Continuous-wave pumped, all-fiber optical parametric oscillator assisted by stimulated Raman scattering

A continuous-wave pumped, all-fiber optical parametric oscillator (OPO) around 1523 nm based on the mixing interaction of parametric process and stimulated Raman scattering (SRS) effects are investigated in this paper. We study and give the detailed analysis of the generation and output characteristics of all-fiber OPO with the influence of the SRS effects. Experimental results show that there exists the saturation output effect for this kind of OPO due to the coexistence of first order SRS effect, second order SRS and the optical parametric oscillation. The maximal output power of OPO is about 100 mW as the pump power reaches 1.4 W. Furthermore, a depolarized L-band super-continuum light source from 1570 nm to 1640 nm based on the combined interaction of parametric process and SRS effects can also be obtained.     

Supercontinuum generation in standard single-mode fiber pumped by a nanosecond-pulse laser

We have experimentally investigated supercontinuum generation in a conventional single-mode fiber pumped with a nanosecond pulse source. The experimental results show that, when pump power increases, the spectral flatness is improved obviously and the spectral broadening only occurs in a red-shifted radiation rather than a blue-shifted radiation. A supercontinuum source is experimentally reported with a flatness of 4.7 dB over 180 nm (ranging from 1555 to >1735 nm) at pump power P _R = 5 W and is predicted to have the flatness of less than 1 dB at P _R > 8 W. The cascade of stimulated Raman scattering (SRS) together with soliton fission plays the key roles in supercontinuum generation.     

Continuous-wave pumping in the anomalous- and normal-dispersion regimes of nonlinear fibers for supercontinuum generation

Supercontinuum (SC) growth in highly nonlinear fibers is compared for cw pumping in the anomalous- and normal-dispersion regimes. For anomalous-dispersion pumping, the combined effects of modulation instability (MI) and stimulated Raman scattering (SRS) contribute to spectral broadening. Furthermore, breakup of the cw light into ultrashort pulses by MI leads to the formation of a Raman pulse that evolves into a soliton, as evidenced by the observation of soliton self-frequency shift. Blueshifted, nonsolitonic radiation associated with the fission of higher-order solitons is also present in the SC spectra. For normal-dispersion pumping, SRS seeds the SC growth by generating several cascaded Stokes orders. When the Stokes orders are shifted into the anomalous-dispersion regime at higher launch powers, MI again causes soliton formation. Broadband continua are generated when the laser is positioned as far away as 191 nm from the zero-dispersion wavelength in normal dispersion.     

Self-aligned fiber-based dual-beam source for STED nanolithography

A fiber-based source that can be exploited in a stimulated emission depletion(STED) inspired nanolithography setup is presented. Such a source maintains the excitation beam pulse, generates a ring-shaped depletion beam, and automatically realizes dual-beam coaxial alignment that is critical for two beam nanolithography. The mode conversion of the depletion beam is realized by using a customized vortex fiber, which converts the Gaussian beam into a donut-shaped azimuthally polarized beam. The pulse width and repetition frequency of the excitation beam remain unchanged, and its polarization states can be controlled. According to the simulated point spread function of each beam in the focal region, the full width at half-maximum of the effective spot size in STED nanofabrication could decrease to less than 28.6 nm.     

温度变化对单模石英光纤受激拉曼散射附加峰的影响

用10 m长单模石英光纤进行受激拉曼散射温度特性研究, 实验中发现在泵浦光和一级Stokes左右出现了附加峰(称为双峰), 其峰强度随温度的升高(80～295 K)呈现先增加后减弱现象。当温度达到295 K时, 一级Stokes双峰消失。由受激四光子混频理论计算可知,这种双峰现象是受激四光子混频的结果。同时对SRS一级Stokes所产生的受激四光子混频的Stokes频移随温度升高由706.9 cm-1增大到712.9 cm-1和其半宽度由1.75 nm增至2.18 nm的现象也进行了解释。     

一种用弱信号光调制的宽带光纤拉曼相干光源

本文考察了光纤中外加可调谐信号光条件下的受激拉曼散射现象,提出一种新的宽带光纤拉曼相干光源的设想。这种光源是利用波长可调的弱信号光来调制固定波长的强泵浦光在光纤中产生的高阶受激拉曼散射,使得高阶受激拉曼散射Stokes峰值波长随弱信号光的波长改变而改变。从而获得一个输出均匀而频带又比信号光宽得多的高强度相干光。     

Pump spectral linewidth influence on stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) and self‐termination behavior of SRS in liquids

The threshold, temporal behavior, and conversion efficiency of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SBS) in three liquids (benzene, hexane, and dimethyl sulfoxide) and two crystals (calcite and barium nitrate) have been investigated under three largely different spectral linewidth conditions. Pumped with 532‐nm and nanosecond duration laser pulses of ≤ 0.01 cm^?1 linewidth, only SBS can be generated in all tested liquids with a high nonlinear reflectivity. However when the pump spectral linewidth is ～0.07 cm^?1 or ～0.8 cm^?1, both SBS and SRS can be observed in benzene while only SRS can be generated in dimethyl sulfoxide; in all these cases SRS is the dominant contribution to the stimulated scattering but the efficiency values are drastically decreased due to the self‐termination behavior of SRS in liquids, which arises from the thermal self‐defocusing of both pump beam and SRS beam owing to Stokes‐shift related opto‐heating effect. In contrast, for SRS process in the two crystals, the thermal self‐defocusing influence is negligible benefitting from their much greater thermal conductivity, and a higher conversion efficiency of SRS generation can be retained under all three pump conditions.

     

351nm激光入射大腔靶的受激Raman散射光谱

 报道了“神光 Ⅱ”装置上Au大柱腔靶产生的受激Raman散射光谱。通过分析实验条件和测量结果，排除了散射光谱来自增强非相干Thomson散射的可能性，发现用丝化不稳定性与受激Raman散射的耦合能合理解释观测到的Raman光谱。考虑到丝化不稳定性与SRS的耦合，测量到的散射光谱依然能用于密度诊断，其结果与对流SRS理论的计算值相差不到10%。     

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.     

神光Ⅱ激光装置的全口径背向散射测量系统

报道了基于神光Ⅱ激光装置的全口径背向散射测量系统。系统通过激光聚焦透镜收集激光等离子体相互作用产生的散射光;两个热能量卡计分别测量波长348~354 nm范围的受激布里渊散射和波长400~700 nm范围的受激拉曼散射产生的散射光。该系统可以测量散射光的能量、功率和光谱图像,提供优于0.5 nm光谱分辨和10 ps时间分辨的光谱物理信息。同时采用PIN二极管阵列对聚焦透镜外侧附近的近背向散射光信息进行测量。     

变色磷灰石的可见光光谱特征研究

磷灰石是珠宝市场上常见的宝石品种,因颜色丰富而广受欢迎。变色磷灰石是稀有品种且价格高昂,该品种在D65光源（色温6 500 K）下呈黄绿色,A光源（色温2 856 K）下呈粉红色,其可见光光谱的谱学特征与变色成因未被详细研究。基于此,将一颗变色磷灰石晶体,沿其平行c轴和垂直c轴方向各切下一个薄片并双面平行抛光,分别测试其可见光光谱与微量元素。结果发现,其可见光光谱中谱峰较多：位于583和578 nm处的吸收双峰强度最强,位于748和738 nm处的吸收双峰强度中等,分别位于688和526 nm处的吸收峰,强度较弱。还有一些非常微弱的吸收峰,分别位于514,483,473和443 nm处。位于748和738 nm处的吸收双峰与583和578 nm处的吸收双峰共同造成了红橙光区的透射窗,583和578 nm处的吸收双峰与526 nm处的吸收峰共同造成了黄绿光区的透射窗。D65光源和A光源由于相对光谱功率分布不同,在不同透射窗的透过有所不同,导致变色磷灰石在不同光源下呈现出不同颜色。D65光源中黄绿光成分较多,透过黄绿光区透射窗的成分较多,D65光源下磷灰石呈黄绿色,A光源中红光成分较多,通过红橙光区透射窗的成分较多,A光源下磷灰石呈粉红色。因此,磷灰石的变色效应与位于748和738 nm处的吸收双峰,位于583和578 nm处的吸收双峰以及位于526 nm处的吸收峰相关。根据微量元素数据与稀土元素的晶体场理论,这些吸收峰是由稀土元素钕（Nd）导致。根据不同晶体方向样品的可见光光谱特征,平行c轴方向变色效果更好,建议加工变色磷灰石晶体时宝石台面应尽量平行c轴。该研究结合微量元素与可见光光谱分析了变色磷灰石的变色成因,并为其加工切割方向提供了指导。     

All-Fiber Optical Signal Inversion Using Stimulated Raman Scattering

We propose a method for all-optical signal inversion by means of stimulated Raman scattering (SRS). The idea is based on the transference of energy from the logic state #x201C;1#x201D; to the second stokes and making the logic state #x201C;0#x201D; into logic state #x201C;1#x201D; by receiving energy from the pump through SRS. According to the numerical simulation, a continuous wave pump source of LOW at 1453 nm and a few hundred meters of highly nonlinear fiber are required to invert an input signal at 1555 nm. The optical signal is inverted with high on-off contrast ratio.     

Utilization of stimulated Raman Scattering as secondary pump on hybrid remotely-pump l-band Raman/erbium-doped fiber amplifier

An L-band remotely-pumped erbium-doped fiber amplifier incorporating a secondary pumping scheme utilizing stimulated Raman Scattering (SRS) was demonstrated. 1423 nm Raman laser was employed to generate SRS which became the secondary pump source. The amplifier displayed excellent gain of up to 27.3 dB at 1570 nm for −30 dBm input. Noise figures were also kept to a minimum, with the highest figure measured at 11 dB which was influenced by imperfection of the C/L coupler utilized in this architecture. Overall transmission performance was measured as well and demonstrated an encouraging outcome with gain as high as 24 dB while the noise figure was maintained at about 11 dB. The L-band signal amplification was also contributed by the stimulated Raman scattering along the transmission fiber. The outcome of this study emphasized the feasibility of secondary pumping scheme using SRS in L-band gain enhancement.     

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.     

Stimulated emission depletion microscopy with a supercontinuum source and fluorescence lifetime imaging

We demonstrate stimulated emission depletion (STED) microscopy implemented in a laser scanning confocal microscope using excitation light derived from supercontinuum generation in a microstructured optical fiber. Images with resolution improvement beyond the far-field diffraction limit in both the lateral and axial directions were acquired by scanning overlapped excitation and depletion beams in two dimensions using the flying spot scanner of a commercially available laser scanning confocal microscope. The spatial properties of the depletion beam were controlled holographically using a programmable spatial light modulator, which can rapidly change between different STED imaging modes and also compensate for aberrations in the optical path. STED fluorescence lifetime imaging microscopy is demonstrated through the use of time-correlated single photon counting.     

Raman Scattering in Light and Heavy Waters

We present the results of an experimental study of the spontaneous and stimulated Raman scattering (SRS) spectra and luminescence spectra of light and heavy waters. Also we investigate various samples of light water. We show the possibility of using spectral methods to detect impurities in water and determine the degree of its structuredness, which can serve as an additional criterion of the water quality. We find a fine structure in the SRS spectrum of heavy water in the low-frequency region.     

Quantitative pupil analysis in stimulated emission depletion microscopy using phase retrieval

The resolution attainable with stimulated emission depletion (STED) microscopy greatly depends on the quality of the STED laser focus. So far, visual inspection of a measured STED focus has been the only convenient means of gauging the source of aberrations. Here we describe a method, requiring no instrument modifications, for obtaining an equivalent to the complex pupil function at the back aperture of the objective and show that it provides quantitative information about aberration sources (including aberrations induced by the objective or sample). We show the accuracy of this field representation to be sufficient for reconstructing the STED focus in three dimensions and determining corrective steps.     

Steady-state Raman gain in visible and near-infrared waveband of Sr WO_4 and Ba WO_4 crystals

The steady-state stimulated Raman scattering （SRS） gain with different excitation wavelengths ranging from 400 to 1100 nm of tungstate crystals, SrWO4 and BaWO4, is systematically researched. As excitation frequency is close to electronic transition frequency, molecular polarizability is not a constant, which has to be taken into account in our work. The experiment and theory agree well with each other and show that SRS gain is not only proportional to Stokes light frequency, but is also inversely proportional to biquadratic excitation frequency.     

超短超强激光与气体靶相互作用中的背向受激Raman散射

 实验诊断测量了超短超强激光与气体靶相互作用产生的背向受激Raman散射,在实验条件下呈现强耦合模式,背向受激Raman散射出现非线性Stokes多峰伴线结构,峰值的频率间隔小于等离子体波的频率,可以大致地推断出激光打靶过程中产生的等离子体密度偏低,其结果与等离子体强耦合理论计算结果一致。     

2 ns, 351 nm激光黑腔靶受激Raman散射实验研究

 介绍了在神光Ⅱ装置上开展的长脉冲2 ns,351 nm激光与黑腔靶相互作用的实验，报道了受激Raman散射光时间分辨谱图及能量测量的实验结果。长脉冲2 ns激光注入小腔靶（Ø700 mm×1 250 mm）时，激光辐照缝靶产生的SRS光能量是激光与全腔靶作用产生的SRS光能量的1.3倍。在2 ns激光与不同尺寸黑腔靶作用的情况下，激光辐照小腔靶产生的SRS光能量比标准腔靶(Ø800 mm×1 350 mm)产生的SRS光能量高1.6倍。由于激光功率密度下降，2 ns激光打靶SRS散射光要弱于短脉冲1 ns激光打靶，但持续时间稍长。实验结果表明：长脉冲2 ns激光与标准腔靶相互作用时，等离子体“堵腔效应”比较严重，标准腔靶尺寸不再合适。