This PDF file contains the front matter associated with SPIE Proceedings Volume 8959, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Single-crystal (SC) fiber optics have been grown for many years for use as passive fibers for the delivery of IR laser radiation and as active fibers useful as minirod lasers. Most of the early work on SC fiber optics involved the growth of unclad sapphire fibers for the transmission of Er:YAG laser radiation at 2.94 μm. More recently there has been a renewed interest in rare-earth (RE) doped oxide crystal fibers for use as high power fiber lasers. By analogy with RE doped-bulk laser crystals it is expected that pure YAG and other crystalline SC fibers should be capable of transmitting extremely high laser energies. SC oxide fibers have some distinct advantages over conventional glass fibers including higher thermal conductivity and low stimulated Brillouin scattering (SBS) gain coefficients. The latter can limit the ultimate power output of glass fiber lasers. To date most of the investigators have used the technique of Laser Heated Pedestal Growth (LHPG) to grow unclad SC fibers with diameters ranging from 30 to 350 μm and in lengths as long as 5 m. The loss for SC sapphire fibers at 2.94 μm has been measured as low as 0.3 dB/m. In this review we discuss the technique of LHPG, the various SC fiber optics that have been grown for both active and passive applications, and methods that may be used to clad the fibers.

Single crystal fibers are an intermediate between laser crystals and doped glass fibers. They have the advantages of both guiding laser light and matching the efficiencies found in bulk crystals, which is making them ideal candidates for high-power laser and fiber laser applications. This work focuses on the growth of a flexible fiber with a core of dopant (Er, Nd, Yb, etc…) that will exhibit good wave guiding properties. Direct growth or a combination of growth and cladding experiments are described. Scattering loss measurements at visible wavelengths along with dopant profile characterization are also presented. Laser characterization for these fibers is in progress.

In the paper, a Yb:YAG single crystal fiber is used for the first time to amplify week image signal. It was longitudinally pumped by a fiber-coupled laser diode with a maximum power of 150W at 940 nm. The image amplifier provided low noise and high gain amplification. A spatially uniform amplification gain of up to 10.2 dB at wavelength of 1030 nm was obtained.

Double-clade crystalline fiber waveguide (CFW) has been produced by using adhesive-free bond (AFB®) technology. The waveguide consists of a 1 at.% Yb:YAG core, un-doped YAG inner cladding and ceramic spinel outer cladding. It is a direct analog of the conventional double-clad glass fiber laser in the crystal domain. Signal gain of 45 or 16.5 dB has been measured in a preliminary master oscillator power amplifier (MOPA) experiment. Due to the high laser gain and the weak Fresnel reflection at the uncoated waveguide ends, the CFW even starts self-lasing above a certain pump power. Laser output power of 4 W in the backward propagation direction has been measured for input pump power of 44 W. After considering the same amount of forward propagated laser power, the laser efficiency to the absorbed pump power is estimated to be about 44%. In principle, CFW can have extremely large single mode area for high efficiency and high power laser applications. So far, Single mode area < 6700 μm² has been demonstrated in Er:YAG CFWs.

We demonstrate cw and Q-switched operation of Er:YAG laser systems emitting eye-safe radiation at 1645 nm. High brightness, narrow bandwidth diode laser modules at 1532 nm are used for resonant pumping, providing an absorption efficiency of 96%. Comparison with broadband diode pumping at 1455 nm shows an increase of the pumping efficiency by a factor of 2.5, leading to an optical efficiency of more than 25% in cw and 10 % in Q-switched mode under cw pumping. In addition, dual-wavelength resonant pumping of the Er:YAG laser is investigated.

We report on a new monolithic high-power diode pumped Er:YAG solid state laser at 2.94 µm. The pulsed laser reaches 30Waverage output power and 150mJ pulse energy. To accomplish these high powers the thermal lensing of Er:YAG was modeled beyond ABCD matrix formalism with FEM simulations. With the well understood thermal lensing e ect alternative laser cavities have been designed for higher brightness. The predicted results t the theoretically simulated performance and with that an improved beam quality factor of M2 = 12 has been achieved with 100mJ pulse energy.

We present laser operation of a 750 μm diameter Er:YAG single crystal fibers pumped at 1470 nm. Laser output performances are numerically simulated, experimentally measured and compared. In Passive Q-switch regime, we obtained pulse energy of 180 μJ around 500 Hz at 1617 nm without any spectral selecting element. Pulse duration is 33 ns. By controlling the saturable absorber temperature, we succeeded to improve the output energy up to 270 μJ. These results show the interesting potential of Er:YAG single crystal fiber for compact and low power consumption rangefinders.

We present an OPO pumped mid IR diamond Raman laser with tuneable output from 3.49 μm to 3.78 μm, which to our knowledge is the longest wavelength produced in a solid state Raman laser. Up to 59 μJ is generated with a conversion efficiency of 10%. We also determine the Raman gain coefficient of diamond at 1.864 μm through measurement of the amplification of a seed signal. With pump and probe polarisations aligned with the <110< crystal axes a value of 4.8 cm/GW is measured, which corresponds to 6.4 cm/GW for polarisation aligned with the <111< crystal axes. Achievable conversion efficiencies were limited by multi-phonon absorption at the Stokes wavelength. Numerical modelling shows that increasing the output coupling factor of the cavity reduces the impact of multi-phonon absorption and leads to higher conversion efficiencies. By reducing the output coupler reflectivity from 55% to 5% and eliminating Fresnel reflections from cavity components, 30% conversion efficiency (44% quantum conversion efficiency) is predicted.

Efficient generation of diode pumped Tm:YLF laser end-pumped by 25-W laser diode bar was demonstrated. Above 5 W of output power has been obtained. The output spectrum was centered at 1908-nm with <15 nm linewidth. In active Q-switching mode, for 20-Hz repetition rate, up to 5.5 mJ output energy with a pulse duration of 11 ns was achieved. Further pulse energy scaling up was limited by the damage of laser elements. The divergence angle was about 3.5 mrad and estimated parameter M² < 1.15. We also report mid-infrared laser generation at 2488 nm with a linewidth of ~60 nm in a polycrystalline Cr:ZnSe active medium. In a free-running regime, for 80 mJ incident pump energy, 3 mJ of output pulse energy was achieved.

Recent efforts have demonstrated efficient Cr²⁺ :II-VI chalcogenide (e.g. ZnSe, ZnS) broadly tunable (1.9-3.3μm) lasers under direct intra-shell Cr²⁺ optical excitation. We report on the spectroscopic study of Cr²⁺+:ZnSe/ZnS under visible excitation into the charge transfer band of Cr²⁺ ions. Polycrystalline samples prepared by thermal diffusion method were studied. Middle-infrared (mid-IR) photo-luminescence (PL) of Cr²⁺ ions was compared under continuous wave (CW) direct 1532nm (5T2→5E) excitation and under 532nm excitation into charge transfer band. The quantum yield of Cr:ZnSe mid-IR luminescence under CW green excitation was estimated as close to 100% at room temperature. To estimate Cr excitation rate via charge transfer band under short pulse excitation, mid-IR PL kinetic measurements were performed with the use of 532nm picosecond and nanosecond pumping. Mid-IR PL kinetics of Cr:ZnSe under pulsed green excitation exhibit a relatively slow growth reaching a peak at ~5-10μs for nanosecond and picosecond excitations, respectively, while PL kinetics in Cr:ZnS reveal shorter measured rise time (<1μs) limited by the response time of the detector. This rise of the PL intensity under 532nm pulsed excitation implies that 5E population continues to grow after the excitation pulse due to slow relaxation processes from higher-lying excited levels of Cr²⁺ to the upper laser level 5E. At the same time for nanosecond excitation the excited level is pumped at a rate faster than it is depleted and, hence, it is reasonable to expect that the population of the 5E level could be inverted. For laser experiments we used 5ns radiation from BBO based optical parametric oscillator tunable over 450-700nm. Cr:ZnSe lasing at 2.5μm induced by 2+→1+→2+ ionization transitions of chromium under visible excitation was achieved.

In this paper a resonantly pumped 1.645 μm injection-seeded single frequency Er:YAG Q-switched laser is reported. Firstly we introduce a monolithic Er:YAG non-planar ring oscillator (NPRO) developed by ourselves. The Er:YAG NPRO generated up to 10.5W 1.645 μm single frequency laser output power, with a slope efficiency of 60% and a linewidth of 18.6 kHz. The slave laser is a resonantly pumped Q-switched Er:YAG laser with a U-shape resonator and a 0.25 at.% Er:YAG crystal as the laser medium. When the pulse repetition rate was 1kHz, the Q-switched pulse energy was 5.4 mJ. Then one fraction of the single frequency laser beam from the Er:YAG NPRO was injection-seeded into a Q-switched slave laser. The Ramp-Hold-Fire technique was used to ensure the single frequency operation. Finally we obtained 1.645 μm single frequency Q-switched laser output with a pulse energy of 4.75mJ, pulse width of 336ns and pulse repetition rate of 200Hz, respectively.

We present a theoretical and experimental analysis of a pulsed 1645 nm optical parametric oscillator (OPO) conducted to prove the feasibility of such a device for a spaceborne laser transmitter in an integrated path differential absorption (IPDA) lidar system. The investigation is part of the French-German satellite mission MERLIN (Methane Remote Sensing Lidar Mission). As an effective greenhouse gas, methane plays an important role for the global climate. The architecture of the OPO is based on a conceptual design developed by DLR, consisting of two KTA crystals in a four-mirror-cavity. One of the cavity mirrors is piezo-driven to provide single frequency operation of the OPO. Using numerical simulations, we studied the performance and alignment tolerances of such a setup with KTP and KTA and investigated means to optimize the optical design by increasing the efficiency and decreasing the fluence on the optical components. For the experimental testing of the OPO, we used the INNOSlab-based ESA pre-development model ATLAS as pump laser at 1064 nm. At a pulse frequency of 25 Hz this MOPA delivers a pump energy up to 45 mJ with a beam quality factor of about M² = 1.3. With KTP as nonlinear crystal the OPO obtained 9.2 mJ pulse energy at 1645 nm from 31.5 mJ of the pump and a pump pulse duration of 42 ns. This corresponds to an optical/optical efficiency of 29%. After the pump pulse was reduced to 24 ns a similar OPO performance could be obtained by adapting the pump beam radius.

The Mercury Laser Altimeter (MLA) is one of the payload instruments on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, which was launched on August 3, 2004. MLA maps Mercury’s shape and topographic landforms and other surface characteristics using a diode-pumped solid-state laser transmitter and a silicon avalanche photodiode receiver that measures the round-trip time of individual laser pulses. The laser transmitter has been operating nominally during planetary flyby measurements and in orbit about Mercury since March 2011. In this paper, we review the MLA laser transmitter telemetry data and evaluate the performance of solid-state lasers under extended operation in a space environment.

We are developing a laser and electro-optic technology to remotely measure Sodium (Na) by adapting existing lidar technology with space flight heritage. The developed instrumentation will serve as the core for the planning of an Heliophysics mission targeted to study the composition and dynamics of Earth’s mesosphere based on a spaceborne lidar that will measure the mesospheric Na layer. We present performance results from our diode-pumped tunable Q-switched self-Raman c-cut Nd:YVO4 laser with intra-cavity frequency doubling that produces multi-watt 589 nm wavelength output. The c-cut Nd:YVO4 laser has a fundamental wavelength that is tunable from 1063-1067 nm. A CW External Cavity diode laser is used as a injection seeder to provide single-frequency grating tunable output around 1066 nm. The injection-seeded self-Raman shifted Nd:VO4 laser is tuned across the sodium vapor D2 line at 589 nm. We will review technologies that provide strong leverage for the sodium lidar laser system with strong heritage from the Ice Cloud and Land Elevation Satellite-2 (ICESat-2) Advanced Topographic Laser Altimeter System (ATLAS). These include a space-qualified frequency-doubled 9W @ 532 nm wavelength Nd:YVO4 laser, a tandem interference filter temperature-stabilized fused-silica-etalon receiver and high-bandwidth photon-counting detectors.

For the CO2 and CH4 IPDA lidar CHARM-F two single frequency Nd:YAG based MOPA systems were developed. Both lasers are used for OPO/OPA-pumping in order to generate laser radiation at 1645 nm for CH4 detection and 1572 nm for CO2 detection. By the use of a Q-switched, injection seeded and actively length-stabilized oscillator and a one-stage INNOSLAB amplifier about 85 mJ pulse energy could be generated for the CH4 system. For the CO2 system the energy was boosted in second INNOSLAB-stage to about 150 mJ. Both lasers emit laser pulses of about 30 ns pulse duration at a repetition rate of 100 Hz.

The demand for laser systems for marking and micromachining using high power UV has created a significant growth of lasers in manufacturing. To further support this growth advanced and cost-efficient technologies are required. Using a cavity dumped laser system based on thin disk technology leads to very short pulses below 10 ns. In addition the pulse width is independent of the chosen pulse repetition rate. This is in contrast to conventional Q-switched lasers. The combination of high average power and short pulses leads to high peak powers, e.g. more than 20 kW at 100 kHz. These short pulses are available even at high repetition rates up to 250 kHz and enable both high quality and high speed marking and micromachining. Using the field proven disk technology allows easy scaling to even higher power while maintaining reasonable costs. In this paper we will present a laser system based on the thin disk technology suitable for both micro machining and precise material processing.

We demonstrate 220 W average power at 355 nm from a diode-pumped acousto-optically Q-switched Nd:YAG laser using intracavity second harmonic generation and sum frequency mixing in a nested sub-cavity design. The laser generates linearly polarised pulses with duration 65 ns at repetition rate 10 kHz. Polarisation multiplexing is used to combine two orthogonal beams giving total output pulse energy 22 mJ with peak power <0.3 MW in an unpolarised multimode beam with smooth gaussian-like transverse intensity distribution. The combination of high peak power and high average power in a multimode beam enables the use of low maintenance, low cost of ownership DPSS lasers for high-throughput industrial processes in the UV.

We have developed high average power MOPA laser system with SHG unit on the table top size (3 × 1.5m). At the wavelength 1064nm has been obtained the max average output power of 715W. We have achieved the average power 180W at the wavelength 532nm, the pulse width of about 100ns, the frequency of 1kHz. And the power efficiency of the SHG from the wavelength of 1064nm to 532nm was obtained about 25.6%.

Thin-disk lasers with multi-kW output power in continuous-wave operation are widely used for industrial materials processing due to their excellent beam quality, high efficiency, and high reliability with low investment and operation costs. We present our latest laboratory results of nanosecond thin-disk lasers with multi-kW average output power. We show that in pulsed laser systems almost the same average power and beam quality as in CW systems can be realized. Utilizing the cavity-dumping principle for pulse generation we demonstrated more than 4 kW of average output power with pulse energies exceeding 180 mJ. The laser generates pulses with a pulse duration of 20 ns which is almost independent of the power level and the repetition rate. The beam parameter product was measured to be better than 4.5 mm•mrad (M² < 14). Deploying intracavity frequency conversion the efficient generation of pulsed laser output in the green spectral range is investigated. Results for a q-switched thin-disk laser with an average power exceeding 1.8 kW and pulse durations between 100 ns and 300 ns are presented. First results for the external second and third harmonic generation of a nanosecond thin-disk laser using the cavitydumping principle are presented. With an incident IR average power of 2.3 kW more than 800 W at 515 nm are demonstrated for the second harmonic generation and more than 500 W at 343 nm are shown for the third harmonic generation with a pulse duration measured to be < 20 ns.

We are developing one joule energy and one picosecond pulse duration laser system at the repetition rate of 120 Hz based on Yb-doped solid-state materials. The amplified output from the thin disk regenerative amplifier is amplified by a cryogenically conduction cooled single slab amplifier. In this work, we also present a new measurement method of a gain distribution insensitive to mode matching. One of the advantages of this method is a fact that it provides real dimensions of the gain distribution. Knowledge about it allows one to find the optimum spatial mode matching to maximize the output energy.

Double-clade crystalline fiber waveguide (CFW) has been produced by using adhesive-free bond (AFB®) technology. The waveguide consists of a 1 at.% Yb:YAG core, un-doped YAG inner cladding and ceramic spinel outer cladding. It is a direct analog of the conventional double-clad glass fiber laser in the crystal domain. Signal gain of 45 or 16.5 dB has been measured in a preliminary master oscillator power amplifier (MOPA) experiment. Due to the high laser gain and the weak Fresnel reflection at the uncoated waveguide ends, the CFW even starts self-lasing above a certain pump power. Laser output power of 4 W in the backward propagation direction has been measured for input pump power of 44 W. After considering the same amount of forward propagated laser power, the laser efficiency to the absorbed pump power is estimated to be about 44%. In principle, CFW can have extremely large single mode area for high efficiency and high power laser applications. So far, Single mode area < 6700 μm2 has been demonstrated in Er:YAG CFWs.

Microwave assisted chemical vapour deposited bulk diamond products have been used in a range of high power laser systems, due to low absorption across a range of wavelengths and exceptional thermal properties. However the application of polycrystalline products has frequently been limited to applications at longer wavelengths or thermal uses outside of the optical path due to the birefringence and scatter that are intrinsic properties of the polycrystalline materials. However, there are some solid state structures, including thin disc gain modules and amplifiers, that will gain significantly in terms of potential output powers if diamond could be used as a heat spreader in the optical path as well as a heat spreader on the rear surface of the disk. Therefore single crystal grades of diamond have been developed that overcome the limitations of the polycrystalline material, with low absorption, low scatter and low birefringence grades for demanding optical applications. We will present new data, characterising the performance of these materials across infra-red and visible wavelengths with absorption coefficient measured by laser calorimetry at a range of wavelengths from 1064 nm to 452 nm.

We are developing a 100-mJ Yb:YAG thin-disk regenerative amplifier operating at 1-kHz repetition rate pumped at zero-phonon-line (968.825-nm¹) and delivering 1-2 ps pulses for EUV plasma sources applicable in science and industry. Recently we achieved the output energy of nearly 50-mJ from a single laser-head cavity with good beam quality (M2<1.2) as well as stable beam-pointing (<4μrad). Applying pulsed pumping with the pulse duration shorter than the upper state lifetime of Yb:YAG helps to reduce the ASE and thermal loading of the thin-disk.

We present the world’s first laser at 515 nm with sub-picosecond pulses and an average power of 445 W. To realize this beam source we utilize an Yb:YAG-based infrared laser consisting of a fiber MOPA system as a seed source, a rod-type pre-amplifier and two Innoslab power amplifier stages. The infrared system delivers up to 930 W of average power at repetition rates between 10 and 50 MHz and with pulse durations around 800 fs. The beam quality in the infrared is M² = 1.1 and 1.5 in fast and slow axis. As a frequency doubler we chose a Type-I critically phase-matched Lithium Triborate (LBO) crystal in a single-pass configuration. To preserve the infrared beam quality and pulse duration, the conversion was carefully modeled using numerical calculations. These take dispersion-related and thermal effects into account, thus enabling us to provide precise predictions of the properties of the frequency-doubled beam. To be able to model the influence of thermal dephasing correctly and to choose appropriate crystals accordingly, we performed extensive absorption measurements of all crystals used for conversion experiments. These measurements provide the input data for the thermal FEM analysis and calculation. We used a Photothermal Commonpath Interferometer (PCI) to obtain space-resolved absorption data in the bulk and at the surfaces of the LBO crystals. The absorption was measured at 1030 nm as well as at 515 nm in order to take into account the different absorption behavior at both occurring wavelengths.

We report on our latest results of near fundamental mode operation of Yb-doped thin-disk lasers. 4 kW of continuous wave output power at M²<1.4 has been achieved by using one disk only. To the best of our knowledge this is the highest cw output power ever extracted from a single disk resonator design aiming for fundamental mode beam quality. Furthermore, a promising optical-to-optical efficiency of up to 56% at peak power has been achieved by pumping at 969 nm. Besides zero phonon line pumping, standard resonator components of our TruDisk thin-disk laser product series have been used such as the laser disk, and the pump optics which allows for 44 passes of the pump light through the laser crystal. It should be noticed that neither aberration correction methods nor a vacuum resonator design have been necessary to achieve this result.

We demonstrate power scaling of an Nd:YAG picosecond master oscillator power amplifier system to over 200 W. The ‘z-slab’ amplifier design is a power scalable, edge-pumped zigzag slab amplifier architecture, and it is demonstrated here in two alternative multi-stage implementations at 1064 nm using a picosecond seed laser. In a simple design, an average power of 225 W was generated with up to 450 μJ pulse energy at 11 ps pulse duration. In a compact multi-pass design, 150 W was generated with M² < 1.75.

We report on the progress of the front end development for a 100 J, 1030 nm amplifier at ELI-Beamlines and discuss requirements for and features of the front end. A particular emphasis is placed on the use of a fiber-based nanosecond pulse generator to produce arbitrarily shaped, stable pulses. Disadvantages of using such a fiber-based seed, such as a wandering baseline, are discussed and solutions are presented. A home-built RF harmonic synthesizer is shown to be capable of controlled sub-nanosecond shaping of optical pulses.

We report on a high-pulse-energy solid-state picosecond Nd:YVO₄ oscillator with cavity-dumping. The laser is end-pumped by an 808 nm laser diode and passively mode-locked with a semiconductor saturable absorption mirror (SESAM). In pure cw-mode-locking, this laser produced 2.5 W of average power at a pulse repetition rate of 40 MHz and pulse duration around 12 ps. A cavity dumping technique using an intra-cavity BBO electro-optic crystal to which bidirectional voltage was applied was adopted, effectively improving the cavity-dumping rate. Tunable high repetition rate from 100 kHz to 1 MHz was achieved. With electro-optic cavity dumper working at 1 MHz repetition rate, we achieved average power 594 mW. The laser includes a 5 mm long, a-cut, 0.5% doped Nd:YVO₄ crystal with a 5-degree angle at one end face. Laser radiation is coupled out from the crystal end face with a 5-degree angle, without requiring insertion of a thin-film polarizer (TFP), thus simplifying the laser structure. This picosecond laser system has the advantages of compact structure and high stability, providing a good oscillator for regenerative amplifiers.

We present a passively Q-switched, diode end-pumped, 1μm Nd:YAG laser with a single pulse energy in excess of 40mJ. To our knowledge, this is the highest single pulse energy reported for a passively Q-switched end-pumped laser. We achieved this with a novel pump scheme, which uses an engineered diffuser to create the necessary uniform gain distribution for efficient passive Q-switching. The system consists of a 3kW, 808nm, diode-laser stack pump source, and a set of collimating optics, with the engineered diffuser, to homogenise and couple the pump beam into the end of a 20mm diameter Nd:YAG laser rod. Q-switching is achieved with a Cr:YAG saturable absorber within a plane-parallel cavity. The 40mJ value was achieved despite a pump coupling efficiency of only 55%; hence we believe higher energies are achievable. The beam parameter product and pulse width were measured to be 12mm mRad and 18ns, respectively, which are consistent with those required for designation. We have investigated the pulse-to-pulse timing jitter of our system, which has been previously cited as the main drawback when implementing passive Q-switching for designation applications. We have achieved a reduction in timing jitter from 16 μs to 3.2 μs by environmental isolation of the laser resonator.

In this work we present a compact, nanosecond pulsed, single frequency, single stage Yb-doped fiber amplifier by using an overall fiber core diameter of 20 μm. The key component is a custom made, compact, ultra-low noise, single frequency ring-cavity solid state laser (SSL) at 1064 nm used as a master oscillator. The SSL can be designed to provide nanosecond pulses with pulse energies in the sub-mJ range. Our ultimate goal is to develop a compact linearly polarized, single frequency, nanosecond pulsed laser source in an all-fiber format. Short (less than 1m), highly Yb-doped fibers have been used in order to suppress non-linear effects.

In this work, we discuss mode-locking results obtained with low-loss, ion-exchanged waveguide lasers. With Yb³⁺-doped phosphate glass waveguide lasers, a repetition rate of up to 15.2 GHz was achieved at a wavelength of 1047 nm with an average power of 27 mW and pulse duration of 811 fs. The gap between the waveguide and the SESAM introduced negative group velocity dispersion via the Gires Tournois Interferometer (GTI) effect which allowed the soliton mode-locking of the device. A novel quantum dot SESAM was used to mode-lock Er³⁺, Yb³⁺-doped phosphate glass waveguide lasers around 1500 nm. Picosecond pulses were achieved at a maximum repetition rate of 6.8 GHz and an average output power of 30 mW. The repetition rate was tuned by more than 1 MHz by varying the pump power.

Yb:CaAlGdO4 (Yb:CALGO) is a very promising material for high power ultrashort pulse generation, due to its broad emission bandwidth and good thermal properties. Here we report, to the best of our knowledge, the highest power and shorter pulses ever demonstrated from a Yb:CALGO-based regenerative amplifier. The system layout consists of a Yb:CALGO oscillator seeding a Yb:CALGO regenerative amplifier followed by a folded grating compressor. The Yb:CALGO oscillator provides approximately 650 mW output power in a 63 MHz repetition rate pulse train of 92-fs long pulses. The related spectrum is 12.5 nm wide (FWHM) and centered around 1050 nm. Average output powers as high as 36 W at 500 kHz are achieved out of the regenerative amplifier while pumping with 116 W at approximately 980 nm. A small roll-over in the regenerative output power is observed at maximum pump power. This is mostly due to a drift of the pump wavelength away from the maximum crystal absorption peak with increasing pump current. After compression, we obtained 28 W in a train of 217-fs long pulses, corresponding to a pulse energy higher than 50 μJ per pulse and a peak power above 0.25 GW. The pulse spectrum is centered at 1046 nm and is approximately 11 nm wide, corresponding to a time bandwidth product of 0.69. The beam quality factor stays below M2=1.15 up to the maximum output power level, confirming the outstanding thermal performances of the Yb:CALGO material. Experiments on further power up-scaling are in progress.

We demonstrate a femtosecond Yb:YAG InnoSlab laser amplifier producing <3mJ pulse energy at 100kHz pulse repetition rate. The minimal pulse duration is <1ps resulting in pulse powers <3GW. High energy and high average power could be obtained with the use of chirped pulse amplification on the power amplifier end. The laser setup consists of a seed laser with 10mW average power at pulse repetition rates of 100kHz to 1MHz, a pre-amplifier stage, a highpower InnoSlab-amplifier stage and a grating based pulse compressor. This laser source is suited for pumping of OPCPA setups und parallelisation of applications in materials processing.

We report on a temporal jitter-compensated Yb:YAG thin-disc laser as a pump source for the front-end OPCPA of ELIBeamlines high energy and repetition rate system. The main advantage of using picosecond Yb:YAG thin-disc lasers is that relatively high energies in the kHz repetition rate range can be easily accessed. Although in our case the pump laser is optically synchronized to the OPCPA seeding Ti:Sapph laser, the stability of the OPCPA output gets heavily affected by delay jitter due to a large number of roundtrips in the regenerative amplifier cavity and slight ambient temperature drifts. Since interacting pulses are only ~1.5 ps in duration, an additional active stabilization of the pump path length corresponding to sub-100 fs delay precision must be implemented. In our work we demonstrate a novel design of a jitter stabilization system which employs a cross-correlation setup employing parametric amplification in two perpendicularly oriented nonlinear crystals. A small fraction of the OPCPA seed signal is being locked between cross-polarized and delayed replicas of a pump pulse. The feedback signal for the delay compensation is acquired by coupling the polarization-separated parts of the parametrically amplified signal into two channels of a balanced photodetector. The delay stabilization is achieved mainly by adjusting the cavity length of the regenerative amplifier with a piezo-mounted mirror. The proposed setup allows reducing the temporal jitter of Yb:YAG thin-disc regenerative amplifier to tens of fs RMS and maintaining it over extended periods of time.

We present the design and challenges of a diode-pumped solid-state (DPSS) system to amplify picosecond pulses to high pulse energies and high average powers. We discuss our implemented solutions to mitigate thermal effects and present the obtained performance of the picosecond pulse amplification at the multi-10-MW level. Our here presented picosecond DPSS laser is well suited for pumping an optical parametric chirped-pulse amplification (OPCPA) system. Several laser technologies have been employed to pump OPCPA systems and we show how our DPSS system compares in performance to the other approaches.

We report the solid-state Cr:ZnS laser mode-locked by CNT-based saturable absorber. The absorber was deposited on a protected silver mirror used as a high reflector mirror in a standard 250-MHz cavity with chirped mirror GDD compensation. Laser pulses with duration of 61 fs were obtained at 2.35 μm wavelength. The output power was limited at 950 mW, corresponding to the pulse energy of 3.8 nJ. We have demonstrated the longest-wavelength mid-IR CNT-mode-locked laser with record parameters, advancing the carbon nanotube mode-locking technology well beyond 2 μm into the mid-IR.

We report a diode-pumped Yb:KGW laser that is capable of operating as a Q-switched oscillator or as a regenerative amplifier with average power of more than 20 W. The laser is based on a dual-crystal configuration where the pump thermal load is distributed over relatively long two crystals. It permits a sufficiently large number of passes with low passive losses and maximizes the energy extraction efficiency. The amplification bandwidth was extended by spectral combining of two Yb:KGW crystals with spectrally shifted gain maxima, that allows to mitigate spectral gain narrowing and provides pulse length down to 200 fs after compression in a stretcher-compressor module. The output power saturated with increasing pump power and output beam quality was defined by aberration of thermal lenses. Optimization of laser cavity allows us to compensate thermal lens partially and provide output beams with quality M²<1.2. Efficient frequency doubling and tripling of high-power femtosecond Yb:KGW laser is demonstrated in a nonlinear BBO crystal. Second or third harmonic generation with respective conversion efficiency of 55% or 24% was achieved in a single-pass configuration.

A Compact Yb:YAG thin-disk regenerative amplifier utilizing a chirped volume Bragg grating (CVBG) as a pump source for anOPCPA amplifier at ELI-Beamlines is demonstrated. As a compact and stable stretcher, a chirped fiber Bragg grating (CFBG) followed by a CVBG is used. After amplification to 24 mJ, pulses are compressed to 2 ps in the CVBG and a short grating compressor; this allows the system to be compact while avoiding the SPM which comes from highly compressed pulses within the CVBG. The compact setup fits into a box 1200x600 mm² large, which includes the regenerative amplifier, grating compressor, and second harmonic assembly.

In this work longitudinal pumping of a continuous wave (CW) Nd:YVO4 laser by high power VCSEL modules was numerically studied. Two VCSEL pump modules (6 W and 15 W) were compared. The maximum output power from a Nd:YVO₄ crystal using these pump modules was calculated to be 2.5 W and 6 W, respectively, using a 10 % output coupler. The slope and optical-to-optical efficiencies in both cases were around 47% and 40%, respectively. The performance of Nd:YVO₄ crystal was found to be better than that of Nd:YAG crystal. Our numerical results indicate that VCSELs can serve as efficient pump sources for the end-pumped CW Nd:YVO₄ lasers.

First step of Er-Yb laser based on PTR glass creation was made. Complex study of luminescent characteristics of PTR glass doped with Er-Yb with recorded holograms was made. Direct measurements of population inversion of main Erbium level ⁴I_13/2 for different Erbium concentrations and pumping power was made. Was shown that spectral and luminescent characteristics of poly-functional PTR glass doped with Er-Yb and recorded holograms are comparable to the traditional laser barium phosphate glass.

We present a single frequency, stable, narrow linewidth, miniature laser sources operating at 532 nm (or 1064 nm) based on a monolithic resonators. Such resonators utilize birefringent filters formed by YVO4 beam displacer and KTP or YVO4 crystals to force single frequency operation at 532 nm or 1064 nm, respectively. In both configurations Nd:YVO4 gain crystal is used. The resonators dimensions are 1x1x10.5 mm3 and 1x1x8.5 mm³ for green and infrared configurations, respectively. Presented laser devices, with total dimensions of 40x52x120 mm³, are fully equipped with driving electronics, pump diode, optical and mechanical components. The highly integrated (36x15x65 mm3) low noise driving electronics with implemented digital PID controller was designed. It provides pump current and resonator temperature stability of ±30 μA@650 mA and ±0,003ºC, respectively. The laser parameters can be set and monitored via the USB interface by external application. The developed laser construction is universal. Hence, the other wavelengths can be obtained only by replacing the monolithic resonator. The optical output powers in single frequency regime was at the level of 42 mW@532 nm and 0.5 W@1064 nm with the long-term fluctuations of ±0.85 %. The linewidth and the passive frequency stability under the free running conditions were Δν < 100 kHz and 3⋅10-9@1 s integration time, respectively. The total electrical power supply consumption of laser module was only 4 W. Presented compact, single frequency laser operating at 532 nm and 1064 nm may be used as an excellent source for laser vibrometry, interferometry or seed laser for fiber amplifiers.

We present design and first performance data of a broadly tunable Alexandrite laser longitudinally pumped by a newly developed high brightness single emitter diode laser module with output in the red spectral range. Replacing the flashlamps, which are usually used for pumping Alexandrite, will increase the efficiency and maintenance interval of the laser. The pump module is designed as an optical stack of seven single-emitter laser diodes. We selected an optomechanical concept for the tight overlay of the radiation using a minimal number of optical components for collimation, e.g. a FAC and a SAC lens, and focusing. The module provides optical output power of more than 14 W (peak pulse output in the focus) with a beam quality of M² = 41 in the fast axis and M² = 39 in the slow axis. The Alexandrite crystal is pumped from one end at a repetition rate of 35 Hz and 200μs long pump pulses. The temperature of the laser crystal can be tuned to between 30 °C and 190 °C using a thermostat. The diode-pumped Alexandrite laser reaches a maximum optical-optical efficiency of 20 % and a slope efficiency of more than 30 % in fundamental-mode operation (M² < 1.10). When a Findlay-Clay analysis with four different output couplers is conducted, the round-trip loss of the cavity is determined to be around 1 %. The wavelength is tunable to between 755 and 788 nm via crystal temperature or between 745 and 805 nm via an additional Brewster prism.

Volume Bragg gratings (VBG) recorded in photo-thermo refractive glass (PTR) have high stability, and high damage threshold, allowing for many applications to the design of high power lasers. Gratings recorded in the transmitting geometry (TBG) have narrow angular selectivity, and can be used as a spatial filter in a resonator. Such gratings have previously been useful for improving the brightness of high power diodes, and increasing the beam quality in rod geometry solid state lasers. As the gratings have narrow angular selectivity, losses for higher order modes in the resonator no longer depend on the cavity length, allowing for the construction of short cavities with large mode areas. In this paper, we explore the design of short 1cm cavities using two TBGs as a spatial filter and no aperture in the cavity. The M² parameter as a function of pump size and angular selectivity of the TBG are explored, using pump diameters ranging from 800um to 2mm and angular selectivity ranging from 11mrad to 1.8mrad. An M² parameter of 1.05 is reported for an 800μm pump diameter, a 6.2mrad TBG, and a 1cm long cavity.

Solar-pumped laser has attracted attention in the area of renewable energy creation. However, since the conversion efficiency from solar energy to laser energy is low, such lasers are not yet in practical use. In this work, we developed Nd³⁺,Cr³⁺ codoped YVO4 and CaYAlO₄ crystals for solar-pumped laser. We succeeded to increase absorption at UV-VIS region with both crystals drastically. The absorption cross section of Nd,Cr:CaYAlO₄ around 400 nm was more than 70 times that of Nd,Cr:YAG crystals. The fluorescence at 1 μm was observed by pumping at 400 nm. It indicates that energy transfer from Cr to Nd occurred effectively.

We present some anisotropy properties of the Yb:CALGO with a spatial mode switching when pumped in the multihundred watts of power. This allows to automatically stabilize a TEM₀₀ mode from highly spatial-multimode regime. This stabilization is achievable thanks to a polarization mode switching allowed by the particular anisotropic spectroscopy and thermal properties of Yb:CALGO.

We report results for antireflective surface structures (ARSS) fabricated directly into the surface of optics and lenses which are important as high energy (multi-kW) laser components, including fused silica windows and lenses, YAG crystals and ceramics and spinel ceramics. Very low reflection losses as well as high laser damage thresholds have been measured for optics with ARSS. Progress to scale up the process for large size windows will also be presented..

Characterizations of linear spectroscopic properties in polarized light have been performed for the highly-concentrated Yb-doped borate family Li6Ln(BO₃)₃ (with Ln: Gd, Y, and labeled Yb:LLnB), in order to start to evaluate their potentiality for high-power laser applications. Modifications in spectral distributions and intensities are reported with respect to crystal orientation and polarization. Chemical composition and crystal shaping are discussed, pointing strong possible dependence with experimental conditions, which has to be considered so as to take sufficient precautions regarding the prediction of potential laser properties in such anisotropic laser crystals.

Measurements of the optic axis dispersion in double tungstate crystals have been performed from 400 nm to 1.58 μm. The measurements have been performed on KGdW, Nd-doped KGdW and Ho-doped KYW crystals. The samples were longer than 1 cm and had a good optical quality. The absolute angle reference was set using the Laue method. This reference allows us to compare more accurately our measurements with the calculations made using refractive index values found in literature. The difference observed between calculated values and measurements is significant. The relative dispersion looks similar for all double tungstate crystals tested. Furthermore first results of Holmium doped KYW laser operation along this axis will be shown.

The output beam profile of a Nd:YAG diode-pumped ceramic regenerative amplifier depends heavily on amplified spontaneous emission (ASE) and amplification competition. A careful pump geometry arrangement is required in a highgain regenerative amplifier to achieve the beam profile that corresponds to the TEM₀₀ mode of the amplifier cavity.

Cobalt doped II-VI wide band semiconductors (e.g. ZnSe, ZnS, CdSe) are promising media for infrared (IR) laser applications. They could be utilized as effective passive Q-switches for cavities of Alexandrite as well as Nd and Er lasers operating over 0.7-0.8, 1.3-1.6, and ~2.8 μm spectral ranges. We report spectroscopic characterization of Co:ZnSe and Co:ZnS crystals. Absorption cross-sections were measured for ⁴A₂(F) → ⁴T₁(P), ⁴A₂(F) → ⁴T₁(F), and ⁴A₂(F) → ⁴T₂(F) transitions with maximum absorption at 768(726), 1615(1500), 2690(2740) nm for ZnSe(ZnS) crystals, respectively. The calculated absorption cross-sections of the above transitions were estimated to be 64(56)×1019, 7.5(7.8)×1019, and 0.52(0.49)×1019 cm2 for ZnSe(ZnS) crystal hosts. In addition to the above applications the cobalt ions could be utilized for excitation of Fe²⁺ ions via resonance energy transfer process. Tunable room temperature lasing of Fe ²⁺ doped binary and ternary chalcogenides has been successfully demonstrated over 3.5-6 μm spectral range. However, II-VI lasers based on Fe²⁺ active ions don’t feature convenient commercially available pump sources (e.g. some Fe doped crystal hosts require pump wavelengths longer than 3 μm). Therefore, the process of energy transfer from Co²⁺ to Fe²⁺ ions could enable utilization of commercially available visible and near-infrared pump sources. We report a spectroscopic characterization of iron-cobalt co-doped ZnS and ZnSe crystals over 14-300K temperature range. Mid-IR laser oscillation at 3.9 μm(3.6 μm) via energy transfer in the Co:Fe:ZnSe (Co:Fe:ZnS) co-doped crystals was demonstrated under cobalt excitation at ⁴A₂(F) → ⁴T₁(P) (~0.7μm) and 4A2(F) → 4T1(F) (~1.56 μm) transitions.

We report on spectroscopic characterization of laser active powders based on iron doped II-VI ternary and quaternary semiconductors for mid-IR laser applications. Iron doped Cd_1-x Mn_xTe, Cd_1-x MnxS, Cd1-xMn_xSe, Cd_0.5Mn0.5S_0.5Se_0.5 , Cd1-xZnxTe compounds with x=0.5-0.25, were prepared by using thermo diffusion technique. The starting binary powders were mixed in the appropriate molar ratios, sealed in evacuated (10-3 Torr) quartz ampoules, and annealed at 800-1000oC for several days. Samples composition, integrity, and grain size were characterized by micro-Raman and Xray diffraction and revealed a variation of the crystal field parameters depending on powder composition. Fe²⁺ photoluminescence was characterized by spectral band position (normalized with respect to the detection platform spectral sensitivity) and lifetime at different temperatures, enabling calculation of the absorption and emission crosssections. Practical utility of the developed powders was demonstrated by a room temperature random lasing of iron doped Cd_0.5Zn_0.5Te powders over 5620-6020 nm spectral range pumped by a 2.94 μm radiation of a Q-switched Er:YAG laser. In summary, the following has been accomplished: (1) It was demonstrated that laser active Fe²⁺ doped ternary and quaternary II-VI materials can be produced by simple annealing of the commercially available binary powders omitting expensive and complicated crystal growth processes; (2) It is possible to effectively shift PL of Fe²⁺ in II-VI host materials towards shorter or longer wavelength by varying composition, type and amount of the second cation in ternary II-VI materials; (3) Major spectroscopic characteristics of Fe²⁺ doped II-VI ternary and quaternary compounds were obtained and their practical utility for mid-IR lasing demonstrated.

Laser design codes utilize laser properties provided by materials manufacturers for performance modeling. Large scale manufacturing of materials during compositional developments for a particular laser design is not economically feasible. Nevertheless, the laser properties derived from the available sample volumes must be reliable and reproducible. In recent years, as a result of the renewed interest in novel glasses for ultrafast laser applications, SCHOTT has developed improved measurements and methodologies for providing the most accurate information possible to laser scientists. Even though the J-O method is robust and time tested for the spectroscopic characterization of Nd3+, the accuracy of the results requires reliable measurements. This paper outlines the J-O approximation for manifold to manifold transitions, measurements needed, and some of the pitfalls to watch for during the collection of data for Nd-doped materials.

Thermal lensing e ects and birefringence in laser crystals strongly in uence beam quality of high power solid state lasers. Particularly, the e ects of birefringence are important in laser ampli ers using Nd:YAG crystals. To study these e ects, a simulation tool was developed, which allows to calculate birefringence in laser crystals for di erent kind of crystal cuts and pumping con gurations. The photoelastic e ects were accurately calculated by FE method. Simulation results are presented for di erent crystals and pumping. We also show that birefringence is not radially symmetric for a 111-cut Nd:YAG crystal, even when the pumping con guration is radially symmetric.

The 1.6 μm emission properties originating from the ³F₃, ³F₄ → ³H₄ transition of Pr³⁺ ions in Pr-doped PbCl₂ crystals were investigated for possible application in resonantly-pumped, eye-safe laser development. Pr: PbCl₂ was synthesized and purified through a combination of zone-refinement and chlorination of the melt before growing crystals by Bridgman technique. Under optical pumping using a ~1.48 μm laser diode, an IR emission band centered at ~1.65 μm with a full width at half maximum of 75 nm was observed at room temperature. The decay transient of the thermally coupled ³F₃ and ³F₄ excited states was single exponential with a value of 170 μs at room temperature. The lifetime increased to 350 μs at 77 K suggesting the existence of some non-radiative decay through multi-phonon relaxation. Emission cross-section spectra were determined using the principle of reciprocity and Ladenburg-Fuchtbauer method and yielded peak values of 3.8x10^-20 cm² and 5.7x10^-20 cm² at room-temperature and 77 K, respectively. The effective gain cross-section spectra of Pr: PbCl₂ will be discussed for possible lasing in the 1.6-1.7 μm region.

We present an edge-pumped Yb:YAG /YAG trapezoid-shape thin disk laser with slanted faces of 30 degree. The crystal consist of a 0.2-mm-thick Yb:YAG crystal as a gain medium and a 1.3-mm-thick un-doped YAG crystal bonded on the gain medium . The crystal is pumped from four sides in such a way that pump light trapped inside the crystal after total reflections. To study this configuration, we performed a detailed simulation of our delivery system using our laser simulation code ASLD^TM software. The simulation include Monte-Carlo ray tracing of pump light and 3-dimentional opto-mechanical analysis. Furthermore, we calculate the optical path difference of the crystal respect to the output power and relating beam quality using Dynamic Multimode Analysis (DMA) method. Finally, we demonstrated that the OPD inside the crystal increased from -3613.74mm to -542.85mm in the output power range of 33W to 250W.

A device with a solid funnel shape is designed for diode-pumped slab amplifier of kW-class cw laser. The solid funnel guides the diode beam in fast axis and expands the pump beam in slow axis. The beam guiding and shaping by the funnel enhance uniformity of absorbed diode beam distribution in the slab Nd:YAG amplifier. The effect of slole angle of the funnel on transmission efficiency is also presented.

This report presents a diode-pumped solid state (DPSS) high peak power laser with output energy of 300 mJ, operating frequency of 300 Hz, and pulse width of less than one nanosecond (FWHM) which corresponds to peak power of 300 MW at one micrometer emission wavelength. The laser system consists of a master oscillator power amplifier (MOPA) configuration with the output energy of one millijoule from the oscillator, the output energy of up to 15 millijoules from the pre-amplifier and the output energy of 300 mJ from the power amplifier. The transversal mode is close to the single mode with M² less than 1.5 and the lasing wavelength is 1064 nm. The MOPA system is well packaged in a compact footprint of 2×3 square feet.

A high peak power low jitter, single frequency eyesafe laser with precisely controllable firing time is presented by a new injection seeding configuration, in which the oscillator can output energy of near tens to more than 100 mJ, with a pulse width of tens of nanoseconds and single transversal mode. Comparing with current existing techniques, this design presents a new approach of using the cavity entrapped, amplified seeding signal, injection seeding method to precisely control the high peak energy launching time within a nanosecond jitter and achieve single frequency operation at the same time. The advantage of the realized regime is that in stable laser operation there is no need to adjust the slave cavity length to match the seeded light longitudinal mode.

Passively Q-switched, closed-loop, self-adaptive resonator with a Nd:YAG as an active medium is presented. For maximal pump energy of 840 mJ Q-switched generation provided 5 pulse series with total energy of 120 mJ. Single pulse width was 24 ns. The beam quality parameter M² was 1.6. Four-wave mixing and linear resonators were compared.

The goal of our research was investigation of diode pumped Nd:YAlO3 (Nd:YAP) laser tuning possibility in spectral range of general interest 1.3 – 1.5 μm. Laser radiation in this region is very required for many applications in medicine, atmospheric physics, and spectroscopy due to high absorption of this radiation in liquid water and water vapors. We use 1.0 at. % doped Nd:YAP active medium φ5 x 8 mm in dimensions. As a pumping source, a fibre-coupled 808 nm laserdiode was utilized. Two particular Nd:YAP laser resonators (one for 1.3 μm spectral region and the second one for 1.4 μm spectral range) consisted of a flat pump mirror and a curved output coupler were designed and constructed. The laser line selection was realized by a single 1.5 mm thick quartz plate placed at the Brewster angle between the output coupler and laser active medium. Six single emission lines were reached within the desired spectral range (1340 nm, 1341 nm, 1342 nm, 1403 nm, 1408 nm, 1433 nm). Moreover, it was possible to realize the laser system generating in dual frequency regime for some line combination. The respective output laser characteristics in terms of output power, beam spatial structure, and temporal profile were also recorded.

We report on spectroscopic and lasing properties of Pr:YAlO₃ crystal down to cryogenic temperature. Fluorescence lifetime, polarization-resolved absorption and emission spectra, and laser characteristics in the green spectral range are described. Using 1-W InGaN laser-diode pumping together with microchip resonator geometry, 37 mW of continuous-wave output power at 547 nm wavelength at 80 K crystal temperature is reported. The corresponding oscillation threshold and slope efficiency related to absorbed pump power were 320 mW and 6.5 %, respectively.

In high power diode pumped solid-state lasers, thermal effects in the laser medium are important factors limiting the power scaling and beam quality. Besides total pump power, pump structure, such as pump geometry, cooling scheme, laser crystal shape and dimension all affect the result of thermal effects. The theoretical modelling and calculations may only conclude approximate results with the consideration of parts of factors. This paper introduces a new technique of measuring Nd:YAG rod thermal lensing by digital holography (DH). Both dynamic and steady state can be measured by this method. The digitally recorded hologram can reveal each part of the thermal effects in the crystal, and detailed variations of thermal effects can be mapped out through digital reconstructions of the captured holograms. It can help to study the uniformity of the pump distribution in the gain medium, find "hot" spots which may result in potential crystal crack. Moreover, an integrated thermal lensing can be accurately determined. DH is an informative tool to understand thermal effects and provide a guidance for laser cavity design and simulations.

We are developing 100-kHz picosecond Yb:YAG thin disk regenerative amplifier with 500-W average power for medical and industrial applications. Especially in case of the next generation of semiconductor lithography, high average power solid-state laser with picosecond pulse duration as pre pulse source is a key element to realize 1-kW EUV lithography source. We compared the output characteristics of CW laser operation pumped at 940-nm and 969-nm, and measured the surface temperature of thin disk. We found that the surface temperature of thin disk pumped at 960-nm was much lower than that pumped at 940-nm. We obtained 83-W output from thin disk regenerative amplifier at the repetition rate of 100-kHz pumped at 969-nm. The measured pulse duration was 1.9-ps.

The Tm:CaF2 (4% of TmF3) and Tm:Ho:CaF2 (2% of TmF3, 0.3% of HoF3) ceramics, prepared using hot pressing, and hot formation technique had been used as an active medium of diode pumped mid-infrared tunable laser. A fibre (core diameter 400 μm, NA = 0.22) coupled laser diode (LIMO, HLU30F400-790) was used to longitudinal pumping. The laser diode was operating in the pulsed regime (6 ms pulse length, 10 Hz repetition rate). The duty-cycle 6% ensures a low thermal load even under the maximum diode pumping power amplitude 25W (ceramics samples were only air-cooled). The laser diode emission wavelength was 786 nm. The 80mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (HR @ 1.85 − 2.15 μm, HT @ 0.78 μm) and a curved (r = 150mm) output coupler with a reflectivity of ∼ 98% @ 1.85 − 2.0 μm for Tm:CaF2 laser or ∼ 99.5% @ 2.0 − 2.15 μm for Ho:Tm:CaF2. Tuning of the laser was accomplished by using a birefringent filter (single 1.5mm thick quartz plate) placed inside the optical resonator at the Brewster angle. Both samples offered broad and smooth tuning possibilities in mid-IR spectral range and the lasers were continuously tunable over ∼ 100 nm. The obtained Tm:CaF2 tunability ranged from 1892 to 1992nm (the maximum output energy 1.8mJ was reached at 1952nm for absorbed pumping energy 78 mJ). In case of Tm:Ho:CaF2 laser tunability from 2016 to 2111nm was reached (the maximum output energy 1.5mJ was reached at 2083nm for absorbed pumping energy 53 mJ). Both these material are good candidates for a future investigation of high energy, ultra-short, laser pulse generation.

The spectroscopic and laser properties of bulk Bridgman-grown Zn_1-xMg_xSe single crystals with the various concentrations of Mg (x=0.19; x=0.27; x=0.38) were investigated in the wide temperature range. The pumping was provided by a 2.94 μm Q-switched Er:YAG laser with a maximal energy of 15 mJ in 120 ns pulse, repetition rate 1 Hz. Q-switched operation was achieved by the Brewster angle cut LiNbO3 Pockels cell placed between the rear mirror and the Er:YAG laser active medium. The pump radiation was directed into the Fe:ZnMgSe crystal placed inside the LN cooled dewar. The 55 mm long plane-concave cavity was formed by a dichroic pumping mirror (T = 92 % @ 2.94 μm and R = 100 % @ 4 - 5 μm) and a output coupler (R = 95 % @ 4.5 μm, r = 200 mm). The strong dependence of output pulse energy on temperature was observed for all samples. The maximum output Fe,Cr:Zn1- xMgxSe laser energy was 230 μJ and 180 μJ (for Mg concentration x=0.19 and x=0.38, respectively) for gain switched operation at 88 K. The central emission wavelength of ~ 4600 nm, ~ 4700 nm and ~ 4800 nm for Mg concentration x=0.19; x=0.27, and x=0.37, respectively at 88 K was obtained. The emission wavelength was found to increase up to ~ 4700 nm and ~ 4900 nm at 250 K for Mg concentration x=0.19 and x=0.38, respectively. This results show the possibility to obtain sufficiently longer oscillation wavelengths compared to previously studied Fe:ZnSe active medium especially at liquid nitrogen temperatures when pumping by free-running Er:YAG laser becomes possible. Fluorescence spectra and lifetimes of Fe2+ ions in different Zn_1-xMg_xSe crystals in the range from 250 K down to 80 K were also measured.

For the non-uniform distribution of pump and temperature in the large aperture, high-power thin disk laser medium, a novel cooling method of multi-annular channel liquid cooling is proposed and examined both experimentally and theoretically. The temperature distribution in the gain medium is getting into uniform utilizing the method of multi-annular channel liquid cooling, which is proved by a numerical model using ANSYS software. In the modeling, the distribution of temperature in the medium varies with the changes of the flow rate and temperature of the coolant in each annular channel. A wonderful uniform temperature distribution could be obtained in the gain medium with arbitrary power and profile of pump light by setting a tailored parameter of the coolant in each annular channel. The highest temperature difference in the gain medium with multi-annular channel liquid cooling reduces about 88% compared with an evenly cooling. And the thermal effect has been suppressed, the experimental result is consistent well with numerical modeling. This method could be a new idea for designing the thin disk laser’s cooling system.