JP2989982B2 - Fiber type optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator for blocking reflected light returning to a semiconductor laser in the field of optical communication and optical measurement, and more particularly to an in-line type which can be incorporated between an optical fiber amplifier and an arbitrary transmission line. And a fiber type optical isolator.

[0002]

2. Description of the Related Art Semiconductor lasers (laser diodes,
In optical communication and optical measurement using an LD as a light source, when light reflected in the middle of a transmission path returns to the active layer of the LD as a light source, the oscillation wavelength and output fluctuate, and accurate signal transmission and the like occur. Measurement becomes impossible. There are various causes of this reflection,
In addition to reflection not only at the optical fiber, optical element, or each input / output surface of the connected device, it is not possible to avoid damage to the optical fiber, stress on the optical fiber, bending of the optical fiber, and uneven refractive index. Some are due to possible factors. Therefore, means for preventing reflected light is indispensable in a transmission path using an LD as a light source. An optical isolator is such an LD
This device prevents reflected light from returning to the device.

Hitherto, a combination of a bulk type Faraday rotator and a polarizer has been used as an optical isolator. When an isolator is incorporated in an optical fiber amplifier or an arbitrary portion in a transmission line, a so-called in-line type or pigtail type in which a lens and an optical fiber are attached to an input / output end of the isolator is used.

FIG. 5 shows a conventional example of an in-line optical isolator (Japanese Patent Publication No. 60-51690), in which a first birefringent plate 6 formed in a parallel flat plate so that an optical axis is inclined with respect to the surface, and a first birefringent plate 6 respectively. It has the same surface and the inclination angle of the optical axis as the birefringent plate, has a thickness of one half of the route of the first birefringent plate, and has the axis of incident light with respect to the first birefringent plate A second birefringent plate 7 and a third birefringent plate 8, each of which is rotated by an angle of 45 degrees;
And a Faraday rotator 9 having a rotation of the polarization plane of 45 degrees. When the incident light beam 10 is incident on the first birefringent plate 6, it is separated into two orthogonal linearly polarized light beams (ordinary light and extraordinary light), travels straight as two parallel light beams, and the Faraday rotator 9 changes the polarization plane to 45 light. Rotate degrees. The light exiting the Faraday rotator 9 enters the second birefringent plate 7 whose optical axis is oriented in the same direction as the direction of rotation of the plane of polarization due to the Faraday effect. The light having a polarization plane parallel to the optical axis of the birefringent plate is emitted from a deviated position and is combined by the third birefringent plate 8 into one light beam. Next, the light coming from the right proceeds through the third birefringent plate 8 and exits the second birefringent plate 7 only in the reverse of the above description. Since the polarization plane is rotated, the light emitted from the Faraday rotator 9 has a polarization plane different from that of the light coming from the left by 90 degrees. Therefore, the light incident on the first birefringent plate 6 from the right is emitted to a position different from that from the left like the emitted light 11 and is blocked.

[0005]

However, the optical isolator having the structure shown in FIG. 5 uses a large number of expensive birefringent plates, so that it is difficult to provide an inexpensive product. The cross section becomes large because the light beam passes through the book. In addition, it is very difficult to separate and combine two light beams and to adjust the optical axis of each optical element and the polarization direction of the light beams, which takes a lot of time to manufacture, and causes an increase in cost. Further, since there are many entrance / exit surfaces by a large number of optical elements, reflection and scattering loss are large. Also, an optical system such as a lens is required for coupling the optical isolator and the optical fiber,
Again, a loss of optical power will occur.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to solve these problems of the prior art and to provide a method for directly adding an optical isolator function to an optical fiber itself.
An optical fiber using an isotropic material, wherein a part of a clad of the optical fiber is removed, a magneto-optical material is loaded on the portion, and an additional waveguide is formed by adjusting the shape of the magneto-optical material. Provided is a fiber-type optical isolator that is a layer and generates non-reciprocal distributed coupling with light in an optical fiber core.

That is, it is known that the propagation constant of light propagating in a forward direction and a backward direction in a waveguide made of a magneto-optical material to which a magnetic field is applied is different. This is called a non-reciprocal phase shift. If the propagation constant is β when no magnetic field is applied, for example, β + Δβ in the forward direction and β−Δ in the reverse direction
Express as β. The magnitude of Δβ is determined by the characteristics of the magneto-optical material and the structure of the waveguide. By utilizing this property, it is possible to configure a branch coupler having different coupling efficiencies in the forward and reverse directions.

[0008] The transition of the power between the two waveguide layers in the distributed coupling is represented by the propagation constant of the waveguide layer A being βa, the propagation constant of the waveguide layer B being βb, the coupling length being L, and the coupling coefficient of the two waveguides being χ. The ratio of the power transferred from the waveguide layer A to the waveguide layer B can be expressed as the following PAB with the initial value being 1.

[0009]

(Equation 1)

From this equation, it can be seen that by changing both or one of the propagation constants βa and βb, the distance at which the power changes and the rate at which the power changes can be changed. , Width, and length), the propagation constant can be changed in the forward and reverse directions, and a branch coupler that couples 0% in the forward direction and 100% in the reverse direction can be realized. That is, this is a circulator,
And an isolator. In the past, optical isolators were considered to be completely separate from optical fibers and connected using connectors, lenses, etc., and there was no idea to use optical fibers themselves as optical isolators. It is the present invention to add the non-reciprocal waveguide coupling properties directly to the transmission optical fiber. This will be described with reference to FIG. A part of the clad 2 of the optical fiber 1 is removed, and a waveguide layer made of a magneto-optical material is formed in that part in parallel with the optical fiber core 3. This will be referred to as an additional waveguide layer 4. A magnetic field is applied to the additional waveguide layer 4 from the outside by a permanent magnet, an electromagnet, or the like in a direction Y perpendicular to the light traveling direction.

[0011]

The light propagating through the core 3 of the optical fiber undergoes distributed coupling with the additional waveguide layer 4 at a certain portion of the additional waveguide layer 4 to exchange optical power. The additional waveguide layer 4 has Y
Since a magnetic field is applied in the direction, a non-reciprocal propagation constant difference occurs between the forward direction and the reverse direction. The forward propagation constant is βb + Δβ
b, the reverse direction is βb−Δβb, where Δβb is due to the magneto-optical effect due to the application of the magnetic field. Optical fiber core 3
Is the propagation constant of βa, the forward direction is the waveguide of βa and β
The distribution coupling of the waveguide of b + Δβb can be regarded as the distribution coupling of the waveguide of βa and the waveguide of βb−Δβb in the opposite direction. FIG. 2 shows the optical power P AB transferred from the optical fiber core 3 to the additional waveguide layer 4 as a function of the coupling length L.

F corresponds to the forward direction and B corresponds to the reverse direction. Looking at the coupling length Lc in this figure, almost 0% coupling occurs in the forward direction and about 100% coupling occurs in the reverse direction. Therefore, assuming that the length of the additional waveguide layer 4 is Lc, the light in the forward direction travels through the core 3 of the optical fiber as it is, and the light in the reverse direction is almost 100.
Since it moves to the% additional waveguide layer 4, it cannot propagate through the core 3 and return in the opposite direction. That is, it is understood that an optical isolator is formed. In this manner, a fiber in-line optical isolator can be manufactured with a very simple configuration. There is no connection point in the optical isolator and there is no worry about reflection. Also, no complicated optical element alignment or lens system is required.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention, in which a part of a cladding 2 of a transmission single mode optical fiber 1 is removed, and an additional waveguide layer 4 made of a magneto-optical material is formed thereon. If the light absorbing substance 5 is attached to the end of the additional waveguide layer 4, reflection and scattering here are eliminated, and the characteristics are improved.

FIG. 3 is an explanatory view of the manufacturing method of the present invention.
The cross section of the optical fiber is shown. First, the clad portion 2 at an arbitrary portion of the optical fiber 1 is removed (FIG. (A)), and a groove portion 12 is formed by electron beam etching or the like (FIG. (B)). The width W, depth T, and length (coupling length) L of the groove 12 are adjusted according to the propagation constant of light in the optical fiber core 3 and the characteristics of the loaded magneto-optical material. When a magnetic garnet or the like is loaded on a silica-based single-mode optical fiber, W and T are about 0.1 μm to 0.3 μm. L is about several mm to 30 mm. Next, a magneto-optical material 13 such as garnet is loaded (FIG. (C)). This may be a conventional thin film forming process, but since it may be a film of 1 μm or less, it can be formed at very low cost by a dip method or the like. Finally, the additional waveguide layer 4 is formed by polishing or the like so that the magneto-optical material 13 remains only in the groove 12 (FIG. (D)). The removal of the clad 2, the formation of the magneto-optical material 13, and the polishing of the finish can be performed simultaneously with a plurality of optical fibers in parallel, and the productivity is very high.

FIG. 4 shows a second embodiment of the present invention, in which an optical isolator is formed in the middle of an optical fiber amplifier. Conventionally, it has been necessary to attach an in-line optical isolator via a connector. However, in the present invention, such a connection portion is not required and an optical isolator function can be directly added to an optical fiber. It is also possible to form an optical isolator in the erbium-doped optical fiber itself.

[0016]

As described above, according to the structure and method of the present invention, since there is no lens or polarizer in a simple structure, no optical alignment is required, and no expensive birefringent plate is required. Further, since the magneto-optical material may have a thickness of 1 μm or less, it can be manufactured at very low cost. Further, the number of components is small, and the optical fiber itself is used as an optical isolator, so that the size is reduced. Furthermore, since multiple fibers can be processed at the same time, productivity is high, and since there is no entrance / exit surface on the way, there is little fear of reflection, and it can be directly added to any transmission optical fiber, so that it can be applied to any fiber. Is high and the utility value is large.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a first embodiment of the present invention.

FIG. 2 is a graph showing non-reciprocal distributed coupling of the present invention.

FIG. 3 is a schematic diagram showing a manufacturing method of the present invention.

FIG. 4 is a schematic diagram showing a second embodiment of the present invention.

FIG. 5 is a schematic view showing a conventional in-line optical isolator.

[Explanation of symbols]

 Reference Signs List 1 optical fiber 2 clad 3 core 4 additional waveguide layer 5 light absorbing material

Claims (1)

(57) [Claims] 1. An optical fiber using an isotropic substance,
A part of the cladding of the optical fiber is removed, a magneto-optical material is loaded on the part, and the shape of the magneto-optical material is adjusted to form an additional waveguide layer, which is non-reciprocal to light in the optical fiber core. A fiber type optical isolator characterized by generating distributed coupling.