Yb:YAG diverging beam amplifier with 20 mJ pulse energy and 1.5 kHz repetition rate.

We have developed a laser system with a combination of record-breaking parameters for rod ytterbium-doped yttrium aluminum garnet (Yb:YAG) lasers with pulse energy 20 mJ, average power 30 W, and beam quality М2 < 1.35. This record was achieved thanks to the Yb:YAG diverging beam amplifier (DBA) geometry, which allows combining efficient amplification with high average power, good beam quality, and high-energy pulse extraction.

Thermally induced depolarization in a silicon (c-Si) single crystal.

Single-crystal silicon is one of the most promising materials for producing test masses for the new generation of laser interferometers intended for the detection of gravitational waves. We studied the thermally induced depolarization of radiation in single-crystal silicon with [001] orientation at a wavelength of 1940 nm at room temperature. The value of the piezo-optical anisotropy ratio was found to be ξ = -0.63 ± 0.05 for one manufacturer and ξ = 0.62 ± 0.05 for another. The physical reason for the different signs was not established, and the search for this reason will continue. For the single-crystal silicon with the negative piezo-optical anisotropy ratio, the directions of the special orientations [C] and [P] for which thermally induced depolarization and thermal lens astigmatism vanish, respectively, were determined. When ξ is close to -0.5, the special orientations are close to [111], making this orientation preferable to the other traditionally used orientation, [001].

Thermo-optical properties of terbium sesquioxide (Tb2O3) ceramics at room temperature.

Thermo-optical properties of several (Tb1-xYx)2O3 ceramic samples were investigated in this Letter. The linear absorption and thermal conductivity coefficients, as well as the power dependence of thermally induced phase and polarization distortions of laser radiation, were measured. In addition, the effective thermo-optical constants Peff and Qeff were estimated. Thermo-optical properties of the studied ceramics were compared with those of the widely used terbium gallium garnet. It was shown that the material under consideration is highly promising for Faraday isolators operating at high average power laser radiation.

Thermo-Optical Studies of Laser Ceramics.

A cycle of works on manufacturing and studying laser and magnetooptical ceramics with a focus on their thermo-optical characteristics performed by the research team is analyzed. Original results that have not been published before such as measurements of the Verdet constant in the Zr:TAG, Re:MgAl2O4, and ZnAl2O4 ceramics are also presented.

High-purity CVD-ZnSe polycrystal as a magneto-active medium for a multikilowatt Faraday isolator.

Prospects for using ZnSe polycrystals synthesized by the chemical vapor deposition (CVD) method for the development of Faraday isolators for high-power radiation at a wavelength of 1076 nm are investigated. A Faraday isolator was built by a conventional scheme for room temperature operation. No thermally induced depolarization was observed in the device for the laser power range up to 1270 W, which is the main limiting factor for powerful isolators. It was demonstrated experimentally that thermally induced depolarization is not expected at powers up to 2.5 kW.

High-power Faraday isolator on a uniaxial CeF3 crystal.

We have created the first high-power Faraday isolator on an anisotropic magneto-optical element (MOE). The isolator is based on one MOE of a uniaxial CeF3 crystal and ensures an isolation degree of 30 dB at a high average laser radiation power of 700 W. The limitations due to the anisotropic nature of the crystal do not impose significantly more stringent requirements, either for the beam or the MOE.

Thin-rod Yb:YAG amplifiers for high average and peak power lasers.

The concept of a high-power thin-rod Yb:YAG laser amplifier with high-brightness diode pumping was proposed. The principle of the amplifier parameter variation aimed at achieving an efficient signal gain at different power levels was developed. Three versions of thin-rod gain modules were implemented, where small and strong signal gains were studied experimentally. The ultrafast laser system with high average power (28 W) and high pulse energy (2.5 mJ) was created on the basis of the unique thin-rod gain modules.

Matrix multi-pass scheme disk amplifier.

Two multi-pass optical schemes of a disk laser amplifier have been proposed. Different variants of both the schemes with the smallest amount of optical elements and a lens in the active element taken into account have been calculated. For 64 passes of radiation through the active element, the average power of ∼50 W with ∼10% optical-to-optical efficiency in a pulse-periodic regime with a repetition rate of 10 kHz and a pulse duration of 2 ns was obtained at the amplifier output. The small signal gain amounted to 200.

Cryogenically cooled CeF3 crystal as media for high-power magneto-optical devices.

The thermally induced depolarization and Verdet constant of CeF3 crystals-their most important characteristics-have been studied in the 79-293 K temperature range. It has been found that thermal effects reduce substantially upon cooling down to 79 K and the Verdet constant grows in inverse proportion to the temperature. It was shown that CeF3 crystals are not inferior to TGG as a medium for Faraday isolators, including cryogenic ones.

Thin-tapered-rod Yb:YAG laser amplifier.

The concept of the thin-tapered-rod Yb:YAG amplifier with waveguide pumping is proposed and implemented. It is shown that such an amplifier demonstrates considerably higher small signal gain and is less affected by thermal effects than the thin-rod ("single-crystal fiber") amplifier. In the developed amplifier, a small signal gain as high as 23 per pass is achieved.

Thermal effects in the DKDP Pockels cells in the 215-300 K temperature range.

The thermal and electro-optical effects in Pockels cells with deuterated potassium dihydrogen phosphate (DKDP) crystals in the 215-300 K temperature range were investigated. Half-wave voltage decreases linearly with cooling, thereby it reduces seven times (up to 1 kW) with cooling to 215 K. The optical power of the thermally induced lens falls twice on cooling to 215 K from 300 K; thermally induced depolarization is almost independent of temperature and determined by input radiation power. A significant reduction of the thermally induced depolarization in a DKDP crystal with the axis inclined to the polarization plane of the beam was demonstrated numerically and experimentally. Thus, the DKDP crystals cooled to 215 K allow the creation of a low-voltage Pockels cell working with a high average and peak laser power.

Faraday rotator based on TSAG crystal with <001> orientation.

A Faraday isolator (FI) for high-power lasers with kilowatt-level average power and 1-µm wavelength was demonstrated using a terbium scandium aluminum garnet (TSAG) with its crystal axis aligned in the <001> direction. Furthermore, no compensation scheme for thermally induced depolarization in a magnetic field was used. An isolation ratio of 35.4 dB (depolarization ratio γ of 2.9 × 10-4) was experimentally observed at a maximum laser power of 1470 W. This result for room-temperature FIs is the best reported, and provides a simple, practical solution for achieving optical isolation in high-power laser systems.

Effect of full compensation of thermally induced depolarization in two nonidentical laser elements.

Thermally induced depolarization of radiation introduced by a system of two optical elements separated by a quartz rotator has been analyzed. The conditions of full compensation of thermally induced depolarization for two nonidentical optical elements have been found. The model experiment has demonstrated that full compensation in two optical elements of different materials is possible without a quartz rotator between them.

Thermo-optical properties of terbium-aluminum garnet ceramics doped with silicon and titanium.

The Verdet constant and thermo-optical characteristics of a Si-doped and Ti-doped terbium aluminum garnet ceramics have been investigated. It is shown that the Verdet constant of the samples is ∼40% higher than that of TGG ceramics at 1064 nm. The best samples of Si:TAG have magneto-optical figures of merit more than 1.5 times greater than those of TGG ceramics. Si:TAG is better than TGG ceramics as a medium for high-power Faraday isolators.

Thermal effects in Yb:YAG single-crystal thin-rod amplifier.

This work presents a new design for the laser gain module based on a ytterbium-doped yttrium aluminum garnet (Yb:YAG) single-crystal thin rod. Thermal effects (temperature, phase, and polarization distortion of laser radiation) and small signal gain are investigated both experimentally and theoretically. We then analyzed the influence of thermal effects and amplified spontaneous emission on the power scaling of the laser based on the gain module. A small signal gain as high as 3.3 per pass was experimentally achieved.

Terbium gallium garnet ceramic Faraday rotator for high-power laser application.

A terbium gallium garnet (TGG) ceramic Faraday rotator (FR) with an isolation ratio of 33 dB was demonstrated at a laser radiation power of 257 W. This FR can be equipped with a large optical aperture by using ceramics technology to prevent laser damage at high-energy pulse operation. The thermal lens of a 257 W laser with a beam diameter of 2.6 mm had a focal length of 9.5 m, which is easily corrected using a spherical lens to suppress the undesirable effects of thermal lensing. The rotation angle of the FR was stabilized by water cooling. The results indicate that the TGG-ceramic-based FR is suitable for high-energy laser systems with high repetition rates.

Improving characteristics of Faraday isolators based on TAG ceramics by cerium doping.

A Faraday isolator (FI) based on a new magneto-optical medium-Ce:TAG ceramics-has been fabricated and studied in experiments. Compensation of thermally induced depolarization in the FI increases the isolation ratio from 31 to 39 dB at the laser power of 300 W. Estimates predict stable operation of the device with kilowatt laser power.

High-power Faraday isolators based on TAG ceramics.

The Faraday isolator based on a new magneto-optical medium--TAG (terbium aluminum garnet) ceramics was implemented and investigated experimentally. The magneto-optical element was temperature-stabilized using water cooling. The device provides a stable isolation ratio of 38 dB at 300 W laser power. Estimates show high performance of the device at a kilowatt laser power.

Study of the thermo-optical constants of Yb doped Y2O3, Lu2O3 and Sc2O3 ceramic materials.

Thermally induced depolarization and thermal lens of three Konoshima Chemical Co. laser-ceramics samples Yb(3+):Lu(2)O(3)(C(Yb) ≈ 1.8 at.%), Yb(3+):Y(2)O(3)(C(Yb) ≈ 1.8 at.%), and Yb(3+):Sc(2)O(3) (C(Yb) ≈ 2.5 at.%) were measured in experiment at different pump power. The results allowed us to estimate the thermal conductivity of the investigated ceramic samples and compare their thermo-optical properties. The thermo-optical constants P and Q and its sign measured for these materials at the first time.

Self-compensation of thermally induced depolarization in CaF2 and definite cubic single crystals.

Compensation of thermally induced depolarization in laser active elements at small birefringence without additional phase elements was proposed and observed experimentally. Requirements to the crystals were formulated. An order of magnitude reduction of depolarization degree was obtained experimentally. A further modification of the scheme was developed.