This PDF file contains the front matter associated with SPIE Proceedings Volume 12399, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

Doped single-crystal YAG fibers used as single-mode lasers require claddings with precise refractive index and high thermal conductivity. Three cladding materials that use coextrusion of green cladding on fiber cores as an initial processing step are described: 1. Undoped YAG cladding, followed by sintering or hot isostatic pressing. 2. Ca₃Ga₂Ge₃O₁₂ garnet cladding that melts beneath 1400°C. 3. LiCa₂Mg₂As_3xV_3-3xO₁₂ garnet cladding that melts beneath 1100°C. Microstructures are characterized by TEM. Equipment and procedures are described. Garnet refractive index models are developed and validated to predict cladding refractive index. Advantages and disadvantages of the different claddings are compared.

This paper briefly highlights advancements in crystalline semiconductor core optical fibers. Such fibers potentially open the doors to marry on-chip optoelectronics with fiber optics, particularly nonlinear fiber optics. However, since crystals are grown, not drawn like glass, their fabrication into practical lengths represented a major limitation. This limitation can be overcome using the molten core method (MCM), which affords long lengths of crystalline core fibers using industry accepted glass fiber draw methods. This paper briefly reviews the process and the crystalline semiconductor core fibers realized to-date.

A new versatile patent-pending technology enabling new operation regimes and a unique set of features in the industrialgrade 30 W-level average power femtosecond hybrid laser is introduced in this work. The developed technology, based on the use of an all-in-fiber active fiber loop (AFL), enabled to form GHz bursts of ultrashort laser pulses with any desired pulse repetition rate and any number of pulses in a burst with identical intra-burst pulse separation. Furthermore, the AFL allowed to tune pulse duration from a few hundred femtoseconds to picoseconds and even up to the nanosecond range.

We apply mode transformation to an Yb fiber laser for direct generation of a first order vortex mode (LG₀₁), yielding LG₀₁ power of 5W at 96 % purity (from modal decomposition) with 16W pumping. The laser used standard single mode Yb doped fibers operating at 1064 nm. A free-space Sagnac interferometer formed one reflector of the cavity by feeding back the internal Gaussian mode of the fiber laser and output coupling a LG₀₁ via interferometric mode transformation. It was stable over hours of operation and days of inactivity, and was insensitive to polarisation. The maximum output power was only limited through heating of a optical element, which could be mitigated with thermal management. We also show that additional spiral phase plates (SPPs) are a route to higher purity, higher order vortex modes than with SPPs alone due to improved intensity matching between LG₀₁ and higher order states.

We demonstrate the simultaneous creation of OAM-carrying, Gaussian beamlets around a perfect vortex envelope. Each beamlet is generated with a unique frequency, allowing beat frequencies to be recovered at the receiver providing sensing information in underwater environments.

We present a readily configurable and scalable 2 μm multi-channel laser source that integrates a waveguide chip laser with an all-fiber cavity. We report here the first experimental realization and characterization of a two-channel holmium-doped zirconium fluoride glass waveguide array laser pumped by a single 1945 nm thulium fiber laser. Specific laser wavelengths are selected by fiber Bragg grating output couplers (2076.7 nm; 2074.4 nm), and single channel powers of <100 mW with slope efficiencies of up to 16 % are achieved. Design, assembly details, performance, channel scaling potential, and considerations for future work are proposed.

For more than 45 years, L3Harris divisions in Hilton Head, SC and Orlando, FL have pioneered the development of erbium/ytterbium (Er/Yb) co-doped phosphate laser glass and Q-switches in “eye-safe” laser transmitters for range finding and illumination/tracking/targeting applications. In this paper we will present how varying the Er to Yb ratio in high-gain phosphate laser glass media has innovation in small laser transmitters emitting at 1.54µm and capable of operating over a wide temperature range from -40°C to 85°C without active cooling. We focused on three different doping ratio regimes: The first ratio – medium Er/high Yb – enabled slightly larger laser 3mJ per shot with a repetition rate of 5Hz. The second ratio – low Er/medium Yb enabled the development of a high repetition rate of 100Hz laser emitting 1mJ of energy in each shot. The third ratio – high Er/ relatively low Yb – was used to produce a very small lightweight laser with an energy output greater than 200μJ per shot with a repetition rate of 10Hz and triple pulse output. In this paper, we will provide a theoretical model for calculating the output from our Er³⁺ mini lasers depending on the Er to Yb co-doping ratio and energy transfer efficiency in phosphate laser glass. The model takes into account the major laser parameters such as number of pump diode, their efficiency, pump time, diode wavelength, optical density of the pump wavelength in the laser gain medium, pump volume (absorption length, pump length, pump width), cavity mirrors, Co²⁺ Spinel Q-switch initial transmission, maximum energy output, and “expected” energy output etc. The pump structure of the laser transmitter and the laser cavity design will be illustrated. The laser output energy over temperature, its pulse width, output beam quality, and shot-to-shot beam pointing stability will also be presented.

We present a crossed-Porro prism resonator with a Ho³⁺:YAG crystal and investigate it with a focus on the alignment stability. Furthermore, we show a single-Porro-ended resonator optimized for Q-switched operation. Both resonators are compared to corresponding mirror resonators. In the crossed-Porro prism resonator, a maximum output power of 30.7 W is reached with a high slope efficiency of 67.4 %. By tilting each of the prism axes one by one and measuring the entailed drop in output power, the alignment sensitivity is determined. In comparison to a corresponding mirror resonator, it is improved by a factor of up to 200. With this design, 170 ns Q-switched pulses with an energy of 0.51 mJ are generated at a repetition rate of 50 kHz. In the single-Porroended resonator significantly shorter pulses with a duration of 55 ns and a maximum pulse energy of 0.8 mJ were achieved.

We discuss the design of a pulsed, Joule class diode-pumped MWIR Fe:ZnSe laser with M2 ≤ 1.3. We go over the challenges in the process of designing the diode-pumped 2.94 μm Er:YAG laser, which is used to pump the Fe:ZnSe laser - from the abnormal behavior in the lifetimes of the upper and lower lasing states, to the self-pumping process that exists in the highly doped Er:YAG crystal, which makes this laser work. We will discuss the process which leads to the size of the laser rod and how to determine the wavelength of the laser diodes, which pump the laser rod. We also review the challenges in the designing and building of the MWIR Fe:ZnSe laser itself - the factor which limits the maximum pulse energy and how to solve it, along with the lasing wavelength versus the temperature of the Fe:ZnSe crystal, the astigmatic issue of this laser and its solution. Finally, we go over the energy scaling prospect of the Er:YAG pump laser and the Fe:ZnSe laser.

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 previously developed such a diode-pumped Yb:YAG micro-laser with an oscillator delivering 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. This oscillator had been amplified to the mJ level by an additional laser module. But there is a large choice of possible dopant concentration and thickness for the Yb:YAG laser crystal, of low power transmittance value for the Cr:YAG passive Q-switching crystal and of pump power and burst duration, and we want to be sure the choice of design we make is the best one. In order to optimize this choice of design for the micro-laser, this paper, we developed a numerical model of laser amplification and passive Q-switch. After presenting the model, we describe how it compares with previous results from our own experimental results, in terms of energy per pulse, pulse duration and repetition frequency of the laser and how we managed to obtain good agreement with the experiments by optimizing the numerical modelling of the overlap between the laser and pump beams in the amplifying medium. Finally, future work to verify the reliability of the numerical model and to use it for optimization of the architecture of the passively Q-switched laser is presented.

Despite the extensive studies of Alexandrite as a potential laser-active medium for future Earth observation space missions in the last years, the corresponding laser systems and especially coated Alexandrite crystals do not exceed Technology Readiness Level (TRL) 4. This means that although the developed Alexandrite-based laser systems have been validated in the laboratory, it has not been published up to now, whether they can withstand the harsh environmental conditions in space. In the course of the Horizon 2020 project GALACTIC, high-quality functionallycoated Alexandrite crystals developed and manufactured within the GALACTIC project will be qualified according to TRL 6 in an environmental test campaign comprising irradiation and thermal cycling tests together with optical performance tests before and afterwards (functional laser tests, LIDT, and transmission measurements). In this work, we present our strategy for testing functionally coated alexandrite crystals for space applications developed, fabricated and tested in a fully European supply chain within the GALACTIC project with the goal of increasing the TRL level to 6.

We report our recent upgrade of the laser and optical transport line for the laser-based ion beam diagnostics (laser wire) system at the linear accelerator (linac) of the Spallation Neutron Source (SNS). A new light source based on diodepumped solid-state laser amplifiers has been developed to provide near diffraction-limited laser beam with a variety of pulse structures. The free-space optical transport line has been modified to include image relay optics with remote control, which greatly improved the laser beam quality and position stability at individual measurement stations. The upgraded system enables novel beam diagnostics/control capabilities including longitudinal profile measurement and high-energy proton beam extraction from the SNS linac.

NASA’s Dragonfly mission is a rotorcraft lander which will explore several geologic locations on Saturn’s moon, Titan and investigate evidence of surface-level prebiotic chemistry as well as search for chemical signatures of water-based and/or hydrocarbon-based life. To perform molecular composition investigations in-situ, the payload includes the Dragonfly Mass Spectrometer (DraMS), being developed at NASA’s Goddard Space Flight Center (GSFC). DraMS will utilize laser desorption mass spectrometry (LDMS) to interrogate surface samples and measure the organic composition. Enabling this science capability is the Throttled Hydrocarbon Analysis by Nanosecond Optical Source (THANOS) laser being developed at NASA-GSFC. The THANOS laser is comprised of a solid state, passively Q-Switched Nd:YAG oscillator which is frequency converted to 266 nm and utilizes a RTP high voltage electro-optic for pulse energy control. The laser outputs <2.0 ns pulses with a maximum energy of approximately 200 uJ which can be emitted in 1 - 50 shot bursts at 100 Hz while performing LDMS science operations. The laser has the capability to throttle its UV pulse energy output from full attenuation to maximum energy to provide varying levels of fluence on samples in the DraMS instrument. We report on the THANOS’ laser technology development and space qualification effort including vibration, thermal vacuum cycling, radiation as well as optical damage testing due to Titan’s atmospheric composition, performed at NASA-GSFC from 2019 through 2022.

Heavy metal halides have been used in variety of optical devices and systems including lasers, acousto-optic devices, wide transparency windows and doped materials for radiation sensing. The performance of optical and electronic materials and devices are affected significantly in space environment especially due to exposure of high energy radiations. We have investigated the effect of γ-ray radiations on multifunctional binary and ternary halide crystals in a laboratory environment. In this paper, the characteristics of mercurous halides (Hg₂X₂, X= Cl, Br) before and after radiation exposure are discussed. These crystals, synthesized with high purity source materials, were exposed with a ¹³⁷Cs γ-ray source for a period of more than 70 hours. It was observed that a ¹³⁷Cs γ-ray source with 5 μ curie source did not affect the optical characteristics of these materials which suggests their potential use in space-based components.

Here, we report on the development of a diode-pumped single-frequency transition-metal-doped crystalline lasers designed in a miniature Fabry-Perot-type resonator by utilizing a narrow-bandwidth volume Bragg grating output coupler. Namely, single-longitudinal-mode operation was achieved from Ti:sapphire and Alexandrite lasers with a maximum output power of 570 mW and 275 mW near 813 nm and 780 nm, respectively. The mode-hop-free laser frequency tunability of up to 30 GHz was achieved by the cavity temperature and length variation with the Ti:sapphire system. The laser linewidth was measured to be in the 180 kHz range when locked to an external reference cavity transmission peak.

Phase aberrations are ubiquitous in optical systems and can lead to the degradation of laser beam quality. The laser beam quality factor is an important parameter that can be used to determine the quality of a laser beam. In this work, we derive analytical expressions for the beam quality factor of Laguerre-Gaussian beams due to different types of astigmatism: 0° astigmatism, 45° astigmatism, x-triangular astigmatism, and y-triangular astigmatism. The results show that the beam waist radius is an important parameter in determining the effect of astigmatism on the beam quality factor of LG optical beams.

Recently, there is a growing interest in understanding the interactions between noble metals and rare earth ions doped into the glass matrix. Metallic or plasmonic particles can improve the spectroscopic properties of RE doped glasses for functional applications. In this present work, the spectroscopic properties of silver nitrate (AgNO₃) and Eu³⁺ co-doped zinc alumino borosilicate (ZABS) glasses are reported. The Ag:Eu³⁺ ZABS glasses were prepared using melt-quenching method at 1320 oC for 2 hours. The optical absorption spectra of Ag:Eu³⁺ glasses reveal the presence of Eu3+ bands and also a small band at 402 nm corresponding to molecule like Ag particles in the glass system. The enhancement of Eu³⁺ emission by resonance energy transfer from the Ag⁺ centers to Eu³⁺ ions confirm the surface plasmon resonance effect taken place in the glass system. The excitation of Ag:Eu³⁺ doped glasses under 393 nm showed a shift in the colour coordinate (x,y) values from deep red region to near red region. The glasses under excitations such as 330 nm and 300 nm resulted in near white light emissions with incorporation of Ag particles. Therefore, the prepared glasses can act as suitable candidate for solid-state devices like W-LEDs and lasers.

We present a temperature influence (in range from 78 up to 300 K) on laser properties of Tm:LuVO₄ vanadate crystal doped with 2 at. % of Tm³⁺. The sample was 2 mm thick plane-parallel face-polished single-crystal without any AR coating. The Tm:LuVO₄ crystal was mounted in temperature-controlled copper holder of the liquid nitrogen cryostat. The 107 mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (T < 95 % @ 788 nm, HR @ 1750-2100 nm) placed inside the cryostat and a curved output coupler (r=150 mm, R=97.5 % @ 1750-2100 nm) placed outside the cryostat. For longitudinal pumping, a fiber coupled laser diode was used. The diode was operating in the pulse regime (5 ms pulse length, 10 Hz repetition rate) at wavelength 788 nm. At 300 K, the laser emission was detected at 1931 nm with negligible power. With a temperature decrease the laser wavelength shifted to 1868 nm and the slope efficiency doubled from 7 % at 200 K up to 15 % at 78 K, while the laser threshold decreased more than five times. The maximum power amplitude of 0.8 W was achieved at 78 K. The quality of laser output beam was very good close to TEM₀₀ mode. Using the SiO₂ birefringent filter, we were able to switch the output wavelength between 1868 and 1931 nm.

In our work, we developed a LIDAR (LIght Detection And Ranging) source with high output energy of over 12 mJ and narrow spectral bandwidths < 30 MHz at a repetition rate of 1 kHz. Our approach contains the amplification of pulses with E ≤ 50 nJ out of a low energy seed source at the central wavelength 1064.475 nm and spectral width < 20 MHz. The seed pulses were amplified in four sequential Nd:YVO₄ amplification stages. The amplification of parasitic ASE (Amplified Spontaneous Emission) from stage to stage due to the high emission cross section of the material had to be suppressed to guarantee maximal population inversion for the seed pulse amplification. By temporal separation of ASE and seed pulses with an AOM between the first and the second amplification stage, ASE could be suppressed sufficiently.

Influence of temperature on gain-switched Ti:Sapphire laser based on a microchip resonator geometry is investigated. Laser performances are described within 78–300K temperature range. A frequency doubled radiation originating from the diode pumped Yb:YAG/Cr:YAG microchip crystal was used as a pump source, providing 2 ns pulses with a maximum energy of 113 μJ. The best output from gain-switched Ti:Sapphire laser was achieved at cryogenic temperature — 16 μJ of the output energy in 2 ns pulse at ∼746 nm wavelength. The corresponding slope efficiency related to incident pump energy was 16 %. Taking into account the upper estimation of the input energy absorbed in the crystal, the slope efficiency with respect to absorbed pump energy was determined to be 27%.

The study of the atomic spectrum via resonant laser excitation provides access to underlying effects caused by the nuclear structure, which is of special interest in short-lived radioisotopes produced at Isotope Separator On-Line (ISOL) facilities. Current implementations of resonant laser ionization techniques often limit the extraction of the nuclear observables due to the low spectral resolution of the pulsed laser systems deployed. Several high-resolution spectroscopy techniques demand spectral widths in the order of hundreds of MHz and below. A proven solution to reduce this linewidth is the pulsed amplification of a narrow-band continuous wave (cw) laser. This work presents the demonstration of a pulsed dye amplifier seeded by a commercially available cw Optical Parametric Oscillator (OPO). The performance of this system was compared with competing setups using a cw dye laser seed source as well as a frequency mixing technique using a combination of an injection-locked titanium:sapphire (Ti:Sa) and a Nd:YVO₄ laser. Spectral bandwidths of the systems were measured using a high finesse Fabry-Perot Interferometer, resulting in comparable optical linewidths between 140 to 156 MHz at a wavelength of 328 nm for the different laser setups. Suitability for on-line experiments was validated by performing high-resolution spectroscopy of radioactive silver isotopes in the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at the Isotope Separator On-Line Device (ISOLDE), at the European Organization for Nuclear Research (CERN). The quality of the hyperfine spectra was similar for the dye and the OPO seed and the deduced hyperfine splitting was in good agreement with literature, while the frequency mixing technique exhibited less precise results attributed to the frequency instabilities and mode-hops of the single-mode Nd:YVO₄ laser.

In this work, the spectroscopy and laser characteristics of the ceramic Er:Y₂O₃ are presented in the temperature range from 80 to 300 K. Er:Y₂O₃ ceramic (5 at. % of Er³⁺, Baikowski Co., Ltd.) had the form of a rectangular block (thickness 9 mm) with plan-parallel polished faces (cross-section 3 × 3 mm) without anti-reflection coatings. The active medium was attached to the copper cold finger of the liquid nitrogen cryostat in a vacuum chamber. The pumping of Er:Y₂O₃ was carried out by radiation from fibre-coupled laser diode (pulse duration 3 ms, repetition rate 16.6 Hz, wavelength 972 nm). All spectroscopy measurements – transmission, fluorescence and up-conversion spectra together with fluorescence decay times at ⁴I_11/2 and ⁴I_13/2 were obtained in the temperature range from 80 to 300 K. The fluorescence decay time of manifold 4 I11/2 (upper laser level) is slightly increasing with rising temperature from 325 μs (80 K) to 342 μs (170 K) and then its decreasing to 295 μs (300 K). On the other hand, the decay time of lower laser lever ⁴I_13/2 is increasing together with a rising temperature from 3.6 ms (80 K) to 4.6 ms (300 K). Moreover, it was obvious that with decreasing temperature, the intensity of green up-conversion radiation (564 nm) increased. For laser experiments a hemispherical resonator (80 mm in length) with a flat pumping mirror (HT < 94 % @ 960 - 980 nm and HR < 99 % @ 2.65 - 3 μm) and spherical output coupler (r = 100 mm, R = 95 % @ 2.65 - 2.95 μm) were used. Er:Y₂O₃ laser was operated in the pulsed regime to prevent damage to the active medium, the highest slope efficiency 9.5 % (with respect to absorbed power) and maximum output power of 83 mW were reached at a duty cycle of 5 %. With rising temperature the slope efficiency was decreasing down to 5 % at 300 K, on the other hand the laser threshold with respect to absorbed mean power was increased from 50 mW (80 K) to 150 mW (300 K). The emitted laser wavelength by the Er:Y₂O₃ laser was slightly changed due to the heating of the active medium from 2744 nm (at 80 K) to 2742 nm (at 300 K). The erbium-doped ceramic active media can be used as an alternative to crystals because of its easier fabrication compared to single-crystal sesquioxides, which require high melting point and special materials for crucible during growing process.

The use of lasers in many industrial or medical applications often requires the delivery of laser radiation from the laser system to the place of an exposure. One of the radiation transfer possibilities is the delivery of its by hollow glass waveguide. The fundamental advantage of this delivery media is the absence of the material through which the laser radiation is transferred. Other hollow waveguides properties are a possibility of delivery radiation in wide range of spectra, lossless delivery, non-toxicity, and low aging effect. This system is possible to use also for radiation which has strong absorption in water and therefore cannot be delivered through conventional glass fiber. Especially Er:YAG radiation which generated wavelength corresponding to maximum absorption in water and therefore it is extremely suitable for medical application. The aim of this study was to investigate radiation transmission for various parameters (repetition frequency, pulse length) of a commercially available diode-pumped Er:YAG laser. The special hollow glass waveguide with a cyclic olefin polymer coated silver layer has a length of 108 cm and an inner diameter of 700 µm. Radiation delivery as a function of laser input power and spatial distribution of output beams were investigated depending on pulse length from 100 µs to 400 µs and repetition frequency 20 Hz or 150 Hz. The transmission was achieved up to 86 % at a maximum amplitude of the output peak power of up to 406 W. The obtained results predetermine the use of this simple compact delivery laser system for further research with subsequent use in medicine.

A compact system for measuring laser-induced damage threshold (LIDT) was developed. As a source of linearly polarized 1064 nm testing radiation, the special Q-switched microchip laser was used. This laser was based on monolith crystal composed of Nd:YAG active laser part (0.9 at.% Nd/Y, 20 mm long, pump mirror deposited) and Cr:YAG saturable absorber (initial transmission 60 %, 1.8 mm long, output coupler with reflection 60 % @ 1064 nm deposited). The laser generates 3.9 ns long pulses with an energy of 0.42 mJ and repetition rate of 50 Hz. The probe beam spatial structure has Gaussian profile (M² = 1.1). At the focal point of the focusing aspheric lens, it is possible to achieve a peak energy density of up to 900 J/cm² (the beam radius at the focal spot was 5.6 μm). The LIDT measurement uses computer-controlled attenuator and sample 3D positioner. To calibrate the spatial parameters of the probe beam, the knife-edge method is used. The system was designed to test transparent samples with a diameter of up to 25 mm. The measurement takes place automatically. First, the probe beam is characterized. Next, the LIDT estimate is determined, and then the LIDT measurement is performed using the S-on-1 method. The system was tested for silica glass and YAG crystal. The optical part of the device has dimensions of 45 × 60 cm. It is assumed that this system will be used for routine control of LIDT of dielectric layers on laser crystals and mirrors.

In this work the results of research of an optimal concentration ratio of Cr²⁺ and Fe²⁺ co-doped active ions for more efficient Cr²⁺ → Fe²⁺ ions energy transfer in a chosen Zn_1-xMn_xSe (x ≈ 0.15) host material are presented. Three cryogenically cooled Cr²⁺,Fe²⁺:Zn_1-xMn_xSe (x ≈ 0.15) single crystals with the same thickness of ~2.7 mm and with different doping ratios of Cr²⁺ and Fe²⁺ ions of ~1:1, ~2:1, and ∼3:1 were investigated under Q-switched Er:YLF laser excitation (wavelength: ∼1.73 μm, pulse energy: ∼10 mJ, pulse duration: ∼150 ns). The temperature dependence of the absorption and fluorescence spectra, the fluorescence decay time as well as the laser output characteristics were measured. The Fe²⁺ ions maximum laser output of ~50 μJ at the wavelength range of ~4.25-4.42 μm was obtained with the crystal sample for which the active ions ratio was ~2:1. A further increase of the chromium ions amount (Cr²⁺:Fe²⁺ ≈ 3:1) led to worse results. Using appropriate resonator cavity mirrors, the samples were also able to generate the ~2.35-2.45 μm laser radiation from Cr²⁺ ions. The laser output beam spatial profiles were close to Gaussian in all cases. In summary, an optimized compact source of mid-infrared ~4.25-4.42 μm (Fe²⁺) and ~2.35-2.45 μm (Cr²⁺) of Cr,Fe:Zn_1-xMn_xSe (x ≈ 0.15) crystal pumped via Cr²⁺ ions by the ~1.73 μm radiation is described.

We fabricated a laser rangefinder for a long range distance measurement. The transmitter is our home-made laser with wavelength of 1535 nm and the peak power of 0.25 MW. The echo pulse laser signals are detected by an avalanche photodiode detector, then the signal is sent to analog to digital circuit board for signal processing. We developed an algorithms based on FPGA to de-noise and improve signal-to-noise ratio. The experimental result shows that our laser rangefinder is capable of measuring distance up to 20 km with false-alarm probability < 1% and the accuracy can reach less than 1 m by range-walk error correction.

We demonstrate sub-100 ps pulses with a Yb³⁺:YAG microchip laser passively Q-switched by a Cr⁴⁺:YAG saturable absorber. By introducing a subcavity, the laser threshold and the saturation energy are decreased which helps to prevent damage and to vary the effective emission and absorption cross sections. Pulse widths of 84 ps, repetition rates of 3.3 kHz and pulse energies of 32 μJ are achieved. This allows direct micromaterial processing e.g. for ophthalmic surgeries. To the best of our knowledge, this is the first sub-100 ps Yb³⁺:YAG/Cr⁴⁺:YAG microchip laser. A new approximation is used to calculate the rate equations for multiple longitudinal modes and to determine the threshold for single-longitudinal-mode operation.

We demonstrated a 100 W class hybrid laser system based on fiber seed laser and two free-space end-pumped Yb:YAG amplifiers capable of delivering record high pulse energy in a rod-type active medium setup operating at room temperature. The achieved output pulse energy was <10 mJ at 10 kHz pulse repetition rate. The output pulses of 1.09 ps duration were close to Fourier transform-limit. The output beam quality remained high (M² < 1.3) despite being affected by thermally induced stress in the gain medium.

Holmium-doped solid state lasers are important direct sources of coherent radiation at 2 μm with applications in medicine, spectroscopy, LIDAR technologies, or conversion to mid-infrared wavelengths. Co-doping with Tm³⁺ sensitizer enables resonant diode-pumping at 1.7 μm with aim to reduce lasing threshold and thermal loading and to increase efficiency and obtainable wavelengths range. The Gd₃(Ga,Al)₅O₁₂ (GGAG) crystal investigated in this work belongs to a class of mixed or disordered garnets. Such crystals are actively researched host material for rare earth ions due to broadening of dopant spectral lines and preserving good thermal and mechanical properties of crystalline garnet hosts. Spectroscopic and laser properties and their doping concentration dependence of Tm, Ho:GGAG crystal were investigated under 1.7 μm diode pumping. The laser material was Tm³⁺ and Ho³⁺ co-doped Gd₃(Ga,Al)₅O₁₂. It was grown by Czocharlski method from melt with initial composition Gd_2.91Ho_0.012Tm_0.075Ga_2.7Al_2.3O₁₂. The grown crystal boule was cut into eight face-polished crystal samples 5.4 mm thick and 8–14 mm wide in diameter. Tm³⁺ and Ho³⁺ content of samples was between 2.1–3.2 at.% Tm/Gd and 0.3–0.5 at.% Ho/Gd. Crystals were pumped at room temperature by fiber-coupled (NA = 0.22, core diameter = 400 µm) 30 W laser diode emitting at 1.7 µm in quasi-continuous regime. A hemispherical laser cavity was tested with OC curvature of -150 mm and reflectivity of 96.5 % at 2090 nm. All lasers emitted at 2084–2090 nm range in the untuned setup. Threshold absorbed power was in 0.1–0.3 W range and generated beam corresponded to TEM00 mode. Both the highest efficiency w.r.t. absorbed power of 37 % and the highest output power amplitude of 3.8 W were obtained for 3.0 at.% (Tm/Gd) 0.5 at.% (Ho/Gd) crystal. The total wavelength tunability range of 1936–2111 nm was obtained, with lower concentration samples tended to result in broader continuous tuning curves.

This paper is about Freetriging. First, we define what is the Freetrig mode, and list its characteristics, and then elaborate the areas one should pay attention to in the selection and use of the Freetrig mode. We also discuss the current status and future applications of the Freetrig mode, and analyze its advantages and disadvantages in various situations. We use calculations to optimize the energy utilization and use experimental data to compare the performance with and without the Freetrig mode.