JP3646158B2 - Dielectric multilayer dispersion compensating reflector - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、誘電体多層膜分散補償反射鏡に関し、特に、短パルスレーザーの発振、増幅、制御を目的とする機器を製造、利用する分野において、分散の補償、すなわち、短光パルスの発生及び伝搬の際の遅延時間の波長依存性の補償あるいは制御に関する。
【０００２】
【従来の技術】
従来作製されている誘電体多層膜分散補償反射鏡について説明する。近年、超短（数〜１００フェムト秒）光パルスを発生することの可能なレーザーが急速に進展している。超短光パルスの超高速性を生かした物質内での超高速現象の計測、高エネルギーでパルス幅が短いことにより電界強度が原子内のクーロン場と同等になることを利用した多価イオン生成、Ｘ線発生などがそのような光パルスを用いることにより行われている。この様な超短光パルスの発生及び伝搬の際、群遅延時間の波長依存性（分散）が問題となる。群遅延時間が波長によって異なると、波長幅の広いスペクトルを持つ超短光パルスは、異なる波長成分が異なる伝搬時間を持つことになり、結果としてパルス幅が広がってしまい、その高速性、高強度性を十分に生かすことができなくなってしまう。分散は、あらゆる物質に存在し、この場合に特に問題となるのは、超短光パルス発生に用いられるモード同期レーザー発振器の共振器内の分散や、超短光パルスを反射させたりするのに用いられる誘電体多層膜反射鏡による分散である。分散の評価は、一般的には、ある波長で群遅延時間を角周波数で微分した値で評価を行い、通常、群遅延時間の１次微分を２次分散、２次微分を３次分散、ｎ次微分を（ｎ＋１）次分散と呼ぶ。もし、２次分散が波長によらずゼロならば、群遅延時間は波長によらず一定となるので、分散によりパルス幅が広がることはない。また、この分散を制御することにより、超短光パルスの時間波形を望ましい形に整形することもある程度可能である。
【０００３】
誘電体多層膜分散補償反射鏡は、このようなモード同期レーザー発振器の共振器内の分散や、超短光パルスを透過させる物質の分散の打ち消し、あるいは、超短光パルスの時間波形を望ましい形に整形するために作製されるものである。この反射鏡は、遠赤外からＸ線領域で使用可能な誘電体多層膜反射鏡が元となっている。誘電体多層膜反射鏡は、光を反射するための平滑なガラス基板面に該所定波長範囲の光に対して透明である誘電体物質で、互いに屈折率の異なる２種類の誘電体薄膜（屈折率の高い方を高屈折率誘電体、低い方を低屈折率誘電体と呼ぶことにする。これらはお互いに相対的なもので、絶対的な屈折率の値で区別するものではない。）を交互に数十層積層することにより構成されている。図４にその構造の例を示す。この各膜のすべての光学的厚みをある波長の１／４にするとその波長付近で高い反射率を得ること、２種類の誘電体の屈折率の差が大きいほど少ない層数で反射率を大きくすることができ、反射波長帯域も広くとることができることが一般に知られている。しかしながら、その分散特性、反射帯域は、用いる誘電体の屈折率、膜厚、膜の層数で一義的に決定されてしまうので、任意の望ましい分散特性を得ることは困難である。また、帯域も上記の誘電体の組み合わせでは、例えば中心波長800 nm付近では帯域幅が200 nm程度で、 10 フェムト秒を下回るパルス幅を持つ超短光パルスでは、そのスペクトルの裾の幅が200 nmを越えるために、もしこのような反射鏡でこのような光パルスを反射させるとパルスのスペクトルが狭まってしまうため結果的にパルス幅が広くなってしまうので、このような超短光パルスを、パルス幅を広げることなく取り扱うことは極めて困難である。単に帯域を広げるのであれば、さきの誘電体の光学的厚みをすべて一定ではなく、例えば、互いに高反射波長域の異なる誘電体多層膜を複数重ねることや、基板から表層にいくに従って膜厚を徐々に増やすあるいは減らすことで実現可能である。しかしながら、これらの方法では高反射帯域内で分散が大きく変動して、これらの膜構成のままでは、超短光パルスを扱うことは極めて困難である。
【０００４】
誘電体多層膜分散補償反射鏡は、これらの誘電体多層膜反射鏡の設計を初期値として、準ニュートン法や焼き鈍し法などの最適化法を用いて、望ましい分散となるように各層の膜厚の最適化をおこない、その結果をもとに作製されたものである。図５に従来の誘電体多層膜分散補償反射鏡の膜構成の例を示す。この図において、横軸の膜ナンバーは、反射鏡の表面にある膜を１として、ガラス基板に近づくにつれ、２、３、４、…としたものである。縦軸は、それぞれの膜の光学的厚みである。これは、近年超短（数〜１００フェムト秒）光パルス発生に用いられているモード同期チタンサファイアレーザー発振器の共振器内分散を補償するために設計された反射鏡の例であり、共振器内をパルスが１往復するとき、共振器内において分散を持つもの、例えば、増幅媒質であるチタンサファイア、出力を取り出すための誘電体多層膜出力鏡、広帯域で分散を補償するための合成石英プリズムの分散とこの反射鏡を５回反射したときの２次分散をあわせたものが650 nmから1000 nmにわたってゼロになるようにしたものである。設計は、基板から表層にいくに従って膜厚を徐々に減らした膜構成を初期値として、望ましい分散になるように各膜厚の最適化を準ニュートン法により行ったものである。
【０００５】
このような誘電体多層膜分散補償反射鏡において良好な分散特性を得るには各膜厚の誤差が所定波長範囲の中心波長のおおよそ1/1000程度にしなければならないことが一般的に知られている。通常このような誘電体多層膜を作成する装置で、膜厚をさきにあげたような精度で制御可能なのは膜の光学的厚みが通常おおよそ100 nm以上の場合とされており、さきにあげた誘電体多層膜分散補償反射鏡の例のそれぞれの膜の光学的厚みは100 nm以上になるよう最適化が行われている。但し、最適化を行う際に最初に膜厚の制限をしてしまうと、この反射鏡のように最適化の対象となるパラメータが数十個にもなる場合には本当に最良な解ではなく、その近辺からみると良い解であるが、最良ではない解に収束してしまうことがある。このようなことを防ぐために、最初は、膜厚制限なしで最適化を行い、そのあと、光学的厚みが100 nm以下の層の膜厚を100 nmにして、且つ、全部の層に対して100 nm以上とする膜厚制限をつけて再び最適化を行い、図５に示す膜構造を得ている。図５に示した構造の反射率と２次分散の波長特性及び２次分散の目標値を図６に示す。この図での反射率は、さきに図５で示した膜構造から計算したもので、２次分散については、同膜構造から計算した位相を各波長での角周波数で２回微分することにより計算した。この図を見ると、２次分散が振動しているのがわかる。この振動は、分散補償の際に光パルスの時間幅を広げてしまったり、モード同期発振器に用いられた際には出力光パルスのスペクトル強度に、この分散の振動に応じてモジュレーションがかかるなどの問題を引き起こすので、これをいかに少なくするかが重要な問題となっている。
【０００６】
【発明が解決しようとする課題】
上記の従来例では、分散の振動があり、これが分散補償の際に光パルスの時間幅を広げてしまったり、モード同期発振器に用いられた際には出力光パルスのスペクトル強度に、この分散の振動に応じてモジュレーションがかかるなどの問題を引き起こすので、これをいかに少なくするかが重要な問題となっている。
本発明は、かかる問題点を解決して、この分散の振動を低減することを目的としている。
【０００７】
【課題を解決するための手段】
本発明では、従来高屈折率誘電体、低屈折率誘電体の２種類の誘電体を用いて作製されていた誘電体多層膜分散補償反射鏡を、従来の高屈折率誘電体、低屈折率誘電体の２種類にそれらの中間の屈折率を持つ誘電体（中間屈折率誘電体と呼ぶことにする）を加えて３種類の誘電体（高屈折率誘電体、低屈折率誘電体、中間屈折率誘電体の区別はお互いに相対的なもので、絶対的な屈折率の値で区別するものではない。）で構成することにより、分散の振動の低減をはかることができる。
【０００８】
【発明の実施の形態】
図４に示すような、低屈折率誘電体と高屈折率誘電体が交互に重なっている構造の場合、各層の光学的厚みがある波長の１／４のときその波長での反射率が最も大きくなり、１／４から小さくなると反射率が小さくなってくる。また、低屈折率誘電体と高屈折率誘電体の屈折率の差が大きいほど反射率が大きい。図５において、最初の膜厚制限なしでの最適化により、対象としている波長で一番短い650 nmの１／４である162.5 nmより薄い層が存在するということは、その膜の付近で、ゼロではないある程度の反射が必要とされていることを示している。もしその膜の付近で強い反射が必要ならば、その膜の光学的厚みはこの反射鏡の対象波長である650 nm、〜1000 nmの１／４、すなわち、162.5 nm〜250 nmの間になるはずであり、もし必要ないならば、最適化の結果、その層の膜厚がゼロになるはずであるからである。その膜厚を膜厚制限で最適の膜厚より大きくしてしまうことは、その膜の付近での反射を必要以上に大きくしてしまい、最適な条件からずれてしまう。そこで、その膜を、高屈折率誘電体と低屈折率誘電体の中間の屈折率を持つ誘電体（中間屈折率誘電体）にすれば、同じ膜厚でも反射が小さくなるので、最適な条件に近づけることが可能となる。
【０００９】
以上に述べたような３種類の誘電体を用いた反射鏡の設計は、以下の手順で行う。まず、高屈折率誘電体、低屈折率誘電体の２種類の誘電体を交互に積層した多層膜で、光学的厚みをすべて所定波長の１／４にした多層膜、または、高反射波長域の異なる誘電体多層膜を複数個重ねた多層膜、あるいは基板から表層にいくに従って膜厚を徐々に増やすあるいは減らすようにした構造の多層膜を初期値とし、但し厚みの制限は与えずに、準ニュートン法や焼き鈍し法などの最適化法を用いて、望ましい分散となるように各層の膜厚の最適化をおこなう。これは、最適化を行う際に最初に膜厚の制限をしてしまうと、この反射鏡のように最適化の対象となるパラメータが数十個にもなる場合には本当に最良な解ではなく、その近辺からみると良い解であるが、最良ではない解に収束してしまうことがあり、このようなことを防ぐために、最初は膜厚制限なしで最適化を行う。この結果得られた膜構成で、光学的厚みが100 nm以下の高屈折率誘電体膜を中間屈折率誘電体膜に置き換えて、光学的厚みを100 nmにする。光学的厚みが100 nm以下の低屈折率誘電体膜については、光学的厚みを100 nmにして、今度は、各層の光学的厚みが100 nm以上となる条件で再び最適化を行って、目的の膜構成を得る。この結果得られる、３種類の誘電体を用いた誘電体多層膜分散補償反射鏡の構成図を図１に示す。図１の中で、高屈折率あるいは中間屈折率誘電体とあるのは、最適化の結果次第で高屈折率誘電体または中間屈折率誘電体に決定されることを示している。
【００１０】
図２に、本発明による３種類の誘電体を用いた誘電体多層膜分散補償反射鏡の実施例の膜構造を示す。モード同期チタンサファイアレーザー発振器の共振器内分散を補償するために設計された反射鏡の膜構造の例である。この図において、横軸の膜ナンバーは、反射鏡の表面にある膜を１として、ガラス基板に近づくにつれ、２、３、４、…としたものである。縦軸は、それぞれの膜の光学的厚みである。これは、共振器内をパルスが１往復するとき、共振器内において分散を持つもの、例えば、増幅媒質であるチタンサファイア、出力を取り出すための誘電体多層膜出力鏡、広帯域で分散を補償するための合成石英プリズムの分散とこの反射鏡を５回反射したときの２次分散をあわせたものが650 nmから1000 nmにわたってゼロになるようにしたものである。本実施例ではまず、図５に示した従来の２種類の誘電体（本実施例では、高屈折率誘電体として二酸化チタン（TiO2）を、低屈折率誘電体として二酸化ケイ素(SiO2)を用いる。）を用いた誘電体多層膜分散補償反射鏡の設計に利用した、基板から表層にいくに従って膜厚を徐々に減らした膜構成を初期値としている。これをもとに、望ましい分散になるように各膜厚の最適化を準ニュートン法により行う。この結果得られた膜構成で、光学的厚みが100 nm以下の高屈折率誘電体膜を中間屈折率誘電体膜（本実施例ではアルミナ（Al2O3）を用いる。）に置き換えて、光学的厚みを100 nmにする。光学的厚みが100 nm以下の低屈折率誘電体膜については、光学的厚みを100 nmにして、今度は、各層の光学的厚みが100 nm以上となる条件で再び最適化を行って、図２の膜構成を得る。図６に、図２の膜構成の場合の反射率と２次分散の波長依存性、２次分散の目標値を示す。この場合にも従来の２種類の誘電体を用いた誘電体多層膜分散補償反射鏡と同じく、分散の振動が存在するが、目標値からの分散のずれの平均値が7.1 fs2で、図５で示した従来の膜構造の場合の値9.4 fs2に比べて小さくなっている。このことは、３種類の誘電体を用いた誘電体多層膜分散補償反射鏡は、従来の２種類の誘電体を用いた誘電体多層膜分散補償反射鏡に比べ、分散の振動が少ないことを示す。
【００１１】
別の実施例として図３に、可視〜近赤外域である525 nm〜675 nmにおいて、光学部品の構成材としてよく用いられる合成石英の厚さ0.5 mm分の分散を打ち消すための誘電体多層膜分散補償反射鏡の実施例の膜構造を示す。本実施例でも、まず、図５に示した従来の２種類の誘電体（本実施例では、高屈折率誘電体として二酸化チタン（TiO2）を、低屈折率誘電体として二酸化ケイ素(SiO2)を用いる。他の高屈折率誘電体として酸化タンタル（V）（Ta2O5）なども用いることが可能である。）を用いた誘電体多層膜分散補償反射鏡の設計に利用した、基板から表層にいくに従って膜厚を徐々に減らした膜構成を初期値としている。これをもとに、望ましい分散になるように各膜厚の最適化を準ニュートン法により行う。この結果得られた膜構成で、光学的厚みが100 nm以下の高屈折率誘電体膜を中間屈折率誘電体膜（本実施例ではアルミナ（Al2O3）を用いる。他の中間屈折率誘電体として酸化イットリウム（Y2O3）なども用いることが可能である。）に置き換えて、光学的厚みを100 nmにする。光学的厚みが100 nm以下の低屈折率誘電体膜については、光学的厚みを100 nmにして、今度は、各層の光学的厚みが100 nm以上となる条件で再び最適化を行って、図３の膜構成を得る。図７に、図３の膜構成の場合の反射率と２次分散の波長依存性、２次分散の目標値を、従来の２種類の誘電体で構成された誘電体多層膜分散補償反射鏡の反射率と２次分散とあわせて示す。この場合にも従来の２種類の誘電体を用いた誘電体多層膜分散補償反射鏡と同じく、分散の振動が存在するが、目標値からの分散のずれの平均値が1.9 fs2で、従来の膜構造の場合の値4.6 fs2に比べて小さくなっている。この実施例でも、３種類の誘電体を用いた誘電体多層膜分散補償反射鏡は、従来の２種類の誘電体を用いた誘電体多層膜分散補償反射鏡に比べ、分散の振動が少ないことを示している。
【００１２】
【発明の効果】
以上説明したように、本発明によれば分散の振動の少ない誘電体多層膜分散補償反射鏡を設計することが可能となり、分散補償の際の光パルスの時間幅伸長の低減や、モード同期発振器に用いられた際にこの分散の振動に応じて起こる出力光パルススペクトル強度のモジュレーションの低減という顕著な効果が得られる。
【図面の簡単な説明】
【図１】本発明の３種類の誘電体を用いた誘電体多層膜分散補償反射鏡の構造を示す説明図である。
【図２】本発明の実施例の一つである３種類の誘電体を用いた誘電体多層膜分散補償反射鏡の膜構成を示す図である。
【図３】本発明のもう一つの実施例である３種類の誘電体を用いた誘電体多層膜分散補償反射鏡の膜構成を示す図である。
【図４】従来の誘電体多層膜反射鏡の構造を示す説明図である。
【図５】従来の２種類の誘電体を用いた誘電体多層膜分散補償反射鏡の膜構成を示す図である。
【図６】本発明の実施例の一つである３種類の誘電体を用いた誘電体多層膜分散補償反射鏡の反射率と２次分散、従来の２種類の誘電体を用いた誘電体多層膜分散補償反射鏡の反射率と２次分散の比較を示す図である。
【図７】本発明のもう一つの実施例である３種類の誘電体を用いた誘電体多層膜分散補償反射鏡の反射率と２次分散、従来の２種類の誘電体を用いた誘電体多層膜分散補償反射鏡の反射率と２次分散の比較を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a dielectric multilayer dispersion compensating reflector, and in particular, in the field of manufacturing and using equipment for the purpose of oscillation, amplification and control of a short pulse laser, dispersion compensation, that is, generation of a short light pulse and The present invention relates to compensation or control of wavelength dependence of delay time during propagation.
[0002]
[Prior art]
A conventional dielectric multilayer dispersion compensating mirror will be described. In recent years, lasers capable of generating ultrashort (several to 100 femtosecond) light pulses have been rapidly developed. Measurement of ultra-high-speed phenomena in a material taking advantage of the ultra-high speed of ultra-short light pulses, and the generation of multiply charged ions utilizing the fact that the electric field strength is equivalent to the Coulomb field in atoms due to high energy and short pulse width. X-ray generation is performed by using such light pulses. When such an ultrashort optical pulse is generated and propagated, the wavelength dependency (dispersion) of the group delay time becomes a problem. If the group delay time varies depending on the wavelength, ultrashort optical pulses with a broad spectrum of wavelengths will have different propagation times for different wavelength components, resulting in a wider pulse width, resulting in higher speed and higher intensity. It becomes impossible to make full use of sex. Dispersion exists in all materials, and in this case, a particular problem is dispersion in the resonator of a mode-locked laser oscillator used for generating an ultrashort optical pulse, or reflecting an ultrashort optical pulse. Dispersion by a dielectric multilayer reflector used. In general, the dispersion is evaluated by a value obtained by differentiating the group delay time with an angular frequency at a certain wavelength. Usually, the first derivative of the group delay time is second-order dispersion, the second-order derivative is third-order dispersion, The nth derivative is called (n + 1) th order variance. If the second-order dispersion is zero regardless of the wavelength, the group delay time is constant regardless of the wavelength, so that the pulse width does not increase due to dispersion. Also, by controlling this dispersion, it is possible to shape the time waveform of the ultrashort optical pulse to a desired shape to some extent.
[0003]
Dielectric multilayer dispersion compensating mirrors have a desirable shape for canceling dispersion in a resonator of such a mode-locked laser oscillator, dispersion of a material that transmits an ultrashort optical pulse, or a temporal waveform of an ultrashort optical pulse. It is produced for shaping. This reflector is based on a dielectric multilayer reflector that can be used in the X-ray region from the far infrared. A dielectric multilayer mirror is a dielectric material that is transparent to light in a predetermined wavelength range on a smooth glass substrate surface for reflecting light. The higher refractive index is called the high refractive index dielectric, and the lower one is called the low refractive index dielectric, which are relative to each other and are not distinguished by absolute refractive index values.) Are formed by alternately stacking several tens of layers. FIG. 4 shows an example of the structure. When all the optical thicknesses of the respective films are set to 1/4 of a certain wavelength, a high reflectance is obtained in the vicinity of the wavelength. The larger the difference in refractive index between the two types of dielectrics, the larger the reflectance with a smaller number of layers. It is generally known that the reflection wavelength band can be widened. However, since the dispersion characteristics and reflection band are uniquely determined by the refractive index, the film thickness, and the number of layers of the dielectric used, it is difficult to obtain any desired dispersion characteristics. In addition, in the combination of the above dielectric materials, for example, the bandwidth is about 200 nm in the vicinity of the center wavelength of 800 nm. In order to exceed nm, if such a light pulse is reflected by such a reflector, the spectrum of the pulse will be narrowed, resulting in a wider pulse width. It is extremely difficult to handle without increasing the pulse width. If the bandwidth is simply widened, the optical thickness of the previous dielectric is not all constant.For example, multiple dielectric multilayers with different high reflection wavelength ranges are stacked on each other, or the film thickness is increased from the substrate to the surface layer. This can be achieved by gradually increasing or decreasing. However, in these methods, dispersion greatly fluctuates within the high reflection band, and it is extremely difficult to handle ultrashort light pulses with these film configurations.
[0004]
Dielectric multilayer dispersion compensating mirrors are designed with these dielectric multilayer reflectors as initial values, and using an optimization method such as quasi-Newton method or annealing method, the film thickness of each layer to achieve the desired dispersion It was made based on the result of optimization. FIG. 5 shows an example of the film configuration of a conventional dielectric multilayer dispersion compensating mirror. In this figure, the film number on the horizontal axis is set to 1, 2, 3, 4,... The vertical axis represents the optical thickness of each film. This is an example of a reflector designed to compensate for intra-cavity dispersion of a mode-locked titanium sapphire laser oscillator that has recently been used to generate ultrashort (several to 100 femtosecond) optical pulses. When the pulse reciprocates once, the resonator has dispersion in the resonator, for example, titanium sapphire as an amplifying medium, a dielectric multilayer output mirror for extracting output, and a synthetic quartz prism for compensating dispersion in a wide band. The total of the dispersion and the second-order dispersion when the reflecting mirror is reflected five times is zero from 650 nm to 1000 nm. In the design, each film thickness is optimized by the quasi-Newton method so as to obtain a desired dispersion, with a film configuration in which the film thickness is gradually reduced from the substrate toward the surface layer as an initial value.
[0005]
It is generally known that in order to obtain good dispersion characteristics in such a dielectric multilayer dispersion compensating reflector, the error of each film thickness must be about 1/1000 of the center wavelength in a predetermined wavelength range. Yes. In general, an apparatus for producing such a dielectric multilayer film can be controlled with the accuracy as described above when the optical thickness of the film is usually about 100 nm or more. Optimization is performed so that the optical thickness of each film in the example of the dielectric multilayer dispersion compensating reflector is 100 nm or more. However, if the film thickness is first limited when performing optimization, it is not the best solution when there are dozens of parameters to be optimized like this reflector. Although it is a good solution from the vicinity, it may converge to a solution that is not the best. In order to prevent such a situation, optimization is performed without limiting the film thickness at first, and then the film thickness of the layer having an optical thickness of 100 nm or less is set to 100 nm, and for all the layers. The film structure shown in FIG. 5 is obtained by optimizing again with a film thickness limitation of 100 nm or more. FIG. 6 shows the reflectance of the structure shown in FIG. 5, the wavelength characteristic of the secondary dispersion, and the target value of the secondary dispersion. The reflectivity in this figure was calculated from the film structure shown in FIG. 5, and the second-order dispersion is obtained by differentiating the phase calculated from the film structure twice by the angular frequency at each wavelength. Calculated. From this figure, it can be seen that the secondary dispersion is oscillating. This vibration increases the time width of the optical pulse during dispersion compensation, and when used in a mode-locked oscillator, the spectrum intensity of the output optical pulse is modulated according to this dispersion vibration. Since it causes problems, how to reduce this is an important issue.
[0006]
[Problems to be solved by the invention]
In the above-described conventional example, there is a dispersion vibration, which widens the time width of the optical pulse at the time of dispersion compensation, or when used in a mode-locked oscillator, the spectral intensity of the output light pulse is reduced. Since it causes problems such as modulation in response to vibration, how to reduce this is an important issue.
An object of the present invention is to solve this problem and reduce the vibration of the dispersion.
[0007]
[Means for Solving the Problems]
In the present invention, a conventional dielectric multilayer dispersion compensating reflector manufactured using two types of dielectrics, a high refractive index dielectric and a low refractive index dielectric, is replaced with a conventional high refractive index dielectric, low refractive index. Three types of dielectrics (high refractive index dielectrics, low refractive index dielectrics, intermediates) are added to two types of dielectrics, dielectrics having an intermediate refractive index (referred to as intermediate refractive index dielectrics). Dispersion of refractive index dielectrics is relative to each other, not absolute refractive index values.), The dispersion vibration can be reduced.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In the case of a structure in which a low refractive index dielectric and a high refractive index dielectric are alternately overlapped as shown in FIG. 4, when the optical thickness of each layer is 1/4 of a certain wavelength, the reflectance at that wavelength is the highest. Increasing and decreasing from 1/4 decreases the reflectivity. Also, the greater the difference in refractive index between the low refractive index dielectric and the high refractive index dielectric, the greater the reflectivity. In FIG. 5, the fact that there is a layer thinner than 162.5 nm, which is a quarter of the shortest 650 nm at the wavelength of interest, by the optimization without the first film thickness limitation, This indicates that some non-zero reflection is required. If strong reflection is required near the film, the optical thickness of the film will be between 650 nm, which is the target wavelength of the reflector, ¼ of 1000 nm, that is, between 162.5 nm and 250 nm. If it is not necessary, the film thickness of the layer should be zero as a result of optimization. Making the film thickness larger than the optimum film thickness by limiting the film thickness makes reflection near the film unnecessarily large and deviates from the optimum conditions. Therefore, if the film is made of a dielectric having an intermediate refractive index between a high refractive index dielectric and a low refractive index dielectric (intermediate refractive index dielectric), reflection is reduced even with the same film thickness. It becomes possible to approach.
[0009]
The design of the reflector using the three types of dielectrics as described above is performed according to the following procedure. First, a multilayer film in which two types of dielectrics, a high refractive index dielectric and a low refractive index dielectric, are alternately stacked, and a multilayer film in which the optical thickness is all ¼ of a predetermined wavelength, or a high reflection wavelength region The initial value is a multilayer film in which a plurality of different dielectric multilayer films are stacked, or a multilayer film having a structure in which the film thickness is gradually increased or decreased from the substrate to the surface layer, without limiting the thickness, By using an optimization method such as a quasi-Newton method or an annealing method, the thickness of each layer is optimized so as to obtain a desired dispersion. This is not really the best solution when there are dozens of parameters to be optimized like this reflector if the film thickness is first limited when performing optimization. Although it is a good solution from the vicinity, it may converge to a solution that is not the best, and in order to prevent such a situation, optimization is initially performed without limiting the film thickness. In the film structure obtained as a result, the high refractive index dielectric film having an optical thickness of 100 nm or less is replaced with an intermediate refractive index dielectric film so that the optical thickness is 100 nm. For low-refractive-index dielectric films with an optical thickness of 100 nm or less, the optical thickness was set to 100 nm, and this time, optimization was performed again under the condition that the optical thickness of each layer was 100 nm or more. The film configuration is obtained. FIG. 1 shows a configuration diagram of a dielectric multilayer dispersion compensating mirror using three types of dielectrics obtained as a result. In FIG. 1, the presence of a high refractive index or intermediate refractive index dielectric means that it is determined to be a high refractive index dielectric or an intermediate refractive index dielectric depending on the result of optimization.
[0010]
FIG. 2 shows a film structure of an embodiment of a dielectric multilayer dispersion compensating reflector using three kinds of dielectrics according to the present invention. It is an example of the film structure of the reflecting mirror designed in order to compensate the intracavity dispersion of a mode-locked titanium sapphire laser oscillator. In this figure, the film number on the horizontal axis is set to 1, 2, 3, 4,... The vertical axis represents the optical thickness of each film. This means that when a pulse reciprocates once in the resonator, it has dispersion in the resonator, for example, titanium sapphire as an amplifying medium, a dielectric multilayer output mirror for taking out the output, and compensates dispersion in a wide band. The combination of the dispersion of the synthetic quartz prism for this and the secondary dispersion when the reflecting mirror is reflected five times is made zero from 650 nm to 1000 nm. In this embodiment, first, two conventional dielectrics shown in FIG. 5 (in this embodiment, titanium dioxide (TiO 2 ) is used as a high refractive index dielectric, and silicon dioxide (SiO 2 ) is used as a low refractive dielectric. The film configuration in which the film thickness is gradually decreased from the substrate to the surface layer, which is used for the design of the dielectric multilayer dispersion compensating reflector using the above-mentioned, is the initial value. Based on this, optimization of each film thickness is performed by the quasi-Newton method so as to obtain desirable dispersion. In the film configuration obtained as a result, the high refractive index dielectric film having an optical thickness of 100 nm or less is replaced with an intermediate refractive index dielectric film (in this embodiment, alumina (Al 2 O 3 ) is used). The optical thickness is 100 nm. For low-refractive-index dielectric films with an optical thickness of 100 nm or less, the optical thickness is set to 100 nm, and this time, optimization is performed again under the condition that the optical thickness of each layer is 100 nm or more. Two membrane configurations are obtained. FIG. 6 shows the reflectance and the wavelength dependence of the secondary dispersion in the case of the film configuration of FIG. 2 and the target value of the secondary dispersion. In this case as well, as in the conventional dielectric multilayer dispersion compensating reflector using two types of dielectrics, dispersion vibration exists, but the average deviation from the target value is 7.1 fs 2 . This is smaller than the value 9.4 fs 2 in the case of the conventional film structure shown in FIG. This means that the dielectric multilayer dispersion compensating reflector using three types of dielectrics has less dispersion vibration than the conventional dielectric multilayer dispersion compensating reflector using two types of dielectrics. Show.
[0011]
As another example, FIG. 3 shows a dielectric multilayer film for canceling the dispersion of 0.5 mm thick synthetic quartz, which is often used as a component of optical components, in the visible to near-infrared region of 525 nm to 675 nm. The film structure of the Example of a dispersion compensation reflective mirror is shown. Also in this example, first, two conventional dielectrics shown in FIG. 5 (in this example, titanium dioxide (TiO 2 ) is used as a high refractive index dielectric, and silicon dioxide (SiO 2 as a low refractive index dielectric). It is also possible to use tantalum oxide (V) (Ta 2 O 5 ) etc. as other high refractive index dielectrics.) The initial value is a film configuration in which the film thickness is gradually reduced from the substrate to the surface layer. Based on this, optimization of each film thickness is performed by the quasi-Newton method so as to obtain desirable dispersion. In the film structure obtained as a result, an intermediate refractive index dielectric film (in this embodiment, alumina (Al 2 O 3 ) is used as the high refractive index dielectric film having an optical thickness of 100 nm or less. Other intermediate refractive index It is possible to use yttrium oxide (Y 2 O 3 ) or the like as a dielectric, and the optical thickness is set to 100 nm. For low-refractive-index dielectric films with an optical thickness of 100 nm or less, the optical thickness is set to 100 nm, and this time, optimization is performed again under the condition that the optical thickness of each layer is 100 nm or more. A film configuration of 3 is obtained. FIG. 7 shows the wavelength dependence of the reflectance and secondary dispersion in the case of the film configuration of FIG. 3, and the target value of secondary dispersion is a dielectric multilayer dispersion compensating reflector made up of two conventional dielectrics. This is shown together with the reflectance and secondary dispersion. In this case as well, as with the conventional dielectric multilayer dispersion compensating reflector using two types of dielectrics, there is a vibration of dispersion, but the average value of the deviation of the dispersion from the target value is 1.9 fs 2 . This is smaller than the value 4.6 fs 2 in the case of the film structure. Also in this embodiment, the dielectric multilayer dispersion compensating reflector using three kinds of dielectrics has less dispersion vibration than the conventional dielectric multilayer dispersion compensating reflector using two kinds of dielectrics. Is shown.
[0012]
【The invention's effect】
As described above, according to the present invention, it is possible to design a dielectric multilayer dispersion compensating reflector with little dispersion vibration, and it is possible to reduce the time width extension of an optical pulse at the time of dispersion compensation and a mode-locked oscillator. When used in the above, a remarkable effect is obtained in that the modulation of the output light pulse spectrum intensity which occurs in response to the vibration of the dispersion is reduced.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the structure of a dielectric multilayer dispersion compensating reflector using three types of dielectrics according to the present invention.
FIG. 2 is a diagram showing a film configuration of a dielectric multilayer dispersion compensating reflector using three types of dielectrics according to one embodiment of the present invention.
FIG. 3 is a diagram showing a film configuration of a dielectric multilayer dispersion compensating reflector using three kinds of dielectrics according to another embodiment of the present invention.
FIG. 4 is an explanatory view showing the structure of a conventional dielectric multilayer film reflecting mirror.
FIG. 5 is a diagram showing a film configuration of a dielectric multilayer dispersion compensating reflector using two types of conventional dielectrics.
FIG. 6 shows the reflectivity and second-order dispersion of a dielectric multilayer dispersion compensating reflector using three types of dielectrics according to one embodiment of the present invention, and a dielectric using two types of conventional dielectrics. It is a figure which shows the comparison of the reflectance of a multilayer film dispersion compensation reflective mirror, and secondary dispersion.
FIG. 7 shows the reflectance and secondary dispersion of a dielectric multilayer dispersion compensating reflector using three types of dielectrics according to another embodiment of the present invention, and a dielectric using two types of conventional dielectrics. It is a figure which shows the comparison of the reflectance of a multilayer film dispersion compensation reflective mirror, and secondary dispersion.
Claims (2)
前記低屈折率誘電体と、前記高屈折率誘電体或いは前記中間屈折率誘電体のいずれかとは交互に積層され、かつ、
前記高屈折率誘電体或いは前記中間屈折率誘電体のいずれかは、望ましい分散となるように行われる各層の膜厚の最適化に従って選択されることから成る誘電体多層膜分散補償反射鏡。Transparent, high refractive index dielectric with respect to light of a predetermined wavelength range on a smooth glass surface for reflecting light, mid refractive index dielectric, a low refractive index dielectric dielectric comprised of three types of dielectric In the multilayer dispersion compensating mirror,
The low refractive index dielectric and the high refractive index dielectric or the intermediate refractive index dielectric are alternately laminated, and
Either the high refractive index dielectric or the intermediate refractive index dielectric is selected according to the optimization of the thickness of each layer performed to achieve a desired dispersion .
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| JP2003107223A (en) * | 2001-09-27 | 2003-04-09 | Sekiji Yamagata | Dielectric multi-layered film mirror |
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| JP5218243B2 (en) * | 2009-04-17 | 2013-06-26 | 日本電気株式会社 | Optical module |
| JP5945886B2 (en) * | 2012-02-27 | 2016-07-05 | 国立大学法人山形大学 | Manufacturing support method for multilayer substrate, manufacturing method for multilayer substrate, failure cause identification method, manufacturing support program for multilayer substrate, and multilayer substrate |
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