This PDF file contains the front matter associated with SPIE Proceedings Volume 10896, including the Title Page, Copyright information, Table of Contents, and the Author and Conference Committee lists.

We have developed a fast wavelength modulated mid-IR source, especially designed for gas spectroscopy. The whole laser source is composed of a picosecond fiber laser emitting a spectrally narrow signal which can be modulated between 1028.3 nm and 1029.3 nm at the kHz range. This fiber laser seeds a Synchronously Pumped Optical Parametric Oscillator. This latter converts the near-IR pump (1028.3 nm) to the mid-IR region (3000-3500 nm) with equivalent modulation parameters i.e. 10 cm^-1 tuning range at the kilohertz modulation frequency. This laser was combined with a photo-acoustic cell for methane detection.

We characterize several configurations of compact, Q-switched, 1.5 um lasers based on Er/Yb doped glass. Such lasers are required for eye-safe laser range-finders (LRF), laser markers, and illuminators for 3D and gated imaging. While 1-3 Hz pulse repetition frequency (PRF) is adequate for LRFs, the other applications require much higher PRFs to achieve near-real-time image refresh rates. Lasers described here utilize Er/Yb glass active elements, side-pumped by a 940 nm laser diode bar, or end-pumped by a fiber-coupled laser diode, and made use of active or passive Q-switching (PQS) techniques. Active Qswitching is implemented with resonant scanning mirror, and PQS utilizes Co:Spinel saturable absorbers. Q-switched pulse energies of 5mJ and 3mJ are achieved with side-pumping and end-pumping, respectively. An optical efficiency of over 3.7%, the highest to our knowledge for a PQS Er/Yb glass laser, is measured for the end-pumped implementation. When configured to generate 1mJ, the endpumped PQS laser operates over a 2-25 Hz PRF range, with nearly constant pulse energy and optical efficiency. Features and advantages of the various laser configurations are compared.

We report on the fabrication and characterization of Er:YGG films suitable for waveguide amplifiers that could in principle be used in integrated path differential absorption lidar systems. Presented is our fabrication technique, comprising pulsedlaser- deposition growth of ~10 μm-thick crystalline films, their channeling via ultraprecision ductile dicing with a diamond-blade, producing optical quality facets and sidewalls, and amplifier performance. Net gain at 1572 nm and 1651 nm is obtained for the first time in Er-doped YGG waveguide amplifiers. Additionally, in a channel waveguide a maximum internal gain of 3.5 dB/cm at the 1533nm peak was realized. Recent crystal film quality improvements promise further performance enhancements needed for the intended application for high-peak power sources in the 1.6-μm spectral region targeting Earth observation systems for monitoring greenhouse gases.

Wide-band tunable 2 µm lasers are sought for remote sensing, eye–safe lasers, laser processing of transparent plastics, medical therapy, ultra¬fast lasers (UFL), accelera¬tion of nuclear particles, and generation of visible output via harmonic conversion [1]. We have previously reported efficient lasing in Tm:Lu2O3 ceramic while tuning over 230-nm range in the vicinity of 2 µm and delivering up to 43 W QCW [2]. Tm:Lu2O3 ceramic gain material has a much lower saturation fluence than the traditionally used Tm:YLF and Tm:YAG materials, thus offering improved energy extraction. Ceramic construction offers size scalability and convenient fabrication of gain medium composites. This paper reports on experimental evaluation of laser gain and q-switched output pulse energy in vicinity of 2-microns in Tm:Lu2O3 ceramic rod end-pumped by 796-nm diodes. Also included is the operation and spectral gain evaluation of a Tm:Lu2O3 ceramic edge-pumped disk laser with multi-passed extraction, which is seeded by the above end-pumped rod laser. This work was supported by the U.S. Department of Energy grant number DE-SC0013762. 1. Drew A. Copeland, John Vetrovec, and Amar S. Litt, "Wide-Bandwidth Ceramic Tm:Lu2O3 Amplifier," SPIE 9834, (2016). 2. John Vetrovec, et al., "2-Micron Lasing in Tm:Lu2O3 Ceramic: Initial Operation," SPIE vol. 10511 (2018)

Mid-Infrared (MIR) lasers emitting in the 2.7 - 3 micrometer range are important for remote sensing, medical, and material processing applications. Erbium ions in various host materials exhibit strong fluorescent transitions between the 4I11/2 and 4I13/2 energy levels suitable for MIR lasing, but the fluorescence lifetime of the lower laser level 4I13/2 is usually much longer than that of the 4I11/2 level. Thus, the 4I11/2 → 4I11/2 laser transition is self-terminating. Typically, lasing around 3 µm is achieved with elevated Er-doping concentrations which helps to deplete the population of the lower laser level by the concentration-dependent energy transfer up-conversion process from the 4I13/2. An alternative method of depleting 4I13/2 is to use another laser transition, 4I13/2 → 4I15/2 at 1.6 µm, which Er-doped materials are well known for. This approach allows for the reduction of Er-doping and the improvement of thermo-optical properties by weakening up-conversion and improving thermal conductivity of the lasing media. By combining two laser processes into a cascade operation 4I11/2 → 4I13/2 + 4I13/2 → 4I15/2 and using resonant pumping at 960–980 nm (4I15/2 → 4I11/2), one can significantly increase the efficiency of MIR lasing. This approach was successfully tested in CW operation of different Er-doped crystalline and fiber lasers, but was rarely tried in the Q-switched lasing. We present our comparative study on active Q-switching in several Er-doped crystals (YAG, YALO and Y2O3) with different Erbium content, at room and cryogenic temperatures. Under resonant, diode-pumping we achieved up to several watts of the average power.

We report on a newly developed Q-switched diode side-pumped Er:YLF solid state laser emitting at 2.81 μm. Efficient short pulse generation is achieved by utilizing the relatively long lifetime of the upper laser level and the inherently linear polarized laser light of the Er:YLF crystal material. By means of an acousto-optic switch, peak powers of 50 kW with corresponding pulse widths of 70 ns and pulse energies of up to 3.5mJ are realized at a repetition rate of 100 Hz. The laser operates efficiently at room temperature and has a compact nature, enabling minimized thermal impact tissue ablation as well as pumping of non-linear crystals for mid-IR generation.

Yttrium Aluminum Garnet (YAG) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level.The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. Specifically the multi-photon effect known as excited state absorption (ESA) is modeled by measuring the cross section in a single crystal Er:YAG sample utilizing a pump and probe technique. The measured ESA cross section is then included in the rate equation modeling that has typically been ignored such an effect when modeling the non-linear ion to ion interactions. By pumping with selective resonant pumps, the interaction dynamics of the upper and the lower laser levels in the Erbium ion reveal the contributions of the non-linear energy transfer mechanisms towards the populations of the levels respectively. This thesis work shows the added accuracy of the rate equation modeling by its inclusion of the measured ESA cross-section and its effect on the population evolution in the Er:YAG system. The model will be tested with measurements made with select Er:YAG crystals being pumped by combinations of resonant infra-red lasers with a chopper spinning at a rate on the order of magnitude of the lower lasing level lifetime.

Iron-doped binary and ternary chalcogenide crystals are very promising for tunable solid-state lasers operating over the 3-6 μm spectral range. The most significant results have been reported for iron doped ZnSe crystals with 9.6 W output power in CW at 77K when pumped by radiation of Cr:ZnSe laser, and 1.4 J at ~150 ns pulse duration at room temperature (RT) when pumped by the radiation of HF laser. The lifetime of the upper laser level 5T2 of the Fe2+ ion in a ZnSe matrix falls with temperature from 52µs at 77 K to 370 ns at RT due to the increase of nonradiative relaxation. It allows effective laser oscillation in the gain-switched regime at RT and operation in Q-switched regime at low temperature. We report on RT gain-switched Fe:ZnSe lasers tunable over 3.60-5.15 µm pumped by radiation of mechanically Q-switched Er:YAG laser operating at 2.94 µm. The maximum output energy was measured to be 5 mJ under 15mJ of pump energy. The long upper level lifetime of Fe:ZnSe gain medium is sufficient for energy storage with pumping by radiation of free running Er:YAG lasers. We demonstrated that Q-switched regime of oscillation could be effectively utilized for Fe:ZnSe lasers. The rotating back mirror was used as a mechanical Q-switcher of a Fe:ZnSe laser. The maximum output energy in single 150 ns pulse was measured to be 3mJ which is ~25% from the theoretical limit. This approach could be attractive for development of high-energy short-pulse solid-state mid-IR systems operating over 3.6-5.0 µm spectral range

We describe mid-IR sources utilizing a CSP Optical Parametric Oscillator (OPO) directly pumped by a high efficiency 1.94 μm Tm:YAP Q-switched laser. The OPOs, constructed using the latest generation CSP crystals with low 1.94 μm absorption, were operated at near-degeneracy with mid-IR output in the 3.6-4.2 μm range. Compact Q-switched Tm:YAP lasers, implemented using both a Cr:ZnS saturable absorber Q-switch or a mechanical Q-switching (MQS) technique, were constructed with the Q-switching method’s impact on OPO performance evaluated. Resonant effects were observed for MQS that were absent in the passively Qswitched (PQS) experiment. It was determined that optimizing the OPO resonator length, relative to the cavity length of the Tm:YAP laser, maximized the OPO conversion efficiency. An optimized OPO, pumped at an incident 1.94μm MQS laser power of 7.6W, generated an average mid-IR power of 4.6 W, corresponding to an optical conversion efficiency of 60%, and an overall optical efficiency for mid-IR generation of 21% relative to diode power incident on the Tm:YAP.

Here we report on low-threshold blue lasing from fully-transparent nanostructured porous silicon (PSi) monolithic microcavities (MCs) infiltrated with a polyfluorene derivative, namely poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO). Single-mode blue lasing is achieved at the resonance wavelength of 466 nm, with line width of ~1.3 nm, lasing threshold as low as 5 nJ (i.e. fluence of 15 μJ/cm²), and good stability under operation (i.e. 40% decay in intensity after about 7 ×10⁵ pulses at 50 nJ).

The huge potential of tunable optical parametric oscillators (OPOs) derives from their exceptional wavelength versatility, as they are in principle not limited by the wavelength coverage dictated by the energy levels and transitions in a laser gain medium. However, while the OPO concept has been experimentally demonstrated already more than half a century ago, the progress in development of practicable and reliable turn-key devices that operate in continuous-wave (cw) mode has been stalled by several technical obstacles. This applies particularly for systems that sought to deliver tunable output across the visible spectral range (VIS), where only relatively recent advances have spurred the development of operationally stable benchtop devices. We discuss the principles and design challenges of such technically practicable cw OPOs, focusing on singly resonant OPO cavity designs that are linked with frequency conversion of the primary OPO output into different ranges of the visible spectrum. In this context, suitable choices and combinations of (quasi-phase-matched) nonlinear crystals are examined. We further discuss the overall performance highlights as well as current limitations of state-of-the art tunable cw OPO designs, and present first measurement results from conceptual approaches to shift and/or extend the wavelength coverage in future design layouts that eventually target commercialization. Last no least, after presenting real-world applications in an illustrative manner, we critically discuss how OPO technology, on the long run, can be expected to perform in the competition with alternatives based on common tunable laser designs.

We demonstrated a 50 mJ, 100 Hz, 355 nm UV laser. The third harmonic generated in a diode-pumped 200 mJ, 500 ps Nd:YAG laser system centered at 1064 nm, which based on a master oscillator power amplifier configuration. The pulse duration and temporal shape from oscillator could be modulated by an Arbitrary-Waveform-Generator and an amplitude modulator. Two LBO nonlinear crystals were employed to second and third harmonic generation respectively. The UV laser system featured an approximately flat-top temporal distribution and a 2.5% energy stability (RMS) within 1 hour.

A compact and simple laser has been developed to generate 5 mJ of energy and < 7ns pulses at 532 nm. A pump cavity has been uniquely designed to directly couple diode light to the laser crystal, thereby eliminating the need for mounting the diodes on sub-mounts and using a fast-axis collimating lens. An Nd:YAG crystal is side pumped by diode bars and the laser components are designed in order to extract the largest fundamental mode for better beam quality. The laser is passively q-switched by a Cr:YAG crystal. A type II KTP crystal is used to generate 532-nm wavelength.

We report an in band pumped single-stage 410 W INNOSLAB laser amplifier based on Nd:YVO4. VBG stabilized diodes are used to pump the crystal from both end faces at 880 nm wavelength. At 3 W@800KHz input power and a pulse duration of 300 ps an extraction efficiency of 44% is achieved. A beam quality of M2<1.5 (4-Sigma Method) and M2<1.3 (10/90 Knife Edge Method) is measured over the whole power range. The output beam is free of self-lasing or CW-background.

In many laser micro machining applications, ultra-short pulse (USP) lasers with a dynamical adaptive pulse repetition frequency (PRF) would allow for a significant increase of processing speed. Machining with ultra-fast resonant scanners or new techniques for fast processing narrow beam paths are two examples. These applications require USP lasers which provide a real-time PRF-synchronization to the deflection speed of the laser beam. Moreover, despite the dynamical change of the PRF between single pulses up to 10 MHz the pulse energy shall remain constant for high quality processing. Even today, this seems to be not easily realized. We present a laser system with constant pulse energies for all PRFs from single pulse to 10 MHz with an extremely fine PRF step size. The laser system consists of a mode-locked diode laser, running at a basic PRF of 4:3 GHz and a succeeding ultrafast pulse picker. This leads to a temporal accuracy of 233 ps, 40 times higher than with a typical solidstate mode-locked laser. To guarantee constant pulse energy over the entire PRF-range, the pulsed signal is superimposed with a cw-signal of the same polarization, serving as an inversion control in subsequent fiber and slab-amplifiers. In the end a SHG-stage serves as the desired cw-filter. The output power at 515nm is up to 109W. The pulse duration is 5 ps. With this setup, first experiments were performed showing the advantage of real USP pulse on demand laser systems.

Understanding gas-phase combustion reactions in turbulent flows and energetic materials requires high-speed multiparameter imaging at kHz-MHz frame rates. Such measurements require energies per pulse in the range of 10s to 100s mJ at MHz rates, which led to development of burst-mode nanosecond lasers with burst durations up to 100 ms and energies per burst of up to 400 J. Burst-mode laser technology have enabled significant advances in measurement capabilities, for accessing multiple species, temperature, and velocity. However, the number of simultaneously measured parameters are limited by number of lasers and interference between modalities having similar output spectral characteristics. Here we report the laser system with three outputs with up to 1 J/pulse at 10 kHz and variable time delays between outputs to avoid interference between modalities. The laser is based on the common all-fiber oscillator and free-space Nd:YAG flashlamppumped preamplifier to reduce the cost and size of the overall system. The oscillator/preamplifier produces up to four pulses with user-selected time delays between pulses. This four-pulse sequence is repeated at user selected repetition rate up to 500 kHz. The pulse sequence is split between three free-space Nd:YAG amplifiers by two Pockels cells with time resolution of 10 ns. Special precaution is taken to minimize the crosstalk between amplifiers with less than 5% prepulse/afterpulse in the fundamental output and less than 1% in the harmonics. The system is designed for measurements of species (via planar laser induced fluorescence, PLIF) and velocity (via particle image velocimetry, PIV) at rates up to 500 kHz and total burst duration of 10 ms, with the feasibility of achieving MHz rates with further developments. The simultaneous measurements of velocity using PIV and PLIF imaging of hydroxyl and formaldehyde in a turbulent jet flame is demonstrated.

Some applications like range finding, optical counter measures or engine ignition, require lasers capable of delivering high repetition rate bursts of nanosecond pulses with hundreds of microjoules to a few millijoules in terms of energy per pulse. We have developed such a diode pumped Yb:YAG micro-laser with an oscillator comprised of a 2-mm long 10% at. doped Yb:YAG crystal followed by a Cr:YAG passive Q-switch with an initial transmittance of 85 %. The laser planoconcave cavity is 5-cm long. This oscillator emits 250 μJ to 300 μJ per pulse, with a 3 – 5 ns pulse duration, with an intra-burst pulse repetition frequency that can be tuned continuously from 1 kHz to 20 kHz by increasing the pump power. The pumping diode laser is operated in quasi continuous wave regime, emitting 1-ms to 10-ms long pulses with up to 20 W peak power This qcw pumping results in the emission of a burst of pulses at high repetition rate for the duration of this pump long pulse. These pump pulses, and consequently the bursts of nanosecond pulses, are repeated at very low frequency, between 1 Hz and 5 Hz, so that the average power to handle doesn’t require active cooling. This oscillator is then amplified to the millijoule level in a second 3-mm long Yb:YAG crystal pumped by a synchronous qcw emitting diode laser.

We present a passively Q-switched Nd³⁺:YAG/Cr⁴⁺:YAG laser with subsequent nonlinear pulse compression. This miniature laser combines both the high pulse energy of several tens of micro Joules and the short pulse duration of <20 ps without any amplification. It is therefore readily usable for many ultrafast applications including micro machining and medical applications. With these parameters, the laser shows a new level of compactness in comparison to other sub-100 picosecond laser sources. We utilize a Nd³⁺:YAG and a Cr⁴⁺:YAG crystal in a flat-Brewster, Brewsterflat configuration, respectively, positioned closely and the Brewster faces adjacent to each other. Pumped with approx. 6W from a fiber-coupled, 808 nm laser diode, the miniature, passively Q-switched laser oscillator delivers a pulse energy of 54 μJ with a pulse width of 339 ps and a repetition rate of 8.5 kHz. These pulses are subsequently coupled into a 20 μm core dia. large mode area PM fiber. Following a fiber propagation of 1.65 m the pulses are spectrally broadened by about a factor 40 due to self-phase modulation. Thereafter the nearly linearly chirped laser pulses are compressed by a chirped volume Bragg grating (CVBG). The optimized laser output pulses have a pulse width of 11.8 ps and a pulse energy of 20 μJ. We measured the polarized (PER<20dB) beam quality to be close to the diffraction limit with an M²≈1.5. A 13 hours continuous, stable laser operation has indicated a good long term stability and reliability.

A laser-diode (LD) pumped Nd:YAG compact laser system that is capable of generating 1064-nm, 1-J output pulses in several tens of nanoseconds pulse duration at 300-Hz repetition rate (300-W average power) was developed. A concept of this laser system is based on a ubiquitous machine that is easy to transport and process test rapidly in laboratory. A footprint is 1.2-m in width and 2.4-m in length. This laser system is a master-oscillator power-amplifier (MOPA) architecture that allows for increasing the output energy by adding amplifiers. It consisted of an acousto-optic Qswitched Nd:YAG oscillator, a ϕ3-mm Nd:YAG preamplifier and three ϕ12-mm Nd:YAG main amplifiers. The oscillator generated 6-mJ pulse energy at 37-ns pulse duration that could be adjusted by changing the cavity length. The main amplifier had a small-signal gain (SSG) of 9 by laser diodes (LDs) pumping with maximum 27-kW peak power. The beam size and divergence were adjusted to compensate for thermal lens effect in each amplifier. Single-pass amplification by three main amplifiers increased the pulse energy to 1 J. The pumping repetition rate was fixed to obtain thermally stable condition. However, the output repetition rate is variable from single shot to 300 Hz by controlling the oscillator for the experiment usability.

A 11.5 J at 40 ns output has been obtained from a diode-pumped cryo-cooled Yb:YAG ceramics active-mirror laser amplifier system. The system consists of two amplifier heads which has four Yb:YAG ceramic disks and two pump LD modules. The Yb:YAG ceramics are cooled by conventional cryostat from rear side and are pumped by LD modules from front-side. A pump pulse is delivered to Yb:YAG ceramics coaxially with a seed pulse to reduce damage risk at a dielectric coating of Yb:YAG ceramics due to simplified coating design. To realize this system design, a LD module has been developed to keep a rectangle pattern with side length of around 37 mm among imaging depth of about 10cm at working distance of about 410 mm. As an experimental result of two pass amplification, a 11.5 J pulse energy was obtained with input energy of 1.0 J and total pump energy of 90.2 J. Then, an optical-to-optical conversion efficiency was 11.6% and an extraction efficiency was estimated to be 42%. In our knowledge, this is the highest output energy with nano second pulse duration in cryo-cooled Yb:YAG active-mirror laser amplification scheme. A repetition rate of 0.05 Hz depends on a limitation of a repetition rate of the seed pulse. A dependence of small-signal-gain on pumping repetition rate of the active-mirror laser head was experimentally evaluated. From the experimental result, we have estimated a feasible repetition rate of over 5 Hz. A 10 Hz operation will be demonstrated to reduce a thermal resistance between Yb:YAG ceramics and cryostat. Finally, this laser amplifier system is installed to a 100-J class laser system as preamplifier.

As the laser pumping source, pulsed xenon flash lamp is widely used in high power laser amplifier system such as inertial confinement fusion projects. The radiation efficiency of pulsed xenon flash lamp is the crucial parameter. In our previous work, it has been proven that the pulsed xenon flash lamp with annular section has higher radiation efficiency as well as laser amplifier efficiency compared with traditional flash lamp. To further improve the characteristics of this kind annular section flash lamp, a multi-electrode system was proposed for annular section flash lamp in this paper. Experimental results shown that the discharge channels established more quickly and expended fast to whole lamp tube space. Also the distribution of the discharge plasma is more uniform. The novel multi-electrode xenon flash lamp has been investigated in the aspects of the electrical characteristics, the discharge plasma channel, radiation and neodymium doped glass fluorescence. Experimental results show that discharge and radiation characteristics of xenon flash lamp with multi-electrode structure are improved. The detail mechanisms of the improvements for the multi-electrode structure will be discussed in this paper.

Ultrashort pulse lasers with pulse durations < 1 ps make it possible to cold process a wide range of materials, while introducing virtually no heat into the workpiece. Industrial ultrashort pulse lasers are currently mainly limited to the wavelength range around 1 μm and below. With optical parametric frequency conversion, however, the addressable wavelength can be extended to the IRB range (1.5 to 3.0 μm). Based on a commercially available laser emitting at a wavelength of 1030 nm, the system presented here generates laser light at a wavelength of 2.06 μm in a two-stage process. First, in an optical parametric generator (OPG), part of the pump power is converted into the degenerated signal and idler field (2.06 μm). In an optical parametric amplifier (OPA), this field is further amplified by the remaining pump power. An optional seeding with a narrow-band diode laser can be used to influence the output bandwidth in a targeted manner. An output power of 18.5 W was generated from approximately 80 W input power. At a pulse repetition rate of 800 kHz, this corresponds to a pulse energy of approximately 23 μJ. Moreover, a beam quality M² of 1.8 and 2.0 in horizontal and vertical direction was achieved. The pulse duration at 2 μm at this operating point is about 600 fs at a pump pulse duration of 900 fs. At an operating point with optimized power, a maximum output power of about 28 W, corresponding to about 35 μJ of pulse energy, was generated. The overall conversion efficiency at this working point was more than 35 percent.

A 1064 nm picosecond hybrid fiber/bulk laser delivering 85 μJ, 30 ps pulses is reported. The whole laser chain is made of a compact fibered mode-lock oscillator, pulse-picker and ytterbium doped amplifiers while high pulse energy operation is achieved thanks to a Nd:YVO₄ crystal amplifier which permits to obtain MegaWatt range peak power pulses without detrimental nonlinear effects. This laser system has been designed in order to efficiently produce LIPSS on metals.

The generation of sub-10 ps pulses around a wavelength of 2 μm with pulse energy at millijoule-level in a compact CPA-free amplifier chain is presented. This laser source covers a broad range of pulse repetition frequencies from 1 to 100 kHz with a pulse peak power from 136 to 17MW, respectively. We used highly doped Ho:YLF crystals to achieve an overall amplification factor of almost 52 dB. A characterization of these crystals regarding upconversion losses and attainable small-signal gain supports this work.

We present an energy-scalable ultrafast Yb:YAG MOPA system for material processing applications. The system consists of a dual-side-pumped Yb:YAG planar waveguide (PWG) amplification stage that is seeded by a commercial laser, pre-amplified by a dual-end-pumped Yb:YAG single crystal fibre (SCF). The SCF is pumped by two 80 W fibre-coupled laser diodes and amplified the seed (344 fs pulses, 10 MHz, 140 nJ) to 1.4 µJ in a single-pass configuration. Thermal lensing, astigmatism and depolarisation within the SCF was analysed and compensated for by careful selection of beam-shaping optics. The 12 mm wide by 13 mm long PWG crystal has a 150 µm core of 2 at.% Yb:YAG, bonded top and bottom to sapphire cladding of 1 mm thickness. The core is side-pumped using two 540 W phase-corrected diode stacks such that a uniformly distributed high gain was achieved. The advanced crystal design suppresses intra-crystal parasitic oscillations and the PWG geometry significantly alleviates thermal lensing. The seed is multi-passed through the PWG crystal and the mirror parameters were carefully chosen to optimise gain extraction. The seed path is scalable up to 7 passes through the crystal for which 8.7 µJ per pulse was achieved at a pump power of 860 W for sub-ps pulses at 10 MHz. Current investigations include suppression of unwanted parasitic oscillations between the multi-pass mirrors to improve the output beam quality. Future work is aimed at the inclusion of kHz burst modes of sub-ps pulses at 1MHz repetition frequency.

We present an amplifier system for 2 µm ultrafast laser pulses for potential material processing applications. The amplifier gain material is a 0.75 at.% doped Ho:YAG slab crystal measuring 10 mm x 1.5 mm x 55 mm. The pump source is an in-house developed continuous wave Tm:YLF slab laser which produces a maximum output power of 340 W, centred at the 1908 nm Ho:YAG absorption peak. The pump beam full widths were 0.2 mm by 5.3 mm in the slab. The seed for the experiment was a mode-locked Tm:LuScO3 laser that produced 200 fs pulses (~23.6 nm spectral bandwidth) centred at 2094 nm. The spectral peak of the seed laser was chosen so as to spectrally overlap both the 2090 and 2097 nm emission peaks of Ho:YAG. The pulse repetition frequency of the seed laser was 115 MHz, and the average power as measured after an optical isolator was ~57 mW. In the initial experiment the seed was focused into the slab using a spherical doublet lens pair to a beam diameter of 0.2 mm. The measured single pass gain was ~10 (up to 0.54 W) when pumped with 280 W. The effective pump power (disregarding transmitted pump light) in the gain volume used for amplification was estimated to be 8.3 W. The spectral bandwidth of the output signal was measured at several output powers and shown to converge to ~11.8 nm. Based on these results and in-house simulations we will implement a pre-amplifier and scale 2 µm ultrashort pulses to >100 W average power at MHz PRFs.

For spectroscopy on muonic helium, we developed a thin-disk laser with an output pulse energy of 147 mJ capable of stochastic triggering (< 500 Hz) with a beam quality of M² = 1.04. We reached these values by implementing Fourier transform propagation from disk to disk. This architecture is passively compensating for phase front distortions caused by thermal lensing in the thin disk. The amplifier was not only very stable against thermal lens effects but was also forgiving misalignment of the disk. Indeed, it was operated for 3 months without realignment. Our design utilizes the gain profile of the active medium (effective soft aperture) to provide transversal mode filtering. As a result, the beam remains in the TEM00 mode during propagation in the amplifier. This implies that modeling of its propagation does not require techniques for higher-order mode propagation. Soft apertures can simply be modeled as lenses with imaginary focal length. We modeled the misalignment related losses for two different multi-pass amplifier designs reproducing the measured behavior. We also realized a simple control system to correct for tilt misalignment of the active medium. Only two servo-controlled folding mirrors inside the amplifier were sufficient to stabilize the beam position of all (eight) passes on the active medium. This active stabilization reduced the sensitivity (decrease of output energy) to tilts of the active medium by a factor of 10 compared to the multi-pass amplifier without active stabilization.

Due to the low absorption of pump light in a thin disk laser, the pump light has to be redirected multiple times onto the active medium in order to achieve high pumping efficiency. Therefore, the pump optics in current systems require a large volume compared to the thin disk itself and multiple optics have to be aligned correctly with each other. Our wedged optical lasing chamber for ytterbium disks (WOLCYD) consists of an optical long-pass filter placed at a small angle directly in front of the thin disk. By this, an in-place multiplication of the number of pump passes is achieved. This results in a compact pump optic without the need of sophisticated alignment efforts. We demonstrate a laser oscillator setup and a laser amplifier setup on the basis of the WOLCYD geometry.

A 5-kW thin disk laser with a beam parameter product (BPP) of ≤ 2.5 mm×mrad (50-μm processing fiber) has been realized. Target applications of this device include high speed laser cutting and remote (wobble) welding. Furthermore, we present an 8-kW thin disk laser system with a BPP of 4 mm×mrad (100-μm processing fiber) based on one disk. We also present results on a 18-kW thin disk laser based on two disks (125-μm processing fiber). A new line of thin disk lasers with output powers of 1-6 kW is introduced: up to four fiber outputs allow for a wide variety of time and energy sharing schemes.

We present a multi-pass amplifier which passively compensates for distortions of the spherical phase front occurring in the active medium. The design is based on the Fourier transform propagation which makes the output beam parameters insensitive to variation of thermal lens effects in the active medium. The realized system allows for 20 reflections on the active medium and delivers a small-signal gain of 30 with M² = 1.16. Its novel geometry combining Fourier transform propagations with 4f-imaging stages as well as a compact array of adjustable mirrors allows for a layout with a footprint of 400 mm × 1000 mm.

TRUMPF Scientific Lasers provides ultrafast laser sources for the scientific community with high pulse energies and high average power. All systems are based on the industrialized TRUMPF thin-disk technology. Regenerative amplifiers systems with multi-millijoule pulses, kilohertz repetition rates and picosecond pulse durations are available. Record values of 220mJ at 1kHz could be demonstrated originally developed for pumping optical parametric amplifiers. A huge step will be to combine high energies, 1J per pulse, with average powers of several hundred watts to a kilowatt. Multipass amplifiers based on the thin-disk technology were successfully used to realize picosecond amplifiers with more than 2kW of average power. Nevertheless, the pulse energy was in the μJ or low mJ range. At TRUMPF Scientific Lasers these experiences will lead the way to set-up a system running at 1kHz repetition rate and a target pulse energy of 1J. Within the paper the roadmap to a Joule system will be presented as well as first results from a laboratory set-up.

We demonstrate a continuous-wave diamond Brillouin laser (DBL) in a ring cavity, operating near 532 nm with a 167 GHz Stokes shift. The DBL is pumped by a narrow-line (<1 MHz), frequency-doubled ytterbium fiber laser with the intracavity power resonantly enhanced via Hänsch-Couillaud locking. The measured threshold enabled the Brillouin gain coefficient in diamond to be determined for the first time, yielding a value of 60 cm/GW for pump and Brillouin polarizations aligned parallel to the 〈111〉 crystallographic direction in diamond (determined by the cut of our diamond in this case). Analysis of diamond’s photoelastic tensor shows that for polarization aligned to 〈110〉 for maximum gain, a coefficient of 115 cm/GW can be deduced, the highest bulk Brillouin gain coefficient reported for any material. The high Brillouin gain coefficient in combination with outstanding optical and thermal properties, indicates great potential for realizing diamond lasers and stimulated Brillouin scattering-enabled devices of performance far exceeding other materials.

In this work a diode-pumped Q-switched Alexandrite laser operating in single longitudinal mode (SLM) at the potassium resonance line is presented. The self-developed laser diode pump device is fiber-coupled ( = 400μm, NA=0.22) and delivers a pump energy of 18 mJ at 636 nm with a pulse duration of 120 μs and a repetition rate of 500 Hz. Pump light not absorbed in single-pass through the 7 mm long crystal is recollimated, polarization adjusted and refocused into the crystal. The Alexandrite laser yields a pulse energy of 1.7 mJ at a repetition rate of 500 Hz with a high pulse-to-pulse stability of 0.2 % (rms) and a beam quality of M² < 1.1 in both spatial directions. The output beam is round and stigmatic without further beam shaping. The electro-optical efficiency of the laser system is 2 % which is approximately two magnitudes higher than of comparable flashlamp-pumped Alexandrite laser systems. By seeding the resonator with a SLM diode laser and actively stabilizing the cavity length, SLM-operation at the resonance line of potassium at 769.898 nm with a linewidth of approximately 10 MHz is achieved. Thereby the laser fulfills all the requirements for a resonance-lidar system. The investigations presented in this publication show the feasibility for pumping a complex ring resonator with a fibercoupled pump module in the red spectral region. This presents an important step to compact lidar systems for autonomous measurements under rough environmental conditions.

We demonstrated continuous-wave dual-wavelength (DW) operation of a Nd:CALGO laser using a single birefringent filter (BRF) plate. Owing to a new mechanism of the BRF loss action, the equal lasing thresholds at two wavelengths could be provided by a single transmission peak of the filter and wavelength separation was not limited to the free-spectral range (FSR) of the filter. A wide range of DW pairs with wavelength separation from sub-nm up to ~4 nm with adjustable spectral intensity ratio was experimentally demonstrated using a single 2 mm-thick BRF plate.

We report what we believe is the first demonstration of a continuous-wave Nd:YLF laser under in-band diode pumping at 908 nm. The laser produced the maximum average output power of ~750 mW at 1047 nm. The maximum slope and optical-to-optical efficiencies were 39.5% and 64.6%, respectively, with respect to the absorbed pump power. The maximum output power was limited by the available absorbed pump power and not the thermal effects. The results that are presented in this work are preliminary and further work is in progress to improve the efficiency of the laser system.

A quasi-continuous-wave external cavity diamond Raman laser with 1.2 kW output power was demonstrated using gated pump pulses of 100 µs duration, which was 7 times longer than the time constant for the establishment of steady-state temperature gradients. An 83% slope efficiency and a 53% optical-to-optical efficiency were obtained in conversion from a 1.064 µm Nd:YAG pump to a 1.24 µm first Stokes. The transient Stokes behavior from the sharp turn-on was consistent with calculations for the first Stokes build-up time. A brightness enhancement factor of 56 was obtained from the M2 = 15 pump beam. An observed increase in the first Stokes beam quality from M2 = 2.95 to M2 = 1.25 with increase of the Raman laser output power indicated the presence of a steady-state thermal lens in diamond.

In the literature one finds several conflicting accounts of the phase difference of stimulated and spontaneous emission, as well as absorption, with respect to an existing (triggering) electromagnetic field. One of these approaches proposes that stimulated emission and absorption occur in phase and out of phase with their driving field, respectively, whereas spontaneous emission occurs under an arbitrary phase difference with respect to an existing field. It has served as a basis for explaining quantum-mechanically the laser linewidth, its narrowing by a factor of 2 around the laser threshold, as well as its broadening due to amplitude-phase coupling, resulting in Henry’s α-factor. Assuming the validity of Maxwell’s equations, all three processes would, thus, violate the law of energy conservation. In semi-classical approaches, we investigate stimulated emission in a Fabry-Pérot resonator, analyze the Lorentz oscillator model, apply the Kramers-Kronig relations to the complex susceptibility, understand the summation of quantized electric fields, and quantitatively interpret emission and absorption in the amplitude-phase diagram. In all cases, we derive that the phase of stimulated emission is 90^o in lead of the driving field, and the phase of absorption lags 90^o behind the transmitted field. Also spontaneous emission must obey energy conservation, hence it occurs with 90^o phase in lead of an existing field. These semi-classical findings agree with recent experimental investigations regarding the interaction of attosecond pulses with an atom, thereby questioning the physical explanation of the laser linewidth and its narrowing or broadening.

Distributed-feedback waveguide lasers based on Bragg-grating resonators generate ultranarrow-linewidth emission. Oscillation at the center of the reflection band ensures maximum reflectivity, hence minimum laser linewidth. The required μ/2 phase shift is often introduced by a distributed change in effective refractive index, e.g. by adiabatically widening the waveguide. Despite careful design and fabrication, the experimentally observed resonance wavelength deviates, thereby placing the resonance and laser emission at a position with lower reflectivity inside the reflection band. This effect is usually incorrectly attributed to fabrication errors. Here we show theoretically and experimentally that the decay of light intensity during propagation from the phase-shift center into both sides of the Bragg grating due to (i) reflection by the periodic grating and (ii) the adiabatic refractive-index change causes an incomplete accumulation of designed phase shift, thereby systematically shifting the resonance to a shorter wavelength. Calculations are performed based on the characteristic-matrix approach. Experimental studies are carried out in a distributed-feedback channel-waveguide resonator in amorphous Al²O³ on silicon with a distributed phase shift introduced by adiabatic widening of the waveguide according to a sin² function. Calculations and experiments show good agreement. Considering in the design the overlap integral between distributed phase shift and light intensity provides the desired performance.

Laser and optical amplifier geometries may be split into categories such as rod and fiber. Rod gain media are susceptible to thermal effects at high power, whereas fiber suffer from detrimental non-linear effects due to their long length and small mode areas. Here we present an application of a hybrid architecture between the two geometries – the Thermally-Guiding Fiber-Rod (TGFR). The TGFR inherits the large mode area of the rod amplifier, the high surface area of a fiber, and exploits thermal lensing to guide modes. We present a successful demonstration of amplification of a radially polarized mode using the TGFR. A 1030 nm continuous-wave radially polarized seed source of high purity and beam quality (M2=1.9±0.1) was constructed using thermal bifocussing in a Yb:YAG crystal to provide mode selection. This seed source was carefully focussed into the 300 µm core of a 10 cm long sample of commercially available triple-clad Yb-doped silica fiber in order to satisfy the thermal guidance condition and avoid waveguiding due to the refractive index step. The TGFR was pumped using a high power 915 nm diode laser. The radially polarized mode was preserved through transmission of the TGFR. The output beam polarization was maintained at 99.1% purity while the M2 factor was measured to be 2.1±0.1. The maximum output power was 12.6 W of radially polarized light, corresponding to a gain of 7.0 dB limited by available pump power. This promising geometry the potential for further power scaling of radially-polarized beams for application in laser processing.

For many Q-switched lasers, it is desired to maximize the pulse energy while minimize the pulse duration. Such high energy, short pulse output can be achieved by pumping with large mode diameters and maintaining a cavity length of a few centimeters. However, increasing the mode area of the resonator while keeping the same cavity length causes the excitation of higher order transverse modes and consequently decreases the brightness of the output radiation. To overcome this drawback, angular mode selection technique with transmitting Bragg grating (TBGs) was implemented inside the cavity in order to suppress the higher order transverse modes, allowing for near diffraction limited and high brightness output. Since a Bragg gratings angular selection works only in one transverse direction, two orthogonal gratings were implemented for mode selection in both transverse directions. A compact high energy passively Q-switched Nd:YAG laser is presented showing output pulses with 1 mJ of energy and 1.5 ns of duration. Due to the compact design requirements and to obtain such high energy, the resulting laser had large mode diameter and was multimode. The laser consists of a 5 mm thick slab of Nd:YAG, a 3 mm thick slab of Cr:YAG with a 65% transmission, and a 40% output coupler. Two TBGs were implemented in the cavity, orthogonal to one another, for mode selection in both transverse directions. Experimental results are presented demonstrating a 5x improvement in the beam quality of the system after implementing the TBG mode selection approach.

Interest in compact, single-frequency fiber amplifiers has increased within many scientific and industrial applications. The main challenge is the onset of nonlinear effects, which limit their power scaling. Here we demonstrate a compact, highpower, single-frequency, polarization-maintaining, continuous-wave fiber amplifier using only one amplification stage. We developed the fiber amplifier using a master oscillator fiber amplifier architecture, where a low-noise, singlefrequency, solid-state laser operating at 1064 nm was used as a seed source. We evaluated the amplifier's performance by using several state-of-the-art, small-core, Ytterbium (Yb)-doped fibers, as well as an in-house-made, highly Yb-doped fiber. An output power of 82 W was achieved with no sign of stimulated Brillouin scattering. A good beam quality and a polarization extinction ratio (PER) of < 25 dB were achieved. The compact fiber amplifier can be a competitive alternative to multi stage designed fiber amplifiers.

High-power and high-beam-quality lasers are used in various fields such as high energy physics, laser fusion, aerospace systems and material processing and are expected to have higher output. Power scalability is important for those applications. Solid-state lasers have such a characteristic. One of major problems in solid-state lasers is heat, which may cause damage to the medium due to thermal stress, deterioration of the beam quality, thermal lens effect, etc. Thus an effective cooling method is required. Solid-state lasers include active mirror lasers, slab lasers, thin disc lasers, rod lasers and the like, as is generally known. We have been developing laser systems using active mirror media such as TRAM (Total-Reflection Active Mirror) and ZiZa-AM (Zig-Zag Active-Mirror). In these media, disk like active materials are bonded to the outside of the non-doped material. The seed laser travels in the medium coaxially with the pumping laser while being totally reflected, and is amplified in the active material layer. Since high reflection coating required for a thin disk laser or the like is unnecessary in the case of these media, this is an advantage for removal of heat. We developed a liquid nitrogen jet impingement cooling system. This is an effective system that enables high-level heat removal performance while keeping the medium at an extremely low temperature and jets subcooled liquid nitrogen directly to the active material layer of the medium. In our previous studies, we have achieved development of a kW-class amplifier using a ZiZa-AM. This report presents the various laser characteristics in the case of multi amplifier chain to confirm the scalability of these amplifier systems.

Laguerre-Gaussian (LG) modes have properties that make them well suited to many applications, particularly laser processing when scaled to high-power. Here we present an approach for generating high-purity LG01 vortex beams in a Nd:YVO4 laser which overcomes the common problems of degenerate handedness and low damage thresholds in previous methods. The obtained modes are scaled in power by application of a novel Thermally-Guiding Fiber-Rod Amplifier (TGFRA). Our approach is based on a novel end-pumping arrangement for efficiently generating the Hermite-Gaussian TEM01 mode in a 1064 nm Nd:YVO4 laser. A fiber-coupled laser diode was spliced to a 50:50 fiber splitter to give two equal outputs. These outputs were bonded to a bulk optic with a small separation for spatially matching the TEM01 mode. An astigmatic mode converter made using two concave mirrors was used to obtain a LG01 mode with controlled handedness. The obtained LG01 mode was propagated through the 300 µm core of a 10 cm long sample of triple-clad Yb-doped silica fiber by utilizing thermal lensing as a waveguiding mechanism. The fiber was pumped using a high-power 915 nm diode laser. This amplifier geometry ensures preservation of the mode while the inheriting good thermal management from a fiber geometry and the large mode area common in rod geometries. The 0.89 W LG01 seed-source was amplified with gain of 2.7 dB. The gain was limited by available pump power and the emission cross-section at 1064 nm. This result provides an avenue to high-power LG01 modes.

High power performance of a continuous-wave Yb:YAP laser was investigated. The laser generated <7 W of output power with 22 W of pump power and slope efficiency of around 70% while maintaining high beam quality. The thermal lensing effect was observed at high pumping power. The combination of high thermal conductivity and broad gain bandwidth make this laser host a suitable candidate for high power CW and ultrashort pulse generation.

Stress-optic measurements of Nd- and Er-doped SCHOTT laser glasses were made at 1064 and 1550 nm, close to their respective lasing wavelengths. In addition, measurements of fused silica at 1064 nm serve as a useful benchmark owing to the existence of multiple studies which report on this material, including several from NBS/NIST. Owing to a currently unexplained discrepancy amongst our fused silica data using three different IR cameras, the results reported herein must be considered preliminary. Nevertheless, we believe the general trends reported herein for thermal lensing will hold, owing to a comparable increase in magnitude of the individual stress-optic parameters K_par and K_perp with increasing wavelength as previously observed for fused silica and as seen in our current data.

Recent progress in iron doped II-VI chalcogenide laser materials enabled important advancements in room temperature high energy, high power laser systems operating over 3.5-6.0 um. However, a lack of efficient and convenient pump sources for direct pumping of Fe(2+) ions limits possible applications of these materials. One viable option is using readily available pump sources to excite iron centers via Förster-Dexter energy transfer between transition metal ions. This paper reports on the characterization of iron-chromium and iron-cobalt energy transfer in Fe:Cr:ZnSe and Fe:Co:ZnSe co-doped crystals. The kinetics photoluminescence and spectroscopic measurements at 5T2-5E chromium and iron transitions indicated an efficient resonance energy transfer between ions even at room temperature. It was demonstrated that an energy transfer rate in Fe-Cr centers could be shorter than the upper level lifetime of Fe(2+) ions in ZnSe with total TM ions concentration larger than 10^19 cm^-3. Therefore, this mechanism can serve as an effective pump pathway for Fe lasing. Analysis of the dipole-dipole coupling between Fe(2+) and Cr(2+) ions demonstrated the for the shortest distance between iron and chromium ions in ZnSe host, the energy transfer rate is smaller than 1 ns. The absence of excited state absorption in Fe:Cr:ZnSe host make this materials more attractive in comparison with Fe:Co:ZnSe where Fe lasing due to excited state absorption of Co(2+) ions was limited only to low (<30K) temperature.

Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of potassium gadolinium ytterbium double tungstate with a high Yb content of 57at.%, representing an Yb³⁺ concentration of ~3.8 × 10²¹ per cubic centimeter, grown onto an un-doped KY(WO₄)₂ substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, firstly, a fraction of Yb³⁺ ions not contributing to stimulated emission, secondly, a reduction of population inversion owing to a parasitic energy-transfer process and, thirdly, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ~800 dB/cm, which was achieved without thermal management, is promising for waveguide amplifier operating without active cooling.

A highly stable and long-lifetime laser system is a key component of the space-based Laser Interferometer Space Antenna (LISA) mission, which is designed to detect gravitational waves from various astronomical sources. We are developing such laser system at the NASA Goddard Space Flight Center (GSFC). Our baseline architecture for the LISA laser consists of a low-power, low-noise small Nd:YAG non-planar ring oscillator (micro NPRO) followed by a diodepumped Yb-fiber amplifier with ~2 W output. In this paper, we will describe our progress to date and plans to demonstrate a technology readiness level (TRL) 6 LISA laser system.

Quantum Key Distribution (QKD) directly exploits the quantum phenomenon of entanglement to allow the secure sharing of a cryptographic key for information encoding. The current generation of QKD devices typically operate over dedicated and expensive private ‘dark fiber’ networks, where they are limited in transmission range to 200-300km due to the lack of quantum repeaters. This paper is concerned with an alternative approach that can lift this range limit by exploiting QKD over free-space links between satellites. Typically, commercial QKD systems rely on phase encoding of information on single photons, and more recently on continuously variable schemes with more powerful lasers. However, these protocols are not suitable for communications through atmosphere. On the other hand, QKD by polarization-entanglement holds great promise for satellite-based QKD encoded communications links if the entangledphoton source can be packaged in a compact, robust and commercially-viable form. This paper will describe the development and packaging of an entangled-photon source utilizing space-qualified telecoms packaging techniques, resulting in a compact device that targets satellite deployment. The key design choices that impact performance in a space environment will be discussed and the results of device characterization in the laboratory environment will be shared.

The spectroscopic and laser properties of Er:YLF crystal, that is appropriate for generation at 2.8 μm, in temperature range 80 - 300 K are presented. The sample of Er:YLF (6 at. % of Er³⁺) had face-polished plan- parallel faces without anti-reflection coatings (thickness 9 mm). During experiments the Er:YLF was attached to temperature controlled copper holder and it was placed in vacuum chamber. The transmission and emission spectra together with the fluorescence decay time were measured depending on temperature. The excitation of Er:YLF was carried out by a laser diode radiation (pulse duration 5 ms, repetition rate 10 Hz, pump wavelength 972 nm). Laser resonator was hemispherical, 100 mm in length with flat pumping mirror (HR @ 2.8 μm) and spherical output coupler (r = 100 mm, R = 95 % @ 2.65 - 2.85 μm). The tunability of laser at 80, 200 and 300 K was tested using MgF₂ birefringent filter and several laser lines in wavelength range from 2709 nm to 2860 nm were observed. The fluorescence decay time of manifold ⁴I_11/2 (upper laser level) became shorter and intensity of up-conversion radiation was increasing with decreasing temperature. In pulsed regime, the highest slope efficiency with respect to absorbed mean power 23 % and the maximum output energy 12.5 mJ were reached. The laser radiation generated by Er:YLF laser (2.81 μm) is close to absorption peak of water (3 μm) thus this wavelength can be use in medicine and spectroscopy.

Passively Q-switched microchip (MC) laser with a compact cavity configuration, allowing a sub-nanosecond pulse generation, is an attractive source for the industrial applications including laser processing. Polarization control in such a laser system can be achieved by arranging a polarization-selective element inside of the cavity, e.g. thin film polarizer, resulting in the linearly-polarized output. However, the arrangement impacts on the cavity-length, which leading to expanded the pulse width of the laser output. In this work, we have successfully demonstrated a compact, sub-nanosecond green pulse laser, based on second harmonic generation (SHG) of the MC laser, in which the polarization-selective photonic crystal grating mirror was employed as an output coupler. This system enables to freely select the polarization direction of the linearly-polarized output by just azimuthally rotating the output coupler and thus can accomplish the second harmonic process via a nonlinear crystal without a half-wave plate. The MC laser, pumped by a fiber-coupled 808 nm quasi-continuous wave laser diode, was comprised of 4 mm long Nd³⁺:YAG crystal with high reflectivity at 1064 nm, Cr⁴⁺:YAG crystal as a saturable absorber, and the photonic crystal grating mirror with 50 % reflectivity for 1064 nm. The resulting millijoule-level, sub-nanosecond laser pulse with 45^o polarization direction to the crystal axis of a KTiOPO4 (KTP) crystal (Type-II, 9 mm long) for the SHG was frequencyconverted to 532 nm laser output. The total length of this laser system (including the MC laser and the KTP crystal) was also around 35 mm.

A innovative modification method based on polypyrrole (PPy) was developed as an effective way to improve the saturable absorption performance of MoS₂. By employing the homemade MoS₂-PPy copolymers as saturable absorber (SA), a diode-end-pumped passively Q-switched Nd:LLF laser at 1.06 μm was demonstrated. Based on a MoS₂-PPy nanosheet saturable absorber, a pulsed laser output with 93 ns pulse width was achieved, which was nearly half of the pulse width using MoS₂ as saturable absorber. Meanwhile, the pulse peak power reached 75 W.

Demonstration of intracavity loss measurement in a diode-pumped continuous-wave Yb:CALGO laser is presented. The characterization method was based on spectroscopic gain measurements and allowed accurate and reliable determination of intracavity losses above the laser threshold. A comparison with traditional Findlay-Clay analysis was also made and highlighted the advantages of the spectroscopic method.

Thermal lensing effect in a Nd:GdVO₄ laser system operating at 1063 nm with in-band pumping at 912 nm was studied. The thermal lensing strength was experimentally measured and the determined focal power was as strong as 9 m^-1 at 11.3 W of output power. The sensitivity factor of the thermal lens was calculated to be as small as M = 0.503 m⁻¹/W for the pump spot size radius of ~197 μm. The present work indicates that in-band pumping at 912 nm can offer further power scaling opportunities for the Nd:GdVO₄ lasers due to the strongly reduced thermal lensing effect.

A direct comparison of continuous-wave diode-pumped Yb:CALGO and Yb:KYW lasers was demonstrated. The lasers were characterized not only in terms of output power but also in terms of thermal lensing. The results showed that in the medium to low power regimes the Yb:KYW laser outperformed the Yb:CALGO laser in terms of optical efficiency, however, in high power regime the latter showed much more favorable thermal behavior than Yb:KYW.

Monoclinic rare-earth silicates, RE₂SiO₅, are the promising hosts for Nd³⁺ doping. We have studied Nd:(Gd,Y)₂SiO₅, Nd:(Lu,Y)₂SiO₅ and Nd:Lu₂SiO₅ crystals for their suitability for ~1.3 μm (⁴F_3/2 → ⁴I1_3/2) lasers. The absorption and stimulated-emission cross-section spectra were determined. The continuous-wave laser operation was studied in a compact plano-plano cavity. A b-cut Nd:(Gd,Y)₂SiO₅ crystal generated up to 0.75 W of linearly polarized emission at 1360.7 nm with a slope efficiency η of 16.9%. For the same crystal operated at the ⁴F_3/2 → ⁴I_11/2 transition, the output power reached 3.84 W at 1077.4 nm with η = 54.5% with a threshold of only 80 mW.

Room temperature lasing of a set of Cd_1-xMn_xTe solid-solution crystals doped with Fe²⁺ ions was obtained under optical pumping by 4.1 μm liquid nitrogen cooled Fe²⁺:ZnSe laser. Oscillation wavelength maxima were found to increase linearly with Mn content x increase at a rate of about 60 nm per each 10 % of Mn content in the sample. The central oscillation wavelength as high as 5940 nm was obtained for sample with the highest Mn content (x = 0.76). The output energy was found to decrease for samples with higher Mn content due to very strong nonradiative fluorescence quenching.

The influence of temperature (from 80 up to 400 K) on emission of microchip lasers based on Nd:YAG crystal together with its spectroscopic properties was investigated. Three microchip lasers primarily designed for emission at 1.06, 1.32, and 1.44 μm, were tested. For all three lasers, the same parameters of Nd:YAG rod were used (Nd-doping » 0:9 at.% Nd/Y, length of 5 mm, diameter 5 mm). Resonator mirrors were deposited directly on the laser crystal faces. The output coupler transmission for desired wavelength was 2-5 %. The microchip lasers were placed in the temperature controlled cupreous holder inside vacuum chamber of the liquid nitrogen cryostat. The lasers were longitudinaly pumped by 808nm fibre-coupled pulsed laser diode (pulse duration 10 ms, repetition rate 10 Hz, maximum pumping power amplitude 10.5 W). The output microchip laser emission wavelength, laser threshold, laser slope efficiency, and laser beam profile were measured in temperature range from 80 up to 400 K. It was found that temperature strongly influenced mainly the laser beam profile and divergence which is rising with temperature (about 120% per 100 K). There exists also a significant influence on laser emission wavelength. Rich set of laser emission lines were obtained with these three lasers: 1061 and 1064nm (1.06 μm laser), 1318, 1332, 1338, and 1354nm (1.32 μm laser), and 1354, 1412, and 1444nm (1.44 μm laser). Generally, a higher temperature caused a longer emission wavelength. The temperature influence on lasers input-output characteristics was not so strong. For all three samples the best results were obtained for temperature in range 200-250 K. The highest laser slope and lowest threshold power were following: 60% and 0.8W (1.06 μm laser), 44% and 1.2W (1.06 μm laser), and 16% and 1.7W (1.44 μm laser).

For the ESA/Roscosmos ExoMars 2020 mission a pulsed UV laser source as part of the Mars Organic Molecule Analyzer (MOMA) instrument was developed, assembled and thoroughly tested concerning thermal, vibrational and shock loads. The characterization was performed before and after integration to a mass spectrometer, which serves as the detector for ionized fragments desorbed from the Martian soil samples due to UV irradiation. The opto-mechanical design of the flight model and the verification of its suitability for the mission requirements are presented here. A longitudinally pumped, passively Q-switched oscillator emits bursts of up to 50 pulses with an output energy of 1.1 mJ at 1064 nm and an intra-burst repetition rate of 100 Hz. Via a two-stage frequency quadrupling with a KTP and a BBO crystal this radiation is converted to 1.5 ns long pulses at 266 nm with an output energy of 130 μJ which can be decreased by temperature tuning of the nonlinear crystals to less than 10% of the nominal energy. The laser head also comprises beam shaping and steering optics to adjust the spot size and position on the sample and the capability to measure the UV energy and the pulse release time. The complex opto-mechanical design is realized within an envelope of less than 220×57×45 mm³ and has a total mass of less than 220 g. To minimize negative effects of the harsh Martian environment on the coatings the laser head is enclosed in a hermetically sealed housing filled with dry synthetic air.

We are investigating the output and temperature characteristics of Yb:YAG TRAM (Total-Reflection Active Mirror) laser using zero-phonon line excitation (969-nm pumping) and direct water jet cooling for efficient heat removal. The TRAM configuration has an advantage of cooling the surface of the Yb:YAG disk without the high-reflection coating. We have developed an efficient hydrodynamic cooling system, where the disk is directly cooled by impinging water jet with flow rate of up to 52 liter/min., while the water temperature can be controlled from 7 to 80 degrees Celsius. For the estimation of operating temperatures of the Yb:YAG, we measured fluorescence spectra from Yb:YAG using a spectrometer. We tested several types of TRAM with different layer thicknesses and doping concentrations, which were designed to absorb more than 80% of the pump power in a single bounce at room temperature. A fiber-coupled CW laser diode (FCLD) with 600 W output power at 969 nm was used as a pump source. The dependences of oscillator output power and the laser medium temperature on the cooling water temperature and flow rate were investigated. The direct impinging water jet at high flow rate was demonstrated to be effective for cooling the laser medium. It was also confirmed that the zero-phonon line excitation at 969-nm resulted in lower laser medium temperature and hence higher output power compared to the 940-nm pumping. In addition, we demonstrated kW-class laser oscillation using the cooling system and achieved slope efficiency of 63 %.

Active mode locking is reported of a hybrid fibre laser using a semiconductor optical amplifier (SOA) as an active medium synchronously pumped with electric pulses. For the first time it was demonstrated that the shape and structure of generated optical pulses can be manipulated by appropriate shaping of electric pumping pulses. The proposed hybrid SOA/fibre cavity topology features relatively high mode-locked generation efficiency in combination with compact cavity dimensions and spectral universality of the laser. Further ways to advance mode-locked lasers relying on hybrid SOA/fibre topology are discussed.

A passive, Q-switched pulsed, Nd:YAG laser system was modeled, designed, and implemented to provide a reliable laser source for portable laser induced breakdown spectroscopy (LIBS) systems instead the ordinary systems that are using laser pulses separated by some delay with microsecond order. The designed system consists of a Nd:YAG rod that is pumped by a flash lamp. Our choice for flash lamp as a pumping source based on the fact that flash lamps are fairly inexpensive and are capable of delivering enough amount of optical power in a short time span. We have successfully implemented a xenon-flashlamp-pumped Nd:YAG laser passively Q-switched with a Cr4+:YAG saturable absorber operates at 1064 nm contains a train of pulses having maximum with total output energy of 700 mJ . The pulse width of the train of pulses is about 37ns and the main output pulse train is within 550 μs. These laser shots were employed to enhance the LIBS signal. System verification was performed by LIBS measurements and analysis on different ancient ceramic samples belongs to different ancient Egyptian Islamic eras. The results showed highly robust and trusted information that can help archeologists in restoring and repairing different precious archeological objects.