Faraday isolator for a 100 J/10 Hz pulsed laser.

We report the first-ever, to the best of our knowledge, demonstration of the optical isolation of a kilowatt average power pulsed laser. A Faraday isolator capable of stable protection of the laser amplifier chain delivering 100 J nanosecond laser pulses at the repetition rate of 10 Hz has been developed and successfully tested. The isolator provided an isolation ratio of 30.46 dB in the course of an hour-long testing run at full power without any noticeable decrease due to the thermal effects. This is the first-ever, to the best of our knowledge, demonstration of a nonreciprocal optical device operated with such a powerful high-energy, high-repetition-rate laser beam, opening up the possibilities for this type of laser to be used for a number of industrial and scientific applications.

Improved stability second harmonic conversion of a diode-pumped Yb:YAG laser at the 0.5 kW level.

We report on efficient and stable, type-I phase-matched second harmonic conversion of a nanosecond high-energy, diode-pumped, Yb:YAG laser. With a frequency-doubling crystal in an enclosed, temperature controller with optical windows, 0.5% energy stability was achieved for approximately half an hour. This resulted in 48.9 J pulses at 10 Hz (489 W) and a conversion efficiency of 73.8%. These results are particularly important for stable and reliable operation of high-energy, frequency-doubled lasers.

Investigation of the lasing performance of a crystalline-coated Yb:YAG thin-disk directly bonded onto a silicon carbide heatsink.

We investigated the use of crystalline coatings as the highly reflective coating of an Yb:YAG thin disk directly bonded onto a silicon carbide heatsink. Compared to commonly used ion-beam-sputtered coatings, it possesses lower optical losses and higher thermal conductivity, resulting in better heat management and laser outputs. We pumped the disk up to 1.15 kW at 969 nm and reached 665 W of average output power, and disk temperature of 107 °C with a highly multi-modal V-cavity. These promising results were reached with this novel design despite the adoption of a cheap silicon carbide substrate having more than 3 times lower thermal conductivity compared to frequently used CVD diamond.

10-µJ few-cycle 12-µm source based on difference-frequency generation driven by a 1-kHz mid-wave infrared OPCPA.

We report on high-energy, few-cycle pulse generation in the long-wave infrared spectral region via difference-frequency generation (DFG) in GaSe and AgGaSe2 nonlinear crystals. The DFG is driven by the signal at 3.5 µm and idler at 5 µm of a high-power mid-wave infrared optical parametric chirped pulse amplification (OPCPA) system operating at a 1-kHz repetition rate. The DFG pulses contain up to 17 µJ of energy and cover a spectrum from 8.5 µm to 14.5 µm. They are generated with a conversion efficiency of 2.1 %. Compression results in 10.2-µJ pulses with sub-150-fs duration, corresponding to less than four optical cycles.

Adaptive optics system for a short wavelength mid-IR laser based on a Shack-Hartmann wavefront sensor and analysis of thermal noise impacts.

We present an adaptive optics (AO) system for a 1.94-µm laser source. Our system consists of a home-made Shack-Hartmann wavefront sensor and silver-coated bimorph deformable mirror operating in a closed-loop control scheme. The wavefront sensor used an uncooled vapor phase deposition PbSe focal-plane array for the actual light sensing. An effect of thermal afterimage was found to be reducing the centroid detection precision significantly. The effect was analyzed in detail and finally has been dealt with by updating the background calibration. System stability was increased by reduction of control modes. The system functionality and stability were demonstrated by improved focal spot quality. By replacing some of the used optics, the range of the demonstrated mid-IR AOS could be extended to cover the spectral range of 1-5 µm. To the best of our knowledge, it is the first AO system built specifically for mid-IR laser wavefront correction.

150 J DPSSL operating at 1.5 kW level.

We report on obtaining output energy of 146 J in 10 ns long pulses at 10 Hz repetition rate from Bivoj, a multi-Joule multi-slab cryogenic gas-cooled diode pumped solid state laser, by overcoming its damage threshold bottleneck. This is a 40% energy and power increase of the laser system in comparison to our previous publication and to the most powerful multi-Joule high power laser system.

Silicon Brewster plate wavelength separator for a mid-IR optical parametric source.

The availability of optical elements for the mid-infrared wavelength range, such as polarizers and wavelength separators, is limited especially when a broadband wavelength range coverage is required. We propose a polarizer based on uncoated silicon Brewster plates. A detailed analysis of the polarizer's contrast and the influence of parasitic reflections, its dependence on wavelength, and the angular misalignment is shown. Two different arrangements of the two- and four-plate polarizers are discussed. With contrast including the influence of parasitic reflections of over 103 for the whole transparency range of silicon (1.2-6.5 µm), the four-plate polarizer is an effective, low-cost, high-power compatible tool providing sufficient contrast for signal and idler beam separation of the broadband mid-infrared Type II optical parametric sources. The proposed polarizers can function as an attenuator assembly without any wave plate.

Experimental study on compression of 216-W laser pulses below 2 ps at 1030 nm with chirped volume Bragg grating.

We report on the characterization of a high-power, chirped volume Bragg grating (CVBG) pulse compressor. It includes measurements of the CVBG's diffraction efficiency, beam profile, beam quality (M2 parameter), pulse spectrum, the CVBG's temperature, and the thermal lens. These parameters were monitored for a wide range of input laser powers and with different clamping forces applied on the CVBG. This analysis was performed with a CPA-based Yb:YAG thin-disk laser system operating at a wavelength of 1030 nm, a 92 kHz repetition rate, 2 ps pulse duration, and an average output power after compression of 216 W (270 W uncompressed), which is, to the best of our knowledge, the highest value reported to date using this pulse compression technique.

New observations on DUV radiation at 257†nm and 206†nm produced by a picosecond diode pumped thin-disk laser.

We report new observations on picosecond deep ultraviolet coherent beams generated in a CLBO as the fourth and fifth harmonics of a diode pumped high average power Yb:YAG thin disk laser operating at 77 kHz repetition rate at 1030 nm. The effects of the two-photon absorption were observed, e.g. the modification of phase matching conditions, lowering of the conversion efficiency. The fifth harmonic generation (4ω+1ω) was studied for different time delays between both pump beams and for the case of excess input power of the fundamental. The latter effect suggests a possibility of increasing DUV output at short crystals. The highest output power obtained at 257 nm was 7.6 W and 1 W at 206 nm. To our knowledge these DUV output powers rank among the highest for picosecond pulses at the repetition rate of the order of magnitude of 100 kHz.

High power picosecond parametric mid-IR source tunable between 1.7 and 2.6 µm.

A high-average-power wavelength-tunable picosecond mid-IR source based on parametric downconversion has been developed. The conversion system consists of two stages, optical parametric generator and optical parametric amplifier (OPA), which are pumped by an Yb:YAG thin-disk laser operated at 77 kHz repetition rate, 1030 nm wavelength, and pulse duration down to 1.3 ps. The signal beam is amplified up to 9.2 W and the idler up to 5.4 W at OPA pumping of 43 W. Tunability between 1.70 and 1.95 µm for the signal and between 2.2 and 2.6 µm for the idler has been achieved. The system is rather simple and power scalable.

Picosecond green and deep ultraviolet pulses generated by a high-power 100 kHz thin-disk laser.

We report on the generation of the second (515 nm) and fourth (257.5 nm) harmonics from a 100 kHz diode-pumped solid-state laser operating at a wavelength of 1030 nm which uses one Yb:YAG thin disk in the regenerative amplifier and delivers 60 W of the average output power in pulses of 4 ps duration. Thirty-five W in green light and 6 W in deep ultraviolet (DUV) were achieved. The sensitivity of the second harmonic generation efficiency toward the lithium triborate crystal temperature is demonstrated in experiment. The overall conversion efficiency from NIR to DUV of 10% was achieved. The ß-barium borate and cesium lithium borate crystals were used as green to DUV convertors and compared regarding the efficiency and spectral bandwidths. The achieved output power is unique for DUV picosecond pulses.

Optimization of beam quality and optical-to-optical efficiency of Yb:YAG thin-disk regenerative amplifier by pulsed pumping.

We demonstrate an optimization method of beam quality and optical-to-optical (O-O) efficiency by using pulsed pumping. By changing the pulse duration and the peak intensity of pump pulse at the repetition rate of 1 kHz, the beam quality and O-O efficiency of the Yb:YAG thin-disk regenerative amplifier can be improved. We applied this method to the regenerative amplifier under the pumping wavelength of both 940 and 969 nm, and found that the method was effective in both pumping wavelengths. Although a Yb:YAG thin disk soldered on a copper tungsten heat sink, which has poor thermal properties compared with a thin disk mounted on a diamond substrate, was applied as a gain media, we obtained 45 mJ output with 19.3% O-O efficiency and nearly diffraction-limited beam.

Suppression of nonlinear phonon relaxation in Yb:YAG thin disk via zero phonon line pumping.

A quantitative comparison of conventional absorption line (940 nm) pumping and zero phonon line (ZPL) (969 nm) pumping of a Yb:YAG thin disk laser is reported. Characteristics of an output beam profile, surface temperature, and deformation of a thin disk under the different pump wavelengths are evaluated. We found that a nonlinear phonon relaxation (NPR) of the excited state in Yb:YAG, which induces nonlinear temperature rise and large aspheric deformation, did not appear in the case of a ZPL pumped Yb:YAG thin disk. This means that the advantage of ZPL pumping is not only the reduction of quantum defect but also the suppression of NPR. The latter effect is more important for high power lasers.

Femtosecond pulse parametric amplification at narrowband high power gas laser pumping.

In ultrashort pulse amplification a narrowband gas pump pulse laser has been used for the first time. An all-stage optical parametric chirped pulse amplifier (OPCPA) was driven by a single-shot iodine photodissociation laser. For the first time a broadband amplification was achieved in potassium dihydrogen phosphate crystal at 800 nm seeding. Ti:sapphire laser pulses stretched from 12.5 fs to 250 ps were amplified and compressed to 27 fs at a 0.5 TW output power. The results suggest using narrowband high power gas lasers as OPCPA drivers to generate petawatt beams.

Thermal-stress-induced birefringence management of complex laser systems by means of polarimetry.

The novel method of the thermally-induced polarization changes driven power losses (TIPCL) analysis in the complex laser systems has been developed. The measurement has been tested on the amplifier chain of the 100 J / 10 Hz laser system 'Bivoj' operated at HiLASE Centre. By the usage of the measured non-uniform Mueller matrix of the amplifier chain, the optimization of the ideal input and output polarization state has been calculated numerically. The results of the optimization have been applied to the laser system, thus reducing the TIPCL from originally observed more than 33% to 7.9% for CW beam and to 9% for pulsed laser beam, respectively. To the best of our knowledge, this result represents the most efficient TIPCL suppression method for complex laser systems so far. The method also allows the definition of the ideal fully polarized non-uniform pre-compensation of input beam consequently suffering from zero TIPCL.

Anti-Reflection Nanostructures on Tempered Glass by Dynamic Beam Shaping.

Reflectivity and surface topography of tempered glass were modified without any thermal damage to the surroundings by utilizing 1.7 ps ultrashort pulsed laser on its fundamental wavelength of 1030 nm. To speed up the fabrication, a dynamic beam shaping unit combined with a galvanometer scanning head was applied to divide the initial laser beam into a matrix of beamlets with adjustable beamlets number and separation distance. By tuning the laser and processing parameters, reflected intensity can be reduced up to 75% while maintaining 90% of transparency thus showing great potential for display functionalization of mobile phones or laptops.

Towards Rapid Fabrication of Superhydrophobic Surfaces by Multi-Beam Nanostructuring with 40,401 Beams.

Superhydrophobic surfaces attract a lot of attention due to many potential applications including anti-icing, anti-corrosion, self-cleaning or drag-reduction surfaces. Despite a list of attractive applications of superhydrophobic surfaces and demonstrated capability of lasers to produce them, the speed of laser micro and nanostructuring is still low with respect to many industry standards. Up-to-now, most promising multi-beam solutions can improve processing speed a hundred to a thousand times. However, productive and efficient utilization of a new generation of kW-class ultrashort pulsed lasers for precise nanostructuring requires a much higher number of beams. In this work, we introduce a unique combination of high-energy pulsed ultrashort laser system delivering up to 20 mJ at 1030 nm in 1.7 ps and novel Diffractive Laser-Induced Texturing element (DLITe) capable of producing 201 × 201 sub-beams of 5 µm in diameter on a square area of 1 mm2. Simultaneous nanostructuring with 40,401 sub-beams resulted in a matrix of microcraters covered by nanogratings and ripples with periodicity below 470 nm and 720 nm, respectively. The processed area demonstrated hydrophobic to superhydrophobic properties with a maximum contact angle of 153°.

Large-Beam Picosecond Interference Patterning of Metallic Substrates.

In this paper, we introduce a method to efficiently use a high-energy pulsed 1.7 ps HiLASE Perla laser system for two beam interference patterning. The newly developed method of large-beam interference patterning permits the production of micro and sub-micron sized features on a treated surface with increased processing throughputs by enlarging the interference area. The limits for beam enlarging are explained and calculated for the used laser source. The formation of a variety of surface micro and nanostructures and their combinations are reported on stainless steel, invar, and tungsten with the maximum fabrication speed of 206 cm2/min. The wettability of selected hierarchical structures combining interference patterns with 2.6 µm periodicity and the nanoscale surface structures on top were analyzed showing superhydrophobic behavior with contact angles of 164°, 156°, and 150° in the case of stainless steel, invar, and tungsten, respectively.

Laser-induced crystallization of anodic TiO2 nanotube layers.

In this study, crystallization of amorphous TiO2 nanotube (TNT) layers upon optimized laser annealing is shown. The resulting anatase TNT layers do not show any signs of deformation or melting. The crystallinity of the laser annealed TNT layers was investigated using X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The study of the (photo-)electrochemical properties showed that the laser annealed TNT layers were more defective than conventional TNT layers annealed in a muffle oven at 400 °C, resulting in a higher charge recombination rate and lower photocurrent response. However, a lower overpotential for hydrogen evolution reaction was observed for the laser annealed TNT layer compared to the oven annealed TNT layer.