Closed-form solution for parametric second Stokes generation inRaman amplifiers

The efficiency of a first Stokes Raman amplifier can be seriously reduced by the generation of a second Stokes beam. The dominating process for this generation is often a Raman four-wave mixing interaction which couples the pump and second Stokes beams. A one-dimensional model for this effect has a closed-form solution, simply described in terms of an equivalent input, under conditions where high first Stokes conversion efficiency is possible. Excellent agreement with numerical integration results has been obtained. Use of the model for maximizing conversion efficiency of pump beams with large temporal or spatial intensity nonuniformity is described, with an example worked out for a triangular pulse shape     

在Ce：KNSBN晶体中双光束耦合的光束整形

本文的工作是利用在光折变晶体Ｃｅ：ＫＮＳＢＮ中，其光生载流子的迁移机制在空间电荷场形成过程中是以扩散为主的特性，使得能量从变形的泵浦光转移到信号光上去，而无相位转移，从而实现泵浦光的整形。     

Adaptive-optics compensation in a Raman amplifier configuration

An experimental study of real-time adaptive compensation through a high-gain Raman amplifier is discussed. Atmospheric turbulence was simulated by selectively aberrated quartz plates designed to produce a turbulence-like phasefront distortion on a transmitted beacon. A 69-channel wavefront sensor measured the beacon's phasefront and commanded a deformable mirror to impose the conjugate phasefront on the Stokes seed to a high-gain, large-Fresnel-number Raman amplifier. After amplification, the output Stokes beam was made to retrace the path of the beacon back through the simulated turbulence. Measurements of the Stokes beam quality indicate a dramatic improvement to near-diffraction-limited performance. The experimental results are in good agreement with theory     

Steady-state coherent Raman beam combining with multiaxial mode lasers

Forward-stimulated Raman scattering can be used to combine multiple broad-band pump beams into a single coherent Stokes wave. In this paper, the equations of motion for the Stokes electric field are derived in the presence of multiple pump waves propagating at slightly different angles with respect to the Stokes propagation direction. The solutions with pump depletion are presented for fully coherent pump waves. It is shown that the Raman gain is lower for incoherent pump waves than for coherent pump waves. Stokes field solutions are used to evaluate the far-field intensity of the Stokes output from a saturated Raman beam combiner. It is shown that the condition for coherent combination depends on the beam diameter, pump coherence length, and the angle between the pump and the Stokes. It is also shown that multiaxial mode Raman shifting can generate Stokes waves with higher beam quality than the pump waves, both with phase perturbed or highly distorted pump beams.     

Shallow angle beam combining using a broad-band XeF laser

Experiments in which two xenon fluoride laser pump beams (353 nm) were crossed at an angle of 2.5 mrad in a 565-cm-long Raman amplifier containing high-pressure (10-atm) hydrogen are described. Bisecting the angle made by these two pump beams was a seed beam at the vibrational Stokes shifted wavelength of 414 nm. This seed beam was generated from a portion of the pump beam that was split off and brought to focus in a separate Raman seed generator. All three beam paths were adjusted so they arrived in the main Raman amplifier at the same time. For the 1.4-Å (FWHM) xenon fluoride pump radiation this required path offset precision to within 0.2 mm over many meters of optical path. The beam quality of the resulting amplified Stokes beam was determined through shearing interferometry techniques to be near the diffraction limit, whereas the pump beams had significant optical distortion     

受激布里渊放大光束发散角实验研究

研究了布里渊放大器Stokes出射光束的发散角随抽运功率密度的变化规律以及与种子光束和抽运光束发散角的关系。抽运功率密度的变化对Stokes出射光束的发散角几乎没有影响,通过控制Stokes种子光束的发散角可以获得高光束质量的输出激光。     

Time-delay characteristics of four-wave mixing among subpicosecondoptical pulses in a semiconductor optical amplifier

We have studied four-wave mixing (FWM) characteristics among subpicosecond optical pulses in a semiconductor optical amplifier for different time-delays between pump and probe beams, for the first time. Our experimental results have shown that an optimum time-delay shifted from the perfect pump-probe overlap condition with an increase in input probe beam energy. An optimum intensity of the pump beam has also been obtained to generate the maximum FWM signal     

双光束Raman组束研究

着重研究了双光束拉曼组束改善准分子激光光束质量。通过理论计算得出，在畸变泵浦光作用下可以获得近衍射极限Ｓｔｏｋｅｓ光的放大。并在稳态和瞬态激光脉冲的条件下，讨论了Ｒａｍａｎ工作介质的选择。在此基础上进行了氯化氙准分子激光在氢气中的双光束Ｒａｍａｎ组束实验研究，近衍射极限Ｓｔｏｋｅｓ光在获得功率放大的同时，其远场分布保持不变。激光聚焦亮度提高约３０００倍。最后讨论了放大自发辐射的抑制。     

光传输模拟计算

利用自相似传输模计算激光束通过空间滤波器以后的光场分布以及象差光束的可聚焦功率,给出了象差光束的焦体分布。计算精度达1％。     

An efficient double-pass Raman amplifier with pump intensity averaging in a light guide

Experimental observations on a double-pass methane Raman amplifier pumped by a KrF laser demonstrate efficient photon extraction (75-85 percent) of multiple pump pulses and high stage gain (10-20) with simultaneous intensity gain or "pulse compression" of about 2. The Stokes beam divergence is about 2.5 times the diffraction limit and is unchanged in the amplifier to within the precision of the experiment. The pump pulses are of poor spatial quality and propagate through the amplifier in a light guide. A simple one-dimensional theoretical model fits the results of the experiment. Analysis using the model permits a projection of the performance of very large final Raman amplifier-compressor stages for fusion laser systems. These systems, with somewhat better controlled pulse shapes than those used in the experiment, can deliver average intensity gains of 2-3 at a net energy conversion efficiency (all losses included) of about two thirds and a stage gain of 30.     

Intensity averaging and four-wave mixing in Raman amplifiers

Intensity averaging of pump beams in a Raman amplifier can be treated using a four-wave mixing model. The model shows explicitly the consequences of finite pump beam correlation length in four-wave mixing processes. Phase conjugate effects in a Raman amplitier are observed and described. Measurements of four-wave mixing effects in a KrF laser-pumped methane Raman amplifier agree with the model.     

基于级联调制器和四波混频效应的24倍频微波信号的光学生成

提出了一种基于级联马赫-曾德尔调制器 (MZM)和半导体光放大器(SOA)的24倍频微波信号 光学生成方案,具有覆盖频段高、杂波抑制效果好等优点。在本方案中,低频微波信号分别 通过两个级联 MZM对连续光源进行调制,调节直流偏置使两个MZM均工作在最大偏置点,以抑制奇数阶光分 量;进一 步调节两个MZM的调制深度,并结合可调谐电相移器(TEPS)和可调谐光相移器(TOPS)引入相 移,完全抑 制第2个MZM输出的±2阶光分量和光载波,得到±4阶光分量;再经过SOA发生四波混频(FWM )效应,形成±12 阶光分量;滤波后拍频可以获得24倍频微波信号。最后,搭建了实验和仿真系统,分别以11．0GHz,11.5GHz和12.0GHz的微波信号为驱动,得到间隔为264、276GH z和288GHz的±12阶光边带,有效验证了方案的可行性。     

Heterodyne detection of tunable FIR sidebands

Continuously tunable far infrared (FIR) laser sidebands have been generated in a Schottky diode by nonlinear mixing the radiation from an optically pumped FIR laser operating between 0.6 and 3 THz with that from a microwave source in the 2-4 GHz range. A very sensitive heterodyne detection is adapted to detect the sidebands generated not only at the sum or difference frequency of the two radiations, but also, for the first time in the FIR region, those related to the mixing of the FIR radiation with the second and third harmonics of the microwaves. The high selectivity of the superheterodyne detection allows easy separation of the various sidebands. The global tunability of this system is 2 × 10 GHz for the more powerful FIR emission lines. To demonstrate the tunability of the device absorption signals of several molecules are presented in the whole range of operation of the system (0.6-3 THz) using either direct or lock-in detection techniques.     

Probe beam amplification via two- and four-wave mixings in a nematic liquid crystal film

We have observed very large probe beam gain (up to 600 percent) in a thin film (100 μm) of nematic liquid crystal for a low pump beam intensity on the order of 2 W/cm². The effect is nonlinearly dependent on the pump intensity and the wave mixing angle.     

Determination of the threshold for instability in four-wave mixingmediated by Brillouin scattering

The threshold for instability in Brillouin-enhanced four-wave mixing has been experimentally determined as a function of both the phase mismatch and the ratio of the pump beam intensities and is shown to agree with theoretical modeling. The effective input noise intensity for four-wave mixing in the unstable regime is compared to the noise in a stimulated Brillouin scattering amplifier and is found to be higher by a factor of three in the forward direction. Competition between two input signals has been investigated, and it is shown that the signal which arrives first dominates the interaction in the unstable regime     

Low-frequency reactance amplifier

A reactance amplifier has been constructed which is capable of low-noise amplification at audio and subaudio frequencies [1]. Stable voltage amplification of 20-30 db and noise figures as low as 0.16 db (50-K source resistance) have been observed for signal frequencies between 2 and 50 cps. The circuit configuration is a double sideband up-converter or modulator in which a band of frequencies from dc to several hundred cycles is used to modulate a 150-kc carrier. An analysis for noise and gain is presented which considers the average, fundamental, and second-harmonic components of the time-varying diode capacitance. The amplifier input is a signal at low audio frequency, while the output is composed of the sum and difference sidebands of the pump. Gain is realized through up-converter action associated with the frequency shift from signal frequency to near pump frequency; the sum and difference sidebands are further amplified in the output tank by the negative resistance resulting from the action of the component of time-varying capacitance at the second harmonic of the pump frequency. Gain and equivalent input noise resistance are discussed as functions of detuning in the passive output tank; measured and calculated results are compared. A scheme is also presented for stabilizing the amplifier at high values of negative resistance gain.     

A model for diffraction-limited high-power multimode fiberamplifiers using seeded stimulated Brillouin scattering phaseconjugation

Diffraction-limited polarized stimulated Brillouin scattering (SBS) Stokes output from a multimode fiber is possible when the Stokes beam is the phase conjugate of diffraction-limited polarized pump light from a narrow-band master oscillator. Net amplification can be obtained by interposing a gain medium, such as a fiber amplifier between the master oscillator and the region of SBS generation. This paper proposes and studies numerically a model which describes the space-time dynamics of SBS generation, including phase conjugation, attenuation, phonon decay, thermal noise, inhomogeneous broadening, and amplifier gain. Noise reduction and phase locking are obtained by seeding the low-power end of the fiber at the Stokes frequency. Simulations are described for the case of 1.064 μm light amplification in a dual-clad Yb-doped multimode fiber amplifier     

IMPATT Pump Sideband Noise and its Effect on Parametric Amplifier Noise Temperature

This paper describes an investigation of the amplitude modulation (AM) noise sidebands of silicon IMPATT microwave oscillators and the effect these noise sidebands have on the excess noise temperature of parametric amplifiers. It is shown that the noise temperature may be affected under both large- and small-signal conditions to an extent which depends on the level of pump sideband noise. Simple relationships between easily measurable amplifier properties and pump noise power are given which enable the performance of any combination of pump and amplifier under the two signal conditions to be predicted. A method which simultaneously eliminates the small-signal effect and reduces the large-signal effect to an acceptable level is proposed and demonstrated to be a practical solution.     

双池布里渊放大器中Stokes波的高效放大

微小Stokes信号光在布里渊放大器中得到放大,当Stokes信号光强度为0.15mJ时,增益系数达到67。Stokes光增益决定于Stokes信号光强度及放大池中泵光与Stokes光的耦合程度,小信号Stokes光将得到较大的增益,并能得到较好的相位共轭效果。     

多泵浦拉曼光纤放大器增益均衡的研究

基于受激拉曼散射原理,根据传输方程,用数值解法计算了单泵浦输入时的拉曼光纤放大器的增益;在此基础上,计算了多信道信号输入时,多泵浦拉曼光纤放大器的增益,并和文献的实验结果做了比较;用遗传算法优化泵浦参数以实现多泵浦拉曼光纤放大器的增益均衡。 1