Victor V. Apollonov

Phase-locking of the 2D structures

Experimental results on 1D and 2D phase-locking of laser diode arrays are presented. Attention is paid to the employment of the arrays consisting of wide aperture lasers diodes. Selection of the “in-phase” supermode, preferable for most of the cases, is attained in the external quarter Talbot (Lc=ZT/4=d2/2λ) cavity due to the output mirror tilt at the angle φm=λ/2d. Analysis of the parameters that influence on the phase-locking is given. Our experiments confirm theoretical predictions of the system stability and adequate selectivity for the laser diode array fill factor (FF) FF=0.6.

Ion — ion recombination in SF6 and in SF6 — C2H6 mixturesfor high values of E/N

Ion—ion recombination coefficients in a decaying SF6 plasma and in SF6 — C2H6 mixtures are measured in the pressure range 15 — 90 Torr for reduced field strengths 100 — 250 Td. The charge composition is analysed and dominating ion — ion recombination channels in such plasmas are determined. Relations for estimating the potential drop at the electrodes are obtained for decaying plasma in strongly electronegative gases. The results of measurements are extrapolated for estimating the ion — ion recombination coefficient in SF6 for nearly critical field strengths. It is concluded that the ion — ion recombination should be taken into account in calculations of the discharge characteristics in non-chain reaction HF lasers.

Intensification of heat transfer in high-power laser diode bars by means of porous metal heat-sink.

To intensify a heat transfer in high-power emitters based on laser diode bars we propose the use of a heat sink from a porous permeable material cooled by a fluid flow [1-3]. The main advantage of this class of materials is the possibility of removing significant heat flows with compact heat sink. An analysis of the characteristic values of the thermal loads and their relations with the material and liquid parameters drawn from an one-dimensional model of stationary one-sided heat exchange shows the possibility of heat flow removal of more than 1.5 kW/cm 2 at room temperature in a liquid. Methods for improving the effectiveness of the strategy are considered.

High-power laser diode array phase locking

The overview of the phase-locking problem for the powerful laser diode arrays is presented. Comparison of the external Talbot cavity configuration with the other ones is held. Attention is paid to both investigation and employment of the laser diode arrays consisting of wide aperture lasers. Such arrays, placed in the external cavity, allow easier solution of the scaling up problem that results in the output lobes with both significantly increased power and low divergence. Our experiments confirm feasibility of both QCW and CW phase- locking of the LDA in the external quarter-Talbot (Lc equals ZT/4) cavity at the output power of more than 10 W with the lobes divergence of (delta) (Psi) approximately equals 0.5 mrad. Employment of the efficient porous heatexchanger along with the system that cancels both induced phase distortions arising due to heat release and existing optical imperfections has allowed a breakthrough in powerful arrays phase-locking.

High-power nonchain HF (DF) lasers initiated by self-sustained volume discharge

Initiation mechanisms of self-sustained volume discharge (SSVD) are analyzed in mixtures of SF6 with hydro-carbons (deuterocarbons). The possibility is shown to obtain SSVD in a large volume of SF6 and hydrocarbon (deuterocarbon) mixtures without preionization in the discharge gaps possessing high edge nonuniformity of electric field. The scaling characteristics of nonchain HF (DF) laser are investigated. The highest generation energy has been achieved, namely, 397 J for HF laser and 312 J for DF laser at the electric efficiency 3.8% and 3%, respectively.

Free-electron laser exploiting a superlattice-like medium

Amplification in free-electron lasers exploiting media with periodically modulated refractive indices is studied in the regime of a large modulation. The conditions for realization of the large-modulation regime in a superlattice-like medium are established. The maximized gain, the corresponding saturation field and efficiency, as well as the optimal electron energy and propagation direction are determined. It is shown that the large-modulation regime makes it possible to extend significantly the operation frequency domain of the FEL employing a low-relativistic electron beam. Relationship with the Cherenkov and stimulated resonance-transition-radiation FELs is discussed. This research is partially supported by RFBR grant 97-02-17783.

Scaling up of nonchain HF(DF) laser initiated by self-sustained volume discharge

Scaling problems are considered for non-chain HF laser operating on mixtures of SF6 and hydrocarbons in which a chemical reaction is initiated by self-sustained volume discharge. The possibility of obtaining a volume discharge in SF6 and in corresponding mixtures without preionization, i.e., self- initiated volume discharge (SIVD) is a new qualitative result in solving the scaling problem for non-chain lasers. The dynamics of SIVD evolution has been investigated. The possibility of obtaining SIVD is determined in SF6 by mechanisms that limit the density of current, are related to specific energy release, and prevent the total energy from flowing through a single channel. A simple mode is developed for calculating the discharge characteristics in non-chain laser. The model provides a good agreement with experiment data. The obtained energy of nonchain HF-laser is 407 J and that of DF-laser is 325 J with the electric efficiency 4.3 percent and 3.4 percent, respectively. A possibility is estimated of creating non-chain HF lasers with the output energy of the order of kilojoules and higher on the basis of experimental data obtained.

Far-infrared generation by CO2 lasers frequencies subtraction in a ZnGeP2 crystal

A pulsed radiation power of approximately 1 W was reached for the first time, with the aid of a ZnGeP2 crystal, at the difference frequency of CO2 lasers radiation in the submillimeter spectral range.

Self-initiated volume discharge in mixtures of SF6 with hydrocarbons to excite nonchain hf lasers

The paper studies a self-sustained volume discharge without preionization-self-initiated volume discharge (SIVD)--to excite non-chain HF lasers on SF6-C2H6 mixtures. Once initiated by a local discharge gap breakdown, SIVD is found to propagate then over the whole gap normally to the applied electric field through successively starting diffuse overlapping channels at a voltage close to the quasi- stationary value. With forming new channels, the current through those previously originated decreases. The volume occupied by SIVD tends to expand with increasing the energy released within the discharge plasma, whereas a discharge bounded by a dielectric surface shows a simultaneous increase both in burning voltage and current. All these features combined allow a concept to be put forward of the existence of certain restriction mechanisms depending on the specific energy released and not permitting the total deposited energy to pass through a single channel. It is suggested that SF6 dissociation by electron impact and the electron attachment to vibrationally excited SF6 molecules are just those mechanisms. Simple analytical models have been developed allowing these mechanisms to be qualitatively described.

Highly efficient heat exchangers for laser diode arrays

Highly efficient heat exchangers with uniform temperature distribution based on microchannel and porous structure are developed. The heat exchange efficiency dependence on construction peculiarities of the devices and cooling liquids properties are shown. Basing on the comparison of both experimental results and the numerical simulation, the ways to obtain the device thermal resistance about 0,2 C/W are discussed. Practical neededs for fabrication both linear diode arrays with power more than 1 kW will be observed.