This PDF file contains the front matter associated with SPIE Proceedings Volume 8235, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Er:YLF diode pumped solid-state laser with high optical efficiency and brightness was designed, built and tested. Pulsed multi-frequency generation at a number of wavelengths in 2.66-2.85 μm range was theoretically analyzed and experimentally investigated. An effective computational model used for Er:YLF laser optimization that accounts for the generating spectrum was developed.

Room temperature iron doped II-VI lasers have demonstrated broad band tunability between 3.5 and 6 μm with efficiency ~40%. However, these lasers require pump sources with a wavelength ~3 μm which could be selected only from a few available. Cobalt ions in the II-VI materials have strong absorption bands at ⁴A₂→⁴T₁(⁴P) and ⁴A₂→⁴T₁(⁴F) transitions located at ~0.75 and 1.5 μm, respectively. A number of different laser sources (including diode lasers) could be potentially used for cobalt excitation followed by energy transfer to iron ions. Here we report materials fabrication and study of energy transfer in Co:Fe:ZnS(ZnSe) crystals. Iron-cobalt co-doped samples were prepared using a two-stage post-growth thermal diffusion procedure with Fe concentrations of 8.5-19x10¹⁸ cm^-3. Kinetics and photoluminescence spectra reveal energy transfer under cobalt excitation at ⁴A₂-⁴T₁(⁴F) transition by 1.56 μm radiation and ⁴A₂-⁴T₁(⁴P) transition by 0.7 μm radiation. Analysis shows effective energy transfer from ⁴T₁(⁴F), ⁴T₂ and ⁴T₁(⁴F) Co²⁺ energy levels to ⁵T₂ excited level of Fe²⁺ ions the first realization of Fe²⁺ ions lasing at 3.6 μm and 3.8μm via Co-Fe energy transfer. Demonstrated effective Co²⁺→Fe²⁺ energy transfer process could result in utilization of a more convenient laser pump sources for the Fe²⁺:II-VI lasers.

Mid-infrared (mid-IR) light sources operating in the 3-μm waveband are useful photonic devices for the spectroscopic detection of trace gases and biomaterials. In order to achieve efficient mid-IR light emissions with a small footprint, we propose an attractive technique for developing a compact intracavity system that enables the reuse of the photons emitted by the pump source. Periodically poled congruent LiNbO₃ (PPCLN) and Nd-YVO₄ crystals were both set in the intracavity. An 805-nm waveband GaAs-based laser diode was used as the signal and pump light sources for differential frequency generation (DFG), since GaAs-based device technology enables the construction of high-power, highefficiency lasers operating in this waveband. We have successfully demonstrated that a 3-μm wavelength-tunable light source using the photon reuse technique is the most effective and compact intracavity system; it possesses useful broadband wavelength tunability characteristics up to approximately 90 nm and offers a small footprint (15 × 30 cm). We obtained high output power of the order of a few milliwatts from the developed light source. In addition, since the optical spectrum shape of the mid-IR light is also important, we propose a useful spectrum shaping technique using a Fabry-Perot (FP) etalon filter included in the intracavity system; we obtained a fine single-peak spectrum in the 3-μm waveband. The developed wavelength-tunable compact intracavity mid-IR light source using the photon reuse and spectrum shaping techniques is attractive for optical communications and biomedical applications.

Single-crystal (SC) fiber optics have been grown for many years for use as passive fibers for the delivery of IR laser radiation and as active fibers useful as minirod lasers. By analogy with doped-YAG, bulk laser crystals it is expected that pure YAG SC fibers would be capable of transmitting extremely high laser energies. In this study we report on the growth of SC yttrium aluminum garnet, Y₃Al₅O₁₂ (YAG), fibers from undoped SC YAG source rods using the Laser Heated Pedestal Growth (LHPG) technique. The YAG transmits IR wavelengths up to approximately 4 μm which is a little beyond the transmission range for SC sapphire fibers. The garnet family of crystals is one of the most commonly used oxide crystal hosts for lasing ions in high power solid-state lasers, with the most commercially common laser host being YAG. The optical losses for 400-μm diameter YAG fibers have been measured to be about 1 dB/m at 2.94 μm. The longest length of YAG fiber grown has been about 65 cm.

We highlight our recent progress in achieving high efficiency solid state laser materials based on hot pressed Lu₂O₃. Experimental details to further enhance the lasing performance from sesquioxide-based ceramic lasers are presented. Synthesis of high purity rare-earth doped sesquioxide ceramic powder primarily based on Lu₂O₃ for high energy solid state lasers is discussed. We also report the effect of the powder quality and fabrication process on the optical quality and lasing performance of these ceramics. Various methods to obtain fine grained ceramics which are desirable for scaling to high power lasers are also presented.

The refractive indices and thermo-optic coefficients for varying concentrations of Er³⁺ doped polycrystalline yttria were measured at a variety of wavelengths and temperatures. A Lorenz oscillator model was employed to model the room temperature indices and thermo-optic coefficients were calculated based on temperature dependent index measurements from .45 to 1.064 microns. Some consequences relating to thermal lensing are discussed.

The current effort reports on the spectroscopic properties (absorption, emission and fluorescence lifetime) as a function of varying Erbium concentration in Y₂O₃. Results show a non-linear behavior in the fluorescence lifetimes and the radiative-emission intensities for the ⁴I_11/2 and the ⁴I_13/2 energy levels.

This work presents a passively Q-switched and mode-controlled Nd:YLF laser that generates 150 kW of peak power at 1053 nm with 1kHz repetition rate. The new resonator design is capable of delivering 1.5 mJ and 10 ns pulses in a very compact, simple and lightweight set-up.

Passive mode-locking of 1.3 μm solid-state lasers is problematic for semiconductor saturable absorber mirrors (SESAMs) not only because of difficulties in their fabrication process but also in relation to the achievable parameters and damage resistivity. In contrast, single-walled carbon nanotube saturable absorbers (SWCNT-SAs) exhibit broadband absorption which is controllable by varying the nanotube diameter and chirality, and require relatively simple manufacturing processes. Here we report on steady-state mode-locked operation of a diode pumped Nd:YVO₄ laser on the ⁴F_3/2→⁴I_13/2 transition at 1.342 μm using a transmitting SWCNT-SA. The SWCNT-SA employed in the present work was fabricated by SWCNTs grown by high-pressure CO conversion technique, showing broad absorption around 1.3 μm. The linear transmission at the laser wavelength was about 99%. The Nd:YVO₄ laser was longitudinally pumped by the unpolarized radiation of a 808 nm fiber-coupled laser diode. The ~1.2-m long cavity was optimized for large fundamental mode size. Above threshold the laser operated first in the CW mode, then had a range of Q-switched modelocked operation before reaching the regime of stable steady-state mode-locking. With an output coupler of 90% reflectivity, the average output power in the steady-state mode-locked regime reached 0.8 W at a slope efficiency of 14.5% with respect to the incident pump power. At a repetition rate of 127 MHz this corresponds to single pulse energy of 6.3 nJ. Such pulse energies are comparable to the best results obtained using SESAMs but the pulse duration of 16.5 ps measured in the present experiment is substantially shorter.

We present advanced 3D ray-tracing code for calculation of amplified spontaneous emission (ASE), stored energy and heat generation in laser active material. New results including spectrally resolved absorption, amplification and super- Gaussian pump beam profile are obtained for 100 J cryogenically cooled multi-slab amplifier.

We present a comparison of two conceptual layouts of 100 J class laser systems for HiLASE and ELI Beamlines projects with respect to the output energy, beam propagation and system performance obtained from MIRO code. Both systems are based on cryogenically cooled Yb:YAG multi-slabs amplifiers technology and operate at 10 Hz repetition rate with average power above 1 kW. First concept of the laser system consists of separate 10 J preamplifier and 100 J power amplifiers, while the second consists of a main amplifier with two identical amplifier heads delivering 100 J.

Solid-state lasers pumped by high-power two-dimensional arrays of vertical-cavity surface-emitting lasers (VCSELs) were investigated. Both end-pumping and side-pumping schemes of Nd:YAG lasers with high power kW-class 808 nm VCSEL pump modules were implemented. For one application 10 mJ blue laser pulses were obtained from a frequencydoubled actively Q-switched VCSEL-array dual side-pumped Nd:YAG laser operating at 946 nm. For another application 10 mJ green laser pulses were obtained from a frequency-doubled passively Q-switched VCSEL-array endpumped Nd:YAG laser operating at 1064 nm. Both QCW and CW pumping schemes were investigated to achieve high average Q-switched power.

We have explored using 808nm Vertical Cavity Surface emitting laser (VCSEL) arrays for end-pumping of Nd:YAG lasers. A variety of laser designs were explored including a compact passively Q-switched lasers that produced a 22mJ pulse having a pulse width of <1.5ns, and an actively Q-switched laser that produced a 40mJ pulse having a 7 ns pulse width. The VCSEL pumped actively Q-switched laser was used as a source for sum frequency generation. Using a 2mm type II KTP and 3mm type I LBO, we generated greater than 5mJ at 355nm with a 21% THG conversion efficiency.

We present an Yb-CALGO thin-disk laser in continuous regime. In a slightly multimode configuration we obtain up to 29.5W of average power, corresponding to a slope efficiency of 40% and an optical-to-optical efficiency of 32%. We then design a monomode cavity, and achieve 25W of average power. The tuning range of the laser emission was from 1018 to 1052nm. In a Q-switch regime, we obtain more than 1 mJ at 100 Hz repetition rate, and 4.5W at 10 kHz repetition rate.

We present recent progress in development of high power directly diode pumped lasers based on the Yb:YAG thin-disk technology which are being constructed at the HiLASE project in the Czech Republic. Up to three laser chains, each delivering more than 1 J in 1 ps pulse with 1 kHz repetition rate should be built until 2015. In order to overcome technological barriers and optimize the amplifiers which are under construction, a new time-resolved numerical model suitable for describing of quasi-three-level active media operateing at room temperature in the thin-disk laser head is being created. The model is based on Monte Carlo ray-tracing and considers decreasing of the upper laser level population due to the amplified spontaneous emission. In the near future, the model will be used for optimizing of a 2- heads Yb:YAG zero phonon line pumped regenerative amplifier delivering up to 100 mJ in 1 ps pulse and 1 kHz repetition rate.

Cryogenic solid-state laser materials offer many improvements in thermal, optical, structural, and lasing properties over their room temperature counterparts. As the temperature of Yb:YAG decreases from room to 80K it transitions from quasi-three-level lasing to a 4-level laser. In this study, we compare Yb:YAG thin-disk laser performance at room 293K and 80K. To achieve this direct comparison we have built two cooling systems based on R134A refrigerant and also on liquid nitrogen (LN₂). We have made an analytical calculation of the small signal laser gain that takes into account the spurious amplified spontaneous emission and photon re-absorption. The cold thin-disk laser clearly outperforms room temperature operation, and the theoretical results shows room temperature gain flattening.

We report on initial testing of an edge-pumped Yb:YAG disk laser. The assembled laser uses two laser disks having a composite ceramic construction with undoped perimetral edge designed to channel pump light while efficiently outcoupling amplified spontaneous emission (ASE). Edge-pumping allows for reduced doping of crystals with laser ions, which translates to a lower lasing threshold in Yb:YAG material and much reduced waste heat flux. This work presents results of initial testing of the laser with one Yb:YAG laser disk and resonator optics configured for power extraction with two passes through the disk.

The effect of gain tailoring upon the optical extraction and OPD in a CW edge-pumped disk amplifier is examined using a two-dimensional model of diode pumping coupled with a two-dimensional, geometric model of optical extraction by a Gaussian profile beam from a Yb:YAG medium¹. The gain medium is described by the well-known quasi-three level model of Beach^2,3. Gain tailoring is accomplished by focusing the diode pump beam using cylindrical lenses. The diode pump beam, optical extraction, and gain medium models are described after which the pump absorption efficiency, energy deposition uniformity, output energy, and maximum peak-to-valley (PV) OPD are examined as a function of the pump lens focal length and output aperture radius as well as amplifier input seed energy, number of roundtrip amplifier passes, and diode pump power. It is shown that using pump beam focusing to tailor the gain radially deposits more energy in the central region of the disk and thus results in improved optical extraction because a Gaussian input optical beam preferentially accesses the central region of the disk. With gain tailoring one can achieve the same amplifier output energies as without gain tailoring but using less pump power and/or amplifier seed energy, resulting in reduced disk heating and diode-pump waste heat. Although the maximum PV OPD is larger, the central region of the thermally-induced OPD remains relatively uniform, allowing one to increase the output energy with only modest increases in the effective OPD.

The Performance of High Power Disk Lasers and Fiber Lasers along with their rapid development to the high power cw regime have been of great interest throughout the last decade. Both technologies are still in the focus of several conferences, workshops, and papers and represent the "state-of-the-art" of industrial high power solid state lasers for material processing. As both laser concepts are considered to be the leading 1 μm light-source, this presentation presents an objective and fair comparison of the two different technologies from a manufacturer who pursued both. From the geometry of the active material, through the resonator design, cooling regime, and pumping method to the point of beam quality and power scaling, the different approaches associated with the advantages, challenge and limits of each technology will be discussed. Based on ROFIN's substantial industrial experience with both laser concepts, an outlook into future trends and chances, especially linked to fiber laser, will be given.

Though the genesis of the disk laser concept dates to the early 90's, the disk laser continues to demonstrate the flexibility and the certain future of a breakthrough technology. On-going increases in power per disk, and improvements in beam quality and efficiency continue to validate the genius of the disk laser concept. As of today, the disk principle has not reached any fundamental limits regarding output power per disk or beam quality, and offers numerous advantages over other high power resonator concepts, especially over monolithic architectures. With about 2,000 high power disk lasers installations, and a demand upwards of 1,000 lasers per year, the disk laser has proven to be a robust and reliable industrial tool. With advancements in running cost, investment cost and footprint, manufacturers continue to implement disk laser technology with more vigor than ever. This paper will explain recent advances in disk laser technology and process relevant features of the laser, like pump diode arrangement, resonator design and integrated beam guidance. In addition, advances in applications in the thick sheet area and very cost efficient high productivity applications like remote welding, remote cutting and cutting of thin sheets will be discussed.

Disk lasers with multi-kW continuous wave (CW) output power are widely used in manufacturing, primarily for cutting and welding applications, notably in the automotive industry. The ytterbium disk technology combines high power (average and/or peak power), excellent beam quality, high efficiency, and high reliability with low investment and operating costs. Fundamental mode picosecond disk lasers are well established in micro machining at high throughput and perfect precision. Following the world's first market introduction of industrial grade 50 W picosecond lasers (TruMicro 5050) at the Photonics West 2008, the second generation of the TruMicro series 5000 now provides twice the average power (100 W at 1030 nm, or 60 W frequency doubled, green output) at a significantly reduced footprint. Mode-locked disk oscillators achieve by far the highest average power of any unamplified lasers, significantly exceeding the 100 W level in laboratory set-ups. With robust long resonators their multi-microjoule pulse energies begin to compete with typical ultrafast amplifiers. In addition, significant interest in disk technology has recently come from the extreme light laser community, aiming for ultra-high peak powers of petawatts and beyond.

An all-solid-state laser system that provides 165-fs to 60-ps FWHM (full-width-at-half-maximum) pulses in a UV to near-IR wavelength range is described. The laser system consists of a femtosecond laser, a highly stable diode-pumped regenerative amplifier, and frequency-conversion crystals. The system can be configured for various photodetector impulse-response measurements, single-shot impulse and frequency response measurements for the photodetector- oscilloscope combinations, streak-camera sweep calibration, and dynamic-range measurements.

We report on power scaling of a modelocked thin disk laser (TDL) based on the broadband mixed sesquioxide material Yb:LuScO₃ (22 nm full width half maximum (FWHM) emission bandwidth). In a first experiment, we could demonstrate pulse durations as short as 195 fs at a moderate average power of 9.5 W. Furthermore, we were able to power scale our TDL while keeping the pulses short reaching 23 W at a pulse duration of 235 fs. A key element to achieve this result was the design of new SESAM structures with multiple quantum wells (QW) and a suitable dielectric topcoating, resulting in SESAMs with appropriate parameters for short pulse geneartion, low two-photon absorption (TPA) and high damage thresholds. We will present SESAM optimization guidelines for short pulse generation from high-power modelocked oscillators.

A picosecond regenerative amplifier with pulse on demand operation is demonstrated. The pulse energy is held constant for arbitrary pulse separation by employing an intracavity loss modulator. The loss modulator is controlled by a digital signal processor monitoring the gain of the laser material in real time. The gain is estimated from a measurement of the emitted fluorescence from the upper laser level, and the signal processor adjusts the loss in such way to keep the total round-trip gain constant regardless of the time separation to the previous pulse. The system is able to completely suppress the usual multistable regimes of a regenerative amplifier operated under a variation of the repetition rate. We achieve constant output energy for a linear change of repetition rates between 0 and 100kHz with a slope of 2MHz/s.

We report on the generation of 94-fs pulses with 12.5-W average output power from an 80-MHz Yb³⁺:CaGdAlO₄(Yb:CALGO) oscillator with pulse energies and peak power levels higher than 150-nJ and 1.45-MW, respectively. To the best of our knowledge, this is the highest average power ever reported for a sub-100-fs bulk oscillator. Average output power levels higher than 10-W were also achieved in lower repetition rate oscillators at 23.7 MHz and 42.5 MHz with pulse durations ranging from 141-fs to 323-fs and corresponding pulse energies as high as 0.42 μJ. In all experiments stable and self-starting mode-locking with transform-limited pulses and beam quality factors better than M²<1.1 were achieved. Continuous tunability in mode-locked operation between 1026-nm and 1052-nm was also demonstrated. We believe that shorter pulses at higher average output powers should be possible and - combined with extended cavities - such laser systems could reach the μJ pulse energy level with peak powers in the 10-MW regime.

Advanced, high-brightness photoinjectors are required for the next generation of linear accelerators and free-electron lasers. Current photoinjector lasers suffer from complexity due to the use of multiple amplifier stages to achieve the desired pulse energy and have issues with power scaling. In this work, we used a liquid-nitrogen-cooled, Yb:YAG, mode-locked laser master-oscillator/power-amplifier as a high-average-power laser source for laser photoinjector applications. Such a laser can provide average powers of 93 W with repetition rates of 1 MHz (93 μJ per pulse). The 10-ps pulses from the oscillator were amplified in a four-pass cryo-Yb:YAG amplifier.

We have obtained 23 J uncompressed laser pulses at a repetition rate of 1 Hz with a spectral width of 41 nm FWHM which could produce 600 TW if compressed ; this is the highest energy obtained to date from a Titanium Sapphire amplifier working at a such repetition rate. This amplifier is part of a 1.3 PW laser system under construction by Thales Optronique for the BELLA project of LBNL aiming laser wakefield acceleration of electrons up to 10 GeV.

Tellurite glass compositions doped with erbium and erbium/ytterbium optimised to support extended gain bandwidth with significant amplification have been fabricated, and their thermal, optical absorption, excitation and luminescence properties investigated. Each rare-earth dopant concentration was set at 1x10²⁰cm^-3. Broad emission crosssection bandwidths up to 50nm FWHM were observed, with fluorescence lifetimes of ~3ms. Collinear pump probe measurements on ~4mm thick bulk samples revealed peak gains of up to 2.1dB/cm at a wavelength of 1535nm in the codoped material, with an incident pump intensity of only I_inc~8kWcm^-2 at a wavelength of 974nm. At equivalent absorbed pump powers between co-doped and single doped materials the relative gain was 1.25dB/cm (I_inc~4kWcm^-2) and 0.9dB/cm (I_inc~8kWcm^-2) respectively, demonstrating efficient energy transfer from the ytterbium to erbium ions. Excited state absorption at longer wavelengths was observed and characterised and its implication on realising sufficient gain in the wavelength band of interest is discussed.

A new heat extraction geometry for resonantly-diode-pumped Er³⁺:YAG lasers has been proposed. With this approach heat extraction from a laser rod is symmetrised and improved significantly, thus thermal lensing and thermo-induced aberrations of the active crystal are reduced. For proposed approach more than 10 W average has been generated both in CW and QCW mode of operation at comparable pumping conditions with nearly diffraction limited beam. Investigations on diffraction effects inside the fiber-like laser rod have been performed and theoretical background of observed phenomena have been defined. Finally results of further investigations on actively Q-switched laser will be presented.

We demonstrated laser operation of a Er:YAG single crystal fiber at 1617 nm. Pumped on both sides by a laser diode at 1532 nm, a 600 μm diameter- 60 mm long- single crystal fiber produced an output power of 5.5 W once the wavelength 1617 nm was selected by an intracavity etalon. In Q-switched operation with an acousto-optic modulator, the laser produced an energy of 0.5 mJ at 100 Hz repetition rate with a pulse duration of 28 ns. The Watt level in average power was achieved for a repetition rate of 3 kHz with a pulse duration maintained around 30 ns.

Continuous-wave (CW) lasing of Ho in KRE(WO₄)₂ (RE=Y, Gd, Lu) crystals has been compared using in-band pumping by a diode pumped Tm:KLu(WO₄)₂ laser under identical conditions at room temperature. The three monoclinic double tungstate hosts perform similarly with the maximum output power obtained for Ho:KY(WO₄)₂, 406 mW with slope efficiency of 59.9%. For Ho:KLu(WO₄)₂ the maximum power reached 392 mW but the slope efficiency was slightly higher, 61.6%. Ho:KGd(WO₄)₂, for which the ionic radius difference between dopant and substituted ions is maximum, generated an output power of 368 mW with slope efficiency of 53.2%.

Recent advances of high power and narrow bandwidth laser diodes emitting at 1.9 μm open the path to direct diode pumping of Ho³⁺:YAG lasers. The usual method to pump such laser is to use thulium fiber laser which has an excellent beam quality with high power and narrow bandwidth emission. The draw back of this system is the low efficiency of this fiber laser and the increased overall complexity. In this paper we present first results of resonantly diode pumping of a Ho³⁺:YAG laser with fiberlike geometry. The fiber coupled diode modules used for pumping in this work (BrightLock^TMUltra-500) produce each 25 W at 1.91 μm with 3 nm linewidth. The fiber has a core diameter of 600 μm with 0.22 numerical aperture. The Ho³⁺:YAG crystal has a diameter of 1.2 mm, a length of 60 mm, a doping concentration of 0.75 at.% and is symmetrically pumped by two diode modules from both ends. Total internal reflection on the polished rod barrel allows a high pump intensity along the rod length. The Ho³⁺:YAG laser cavity is composed of a high reflective flat mirror and a concave output coupler with a radius of curvature of 500 mm. With an output coupler of 50 % we measured a threshold of 11 W. The maximum output power was 11.87 W with a wavelength of 2.09 μm. The incident power to output power slope efficiency was 0.38 at currently 4 % of internal losses.

A low cost design concept for Fibre-based Coherent Laser Doppler Wind Profiler is presented for supporting an active pitch control of Wind Turbines (WTs). The system is based on a 1.5μm Continuous-Wave (CW) semiconductor laser source plus an erbium-doped fibre amplifier (EDFA) with an output power of 1W. A coherent detection method is used for Doppler frequency measurement. In addition, a concept of wind turbine predictive pitch control system is proposed for reducing the damage caused by wind turbulence. A mathematical simulation and the experimental result based on a lab setup are presented to show the calibration of such a system.

We show the spectroscopic properties of a new potential laser crystal in the visible range. The Eu-doped borate compound has the structure Eu³⁺:Li₆Y(BO₃)₃ with a doping concentration of about 25%. We studied the absorption and emission spectra of the crystal and calculated the gain cross sections. The compound shows a strong emission peak at 613 nm and absorption at 394 nm, which is compatible with pumping by new blue laser diodes. These features make the Eu:LYB crystal a very interesting candidate for the development of DPSS lasers emitting in the visible range.

More than 370 W average power at 515 nm were achieved by frequency doubling of sub-picosecond pulses of an Innoslab based Yb:YAG MOPA system. At 50 MHz repetition rate this corresponds to a pulse energy of 7 μJ in the green. The pulse duration of 680 fs and the transformation limit of the infrared source could be maintained, thus the laser provides more than 10 MW pulse power. The beam quality at 280 W was M² < 1.4, at more than 370 W it was measured as M² < 1.7. Extensive numerical modeling was applied to design the conversion stage.

In this paper the development and optimization process of single frequency, diode pumped Nd:YVO₄/YVO₄/KTP microchip laser operating at 532nm is presented. A birefringent filter formed by beam displacer (undoped YVO₄ crystal) and KTP crystal was used to obtain single mode operation. As a final result of optimization miniature, monolithic laser resonator was elaborated. The laser generated Gaussian beam with quality better than M²<1.2 and maximum single frequency output power at the level of 160mW @ 532 nm. The optical to optical efficiency (532nm to 808nm) was at the level of 16,8% at the maximum single frequency output power.

Laser stability is critical to many industrial applications but is often a source of confusion when specifying and comparing different laser systems. This can be due to the variety of parameters characterized, the range of measurement techniques available and the many alternative methods that can be employed to analyse and present the stability data. High throughput semiconductor applications with sensitivity to individual pulse variations require high average power systems with optimised pulse energy stability. We report stability characterisation and optimisation for a range of frequency doubled Q-switched Nd:YAG laser systems with average power up to 650 W and peak power up to 0.9 MW. The techniques used to refine the stability of the lasers are described and the stability of the lasers is compared before and after optimisation. Stabilised industrial 532 nm laser systems are presented with pulse energy up to 63 mJ and peak to peak pulse energy variation reduced by a factor of two compared to standard systems at 10 kHz repetition rate.

A single-pass frequency doubled high-power tapered diode laser emitting nearly 1.3 W of green light suitable for direct pumping of Ti:sapphire lasers generating ultrashort pulses is demonstrated. The pump efficiencies reached 75 % of the values achieved with a commercial solid-state pump laser. However, the superior electro-optical efficiency of the diode laser improves the overall efficiency of the Ti:sapphire laser by a factor > 2. The optical spectrum emitted by the Ti:sapphire laser shows a spectral width of 112 nm (FWHM). Based on autocorrelation measurements, pulse widths of less than 20 fs are measured. These results open the opportunity of establishing diode laser pumped Ti:sapphire lasers for e.g. biophotonic applications like retinal optical coherence tomography or pumping of photonic crystal fibers for CARS microscopy.

A mode hop free tunable blue laser at 465 nm based on an external cavity system is investigated. The single longitudinal mode second-harmonic generation (SHG) blue laser was generated using quasi-phase matching (QPM) based MgO: PPLN pumped by infrared diode laser at 930 nm with one lasing longitudinal mode. The wide turning rang in excess of 100 GHz is achieved by using combination the etalon, silica glass plate and narrow band filter into the external cavity, which only allow one longitudinal mode running and operating wavelength tuning. 30 mw blue light was obtained at wavelength of 465 nm with beam quality better then M² =1.3.

The passive-alignment-packaging technique presented in this work provides a method for mounting tolerance-insensitive optical components e.g. non-linear crystals by means of mechanical stops. The requested tolerances for the angle deviation are ±100 μrad and for the position tolerance ±100 μm. Only the angle tolerances were investigated, because they are more critical. The measurements were carried out with an autocollimator. Fused silica components were used for test series. A solder investigation was carried out. Different types of solder were tested. Due to good solderability on air and low induced stress in optical components, Sn based solders were indicated as the most suitable solders. In addition several concepts of reflow soldering configuration were realized. In the first iteration a system with only the alignment of the yaw angle was implemented. The deviation for all materials after the thermal and mechanical cycling was within the tolerances. The solderability of BBO and LBO crystals was investigated and concepts for mounting were developed.

The ability to simultaneously produce pulsed laser output over multiple discrete wavelengths can mitigate many of the timing and jitter issues associated with the use of multiple laser systems. In addition, Fourier-transform limited laser output on every pulse is required for many applications such as with pump-probe detection, non-linear frequency mixing, differential absorption lidar (DIAL), and resonance ionization. As a matter of practice, such lasers need to be capable of operating within uncontrolled or noisy environments. We report on a novel Ti:sapphire ring laser that has been developed to produce Fourier-transform limited 20-ns laser pulses at multiple discrete wavelengths, simultaneously, utilizing a Ramp-Hold-Fire (RHF) seeding technique. Resonance of the seed light is achieved by using a KD*P crystal to modify the phase of the light circulating within the slave oscillator cavity where the fast response of the crystal results in a seeding technique that is immune to noise throughout the acoustic regime.

Since organic laser materials offer broad optical gain spectra they are predestined for the realization of widely tunable laser sources. Here we report on a compact organic laser device that allows for voltage controlled continuously wavelength tuning in the visible range of the spectrum by external deformation. The device consists of an elastomeric distributed feedback (DFB) laser and an electro-active elastomer actuator also known as artificial muscle. Second order DFB lasing is realized by a grating line structured elastomer substrate covered with a thin layer of dye doped polymer. To enable wavelength tuning the elastomer laser is placed at the center of the electro-active elastomer actuator. Chosen design of the actuator gives rise to homogeneous compression at this position. The voltage induced deformation of the artificial muscle is transferred to the elastomer laser and results in a decrease of grating period. This leads to an emission wavelength shift of the elastomer laser. The increase of actuation voltage to 3.25 kV decreased the emission wavelength from 604 nm to 557 nm, a change of 47 nm or 7.8%.

A complete electrical and optical characterization for the high power diode laser module operating at 808 nm was performed. The laser diode operating at room temperature 25^oC gave a higher slope efficiency with low consumption electrical power and low threshold current lasing of 1.3 A. Maximum slope efficiency of 42% with a final electrical to optical conversion efficiency of 28% at 807.96 nm central wavelength with line width of 3.59 nm at FWHM was investigated. The emitted laser wavelength was measured and is affected by the temperature increasing; the diode peak wavelength was shifted by 0.35nm/^oC. Moreover, the appropriate wavelength for pumping Nd³⁺ doped materials (around 808nm) was obtained at the temperature range of (20-25^oC). Typical 808nm diode laser output was obtained with different driving input current at constant temperature of 25^oC. To illustrate the effect of changing diode temperature on the DEPSS output laser wavelength, the diode operating temperature must varied to be (12, 25, and 36^oC) and consequently, the pumping central wavelength will be changed, we presented that the output DEPSS (Nd:YVO₄) wavelength does not change because the broad band absorbance property for the Nd:YVO₄ crystals. DEPSS (Nd:YVO₄) laser system was implemented, and its optical characteristics were measured. Stable wavelength of 1066.08nm with a linewidth of 1.48nm at FWHM at different diode pumped powers (1, 1.4, and 1.75 Watt) and at constant temperature of 25^oC was measured. With diode pumping power of 6.6W, the output power was 3.273W with an output coupler reflectivity of 90%. The Fundamental mode (TEM₀₀) was examined, and the measured DEPSS divergence angle was 11±0.5 milliradian.

Diode end-pumped solid-state lasers have the potential to yield high quality laser beams with high efficiency and reliability. This paper presents a comparison between high-efficiency Nd: YVO₄ and Nd: YAG laser rods in the 20W diode end-pumped laser systems. Comparison done by using a special finite element analysis software program (professional LasCad version 3.6.1) for characterizing the all effective parameters inside the cavity like; optimized rod length, optimized rod diameter, beam waist spot size, optimized resonator length. Theoretical modeling of the absorbed power density for end pumped rods is studied. Stability and analyzed temperature distributions, temperature effects on the solid state crystal (thermal fracture) and thermal lensing effects in 3-D numerical model are investigated. It is important to simulate the temperature gradient on the front face and inside the pumped Nd: YAG and Nd: YVO₄ crystal to avoid thermally induced fracture and evaluates the maximum pumped power. This paper shows that Nd: YVO₄ laser system has higher optical efficiency of 40% than that of Nd: YAG laser system with optical efficiency of 25%, and the optimized output powers for Nd: YVO₄ and Nd: YAG of 8.63 W and 5.4 W, respectively for same parametric study like; rod length of 6 mm, rod diameter of 2 mm, beam waist of 150 μm and resonator length of 56.16 mm.

Amplified spontaneous emission (ASE) and parasitic oscillations (PO) in disk lasers with unstable resonators are investigated theoretically. The efficiency loss due to ASE/PO is essentially determined by the product of disk diameter D and "self-maintain gain" of ASE/PO. Threshold to establish the laser oscillations in presence of ASE/PO is explored as the self-maintain gain of ASE/PO is higher than threshold gain _gt of the resonator. The criterion to ignore ASE/PO in disk laser is given by _gtD < 2.0 as an ASE absorber is clad, _gtD < 0.55 as the disk side is roughened, and _gtD <0.8 as the effective ASE path is half reduced. The time evolution of laser output exhibits a strenuous relaxation oscillation, in which ASE plays a role of strong damping, and consume the energy before each of sub-pulses.

Influence of Ce³⁺-ions in Pr,Ce:YAlO₃ crystal on spectroscopic and laser characteristics has been investigated. Spectroscopic properties and preliminary laser results of GaN-diode pumped Pr,Ce:YAlO₃ active medium at room temperature have been reported and compared with the same crystal without Ce³⁺-ion. Co-doping of the active medium by Ce³⁺-ions was proposed for crystal property improvement in terms of color center formation caused by UV radiation.

In this report, we present results of dynamic laser oscillation over a 10 km distance using a coupled-cavity structure assisted by four wave mixing (FWM). The system consists of a master cavity and a slave cavity that are coupled together through FWM in a nonlinear medium. The master and slave laser beams each form two arms of the four wave mixing structure. The master laser consists of a cavity formed by two highly reflective mirrors, in which there is a gain medium and the FWM nonlinear medium. For the slave laser cavity, one of the end mirrors is the grating formed inside the FWM medium, and the other mirror is the remote moving target. In this case, this target is a retro-reflector located on a moving airplane. The laser tracking was completed with a rotating gimbal mirror system that locked on the target. Successful lasing field tests at distances over 10 km have been achieved, and further results can potentially be obtained at longer distances. The laser oscillations are free running with a pump pulse width of 2-5 milliseconds. Based on the time separations between the pulses from the master and slave cavities, the target distance can be precisely calculated.

Passive Q-switching of a diode-pumped (Tm,Yb):KLu(WO₄)₂ laser, operating between 1910 and 1950 nm, has been achieved using polycrystalline Cr²⁺:ZnS as a saturable absorber in a hemispherical L-shaped cavity. The dependence on the doping levels of Tm and Yb and the low-signal absorption of the saturable absorber has been studied. The highest average output power reached 272 mW at a repetition rate of 2.04 kHz corresponding to a pulse energy of 133 μJ. This was achieved with (8.8 at. % Tm, 2.3 at. % Yb):KLu(WO₄)₂ active element and a saturable absorber with 85% lowsignal transmission. The highest pulse energy, 148 μJ, was achieved with the same combination at lower (0.94 kHz) repetition rate. The obtained pulse energy with the different samples is compared with theoretical analysis and the pulse duration and peak power are estimated from the calculations.

We present recent progress in development of a compact regenerative laser amplifier based on the Yb:YAG thin-disk technology delivering up to 100 mJ picosecond pulses with a repetition rate of 1 kHz, designed for seeding a kW-class multipass amplifier for industrial and scientific applications. System utilizes two laser heads pumped at zero-phonon line (968.825 nm¹) by stabilized high-power pump diodes operated in pulsed regime. The concept is based on the CPA technique where seed pulses produced by fiber oscillator at 1030 nm are stretched and compressed by use of transmission gratings.

One of the promising active ions giving the possibility of laser radiation generation in 4 - 5 μm region is trivalent dysprosium in lead thiogallate crystal. On the basis of our previous Dy³⁺:PbGa₂S₄ laser study, this work is showing a several wavelengths generation possibility. The laser was working at the room temperature and it was in-band pumped by the Er:YLF laser radiation with the wavelength 1.73 μm. The investigated crystal Dy³⁺:PbGa₂S₄ was synthesized using Bridgman technique from the melt. Dimension of the sample was 16 mm long and 19 mm in diameter and the nominal Dy³⁺ ion concentration was ~ 0.7 at. %. The laser operation was tested with three resonator configurations. The first was the non-selective resonator with the length 41 mm. To perform the wavelength tuning, in the second case the MgF₂ Lyot filter was inserted under Brewster angle inside the resonator which length was 100 mm. In both these cases the laser resonator was formed by the incoupling flat dichroic mirror with low reflectivity at pumping wavelength (T = 90%@1.73 μm) and high reflectivity (R~100%) within the 3.5 - 4.5 μm spectral range, and by an out-coupling concave (r = 500 mm) mirror with reflectivity of 95 % or 98 % in the same region of wavelengths. For the third case the selective mirrors supporting the 3.0 - 4.0 μm spectral region were chosen. The generation possibility on three lines 4 μm, 4.3 μm, and 4.6 μm were found without continuous tuning between them.

Bulk Erbium-doped lasers are widely used for long-distance telemetry and ranging. In some applications such as coherent Doppler radars, laser outputs with a relatively long pulse width, good beam profile and pulse shape are required. High energy Q-switched Er:glass lasers were demonstrated by use of electro-optic (E/O) Q-switching or frustrated total internal reflection (FTIR) Q-switching. However, the output pulse durations in these lasers were fixed to relatively small values and extremely hard to tune. We report here on developing a novel and compact Q-switched Er:Yb co-doped phosphate glass laser at an eye-safe wavelength of 1.5 μm. A rotating mirror was used as a Q-switch. Co-linear pump scheme was used to maintain a good output beam profile. Near-perfect Gaussian temporal shape was obtained in our experiment. By changing motor rotation speed, pulse duration was tunable and up to 240 ns was achieved. In our preliminary experiment, output pulse energies of 44 mJ and 4.5 mJ were obtained in free-running and Q-switched operation modes respectively.

Two samples of Er-Yb doped phosphate glass were tested as a gain medium of longitudinally diode pumped laser. One sample was a simple Er:Yb:glass rod (length 2.8 mm), second sample was composite rod consisting of 2.8mm long Er:Yb:glass and 6mm long YAG crystal. Diameter of both samples was 6 mm. Dopant concentration for Er:Yb:glass was 0.75 × 10²⁰ cm⁻³ Er and 1.7 × 10²¹ cm⁻³ Yb. The goal of the experiment was to investigate an effect of the undoped YAG cap on the Er:Yb:glass laser operation. The active medium, fixed in cupreous heatsink, was placed inside the 150mm long resonator consisted of a flat pumping mirror (HR @ 1.52−1.65 μm, HT @ 0.97 μm) and curved output coupler (r = 150 mm, R = 97% @ 1.52−1.61 μm). For Er:Yb:glass pumping a fiber coupled laser diode, operating in pulsed regime, was used. The pumping pulse width, energy, and wavelength were 1 ms, 10 mJ, and 975 nm, respectively. The decrease of Er:Yb:glass laser output pulse energy with increasing pumping repetition rate was observed for both samples. In case of simple Er:Yb:glass the energy dropped from 1.4mJ to 0.6mJ after pumping duty cycle increase from 0.5% to 6 %. In case of composite YAG/Er:Yb:glass active medium the relative output energy decrease was only 20% for pumping duty cycle increase from 0.5% to 10%. This result showed that the slope of the output energy decrease with increasing duty cycle was approximately four times slower for composite active media in comparison with simple Er:Yb:glass.

Mid-IR lasers based on Cr²⁺ and Fe²⁺ ions in II-VI tetrahedral crystal hosts have been proven useful as tunable solid-state lasers operating over the 1.9-6 μm spectral range. However, the development of new effective gain media promising for lasing over 3-4 μm and with wavelengths longer than 6 μm is still under progress. In this paper we report spectroscopic characterization of Titanium doped AgGaS₂ and Iron doped MgAl₂O₄ crystals with coordination number four. Polarized absorption and luminescence spectra of titanium doped AgGaS₂ crystals were measured at room and low temperatures. It was shown that Ti ions in the AgGaS₂ crystal were in both 1+ and 3+ valence states substituting silver and gallium sites, correspondingly. Ti³⁺ ions in AgGaS₂ and Fe²⁺ in MgAl₂O₄ crystals feature broad absorption bands with a maximums near 2μm. These ions have no second excited state absorption bands and could potentially be used as effective saturable absorbers for passive Q-switching of 2.1 μm holmium laser cavities. Broad emission covering the 3-6 μm spectral range was observed under 1.9 μm excitation into Ti³⁺ absorption band. A luminescence signal between 3 and 6 μm was also observed in the Fe:MgAl₂O₄ crystal.

Currently, a majority of Volumetric Bragg gratings (VBG) use photorefractive glasses with a transmission band between 0.3 and 2.7 μm. We have proposed and realized VBGs for mid-IR spectral range based on LiF color center crystals (LiF:CC). γ-irradiated LiF:CC crystals feature strong absorption bands in the visible and near-IR spectral range, where selective color center photo-bleaching allows for the LiF refractive index modification. The absence of active absorption in LiF:CCs at wavelengths longer than 1.3 μm results in a VBG that is stable under mid-IR irradiation. Our calculations predict that ~60% diffraction efficiency over 1-6 μm spectral range could be realized in ~1 cm long VBG. To verify this calculation, we fabricated periodic structures in LiF:CC crystals with 24 and 12 μm spacings by CCs photo-bleaching using femtosecond Ti:sapphire laser pulses. Periodic structures exhibit diffraction in multiple orders in the Raman-Nath regime at 0.532, 0.632, and 1.56 μm. The first order diffraction efficiencies were stronger for a visible radiation due to a bigger refractive index variations and additional amplitude modulation. The demonstrated diffraction at 1.56 μm is a clear manifestation of a phase grating in LiF and serves as a proof of feasibility of these LiF:CC crystals for mid-IR VBG applications.

A compact diode pumped Q-switched lasers, operating at wavelength 1031 nm, were based on the composite crystal Yb:YAG/Cr:YAG. This composite crystal (diameter 3mm) consisted of diffusion bounded 3mm long Yb:YAG (10 at.% Yb/Y) and 1.6mm long Cr:YAG crystal (initial transmission 85% @ 1031 nm). External resonator allowing to tune generated Q-switched pulse parameters or microchip configuration offering the shortest pulses and highest peak power were tested for this device. For longitudinal pumping of Yb:YAG gain medium, fibre coupled (core diameter 200 μm, NA= 0.22) laser diode, operating at wavelength 968 nm, was used. In the first case, Yb:YAG/Cr:YAG composite crystal was AR-coated and placed inside the 150mm long semihemispherical resonator consisted of a flat pumping mirror (HR @ 1.01 − 1.09 μm, HT @ 0.97 μm) and curved output coupler (r = 150mm) with reflectivity 70% @ 1031 nm. Generated pulses with the peak power 23 kW were 17 ns long (FWHM). The highest generated pulse energy was 0.38 mJ. In the second case, the resonator mirrors were deposited directly on the crystal faces. The output coupler reflectivity was 85% @ 1031 nm. In this compact microchip configuration, 140 ps long (FWHM) pulses with energy 0.13mJ and peak power 0.92MW were generated at wavelength 1031 nm.

A mode-locked laser system achieves average output power of more than 18 watts, in a single transversal mode, with a pump power of 70 watts. The optical to optical conversion efficiency is about 25%. The output wavelength is 1053 nm and the mode locked frequency is 150 MHz; the pulse width is less than 80 ps. The peak power reaches kilowatts per pulse. The wavelength can be converted into 527nm and 351 nm with a nonlinear conversion.

The goal of this study is to demonstrate how a DPSS laser beam's quality parameters can be simultaneously optimized through pump current tuning. Two DPSS lasers of the same make and model were used where the laser diode pump current was first varied to ascertain the lowest RMS noise region. The lowest noise was found to be 0.13% in this region and the best M² value of 1.0 and highest laser output power were simultaneously attained at the same current point. The laser manufacturer reported a M² value of 1.3 and RMS noise value of .14% for these lasers. This study therefore demonstrates that pump current tuning a DPSS laser can simultaneously optimize RMS Noise, Power and M² values. Future studies will strive to broaden the scope of the beam quality parameters impacted by current tuning.

Adaptive optics (AO) systems are used to compensate atmospheric perturbations on a propagating laser beam. However, AO needs a beacon to obtain the phase information. This paper presents a possible formation of beacon in target-in-theloop (TIL) geometry which is analog to a laser cavity. The TIL laser cavity is formed with a high reflectivity mirror on one end and an optical phase conjugate mirror as the second mirror. The TIL laser is initialized by a single frequency 10 ns Q-switched laser pulse. This is very similar to how an injection seeding or regenerative amplifier scheme starts a laser oscillation. With a cavity length of around 11 meters and an initial laser pulse of 10 ns, we have been able to isolate laser field images related to each round-trip pulse. Furthermore, by replacing the first mirror with a rough-surface target to simulate an uncooperative target and adding phase distortion elements to simulate atmospheric effects, we can observe the image status under such conditions.