G. I. Freidman’s research while affiliated with Russian Academy of Sciences and other places

A new scheme of a cascaded converter of the first harmonic of broadband cw laser radiation into the second harmonic (SH) with compensation for the group walk-off in cascades is proposed and investigated. The conditions under which high conversion coefficients of broadband ({approx}33 cm{sup -1}) single-mode fibre laser radiation with low peak power ({approx}300 W) into the SH are determined for frequency doublers based on the most promising LBO crystal. Conversion of cw radiation with an average power of 300 W and efficiency {eta} = 4.5 % into the SH is obtained in a single LBO crystal. Effect of coherent addition of SH radiation excited in different cascades is demonstrated for two- and three-stage schemes. The expected conversion efficiencies, calculated disregarding loss but taking into account real aberrations of elements, are 18 % and 38 %, respectively. The effect of pumping depletion begins to manifest itself in the third cascade of a three-stage converter; it may reduce the latter value to {approx}30 %. (nonlinear optical phenomena)

This study reports the construction of a Russian OPCPA-based laser complex. In this system, a 100 TW laser is taken as a front-end system, supplemented with a KD*P crystal with clear aperture of 20times20 cm, as another cascade of parametric amplification. The pump laser comes from one channel of the "Luch" facility with 2 kJ output pulse energy at the wavelength of 1054 nm. In addition, a 1 kJ pulse with 2.3 ns duration at the second harmonic is used to pump the final stage of the parametric amplifier. Data reveals that the maximum energy obtained in a chirped pulse is more than 100 J. Autocorrelation function on compression is also shown. If the pulse shape is regarded to be Gaussian, then its pulse duration is 70 fs, which corresponds to the power of 1 PW at compressor output. Lastly, beam divergence shows a value close to the diffraction limit.

Conditions of the applicability of equations in the quasi-static approximation for studying the parametric interaction of frequency-modulated light beams in multistage amplifiers are considered. This approximation is used to simulate numerically processes in a multistage DKDP crystal amplifier with the output power exceeding 10 PW and suppressed luminescence. (lasers and amplifiers)

The process of head-on stimulated scattering is analyzed by approximate and numerical methods. It is shown that the formation of quasisoliton pulses of duration τ≈10−1T2, characterized by high conversion efficiency (~50–70%), is possible if the leading edges of the pulses are sufficiently steep.

An experimental investigation was made of the energy, frequency, and spatial characteristics of the resonance wave radiation from a single-resonator optical parametric oscillator (OPO) utilizing an LiIO3 crystal pumped by a train of pulses of duration τ = 10–2–1 nsec. When pumped by λ = 0.53 μ radiation, the power of the resonance wave from the OPO reached 12 MW in the 0.61–2.7 μ range. When pumped by λ = 1.06 μ radiation, the radiation power reached 30–50 MW in the 1.4–2.7 μ range. In feedback OPO systems pumped by a sequence of subnanosecond pulses a stable efficiency of several tens of percent was achieved for a resonance wave radiation divergence close to the diffraction limit and a line width Δν ~ 1/τ.

Numerical solutions are used in an investigation of some features of the nonlinear interaction of wave packets (pulses or beams) in quadratic media. Three types of interaction are considered: the interaction of escaping waves with a pump wave, the interaction of concurrent and escaping waves, and the interaction of spatially confined waves. It is shown that a near-100% conversion of the pump pulse energy into the energy of low-frequency fields can be achieved for all three types of interaction. A study is made of the possibility of generation of ultrashort pulses in the three-frequency interaction. It is shown that, in particular, when confined fields with a low initial amplitude are amplified in the field of a long pump pulse, this pump pulse becomes shortened and its maximum intensity increases considerably.

An investigation was made of the intensity, frequency spectrum, and angular distribution of the spontaneous anti-Stokes scattering of ruby laser radiation in a lithium iodate crystal. When the pump field was a superposition of ordinary and extraordinary waves, the strongest spontaneous radiation was due to a two-stage process involving up-conversion of the frequency of three-photon parametric luminescence. When only an ordinary or extraordinary pump wave was scattered, the output radiation had angular and frequency distributions which—within the limits of the experimental error—were identical with the distributions predicted theoretically for four-photon parametric luminescence. It was found that, in the investigated wavelenght range 0.45–0.54 μ, the component χ35 of the cubic nonlinearity tensor of the LiIO3 crystal was of the same order as the component χ11.

A theoretical and experimental investigation is reported of the influence of the polychromaticity and multimode nature of a pump wave on the resolution, contrast, and conversion efficiency of a parametric image converter. It is shown that if the width of the pump spectrum Δωp (or the divergence Δθp) exceeds the phase-matching width σωp (or the angle σΨp), a wide base appears in the distribution of the intensity of the converted image of a point or line object. It is also shown that when Δωp or Δθp is increased, the depencence of the conversion efficiency on the pump intensity changes from oscillatory (for example in the case Δθp < σΨp ) to monotonic. If Δθp/σp 1, the maximum conversion efficiency is 50%. The results of an experimental investigation are in agreement with the theoretical predictions. It is shown that in the most-difficult- to-analyze case of high conversion efficiencies and multimode pumping the converter resolution is practically independent of the pump intensity.

It is shown that the efficiency of conversion of pump to long-wavelength radiation can be increased by a multistage process in a single nonlinear element. The plane-wave approximation is used in a study of the characteristics of multistage conversion in an oscillator with low-frequency feedback. The dependences are obtained of the conversion efficiency on the number of conversion stages and on the excess of the pump power density over the self-excitation threshold. It is shown that the phase-matching conditions of the multistage process of parametric conversion are satisfied by a proustite crystal in the interaction of the e–eo waves in each stage and also by systems in which far infrared radiation is generated as a result of stimulated scattering by polaritons in an LiIO3 crystal when the interaction of waves is of the e–ee type.

An analysis is made of the self-interaction and of the processes governing the angular and frequency spectra of a single-resonator pulse optical parametric oscillator with a noncollinear interaction. It is shown that at high pump powers the structure of the excited oscillations and the widths of the angular and frequency spectra are all governed largely by the eigenmodes formed as a result of parametric amplification in pump beams. Theoretical and experimental investigations are reported of the characteristics of these modes and of their excitation conditions. An analysis is given of the special features of the formation of the angular and frequency spectra in a conventional single-resonator optical parametric oscillator and of the selection mechanisms specific to parametric oscillation in the case of nonresonant reflection of pump radiation and also when a dispersive resonator is used.