JPH07218880A - Optical semiconductor element and optical communication device - Google Patents
Optical semiconductor element and optical communication deviceInfo
- Publication number
- JPH07218880A JPH07218880A JP902494A JP902494A JPH07218880A JP H07218880 A JPH07218880 A JP H07218880A JP 902494 A JP902494 A JP 902494A JP 902494 A JP902494 A JP 902494A JP H07218880 A JPH07218880 A JP H07218880A
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- optical
- absorption
- layer
- semiconductor element
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Abstract
Description
【０００１】[0001]
【産業上の利用分野】本発明は、光通信等に用いる光半
導体素子及び光半導体素子を用いた光通信装置の構成方
法に関する。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical semiconductor device used for optical communication and the like, and a method for constructing an optical communication device using the optical semiconductor device.
【０００２】[0002]
【従来の技術】光ファイバ通信システムは近年めざまし
い発展を遂げ、４００Ｍｂ／ｓ、１．６Ｇｂ／ｓ等の大
容量の光通信システムが日本国内をはじめ、海底ケーブ
ルを通じて外国との間にも導入されている。この光通信
システムも、従来は半導体レーザを直接変調する方式で
行っていた。しかしながら、将来の１０Ｇｂ／ｓ、４０
Ｇｂ／ｓ等の超高速・大容量光伝送システムの実現のた
め、半導体レーザの直接変調方式から光の吸収変化や光
の屈折率変化を動作原理とする光半導体素子を用いた外
部変調器方式に移行されつつある。このため、より高性
能な外部変調器の開発が重要となってきている。また、
光ファイバ通信システムが広く実用化されるに伴って、
交換機系の光化も要望が高まりつつある。光交換機を実
現するための最も重要な光半導体素子としては光スイッ
チがある。2. Description of the Related Art Optical fiber communication systems have made remarkable progress in recent years, and large-capacity optical communication systems such as 400 Mb / s and 1.6 Gb / s have been introduced not only in Japan but also through foreign countries through submarine cables. ing. Also in this optical communication system, conventionally, a method of directly modulating a semiconductor laser has been performed. However, future 10 Gb / s, 40
In order to realize an ultra-high-speed, large-capacity optical transmission system such as Gb / s, an external modulator method using an optical semiconductor element whose operation principle is a change in absorption of light or a change in refractive index of light from a direct modulation method of a semiconductor laser Is being moved to. Therefore, the development of higher performance external modulators has become important. Also,
With the widespread practical use of optical fiber communication systems,
There is a growing demand for optical switching systems. An optical switch is one of the most important optical semiconductor elements for realizing an optical switch.
【０００３】外部変調方式に用いられる光半導体素子と
して代表的な電界吸収型光変調素子は、光通信装置の小
型化、容易性といった面で優れており、これまでに多く
の報告（例えば、電子情報通信学会の光量子エレクトロ
ニクス研究会の技術報告；ＯＱＥ９１−５７，ｐｐ６９
−７４）があり、一部は商用レベルに到達しつつある。
電界吸収型光変調素子の基本動作原理としては、図７
（ｂ）に示したバルク半導体層のフランツ−ケルディシ
ュ効果や図７（ｃ）に示したタイプＩ超格子構造の量子
閉じ込めシュタルク効果（ＱＣＳＥ）が良く知られてい
る。ここでタイプＩ超格子構造とは、半導体材料１、２
からなる超格子構造において、各々の電子親和力χ1 、
χ2 、禁制帯エネルギＥg1、Ｅg2が χ1 ＜χ2 χ1 ＋Ｅg1＞χ2 ＋Ｅg2 なる関係を有する超格子構造を指す。An electro-absorption optical modulator, which is a typical optical semiconductor element used in an external modulation system, is excellent in terms of downsizing and easiness of an optical communication device, and many reports have been published so far (for example, electronic Technical report of the Photonics Electronics Workshop of the Institute of Information and Communication Engineers; OQE91-57, pp69
-74) and some are reaching commercial level.
The basic operation principle of the electro-absorption optical modulator is shown in FIG.
The Franz-Keldysh effect of the bulk semiconductor layer shown in (b) and the quantum confined Stark effect (QCSE) of the type I superlattice structure shown in FIG. 7C are well known. Here, the type I superlattice structure means semiconductor materials 1, 2
In the superlattice structure consisting of, each electron affinity χ 1 ,
χ 2 and forbidden band energies E g1 and E g2 refer to a superlattice structure having a relation of χ 1 <χ 2 χ 1 + E g1 > χ 2 + E g2 .
【０００４】フランツ−ケルディシュ効果やＱＣＳＥを
用いた従来の電界吸収型光変調素子の構成を図７（ａ）
に示す。概略的には、ノンドープ光吸収層をｐ型クラッ
ド層及びｎ型クラッド層で挟み込んだｐｉｎ構造を基本
構造とし、図７（ｂ）や図７（ｃ）のように、電界印加
によって光吸収端近傍波長（図の変調波長）での光吸収
の程度を制御することで光の変調を行う。FIG. 7 (a) shows the structure of a conventional electro-absorption optical modulator using the Franz-Keldysh effect or QCSE.
Shown in. In general, a pin structure in which a non-doped light absorption layer is sandwiched between a p-type clad layer and an n-type clad layer is used as a basic structure, and a light absorption edge is generated by applying an electric field as shown in FIGS. 7B and 7C. The light is modulated by controlling the degree of light absorption at a near wavelength (modulation wavelength in the figure).
【０００５】一方、光交換機に用いられる光スイッチも
これまでに多くの報告がある。その動作原理として、キ
ャリア注入プラズマ効果による屈折率変化を用いた報告
（例えば、電気学会の光・量子デバイス研究会ＯＱＤ−
９０−６１、ｐｐ２７−３１）や前記ＱＣＳＥに伴う屈
折率変化を用いた報告がある。キャリア注入プラズマ効
果を動作原理とした交差型光スイッチの構成を図８に示
す。概略的には、ノンドープ光ガイド層をｐ型クラッド
及びｎ型クラッドで挟み込んだｐｉｎ構造を有してお
り、電流を交差部に注入することで注入部の屈折率変化
（△ｎ／ｎ）が負となるようにし、光の全反射条件を満
たすときに光路が切り換わる。On the other hand, there have been many reports on optical switches used in optical switches. As a principle of its operation, a report using the change in the refractive index due to the carrier injection plasma effect (for example, OQD-
90-61, pp27-31) and a report using the change in refractive index associated with the QCSE. FIG. 8 shows the structure of a cross-type optical switch based on the carrier injection plasma effect as an operating principle. In general, it has a pin structure in which a non-doped optical guide layer is sandwiched between a p-type cladding and an n-type cladding, and when a current is injected into the intersection, the change in the refractive index (Δn / n) of the injection portion is reduced. The optical path is switched when the total reflection condition of the light is satisfied so as to be negative.
【０００６】[0006]
【発明が解決しようとする課題】以上述べたような従来
の吸収型光変調素子や光スイッチには次のような問題が
あった。The conventional absorption type optical modulator and optical switch as described above have the following problems.
【０００７】図７（ａ）に示した吸収型光変調素子で
は、図７（ｂ）（ｃ）のように、電界印加時に光吸収端
波長が長波長側にシフト（レッドシフト）するが、特に
フランツ−ケルディッシュ効果を用いた素子ではかなり
の高電界を必要とするという問題がある。また、ＱＣＳ
Ｅを用いた素子は比較的低電界で動作可能であるが、Ｑ
ＣＳＥは超格子井戸層内の電界印加時のキャリア分極に
よる遷移確率の低減によって生ずるため、キャリアの空
間的分離が十分でないと遷移確率の低減が十分でなく、
光吸収端のシフトが十分でなくなるという問題点、光吸
収により光キャリアが発生しホールパイルアップが生じ
るという問題点がある。In the absorption type optical modulator shown in FIG. 7A, the light absorption edge wavelength shifts to the long wavelength side (red shift) when an electric field is applied as shown in FIGS. 7B and 7C. In particular, an element using the Franz-Keldysh effect has a problem that a considerably high electric field is required. Also, QCS
The element using E can operate in a relatively low electric field, but Q
Since CSE is caused by a reduction in transition probability due to carrier polarization when an electric field is applied in the superlattice well layer, the transition probability is not sufficiently reduced unless the carriers are spatially separated,
There are problems that the shift of the light absorption edge becomes insufficient and that light carriers are generated by light absorption to cause hole pile-up.
【０００８】さらに、図９（ａ）のような従来の半導体
レーザの直接変調を行う光通信装置は、駆動回路の出力
がオン時に光出力がオンされ、駆動回路の出力がオフ時
には光出力がオフのいわゆるノーマリーオフの装置構成
であるのに対して、図９（ｂ）のような従来型の外部変
調素子を用いた光通信装置では、駆動回路の出力がオフ
で外部変調器の印加電界が零の場合には光変調器での光
吸収は無く光出力はオンで、駆動回路の出力がオンで外
部変調器の印加電界がある場合には光変調器での光吸収
によって光出力はオフとなるいわゆるノーマリーオンの
装置構成である。従って、従来の半導体レーザの直接変
調を用いた光通信装置の駆動方法と外部変調器を用いた
光通信装置の駆動方法が整合しないという問題点があ
る。Further, in the conventional optical communication device for directly modulating a semiconductor laser as shown in FIG. 9A, the optical output is turned on when the output of the drive circuit is on, and the optical output is turned on when the output of the drive circuit is off. In contrast to the so-called normally-off device configuration of OFF, in the optical communication device using the conventional external modulation element as shown in FIG. 9B, the output of the drive circuit is OFF and the external modulator is applied. When the electric field is zero, there is no light absorption in the optical modulator and the optical output is on. When the output of the drive circuit is on and the applied electric field of the external modulator is present, the optical output is due to the optical absorption in the optical modulator. Is a normally-on device configuration in which it is turned off. Therefore, there is a problem that the conventional method for driving an optical communication device using direct modulation of a semiconductor laser does not match the method for driving an optical communication device using an external modulator.
【０００９】次に、図８に示した従来の全反射型の光ス
イッチは、電流注入によって屈折率変化を生じさせるた
め、消費電力が大きい、動作速度が電界制御素子に比べ
て遅いという問題点がある。また、従来のタイプＩ超格
子構造のＱＣＳＥによっても屈折率変化が得られるが、
ＱＣＳＥを用いた全反射型の光スイッチでは、図７
（ｄ）の屈折率変化スペクトル図のように、負の大きな
屈折率変化は吸収端近傍波長λ6 でしか得られないため
に、ガイド層にこの屈折率変化を生じさせる超格子をそ
のまま用いると吸収損失が大きくなるといった問題点が
ある。Next, since the conventional total reflection type optical switch shown in FIG. 8 causes a change in the refractive index due to current injection, it consumes a large amount of power and operates at a slower speed than an electric field control element. There is. Further, although the change in refractive index can be obtained by the conventional QCSE of the type I superlattice structure,
In the total reflection type optical switch using QCSE,
As shown in the refractive index change spectrum diagram of (d), a large negative refractive index change can be obtained only at the wavelength λ 6 near the absorption edge. Therefore, if the superlattice that causes this refractive index change is used as it is in the guide layer. There is a problem that absorption loss becomes large.
【００１０】[0010]
【課題を解決するための手段】本発明は上記問題点を解
決した光半導体素子、及び光通信装置の構成方法を提供
するもので、半導体基板上に導電型クラッド層、ガイド
層、逆導電型クラッド層を基本構造として有し、印加電
界による吸収係数または屈折率変化を動作原理とする半
導体素子において、前記ガイド層の一部が半導体材料
１、２からなる超格子構造であり、かつ各々の電子親和
力χ1 、χ2 、禁制帯エネルギＥg1、Ｅg2が χ1 ＜χ2 χ1 ＋Ｅg1＜χ2 ＋Ｅg2 なる関係を有する、いわゆるタイプＩＩ超格子であるこ
とを特徴とする光半導体素子を第１の発明とする。The present invention provides an optical semiconductor device and a method for constructing an optical communication device, which solve the above-mentioned problems. A conductive clad layer, a guide layer, and a reverse conductive type are provided on a semiconductor substrate. In a semiconductor device having a cladding layer as a basic structure and operating principle of absorption coefficient or refractive index change due to applied electric field, a part of the guide layer is a superlattice structure made of semiconductor materials 1 and 2, and An optical semiconductor characterized by a so-called type II superlattice in which electron affinities χ 1 and χ 2 and forbidden band energies E g1 and E g2 have a relation of χ 1 <χ 2 χ 1 + E g1 <χ 2 + E g2. The element is the first invention.
【００１１】第１の発明において、光半導体素子が屈折
率変化によって生ずる位相変化を動作原理とするマッハ
ツェンダ型に構成されていることを特徴とする光半導体
素子を第２の発明とする。A second invention is an optical semiconductor element characterized in that, in the first invention, the optical semiconductor element is of a Mach-Zehnder type in which a principle of operation is a phase change caused by a change of a refractive index.
【００１２】第１の発明において、光半導体素子が少な
くとも２本の交差する導波路の交点に位置する一部の半
導体層の屈折率を変化させ、全反射によって光路を切り
換える全反射型光スイッチとして構成されていることを
特徴とする光半導体素子を第３の発明とする。In the first invention, an optical semiconductor element is a total reflection type optical switch in which the refractive index of a part of semiconductor layers located at the intersection of at least two intersecting waveguides is changed and the optical path is switched by total reflection. An optical semiconductor element having the above structure is defined as a third invention.
【００１３】第１の発明に記載された光半導体素子を吸
収型の外部変調素子として含むことを特徴とする光通信
装置を第４の発明とする。A fourth invention is an optical communication device characterized by including the optical semiconductor element described in the first invention as an absorption type external modulation element.
【００１４】[0014]
【作用】本発明の光半導体素子は、上述の手段をとる事
により従来技術の課題を解決した。The optical semiconductor device of the present invention has solved the problems of the prior art by taking the above-mentioned means.
【００１５】その原理を図１を用いて説明する。図１は
本発明の基本をなすタイプＩＩ半導体超格子構造のバン
ド図である。タイプＩＩ超格子は、半導体材料１、２の
各々の電子親和力χ1 、χ2、禁制帯エネルギＥg1、Ｅ
g2が χ1 ＜χ2 χ1 ＋Ｅg1＜χ2 ＋Ｅg2 なる関係を有する。タイプＩＩ超格子構造では、無電界
時の吸収は最低禁制帯エネルギーを有する半導体層１、
２ヘテロ界面近傍のみで起こると考えられるが、電子正
孔分布確率が空間的に完全に分離されている（図１
（ａ））ため、極めて遷移確率が低く、間接遷移型に近
い吸収特性を有する。The principle will be described with reference to FIG. FIG. 1 is a band diagram of a type II semiconductor superlattice structure which is the basis of the present invention. The type II superlattice has electron affinities χ 1 and χ 2 of the semiconductor materials 1 and 2 and forbidden band energies E g1 and E, respectively.
g2 has a relationship of χ 1 <χ 2 χ 1 + E g1 <χ 2 + E g2 . In the type II superlattice structure, the absorption in the absence of electric field has the lowest forbidden band energy in the semiconductor layer 1,
It is thought that this occurs only near the two-hetero interface, but the electron-hole distribution probabilities are completely separated spatially (Fig. 1).
Because of (a), the transition probability is extremely low and the absorption characteristics are close to those of the indirect transition type.
【００１６】ところが、有電界時には電子正孔分布確率
の空間分布がオーバーラップ（図１（ｂ））し、遷移確
率が高まるため直接遷移型に近い吸収特性を示す。従っ
て、タイプＩＩ超格子構造は、タイプＩ超格子構造に比
べて無電界時から有電界時の吸収変化が大きく、吸収端
のシフトも大きい。However, when the electric field is applied, the spatial distributions of electron-hole distribution probabilities overlap (FIG. 1B) and the transition probability increases, so that the absorption characteristics are close to those of the direct transition type. Therefore, the type II superlattice structure has a larger absorption change from no electric field to an electric field and a larger shift of the absorption edge than the type I superlattice structure.
【００１７】タイプＩＩ超格子をガイド層とする導波路
を用いた著者らの実験によれば、図２（ａ）に示す様
に、タイプＩＩ超格子構造の光吸収スペクトルは印加電
界強度の増加に伴い、吸収端が短波長側にシフト（ブル
ーシフト）することが観測されている。これは電界印加
によって光の遷移波長がλ0 からλ1 （λ1 ＜λ0 ）に
変化する（図１（ａ）（ｂ）参照）からである。この
時、タイプＩＩ超格子構造の屈折率変化は図２（ｂ）の
ように負に大きく変化することも我々の実験で観測され
ている。According to the experiments performed by the authors using a waveguide having a type II superlattice as a guide layer, as shown in FIG. 2A, the optical absorption spectrum of the type II superlattice structure shows an increase in applied electric field strength. It has been observed that the absorption edge shifts toward the short wavelength side (blue shift). This is because the transition wavelength of light changes from λ 0 to λ 1 (λ 1 <λ 0 ) by applying an electric field (see FIGS. 1A and 1B). At this time, it is also observed in our experiment that the change in the refractive index of the type II superlattice structure largely changes negatively as shown in FIG. 2 (b).
【００１８】図２（ｃ）も我々の実験結果で、ｐｉｎダ
イオード構造を有する面型素子の光吸収スペクトルの電
界強度依存性を示す図である。印加電界強度の増加に伴
い吸収端はブルーシフトする。また、図２（ｃ）に示し
た様に、吸収係数が増加する波長λ5 と吸収係数が減少
する波長λ3 がある。この時、屈折率変化スペクトルは
クラマース・クローニッヒの関係から求めることがで
き、図２（ｄ）の様になる。図２（ｃ）（ｄ）から、タ
イプＩＩ超格子構造では、吸収変化がほとんどない波長
域で大きな負の屈折率変化が得られることがわかる。こ
の点が図７（ｃ）（ｄ）に示した従来のタイプＩ超格子
構造の様相と大きく異なっている。FIG. 2C is also the result of our experiment, and is a diagram showing the electric field intensity dependence of the light absorption spectrum of the planar element having the pin diode structure. The absorption edge undergoes a blue shift as the applied electric field strength increases. Further, as shown in FIG. 2C, there are wavelength λ 5 where the absorption coefficient increases and wavelength λ 3 where the absorption coefficient decreases. At this time, the refractive index change spectrum can be obtained from the Kramers-Kronig relationship, as shown in FIG. From FIGS. 2C and 2D, it can be seen that the type II superlattice structure has a large negative refractive index change in the wavelength region where there is almost no absorption change. This point is largely different from the aspect of the conventional type I superlattice structure shown in FIGS.
【００１９】以上の様に、本発明のタイプＩＩ超格子構
造の吸収係数はある適当な波長域では印加電界の増加と
共に減少し、別の波長域では増加するという特徴を有し
ている。また、タイプＩＩ超格子構造の屈折率変化は、
バルク半導体層のフランツーケルディシュ効果と比べる
と大きく、タイプＩ超格子構造のＱＣＳＥと比べると、
屈折率変化の大きさは同程度であるが、その符号は反対
であり、吸収端から比較的離れた長波長側で負の屈折率
変化が得られるという特徴を有している。As described above, the absorption coefficient of the type II superlattice structure of the present invention is characterized in that it decreases with an increase in the applied electric field in a certain suitable wavelength range and increases in another wavelength range. In addition, the change in the refractive index of the type II superlattice structure is
Compared to the Franz-Keldish effect of the bulk semiconductor layer, it is larger than that of the type I superlattice structure QCSE.
The magnitudes of the changes in the refractive index are similar, but the signs are opposite, and the feature is that a negative change in the refractive index is obtained on the long wavelength side relatively far from the absorption edge.
【００２０】従って、本発明の第一の作用として、タイ
プＩＩ超格子構造を用いた吸収型光半導体素子では、低
電界動作で大きな吸収係数変化を得ることが可能であ
り、電界印加によって光吸収が減少するような波長域を
用いた吸収型の光半導体素子においては、光吸収により
光キャリアが発生しホールパイルアップが生じるという
問題点は生じない。Therefore, as the first effect of the present invention, in the absorption type optical semiconductor device using the type II superlattice structure, it is possible to obtain a large change in the absorption coefficient by the operation in the low electric field, and the optical absorption can be achieved by applying the electric field. In the absorption-type optical semiconductor element using a wavelength range in which the light absorption is reduced, there is no problem that hole pileup occurs due to generation of optical carriers due to light absorption.
【００２１】また、本発明の第二の作用として、タイプ
ＩＩ超格子構造を用いた吸収型光半導体素子では、電界
印加によって光吸収が急激に増加するような波長域も利
用でき、この場合には従来のタイプＩ超格子構造の光半
導体素子よりも低電圧で動作が可能である。As a second action of the present invention, in the absorption type optical semiconductor device using the type II superlattice structure, a wavelength range in which the light absorption sharply increases by applying an electric field can be used. Can operate at a lower voltage than conventional type I superlattice optical semiconductor devices.
【００２２】次に、第三の作用として、本発明の光半導
体素子を第一の作用で用いた場合には、従来の半導体レ
ーザを用いた光通信装置と整合するノーマリオフの光通
信装置の構成が可能である。Next, as a third action, when the optical semiconductor element of the present invention is used in the first action, a configuration of a normally-off optical communication device that matches an optical communication device using a conventional semiconductor laser is constructed. Is possible.
【００２３】さらに、第四の作用として、タイプＩＩ超
格子構造の電界効果を屈折率制御型光半導体素子に用い
た場合、吸収端から比較的離れた長波長側で負の屈折率
変化が得られ、かつ吸収変化が少ないため、従来よりも
小型で低損失の光半導体素子が得られ、その際の消費電
力は小さくかつ高速動作が可能である。Further, as a fourth effect, when the electric field effect of the type II superlattice structure is used in the refractive index control type optical semiconductor element, a negative refractive index change is obtained on the long wavelength side relatively far from the absorption edge. In addition, since the absorption and the change in absorption are small, it is possible to obtain an optical semiconductor element that is smaller in size and lower in loss than conventional ones, consumes less power at that time, and can operate at high speed.
【００２４】[0024]
【実施例】本発明の光半導体素子の実施例を図面に用い
て詳細に説明する。Embodiments of the optical semiconductor device of the present invention will be described in detail with reference to the drawings.
【００２５】図３（ａ）は本発明の第１の実施例（第一
の作用に対応）である電界吸収型の光半導体素子の斜視
図である。この実施例は、ｎ型ＩｎＰ基板１上に、ｎ型
クラッド層２（層厚０．５μｍ、キャリア濃度５×１０
17ｃｍ-3）、ノンドープＩｎＰ／ＩｎＡｌＡｓ超格子ガ
イド層３、ｐ型ＩｎＰクラッド層４（層厚１．５μｍ、
キャリア濃度５×１０17ｃｍ-3）、ｐ型ＩｎＧａＡｓコ
ンタクト層５（層厚０．２μｍ、キャリア濃度１×１０
19ｃｍ-3）をガスソース分子線エピタキシャル成長法で
順次積層する。その後反応性イオンビームエッチング
（ＲＩＢＥ）によってｎ型クラッド層２の途中までエッ
チングし、リブ導波路を形成する。ここで、ノンドープ
ＩｎＰ／ＩｎＡｌＡｓ超格子ガイド層３は本発明による
もので、ＩｎＰ（７０オングストローム）／ＩｎＡｌＡ
ｓ（７０オングストローム）を３０周期積層してあり、
この時、伝導帯不連続量△ＥV ＝０．３８ｅＶ、価電子
帯不連続量△ＥC ＝０．２６ｅＶを有する図３（ｂ）に
示した様なバンド構造のタイプＩＩ超格子構造となる。
リブ導波路上部と基板の裏面に電極を形成して素子は完
成する。FIG. 3A is a perspective view of an electro-absorption type optical semiconductor device according to the first embodiment (corresponding to the first operation) of the present invention. In this embodiment, an n-type clad layer 2 (layer thickness 0.5 μm, carrier concentration 5 × 10) is formed on an n-type InP substrate 1.
17 cm −3 ), non-doped InP / InAlAs superlattice guide layer 3, p-type InP clad layer 4 (layer thickness 1.5 μm,
Carrier concentration 5 × 10 17 cm −3 ), p-type InGaAs contact layer 5 (layer thickness 0.2 μm, carrier concentration 1 × 10 5
19 cm −3 ) are sequentially laminated by the gas source molecular beam epitaxial growth method. After that, reactive ion beam etching (RIBE) is performed until the n-type cladding layer 2 is etched halfway to form a rib waveguide. Here, the non-doped InP / InAlAs superlattice guide layer 3 is according to the present invention, and is InP (70 angstrom) / InAlA.
30 cycles of s (70 Å) are stacked,
At this time, a band-type type II superlattice structure having a conduction band discontinuity ΔE V = 0.38 eV and a valence band discontinuity ΔE C = 0.26 eV as shown in FIG. Become.
The device is completed by forming electrodes on the upper portion of the rib waveguide and the back surface of the substrate.
【００２６】図３（ａ）に示した光半導体素子に、タイ
プＩＩ超格子の吸収端に近い波長λ3 ＝１．１２５μｍ
の光を入射し素子に電圧を印加すると、図２（ａ）の様
に、印加電界強度の増加と共に吸収係数が減少する。図
２（ａ）から分かるように、Ｅ0 のような無電界時には
光の吸収が大きく光出力はオフとなり、Ｅ2 のような有
電界時には吸収端のブルーシフトによって光の吸収は少
なくなり、光出力はオンの状態となる。我々の実験で
は、印加電圧３Ｖで消光比（＝光出力オン／光出力オ
フ）は１３ｄＢ以上であった。このように、本発明では
低電圧で大きな吸収係数変化が得られる。また、有電界
時の光吸収が無いため、従来のタイプＩ超格子で問題と
なるホールパイルアップによって実効的な電界強度が弱
まる事はなく、キャリアの引き抜きも早いために高速変
調時に応答劣化が生じる事もない。In the optical semiconductor device shown in FIG. 3A, the wavelength λ 3 = 1.125 μm near the absorption edge of the type II superlattice.
2A is applied and a voltage is applied to the element, the absorption coefficient decreases as the applied electric field strength increases, as shown in FIG. As can be seen from FIG. 2A, when there is no electric field such as E 0 , the light absorption is large and the light output is off, and when there is an electric field such as E 2 , the absorption of the light decreases due to the blue shift at the absorption edge. The light output is turned on. In our experiment, the extinction ratio (= optical output on / optical output off) was 13 dB or more at an applied voltage of 3V. Thus, in the present invention, a large change in absorption coefficient can be obtained at a low voltage. In addition, since there is no light absorption when there is an electric field, the effective electric field strength does not weaken due to hole pile-up, which is a problem with conventional type I superlattices, and the extraction of carriers is fast, resulting in degraded response during high-speed modulation. It never happens.
【００２７】図４は本発明の第２の実施例（第二の作用
に対応）である電界吸収型の光半導体素子の断面図であ
る。層構造は図３の従来例と同様であるが、この実施例
は面型の変調器であり、ＩｎＧａＡｓコンタクト層５で
の光の吸収が生じないようにＩｎＧａＡｓコンタクト層
５の一部を硫酸系（Ｈ2 ＳＯ4 、Ｈ2 Ｏ、Ｈ2 Ｏ2 の混
合液）の選択エッチャントによって除去し、電極も光の
入出射を行う部分は除去する。この素子に、波長λ5 ＝
１．０８μｍの光を入射する。図２（ｃ）で明らかなよ
うに、波長λ5 の光に対しては、光吸収は電界印加によ
って急激に増加する。この時の電界印加による吸収変化
は、従来のタイプＩ超格子のＱＣＳＥよりも大きいた
め、低電圧で大きな消光比が得られる。我々の実験で
は、図４のような面型変調器で消光比１３ｄＢ以上が得
られている。本実施例のような面型変調器は光の並列性
を使うことができるため、光コンピュータ等への適用も
可能である。FIG. 4 is a sectional view of an electroabsorption type optical semiconductor device according to a second embodiment (corresponding to the second operation) of the present invention. The layer structure is the same as that of the conventional example of FIG. 3, but this example is a surface-type modulator, and a part of the InGaAs contact layer 5 is made of a sulfuric acid-based material so that the InGaAs contact layer 5 does not absorb light. It is removed by a selective etchant of (a mixed solution of H 2 SO 4 , H 2 O and H 2 O 2 ), and the electrode is also removed at the portion where light enters and exits. This element has a wavelength λ 5 =
Light of 1.08 μm is incident. As is clear from FIG. 2C, the absorption of light for the light having the wavelength λ 5 is sharply increased by the application of the electric field. Since the change in absorption due to the application of an electric field at this time is larger than that of the conventional type I superlattice QCSE, a large extinction ratio can be obtained at a low voltage. In our experiment, an extinction ratio of 13 dB or more was obtained with a surface modulator as shown in FIG. Since the surface modulator as in this embodiment can use the parallelism of light, it can be applied to an optical computer or the like.
【００２８】図９（ｃ）は本発明の第３の実施例（第三
の作用に対応）である光通信装置の構成図である。図３
（ａ）の様な本発明の第一の作用で動作する吸収型の光
半導体素子を含む光通信装置では、駆動回路Ａの出力が
オフの無電界時には光吸収があるため光出力はオフとな
り、駆動回路Ａの出力がオンの電界印加時には光吸収は
減少し光出力はオンとなるノーマリーオフの装置構成が
可能である。この時の駆動回路Ａは図９（ａ）の半導体
レーザ直接変調型光通信装置の駆動回路Ａと同じとな
り、駆動回路を新たに設計変更する必要はない。これに
対して、図７（ｂ）（ｃ）に示したフランツケルディッ
シュ効果やＱＣＳＥを用いる従来の吸収型外部変調器を
用いた図９（ｂ）のような光通信装置では、従来型外部
変調器が電界印加によって図７（ｂ）（ｃ）に示されて
いるような吸収変化を生ずるため、光通信装置の駆動回
路Ｂの出力がオフの無電界時には光吸収がなく、光出力
はオンとなり、駆動回路Ｂがオンの有電界時には光吸収
が生じ、光出力はオフとなるノーマリーオンの装置構成
となる。この場合には、図９（ａ）のような半導体レー
ザ直接変調型光通信装置と互換性がなく、根本的な駆動
回路の設計変更が必要となる。FIG. 9C is a block diagram of an optical communication device according to a third embodiment (corresponding to the third operation) of the present invention. Figure 3
In the optical communication device including the absorption type optical semiconductor element that operates according to the first operation of the present invention as in (a), the optical output is turned off because there is optical absorption when the output of the drive circuit A is off and there is no electric field. A normally-off device configuration is possible in which light absorption is reduced and light output is turned on when an electric field is applied when the output of the drive circuit A is turned on. The drive circuit A at this time is the same as the drive circuit A of the semiconductor laser direct modulation optical communication device of FIG. 9A, and there is no need to newly design the drive circuit. On the other hand, in the optical communication device as shown in FIG. 9B using the conventional absorption type external modulator using the Franz-Keldish effect and QCSE shown in FIGS. Since the modulator causes an absorption change as shown in FIGS. 7B and 7C when an electric field is applied, there is no light absorption when the output of the drive circuit B of the optical communication device is off and there is no optical output. When the drive circuit B is turned on and the drive circuit B is turned on, light absorption occurs, and the light output is turned off, thus forming a normally-on device configuration. In this case, it is not compatible with the semiconductor laser direct modulation type optical communication device as shown in FIG. 9A, and a fundamental design change of the drive circuit is required.
【００２９】図５（ａ）（ｂ）はそれぞれ本発明の第４
の実施例（第四の作用に対応）であるマッハツェンダ型
光半導体素子の上面図と断面図である。基本的な製造方
法は図３の電界吸収型光半導体素子と同じであるが、一
本の光導波路を二本の位相変調光導波路に分岐し、これ
を一本の光導波路に合流したマッハツェンダ型の構成と
なっている。但し、リッジ２０形成時のＲＩＢＥエッチ
ングはｐ型ＩｎＰクラッド１５の途中で止めている。素
子は二本の位相変調光導波路に各々独立の電極を設け、
基板裏面電極を設けて完成する。FIGS. 5 (a) and 5 (b) respectively show the fourth embodiment of the present invention.
4A is a top view and a cross-sectional view of a Mach-Zehnder optical semiconductor element that is an example (corresponding to the fourth action) of FIG. The basic manufacturing method is the same as that of the electro-absorption type optical semiconductor device of FIG. 3, but one optical waveguide is branched into two phase modulation optical waveguides, and this is merged into one optical waveguide. It has a structure of. However, the RIBE etching when forming the ridge 20 is stopped in the middle of the p-type InP clad 15. The device has two phase modulation optical waveguides with independent electrodes,
A substrate backside electrode is provided to complete the process.
【００３０】図５の光半導体素子に波長λ4 ＝１．３μ
ｍの光を入射し、一方の位相変調光導波路に電圧を印加
すると、その光導波路では図２（ｂ）（または図２
（ｄ））のような屈折率変化が生じ、合流した出力光導
波路では二本の位相変調光導波路の光に位相差が生じる
ことによって光の変調が行える。図５（ａ）（ｂ）に示
した本発明のマッハツェンダ型光半導体素子の印加電圧
に対する消光特性を図５（ｃ）に示す。印加電圧３．５
Ｖで１５ｄＢ以上の大きな消光比が得られている。図２
（ａ）で明らかなように、本素子ではλ4 ＝１．３μｍ
の光に対する光吸収がない。従って、光キャリアの発生
がなくホールパイルアップの様な現象は生じないため、
本質的な高速動作が可能である。また、吸収の変化がな
いために一方の位相変調部の損失変化によって、消光比
が劣化するという問題もない。なお、本実施例ではＣＲ
時定数で制限される周波数帯域拡大のために、電極パッ
ド下にポリイミド１６を用いて素子容量０．６ｐＦ以下
の低容量化を実現している。In the optical semiconductor device of FIG. 5, the wavelength λ 4 = 1.3 μ
When light of m is incident and a voltage is applied to one of the phase-modulating optical waveguides, the optical waveguide of FIG.
In the output optical waveguide that merges due to the change in the refractive index as shown in (d), the light can be modulated by the phase difference between the lights of the two phase modulation optical waveguides. FIG. 5C shows the extinction characteristic with respect to the applied voltage of the Mach-Zehnder optical semiconductor element of the present invention shown in FIGS. 5A and 5B. Applied voltage 3.5
A large extinction ratio of 15 dB or more at V is obtained. Figure 2
As is clear from (a), λ 4 = 1.3 μm in this device.
There is no light absorption for the light. Therefore, since there is no generation of optical carriers and a phenomenon like hole pile-up does not occur,
Intrinsic high speed operation is possible. Further, since there is no change in absorption, there is no problem that the extinction ratio is deteriorated due to a change in loss of one phase modulation section. In this embodiment, CR
In order to expand the frequency band limited by the time constant, polyimide 16 is used under the electrode pad to realize a low device capacitance of 0.6 pF or less.
【００３１】図６は本発明の第５の実施例（第四の作用
に対応）である全反射型光スイッチの斜視図とＡ−Ａ′
での断面図である。基本的な製造方法は図５のマッハツ
ェンダ型の光半導体素子と同じであり、ｐ型ＩｎＰクラ
ッド層４の途中でＲＩＢＥによるエッチングを止めてリ
ッジ２０を形成している。本実施例の素子は、二本の交
差する導波路の中央に屈折率を変化させる電界印加領域
への電界印加のための電極を設け、基板裏面電極を形成
することで完成する。FIG. 6 is a perspective view of a total reflection type optical switch according to a fifth embodiment of the present invention (corresponding to the fourth operation) and AA '.
FIG. The basic manufacturing method is the same as that of the Mach-Zehnder type optical semiconductor element of FIG. 5, and etching by RIBE is stopped in the middle of the p-type InP clad layer 4 to form the ridge 20. The element of this example is completed by providing an electrode for applying an electric field to the electric field application region where the refractive index is changed in the center of the two intersecting waveguides, and forming the substrate back surface electrode.
【００３２】この素子に波長λ4 ＝１．３μｍの光を図
のＰinから入射すると、無電界時には図６のＰ2 から出
射するが、電界印加領域に電圧を印加すると、電界印加
領域下のガイド層に、図２（ｂ）の様な負の屈折率変化
が生じ、この電界印加領域と電界が印加されないガイド
層の間に全反射条件が満たされると、電界印加領域下を
全反射面として光路が切り替わり、図６のＰ1 から出射
する。この屈折率の変化を用いる点は、図８に示した従
来例のキャリア注入プラズマ効果によって負の屈折率変
化が生じ、全反射を起こす光スイッチと同じである。と
ころが、本発明は、電圧制御によって全反射を生じさ
せ、光路の切り換えを行う光スイッチであることが従来
例とは大きく異なる。本発明の消費電力は逆バイアスを
用いた電圧駆動素子のため小さく、応答速度は電流注入
型の光路切り換え光スイッチよりもはるかに高速であ
る。更に、光の吸収がないためホールパイルアップが無
く、この点からも高速動作が可能である。また、ガイド
層における光の吸収も少ないため、従来よりも伝搬損失
の低減が可能である。When light of wavelength λ 4 = 1.3 μm enters this device from Pin in the figure, it is emitted from P 2 in FIG. 6 when there is no electric field, but when voltage is applied to the electric field application area, it is below the electric field application area. When a negative refractive index change occurs in the guide layer as shown in FIG. 2B and the total reflection condition is satisfied between the electric field application region and the guide layer to which no electric field is applied, the total reflection surface is formed below the electric field application region. As a result, the optical path is switched and the light is emitted from P 1 in FIG. The point of using this change in the refractive index is the same as that of the conventional optical switch shown in FIG. 8 in which a negative refractive index change is caused by the carrier injection plasma effect and total reflection occurs. However, the present invention is significantly different from the conventional example in that it is an optical switch that switches the optical path by causing total reflection by voltage control. The power consumption of the present invention is small because of the voltage driving element using the reverse bias, and the response speed is much faster than that of the current injection type optical path switching optical switch. Furthermore, since there is no light absorption, there is no hole pile-up, and high speed operation is possible from this point as well. Further, since the light absorption in the guide layer is small, the propagation loss can be reduced as compared with the conventional case.
【００３３】以上、本発明の実施例について説明してき
たが以下で若干の補足をする。実施例では何れもＩｎＰ
／ＩｎＡｌＡｓタイプＩＩ超格子構造を適用している
が、原理的にタイプＩＩ超格子構造を有していればどん
な材料でも良く、例えば他に、ＩｎＧａＡｓＰ／ＩｎＧ
ａＡｌＡｓ、ＩｎＧａＡｌＡｓ／ＧａＡｌＡｓＳｂ超格
子系に関しても適用可能である。また、このタイプＩＩ
超格子構造を用いて形成される光半導体素子の導波路構
造をどのような形態で形成するか、電界印加手段をどの
位置にどのような形態で形成するかは、利用される光の
波長、行われるべき機能等によって適当に決定すれば良
い。The embodiments of the present invention have been described above, but some supplements will be given below. In the examples, all are InP
Although the / InAlAs type II superlattice structure is applied, any material may be used as long as it has a type II superlattice structure in principle. For example, InGaAsP / InG
It is also applicable to aAlAs and InGaAlAs / GaAlAsSb superlattice systems. Also, this type II
The form of the waveguide structure of the optical semiconductor element formed by using the superlattice structure, the position and the form of the electric field applying means are determined by the wavelength of the light used, It may be appropriately determined according to the function to be performed.
【００３４】[0034]
【発明の効果】以上説明したように、本発明によって得
られた光半導体素子は、タイプＩＩ超格子層を光吸収層
として用いることにより、吸収係数変化の大きな即ち消
光比が大きな吸収型光半導体素子が得らる。また、タイ
プＩＩ超格子層を屈折率変化部に用いる事により、光吸
収変化が無く屈折率変化が負に大きい屈折率制御型光半
導体素子が得られる。これらの素子は何れも低電力・高
消光比・高速である。更に、吸収型光半導体素子を用い
た光通信装置ではノーマリーオフの装置構成も可能であ
る。As described above, the optical semiconductor device obtained by the present invention uses the type II superlattice layer as the light absorbing layer, so that the optical semiconductor device has a large change in absorption coefficient, that is, a large extinction ratio. The element is obtained. Further, by using the type II superlattice layer in the refractive index changing portion, it is possible to obtain a refractive index control type optical semiconductor element having no large light absorption change and a large negative refractive index change. All of these devices have low power consumption, high extinction ratio, and high speed. Furthermore, in an optical communication device using an absorption type optical semiconductor element, a normally-off device configuration is possible.
【図１】本発明の基本原理でタイプＩＩ超格子構造のバ
ンド構造図を示す。FIG. 1 shows a band structure diagram of a type II superlattice structure according to the basic principle of the present invention.
【図２】本発明の作用を説明するための図で、（ａ）
（ｃ）は吸収係数スペクトルの電界依存性、（ｂ）
（ｄ）は屈折率変化スペクトルの電界依存性を示す。FIG. 2 is a diagram for explaining the operation of the present invention, (a)
(C) is the electric field dependence of the absorption coefficient spectrum, (b)
(D) shows the electric field dependence of the refractive index change spectrum.
【図３】本発明による電界吸収型光半導体素子の斜視図
と超格子ガイド層のバンド図を示す。FIG. 3 shows a perspective view of an electro-absorption type optical semiconductor device according to the present invention and a band diagram of a superlattice guide layer.
【図４】本発明による面型の電界吸収型光半導体素子の
断面図を示す。FIG. 4 is a sectional view of a surface-type electro-absorption type optical semiconductor device according to the present invention.
【図５】本発明によるマッハツェンダ型の光半導体素子
の上面図（ａ）と断面図（ｂ）とその印加電圧に対する
消光特性図（ｃ）を示す。FIG. 5 shows a top view (a), a cross-sectional view (b), and an extinction characteristic view (c) of an applied voltage of a Mach-Zehnder type optical semiconductor device according to the present invention.
【図６】本発明による全反射型の光スイッチの斜視図
（ａ）とＡ−Ａ′で切断した図（ｂ）を示す。FIG. 6 shows a perspective view (a) of a total reflection type optical switch according to the present invention and a view (b) taken along the line AA ′.
【図７】従来のバルク又はタイプＩ超格子構造を用いた
電界吸収型光半導体素子の斜視図（ａ）とその原理を説
明する吸収係数スペクトルの電界依存性（ｂ）（ｃ）と
屈折率変化スペクトルの電界依存性（ｄ）を示す図。FIG. 7 is a perspective view of an electro-absorption type optical semiconductor device using a conventional bulk or type I superlattice structure (a) and electric field dependence (b) and (c) of an absorption coefficient spectrum and a refractive index for explaining the principle thereof. The figure which shows the electric field dependence (d) of a change spectrum.
【図８】従来の全反射型光スイッチの上面図。FIG. 8 is a top view of a conventional total reflection type optical switch.
【図９】従来の半導体レーザを用いた光通信装置の構成
図（ａ）と従来の吸収型光半導体素子を外部変調器とし
て有する光通信装置の構成図（ｂ）と本発明による光半
導体素子を用いた光通信装置の構成図（ｃ）を示す。FIG. 9 is a configuration diagram (a) of an optical communication device using a conventional semiconductor laser, a configuration diagram (b) of an optical communication device having a conventional absorption type optical semiconductor element as an external modulator, and an optical semiconductor element according to the present invention. FIG. 2C is a configuration diagram (c) of an optical communication device using the.
１ ｎ型ＩｎＰ基板 ２ ｎ型ＩｎＰクラッド層 ３ ノンドープＩｎＰ／ＩｎＡｌＡｓタイプＩＩ超格子
ガイド層 ４ ｐ型ＩｎＰクラッド層 ５ ｐ型ＩｎＧａＡｓコンタクト層 ６ ｐ電極 ７ ｎ電極 ８ 入射光 ９ 出射光 １０ ＳｉＯ2 膜 １１ ＩｎＰ基板 １２ ＩｎＰ／ＩｎＡｌＡｓタイプＩＩ超格子ガイド層 １３ ノンドープＩｎＰ １４ ＳｉＯ2 １５ ＩｎＰクラッド層 １６ ポリイミド １７ Ｔｉ／Ａｕ電極 １８ ＩｎＧａＡｓコンタクト層 ２０ リッジ1 n-type InP substrate 2 n-type InP clad layer 3 undoped InP / InAlAs type II superlattice guide layer 4 p-type InP clad layer 5 p-type InGaAs contact layer 6 p-electrode 7 n-electrode 8 incident light 9 outgoing light 10 SiO 2 film 11 InP Substrate 12 InP / InAlAs Type II Superlattice Guide Layer 13 Undoped InP 14 SiO 2 15 InP Clad Layer 16 Polyimide 17 Ti / Au Electrode 18 InGaAs Contact Layer 20 Ridge
Claims (5)
層、逆導電型クラッド層を基本構造として有し、吸収係
数変化または屈折率変化を生じさせる電界印加手段を有
する半導体素子において、前記ガイド層の一部が半導体
材料１、２からなる超格子構造であり、かつ半導体材料
１、２の各々の電子親和力χ1 、χ2 、禁制帯エネルギ
Ｅg1、Ｅg2が χ1 ＜χ2 χ1 ＋Ｅg1＜χ2 ＋Ｅg2 なる関係を有することを特徴とする光半導体素子。1. A semiconductor device having a conductive clad layer, a guide layer, and a reverse conductive clad layer as a basic structure on a semiconductor substrate and having an electric field applying means for causing a change in absorption coefficient or a change in refractive index. A part of the layer has a superlattice structure composed of semiconductor materials 1 and 2, and the electron affinity χ 1 , χ 2 of each of the semiconductor materials 1 and 2 and the forbidden band energies E g1 and E g2 are χ 1 <χ 2 χ. An optical semiconductor device having a relationship of 1 + E g1 <χ 2 + E g2 .
位相変化を用いるマッハツェンダ型に構成されているこ
とを特徴とする請求項１記載の光半導体素子。2. The optical semiconductor element according to claim 1, wherein the optical semiconductor element is of a Mach-Zehnder type that uses a phase change caused by a change in refractive index.
導波路の交点に位置する一部の半導体層の屈折率を変化
させ、全反射によって光路を切り換える全反射型光スイ
ッチとして構成されていることを特徴とする請求項１記
載の光半導体素子。3. An optical semiconductor element is configured as a total reflection type optical switch that changes the refractive index of a part of semiconductor layers located at the intersection of at least two intersecting waveguides and switches the optical path by total reflection. The optical semiconductor element according to claim 1, wherein:
型の外部変調器として含むことを特徴とする光通信装
置。4. An optical communication device comprising the optical semiconductor element according to claim 1 as an absorption type external modulator.
器とその駆動回路とからなり、前記外部変調器が請求項
１記載の光半導体素子からなる吸収型変調器であり、ノ
ーマリオフ型であることを特徴とする光通信装置。5. A semiconductor laser and its drive circuit, an external modulator and its drive circuit, wherein the external modulator is an absorption type modulator comprising the optical semiconductor element according to claim 1 and is a normally-off type. An optical communication device characterized by the above.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP6009024A JP3001365B2 (en) | 1994-01-31 | 1994-01-31 | Optical semiconductor element and optical communication device |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP6009024A JP3001365B2 (en) | 1994-01-31 | 1994-01-31 | Optical semiconductor element and optical communication device |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH07218880A true JPH07218880A (en) | 1995-08-18 |
| JP3001365B2 JP3001365B2 (en) | 2000-01-24 |
Family
ID=11709096
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP6009024A Expired - Fee Related JP3001365B2 (en) | 1994-01-31 | 1994-01-31 | Optical semiconductor element and optical communication device |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP3001365B2 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6122414A (en) * | 1997-05-28 | 2000-09-19 | Nec Corporation | Semiconductor Mach-Zehnder modulator |
| WO2005081050A1 (en) * | 2004-02-20 | 2005-09-01 | Nec Corporation | Modulator-integrated light source and its manufacturing method |
| JP2012098533A (en) * | 2010-11-02 | 2012-05-24 | Fujitsu Optical Components Ltd | Optical transmitter and optical transmission device |
-
1994
- 1994-01-31 JP JP6009024A patent/JP3001365B2/en not_active Expired - Fee Related
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6122414A (en) * | 1997-05-28 | 2000-09-19 | Nec Corporation | Semiconductor Mach-Zehnder modulator |
| WO2005081050A1 (en) * | 2004-02-20 | 2005-09-01 | Nec Corporation | Modulator-integrated light source and its manufacturing method |
| JPWO2005081050A1 (en) * | 2004-02-20 | 2008-01-10 | 日本電気株式会社 | Modulator integrated light source and manufacturing method thereof |
| JP2012098533A (en) * | 2010-11-02 | 2012-05-24 | Fujitsu Optical Components Ltd | Optical transmitter and optical transmission device |
Also Published As
| Publication number | Publication date |
|---|---|
| JP3001365B2 (en) | 2000-01-24 |
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