Table of contents

The semiconductor disk laser, a relatively novel type of light oscillators, is now under intensive development. These lasers produce an excellent beam quality in conjunction with a scalable output power. This paper presents recent achievements in power scalability, mode-locking and frequency conversion with optically-pumped semiconductor disk lasers. A novel concept for power scaling described here allows the thermal load of the gain material to be reduced, increasing the threshold of rollover and extending the capability for boosting the output power without degradation in the beam quality. The proposed technique is based on the multiple gain scheme. The total power of over 8 W was achieved in dual-gain configuration, while one-gain lasers could produce separately up to 4 W, limited by the thermal rollover of the output characteristics. The results show that the reduced thermal load to a gain element in a dual-gain cavity allows extending the range of usable pump powers boosting the laser output. Orange—red radiation required for a number of challenging applications can be produced through frequency-doubling using a GaInNAs/GaAs laser. Using such a disk laser operating at a fundamental wavelength of 1224 nm, we demonstrate an output power of 2.68 W in the visible region with an optical-to-optical conversion efficiency of 7.4%. The frequency-converted signal could be launched into a single-mode optical fibre with 70%—78% coupling efficiency, demonstrating good beam quality for the visible radiation. Using a Fabry—Perot glass etalon, the emission wavelength could be tuned over an 8-nm spectral range. We report on optically-pumped disk lasers passively mode-locked with a semiconductor saturable-absorber mirror. The potential of harmonic mode-locking in producing pulse trains at multigigahertz repetition rates has been explored. The mode-locked disk laser is investigated for different designs of the gain medium that allow bistable mode-locking to be controlled. An explanation for hysteresis of mode-locking is given based on a laser model including saturable absorption and unsaturated gain in a semiconductor oscillator operating in pulsed regime.

The spatial density distribution of the absorbed energy in ZnSe semiconductor lasers excited by electrons with energies from 2 keV to 1 MeV is calculated by the Monte-Carlo method. Approximate analytic expressions determining the absorbed energy of electrons in ZnSe are presented. The pump power threshold in a semiconductor quantum-well ZnSe structure is experimentally determined. The lasing threshold in such structures is estimated as a function of the electron energy.

Vibrational distributions of I₂(X, v) (0 ≤ v ≤ 45) molecules are calculated and analysed in the active medium of an oxygen—iodine laser. It is found by comparing the calculated values with the experimental data that during the I(²P_1/2) + I₂(X) → I(²P_3/2) + I₂(X, v > 10) reaction the probability of the production of I₂(X, v > 23) molecules is 0.1 and the total probability of the direct excitation of iodine molecules at the vibrational levels from 15 to 23 is 0.9. Based on the data obtained, the dissociation mechanisms of iodine molecules in the active medium of the oxygen—iodine laser are analysed.

Stationary distributions of temperature and thermoelastic stresses (thermal tensions) are obtained in an active disk element with an arbitrary optical density upon a double-pass pumping. It is shown that the temperature distribution is determined by the sum of three terms: two exponential and a linear one, the exponential terms being preserved with changing the boundary conditions while the linear term producing no thermal tensions changes. It is found that thermal tensions decrease with increasing the absorption coefficient both for the constant thickness of the disk and for the constant optical density. The assessed values of the temperature are calculated during the local heating of a thin disk when the diameter of the pumped region is comparable with the disk thickness.

We present the results of an experimental study of a Tl vapour/Ar mixture as an active medium. Under optical excitation of thallium to the 6d ²D_3/2 state, we have achieved lasing at λ = 1.3 μm (7p ²P⁰_1/2 — 7s ²S_1/2 transition) and 535 nm (7s ²S_1/2 — 6p ²P⁰_3/2) through cascade transitions from the 6d ²D_3/2 state to the upper laser levels. The electron deexcitation rate of the 6p ²P⁰_3/2 metastable state is no slower than 1.5×10^-8 cm³ s^-1, meaning that this process cannot prevent self-terminating laser action at the 7s ²S_1/2 — 6p ²P⁰_3/2 transition under electron-beam pumping at pulse repetition rates up to 100 kHz.

A continuous-wave oscillation is obtained for the first time in a Fe²⁺:ZnSe laser. The laser wavelength was in the range from 4.04 to 4.08 μm. A liquid-nitrogen-cooled active element was pumped by a Cr²⁺:CdSe laser at 2.97 μm. The maximum output power of the laser was 160 mW with the 56% slope efficiency. The minimum absorbed pump power threshold was 18 mW. The intrinsic losses in the Fe²⁺:ZnSe crystal did not exceed 0.024 cm^-1 during lasing.

A short-pulse 'green' 532-nm Nd³⁺:YVO₄ and KTiOPO₄ microchip laser with intracavity second-harmonic generation, which is pumped by a 809-nm semiconductor laser diode, is developed.

Repetitively pulsed multipass copper vapour amplifiers are studied experimentally. A considerable increase in the peak power of laser pulses was achieved by using a special scheme of the amplifier. It is found that the main reasons preventing an increase in the peak power during many passages of the beam are the competitive development of lasing from spontaneous seeds in a parasitic resonator formed by the fold mirrors of a multipass amplifier, a decrease in the amplification during the last passages, and an increase in the pulse width at the amplifier output.

The propagation of focused high-power femtosecond laser pulses in air is numerically simulated. The dependences of the effective average size of a focal spot and the maximum achievable radiation intensity in the focal beam waist on the peak power of incident radiation are studied. It is shown that in the regime of nonstationary self-action of radiation, due to photoionisation of the medium and formation of plasma, it becomes impossible to focus radiation into a spot of diffraction-limited size predicted by a linear theory.

It is shown that four-mode interaction in quasi-synchronous cascade frequency conversion on a quadratic nonlinearity can be described in terms of an effective cubic nonlinearity, which reduces the problem to solving the system of two coupled nonlinear Schrödinger equations (NSEs) with respect to the amplitudes of waves involved in both nonlinear processes. Analytic solutions of a new type found for this system have the form of cnoidal waves with components representing the sum and difference of the identical fundamental solutions of the NSE with shifted arguments. The obtained solutions cover the entire range of variation of boundary conditions, allowing the optimisation of the conversion efficiency in any particular situation.

The properties of the optical connection of single-mode fibres by laser beams counterpropagating from the ends of these fibres are considered numerically and experimentally. It is shown that effective connection of single-mode fibres at the wavelength of 1.55 μm is possible during self-channelling and interaction of optical beams at 0.63 μm in a photopolymerising medium. In this case, the long exposure can lead to a decrease in the transmission coefficient, especially when the ends of the fibres being connected are positioned off-axially. It is obtained experimentally that for the 1–2-mm-long polymer connector and the ∼15-μm radial displacement, the efficiency of the optical connection of the SMF-28 and CS-980 single-mode fibres in a OCM-2 photopolymerising composition can achieve 80%—90% and 50%—60%, respectively.

We report a Raman scattering study of photoinduced and thermal reactions between H₂ and germanosilicate optical fibres with 22 mol % and 97 mol % GeO₂ in the core (F1 and F2, respectively) after H₂ loading at 150 MPa (1500 atm). The mechanisms of photoreactions are investigated in a wide range of incident laser wavelengths (244, 333, 354, 361 and 514 nm). Thermal reactions are studied at 500 °C. The results indicate that the main mechanism behind the formation of hydrogen-containing defects with Raman bands at 700, 750, 2190, 3600 and 3680 cm^-1 involves ≡Ge—O—Ge≡ or ≡Ge—O—Si≡ bond breaking and formation of hydride and hydroxyl species: =GeH₂ (700, 750 cm^-1), ≡Ge—H (2190 cm^-1), ≡GeO—H (3600 cm^-1) and ≡SiO—H (3680 cm^-1). The key features of the reactions in the F1 and F2 fibres are analysed. In particular, photoinduced reactions give ≡Si—OH groups only in the F1 fibres, whereas the formation of germanium nanoclusters at a relatively low temperature (∼500 °C) or ≡GeO—H and ≡Ge—H defects under 514-nm irradiation has only been observed in the F2 fibres.

The light pressure force acting on a spherical dielectric particle in the interference field of two plane monochromatic electromagnetic waves is studied in detail for different particle radii and angles of incidence of waves.

The mechanisms of the transport of cold atomic ensembles and transformation of their parameters in potential channels and wells treated by analogy of electromagnetic microwave waveguides and hollow resonators are considered. The possibility of performing various manipulations with such ensembles, in particular, the isothermal phase transition to a Bose—Einstein condensate is pointed out.

A simple method for suppressing speckles in images produced by laser projectors is proposed. The coherence of the laser beam and, therefore, speckles can be destroyed when the beam passes through an electrooptical cell in which a special ferroelectric liquid crystal is used as a modulating medium. The effect is achieved due to the spatially inhomogeneous phase modulation of light when specially shaped bipolar electric pulses are applied to the cell.

A highly-efficient phase photothermal method is developed for quantitative measurements of the small optical absorption coefficient in thin plates made of highly transparent materials in which bulk losses significantly exceed surface losses. The bulk absorption coefficient at 10.6 μm is estimated in polycrystalline diamond plates grown from the vapour phase (a CVD diamond). The results are compared with those for natural and synthetic diamond single crystals and with the concentrations of nitrogen and hydrogen impurities. The absorption coefficient of the best samples of the CVD diamond did not exceed 0.06 cm^-1, which, taking into account the high thermal conductivity of the CVD diamond (1800–2200 W mK^-1 at room temperature), makes this material attractive for fabricating output windows of high-power CO₂ lasers, especially for manufacturing large-size optics.

An ecologically perfect generator of singlet oxygen O₂ (a¹Δ_g) is proposed which fundamentally differs from existing singlet-oxygen generators. Excited O₂ (a¹Δ_g) molecules are generated due to interaction of the O₂ (X³Σ^-_g) molecules with a quasi-monochromatic field, which is supplied from an external source to a closed volume — an optical boiler containing oxygen. It is shown that, by pumping continuously the optical boiler by the light field of power ∼3×10⁵ W, it is possible to accumulate up to 40% of singlet oxygen (O₂(b¹Σ⁺_g)) + (O₂ (a¹Δ_g)) in the boiler volume during ∼10^-2 s.