This PDF file contains the front matter associated with SPIE Proceedings Volume 11664, including the Title Page, Copyright information, and Table of Contents.

Welcome and Introduction to SPIE Photonics West LASE conference 11664: Solid State Lasers XXX: Technology and Devices

Extreme-light laser is a universal source providing a vast range of high energy radiations and particles along with the highest field, highest pressure, temperature and acceleration. It offers the possibility to shed light on some of the remaining unanswered questions in fundamental physics like the genesis of cosmic rays with energies in excess of 1020 eV or the loss of information in black-holes. Using wake-field acceleration some of these fundamental questions could be studied in the laboratory. In addition extreme-light makes possible the study of the structure of vacuum and particle production in "empty" space which is one of the field’s ultimate goal, reaching into the fundamental QED and possibly QCD regimes. Looking beyond today’s intensity horizon, we will introduce a new concept that could make possible the generation of attosecond-zeptosecond high energy coherent pulse, de facto in x-ray domain, opening at the Schwinger level, the zettawatt, and PeV regime; the next chapter of laser-matter interaction.

LASE Hot Topics presentation on Development of High-Power Ultrafast Lasers: Status and Perspectives

LASE Hot Topics presentation on Photonic-based Quantum Computing

Autonomous driving and automated ships are in increasing demand and the performance for pattern recognition and safety functions are essential. Harsh weather conditions lower the atmospheric transmission for useable eye-safe wavelengths and water vapor peaks prevent most laser light propagation for longer distances. In LIDAR application this problem can be solved with increased peak power. However, high peak power solutions with high repetition rate do not exist or they are bulky and expensive. Moreover, not all applications need the highest peak powers or high repetition rates. We present a collection of eye-safe LIDAR laser sources with varying laser pulse parameters for demanding conditions. Wavelengths ranging from 1400 nm to 1560 nm with peak powers from a few Watts to up to 18 kW and repetition rates up to 100 kHz. Naturally, for a good temporal resolution a short pulse operation is necessary and best sources presented here produce sub-10 ns pulses. Such high peak power, high repetition rate lasers present the state-ofthe-art performance for long range LIDAR, enabling hundreds of meters detection distance in poor weather, currently unavailable in earlier commercial solutions.

We have demonstrated pulse-on-demand operation of a 2 μm AO Q-switched Tm: YAG laser. Burst pulse operation, the number of pulses in a packet, and pulse interval can be controlled by RF power modulation and pump control. The burst packet included up to 10 pulses at a pulse interval of 138 s. The pulse energy and pulse width of a single pulse in the packet were ~0.7 mJ and 70–150 ns, respectively. In addition, we have demonstrated that the envelope of the burst pulse train could be designed as required.

We report mechanically Q-switched 2.94 µm Er:YAG laser based on spinning mirror with 805 mJ output energy in a single 61 ns pulse at 1 Hz repetition rate and 670 Hz rotational rate of the spinning mirror. This record output energy was achieved with the use of 300 mm long MQS Er:YAG laser cavity consisted of 70% output coupler, 7x120 mm AR coated Er(50%):YAG crystal and spinning HR mirror. The maximum output energy was limited by the optical damage of the Er:YAG AR coatings.

We describe a compact mid-IR source utilizing an intracavity, non-critically phase matched potassium titanyle arsenate (KTA) optical parametric oscillator (OPO) placed inside a passively Q-switched (PQS), 1.064 µm Nd:YAG laser resonator. A 45-degree dichroic beam splitter was employed to split the 1.064 µm resonator leg from the 1.5 μm/3.5 μm KTA OPO cavity that was singly resonant for the 1.5 μm signal. The 20 mm long KTA crystal was placed in the shared-path section of both cavities, with the KTA OPO output coupler partially reflective at 1.5 μm, and highly transmitting at 3.5 μm. With the Nd:YAG pumped by a 3-λ 12-bar diode stack at 5 Hz PRF, the KTA OPO generated 23.1 mJ signal pulses at 1.5 μm and 9.8 mJ idler pulses at 3.5 μm. To further increase the 3.5 μm energy, a cadmium silicon phosphide (CSP) optical parametric amplifier (OPA), phase matched for 1.5 μm-pumped amplification of 3.5 μm radiation, was placed immediately after the KTA OPO output coupler. A maximum 3.5 μm pulse energy of 13.2 mJ was measured after the OPA, with an additional 4.2 mJ generated at 2.8 μm. The 3.5 μm pulses had a measured temporal duration (FWHM) of 27 ns, corresponding to a peak-power of approximately 490 kW. This paper will detail the Nd:YAG pumped intracavity KTA OPO / CSP OPA and optimization of its performance as a pulsed midIR source.

A 1908 nm Tm:YLF laser, passively Q-switched (PQS) using a Cr:ZnS saturable absorber, is shown to generate significantly higher pulse energy and peak power than previously reported. Using a compact 13 cm-long, plano-concave resonator cavity, the end-pumped Tm:YLF laser generated 15.6 mJ, 26ns FWHM pulses, corresponding to a peak power of 600 kW, with a 0.5kHz PRF, and 7.8W average power. A 10 cm-long laser generated 10 mJ, 20 ns long pulses, with a 1 kHz PRF and 10 W average power. A laser utilizing the shortest, 4.5 cm-long cavity, generated 7 ns pulses with a pulse energy of 3.7mJ. All of the laser configurations incorporated a Volume Bragg Grating (VBG) high reflectivity cavity mirror to lock the laser wavelength and polarization. To minimize the energy fluence at Cr:ZnS saturable absorbers (To=88.6% and To=91.6%), they were placed near the concave output coupler where that laser mode field area of the plano-concave laser resonator was at its maximum.

We report on mid-IR Fe:ZnSe master oscillator power amplifier (MOPA) laser system operating at room temperature (RT) pumped by a radiation of mechanically Q-switched Er:YAG laser operating at 2.94 m, at 3Hz repetition rate. The maximum output energy was as high as 250mJ in ~250 ns pulses. The RT gain-switched Fe:ZnSe master oscillator demonstrated tunability over 3.60-5.15 µm spectral range with a maximum output energy of ~3 mJ. The output energies of 12 (8), 34 (25) and 60 (48) mJ were demonstrated at 4.4 (4.1) µm in the 1st, 2nd and 3rd stages of amplifier, respectively, with a total pump energy of 200 mJ.

We report the first room temperature gain switched Fe:ZnSe hot-pressed ceramic laser pumped by 2.94 μm radiation of mechanically Q-switched Er:YAG laser. The maximum output energy was obtained to be 2.5 mJ at 32 mJ of pump energy. The measured slope efficiency was 8% with respect to the pump energy. In this experiment, 3.2 mm hot-pressed Fe:ZnSe ceramic sample was used which had 6% active transmission at the pump wavelength. By using absorption cross-section, the concentration of Fe2+ ions was calculated to be N=9*10^18 cm-3.

YAG and sapphire single crystal fibers (SCF) have the potential to be good candidates for medical, dental, industrial, and military applications requiring robust, highly transmissive, passive delivery systems capable of handling 10s - 100s Watt power levels in the 2-5 micron spectral region. This work will present latest results of SCF grown by the Laser Heated Pedestal Growth (LHPG) technology developed at Phoenician Photonics and will address factors influencing their transmission and power handling performances in the Mid IR.

We report on UV-pumping of a visible Tb:LiLuF4 laser emitting at 544 and 588 nm. Pumping with a frequency-doubled Ti:sapphire laser at 359 nm significantly improves the absorption efficiency compared to conventional in-band pumping at ~488 nm and cross relaxation from the excited level 5D3 efficiently populates the upper laser level 5D4. In this way, optical efficiencies of 29% and 12%, respectively, are obtained with respect to the incident pump power. A passively Q-switched Tb:LiLuF4 laser at 544 nm using a Co2+-doped MgAl2O4 as a saturable absorber yields 23-µJ pulses at 3.5 kHz with a pulse duration of ~200 ns.

External cavity diamond Raman laser pumped by 8.3 ns (10 Hz) single frequency 532 nm pump was studied. The output energy of 4.2 mJ and 2 mJ at first (573 nm) and second (620 nm) Stokes, respectively, was demonstrated for 12 mJ of pump energy with the total conversion efficiency of 51%. The minimum first Stokes pulse duration of 420 ps in backward and 500 ps in forward directions were measured under 3.5 mJ of pump energy and 7.5 mJ (9 mJ) of output energy in backward (forward) Raman signal.

Green disk lasers are the superior tool among industrial lasers operating in the visible wavelength regime, whenever a combination of high power and very good beam quality is desired. We report latest lab and application results, showing the potential of the technology beyond today´s commercially available products and use cases. TRUMPF´s green TruDisk lasers are based on a well-established and proven thin disk oscillator design with an added internal-resonator nonlinear crystal for the generation of second harmonic of 515 nm. Power level and beam parameter product of these devices have significantly been improved recently, leading to a commercially available product of 2 kW in combination with a 100 µm fiber core diameter and NA 0.1. Here, we report results on our new 3 kW green TruDisk laser, showing the progress achieved in making this power-enhanced laser a stable, reliable tool for industrial applications. Moreover, lab results will be presented that indicate the potential for further power scaling beyond 3 kW output power. The benefits of visible laser light in comparison to standard IR laser wavelengths around 1000 nm in terms of machining highly reflective materials are demonstrated. Due to the much higher absorption at room temperature, process stability, efficiency and reproducibility are significantly improved. The green laser light enables spatter-free weld connections of copper parts, and welding in heat conduction mode, such as on multiple layers of copper foils. This makes green TruDisk lasers an extremely versatile and easy-to-use tool for a range of applications, for example in the context of eMobility and power storage.

There is a growing demand for MHz-repetition rate industrial ultrafast laser sources that operate from the ultraviolet to the near IR. This paper discusses third harmonic generation (THG) of a high repetition-rate, femtosecond dual function Yb:YAG thin slab amplifier. Both pre-amplifier and power amplifier functions take place in a single crystal and homogeneous pumping of the thin slab is achieved by means of novel diode bar imaging. Fundamental output powers of <130 W at a wavelength of 1030 nm are maintained across a range of pulse repetition frequencies from 1 MHz to 40 MHz with a pulse duration of 900 ± 100 fs and M2 ~ 1.2. Second harmonic generation (SHG) at 515 nm with a conversion efficiency of 76% with respect to the incident fundamental is achieved using type-1 critically phase-matched LBO resulting in average powers of <90 W, a pulse energy of <90 µJ, with an M2< 1.2 and pulse durations of 800 ± 100 fs. The fundamental and second harmonic outputs were frequency mixed in a critically phase matched LBO crystal to produce third harmonic generation (THG) at 343 nm. Several lengths of type-2 THG LBO were investigated alongside type-1 THG LBO, with <50 W of UV produced at single-pass conversion efficiencies of <40% with respect to incident fundamental power at the SHG stage, corresponding to pulse energies of <50 µJ. Beam quality and pulse durations at 343 nm were investigated across 1–40 MHz.

We study the decay kinetics of excited state of GR1 center in diamond using 633 nm pump pulse and 658 nm CW probe beam. The absorption saturation followed by a strong absorption of the probe below steady-state transmission level was observed. The recovery of the centers to the original level occurs slowly with ~500 ns time scale and reveals multi-exponential rates with no long-term bleaching or residual absorption. The branching ratio for relaxation process to the ground state and to the metastable state was estimated to be ~ 0.50.

Long-time operation of passively Q-switched monolithic Nd:YAG/Cr⁴⁺:YAG ceramic microchip lasers was carried out to test the durability as a laser ignitor of internal combustion engines. The pulse energies of three microchip lasers were monitored, operating at three different configurations of repetition rates and temperatures: 20 Hz/room temperature, 20 Hz/90°C, and 80 Hz/ room temperature. No significant degradation of 3 mJ level output energies was observed over 32,000 hours or 3.6 years for the all operating conditions.

We report technological advances in thin-disk laser technology enabling further scaling of average output power and beam quality. A newly developed resonator design serves as a universal building block for industrial-grade thin-disk lasers from 6 to 24 kW. The robust resonator design allows for power levels beyond 12 kW from a single disk with a beam parameter product (BPP) of ~ 4 mm*mrad. By polarization combining of the output of two resonators, i.e. two laser disks, the power can be doubled to up to 24 kW while maintaining the good beam quality. The mentioned properties render the new TruDisk lasers ideal for high-throughput laser material processing. With slight modifications of our setup we also achieve a BPP of ~ 2 mm*mrad with < 8 kW output power. The optical setup provides two fiber outputs, switchable on a < 100 ms timescale, that can be equipped with different types of exchangeable processing fibers for maximum productivity and flexibility. It is possible to use TRUMPF’s BrightLine Weld (BLW) technology in combination with a 50/200 µm dual-core fiber at each fiber port. The BLW technology allows for distribution of the laser power between the 50 µm fiber core and a 200 µm ring, enabling a significant increase in productivity and quality in welding. We use BLW with a 50/200 µm dual-core fiber for welding of stator hairpins for electric drives at a quality and speed unattained so far.

We describe sub-TW class modular laser with multiple outputs in the visible and near-infrared wavelength range based on Optical Parametric Chirped Pulse Amplification (OPCPA) and Transient Stimulated Raman Chirped Pulse Amplification (TSRCPA). It provides 1.2 ps pulses with an energy of ~20 mJ at 1030 nm from two-stage double-pass Yb:YAG chirped pulse amplifier; ~20 fs pulses with an energy of <2 mJ at a central wavelength of 790 nm from OPCPA and 145 fs pulses with an energy of 0.6 mJ at a central wavelength of 1135 nm from TSRCPA. White light supercontinuum extending from 500 nm to 2300 nm in the YAG was used for seeding both a three-stage OPCPA based on BBO and a two-stage TSRCPA based on KGW crystals. The wide OPCPA bandwidth was maintained by the temporal shaping of pump pulses. We demonstrate the expansion of the spectrum of pulses amplified in the TSRCPA by ~10 times in comparison with the spectral bandwidth of the pump pulses. A maximum conversion efficiency of 55% was achieved in the second TSRCPA stage. The amplified pulses after compression were more than 8 times shorter than the pump pulses.

Driving nonlinear processes in scientific and upcoming industrial applications has been a topic with increasing interest and activities in the last years. Examples are the production of very short wavelengths via direct driven plasma light-sources (incoherent) or high-harmonic generation (coherent), optical parametric chirped pulse amplification to different wavelengths and shorter pulses and direct pulse shortening via self-phase modulation and subsequent compression down to the few-cycle pulse duration regime. We report on multi100W ultrafast laser sources with 1ps pulse durations and below and <10mJ pulse energies based on the InnoSlab laser-concept. Achieved beam qualities are M2<1.2 at average power stabilities in the 0.1% regime. Measured pulse stabilities are around 1% (rms) and pulse intensity contrasts well exceed 50dB for preceding or following pulses. These stability values together with the high average pulse power are very well suited for use as drivers of nonlinear optical processes. We show that these sources can be integrated into very compact housings with full computer control which additionally eases the practical use for further processing of the radiation.

We present a new concept of thick-disk TiSa amplifier developed for increasing repetition rate of multi-TW laser at 100Hz repetition rate. Thanks to the management of thermal lens and transverse lasing effect, the design of 300mJ amplifier at 100Hz repetition rate is presented. The full characterization of the gain module is done, and the upgrade for J class amplifier is presented. Those amplifiers are pumped by new laser developed by Thales, THEIA family, emitting more than 700mJ at 532nm for 100 and 200Hz repetition rate.

We demonstrate the 6.5-fold compression of 1-mJ 200-fs pulses by spectral broadening in a gas-filled multi-pass cell and subsequent chirped-mirror compression. A coherently-combined fiber laser source is compressed with an overall power efficiency of ~95% resulting in a record compressed power of >950 W. In a second experiment, the same overall spectral broadening is obtained with the nonlinear phase per medium pass enhanced to well beyond 3 rad while still retaining all the favorable properties of the multi-pass cell configuration. This may enable an even higher power efficiency beyond 95%.

We report on femtosecond lasers with more than one kilowatt of average output power and pulse energies around 10 mJ. Power scaling is enabled by scaling the crystal dimensions of the slab-like amplifier crystal as well as by applying multi-stage booster amplifier configurations. The demonstrated femtosecond laser is aimed to serve in high throughput industrial applications in the aerospace industries in order to generate hybrid laminar flow controlled structures. Further industrial and scientific applications will be reviewed as well.

We report a full experimental comparison study on the injection of a Ti:Sa multi-TW amplifier chain with a standard 15 fs Ti:Sa oscillator and a 35 fs frequency doubled fiber oscillator. The study highlights that the Ti:Sa oscillator with high performances in term of pulse duration and spectral width can be replaced by the frequency doubled fiber oscillator to seed Ti:Sa amplifier chains without almost any compromise on the output pulse duration and the picosecond contrast. Finally, we demonstrate for the first time of our knowledge a 30 TW and 33 fs Ti:Sa amplifier injected by a fiber oscillator.

Ultralow phase-noise mode-locked lasers are crucial for real-world applications. We present two accomplishments: (1) short-term FF CEP stabilization of a SESAM mode-locked Er:Yb:glass laser at 1.55 um with timing jitter below 3 as (1-3 MHz) and (2) a hybrid solution adding a FB technique addressing slowly varying sources of interference to the FF system that demonstrates 75 hours of stabilization with a minimally detrimental effect amounting to a timing jitter of 11 as.

The engineering of spatial modes via conventional methods of phase transformation can only be efficiently achieved at a specific wavelength due to the monochromatic nature of a complex phase profile. Nevertheless, holographic phase masks – i.e. complex phase structures coupled with the transmissive volume Bragg grating, possess a high degree of tunability and achromatism. Such complex phase elements, holographically recorded in the volume of photo-thermo-refractive glass, allow them to withstand high average-power operating conditions uniquely suited for intra-cavity mode conversion. Here we demonstrate a linear, wavelength-tunable, continuous-wave Yb3+:KYW laser capable of emitting customizable spatial modes by mean of holographic phase masks.

Many applications such as high power laser locking and seeding, atom cooling and trapping or optical heterodyning and coherent communication, require a single-frequency emission with high frequency stability and low noise. Narrow linewidth single-frequency emission at 1.064µm is well known and usually monolithic ring cavities (such as Non Planar Ring Oscillators) is a good solution. In this paper, we demonstrate a stable single-frequency emission at 1.064μm with noise reduction at a power of 500mW based on a linear monolithic cavity with Nd:YAG amplifier. In the past, we have already demonstrated the efficiency and the reliability of monolithic cavities used in our standard product line (LaserBoxx LCX-532S, LCX-553S and LCX-561S). In this presentation, single-frequency operation is achieved by a double Lyot filter, the first filter selecting the emission band of Nd:YAG and the second filter selecting the longitudinal mode and achieves the single-frequency operation. We report narrow linewidth <100kHz. By introducing a nonlinear crystal (KTP for second harmonic generation (SHG) at 532nm), we modify the laser dynamics and reduce the oscillation relaxation and consequently the laser noise (<0.2% RMS). We also report a good laser frequency stability due to our monolithic cavity (<100 MHz for short-term behavior and <1pm for long-term behavior), a frequency tuning capabilities (up to 10pm without mode hopping and up to 1nm with mode hopping), a power stability less than 2% for a laser base temperature from 15°C to 45°C at 500mW, a good optical efficiency (<25%) and a high beam quality (M²<1.2).

Optical parametric amplification is a coherent mechanism whereby an optical signal is amplified by a pump via the generation of an idler field and it is the key ingredient of parametric oscillators. Here we demonstrate optical parametric amplification by monolayer transition-metal dichalcogenides, showing that amplification can be attained over a propagation through an atomic layer. The surface-like second-order nonlinear interaction bypasses phase-matching constraints, enabling ultrabroadband collinear amplification, generally unattainable due to material dispersion. Moreover, the amplification process is invariant over signal and pump in-plane polarizations. Our experimental findings pave the way for innumerable applications in nanophotonics and quantum information technology.

Two novel cryogenically cooled non-cubic wurtzite structure Cr²⁺,Fe²⁺:Zn_1-xMg_xSe (x ≈ 0.2 and x ≈ 0.3) single crystals co-doped with Cr²⁺ and Fe²⁺ ions with thickness of 2.5 and 5 mm, respectively, were investigated under two excitation wavelengths of the Q-switched Er³⁺:YLF (λ ≈ 1.73 μm) and Er³⁺:YAG (λ ≈ 2.94 μm) lasers. Absorption and fluorescence spectra, fluorescence lifetimes as well as laser output characteristics for both Cr²⁺ and Fe²⁺ doping ions were measured at 78 K. Both Cr²⁺,Fe²⁺:Zn_1-xMg_xSe laser systems were able to generate radiation from Cr²⁺ as well as Fe²⁺ active ions depending on appropriate pumping wavelength and a set of laser cavity mirrors. Moreover, Fe²⁺ ions mid-IR lasing using the Cr²⁺ → Fe²⁺ ions energy transfer at ~4.57 μm and ~4.8 μm for magnesium content of x ≈ 0.2 and x ≈ 0.3, respectively were realized for ~1.73 μm Q-switched Er³⁺:YLF laser pumping. The results present an opportunity to develop novel mid-IR 4.4 – 4.9 μm coherent laser sources based on non-cubic AIIBVI matrices using direct Fe²⁺ ions pumping at ~2.94 μm as well as excitation via co-doped Cr²⁺ ions at ~1.73 μm.

Alexandrite is a promising, highly efficient laser material enabling wavelength tunability for applications in the field of spaceborne Earth observation. The Horizon 2020 project GALACTIC has been initiated to establish a fully Europeanbased supply chain for high-quality functionally coated Alexandrite laser crystals. To reach this goal, the project consortium, consisting of Optomaterials S.r.l., Altechna Coatings and the Laser Zentrum Hannover e.V., works closely together to firstly develop and improve the crystal manufacturing and coating technologies and secondly to characterize the developed coated laser crystals. Finally, the Technology Readiness Level (TRL) of 6 will be achieved for typical Earth observation space missions. The necessary qualification test campaign will conclude the development process and will enable European non-dependence from the Alexandrite laser crystal market currently dominated by non-European suppliers.

Optical parametric oscillators (OPOs) with broadband output in the mid-infrared are well suited for high resolution, high sensitivity multi-species infrared spectroscopy. Applications often require remote delivery, either over open-paths or via fiber delivery to inaccessible environments. Here we explore a flexible, easy to use, high-resolution technique in a challenging, spectroscopically cluttered landscape containing both narrow line-like and broad continuum-like features. Quantitative, high resolution, time-resolved, simultaneous measurement of water, methane, methanol and C2H7NO (MEA) concentrations are performed via fiber delivery of the OPO light. Detection limits, and the impact of the presence of one species on the others, are also explored.

We present the dependence of the main spectroscopic properties of Ho:YAG on Ho doping contraction estimated at room temperature. Five Ho:YAG crystals with various Ho-doping concentrations were grown by Czochralski method: 0.34 at.% Ho/Y, 0.51 at.% Ho/Y, 0.90 at.% Ho/Y, 1.61 at.% Ho/Y, and 1.98 at.% Ho/Y. Detailed absorption spectra with high resolution were measured in the range from 185 up to 6500 nm. The emission spectra were measured in the range from 500 up to 3500 nm under excitation in UV (450 nm) and IR (1862 nm) range. The ⁵I₇ upper-laser-level lifetime was measured using the confocal method. From the measured transmission data, the absorption cross-sections in all investigated spectral ranges were determined. Using the absorption spectrum data, the Judd-Ofelt analysis was performed to reach nett spontaneous radiative lifetime of the ⁵I₇ level. Based on these data and measured fluorescence spectra, the emission cross-sections for 2.1 μm laser band were determined together with non-radiative relaxation rates. It was found that the Ho-doping concentration significantly influenced mainly the upper laser level lifetime, which drops from 7 ms for the lowest doping to 5.8 ms for the highly doped Ho:YAG sample.

Lasing of Tm³⁺-doped yttrium aluminum perovskite (Tm:YAlO3) microchip laser was investigated. A novel 1.7 μm in-band diode pumping was compared with traditional 0.8 μm pumping of thulium-doped lasers. The sample was b-cut (Pbnm) cylindrical microchip, 5 mm in length and 3 mm in diameter, with its planar surfaces polished and coated: HT for pumping wavelengths (T < 95 % at 1690 nm, T < 95% at 790 nm) and HR R < 99.8 % at 1900–2050 nm at the pumping end and R=97.5–98.5 % at 1900–2050 nm at the output end. The doping concentration was 4 at.% (Tm/Y). The sample was wrapped in an indium foil and held in a water-cooled (11°C) copper holder. For both pumping wavelengths, the sample was longitudinally pumped using a fiber (core diameter 400 μm, NA=0.22) coupled laser diode operating in QCW (25% duty cycle) and CW regime. Under the 1.7 μm diode (delivering up to 30 W at 1680 nm) pumping, obtained slope efficiency (with respect to absorbed power) was 57%. The highest obtained output power amplitude was 14.4 W in QCW and 9.4 W in CW. Under the 0.8 μm diode (delivering up to 20 W at 793 nm) pumping, obtained slope efficiency (with respect to absorbed power) was 58%. The highest obtained output power amplitude was 6.6 W in QCW and 6.9 W in CW. Laser emitted at 1988 nm under all pumping regimes.

A detailed modeling of thermal lensing in a microchip Nd:YAG laser is presented. A range of different pump spot sizes as well as pump power levels was explored to determine their effect on thermal lensing, cavity stability and mode-matching was explored. Optimum pump focusing and pump power level conditions were estimated and were shown to strongly depend on the pump focusing condition. This can be used as an additional degree of freedom for optimization of microchip lasers. These results can enable power scaling of microchip lasers.

Efficient diode-pumped continuous-wave Yb:CaF₂ laser was demonstrated with multi-watt output power at room temperature. The laser produced up to 9.6 W of output power in the fundamental mode with 64% of optical-to-optical efficiency and up to 71% slope efficiency with respect to the absorbed pump power. Wavelength tuning range was measured to be 60 nm (1011-1071 nm). To the best of our knowledge these are the highest efficiency factors for directly diode-pumped Yb:CaF₂ lasers.

A simple approach to estimate the values of thermal lensing in diode end-pumped Nd:YVO₄ and Nd:GdVO₄ crystals for any experimental conditions is presented. It is based on the fact that dioptric power of thermal lensing linearly depends on the generalized thermo-optic coefficient. Knowledge of this coefficient allows not only to predict thermal lensing values but also to compare them from different experiments.

Application of new nonlinear-optical crystals for development of novel methods for nonlinear-optical conversion of solid-state laser radiation into mid-infrared range presents an important task of modern infrared photonics. Significance of this challenge is caused not only by limited choice of solid-state sources of coherent radiation in mid-infrared range, but also by potential applications of such sources in science, technology, medicine, and biology. Efficient method of optical frequency down-conversion is the difference-frequency generation (DFG) allowing the single-pass conversion of the pump and signal optical frequencies lying in the near-IR range into the mid-IR idler wave. The narrowband, frequency stable signal wave for the DFG is generated in our setup by the stimulated Raman scattering (SRS) in a cubically nonlinear crystal (CaCO₃, BaWO₄, or diamond). In order to present a comparative study, the LiGaSe₂ and LiGaS₂ crystals with the equal length of 8 mm were used. Narrowband idler waves at the discrete wavelengths of 4.6 / 5.4 / 7.5 / 9.2 μm and high pulse energies in the range 10 - 50 μJ were generated. The measured linewidths were close to the monochromator resolution limit of < 2 cm^-1 (~10 nm @ 7.5 μm) and they can be even narrower. It can be supposed that the idler wave linewidth should be comparable with the Raman mode linewidth (ΔνR = 1.2 2.7 cm^-1). Generation at 10.8 μJ was achieved in LiGaSe₂ only and the output energy was at in the order of ~100 μJ (close to the measuring probe resolution limit).

We demonstrated continuous-wave dual-wavelength operation of a Nd:CALGO laser with intracavity conerefringent element. The laser produced conically refracted dual-wavelength radiation output with more than 100 mW of power. Dual-wavelength radiation was generated owing to the broad gain bandwidth of the Nd:CALGO crystal.

Dual-wavelength operation of a diode-pumped Yb:YAP laser using a 1 mm-thick birefringent filter was demonstrated. Multiple wavelength pairs with wavelength separation ranging from 1.8 nm up to 11.3 nm could be generated. The gain balancing for oscillating wavelength pairs was based on the differential loss around a single transmission peak of the filter. In this regime of operation, separation of the wavelength pairs was not limited to the free-spectral range of the filter. This laser can be used to generate THz radiation using a photomixing technique.

Dual-wavelength operation of a Nd:YVO laser based on two-crystal geometry with the a- and c-cuts was demonstrated. The laser delivered continuous-wave radiation at 1064.4 nm and 1066.6 nm with more than 450 mW of output power. Optical-to-optical efficiency reached more than 50%. The proposed design is suitable for diode-pumping and can lead to compact and efficient dual-wavelength lasers.