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The gain properties of multicomponent fibers with a heavily ${\rm{Er}^{3 + }}$-doped phosphate-based core in a silica cladding were investigated in an all-fiber amplifier configuration that included spliced joints of the investigated fibers with silica ones. The maximum value of small-signal gain reached about 38 dB. The maximum small-signal gain per unit length of 3.1 dB/cm was demonstrated in 10 cm long 3 wt.% ${\rm{Er}^{3 + }}$-doped fiber.

In this Letter, we present the results on fabrication and investigation of a holmium (Ho) distributed fiber laser based on a π-phase-shifted FBG inscribed in a heavily Ho-doped fiber by IR femtosecond laser pulses. Single-polarization and single-transverse mode operation regimes were observed with a linewidth of the order of 10 kHz and output power up to 53 mW at 2.07 μm. Lasing regimes are compared for room and cryogenic (77 K) temperatures of the laser cavity. To the best of our knowledge, this is the first realization of such type of fiber laser based on holmium active medium.

We report a study of composite fibres as gain media of all-fibre amplifiers. A distinctive feature of the fibres is an erbium-ytterbium doped phosphate core in a double silica cladding. The high Yb3+ concentration (above 1021 cm-3) ensures effective pump absorption from the cladding at fibre lengths under 40 cm and effective erbium ion sensitisation. The silica cladding facilitates fusion splicing to standard fibres. The composite fibres offer a large gain coefficient: about 0.4-0.5 dB cm-1 at a signal wavelength of 1550 nm. The maximum gain at a fibre length of just 36 cm and an input signal power of 0.37 mW at l = 1550 nm is about 13.5 dB. Under such conditions, the amplified spontaneous emission level is lower than the input signal level by 10 dB.

This paper reports an all-fibre active Q-switching scheme for a holmium fibre laser. We have obtained repetitively pulsed lasing at a wavelength of 2098 nm with pulse durations from 0.8 to 1.56 us, repetition rates of 11.2 and 22.4 kHz, a maximum peak power of 3.9 W and pulse energy of [?]3.5 uJ.

An all-fibre pulsed fibre laser system emitting at a wavelength of 1093 nm with an average output power up to 10 W is presented. The system is assembled according to the master oscillator/fibre amplifier scheme. Pulses were generated with passive mode locking due to nonlinear polarisation rotation in a standard single-mode fibre. The main fibre amplifier was pumped by fibrecoupled semiconductor laser diodes at a wavelength of 976 nm with a maximum total power up to 50 W. The measured pump duration did not exceed 60 ps at a pulse repetition rate of about 1 MHz. The pulse energy was 10 uJ.

The impact of holmium fibre laser radiation on the samples of biologic tissues (dura mater of spinal cord and adipose tissue with interlayers of muscle) is studied. The experimental results are evaluated by the size of carbonisation and coagulation necrosis zones. The experiment shows that in the case of irradiation of the spinal cord dura mater samples the size of carbonisation and coagulation necrosis zones is insignificant. In the adipose tissue the carbonisation zone is also insignificant, but the region of cellular structure disturbance is large. In the muscle tissue the situation is opposite. The cw laser operation provides clinically acceptable degree of destruction in tissue samples with a minimal carbonisation zone.

The impact of holmium fibre laser radiation on the samples of biologic tissues (dura mater of spinal cord and adipose tissue with interlayers of muscle) is studied. The experimental results are evaluated by the size of carbonisation and coagulation necrosis zones. The experiment shows that in the case of irradiation of the spinal cord dura mater samples the size of carbonisation and coagulation necrosis zones is insignificant. In the adipose tissue the carbonisation zone is also insignificant, but the region of cellular structure disturbance is large. In the muscle tissue the situation is opposite. The cw laser operation provides clinically acceptable degree of destruction in tissue samples with a minimal carbonisation zone.

The review summarizes and analyzes recent published data on topoisomerase I and II inhibitors as potential antitumour agents. Functions and the mechanism of action of topoisomerases are considered. The molecular mechanism of interactions between low-molecular-weight compounds and these proteins is discussed. Topoisomerase inhibitors belonging to different classes of chemical compounds are systematically covered. Assays for the inhibition of topoisomerases and the possibilities of using the computer-aided modelling for the rational design of novel drugs for cancer chemotherapy are presented. The bibliography includes 127 references.

A system for maintaining a constant phase difference between several laser channels is designed and an algorithm of its operation under a slow (with a characteristic time of no less than 100 ms) variation in the radiation phase in a channel is developed. Different possible regimes of operation of the measurement system and versions for phase difference maintaining are analysed and an optimal algorithm is elaborated. Long-term stable operation of the system under thermal and mechanical effects on the active medium is experimentally demonstrated by the example of a seven-channel fibre amplifier with a working wavelength of 1.064 um.

We present the results of interference studies of a diode-pumped active disk element made of a gadolinium -- gallium -- garnet (GGG:Nd) crystal. The disk is cut perpendicular to the crystallographic axis [001], along which the pump beam propagates. With absorbing pump radiation, a thermal lens, which has been investigated by the interference methods using linearly polarised probe radiation, is formed in the disk. At the absorbed pump power up to 12 W, the interference pattern near the disk surface is a system of concentric rings. With increasing absorbed pump power up to 24 W, the outer rings transform into hexagons. It is shown that such an interference pattern in the garnet crystal is caused by thermally induced birefringence.

We report a passively mode-locked, all-fibre holmium laser based on nonlinear polarisation rotation. As a pump source use is made of an 1125-nm ytterbium-doped fibre laser. The pulse repetition rate of the holmium laser is 7.5 MHz, and the pulse duration does not exceed 52 ps at wavelengths of 2065 and 2080 nm. The average laser output power reaches 5 mW.

Optimal conditions are determined for the development of efficient diode-pumped 1-mm solid-state lasers with output powers up to a few tens of kilowatts. The thermal operating conditions are analysed for various Yb3+- or Nd3+-doped active media used in high-power laser systems. The advantages and disadvantages of these active ions and various crystal matrices are discussed. The theoretical analysis and experimental simulations allow one to determine the application fields of various laser crystals. A new concept of a multibeam (multipoint) pumping of active media is proposed.

We have produced planar optical waveguides having a crystalline CaF2 - YF3 - NdF3 mixed yttrofluorite core and two reflective claddings in order to improve waveguide excitation efficiency. Under diode pumping, lasing has been achieved at a wavelength of 1064 nm with a slope efficiency near 15%.

We demonstrate the employment of single-crystal optical fibres based on lithium niobate for doubling the laser radiation frequency. The measured characteristics of the fibre confirm its high quality and spatial homogeneity. Parameters of the frequency doublers for neodymium laser radiation (l = 1 mm) based on fibre and bulk single crystals are compared. Single crystals are grown by the method of laser-heated pedestal growing with heating by radiation of a CO2 laser (LHPG-method).

We report on a diode-pumped 531-nm self-frequency-doubling Nd:YAB laser passively Q switched by a Cr^4+:YAG saturable absorber. Pumped by 1.4 W of 807-nm GaAlAs diode laser radiation, the Nd:YAB laser system generates 5.6-ns-long TEM_00 light pulses with an energy of 1.0 μJ at a repetition rate of 45 kHz. The experimental data are compared with the results of a numerical analysis based on rate equations that consider the optical properties of the Nd:YAB crystal and of the Cr^4+:YAG saturable absorber.

Supercontinuum generation in thulium-doped fibres under pumping at 1.59 um is investigated. Amplification of supercontinuum in the range of 1.8--2.0 um is found for a fibre doped to a level of 2 x 1019 cm-3. For a fibre with an activator concentration of 2 x 1020 cm-3 amplification is also observed in the (2.1 -- 2.45)-um band, which suggests the occurrence of the 3H4 - 3H5 optical transition in the fibre. The occupation of the 3H4 level can be explained by cooperative effects.

A high-power single-frequency laser with an output power of 2.5 W in the cw regime at a wavelength of 1064 nm has been developed using a hybrid scheme based on a master singlefrequency semiconductor laser (wavelength 1064 nm, lasing linewidth less than 3 MHz) and a two-cascade fibre amplifier pumped by high-power laser diodes. At pump powers of 4.8 W in the first cascade and 6.8 W in the second cascade the total gain is about 100.

Experimental evidence is presented that standard optical fibres can be used to efficiently convert nanosecond pulses at 1.59 um to a supercontinuum in the range 1.6 -- 2.4 um. The highest efficiency of conversion to wavelengths above 2 um, 38%, has been offered by a multimode graded-index fibre. The spectral density of average power in the range 2 -- 2.35 um in this fibre is about 1 mW nm-1 and that of peak power is about 10 W nm-1. In all the fibres studied, the practical long-wavelength limit of generation is 2.35 um.

A simple scanning interferometer is implemented for measuring the emission linewidth of single-frequency semiconductor lasers. The free dispersion region of the interferometer is 28 MHz, the spectral resolution being 470 kHz.

We report holmium-doped fibre lasers with a Ho3+ concentration of 1.6 x 1019 cm-3 and lasing wavelengths of 2.02, 2.05, 2.07 and 2.1 um at a pump wavelength of 1.15 um. The slope efficiency of the lasers has been measured. The maximum efficiency, 0.455, has been obtained at a lasing wavelength of 2.05 um. The laser efficiency is influenced by both the optical loss in the wing of a vibrational absorption band of silica and active-ion clustering.

The review summarizes and analyzes recent published data on topoisomerase I and II inhibitors as potential antitumour agents. Functions and the mechanism of action of topoisomerases are considered. The molecular mechanism of interactions between low-molecular-weight compounds and these proteins is discussed. Topoisomerase inhibitors belonging to different classes of chemical compounds are systematically covered. Assays for the inhibition of topoisomerases and the possibilities of using the computer-aided modelling for the rational design of novel drugs for cancer chemotherapy are presented.

A system for maintaining a constant phase difference between several laser channels is designed and an algorithm of its operation under a slow (with a characteristic time of no less than 100 ms) variation in the radiation phase in a channel is developed. Different possible regimes of operation of the measurement system and versions for phase difference maintaining are analysed and an optimal algorithm is elaborated. Long-term stable operation of the system under thermal and mechanical effects on the active medium is experimentally demonstrated by the example of a seven-channel fibre amplifier with a working wavelength of 1.064 μm.

Supercontinuum generation in thulium-doped fibres under pumping at 1.59 μm is investigated. Amplification of supercontinuum in the range of 1.8--2.0 μm is found for a fibre doped to a level of 2 × 1019cm-3. For a fibre with an activator concentration of 2 × 1020cm-3amplification is also observed in the (2.1 — 2.45)-μm band, which suggests the occurrence of the 3H4→ 3H5optical transition in the fibre. The occupation of the 3H4level can be explained by cooperative effects.

A high-power single-frequency laser with an output power of 2.5 W in the cw regime at a wavelength of 1064 nm has been developed using a hybrid scheme based on a master singlefrequency semiconductor laser (wavelength 1064 nm, lasing linewidth less than 3 MHz) and a two-cascade fibre amplifier pumped by high-power laser diodes. At pump powers of 4.8 W in the first cascade and 6.8 W in the second cascade the total gain is about 100.

Experimental evidence is presented that standard optical fibres can be used to efficiently convert nanosecond pulses at 1.59 μm to a supercontinuum in the range 1.6 — 2.4 μm. The highest efficiency of conversion to wavelengths above 2 μm, 38%, has been offered by a multimode graded-index fibre. The spectral density of average power in the range 2 — 2.35 μm in this fibre is about 1 mW nm-1and that of peak power is about 10 W nm-1. In all the fibres studied, the practical long-wavelength limit of generation is 2.35 μm.

We present the results of interference studies of a diode-pumped active disk element made of a gadolinium — gallium — garnet (GGG:Nd) crystal. The disk is cut perpendicular to the crystallographic axis [001], along which the pump beam propagates. With absorbing pump radiation, a thermal lens, which has been investigated by the interference methods using linearly polarised probe radiation, is formed in the disk. At the absorbed pump power up to 12 W, the interference pattern near the disk surface is a system of concentric rings. With increasing absorbed pump power up to 24 W, the outer rings transform into hexagons. It is shown that such an interference pattern in the garnet crystal is caused by thermally induced birefringence.

A simple scanning interferometer is implemented for measuring the emission linewidth of single-frequency semiconductor lasers. The free dispersion region of the interferometer is 28 MHz, the spectral resolution being 470 kHz.

We report holmium-doped fibre lasers with a Ho3+concentration of 1.6 × 1019cm-3and lasing wavelengths of 2.02, 2.05, 2.07 and 2.1 μm at a pump wavelength of 1.15 μm. The slope efficiency of the lasers has been measured. The maximum efficiency, 0.455, has been obtained at a lasing wavelength of 2.05 μm. The laser efficiency is influenced by both the optical loss in the wing of a vibrational absorption band of silica and active-ion clustering.

Optimal conditions are determined for the development of efficient diode-pumped 1-μm solid-state lasers with output powers up to a few tens of kilowatts. The thermal operating conditions are analysed for various Yb3+- or Nd3+-doped active media used in high-power laser systems. The advantages and disadvantages of these active ions and various crystal matrices are discussed. The theoretical analysis and experimental simulations allow one to determine the application fields of various laser crystals. A new concept of a multibeam (multipoint) pumping of active media is proposed.

The efficient technology is developed for growing high-quality doped single-crystal fibres by the mini-pedestal method with laser heating. The technology can be used to fabricate high-quality doped single-crystal fibres with a controllable radial gradient of the refractive index. The refractive-index profile in single-crystal fibres is studied experimentally. The efficient operation of a Nd3+: YAG single-crystal fibre laser in the waveguide regime was demonstrated upon longitudinal pumping by a laser diode.

The efficient technology is developed for growing Cr- and Ca-doped gadolinium—gallium garnet single-crystal films of thickness up to 100 μm on large substrates (76 mm) by the method of liquid-phase epitaxy from a solution—melt. The dependences of the absorption spectra of films and optical losses at 1 μm on the growth conditions and the solution—melt composition are studied. It is shown that Cr4+ions are formed in the films, and these films can be used as passive Qswitches in lasers.