Efficient generation of vibrational stimulated Raman emission in the ultraviolet region using a femtosecond pump beam

The third harmonic emission (261 nm, 30 μJ) of a femtosecond Ti:sapphire laser is focused into molecular hydrogen to generate vibrational stimulated Raman emission. High-order emission lines are efficiently generated by using a high-beam-quality nearly-transform-limited laser and high-pressure hydrogen (40 atm) as a Raman medium.     

Light-induced frequency shift of stimulated Raman scattering in Bessel light beams

The medium susceptibility related to stimulated Raman scattering (SRS) is known to be a complex value whose imaginary part determines the SRS gain coefficient whereas the real part constitutes a small fraction of the refractive index at the frequencies of the scattered emission that is proportional to the intensity of the exciting beam. Strong transverse nonuniformity of this beam causes focusing of high-frequency components of the scattered light and defocusing of low-frequency ones that inevitably affect their amplification efficiency. The effect for Gaussian pumping is responsible for a slight shift of the SRS gain spectrum towards higher frequencies. The present work analyzes theoretically SRS excited by a Bessel light beam and demonstrates that the axial component of the scattered emission experiences a frequency shift that is much greater than that for the case of Gaussian beam pumping. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 336–341, May–June, 2008.     

Formation of Bessel light pulses by means of a conical mirror

The feasibility of forming diffractionless Bessel light pulses by means of a conical mirror is demonstrated. The dependence of the spatial-temporal and energy characteristics of these pulses on the mirror parameters is examined.     

Efficiently generated rotational and vibrational stimulated raman emission by means of a two-color laser beam

When a two-color laser beam is introduced into pressurized hydrogen, about 40 laser emission lines are generated from the ultraviolet to the visible regions. This phenomenon is ascribed to the stimulated Raman effect due to a combination ofJ=1 J=3 rotational andv=0 v=1 vibrational transitions. By introducing the two-color laser beam, the threshold for generation of the rotational line is substantially reduced. The present phenomenon is attributed to four-wave mixing, which allows efficient generation of higher-order rotational and vibrational Raman lines.     

Transient Raman gain stimulated by a noise pump

We predict a strong transient Raman gain stimulated by a Gaussian noise field in dispersive media. Our theory is based on the method of successive approximations, the first of which is the Markovian approximation. It is shown that the transient behavior will take place near critical pump intensity both for “short” (τ_p > T₂) and “ultrashort” (τ_p < T₂) laser pulses (τ_p is the pump pulse duration, T₂ is the relaxation time of optical phonons). Solutions for average intensities of the Stokes wave and optical phonons are obtained for arbitrary form of the given pump pulse. We show that transient Raman gain allows to form a very steep leading edge of the Stokes pulse.The steady-state increments of the second approximation describe the effects of a strong noise field. We show that after the intensity of the pump has exceeded a value beyond which the framework of validity of the Markovian approximation does not hold anymore, the steady-state Raman gain is saturated.We give a review of experimental results and make estimations for conditions of observing transient scattering in the field of a noise pump. Methods of obtaining the powerful femtosecond pulses based on stimulated Raman scattering of broad-band excimer lasers and supercontinuum emission are suggested.     

Internal conical refraction of Bessel light beams

The internal conical refraction of a Bessel beam of arbitrary order is studied theoretically. It is shown that a beam of the order m of arbitrary polarization in the general case is partially converted to Bessel beams of the order m ± 1 with orthogonal circular polarizations. Conditions for complete conversion of a beam of the order m to a Bessel beam of the order m?1 or m+1 are determined.     

Efficient generation of Bessel beam arrays by means of an SLM

We use a Spatial Light Modulator (SLM) to produce arrays of Bessel beams by using multiple axicon phase-masks on the SLM. This approach utilises the whole of the SLM, rather than just a thin annular region (which is the case if the SLM is in the far-field of the generated Bessel beams). Using the whole SLM rather than just an annular region means that the required intensity on the SLM is an order of magnitude lower for a given power in the Bessel beams. Spreading the power over the whole SLM is important for high-power applications such as laser micromachining. We allow the axicons to overlap and interfere in the hologram, so the axial length of the Bessel beam core is maintained as we add more beams to the array.     

Efficient compression of femtosecond pulses by stimulated Raman scattering

Femtosecond pulses at 496 nm were Raman-shifted in methane with 20% efficiency. The pulse duration could be reduced up to 6.5 times from 560 fs at the fundamental to 85 fs at the Stokes frequency (580nm), which is the shortest pulse duration generated in this way. It was shown experimentally that chirped-pulse Raman scattering avoids the limitations arising from self-phase modulation.     

产生二步混合激发受激辐射的光延时效应研究

本文对双光束二步混合激发气体样品产生受激辐射的光延时效应进行了研究.通过三能级模型计算了激发能级上粒子数布居随时间的变化,并以实验进行验证.计算得到与实验值符合较好.     

A controlled conical lens for the formation of Bessel light beams

The possibility of the formation of zero-order Bessel light beams using a uniaxial crystal with cylindrical electrodes is investigated. It is shown that, if a given polarization distribution is provided in the incident beam, a Bessel light beam with the cone angle controlled by an external electric field is formed. It is proposed to use this result for the matching of the axicon parameters and creation of conical lenses with an adjustable cone angle.     

Atom funnel running with evanescent light generated by a thick hollow light beam

The use of a hollow light beam reduces the loss of atoms inside and outside an evanescent-light funnel. Therefore, we superimpose an LG₀₁ beam with a small hole onto a doughnut beam with a large outer diameter and couple the resultant hollow light beam to the 240-μm-wide outlet of an inverted trigonal-pyramidal funnel. Compared to the case where a Gaussian light beam is employed for generation of evanescent light, the flux intensity of the output cold ⁸⁷Rb atoms rises 13-fold when the inner diameter of the 4.5-mm wide hollow light beam is 1.7 times as large as the size of the outlet under optimal blue-detuning conditions.     

High frequency fiber laser emission generated by pump spiking

A stable and short pulse train of ∼100 MHz repetition frequency was obtained from an erbium doped fiber laser excited by a “continuous” semiconductor laser and by using a linear cavity defined by a Bragg grating pair. The operation frequency of the fiber laser was greater (∼5-15 times) than the cavity round-trip frequency. Emission properties obtained from the erbium doped fiber laser were correlated with those taken from the pump laser, which presented a particular optical noise (very short pulses) added to the continuous emission. From the temporal and radio-frequency analysis of both systems, we conclude that the pump emission characteristics are the responsible of the fiber laser pulsed behaviour.     

Scattering of a Bessel beam by a sphere

The exact scattering by a sphere centered on a Bessel beam is expressed as a partial wave series involving the scattering angle relative to the beam axis and the conical angle of the wave vector components of the Bessel beam. The sphere is assumed to have isotropic material properties so that the nth partial wave amplitude for plane wave scattering is proportional to a known partial-wave coefficient. The scattered partial waves in the Bessel beam case are also proportional to the same partial-wave coefficient but now the weighting factor depends on the properties of the Bessel beam. When the wavenumber-radius product ka is large, for rigid or soft spheres the scattering is peaked in the backward and forward directions along the beam axis as well as in the direction of the conical angle. These properties are geometrically explained and some symmetry properties are noted. The formulation is also suitable for elastic and fluid spheres. A partial wave expansion of the Bessel beam is noted.     

Threshold features of a Brillouin Stokes comb generated in a distributed fiber Raman amplifier

We study threshold features of a Brillouin-shifted Stokes comb generated in a distributed fiber Raman amplifier. When the input power of Brillouin pump is linearly increased in high Raman gain, the first Stokes wave grows exponentially at much lower threshold power and then experiences an appreciable power decrease in the vicinity of the Brillouin comb's threshold. This power reduction of the first Brillouin Stokes, which we did not see mentioned in previous reports, was caused by a power transfer to higher-order lines and initiated Brillouin comb generation. Moreover, the effects of Raman pump power and pumping direction on the threshold of a Brillouin comb are investigated.     

Theory of stimulated Raman scattering in a chaotic incoherent pump field

A novel theory of forward and backward Raman amplification in a dispersive medium pumped by a markovian chaotic field of arbitrary bandwidth is reported. Calculations are presented for the average intensity, spectrum and two-time intensity correlation of the Stokes output.     

非稳腔主动式直接获取纳秒近似无衍射贝塞尔绿光

通过对基于轴棱锥的贝塞尔谐振腔和贝塞尔-高斯谐振腔的研究,设计了一台腔内倍频Nd:YAG纳秒近似无衍射贝塞尔绿光激光器. 非稳激光器谐振腔由轴棱锥和凸面镜组成. 实验采用单灯脉冲抽运激光增益介质Nd:YAG晶体,倍频晶体选用KTP. 当抽运电压为350 V时,由主动式直接产生纳秒近似零阶贝塞尔绿光,其脉冲宽度为55.1 ns,波长为532 nm,线宽为0.8 nm,近似无衍射零阶贝塞尔绿光的中心光斑直径为192 μm. 利用Fresnel-Kirchhoff 衍射积分和Fox-Li迭代法,通过数值计算得出 关键词： 主动式 纳秒贝塞尔光束 腔内倍频 轴棱锥     

Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification

Optical parametric amplification of multi-frequency seed pulses generated in a mixture of compressed hydrogen and methane by stimulated Raman scattering of 1 ps, 1 kHz laser pulses at 395.8 nm has been studied. Efficient generation of spectrally narrow ultrashort pulses with a spatial distribution close to the Gaussian profile of the pump beam was obtained in the visible and near infrared ranges.