JPH04349421A - Optical isolator - Google Patents

Abstract

PURPOSE:To eliminate the angle deviation between an incidence side and a projection side optical fiber by cutting a slanting groove halfway in the optical fiber integrated with a substrate and arranging polarization beam splitters, a Faraday rotator, and a lambda/2 wavelength plate in the groove. CONSTITUTION:The groove 8 is cut slantingly halfway in the optical fiber 4 embedded in the substrate 2, the polarization beam splitters 10 and 14, Faraday rotator 16, and 1/2-wavelength plate 12 are arranged therein, and a magnet is fitted. The incidence side and projection side optical fibers are present on the same optical axis and their end surfaces are oblique to the optical axis. Therefore, light is refracted and projected slantingly from the incidence side optical fiber 4 and reaches the projection side optical fiber 4 after being made by the polarization beam splitters 10 and 14 to shift laterally by a constant quantity in parallel to the original light. For the purpose, the optical isolator is designed by adjusting the angle theta of the groove 8 to the optical axis and the thicknesses of the polarization beam splitters 10 and 14, preferably, so that the refraction angle of the optical fiber 4 and the quantity of lateral deviation by the polarization beam splitters cancel each other.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to an optical isolator for preventing reflected light from an optical system in optical fiber communication using a semiconductor laser, input/output of an optical disk, etc.
In particular, the present invention relates to a polarization-independent optical isolator that is attached to a fiber-type optical amplifier or the like and is not affected by the polarization direction of incident light and is embedded in an optical fiber.

0002

[Prior Art] Semiconductor lasers, which are the main light source for optical fiber communications and optical disk input/output, are used at the end faces of optical fibers coupled to them, at the connection points between optical fibers, or at optical fibers such as coupling lenses and optical connectors. It is known that oscillation becomes unstable when reflected light from the system is received, resulting in significant deterioration of operating characteristics such as increased noise and output fluctuations. Various optical isolators have been developed to eliminate the instability of semiconductor laser operation caused by this reflected light and to realize a stable light source for optical communications.

[0003] A Rochon prism is used as a polarizer and an analyzer, and a Faraday rotator made of YIG (yttrium iron garnet) single crystal or Bi-substituted garnet is used, and an SmCo material is used to magnetize the Faraday rotator in the forward direction. Optical isolators constructed using perforated permanent magnets such as
Also, there is a drawback that only a certain plane of polarization is transmitted through the polarizer, and when light that does not match the transmission direction of the polarizer is incident, the amount of transmitted light is significantly reduced. Furthermore, when this optical isolator is used by being inserted between optical fibers, since the polarization plane of the light beam propagating through the optical fibers is generally not constant, the amount of light passing therethrough varies. When inserted between optical fibers, an optical isolator without polarization dependence is desirable.

[0004] Therefore, in order to achieve a configuration in which the loss in the forward direction is almost zero for all polarization planes without depending on the polarization direction, a tabular birefringent crystal such as calcite or an artificial anisotropic crystal is used instead of the Rochon prism. Various polarization-independent optical isolators have been proposed that utilize the separation/synthesis of ordinary and extraordinary light using a medium. For example, examples using artificial anisotropic media include Kazuo Shiraishi and Shojiro Kawakami, "Polarization independence of fiber-embedded isolators using artificial anisotropic media," 1990 Institute of Electronics, Information and Communication Engineers Spring National Conference C. -2
90, P. As described in 4-343, a polarization-independent system is constructed by combining a beam expanding fiber, three dielectric multilayer film polarizing beam splitters, and one Bi-substituted rare earth garnet Faraday rotator in a row. An optical fiber-embedded optical isolator has been proposed. When such an optical fiber embedded optical isolator is used, there is no need for a lens system, and in particular, there is no need to use a fiber collimator configured by strictly adjusting a single mode fiber and a microlens. Therefore, it is expected that it will be much easier to assemble compared to conventional products, and it is also expected that it will be smaller, more stable, and have lower loss.

[0005] The beam expanding fiber described above is a single mode optical fiber that is heat-treated at one end to diffuse the dopant and enlarge the mode field diameter. When two beam expanding fibers are placed facing each other, the coupling loss between the fibers is reduced. can be made smaller. For example, a mode field diameter of approximately 10 μm is expanded to approximately 40 μm, and the length of the portion where the mode field diameter is expanded is several cm.
It is. The dielectric multilayer optical beam splitter described above is an artificial anisotropic medium consisting of alternating multilayer films of a-Si (amorphous silicon) and SiO2 with a thickness of about 100 nm, and has a large anisotropy of refractive index. It has a large polarization separation angle compared to calcite and rutile. The above-mentioned document shows that a polarization separation angle of about 23 degrees can be obtained using an optimally designed dielectric multilayer beam splitter made of alternating multilayer films of a-Si and SiO2.

[0006]

[Problem to be Solved by the Invention] However, in this type of optical isolator, the light propagating in the forward direction reaches the output fiber after being parallel to and laterally shifted by a certain amount with respect to the optical axis of the incident light due to the polarization beam splitter. do. Therefore, the optical fiber on the input side and the optical fiber on the output side need to be arranged not on the same axis but with a certain amount of horizontal displacement. However, arranging them so that they are shifted laterally by a certain amount involves considerable difficulty in the assembly process. Moreover, if an angular deviation occurs between the input side optical fiber and the output side optical fiber at this time, coupling loss will increase. In particular, when a beam expanding fiber is used, coupling to obliquely incident light is extremely poor.
There was a problem to be solved in that when the above-mentioned angular deviation occurred, light would no longer be transmitted at all.

[0007]

[Means for Solving the Problems] In view of the above-mentioned points, it is an object of the present invention to provide a polarized light beam that eliminates the occurrence of angular deviation between the input side optical fiber and the output side optical fiber, and that can be easily assembled without adjustment. The object of the present invention is to provide an optical isolator that is independent and has an embedded optical fiber.

In order to achieve the above object, the present invention provides a substrate in which one optical fiber is embedded, and an optical fiber at a predetermined angle with respect to the optical axis of the optical fiber in the middle of the optical fiber embedded in the substrate. a groove for cutting the optical fiber obliquely, a polarizing beam splitter, a Faraday rotator, and a λ/2 wavelength plate inserted into the groove, and a magnet for magnetizing the Faraday rotator. Features.

[0009] Furthermore, a preferred aspect of the present invention is characterized in that the optical fiber around the groove is a beam expanding fiber.

[0010] Further, as another preferred embodiment of the present invention,
The angle that the groove makes with the optical axis of the optical fiber and the thickness of the polarizing beam splitter are set in advance so that the refraction angle of the optical fiber and the amount of lateral shift caused by the polarizing beam splitter cancel each other out. Features.

[0011] Further, as another preferred embodiment of the present invention,
The angle that the groove makes with the optical axis of the optical fiber is 2° to 10°.
It is characterized by a range of °.

0012

[Operation] As described above, in the optical fiber embedded type optical isolator of the present invention, a groove for inserting an optical element is cut diagonally at a predetermined angle in the middle of one optical fiber embedded in a substrate.
A polarizing beam splitter, a Faraday rotator, and a λ/2 wavelength plate are arranged in this groove. In this way, the input side optical fiber and the output side optical fiber are composed of a single optical fiber embedded in the substrate and cut in the middle by a groove, so that they always exist on the same optical axis. Furthermore, since the groove is cut diagonally, the end face of the optical fiber is diagonal to the optical axis. Therefore, the light is refracted and emitted obliquely from the input side optical fiber, and this light reaches the output side optical fiber at the polarizing beam splitter parallel to the original light and laterally shifted by a certain amount. Therefore, the refraction angle of the optical fiber and the amount of lateral shift caused by the polarizing beam splitter cancel each other out (
If the angle at which the groove becomes the optical axis and the thickness of the polarizing beam splitter are adjusted in advance, the light emitted from the polarizing beam splitter will be output on the same optical axis as the input optical fiber. Coupled to side optical fiber. Therefore, a polarization-independent optical isolator can be easily assembled without adjustment simply by inserting a polarization beam splitter and a Faraday rotator with a predetermined thickness into the groove.

[0013] Also, since the present invention has a configuration of two normal polarizing beam splitters, one Faraday rotator (F.R.), and one λ/2 wavelength plate, the total thickness of these optical crystals can be reduced. Can be made thinner. For example, when using rutile crystal as a polarizing beam splitter to achieve a separation width of 70 μm, configuration 1: rutile (707 μm) + F. R. (290μm
) + Rutile (500μm) + Rutile (500μm) = 1
997 μm Configuration 2; Rutile (707 μm) + F. R. (290μm
) + crystal λ/2 wavelength plate (32μm) + rutile (707μm)
m) = 1736 μm, which is 26 μm for configuration 2 compared to configuration 1.
It becomes thinner by about 0 μm. Furthermore, in order to achieve a separation width of 50 μm, even when a dielectric multilayer film is used as a polarizing beam splitter, the configuration 3: dielectric multilayer film (121 μm) + F. R. (29
0μm) + dielectric multilayer film (86μm) + dielectric multilayer film (
86 μm) = 583 μm Configuration 4: Dielectric multilayer film (121 μm) +F. R. (29
0 μm) + crystal λ/2 wavelength plate (32 μm) + dielectric multilayer film (121 μm) = 564 μm, configuration 4 is better than configuration 3.
It can be made thinner by about 20 μm compared to .

[0014] If the total thickness of the optical crystal can be reduced in this way, the MFD of the expanding section of the beam expanding fiber does not have to be so large, which is not only advantageous in terms of manufacturing, but also
This is desirable because it is insensitive to increased loss caused by fiber inclination. Note that the wavelength plate does not necessarily have to be a λ/2 wavelength plate, and may be an (n+1/2) λ wavelength plate, but it is preferable that n (an integer including zero) be small.

[0015]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a schematic configuration of an optical isolator according to an embodiment of the present invention. As shown in this figure, a groove 8 into which an optical element 6 is inserted is cut diagonally at a predetermined angle θ approximately in the middle of one optical fiber 4 embedded in a substrate 2.
As shown in FIG. 2, a plurality of parallel plate-shaped polarizing beam splitters 10, 14, a Faraday rotator 16, and a crystal λ/2 wavelength wave 12 constituting the optical element 6 are arranged, and furthermore, this Faraday rotator A cylindrical magnet (see FIG. 4C) is assembled to cover the optical fiber 16, thereby forming an optical fiber-embedded polarization-independent optical isolator. In FIG. 2, 4A is an optical fiber on the input side, 4B is an optical fiber on the output side, and 2B is an upper substrate.

Angle θ(
e.g. 2° to 10°) and a polarizing beam splitter 10,1
The thickness of 4 (10 and 14 have the same thickness) is designed in advance so that the refraction angle of the optical fiber and the amount of lateral shift caused by the polarizing beam splitter cancel each other out. Therefore, the light emitted from the polarizing beam splitter 14 on the output end side enters the output side optical fiber 4B, which is on the same optical axis as the input side optical fiber 4A, as described above.

Although rutile crystal or calcite plates can be used as the polarizing beam splitters 10 and 14, a-Si
An artificial anisotropic medium made of a dielectric multilayer film such as an alternating multilayer film of SiO2 and SiO2 is more preferable because it can obtain a larger polarization separation angle than calcite or rutile and can reduce the element length (thickness). As the Faraday rotator 16, Bi-substituted garnet or RIG (rare earth iron garnet) such as (YbTbSi)2Fe5O12 is preferable because the dimensions can be reduced, but various magneto-optical materials such as YIG (yttrium iron garnet single crystal) can also be used. Of course you can use it. As the magnet described later for magnetizing the Faraday rotator 16, an annular perforated permanent magnet such as SmCo can be used.

Furthermore, when a beam expanding fiber is used as the optical fiber 4 around which the groove 8 is cut, it is possible to take advantage of the fact that the input side fiber 4A and the output side fiber 4B are completely on the same optical axis. . Furthermore, in this case, there is an advantage that the mode fields of the input side fiber 4A and the output side fiber 4B are strictly equal.

Next, a manufacturing process of an embodiment of the present invention will be explained.

■Beam expanding fiber manufacturing process As shown in FIG. 3, the core 18 is first made of GeO2 doped SiO2.
, the cladding 20 is made of SiO2, and the core diameter D1
A single mode fiber was prepared with a cladding diameter D2 of 8.5 μm, a cladding diameter D2 of 125 μm, and a core refractive index difference Δn=0.32%. The mode field diameter (diameter) at this initial stage was 9 μm. A length of 5 m of this single mode fiber was taken, and a 10 cm resin strip was removed from the middle portion of the fiber, and the stripped portion was heated at 1300° C. for about 15 hours. It was confirmed that the mold field diameter (MFD) of this heated portion was 35 μm using a fiber made under the same conditions.

■Embedding the fiber in the substrate and cutting the groove As shown in FIG. 4(A), a V-groove 22 with a depth of 130 μm is cut in the silicon chip 2A measuring 4 mm x 20 mm x 2 t.
The beam expanding fiber 4 obtained by the above heat treatment is embedded in this V-groove 22, and a 4 mm x 20 mm x
A 2t silicon chip 2B was placed on it and fixed with resin or the like.

Next, as shown in FIG. 4B, a groove 8 having a width of 2 mm was cut at the center of the silicon chips 2A and 2B at an angle of 5° with respect to the optical axis. The cutting depth at this time was such that the embedded fiber was completely cut.

■ Insertion process of optical element Next, as shown in FIGS. 1 and 2, insert 7 into the groove 8.
Two rutile crystal plates 10 with thicknesses of 07 μm and 707 μm
, 14, Bi-GdYIG 16 with a thickness of 290 μm, and a crystal λ/2 wavelength plate 12 with a thickness of 32 μm were inserted to the bottom of the groove 8 and fixed with resin. At this time, Bi-GdYI
The G Faraday rotator 16 was placed next to the first rutile crystal plate 10 on the incident side, and the λ/2 wavelength plate 12 was placed next to this Faraday rotator 16.

(2) Magnet assembly process Finally, as shown in FIG. 4C, the above chip was passed through the cylindrical magnet 24 and both were fixed together.

It was confirmed that the polarization-independent optical isolator produced as described above had a forward coupling loss of 1.0 dB and an isolation of 39 dB.

[0027]

[Effects of the Invention] As explained above, according to the present invention,
A groove for placing an optical element is diagonally cut in the middle of a single optical fiber embedded in the substrate, and a polarizing beam splitter, a Faraday rotator, and a λ/2 wavelength plate are arranged in this groove. This configuration eliminates angular misalignment between the input optical fiber and the output optical fiber, and the light emitted from the polarizing beam splitter is coupled to the output optical fiber, which is on the same optical axis as the input optical fiber. Therefore, the effect of easy assembly without any adjustment can be obtained. Furthermore, since the total thickness of the optical crystal is reduced, it is advantageous in terms of manufacturing and characteristics.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing a schematic configuration of an optical isolator according to an embodiment of the present invention.

FIG. 2 is a top plan view of the optical isolator of FIG. 1;

FIG. 3 is a diagram showing a refractive index profile of an optical fiber used in an embodiment of the present invention.

FIG. 4 is a perspective view showing a manufacturing process of an optical isolator according to an embodiment of the present invention.

[Explanation of symbols]

2, 2A, 2B board 4, 4A, 4B Optical fiber 6 Optical element 8 groove 10, 14 Polarizing beam splitter 12　λ/2 wavelength plate 16 Faraday rotator 18 core 20 Clad 22 V groove 24 Magnet

Claims (6)

[Claims] Claim: 1. A substrate in which one optical fiber is embedded, and the optical fiber is installed obliquely at a predetermined angle with the optical axis of the optical fiber between the optical fibers embedded in the substrate. An embedded optical fiber characterized by comprising a groove for cutting, a polarizing beam splitter, a Faraday rotator and a λ/2 wavelength plate inserted into the groove, and a magnet for magnetizing the Faraday rotator. optical isolator. 2. The optical fiber-embedded optical isolator according to claim 1, wherein the optical fiber around the groove is a beam expanding fiber. 3. The angle that the groove makes with the optical axis of the optical fiber and the thickness of the polarizing beam splitter are set in advance so that the refraction angle of the optical fiber and the amount of lateral shift caused by the polarizing beam splitter cancel each other out. 3. The optical fiber-embedded optical isolator according to claim 1, wherein the optical isolator is embedded in an optical fiber. 4. The optical fiber-embedded optical isolator according to claim 3, wherein the angle between the groove and the optical axis of the optical fiber is in the range of 2° to 10°. 5. The optical fiber-embedded optical isolator according to claim 1, wherein the polarizing beam splitter is made of a dielectric multilayer film. 6. Claims 1 to 4, wherein the polarizing beam splitter is made of a birefringent crystal such as rutile.
The optical fiber-embedded optical isolator according to any one of the above.