JP4998990B2 - Optical bistable element - Google Patents
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本発明は、光通信、全光論理スイッチ、光フリップ・フロップ、光バッファメモリ、全光ルータ等において用いることのできる光双安定素子に関する。 The present invention relates to an optical bistable element that can be used in optical communication, an all-optical logic switch, an optical flip-flop, an optical buffer memory, an all-optical router, and the like.
今後、光通信容量の拡大を続けていくために、ルーティング処理の光化（電気信号へ変換しないで光のままで処理すること）が必要であり、そのために必要な光バッファメモリの構成要素として、光双安定素子は近年、注目を集めている。ひとつの双安定素子は１ビットに過ぎず、多ビットの集積化に展開できるように、単一素子の小型化が求められている。 In the future, in order to continue to expand the optical communication capacity, it is necessary to make the routing process optical (process it as light without converting it into an electrical signal), and as a component of the optical buffer memory necessary for that purpose In recent years, optical bistable elements have attracted attention. One bistable element is only one bit, and a single element is required to be miniaturized so that it can be developed for multi-bit integration.
図７は、従来技術による光双安定素子を説明する図である。図７に示すブラッグ格子を刻んだ半導体メサ型導波路は、光集積回路に実装可能な、既存の光双安定素子である(例えば、非特許文献１参照)。半導体微細加工技術によって、メサ型導波路の外形に周期的変調を持たせた構造は、ブラッグ結合に由来する分布帰還の機構によって光双安定動作を示すことが知られている。このような構造は、前進波と後進波間のブラッグ結合が強まる周波数域（ストップバンド）において、光透過率が零に近くなる。入力光の波長をストップバンド内に選び、光強度を上げ下げすると、出力光強度は不連続に飛躍し、かつヒステリシスを持つ。これを双安定動作と呼ぶ。双安定動作は、ブラッグ結合による帰還作用と半導体材料の非線形光学特性から生じる。 FIG. 7 is a diagram for explaining an optical bistable element according to the prior art. A semiconductor mesa waveguide engraved with a Bragg grating shown in FIG. 7 is an existing optical bistable element that can be mounted on an optical integrated circuit (see, for example, Non-Patent Document 1). It is known that a structure in which the external shape of a mesa waveguide is periodically modulated by a semiconductor microfabrication technique exhibits an optical bistable operation by a distributed feedback mechanism derived from Bragg coupling. In such a structure, the light transmittance is close to zero in the frequency band (stop band) where the Bragg coupling between the forward wave and the backward wave is strengthened. When the wavelength of the input light is selected within the stop band and the light intensity is raised or lowered, the output light intensity jumps discontinuously and has hysteresis. This is called bistable operation. Bistable operation results from the feedback effect due to Bragg coupling and the nonlinear optical properties of the semiconductor material.
しかし、メサ型ブラッグ導波路では、ブラッグ結合、即ち帰還が弱いため、双安定動作を起すのに、1mm程度の導波路長を必要とすることが、素子寸法の小型化にとって障害となっている。
本発明は、帰還作用の強い導波路構造を用いて、導波路長を短くし、これによって、集積化をねらって素子の小型化を進めることを目的としている。 An object of the present invention is to reduce the length of a waveguide by using a waveguide structure having a strong feedback action, thereby promoting the miniaturization of elements for integration.
本発明の光双安定素子は、入力光強度の変化に対して出力光強度を不連続に変化させて、分布帰還型の双安定動作を行う。この光双安定素子は、半導体２次元フォトニック結晶薄板上に線欠陥導波路を設け、該線欠陥導波路は、所定の透過帯域を持ち、その低周波数側には、透過率が小さい周波数域のモードギャップを有し、前記入力光波長を、このモードギャップの周波数域におさまるように選択した。 The optical bistable element of the present invention performs a distributed feedback type bistable operation by discontinuously changing the output light intensity with respect to the change of the input light intensity. In this optical bistable element, a line defect waveguide is provided on a semiconductor two-dimensional photonic crystal thin plate, the line defect waveguide has a predetermined transmission band, and a frequency region having a small transmittance on the low frequency side thereof. And the input light wavelength was selected to fall within the frequency range of this mode gap.
モードギャップは、その透過率が導波帯域のそれの1/10以下になる範囲を指す。線欠陥導波路は、２次元フォトニック結晶薄板に設けた空気穴の周期配列を三角格子とし、かつ入射させる光の偏波を薄板面に平行として、空気穴の周期格子から、穴を一列無くして構成した。また、この線欠陥導波路の両側に、該導波路よりも格子定数の大きな別の線欠陥導波路を接続することができる。 The mode gap refers to a range in which the transmittance is 1/10 or less of that of the waveguide band. A line defect waveguide has a periodic arrangement of air holes provided in a two-dimensional photonic crystal thin plate made of a triangular lattice and the polarization of incident light is made parallel to the thin plate surface, so that one row of holes is eliminated from the periodic lattice of air holes. Configured. Further, another line defect waveguide having a larger lattice constant than that of the waveguide can be connected to both sides of the line defect waveguide.
本発明の光双安定素子は、帰還作用が強いため、双安定動作を担う導波路の長さを、メサ型導波路を基にした従来技術の約1/100以下に小型化できる。
従来技術の、メサ型導波路の外形に周期的変調を持たせた構造には、ブラッグ結合に由来する前進波と後進波の変換機構（帰還）があり、やはり光双安定動作を示すことは、上述の通りである。１に近い反射率、ゼロに近い透過率を示す周波数帯域（ストップバンド）幅は、本発明のモードギャップ帯域幅の約1/100と狭い。帰還の強さをκ、素子長をLとすると、ストップバンドやモードギャップ帯域幅は、κに比例するので、このことは、既存の構造で、双安定に適切とされる積κL= 2.5を得るには、素子長Lは上の例の100倍必要であることを意味している。本発明は、モードギャップを利用して、より大きな帰還κを得る結果、双安定素子の小型化をはかることが可能になる。
Since the optical bistable element of the present invention has a strong feedback action, the length of the waveguide responsible for the bistable operation can be reduced to about 1/100 or less of the conventional technology based on the mesa waveguide.
The structure of the prior art with a periodic modulation on the outer shape of the mesa-type waveguide has a forward wave and backward wave conversion mechanism (feedback) derived from Bragg coupling, and also shows optical bistable operation. , As described above. The frequency band (stop band) width showing reflectivity close to 1 and transmittance close to zero is as narrow as about 1/100 of the mode gap bandwidth of the present invention. Assuming that the feedback strength is κ and the element length is L, the stop band and mode gap bandwidth are proportional to κ, which means that the existing structure has a product κL = 2.5, which is appropriate for bistability. This means that the element length L needs to be 100 times that of the above example. In the present invention, as a result of obtaining a larger feedback κ using the mode gap, it is possible to reduce the size of the bistable element.
以下、例示に基づき、本発明を説明する。図１及び図２は、本発明の光双安定素子の第１の実施形態を説明する図である。図１を参照して、半導体２次元フォトニック結晶薄板上に設けた線欠陥導波路、及びその機能を説明する。半導体薄板に、空気穴の周期配列を設けたものを、２次元フォトニック結晶薄板と呼び、空気穴のパターンによって、光の薄板面内での伝播を制御できる。 Hereinafter, the present invention will be described based on examples. 1 and 2 are diagrams for explaining a first embodiment of an optical bistable element according to the present invention. A line defect waveguide provided on a semiconductor two-dimensional photonic crystal thin plate and its function will be described with reference to FIG. A semiconductor thin plate provided with a periodic array of air holes is called a two-dimensional photonic crystal thin plate, and the propagation of light within the thin plate surface can be controlled by the air hole pattern.
周期配列を三角格子とし、入射させる光の偏波を薄板面に平行として、空気穴の周期格子から、穴を一列無くして生じる構造は線欠陥と呼ばれ、光導波路として機能する。線欠陥導波路は、一定の透過帯域を持ち、その低周波数側に隣接して、透過率が小さい周波数域が存在する。この周波数域では、実数としての通常の伝播定数を割り当てることができず（図1左）、ここはモードギャップと呼ばれる。この周波数域では、入射波である前進波が、効率的に後進波に変換される結果、１に近い反射率、ゼロに近い透過率を与える。 A structure in which the periodic arrangement is a triangular lattice and the polarization of incident light is parallel to the thin plate surface and the holes are not formed in a row from the periodic lattice of air holes is called a line defect and functions as an optical waveguide. The line defect waveguide has a certain transmission band, and a frequency region with a small transmittance exists adjacent to the low frequency side. In this frequency range, a normal propagation constant as a real number cannot be assigned (left in Fig. 1), and this is called a mode gap. In this frequency range, the forward wave, which is an incident wave, is efficiently converted into a backward wave, resulting in a reflectance close to 1 and a transmittance close to zero.
図２は、モードギャップに基づいた双安定動作を説明する図である。格子定数ａを有する線欠陥導波路には、光源からの波長λの入力光が入射され、そこからの出力光は、受光器により検出される。モードギャップは、導波帯域の低周波数側に隣接し、その透過率が、導波帯域のそれの1/10以下になる、周波数帯域である。光の周波数がモードギャップにおさまるように、格子定数ａまたは入射光波長λを選ぶと、双安定動作が起きる。 FIG. 2 is a diagram illustrating a bistable operation based on the mode gap. Input light having a wavelength λ from a light source is incident on a line defect waveguide having a lattice constant a, and output light therefrom is detected by a light receiver. The mode gap is a frequency band adjacent to the low frequency side of the waveguide band and having a transmittance of 1/10 or less of that of the waveguide band. When the lattice constant a or the incident light wavelength λ is selected so that the light frequency falls within the mode gap, bistable operation occurs.
光の周波数がモードギャップ内にあると、このように反射率は１に近くなる。導波路内部での光の強度は、その場合でも零になる訳ではなく、入射端から出射端にかけて指数関数的に減衰するように分布する。光学的非線形性のために、導波路の屈折率は、光の強度に依存する。前進波が後進波に変換される効率が、屈折率に依存するため、図２のグラフに示すように、入力光の強度を上げてゆくと、ある強度で、後進波への変換効率が不連続に減少し、その結果、反射率がゼロに近く、透過率が1に近い状態が出現する。ここから、入力光強度を減少させてゆくと、先に跳躍が起きた入力強度では、元の状態には戻らず、より小さな、ある入力光強度で、元の状態に戻る。透過率の不連続な変化が起きる、２つの入力閾値に挟まれる光強度においては、取り得る透過率が高低２種類あることを指して、双安定性と呼ぶ。双安定性の発現には非線形性と帰還、そして適切な導波路長が必要である。帰還の強さは、前進波と後進波の間の結合係数κで表され、κは長さの逆数の次元を持ち、その大きさは計算からκ＝2.5×103cm-1と予想される。双安定動作が起きるためには、κL >1.25でなければならないことが知られている。一方、κLが大きすぎると、下向き遷移の入力閾値がゼロに近づいてしまうため、信号処理の用途では、1.5< κL <5となるように素子長Lを選ぶ。例えば、Lを10μmすると κL= 2.5が得られる。 When the frequency of light is within the mode gap, the reflectance is close to 1. Even in this case, the intensity of light inside the waveguide does not become zero, but is distributed so as to attenuate exponentially from the incident end to the output end. Because of optical nonlinearity, the refractive index of the waveguide depends on the light intensity. Since the efficiency of conversion of the forward wave into the backward wave depends on the refractive index, as shown in the graph of FIG. 2, when the intensity of the input light is increased, the conversion efficiency into the backward wave is reduced at a certain intensity. As a result, a state in which the reflectance is close to zero and the transmittance is close to 1 appears. From this point, when the input light intensity is decreased, the input intensity at which the jump occurred first does not return to the original state but returns to the original state with a certain smaller input light intensity. With respect to the light intensity between two input thresholds where discontinuous change in transmittance occurs, it refers to the fact that there are two types of transmittance that can be taken, which is called bistability. The development of bistability requires nonlinearity, feedback, and an appropriate waveguide length. The strength of the feedback is expressed by the coupling coefficient κ between the forward wave and the backward wave. Κ has the dimension of the reciprocal of the length, and the magnitude is predicted from the calculation as κ = 2.5 × 10 3 cm -1 The It is known that in order for bistable operation to occur, κL> 1.25 must be satisfied. On the other hand, if κL is too large, the input threshold value for the downward transition approaches zero. Therefore, in the signal processing application, the element length L is selected so that 1.5 <κL <5. For example, when L is 10 μm, κL = 2.5 is obtained.
図３は、本発明の光双安定素子の第２の実施形態を説明する図である。図３に示す導波路Ａ自体は、第１の実施形態について説明したモードギャップ結合を利用する導波路と同一構成にすることができる。図３に示す導波路構成は、格子定数ａ1の導波路Ａの両側に、格子定数ａ2の導波路Ｂを接続することにより構成される（ａ1<ａ2）。即ち、双安定動作を担う短い線欠陥導波路Ａを保持し、光の入出力を容易にするために、基本周期（格子定数）の大きな導波路Ｂを接続する。格子定数が大きくなると、分散曲線は下方に移動するので、Ｂの導波帯域であり、かつＡのモードギャップに当たる周波数域に、動作周波数を選ぶことができる。 FIG. 3 is a diagram for explaining a second embodiment of the optical bistable element of the present invention. The waveguide A itself shown in FIG. 3 can have the same configuration as the waveguide using the mode gap coupling described in the first embodiment. Waveguide structure shown in FIG. 3, on both sides of the waveguide A for the lattice constants a 1, constituted by connecting the waveguide B of the lattice constant a 2 (a 1 <a 2 ). That is, in order to hold the short line defect waveguide A that is responsible for the bistable operation and to facilitate the input and output of light, the waveguide B having a large fundamental period (lattice constant) is connected. As the lattice constant increases, the dispersion curve moves downward, so that the operating frequency can be selected in the frequency range corresponding to the B waveguide band and the A mode gap.
モードギャップ結合を利用する導波路Ａの長さには、双安定動作にとっての最適長が存在し、それは数10μm以下と短い。この寸法の導波路を保持する構造が必要である。このため、格子定数の大きな線欠陥導波路Ｂを接続することで、短尺の導波路Ａの保持と光入出力を容易にすることが可能となる。格子定数と空気穴径の比を一定に保つ場合、図３のグラフに示すように、導波路Ｂの分散曲線は、導波路Ａのそれを低周波数側に移動したものとなるので、導波路Ａのモードギャップ帯域であり、かつ導波路Ｂの導波帯域である周波数域に、光の周波数を選ぶことができる。または、意図する光周波数がその周波数領域に入るよう、導波路Ａ、Ｂの格子定数を選ぶことができる。 The length of the waveguide A using mode gap coupling has an optimum length for bistable operation, which is as short as several tens of μm or less. A structure is needed to hold a waveguide of this size. For this reason, by connecting the line defect waveguide B having a large lattice constant, it is possible to easily hold the short waveguide A and input / output light. When the ratio of the lattice constant to the air hole diameter is kept constant, the dispersion curve of the waveguide B is obtained by moving that of the waveguide A to the low frequency side as shown in the graph of FIG. The frequency of light can be selected in the frequency range that is the mode gap band of A and the waveguide band of the waveguide B. Alternatively, the lattice constants of the waveguides A and B can be selected so that the intended optical frequency falls in the frequency region.
図４は、GaAsのフォトニック結晶薄板上(厚さ260nm)に作製した、格子定数a１=385nm（導波路Ａ）, a2=420nm（導波路Ｂ）, 穴半径と格子定数の比 r/ a1,2=0.3の導波路構造の電子顕微鏡像を示している。双安定を担う導波路Ａは空気穴24列分、長さ9μmである。ここに接続する導波路Ｂは、双安定を担う導波路Ａへの光入出力のために設けた。これによって、双安定動作を担う、導波路Ａを、入出力用導波路Ｂで挟んだ構造となる。導波路Ｂは、1750nmより長波長側にそのモードギャップを持つが、それより短波長側は導波帯域である。格子定数と空気穴半径の比は、いずれも0.3である。 FIG. 4 shows the lattice constant a 1 = 385 nm (waveguide A), a 2 = 420 nm (waveguide B) fabricated on a GaAs photonic crystal thin plate (thickness 260 nm), the ratio of the hole radius to the lattice constant r An electron microscope image of the waveguide structure with / a 1,2 = 0.3 is shown. The waveguide A responsible for bistability has a length of 9 μm for 24 rows of air holes. The waveguide B connected to this is provided for optical input / output to the waveguide A responsible for bistability. As a result, the waveguide A, which is responsible for bistable operation, is sandwiched between the input / output waveguide B. The waveguide B has its mode gap on the longer wavelength side than 1750 nm, but the shorter wavelength side is the waveguide band. The ratio between the lattice constant and the air hole radius is 0.3.
図５は、試作した素子の透過スペクトルを示す図である。透過域の長波長端は1500nm付近にあり、ここより長波長側では、透過率が落ち込んでおり、双安定動作に利用するモードギャップが広がっている。矢印は、双安定動作の観測に用いた波長1503.036nmを示しており、この波長での透過率は導波帯域での透過率の1/20である。 FIG. 5 is a diagram showing a transmission spectrum of the prototyped device. The long wavelength end of the transmission region is in the vicinity of 1500 nm, and on the longer wavelength side from here, the transmittance is reduced, and the mode gap used for bistable operation is widened. The arrow indicates the wavelength 1503.036 nm used for the observation of bistable operation, and the transmittance at this wavelength is 1/20 of the transmittance in the waveguide band.
図６は、モードギャップ内部の波長で、観測された双安定応答を示す図である。モードギャップ内にある1503.036nmの波長の光について、入力光の強度を上げ下げして、双安定動作を確認した。 FIG. 6 shows the observed bistable response at wavelengths within the mode gap. For light with a wavelength of 1503.036 nm in the mode gap, the input light intensity was raised and lowered to confirm bistable operation.
Claims (3)
半導体２次元フォトニック結晶薄板上に前記光導波路として機能する線欠陥導波路を設け、
該線欠陥導波路は、所定の透過帯域を持ち、その低周波数側には、透過率が小さい周波数域のモードギャップを有して、前記入力光波長を、このモードギャップの周波数域におさまるように選択し、
前記線欠陥導波路の両側に、該導波路よりも格子定数の大きな別の線欠陥導波路を接続したことから成る光双安定素子。 In optical bistable element for bistable operation of the distributed feedback type by Rukoto discontinuously changing the output light intensity with respect to the change of the input light intensity to be input into the optical waveguide,
A line defect waveguide functioning as the optical waveguide is provided on a semiconductor two-dimensional photonic crystal thin plate,
該線defect waveguide has a predetermined transmission band, on its lower frequency side, with a mode gap is small frequency range transmission, the input light wavelength, to fit the frequency range of this mode gap selected,
An optical bistable element comprising: another line defect waveguide having a lattice constant larger than that of the waveguide connected to both sides of the line defect waveguide .
In the line defect waveguide, a periodic array of air holes provided in a two-dimensional photonic crystal thin plate is a triangular lattice, and the polarization of incident light is parallel to the thin plate surface. The optical bistable element according to claim 1, which is configured without the optical element.
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