JP2000347135A - Magneto-optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To greatly improve the productivity of a Faraday rotator, to improve the extinction ratio of the Faraday rotator and to make its distribution smaller. SOLUTION: This magneto-optical waveguide is constituted by growing a lower clad layer consisting of a magnetic material crystal thin film on a substrate, growing a core layer consisting of the magnetic material crystal thin film on the lower clad layer, shaping and processing part of at least the core layer and further, growing an upper clad layer consisting of the magnetic material crystal thin film on the waveguide substrate. The upper clad layer is used as an optical waveguide to induce a magneto-optical effect in propagated light under an external magnetic field. In such a case, the surface of at least one layer of the lower clad layer and the core layer is mechanically polished.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical waveguide such as an optical isolator and an optical circulator used for optical communication and optical signal processing.

[0002]

2. Description of the Related Art A conventional bulk-type optical isolator utilizing the magneto-optical effect is a gadolinium gallium garnet (GGG: Gd ₃ Ga _5).
A garnet-type magnetic thin film formed by a liquid phase epitaxial growth method on a garnet crystal wafer of O ₁₂ ) or a partially substituted element thereof is mainly used as a magneto-optical material, and after growing to a predetermined film thickness, the substrate is removed. It was removed by polishing or the like and used as a Faraday rotator.

In recent years, a waveguide type Faraday rotator that enables in-plane optical integration has been proposed. This is because the Faraday rotator is converted into a waveguide with a light wave confinement structure, so that it can be butted with other waveguides without a lens, and the magnetization direction is set to the in-plane parallel direction, which is the easy magnetization direction. Enables the applied magnetic field strength to be reduced,
This is realized by increasing the shape selectivity of the external magnetic circuit and reducing the volume of the external magnetic circuit, thereby enabling the incorporation into a planar circuit.

[0004] This waveguide type Faraday rotator forming step includes forming a lower cladding layer and a core layer on a substrate, forming a core ridge, forming an upper cladding layer, cutting out, forming a mirror end face, and preventing reflection. I have. In the core ridge forming step in this forming step, photolithography and etching are usually used in combination to form a core ridge having a width and height on the order of several μm. Conventionally, a lower cladding layer of several μm is formed on a GGG substrate having a thickness of several hundred μm to several mm by a liquid phase epitaxial method (LPE method) and
Is formed by the LPE method. After a while
The core shape is adjusted, a core ridge is formed, and an upper clad is formed by the LPE method to complete a buried structure.

After that, the magnetic waveguide is cut into strips from the substrate so as to have a desired length, and the end face of the waveguide is optically polished, coated with an antireflection coating, cut into a desired width, and then cut. Used as a Faraday rotator. Normally, only rotors having magnetic properties that meet their initial purpose are taken out of the rotor created in this way and used.
The yield is poor, and in such a production process, improvement of production efficiency has been a major problem.

An object of the present invention is to provide a magneto-optical waveguide having improved productivity and magneto-optical characteristics in view of the above situation.

[0007]

Means for Solving the Problems and Embodiments of the Invention The present inventors have conducted intensive studies in order to achieve the above object, and as a result, have found that at least either after forming the lower cladding layer or after forming the core layer. Alternatively, preferably, after the formation of the lower cladding layer and the core layer, the surface is mechanically polished to mechanically remove flux defects and triangular projections generated by the LPE film formation. It has been found that the waveguide can be obtained with high productivity and high magneto-optical characteristics.

That is, conventionally, a large number of flux defects and triangular projections exist on the surface of the lower cladding layer and the core layer formed by LPE film formation. In these defects, the core shape is locally deformed in the vicinity where the defects exist. The deformation of the core layer deteriorates the insertion loss of the waveguide. Furthermore, since an extra stress is generated due to the deformation of the core, this causes deterioration of the extinction ratio of the magneto-optical waveguide and the degree of polarization dependence of the extinction ratio. Further, defects in the core layer cause deterioration of insertion loss of the magneto-optical waveguide.

According to the present invention, in order to solve the above-mentioned problems, the surface is mechanically polished at least after formation of the lower cladding layer or the core layer, so that the LP is formed.
Faraday processed by using the lower clad layer and core layer in which flux defects and triangular protrusions on the surface of the lower clad layer and core layer generated by E film formation have been mechanically removed by polishing. The rotor is excellent in magneto-optical characteristics such as insertion loss and extinction ratio because there is no deformation of the core and no extra stress. Further, the yield of a magneto-optical waveguide having excellent magneto-optical characteristics is dramatically improved as compared with the related art.

Therefore, according to the present invention, first, a lower clad layer made of a magnetic crystalline thin film is grown on a substrate, and a core layer made of a magnetic crystalline thin film is grown on the lower clad layer. A part of the core layer is shape-processed, and an upper clad layer made of a magnetic crystal thin film is further grown on the waveguide substrate and used as an optical waveguide, and a magneto-optical effect is induced in the propagated light under an external magnetic field. In the optical waveguide, at least one surface of the lower cladding layer and the core layer is mechanically polished, and secondly, a magnetic crystalline thin film on the substrate Growing a lower clad layer made of a magnetic crystal thin film on the lower clad layer, processing at least a part of the core layer, and further forming a magnetic material on the waveguide substrate. Crystal thin In the magneto-optical waveguide for growing the upper clad layer made of and using it as an optical waveguide and inducing a magneto-optical effect on the propagating light under an external magnetic field, a protrusion or a pit in at least one of the lower clad layer and the core layer is provided. A magneto-optical waveguide characterized by having a defect density of 2 / cm ² or less.

Hereinafter, the present invention will be described in more detail.
The magneto-optical waveguide of the present invention is obtained by forming a lower cladding layer on a substrate, forming a core layer thereon, processing a part of the core layer, and forming an upper cladding layer thereon. And to induce a magneto-optical effect on propagating light under an external magnetic field.

In this case, the substrate is gadolinium.
A known substrate such as a gallium garnet (GGG) substrate is used. The lower cladding layer formed on the substrate is made of a magnetic crystalline thin film, and can be formed of a known material used for forming a lower cladding layer of this type of magneto-optical waveguide. This can be usually formed to a thickness of 4 to 100 μm by a liquid phase epitaxial method (LPE method). Further, the core layer formed on the lower clad layer is also made of a magnetic crystal thin film, which can also be formed of a known material used for forming the lower clad layer of the magneto-optical waveguide, It can be formed to a thickness of 4 to 30 μm by the LPE method.

The shape processing (formation of a core ridge) of the core layer can be performed by a known method. For example, after forming an SiO ₂ film on the core layer, a photoresist layer is formed, and a titanium thin film is further formed. After removing the titanium thin film, the photoresist layer, and the SiO ₂ film at the required locations by etching, a method of dry-etching the core layer and then removing the remaining titanium film, resist layer, and SiO ₂ film can be adopted. .

Thereafter, the upper clad layer is formed. The upper clad layer is also formed of a known material used for forming the upper clad layer of the magneto-optical waveguide, usually, the same material as the lower clad layer. Can be L
It is usually formed to a thickness of 4 to 100 μm by the PE method.

According to the present invention, after the lower clad layer is formed, the surface thereof is formed, and after the core layer is formed, one of the surfaces, preferably each surface is mechanically polished. Thereby, when these lower cladding layers and core layers are grown and formed by the LPE method, defects such as flux defects and triangular projections, and LP
E. Protrusions generated around the place where the substrate is held by the jig during the growth are removed.

In this case, as the mechanical polishing method, a known mechanical polishing method such as buffing or lapping can be adopted, and the above-mentioned defects can be removed, preferably, protrusions and pits on the surface of the lower cladding layer or the core layer. Defect density of 2 /
It is preferable that the polishing is performed so as to be not more than cm ² , particularly not more than 0.5 pieces / cm ² .

In the present invention, mechanical polishing is performed at least after the formation of the lower clad layer and the core layer as described above, and the back surface of the substrate is polished after the core layer processing and the formation of the upper clad layer. The end face is polished and used as a Faraday rotator. However, as described above, the surface of the lower clad layer and / or the core layer is mechanically polished to obtain a defect on the surface. Is removed, no crack is generated from the projection of the lower cladding layer or the core layer as a starting point during polishing of the back surface of the substrate, thereby improving the yield and increasing the productivity. In addition, there are almost no unnecessary projections or crystal defects on the surface of the lower cladding layer or the core layer, so that the extinction ratio is improved and the distribution is reduced, resulting in high magneto-optics. It is what gives sex.

[0018]

EXAMPLES The present invention will be described below in detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[Embodiment] Substrate diameter 50 mm, thickness 1 mm
Liquid phase epitaxial growth method (L
A lower cladding layer of 13 μm was formed by PE method. At this time, the melt composition at the time of forming the lower clad layer is La ₂ O ₃ : Y ₂ O ₃ : Fe ₂ O ₃ : Ga ₂ O ₃ : PbO: B in weight ratio.
₂ O ₃ = 0.202: 0.690: 7.034: 0.25
5: 100: 2.450 and the growth temperature was 899 ° C. When the lattice constant of this lower cladding layer was measured,
The value was smaller by -0.0003 ° than the lattice constant of the substrate.

When the surface of the lower cladding layer was observed with a microscope, a flux defect having a diameter of 100 μm to 3000 μm, a height of about 5 μm, and 0.1 to 1 μm.
Many △ -shaped crystal defects having a size of about μm were observed. When the number of these defects was counted and the number of defects per 1 cm ² (hereinafter referred to as defect density) was examined, the defect density was 15 / cm ² .

During LPE growth, projections having a height of 10 μm and a diameter of about 3 mm were formed at four locations around the wafer where the wafer was held by the jig.

Next, the surface of the lower clad layer was mechanically polished by buffing. The thickness of the lower cladding layer was measured and found to be 10 μm. Observation of the surface of the lower clad layer with a microscope revealed that there were no flux defects and four crystal defects were observed. Defect density is the number of defects per 1 cm ² was as small as 0.2 pieces / cm ^2. Further, the protrusions around the wafer were also removed at the same time.

Next, a core layer having a thickness of 5 μm was formed on the buff-polished GGG substrate with a lower cladding layer by the LPE method. The melt composition at the time of forming the core layer was La ₂ in weight ratio.
O ₃ : Y ₂ O ₃ : Fe ₂ O ₃ : Ga ₂ O ₃ : PbO: B ₂ O ₃ =
0.196: 0.701: 7.097: 0.170: 1
00: 2.451 and the growth temperature was 896 ° C.

When the surface of the core layer was observed, it had a diameter of 100 μm to 3000 μm and a height of about 5 μm.
Flux defect and a size of about 0.1 to 1 μm
Many shape crystal defects were observed. When the defect density was examined, the defect density was 15 defects / cm ² .

Also, at the time of LPE growth, projections having a height of 4 μm and a diameter of about 3 mm were formed at four locations around the wafer where the wafer was held by the jig.

Next, the surface of the core layer was mechanically polished by buffing. When the thickness of this core layer was measured,
It was 4 μm. When the surface of the core layer was observed with a microscope, no flux defects were found, and four triangular projecting crystal defects were observed. The defect density was reduced to 0.2 defects / cm ² . Further, the protrusions around the wafer were also removed at the same time.

Then, a 0.2 μm thick Si layer is formed on the core layer.
An O ₂ film was formed, and a photoresist having a thickness of about 4.5 μm was spin-coated. Next, after hard-baking the photoresist at about 200 ° C., a titanium thin film was formed on the entire surface of the substrate to a thickness of about 1 μm while cooling the substrate by DC magnetron sputtering. Thereafter, the titanium thin film was processed by argon ion beam etching. Furthermore, by switching the gas type to oxygen and processing the resist under the titanium thin film,
4 μm width, 5 height made of titanium thin film and photoresist
A two-layer laminated mask pattern having a thickness of μm was obtained.

Thereafter, the gas type was switched to argon again, and the garnet film was processed by the ion beam.
Etching was performed to a depth of μm. After the remaining resist and SiO ₂ film were completely removed by oxygen plasma ashing, hot phosphoric acid and buffered hydrofluoric acid, an upper clad layer of 10 μm was formed again by the LPE method. At this time, the melt composition at the time of forming the upper clad layer was the same as the lower clad composition, and the growth temperature was 899 ° C.

At the time of growing the upper cladding layer, a height of 10 μm was set at four locations around the wafer where the wafer was held by the jig.
m, a projection having a diameter of about 3 mm was formed.

Thereafter, the back surface of the substrate of the waveguide wafer is
Polishing was performed until the thickness of the GG became 400 μm. At this time, no crack occurred in the wafer.

Using a dicing saw, a rectangular strip is cut out from the prepared waveguide wafer in a direction perpendicular to the waveguide.
The end face of the waveguide was polished to obtain a waveguide type rotor array piece having a waveguide length of 3 mm and a rotation angle of the polarization plane of 45 °. An antireflection coating was applied to the end face of the waveguide type rotor array piece.

Of the array pieces, the widest one was 45 mm wide, and 400 Faraday rotators could be used from this array piece.

The extinction ratio of each rotator in the above-mentioned waveguide type Faraday rotator array piece was measured, and its distribution was evaluated.
FIG. 1 shows a histogram of the extinction ratio.

[Comparative Example] Substrate diameter 50 mm, thickness 1 mm
Liquid phase epitaxial growth method (L
A lower cladding layer of 10 μm was formed by PE method. At this time, the melt composition at the time of forming the lower clad layer is La ₂ O ₃ : Y ₂ O ₃ : Fe ₂ O ₃ : Ga ₂ O ₃ : PbO: B in weight ratio.
₂ O ₃ = 0.202: 0.690: 7.034: 0.25
5: 100: 2.450 and the growth temperature was 899 ° C. When the lattice constant of this lower cladding layer was measured,
The value was smaller by -0.0003 ° than the lattice constant of the substrate.

When the surface of the lower cladding layer was observed with a microscope, it was found that flux defects having a diameter of 100 μm to 3000 μm and a height of about 5 μm and 0.1 to 1 μm.
Many △ -shaped crystal defects having a size of about μm were observed. When the number of these defects was counted and the number of defects per 1 cm ² (hereinafter referred to as defect density) was examined, the defect density was 15 / cm ² .

During LPE growth, protrusions having a height of 10 μm and a diameter of about 3 mm were formed at four locations around the wafer where the wafer was held by the jig.

Next, a 4 μm core layer was formed on the GGG substrate with the lower cladding layer by the LPE method. The melt composition at the time of forming the core layer is La ₂ O ₃ : Y ₂ O ₃ : Fe in weight ratio.
₂ O ₃ : Ga ₂ O ₃ : PbO: B ₂ O ₃ = 0.196: 0.7
01: 7.097: 0.170: 100: 2.451 and the growth temperature was 896 ° C.

When the surface of the core layer was observed, it had a diameter of 100 μm to 3000 μm and a height of about 5 μm.
Flux defect and a size of about 0.1 to 1 μm
Many shape crystal defects were observed. When the defect density was examined, the defect density was 15 defects / cm ² .

During LPE growth, projections having a height of 15 μm and a diameter of about 3 mm were formed at four locations around the wafer where the wafer was held by the jig.

Thereafter, a 0.2 μm thick Si layer is formed on the core layer.
An O ₂ film was formed, and a photoresist having a thickness of about 4.5 μm was spin-coated. Next, after hard-baking the photoresist at about 200 ° C., a titanium thin film was formed on the entire surface of the substrate to a thickness of about 1 μm while cooling the substrate by DC magnetron sputtering. Thereafter, the titanium thin film was processed by argon ion beam etching. Furthermore, by switching the gas type to oxygen and processing the resist under the titanium thin film,
4 μm width, 5 height made of titanium thin film and photoresist
A two-layer laminated mask pattern having a thickness of μm was obtained.

Thereafter, the gas type was switched to argon again, and the garnet film was processed by the ion beam.
Etching was performed to a depth of μm. After the remaining resist and SiO ₂ film were completely removed by oxygen plasma ashing, hot phosphoric acid and buffered hydrofluoric acid, an upper clad layer of 10 μm was formed again by the LPE method. At this time, the melt composition at the time of forming the upper clad layer was the same as the lower clad composition, and the growth temperature was 899 ° C.

At the time of growing the upper clad layer, a height of 26 μm was set at four places around the wafer where the wafer was held by the jig.
m, a projection having a diameter of about 3 mm was formed.

Thereafter, the back surface of the substrate of this waveguide wafer is
Polishing was performed until the thickness of the GG became 400 μm. At this time, cracks occurred in the wafer starting from four locations around the wafer where the wafer was held by the jig during the growth by LPE.

Using a dicing saw, a rectangular strip is cut out from the prepared waveguide wafer using a dicing saw.
The end face of the waveguide was polished to obtain a waveguide type rotor array piece having a waveguide length of 3 mm and a rotation angle of the polarization plane of 45 °. An antireflection coating was applied to the end face of the waveguide type rotor array piece.

Among the array pieces, the widest one has a width of 45 mm. However, since there are cracks generated around the wafer, 250 Faraday rotators can be used from this array piece.

The extinction ratio of each rotator in the above-mentioned waveguide type Faraday rotator array piece was measured, and its distribution was evaluated.
FIG. 2 shows a histogram of the extinction ratio.

From the above results, the following is confirmed. In the case of the embodiment, when the back surface of the substrate of the waveguide wafer is polished until the thickness of the GGG becomes 400 μm after the formation of the upper cladding layer, no crack is generated from the projections around the wafer as a starting point. As a result, the number of available Faraday rotators increases. On the other hand, in the case of the comparative example in which the surfaces of the lower clad layer and the core layer are not polished, cracks are generated starting from the protrusions around the wafer, so that the productivity of the waveguide type Faraday rotator decreases. In the present invention, the extinction ratio of the Faraday rotator is smaller than the center value of 42 dB of the comparative example by 4% in the comparative example because there are almost no unnecessary protrusions and crystal defects on the surfaces of the lower cladding layer and the core layer.
Improved to 5dB. In the comparative example, the distribution of the extinction ratio with respect to the center value is 2.5 dB in sigma value,
In the example, the distribution was reduced to a sigma value of 1.8 dB.

[0048]

According to the present invention, the productivity of the Faraday rotator is greatly improved, the extinction ratio of the Faraday rotator is improved, and the distribution thereof is reduced.

[Brief description of the drawings]

FIG. 1 is a histogram showing an extinction ratio distribution of a Faraday rotator of an example.

FIG. 2 is a histogram showing an extinction ratio distribution of a Faraday rotator of a comparative example.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Toshihiko Nagao 2-13-1 Isobe, Annaka-shi, Gunma F-term in Precision Functional Materials Research Laboratories, Shin-Etsu Chemical Co., Ltd. 2H079 AA03 BA02 CA06 DA13 DA22 DA25 EA03 EA08 HA13 JA07 JA08 2H099 BA03 BA07

Claims (2)

[Claims] A lower clad layer made of a magnetic crystalline thin film is grown on a substrate, a core layer made of a magnetic crystalline thin film is grown on the lower clad layer, and at least a part of the core layer is shaped. An upper clad layer made of a magnetic crystalline thin film is further grown on the waveguide substrate and used as an optical waveguide. In the magneto-optical waveguide for inducing a magneto-optical effect on propagating light under an external magnetic field, the lower clad layer A magneto-optical waveguide, wherein the surface of at least one of the layer and the core layer is mechanically polished. 2. A lower cladding layer made of a magnetic crystal thin film is grown on a substrate, a core layer made of a magnetic crystal thin film is grown on the lower cladding layer, and at least a part of the core layer is shaped. An upper clad layer made of a magnetic crystalline thin film is further grown on the waveguide substrate and used as an optical waveguide. In the magneto-optical waveguide for inducing a magneto-optical effect on propagating light under an external magnetic field, the lower clad layer A magneto-optical waveguide, wherein the density of defects such as protrusions and pits in at least one of the layer and the core layer is 2 / cm ² or less.