Polarization decay of pulses of electromagnetically induced transparency on J=0→J=1→J=2 degenerate quantum transitions

The evolution of radiation under conditions of electromagnetically induced transparency in the scheme of degenerate quantum transitions J = 0 → J = 1 → J = 2 in the pulsed interaction regime of the fields and with allowance for the Doppler broadening of spectral lines has been analyzed numerically. It has been shown that, if the input coupling radiation is linearly polarized, the circularly polarized input probe pulse splits in the medium into pulses with mutually perpendicular linear polarizations. The direction of polarization of one of these pulses coincides with the direction of polarization of the input coupling field. The distance that the probe pulse travels in the medium until it completely decays decreases with a decrease in both the duration of the input probe pulse and the intensity of the input coupling radiation. A change in the power of the input probe pulse hardly affects the distance required for the decay and the velocity of propagation of linearly polarized pulses in the medium. An increase in the Doppler broadening of spectral lines leads to a decrease in this distance and, simultaneously, to an increase in the energy losses of the probe radiation. Qualitative considerations that explain the physical reason for the investigated effects have been presented.     

Compression of single-cycle mid-infrared pulses by Raman-active molecular modulators

We show theoretically how high-order stimulated Raman scattering in the impulsive pump-probe regime can be used for generation of single mid-infrared (MIR) single-cycle pulses. The propagation of MIR probe pulses in a hollow waveguide filled with a Raman-excited gaseous medium, with a probe delay in the maximum of the molecular oscillations, results in spectral broadening covering almost 2 octaves. The spectral phases of this broadening can be compensated for by use of an output glass window with anomalous dispersion in the MIR. The spectral and temporal characteristics of the output pulses and the mechanism of pulse compression are studied by use of numerical and analytical solutions, and compression of a 70-fs input pulse at 4 microm to a single-cycle 6.5-fs output pulse is shown.     

The spectral broadening of a blue optical pulse and the modulation of a pulse waveform in a photonic crystal fiber by induced-phase modulation

By copropagating a fundamental pulse and a blue second-harmonic pulse from a Ti:Sapphire oscillator in a photonic crystal fiber (PCF), the spectral broadening of the blue second-harmonic pulse from 380 to 600 nm has been observed by use of induced-phase modulation (IPM) at a 78-MHz repetition rate. From the experimental and the calculated delay time dependence of spectral intensities, it was inferred that the largest spectral broadening was observed when the second-harmonic pulse interacted with the fundamental pulse near the input end of a PCF, where the fundamental pulse was compressed temporally due to self-phase modulation and negative group velocity dispersion. From the simulation, the mechanism of spectral broadening was clarified and the fission process of the fundamental pulse was shown to be influenced strongly by IPM.     

A study of the influence of the input electrical power on the spectral width of the 510.6 nm line of an atomic copper vapor laser

The effect of the input electrical power on the spectral width of the 510.6 nm line of an atomic copper vapor laser (CVL) is investigated. An analysis of the gas temperature inside the discharge tube and the line broadening mechanism of the CVL is reported. The input electrical power was varied from 2.0 to 4.2 kW in a cylindrical discharge tube of inner radius 2.35 cm and length 150.0 cm. A Fabry–Perot etalon and imaging camera-based setup interfaced with personal computer was used to measure the spectral width of the 510.6 nm (green) laser line. The Doppler broadened spectral profile of the laser emission varies with input electrical power and an additional broadening of almost 1 GHz at the highest operating input power was observed.     

Delay limit of slow light in an optical fiber

We show that coherent population oscillations effect produces a very narrow spectral hole in the absorption spectrum. The large dispersion of the refractive index associated with this hole permits us to achieve a group velocity as low as 1496.25 m/s at room temperature in an erbium-doped fiber. When the input intensity is equal to the saturation intensity, the dispersion is optimal. The optimal dispersion corresponds to the maximum fractional delay. Therefore, the input intensity can be used as a control parameter to increase the fractional delay. Our theoretical results based on population oscillation agree very well with the experimental data. In addition, we confirm that the spectral hole experiences power broadening for optical fibers of different lengths.     

Spectral broadening of femtosecond pulses in an nonlinear optical fiber amplifier

A promising method for the generation of a supercontinuum with a high spectral power density based on the spectral broadening of ultrashort pulses in a fiber amplifier is considered. The advantage of the method, as compared to the conventional way of the supercontinuum generation in a microstructure fiber, is a lower pulse spectral broadening rate, which allows one to achieve higher SC spectral power densities. The initial stage of the supercontinuum generation in an ytterbium fiber amplifier (a fiber core diameter of 7 μm) with side pumping from an array of laser diodes with a total power of 8 W at a wavelength of 976 nm is experimentally studied. Yb:KYW laser pulses with a duration of 250 fs, a central wavelength of 1046 nm, and an average power of 150 mW have been supplied to the input of the amplifier. In this case spectrally broadened radiation with an average spectral power density of higher than 65 mW/nm and a spectrum width of 50 nm has been obtained.     

Optimization of supercontinuum generation in photonic crystal fibers for pulse compression

A theoretical study of super generation in photonic crystal fiber and its application to pulse compression is presented. The evolution of the spectrum can be divided into three stages: initial broadening below a certain threshold propagation distance, dramatic broadening to a supercontinuum at a threshold distance, and, finally, saturation of the spectral width on propagation. It is found that the group delay and group-delay dispersion of the supercontinum are sensitive to the input pulse peak power after further propagation at the third stage. Fluctuations from the input pulse are amplified and translated into fluctuations and time shift of the compressed pulses. There exists an optimum compressed distance at which compressed pulses with negligible fluctuation and time shift can be obtained.     

Recombination emissions and spectral blueshift of pump radiation from ultrafast laser induced plasma in a planar water microjet

We report spectroscopic investigations of an ultrafast laser induced plasma generated in a planar water microjet. Plasma recombination emissions along with the spectral blueshift and broadening of the pump laser pulse contribute to the total emission. The laser pulses are of 100 fs duration, and the incident intensity is around 10¹⁵ W/cm². The dominant mechanisms leading to plasma formation are optical tunnel ionization and collisional ionization. Spectrally resolved polarization measurements show that the high frequency region of the emission is unpolarized whereas the low frequency region is polarized. Results indicate that at lower input intensities the emission arises mainly from plasma recombinations, which is accompanied by a weak blueshift of the incident laser pulse. At higher input intensities strong recombination emissions are seen, along with a broadening and asymmetric spectral blueshift of the pump laser pulse. From the nature of the blueshifted laser pulse it is possible to deduce whether the rate of change of free electron density is a constant or variable within the pulse lifetime. Two input laser intensity regimes, in which collisional and tunnel ionizations are dominant respectively, have been thus identified.     

Dynamics of the spectral width of intense laser pulses consisting of several field oscillations in optical waveguides

The dependence of the rms spectral width of a light pulse consisting of several light-field oscillations on the distance passed in an optical waveguide with arbitrary dispersion and nonresonant electronic nonlinearity has been derived. This dependence allows one to rapidly predict the scenarios of the initial evolution of the spectrum (broadening, distance independence, or compression) by using the input pulse parameters and waveguide characteristics. It is shown that the pulse spectral width increases when the enrichment of the spectrum due to the generation of multiple harmonics is taken into account. In this case, for pulses with the spectrum in the region of the anomalous group dispersion of the waveguide, there is the intensity range for which the self-narrowing of the main spectral peak around the central radiation frequency is characteristic.     

High-Power Extracavity Pulse Compression in Solid Materials

A novel technique for high-power extracavity pulse compression with a nonlinear solid material is demonstrated. Before spectral broadening by self-phase modulation in the solid material, a short filament generated in argon is used as a spatial filter, which works for a uniform spectrum broadening over the spatial profile. Compensated by chirped mirrors, a 15-fs pulse is generated from a 32-fs input laser pulse. A total transmission larger than 80% after the solid material is achieved.     

不同微场分布函数下的光谱线型

根据光谱学,总的光谱线型是各种加宽机制的卷积结果。考虑到等离子体中的离子碰撞,Stark加宽本质上是一种非对称的光谱线型, 其中微场分布函数对光谱线型起着非常关键的作用。该文利用不同的微场分布函数研究了微场分布函数对总的光谱线型的影响。研究结果表明,在电子加宽参数和离子加宽参数很小时,不同微场分布函数对整个光谱线型的影响基本相似,但随着离子加宽参数的增加,3种不同的微场分布函数对Stark光谱线型的影响逐渐增加; 随着电子加宽参数的增加,不同的微场分布函数对Stark光谱线型的影响也逐渐增加; 但总体上Holtsmark分布和Nearest-Neighbor场分布下的光谱线型差别较小,但是Mayer模型对光谱线型影响较大。特别是,当离子加宽参数较大时,Mayer模型对光谱线型的影响异常明显,这也正说明离子间碰撞剧烈时对光谱线型的影响很大。该结果对等离子体参数诊断有一定参考价值。     

Generation of intense ultrabroadband optical pulses by induced phase modulation in an argon-filled single-mode hollow waveguide

We experimentally demonstrate the generation of intense ultrabroadband optical pulses whose spectrum ranges from 300 to 1000 nm (700-THz bandwidth) with a well-behaved spectral phase and 23-muJ pulse energy by a novel, simple setup utilizing induced phase modulation (IPM) in an argon-filled single-mode hollow waveguide. Fundamental as well as second-harmonic pulses produced by one common femtosecond pulse from a Ti:sapphire laser-amplifier system are copropagated in the hollow waveguide. The effect of the delay time between the two input pulses on the IPM spectral broadening is clarified and confirmed to agree with the theoretical result. It is found that the compressed pulse duration from this pulse is 1.51 fs if its phase is completely compensated for.     

钕玻璃宽带激光放大能量提取效率研究

基于钕玻璃宽带脉冲激光放大模型,采用数值模拟的方法,研究了钕玻璃放大器对不同输入光通量、不同脉宽以及不同带宽(或波长分布)脉冲激光放大的能量提取效率。计算结果表明,钕玻璃宽带放大能量提取效率随着输入光通量的增加而提高,并最终趋于某一定值。对于以均匀加宽为主的介质,随着带宽的增加,能量提取效率逐渐下降;对于以非均匀加宽为主的介质,随着带宽的增加,能量提取效率先逐渐升高,达到最大值后开始下降。对于均匀加宽与非均匀加宽线宽比为0.1的混合加宽介质,在饱和通量输入条件下,使用宽带激光能够带来大约80%的效率提升。     

Optimal design for a flat-top AWG demultiplexer by using a fast calculation method based on a Gaussian beam approximation

Passband broadening of an AWG (array waveguide grating) demultiplexer with an MMI (multimode interference) coupler connected at the end of a tapered input waveguide is considered. An explicit formula based on the field propagation of an approximate Hermit-Gaussian beam is used to calculate quickly and reliably the spectral response of the AWG demultiplexer. The widths of the input waveguide, the output waveguides and the MMI coupler are optimized. The optimal design is verified with the experimental measurement.     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。     

Generation of single and multiple dissipative soliton in an erbium-doped fiber laser

We experimentally report on the generation of single and multiple dissipative soliton via nonlinear polarization rotation technique. The spectrum of the mode-locked dissipative soliton exhibits typical steep edges with a flat top; the pulse duration is 10.07 ps. It is found that with the pump power increasing from 110 mW to 161 mW, the top of the mode-locked spectrum becomes flater and the 3-dB spectral bandwidth is broadened, which indicates that the gain-dispersion effect is lowered under stronger pump. However, the full bandwidth of the spectrum is narrowed, which proves that the spectral filter effect increases and overcomes the effect of self-phase modulation induced spectral broadening. Such a phenomenon was not noticed nor reported before. Our experiment also demonstrates that the pulse interval is highly dependent on the input pump power: with pump power increasing, the pulse interval tends towards more uniform. So our observation qualitatively analyzes the relationship between mode-locked pulse characteristics and input pump power.     

Infrared extension of femtosecond supercontinuum generated by filamentation in solid-state media

We carried out experimental and numerical study of the ultrafast supercontinuum generation by filamentation of 120 fs, 800 nm laser pulses in sapphire under variable focusing conditions. We demonstrate that the supercontinuum spectra produced in loose focusing conditions have much larger infrared extent, while the spectral blueshift remains fairly the same. Our numerical simulations reproduce the experimental results in great detail and disclose that the input beam focusing conditions affect only the nonlinear propagation of the leading sub-pulse, which emerges after the pulse splitting event, and which is responsible for the redshifted spectral broadening.     

Blue light and infrared continuum generation by soliton fission in a microstructured fiber

The nonlinear propagation of ultrashort pulses in a microstructured fiber is experimentally investigated. By working around 800 nm, in the anomalous dispersion region, clear evidence of pulse break-up and soliton propagation is obtained. This is consistent with the recently suggested mechanism of spectral broadening based upon the fission of higher order solitons into red-shifted fundamental solitons and blue-shifted dispersion waves. When 190-fs pulses at high input intensities are used, the output spectrum is made of a broad infrared supercontinuum coexisting with a sharp and very intense blue peak that takes up to 24% of the input power. We tentatively propose an explanation of this effect by invoking pulse-trapping phenomena controlled by the group-velocity matching of infrared and visible pulses. PACS 42.65.Tg; 42.81.Dp     

不同入射脉冲强度线性啁啾对BaF2晶体交叉偏振波输出特性影响的数值模拟研究

在超短飞秒激光技术中, 基于BaF₂晶体三阶非线性效应的交叉偏振波技术是目前最重要的提高脉冲时域对比度的手段之一. 针对不同入射脉冲强度下入射脉冲的线性啁啾对BaF₂晶体交叉偏振波的输出特性, 包括转换效率、光谱加宽和时域压缩的影响进行了详细的数值模拟和讨论.特别地, 详细对比研究了非饱和及强入射脉冲情况下线性啁啾对交叉偏振波输出脉冲光谱加宽的影响. 结果表明, 在强入射脉冲情况下, 交叉偏振波的光谱加宽量从非饱和情况下相对于零点线性啁啾时的对称结构, 变得不再具有对称性, 并对产生非对称性的原因进行了详细的物理解释.     

Enhancement of filamentation postcompression by astigmatic focusing

The energy scaling up of pulse postcompression is still an open issue. In this work we analyze the use of astigmatic focusing to improve the output pulses in a filamentation based postcompression setup. Unlike spherical conditions, astigmatic focusing enhances the output energy and the spectral broadening of the filament. This is due to the increase of critical power, allowing a considerable improvement of the postcompression energy and stability in a simple way. We demonstrated compression from FWHM 100 fs, 10 nm, 3 mJ input pulses to 13 fs, 142 nm, near 1 mJ pulses.