JP6274541B2 - Mode-locked laser, high-speed optical signal processing device, and spectrum spectroscopic measurement device - Google Patents
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Classifications
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/015—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on semiconductor elements with at least one potential jump barrier, e.g. PN, PIN junction
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/03—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on ceramics or electro-optical crystals, e.g. exhibiting Pockels effect or Kerr effect
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/35—Non-linear optics
- G02F1/365—Non-linear optics in an optical waveguide structure
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
- H01S3/083—Ring lasers
Description
本発明は、光時分割多重伝送で用いられる、また、高速光信号処理やスペクトル分光計測用光源として用いられる、ＧＨｚ帯高繰り返しフェムト秒パルスを出力するモード同期レーザ、ならびに、それを搭載した高速光信号処理装置およびスペクトル分光計測装置に関するものである。 The present invention relates to a mode-locked laser that outputs a high-frequency repetitive femtosecond pulse for use in optical time-division multiplex transmission and that is used as a light source for high-speed optical signal processing and spectrum spectroscopy measurement, and a high-speed mounted with the mode-locked laser. The present invention relates to an optical signal processing device and a spectrum spectrometer.
基幹光伝送網の大容量化に向けた取り組みとして、波長多重（WDM: Wavelength Division Multiplexing）伝送システムの高密度化が、近年著しく進展している。その一方で、波長制御の容易さならびに低消費電力化の点からは、１波長あたりの伝送速度を高速化することにより波長多重数をできるだけ小さくすることが重要である。特に、超短光パルスを光領域で時間多重する光時分割多重（OTDM: Optical Time Division Multiplexing）は、光の高速性を活かして、電子回路の動作速度を上回る超高速伝送を実現可能な方式として、精力的に研究されている。 As an effort to increase the capacity of the backbone optical transmission network, the density of wavelength division multiplexing (WDM) transmission systems has been remarkably advanced in recent years. On the other hand, from the viewpoint of easy wavelength control and low power consumption, it is important to reduce the number of wavelength multiplexing as much as possible by increasing the transmission speed per wavelength. In particular, Optical Time Division Multiplexing (OTDM), which time-multiplexes ultrashort optical pulses in the optical domain, is a method that can realize ultrahigh-speed transmission that exceeds the operating speed of electronic circuits by taking advantage of high-speed light. As energetically studied.
OTDM伝送用パルス光源としては、モードロッカーとして光変調器を用いた能動モード同期レーザが使用されている。能動モード同期レーザでは、光変調器の駆動周波数によりレーザ出力パルスの繰り返し周波数を任意に変えることができるため、GHz帯の高い繰り返し周波数を有する光パルス列を容易に生成できる。例えば、LiNbO3光位相変調器を有する能動モード同期ファイバレーザにより、繰り返し周波数が40 GHz、パルス幅が0.85 ps（850フェムト秒）である光パルスが生成されている（例えば、非特許文献１参照）。As a pulse light source for OTDM transmission, an active mode-locked laser using an optical modulator as a mode locker is used. In the active mode-locked laser, the repetition frequency of the laser output pulse can be arbitrarily changed depending on the drive frequency of the optical modulator, so that an optical pulse train having a high repetition frequency in the GHz band can be easily generated. For example, an active mode-locked fiber laser having a LiNbO 3 optical phase modulator generates an optical pulse having a repetition frequency of 40 GHz and a pulse width of 0.85 ps (850 femtoseconds) (for example, see Non-Patent Document 1). ).
しかしながら、モードロッカーとして使用する光変調器の変調指数により、能動モード同期レーザから出力される光パルスの時間幅は、サブピコ秒に制限されてしまう。そのため、1 Tbit/s以上の高速OTDM伝送では、レーザ外部に光パルス圧縮回路を配置する必要があり、その結果、光パルスの裾野が圧縮されずに残留してしまう問題や、主となるパルス以外に小さな寄生パルスが発生するなどの問題が生じてしまう。また、光パルス圧縮する際に光パルスの信号強度を増幅する必要があるため、その光増幅の過程において放出される自然放出光の影響により、光パルスのS/Nが劣化するといった問題がある。 However, due to the modulation index of the optical modulator used as the mode locker, the time width of the optical pulse output from the active mode-locked laser is limited to sub-picoseconds. Therefore, in high-speed OTDM transmission of 1 Tbit / s or more, it is necessary to place an optical pulse compression circuit outside the laser. As a result, there is a problem that the base of the optical pulse remains uncompressed, and the main pulse In addition to this, problems such as generation of small parasitic pulses occur. Moreover, since it is necessary to amplify the signal intensity of the optical pulse when compressing the optical pulse, there is a problem that the S / N of the optical pulse deteriorates due to the influence of spontaneous emission light emitted in the process of the optical amplification. .
本発明はこのような課題を解決するためのものであり、光パルス圧縮回路がなくとも、GHz帯の高繰り返し周波数を有するフェムト秒パルスを直接出力することが可能なモード同期レーザ、ならびに、それを搭載した高速光信号処理装置およびスペクトル分光計測装置を提供することを目的とする。 The present invention has been made to solve such problems. A mode-locked laser capable of directly outputting a femtosecond pulse having a high repetition rate in the GHz band without an optical pulse compression circuit, and It is an object to provide a high-speed optical signal processing apparatus and a spectrum spectroscopic measurement apparatus equipped with the
かかる目的を達成するために、本発明に係るモード同期レーザは、モード同期光回路として光変調器を用いたＧＨｚ帯の高繰り返し能動モード同期レーザであって、レーザ共振器内に可飽和吸収体を設け、該可飽和吸収体によるパルス狭窄効果を併用することでＧＨｚ帯高繰り返しフェムト秒パルスを出力することを特徴とする。 In order to achieve such an object, a mode-locked laser according to the present invention is a high-frequency active mode-locked laser in the GHz band using an optical modulator as a mode-locked optical circuit, and a saturable absorber in the laser resonator. And a high frequency repetition femtosecond pulse is output by using the pulse constriction effect by the saturable absorber.
また、本発明に係るモード同期レーザは、レーザ共振器内にモード同期光回路として光変調器を用いた、ＧＨｚ帯の高繰り返し能動モード同期レーザであって、前記レーザ共振器内に、高次光ソリトンパルスを生成可能に設けられた非線形光学素子および異常分散媒体と、飽和効果により光パルスの狭窄を誘起する可飽和吸収体とを有し、前記ソリトンパルスを前記可飽和吸収体に入射することにより、ＧＨｚ帯高繰り返しフェムト秒パルスを出力するよう構成されていることが好ましい。 A mode-locked laser according to the present invention is a high-frequency active mode-locked laser of GHz band using an optical modulator as a mode-locked optical circuit in a laser resonator, and includes a high-order optical soliton in the laser resonator. A non-linear optical element and an anomalous dispersion medium provided so as to be capable of generating a pulse, a saturable absorber that induces a narrowing of an optical pulse by a saturation effect, and the soliton pulse is incident on the saturable absorber It is preferably configured to output a GHz band high repetition femtosecond pulse.
この場合、本発明に係るモード同期レーザは、レーザ共振器内に設けられた非線形光学素子と異常分散媒質とにより、高次光ソリトンパルスが生成され、そのソリトン効果によりパルス圧縮効果を得ることができる。また、レーザ共振器内を周回する光パルスのピーク電力を、可飽和吸収体の飽和電力以上となるように設定することにより、可飽和吸収体の飽和効果を誘発し、パルス狭窄効果を得ることができる。本発明に係るモード同期レーザは、このパルス圧縮効果とパルス狭窄効果とを併用することにより、GHz帯の高繰り返し周波数を有するフェムト秒パルスを直接出力することができる。 In this case, in the mode-locked laser according to the present invention, a high-order optical soliton pulse is generated by the nonlinear optical element and the anomalous dispersion medium provided in the laser resonator, and a pulse compression effect can be obtained by the soliton effect. Also, by setting the peak power of the optical pulse that circulates in the laser resonator to be equal to or higher than the saturation power of the saturable absorber, the saturation effect of the saturable absorber is induced and the pulse constriction effect is obtained. Can do. The mode-locked laser according to the present invention can directly output a femtosecond pulse having a high repetition rate in the GHz band by using both the pulse compression effect and the pulse constriction effect.
本発明に係るモード同期レーザは、レーザ共振器内でパルス圧縮効果およびパルス狭窄効果が得られるため、外部に光パルス圧縮回路を配置する必要がない。このため、例えば、本発明に係るモード同期レーザをOTDM伝送用パルス光源として用いることにより、送信部において光パルス圧縮回路が不要となり、その結果、送信信号の品質やSN比を大幅に改善することができる。 Since the mode-locked laser according to the present invention can obtain a pulse compression effect and a pulse constriction effect in the laser resonator, it is not necessary to provide an optical pulse compression circuit outside. For this reason, for example, by using the mode-locked laser according to the present invention as a pulse light source for OTDM transmission, an optical pulse compression circuit becomes unnecessary in the transmission unit, and as a result, the quality of the transmission signal and the SN ratio are greatly improved. Can do.
本発明に係るモード同期レーザで、前記非線形光学素子は、バルクガラス、光半導体材料、シリコン細線導波路、フォトニック結晶ファイバまたは高Δファイバから成ることが好ましい。また、前記異常分散媒体は、石英ガラスファイバ、バルクガラス、またはグレーティングから成ることが好ましい。これらの場合、非線形光学素子に高密度に光を閉じ込めることができ、効果的に高次光ソリトンパルスを生成することができる。また、これにより、優れたパルス圧縮効果を得ることができる。 In the mode-locked laser according to the present invention, the nonlinear optical element is preferably made of bulk glass, an optical semiconductor material, a silicon fine wire waveguide, a photonic crystal fiber, or a high Δ fiber. The anomalous dispersion medium is preferably made of quartz glass fiber, bulk glass, or grating. In these cases, light can be confined in the nonlinear optical element with high density, and high-order optical soliton pulses can be generated effectively. Thereby, an excellent pulse compression effect can be obtained.
本発明に係るモード同期レーザで、前記レーザ共振器は、空間結合型もしくはファイバ型のファブリー・ペロー共振器もしくはリング共振器から構成されていることが好ましい。 In the mode-locked laser according to the present invention, the laser resonator is preferably composed of a spatially coupled or fiber Fabry-Perot resonator or a ring resonator.
また、本発明に係るモード同期レーザは、前記レーザ共振器のレーザ出力光の一部から、共振器長に対応した基本周波数の整数倍の周波数のクロック信号を抽出し、該クロック信号で前記光変調器を駆動する再生モード同期ループを有していてもよい。この場合、レーザ共振器の共振周波数と、光変調器による光強度変調または光位相変調の変調周波数とを常に一致させることができ、安定したパルス発振動作を実現することができる。 Further, the mode-locked laser according to the present invention extracts a clock signal having a frequency that is an integral multiple of the fundamental frequency corresponding to the resonator length from a part of the laser output light of the laser resonator, and uses the clock signal to extract the light. You may have the reproduction | regeneration mode locked loop which drives a modulator. In this case, the resonance frequency of the laser resonator and the modulation frequency of the light intensity modulation or the optical phase modulation by the optical modulator can always be matched, and a stable pulse oscillation operation can be realized.
さらに、本発明に係るモード同期レーザは、前記レーザ共振器のレーザ出力光パルスの繰り返し周波数および光周波数、あるいはいずれか一方の周波数を安定化する周波数安定化機構（負帰還制御機構）を有していてもよい。この場合、レーザ出力光パルスの繰り返し周波数および／または光周波数を安定化させることができ、レーザの機能性を高めることができる。 Furthermore, the mode-locked laser according to the present invention has a frequency stabilization mechanism (negative feedback control mechanism) that stabilizes the repetition frequency and / or optical frequency of the laser output light pulse of the laser resonator. It may be. In this case, the repetition frequency and / or optical frequency of the laser output light pulse can be stabilized, and the functionality of the laser can be improved.
本発明に係る高速光信号処理装置は、本発明に係るモード同期レーザを搭載していることを特徴とする。また、本発明に係るスペクトル分光計測装置は、本発明に係るモード同期レーザを搭載していることを特徴とする。 The high-speed optical signal processing apparatus according to the present invention is equipped with the mode-locked laser according to the present invention. Moreover, the spectral spectroscopy measurement apparatus according to the present invention is equipped with the mode-locked laser according to the present invention.
本発明によれば、光パルス圧縮回路がなくとも、GHz帯の高繰り返し周波数を有するフェムト秒パルスを直接出力することが可能なモード同期レーザ、ならびに、それを搭載した高速光信号処理装置およびスペクトル分光計測装置を提供することができる。 According to the present invention, a mode-locked laser capable of directly outputting a femtosecond pulse having a high repetition rate in the GHz band without an optical pulse compression circuit, and a high-speed optical signal processing apparatus and spectrum equipped with the mode-locked laser A spectroscopic measurement device can be provided.
本発明の第１の実施形態のモード同期レーザを、図１に示す。図１において、光増幅器１、可飽和吸収体２、レーザ共振器内を周回する光パルスのパワーの一部を出力光として取り出す光分岐器３、光アイソレータ４、モード同期光回路として用いる光変調器５をリング状に結合してモード同期レーザを構成する。光増幅器１としては、例えば波長1.55 μm帯では、エルビウム添加光ファイバやエルビウム添加ガラス、あるいは半導体を利用した光増幅器を用いることができる。可飽和吸収体２としては、例えば半導体やカーボンナノチューブ、グラフェンなどの光学材料を用いることができる。光変調器５は、LiNbO3や半導体中のポッケルス効果を利用した進行波型強度ならびに位相変調器や、半導体中のQCSE（Quantum Confined Stark Effect）効果を利用した強度ならびに位相変調器を用いることができる。A mode-locked laser according to the first embodiment of the present invention is shown in FIG. In FIG. 1, an optical amplifier 1, a saturable absorber 2, an optical branching device 3 for extracting a part of the power of an optical pulse circulating in the laser resonator as output light, an optical isolator 4, and an optical modulation used as a mode-locked optical circuit. A mode-locked laser is constructed by connecting the devices 5 in a ring shape. As the optical amplifier 1, for example, an erbium-doped optical fiber, an erbium-doped glass, or an optical amplifier using a semiconductor can be used in a wavelength band of 1.55 μm. As the saturable absorber 2, for example, an optical material such as a semiconductor, a carbon nanotube, or graphene can be used. The optical modulator 5 uses a traveling wave type intensity and phase modulator using the Pockels effect in LiNbO 3 or a semiconductor, or an intensity and phase modulator using a QCSE (Quantum Confined Stark Effect) effect in the semiconductor. it can.
以上のモード同期レーザにおいて、光増幅器１の励起電力を高め、ソリトン効果によるパルス圧縮効果を用いることにより、レーザ共振器内を周回する光パルスのピーク電力が可飽和吸収体２の飽和電力以上となるように設定し、その結果誘発された可飽和吸収効果によりフェムト秒に狭窄化された光パルスを出力することができる。 In the above mode-locked laser, the pump power of the optical amplifier 1 is increased and the pulse compression effect due to the soliton effect is used, so that the peak power of the optical pulse that circulates in the laser resonator is equal to or higher than the saturation power of the saturable absorber 2. It is possible to output an optical pulse narrowed to femtoseconds by the saturable absorption effect induced as a result.
即ち、図２に示すように、レーザ共振器内に非線形光学素子６と異常分散媒質７とを挿入することにより、それら素子により誘発されるソリトン効果を利用して短パルス化を図り、レーザ共振器内を周回する光パルスのピーク電力を可飽和吸収体２の飽和電力以上にするようにレーザを構成することができる。非線形光学素子６としては、例えば高密度に光を閉じ込めることが可能なバルクガラスや光半導体材料、シリコン細線導波路、フォトニック結晶ファイバ、高Δファイバを用いることができる。異常分散媒質７としては、石英ガラスファイバやバルクガラス、グレーティングを用いることができる。 That is, as shown in FIG. 2, by inserting the nonlinear optical element 6 and the anomalous dispersion medium 7 into the laser resonator, the pulse is shortened using the soliton effect induced by these elements, and the laser resonance The laser can be configured so that the peak power of the light pulse that circulates in the chamber is equal to or higher than the saturation power of the saturable absorber 2. As the nonlinear optical element 6, for example, bulk glass, an optical semiconductor material, a silicon fine wire waveguide, a photonic crystal fiber, or a high Δ fiber capable of confining light at high density can be used. As the anomalous dispersion medium 7, quartz glass fiber, bulk glass, or grating can be used.
また、図３に示すように、レーザ外部に配置した光分岐器８およびクロック抽出器９を用いて、レーザ出力光の一部より共振器長に対応した基本周波数の整数倍の周波数のクロック信号を抽出し、移相器１０および電気アンプ１１を用いてその位相と振幅とを調整した後に、抽出したクロック信号で光変調器５を駆動する再生モード同期ループを構成してもよい。この場合、レーザ共振器の共振周波数と、光変調器５による光強度変調または光位相変調の変調周波数とを常に一致させることができ、安定したパルス発振動作を実現することができる。 Further, as shown in FIG. 3, by using an optical branching device 8 and a clock extractor 9 arranged outside the laser, a clock signal having a frequency that is an integral multiple of the fundamental frequency corresponding to the resonator length from a part of the laser output light. After adjusting the phase and amplitude using the phase shifter 10 and the electric amplifier 11, a reproduction mode locked loop for driving the optical modulator 5 with the extracted clock signal may be configured. In this case, the resonance frequency of the laser resonator and the modulation frequency of the light intensity modulation or the optical phase modulation by the optical modulator 5 can always be matched, and a stable pulse oscillation operation can be realized.
さらに、図４に示すように、レーザ出力光パルスの繰り返し周波数と光周波数、あるいはいずれか一方の周波数を安定化するための周波数安定化機構１２を設けてもよい。この場合、レーザ出力光パルスの繰り返し周波数および／または光周波数を安定化させることができ、レーザの機能性を高めることができる。周波数安定化機構１２としては、例えばレーザ共振器長や光増幅器の励起電力、再生モード同期ループ内のループ長などの制御回路が有効である。 Furthermore, as shown in FIG. 4, a frequency stabilization mechanism 12 may be provided for stabilizing the repetition frequency and / or optical frequency of the laser output light pulse. In this case, the repetition frequency and / or optical frequency of the laser output light pulse can be stabilized, and the functionality of the laser can be improved. As the frequency stabilization mechanism 12, for example, a control circuit such as a laser resonator length, an excitation power of an optical amplifier, or a loop length in a reproduction mode locked loop is effective.
次に、図３に示す本発明の第１の実施形態によるハイブリッドモード同期ソリトンレーザの発振特性について述べる。光増幅器１としてエルビウム添加光ファイバ増幅器、可飽和吸収体２として半導体可飽和吸収体鏡、光変調器５としてLiNbO3光位相変調器、非線形光学素子６として高Δファイバ、異常分散媒質７として平均分散値が2.8 ps/nm/kmの単一モード光ファイバを使用している。レーザの共振器長は8.6 mであり、共振器長で決まる基本縦モード間隔は24 MHzである。図３に示す再生モード同期ループを用いて抽出した9.2 GHzのクロック信号（約380次の高次ビート信号）で、LiNbO3光位相変調器を駆動している。Next, the oscillation characteristics of the hybrid mode-locked soliton laser according to the first embodiment of the present invention shown in FIG. 3 will be described. The optical amplifier 1 is an erbium-doped optical fiber amplifier, the saturable absorber 2 is a semiconductor saturable absorber mirror, the optical modulator 5 is a LiNbO 3 optical phase modulator, the nonlinear optical element 6 is a high Δ fiber, and the anomalous dispersion medium 7 is average. A single-mode optical fiber with a dispersion value of 2.8 ps / nm / km is used. The cavity length of the laser is 8.6 m, and the fundamental longitudinal mode interval determined by the cavity length is 24 MHz. The LiNbO 3 optical phase modulator is driven by a 9.2 GHz clock signal (about 380th order higher order beat signal) extracted using the reproduction mode locked loop shown in FIG.
光ファイバ中を伝搬する光信号は、光カー効果により自己位相変調を受ける。これらの事象が同時に存在することから、光ファイバ中を伝搬する光信号は、非線形シュレディンガ方程式
ここで、光ファイバが異常分散を有する場合、式（１）はソリトンと呼ばれる安定解をもつ。その安定解の中で次数の最も低い基本ソリトンは、双曲線正割関数（sech関数）
共振器内でピーク電力の高いパルスを得るためには、高次ソリトンを励振すればよい。このソリトン効果を導入し、さらに可飽和吸収体２の条件を最適に設定した際のレーザ出力パルスの自己相関波形および光スペクトル波形を、それぞれ図５（ａ）および（ｂ）に示す。図５に示すように、レーザ出力光のパルス幅は440 fs、スペクトル幅は710 GHzであり、時間バンド幅積が0.32であるsech型パルスが出力されている。ここで、本レーザ共振器から半導体可飽和吸収体鏡を除去した場合に得られる最短パルスの時間幅は、880 fsである。また、ソリトン圧縮効果を用いなければ、パルス幅は1〜2 ps程度である。これらの結果は、本発明の第１の実施形態のモード同期レーザが500 fs以下のフェムト秒パルスを生成するために有効であることを実証している。 In order to obtain a pulse with a high peak power in the resonator, a high-order soliton may be excited. FIGS. 5A and 5B show the autocorrelation waveform and the optical spectrum waveform of the laser output pulse when the soliton effect is introduced and the condition of the saturable absorber 2 is optimally set. As shown in FIG. 5, a sech type pulse with a pulse width of 440 fs, a spectral width of 710 GHz, and a time bandwidth product of 0.32 is output. Here, the time width of the shortest pulse obtained when the semiconductor saturable absorber mirror is removed from the laser resonator is 880 fs. If the soliton compression effect is not used, the pulse width is about 1 to 2 ps. These results demonstrate that the mode-locked laser of the first embodiment of the present invention is effective for generating femtosecond pulses of 500 fs or less.
次に、本発明の第１の実施形態により、GHz帯高繰り返しフェムト秒パルスがレーザから直接発生できることを数値シミュレーションにより示す。図６は図５に示す発振特性を有するモード同期レーザにおいて、自然放出光雑音を種光としてレーザ共振器内で周回ごとにパルスが生成されていく様子を解析した結果である。ここで、図６（ａ）はレーザ共振器内に可飽和吸収体２を挿入しない場合、図６（ｂ）は可飽和吸収体２を挿入した場合にそれぞれ対応している。 Next, it will be shown by numerical simulation that a GHz band high repetition femtosecond pulse can be generated directly from a laser according to the first embodiment of the present invention. FIG. 6 is a result of analyzing the state in which pulses are generated every round in the laser resonator using spontaneous emission light noise as seed light in the mode-locked laser having the oscillation characteristics shown in FIG. 6A corresponds to the case where the saturable absorber 2 is not inserted into the laser resonator, and FIG. 6B corresponds to the case where the saturable absorber 2 is inserted.
図６（ａ）に示すように、可飽和吸収体２を挿入しない場合は、周回数によって周期的にパルス形状が変化する高次ソリトンパルスが生成される。一方、図６（ｂ）に示す可飽和吸収体２を挿入した場合は、高次ソリトンパルスの裾野部分が可飽和吸収効果により除去され、余分な裾野成分をもたない基本ソリトンパルスが生成されていく様子を示している。これらの結果は、本発明の第１の実施形態のモード同期レーザにおいて、高次ソリトンの励振によるパルス圧縮効果と可飽和吸収体によるパルス狭窄効果とを併用することでフェムト秒パルスを生成できることを理論的に証明している。 As shown in FIG. 6A, when the saturable absorber 2 is not inserted, a high-order soliton pulse whose pulse shape periodically changes depending on the number of turns is generated. On the other hand, when the saturable absorber 2 shown in FIG. 6B is inserted, the base portion of the high-order soliton pulse is removed by the saturable absorption effect, and a basic soliton pulse having no extra base component is generated. It shows how it goes. These results show that in the mode-locked laser according to the first embodiment of the present invention, femtosecond pulses can be generated by combining the pulse compression effect by excitation of higher-order solitons and the pulse constriction effect by the saturable absorber. Proof in theory.
図７は、本発明の第２の実施形態のモード同期レーザを示すブロック図である。図７において、光増幅器１、可飽和吸収体２、光変調器５を一対の反射鏡１３の間に配置してファブリー・ペロー共振器を形成し、この共振器内で発生する光パルスのパワーの一部を反射鏡１３の透過光として外部に取り出すモード同期レーザを構成している。本実施形態の動作原理は、第１の実施形態と同じである。 FIG. 7 is a block diagram showing a mode-locked laser according to the second embodiment of the present invention. In FIG. 7, an optical amplifier 1, a saturable absorber 2, and an optical modulator 5 are arranged between a pair of reflecting mirrors 13 to form a Fabry-Perot resonator, and the power of an optical pulse generated in the resonator. A mode-locked laser that extracts a part of the light as the transmitted light of the reflecting mirror 13 is constructed. The operation principle of this embodiment is the same as that of the first embodiment.
図８に示すように、第２の実施形態のレーザ共振器内に非線形光学素子６と異常分散媒質７とを挿入し、それら素子により誘発されるソリトン効果を利用して短パルス化を図り、レーザ共振器内を周回する光パルスのピーク電力を可飽和吸収体２の飽和電力以上するようなレーザを構成してもよい。これにより、GHz帯の高繰り返し周波数を有するフェムト秒パルスを直接出力することができる。 As shown in FIG. 8, the nonlinear optical element 6 and the anomalous dispersion medium 7 are inserted into the laser resonator of the second embodiment, and the pulse is shortened using the soliton effect induced by these elements. You may comprise the laser which makes the peak electric power of the optical pulse which circulates in the laser resonator more than the saturation electric power of the saturable absorber 2. FIG. As a result, femtosecond pulses having a high repetition rate in the GHz band can be directly output.
また、図９に示すように、一方の反射鏡１３からのレーザ出力光とクロック抽出器９とにより、共振器長に対応した基本周波数の整数倍の周波数のクロック信号を抽出し、移相器１０および電気アンプ１１を用いてその位相と振幅とを調整した後に、抽出したクロック信号で光変調器５を駆動する再生モード同期ループを構成してもよい。 Also, as shown in FIG. 9, a clock signal having a frequency that is an integral multiple of the fundamental frequency corresponding to the resonator length is extracted by the laser output light from one of the reflecting mirrors 13 and the clock extractor 9, and a phase shifter is extracted. 10 and the electric amplifier 11 may be used to adjust the phase and amplitude, and then a reproduction mode locked loop for driving the optical modulator 5 with the extracted clock signal may be configured.
さらに、図１０に示すように、レーザ出力光パルスの繰り返し周波数と光周波数、あるいはいずれか一方の周波数を安定化するための周波数安定化機構１２を設けてもよい。 Furthermore, as shown in FIG. 10, a frequency stabilization mechanism 12 for stabilizing the repetition frequency and / or optical frequency of the laser output light pulse may be provided.
以上詳細に説明したように、本発明では、GHz帯の高い繰り返し周波数を有するフェムト秒パルスを出力するモード同期レーザを提供する。レーザ外部でパルス圧縮することなく、レーザよりSN比の高いフェムト秒パルスを直接出力することができるため、本発明のレーザは、OTDM伝送用パルス光源として有用である。このように、本発明に係るモード同期レーザは、高速光信号処理装置やスペクトル分光計測装置などに搭載して、光源として利用することができる。 As described above in detail, the present invention provides a mode-locked laser that outputs femtosecond pulses having a high repetition rate in the GHz band. Since the femtosecond pulse having a higher S / N ratio than the laser can be directly output without pulse compression outside the laser, the laser of the present invention is useful as a pulse light source for OTDM transmission. As described above, the mode-locked laser according to the present invention can be used as a light source by being mounted on a high-speed optical signal processing device, a spectral spectroscopy measurement device, or the like.
１ 光増幅器
２ 可飽和吸収体
３ 光分岐器
４ 光アイソレータ
５ 光変調器
６ 非線形光学素子
７ 異常分散媒質
８ 光分岐器
９ クロック抽出器
１０ 移相器
１１ 電気アンプ
１２ 周波数安定化機構
１３ 反射鏡
DESCRIPTION OF SYMBOLS 1 Optical amplifier 2 Saturable absorber 3 Optical splitter 4 Optical isolator 5 Optical modulator 6 Nonlinear optical element 7 Anomalous dispersion medium 8 Optical splitter 9 Clock extractor 10 Phase shifter 11 Electric amplifier 12 Frequency stabilization mechanism 13 Reflective mirror
Claims (8)
前記レーザ共振器内に、高次光ソリトンパルスを生成可能に設けられた非線形光学素子および異常分散媒体と、飽和効果により光パルスの狭窄を誘起する可飽和吸収体とを有し、
前記ソリトンパルスを前記可飽和吸収体に入射することにより、ＧＨｚ帯高繰り返しフェムト秒パルスを出力するよう構成されていることを
特徴とするモード同期レーザ。A high repetition active mode-locked laser in the GHz band using an optical modulator as a mode-locked optical circuit in the laser resonator,
In the laser resonator, having a nonlinear optical element and an anomalous dispersion medium provided so as to be able to generate a high-order optical soliton pulse, and a saturable absorber that induces constriction of the optical pulse by a saturation effect,
A mode-locked laser configured to output a high repetition femtosecond pulse in a GHz band by making the soliton pulse incident on the saturable absorber.
A spectrum spectroscopic measurement apparatus, comprising the mode-locked laser according to any one of claims 1 to 6.
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