Propagation of shape-preserving optical pulses in inhomogeneously broadened multi-level systems

We study stable propagation of multiple shape-preserving optical pulses in an inhomogeneously broadened multi-level atomic medium. By analytically solving the Maxwell-Schr?dinger equations governing the evolution of N coupled optical fields and atomic amplitudes we show that N pulsed optical waves coupling to (N+1)-levels can be automatically matched with the same soliton waveform and identical yet very slow propagation velocity. Several sets of coupled soliton solutions for two different (N+1)-level models are given and their stability is studied by using a numerical simulation.     

Soliton-induced transparency of inhomogeneously broadened three-level atoms

The effect of soliton-induced transparency (SOIT) that arises for a weak pulse that interacts with one of allowed transitions of inhomogeneously broadened three-level atoms with the V configuration of the levels under the action of a self-induced transparency soliton that simultaneously interacts with the adjacent transition is studied theoretically. The effect is explained by phasing of the ensemble of dipoles oscillating with different frequencies in the soliton field. The weak pulse is locked by the soliton, so that both pulses propagate with the same velocity independently of the ratio of the propagation constants. If the latter are equal, the weak pulse loses no energy; when they are different, the rate of energy loss is considerably slower than it follows from Beer’s law.     

Excitation performance in inhomogeneously broadened systems

The problem of predicting through numerical simulation the performance of pulse sequences applied to inhomogeneously broadened spin systems is examined. Numerical or analytical calculations of the dependence of magnetization on frequency do not lead immediately to the actual form of the spectrum in the inhomogeneously broadened case. To predict the spectrum it is usually necessary to proceed through the time domain and to take into account the fact that the experimental data of negative times are usually not accessible. These results are particularly relevant to the optimization of pulse sequences for multislice imaging at zero gap. Experimental results from high-resolution NMR and magnetic resonance imaging verify these ideas.     

Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification

Optical parametric amplification of multi-frequency seed pulses generated in a mixture of compressed hydrogen and methane by stimulated Raman scattering of 1 ps, 1 kHz laser pulses at 395.8 nm has been studied. Efficient generation of spectrally narrow ultrashort pulses with a spatial distribution close to the Gaussian profile of the pump beam was obtained in the visible and near infrared ranges.     

Reaching the nonlinear regime of Raman amplification of ultrashort laser pulses

The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements.     

Multiple spin echoes in inhomogeneously broadened NMR systems

New types of spin echoes produced by refocusing the magnetic moments dephased during a long rf excitation pulse are observed in inhomogeneously broadened NMR systems. The experimental results are in good agreement with the theoretical predictions.     

Divided-pulse amplification of ultrashort pulses

We propose and demonstrate a new approach, to the best of our knowledge, for avoiding nonlinear effects in the amplification of ultrashort optical pulses. The initial pulse is divided longitudinally into a sequence of lower-energy pulses that are otherwise identical to the original, except for the polarization. The low-intensity pulses are amplified and then recombined to create a final intense pulse. This divided-pulse amplification complements techniques based on dispersion management.     

Quantum theory of stimulated raman scattering in an inhomogeneously broadened three-level gaseous system

A quantum-statistical treatment of stimulated Raman scattering in a gaseous system is presented using a density-matrix formalism. The molecular (atomic) system is described by three energy levels. Both atomic system and the radiation fields are quantized. The effects of atomic motion and detuning are incorporated in the analysis. Higher order nonlinearities and loss terms are included to render the problem more realistic. The equations of motion describing the photonstatistics of pump and Stokes fields are obtained. The equation, without detailed balance, is solved in the steady-state by a slowly varying function technique in the case of two variables. The steady state characteristics of the Stokes field are studied. The coherence properties, occurrence of antibunching phenomena are studied for different initial distributions.     

Sliding pump method for generation and amplification of ultrashort light pulses

A method for the generation of an ultrashort Raman pulse train with peak power considerably in excess of the pumping one is proposed and theoretically developed. It is shown that it can be used for obtaining tunable picosecond pulses in Raman fiber laser.     

Nonresonant echo spectroscopy of inhomogeneously broadened two-level systems

Effects which arise in inhomogeneously broadened systems upon nonresonant excitation are examined theoretically. The effect of such excitation on the signals representing a decaying free polarization and a two-pulse echo is discussed. The onset of echo signals generated in two-pulse retarded nutation during the application of a field is studied. The response can acquire a multicomponent structure. This structure is manifested in the appearance of a single-pulse echo, eight satellites in the two-pulse echo, and four nutation echo signals in the retarded two-pulse nutation. An experimental study of these effects is reported. The possibility of using them to determine relaxation times is analyzed.Institute of Solid-State and Semiconductor Physics, Academy of Sciences of Belarus. Scientific-Research Institute of Problems of Semiconductor Physics, Belarus State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 59–71, July, 1993.     

Dynamics of pulsed saturation in inhomogeneously broadened two-level systems

On the basis of Bloch's equations an analytical expression is obtained for the pulsed saturation signal of a two-level quantum system with allowance for inhomogeneous line broadening. It is shown that in this case the signal drop is exponential over the initial portion and nonexponential for long times. It is found that starting from a certain time, which depends on the field magnitude, relaxation parameters, and inhomogeneous width of the line, a through appears in the line contour, the depth and width of which increases with an increase in pulse duration. At smaller durations of the pulse, when the line is bell-shaped, an exponential decay of the signal indicates saturation of homogeneously broadened line. Comparison of the results obtained in an experiment on pulsed saturation in ruby shows that at large amplitudes of the field and considerable inhomogeneous broadening, one observes a deviation from the Bloch theory manifesting itself as a biexponential drop in the pulsed saturation signal. A conclusion is drawn that the Bloch theory satisfactorily describes the experiment until the effective time of energy transfer to the lattice is smaller than the time of the spectral propagation of excitation. Institute of Solid-State and Semiconductor Physics, Academy of Sciences of Belarus, 17, P. Brovka St., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 197–203, March–April, 1997.     

Exact solution for the gigantic amplification of ultrashort pulses in counterpumped Raman amplifiers

We exactly solve the initial-boundary value problem of interaction of three waves in the limit when one of these waves is strongly damped. The solution is applied to the characterization of transient effects in Raman amplifiers, with a special emphasis on the possibility of generating Stokes pulses with peak powers that are orders of magnitude higher than the input power of the pump beam.     

Ultraslow light propagation in an inhomogeneously broadened rare-earth ion-doped crystal

We show that coherent population oscillations effect allows us to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits us to achieve a group velocity as low as 2.7 m/s with a transmission of 40%. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40% to 90%, and the light group velocity from 2.7 m/s to 100 m/s.     

Correlation echoes in the stochastic excitation of inhomogeneously broadened two-level systems

The third-order cross-correlations between a free induction signal of an inhomogeneously broadened two-level system and white Gaussian noise exciting this system are studied. The temporal properties of the third-order cross-correlation functions are found to correspond to the characteristics of ordinary two-and three-pulse spin and light (photon) echoes excited by determinate radio pulses. The nonlinear properties of correlation echoes are studied as functions of the noise pulse parameters. It is established that the correlation echo amplitude is determined not only by the noise pulse parameters but also by the position on the time axis of the noise counts that form the given type of echo. Finally, the behavior of the spin and light correlation echoes in the appropriate ranges is discussed. Zh. éksp. Teor. Fiz. 112, 63–77 (July 1997)     

Quantum theory of coherent anti-Stokes Raman scattering in an inhomogeneously broadened gaseous system

A quantum-statistical treatment of coherert anti-Stokes Raman scatterirg in a gaseous system is presented. The atomic system is described by two energy levels. Both the atomic system and the radiation fields are quartized. The effects of atomic motion ard the detuning are incorporated into the analysis. The equations of motion describing the photor statistics of pump, Stokes and anti-Stokes fields are obtained considering the process as a two-step one. The time-evolution of average photon number and the second-order coherence of the anti-Stokes fields for different initial distributions are studied. The occurrence of antibunching is investigated.     

Optical power limiting of ultrashort hyper-Gaussian pulses in cascade three-level system

Propagation of strong femtosecond hyper-Gaussian pulses in a cascade three-level molecular system is studied by solving numerically the Maxwell–Bloch equations by the iterative predictor-corrector finite-difference time-domain method.Optical power limiting behavior induced by strong nonlinear two-photon absorption is observed for different orders of the femtosecond hyper-Gaussian pulses. Pulses of a higher order temporal profile are found to have a wider power range of optical limiting but a larger output saturation intensity. Both the output saturation value and the damage threshold of optical power limiting decrease with pulse duration increasing. The decrease of the pulse area along the pulse propagation is much slower than that obtained from the two-photon area theorem due to invalidity of the slowly varying amplitude approximation and the monochromatic field hypothesis.     

Theoretical investigation of noncollinear phase-matched parametric four-photon amplification of ultrashort light pulses in isotropic media

The amplification of light signals (angular frequency _S in some isotropic media (D₂O, fused silica, and Schott type SF10 glasses) by noncollinear phase-matched parametric four-photon interaction ₁+_2S+₁ is studied theoretically. Computer simulations are carried out for fundamental and second-harmonic pump pulses of a mode-locked Nd: glass laser. Degenerate interaction (wavelength ₁=₂=1054nm or 527 nm) and nondegenerate interaction (₁=1054nm, ₂=527 nm are considered. Characteristic phase-matching parameters and gain parameters versus wavelength are determined. Limitations by spectral bandwidth, optical absorption, optical damage, self-phase modulation, self-focusing and stimulated Raman scattering are analysed.     

Influence of gas circulation on stimulated Raman scattering and amplification of ultrashort laser pulses in methane

Applying gas recirculation in a high pressure cell, laser pulses of 1 ps at 400 nm and with a repetition rate of 1 kHz were frequency shifted by stimulated Raman scattering and amplification in methane gas at high pressure. We studied the influence of gas recirculation on the conversion efficiencies into the Stokes and anti-Stokes components as well as on their spatial distributions and spectral shapes using generator and generator-amplifier arrangements. For high pump energies, recirculation in the generator cell decreases conversion efficiency into the first Stokes component whereas it increases conversion into higher Stokes and anti-Stokes components. It results in a significantly improved spatial characteristics of the frequency-shifted radiation, however, is accompanied by a substantial spectral broadening. Using gas recirculation in the generator-amplifier arrangement we achieved a conversion efficiency into the first Stokes component of about 50% with highly improved spatial and spectral characteristics.