JP2002258229A - Optical attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low cost optical attenuator to reduce the number of components. SOLUTION: The optical attenuator comprises a two core optical fiber, a walk-off crystal, a lens and a variable Faraday rotor having a totally reflecting film on one optical face sequentially arranged in the direction of an optical path. The variable Faraday rotor having the totally reflecting film on one optical face can be replaced with the variable Faraday rotor and a totally reflecting mirror. The variable Faraday rotor may be provided with a Faraday rotating material, an electromagnet and a permanent magnet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical attenuator for attenuating and adjusting light intensity in an optical communication system or the like, and more particularly to a variable attenuator having a Faraday rotator suitable for use in a wavelength division multiplexing optical communication device. Related to optical attenuator.

[0002]

2. Description of the Related Art An optical attenuator that controls Faraday rotation angle by controlling current to attenuate light operates as a highly reliable optical attenuator because it has no mechanical movable parts. In particular, an optical attenuator using a Faraday rotator that controls the Faraday rotation angle by changing the angle between the magnetization direction and the light transmission direction while keeping the magnetization of the Faraday rotation crystal saturated, is a light scattering due to a change in the magnetic domain. , And operates as an attenuator with little polarization dependence and excess loss.

FIG. 5 shows a configuration of a general optical attenuator conventionally used. The light emitted from the optical fiber 51a is converted into a parallel light beam by a lens 52a, and is incident on a wedge-shaped birefringent crystal 53a. When passing through this crystal, the ordinary light component and the extraordinary light component of light undergo refraction according to their respective refractive indexes. Next, the light of each component undergoes rotation of the polarization plane by about 0 to 90 ° by the variable Faraday rotator 54, and then passes through the wedge-shaped birefringent crystal 53b, and is converted into a convergent light flux by the lens 52b. Thus, some light is coupled to the optical fiber 51b.

Here, wedge-shaped birefringent crystals 53a and 53a
The direction of the wedge in b is opposite, and the direction of the c-axis (optical axis) is substantially orthogonal. Therefore, when the Faraday rotation angle of the variable Faraday rotator 54 is approximately 90 °, light that has passed through the wedge-shaped birefringent crystal 53a as ordinary light also passes through the wedge-shaped birefringent crystal 53b as ordinary light. As a result, the wedge-shaped birefringent crystals 53a and 53b
The change in the traveling direction of the light received at the step cancels out, and all ordinary light components are coupled to the optical fiber 51b. The situation is the same for light passing through the wedge-shaped birefringent crystal 53a as extraordinary light.

On the other hand, when the Faraday rotation angle is 0 °, light passing through one of the wedge-shaped birefringent crystals 53a and 53b as ordinary light passes through the other as extraordinary light, so that two wedge-shaped birefringent crystals 53a and 53b pass through. Changes in the direction of travel when passing through the crystal are not canceled out. As a result, light does not couple to the optical fiber 51b.

Accordingly, when the Faraday rotation angle is 90 °, the attenuation is zero, and the attenuation increases as the Faraday rotation angle approaches 0 °.

[0007]

In the optical attenuator described as the conventional example, two wedge-shaped birefringent crystals are used. Further, in many applications of the optical attenuator, it is necessary to make the polarization dispersion zero, and in that case, another polarization dispersion compensator, which is a birefringent crystal, is required.

In general, the manufacturing cost of a birefringent crystal element or lens cannot be easily reduced. Therefore, the use of a large number of birefringent crystal elements and lenses increases the manufacturing cost of the optical attenuator.

Therefore, the present invention reduces the number of parts,
It is an object to provide a low-cost optical attenuator.

[0010]

In order to achieve the above object, an optical attenuator according to the present invention reduces the number of parts by reciprocally transmitting light to a walk-off crystal, a lens and a variable Faraday rotator. ing.

That is, in the optical attenuator of the present invention, a two-core optical fiber, a walk-off crystal, a lens, and a variable Faraday rotator having a total reflection film on one of the optical surfaces are arranged in this order in the optical path direction. It is composed.

Further, the optical attenuator of the present invention may include a two-core optical fiber, a walk-off crystal, a lens, a variable Faraday rotator, and a total reflection mirror.

Further, the variable Faraday rotator may include a Faraday rotation material, an electromagnet, and a permanent magnet.

[0014]

Next, an embodiment of the present invention will be described with reference to FIGS.

(Embodiment 1) FIG. 1 is a schematic diagram showing an optical configuration of an optical attenuator according to Embodiment 1 of the present invention. Reference numeral 11 denotes a two-core optical fiber in which an input optical fiber 18 and an output optical fiber 19 are integrated. 12 is a walk-off crystal, 13 is a convex lens,
A variable Faraday rotator 14 has a total reflection film 15 formed on one of its optical surfaces.

Here, the variable Faraday rotator 14 used will be described with reference to FIG. In FIG. 3, reference numeral 32 denotes a Bi-substituted rare earth iron garnet crystal.
1a and 31b are electromagnet yokes, and 33 is a permanent magnet. The light travels along the optical path 34, is reflected by the total reflection film formed on the optical surface on the side of the permanent magnet 33, and travels backward in the optical path 34.

Also, Bi-substituted rare earth iron garnet crystal 3
2 is 0.8 × 0.8 × 0.4 mm, a fixed magnetic field in the direction of the optical path 34 is applied by the permanent magnet 33, and the variable magnetic field orthogonal to the optical path 34 is provided by the yokes 31 a and 31 b of the electromagnets. Is applied.

By controlling the current of this electromagnet,
The direction of the saturation magnetization of the Bi-substituted rare-earth iron garnet crystal 32 is controlled to obtain a variable Faraday rotation angle of about 0 to 90 ° in a reciprocating manner.

As the walk-off crystal 12 shown in FIG. 1, a parallel plate of rutile single crystal can be used, and a crystal having a thickness of about 0.75 mm is easy to use. The convex lens 13 having a focal length of about 2 mm is easy to use.

The progress of light in the optical attenuator according to the present embodiment will be described with reference to FIG. First,
A case where the coupling efficiency from the input side to the output side of the two-core optical fiber is almost 100% will be described with reference to FIG.
Light emitted from the upper fiber of the two-core optical fiber enters the walk-off crystal 12, is separated into optical paths 16a and 17a depending on whether it is ordinary light or extraordinary light, and proceeds after being emitted. Proceed with the directions parallel. afterwards,
The two lights are converted into a parallel light beam by the convex lens,
When the light enters the variable Faraday rotator 14, is reflected by the total reflection film 15, and exits, it undergoes a 90 ° Faraday rotation. Next, the two light beams are converted into convergent light beams by the convex lens 13, and the traveling directions of the light beams become parallel again after exiting, travel along the optical paths of 16b and 17b, and the walk-off crystal. It is incident on 12.

At this time, the light that was ordinary light on the outward path passes through the walk-off crystal 12 as extraordinary light because of the 90 ° polarization plane rotation received by the variable Faraday rotator. Light that was extraordinary light on the outward path passes through the return path as ordinary light. As a result, the two lights exit to the same optical path and couple to the lower optical fiber of the two-core optical fiber.

Next, a case where there is almost no coupling from the input side to the output side of the two-core optical fiber will be described with reference to FIG. In this case, due to the magnetic field of the electromagnet, the direction of magnetization of the Faraday rotation material and the traveling direction of light are substantially perpendicular, and the Faraday rotation angle is substantially zero. However, on the return path, the optical paths of the two lights coincide with those in FIG. 4A until the light enters the walk-off crystal 12 again.

However, in the walk-off crystal 12, the light that was extraordinary light on the outward path is extraordinary light even on the return path, and the light that was ordinary light on the outward path is also ordinary light on the return path. In this case, the two lights are further separated from each other and are incident on a point away from the core of the lower optical fiber. That is, most of the light does not couple to the output side optical fiber.

Therefore, in order to obtain a predetermined amount of attenuation, the current applied to the electromagnet is controlled, and the direction of the combined magnetic field of the electromagnet and the permanent magnet is controlled. Adjust the angle with the traveling direction to adjust the Faraday rotation angle. Therefore, when the current of the electromagnet is zero, the amount of attenuation is almost zero, and the amount of attenuation increases as the current of the electromagnet increases.

By the way, regarding the assembling, a collimator is composed of a two-core optical fiber, a walk-off crystal, and a convex lens, and thereafter, the distance between the convex lens and the total reflection film of the variable Faraday rotator, and the central axis of the convex lens and the variable Faraday rotator. The angle between the rotor and the total reflection film may be adjusted.

(Embodiment 2) FIG. 2 is a schematic diagram showing an optical configuration of an optical attenuator according to Embodiment 2 of the present invention. 21 is a two-core optical fiber, 22 is a walk-off crystal, 23 is a convex lens, 24 is a variable Faraday rotator, and 25 is a total reflection mirror.

Here, the variable Faraday rotator 24 can be constituted by disposing a total reflection mirror 25 between the Bi-substituted rare earth iron garnet crystal 32 and the permanent magnet 33 shown in FIG.

The light traveling operation is the same as in the first embodiment. The only difference is that the variable Faraday rotator 24 and the total reflection mirror 25 are separated, so that the degree of freedom in optical path adjustment is increased.

In the above two embodiments, one walk-off crystal, one lens, and one Bi-substituted rare earth iron garnet crystal may be used. Also, B
The i-substituted rare earth iron garnet crystal may be half the thickness of the conventional example.

Further, in the optical attenuator of the present invention, since the input and output optical fibers are drawn out to one side, the space for routing the optical fibers can be reduced.

Further, in the optical attenuator of the present invention, the two optical paths separated according to the polarization have the same optical path length, so that no polarization dispersion compensator is required.

Therefore, in the optical attenuator of the present invention, the cost can be reduced by reducing the number of parts. Also, since the assembling method is as described in the first embodiment, the number of steps is small.

[0033]

As described above, according to the present invention,
An optical attenuator having a small number of parts, a small size, and a low cost can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing an optical configuration of an optical attenuator according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an optical configuration of an optical attenuator according to a second embodiment of the present invention.

FIG. 3 is a perspective view showing a main part of a variable Faraday rotator in the optical attenuator according to the first embodiment of the present invention.

FIG. 4 is a diagram showing a light traveling operation in the optical attenuator according to the first embodiment of the present invention. FIG. 4A is a diagram illustrating a case where the entire amount of light is coupled between input and output, and FIG. 4B is a diagram illustrating a case where the entire amount of light is attenuated.

FIG. 5 is a diagram showing an optical configuration of a conventional optical attenuator.

[Explanation of symbols]

 11, 21 Two-core optical fiber 12, 22, Walk-off crystal 13, 23 Convex lens 14, 24 Variable Faraday rotator 15 Total reflection film 16a, 16b, 17a, 17b, 34 Optical path 18 Input optical fiber 19 Output optical fiber 25 All Reflector 31a, 31b Electromagnet yoke 32 Bi-substituted rare earth iron garnet crystal 33 Permanent magnet

Claims (3)

[Claims] 1. A two-core optical fiber, a walk-off crystal, a lens, and a variable Faraday rotator having a total reflection film on one of optical surfaces are arranged in the optical path direction in this order. Optical attenuator. 2. A two-core optical fiber, a walk-off crystal, a lens, a variable Faraday rotator, and a total reflection mirror,
An optical attenuator, which is arranged in this order in an optical path direction. 3. The optical attenuator according to claim 1, wherein the variable Faraday rotator includes a Faraday rotation material, an electromagnet, and a permanent magnet.