New regime of fs-pulse stimulated Raman scattering

We present the results of experimental and theoretical studies of fs-pulse Raman-frequency conversion in gases in the regime of medium pumping with pulses shorter than the molecular vibrational period T_v. The spectral transformation of both intense excitation pulses and variably delayed injection pulses is analyzed as a function of laser parameters and gas pressure. For intense 40 fs pump pulses propagating in a SF₆-filled hollow waveguide, we observed a continuous (pressure-dependent) redshift of the carrier frequency, amounting to several thousand cm^у. By transmitting a weak injection pulse with a narrow spectrum through impulsively excited SF₆, we observed multiple Stokes and anti-Stokes lines of the injection radiation corresponding to scattering on the symmetric vibrational mode of SF₆. Since the pumping and scattering processes are decoupled in this delayed-pulse scheme, the effect of Kerr self-action on the injection field scattering was completely eliminated. Our physical model of the fs-pulse stimulated Raman scattering is in good quantitative agreement with the observed effects.     

Fluctuations in the nonlinear regime of stimulated Raman scattering

We investigate the distribution of Stokes pulse energies in the transient nonlinear regime of stimulated Raman scattering. Nonlinear effects lead to a stabilization of Stokes pulse energies. The character of the transition from the chaotic linear regime to the nonlinear regime is studied in detail. The results compare nicely with recent experiments of Walmsey and Raymer.     

Driven spatially autoresonant stimulated Raman scattering in the kinetic regime

The autoresonant behavior of Langmuir waves excited by stimulated Raman scattering (SRS) is clearly identified in particle-in-cell (PIC) simulations in an inhomogeneous plasma. As previously shown via a 3-wave coupling model [T. Chapman et al., Phys. Plasmas 17, 122317 (2010)], weakly kinetic effects such as trapping can be described via an amplitude-dependent frequency shift that compensates the dephasing of the resonance of SRS due to the inhomogeneity. The autoresonance (AR) leads to phase locking and to growth of the Langmuir wave beyond the spatial amplification expected from Rosenbluth's model in an inhomogeneous profile [M. N. Rosenbluth, Phys. Rev. Lett. 29, 565 (1972)]. Results from PIC simulations and from a 3-wave coupling code show very good agreement, leading to the conclusion that AR arises even beyond the so-called weakly kinetic regime.     

Temporal characteristics of phase conjugation fidelity in stimulated Brillouin scattering

The pulse compression and phase conjugation characteristics of stimulated Brillouin scattering (SBS) in CS₂ were investigated in a focusing geometry using an iodine laser as pumping source. The highly compressed front part of the Stokes pulse was found to have better phase conjugation fidelity than the uncompressed tail part. This result is explained by considering the difference in the interaction length and spatial structure of the front and tail parts of the Stokes pulse.     

Stabilization of stokes pulse energies in the nonlinear regime of stimulated Raman scattering

An experimental study of quantum mechanical pulse energy fluctuations in stimulated Raman scattering from hydrogen is presented. The probability density function P(W) for Stokes pulse energy W is measured for highly transient scattering in both the linear and nonlinear gain regimes. While large pulse energy fluctuations (100%) occur in the linear gain regime, the fluctuations are reduced to about 20% in the nonlinear regime where the laser pulse is depleted significantly. The results are in excellent agreement with the theoretical predictions of Lewenstein.     

The buildup of stimulated Raman scattering from spontaneous Raman scattering

We present a quantum electrodynamic theory which shows for the first time how stimulated Raman scattering builds up, both spatially and temporally, from spontaneous Raman scattering. We analytically solve operator Maxwell-Bloch equations to find the Stokes field operator, and thus the Stokes intensity, for both monochromatic and broad-band (phase diffusion) pump lasers.     

Low-power phase conjugation based on stimulated Brillouin scattering in fiber amplifiers

Phase-conjugating stimulated Brillouin scattering (SBS) mirrors are frequently used to improve the brightness of solid-state lasers. With a scheme for designing fiber amplifiers as SBS phase conjugators the power requirements can be decreased by orders of magnitude; yet only low coherence is required, and high fidelity is reached. A peak power of 5 W was demonstrated to be sufficient for optical SBS phase conjugation.     

Effect of stimulated Brillouin scattering on the gain saturation of distributed fiber Raman amplifier and its suppression by phase modulation

For distributed fiber Raman amplifiers(DFRAs), stimulated Brillouin scattering(SBS) can deplete the pump once occurring and consequently generate gain saturation. On the basis of such a theory, theoretical gain saturation powers in DFRAs with various pump schemes are obtained by calculating SBS thresholds in them, and the experimental results show that they are in excellent agreement with the calculation results. The saturation power of the DFRA with a 300 m W forward pump is as low as 0 d Bm, which needs to be enhanced by phase modulation, and the effect is quantitatively studied. A simple model taking both modulation frequency and index into consideration is presented by introducing a correction factor to evaluate the effect of phase modulation on the enhancement of saturation power. Experimentally, it is shown that such a correction factor decreases as the modulation frequency increases and approaches zero when the modulation frequency becomes high enough. In particular, a phase modulation with a modulation frequency of 100 MHz and a modulation index of 1.380 can enhance the saturation power by 4.44 d B, and the correction factor is 0.25 d B, in which the modulation frequency is high enough. Additionally, the factor is 1.767 d B for the modulation frequency of 25 MHz. On this basis,phase modulations with various indexes and a fixed frequency of 25 MHz are adopted to verify the modified model, and the results are positive. To obtain the highest gain saturation power, the model is referable. The research results provide a guide for the design of practical DFRAs.     

Mechanism of phase conjugation via stimulated Brillouin scattering in narrow band gap semiconductors

We develop a theoretical model to study optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in narrow band gap transversely magnetized semiconductors. Threshold value of pump electric field and reflectivity of the image radiation for the onset of OPC-SBS are estimated. The analysis is applied to both cases viz. centrosymmetric (CS) and non-centrosymmetric (NCS) crystals. Numerical estimates made for n-type InSb crystal at liquid nitrogen temperature duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser shows that high OPC-SBS reflectivity (90%) can be achieved in NCS crystals at moderate pump electric fields if the crystal is used as an optical waveguide with relatively large interaction length (L = 5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.     

Chaos in stimulated Raman scattering

Coherent state technique and maximal Lyapunov exponent method are used for the study of deterministic chaos in a system governing stimulated Raman scattering. The influence of the external pumping on dynamics of the system is investigated. Phase portraits and FFT spectra are presented.The author would like to express his sincere thanks to prof. J. Peina for his comments and stimulating discussions.     

Applications of stimulated Raman scattering

The applications of stimulated Raman scattering reviewed in this paper include: determination of spontaneous Raman parameters and nonlinear susceptibilities by gain measurements and coherent anti-Stokes Raman spectroscopy, excitation of molecular vibrations and lattice waves and measurement of their relaxation times, generation of intense tunable light in the infrared region of the spectrum. The main results of the theory of stimulated Raman scattering relevant to the understanding of these applications are discussed.     

Studies of stimulated Raman scattering

The dependence of the excitation threshold and the intensity of the first Stokes component on the properties of the scattering material, the concentration of its molecules, and the exciting light intensity has been studied. It has been found that the intensity of stimulated Raman scattering lines depends exponentially on the intensity of the exciting light and the concentration of the scattering molecules. This dependence is in agreement with the theory developed.The author are gratful to P. A. Bazhuln, N. G. Basov, and A. M. Prokhorov for the present study.     

Influence of the non linear refraction index on the phase conjugation efficiency in stimulated scattering effects

The efficiency of the phase conjugated wave produced by stimulated backward scatterings has been calculated with the non linear propagation equation formalism. In the absence of optical Kerr effect, the phase conjugated efficiency remains smaller than 1. However, the calculations show that this efficiency may become much higher than 1 if the Kerr effect is taken into account.     

Soliton experiments in stimulated Raman scattering

Experimental observations of solitons in stimulated Raman scattering are reported. Soliton formation resulted from the introduction of a phase shift in the incident Stokes beam as predicted by theory. Pulse sharpening and retardation on propagation in the Raman medium have been observed along with amplitude diminution. The first two features were predicted and the third was not. Spontaneous soliton formation has been observed in the absense of any amplitude modulation or apparent phase shift in the optical fields, indicating that additional sets of initial conditions may result in soliton formation.Work performed under the auspices of the U.S. Department of Energy.     

Efficient stimulated Raman scattering in silicon

Stimulated Raman scattering in silicon was investigated at liquid helium temperatures. A Q-switched single mode Nd: YAG laser was used to generate Stokes radiation at 1.127 μm. Time resolved measurements give a maximum power conversion efficiency of 20%. The effects of intensity induced losses are discussed.     

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.