飞秒激光在双折射微结构光纤中模式控制的四波混频效应的实验研究

利用飞秒激光脉冲在长度为10cm，包层具有大空气比的双折射微结构光纤中通过高阶模相位 匹配的四波混频获得了波长可调谐的反斯托克斯波.实验中脉冲宽度为35fs，中心波长820nm ，单脉冲能量4nJ的飞秒激光脉冲耦合到长轴直径为5μm，短轴为46μm的双折射微结构光 纤中.在高阶模传输情况下，通过调制耦合光的偏振方向，获得了具有不同中心波长的反斯 托克斯波.通过对比分析，讨论了输入光的偏振态对双折射微结构光纤中高阶模式下四波混 频效应的影响情况.理论计算分析很好的解释了实验结果. 关键词： 微结构光纤 飞秒脉冲激光 四波混频     

Femtosecond spectroscopy of coherent anti-Stokes Raman scattering with frequency-tunable chirped pulses generated in a microstructure fiber

A new scheme of chirped-pulse femtosecond coherent anti-Stokes Raman scattering spectroscopy is proposed and experimentally implemented. A theory of this modification of coherent nonlinear spectroscopy is developed. We use this approach to show that a linear time-frequency mapping defined by linearly chirped pulses allows the spectra of nonlinear response of a medium to be measured by varying the delay time between the pump pulses. Microstructure fibers with a special dispersion profile are at the heart of the experimental implementation of this technique. Such fibers are ideally suited for the generation of frequency-tunable pulses with a smooth envelope and a controlled chirp. We present the results of experimental characterization of the envelope, spectrum, and chirp of anti-Stokes pulses generated in microstructure fibers by femtosecond Cr:forsterite-laser pulses. These frequency-tunable anti-Stokes pulses produced and shaped in microstructure fibers are then employed for coherent anti-Stokes Raman scattering spectroscopy of toluene solution.     

Frequency conversion of subnanojoule femtosecond pulses in microstructure fibers

We experimentally demonstrate highly efficient multiplex frequency conversion of unamplified subnanojoule femtosecond pulses of Ti:sapphire laser radiation in fused silica microstructure fibers. Nonlinear optical spectral transformation of femtosecond pulses in an array of fused silica threadlike channels in these microstructure fibers results in the generation of isolated anti-Stokes spectral components within the wavelength range of 400–500 nm. An efficiency of frequency conversion of about 20% is achieved for 800-nm pump pulses with an energy of 0.7 nJ and a pulse duration of 70 fs.     

Generation of femtosecond anti-stokes pulses through phase-matched parametric four-wave mixing in a photonic crystal fiber

Phase-matched parametric four-wave mixing in higher-order guided modes of a photonic crystal fiber is shown to result in an efficient decay of 40-fs 800-nm Ti:sapphire laser pump pulses into an anti-Stokes signal with a central wavelength around 590-600 nm and a Stokes signal centered at 1.25 microm. The photonic crystal fiber is designed in such a way as to minimize the group-velocity dispersion at the pump wavelength, phase match the parametric four-wave-mixing process, and reduce the group delay between the pump and the anti-Stokes pulses. The duration of the anti-Stokes pulse under these conditions, as shown by cross-correlation frequency-resolved optical gating measurements, is less than 200 fs.     

Microstructure fibers as frequency-tunable sources of ultrashort chirped pulses for coherent nonlinear spectroscopy

Microstructure fibers are shown to allow the creation of new tunable sources for femtosecond nonlinear spectroscopy. These fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr:forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp. These pulses from a microstructure fiber were used to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene solution by cross-correlating these pulses with the femtosecond second-harmonic output of the Cr:forsterite laser in boxcars geometry (XFROG CARS). PACS 42.65.Wi; 42.81.Qb     

Influence of the scattering matrix elements on the gain of Stokes and anti-Stokes components in the nonstationary regime

The influence of the scattering matrix elements that are responsible for the amplification of the Stokes and anti-Stokes components on the process of stimulated Raman scattering in the essentially nonstationary regime is analyzed. It is demonstrated that the optimum proportion between the scattering matrix elements, which maximizes the anti-Stokes component, is determined by the ratio between the optical frequencies of the components and the initial proportion between the intensities of the pump and initiating pulses. In the case of the optimum proportion between the scattering matrix elements, the variation in the excitation of a medium compensates the self-induced modulation of gain, and the efficiency of converting the radiation energy into the Stokes and anti-Stokes components increases substantially.     

Experimental study on the Stokes effect in disordered birefringent microstructure fibers

In this paper, a 120-fs pulse transmission experiment is carried out using disordered birefringent microstructure fibers with cladding ventages. Through this experiment, it is found for the first time that remarkable Stokes and anti-Stokes waves can also be produced when the central wavelength of the incident pulse is in the normal dispersion regime of the microstructure fiber. The generation of the two waves can be explained by the four-wave mixing phase matching theory. Properties of the two waves under the action of femtosecond laser pulses with different parameters are studied. The results show that the central wavelength of anti-Stokes waves and Stokes waves produced under the two orthogonal polarization states shift by 63 nm and 160 nm, respectively. The strengths and central positions of the two waves in birefringent fibers can be controlled by adjusting the phase match condition and the polarization directions of incident pulses.     

Generation of anti-Stokes radiation upon amplification of stokes pulses in transient stimulated Raman scattering

An analytical solution is obtained for equations of transient stimulated Raman scattering for short samples and a weak seed Stokes wave in the approximation of a given pump field, and a numerical solution for more extended media or more intense input Stokes pulses with allowance for pump depletion. The dependence of the anti-Stokes wave energy on its spatial mismatch with the Stokes and laser waves is studied. The optimum angle of the anti-Stokes generation is found as a function of the length of the system, transverse relaxation rate, laser pulse energy, and intensity ratio of the Stokes and pump pulses at the entrance of the sample.     

Non-Degenerate Four-Wave Mixing in Microstructure Fibres

Non-degenerate four wave mixing based on third-order susceptibility X^3 in high nonlinearity microstructure fibres is experimentally demonstrated. The Stokes and anti-Stokes peaks are observed simultaneously by launching 10-fs pulses from an 80Ohm Ti：sapphire laser into the fibre.     

Chirped-pulse four-wave Raman mixing in molecular hydrogen

Four-wave Raman mixing (FWRM) in molecular hydrogen was studied using chirped pump and Stokes pulses emitting at 802 and 1,203 nm, respectively. The group delay dispersion (GDD) of the anti-Stokes pulse was examined employing a frequency-resolved optical gating system at different GDDs of the pump and Stokes pulses (0 or ±1,000 fs²). As a result, the energy and the sign of GDD for the anti-Stokes pulse remained unchanged, when the pump and Stokes pulses had the GDD with the same sign. When the sign was not the same, the energy decreased and only the portion useful for resonant FWRM was converted into a Raman emission. This technique has a potential for use in compensation of dispersion by passing the negatively chirped high-order Raman sidebands through the optics with positive chirps in the spectral region from the deep-ultraviolet to the near-infrared, to generate multiple transform-limited Raman pulses and then to produce an ultrashort optical pulse by a Fourier synthesis of these Raman emissions.     

Multiwave stimulated raman scattering under quasi-phase-matching conditions

The effect of the generation of higher Stokes and anti-Stokes components of steady-state and transient stimulated Raman scattering (SRS) on the realization of quasi-phase-matching conditions in the course of generation of the first anti-Stokes SRS component is numerically analyzed by using a system of coupled equations derived with regard to diffraction effects. The calculations show that the application of a periodic layered structure in order to realize quasi-phase-matching conditions virtually does not reduce the anti-Stokes SRS efficiency and allows efficient conversion of pulses with a duration of 3 ns or longer. It is found that focusing of waves inside the medium allows partial compensation for diffraction spreading and provides for an increase in the efficiency of anti-Stokes SRS conversion, and, at a Fresnel number above three, the effect of diffraction on the realization of quasi-phase-matching conditions during SRS can be disregarded.     

Multifrequency parametric infrared Raman generation in KGd(WO(4))(2) crystal with biharmonic ultrashort-pulse pumping

Mutlifrequency parametric Raman generation was carried out in a KGd(WO(4))(2) crystal by use of a dual-wavelength Ti:sapphire laser system. It was found that with femtosecond pump pulses the efficiency of Raman generation is low because of the onset of self-phase modulation. The mechanism for suppression of stimulated Raman scattering by self-phase modulation is discussed. Employing 2-ps-long chirped pulses generated four Stokes and one anti-Stokes component.     

微结构光纤次芯中的四波混频过程

利用Ti:sapphire飞秒激光脉冲在微结构光纤包层的次芯中通过参量四波混频效应获得480—550nm的反斯托克斯波,转换效率可高达28%. 通过改变输入光的偏振方向可以调节反斯托克斯波的中心波长. 理论模拟了飞秒激光在次芯中的模式特性和色散特性,较好地解释了实验结果. 关键词： 微结构光纤 飞秒激光脉冲 参量四波混频过程     

Parametric solitons on Raman resonance and self-induced transparency in silica optical fibre

An analytical and numerical analysis of the dynamics of parametric solitons on Raman resonance is performed. The results of calculations describe an effect of self-induced transparency for parameteric solitons and the influence of the initial phase on Raman Stokes gain and anti-Stokes absorption. This can be used to maintain the process of energy transfer between pulses.     

Spectrally and temporally isolated Raman soliton features in microstructure fibers visualized by cross-correlation frequency-resolved optical gating

We study soliton phenomena accompanying the propagation of femtosecond Cr: forsterite-laser pulses through a microstructure fiber in the regime of efficient anti-Stokes frequency conversion. The dispersion of the fiber is designed in such a way as to minimize the group delay of the 1.25-m pump and the Stokes pulse within the length of soliton pulse compression in the regime of anomalous dispersion. Spectrally and temporally isolated solitonic features, resulting from soliton self-frequency shift, are detected at the output of such a microstructure fiber by means of cross-correlation frequency-resolved optical gating. PACS 42.65.Wi; 42.81.Qb     

Generation of Anti-Stokes Line in Fundamental Mode of Photonic Crystal Fibre

A photonic crystal fibre （PCF） with zero-dispersion wavelength around 800 nm is designed and fabricated. Simulated results show that the zero-dispersion wavelength of fundamental mode for this PCF is at 826nm, and phase-matched four-wave mixing can be achieved in fundamental mode. Using 20ors Ti：sapphire laser with central wavelength at 810nm as pump, the anti-Stokes line around 610hm ＆ generated efficiently. The output signal has a Gaussian-like profile, which indicates that the anti-Stokes signal is in the fundamental mode of the PCF. The energy of anti-Stokes signal is higher than that of residual pump laser and the maximum ratio of the anti-Stokes signal to the pump component in the output spectrum is estimated to be 1.2.     

Influence of gas circulation on stimulated Raman scattering and amplification of ultrashort laser pulses in methane

Applying gas recirculation in a high pressure cell, laser pulses of 1 ps at 400 nm and with a repetition rate of 1 kHz were frequency shifted by stimulated Raman scattering and amplification in methane gas at high pressure. We studied the influence of gas recirculation on the conversion efficiencies into the Stokes and anti-Stokes components as well as on their spatial distributions and spectral shapes using generator and generator-amplifier arrangements. For high pump energies, recirculation in the generator cell decreases conversion efficiency into the first Stokes component whereas it increases conversion into higher Stokes and anti-Stokes components. It results in a significantly improved spatial characteristics of the frequency-shifted radiation, however, is accompanied by a substantial spectral broadening. Using gas recirculation in the generator-amplifier arrangement we achieved a conversion efficiency into the first Stokes component of about 50% with highly improved spatial and spectral characteristics.     

Ionization penalty in nonlinear Raman neuroimaging

Light-assisted ionization accompanying coherent anti-Stokes Raman scattering (CARS) of ultrashort laser pulses in brain tissue is shown to manifest itself in a detectable blueshift of the anti-Stokes signal. This blueshift can serve as an indicator of ionization processes in CARS-based neuroimaging.     

New results on vibrational population decay in simple liquids

The population lifetimes of the symmetric CH₂-stretching mode of several methylene halides were measured after infrared excitation by spontaneous anti-Stokes Raman scattering of delayed picosecond probe pulses. Data ranging from 6.5 to 45 ps are reported. The results give evidence for vibrational energy transfer via anharmonic coupling to neighbouring vibrational combination bands and overtones.