JP2001042263A - Faraday rotation mirror - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Faraday rotation mirror of a small size and a simple structure which make alignment of the optical axes of optical parts easy and which has low insertion loss, extremely small unnecessary reflected light and high characteristics. SOLUTION: The Faraday rotation mirror 1 consists of an optical fiber 2, a Faraday rotator 4 disposed at the output end of the optical fiber 2 and a total reflection mirror disposed on the opposite side of the Faraday rotator 4 with respect to the optical fiber 2. The optical fiber 2 has an end face perpendicular to the optical axis. The face of the Faraday rotator 4 facing the optical fiber 2 is formed as a convex curved plane, and the face of the rotator opposite to the optical fiber 2 is formed to be perpendicular to the optical axis, and the total reflection film mirror 5 is formed on that face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Faraday rotating mirror used in an optical fiber sensor system or an optical amplifier system. Faraday rotating mirrors are optical passive components used to stably operate these systems.

[0002]

2. Description of the Related Art An optical fiber sensor is one in which a system path is mainly constituted by an optical fiber and has a sensing element somewhere in an optical path. The sensing element undergoes some optical characteristic change depending on an amount to be measured. It is. When using a single mode fiber as the sensing element, vibration, pressure,
External force such as temperature, electric field, magnetic field, sound wave, etc. can be detected. Since the optical path length of the optical fiber changes due to these external forces, the change in the optical path length is detected by a fiber interferometer.

In such an optical fiber sensor, there is a problem in that the incident interference pattern fluctuates and the signal disappears due to an accidental change in the polarization state of light due to birefringence in the optical fiber. For such a problem, 199
One year, A. D. It has been proposed by KERSEY et al. To use a Faraday rotating mirror in a fiber interferometer. A 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. Each time light passes through the Faraday rotating mirror element, the polarization state is non-reciprocally 45 °.
Rotate. At this time, together with the left and right conversion of the polarization plane by reflection on the mirror surface, the incident polarization plane is canceled to return to the same polarization state as the original signal.

FIG. 7 is a schematic view showing the structure of a conventional Faraday rotation mirror. Faraday rotating mirror 11 is optical fiber 1
2, a configuration using a coupling lens 13, a Faraday rotator 14, a total reflection mirror 15, and a magnet 6.

[0005] The Faraday rotator 14 is made 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 6 in a direction parallel to the optical axis direction. The thickness is adjusted so as to rotate the polarization direction of the incident light by 45 °. The total reflection mirror 15 and the coupling lens 13 are installed such that the light emitted from the optical fiber 12 is reflected by the total reflection mirror and is efficiently coupled to the optical fiber 12 again.

FIG. 8 is a view for explaining the operation principle of the conventional Faraday rotation mirror 11 as viewed from the direction of the optical fiber 12. For convenience, light emitted from the optical fiber 12 is referred to as incident light, and light reflected by the total reflection mirror is referred to as reflected light. In addition, although the incident polarization direction is linearly polarized, the present invention is not limited to this, and is applicable to any polarization direction.

The polarization direction a of the incident light emitted from the optical fiber 12 is transmitted through the Faraday rotator 14, and the polarization direction is rotated clockwise by 45 ° with respect to the traveling direction of the light to become the polarization direction b. . Thereafter, the reflected light reflected by the total reflection mirror 15 is again reflected in the Faraday rotator 1 in the polarization direction c from the opposite direction.
4 is incident. The reflected light that has passed through the Faraday rotator 14 in the opposite direction is further rotated by 45 ° in the polarization direction, and enters the optical fiber 12 in the polarization direction d. As a result, the reflected light of the Faraday rotation mirror 11 has a polarization direction orthogonal to the incident light, and undergoes birefringence just opposite to the incident light, so that the output polarization is changed with respect to an arbitrary input polarization state. The wave state is stabilized to a state orthogonal thereto.

Further, the conventional Faraday rotation mirror 11 is applied to an optical fiber amplification system in addition to an optical fiber sensor system. In an optical fiber amplifier,
An excitation light source that oscillates excitation light, an amplification optical fiber that can amplify signal light by the excitation light, and an excitation light and the signal light from the excitation light source that are provided at one end of the amplification optical fiber. An optical coupler that guides the light into the amplification optical fiber, and a Faraday rotation mirror as light reflection means provided at the other end of the amplification optical fiber and reflecting at least the signal light to one end of the amplification optical fiber. Can be provided.

Since an optical fiber amplification system generally uses an erbium-doped single mode fiber of several tens to several hundreds of meters, the polarization state changes due to birefringence in the optical fiber. Another problem is the existence of polarization mode dispersion that causes signal waveform degradation in long-haul optical fiber communication systems. These problems are compensated for by using the Faraday rotation mirror 11, and a stable output can be obtained.

[0010]

However, in the conventional Faraday rotation mirror 11 as described above, the number of parts is large, not only the size of the apparatus is increased, but also each component must be precisely optically adjusted. In addition, there is a problem that the number of steps is large, the assembly is complicated, and it takes time.

The present invention has been made in view of the above points, and has as its object to provide a Faraday rotation mirror which is small in size, has a simple structure, has a small insertion loss, and can easily adjust the optical axis of each optical component. It is to be.

[0012]

SUMMARY OF THE INVENTION In order to solve these problems of the prior art, the present invention provides an optical fiber and a Faraday rotator disposed at an output end thereof, and an optical fiber of the Faraday rotator opposed to the optical fiber. In a Faraday rotation mirror consisting of a total reflection mirror arranged on the side, the optical fiber has an end surface perpendicular to the optical axis, and the Faraday rotator has a surface facing the optical fiber with a convex curved surface, The surface on the reflection side is perpendicular to the optical axis, and a total reflection film mirror is formed on this surface.

[0013] Further, it is characterized in that the incident light from the optical fiber is set to pass through the Faraday rotator, be totally reflected by the total reflection mirror film, and travel in the opposite direction on the incident optical path.

[0014]

According to the present invention, by using a Faraday rotator having a lens function and forming a total reflection mirror film on one surface of the Faraday rotator, the optical axis can be easily adjusted and the number of parts can be reduced. .

Further, a high-performance Faraday rotating mirror with little unnecessary reflected light and very small insertion loss is realized.

Further, by bringing the optical fiber and the Faraday rotator into close contact with each other via an optical adhesive, a small-sized Faraday rotating mirror which is easy to assemble is realized.

[0017]

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

FIGS. 1 and 2 show the configuration of a Faraday rotation mirror according to the present invention. The Faraday rotating mirror 1 includes an optical fiber 2
(2 'is an optical fiber coating), a ferrule 3, a Faraday rotator 4, a total reflection mirror 5, a magnet 6, and an optical adhesive 7. The magnet 6 is a cylindrical magnet and has an internal Faraday rotator 4.
Consider a saturation magnetic field parallel to the optical axis. The ferrule 3 holds the optical fiber 2, and the optical fiber 2 and the ferrule 3 have their connection end faces with the Faraday rotator 4 polished flat with respect to a plane perpendicular to the optical axis.

The Faraday rotator 4 has a flat surface 4b on one side and a convex surface 4a on the other side facing the optical fiber 2 acting as a lens. The material is a bismuth-substituted garnet crystal or the like. Is adjusted so that the polarization direction of the incident light is rotated by 45 °.

The total reflection mirror film 5 is made of a multilayer dielectric,
It is formed directly on the plane 4 b of the Faraday rotator 4. The total reflection mirror film 5 made of a multilayer dielectric has a small loss of light and has a reflectance of 99% or more. By integrating the Faraday rotator 4 and the total reflection mirror film 5 in this manner,
The number of parts can be reduced.

The end face 2a of the optical fiber 2 is polished so as to be perpendicular to the optical axis of the optical fiber 2, and the center of the convex curved surface 4a of the Faraday rotator 4 is located on the center optical axis of the end face 2a. It is installed to be. The plane 4b of the Faraday rotator 4 is a plane perpendicular to the optical axis of the optical fiber 2. Therefore, the incident light 8 passes through the Faraday rotator 4 in parallel with the optical axis, and the light transmitted through the surface 4a is
It is set and arranged so that the light is totally reflected by the total reflection mirror film 5 on the surface 4b in parallel with the optical axis and travels on the incident optical path in the opposite direction and enters the optical fiber 2 again.

The ferrule 3, the end face 2 a of the optical fiber 2 held by the ferrule 3, and the convex curved surface 4 a of the Faraday rotator 4 are in close contact with each other via an optical adhesive 7. The optical adhesive 7 is preferably optically transparent and has a refractive index close to that of the core of the optical fiber 2. As the optical adhesive, for example, a photo-curable adhesive,
There is a thermosetting adhesive, a low-melting glass, or the like.

As described above, when the optical fiber 2 and the Faraday rotator 4 are brought into close contact with each other, the insertion loss of the Faraday rotation mirror 1 can be further reduced because the distance between the fibers becomes short. Further, since the Faraday rotator 4 is directly adhered to the end face of the flat-polished ferrule 3, the Faraday rotator 4
There is no need to use a holder for holding the Faraday mirror, and no assembly adjustment is required, and a Faraday rotation mirror with a smaller number of parts can be configured.

Next, the operation of the Faraday rotation mirror 1 of the present invention will be described.

When the incident light 8 passes through the optical adhesive 7 from the end face 2a of the optical fiber 2, the optical adhesive 7 and the convex curved surface 4
When the light enters the interface a, the light is partially reflected at the interface at that surface and returns to the optical adhesive 7, and the other portion is transmitted and refracted at the interface and proceeds to the Faraday rotator 4. Assuming that the light incident on the convex curved surface 4a is incident light 8, and the returned light is unnecessary reflected light 17, the unnecessary reflected light 17 travels in parallel to the surface formed by the normal of the convex curved surface 4a and the direction of the incident light 8; Since the unnecessary reflected light 17 on the convex curved surface 4a is reflected with an angle shift of twice with respect to the optical axis, it is hardly coupled to the optical fiber 2 and the unnecessary reflected light 17 is removed.

By forming the convex curved surface 4a as described above,
In addition to providing a lens function, the function of removing unnecessary reflected light 17 can be achieved. The convex curved surface 4a may have, for example, a spherical shape or an aspherical shape.

Each time the incident light 8 passes through the Faraday rotator 4, the polarization direction is non-reciprocally rotated by 45 ° with respect to the progress of the light, and is totally reflected by the parallel total reflection mirror film 5 to be reflected light 9.
Becomes The reflected light 9 passes through the same optical path as that of the incident light 8 in the Faraday rotator 4 and is transmitted after the polarization direction is further rotated by 45 °. Further, the reflected light 9 is transmitted and refracted at the boundary between the convex curved surface 4a and the optical adhesive 7, and the reflected light 9 travels through the optical adhesive 7, enters the end face 2a, and is inserted into the optical fiber 2.

As a result, the reflected light 9 has a polarization direction orthogonal to the incident light 8 and undergoes a birefringence just opposite to that of the incident light 8. The polarization state is stabilized to a state orthogonal thereto.

As described above, the reflected light 9 from the flat surface 4b passes through the same optical path as the incident light 8 and does not cause an incident axis shift or an angle shift due to refraction. Unnecessary reflected light 17 generated on the convex curved surface 4a is hardly coupled to the optical fiber 2 because the unnecessary reflected light 17 is reflected with an angle shift of twice with respect to the optical axis. Accordingly, the Faraday rotation mirror 1 with high insertion characteristics and a high characteristic of the unnecessary reflection light 17 having very small unnecessary reflection light 17 is realized.

In another embodiment of the present invention, the core at the end face of the optical fiber 2 can be enlarged.

FIG. 3 is a sectional view showing the core-enlarged optical fiber. The optical fiber 2 includes a core 21 and a clad 22, and the diameter of the core 21 before expansion is D, and the diameter of the core 21 after expansion is W. In the structure of the optical fiber 2 in which the core is enlarged, the core diameter of the single-mode fiber is generally enlarged in a tapered shape, and the core diameter W is three to four times the transmission path at the end.
have. This increase in the core diameter is caused by the optical fiber core 21.
Is realized by thermal diffusion of the dopant contained in the core, and the refractive index distribution of the core enlarged region becomes smaller than that of the unexpanded portion.

If the Faraday rotation mirror 1 of the present invention is constituted by using such an optical fiber 2 having an enlarged core,
Further, the characteristics can be improved.

The insertion loss of the Faraday rotation mirror 1 is determined by the core diameter W at the end of the optical fiber 2, the inter-fiber connection distance Z, the loss L of the Faraday rotator 4, and the reflectance R of the total reflection mirror film 5. Can be calculated by

For example, the optical adhesive 7 is a light-curing adhesive, the convex surface 4a of the Faraday rotator 4 having an effective thickness of 170 μm has a radius of curvature of 350 μm, and the fiber connection distance Z
= 340 μm, core diameter W = 40 μm, loss L = 0.1,
When the Faraday rotator mirror 1 shown in FIG. 1 is configured with the reflectance R = 99%, the insertion loss is about 0.5 dB, and it can be seen that a sufficiently low insertion loss can be realized without using a lens as in the related art. .

FIG. 4 shows an example of an optical fiber connection experiment, in which the cores of two optical fibers 2 having the same core diameter are expanded so that the core expansion regions of the optical fibers are opposed to each other, and the inter-fiber distance Z is changed. FIG. 5 shows the relationship between the inter-fiber distance Z and the connection loss when light having a wavelength of 1.55 μm is coupled.

In FIG. 5, each curve represents the core expansion ratio W / D of the optical fiber obtained by enlarging the core of the optical fiber 2, which is 1 ×, 2 ×
These are the results calculated for the case of double, triple and quadruple. From this result, it is understood that the connection loss becomes smaller as the core enlargement ratio is larger, and the tolerance characteristic of the distance Z between the optical fibers is improved. In the case of a normal single mode fiber having a core magnification of 1, the splice loss increases sharply as Z increases, and it is understood that it is necessary to reduce the loss by using a lens. Also, when a quadruple core enlarged fiber is used, the connection loss is about 1 dB even when the distance between the fibers is Z = 800 μm, and it can be seen that coupling with a small connection loss is possible without using a lens.

As described above, when the core expanded fiber is used as the optical fiber 2, even if the Faraday rotator 4 is inserted between the two optical fibers 2, the connection loss due to the distance between the optical fibers can be suppressed to a small value. . In addition, since the configuration is made without a lens, optical adjustment is easy and the number of parts is reduced.

FIG. 6 is a schematic perspective view showing a third embodiment of the present invention, and is a schematic perspective view of a Faraday rotation mirror 20 having a plurality of input / output ports. The substrate 30 is a substrate for holding the plurality of optical fibers 2 and includes a V-groove substrate 30a and a cover 30b. Also, the substrate 30
The contact end face of the Faraday rotator 4 with the Faraday rotator 4 has a surface perpendicular to the optical axis.
And the opposing surface of the total reflection mirror film 5 is in close contact with the optical adhesive 7.

According to the present embodiment, by assembling the Faraday rotation mirror 20 by holding the plurality of optical fibers 2 on the substrate 30, the assembly can be simplified, and when the Faraday rotation mirror 20 is incorporated in a system. Space saving is realized. Furthermore, since the Faraday rotator 4 corresponding to the plurality of optical fibers 2 can be formed as an integral structure, the end surface of the core-enlarged fiber is kept in a polished state, uniform characteristics can be obtained at each port.

[0040]

As described above, according to the Faraday rotation mirror of the present invention, the number of parts can be reduced by integrating the Faraday rotator and the total reflection mirror.
Further, by using the core-expanded optical fiber, the connection loss caused by the distance between the optical fibers can be reduced. Further, since the lens is configured without a lens, optical adjustment is easy, and the number of parts can be reduced.

Furthermore, by using a Faraday rotator having a convex curved surface, a high-performance Faraday rotator with very little unnecessary reflected light and small insertion loss is realized.

Further, by forming the Faraday rotation mirror by holding the coated optical fiber by the substrate, the assembly can be simplified, and the space can be saved when the Faraday rotation mirror is incorporated in the system.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view of a main part of a Faraday rotation mirror according to the present invention.

FIG. 2 is a vertical cross-sectional view of the Galade rotating mirror of the present invention.

FIG. 3 is a cross-sectional view showing an enlarged core optical fiber.

FIG. 4 is a diagram showing an example of an optical fiber connection experiment.

FIG. 5 is a graph showing a relationship between a distance Z between optical fibers and a connection loss.

FIG. 6 is a schematic perspective view showing another embodiment of the present description.

FIG. 7 is a schematic configuration diagram of a conventional Faraday rotation mirror.

FIG. 8 is a diagram illustrating the operation principle of a conventional Faraday rotation mirror.

[Explanation of symbols]

 1: Faraday rotating mirror 2: Optical fiber 3: Ferrule 4: Faraday rotator 5: Total reflection mirror film 6: Magnet 7: Optical adhesive 8: Incident light 9: Reflected light 17: Unnecessary reflected light 21: Core 22: Cladding 30: substrate 30a: V-groove 30b: cover

Claims (6)

[Claims] 1. A Faraday rotator comprising: an optical fiber; a Faraday rotator disposed at an output end of the optical fiber; and a total reflection mirror disposed on an opposite side of the Farade rotator to the optical fiber. The optical fiber has an end surface perpendicular to the optical axis, and the Faraday rotator has a convex surface on the surface facing the optical fiber, and the optical fiber and the reflection side surface are perpendicular to the optical axis, A Faraday rotation mirror characterized in that a total reflection film mirror is formed on this surface. 2. The apparatus according to claim 1, wherein the incident light from the optical fiber is set so as to pass through the Faraday rotator, be totally reflected by the total reflection mirror film, and travel in the opposite direction on the incident optical path. A Faraday rotating mirror according to 1. 3. The optical fiber is held by a ferrule.
The Faraday rotation mirror according to claim 1, wherein the ferrule and the Faraday rotator are closely bonded to each other via an optical adhesive. 4. A method according to claim 1, wherein a plurality of said optical fibers are held on an alignment substrate having a V-groove, and said Faraday rotator is closely bonded to an end face of said alignment substrate. The described Faraday rotating mirror. 5. The Faraday rotation mirror according to claim 4, wherein said Faraday rotator is integrally formed. 6. The Faraday rotation mirror according to claim 1, wherein the optical fiber is a core-expanded fiber.