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A stable operation of 1.1 J/100 Hz 10 ns laser pulses were achieved from a single cryogenically cooled Yb:YAG rod amplifier with ink-cladding. The efficiency and gain coefficient were 44% and 383, respectively.

We have developed a conductive-cooled Yb:YAG active-mirror amplifier with an diameter of 50 mm and a liquid-nitrogen circulation system. The high thermal capability that enables 10 J, 50 Hz laser amplification was demonstrated successfully.

This study demonstrates fast pulse train switching using a coherent polarization beam combiner with a phase difference detection (PDD) algorithm. Based on the highly stable feedback control obtained with the PDD algorithm, stable burst-mode extraction with an arbitrary duty cycle was achieved with a contrast ratio of 800:1 for a third-harmonic light (347 nm) of a 5 MHz pulsed Yb-doped fiber laser. The proposed method can be effectively used for high-speed switching in high-average-power pulsed laser systems.

Development of high-energy and high-repetition rate laser using Yb:YAG is a subject of growing attention to use many scientific and industrial applications. Especially, The Laser Wake Field Acceleration (LWFA) is receiving a lot of attention in the world [1–3]. Laser driven plasma accelerator (LPA) has potential to accelerate electron bunches to high energies in mm-size acceleration length, and still provides high quality bunch characteristics like: high bunch charge, femtosecond duration bunch length, and high pointing stability [4]. However, the accelerated electron charge is no many compare to radio frequency accretion. Therefore, the high repetition rate and high peak power laser is required. We have developed 1 J, 10 Hz cryogenically-cooled active mirror amplifier using Total-reflection active-mirror (TRAM) architecture [5]. At present, we are developing the 100 J, 100 Hz cryogenically-cooled active-mirror amplifier using Yb:YAG. The front end for the laser system is consists of CW distributed feedback laser diode, EO pulse slicer, which generates laser pulse of several ns duration at a pulse energy of few nJ. This pulse amplified up to around 200 mJ in the regenerative amplifier and multi-pass amplifiers using by cryogenically-cooled Yb:YAG. This pulse is amplified energy of 100J through two stage cryogenically-cooled active-mirror amplifier. We are developing the conductive cooled active mirror using the liquid nitrogen circulation. It is possible to 10kW (100J, 100Hz) operation. In order to conductive cooled active mirror, it is necessary to develop a cooling method for suppresses wave front changes in cooling. Therefore, we are developing Key technologies about cooling structure, bonding of amplifier medium and heat sink, wave front compensation. Also, we made the wave front measurement system of cryogenically-cooled active mirror. Figure. 1 (a) show the layout of wave front measurement system for cryogenically-cooled active mirror. The reference laser is fibre coupled CW distributed feedback laser diode at 1030 nm of wavelength. The reference laser is expanded to 50 m diameter by beam expander. The Yb:YAG coated AR and HR coat for 1030 nm. The Yb:YAG bonding at Heat-sink and cooling by cryocooler (Stirling cooler). The refracted laser is reduced to 5 mm diameter by beam reducer and input to the wave front sensor (SID4, PHASICS). Figure.1(b) show the measurement result of wave front shift. The wave front shift is change amount (P-V change) from the reference at room temperature (300K). The wave front shift increases with cooling, it is small value. Also, we measured the Zernike coefficient. It is dominant in low order and easy to compensation.

We report, to the best of our knowledge, the highest power conductive-cooled active-mirror amplifier (CcAMA) using Yb:YAG with a pulse energy of 10 J. By using four liquid-nitrogen circulating cooled laser heads, we achieved a repetition rate, pulse energy, and average power of 33.3 Hz, 9.3 J, and 310 W, respectively. The problem of wavefront distortion, which is difficult to solve with a large-aperture active-mirror laser, is suppressed by using reinforcing materials with the same thermal expansion coefficient. We have confirmed that the wavefront distortion is small ( 0.15 λ P-V per head) at 100 Hz operation, which paves the way for 100 Hz operation with the CcAMA concept.

We had developed a unique porous thin films by a special coating method1. In this technique, two dielectric materials A and B having different refractive indices nA and nB ,where nA＞nB are simultaneously deposited in vacuum on a substrate such as fused silica or optical glasses. Then the coated surface is processed in ultra-pure water which preferentially dissolves the material B. These processes result in a porous thin film which has gradient refractive index and has the antireflection (AR) property over broad bandwidth. The porous coating obtained by this method cannot apply depositing a multilayered dielectric thin film. We have developed a novel method. The present technique, a dielectric material D and a plastic P are simultaneously deposited in vacuum on a heated-substrate such as fused silica, ceramic or optical glasses. Then the coated surface forms an adaptively mixed thin film ( AMTF ) with dielectric material and plastic. In this coating process, plastics partially evaporate due to the heated-substrate. The refractive index of the coated AMTF mainly decided by the mixing ratio of the dielectric material and plastic. In our samples the damage threshold was confirmed to be 115 J/cm2 at 10 ns and λ=1064 nm. The band width of AMTF with MgF2 and Teflon (AMTF: MgF2 ) was confirmed to cover from 200 to 8000 nm. This AMTF: MgF2 can be applicable not only to AR thin film, but to a high reflectance mirror and polarizer in various high intensity laser syetems. 1K.Yoshida, H.Yoshida, Y.Kato, and C.Yamanaka, Appl.Phy.Lett.47,911(1985)

We report the absorption characteristics of ceramics for amplified-spontaneous-emission-absorbing cladding and their temperature dependence. We measured YAG ceramics doped with Cr4+, Co3+, Co2+, and Sm3+. Our results indicate that Cr4+- and Co3+-doped YAG ceramics are suitable for 1-μm lasers and that Co3+ have higher temperature robustness than Cr4+.

We have demonstrated an efficient high energy 2 μm laser generation with a 36 mm long large aperture 5 mol% MgO-doped periodically poled LiNbO3 (PPMgLN) nonlinear optical crystal. A high power Q-switched Nd:YAG laser (1.064 μm) was used to pump the quasi-phase matched (QPM) optical parametric oscillator (OPO). A total output energy of 186 mJ with 58% slope efficiency was obtained in two separate beams at 2 μm.

We have proposed the cryogenic TGG ceramics based Faraday rotator for High peak and high average power laser. As the first demonstration of Faraday effect in a TGG ceramics, its Verdet constant at 1053 nm is evaluated to be 36.4 rad/Tm at room temperature which is same as that of the single crystal. In addition, the temperature dependence of Verdet constant is obtained experimentally. At liquid helium temperature, it is 87 times greater than that at room temperature. The measured thermal conductivity was corresponding to TGG single crystal.

Experimental results are reported on the compensation of thermal lensing caused by the radial thermal gradient in an optically pumped Nd:YAG laser. Poly(methyl methacrylate) (PMMA) with a higher negative dn/dT, a high damage threshold and low absorbance is very useful for compensating thermal effects in the amplifier. The results revealed that the use of PMMA resulted in the complete reduction of the thermal lens effect by an order of magnitude.

A liquid-cooled ceramic Nd:YAG split-disk laser amplifier has been designed for a high-average-power laser system. The maximum average output power of 17.1 W (1.71 J at 10 Hz) and the beam size of far-field pattern with 1.5 times diffraction limit was obtained. The combination of liquid cooling for disk laser material and wavefront compensation with a phase conjugation mirror was demonstrated. The wavefront degradation by liquid cooling was observed and measured with Michelson interferometer and far-field patterns.

A prototype split-disk amplifier consisted of four Nd:YAG ceramics and single crystals was demonstrated with a 3.7 cm square clear aperture. A single-pass small-signal gain of 2.0 and a total stored energy of 11.1 J in the eight YAG disks were obtained. The maximum output energy of 10.4 J with the YAG disk amplifier was achieved in a single-shot operation. A near-field pattern of 31 mm × 31 mm square with super Gaussian shape was measured.

We have demonstrated the short pulse generation of Nd:YAG laser at the fundamental wavelength by using stimulated Brillouin scattering (SBS) in liquid FC-75 at the energy of 1.5 J with a compact single set-up consists of generator and amplifier. Minimum pulse width obtained without attenuator and with attenuator are 770 ps and 600 ps respectively. SBS Conversion efficiencies are 90% without attenuator and 55% with attenuator. The brightness of the compressed pulse was about twelve fold higher than that of incident pulse.

The dependence of the bulk damage threshold of nonlinear crystals, such as KDP, KTP, LBO, BBO, CLBO, LiNbO3, and LiTaO3 on laser irradiation direction and their polarization under a single-shot operation have been investigated. We found that the intrinsic bulk-damage threshold for nonlinear crystals was mainly caused by dielectric breakdown from self-focusing with nonlinear refractive index. LBO single crystals have the highest value studied among nonlinear crystals. For KDP, BBO, and CLBO crystals, the damage threshold when an incident laser was propagated along the axis is approximately twofold that in the {100} or {010} plane at a 1.064 µm wavelength. The damage crack pattern is also suggested to be in the mechanically weak direction due to inner pressure caused a laser-induced plasma in the crystal as a result of the laser breakdown due to electron avalanche breakdown.

In order to improve the measurement performance of incoherent Thomson scattering diagnostics, a high performance phase conjugate mirror based on stimulated Brillouin scattering (SBS-PCM) is applied to a Thomson scattering system for the first time in the JT-60U tokamak. We have demonstrated that a SBS-PCM which uses heavyfluorocarbon liquid showed a high reflectivity of 95% at a high input-power of 145 W. Using the SBS-PCM, two newly developed methods were employed to increase the amount of scattered light. In the first method, we first developed a new optical design to provide a double-pass scattering scheme with the SBS-PCM. In this new optical design, a laser beam passing through the plasma is reflected by the SBS-PCM, and the reflected beam is returned via the same path by means of the phase conjugate effect, and is then passed through the plasma again, in order to increase the scattered light. A double-pass Thomson scattering scheme using the SBS-PCM was demonstrated in JT-60U ohmic plasma, resulting in an increase of the scattered light by a factor of 1.6, and the reduction of relative error by 2/3 for electron temperature measurement in contrast to single-pass scattering. A multi-pass Thomson scattering scheme is also proposed based on the results of double-pass scattering. It is estimated that multi-pass scattering allows the generation of several times the amount of scattered light, and the reduction of the relative error for electron temperature measurement by 37% in contrast to single-pass scattering. Regarding the second method, a high average-power of YAG laser system was developed by applying the SBS-PCM to a existent diagnostic laser. As a result, the average-power was increased by over 8 times in contrast to the average power of the original system, achieving up to 368 W (7.4 J × 50 Hz).

In the Leading Project of MEXT from 2003, Japan, high average power LD-pumped YAG lasers are being developed with specifications of > 1 J, > 5 kHz, and 1-10-ns pulse. All fiber front end system consists of cw 40-mW Yb doped fiber laser (YbDFL) with a fiber Bragg grating cavity, a pulse shaper of LiNbO3, and three stages of YbDFL amplifiers, delivering 2-mW average power of 1-20 ns pulses in 5-kHz repetition rate. The design and test of 500-W class LD-pumped booster amplifier head is discussed. Some key elements of technology for high-peak and high-average power system are reviewed such as a ten-cm silica laser material, a repetitive work of stimulated Brillouin scattering phase conjugate mirror for 10-kW capability, a ceramic material for Faraday rotator for high-average power level, so on.

Fused-quartz glass is found to be an ideal medium for high-power stimulated Brillouin-scattering experiments, because it gives a high reflectivity of over 95% at an incident energy of 2.3 J, has pulse duration of 16 ns, and has excellent fidelity in phase conjugation. The low optical homogeneity of the fused-quartz material selected resulted in a low focusability at the focus, resulting in a reduction in laser-damage occurrence, even in high-energy operations. Despite this fact, its conjugation fidelity is good for cancellation as a phase-conjugated mirror. All solid-state lasers with high-beam qualities, based on phase-conjugated mirrors with fused-quartz glass are highly suitable for use as laser sources in many applications.

An improvement of the output energy of a multistage YAG laser system by using a stimulated Brillouin scattering (SBS) phase conjugation mirror (PCM) was achieved. The phase conjugation of the optically nonlinear SBS process in a liquid material effectively compensated thermal degradation at an average/peak power amplifier, resulting in an average power increase from 1.5 J at 30 Hz repetition rate to 2.6 J at 50 Hz drive. The beam quality was also recovered without wave front deformation and depolarization resulting in a transfer-limited divergence with a good flat-top pattern in a near field.

A uniform laser ablation was observed in a polystyrene film coated with a photovoltaic perylene/phthalocyanine bilayer when an incident took place at an intensity range of 10 9 –10 10 W/cm 2 ( λ =1064 nm, 1.1 ns FWHM). Without the bilayer coating, the laser pulse formed spiky structures in the polystyrene film as self-focusing traces of the laser pulse, while for the coated film, the uniform surface ablation trace without the spiky interior structures was observed. In the case of incident of 532 nm where the coating material exhibits no reflection, such difference depending the coating was not observed. These phenomena were coupled with the reflection and conduction properties via its photovoltaic effect, and agreed with the required ablation to achieve high-density compression of the fuel capsule for inertial fusion energy (IFE).

Recently, several petawatt (PW, 1015 W) lasers with pulse duration of ~20–30 fs have been introduced throughout the world, pushing the upper limit on laser peak power. However, besides well-known spatio-temporal coupling effects, such as residual spatial/angular chirps and pulsefront tilt/curvature, the spatio-temporal/spectral coupling in compressors induced by wavefront errors of gratings, which could dramatically distort ultra-intense pulses, has been neglected. In this work, for the first time we analyzed this phenomenon and the peak power/intensity degradation induced by it. Our results suggest that the actual performance of femtosecond PW lasers may be worse than previously estimated.

The spatial distribution of the optical parametric fluorescence generated in a negative uniaxial nonlinear crystal is asymmetric with respect to the pump when the pump beam has a slight divergence angle. The formation mechanism of this phenomenon and the influence of parameters were analyzed and discussed from a theoretical standpoint. Moreover, two potential applications of this phenomenon were experimentally demonstrated, showing the temporal contrast improvement of ultra-intense lasers and the intensity enhancement of special light sources induced by the optical parametric generation.

We designed and demonstrated two new schemes to improve the performance of stimulated Brillouin scattering (SBS) cells for a high average power laser. By introducing a lamellar flow cell, thermallyinducedefocusingand convection via laser light absorption were removed. By inserting a rotational wedge plate, the spot wasrotated in time and locate heat sources were diffused. As a result, the SBS reflectivity and fidelity wereimproved dramatically with acetone. Using the cell, the operational power can be improved by as much asseveral hundred times the closed-type. If a medium with a small absorption coefficient such asheavy fluorocarbonsis used, the phase conjugate performance enables the incident power to reach a kW level.

Bulk laser-induced damage in KDP crystal was measured using a single-shot 1-ns pulse Nd:YAG laser in a transverse and longitudinal single mode. It is found that the damage threshold of KDP single crystal depends on the laser irradiation direction, polarization direction and laser wavelength. The damage threshold in the direction of c axis is about two times higher than that of in the a(b) axis at 0.532 and 1.064 μm wavelength. This result is consistent with the mechanical strength tests for various directions of KDP crystal.

We report a high average power and high pulse energy diode-pumped Nd:glass master oscillator power amplifier system that is consisted of a thermally-edge-controlled zigzag slab amplifier and stimulated Brillouin scattering (SBS) mirror for use of pumping source of ultra intense laser system. This phase conjugated system produces an average power of 213 W at 10 Hz in 8.9-ns pulse (2.4 GW peak power) with an optical-to-optical conversion efficiency of 11.7 % and a near diffraction limited beam quality. To the best of our knowledge these results represent the highest average power achieved from a Nd:glass based laser amplifier system.

A compact and two dimensional detector has been developed for the JT-60U Thomson scattering system, that consists mainly of a proximity focused image intensifier, a 20×12 spatially and spectrally arrayed silicon photodiode, and a data processing unit enabling high repetitive measurement at 1 kHz. To secure a wide spectral bandwidth for an effective detection of the Thomson scattering spectrum, we have introduced a spectrometer and lens systems with enhanced transmission in a longer wavelength, which can compensate for the decreasing characteristic of quantum efficiency of a transmission-mode photocathode used in the image intensifier. The spectral divide of the photodiode array is designed to give its full performance in a wide electron temperature (Te) range of 0.2–15 keV with 12 spectral channels. The application of this newly developed detector to the JT-60U core plasma measurement is also presented.

Effect of ion beam etching on laser-induced surface damage was investigated in CsLiB6O10 (CLBO) crystaland fused silica. In high-power UV lasers, these optical surfaces were damaged due to absorption of thepolishing compound embedded inside the surface. From SIMS analysis, the polishing compounds were foundto be embedded in the CLBO and fused silica surfaces to a depth of 60 nm. These polishing compounds couldbe removed by using Ar ion beam etching. No ion beam-induced surface damage was observed when theapplied ion beam voltage was less than 200 V. Due to the effect of polishing compounds removal on damageresistivity, the laser-induced surface damage threshold of CLBO was improved up to 16 J/cm2 (wavelength: 355 nm, pulse width: 0.85 ns) as compared with that of the as-polished surface 11 J/cm2. The laser-inducedsurface damage of fused silica also increased from 7.5 J/cm2 to 14.5 J/cm2.

The design and operation performance of the latest ruby Thomson scattering system for the JT-60U are described, which includes many novel approaches to attain the repetitive measurement of 60 spatial points with high resolution (8 mm), precision, and reliability especially for a high temperature and large size tokamak device. A beam combiner composed of a polarizer and a Faraday rotator for two ruby lasers has provided the transient measurement with the minimum time interval of 2 ms in burst operation and the multitime-slice measurement with the repetition rate of 0.5 Hz in normal operation. A newly developed photodiode array detector with high repetitive ability of 1 ms has been used for the high temperature core plasma measurement. A quantitative alignment of collection fiber object field has been utilized for the reliable ne profile measurement. By using Rayleigh scattering light, an in situ inferring method for a coated viewing window transmission has been found to have an effect on the precise correc...

A YAG laser Thomson scattering system has been developed to measure the time evolution of electron temperature (Te) and density (ne) profiles in the JT-60U. Presently this system provides Te and ne periodically every 100 ms (20 ms at the minimum) at 15 spatial channels. A six spectrum channel polychromator, which is a successive interference filter type, was designed to measure a wide Te range (20 eV–20 keV) by using five channels. The sixth channel is used for calibration by Rayleigh scattering. However, we also used the Rayleigh channel to measure Te and ne since the stray light level was very low. As a result, we were able to measure temperatures below 20 eV. The temperature of the silicon avalanche photodiode (APD) is stabilized by a Peltier element so that the gain is kept constant within ±0.5%. The stable performance of the APD enables accurate Te and ne measurement. During plasma operation, the electron temperature is measured over a wide range from the low temperature plasmas in the runaway plasma...

Efficient compensation of thermally induced birefringence with thermal lensing in a rod geometry Nd:YAG laser amplifier was achieved by an in-cavity 90° polarization rotator with three sets of image relaying optics between the laser rods. The measured birefringence loss per transit was 2.8% for pumping power of 580 W per rod. A nearly diffraction limited output beam of 38 W was observed.

We have designed a compact solid compressor by stimulated Brillouin scattering (SBS) consisting of two right-angle prisms and a fused-silica glass block. A 13 ns Nd:YAG laser pulse was temporally compressed to a 1.2 ns phase-conjugated pulse in fused-silica glass at a 1064 nm wavelength. The maximum reflectivity of the SBS was over 95% without optical damage. The compressed pulse brightness was about 10-fold higher than that of the incident pulse.

Self-controlled sub-nanosecond pulse generator was demonstrated with an ytterbium-doped fiber. This fiber laser consisted simply of all non-polarization fiber without any devices for polarization control and birefringence compensation. The self-pulse operation system gave an average output power of 0.9 mW in 800-ps duration pulses.

A flashlamp-pumped highly efficient 0.1 at. % Cr3+ and 1.0 at. % Nd3+ codoped Y3Al5O12 ceramic laser was demonstrated. A 10.4 J output energy at 1064 nm was obtained, with a laser efficiency of 4.9%. The laser efficiency of the Cr3+ and Nd3+ codoped Y3Al5O12 ceramic rod was found to be more than twofold that of a 1.0 at. % Nd3+-doped Y3Al5O12 ceramic rod.

The design concept and operation performance of a novel alignment system for accurate electron density profile measurement is presented, which has been developed on the basis of the original method for adjusting quantitatively and actively an object field of collection fiber optics to a laser-beam axis. The quantitative expression needed for an appropriate alignment was able to be formulated using the beam width measured in situ by this alignment system and the four laser scattering intensity signals from special alignment fiber bundles. The effectiveness of this alignment system for precise and reliable measurement of electron density has been successfully demonstrated in the JT-60U Thomson scattering diagnostic through direct monitoring and quick restoration of alignment. The important and distinctive feature of this alignment system is to attain compatibility between a stable and accurate maintenance of alignment and an improvement of the signal-to-noise ratio of Thomson scattering to plasma light acco...

Arbitrary temporal waveform control can be achieved by means of amplifier gain control with chirped laser pulse. As lasers for fusion research are broad-band glass lasers, a chirping method of the THz region is required to obtain arbitrary temporal waveform control. In order to precisely simulate amplification properties of chirped pulse, we have developed the simulation code for an inhomogeneous glass laser amplifier. Included physics are as follows: temporal chirping; relaxation of lower level; and change of distribution density between sub-levels. Chirping (0.9 THz) applied to a Gaussian pulse increased the peak intensity of output in 20% to the case without chirping.

Organic L-Arginine Phosphate Monohydrate (LAP) crystal showed much lower threshold (6mJ/18ns) stimulated Brillouin scattering (SBS) at 1.06μm wavelength than fused silica, and higher SBS reflectivity of approximately 65% (inherently 93%) limited due to its absorbance. The reason why LAP crystal has a high damage threshold at Q-switched pulse operation is explained due to the SBS backward reflectance. We have demostrated the improvement of wavefront distortion induced by thermal effects in solid-state laser materials. Deuterated LAP crystal is very interesting not only as a frequency converter but also as a phase conjugated SBS mirror for all solid-state high-energy lasers because of high SBS gain coefficient and its low absorbance.

A beam combine method of two ruby lasers has been developed for the JT‐60 Thomson scattering system, that is mainly composed of a polarizer plate and a Faraday rotator. By this method we have achieved a minimum time interval of 2 ms between the two laser pulses homogeneously combined. Also a high energy transmission of more than 95% has been attained. As a result, transient phenomena measurement can be available especially for pellet‐injected plasmas by using the Thomson scattering system. The beam combine method applied to JT‐60 is very attractive in terms of upgrading a Thomson scattering system to a high repetition diagnostic without any essential change of the existing equipment, and has been working with no serious trouble since 1989. The application of this method to a multilaser system with three or more lasers is also discussed for the practical purpose of improving a repetition rate of high power pulsed laser.

We investigate the effect of dispersion compensation on temporal characteristics in mode-locking by nonlinear polarization rotation in an ytterbium-doped fiber (YDF) oscillator with intracavity and external grating pairs. A short fixed length YDF was spliced with a longer single-mode fiber (SMF). Using experimentally measured dispersion characteristics of the YDF, SMF and cavity optics, we control the group velocity dispersion (GVD) and spectral broadening in a cavity by changing the SMF length. As a result, the oscillator generated 29.4-fs transform-limited wing-free pulses, which are to our knowledge the shortest and cleanest pulses achieved without the use of additional optics like a prism pair for high-order dispersion compensation. The results show that a precise balance of higher order terms of the GVD and self-phase modulation is essential for shortening pulse duration.

Phase conjugation via stimulated Brillouin scattering (SBS) is useful to compensate thewavefront distortion due to the thermally induced aberrations and depolarization of solid statelaser medium. Principle of phase conjugation by SBS, reflectivities and fidelities of SBS forliquid and gaseous medium are discussed and several applications to compensate aberations inlaser oscillator and MOPA system are described.

Phase-conjugate mirror (PCM) is very important to improve the wavefront distortion due to the optical pumping of solid state laser materials and the inhomogeneity of optical components. In this paper, the reflection experiment of PCM based on stimulated Brillouin scattering and the oscillation characteristics of YAG laser using PCM as a cavity mirror are described.

We have achieved a high compression ratio by stimulated Brillouin scattering (SBS) consisting of two long cells. A 13-ns Nd:YAG laser pulse was temporally compressed to about 160-ps phase-conjugated pulse in heavy fluorocarbon FC-40 liquid at a 1064 nm wavelength. The maximum reflectivity of SBS process was over 80 % without an optical damage. The compressed pulse brightness was about 65-fold higher than that of the incident pulse.

Optical parametric oscillator based on a large aperture periodically poled MgO:LiNbO3 was used to generate 2 µm laser. Total output of 179 mJ with 53% slope efficiency was achieved at low pump fluence.

We have been developing a high repetition (5-100 kHz) and high power Nd: YAG laser system pumped by cw LDs. Average power of 4.6 kW was obtained at 28.8 kW of pumping power.