A broadly tunable NH3Raman FIR laser

The generation of broadly tunable FIR laser emission by pumping Raman transitions in NH3with 300 mJ, 50 ns duration pulses from a continuously tunable CO2laser is reported. FIR laser outputs up to 4 mJ and a photon conversion efficiency as high as 16 percent have been obtained.     

Sub-T2optical pulse generation: Application to optically pumped far-infrared lasers

Several polar molecules possessing strong far-infrared (FIR) transitions have been optically pumped by a mode-locked CO2TEA laser in a single-pass configuration. All the resulting FIR pulses are shorter than the sub-T2pump pulses; subnanosecond FIR pulses are observed. These short FIR pulses exhibit lethargic gain, stimulated Raman emission, and multiple-pulse superradiance. The experimental results are modeled by numerical solution of the Maxwell-density matrix equations for two fields interacting with a three-level system. Level degeneracy is included, and the pump is described by a multimode power spectrum with a parabolic envelope, resulting in a periodic train of pulses. Variation of the detuning of the pump from resonance allows simulation of the superradiant or Raman cases. This fully coherent model, which is the major contribution of this paper, explains key experimental observations such as the noncoincidence of the Raman peak with the pump modulation minimum and the enhancement of multiple-pulse superradiance due to remanent polarization.     

抽运功率配置对双向喇曼放大器性能的影响

为了研究抽运功率配置对双向抽运光纤喇曼放大器性能的影响,基于已有的耦合方程,采用数值仿真方法,分析了不同抽运功率配置对双向抽运光纤喇曼放大器的增益、增益饱和以及抽运功率转换效率的影响。结果表明,双向抽运光纤喇曼放大器的增益、增益饱和以及抽运功率转换效率特性均介于同向和反向抽运光纤喇曼放大器之间,并且随着同向抽运功率在抽运总功率中所占比例的升高,增益、增益饱和功率和抽运功率转换效率的数值增加;大信号、抽运功率较大时,抽运功率配置对双向抽运光纤喇曼放大器性能的影响显著。这对双向抽运光纤喇曼放大器和光纤激光研究具有一定的参考价值。     

Low frequency characteristics of a quasioptical Schottky diode detector part II: Experimental results

A detailed experimental study of the low frequency (video) response of a quasioptical Schottky diode detector over the microwave and FIR wavelength range is presented. An optimization of the responsivity versus the bias current is proposed and a generalized curve of the saturation power versus the FIR wavelength is given. This curve defines for any antenna point-contact Schottky diode detector, suitable for FIR detection, the power range for a linear detector response. A simple method is also described to calculate the coupling efficiency of the laser radiation into the antenna reception pattern.     

一种新的SOI波导拉曼激光器模型及其应用

通过考虑腔中的自发拉曼散射,提出一种新的绝缘体上硅(SOI)波导拉曼激光器物理模型。仿真结果表明该模型能较好地描述其小信号输出特性,实现对该类激光器的快速分析、设计与优化。利用模型对基于多种SOI波导的拉曼激光器进行分析,结果表明室温下只有采用拥有较大拉曼系数,较小线性损耗和较短的有效载流子寿命的SOI波导才能使拉曼激光器达到阈值。提出采用优化SOI波导截面几何尺寸的方法来降低有效载流子寿命,进而提高整体拉曼增益。结果显示该方法可大幅提高SOI波导拉曼激光器的输出功率和能量转换效率。     

Higher order mode Raman gain coefficients

The Raman gain coefficients for higher order Hermite-Gaussian modes are derived. The results indicate that the Raman intensity gain coefficient (collimated beam) is relatively insensitive to, while the Raman power gain coefficient (tight focus) is very sensitive to the mode structure of a beam. Thus, for high power lasers in which high gain is attainable with a collimated beam, the conversion efficiency would not be sensitive to the laser mode, while for low power lasers in which high gain is attainable only under focused configuration, the conversion efficiency would be quite sensitive to the laser mode. In addition, the coupling coefficients between modes and their contributions to the gain are discussed.     

High power ns FIR Raman pulses in NH3 pumped by a short pulse tail-free CO2 oscillator

By using a short pulse CO₂, tail free, broadband laser it is possible to produce a compressed, high power FIR pulse in a pure Raman system when a suitable pressure and Raman cell length are used. In this way we have obtained 4ns (70KW) emission around the sR(3,0) NH₃ line pumped by 130 mJ (23ns) 9P20 CO₂ line. More resonant transitions can be also compressed at a suitable high pressure also if they show a large optical pumping and cascade processes at low pressure.     

Far-infrared CW Raman and laser gain of14NH3

The FIR Raman and laser gain properties of¹⁴NH3optically pumped by the CO29R(30)and the N2O10P(13)laser lines, respectively, have been measured and calculated using the quantum mechanical theory of three-level systems. The laser gain is about two orders of magnitude higher than for usual FIR laser transitions. The Raman gain shows two features characteristic for the two-photon process: extremely high saturation intensity and high pressure maximum. Very satisfactory agreement between theoretically calculated and experimentally measured gain properties is found.     

同向抽运光纤拉曼放大器功率转换效率的理论分析

为了对同向抽运光纤拉曼放大器的功率转换效率进行研究,由耦合方程出发,首先理论推导出功率转换效率的解析解,然后采用数值计算的方法详细分析不同物理因素对功率转换效率的影响。结果表明：功率转换效率先随着光纤长度和单位面积的拉曼增益系数增加而增加,当增加到最大值时保持数值不变;小信号时,初始信号光功率对功率转换效率的影响较小;大信号时,功率转换效率随着初始信号光功率增加而快速增加;功率转换效率和初始抽运光功率呈抛物线曲线关系;抽运光与信号光的频率比增加时,功率转换效率减小。     

Wide-range all-optical wavelength conversion usingdual-wavelength-pumped fiber Raman converter

A wavelength conversion scheme based on a fiber Raman converter is proposed, in which an externally injected high power pump laser and the associated Stokes laser are used to assist the Raman conversion process of signal light coded with optical information. Because the Raman gain spectrum in fibers is extremely broad, a wavelength conversion device with wide-range tunability is feasible. We numerically demonstrate that wavelength conversion from 1.31 to 1.42 μm can be realized using a fiber Raman converter at up to 10 Gb/s with an efficiency of 18%. It is also demonstrated that wide range conversion from 1.31 to 1.55 μm for optical fiber communication is feasible at up to 5 Gb/s when the fiber Raman converters are cascaded twice     

Raman pulse compression of excimer lasers for application to laser fusion

Application of efficient ultraviolet excimer lasers such as the 248 nm KrF laser to laser fusion requires that long laser pulses be efficiently converted to short pulses at high intensity. The backward Raman amplifier is shown to be a promising candidate for this application. Gain, saturation, and limits to amplifier performance are described. It is shown that pump beams of poor spatial quality may be converted to output beams of high spatial quality. Several common gaseous vibrational Raman scatterers are discussed, and it is shown that a simple KrF-pumped backward Raman amplifier using methane at atmospheric pressure will have a saturation fluence near 1 J/cm²and can produce an output five times as intense as the pump in a ten times shorter pulse with an efficiency of about 50 percent. Design tradeoffs and possible techniques for further improving the performance of such amplifiers are discussed.     

Far-infrared raman laser with high intensity laser pumping

Theoretical and experimental results are presented for a pulsed far-infrared (FIR) molecular gas laser with high intensity laser pumping. In these FIR lasers, high intensity pumping is found to produce stimulated Raman emission at very large offsets (up to 30 GHz) from resonance with the intermediate state. A theoretical, density matrix model is developed for these lasers to account for simultaneous Raman emission on rotational levels in the ground and excited vibrational states (double Raman resonance). This theoretical approach is necessary in the case of off-resonant, high intensity pumping. Theory predicts the FIR emission frequency, the FIR laser gain, and the pump threshold intensity as a function of pump laser frequency. Experimental results are obtained onP-,Q-, andR-branch transitions in¹²CH3F and¹³CH3F using a single-mode, grating tuned CO2TEA pump laser with an intensity of up to 40 MW/cm². Good agreement is obtained between theory and experiment for the observed values of FIR emission frequency and pump threshold intensity. These results indicate that a widely tunable (150-1200 mum), pulsed FIR CH3F laser could be constructed with a tunable, multiatmospheric CO2pump laser of modest power (about 2-5 MW).     

同向抽运喇曼放大器功率转换效率的数值分析

为了对同向抽运光纤喇曼放大器的功率转换效率进行研究,由耦合方程出发,采用龙格-库塔算法的数值模拟方法,详细分析了所有物理因素对同向抽运光纤喇曼放大器功率转换效率的影响. 结果表明,功率转换效率先随着光纤长度的增加而增加,当增加到最大值时保持数值不变;并且功率转换效率也随着初始信号光功率、光纤喇曼增益系数、信号光损耗系数的增加而增加,但随着光纤有效面积、抽运光损耗系数、抽运光与信号光的频率比的增加而减小;功率转换效率和初始抽运光功率呈抛物线曲线关系. 这对同向抽运光纤喇曼放大器功率转换效率的进一步研究以及光纤喇曼放大器的相关研究有一定参考意义.     

Theoretical analysis of wavelength conversion based on four-wave mixing in light-holding SOAs

We propose to use an additional injection beam of short wavelength to enhance the wavelength conversion that utilizes the four-wave-mixing (FWM) effect in a semiconductor optical amplifier (SOA). With this scheme, the assist light can increase the saturation intensity without sacrificing the gain of an SOA, and this leads to an increase in conversion efficiency. A numerical method dealing with various FWM mechanisms, such as amplified spontaneous emission (ASE) noise, longitudinal spatial hole burning, and wavelength-dependent gain spectrum, is developed to predict the static characteristics of our scheme. The carrier densities are nonuniformly distributed along the longitudinal direction of the SOA as a result of the ASE effect, which affects the measurement of the wavelength-dependent transparent current. The effects of an assist light on saturation output power and conversion efficiency are analyzed in detail. The analysis shows that using an assist light can improve both the conversion efficiency and signal-to-background-noise ratio (SBR) for SOAs of different lengths. The degree of improvement depends on the bias condition, assist light wavelength, and the device geometry. The study for the device optimization reveals that a compromise between conversion efficiency and SBR must be made to choose the device length.     

New far infrared CW optically pumped cis-C2H2F2laser

Lasing on 6 FIR lines is reported for the new laser molecule cis-C2H2F2with high conversion efficiencies up to 7 percent of the maximum theoretical limit. A set of molecular selection criteria was used to predict efficient FIR lasing from this molecule, and analysis of the laser performance has increased the basic understanding of the molecular parameter contribution to high conversion efficiency. From this work a refined set of selection criteria has evolved that may be used to predict additional efficient laser molecules.     

高转换效率LD抽运Cr4+:YAG被动调Q内腔式SrWO4拉曼激光器实验研究

采用Cr⁴⁺:YAG作为饱和吸收体,实现了结构 紧凑的全固态半导体泵浦被动调Q内腔式钨酸锶(SrWO₄) 拉曼激光器,获得了稳定的、高效率的一阶斯托克斯拉曼光,并研究了激光器运转中拉曼光 的偏振特性。泵 浦抽运功率为5.8W时,获得的拉曼激光输出功率为968mW,调Q 脉冲重复率为49kHz,脉宽为7ns, 抽运光到一阶斯托克斯光的转换效率为16.7%,斜效率为18. 6%。这是目前报道的被动调 Q内腔式固 体拉曼激光器所获得的最高转换效率。     

基于掺TiO2光纤的拉曼放大器特性研究

为探索能够应用于集总式拉曼放大的新型光纤材料,采用掺TiO₂光纤作为增益介质分别设计了一阶、二阶拉曼放大器。给出了高功率转换效率且增益平坦的泵浦参数配置方案,在总泵浦光功率不变的情况下对比了掺TiO₂光纤的一阶、二阶拉曼放大器与掺GeO₂光纤的二阶拉曼放大器的增益特性。仿真结果表明,在L波段60nm的谱宽范围内,32dBm的泵浦光前向注入长达6 km的掺TiO₂光纤对3 dBm的信号光进行二阶拉曼放大,其功率转换效率可达41.57%,增益平坦度仅为1.14 dB,对比掺GeO₂光纤的二阶拉曼放大器具有更平稳的输出增益。     

基于金刚石拉曼转换的光束亮度增强研究进展

具有不同波长的高亮度激光在国防、工业、生命科学等诸多领域发挥着重要作用。但是受限于现有工作物质固有的光谱特性和热物性，传统粒子数反转激光器的波长和输出功率难以兼顾，甚至导致激光在功率提升时光束亮度不升反降。为了克服该难题，近几年人们利用非线性光学技术对光束净化开展了大量研究，即将粒子数反转激光器输出的低光束质量的光束，通过受激拉曼或受激布里渊散射等效应转变为高光束质量激光输出。其中，金刚石晶体以其高拉曼增益系数、极高的热导率和极宽的光谱透过范围等性质，在实现高效率拉曼波长转换的同时展现出优异的光束亮度增强特性，为人们获得高功率、高亮度激光光束提供了新的技术路径。文中对基于金刚石的一阶和级联拉曼转换的光束亮度增强研究进行了综述，并围绕其潜在的应用进行了探讨。     

多阶级联拉曼光纤激光器的理论分析与优化设计

由稳态条件下描述光纤中受激拉曼散射(SRS)效应的光功率耦合方程出发,采用解析方法对多阶级联拉曼光纤激光器(CRFLs)进行了理论分析。根据拉曼光纤激光器级联阶数的奇偶性分别推导出了多阶级联拉曼光纤激光器的输出功率、光-光转换效率最大时的拉曼光纤长度和输出耦合器反射率。通过忽略反射回谐振腔输入端的剩余抽运光功率,计算了5阶级联拉曼光纤激光器的输出特性和光-光转换效率随光纤长度和输出耦合器反射率的变化。利用已有的5阶级联掺锗拉曼光纤激光器输出特性实验数据与理论分析结果进行了对比。     

百瓦级1.2/1.5 μm双波长金刚石拉曼激光器（特邀）

具有高功率、高光束质量的双波长激光器在精密光谱、共振干涉测量和激光雷达等领域有着重要的应用。但是受到激光工作物质固有的光谱和增益特性制约,通过传统的粒子数反转激光器难以直接获得高功率的双波长激光输出,因此通常需要结合非线性光学频率变换技术将常规的单一波长高功率激光拓展至一个或若干个特殊波段。受激拉曼散射作为一种三阶非线性效应,具有频移大、自相位匹配和光束净化等优点,是实现高效率、高光束质量波长转换有效手段。利用具有宽光谱透过范围（>0.23 μm）、超高热导率（>2 000 W·m?1·K?1）和大拉曼频移（1 332 cm?1）等优异特性的金刚石晶体作为拉曼增益介质,通过外腔振荡结构实现了1 μm泵浦光直接向1.2 μm和1.5 μm双波长激光的高效转换,在最高稳态泵浦功率414 W的条件下获得了1.2 μm和1.5 μm功率分别为72 W和110 W的输出。该研究为实现高功率的双波长激光输出开辟了新的途径。