Stimulated Raman scattering in the visible with a multipass cell

In experimental studies of stimulated Raman scattering in a multipass cell (MPC) with a frequency-doubled single-mode Nd:YAG laser at 532 nm, it is found that only forward Stokes generation is observed and that the growth and saturation of the Stokes beam are in agreement with a simple plane wave, transient theory scaled to account for the focusing, and the multiple passes. The absence of second Stokes generation, backward Stokes generation, and anti-Stoke generation is discussed in terms of the low gain per pass in the MPC. It is concluded that the MPC provides an ideal system for studying stimulated Stokes scattering with visible lasers without the four-wave processes usually seen in a single-focus geometry     

Synchronisation effects in broadband FIR raman scattering in NH3

By using a short pulse broad band (～2 GHz) CO₂ laser around a single well detuned line (15 GHz), it is possible to analyse, with sub-ns resolution, the dynamics of a broadband FIR Raman scattering. When the Raman gain is large, well modulated ns emissions are detected also for resonant lines. Generally the pseudo cavity effects, are always present. They have important effects on the gain, in particular an aligned input window has a large positive effect on the Raman gain and on the generation of mode-locked FIR forward emissions     

Amplification Effect on Rayleigh Scattering and SBS in 25 km Distributed Fiber Raman Amplifier

The amplification effect on stimulated Brillouin scattering （SBS） and Rayleigh scattering in the backward pumped G652 fibers Raman amplifier have been researched. The signal source is a tunable narrow spectral bandwidth （〈10 MHz） ECL laser and is pumped by the tunable power 1427.2 nm fiber Raman laser. The Rayleigh scattering lines are amplified by fiber Raman amplifier, and Stokes stimulated Brillouin scattering lines are amplified by fiber Raman amplifier and fiber Brillouin amplifier. The SBS lines total gain is a production of the gain of Raman and the gain of Brillouin amplifier. In experiment, the gain of SBS is about 42 dB and the saturation gain of 25 Ion G652 backward FRA is about 25 dB, so the gain of fiber Brillouin amplifier is about 17 dB.     

Saturation and depletion effect of Raman scattering in opticalfibers

The analytical solution of nonlinear power coupled equations involving three waves is given. The study reveals that the Raman gain saturation effect caused by the generation of the second Stokes has to be taken into consideration in light amplification by stimulated Raman scattering, though it can be ignored in spontaneous stimulated Raman scattering. It is also shown that it is advantageous to feed the fiber with a small low-order Stokes signal in amplifying the higher-order Stokes signal for easing the requirement of high pump power and long fiber length. A model to calculate higher-order Stokes radiation in low-loss optical fibers is presented     

Nonlinearity in fiber transmission

Procedures are presented for estimating critical powers for nonlinear optical processes in single-mode fiber transmission systems. Crosstalk due to Raman gain in multiplexed systems can appear at powers of a few mW. The effects of self-phase modulation and stimulated Brillouin scattering can appear around 100 mW while typical stimulated Raman threshold powers are a few watts.     

Influence of the Stimulated Raman Scattering on the Four-Wave Mixing Process in Birefringent Fibers

The influence of the stimulated Raman scattering in the four-wave mixing process in high birefringent fibers is theoretically analyzed. We consider the dual pump configuration in both co-polarized and orthogonal polarization schemes and treat simultaneously the stimulated Raman scattering and the four-wave mixing processes. The obtained results are valid even when the shift between the pumps and signal lies inside the Raman band. Results show that when the phase-matching condition is achieved the generation of the idler wave is mostly dependent on the real part of the fiber nonlinear response function, whereas the amplification of the signal wave is mostly dependent on the imaginary part. Results also show that when the phase-matching condition is satisfied the optical power evolution of the signal and idler waves for small frequency detunings are mostly described by the anisotropic Raman response of the fiber, whereas for high frequency detunings the isotropic response dominates. Our analysis also shows that the stimulated Raman scattering can increase the efficiency of the four-wave mixing process, for certain frequency detunings.     

脉冲光布里渊散射信号的拉曼放大研究

实验研究了前向拉曼泵浦方式下脉冲信号光产生的自发布里渊散射信号和受激布里渊散射(SBS)信号的拉曼放大规律。拉曼泵浦放大自发布里渊散射信号时,随泵浦功率增大会出现SBS现象,对散射信号的放大由拉曼放大和布里渊放大两部分引起,因此增益较大,当拉曼泵浦功率为1 000 mW时Stokes光增益可达54 dB。拉曼泵浦放大SBS信号时,放大过程中只存在拉曼放大。且当泵浦功率增大至600 mW时,会引起多级布里渊散射,致使一级Stokes和泵浦能量会转移到下一级布里渊散射,一级Stokes光增益饱和并下降。     

一种借助散射截面获得拉曼增益系数的理论方法

从拉曼散射几率出发，推导出拉曼散射增益系数和散射截面的关系，并通过仿真模拟，调节参数，得出了增益系数和频移之间的关系曲线，并将其与实验曲线作比较，给出结果的定性分析。     

Raman amplification for fiber communications systems

Raman amplification has enabled dramatic increases in the reach and capacity of lightwave systems. This tutorial explains why, starting with the fundamental properties of gain from stimulated Raman scattering. Next, noise accumulation from amplified spontaneous emission is reviewed, and the merits of distributing Raman gain along a transmission fiber are explained. Other sources of noise that are particularly relevant for Raman amplifiers are summarized. Finally, novel Raman pumping schemes that have recently been developed are highlighted.     

宽带拉曼光纤放大中的多泵浦源选择方法

理论分析了拉曼光纤放大谱与多波长泵浦光功率和波长的关系.考虑到泵浦光与泵浦光,泵浦光与信号光的拉曼相互作用,得出一个增益数组,增益数组就是离散的宽带拉曼光纤放大增益谱.对任何形态的宽带拉曼光纤放大增益谱,可以设定对应的增益数组,把泵浦光功率和波长组成二维坐标系,遍取坐标系中的各点,求出各组点对应的增益数组,找出最接近设定的增益数组,这组泵浦光功率及波长就是最优的解决方案.     

G652光纤中背向自发拉曼散射的增益特性

讨论了背向自发Raman散射脉冲在自生分布式G652光纤Raman放大器中传输的增益特性。实验发现，反Stokes Raman（ASR）和Stokes Raman（SR）自生分布式脉冲光纤Raman放大器的阈值抽运峰值功率是25．4W和18．0W。在入射功率为52W时，ASR和SR的增益分别为5．0dB和8．6dB。放大的反Stokes和Stokes背向自发Raman散射光时域反射（OTDR）曲线上放大的阈值时间位置随激发功率的增高前移并具有规律性。放大的ASR背向自发散射强度受光纤温度调制，具有温度效应，已应用于远程分布光纤Raman温度传感器系统。     

Amplified spontaneous Raman scattering in fiber Raman amplifiers

Amplified spontaneous Raman scattered light power in forward and backward Raman amplifiers is theoretically and experimentally studied under the assumption of the constant Raman gain coefficient in the bandwidth of an optical filter. On the basis of the results, the power ratio of the amplified signal to amplified spontaneous scattering is discussed. As a result, it is clarified that the effect of amplified spontaneous Raman scattering on the signal to noise ratio for backward Raman scattering is larger than for forward Raman scattering.     

Gain reduction measurements in transient stimulated Ramanscattering

Threshold energy measurements of transient rotational stimulated Raman scattering are compared to Raman conversion calculations from semiclassical theories using a simple concept of a gain reduction factor which expresses the reduction of the gain from its steady-state value due to transient scattering. Excellent agreement was found with theoretically derived gain reduction in the transient and highly transient case     

Near- to far-infrared tunable Raman laser

The importance of stimulated Raman scattering of tunable laser beams in molecular fluids for generating powerful infrared radiation tunable from the near to the far infrared (FIR) is demonstrated. Main criteria (frequency shift, transparency, and Raman gain) leading to the choice of the best Raman medium for a given tunable laser are discussed. For the high power (400-1000 MW), monochromatic (0.02 cm^-1), and broadly tunable (0.72-1.09 mum) ruby pumped dye laser described here, gaseous hydrogen is chosen for the1-10 mum wavelength region yielding powers ranging from 1 to 200 MW. For mid-infrared (IR) generation (10-18 mum), one more Raman shifting in liquid nitrogen permits one to obtain up to 1.7 mJ in the 16 μm region. For longer wavelengths the use of resonantly enhanced stimulated Raman scattering in hydrogen fluoride gives rise to the generation of powerful radiation (2 times 10^{17}photons per pulse of 2.5 ns duration) tunable over half of the40-250 mum spectral range.     

硫化铅掺杂石英玻璃光纤喇曼增强特性研究

普通单模光纤的喇曼增益低,严重制约了喇曼放大器的发展。因此,研究高喇曼增益的光纤具有重要意义。研究了硫化铅掺杂石英玻璃光纤的喇曼散射增强特性。采用改进的化学气相沉积(MCVD)法分别制备出硫化铅掺杂石英玻璃光纤和普通单模光纤样品,并测得其传输损耗谱和喇曼光谱,实验结果表明:硫化铅掺杂石英玻璃光纤具有更强的喇曼散射强度。在不同的泵浦功率条件下,分别进行了喇曼放大实验,相比于普通单模光纤,硫化铅掺杂石英玻璃光纤具有更大的喇曼增益。     

Subnanosecond pulse generation in the VUV by dual-wavelength pumpedstimulated rotational Raman scattering

A novel scheme to generate a subnanosecond pulse in VUV spectral region is proposed, by applying a dual-wavelength pumping method in conjunction with polarization switching technique to the stimulated rotational Raman scattering process. Optimum conditions for the efficient conversion and short pulse generation are presented through numerical model calculations. It is theoretically shown that the duration of the first anti-Stokes pulse of a VUV pump laser can be shorten to 0.72 ns. Since the anti-Stokes pulse is generated at a frequency within the gain profile of the pump laser, the proposed process is applicable to a seed pulse generation for subnanosecond pulse amplification by the pump laser medium in the VUV region     

Gain saturation in fiber Raman amplifiers due to stimulatedBrillouin scattering

The effects of stimulated Brillouin scattering (SBS) on Raman gain are explored. Measurements of gain in a fiber Raman amplifier show a saturation at low gain levels. Experimental data and a theoretical model are presented, demonstrating that this saturation is due to pump depletion by SBS. This effect also leads to Raman gain fluctuations arising from mode partitioning in a multimode pump laser. Two ways to avoid the deleterious effects of SBS on the performance of Raman amplifiers are suggested. One is to use a multimode semiconductor laser with a modal linewidth of the order of a few hundred megahertz, which leads to a strongly reduced Brillouin gain coefficient. The second approach is to use very short pump pulses in a backward configuration     

On the permanent dipole-induced two-level Raman scattering insemiconductor quantum wells in electric field

The permanent dipole-induced two-level Raman scattering in semiconductor quantum wells in a static electric field is analyzed. Calculations performed for GaAs/Al_xGa_1-xAs multiple quantum wells (MQWs) indicate that reasonable, but not high, Stokes wave amplification could be obtained, and eventually used for IR generation. The gain is limited primarily by reststrahlen absorption and a rather large transition linewidth     

铌酸锂掺杂石英光纤拉曼增强特性研究

为了获得高拉曼增益特种石英光纤,提高拉曼光纤 放大器增益,本文研究了铌酸锂掺杂石英光纤的拉曼散射增强特性,采用改进化学气相沉积(MCVD)法制备出了铌酸锂掺杂石英光纤样品,其在1550nm 波长处的传输损耗为5dB/km。测得了铌酸锂掺杂石英光纤的拉曼光谱,并与相同长度的常 规单模光纤拉曼 光谱进行了对比,发现铌酸锂掺杂石英光纤具有更高的拉曼散射强度,在几个主要峰位处, 铌酸锂掺杂石 英光纤的拉曼散射强度为常规单模光纤的3.8～4.8倍。同时,搭建了 拉曼光纤放大器系统,分别测得铌酸 锂掺杂石英光纤和常规单模光纤的拉曼散射放大增益。实验结果表明,铌酸锂掺杂石英光纤 拉曼放大增益 约是常规单模光纤的2倍。因此,铌酸锂掺杂石英光纤作为拉曼光纤放大器的增益介质将可 取代常规单模 光纤,并能有效地提高拉曼光纤放大器的放大效果。     

A Theory of the Multimode Resonant Raman Scattering Compared with the Experiment

The dynamics of the Resonant Raman Scattering (RRS) of multimode broadband lasers is described and resolved without neglecting the phase and cavity effects. The multimode pump structure leads to an equivalent Raman structure by mixing interaction. When the stokes and the pump modes have the same phase, the forward multimodal Raman scattering shows a gain equivalent to the gain of a single mode pump with an equivalent mean power, so that in a Raman resonator the Raman modes can be synchronously amplified to the saturated power until the de phasing destroys the Raman conversion. All these effects have been experimentally observed. Besides an incoherent scattering process is always present in competition with the coherent one.