Table of contents

Tellurium-doped germanosilicate fibre has been fabricated by the MCVD process. In contrast to Te-containing glasses studied earlier, it has a broad luminescence band (full width at half maximum of ∼350 nm) centred at 1500 nm, with a lifetime of ∼2 μs. The luminescence of the fibre has been studied before and after gamma irradiation in a ⁶⁰Co source to 309 and 992 kGy. The irradiation produced a luminescence band around 1100 nm, with a full width at half maximum of ∼400 nm and lifetime of ∼5 μs.

We consider the simplest process of stimulated emission, i.e., evolution of an excited two-level atom interacting with a quantized electromagnetic field being initially in a one-photon state under different boundary conditions imposed on the field. We study the kinetics of evolution, spectral properties and modal composition of the radiation field emerging in such processes. It is shown that in the general case this field is only partly composed of doubly occupied modes.

The dynamics of a model Rydberg atom in a strong nonclassical electromagnetic field is investigated. The field-induced transitions to the continuum involving different numbers of photons (with intermediate states in the discrete spectrum) are taken into account and the specific features of ionisation in 'squeezed' field states are considered in comparison with the case of classical light. A significant decrease in the ionisation rate is found, which is caused by the interference stabilisation of the atomic system. The entanglement of the atomic and field subsystems, the temporal dynamics of the correlations found, and the possibility of measuring them are analysed.

The efficiency of diode-pumped lasers with gain elements made from yttrium, gadolinium, yttrium — gadolinium and yttrium — scandium orthovanadate crystals has been shown for the first time to be influenced by structural imperfections (quality) of the crystals. This allows one to predict lasing parameters of such crystals in a preliminary step, without fabricating gain elements.

High-peak power, passively Q-switched, composite Nd : YAG/Cr⁴⁺ : YAG lasers consisting of all-polycrystalline bonded Nd : YAG and Cr⁴⁺:YAG ceramics are developed, and two applications of such lasers are discussed. A 1.1-at. %-doped Nd : YAG/Cr⁴⁺:YAG ceramic laser is fabricated, which is quasi-cw pumped by a diode laser in the Hz-range, delivering laser pulses of 2.5-mJ energy and 1.9-MW peak power. By frequency doubling the laser output in a LiB₃O₅ (LBO) nonlinear crystal at room temperature, 0.36-mJ, 0.3-MW green laser pulses with 27 % conversion efficiency are produced at 532 nm. Furthermore, a highly doped (1.5-at. %) Nd : YAG/Cr⁴⁺:YAG ceramic laser operates successfully in the range of pulse repetition rates from 50 to 500 Hz, yielding 0.8-to-1.0 mJ pulses with a peak power around 1 MW. The laser output beam is amplified in a master-oscillator — power-amplifier (MOPA) system to generate laser pulses with 11-mJ energy at a 250-Hz repetition rate.

We present an experimental demonstration of additive pulse mode locking based on a nonlinear loop mirror ring laser. The proposed design uses nonlinear phase shifts induced by a loop mirror. The results show that interference between two overlapping pulses from two coupled fibres, containing a nonlinear medium for power-dependent phase modulation, leads to pulse compression, and can provide mode locking with different repetition rates depending on the interplay or combination between the modulated frequency (active mode locking) and the nonlinearity (passive mode locking) generated in the loop mirror.

Coherent beam combining of 2-μm fibre lasers is studied numerically. Such lasers are a suitable candidate for creation of a beam combining module due to their higher nonlinear effect and optical damage threshold compared to their 1-μm counterparts. The calculations results show that radiation of a 1-μm fibre laser has a better focusing property in free-space due to a smaller wavelength, whereas a 2-μm laser beam has a better focusing capability in the target-plane under conditions of real atmospheric turbulence.

A new method has been proposed for achieving wave vector matching in four-wave interactions of frequency components upon SRS in birefringent SRS-active crystals. The method ensures anti-Stokes wave generation and enables a substantial reduction in higher order Stokes SRS generation thresholds. Phase matching directions in BaWO₄ SRS-active negative uniaxial crystals and SrWO₄ SRS-active positive uniaxial crystals have been found in the wavelength range 0.4 — 0.7 μm.

Tunable mid-IR radiation is obtained during second harmonic generation of tunable CO₂-laser radiation using a CdGeAs₂ crystal. Its angular tuning characteristics at the CO₂- laser wavelength, angular acceptance angle and spectral acceptance are measured. For second harmonic generation at 10.6 μm, the conversion efficiency in the CdGeAs₂ crystal is 90 times higher than that in the ZnGeP₂ crystal.

The process of nonlinear rotation of the polarisation ellipse of laser radiation, occurring simultaneously with the broadening of the pulse spectrum due to nonlinear self-phase modulation in a gas-filled capillary, is studied. It is shown that the maximal rotation of the polarisation ellipse is experienced by the spectral components, shifted towards the short-wavelength side with respect to the central wavelength of the initial laser pulse. Using the effect of polarisation ellipse rotation, an eightfold increase in the energy contrast ratio of a 28-fs light pulse, obtained by compression of the radiation pulse from an ytterbium laser with the duration 290 fs, is implemented.

This paper examines the effect of an intermediate layer between the core and periodic cladding of a Bragg fibre on its optical properties. It is shown that any TE_0κ mode of a standard Bragg fibre can be transformed into the fundamental mode of a Bragg fibre with an intermediate layer by adjusting the thickness of the first cladding layer. Varying the thickness of the intermediate layer, one can tune the mode composition of the transmitted light in a periodic manner. The periodic variation of the optical loss with the thickness of the intermediate layer is due to resonances and antiresonances in this layer. The resonances correspond to two-mode light propagation through the fibre, whereas the antiresonances, to single-mode light propagation.

A wideband and flat gain erbium-doped fibre amplifier (EDFA) is demonstrated using a hybrid gain medium of a zirconiabased erbium-doped fibre (Zr-EDF) and a high concentration erbium-doped fibre (EDF). The amplifier has two stages comprising a 2-m-long ZEDF and 9-m-long EDF optimised for C- and L-band operations, respectively, in a double-pass parallel configuration. A chirp fibre Bragg grating (CFBG) is used in both stages to ensure double propagation of the signal and thus to increase the attainable gain in both C- and L-band regions. At an input signal power of 0 dBm, a flat gain of 15 dB is achieved with a gain variation of less than 0.5 dB within a wide wavelength range from 1530 to 1605 nm. The corresponding noise figure varies from 6.2 to 10.8 dB within this wavelength region.

The results of heterodyning of broadband radiation in the near-IR range are presented. A stabilised DFB laser was used as a local oscillator, while the role of a broadband radiation source was played by another DFB laser, excited by the pump current below its threshold value. A fibre-optical Y-coupler based on the single-mode silica fibre served as a diplexer. The radiation mixed in the singlemode fibre was incident on the photodetector, the role of which was played by a p — i — n photodiode. The signal from the photodiode was amplified by the preamplifier with the feedback resistance 240 kΩ and the bandwidth ∼1 MHz and then digitised using a 16-digit ADC. The frequency scanning was implemented via the variation of the local oscillator frequency. The developed registration system provides the sensitivity ∼1.4 % of the shot noise value at the acquisition time ∼40 min.

We report high-precision frequency measurements of the separate hyperfine structure (HFS) components of the emission B — X system transitions of ¹²⁷I₂ molecules in the 982 — 985 nm range. To resolve the HFS of the emission lines, advantage was taken of the method of three-level laser spectroscopy. The function of exciting radiation was fulfilled by the second harmonic of a cw Nd : YAG laser, and the probe radiation in the 968 — 998 nm range was generated by an external-cavity diode laser. The output Nd : YAG laser frequency was locked to an HFS component of the absorption transition and the probing laser radiation to the emission transition component. When both frequencies were locked to HFS components with a common upper level, the output diode laser frequency was precisely equal to the emission transition frequency. The output frequency of the thus stabilised diode laser was measured with the help of a femtosecond optical frequency synthesiser based on a Ti : sapphire laser. We present the results of the absolute frequency measurements of 20 HFS components belonging to six vibrational — rotational transitions of the B — X system of iodine [R56(32 — 48)a1, P58(32 — 48)a1, P85(33 — 48)a1, R87(33 — 48a1, R88(33 — 48)a10] and all 15 components of the R86(33 — 48) line. The relative measurement uncertainty is equal to 7×10^-10 and is determined by the frequency instability of the diode laser radiation.

Thin ZnO nanostructured films were produced by pulsed laser deposition (PLD) for surface enhanced Raman spectroscopy (SERS) studies. The experimental conditions used for preparation of the samples were chosen to obtain different types of ZnO nanostructures. The Raman spectra of rhodamine 6G (R6G) were measured at an excitation wavelength of 785 nm after coating the ZnO nanostructures with a thin Au layer. The influence of the surface morphology on the Raman signal obtained from the samples was investigated. High SERS signal enhancement was observed from all Au-coated ZnO nanostructures.

The review of femtosecond laser installations for medical applications is given and a new femtosecond ophthalmologic system for creation of a flap of corneal tissue during the LASIK operation is described. An all-fibre femtosecond laser emitting ∼400-fs pulses at 1067 nm is used. The pulse repetition rate can vary from 200 kHz up to 1 MHz. The output energy of the femtosecond system does not exceed 1 μJ. A specially developed objective with small spherical and chromatic aberrations is applied to focus laser radiation to an area of an eye cornea. The size of the focusing spot does not exceed 3 μm. To process the required area, scanning by a laser beam is applied with a speed no less than 5 m s^-1. At a stage of preliminary tests of the system, the Κ8 glass, organic PMMA glass and specially prepared agarose gels are used as a phantom of an eye. The femtosecond system is successfully clinically tested on a plenty of eyes of a pig and on several human eyes. The duration of the procedure of creation of a corneal flap does not exceed 20 s.

The applicability of the optoacoustic (OA) method for monitoring temperature during thermal impact on biological tissues is studied experimentally. Tissues under study were chicken breast (as a model of muscle), porcine lard (as a model of fatty tissue) and porcine liver (as a model of richly perfused tissue). The temperature dependences of the amplitude of the OA signals excited in biological tissues were measured in-vitro in the temperature range of 20 — 80 °C under the narrow laser beam conditions. Measurements were performed in two regimes: during heating and cooling. Similarities and differences in the behaviour of the dependences in different temperature ranges associated with different structural composition of the samples were obtained. The accuracy of temperature reconstruction from experimental data for the investigated tissue types was evaluated. It is shown that during tissue coagulation its temperature can be determined with an accuracy of about 1 °C.

We discuss the concept of remote inspection of the isotopic composition of objects in space by products of nuclear reactions (including the fission of heavy isotopes), initiated by directed accelerated beams of ultracold neutrons. Despite the partial unavailability of quantitative estimates of specific situations, the proposed approach is internally consistent. We emphasise the urgency of development the problem in question and offer directions for further research.

Academician Oleg Nikolaevich Krokhin, prominent physicist and organiser of science, laureate of Lenin and State Prizes, Editor-in-Chief of Quantum Electronics was 80 on 14 March 2012.

An outstanding Russian scientist, Academician Vyacheslav Vasil'evich Osiko was 80 on 28 March 2012.

Doctor of Physicomathematical sciences, professor Lev Abramovich Rivlin, an outstanding physicist, a scientific head of the Laboratory of Applied Physics of Moscow State Institute of Radio Engineering, Electronics and Automation was 90 on 14 March 2012.