保偏光纤中在不同频率区域拉曼效应和参量放大增益谱

根据考虑拉曼效应后的双折射光纤所满足的非线性相干耦合薛定谔方程, 推导出了当沿两个偏振轴入射两束不同波长的激光脉冲时所产生的增益表达式. 通过与入射相同频率的光脉冲所产生增益的对比, 在考虑拉曼效应的情况下, 讨论了入射不同频率光脉冲对增益谱的影响. 结果表明, 在正常色散区和反常色散区, 当输入两束激光脉冲频率不同时, 增益谱较输入相同频率激光脉冲时产生了明显的变化, 其外侧的斯托克斯部分和反斯托克斯部分增益峰, 随着群速度失配的增加强度明显加强、偏离中心频率, 可以用于提取太赫兹脉冲.当两偏振模处于不同色散区时, 增益谱与不考虑拉曼效应时也存在明显的不同, 增益谱的对称性遭到破坏, 斯托克斯部分的增益峰强度要明显高于反斯托克斯部分. 关键词： 不同频率区域 保偏光纤 拉曼效应 参量放大     

Continuous-wave rotational Raman laser in H(2)

A diode-pumped, far-off-resonance cw Raman laser in H(2) with rotational Stokes emission is reported for the first time to our knowledge. The Raman laser can produce single-wavelength emission at either 830 nm (rotational Stokes) or 1180 nm (vibrational Stokes) depending on the frequency tuning of the pump laser. The mirrors for the rotational cw Raman laser are easier to produce; the laser also exhibits a wider continuous tuning range and is less sensitive to thermal effects than the previously studied vibrational Raman laser [Opt. Lett. 26, 426 (2001) and references therein].     

Observation of 280 nm and 298 nm ultraviolet emissions in sodium vapor

Dye laser output around 578 nm is utilized to pump sodium vapor confined in a heatpipe. Fixed wavelength ultraviolet emissions at 280 nm and 298 nm are observed for the first time and can be explained by anti-Stokes Raman scattering and two photon emission respectively. In addition, there are tunable emission lines around 280 nm and 298 nm, the frequencies of which vary with pump laser wavelength. These tunable lines can be explained by four-wave mixing schemes.     

Comparison of Brillouin-Raman comb fiber laser in two different configurations

The properties of a Brillouin-Raman comb fiber laser are compared for two different configurations: co-propagating and counter-propagating Raman pump. The optical spectrum is compared for changing the Raman pump power and the power or the wavelength of seed laser. A Brillouin-Raman comb with 400 linewidth lasers in a flat-amplitude bandwidth of 32 nm between 1538 and 1570 nm, with an average optical power 20 dB above the nearby frequencies was generated. The lasers in the comb had an OSNR of 20 dB and a wavelength spacing of 0.08 nm. The results for the counter-propagating configuration were observed to have better quality.     

窄线宽纳秒脉冲光纤拉曼放大器的理论模型和数值分析

窄线宽纳秒脉冲光纤拉曼放大器在非线性频率变换、遥感探测和量子信息等领域有广泛的应用前景.综合考虑受激拉曼散射(stimulated Raman scattering,SRS)、受激布里渊散射(stimulated Brillouin scattering,SBS)、自相位调制(self-phase modulation)和交叉相位调制(cross-phase modulation)等非线性效应,建立了窄线宽纳秒脉冲光纤拉曼放大器的非线性动力学模型.仿真分析了放大器中脉冲激光的时频演化特性,对比研究了抽运脉冲宽度、光纤长度和信号光功率等因素对放大器性能的影响.研究发现,上述因素会影响放大器的SRS阈值、SBS阈值、输出激光线宽、激光转换效率等.例如,当脉冲宽度为800 ns时,SBS随着抽运功率的增加而发生,限制了激光功率的提升;减短抽运脉宽可以抑制SBS,但是输出激光的线宽易于展宽到数百MHz以上;增加光纤长度可以获得更低的SRS阈值和更高的转换效率,但是SBS效应和光谱展宽程度也随之增强.系统搭建中需要平衡各非线性效应,选择合适的系统参数.研究内容可以为窄线宽纳秒脉冲光纤拉曼放大器的设计搭建提供参考.     

Development of Raman-shifted probe laser beam for plasma diagnosis using polaro-interferometer

Optical diagnostics of laser-produced plasma requires a coherent, polarized probe beam synchronized with the pump beam. The probe beam should have energy above the background emission of plasma. Though the second harmonic probe beam satisfies most of the requirements, the plasma emission is larger at the harmonic frequencies of the pump. Hence, at high intensities we need a probe beam at non-harmonic frequencies. We have set up a Raman frequency shifted probe beam using a pressurized hydrogen cell that is pumped by the second harmonic of the Nd glass laser that operates at only one Stokes line of 673.75 nm.     

Generation of terahertz radiation in cubic non-centrosymmetric crystals

The conditions of parametric radiation generation on polaritons in cubic noncentrosymmetric crystals are studied. The possibility of such generation is theoretically justified. The polariton radiation frequencies are calculated for GaP, ZnSe, ZnTe, and GaAs crystals. The obtained generation frequencies are compared to the experimental results on Raman scattering on polaritons. The block diagram of the terahertz radiation generator operation using a GaP crystal and a pulsed laser with high peak power at low energy of laser pump pulses is presented. The lasing frequency shift is analyzed depending on the scattering geometry. The coefficient of exciting radiation conversion to the terahertz range is determined.     

Superradiant linear Raman amplification in plasma using a chirped pump pulse

A theoretical and numerical investigation of small-signal Raman backscattering from a chirped pump pulse in plasma shows that an ultrashort probe pulse will grow superradiantly, i.e., with an amplitude that scales with the propagation length while contracting self-similarly. These features are commonly associated with the nonlinear stages of Raman amplification in the pump depletion and Compton regimes. We show that the superradiant scaling results in very broad-bandwidth amplification due to gain distributed in frequency as well as spatially. Since different frequencies excite the plasma at different positions, wave breaking is avoided, and prepulses and pedestals are substantially suppressed. Linear chirped pulse amplification in plasma could provide a very broad-bandwidth alternative to solid state laser amplifiers, potentially usable for optical pulses a few cycles in duration.     

On the interaction of stimulated spin-flip raman scattering and stimulated recombination radiation in InSb

We have studied the interaction of stimulated spin-flip Raman scattering and stimulated recombination radiation in n-InSb using a CW CO pump laser. A change in linewidth and output power of second Stokes spin-flip radiation is observed when the tuning curve of the first Stokes stimulated spin-flip radiation and the stimulated recombination radiation cross. The observations are explained using a simple rate equation model.     

Off-resonance dispersion in molecular double resonance experiments

This paper analyses effects related to the real part of the gaseous electric susceptibility in microwave-microwave and infrared-microwave double resonance experiments on two transitions with a common level. The most interesting features are obtained when the gaseous refractive index is probed far off resonance, i.e. in the tail of the dispersion characteristic and when the pump frequency is scanned around the resonance: a resonant change of the refractive index for the probe field is observed, the shape of which is that of the saturated pump line. A calculation using a model of strong collisions gives results in good agreement with the experimental ones. Applications of these phenomena to the determination of frequencies and transition matrix elements of infrared lines are discussed in the general case of lack of overlapping of the laser gain profile and of the infrared line. It is proposed to use these dispersion effects to detect Doppler-free infrared two-photon molecular absorption.     

Effect of stimulated raman scattering on the formation of the random lasing spectrum of dyes

The nature of the line structure of the random lasing spectrum of vesicular films activated by dyes (rhodamine 6G, pyrromethene 597) has been analyzed. The spectral lines appear above the random lasing threshold and are manifested only within the spectrum of the amplified spontaneous radiation of dye molecules against the continuous-pedestal background. Their intensities are proportional to the product of the intensities of the pump and continuous spectrum at the frequencies of these lines, and the frequencies are exactly reproduced from pulse to pulse. The shifts of the lines are strongly correlated with the pump frequency and the frequencies of these lines coincide with the frequencies of the Raman scattering lines of dye molecules. Using these properties, it has been shown that the observed lines are due to stimulated resonant Raman scattering by dye molecules, which occurs simultaneously with the stimulated emission of these molecules. These two processes affect each other and jointly form a united nonlinear process where all of the oscillations active in Raman scattering are manifested.     

Ultrafast coherent population transfer driven by two few-cycle laser pulses

We investigate ultrafast coherent population transfer driven by few-cycle pump and Stokes laser pulses in the Λ-type three-level system with the stimulated Raman adiabatic passage technique beyond the rotating-wave approximation. In contrast to the case with the rotating wave approximation, the most efficient population transfer may be realized without the satisfaction of the one-photon resonances or two-photon resonance and the transfer efficiency depends critically on the Rabi frequencies and initial optical phases of the two laser fields when the peak Rabi frequencies are much larger than the respective transition frequencies. Moreover, complete and robust population transfer can still be obtained with the variations of the Rabi frequencies, pulse durations, and one-photon or two-photon detuning in a moderate range, though a considerable transient population may reside in the excited state. These abnormal behaviors result from the counterrotating terms, which are not taken into account in the traditional rotating wave approximation.     

Investigation and suppression of the pump-to-Stokes relative intensity noise transfer in chalcogenide waveguide Raman laser

The characteristic of pump-to-Stokes relative intensity noise (RIN) transfer is comprehensively investigated for integrated As?Se? waveguide Raman laser (As?Se?-WRL). It is found that, compared to its silicon counterpart, the RIN transfer is 5 dB lower across all frequencies for As?Se?-WRL, mainly due to its relatively smaller Raman gain coefficient. A bidirectional pumping scheme is proposed and verified as an effective configuration to suppress RIN transfer because doubling of the inverse round trip time eliminates the RIN transfer peak at the odd multiples of the resonance frequency. The optimization of waveguide length on RIN transfer is also performed, in which two distinct regions are identified due to different dominant physical processes. In addition, we show that RIN transfer in As?Se?-WRL can be further reduced by using a high cavity for both pump and Stokes waves.     

Stimulated Raman backward scattering of a laser beam in a magnetized plasma

Stimulated Raman scattering of a laser beam is investigated in the plasma with strong self generated magnetic field. The magnetized plasma supports various localized radial and azimuthal modes of lower hybrid frequencies. The density fluctuations due to lower hybrid modes couple with the oscillating velocity due to the pump, and drive the scattered wave. Equations describing the Raman process are derived and effects of various modes are studied on the growth rate analytically. Self generated magnetic field has a strong localization effect on the Raman process and growth rate is maximum for radial eigen mode number q = 0 and azimuthal eigen mode number l = 3. The frequency shift has signatures of self generated magnetic field and could serve as a diagnostic.     

Narrowband switchable dual-passband atomic filter with four-wave mixing optical amplification

By using Faraday optical filter combined with four-wave mixing (FWM) amplifier, a narrow bandwidth optical amplifying atomic filter with switchable dual-passband is demonstrated experimentally. The two transmission peaks of the filter correspond to the Stokes and anti-Stokes frequencies, exhibiting a Raman gain in 13- and 17-fold, respectively, with bandwidth of ～120 MHz. By properly setting pump laser detuning, switching between filter passbands is realized. We also investigate the dependence of peak transmission on both pump laser intensity and Rb cell temperature. This atomic filter can find practical applications in long-distance laser communications and laser remote-sensing systems.     

Coupled electron-phonon modes in optically pumped resonant intersubband lasers

Intersubband lasing at 12-16 microm based on a CO2 laser pumped stimulated resonant Raman process in GaAs/AlGaAs three-level double-quantum-well structures is reported. The presence, or lack of, lasing action provides evidence for resonantly coupled modes of collective electronic intersubband transitions and longitudinal optical phonons. An anticrossing behavior of these modes is clearly seen when the difference between the pump and lasing energies (i.e., Stokes Raman shift) is compared with the subband separation. This work reveals the significance of the strong coupling between intersubband transitions and phonons and raises a new possibility of realizing a phonon "laser."     

Nonlinear interference between two sets of frequency conversion processes with widely spaced pump wavelengths in an optical fiber

The processes of nonlinear interaction of two high-power pump waves with widely spaced frequencies are studied in an optical fiber. Substantially increased output power of the Stokes components, generated via stimulated Raman scattering or stimulated four-wave mixing by one of the pump waves have been obtained in the field of the other pump wave and its Stokes components. For the first time the possibility to excited new spectral lines in the spectrum of one of the pump waves with frequency shifts characteristic for the other one, is experimentally demonstrated.     

Numerical simulations of pulsed and quasi-cw regimes of synchronously pumped Raman oscillators

Synchronously pumped Raman oscillators allow for an efficient frequency conversion of pump pulses whose power is well below the threshold for extracavity stimulated Raman scattering (SRS). Numerical simulations show that the cavity mismatch of pump laser and Raman oscillator is critical for maximum conversion efficiency and minimum pulse duration for transient and stationary SRS. At a certain mismatch the simulations predict a quasi-cw Raman oscillator output even at moderate cavity quality factors. Optimum operation conditions are discussed for pump pulse trains of finite lengths. The results of the numerical calculations are in qualitative agreement with a simple phenomenological model. PACS 42.55.Ye; 42.65.Dr; 42.55.Lt     

Semi-analytical model of Raman generation in silicon-on-insulator rib waveguide with DBR/F-P resonator

An approximate method of modelling of Raman generation in silicon-on-insulator (SOI) rib waveguide with DBR/F-P resonator including spatial field distribution and nonlinear effects such as Raman amplification and two photon absorption (TPA), is developed. In threshold analysis of steady-state Raman laser operation, an analytical formula relating threshold pump power to the system parameters is obtained. The analysis of the above threshold operation is based on an energy theorem. In exact energy conservation relation, we approximate the Stokes field distributions by that existing at the threshold, whereas the approximate pump field distributions are obtained by integrating the equations for the pump signal using the linear (threshold) pump field distributions and the threshold Stokes field distributions. An approximate, semi-analytical expression related the Raman output power to the pump power and system parameters is derived. Our calculations remain in a good agreement with the exact numerical solutions.     

远红外可调谐喇曼激光器理论

本文用缀饰原子方法解出四能级系统远红外激光增益系数的解析表达式。结果表明,远红外增益谱一般由四个共振峰组成,取合适的失谐量,可能得到两个正的增益极大,并分别对应于喇曼增益和线中心增益。