Induction of the maximum Raman coherence in an extended medium through fractional adiabatic passage

The effect of fractional stimulated Raman adiabatic passage in an optically dense medium of three-level Λ atoms is shown to result in the maximum coherence on the Raman transition on a length of the medium that considerably exceeds the length of linear resonant absorption. The general case of unequal oscillator strengths of the adjacent transitions is analyzed. The results are of interest for coherent anti-Stokes Raman spectroscopy with delayed pulses.     

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.     

Transition from the Fluctuation Mode of Stimulated Resonant Coherent Raman Scattering to the Energy–Stabilized Mode

The equations of the semiclassical theory for the stimulated resonant coherent Raman scattering (SRS) in an extended medium have been solved numerically. The distribution function of the energy of Stokes pulses at different detunings and energies of a laser has been determined. Just as for the nonresonant coherent SRS, in the situation considered there occurs transition from the large–scale fluctuation mode to the mode of relatively small straggling of the energies of Stokes pulses with increase in the pumping energy. With approach to the resonance, the energy required to stabilize the Stokes radiation decreases.     

皮秒激光瞬态受激喇曼散射的实验研究

本文采用对撞脉冲锁模Nd:YAG激光器输出10ps脉冲串,经KTP非线性晶体腔外倍频,泵浦喇曼介质为二甲亚砜(DMSO)液体.实验研究了不同透镜焦长、焦点位置及不同喇曼介质氏度对瞬态受激喇曼散射的影响,获得了能量转换效率分别为45.6%和10.5%的前、后向一阶斯托克斯-喇曼散射光,并对实验结果进行了讨论.     

光纤中的受激喇曼散射及泵浦脉冲和斯托克斯脉冲的分离

本文报道在单模光纤中的受激喇曼散射的实验结果,用条纹相机测量了泵浦脉冲和喇曼斯托克斯脉冲时间上的相对延迟;结果表明,受激喇曼散射的喇曼斯托克斯脉冲大约在距光纤输入端一个分离距离的位置上形成。     

Subfemtosecond pulse generation and multiplicative increase of pulse spacing in high-order stimulated Raman scattering

We demonstrate a technique for multiplicatively increasing the pulse spacing of subfemtosecond pulse trains generated by high-order stimulated Raman scattering, which uses pairs of off-resonant two-photon excitation of a molecular Raman mode to establish strong molecular modulation in the medium. Every laser pair has two single-mode laser beams tuned slightly off two-photon resonance, and the laser pairs have their corresponding carrier frequencies shifted from each other by one half, one third, or one fourth of the selected molecular Raman mode frequency. Theoretical analyses and calculations based on the fundamental vibrational transition of solid hydrogen show that the pulse spacing of the subfemtosecond pulse trains thus generated can readily be doubled, tripled, or even quadrupled.     

Control of Raman lasing in the nonimpulsive regime

We explore coherent control of stimulated Raman scattering in the nonimpulsive regime. Optical pulse shaping of the coherent pump field leads to control over the stimulated Raman output. A model of the control mechanism is investigated.     

Theoretical investigation on the pumping effect of stimulated Brillouin scattering on stimulated Raman scattering in water

The pumping effect of stimulated Brillouin scattering on stimulated Raman scattering is investigated theoretically through the coupled wave equations of stimulated Brillouin scattering and stimulated Raman scattering. The numerical simulations are in agreement with the experimental results. They indicate that the backward stimulated Raman scattering is excited and amplified collectively by both pump laser and stimulated Brillouin scattering.     

Determination of the composition of mixtures of organic liquids by CARS spectroscopy

Two methods of realization of coherent anti-Stokes Raman scattering (CARS) spectroscopy of organic liquids are described: with a tunable dye laser and by means of biharmonic laser pumping based on stimulated Raman scattering. The second method is applied to liquids for the first time. The possibilities of the two methods are compared. CARS spectroscopy is applied for the first time to the analysis of a mixture of two organic liquids.     

Generation of three-primary-color Raman comb in the continuous-wave regime

We demonstrate the generation of continuous-wave Stokes and anti-Stokes Raman sidebands including the three-primary-color (683, 532, and 436 nm) components. The Raman sidebands are generated through both stimulated and coherent anti-Stokes Raman scattering in a broadband high-finesse optical cavity filled with gas-phase hydrogen as a Raman-active medium and covering the entire visible spectral range (420–680 nm). The blue emission is considerably enhanced by matching the frequency with one of the longitudinal modes of the optical cavity, and high conversion efficiencies are observed when the coherent length corresponds to an integral multiple of the round-trip length of the optical cavity. This indicates that phase-matching plays a critical role in determining the efficiency of the Raman comb generation.     

Slow-light spatial solitons

We numerically and experimentally report the observation of slow-light spatial solitons in a Kerr medium owing to light amplification by stimulated Raman scattering. This was achieved in a CS2 nonlinear planar waveguide that possesses both a strong self-focusing nonlinearity to generate the spatial Raman soliton and a Raman susceptibility sharp enough to induce the slow-light process simultaneously. We show that the Raman Stokes component is optically delayed by more than 120 ps for a 140 ps Raman pulse duration and only 3 cm of propagation length, while propagating as a spatial soliton beam.     

喇曼光纤放大器结构对性能的影响

详细分析了拉曼光纤放大器的结构对性能的影响.基于受激喇曼散射效应中泵浦光与信号光之间相互作用的常微分方程,进行了数值模拟.模拟结果表明,喇曼光纤放大器结构对性能有着非常大的影响作用.后向泵浦结构,输出信号的光功率相对高于前向泵浦输出信号的光功率.当不考虑泵浦波动时,前向泵浦结构输出信号的信噪比高于后向输出信号的信噪比.在任何一种泵浦结构中都可获得高于41dB信噪比.设计了一种后向泵浦、增益平坦的放大器.该放大器可以实现90km、40×10Gbit/s复用信号的无损传输,增益波动小于1.2dB.     

Generation of frequency shifted picosecond pulses with low temporal jitter

Transient stimulated Raman scattering is used for the generation of a frequency shifted picosecond light pulse; part of this Raman shifted pulse is subsequently coherently scattered at a material excitation of a second Raman cell. Starting with the second harmonic pulse (t_p = 4 ps) of a mode-locked Nd : glass laser system, both the stimulated and the coherently produced pulses have durations of 2.3 ps at different wavelengths. By the appropriate choice of the Raman medium pulses between 13 000 and 21 000 cm^-1 can be generated. The coherent generation process minimizes the temporal jitter between the two pulses and allows to obtain a high time resolution of better than 0.3 ps in excite and probe experiments.     

Generation of a compact high-power high-efficiency normal-dispersion pumping supercontinuum in silica photonic crystal fiber pumped with a 1064-nm picosecond pulse

Broadband normal dispersion pumping supercontinuum （SC） generation in silica photonic crystal fiber （PCF） is investigated in this paper. A 1064-nm picosecond fiber laser is used to pump silica PCF for the SC generation. The length of PCF is optimized for the most efficient stimulated Raman scattering process in the picosecond pump pulse region. The first stimulated Raman Stokes peak is located in the anomalous dispersion regime of the PCF and near the zero dispersion wavelength; thus the SC generation process can benefit from both a normal dispersion pumping scheme and an anomalous dispersion pumping scheme. The 51.7-W SC spanning from about 700 nm to beyond 1700 nm is generated with an all-fiber configuration, and the pump-to-SC conversion efficiency is up to 90%. In order to avoid the output fiber end face damage and increase the stability of the system, an improved output solution for the high power SC is proposed in our experiment. This high-efficiency near-infrared SC source is very suitable for applications in which average output power and spectral power density are firstly desirable.     

Interference of coherently scattered signals in the laser SRS-CARS monitoring of hydrogen concentration in gas mixtures

The possibility of measuring the hydrogen impurity concentration in dense gas mixtures by coherent anti-Stokes Raman scattering (CARS) is studied. In this technique, biharmonic laser pumping based on stimulated Raman scattering (SRS) in compressed hydrogen is used. Because of the interference between the coherent scattering components from buffer gas molecules and molecules of the impurity to be detected, the signal recorded may depend on the hydrogen concentration by a parabolic law, which has a minimum and makes the results uncertain. It is shown that this uncertainty can be removed if the frequency of the biharmonic laser pump, which is produced by the SRS oscillator, somewhat differs from the frequency of molecular oscillations of hydrogen in the test mixture. A sensitivity of 5 ppm is obtained as applied to the hydrogen-air mixture under normal pressure. The coefficients of the nonresonance cubic hyperpolarizability of molecules and atoms are measured relative to nitrogen in a number of gas media.     

Numerical analysis of stimulated inelastic scatterings in ytterbium-doped double-clad fiber amplifier with multi-ns-duration and multi-hundred-kW peak-power output

We investigate the stimulated Brillouin scattering (SBS) and the stimulated Raman scattering (SRS) in an ytterbium-doped double-clad fiber amplifier that outputs optical pulses with multi-ns-duration and multi-hundred-kW peak-power. The ytterbium-doped double-clad fiber amplifier is simulated by a model which is composed of a set of propagation-rate equations. The simulated results show that SBS and SRS will deform the output signal pulse in both the time domain and the spectral domain, and degrade the performance of the ytterbium-doped double-clad fiber amplifier seriously. It is shown in our simulation that the troublesome SBS can be effectively suppressed by broadening the signal linewidth to a critical value of 0.07 nm in our calculation, and the effect of SRS can be suppressed using a large-mode-area fiber with proper length. The model and the simulated results are very useful for designing an ultra-high-power ytterbium-doped double-clad fiber amplifier.     

利用CARS方法研究超短脉冲激光的脉冲宽度

在理论上提出了利用两光束CARS/CSRS方法测量超短脉冲激光的三阶相关函数。实验上在几种样品中测量了CARS/CSRS的时间特性,得到了泵浦光脉冲,SCDL输出光脉冲以及受激喇曼散射光脉冲的宽度。     

Scattering-induced changes in the temporal coherence length and the pulse duration of a partially coherent plane-wave pulse

The scattering of a partially coherent plane-wave pulse on a Gaussian-correlated, quasi-homogeneous random medium is investigated. The analytical expressions for the temporal coherence length and the pulse duration of the scattered field are derived. We demonstrate that the scattering-induced changes in the temporal coherence length and the pulse duration may be used to determine the correlation function of the scattering potential of the medium.     

Transition from amplified spontaneous emission to laser action in strongly scattering media

In an active random medium, the combination of multiple scattering with light amplification may lead to random laser action. However, it is crucial but sometimes difficult to distinguish between amplified spontaneous emission and lasing. By varying the amount of scattering in an amplifying random medium, we have observed the transition from amplified spontaneous emission to lasing with coherent feedback. We have found out when the transition occurs through the measurement of the scattering mean free path. Our numerical simulation based on the direct solution to Maxwell equations clearly illustrates the transition from light amplification to laser oscillation due to an increase of the amount of scattering in active random medium.     

β-胡萝卜素分子的黄琨因子的温度特性

β-胡萝卜素具有光采集、光防护功能, 又是重要的光电材料, 它在外场下的分子结构和性能变化既有理论意义也有应用价值。测量了β-胡萝卜素在环己醇中68～26 ℃温度范围内的紫外-可见吸收、拉曼光谱。实验结果表明随着温度的降低, 黄琨因子和碳碳键每个振动模的电子-声子耦合常数减小, 紫外-可见吸收光谱红移, 碳碳键拉曼散射截面增加。用线性链状多烯分子的“相干弱阻尼电子-晶格振动模型”、“有效共轭长度模型”等理论给予了解释。随着温度的降低,β-胡萝卜素分子的热无序减小,分子结构有序性增加,π电子离域扩展,有效共轭长度增加,导致紫外-可见吸收光谱红移和强的拉曼活性。相干弱阻尼电子-晶格振动增强,使碳碳键拉曼散射截面增加。引用带有量纲的电子-声子相互作用常数,既可以与黄昆因子建立关系式,计算出碳碳键每个振动模的数值,也可以表征分子的有效共轭长度,π电子离域程度及拉曼散射截面的大小等。