JP3953812B2 - Hard magnetic garnet material, Faraday rotator, optical isolator and optical communication system - Google Patents

Description

【００３８】
図５に示すように、ファラデー回転能は、外部磁場を除去しても変わらず、角形ヒステリシスをしめしている。
表１において、試料Ｎｏ．１は希土類元素としてＧｄ、ＴｂおよびＹｂを含む本発明のビスマス置換型希土類鉄磁性ガーネット単結晶膜である。一方、Ｎｏ．２〜４は各々Ｇｄ、ＴｂおよびＹｂを含まない点で本発明によるＮｏ．１と相違する。また、Ｎｏ．５〜８は各々前述した特開平６−２２２３１１号公報、特開平９−１８５０２７号公報、特開平９−３２８３９８号公報および特開平１０−３１１１２号公報に開示された単結晶膜に相当する単結晶膜である。
Ｎｏ．１と、Ｎｏ．２〜４を比較すると以下のことが言える。すなわち、Ｇｄを含まないＮｏ．２は、本発明によるＮｏ．１に比べて挿入損失が劣っている。また、Ｔｂを含まないＮｏ．３は、本発明によるＮｏ．１に比べて温度特性および波長特性が劣っている。さらに、Ｙｂを含まないＮｏ．４は、ファラデー回転能が不足している。
【００３９】
また、Ｎｏ．５は波長特性が、Ｎｏ．６は温度特性および波長特性が、Ｎｏ．７は挿入損失が、さらにＮｏ．８は 温度特性が本発明による試料Ｎｏ．１に比べて劣っている。
以上のＮｏ．２〜８に比べて本発明によるＮｏ．１の単結晶膜は、ファラデー回転能、温度特性、波長特性および挿入損失ともに優れた値を示しており、希土類元素としてＧｄ、ＴｂおよびＹｂを必須の構成元素とすることの必要性を認識することができる。
【００４０】
（実施例２）
実施例１と同様にして、表２に示す４種類のビスマス置換型希土類鉄磁性ガーネット単結晶膜を得た。なお、表２中の試料Ｎｏ．１は実施例１において作成した単結晶膜である。これら単結晶膜について、実施例１と同様にしてファラデー回転能、温度特性、波長特性および挿入損失を測定した。その結果を表２に示す。
【００４１】
【表２】

【００４２】
Ｎｏ．１、Ｎｏ．９およびＮｏ．１０を組成面から比較すると、Ｎｏ．１はＮｏ．９およびＮｏ．１０に比べてＴｂの含有量が多い。また、Ｎｏ．９は、Ｎｏ．１およびＮｏ．１０に比べてＧｄの含有量が多い。さらに、Ｎｏ．１０は、Ｎｏ．１およびＮｏ．９に比べてＢｉの含有量が多い。
以上の組成面の相違を念頭において測定された特性を比較してみると、まず、Ｔｂの含有量を多くすると（Ｎｏ．１）、温度特性および波長特性を向上させることができる。また、Ｇｄの含有量を多くすると（Ｎｏ．９）、挿入損失を向上させることができる。さらに、Ｂｉ含有量を多くすると（Ｎｏ．１０）、ファラデー回転能を向上させることができる。
Ｎｏ．１１は、Ｇａの含有量が０．１と低い材料であるが、角形ヒステリシスを示さず、本発明で言うところの硬磁性を具現しない。
以上のとおりであり、本発明のビスマス置換型希土類鉄磁性ガーネット単結晶膜は、構成元素の含有量を適宜調整することにより、ファラデー回転子に要求される種々の特性にマッチした材料とすることができる。
【００４３】
（実施例３）
実施例１と同様にして、表３に示す種々のビスマス置換型希土類鉄磁性ガーネット単結晶膜を得た。これら単結晶膜について、実施例１と同様にしてファラデー回転能、温度特性、波長特性および挿入損失を測定した。その結果を表３に示すが、Ｎｏ．１２〜２０の試料は、いずれも−４０℃〜＋８５℃の温度範囲、波長１５５０ｎｍにおけるファラデー回転能が７００°／ｃｍ以上であるとともに、ファラデー回転角の温度特性がその狙い値の１３％以下、室温かつ波長１５５０〜１６００ｎｍにおけるファラデー回転角の波長特性が狙い値の８％以下、挿入損失が０．１ｄＢ以下という特性を満足していることがわかる。
【００４４】
【表３】

【００４５】
【発明の効果】
以上説明したように、本発明によれば、ファラデー回転能、温度特性、波長特性および挿入損失に優れた硬磁性ビスマス置換型希土類鉄ガーネット材料が提供される。したがって、本発明の硬磁性ビスマス置換型希土類鉄ガーネット材料を用いたファラデー回転子は、小型・低コスト化を図れるとともに、例えば光アイソレータに用いられた場合に、高性能なアイソレーションを実現する。また、このような光アイソレータを用いた本発明の光通信システムは、高品質な光通信の実現に寄与することができる。
【図面の簡単な説明】
【図１】 本実施の形態による光通信システムの構成を示す図である。
【図２】 本実施の形態によるＬＤモジュールの構成を示す斜視図である。
【図３】 本実施の形態による光アイソレータの構成を示す図である。
【図４】 本実施例による磁性ガーネット材料の保磁力と磁化の関係を示す図である。
【図５】 本実施例による磁性ガーネット材料の外部磁場とファラデー回転角の関係を示す図である。
【図６】 従来の光アイソレータの構成を示す図である。
【図７】 光アイソレータの原理を説明するための図である。
【図８】 ＬＰＥ法を説明するための図である。
【符号の説明】
１…光通信システム、２…光送信器、２１…電子回路、２２…ＬＤモジュール、２２２…ＬＤ、２２３…レンズ、２２４…光アイソレータ、２２４ａ，２２４ｃ…偏光子、２２４ｂ…ファラデー回転子、３…光受信器、３１…ＰＤモジュール、３２…電子回路、４…光伝送ライン、５…光増幅器、１０…光アイソレータ、１１…ファラデー回転子、１２…永久磁石、１３，１４…偏光子、４０…坩堝、４１…加熱コイル、４２…メルト、４３…ＬＰＥ基板、４４…単結晶膜[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a hard magnetic garnet material used in an optical communication system, an Faraday rotator, an optical device such as an optical isolator using the Faraday rotator, and an optical communication system including the optical device.
[0002]
[Prior art]
Currently, the spread of optical communication is accelerating against telecommunication with a small transmission capacity. The reason is that, as explained below, optical communication is capable of high-speed and large-capacity transmission, is advantageous for long-distance transmission because it requires fewer repeaters, and is not affected by electromagnetic noise. To be aggregated.
The light coincides with the electromagnetic wave used in TV / radio broadcasting or wireless communication in that it is an electromagnetic wave. However, the frequency of electromagnetic waves used in optical communication is about 200 THz, which is about 20000 times that of satellite broadcasting (about 10 GHz). A high frequency means that the wavelength is short, and so many signals can be transmitted at high speed. Incidentally, the wavelengths (center wavelength) of electromagnetic waves used in optical communication are 1.31 μm (1310 nm) and 1.55 μm (1550 nm).
As is well known, an optical fiber used for optical communication has a double structure of glass having different refractive indexes. The light passing through the central core is repeatedly reflected inside the core, so that even if the optical fiber is bent, the signal is transmitted accurately. Moreover, since high-purity quartz glass with high transparency is used for the optical fiber, optical communication attenuates only about 0.2 dB per 1 km. Therefore, transmission of about 100 km is possible without using an amplifier, and the number of repeaters can be reduced as compared with telecommunication.
In telecommunications, EMI (electromagnetic interference) is a problem, but communication using an optical fiber is not affected by noise caused by electromagnetic induction. Therefore, extremely high quality information transmission is possible.
[0003]
In the current optical communication system, an electrical signal is converted into an optical signal by an LD (laser diode) of an optical transmitter, and the optical signal is transmitted through an optical fiber, and then is transmitted by an optical receiver PD (photo diode). Convert to electrical signal. Thus, the essential elements for the optical communication system are LD, PD, optical fiber and optical connector. In addition to the above elements, in addition to the above elements, optical transmission devices such as optical amplifiers and optical distributors, optical isolators applied to these devices, apart from relatively low-speed and short-distance communication systems, Optical components such as an optical coupler, an optical demultiplexer, an optical switch, an optical modulator, and an optical attenuator are required.
[0004]
An optical isolator is particularly important in high-speed, long-distance transmission or multi-branch optical communication systems. In current optical communication systems, optical isolators are used in LD modules and repeaters of optical transmitters. An optical isolator is an optical component that transmits electromagnetic waves only in one direction and blocks electromagnetic waves that are reflected and returned in the middle. The optical isolator applies the Faraday effect, which is a kind of magneto-optical effect. The Faraday effect refers to a phenomenon in which the polarization plane of light transmitted through a Faraday rotator such as a material exhibiting the Faraday effect, that is, a rare earth iron garnet single crystal film, rotates. The property of rotating the polarization direction of light like the Faraday effect is called optical rotation, but unlike the normal optical rotation, in the Faraday effect, it does not return to its original state even if the light traveling direction is reversed. Furthermore, the polarization direction rotates. An element that utilizes the phenomenon that the polarization direction of light rotates due to the Faraday effect is called a Faraday rotator.
[0005]
The function of the optical isolator will be described using an LD module as an example.
The LD is incorporated in the optical transmitter as an LD module integrated with the optical fiber. The optical isolator is disposed between the LD and the optical fiber, and fulfills the function of preventing reflected return light to the LD by applying the Faraday effect. The reflected return light refers to light that is returned from the light emitted from the LD after being slightly reflected by a component such as an optical connector. The reflected return light causes noise for the LD. An optical isolator that passes light in only one direction removes this noise and maintains communication quality.
[0006]
In the case of an LD of an optical transmitter, the vibration direction (polarization direction) of light emitted from the LD is determined to be one direction, and thus a polarization-dependent optical isolator having a simple structure is used. A basic configuration of a conventional polarization-dependent optical isolator 10 is shown in FIG. The optical isolator 10 includes a Faraday rotator 11 composed of a garnet single crystal film, a cylindrical permanent magnet 12 surrounding the Faraday rotator 11 and magnetizing the Faraday rotator 11, and both front and back surfaces of the Faraday rotator 11. The polarizers 13 and 14 are arranged on the surface. The polarizers 13 and 14 are arranged with a relative angle of 45 °.
In the optical isolator 10, the direction in which light travels is referred to as a forward direction, and the direction in which light is reflected back is referred to as a reverse direction.
[0007]
Next, based on FIG. 7 (the description of the permanent magnet is omitted), a mechanism in which the optical isolator 10 prevents passage of light in the reverse direction will be described. 7A shows a state in which forward light passes through the optical isolator 10, and FIG. 7B shows a state in which light in the reverse direction is blocked from passing through the optical isolator 10.
As shown in FIG. 7A, in the forward direction, the linearly polarized light that has passed through the polarizer 13 is rotated by 45 ° by the Faraday rotator 11 and passes through the polarizer 14 that is disposed at a relative angle of 45 °. On the other hand, as shown in FIG. 7B, in the reverse direction, the linearly polarized light that has passed through the polarizer 14 is further rotated by 45 ° by the Faraday rotator 11, so that it cannot pass through the polarizer 13.
The polarization-dependent optical isolator used in the LD module has been described above. However, there is a polarization-independent type optical isolator used in the optical amplifier. In the case of an optical amplifier, since light is directly incident on the optical isolator from the optical fiber, the polarization direction cannot be specified. For this purpose, a polarization-independent optical isolator has been developed. Since the basic configuration is well known, a description thereof is omitted here. Further, in the present invention, when simply called an optical isolator, it has a concept including both a polarization dependent type and a polarization independent type.
[0008]
The Faraday rotator determines the performance of the optical isolator. Therefore, the characteristics of the material constituting the Faraday rotator are important for obtaining a high-performance optical isolator. What is important in selecting the material constituting the Faraday rotator is that the Faraday rotation angle at the wavelength used (1310 nm and 1550 nm in the case of an optical fiber) is large and the transparency is high. Initially, YIG (yttrium iron garnet, Y_ThreeFe_FiveO₁₂) Was used, but was insufficient in terms of mass productivity and miniaturization.
Subsequently, it was found that when the rare earth site of the garnet-type crystal is replaced with bismuth (Bi), the Faraday rotation ability is dramatically improved. Thereafter, this Bi-substituted rare earth iron garnet single crystal is used for the Faraday rotator. became.
[0009]
[Problems to be solved by the invention]
By the way, the conventional bismuth-substituted rare earth iron garnet single crystal has a constant Faraday rotation angle in a magnetic field higher than a saturation magnetic field. In a magnetic field with a magnitude smaller than the saturation magnetic field, the Faraday rotation angle is proportional to the magnitude of the magnetic field, and the Faraday effect disappears when the external magnetic field is removed. Therefore, as shown in FIG. 6, in the conventional optical isolator 10, a permanent magnet 12 for applying a magnetic field equal to or higher than the saturation magnetic field to the Faraday rotator 11 is provided.
The optical isolator 10 also has a demand for downsizing and cost reduction as with other devices and components. However, it can be said that the presence of the permanent magnet 12 has prevented the optical isolator 10 from being reduced in size and cost.
[0010]
A conventional bismuth-substituted rare earth iron garnet single crystal can be called a soft magnetic material because the Faraday effect disappears when the external magnetic field is removed. Therefore, the arrangement of permanent magnets is indispensable. However, if the bismuth-substituted rare earth iron garnet single crystal can be imparted with hard magnetism, that is, the ability to maintain the Faraday rotation angle even when the external magnetic field is removed (latching), the arrangement of the permanent magnets can be omitted. Can do. The omission of permanent magnets leads to downsizing and cost reduction of various devices and parts utilizing the optical isolator or Faraday effect. For this reason, development of bismuth-substituted rare earth iron garnet single crystals has been underway.
[0011]
For example, Japanese Patent Laid-Open No. Hei 6-222231 discloses a bismuth-substituted rare earth iron garnet single crystal film grown by an LPE (Liquid Phase Epitaxial) method, wherein the single crystal film has a chemical composition of Gd._xR_yBi_3-xy Fe_5-z (AlGa)_z O₁₂(However, R is at least one of La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Er, Tm, Yb, Lu, and Y, and x, y, and z are 1.0 ≦ x, respectively. ≦ 2.5, 0 ≦ y ≦ 1.9, 0.5 ≦ z ≦ 2.0.), And applying an external magnetic field in the direction intersecting the single crystal film surface to cause magnetic saturation. Subsequently, a bismuth-substituted rare earth iron garnet single crystal film that retains the Faraday rotation effect when magnetically saturated even when the external magnetic field is removed is disclosed. This single crystal film has been shown to maintain the Faraday rotation angle when an external magnetic field equal to or higher than saturation magnetization is applied, even if the external magnetic field is removed. However, this bismuth-substituted rare earth iron garnet single crystal film is specifically disclosed as Gd_1.8Bi_1.2 Fe_4.0 Al_0.5Ga_0.5O₁₂There is only a single crystal film and no other specific single crystal film is disclosed.
[0012]
Japanese Laid-Open Patent Publication No. 9-185027 discloses a hard magnetic bismuth-substituted rare earth iron garnet single crystal film having the following composition. In JP-A-9-185027, it is pointed out that the inclusion of Gd, Tb and Dy should be avoided.
Bi₁Eu₁Ho₁Fe_Four Ga₁O₁₂
Bi_0.75Eu_1.5Ho_0.75Fe_4.1 Ga_0.9O₁₂
Bi₁Eu₂Fe_Four Ga_0.5Al_0.5O₁₂
[0013]
Furthermore, JP-A-9-328398 discloses Tb grown by the LPE method._3-xBi_xFe_5-yzGa_yAl_z O₁₂(Wherein 1.1 ≦ x ≦ 1.5, 0.65 ≦ y + z ≦ 1.2, z ≦ y) a square formed by magnetizing a bismuth-substituted rare earth iron garnet single crystal film represented by a chemical composition A Faraday rotator exhibiting hysteresis is disclosed in Japanese Patent Application Laid-Open No. 10-31112._3-xyHo_xBi_yFe_5-xwGa_zAl_w O₁₂(Wherein, 0.40 ≦ x ≦ 0.70, 1.30 ≦ y ≦ 1.55, 0.7 ≦ z + w ≦ 1.2, 0 ≦ w / z ≦ 0.3) Bismuth represented by the chemical composition There has been disclosed a Faraday rotator exhibiting a square hysteresis formed by magnetizing a substitutional rare earth iron garnet single crystal film.
[0014]
As described above, a bismuth-substituted rare earth iron garnet single crystal film having hard magnetism has been proposed. However, the above-described bismuth-substituted rare earth iron garnet single crystal film has been studied only as to whether or not it exhibits hard magnetism, and the Faraday rotation capability and temperature characteristics of the Faraday rotation angle required as a Faraday rotator. There is no mention of wavelength characteristics or insertion loss.
The above-mentioned Faraday rotation capability, temperature characteristics, wavelength characteristics, and insertion loss are important characteristics for practical use of a Faraday rotator. Therefore, an object of the present invention is to provide a hard magnetic bismuth-substituted rare earth iron garnet material having excellent Faraday rotation capability, temperature characteristics, wavelength characteristics, and insertion loss.
Another object of the present invention is to provide a Faraday rotator composed of a hard magnetic bismuth-substituted rare earth iron garnet material having excellent Faraday rotation capability, temperature characteristics, wavelength characteristics, and insertion loss.
Another object of the present invention is to provide an optical isolator including a Faraday rotator made of a hard magnetic bismuth-substituted rare earth iron garnet material having excellent Faraday rotation capability, temperature characteristics, wavelength characteristics, and insertion loss.
Furthermore, this invention makes it a subject to provide the optical communication system provided with such an optical isolator.
[0015]
[Means for Solving the Problems]
As described above, the Faraday rotability, temperature characteristics, wavelength characteristics, and insertion loss are important characteristics for practical use of the Faraday rotator.
Here, the Faraday rotation angle is proportional to the thickness of the material constituting the Faraday rotator, and the rotation angle per unit thickness is called Faraday rotation ability. Since the rotation angle of the Faraday rotator used in the optical isolator is 45 °, the greater the Faraday rotation capability, the thinner the Faraday rotator, which is advantageous for miniaturization.
The optical isolator is not used in a constant temperature state, and requires an operation guarantee in a temperature range of −40 ° C. to + 85 ° C., for example. The smaller the temperature dependency of the Faraday rotation angle, the wider the temperature range can be used. Therefore, the temperature dependence of the Faraday rotation angle, that is, the temperature characteristic is also an important characteristic.
The wavelength of light currently used in optical isolators is 1310 nm or 1550 nm, but this is only the center wavelength. That is, there is a predetermined width in the wavelength of light actually emitted from the LD. Therefore, the wavelength dependency of the Faraday rotation angle, that is, the wavelength characteristic is also a necessary characteristic. In particular, it becomes a very important characteristic when adopting a large-capacity transmission technology by wavelength multiplexing of optical communication.
The attenuation of the outgoing light with respect to the incident light is called insertion loss. In order to ensure high-quality information transmission, the Faraday rotator is required to reduce insertion loss. The insertion loss in the Faraday rotator is composed of the light absorption loss of the material constituting the Faraday rotator and the reflection loss of the interface due to the difference in refractive index between the material and air. However, the reflection loss can be reduced to a negligible level by applying a non-reflective coating to the surface of the Faraday rotator. Therefore, the insertion loss of the optical isolator becomes the light absorption loss of the Faraday rotator. This light absorption loss is determined by light absorption of ions constituting the bismuth-substituted rare earth iron garnet material.
[0016]
  The present inventor conducted studies for realizing hard magnetism and realizing excellent Faraday rotation performance, temperature characteristics, wavelength characteristics, and insertion loss. As a result, it has been found that in bismuth-substituted rare earth iron garnet materials, an unprecedented novel composition that always contains Gd, Tb and Yb in the rare earth sites is effective. The present invention is based on this finding, and (Bi_3-xyzGd_xTb_yYb_z) Fe_{( 5-w )}M_wO₁₂Chemical composition (where M = Ga, Al, Ge, Sc, In, Si and Ti, or two or more of them)0.1 ≦ x ≦ 1.5, 0.3 ≦ y ≦ 2.0, 0.1 ≦ z ≦ 1.5,0.5 ≦ x + y + z ≦ 2.5,0.4 ≦ w ≦ 1.5And is a hard magnetic garnet material characterized by exhibiting a square magnetic hysteresis.
  In the hard magnetic garnet material of the present invention,1.0 ≦ x + y + z ≦ 2.3Is desirable.
[0017]
The hard magnetic garnet material of the present invention can have a Faraday rotation capability of 700 ° / cm or more at a temperature range of −40 ° C. to + 85 ° C. and a wavelength of 1550 nm. Further, the hard magnetic garnet material of the present invention substantially maintains the Faraday rotation ability even after the external magnetic field is removed after applying an external magnetic field having a magnitude greater than or equal to the saturation magnetization exhibited by the hard magnetic garnet material. can do. In the hard magnetic garnet material of the present invention, the temperature characteristic of the Faraday rotation angle in the temperature range of −40 ° C. to + 85 ° C. can be set to 13% or less of the target value. Furthermore, the hard magnetic garnet material of the present invention can make the wavelength characteristic of the Faraday rotation angle at room temperature and a wavelength of 1500 to 1600 nm 8% or less of the target value. Furthermore, the hard magnetic garnet material of the present invention can have an insertion loss of 0.1 dB or less.
[0018]
The present invention provides the following Faraday rotator to which the above hard magnetic garnet material is applied. That is, the Faraday rotator of the present invention is a Faraday rotator that uses a bismuth-substituted rare earth iron garnet single crystal film and rotates the plane of polarization of incident light. And Yb must be present, substantially exhibiting a square magnetic hysteresis, a temperature range of −40 ° C. to + 85 ° C., a Faraday rotation ability at a wavelength of 1550 nm is 700 ° / cm or more, and a temperature characteristic of the Faraday rotation angle is It is characterized by 13% or less of the target value, wavelength characteristics of the Faraday rotation angle at room temperature and wavelengths of 1500 to 1600 nm, 8% or less of the target value, and insertion loss of 0.1 dB or less.
The single crystal film in the Faraday rotator of the present invention may include one or more of La, Pr, Sm, Eu, Dy, Lu, Tm, Er, Ho, Y, and Ca.
The Faraday rotator of the present invention has a Faraday rotatory power of 800 ° / cm or more, a temperature characteristic of 11% or less of the target value, a wavelength characteristic of 7% or less of the target value, and an insertion loss of 0.07 dB or less. Can do.
[0019]
  The present invention also provides the following optical isolator using the above Faraday rotator. An optical isolator according to the present invention includes a first polarizer that receives forward light, and a second polarizer that is disposed to face the first polarizer at a predetermined interval and emits forward light. And between the first polarizer and the second polarizer, the polarization plane of the light transmitted through the first polarizer is rotated and emitted toward the second polarizer. In addition, a Faraday rotator that blocks transmission of light in the reverse direction that has passed through the second polarizer is a basic component. In the optical isolator of the present invention, the Faraday rotator is (Bi_3-xyzGd_xTb_yYb_z) Fe_{( 5-w )}M_wO₁₂Chemical composition (where M = Ga, Al, Ge, Sc, In, Si and Ti, or two or more of them)0.1 ≦ x ≦ 1.5, 0.3 ≦ y ≦ 2.0, 0.1 ≦ z ≦ 1.5,0.5 ≦ x + y + z ≦ 2.5,0.4 ≦ w ≦ 1.5), And the magnetic hysteresis of the bismuth-substituted rare earth iron garnet single crystal film exhibits a square shape.
[0020]
  As described above, the optical isolator is applied to an optical transmitter in an optical communication system. The present invention provides an optical isolator according to the present invention in this optical communication system.ofSuggest application. The proposal includes an optical transmitter that emits an optical signal converted from an electrical signal, an optical transmission line that transmits the optical signal emitted from the optical transmitter, and the optical signal transmitted through the optical transmission line. And an optical receiver that converts the received optical signal into an electrical signal, wherein the optical transmitter converts the electrical signal into the optical signal. A Faraday rotator that includes an element and an optical isolator disposed between the electro-optical conversion element and the optical transmission line._3-xyzGd_xTb_yYb_z) Fe_{( 5-w )}M_wO₁₂(Wherein, M = Ga, Al, Ge, Sc, In, Si and Ti, or one or more of them,0.1 ≦ x ≦ 1.5, 0.3 ≦ y ≦ 2.0, 0.1 ≦ z ≦ 1.5,0.5 ≦ x + y + z ≦ 2.5,0.4 ≦ w ≦ 1.5Bismuth-substituted rare earth iron garnet single crystal film, and the magnetic hysteresis of the bismuth-substituted rare earth iron garnet single crystal film is square.
[0021]
In an optical communication system, for example, an optical amplifier may be arranged on an optical transmission line composed of an optical fiber. An optical isolator is also used for this optical amplifier. This optical isolator may be an optical isolator according to the present invention. That is, in the optical communication system of the present invention, an optical amplifier is disposed on the optical transmission line, and the optical amplifier receives an optical signal transmitted through the optical transmission line and rotates a polarization plane of the received optical signal. The Faraday rotator which has an optical isolator and an amplification means for amplifying an optical signal transmitted through the optical isolator, and which constitutes the optical isolator is (Bi_3-xyzGd_xTb_yYb_z) Fe_(5-w)M_wO₁₂(Where M = Ga, Al, Ge, Sc, In, Si and Ti, one or more of 0.5, 0.5 ≦ x + y + z ≦ 2.5, 0.2 ≦ w ≦ 2.5) A bismuth-substituted rare earth iron garnet single crystal film can be used. The magnetic hysteresis of the bismuth-substituted rare earth iron garnet single crystal film is desirably square.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail and specifically.
First, an optical communication system 1 to which the present invention is applied will be described with reference to FIG.
The optical communication system 1 is a system for transmitting information by an optical signal between a transmission side and a reception side. An optical transmitter 2 is disposed on the transmission side, and an optical receiver 3 is disposed on the reception side. The optical transmitter 2 and the optical receiver 3 are connected by an optical transmission line 4 made of an optical fiber. An optical amplifier 5 is interposed on the optical transmission line 4. The number of the optical amplifiers 5 is provided according to the length of the optical transmission line 4.
The optical transmitter 2 includes an electronic circuit 21 and an LD module 22. The electronic circuit 21 that has received the data to be transmitted as an electrical signal performs predetermined processing and outputs it to the LD module 22. The LD module 22 converts the received electrical signal into an optical signal and transmits the optical signal to the optical transmission line 4.
The optical receiver 3 includes a PD module 31 and an electronic circuit 32. The PD module 31 that has received the optical signal transmitted from the optical transmission line 4 converts it into an electrical signal and outputs it to the electronic circuit 32. The electronic circuit 32 outputs the received electrical signal to the receiving side.
An optical amplifier 5 disposed on the optical transmission line 4 amplifies the optical signal transmitted through the optical transmission line 4 in order to prevent attenuation.
[0023]
FIG. 2 is a diagram illustrating a configuration of the LD module 22. The LD module 22 includes an LD 222 disposed in the case, a lens 223 through which light (signal) output from the LD 222 passes, and an optical isolator 224 that rotates the polarization plane of the light (signal) transmitted through the lens 223. I have.
FIG. 3 is a diagram illustrating the configuration of the optical isolator 224. As shown in FIG. 3, the optical isolator 224 has a configuration in which a Faraday rotator 224b is disposed between two polarizers 224a and 224c. The two polarizers 224a and 224c are arranged to face each other with a predetermined interval. Assuming that forward light is incident on the polarizer 224a, the forward light is emitted from the polarizer 224c toward the optical transmission line 4. Known materials can be used for the polarizers 224a and 224c. For example, a polar core (trade name) manufactured by Corning is desirable, but not limited thereto.
[0024]
The Faraday rotator 224b rotates the polarization plane of the light transmitted through the polarizer 224a and emits the light toward the polarizer 224c. Further, the Faraday rotator 224b blocks the transmission of light in the reverse direction from the polarizer 224c. Since the reason why light transmission in the reverse direction can be prevented is as described above, description thereof is omitted here.
In the present invention, this Faraday rotator 224b is replaced with (Bi_3-xyzGd_xTb_yYb_z) Fe_(5-w)M_wO₁₂(Where M = Ga, Al, Ge, Sc, In, Si and Ti, one or more, 0.5 ≦ x + y + z ≦ 2.5, 0.2 ≦ w ≦ 2.5) And a bismuth-substituted rare earth iron garnet single crystal film exhibiting a square magnetic hysteresis. This single crystal film exhibits hard magnetism.
[0025]
The bismuth-substituted rare earth iron garnet single crystal film of the present invention is characterized in that it always contains three elements of Gd, Tb and Yb as rare earth elements.
In the present invention, since Gd has the largest magnetic moment among the rare earth elements, it is effective in reducing saturation magnetization (4πMs). GdBi-based garnet is advantageous for hard magnetism because its magnetization reversal temperature is about −10 ° C., which is closer to room temperature than −50 ° C. of TbBi-based garnet. Furthermore, since Gd does not absorb light having a wavelength of 1200 nm or more, it is advantageous for insertion loss.
[0026]
In the present invention, Tb is an element effective for ensuring temperature characteristics and wavelength characteristics. Gd is an element having a large magnetic anisotropy and effective in increasing the coercive force, but Tb contributes more to the coercive force.
The bismuth-substituted rare earth iron garnet single crystal film of the present invention is premised on being formed by the LPE method, but Yb is contained for matching the lattice constant of the single crystal film with the lattice constant of the substrate. . In order to increase the Faraday rotation capability, it is desired to use a crystal containing a large amount of Bi. Here, a substrate (hereinafter referred to as an LPE substrate) used in the LPE method has a predetermined lattice constant. Since Bi has a large ion radius, simply increasing the amount of Bi makes it impossible to match the lattice constant of the crystal film to be obtained with the lattice constant of the substrate. Therefore, by increasing the amount of Bi and adding Yb having a small ion radius, the lattice constant of the crystal film to be obtained and the lattice constant of the substrate are matched. And since Yb does not absorb light in the wavelength region of light used for optical communication, insertion loss is not deteriorated.
[0027]
In the bismuth-substituted rare earth iron garnet material of the present invention, M is an element that substitutes part of Fe, and is selected from one or more of Ga, Al, Ge, Sc, In, Si, and Ti. Among these, Ga is the most desirable element from the viewpoint of reducing saturation magnetization.
[0028]
In the bismuth-substituted rare earth iron garnet material of the present invention, x, y, and z indicating the contents of Gd, Tb, and Yb are set to 0.5 ≦ x + y + z ≦ 2.5. When x + y + z is less than 0.5, the amount of Bi having a large ion radius becomes relatively large, and the lattice constant consistency with the LPE substrate for growing a single crystal film by the LPE method cannot be obtained. On the other hand, when x + y + z exceeds 2.5, the amount of Bi is conversely decreased, and the Faraday rotation capability is decreased. As a result, the thickness of the single crystal film must be increased, and it becomes difficult to grow the single crystal film by the LPE method, resulting in a decrease in yield. A desirable range of x + y + z is 1.0 ≦ x + y + z ≦ 2.3.
Further, each of x, y, and z is preferably 0.1 ≦ x ≦ 1.5, 0.3 ≦ y ≦ 2.0, and 0.1 ≦ z ≦ 1.5. The reason why x (Gd amount) is 0.1 or more is to sufficiently secure magnetic anisotropy for obtaining hard magnetism. On the other hand, when x exceeds 1.5, the amount of Tb and Bi is insufficient, and thus high Faraday rotation ability cannot be obtained.
The reason why y (amount of Tb) is 0.3 or more is to sufficiently secure magnetic anisotropy for obtaining hard magnetism. On the other hand, when y exceeds 1.5, the insertion loss in light with a wavelength exceeding 1500 nm increases.
The reason why z (amount of Yb) is 0.1 or more is that if it is less than 0.1, sufficient Faraday rotation capability cannot be ensured, and it becomes necessary to increase the film thickness. On the other hand, if z exceeds 1.5, the amounts of Gd, Tb, and Bi are insufficient, and high Faraday rotation ability cannot be ensured.
[0029]
In the bismuth-substituted rare earth iron garnet material of the present invention, w, which is the amount of substitution of M with respect to Fe, is 0.2 ≦ w ≦ 2.5. When w is less than 0.2, the obtained single crystal film cannot maintain a square hysteresis. On the other hand, if w exceeds 2.5, unnecessary crystal nuclei are generated in the molten portion during the growth of the single crystal, making it difficult to grow the single crystal soundly. A desirable range of w is 0.3 ≦ w ≦ 2.0, and a more desirable range of w is 0.4 ≦ w ≦ 1.5.
[0030]
According to the present invention as described above, a bismuth-substituted rare earth iron garnet material that realizes the Faraday effect in a temperature range of −40 ° C. to + 85 ° C. and has a Faraday rotation ability at a wavelength of 1550 nm of 700 ° / cm or more can be obtained. This garnet material substantially maintains the above Faraday rotation ability even after the external magnetic field is removed after the external magnetic field having a magnitude equal to or greater than the saturation magnetization exhibited by the material is applied. Moreover, this garnet material can obtain the characteristics that the Faraday rotation ability at a wavelength of 1310 nm is 1000 ° / cm or more.
Further, according to the present invention described above, the temperature characteristics of the Faraday rotation angle at a temperature range of −40 ° C. to + 85 ° C. and a wavelength of 1550 nm can be made 13% or less of the target value. Moreover, this material can make the temperature characteristic of the Faraday rotation angle in a temperature range of −40 ° C. to + 85 ° C. and a wavelength of 1310 nm be 10% or less of the target value. Although the temperature characteristic may be a negative value, the temperature characteristic defined in the present invention is an absolute value. The same applies to the wavelength characteristics. In the present invention, the target value has the following significance. In other words, the current optical isolator has a rotation angle of 45 ° as described above. This 45 ° is the target value in the present invention. In the case of the current optical isolator, the temperature characteristic is obtained with reference to this 45 °. For example, when the rotation angle is 60 °, the temperature characteristics are obtained with reference to 60 °. This is also true for the wavelength characteristics.
The garnet material of the present invention can reduce the wavelength characteristic of the Faraday rotation angle at room temperature and a wavelength of 1500 to 1600 nm to 8% or less of the target value, but the wavelength characteristic at a wavelength of 1250 to 1360 nm is 3% of the target value. % Or less.
In the present invention, when temperature characteristics and wavelength characteristics are emphasized, the amount of Tb may be increased. Further, when importance is attached to insertion loss, the amount of Gd may be increased. Furthermore, when the Faraday rotation capability is increased to emphasize downsizing, the amount of Bi may be increased.
[0031]
The Faraday rotator according to the present invention can be composed of a single crystal film obtained by growing the bismuth-substituted rare earth iron garnet material described above by the LPE method. This single crystal film necessarily contains Gd, Tb, and Yb as rare earth elements, exhibits a substantially square magnetic hysteresis, and has a Faraday rotation capability of 700 ° / cm at a temperature range of −40 ° C. to + 85 ° C. and a wavelength of 1550 nm. In addition to the above, the temperature characteristic of the Faraday rotation angle is 13% or less of the target value, the wavelength characteristic of the Faraday rotation angle at room temperature and wavelengths of 1550 to 1600 nm is 8% or less of the target value, and the insertion loss is 0.1 dB or less. It has a configuration.
[0032]
The single crystal film according to the present invention uses Gd, Tb and Yb as essential constituent elements as rare earth elements, but does not exclude the inclusion of other rare earth elements. Therefore, in addition to Gd, Tb and Yb, La, Pr, Sm, Eu, Dy, Lu, Tm, Er, Ho, Y, and Ca may be included in the scope of the present invention. Is done. However, the content should be suppressed to a level that does not impair the effects of the present invention. However, it is not easy to determine a specific content.
The single crystal film according to the present invention is characterized in that Gd, Tb and Yb are essential constituent elements as rare earth elements. Of these three rare earth elements, Gd and Tb are contained for the purpose of improving the temperature characteristics and wavelength characteristics of the Faraday rotation ability. On the other hand, Yb is contained for matching the lattice constant with the LPE substrate. Among the rare earth elements whose inclusion is not excluded in the present invention, Sm, Eu and Dy have a large magnetic anisotropy like Gd and Tb. Lu, Tm, Er, Ho, and Y are common in that the ion radius is smaller than Y than Dy.
[0033]
The single crystal film according to the present invention can be grown by the LPE method. FIG. 8 shows a state in which a single crystal film is grown by the LPE method. As shown in FIG. 8, for example, a raw material and flux of a single crystal film to be obtained are put into a platinum crucible 40. As a material of the crucible 40, gold (Au) or Ir can be used. The raw material and flux charged in the crucible 40 are heated and melted by energizing the heating coil 41 to form a melt 42. When the temperature of the melt 42 is lowered to bring it into a supercooled state and the LPE substrate 43 is rotated and brought into contact with the melt 42, the single crystal film 44 is epitaxially grown on the LPE substrate 43. In the grown single crystal film 44, impurities are inevitably mixed from the flux and the crucible 40, but it goes without saying that the present invention allows such unavoidable impurities to be mixed. Of course, it goes without saying that it is desirable to reduce the contamination of these impurities in order to achieve the effect of the present invention.
[0034]
The single crystal film obtained by the LPE method is grown slightly thicker than the thickness of the Faraday rotator to be finally obtained. This is because it is used as a Faraday rotator after being subjected to polishing. The Faraday rotator uses a single crystal film whose rotation angle is 45 ° with respect to the wavelength of light used. In other words, the single crystal film obtained by the LPE method is polished until the Faraday rotation angle becomes 45 °. The Faraday rotator has a thickness of about 500 μm. After polishing, it is desirable to apply a non-reflective coating to the surface of the Faraday rotator in order to reduce insertion loss.
[0035]
【Example】
Specific examples of the present invention will be described below.
Example 1
Bismuth oxide (Bi₂O_Three, 4N), ferric oxide (Fe₂O_Three, 4N), gadolinium oxide (Gd₂O_Three, 5N), terbium oxide (Tb₂O_Three, 3N), ytterbium oxide (Yb₂O_Three, 4N), gallium oxide (Ga₂O_Three, 4N) as raw materials, four types of bismuth-substituted rare earth iron magnetic garnet single crystal films were grown by epitaxial growth using the apparatus shown in FIG. The LPE substrate used was (111) garnet single crystal ((GdCa)_Three(GaMgZr)_FiveO₁₂). The lattice constant of this substrate is 1.2497 ± 0.0002 nm. In addition to the above raw materials, lead oxide (PbO, 4N) and boron oxide (B₂O_Three, 5N) as a flux was introduced into a platinum crucible 40.
[0036]
Composition analysis was performed on the obtained single crystal film. The results are shown in Table 1.
Further, after polishing this single crystal film to 500 μm, the magnetic properties were measured using a VSM (vibrating sample magnetometer). As a result, the saturation magnetization (4πMs) was 60 G, the coercive force was 600 Oe, and good nucleated hysteresis was shown as shown in FIG.
Further, after applying an external magnetic field having a magnitude greater than or equal to the saturation magnetization, the Faraday rotation angle, temperature characteristics, wavelength characteristics, and insertion loss were measured using a Faraday rotation angle measurement device. The results are also shown in FIG. 5 and Table 1. This single crystal film is formed so as to have a rotation angle of 45 °. Therefore, the target value in the temperature characteristic and the wavelength characteristic is 45 °. Further, the Faraday rotation capability and its temperature characteristics are a temperature range of −40 ° C. to + 85 ° C., values at a wavelength of 1550 nm, wavelength characteristics of Faraday rotation angles are values at room temperature and wavelengths of 1500 to 1600 nm, and insertion loss is −40 ° C. to +85 It is a value at ° C. (the same applies to Examples 2 and 3). Further, with respect to temperature characteristics and wavelength characteristics, values (° / ° C.) and (° / nm) that are often used for the evaluation are also shown (the same applies to Examples 2 and 3).
[0037]
[Table 1]

[0038]
As shown in FIG. 5, the Faraday rotation ability does not change even when the external magnetic field is removed, and exhibits a square hysteresis.
In Table 1, Sample No. Reference numeral 1 denotes a bismuth-substituted rare earth iron magnetic garnet single crystal film of the present invention containing Gd, Tb and Yb as rare earth elements. On the other hand, no. Nos. 2 to 4 are each No. 2 according to the present invention in that they do not contain Gd, Tb and Yb. 1 and different. No. 5-8 are single crystals corresponding to the single crystal films disclosed in Japanese Patent Laid-Open Nos. Hei 6-222231, Hei 9-185027, Hei 9-328398 and Hei 10-31112, respectively. It is a membrane.
No. 1 and no. The following can be said by comparing 2-4. That is, no. 2 is No. 2 according to the present invention. Insertion loss is inferior to 1. In addition, no. No. 3 according to the present invention. Compared to 1, temperature characteristics and wavelength characteristics are inferior. Furthermore, no. 4 has insufficient Faraday rotation capability.
[0039]
No. No. 5 has a wavelength characteristic of No. 5. No. 6 has a temperature characteristic and a wavelength characteristic of No. 6. No. 7 has an insertion loss. Sample No. 8 according to the present invention has a temperature characteristic. It is inferior to 1.
No. above. No. 2-8 according to the present invention. The single crystal film 1 has excellent values for Faraday rotation capability, temperature characteristics, wavelength characteristics, and insertion loss, and recognizes the necessity of using Gd, Tb, and Yb as essential constituent elements as rare earth elements. be able to.
[0040]
(Example 2)
In the same manner as in Example 1, four types of bismuth-substituted rare earth iron magnetic garnet single crystal films shown in Table 2 were obtained. In Table 2, the sample No. Reference numeral 1 denotes a single crystal film prepared in Example 1. For these single crystal films, the Faraday rotatory power, temperature characteristics, wavelength characteristics, and insertion loss were measured in the same manner as in Example 1. The results are shown in Table 2.
[0041]
[Table 2]

[0042]
No. 1, no. 9 and no. When comparing No. 10 from the viewpoint of composition, no. 1 is No. 1. 9 and no. Compared to 10, the content of Tb is large. No. No. 9 1 and no. Compared to 10, the content of Gd is large. Furthermore, no. 10 is No. 1 and no. Compared to 9, the Bi content is high.
Comparing the characteristics measured with the above compositional differences in mind, first, when the Tb content is increased (No. 1), the temperature characteristics and the wavelength characteristics can be improved. Further, when the content of Gd is increased (No. 9), the insertion loss can be improved. Further, when the Bi content is increased (No. 10), the Faraday rotation ability can be improved.
No. 11 is a material having a low Ga content of 0.1, but does not exhibit square hysteresis and does not embody the hard magnetism referred to in the present invention.
As described above, the bismuth-substituted rare earth iron magnetic garnet single crystal film of the present invention should be a material that matches various characteristics required for a Faraday rotator by appropriately adjusting the content of constituent elements. Can do.
[0043]
(Example 3)
In the same manner as in Example 1, various bismuth-substituted rare earth iron magnetic garnet single crystal films shown in Table 3 were obtained. For these single crystal films, the Faraday rotatory power, temperature characteristics, wavelength characteristics, and insertion loss were measured in the same manner as in Example 1. The results are shown in Table 3. Each of the samples 12 to 20 has a temperature range of −40 ° C. to + 85 ° C., the Faraday rotation ability at a wavelength of 1550 nm is 700 ° / cm or more, and the temperature characteristic of the Faraday rotation angle is 13% or less of the target value. It can be seen that the wavelength characteristics of the Faraday rotation angle at room temperature and wavelengths of 1550 to 1600 nm satisfy the characteristics of 8% or less of the target value and the insertion loss of 0.1 dB or less.
[0044]
[Table 3]

[0045]
【The invention's effect】
As described above, according to the present invention, a hard magnetic bismuth-substituted rare earth iron garnet material excellent in Faraday rotation capability, temperature characteristics, wavelength characteristics, and insertion loss is provided. Therefore, the Faraday rotator using the hard magnetic bismuth-substituted rare earth iron garnet material of the present invention can be reduced in size and cost, and realizes high-performance isolation when used in an optical isolator, for example. The optical communication system of the present invention using such an optical isolator can contribute to the realization of high-quality optical communication.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an optical communication system according to the present embodiment.
FIG. 2 is a perspective view showing a configuration of an LD module according to the present embodiment.
FIG. 3 is a diagram showing a configuration of an optical isolator according to the present embodiment.
FIG. 4 is a graph showing the relationship between coercive force and magnetization of a magnetic garnet material according to the present example.
FIG. 5 is a diagram showing the relationship between the external magnetic field and the Faraday rotation angle of the magnetic garnet material according to the present example.
FIG. 6 is a diagram showing a configuration of a conventional optical isolator.
FIG. 7 is a diagram for explaining the principle of an optical isolator.
FIG. 8 is a diagram for explaining an LPE method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Optical communication system, 2 ... Optical transmitter, 21 ... Electronic circuit, 22 ... LD module, 222 ... LD, 223 ... Lens, 224 ... Optical isolator, 224a, 224c ... Polarizer, 224b ... Faraday rotator, 3 ... Optical receiver 31 ... PD module 32 ... Electronic circuit 4 ... Optical transmission line 5 ... Optical amplifier 10 ... Optical isolator 11 ... Faraday rotator 12 ... Permanent magnet 13,14 ... Polarizer 40 ... Crucible, 41 ... heating coil, 42 ... melt, 43 ... LPE substrate, 44 ... single crystal film

Claims (14)

_{(Bi 3-x-y-} z Gd x Tb y Yb z) Fe (5-w) chemical composition of M _w O ₁₂ (provided that, M = Ga, Al, Ge , Sc, In, one of Si and Ti, or Two or more, 0.1 ≦ x ≦ 1.5, 0.3 ≦ y ≦ 2.0 , 0.1 ≦ z ≦ 1.5, 0.5 ≦ x + y + z ≦ 2.5, 0.4 ≦ w ≦ 1.5 ), and exhibits a square magnetic hysteresis. 2. The hard magnetic garnet material according to claim 1 , wherein a Faraday rotation ability at a temperature range of −40 ° C. to + 85 ° C. and a wavelength of 1550 nm is 700 ° / cm or more. In After removal of the external magnetic field after applying an external magnetic field of the saturation magnetization or the size of the hard magnetic garnet material also shows, according to claim 1, characterized in that the Faraday rotational capacity is substantially maintained or 2. Hard magnetic garnet material according to 2. The hard magnetic garnet material according to any one of claims 1 to 3 , wherein a temperature characteristic of a Faraday rotation angle at a temperature range of -40 ° C to + 85 ° C and a wavelength of 1550 nm is 13% or less of the target value. . The hard magnetic garnet material according to any one of claims 1 to 4 , wherein a wavelength characteristic of a Faraday rotation angle at room temperature and a wavelength of 1500 to 1600 nm is 8% or less of a target value. Hard magnetic garnet material according to any one of claims 1 to 5, the insertion loss in the temperature range of -40 ℃ ~ + 85 ℃ is equal to or is less than 0.1dB. A Faraday rotator that rotates a polarization plane of incident light using a bismuth-substituted rare earth iron garnet single crystal film,
The single crystal film is
The rare earth element always contains Gd, Tb, and Yb, exhibits substantially a square magnetic hysteresis, has a temperature range of −40 ° C. to + 85 ° C., and has a Faraday rotation capability at a wavelength of 1550 nm of 700 ° / cm or more. The temperature characteristic of the rotation angle is 13% or less of the target value,
The wavelength characteristic of the Faraday rotation angle at room temperature and a wavelength of 1500-1600 nm is 8% or less of the target value,
A Faraday rotator having an insertion loss of 0.1 dB or less. The single crystal film includes one or more of La, Pr, Sm, Eu, Dy, Lu, Tm, Er, Ho, Y, and Ca in addition to Gd, Tb, and Yb. Item 8. The Faraday rotator according to Item 7 . The Faraday rotator according to claim 7 or 8 , wherein the Faraday rotation ability in a temperature range of -40 ° C to + 85 ° C and a wavelength of 1550 nm is 800 ° / cm or more. The Faraday rotator according to any one of claims 7 to 9 , wherein the temperature characteristic is 11% or less of a target value. The Faraday rotator according to any one of claims 7 to 10 , wherein the wavelength characteristic is 7% or less of a target value. The Faraday rotator according to any one of claims 7 to 11 , wherein the insertion loss is 0.07 dB or less. A first polarizer on which forward light is incident;
A second polarizer disposed opposite to the first polarizer at a predetermined interval and emitting the light in the forward direction;
The first polarizer and the second polarizer are disposed between the first polarizer, the polarization plane of the light transmitted through the first polarizer is rotated and emitted toward the second polarizer, and A Faraday rotator that blocks transmission of light in the reverse direction that has passed through the second polarizer,
The Faraday rotator is
_{(Bi 3-x-y-} z Gd x Tb y Yb z) Fe (5-w) chemical composition of M _w O ₁₂ (provided that, M = Ga, Al, Ge , Sc, In, one of Si and Ti, or Two or more, 0.1 ≦ x ≦ 1.5, 0.3 ≦ y ≦ 2.0 , 0.1 ≦ z ≦ 1.5, 0.5 ≦ x + y + z ≦ 2.5, 0.4 ≦ w ≦ 1.5 ) and an optical isolator characterized in that the magnetic hysteresis of the bismuth-substituted rare earth iron garnet single crystal film has a square shape. An optical transmitter that emits an optical signal converted from an electrical signal;
An optical transmission line for transmitting the optical signal emitted from the optical transmitter;
An optical communication system comprising: an optical receiver that receives the optical signal transmitted through the optical transmission line and converts the received optical signal into an electrical signal;
The optical transmitter is:
An electro-optical conversion element that converts the electrical signal into the optical signal; and an optical isolator disposed between the electro-optical conversion element and the optical transmission line;
The Faraday rotator constituting the optical isolator is:
_{(Bi 3-x-y-} z Gd x Tb y Yb z) Fe (5-w) chemical composition of M _w O ₁₂ (provided that, M = Ga, Al, Ge , Sc, In, one of Si and Ti, or Two or more, 0.1 ≦ x ≦ 1.5, 0.3 ≦ y ≦ 2.0 , 0.1 ≦ z ≦ 1.5, 0.5 ≦ x + y + z ≦ 2.5, 0.4 ≦ w ≦ 1.5 ) and a bismuth-substituted rare earth iron garnet single crystal film, and the magnetic hysteresis of the bismuth-substituted rare earth iron garnet single crystal film exhibits a square shape.

Images