JP2006276506A - Faraday rotation mirror - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a smaller profile, excellent productivity, and further good optical characteristics for a Faraday rotation mirror. <P>SOLUTION: The Faraday rotation mirror in which a graded index fiber, a coreless fiber, a Faraday rotator, a glass body, and a total reflection mirror are sequentially located on one end of an optical fiber with an inclined face, is characterized in that the end part of the coreless fiber on the Faraday rotator side has an inclined face; the glass body has two non-parallel faces where the optical axis passes through; the Faraday rotator is spliced to one of the two faces on the coreless fiber side and also the total reflection mirror is formed on the other face; and the angle β between the inclined face of the coreless fiber and a plane perpendicular to the optical axis is set to the angle α or larger formed between one face and the other face of the glass body. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a Faraday rotating mirror of an optical passive component used for 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 optical path length of the optical fiber due to these external forces can be detected by a fiber interferometer. Detect by.

  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.

  For such problems, it has been proposed to use a Faraday rotating mirror as a part of the fiber interferometer. This Faraday rotating mirror is an optical component that removes fluctuations in the polarization state caused by birefringence in an optical fiber and preserves an arbitrary input polarization state.

  FIG. 3 shows the configuration of a conventional Faraday rotating mirror 10. The Faraday rotation mirror 10 includes an optical fiber 11, a coupling lens 12, a Faraday rotator 14, a total reflection mirror 15, and a magnet 13.

  The Faraday rotator 14 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 in a direction parallel to the optical axis direction by the magnet 13. The thickness is adjusted to rotate the polarization direction of incident light by 45 °. The optical fiber 11 is a single mode fiber.

  The total reflection mirror 15 is disposed so that the light emitted from the optical fiber 11 is reflected by the total reflection mirror 15. The coupling lens 12 is disposed so that the light reflected by the total reflection mirror 15 is efficiently coupled to the optical fiber 11 again.

  FIG. 4 is a diagram for explaining the state of polarization of light in the Faraday rotation mirror 16 as viewed from the direction of the optical fiber 11. Hereinafter, the operating principle of the Faraday rotating mirror 16 will be described with reference to FIG. For convenience, the light emitted from the optical fiber 11 is referred to as incident light, the light reflected by the total reflection mirror 15 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. 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. 4-a) emitted from the optical fiber 11 is transmitted through the Faraday rotator 14, and the polarization direction thereof is rotated 45 ° clockwise as viewed from the forward direction (FIG. 4-b). . Thereafter, the reflected light reflected by the total reflection mirror 15 enters the Faraday rotator 14 from the opposite direction again (FIG. 4-c). The reflected light transmitted through the Faraday rotator 14 in the reverse direction is further rotated 45 ° clockwise when the polarization direction is viewed from the forward direction, and enters the optical fiber 11 (FIG. 4D).

  As a result, the reflected light of the Faraday rotation mirror 16 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.

  A conventional Faraday rotating mirror 10 as shown in FIG. 3 is used not only in an optical fiber sensor system but also in an optical fiber amplification 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 degradation. However, by using the Faraday rotating mirror 10, these can be compensated and a stable output can be obtained.

  In Patent Document 1, the applicant of the present application uses a core expansion fiber having the same outer diameter as that of the optical fiber instead of the coupling lens 12. In addition, to reduce the number of processes and simplify assembly, a proposal is made to form a total reflection mirror film on one side of the Faraday rotator, or to adhere the end face of the core expansion fiber and the Faraday rotator via an optical adhesive. I am doing.

  FIG. 5A is a schematic configuration diagram showing the Faraday rotation mirror 20 of Patent Document 1. The Faraday rotation mirror 20 includes a core expansion fiber 21, a Faraday rotator 14, a total reflection mirror film 15, and a magnet 13. The magnet 13 is a ring-type magnet and gives a saturation magnetic field parallel to the optical axis to the internal Faraday rotator 14. For the Faraday rotator 14, 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 °. The total reflection mirror film 15 is made of a multilayer dielectric and is directly formed on one surface of the Faraday rotator 14. The total reflection mirror film 15 made of this multilayer dielectric has a small light loss and a reflectivity of 99% or more. Here, the total reflection mirror film 15 is directly formed on the Faraday rotator 14, and the number of parts is reduced.

  FIG. 5B is a cross-sectional view of the core expansion fiber 21. The core expansion fiber 21 includes a core 21a and a clad 21b. The core expansion fiber 21 is manufactured by locally heating a general single mode fiber and thermally diffusing Ge or the like doped in the core 21a. .

  The divergence angle of the radiation beam from the optical fiber 21 decreases as the core diameter increases and approaches the parallel light. When the divergence angle is large, the reflected light is difficult to be coupled to the optical fiber 21.

  Here, the decrease in the coupling efficiency is suppressed by increasing the core diameter by 3 to 4 times.

Further, in Patent Document 2, the present applicant has proposed a Faraday rotating mirror that suppresses unnecessary reflected light by making the shape of the Faraday rotator tapered.
JP 1997-21608 JP 1997-26556 A

  However, as described above, the conventional Faraday rotating mirror has the following problems.

  The number of parts is large, precise optical adjustment is required for each part, man-hours are large, assembly is complicated, and production takes time. The reflected light was mixed with unnecessary reflected light from the end faces of the optical components, which caused the characteristics of the Faraday rotating mirror to deteriorate.

  Further, Patent Document 1 proposes a reduction in the number of steps and simplification of the assembly along with downsizing of the Faraday rotator mirror 20. However, since light rays enter the Faraday rotator 14 almost perpendicularly, Even if the anti-reflection coating is applied, the unnecessary reflection cannot be sufficiently suppressed, and the reflection attenuation amount is only about 30 dB. This is because the core expansion fiber 21 is used and the NA is small, and since the Faraday rotator 14 and the total reflection mirror 15 are integrated, the Faraday rotator 14 cannot be inclined. .

  Thus, although the Faraday rotation mirror 21 proposed in Patent Document 1 can be reduced in size, it has a structure in which the return loss cannot be improved.

  In Patent Document 2, the Faraday rotator is tapered, and unnecessary reflection on the surface thereof is suppressed. However, polishing an expensive optical crystal into a tapered shape causes high costs.

  In the present invention, in order to solve the above problems, a graded index fiber, a coreless fiber, a Faraday rotator, a glass body, and a total reflection mirror are sequentially arranged on one end side of an optical fiber having an inclined surface. In the Faraday rotating mirror, an inclined surface is provided at the end of the coreless fiber on the Faraday rotator side, and the glass body is disposed on one surface located on the coreless fiber side of the two non-parallel surfaces through which the optical axis passes. The Faraday rotator is bonded, the total reflection mirror is formed on the other surface, and an angle β formed between the inclined surface of the coreless fiber and a surface perpendicular to the optical axis of the coreless fiber is the glass body. The angle between the one surface and the other surface is set to an angle α or more.

  Further, the angle β is set to 3 to 9 degrees.

  As described above, according to the present invention, in the Faraday rotation mirror in which the graded index fiber, the coreless fiber, the Faraday rotator, the glass body, and the total reflection mirror are sequentially arranged at one end of the optical fiber having the inclined surface. The end of the coreless fiber on the side of the Faraday rotator has an inclined surface, and the glass body has two non-parallel surfaces through which the optical axis passes. A Faraday rotator is joined, a total reflection mirror is formed on the other surface, and an angle β formed between the inclined surface of the coreless fiber and a surface perpendicular to the optical axis is set to one surface of the glass body. Since the other surface is set to an angle α or more, when the total reflection mirror is aligned so that the insertion loss is minimized, the Faraday rotator is simultaneously optical fiber. Since it is arranged at a desired optimum angle with respect to the light emitted from the light, the return loss can be suppressed. In other words, by arranging the Faraday rotator at an angle, unwanted reflections on the surface of the Faraday rotator mirror are effectively suppressed. Therefore, both the insertion loss and return loss characteristics of the Faraday rotator mirror can be simplified. At the same time, it becomes possible to adjust to a good value.

  Moreover, the arrangement angle of the Faraday rotator can be freely changed by simply changing the shape of an inexpensive glass body.

  In addition, the process of joining the glass body and the Faraday rotator is uncentered and can be performed in advance in a separate process, which not only improves productivity, but also significantly reduces the number of parts and excels in mass productivity. Become a Faraday rotator.

  Since the angle β is set to 3 to 9 degrees, the member arrangement conforms to the refraction angle generated due to the magnitude relationship of the refractive index of the member, and the low return loss and the high return loss are obtained. It becomes the Faraday rotator it has.

  Embodiments of the present invention will be described with reference to the drawings. In addition, the same code | symbol is used about the code | symbol similar to a prior art.

  FIG. 1 is a diagram showing a configuration of a Faraday rotating mirror according to the present invention.

  As shown in the figure, the Faraday rotation mirror 1 includes an optical fiber 2, a graded index fiber 3, a Faraday rotator 4, a glass body 5, a total reflection mirror 6, a magnet 7, and a coreless fiber 8.

  Reference numeral 3 denotes an optical fiber type lens having substantially the same diameter as that of the optical fiber 2, which corresponds to a graded index fiber, a green lens, a core expansion fiber, or the like. The graded index fiber has an axially symmetrical refractive index distribution of approximately square, and can be used as a gradient index lens. By cutting with the optimum length Z for the wavelength used, the light emitted from the optical fiber 2 can be collimated or condensed without diverging, and the coupling of the Faraday rotating mirror 1 can be made efficient. Can do.

  The magnet 7 is a ring-type magnet and gives a saturated magnetic field parallel to the optical axis to the internal Faraday rotator 4.

  The Faraday rotator 4 uses a bismuth-substituted garnet crystal or the like, and its thickness is adjusted so that the polarization direction of incident light rotates by 45 °.

  The total reflection mirror 6 is made of a dielectric multilayer film and is formed on one surface of the glass body 5. The total reflection mirror 6 made of this multilayer dielectric has a small light loss and a reflectivity of 99% or more.

  The Faraday rotator 4 is desirably provided with an antireflection film in order to prevent reflection of light. With this antireflection film, the insertion loss of the Faraday rotating mirror 1 is reduced, and unnecessary reflections that cause deterioration in characteristics can also be reduced.

  However, in order to increase the return loss to 50 dB or more, not only the end face of the coreless fiber 8 is obliquely polished at an angle β of 3 to 9 degrees, but also the refractive index of the coreless fiber 8, the Faraday rotator 4, and the glass body 5 is increased. The angle α of the inclined surface of the glass body 5 must be determined in accordance with the refraction angle caused by the magnitude relationship. The optimum value of the tilt angle is obtained from Snell's law.

  That is, a material having a low refractive index is selected for the coreless fiber 8 so as to supplement the lens effect of the graded index fiber 3, and a refractive index of 1.46, for example, is suitable. On the other hand, as the material of the glass body 5, an optical glass having a refractive index of 1.5 to 1.8, such as BK7, which is excellent in translucency and scientifically stable is selected.

  When a material having a refractive index as described above is selected, if the angle β is made larger than the angle α, the insertion loss of the Faraday rotating mirror 1 can be suppressed to 0.8 dB or less.

  Here, the Faraday rotator 4 and the glass body 5 are joined in advance in a self-aligning manner to form an optical element block 17. If the optical element block 17 is moved during assembly to adjust the angle of the total reflection mirror 6 so that the insertion loss is minimized, the Faraday rotator 4 is fixed at an angle according to the inclination angle α of the glass body. . At this time, if the optical element block 17 is bonded to the end face of the optical fiber type lens 3, the support member can be largely omitted. Also, the length can be shortened because there is no space.

  FIG. 2 is a sectional view showing the structure of the tip of the Faraday rotating mirror of the present invention. The description of the above-described symbols is omitted. In addition, the layer by an adhesive agent is not illustrated.

  M is a line schematically representing the central axis of the optical path. At this time, symbols A to D each represent the size of the refraction angle. At this time, if the angle β is selected in the range of 3 to 9 degrees, the return loss becomes 50 dB or more, and a high-performance Faraday rotating mirror is obtained.

  As described above, according to the Faraday rotating mirror 1 shown in the present embodiment, it is more compact, excellent in productivity, and more excellent optical characteristics can be obtained.

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

  The Faraday rotator 4 has a thickness of about 400 μm adjusted so that the polarization direction of incident light is rotated by 45 ° at 1550 nm, and the total reflection mirror 6 is a dielectric multilayer film, and includes a glass body 5. The one having a reflectance of 99% or more was used. A surface of the Faraday rotator 4 and a surface opposite to the total reflection mirror film of the glass body are provided with an antireflection film of an adhesive.

  The magnet 7 is a ring type having an outer diameter of 2.5 mmφ and applies a saturated magnetic field parallel to the optical axis to the internal Faraday rotator 4.

  The optical fiber 2 is a single mode fiber for 1550 nm and has a mode field diameter of 8 μm.

  The Faraday rotator 4 and the glass body 5 are joined in advance in a self-aligning manner to form an optical element block 17.

  When the refractive index of the coreless fiber 8 is 1.46, the refractive index of the Faraday rotator 4 is 2.4, and the refractive index of the glass body is 1.5, the end face inclination angle β of the optical fiber 2 is 8 degrees, and the glass The inclination angle of the body 5 was 7.7 degrees.

  The total reflection mirror 6 was aligned by moving the glass body 17 so that the insertion loss was 0.5 dB. At this time, since the inclination angle of the glass body is set to 7.7 degrees, the Faraday rotator 4 is fixed at an angle of about 7.7 degrees with respect to the optical axis, the insertion loss is 0.5 dB, and the return loss is 55 dB. The Faraday rotator was obtained.

  Since the conventional Faraday rotating mirror shown in FIG. 5 has an insertion loss of 1.0 dB and a return loss of 28 dB, significant performance improvement is achieved by the present invention. Further, the conventional Faraday rotating mirror of FIG. 3 has a number of parts exceeding 15, but the Faraday rotator according to the present invention can reduce the number of parts to 10 or less, and there are 1 centering places. I was able to simplify the location.

  Next, the reliability of the fabricated Faraday rotating mirror was evaluated. The tests were conducted vibration test, impact test, temperature cycle test, high temperature holding test, low temperature holding test, high temperature high humidity test shown in Telcordia 1221. In all tests, the amount of change in insertion loss is ± 0.2 dB or less, Good results were obtained with a change in return loss of ± 3 dB or less.

  With the above trial manufacture, it is possible to provide a Faraday rotating mirror that is small in size, excellent in productivity, excellent in optical characteristics, and excellent in reliability.

It is sectional drawing which shows the structure of the Faraday rotation mirror of this invention. It is sectional drawing which shows the structure of the front-end | tip part of the Faraday rotation mirror of this invention. It is sectional drawing which shows the structure of the conventional Faraday rotation mirror. It is a schematic diagram which shows the function of a Faraday rotation mirror. It is another structure of the conventional Faraday rotation mirror, (a) is a Faraday rotation mirror, (b) is sectional drawing of a core expansion fiber.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 10, 16, 20 Faraday rotation mirror 2, 11 Optical fiber 3 Graded index fiber 4, 14 Faraday rotator 5 Glass body 6, 15 Total reflection mirror 7, 13 Magnet 12 Coupling lens 17 Optical element block 8 Coreless fiber 21 Core expansion fiber 21a Core 21b Clad α Inclination angle of glass body β Inclination angle A, B, C, D Refraction angle L Optical path M Optical axis central axis

Claims (2)

In the Faraday rotation mirror in which the graded index fiber, the coreless fiber, the Faraday rotator, the glass body, and the total reflection mirror are sequentially arranged on one end side of the optical fiber having the inclined surface,
An inclined surface is provided at the Faraday rotator side end of the coreless fiber,
The glass body has the Faraday rotator bonded to one surface located on the coreless fiber side of the two non-parallel surfaces through which the optical axis passes, and the total reflection mirror is formed on the other surface,
Further, the angle β formed by the inclined surface of the coreless fiber and the surface perpendicular to the optical axis of the coreless fiber is set to be equal to or greater than the angle α formed by one surface and the other surface of the glass body. mirror. 2. The Faraday rotating mirror according to claim 1, wherein the angle [beta] is set to 3 to 9 degrees.

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