This PDF file contains the front matter associated with SPIE Proceedings Volume 9513, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

In this paper we provide an overview of the design of DiPOLE100, a cryogenic gas-cooled DPSSL system based on Yb:YAG multi-slab amplifier technology, designed to efficiently produce 100 J pulses, between 2 and 10 ns in duration, at up to 10 Hz repetition rate. The current system is being built at the CLF for the HiLASE project and details of the front end, intermediate 10J cryo-amplifier and main 100J cryo-amplifier are presented. To date, temporal and spatial pulse shaping from the front end has been demonstrated, with 10 ns pulses of arbitrary shape (flat-top, linear ramps, and exponentials) produced with energies up to 150 mJ at 10 Hz. The pump diodes and cryogenic gas cooling system for the 10J cryo-amplifier have been fully commissioned and laser amplification testing has begun. The 100J, 940 nm pump sources have met full specification delivering pulses with 250 kW peak power and duration up to 1.2 ms at 10 Hz, corresponding to 3 kW average power each. An intensity modulation across the 78 mm square flat-top profile of < 5 % rms was measured. The 100J gain media slabs have been supplied and their optical characteristics tested. Commissioning of the 100J amplifier will commence shortly.

We report on an hybrid fiber/crystal ultra-short pulsed laser delivering high pulse energy and high peak power in the picosecond regime. The laser is composed of a mode-lock fiber oscillator, a pulse picker and subsequent fiber amplifiers. The last stage of the laser is a single pass Nd:YVO₄ solid-state amplifier. We believe that this combination of both technologies is a very promising approach for making efficient, compact and low cost lasers compatible with industrial requirements.

High average power picosecond lasers have become an import tool in many fields of science and industry. We report on progress in development of 100 kHz, 100 W picosecond Yb:YAG thin disk laser amplifier with fundamental spatial mode at the HiLASE laser center. More efficient direct pumping to an upper laser level has been employed in order to suppress thermal loading of the thin disk active medium and to increase system stability. We also carefully analyzed and described all benefits of this so called zero phonon line pumping (ZPL) for fundamental spatial mode cavity design and successfully increased extraction efficiency of the amplifier to > 28 %. A novel approach of high-power picosecond pulse compression using a space saving and easy-to-align chirped-volume Bragg grating (CVBG) with high dispersion and high net efficiency approaching 88 % allowed us to build a robust and highly compact pulse compressor. A 100 kHz train of sub-1-milijoule pulses compressed bellow 2 ps (FWHM) in almost diffraction limited Gaussian beam has been successfully generated from this highly compact (900 x 1200 mm) thin-disk-based Yb:YAG regenerative amplifier.

We report results of design and optimization of high average power picosecond and nanosecond laser operating at 1342 nm wavelength. This laser is comprised of master oscillator, regenerative amplifier and output pulse control module. Passively mode-locked Nd:YVO4 master oscillator emits ~ 10 ps pulses at repetition rate of 55 MHz with average output power of ~ 100 mW. These pulses were used to seed regenerative amplifier based on composite diffusion-bonded Nd:YVO₄ rod with variable Nd doping concentration pumped at 880 nm wavelength. Laser produces 10.9 ps pulses at 300 kHz repetition rate with average output power of 11 W and nearly diffraction limited beam quality M² ~ 1.03. Fraction of laser output was converted to the second harmonics with 60 % efficiency providing the average power of 5 W at 671 nm wavelength. Without seeding the regenerative amplifier transforms to electro-optically cavitydumped Q-switched laser delivering 10 ns pulses at high repetition rates with beam propagation factor of M² ~ 1.06.

We present a method of aligning a four-grating compressor of optical pulses developed for the multipetawatt OPCPA laser complex PEARL-X under construction at the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS). The method was successfully used in the compressor of the petawatt laser system FEMTA. The operating planes and the direction of grating grooves were aligned parallel to the vertical axis of rotation by means of silica glass alignment cubes situated on top of the diffraction gratings. Fine alignment of compressor gratings located opposite each other was done by movable mirrors which enabled observing far field images of two beams: mirror-reflected from one grating and reflected from the other grating.

By using the SG-II laser and the ninth-beam as the pump source, the Shenguang-II multi petawatt laser system with three OPCPA stages is designed. Based on the largest size of the commercial gratings provided now by the JY Company, chirped pulses with 260J energy will be delivered after the third stage of OPCPA. When compressed by a four pass compressor, a laser pulse of 150J 30fs (5PW) will be obtained. This laser system is under construction and is expected to be finished in the late of 2015. The paper presents the details of its design and the progress has achieved.

We present recent progress in development of low-jitter all-normal dispersion (ANDi) Yb-doped fiber laser. We developed a timing jitter measurement scheme employing a broadband lock-in amplifier working up to 600 MHz operational frequency which allowed us to measure single side band (SSB) phase noise and long-term drift in laser repetition rate. Following the measurement, we were able to determine timing jitter of our oscillator in the most critical frequency range (10 Hz – 10 kHz) to 1.6 ps. Long-term drift has been successfully suppressed by applying a piezo motor-based active feedback in the cavity.

In large scale laser facility dedicated to laser-matter interaction including inertial confinement fusion, such as LMJ or NIF, high-energy main amplifier is injected by a laser source in which the beam parameters must be controlled. For many years, the CEA has developed nano-joule pulses all-fiber front end sources, based on the telecommunications fiber optics technologies. Thanks to these technologies, we have been able to precisely control temporal shaping and phase-modulated pulse. Nowadays, fiber lasers are able to deliver very high power beams and high energy pulses for industrial needs (laser marking, welding,…). Therefore, we have currently developed new nanosecond pulses fibered amplifiers able to increase output pulse energy up to the mJ level. These amplifiers are based on flexible fibers and not on rod type. This allows us to achieve a compact source. Nevertheless the intensity profile of theses fibers usually has a Gaussian shape. To be compatible with main amplifier section injection, the Gaussian intensity profile must then be transformed into ‘top-hat’ profile. To reach the goal, we have recently developed an elegant and efficient solution based on a single-mode fiber which directly delivers a spatially coherent ‘top-hat’ beam. In the conference, we will present this mJ-class top-hat all-fiber laser system, the results and the industrial prototype which can be used as a front-end of high-power lasers or as a seeder for other types of lasers.

High-energy class laser systems operating at high average power are destined to serve fundamental research and commercial applications. System cost is becoming decisive, and JENOPTIK supports future developments with the new range of 500 W quasi-continuous wave (QCW) laser diode bars. In response to different strategies in implementing high-energy class laser systems, pump wavelengths of 880 nm and 940 nm are available. The higher power output per chip increases array irradiance and reduces the size of the optical system, lowering system cost. Reliability testing of the 880 nm laser diode bar has shown 1 Gshots at 500 W and 300 μs pulse duration, with insignificant degradation. Parallel operation in eight-bar diode stacks permits 4 kW pulse power operation. A new high-density QCW package is under development at JENOPTIK. Cost and reliability being the design criteria, the diode stacks are made by simultaneous soldering of submounts and insulating ceramic. The new QCW stack assembly technology permits an array irradiance of 12.5 kW/cm². We present the current state of the development, including laboratory data from prototypes using the new 500 W laser diode in dense packaging.

The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the visible ~530nm, by tuning the indium content of the laser GaInN quantum well. Low defectivity and high uniformity GaN substrates allows arrays and bars of AlGaInN lasers with up to 20 emitters to be fabricated to obtain optical powers up to 4W at 395nm. AlGaInN laser bars are suitable for optical pumps and novel extended cavity systems for a wide range of applications. An alternative package configuration for AlGaInN laser arrays allows for each individual laser to be addressed individually allowing complex free-space and/or fibre optic system integration with a very small form-factor.

A new ultra stable CO2 laser in carbon fibre resonator technology with an average power of more than 600W has been developed especially as basis for the use with AOMs. Stability of linear polarisation and beam pointing stability are important issues as well as appropriate shaping of the incident beam. AOMs are tested close to the laser-induced damage threshold with pulses on demand close to one megahertz. Transversal and rotational optimization of the AOMs benefits from the parallel-kinematic principle of a hexapod used for this research.

ISP SYSTEM has developed a range of large aperture electro-mechanical deformable mirrors (DM) suitable for ultra short pulsed intense lasers. The design of the MD-AME deformable mirror is based on force application on numerous locations thanks to electromechanical actuators driven by stepper motors. DM design and assembly method have been adapted to large aperture beams and the performances were evaluated on a first application for a beam with a diameter of 250mm at 45° angle of incidence. A Strehl ratio above 0.9 was reached for this application. Simulations were correlated with measurements on optical bench and the design has been validated by calculation for very large aperture (up to Ø550mm). Optical aberrations up to Zernike order 5 can be corrected with a very low residual error as for actual MD-AME mirror. Amplitude can reach up to several hundreds of μm for low order corrections. Hysteresis is lower than 0.1% and linearity better than 99%. Contrary to piezo-electric actuators, the μ-AME actuators avoid print-through effects and they permit to keep the mirror shape stable even unpowered, providing a high resistance to electro-magnetic pulses. The MD-AME mirrors can be adapted to circular, square or elliptical beams and they are compatible with all dielectric or metallic coatings.

The ELI-beamlines facility¹ is being built within the Extreme Light Infrastructure (ELI) project based on the European ESFRI (European Strategy Forum on Research Infrastructures) process. The alignment of the high power lasers is an essential operation to be performed before shooting. A critical part of the alignment procedure is the definition of the references for the alignment. The most common procedure is to insert a cross shaped mask into the beam path. The centre of the cross defines the optical axis. Because of the difficulties in automatizing this procedure, a semi-automatic procedure is being used in many facilities. During such procedure an operator has to interact with the alignment system. The purpose of this work is to present the alignment process and to show how to use light sources as references for a fully automated alignment system.^{1, 2}

A diffraction grating based on all-dielectric multi-layer structure is designed for compression of ultrafast pulses with spectrum centered at 900 nm. The grating at Littrow angle with an out-of-plane configuration shows more than 96% efficiency over the reflective band of 100 nm for the angle of incidence 41 degrees. We suggest grating grooves and the very first layer under the grooves to be made of fused silica. Reflective mirror under corrugated layer is designed as a stock of three types of dielectric nanolayers. Tolerances for groove depth and angle of incidence are estimated and, optimal duty-cycle parameter is found out. Electric field distribution inside of the grating is also numerically studied. The model is simulated by two methods: numerical Fourier Modal Method in LightTrans Virtual Lab and semi-analytical Volume Integral Equation Method. The results obtained by both methods show an excellent agreement.

Dependence of thermally induced depolarization on the diameter of the laser beam and the ratio of the length of the optical element to its diameter is investigated experimentally and numerically. The conditions under which the thermal depolarization depends on the diameter of the beam for face-end and side heatsink are determined. Numerical modeling is supported by experimental results.

Yb:doped Lu₂SiO₅ (Lutetium orthosilicate, LSO) is an optically biaxial crystal with laser emission in the range 1000- 1100 nm. It features different absorption and emission spectra for polarization along its three dielectric axes. In this work we have characterized the laser emission properties of Yb:LSO along all the three dielectric axis, evidencing differences that can be exploited in the design of ultrafast laser sources. The material was tested in a longitudinally pumped laser cavity. The laser emission efficiency was found similar along all the three dielectric axes, with slope efficiencies around 90% in most cases. Regarding the tuning range, for the most favourable polarization direction we obtained a continuously tunable emission between 993 and 1088 nm (i. e. 95 nm) peaked at 1040 nm. The tuning curves along the three dielectric axes spanned similar ranges but with relevant differences in the shape.

Significant improvements in efficiency in high power, high repetition rate laser systems should come from the use of ceramic laser active elements suitably designed to mitigate the thermal and thermo-mechanical effects (TEs and TMEs) deriving from the laser pumping process. Laser active media exhibiting a controlled and gradual distribution of the active element(s) could therefore find useful applications in the laser-driven inertial confinement fusion systems, which are considered among the most promising energy source of the future (ultraintense laser pulses), and in medical applications (ultrashort laser pulses) The present work explores the flexibility of the ceramic process for the construction of YAG (Y₃Al₅O₁₂) ceramic laser elements with a controlled distribution of the Yb doping, in view of the realization of structures modelled to respond to specific application. Two processing techniques are presented to prepare layered structures with a tailored modulation of the doping level, with the goal of reducing the peak temperature, the temperature gradients and also the thermally-induced deformation of the laser material, thus mitigating the overall thermal effects. Tape casting in combination with thermal compression of ceramic tapes with a varying doping level is one of the presented techniques. To make this process as more adaptable as possible, commercial micrometric ceramic powders have been used. The results are compared with those obtained using nanometric powders and a shaping process based on the subsequent pressing of spray dried powders with a different doping level. Laser performance has been characterized in a longitudinally diode pumped laser cavity. The laser efficiency under high thermal load conditions has been compared to those obtained from samples with uniform doping, and for samples obtained with press shaping and tape casting, under the same conditions.

We present a detailed investigation of different compositions of Yb3⁺-doped alumino-silicate glasses as promising materials for diode-pumped high-power laser applications at 1030 nm due to their beneficial thermo-mechanical properties. To generate comprehensive datasets for emission and absorption cross sections, the spectral properties of the materials were recorded at temperatures ranging from liquid nitrogen to room temperature. It was found that the newly developed materials offer higher emission cross sections at the center laser wavelength of 1030 nm than the so far used alternatives Yb:CaF₂ and Yb:FP-glass. This results in a lower saturation fluence that offers the potential for higher laser extraction efficiency. Fluorescence lifetime quenching of first test samples was analyzed and attributed to the hydroxide (OH) concentration in the host material. Applying a sophisticated glass manufacturing process, OH concentrations could be lowered by up to two orders of magnitude, rising the lifetime and the quantum efficiency for samples doped with more than 6.10²⁰ Yb³⁺ -ions per cm³. First laser experiments showed a broad tuning range of about 60 nm, which is superior to Yb:CaF₂ and Yb:FP-glass in the same setup. Furthermore, measurements of the laser induced damage threshold (LIDT) for different coating techniques on doped substrates revealed the appropriateness of the materials for short pulse high-energy laser amplification.

We report on the development of a 2.74-mJ, ~4.2 ps, ~258 nm deep-ultraviolet (DUV) source at 1 kHz based on frequency quadrupling of ~32 mJ, 8.4 ps, ~1030 nm near-infrared (NIR) laser pulses with an excellent beam profile, generated from a diode-pumped, ultrafast hybrid Yb-doped chirped-pulse amplification laser system. We have used a two-stage second harmonic generation scheme at LBO (NIR-to-green) and BBO crystals (green-to-DUV), respectively, to achieve the fourth-harmonic generation (FHG). The NIR-to-DUV conversion efficiency of ~10% in the FHG is obtained. The peak power of the produced DUV laser pulses is as high as 0.56 GW. The beam profiles at near-field and far-field are found to be excellent and the M² value is measured as ~2.6. We also present the systematic parameter study on the optimization of DUV generation. To our best knowledge, this is the most energetic DUV generation from a diodepumped solid-state laser at kHz repetition rates.

We report on the generation of 100 kHz 0.1mJ-level deep ultraviolet pulses based on frequency-quadrupled (257.5 nm) beam of a diode pumped Yb:YAG thin disk laser at the HiLASE Centre. The 100-kHz beamline used for the generation of the harmonic frequencies is operated at an average output power of 100 W level and 2 picosecond duration of pulses. The amplification of the oscillator beam is performed in a regenerative amplifier where the thin disk serves as an active mirror. The CPA technique is used for achieving high average output power of the whole system. The outcoming laser beam at 1030 nm wavelength is frequency-doubled in an LBO crystal and then frequency-quadrupled in BBO crystal, conversion efficiencies being 40% and 19%, resp. The basic characteristics of the harmonics generation in both crystals are given.

We present benchmarking of the home-made MATLAB model with the experimental data obtained for the 10 J/10 Hz cryogenically cooled multi-slab laser systems being developed in the Rutherford Appleton Laboratory, STFC, UK. The laser head of each system consists of four, 5 mm thick slabs separated by 2 mm gaps. Each slab is composed of 35 mm diameter Yb:YAG active material surrounded by 10 mm thick absorptive cladding. The slabs are pumped from both sides by diode arrays which in total deliver up to 40 kW of pump power in 1 ms pulses with a central wavelength of 940 nm, bandwidth 6 nm and pump spot 20 mm x 20 mm. The laser head has been modeled to predict gain in the slabs and the amplification of the seed beam for the temperatures of operation ranging from 100 K to 180 K. Output energy for different pump pulse durations has been calculated. It was determined that the maximum output energy obtained after 6 passes for the amplifier operating at the temperature of 120K and repetition rate 1 Hz was 9 J for the seed energy of 20 mJ. For higher temperature of 160 K the output energy was 8 J. The corresponding maximum single pass gain in the amplifier head was 8 for 120 K and 4 for 160 K. Results of the simulations are in a very good agreement with the measured data presented in [1].

In a high-power laser system, a beam splitter refers to the mirror which locates at the cross point of the path of highpower beam and the weak light section. Because of the thermo-optic effect and elasto-optic effect, a beam splitter deforms under intense laser radiation. This deformation adds extra phase on the incident waves and deliveries inaccurate information to the wavefront sensor. Consequently, the output laser focuses at finite distance and gets divergent when arrives at the target. To settle the above problem, this paper presents a new method for real-time correction of the thermal distortion of beam splitter, based on algorithm of the data fusion of two Shack-Hartmann wavefront sensors (SH-WFS). Different from the traditional AO system, which contains a wavefront sensor, a corrector and a servo controller, two extra Shack-Hartmann wavefront detectors are adopted in our AO system, to detect the transmitted and reflected aberrations of beam splitter mirror. And these aberrations are real-timely delivered to the wavefront sensor. Based on coordinate conversion and data fusion algorithm, it makes the wavefront sensor of AO can “see” the aberrations of splitter mirror by itself. Thus, the servo system controls the corrector to compensate these aberrations correctly. In this paper, the theoretical model of data fusion algorithm is carried out. A closed-loop AO system, which consists of a typical AO system and two extra Shack-Hartmann wavefront detectors, is set up to validate the data fusion algorithm. Experimental results show that, the distortion of a CaF2 beam splitter can be real-time corrected when the AO closedloop control is on. The beam quality factor of output laser decreases from 4 to 1.7 times of diffraction limit.

We propose and demonstrate a wavelength-tunable and narrow-linewidth erbium-doped fiber (EDF) laser using siliconon- insulator (SOI) based micro-ring. We discuss the wavelength selection and wavelength-tunable operation of the proposed fiber laser. The SOI based micro-ring is fabricated on a SOI wafer with a 0.22 um thick top silicon layer and a 2 um thick burial oxide (BOX) layer. In order to enhance the coupling efficiency between the SOI based micro-ring and the EDF, a pair of uniform period grating couplers are used. In the experiment, the lasing wavelengths can be tuned in the wavelengths range from 1532 nm to 1567.2 nm with a tuning step of 2 nm. The wavelength range and the tuning step are determined by the EDFA gain-bandwidth and the FSR of the SOI based micro-ring respectively. The OSNR of each lasing wavelength is > 42 dB. By using a double-ring configuration, a narrow laser linewidth of 50 kHz can be achieved.

In high power solid state lasers, thermal lens effect always give rise to the multi-modes oscillation in the resonator. The beam quality will deteriorate with the increase of output power. In this paper, an intra-cavity beam shaper is introduced to actively compensate the thermal lens in the laser resonator. One round trip ABCD matrix of the resonator with an intra-cavity beam shaper and thermal lens is calculated. The design parameters with wide stable zone are concluded through the ABCD matrix. The mode size and stability diagram of the resonator are calculated under different focal length of the thermal lens. The relationship between the adjustment of the intra-cavity beam shaper and the mode size under different thermal lenses are concluded, and general method to actively control the modes contents by adjusting the intra-cavity beam shaper is introduced. The effectiveness and performance of active mode control with the intra-cavity beam shaper are verified by simulations of the output modes of resonators. It shows that the M2 factor is well maintained below 1.6 even the focal length of the thermal lens changes from 5m to 0.5m.

In recent years, there has been a vast development of high energy class lasers of the order of 100 J to kJ level which have potential applications in the field of science and technology. Many such systems use the gain media cooled at cryogenic temperatures which will help in enhancing the spectroscopic and thermo-optical properties. Nevertheless, parasitic effects like amplified spontaneous emission enhance and affect the overall efficiency. The best way to suppress this effect is to use cladding element attached to the gain material. Based on these facts, this work was focused on the systematic investigation of temperature dependent absorption of several materials doped with transition metals, which can be used as cladding, as laser gain material, or as passive Q-switching element. The Ti:sapphire, Cr:YAG, V:YAG, and Co:MALO samples were measured in temperature range from 80 K to 330 K by step of 50 K. Using Beer-Lambert law we estimated the absorption coefficient of these materials.

We have been studying a high finesse enhancement cavity for photon target of Laser Compton Scattering X-ray generation. It is very important to develop an extremely stable external optical cavity for laser Compton scattering. At the same time, a stable seed laser oscillator for an incident laser of an optical cavity is also very important tissue. Thus, we have been developing a stabil mode-locked Yb-doped fiber laser based on Non-Linear Polarization Rotation. We have generated laser pulses which have 102.9mW average power at repetition rate of 119MHz. Furthermore, we started accumulating lasers in the optical cavity, and we have already confirmed that our oscillator is able to accumulate in the cavity.

We have been developing a pulsed-laser optical enhancement cavity for laser-Compton scattering (LCS). LCS can produce high brightness X-ray through the collision between relativistic electrons generated from the accelerator and high power laser photons with a compact facility. In order to increase the number of collisions/sec, high repetition rate accelerator and laser are required. For the laser system, an optical enhancement cavity is the most powerful tool for LCS, thus we have been developing the cavity for storing 1 micron laser pulse. On the other hand, the resulting X-ray energy can be changed by the collision laser wavelength. If we have another optical cavity with different wavelength, the multicolor, quasi-monochromatic, high brightness and compact X-ray source can be realized. Therefore, we started to develop an optical cavity at 10 micron wavelength with CO₂ laser. At this wavelength region, the absorption loss is dominant compared with scattering loss. Thus we carefully chose the optical mirrors for enhancement cavity. We demonstrated a more than 200 enhancement factor with 795 finesse optical cavity at 10 micron CO₂ laser. Moreover, 2.3 kW storage in the optical cavity was successfully demonstrated. The design of optical cavity, first experimental results and future prospects will be presented at the conference.

In this work, we investigate the passive mode-locked Ytterbium-doped fiber laser based on the nonlinear polarization rotation technique. With the grating pairs inside laser cavity for the GVD compensation, the total cavity dispersion is operated within anomalous dispersion region. As the laser operates at fundamental mode locking, it generates 35 MHz repetition rate mode-locked pulse with 3.3 ps pulsewidth and the optical spectrum reveals obvious Kelly side. After adjusting the waveplate or increase the pump power, the 2^nd HML to 6^th harmonic mode locking (HML) is demonstrated in this laser. Besides, we also generated the bound states of multiple solitons whose separation of each pulse is about several tens of picosecond. The number of solitons and the separation between sequential pulses could be controlled so that it could be used in optical communication.

While considering long pulse or short pulse high power laser facilities, optical components performances and in particular laser damage resistance are always factors limiting the overall system performances. Consequently, getting a detailed knowledge of the behavior of these optical components under irradiations with large beam in short pulse range is of major importance. In this context, a Laser Induced Damage Threshold test facility called DERIC has been developed at the Commissariat à l’Energie Atomique et aux Energies Alternatives, Bordeaux. It uses an Amplitude Systemes laser source which delivers Gaussian pulses of 500 fs at 1053 nm. 1-on-1, S-on-1 and RasterScan test procedures are implemented to study the behavior of monolayer and multilayer dielectric coatings.

The highly-stable Q-switched longitudinally diode-pumped microchip laser, emitting radiation at wavelength 1031 nm, was designed and realized. This laser was based on monolith crystal which combines in one piece an active laser part (YAG crystal doped with Yb³⁺ ions, 10 at.% Yb/Y, 3mm long) and saturable absorber (YAG crystal doped with Cr³⁺ ions, 1.36mm long). The diameter of the diffusion bonded monolith was 3 mm. The initial transmission of the Cr:YAG part was 90% @ 1031 nm. The microchip resonator consisted of dielectric mirrors directly deposited on the monolith surfaces. The pump mirror (HT for pump radiation, HR for generated radiation) was placed on the Yb:YAG part. The output coupler with reflection 55% for the generated wavelength was placed on the Cr³⁺-doped part. Q-switched microchip laser was tested under CW diode pumping. For longitudinal pumping of Yb:YAG part, a fibre coupled (core diameter 100 μm, NA= 0.22) laser diode, operating at wavelength 968 nm, was used. The laser threshold was 3.3W. The laser slope efficiency calculated for output mean power in respect to incident CW pumping was 17%. The wavelength of linearly polarized laser emission was fixed to 1031 nm. The generated transversal intensity beam profile was close to the fundamental Gaussian mode. The generated pulse length was equal to 1.6 ns (FWHM). This value was mostly stable and independent on investigated pumping powers in the range from the threshold up to 9.3W. The single pulse energy was linearly increasing with the pumping power. Close to the laser threshold the generated pulse energy was 45 μJ. For maximum investigated CW pumping 9.3W the pulse energy was stabilized to 74 μJ which corresponds to the Q-switched pulse peak power 46 kW. The corresponding Q-switched pulses repetition rate was 13.6 kHz. The maximum Yb:YAG/Cr:YAG microchip laser mean output power of 1W was reached without observable thermal roll-over.

High-power laser propagation through the inner optical path will produce a significant thermal effect on the beam-splitter mirror which will cause phase aberrations. Based on the three-dimensional transient heat conduction equation and the thermal elastic stress-strain equation, a simulation model of reflector mirror was built with three-dimensional finite element method (FEM). The temperature increase and thermal displacements of two kinds of mirror substrates (Al₂O₃ crystal and spinel) were especially investigated with different laser intensity, output duration and absorption coefficient. The effects of mirror thermal distortion on laser beam phase aberrations were also evaluated on both reflection and transmission directions. The experiments of high–power laser propagation through two kind materials of beam-splitter mirrors samples with diameters of 50mm and thicknesses of 5mm were carried out to measure the thermal effects induced by the absorbed laser energy. Both two kinds mirror samples were deposited the same film layer of same reflectance. A high power semiconductor laser was expanded to a beam of 35mm diameter, and double Shack-Hartmann wavefront sensors were used to detect both reflection and transmission thermal distortions of the mirror samples. The measurements showed that reflection aberrations of spinel mirror sample were larger than those of Al₂O₃ crystal mirror sample while its transmission aberrations were slightly less than Al₂O₃ crystal mirror sample. The results of experiments and simulations showed a very good consistency.

Ultrafast all-fiber oscillators are currently one of the most rapidly developing laser technologies. Many advantages like: environmental stability, low sensitivity to misalignment, excellent beam quality (intrinsic single transverse mode operation), high energy and an excellent active medium efficiency make them the lasers of choice for a variety of applications. In this paper the designs of all-fiber all-normal dispersion femtosecond lasers are described. Due to large positive chirp, the pulses inside the cavity are highly stretched in time and they can achieve higher energies with the same peak power as shorter pulses. High insensitivity to mechanical perturbations or temperature drift is another highly valued property of presented configurations. Two of reported lasers are extremely stable due to the fact that their cavities are built entirely of polarization maintaining fibers and optical elements. We used highly Yb³⁺ ions doped fibers as an active medium pumped by a fiber coupled 976 nm laser diode. The central wavelength of our laser oscillators was 1030 nm. Three methods of passive mode-locking in all-fiber cavities were studied. In particular, the designs with Nonlinear Polarization Evolution (NPE), Nonlinear Optical Loop Mirror (NOLM) and Nonlinear Amplifying Loop Mirror (NALM) as artificial saturable absorbers were investigated. The most attention was paid to all-PM-fiber configurations. We present two self-starting, high energy, all-fiber configurations: one delivering pulses with energy of 4.3 nJ and dechirped pulse duration of 150 fs based on the NALM and another with a 6.8 nJ, 390 fs pulses in configuration with the NOLM. The influence of different artificial saturable absorber on output pulse characteristics were studied and analyzed.

The performance parameters of reflecting mirrors such as absorption coefficient or thermal distortion determine the beam quality of the output laser, so the quality of mirrors is one of the most important factors affecting the capability of the whole laser system. At the present time, there was obviously insufficient in test methods for the mirrors performance. The reflection coefficient, absorption coefficient and scattering coefficient of mirrors could be measured by a lot of test methods such as cavity ring-down method, photothermal deflection method, surface thermal lens method and laser calorimetry. But these methods could not test under high power density radiation. So the test data and results could not indicate the real performance in a real laser system exactly. Testing in a real laser system would be expensive and time consuming. Therefore, the test sequence and data would not be sufficient to analyze and realize the performance of mirrors. To examine the performance of mirrors under high power density radiation, the working principle of intra-cavity was introduced in this paper. Utilizing an output mirror with a low output coupling ratio, an intra-cavity could produce high-power density laser in the resonant cavity on the basis of a relatively small scale of gain medium, and the consumption and cost were very low relatively. Based on a discharge-drived CW DF chemical laser, an intra-cavity device was established. A laser beam of 3kw/cm² was achieved in the resonant cavity. Two pieces of 22.5 degree mirrors and two pieces of 45 degree mirrors could be tested simultaneously. Absorption coefficient and thermal distortion were measured by calorimetry and Hartmann wavefront sensor respectively. This device was simple, convenient, low-maintenance, and could work for a long time. The test results would provide support for process improvement of mirrors.