Generation of high-order rotational lines by four-wave Raman mixing using a high-power picosecond Ti:Sapphire laser

More than thirty rotational lines equally spaced by 587 cm^–1 are generated simultaneously in the vicinity of the fundamental line by four-wave Raman mixing using a high-power picosecond Ti:Sapphire laser as a pump source and hydrogen as a Raman medium. Since the wavelength of this multifrequency laser emission extends from the near-infrared to the near-ultraviolet, it can be utilized as a tunable light source for picosecond spectroscopy. Because of the wide spectral bandwidth available, this procedure has great potential for the generation of ultrashort laser pulses by mode-locking these emission lines.     

Multiplication of spectral lines generated by two-color stimulated raman effect

A dye laser emitting at four different frequencies separated by 293.5 (=587/2) cm^-1 is made by inserting an etalon in the resonator cavity of the laser. This laser beam is focused into molecular hydrogen (rotational Raman shift frequency, 587 cm^-1) to generate a multicolor laser beam consisting of more than 10 rotational lines in the vicinity of the fundamental lines by four-wave Raman mixing. Such rotational lines also occur in the vicinity of the vibrational Raman lines. Thus more than 25 emission lines appear simultaneously. This approach is useful to multiply the line density, i.e., the emission lines within a specified wavelength region.     

Thermal loss mechanism in the generation of multifrequency laser emission via stimulated Raman scattering and four-wave Raman mixing studied by photothermal refraction spectroscopy

The heat produced in conjunction with the processes of stimulated Raman scattering and four-wave Raman mixing in hydrogen was measured by photothermal refraction spectroscopy. Many vibrational, rotational, and vibrationally shifted rotational Raman lines are exclusively/simultaneously generated by changing the polarization of the laser beam and the hydrogen pressure. Thermal loss occurs predominantly from vibrational Raman scattering, which can be ascribed to a large Raman shift frequency of 4155 cm^-1 for the vibrational transition. In contrast to stimulated Raman scattering, little or no thermal loss is observable during the process of four-wave Raman mixing. Received: 12 April 1999 / Revised version: 12 July 1999 / Published online: 20 October 1999     

Stimulated Raman scattering and four-wave Raman mixing seeded by a supercontinuum generated in dibromomethane using picosecond and femtosecond laser sources

Stimulated Raman emission from liquid dibromomethane (vibrational Raman shift frequency, 588 cm⁻¹) is introduced into hydrogen gas (rotational Raman shift frequency, 587 cm⁻¹) as a seed beam, in order to generate numerous rotational lines by four-wave Raman mixing. Unexpectedly, a supercontinuum, which is generated by self-phase modulation in dibromomethane, acted as a seed beam to exclusively generate vibrational lines; the rotational lines are generated only when the supercontinuum is minimal. The former is explained by a competition between the high-gain vibrational and low-gain rotational Raman effects when strongly seeded by a supercontinuum. The latter is explained by stimulated Raman gain under the seed effect exclusively to the first-Stokes rotational line.     

Experimental investigation of supercontinuum generation in highly nonlinear dispersion-shifted fiber pumped by spectrum-sliced amplified spontaneous emission

Spectral broadening of spectrum-sliced amplified spontaneous emission (SS-ASE) in highly nonlinear, dispersion-shifted fiber in different dispersion regimes is investigated experimentally. We find that, the spectral noise of the amplified SS-ASE pump from Er³⁺-doped fiber amplifier seeds the spectral broadening via four-wave mixing or modulation instability. Stimulated Raman scattering, red-shifted Raman solitons, and blue-shifted dispersion waves all enhance the broadening of the spectrum. The effect of the polarization state of pump on supercontinuum generation is also investigated, and it is found that, linear polarization is more efficient than random polarization for pumping supercontinuum. Supercontinuum with −10 dB bandwidth of 200 nm is generated by launching linearly polarized pump with 33.5 dB m power into anomalous dispersion regime near to zero dispersion wavelength of fiber.     

Polarization of multiple rotational Raman sidebands from hydrogen gas by delayed four-wave Raman mixing in the femtosecond regime

We demonstrate delayed four-wave Raman mixing in hydrogen gas and discuss the polarization of multiple rotational Raman radiation in the sub-100-fs regime. The mechanism of sideband generation through the interaction between a probe pulse and coherence of molecular motions induced by a pump pulse in hydrogen is revealed. One can artificially control the ellipticity of the polarization of the rotational Raman sidebands by changing the pump pulse polarization.     

Cherenkov phase-matching in Raman-seeded four-wave mixing by a femtosecond Bessel beam

It is demonstrated experimentally that the angle vs. wavelength dependence of the emission generated by multi-step four-wave mixing process seeded by stimulated Raman scattering in water under femtosecond Bessel beam excitation is determined by the longitudinal phase-matching from IR to near UV. It is shown that if on-axis phase velocity of the pump Bessel beam is equal to the phase velocity of the Stokes axial wave, then, similar to Cherenkov radiation, all the other anti-Stokes beams too acquire that axial velocity.     

Generation of a sub-20-fs single optical pulse by four-wave Raman mixing using two Raman cells filled with molecular hydrogen

To attempt to expand the spectral domain for pulse shortening, we generated several emission lines by stimulated Raman scattering and subsequent four-wave Raman mixing. The efficiency of generation of the Raman emission was improved by passing the beam through two Raman cells that were connected in series. The group-velocity dispersion induced by a Raman cell window and hydrogen was compensated for by means of a pair of chirped mirrors for pulse compression. By a phase lock of the emission lines in the process of four-wave Raman mixing, the pulse width was reduced from 114 to 17 fs in the second Raman cell.     

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.     

Double line stimulated Raman scattering in benzene

Simultaneous stimulated Raman scattering at the 992 cm^–1 and the 3063 cm^–1 line of benzene is observed by mode-locked ruby laser pulse excitation. The double line stimulated Raman scattering is initiated by self-focusing. The influence of small-scale self-focusing, self-phase modulation, and cross-phase modulation on the double line stimulated Raman scattering is discussed. At low pump pulse intensities, before the onset of small-scale self-focusing, the steady-state Raman gain factors of both Raman lines are determined by Raman energy conversion efficiency measurements.     

Polarization-Dependent Light-Gated Phase Conjugation in Methyl-Orange Doped Polymer Film

We have investigated the polarization state of a gated phase conjugate beam using a methyl-orange doped poly-vinyl alcohol (MO/PVA) film in 16 different combinations of linear and circular polarization states of a probe beam and pump beams. The experimental results agree with the theoretical prediction based on the Jones matrix method. Of the third-order nonlinear susceptibility tensors X_ijkl of the MO/PVA film, the X₁₂₂₁ at the 647 nm wavelength in degenerate four-wave mixing under exposure of a gating 515 nm light takes a very small value compared with X₁₁₁₁ and X₁₁₂₂. On the other hand, in the configuration of a probe beam and pump beams having linearly crossed polarizations and without gating light, the MO/PVA film generates a strong linearly polarized PC beam, so that X₁₂₂₁ takes a large value at 515 nm.     

Phase locking in four-wave Raman mixing for generation of an ultrashort laser pulse

The emission lines generated by four-wave Raman mixing were coherently phased and provided an approximately 30-fs pulse without any loss of energy simply by passage of the pulse beam through molecular hydrogen.     

Giant Raman optical activity of the 128-cm-1 mode in α-quartz

The threshold intensity of stimulated Raman scattering along the c-axis in α-quartz was measured for the 128-cm^-1 optical lattice vibration as a function of pump laser polarization at T = 10 K. In a right-handed, optically active quartz crystal the Raman threshold intensity for left-handed, circularly polarized pump light was lower by a factor of about 1.5 than for right-handed, circularly polarized light. The difference in threshold intensity is discussed in terms of Raman optical activity.     

Improved measurements of thermally induced birefringence effects in a laser material using a half-wave plate

The polarization dependence of a probe beam for use in measuring thermally induced birefringence effects in a laser-diode end-pumped Nd:YAG ceramic laser has been investigated. The variation in intensity of the probe beam was found to be 10.2% for a linearly polarized probe beam, 20.6% for a circularly polarized probe beam, and 31.4% for a circularly polarized probe beam using a half-wave plate with a pump power of 12 W. The angle between the analyzer and the inclination of the major axis of the elliptical polarization with respect to the x axis was controlled using a half-wave plate. By combining a half-wave plate with a circularly polarized probe beam, the variation in intensity increased compared with conventional methods.     

采用圆偏振啁啾飞秒激光脉冲操控CS2分子的相干拉曼散射过程

采用两束圆偏振啁啾飞秒激光脉冲,非共线相干激发三原子分子CS₂液体. 在相位匹配的方向上,探测到由CS₂频率为397 cm^-1的振动模式产生的强度对称分布的相干反斯托克斯拉曼散射(CARS)信号和相干斯托克斯拉曼散射(CSRS)信号. 当调整两束激发光的圆偏振状态时,CARS,CSRS信号的强度、偏振、波长均发生规律性的改变:CARS,CSRS信号的强度分布反映了CS₂ 在不同极化状态下的受激拉曼散射截面大小;信号光的 关键词： 啁啾脉冲 相干反斯托克斯拉曼散射(CARS) 相干斯托克斯拉曼散射(CSRS) 2')" href="#">CS₂     

强紫外激光在空气中长程传输受激旋转拉曼散射效应

 采用瞬态受激旋转拉曼散射（SRRS）模型，详细考虑了激光脉冲波形、介质的非线性极化、泵浦光的衰减、斯托克斯光的非线性放大以及拉曼线宽等诸多因素，对强紫外激光束在空气中长程传输过程产生的SRRS效应进行了研究。讨论了产生SRRS效应的阈值条件，详细分析了SRRS效应对激光光束质量的影响。     

Rotation of a Fine Particle with Circularly Polarized Anti-Parallel Collinear Laser Beams

We have succeeded in the operation of rotating a fine particle which was held without mechanical contact by circularly polarized laser beams with opposite propagation directions. A polystyrene latex particle 11.9 μm in diameter deformed in a disk-like shape was dispersed in water, and was trapped using the anti-parallel collinear laser beams which were set up so that they had the circular polarization rotating in the same direction. It was observed that the trapped particle rotated in the same direction as the rotation of the circular polarization of light. Inversion of the rotational direction of the circular polarization of light reversed the direction of the rotation of the particle. Although the rotational speed of a particle varied significantly from particle to particle, it was found to be proportional to the laser beam intensity. A typical value for the rotational speed per beam intensity was 2.1 Hz/W.     

Single line high efficiency tunable fir generation by resonant four-wave mixing in CH3 pumped by a multiatmosphere tunable CO2 laser

By pumping CH₃F with a high pressure tunable TE-CO₂ laser, the resonant four-wave mixing process (RFWM) generates a very efficient tunable single line FIR emission at the Raman frequency. This result is strictly related to the spectroscopic structure of the CH₃F molecule. By means of this process, a tunable FIR emission on a 0.1 cm^–1 bandwidth 150 kW (8 mJ) single line, is obtained which can be used for many FIR multiphoton applications.     

Free-space evolution of focused Gaussian beams transformed by conical diffraction in a biaxial crystal

The intensity and polarization pattern of circularly and linearly polarized Gaussian beams transformed by the effect of conical diffraction in a biaxial crystal has been studied experimentally, and compared with the predictions of paraxial theory. Some general regularities in the relative intensity and geometry of beam structures were analyzed. The formation of Mathieu beam profiles from linearly polarized input beams was demonstrated.     

Phase conjugated wavefronts by stimulated brillouin and Raman scattering

Experimental investigations were made of wavefront reproduction (WFR) by backward stimulated Raman scattering (SRS) and backward stimulated Brillouin scattering (SBS) in CS₂ with a linearly polarized ruby pump beam. The experimental studies were carried out as a function of the optical waveguide diameter and photographic records comparing the pump beam with the backscattered SBS and SRS beams are presented. In all of the cases studied there was a high degree of replication between the SBS and pump beams; however, a deterioration in the quality of the replicated SRS beam was observed, and it increased as the cross-sectional area of the waveguide increased.