JP3764825B2

JP3764825B2 - Optical attenuator

Info

Publication number: JP3764825B2
Application number: JP36475498A
Authority: JP
Inventors: 博貴河合; 英則中田
Original assignee: FDK Corp
Current assignee: FDK Corp
Priority date: 1998-12-22
Filing date: 1998-12-22
Publication date: 2006-04-12
Anticipated expiration: 2018-12-22
Also published as: JP2000187193A

Description

【０００１】
【発明の属する技術分野】
本発明は、偏光子とファラデー効果を有するガーネット単結晶と検光子を組み合わせた光アッテネータに関し、更に詳しく述べると、ガーネット単結晶に対して電磁石によって光軸に平行な方向に磁界を印加すると共に永久磁石によって光軸に垂直な方向に磁界を印加し、それらの合成磁界によりファラデー回転角を変化させ、透過光量を制御する光アッテネータに関するものである。
【０００２】
【従来の技術】
光通信システムなどでは、光の透過量を制御するために光アッテネータが用いられている。光アッテネータとしては、通常、偏光子と検光子の間にファラデー回転角可変装置を組み込む構成が採用されている。このファラデー回転角を可変する装置では、ファラデー効果を有するガーネット単結晶に異なる二方向以上から外部磁界を印加し、それらによる合成磁界を可変することにより、ガーネット単結晶を透過する光のファラデー回転角を制御している。
【０００３】
例えば、特開平６−５１２５５号公報には、永久磁石によって光軸に平行な方向に、ガーネット単結晶の飽和磁界以上の固定磁界を印加して前記ガーネット単結晶を飽和状態にしておき、その状態で垂直方向に電磁石により可変磁界を印加して合成磁界ベクトルを変化させ、ガーネット単結晶の磁化方向を変えてファラデー回転角を変化させ、出射側のファイバに結合する光量を制御する光アッテネータが開示されている。ガーネット単結晶が未飽和の状態だと、磁区が発生することによる消光比の劣化、光の散乱が生じ、印加磁界に対するファラデー回転角の再現性が悪いからである。
【０００４】
【発明が解決しようとする課題】
ところが、本発明者等が上記のようなことを考慮してガーネット単結晶を飽和状態にして複数の光アッテネータを作製したところ、最大減衰量を得るための光軸と垂直方向の電磁石の磁界が、作製した各デバイス毎に大きくばらつくという問題が発生した。
【０００５】
本発明の目的は、各デバイス毎の特性のばらつきを少なくでき、且つ電磁石による必要磁界を小さく、デバイスの消費電力を小さくでき、波長依存性及び温度依存性が小さい光アッテネータを提供することである。
【０００６】
【課題を解決するための手段】
本発明は、偏光子、ファラデー効果を有するガーネット単結晶、検光子が、その順序で配列され、前記ガーネット単結晶には電磁石によって光軸に平行な方向に磁界を印加すると共に永久磁石によって光軸に垂直な方向に磁界を印加し、それらの合成磁界によってファラデー回転角を変化させ、透過光量を制御する構造の光アッテネータである。本発明においては、光軸はガーネット単結晶の〈１１１〉方向であり、電磁石の磁界も〈１１１〉方向である。永久磁石の磁界は、光軸に垂直なガーネット単結晶の（１１１）面を中心としたステレオ投影図において、中心の（１１１）面と、最外周円上の（１１０）面と等価な面と、その（１１０）面と等価な面から中心の（１１１）面から７０度の位置にある（１１１）面に近い方に１０度以内の範囲にあり、その方向は中心の（１１１）面から最外周円上に向いて与えられる。そして、合成磁界ベクトルの変位経路は、ガーネット単結晶の（１１１）を中心としたステレオ投影図において、中心の（１１１）面と、最外周円上の（１１０）面と等価な面と、その（１１０）面と等価な面から中心の（１１１）面から７０度の位置にある（１１１）面に近い方に１０度以内の範囲内に設定されている。
【０００７】
図１はガーネット単結晶の（１１１）面を中心としたステレオ投影図である。隣り合う同心円は互いに１０度ずつ異なっている面を意味し、隣り合う径方向の線は互いに１０度ずつ異なっている面を意味する。従ってガーネット単結晶の任意の面は、このステレオ投影図内の点として示すことができる。最外周円上には（１１０）面と等価な面が６０度毎に現れ、また中心から７０度に位置する円上には（１１１）面と等価な面が１２０度毎に現れる。中心の（１１１）面と、最外周円上の（１１０）面と等価な面と、その（１１０）面と等価な面から中心の（１１１）面から７０度の位置にある（１１１）面に近い方に１０度以内の範囲とは、斜線を付した６箇所の扇形領域である。ここで最外周円上の（１１０）面と等価な面とは、（-1０１）面、（-1１０）面、（０１-1）面、（１０-1）面、（１-1０）面、（０-1１）面のことである。また中心から７０度に位置する円上の（１１１）面と等価な面とは、（-1１１）面、（１１-1）面、（１-1１）面のことである。（なお結晶の面の表記法では、負の指数については、その指数の上に横棒を引いて表すが、本明細書ではそれができないために指数にマイナス記号を付すことで表記している。）
【０００８】
ガーネット単結晶に印加する外部磁界を、ガーネット単結晶の方位を考慮して固定することにより、印加磁界に対するファラデー回転角の再現性を良くすることができる。その際、永久磁石により光軸と垂直方向にガーネット単結晶の磁化を向かせておき、電磁石により光軸方向に回転させることにより、電磁石の磁界は小さくて済む。更に永久磁石の磁界が、光軸に垂直なガーネット単結晶の（１１１）面を中心としたステレオ投影図において、中心の（１１１）面と、最外周円上の（１１０）面と等価な面と、その（１１０）面と等価な面から中心の（１１１）面から７０度の位置にある（１１１）面に近い方に１０度以内の範囲にあり、その方向が中心の（１１１）面から最外周円上に向いて与えられることにより、電磁石の磁界がゼロのとき、ガーネット単結晶の磁化が小さい磁界で光軸と垂直方向に近づき、ファラデー回転角を１０度以下にでき、最大減衰量２０ｄＢ以上を得ることができる。
【０００９】
【発明の実施の態様】
特性面では、特に、永久磁石の磁界が、光軸に垂直なガーネット単結晶の（１１１）面を中心としたステレオ投影図において、中心の（１１１）面から最外周円上の（１１０）面と等価な面の方向であり、合成磁界ベクトルの変位経路が、ガーネット単結晶の（１１１）面を中心としたステレオ投影図において、中心の（１１１）面と最外周円上の（１１０）面と等価な面を結んだ線上にすることが望ましい。しかし、組み立て精度などを考慮すると、その直線から２度程度以内の狭い範囲内に収めれば、最良状態を維持できる。
【００１０】
本発明で用いるガーネット単結晶は、例えば液相エピタキシャル（ＬＰＥ）法で作製した（ＲＢｉ）₃Ｆｅ₅Ｏ₁₂又は（ＲＢｉ）₃（ＦｅＭ）₅Ｏ₁₂（但し、Ｒはイットリウムを含む希土類元素から選ばれた１種以上の元素、Ｍは鉄と置換できる１種以上の元素）がよい。Ｍとしては、例えばＧａ，Ｉｎ，Ａｌ等の元素がある。その他、ガーネット単結晶は、Ｙ₃Ｆｅ₅Ｏ₁₂でもよい。ガーネット単結晶の製造方法は、量産性などの観点から上記ＬＰＥ法が望ましいが、その他、フローティング・ゾーン（ＦＺ）法やフラックス法などでもよい。
【００１１】
【実施例】
（実施例１）
光アッテネータの消費電力を小さくするためには、小さな磁界でガーネット単結晶の磁化を回転させ、ファラデー回転角を可変できなければならない。そこで本発明者等は、図２（ａ）に示する構成の光アッテネータを作製し、ファラデー回転角、減衰量と電磁石の磁界の関係を調べた。光ファイバ１０から出射した光は、レンズ１２を介して偏光子１４、ファラデー効果を有するガーネット単結晶１６、検光子１８を通過し、レンズ２０を介して光ファイバ２２に結合する。ガーネット単結晶１６には電磁石２４により光軸方向に磁界が与えられ、更に一対の永久磁石２６，２８によって光軸と垂直な方向に磁界が与えられる。結晶位置での永久磁石２６，２８による磁界は１５０エルステッドである。ここで偏光子１４と検光子１８は、偏光分離膜を介して２個の直角三角形プリズムを接合した複合偏光プリズムであり、それぞれを通過する偏光面のなす角度が９０度になるように配置した。
【００１２】
ガーネット単結晶は次のように作製した。はじめにＰｂＯ−Ｂ₂Ｏ₃−Ｂｉ₂Ｏ₃を融剤として、液相エピタキシャル（ＬＰＥ）法により、格子定数が１２．４９６Å、組成が（ＣａＧｄ）₃（ＭｇＺｒＧａ）₅Ｏ₁₂、形状が直径１インチで厚み５００μｍの非磁性ガーネット基板の（１１１）面上に、Ｂｉ置換希土類鉄ガーネット単結晶（ＬＰＥ膜、組成Ｔｂ_1.00Ｙ_0.65Ｂｉ_1.35Ｆｅ_4.05Ｇａ_0.95Ｏ₁₂、膜厚４５０μｍ）を育成した。得られたＬＰＥ膜を３mm角に切断し、研磨により基板を除去した後、大気中で１１００℃、８時間熱処理した。熱処理するのは、成長誘導による一軸磁気異方性を低減するためである。そして、再度研磨し、３×３×０．３１mmの形状に鏡面研磨し、反射防止膜を蒸着した。最後に、１×１×０．３１mmに切断した。ＶＳＭ（振動試料型磁力計）によりガーネット単結晶の飽和磁化４πＭｓを測定したところ１２０ガウスであったため、永久磁石による磁界を１５０エルステッドに設定してガーネット単結晶を飽和させた。そして、光がガーネット単結晶の反射防止膜を蒸着した面、即ち（１１１）面に対して垂直に入射するようにして測定を行った。
【００１３】
まずＸ線回折によりガーネット単結晶の方位を調べた。その結果、図４（ａ）で示されるガーネット単結晶のＡ面〜Ｅ面は、それぞれ図４（ｂ）の（１１１）面を中心としたステレオ投影図のＡ′面〜Ｅ′面に相当することが分かった。そこで図５に示すように、３個のガーネット単結晶１６を方位を揃えて並べ、光を矢印方向に入射して、永久磁石と電磁石による合成磁界ベクトルの変位経路を図６の０〜１１０度（即ちａ〜ｌ）に１０度ずつ変えて測定した。図６は図１と基本的に同じであり、それに合成磁界ベクトルの変位経路と角度を描き加えたものである。経路ａ〜ｆまでのファラデー回転角と減衰量の測定結果を図７に、経路ｇ〜ｌまでのファラデー回転角と減衰量の測定結果を図８に示す。また経路ａ〜ｌにおける電磁石の磁界がゼロのときの減衰量を表１に示す。なお、１２０〜２３０度、２４０〜３５０度の場合も測定したが、０〜１１０度までの測定結果と同じであったため記載を省略する。測定では光源に１．５５μｍの半導体レーザを使用した。
【００１４】
【表１】

【００１５】
電磁石の磁界がゼロのところに着目すると、外部印加磁界による合成磁界ベクトルの変位経路がｄとｊのとき、ファラデー回転角がゼロになり、減衰量が最大となった。経路ａとｇのとき、ファラデー回転角が−や＋の最大値になり減衰量が最小になった。これは、経路ａでは図９（ａ）、経路ｇでは図９（ｂ）のようになっており、異方性磁界の影響を受け、ガーネット単結晶の磁化が外部磁界と異方性磁界が合成された磁界ベクトル方向を向いているためと思われる。これらの結果から、特性のばらつきを抑えるためには、外部印加磁界による合成磁界ベクトルの変位経路を固定する必要があり、更に最大減衰量を大きくするためには、外部印加磁界による合成磁界ベクトルの変位経路を限定する必要があることが分かる。表１より、経路ｄ又はｊから中心の（１１１）面から７０度の位置にある（１１１）面に近い方に１０度以内においては（図１の斜線領域）、最大減衰量２０ｄＢ以上が得られており、その範囲において良好な特性の光アッテネータが得られることが分かる。また、この構成の光アッテネータは、ファラデー回転角が最小のとき最大減衰量が得られるため、波長依存性、温度依存性が小さい。
【００１６】
経路ａ等では電磁石の磁界を印加していくと、はじめ減衰量が大きくなりファラデー回転角がゼロになるところで最大減衰量が得られるが、このような挙動は減衰量を制御することが困難になり好ましくない。図１の斜線を付した扇形領域を外れたところでも、永久磁石の磁界を大きくすれば、電磁石の磁界がゼロのときガーネット単結晶の磁化は光軸に垂直に近づき、ファラデー回転角がゼロに近づく。そして、電磁石で大きな磁界を印加すれば経路ｄ又はｊと同様の結果が得られる。しかし、そうすると磁化を回転させるのに必要な電磁石の磁界が大きくなるため、デバイスが大きくなったり、消費電力が大きくなり好ましくない。
【００１７】
（実施例２）
図２（ｂ）に示す構造の光アッテネータを作製した。これは、図２（ａ）に示すものとほぼ同様の構成であるが、永久磁石を片側のみに設けたものである。それ故、対応する各部材には同一符号を付し、それらについての説明は省略する。ここでは永久磁石２６はガーネット単結晶１６の側がＮ極になっており、結晶位置での永久磁石による磁界は１５０エルステッドである。偏光子１４及び検光子１８には楔形複屈折結晶（材質はルチル）を用いた。ファラデー回転角が７５度のとき出射側の光ファイバ２２に最も結合しないように、偏光子１４と検光子１８を配置した。外部から印加する合成磁界ベクトルの変位経路を図６の経路ｄとしたときの電磁石の磁界に対するファラデー回転角と減衰量の関係を図１０に示す。電磁石の磁界が約２５０エルステッドのときに最大減衰量が得られた。
【００１８】
（比較例）
比較のために、図３に示す従来と同じ構成の光アッテネータを作製した。図３の光アッテネータは、図２（ａ）に示す光アッテネータの電磁石と永久磁石の関係を逆にしたもので、一対の永久磁石２６，２８により光軸方向に磁界を与え、電磁石２４により光軸と垂直方向に磁界を与える。ファラデー回転角が１５度のとき、出射側の光ファイバ２２に最も結合しないように、偏光子１４と検光子１８を配置した。結晶位置での永久磁石２６，２８による磁界は１５０エルスデッドである。外部から印加する合成磁界ベクトルの変位経路を図６の経路ｄとしたときの電磁石の磁界に対するファラデー回転角と減衰量の関係を図１１に示す。電磁石の磁界が約８００エルステッドのときに最大減衰量が得られた。この値は、実施例２に示す光アッテネータの３倍以上の値であった。
【００１９】
【発明の効果】
本発明は上記のように、ガーネット単結晶に対して電磁石によって光軸に平行な方向に磁界を印加すると共に永久磁石によって光軸に垂直な方向に磁界を印加し、それらの合成磁界によってファラデー回転角を変化させる際に、永久磁石の磁界方向と合成磁界ベクトルの変位経路を特定しているため、印加磁界に対するファラデー回転角の再現性が良くなり各デバイス毎の特性のばらつきを抑えることができ、且つ電磁石の磁界が小さくて済み消費電力の低減とデバイスの小型化を図ることができる。また、ファラデー回転角が最小の時に最大減衰量が得られるため、波長依存性及び温度依存性が小さくなる。
【図面の簡単な説明】
【図１】本発明範囲を示す（１１１）面を中心としたステレオ投影図。
【図２】本発明における光アッテネータの基本構成図。
【図３】従来の光アッテネータの構成図。
【図４】ガーネット単結晶の実際の面と（１１１）面を中心としたステレオ投影図を示す説明図。
【図５】ガーネット単結晶の配列状態を示す説明図。
【図６】外部合成磁界ベクトルの変位経路と角度を示すステレオ投影図。
【図７】経路ａ〜ｆにおける電磁石の磁界に対するファラデー回転角と減衰量の関係を示すグラフ。
【図８】経路ｇ〜ｌにおける電磁石の磁界に対するファラデー回転角と減衰量の関係を示すグラフ。
【図９】経路ａと経路ｇにおけるガーネット単結晶の結晶磁気異方性の影響を示す説明図。
【図１０】本発明の最良モードの一つにおける電磁石の磁界に対するファラデー回転角と減衰量の関係を示すグラフ。
【図１１】従来技術における電磁石の磁界に対するファラデー回転角と減衰量の関係を示すグラフ。
【符号の説明】
１０，２２光ファイバ
１２，２０レンズ
１４偏光子
１６ガーネット単結晶
１８検光子
２４電磁石
２６，２８永久磁石[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical attenuator in which a polarizer, a garnet single crystal having a Faraday effect, and an analyzer are combined. More specifically, the present invention applies a magnetic field to a garnet single crystal in a direction parallel to the optical axis by an electromagnet and is permanent. The present invention relates to an optical attenuator that applies a magnetic field in a direction perpendicular to the optical axis by a magnet, changes the Faraday rotation angle by the combined magnetic field, and controls the amount of transmitted light.
[0002]
[Prior art]
In an optical communication system or the like, an optical attenuator is used to control the amount of transmitted light. As the optical attenuator, a configuration in which a Faraday rotation angle varying device is normally installed between a polarizer and an analyzer is adopted. In this device for changing the Faraday rotation angle, an external magnetic field is applied to the garnet single crystal having the Faraday effect from two or more different directions, and the resultant magnetic field is changed to change the Faraday rotation angle of the light transmitted through the garnet single crystal. Is controlling.
[0003]
For example, in Japanese Patent Laid-Open No. 6-512255, the garnet single crystal is saturated by applying a fixed magnetic field equal to or higher than the saturation magnetic field of the garnet single crystal in a direction parallel to the optical axis by a permanent magnet. An optical attenuator is disclosed that controls the amount of light coupled to the outgoing fiber by applying a variable magnetic field by an electromagnet in the vertical direction to change the resultant magnetic field vector, changing the magnetization direction of the garnet single crystal, and changing the Faraday rotation angle. Has been. This is because when the garnet single crystal is in an unsaturated state, the extinction ratio is deteriorated due to the generation of magnetic domains, light scattering occurs, and the reproducibility of the Faraday rotation angle with respect to the applied magnetic field is poor.
[0004]
[Problems to be solved by the invention]
However, in consideration of the above, the present inventors made a garnet single crystal in a saturated state to produce a plurality of optical attenuators. As a result, the magnetic field of the electromagnet in the direction perpendicular to the optical axis for obtaining the maximum attenuation is obtained. As a result, there was a problem that the devices produced varied widely.
[0005]
It is an object of the present invention to provide an optical attenuator that can reduce variations in characteristics among devices, reduce a required magnetic field by an electromagnet, reduce power consumption of the device, and reduce wavelength dependency and temperature dependency. .
[0006]
[Means for Solving the Problems]
In the present invention, a polarizer, a garnet single crystal having a Faraday effect, and an analyzer are arranged in this order, and a magnetic field is applied to the garnet single crystal in a direction parallel to the optical axis by an electromagnet, and the optical axis by a permanent magnet. This is an optical attenuator having a structure in which a magnetic field is applied in a direction perpendicular to and a Faraday rotation angle is changed by the combined magnetic field to control the amount of transmitted light. In the present invention, the optical axis is the <111> direction of the garnet single crystal, and the magnetic field of the electromagnet is also the <111> direction. In the stereo projection diagram centered on the (111) plane of the garnet single crystal perpendicular to the optical axis, the magnetic field of the permanent magnet is the center (111) plane and a plane equivalent to the (110) plane on the outermost circumference circle. It is within a range of 10 degrees closer to the (111) plane located 70 degrees from the center (111) plane than the plane equivalent to the (110) plane, and the direction is from the center (111) plane. Given towards the outermost circle. And the displacement path of the synthetic magnetic field vector is the stereo projection centered on (111) of the garnet single crystal, the center (111) plane, the plane equivalent to the (110) plane on the outermost circumference circle, It is set within a range of 10 degrees or less from the plane equivalent to the (110) plane and closer to the (111) plane at a position of 70 degrees from the central (111) plane.
[0007]
FIG. 1 is a stereo projection centered on the (111) plane of a garnet single crystal. Adjacent concentric circles refer to surfaces that differ by 10 degrees from each other, and adjacent radial lines mean surfaces that differ from each other by 10 degrees. Thus, any surface of the garnet single crystal can be shown as a point in this stereographic projection. A plane equivalent to the (110) plane appears every 60 degrees on the outermost circle, and a plane equivalent to the (111) plane appears every 120 degrees on a circle located 70 degrees from the center. The center (111) plane, the plane equivalent to the (110) plane on the outermost circle, and the (111) plane at a position 70 degrees from the center (111) plane from the plane equivalent to the (110) plane The range within 10 degrees closer to is the six fan-shaped regions with diagonal lines. Here, the plane equivalent to the (110) plane on the outermost circle is the (−101) plane, the (−110) plane, the (01-1) plane, the (10−1) plane, and the (1−10) plane. , (0-11) plane. In addition, planes equivalent to the (111) plane on the circle located 70 degrees from the center are the (−111) plane, the (11-1) plane, and the (1-111) plane. (Note that in the notation of crystal plane, a negative index is expressed by drawing a horizontal bar on the index, but in this specification it is not possible to do so by adding a minus sign to the index. .)
[0008]
By fixing the external magnetic field applied to the garnet single crystal in consideration of the orientation of the garnet single crystal, the reproducibility of the Faraday rotation angle with respect to the applied magnetic field can be improved. At that time, the magnetization of the garnet single crystal is directed in the direction perpendicular to the optical axis by the permanent magnet and rotated in the optical axis direction by the electromagnet, so that the magnetic field of the electromagnet can be small. Furthermore, in the stereo projection view centered on the (111) plane of the garnet single crystal perpendicular to the optical axis, the magnetic field of the permanent magnet is equivalent to the (111) plane at the center and the (110) plane on the outermost circle. And within the range within 10 degrees closer to the (111) plane located 70 degrees from the (111) plane at the center from the plane equivalent to the (110) plane, and the direction is the center (111) plane When the magnetic field of the electromagnet is zero, the magnetization of the garnet single crystal approaches the direction perpendicular to the optical axis with a small magnetic field, and the Faraday rotation angle can be reduced to 10 degrees or less. An amount of 20 dB or more can be obtained.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
In terms of characteristics, in particular, in the stereo projection diagram centered on the (111) plane of the garnet single crystal perpendicular to the optical axis, the magnetic field of the permanent magnet is the (110) plane on the outermost circle from the central (111) plane. In the stereo projection view centering on the (111) plane of the garnet single crystal, the (111) plane at the center and the (110) plane on the outermost circle. It is desirable to be on a line connecting planes equivalent to. However, in consideration of assembly accuracy, the best state can be maintained if it falls within a narrow range of about 2 degrees or less from the straight line.
[0010]
The garnet single crystal used in the present invention is, for example, (RBi) ₃ Fe ₅ O ₁₂ or (RBi) ₃ (FeM) ₅ O ₁₂ (wherein R is a rare earth element containing yttrium), which is prepared by a liquid phase epitaxial (LPE) method. One or more selected elements, M is preferably one or more elements that can replace iron. Examples of M include elements such as Ga, In, and Al. In addition, the garnet single crystal may be Y ₃ Fe ₅ O ₁₂ . The method for producing a garnet single crystal is preferably the above LPE method from the viewpoint of mass productivity, but may also be a floating zone (FZ) method or a flux method.
[0011]
【Example】
Example 1
In order to reduce the power consumption of the optical attenuator, the Faraday rotation angle must be variable by rotating the magnetization of the garnet single crystal with a small magnetic field. Therefore, the present inventors manufactured an optical attenuator having the configuration shown in FIG. 2A, and investigated the relationship between the Faraday rotation angle, attenuation, and the magnetic field of the electromagnet. The light emitted from the optical fiber 10 passes through the lens 12, the polarizer 14, the garnet single crystal 16 having the Faraday effect, and the analyzer 18, and is coupled to the optical fiber 22 through the lens 20. A magnetic field is applied to the garnet single crystal 16 in the direction of the optical axis by the electromagnet 24, and further, a magnetic field is applied in a direction perpendicular to the optical axis by the pair of

permanent magnets

26 and 28. The magnetic field generated by the

permanent magnets

26 and 28 at the crystal position is 150 oersted. Here, the polarizer 14 and the analyzer 18 are composite polarizing prisms in which two right-angled triangular prisms are joined via a polarization separation film, and are arranged so that an angle formed by a polarization plane passing through each is 90 degrees. .
[0012]
The garnet single crystal was produced as follows. First, a liquid phase epitaxy (LPE) method using PbO—B ₂ O ₃ —Bi ₂ O ₃ as a flux, the lattice constant is 12.496 組成, the composition is (CaGd) ₃ (MgZrGa) ₅ O ₁₂ , and the shape has a diameter of 1 A Bi-substituted rare earth iron garnet single crystal (LPE film, composition Tb _1.00 Y _0.65 Bi _1.35 Fe _4.05 Ga _0.95 O ₁₂ , film thickness 450 μm) was grown on the (111) plane of a nonmagnetic garnet substrate having a thickness of 500 μm. The obtained LPE film was cut into 3 mm square, the substrate was removed by polishing, and then heat-treated in air at 1100 ° C. for 8 hours. The heat treatment is performed to reduce uniaxial magnetic anisotropy due to growth induction. And it grind | polished again, it mirror-polished in the shape of 3x3x0.31mm, and the antireflection film was vapor-deposited. Finally, it was cut into 1 × 1 × 0.31 mm. The saturation magnetization 4πMs of the garnet single crystal measured by VSM (vibrating sample magnetometer) was 120 gauss, so the magnetic field by the permanent magnet was set to 150 oersted to saturate the garnet single crystal. The measurement was performed such that light was incident perpendicularly to the surface on which the garnet single crystal antireflection film was deposited, that is, the (111) plane.
[0013]
First, the orientation of the garnet single crystal was examined by X-ray diffraction. As a result, the A-plane to E-plane of the garnet single crystal shown in FIG. 4 (a) correspond to the A′-plane to E′-plane of the stereo projection centered on the (111) plane of FIG. 4 (b), respectively. I found out that Therefore, as shown in FIG. 5, three garnet single crystals 16 are aligned with their orientations aligned, light is incident in the direction of the arrow, and the displacement path of the combined magnetic field vector by the permanent magnet and the electromagnet is 0 to 110 degrees in FIG. (I.e., a to l) was changed by 10 degrees and measured. FIG. 6 is basically the same as FIG. 1 and shows the displacement path and angle of the combined magnetic field vector. FIG. 7 shows the measurement results of the Faraday rotation angle and the attenuation amount for the paths a to f, and FIG. 8 shows the measurement result of the Faraday rotation angle and the attenuation amount for the paths g to l. Table 1 shows the amount of attenuation when the magnetic field of the electromagnet in the paths a to l is zero. In addition, although it measured also in the case of 120-230 degree | times and 240-350 degree | times, since it was the same as the measurement result to 0-110 degree | times, description is abbreviate | omitted. In the measurement, a 1.55 μm semiconductor laser was used as the light source.
[0014]
[Table 1]

[0015]
Focusing on the location where the magnetic field of the electromagnet is zero, when the displacement path of the composite magnetic field vector by the externally applied magnetic field is d and j, the Faraday rotation angle becomes zero and the attenuation amount becomes maximum. In the paths a and g, the Faraday rotation angle became the maximum value of-and +, and the attenuation amount was minimized. This is as shown in FIG. 9A for the path a and FIG. 9B for the path g. Under the influence of the anisotropic magnetic field, the magnetization of the garnet single crystal is changed between the external magnetic field and the anisotropic magnetic field. This is probably because the direction of the synthesized magnetic field vector is facing. From these results, it is necessary to fix the displacement path of the composite magnetic field vector due to the externally applied magnetic field in order to suppress the variation in characteristics, and to further increase the maximum attenuation, It can be seen that the displacement path needs to be limited. From Table 1, the maximum attenuation of 20 dB or more is obtained within 10 degrees closer to the (111) plane located 70 degrees from the center (111) plane from the path d or j (shaded area in FIG. 1). It can be seen that an optical attenuator with good characteristics can be obtained in that range. Further, the optical attenuator having this configuration has a small wavelength dependency and temperature dependency because the maximum attenuation can be obtained when the Faraday rotation angle is minimum.
[0016]
In the path a etc., when the magnetic field of the electromagnet is applied, the attenuation becomes large at the beginning and the maximum attenuation is obtained when the Faraday rotation angle becomes zero, but such behavior makes it difficult to control the attenuation. It is not preferable. Even if the magnetic field of the permanent magnet is increased even if it is outside the hatched area in FIG. Get closer. And if a big magnetic field is applied with an electromagnet, the result similar to the path | route d or j will be obtained. However, when doing so, the magnetic field of the electromagnet required to rotate the magnetization becomes large, which is not preferable because the device becomes large and the power consumption becomes large.
[0017]
(Example 2)
An optical attenuator having the structure shown in FIG. This is substantially the same configuration as that shown in FIG. 2A, but a permanent magnet is provided only on one side. Therefore, corresponding members are denoted by the same reference numerals, and description thereof will be omitted. Here, the permanent magnet 26 has an N pole on the garnet single crystal 16 side, and the magnetic field generated by the permanent magnet at the crystal position is 150 oersted. For the polarizer 14 and the analyzer 18, wedge-shaped birefringent crystals (made of rutile) were used. The polarizer 14 and the analyzer 18 are arranged so as not to be coupled to the optical fiber 22 on the emission side when the Faraday rotation angle is 75 degrees. FIG. 10 shows the relationship between the Faraday rotation angle and the attenuation with respect to the magnetic field of the electromagnet when the displacement path of the composite magnetic field vector applied from the outside is the path d in FIG. Maximum attenuation was obtained when the electromagnet magnetic field was approximately 250 oersted.
[0018]
(Comparative example)
For comparison, an optical attenuator having the same configuration as that shown in FIG. The optical attenuator shown in FIG. 3 is obtained by reversing the relationship between the electromagnet and permanent magnet of the optical attenuator shown in FIG. 2A. A magnetic field is applied in the optical axis direction by a pair of

permanent magnets

26 and 28, and light is emitted by the electromagnet 24. Apply a magnetic field perpendicular to the axis. When the Faraday rotation angle is 15 degrees, the polarizer 14 and the analyzer 18 are arranged so as not to be most coupled to the optical fiber 22 on the emission side. The magnetic field generated by the

permanent magnets

26 and 28 at the crystal position is 150 els dead. FIG. 11 shows the relationship between the Faraday rotation angle and the attenuation with respect to the magnetic field of the electromagnet when the displacement path of the composite magnetic field vector applied from the outside is the path d in FIG. Maximum attenuation was obtained when the electromagnet magnetic field was about 800 oersted. This value was three times or more that of the optical attenuator shown in Example 2.
[0019]
【The invention's effect】
In the present invention, as described above, a magnetic field is applied to a garnet single crystal in a direction parallel to the optical axis by an electromagnet, and a magnetic field is applied in a direction perpendicular to the optical axis by a permanent magnet. When changing the angle, the magnetic field direction of the permanent magnet and the displacement path of the combined magnetic field vector are specified, so the reproducibility of the Faraday rotation angle with respect to the applied magnetic field is improved, and variations in the characteristics of each device can be suppressed. Moreover, the magnetic field of the electromagnet is small, so that power consumption can be reduced and the device can be downsized. In addition, since the maximum attenuation is obtained when the Faraday rotation angle is minimum, the wavelength dependency and temperature dependency are reduced.
[Brief description of the drawings]
FIG. 1 is a stereo projection view centered on a (111) plane showing the scope of the present invention.
FIG. 2 is a basic configuration diagram of an optical attenuator in the present invention.
FIG. 3 is a configuration diagram of a conventional optical attenuator.
FIG. 4 is an explanatory view showing a stereo projection centered on an actual plane and a (111) plane of a garnet single crystal.
FIG. 5 is an explanatory view showing an arrangement state of garnet single crystals.
FIG. 6 is a stereo projection view showing a displacement path and an angle of an external composite magnetic field vector.
FIG. 7 is a graph showing the relationship between the Faraday rotation angle and the amount of attenuation with respect to the magnetic field of the electromagnet in paths a to f.
FIG. 8 is a graph showing the relationship between the Faraday rotation angle and the attenuation with respect to the magnetic field of the electromagnet in the paths g to l.
FIG. 9 is an explanatory diagram showing the influence of magnetocrystalline anisotropy of a garnet single crystal in paths a and g.
FIG. 10 is a graph showing the relationship between the Faraday rotation angle and the attenuation with respect to the magnetic field of the electromagnet in one of the best modes of the present invention.
FIG. 11 is a graph showing the relationship between the Faraday rotation angle and the attenuation with respect to the magnetic field of the electromagnet in the prior art.
[Explanation of symbols]
DESCRIPTION OF

SYMBOLS

10, 22

Optical fiber

12, 20 Lens 14 Polarizer 16 Garnet single crystal 18 Analyzer 24

Electromagnet

26, 28 Permanent magnet

Claims

A polarizer, a garnet single crystal having a Faraday effect, and an analyzer are arranged in that order. A magnetic field is applied to the garnet single crystal in a direction parallel to the optical axis by an electromagnet, and a direction perpendicular to the optical axis by a permanent magnet. Is an optical attenuator that controls the amount of transmitted light by changing the Faraday rotation angle by applying a magnetic field to
The optical axis is the <111> direction of the garnet single crystal, and the magnetic field of the permanent magnet in the stereo projection centered on the (111) plane of the garnet single crystal perpendicular to the optical axis, A surface equivalent to the (110) plane on the outermost circle, and within 10 degrees closer to the (111) plane at a position of 70 degrees from the center (111) plane from the plane equivalent to the (110) plane The direction is given from the central (111) plane toward the outermost circle,
Further, the displacement path of the synthetic magnetic field vector includes a center (111) plane, a plane equivalent to the (110) plane on the outermost circle, and its ( The optical attenuator is within a range of 10 degrees or closer to the (111) plane located 70 degrees from the central (111) plane from the plane equivalent to the (110) plane.

In the stereo projection view centered on the (111) plane of the garnet single crystal perpendicular to the optical axis, the direction of the plane equivalent to the (110) plane on the outermost circle from the center (111) plane And the displacement path of the combined magnetic field vector connects a plane equivalent to the (111) plane at the center and the (110) plane on the outermost circle in the stereo projection view centered on the (111) plane of the garnet single crystal. 2. The optical attenuator according to claim 1, which is on an elliptical line.

The optical attenuator according to claim 1 or 2, wherein the garnet single crystal is Y ₃ Fe ₅ O ₁₂ .

The garnet single crystal is (RBi) ₃ Fe ₅ O ₁₂ or (RBi) ₃ (FeM) ₅ O ₁₂ (where R is one or more elements selected from rare earth elements including yttrium, and M can be replaced with iron 1 The optical attenuator according to claim 1 or 2, wherein the optical attenuator is an element of at least a species.

The optical attenuator according to claim 1, wherein the polarizer and the analyzer are composite polarizing prisms.

The optical attenuator according to any one of claims 1 to 4, wherein the polarizer and the analyzer are birefringent crystals.