Saturation and tuning behavior of stimulated electronic Raman scattering

The saturation behavior of stimulated electronic Raman scattering (SERS) in atomic vapors is studied theoretically and experimentally. Limits to the SERS output energy are imposed by both pump depletion and saturation of the Raman transition. The effects of these on the performance of tunable infrared lasers based on SERS are examined.     

背向受激喇曼散射(BSRS)

采用染料Q开关YAP倍频输出的光脉冲(波长0.5398μm、脉宽～30ns),聚焦注入CS2液体盒.在背向获得了四级斯托克斯散射线和两级反斯托克斯散射线,其脉宽压缩至约5ns.     

Analysis of stimulated Raman scattering in waveguides

Stokes radiation, excited by a giant pulse radiation propagating along a rectangular wavegulde, is analyzed in this paper. Equations are established and solved for an arbitrary shape of the giant pulse. At high gains the stimulated radiation is emitted at all angles. The maximum output is almost perpendicular to the exciting beam, and it decreases as the forward and backward directions are approached. Forward-backward ratios are sensitive to the detailed shape and magnitude of the laser pulse. The large forward-backward ratio for intense short laser pulses accounts for the observed results, which are illustrated for a square and a parabolic pulse. In practice, a large number of waveguide modes are simultaneously excited, resulting in intense excitation of vibrational modes in the medium, and enhancement of the inverse Raman effect and of anti-Stokes generation.     

Optimization of the stimulated Raman scattering threshold

A theoretical analysis of the threshold condition for stimulated Raman oscillation is presented. The treatment, similar to that applied recently to optical parametric generation, assumes that a low loss resonator is provided for the Stokes radiation and accordingly pump depletion is neglected in the threshold calculation. Diffraction effects are included in detail, within the paraxial ray limits of Gaussian beam theory. The oscillation threshold for the fundamental resonator mode is expressed as a function of resonator losses, focusing conditions of pump and Stokes waves, and the length of the active medium.     

Transient stimulated Raman scattering in lead vapor

An investigation of stimulated Raman scattering (SRS) of short-pulse (6-ns) XeCl-laser radiation in Pb vapor is reported. Conversion efficiencies, based on peak power, up to 15% have been obtained for Pb-oven temperatures of 1260°C. The functional dependence of conversion efficiency, in the short-pulse regime, upon buffer-gas pressure and species differs significantly from that observed for long-pulse (>20-ns) pumping, corresponding to steady state behavior. Analysis of the data and comparison with transient SRS in H ₂ yield an estimate of T₂≈1ns for the transient response time T₂ of Pb vapor. The observed pressure effects are explained in terms of a reduction in T₂, and hence Raman gain, by collisions with buffer-gas atoms     

Surface vibrational spectroscopy using stimulated Raman scattering

We propose a new method for studying surface vibrational spectroscopy based on an extension of the picosecond stimulated Raman gain technique. This technique is extremely surface sensitive and can detect coverages of 1 percent of a monolayer. This technique does not require ultrahigh vacuums (as do electron spectroscopies), is an extremely sensitive probe of surface local fields, has very high frequency resolution (<1 cm^-1) and very short time resolution (∼ 1 ps).     

玻璃光纤中的瞬态受激喇曼散射

用峰功率大于10~7瓦的0.53微米微微秒光脉冲序列激发一根20米长的锗磷硅玻璃光纤,观测到稳定的15级以上斯托克斯和8级以上反斯托克斯散射。喇曼频移为430厘米~(-1)。散射光频谱范围为0.45～1.00微米。受激喇曼散射的转换效率大于50％。     

Novel delayed pulse techniques for stimulated Raman scattering

The outputs of two independent, incoherent lasers are spatially overlapped in a Raman-active medium. When the probes are separated in time, the stimulated Raman scattering (SRS) efficiency of the delayed pulse is increased from 14 to 27%. This occurs because of the presence of the decaying coherent excitation induced by the SRS of the first pulse. The influence of saturable losses is removed by prepulsing the medium before applying the pulse pair. The temporal evolution of the enhancement and phase-matching conditions are investigated. Surprisingly, the pulses interact only when they are separated in time, with the enhancement vanishing when they are temporally overlapped. The outputs of several independent lasers may be combined to generate significantly higher SRS efficiencies, without increasing the pump intensity incident on the medium. The technique provides a means for recovering efficient SRS in the transient (short-pulse) regime     

Steady-state stimulated Raman scattering by a multimode laser

The development of the Stokes spectrum and the dependence of Raman gain upon pump laser bandwidth in a narrow linewidth Raman medium are analyzed in terms of a closed set of multimode equations. The analysis predicts that the Stokes gain coefficient takes on its maximum value, which is independent of bandwidth, when the Stokes spectrum replicates the pump spectrum. It is also shown that in the absence of dispersion the spectrum of a weak Stokes wave is driven to duplicate the pump spectrum, and that for noise input the average power generated in the Stokes wave is independent of the pump bandwidth.     

共振耦合双波受激喇曼散射的实验研究

利用两束波长不同的毫微秒量级强脉冲激光同时辐照方解石或苯样品,当两入射光频率差正好与样品喇曼跃迁频率发生共振时,可观察到一种新形式的双波受激喇曼散射现象。本文简要报导上述现象产生的条件、特点和实验规律性,并对产生机制给出解释。     

高功率皮秒激光泵浦受激拉曼散射腔研究

为了获得高平均功率的皮秒宽谱段激光,同时 减小整个光学系统的体积,本文从理论和实验两方面研究了高功率皮秒激光泵浦受激拉曼 散射(SRS)振荡腔。 通过分析SRS过程中同时发生的物理过程,将高功率泵浦下的拉曼晶体内多种 非线性效应简化 为晶体在简单等效透镜下的SRS,并通过实验测量单通SRS光的发散角,计算得到了高 功率泵浦下的拉曼晶体等效透镜的曲率半径,在此基础上设计了稳定高效的SRS振荡腔,通 过对比实验验证了此拉曼振荡腔的高效性,实现了波段覆盖532～698nm、最高输出功率为1.32W的皮秒SRS同轴激光的输出。在相同波 段覆盖范围内,是已报道的最高平均输出功率。     

Improved short-pulse pumping of stimulated Raman scattering in leadvapor

Improved efficiency for short pulse stimulated Raman scattering (SRS) in Pb vapor is obtained using accumulation of gain between a closely spaced pulse pair. The peak power conversion efficiency of the delayed pulse is increased to 40%, in contrast to 15% for a single pulse. Numerical modeling gives good agreement with observation, and suggests that this enhancement is caused by the continued presence of coherent excitation produced by the first pulse. The dependence of the enhancement in delayed-pulse efficiency on Xe buffer gas pressure in the Pb-vapor cell is attributed to changes in the transient response time T₂ of the Pb medium with Xe pressure     

Fibre-length dependence of critical power for stimulated Raman scattering

The fibre-length dependence of critical powers, at which Stokes lights begin to build up, is investigated on stimulated Raman scattering in single-mode fibres. A mode-locked and Q-switched Nd:YAG laser is used as a pumping laser. The critical powers for the first five Stokes decrease with an increase in the length of fibre and become constant above 200 m long. The critical powers of 8 W, 20 W, 30 W, 53 W and 73 W for 1st, 2nd, 3rd, 4th and 5th Stokes, respectively, are obtained for fibres longer than 200 m.     

The effect of weak dispersion on stimulated Raman scattering

The Raman gain of an undepleted plane-wave multimode pump laser is calculated. The Stokes and pump fields are represented as vector states in a Hilbert space, and the Dirac bracket notation is introduced to simplify the form of the equations. It is shown that in the absence of dispersion the Stokes vector component correlated with the pump grows exponentially with the monochromatic-pump gain, while all other Stokes components experience zero growth. Perturbation theory is used to second order to determine the gain decrease resulting from relative dispersion between pump and Stokes, and the results are compared with previous works.     

Up-conversion of excimer lasers via stimulated anti-Stokes Raman scattering

Up-conversion schemes for various excimer laser systems based upon anti-Stokes Raman laser action are proposed. Four atomic systems, TI, In, I, and Br, are considered as host media for the Raman laser up-converter. Gain calculations indicate that anti-Stokes Raman up-converters should permit a variety of new tunable laser sources in the 150-325 nm spectral region.     

Control of Stokes pulse duration in stimulated Raman scattering

The generation of long Stokes pulses by stimulated Raman scattering, which is made efficient by a short seed pulse at the Stokes frequency, is demonstrated both in experiments and in simulations for pump powers below the threshold power for unseeded scattering. It is observed that a second seed pulse with appropriate phase and intensity can extend, switch-off or briefly interrupt the Stokes pulse     

The stimulated Raman scattering threshold for anondiffraction-limited pump beam

The threshold power of stimulated Raman scattering (SRS) has been calculated for various nondiffraction-limited pump beams using a numerical model. These calculations were performed for the following pump beam profiles: Gaussian-Hermite, Gaussian-Laguerre, Gaussian-Schell-model (GSM), and several superpositions of Gaussian-Hermite modes. It is shown that the threshold power, for a tightly focused beam, depends on the M² factor, which is a dimensionless factor that compares the divergence of a given beam with the divergence of a (diffraction-limited) Gaussian beam, related to the pump beam quality. The influence of the exact pump beam profile on the threshold is small. It is shown that the measured SRS thresholds in methane for two pump beams, one with an M² of 1.12 and the other with M²=8.7, correspond with the calculated thresholds for two GSM beams with the same M²     

乙醇溶液中后向受激喇曼散射的瞬态效应——亚皮秒喇曼光脉冲的产生

本文报道了在乙醇溶液中实现后向受激喇曼散射光脉冲产生的实验装置和结果,得到了脉宽为280fs、压缩比达100倍的喇曼光脉冲,提出了喇曼超压窄区的概念。     

Dynamics of stimulated Raman scattering in double-clad fiber pulse amplifiers

A systematic analysis on dynamics of stimulated Raman scattering (SRS) in highly pumped ytterbium-doped double-clad fiber amplifiers during amplification of nanosecond pulses at tens-of-kilohertz repetition rates is presented. Energy and waveform evolutions of signal and Raman pulses in the amplifiers under forward, backward and bidirectional pump are depicted respectively. In consideration of the SRS components and incurred energy loss at the amplifier output, the detrimental effects of SRS on signal amplification for three pumping methods are quantitatively compared and comprehensively illustrated upon different signal, fiber and pump conditions. To avoid the performance deterioration, some limitations are also discussed.     

液氮的受激喇曼散射研究

本文使用染料测Q红宝石激光激励液氮的受激喇曼散射，观察到一级斯托克斯、一级反斯托克斯和二级反斯托克斯线，测得散射光的喇曼频移、线宽、能量转换率和退偏振度等参数．并对所得结果进行了讨论。