Impact of fibre nonlinearities in optical DWDM transmission systems at different data rates

The impact of major fibre nonlinearities, like stimulated Raman scattering (SRS) and four wave mixing (FWM), in cascaded amplifier dense wavelength division multiplexing (DWDM) transmission system, has been studied at different data rates. SRS has been calculated considering pulse walk off effect. Analysis has been carried out to evaluate signal to noise ratio (SNR) considering the combined effect of SRS and FWM in the presence of amplified spontaneous emission (ASE) noise to achieve minimum noise at different data rates.     

Effect of pulse walk off and data formats on stimulated Raman scattering in fiber optic communication systems

A novel numerical method has been presented here which is capable of taking into account pulse walk off effect and different data formats such as non return to zero (NRZ), return to zero (RZ) and optical Manchester while evaluating the effect of stimulated Raman scattering (SRS) in dense wavelength division multiplexed (DWDM)-based fiber optic communication systems. Power tilt because of SRS effect in the DWDM systems has been calculated at different data rates and data formats.     

Study of the competition between forward and backward stimulated Raman scattering in methane

Simultaneous forward and backward stimulated Raman scattering (SRS) in methane pumped by a single longitudinal mode Nd:YAG laser at 355 nm has been investigated both experimentally and numerically. For forward SRS, a maximum energy-conversion efficiency of 55% was obtained under 24 atm, while for backward SRS the corresponding figure was 58% under 4.7 atm. Under low pressures or at low pump energies, the conversion efficiency for backward SRS was found to be much higher than that for forward SRS, while the situation is quite the contrary under high pressures and at high pump energies. A quasi-two-dimensional numerical model is introduced to elucidate the competition between forward and backward SRS; we conclude that the high-pressure and high-pump-energy experimental results should be caused by the thermal defocusing effect in the SRS process. This is confirmed by the experimental fact that a decrease of the pulse-repetition rate increases the conversion efficiency for backward SRS but decreases that for forward SRS under high pressures. Our calculated results are in good agreement with the experimental results at a pulse-repetition rate of 2 Hz. PACS 42.55.Ye; 42.65.Dr; 42.65.Es     

Features of image reconstruction with phase conjugation in stimulated scattering within an extended active volume and at the surface

The important role of the near-surface layer and the active-medium surface for stimulated scattering and for image reconstruction in stimulated-scattering beams is demonstrated.     

Investigation of wavefront reconstruction by a two-lens system

A two-lens system was used to obtain wavefront reconstruction (WFR) by stimulated Raman and Brillouin backscattering with the beam focus varied over a large range of distances. A fixed scattering object was placed between two lenses, and the focus of the two-lens system was varied continuously from near the second lens, through the image of the scattering object on to infinity by moving the first lens accordingly. In all prior experiments reported in literature the image of the object was formed beyond the lens focus. Images reconstructed by stimulated Raman and Brillouin backscattering were obtained and their positions and magnifications were in agreement with the holographic model.     

Spontaneous and backward stimulated Raman scattering of light in methane

The initial phase of development of stimulated Raman scattering (SRS) in the backward direction in compressed methane (the excess of the SRS intensity over the signal of spontaneous scattering in the experiment is ～100) has been studied theoretically and experimentally. The theory, developed on the basis of simple assumptions about the mechanism of SRS formation from spontaneous scattering, shows good agreement with experiment. The implicit consideration of backward SRS is shown to be significant for the conditions of depletion of the pump energy due to its continuous loss in the medium and conversion into SRS.     

Pump spectral linewidth influence on stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) and self‐termination behavior of SRS in liquids

The threshold, temporal behavior, and conversion efficiency of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SBS) in three liquids (benzene, hexane, and dimethyl sulfoxide) and two crystals (calcite and barium nitrate) have been investigated under three largely different spectral linewidth conditions. Pumped with 532‐nm and nanosecond duration laser pulses of ≤ 0.01 cm^?1 linewidth, only SBS can be generated in all tested liquids with a high nonlinear reflectivity. However when the pump spectral linewidth is ～0.07 cm^?1 or ～0.8 cm^?1, both SBS and SRS can be observed in benzene while only SRS can be generated in dimethyl sulfoxide; in all these cases SRS is the dominant contribution to the stimulated scattering but the efficiency values are drastically decreased due to the self‐termination behavior of SRS in liquids, which arises from the thermal self‐defocusing of both pump beam and SRS beam owing to Stokes‐shift related opto‐heating effect. In contrast, for SRS process in the two crystals, the thermal self‐defocusing influence is negligible benefitting from their much greater thermal conductivity, and a higher conversion efficiency of SRS generation can be retained under all three pump conditions.

     

Effect of four-wave mixing in WDM optical fibre systems

Wavelength division multiplexing (WDM) can both significantly enhance transmission capacity and provide more flexibility in optical network design. Through the use of erbium-doped fibre amplifiers (EDFAs), it is possible to build long-distance transparent optical transmission links without electrical regenerators. In such systems, fibre nonlinearities are likely to impose a transmission limit due to increased total interaction length. There are a number of optical nonlinear effects in optical fibres, such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), carrier-induced phase modulation and four-wave mixing (FWM). Out of these SRS and FWM are the dominant effects. In this paper, an algorithm has been suggested to study the effect of FWM in the total system noise considering the combined effect of SRS and FWM in the presence of amplified spontaneous noise (ASE). It has been found from the study that to maximize the signal-to-noise ratio of the transmitted signal in a WDM system FWM noise needs to be reduced as this is the dominant noise factor.     

Dependence of parabolic pulse amplification on stimulated Raman scattering and gain bandwidth

An analytical model has been developed and verified by numerical simulations to determine limits induced by stimulated Raman scattering (SRS) on parabolic pulse evolution in high-power, high-energy Yb-fiber amplifiers. Our results show that the maximum achievable parabolic pulse energies are limited by SRS at low amplifier gains and by the finite gain bandwidth at high gains. Therefore, an optimum gain value exists that maximizes the achievable output pulse energy.     

Transition from the Fluctuation Mode of Stimulated Resonant Coherent Raman Scattering to the Energy–Stabilized Mode

The equations of the semiclassical theory for the stimulated resonant coherent Raman scattering (SRS) in an extended medium have been solved numerically. The distribution function of the energy of Stokes pulses at different detunings and energies of a laser has been determined. Just as for the nonresonant coherent SRS, in the situation considered there occurs transition from the large–scale fluctuation mode to the mode of relatively small straggling of the energies of Stokes pulses with increase in the pumping energy. With approach to the resonance, the energy required to stabilize the Stokes radiation decreases.     

Spectral power handling capability caused by stimulated Raman scattering effect in silica optical fibers

Spectral characteristics of the stimulated Raman scattering (SRS) in silica fibers was studied for the purpose of obtaining higher power transmission capability in single-mode silica optical fibers. When the input power exceeds a critical input power, which depends on the wavelength, nonlinear transmission loss caused by SRS gives rise to excess loss and signal waveform distortion. The SRS effect is reduced under the condition of sufficiently large spectral loss at the first Stokes wavelength.     

Enhanced stimulated Raman scattering in H<Subscript>2</Subscript> by seeding an SRS line into an H-ɛ Balmer line: Amplification-coupled laser wavelength shifter

The stimulated Raman scattering (SRS) in H₂ gas above the dissociation energy limit was recorded using a 266-nm UV laser. All of the observed Stokes and anti-Stokes SRS lines showed a normal behavior except the third Stokes SRS lines at 397.8 nm, which showed a substantial intensity enhancement of about a 36%-conversion efficiency of the pump energy. This enhancement in the SRS line is attributed to the seeding of the SRS line into the Balmer H-? line at 397 nm in molecular hydrogen. To the best of our knowledge, there is no report of any work on enhanced stimulated Raman scattering in H₂ by the seeding of the H-? Balmer line into the SRS line and attaining a very high intensity at the third Stokes SRS lines at 397.8 nm. The cell pressure and the laser pulse energy dependence of these SRS lines substantiate our explanation.     

Brechzahländerungen durch Schwingungsanregung beim stimulierten Raman-Effekt

The influence of stimulated Raman scattering (SRS) on refractive index due to excitation of vibrational levels of molecules has been estimated for Hydrogen and carbon disulphide. A comparison is given between the changes of refractive index caused by SRS, orientation of molecules and electrostriction. There are shown conditions under which it should be possible to observe such changes and therefore polarizabilities of excited levels.     

1064 nm激光抽运单模光纤受激喇曼散射的理论分析

基于已有理论模型,利用1 064 nm 连续波抽运源在不同抽运光强下对不同长度光纤抽运所产生的受激喇曼散射现象进行数值模拟,并对模拟结果作了分析.研究发现:发生受激喇曼散射现象时,抽运光强和光纤长度发生兑换;能量红移现象普遍存在,包括同阶Stokes光谱内部和不同阶的Stokes光谱之间.抽运光强越大,能量红移现象越明显.     

Raman Microscopy and Associated Techniques for Label-Free Imaging of Cancer Tissue

Abstract

Raman spectroscopy can identify cancerous from healthy tissue, with a chemical analysis from the measurement of vibrational bond frequencies. However, to detect small tumors a form of Raman imaging is required. Such imaging—by acquiring a Raman spectrum at each imaging pixel—can detect tumors but is rather slow. Multiphoton versions of Raman—anti-Stokes Raman scattering (CARS) microscopy and stimulated Raman scattering (SRS) microscopy—offer similar accuracies in identifying cancerous tissue and tumor margins but with a far higher speed, which is beneficial for diagnosis of small tumors in tissue. SRS microscopy can also be used to image extrinsic molecules in living cells, such as anti-cancer drugs at typical concentrations.     

三倍频Nd∶YAG激光抽运氧气中的受激拉曼和布里渊散射

报道了三倍频脉冲Nd∶YAG激光(355 nm)在两种不同带宽模式下抽运氧气中受激拉曼散射(SRS)和受激布里渊散射(SBS)的实验研究。在宽带(约1 cm-1)抽运模式下,只测到了前向受激拉曼散射,而没有观察到后向散射,其一级和二级斯托克斯最大能量转换效率可达22%和8%。在窄带(约0.003 cm-1)模式下,前向、后向受激拉曼散射和受激布里渊散射都测量到了,但大部分抽运能量都转换到受激布里渊散射,其转换效率可达67%。测量了两种带宽模式下各散射组分在它们最佳转换时的波形;窄带情况下后向受激拉曼散射和受激布里渊散射的脉宽分别可压窄至1.5 ns和2.3 ns,不到抽运脉宽的三分之一,使得受激布里渊散射峰值功率可大大高于抽运功率。对氧气中前向、后向受激拉曼散射和受激布里渊散射之间的竞争进行了讨论。     

三倍频Nd:YAG激光抽运氧气中的受激拉曼和布里渊散射

报道了三倍频脉冲Nd：YAG激光（355nm）在两种不同带宽模式下抽运氧气中受激拉曼散射（SRS）和受激布里渊散射（SBS）的实验研究。在宽带（约1cm^-1）抽运模式下,只测到了前向受激拉曼散射,而没有观察到后向散射,其一级和二级斯托克斯最大能量转换效率可达22％和8％。在窄带（约0．003cm^-1）模式下,前向、后向受激拉曼散射和受激布里渊散射都测量到了,但大部分抽运能量都转换到受激布里渊散射,其转换效率可达67％。测量了两种带宽模式下各散射组分在它们最佳转换时的波形;窄带情况下后向受激拉曼散射和受激布里渊散射的脉宽分别可压窄至1．5ns和2．3ns,不到抽运脉宽的三分之一,使得受激布里渊散射峰值功率可大大高于抽运功率。对氧气中前向、后向受激拉曼散射和受激布里渊散射之间的竞争进行了讨论。     

Crostalk in WDM SCM Systems due to SRS

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Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.     

Modification of spatial and temporal gains of stimulated brillouin and raman scattering by polarization smoothing

Significant reductions of stimulated Brillouin (SBS) and Raman (SRS) scattering are measured by adding polarization smoothing (PS) to a random phase plate smoothed beam. The associated plasma waves, as measured by Thomson scattering, are spatially and temporally modified and reveal that the interplay between SBS and SRS has to be taken into account to understand the effect of the smoothing. The results also support the numerical simulations [S. Huller et al., Phys. Plasmas 5, 2706 (1998); R. Berger et al., Phys. Plasmas 6, 1043 (1999)] predicting a reduction of self-focusing with PS, resulting in a decrease of the maximum laser intensity and consequently of SBS and SRS gains.