拉曼泵浦光源退偏技术研究

文中利用琼斯(Jones)矩阵和斯托克斯参量分别对几种拉曼泵浦光源退偏方式进行了理论分析,并对实际的激光器光谱进行了偏振度的数值计算.在采用基于偏振合光(PBC)的方法进行退偏时,只有当两激光器的光功率相同时才可以得到最小的偏振度;在采用偏振合光加双折射晶体退偏(PBCD)时,计算表明,随着晶体长度的变化,偏振合波后的偏振度总小于单只激光器通过晶体后的偏振度,因此晶体长度的设计应为使单只激光器退偏所要求的晶体长度,这样可保证不同泵浦情况下的偏振度.文中的设计方法和结论对拉曼放大器的泵浦光源退偏设计具有一定参考作用.     

Impulsive pump depletion in saturated Raman amplifiers

The analytical results available for counter-propagating pump distributed Raman amplifiers in the undepleted pump approximation are shown to be easily extended to the signal-saturated case by using effective values of both input pump and signals     

Dispersion-compensating Raman/EDFA hybrid amplifier recycling residual Raman pump for efficiency enhancement

We experimentally demonstrate a novel concept of the dispersion-compensating Raman/erbium-doped fiber amplifier hybrid amplifier recycling residual Raman pump for increase of overall power conversion efficiency. The proposed dispersion-compensating hybrid amplifier system has only one pump source for Raman amplification in the dispersion-compensating fiber (DCF) and the residual pump power after the DCF is recycled for secondary signal amplification in an erbium-doped fiber cascaded to the DCF. Using the proposed scheme, we achieve the significant enhancement of both signal gain and effective gain-bandwidth by 15 dB (small signal gain) and 20 nm, respectively, compared to the performance of the Raman-only amplifier.     

Multiple pass stimulated Raman conversion with pump depletion

In this work we derive an analytical expression for the efficiency of stimulated Raman conversion applicable to Gaussian pump and Stokes beams in a multiple-pass cell. Pump depletion and mirror reflectivity are taken into account. The result provides a simple expression for calculating the conversion efficiency versus the number of passes through a Raman cell. It is especially useful in the design of multiple-pass cells for infrared Raman conversion in which the gain is low and many passes are required.     

New Raman pump module for reducing pump-signal four-wave-mixing interaction in co-pumped distributed Raman amplifiers

We propose a new Raman pump module that effectively reduces four-wave-mixing (FWM) nonlinear interaction among longitudinal pump modes and wavelength-division-multiplexed (WDM) signals in co-pumped distributed Raman amplifiers based on nonzero dispersion-shifted fibers (NZ-DSFs). Theory and experiments confirm that the proposed pump module structure, based on polarization- and wavelength-multiplexed Fabry-Perot lasers, allows distributed amplification with high co-propagating on-off Raman gain, providing a key technology for all-Raman-based dense-WDM ultralong-haul transmission.     

皮秒多脉冲泵浦KGW红外多波长拉曼激光器

报道了多脉冲机制泵浦KGW晶体的皮秒红外多波长拉曼激光器。建立数学模型研究了多脉冲泵浦机制对拉曼活性分子振动模式的影响,模拟结果显示了拉曼活性分子对多脉冲泵浦机制的响应相比于传统的单脉冲泵浦机制更加活跃与持久,从而促进衰减的分子振荡多次回到本征振动频率。多脉冲的增强效果有利于改善拉曼增益,降低拉曼阈值,提高拉曼转换效率。设计了皮秒多脉冲泵浦KGW拉曼晶体实验,结果表明,三脉冲泵浦机制将拉曼增益改善了2倍以上,拉曼阈值降低了50%以上,768 cm–1和901 cm–1拉曼模式的转换效率分别提高了16%和22%以上。基于三脉冲泵浦机制,设计了1 kHz毫焦级皮秒多脉冲泵浦KGW晶体红外多波长拉曼激光器,对于768 cm–1和901 cm–1两个振动模式,获得脉冲能量分别为1.39 mJ和1.38 mJ,最大拉曼转换效率分别为29.6%和25.7%。此外,对于两种拉曼模式,此拉曼激光器都可以同时输出多达8条红外拉曼光谱,涵盖范围800~1 700 nm。     

Raman beam cleanup of a severely aberrated pump laser

Distortion-free amplification of a diffraction-limited (D.L.) Stokes beam in a hydrogen Raman amplifier pumped by a severely aberrated XeCl laser (120 × D.L.) has been observed with an attendant power conversion efficiency of the order of 30 percent. The corresponding increase in available far field intensity over that from the aberrated pump beam is 5000. An optical integrator was used to focus the poor quality pump beam into the amplifier and to remove all near-axial components in the pump field. Numerical study of this process using a two-dimensional propagation code shows that the presence of near-axial pump components can cause phase matched four-wave mixing interactions with the Stokes, leading to increased angular divergence of the amplified Stokes beam and the development of secondary sidebands in the far field. When a moderately aberrated pump beam (20 × D.L.) was used, spatial sidebands of the Stokes beam were generated due to increased coherence length for the mixing process, significantly reducing the far field Stokes intensity.     

Novel methods for fast fiber Raman amplifier pump configuration

Traditional heuristic algorithms are time consuming in searching the pump configuration to obtain a wide-band flat-gain fiber Raman amplifier (FRA). In this paper, two methods are proposed to deal with this problem. One is called combined generic algorithm (CGA). By using the quadratic programming algorithm instead of generic algorithm (GA) to solve the pump integral, the search space dimension is reduced by half, and obviously, this method will converge faster. Based on a linear time-invariant (LTI) system model, another method is also presented under the assumption that the effective area of fiber is weakly dependent on stokes wave. Then, the amplifier gain can be viewed as the output of a special LTI system, which characterizes the normalized Raman gain profile as its unit impulse response and the pump power integral impulse sequence in frequency domain as its input. By using the nonnegative constraint least-square error (LSE) and clustering technique, we will solve this problem quickly. Numerical simulations shows that the total computation time will be halved at the cost of a slight deterioration in gain flatness.     

The dependence of Raman gain on pump laser bandwidth

The dependence of Raman gain upon laser bandwidth is analyzed in terms of a set of coupled mode equations. The analysis predicts that the Raman gain is independent of pump laser bandwidth within limitations set by dispersion in the Raman medium. These results are verified for rotational and vibrational transitions in H2using a variable bandwidth Nd:YAG laser.     

The stimulated Raman scattering threshold for anondiffraction-limited pump beam

The threshold power of stimulated Raman scattering (SRS) has been calculated for various nondiffraction-limited pump beams using a numerical model. These calculations were performed for the following pump beam profiles: Gaussian-Hermite, Gaussian-Laguerre, Gaussian-Schell-model (GSM), and several superpositions of Gaussian-Hermite modes. It is shown that the threshold power, for a tightly focused beam, depends on the M² factor, which is a dimensionless factor that compares the divergence of a given beam with the divergence of a (diffraction-limited) Gaussian beam, related to the pump beam quality. The influence of the exact pump beam profile on the threshold is small. It is shown that the measured SRS thresholds in methane for two pump beams, one with an M² of 1.12 and the other with M²=8.7, correspond with the calculated thresholds for two GSM beams with the same M²     

Time-division multiplexed Raman pump experiment using a tunableC-band laser

In this letter, we report on the first experimental demonstration of a time-division multiplexed frequency-swept Raman pump used to adaptively flatten the gain of a Raman amplifier. By means of a single, but tunable Raman pump laser operating in the C-band, an ON-OFF Raman gain flatness of 0.6 dB over 15 nm (or 1.5 dB over 30 nm) at an average ON-OFF Raman gain of 22 dB could be achieved for a pump wavelength range of 22 nm     

Spin saturation and pump depletion in continuous spin-flip Raman oscillation

A saturation model of stimulated Raman scattering, of general applicability, is put forward. The model is developed with reference to spin-flip electronic Raman scattering in InSb, where the number of excitations available for Raman scattering may be small by comparison with the incident photon numbers. A rate equation technique is used to evaluate the steady-state Stokes intensity and output powers for a Gaussian pump beam. The model is extended to take account of pump depletion and the relative importance of depletion, and saturation in limiting the conversion efficiency under different conditions is brought out. By comparison with continuous wave spin-flip measurements, values are obtained for the spin-relaxation time (τs) associated with spin reversal in InSb. For 10¹⁶free carriers per cubic centimeter at 36 kG,tau_{s} sim 12ns and for 10¹⁵cm^-3at 16.9 kG,tau_{s} sim 1.3ns.     

多波长激光器用做拉曼泵浦的初步研究

借助简化的拉曼光纤放大器数学模型,利用平均功率法和打靶法分别计算出了不同参数下多波长泵浦的拉曼增益。在对增益图分析的基础上,可以得到多波长激光器作为拉曼泵浦,它的三个典型参数(波长间隔、初始功率和光波个数)中的任一参数的变化都会引起拉曼增益的波动,并且为了达到增益均衡的目的,波长间隔不能太小、光波个数不易太少以及不能片面追求高初始功率。与多泵浦系统相比,它需要优化的参数大为减少,运算速度提高,并用一个泵浦源就能满足需求,降低了成本,具有发展的潜力。     

Dispersion compensating Raman amplifiers with pump reflectors for increased efficiency

The effect of retroreflecting the pump in discrete Raman amplifiers is investigated. Simulations are used to estimate the increase in gain and efficiency and decrease in multipath interference due to double Rayleigh backscatter for typical module configurations. The effect of reflecting only a portion of the pump spectrum is considered. Experimental measurements confirm the expected increase in gain. Finally, the effect of pump-signal relative intensity noise transfer is investigated experimentally.     

RIN transfer analysis in pump depletion regime for Raman fibreamplifiers

A numerical analysis is presented that models the relative intensity noise transferred from a Raman pump to a signal laser in an optical fibre. The analysis is valid in both the pump-depletion and nondepletion regimes. The RIN requirements for a 0.1 dB Q-penalty in the pump depletion regime are similar to those in the nondepletion regime, exhibiting a RIN requirement reduction of only 3-4 dB/Hz     

Calculating the Raman pump distribution to achieve minimum gain ripple

A procedure is presented for calculating the optimum Raman pump wavelengths and powers for a discrete set of lasers and for a pump continuum. Theoretical results in the continuum case shown that broad-band gain with extremely low ripple can be obtained.     

基于双向二级喇曼抽运的偏振控制研究

为了发展具有实用价值的受激喇曼全光偏振控制方法,根据光纤中受激喇曼散射矢量理论和光纤随机双折射效应设计了基于双向二级喇曼抽运的全光偏振控制方案,构建了偏振吸引理论模型,采用严格数值迭代算法对该偏振控制方案进行了仿真分析,并完成了偏振控制方案最佳工作性能的优化设计。结果表明,当信号光波长为1550nm、功率为0.1mW、一级抽运光和二级抽运光的功率分别为1W和4W、其偏振态都为（0,1,0）、光纤长度为3km时,信号光偏振度可达0.85以上,且信号光强度波动低于35%。该方案有效降低了作用光波功率、减弱了信号光输出强度噪声,并提高了信号光输出偏振度。     

Raman amplifier based long-distance, remote FBG strain sensor with EDF broadband source recycling residual Raman pump

A novel and simple Raman amplifier based long-distance fibre Bragg grating (FBG) sensing system without any additional broadband light source is demonstrated. With the simplified system configuration recycling residual Raman pump power for generation of amplified spontaneous emission in an erbium-doped fibre (EDF), the high quality of remote strain sensing operation at a location of 50 km is achieved.     

抽运功率配置对双向喇曼放大器性能的影响

为了研究抽运功率配置对双向抽运光纤喇曼放大器性能的影响,基于已有的耦合方程,采用数值仿真方法,分析了不同抽运功率配置对双向抽运光纤喇曼放大器的增益、增益饱和以及抽运功率转换效率的影响。结果表明,双向抽运光纤喇曼放大器的增益、增益饱和以及抽运功率转换效率特性均介于同向和反向抽运光纤喇曼放大器之间,并且随着同向抽运功率在抽运总功率中所占比例的升高,增益、增益饱和功率和抽运功率转换效率的数值增加;大信号、抽运功率较大时,抽运功率配置对双向抽运光纤喇曼放大器性能的影响显著。这对双向抽运光纤喇曼放大器和光纤激光研究具有一定的参考价值。     

Interaction of spatially modulated pump and FIR waves in Raman laser

The propagation of spatially modulated IR pump and FIR generated waves interacting in the active gas medium of Raman laser is simulated numerically. Stationary nonlinear diffraction of monochromatic beams is studied for the isolated homogeneously broadened three-level system without any limitations on the values of field strength and frequency offset. It is shown that FIR radiation is trapped in the active wave guide induced owing to the nonlinear refractive index mechanism and gain inhomogeneity across the pump wave beam. Nonlinear focus positions and on-axis focal intensities for both fields are calculated.