °.

After that, the Faraday rotator 5 was connected to the end face of the processed ferrule 9a with an optical adhesive, and fixed by irradiating with ultraviolet rays. Further, the total reflection mirror 6 was adhered with an optical adhesive. Before the adhesive was cured, alignment was performed so that the insertion loss was 0.7 dB (reflectance 85%) or less, and ultraviolet rays were irradiated and cured at the optimum position. The return loss was -62 dB, which was equivalent to the conventional technology.

  According to the above examples, the rough estimation of the process time was estimated, and as shown in Table 1, the mass productivity was excellent, and the effectiveness of the present invention was confirmed.

  Similarly, a Faraday rotator having the configuration shown in FIG. In addition, by filling an optical resin having a refractive index of 1.5 between the Faraday rotator 5 and the total reflection mirror 6, the alignment time of the total reflection mirror is reduced to about 1/3, and the effectiveness of the present invention is further improved. It could be confirmed.

BRIEF DESCRIPTION OF THE DRAWINGS It is schematic structure of the Faraday rotation mirror of this invention, (a) is side surface sectional drawing, (b) is overhead sectional drawing, (c) is a schematic diagram which shows refractive index distribution of GI fiber, (d) is inside GI fiber It is a schematic diagram of the meandering period. It is embodiment of another Faraday rotation mirror of this invention, (a) is side sectional drawing, (b) is an overhead sectional drawing. It is a graph which shows the processing angle of a ferrule and the return loss amount of a Faraday rotation mirror. FIG. 2 is a Faraday rotation mirror of Patent Document 1, wherein (a) is a schematic cross-sectional view of a Faraday rotation mirror, and (b) is a schematic cross-sectional view of a core expansion fiber alone. It is a schematic diagram explaining the polarization state of the light in a Faraday rotation mirror.

Explanation of symbols

1, 11, 101, 111: Faraday rotating mirror 2: Single mode fiber 3: Graded index fiber 3a: Core 3b: Cladding 4: Coreless fiber 5, 15: Faraday rotators 6, 16: Total reflection mirrors 7, 17: Magnet 8: Optical adhesives 9a, 9b, 9c: Ferrule 13: Core expansion fiber 18: Optical adhesive

Claims (3)

A single mode fiber, a graded index fiber, a coreless fiber, a flat Faraday rotator, and a flat total reflection mirror are sequentially arranged, and the graded index fiber, the single mode fiber, and the coreless fiber, In the Faraday rotating mirror disposed in the through hole provided in the ferrule with the facing end surfaces in contact with each other, the ferrule intersects the optical axis on one end surface or outer peripheral surface on the Faraday rotator side, and the An inclined surface that is inclined with respect to a surface orthogonal to the optical axis, and a vertical surface that intersects perpendicularly with respect to the optical axis, the Faraday rotator being in close contact with the inclined surface, and the total reflection A Faraday rotating mirror, wherein at least a part of the mirror is arranged substantially parallel to the vertical plane. The Faraday rotating mirror according to claim 1, wherein an angle formed by the inclined surface and the vertical surface is set to 1.5 ° to 8 °. The resin or glass having substantially the same refractive index as that of the optical fiber is filled between the Faraday rotator and the total reflection mirror. Faraday rotating mirror.

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