JP2003057405A - Antireflection film for faraday rotator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an antireflection film for a Faraday rotator having high durability in a small number of layers. SOLUTION: The antireflection film consists of a three-layer film in which a first layer in the air side and a third layer in the side of a magnetic garnet single crystal are made of SiO2 and a second layer interposed between the first layer and the third layer is made of one kind of HfO2 , ZrO2 , Ta2 O5 and TiO2 , and which satisfies the condition that the reflectance R at near the designed center wavelength λ0 determined from formulae (1) and (2) is minimized. Further, the physical film thickness of the first SiO2 layer is >=160 nm.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical isolator,
The present invention relates to a Faraday rotator made of a magnetic garnet single crystal that constitutes one part of an optical circulator or the like, and more particularly to improvement of an antireflection film formed on the surface of a Faraday rotator.

[0002]

2. Description of the Related Art Conventionally, a surface of a magnetic garnet single crystal incorporated in an optical isolator or the like is provided with an antireflection film to prevent reflected light from returning to a semiconductor laser.

By the way, when the refractive index of the antireflection film formed on the surface of the magnetic garnet single crystal is n _f , the refractive index of the magnetic garnet single crystal is n _s , and the refractive index of the medium is n ₀ , If the conditional expression for antireflection shown in (3) is satisfied, the reflectance can be made zero with the single layer film.

[0004]

[Equation 3] However, since the refractive index n _s of the magnetic garnet single crystal is normally 2.2 to 2.4, when the medium is air (n
_{In 0} = 1), there was no stable optical thin film material having a refractive index satisfying the above antireflection conditional expression.

Therefore, SiO ₂ is used for the first layer on the air side and the third layer on the magnetic garnet single crystal side, and Al ₂ O ₃ , ZrO ₂ is used for the second layer sandwiched between the first layer and the third layer. TiO ₂ , Ta _{2
O 5, HfO 2, Y 2} 3 using one selected from among O ₃
A layer equivalent film is used as an antireflection film (Japanese Patent Laid-Open No. 4-23 / 1992).
No. 0701), SiO ₂ is used for the first layer on the air side and the third layer on the magnetic garnet single crystal side, and Al ₂ O ₃ is used for the second layer sandwiched between the first layer and the third layer. A three-layer film other than the three-layer equivalent film used was used as an antireflection film (see Japanese Patent Laid-Open No. 10-115815).

However, recently, there has been an increasing demand for an antireflection film having more excellent durability in addition to good reflection characteristics.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of such a requirement, and its object is to maintain the same reflection characteristics as an antireflection film by the conventional three-layer equivalent film method. At the same time, it is to provide a highly durable antireflection film for a Faraday rotator with the smallest possible number of layers.

[0008]

That is, the invention according to claim 1 is premised on an antireflection film of a Faraday rotator which is made of a magnetic garnet single crystal and is used in the air. The third layer on the garnet single crystal side is SiO
₂ , the second layer sandwiched between the first layer and the third layer is HfO ₂ , Zr
A condition that the reflectance R is the lowest in the vicinity of the design center wavelength λ ₀ obtained from the following mathematical formulas (1) and (2), as well as being any one selected from O ₂ , Ta ₂ O ₅ , and TiO _2. Which is a first layer composed of a three-layer film that satisfies
The physical film thickness of ₂ is 160 nm or more,

[0009]

[Equation 4]

[0010]

[Equation 5] However, σ _j = 2πn _{j dj} / (λ / λ ₀ ), i ² = −1, λ is the wavelength for which the reflectance R is to be calculated, and _dj is the j-th wavelength.
The physical thickness of the layer, n _j is the refractive index of the j-th layer, n _s is the refractive index of the magnetic garnet single crystal, and n ₀ is the refractive index of air.

The invention according to claim 2 is based on the antireflection film for a Faraday rotator according to claim 1, and the total optical film thickness of the three layers in the above three-layer film is 0.2.
It is characterized by satisfying the range of 5λ _{0 to} 0.5λ ₀ .

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below.

First, in the present invention, SiO ₂ is used for the first layer exposed on the surface on the air side, because a dense and smooth film can be formed and the hardness is relatively high, and the weather resistance is excellent. This is because scratches are unlikely to occur, and SiO ₂ is also used in the third layer in contact with the magnetic garnet single crystal because a high adhesive force is obtained with respect to the magnetic garnet single crystal.

Further, the first antireflection film is made of SiO ₂ .
Layer and a third layer, and HfO ₂ , Z sandwiched between the first layer and the third layer
The reason why the three-layer film is composed of the second layer made of one kind selected from rO ₂ , Ta ₂ O ₅ , and TiO ₂ is because productivity is taken into consideration.

By the way, an antireflection film comprising a three-layer equivalent film in which SiO ₂ is used for the layer in contact with the magnetic garnet single crystal and the layer exposed on the air side surface is disclosed in the above-mentioned JP-A-4-23.
As disclosed in Japanese Patent Publication No. 0701, the optical thickness of the conventional antireflection film formed by the three-layer equivalent film method is about 0.
25λ ₀ , and SiO ₂ of the surface layer (third layer) is 0.08
λ is _{0 ~0.11λ 0} about. That is, SiO on the surface
The physical film thickness of the _two layers is at most about 120 nm at the wavelength used for optical communication.

Further, in JP-A-10-115815, as described above, although SiO ₂ is used for the layer in contact with the magnetic garnet single crystal and the layer exposed on the air side surface, the magnetic garnet single crystal is used. physical film thickness of SiO ₂ of about 44nm a layer in contact with the crystal, the surface layer a physical thickness of about 138nm of SiO _2, physical film thickness of the intermediate layer is about 88n
An antireflection film which is not a three-layer equivalent film made of Al ₂ O _{3 of} m is disclosed.

The inventors of the present invention have made earnest studies and found that in order to further improve the durability of the antireflection film, it is disclosed in JP-A-4-23.
It was found that the physical film thickness of the surface layer described in JP-A-07011 and JP-A-10-115815 is not sufficient, and the physical film thickness of SiO ₂ of the surface layer should be 160 nm or more. It was

[0018] Here, the optical thickness of the three layers total is set in the range of 0.25λ ₀ ~0.5λ ₀ when the design center wavelength and lambda ₀ is low at less than 0.25 [lambda ₀ This is because it may be difficult to form an antireflection film having a reflectance, and if a thick film having a thickness of more than 0.5λ ₀ is used, the productivity when forming the antireflection film is slightly difficult, and This is because the film thickness of the formed film is likely to be distributed, so that the reflectance as designed may not be obtained.

The reflectance R when three optical thin films of arbitrary thickness are laminated is "Thin-film optical filters 2nd
edn "(by HAMacleod, Bristrol Adam Hilger Ltd. 19
As stated in 86), it is calculated as follows.

[0020]

[Equation 6]

[0021]

[Equation 7] Here, σ _j = 2πn _j d _j / (λ / λ ₀ ), i ² = −1, λ ₀ is the design center wavelength, λ is the wavelength for which the reflectance is to be calculated, and _dj is the physical property of the j-th layer. Film thickness, n _j is the refractive index of the j-th layer,
n _s is the refractive index of the magnetic garnet single crystal, and n ₀ is the refractive index of the medium and is 1.0 in the case of air.

Therefore, based on the above equations (1) and (2), the first and third layers are made of SiO ₂ , the second layer is made of HfO ₂ , and Z.
As one kind selected from rO ₂ , Ta ₂ O ₅ , and TiO ₂ , the physical thickness of each layer is changed little by little to design the center wavelength λ.
_The antireflection film is designed by determining the condition that the reflectance around ₀ (for example, 1550 nm) becomes the lowest.

In order to form the antireflection film of the present invention,
Although the vacuum deposition method, sputtering method, ion plating method, etc. can be applied, an electron beam that irradiates a substrate (magnetic garnet single crystal) before or during deposition with a low energy ion beam and heats the deposited material with the electron beam The ion-assisted vapor deposition method is effective and suitable for cleaning the surface of the substrate (magnetic garnet single crystal), improving the adhesive force of the formed film, and improving the packing density.

[0024]

EXAMPLES Examples of the present invention will be specifically described below.

The light having a wavelength of 1310 nm has a refractive index of 2.35, and the light having a wavelength of 1550 nm has a refractive index of 2.35.
As an antireflection film for a Faraday rotator composed of a magnetic garnet single crystal having a composition of (YbTbBi) ₃ Fe ₅ O ₁₂ , which is No. 34, SiO ₂ / Ta ₂ O ₅ / Si by the above-mentioned method.
O ₂ (Examples 1 to 2), SiO ₂ / HfO ₂ / SiO ₂ (Examples 3 to 4), SiO ₂ / ZrO ₂ / SiO ₂ (Example 5), SiO ₂ / TiO ₂ / SiO ₂ (Implementation) A three-layer antireflection film according to the combination of Examples 6 to 7) was designed.

The results are shown in Table 1 below.

For comparison, SiO ₂ / Ta ₂ O ₅ / SiO ₂ (Comparative Example 1), Si, which was designed by the three-layer equivalent film method, was used.
O ₂ / HfO ₂ / SiO ₂ (Comparative Example 2), SiO ₂ / ZrO
₂ / SiO ₂ (Comparative Example 3), SiO ₂ / TiO ₂ / SiO ₂
Table 2 also shows the results of the antireflection film according to the combination of (Comparative Example 4).

[0028]

[Table 1]

[0029]

[Table 2] Next, the calculated values of the reflectance in air when the antireflection films according to Example 1 and Comparative Example 1 are applied are shown in FIG. 1 (Example 1) and FIG. 2 (Comparative Example 1). .

From the spectral reflection characteristics shown in FIGS. 1 and 2, it can be confirmed that the film structure according to the present invention has antireflection characteristics comparable to the conventional three-layer equivalent film.

Next, in order to investigate the superiority of durability of the antireflection film according to the present invention, the antireflection films according to Example 1 and Comparative Example 1 were formed by the electron beam ion assisted vapor deposition method,
The durability was evaluated.

First, a 10 mm square magnetic garnet single crystal was set in a vapor deposition apparatus and then heated to 200 ° C. while being heated to 3
After evacuation to × 10 ⁻⁴ Pa, SiO ₂ was vapor-deposited in that state.

Next, oxygen is introduced up to 1 × 10 ^-2 Pa and T
Deposition of a ₂ O ₅ was performed.

After the introduction of oxygen was stopped, SiO ₂ was vapor-deposited again. A mixed gas of argon and oxygen was used for the ion assist, and the acceleration voltage was 800 eV.

Each of the samples thus obtained on which the antireflection coatings of Example 1 and Comparative Example 1 were formed was left in an environment of temperature 85 ° C. and humidity 85% for 5000 hours, and then both The surface of the sample was observed under a microscope of 50 times, but no peeling was observed in either film.

However, after that, when left for 1000 hours in an environment of temperature 85 ° C. and humidity 98%, which is stricter than general durability test conditions, white turbidity was observed in the antireflection film according to Comparative Example 1. No change was observed in the antireflection film according to Example 1.

From this, the superiority of durability of the antireflection film according to the present invention was confirmed.

[0038]

According to the antireflection film for a Faraday rotator according to the present invention, the first layer on the air side and the third layer on the magnetic garnet single crystal side are made of SiO ₂ , and the first layer is formed. And the second layer sandwiched between the third layer is HfO ₂ , ZrO ₂ , Ta ₂ O ₅ ,
In addition to any one selected from TiO ₂ ,
A physical film thickness of SiO ₂ which is a first layer and which is composed of a three-layer film satisfying the condition that the reflectance R near the design center wavelength λ ₀ obtained from the above formulas (1) and (2) is the lowest. Is 160 nm or more, there is an effect that an antireflection film having higher durability can be provided as compared with the conventional example composed of a three-layer equivalent film.

[Brief description of drawings]

FIG. 1A is SiO ₂ / Ta ₂ O according to Example 1.
Chart showing spectral reflectance characteristics of ₅ / SiO ₂ trilayer film, Fig. 1
1B is a chart showing the refractive index of a medium, each layer, and a substrate (magnetic garnet single crystal), and FIG. 1C is a graph showing the spectral reflectance characteristics of the above three-layer film.

FIG. 2A is a SiO ₂ / Ta ₂ O film according to Comparative Example 1.
Chart showing spectral reflectance characteristics of ₅ / SiO ₂ three-layer equivalent film, FIG. 2 (B) shows medium, each layer, substrate (magnetic garnet single crystal)
FIG. 2C is a graph showing the spectral reflectance characteristics of the above three-layer equivalent film.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G02F 1/09 505 G02B 1/10 AF term (reference) 2H049 BA08 BA42 BB65 BC25 2H079 AA03 AA12 BA02 DA13 EA27 2H099 AA02 BA02 BA06 DA00 DA01 2K009 AA06 BB04 CC03 DD03 4F100 AA17B AA20A AA20C AA21B AA27B BA03 BA06 BA10A BA10C BA13 GB41 JN06 JN18

Claims (2)

[Claims] 1. An antireflection film for a Faraday rotator which is composed of a magnetic garnet single crystal and is used in air, wherein the first layer on the air side and the third layer on the magnetic garnet single crystal side are S.
The second layer sandwiched between the first layer and the third layer is HfO ₂ with iO ₂ ,
A condition that the reflectance R is the lowest in the vicinity of the design center wavelength λ ₀ obtained from the following mathematical formulas (1) and (2) while being any one selected from ZrO ₂ , Ta ₂ O ₅ , and TiO _2. S, which is a first layer and is composed of a three-layer film
An antireflection film for a Faraday rotator, wherein the physical film thickness of iO ₂ is 160 nm or more. [Equation 1] [Equation 2] However, σ _j = 2πn _{j dj} / (λ / λ ₀ ), i ² = −1, λ is the wavelength for which the reflectance R is to be calculated, and _dj is the j-th wavelength.
The physical thickness of the layer, n _j is the refractive index of the j-th layer, n _s is the refractive index of the magnetic garnet single crystal, and n ₀ is the refractive index of air. 2. The antireflection for a Faraday rotator according to claim 1, wherein the total optical thickness of the three layers in the three-layer film satisfies the range of 0.25λ _{0 to} 0.5λ _0. film.