We set up a single rod amplifier based on a YA1O3-crystal with 9.5 mm diameter, 152 mm length and 1.0 at.% neodymium doping as the active material. This Nd:YALO crystal is optically highly anisotropic and shows therefore no remarkable stress birefringence. On the other hand, the thermal lensing of this material is roughly two times higher than for Nd:YAG. This disadvantage of a higher thermal lensing could be compensated with the phase conjugating mirror

Lasers with high average output power suffer from thermal lensing, stress birefringence and other phase distortions, which reduce the beam quality significantly. Self-pumped phase conjugating mirrors based on stimulated Brillouin scattering (SBS) were applied in different oscillators and master oscillator power amplifier systems (MOPA) to realize high beam qualities at average output powers up to 500 Watt. SBS-mirrors were applied from the mid-infrared down to the ultraviolet range. As an alternative to widely used SBS-materials like liquids and gases we developed a new kind of self-pumped SBS-mirror based on multimode quartz fibers.

The beam quality of high average power solid state lasers degrades due to thermal lensing and stress birefringence, similarly, discharge inhomogenities limit the performance of excimer lasers. To improve the beam quality, self-pumped phase conjugating mirrors based on stimulated Brillouin scattering (SBS) were applied in different oscillator and master oscillator power amplifier systems (MOPA) producing radiation from the mid infrared down to the ultraviolet range.Pulsed Nd-lasers for industrial applications with up to 500 W average output power at 1 micron wavelength and high beam quality were realized. Due to phase conjugation the beam diameter and divergence do not depend on the average output power. Also for medical applications, high beam quality is essential. Therefore we investigated a MOPA-system with phase conjugating mirror at a wavelength of 2.8 micron. In the ultraviolet range, a commercial XeCl laser from Lambda Physik was equipped with an SBS-mirror resulting in a self starting oscillator with phase conjugation at a wavelength of 308 nm.As an alternative to widely used SBS-materials like liquids and gases we developed a new kind of self-pumped SBS-mirror based on multimode quartz fibers. These exhibit lower SBS thresholds and therefore continuously pumped Q-switched laser systems with SBS-mirror can be realized.

Industrial and scientific applications require lasers with high beam quality in Q-switched operation. In oscillators the high reflecting mirror can be substituted by a phase conjugating mirror based on stimulated Brillouin scattering (SBS). Due to the nonlinear response of such a mirror not only the beam quality can be improved also Q-switching can be obtained with the same element. Up to now toxic liquids or gases under high pressure were used as SBS-media in pulsed solid state oscillators. These media can be substituted by a simple multimode fiber.A pulsed oscillator with a multimode fiber inside was realized. Nd:YALO was used as active medium. The flashlamp duration is about 140 mu s and the threshold energy for laser operation is 5 J. The repetition rate can be varied from 1 Hz up to 25 Hz. The energy per shot is about 16 mi. Due to the relatively low SBS threshold a burst of Q-switched pulses per flashlamp shot occurs. The pulse width can be varied between 0.5 ns and 10 ns which depends on the used resonator configuration and the fiber.

High average output power in conjunction with high beam quality is a crucial point for future industrial applications of solid state lasers. Phase conjugation based on stimulated Brillouin scattering in multi amplifier systems results in average output powers up to the kW-range with high beam quality. A solid state laser system containing six amplifiers and a master oscillator was developed. The system consists of a parallel arrangement of two double pass amplification stages with phase conjugating mirrors. To increase the average output power, both beams are amplified in additional single pass amplifiers again and combined using a thin film polarizer. Optical systems between the amplifiers guarantee the variability of average pumping power without damage of optical components. Therefore the average output power can be tuned between 50 W and 520 W. The beam quality was measured using the moving edge method and corrected for the second momentum of the laser beam. The beam quality is 1.8 times the diffraction limit at minimum pumping power and 5.2 times the diffraction limit at 85% of maximum pumping power.

Different schemes were investigated to reduce the power threshold of phase conjugating mirrors based on stimulated Brillouin scattering (SBS). Two methods will be discussed in detail: A phase conjugating SBS-mirror based on multimode glass fibers and an SBS-based beam clean up arrangement.With a fiber phase conjugator we have investigated a continuously pumped Q-switched MOPA-system with SBS phase conjugation. The used fibers were characterized with respect to their reflectivity and power threshold at different repetition rates up to 20 kHz. Typical power thresholds are in the range of 200 W. The master oscillator consists of a ring resonator with an acousto-optical modulator to realize the Q-switch and a high coherence length of six meters. The beam is amplified in a double pass scheme with the fiber phase conjugator after the first pass to compensate phase distortions introduced by the amplifier. To remain the system relatively simple Nd:YALO was used as active medium to avoid depolarization effects.The beam clean up arrangement improves the beam quality outside the laser system. It is based on two wave mixing in an SBS-medium. An oscillator amplifier system was realized with such a beam clean up system based on carbon disulfide.

Summary form only given. Diode-laser-pumped systems with phase conjugating mirrors and Nd:YAG slab laser technology were realized for high average output powers, but industrial use is prevented by high cost. Therefore, we developed a flashlamp-pumped master oscillator power amplifier (MOPA) system that is competitive with common industrial laser systems regarding financial efforts and reliability. The system consists of one master oscillator and two parallel arranged amplifier chains.

Stimulated backscattering leads to efficient phase conjugation multimode quartz fibers with core diameters between 50 and 400 mu m Such fiber phase conjugators with a length of more than 2 m have shown, up to the present time, a reflectivity of more than 60%, maximum fidelity up to 93%, and less than 2 kW power threshold at wavelengths of 1064, 532, and 355 nm. Depolarization of the reflected phase-conjugated light could be reduced to less than 0.5% by proper configuration of the fiber. High power densities of more than 500 MW/cm(2) do not damage the fiber phase conjugator. Furthermore, fiber phase conjugators are harmless to the environment, in contrast to fluid and gaseous SBS media at high pressure. Applied to solid-state MOPA systems, 100 W average output power with high beam quality was achieved.

The beam quality of high average power solid-state lasers can be significantly improved using phase conjugation based on stimulated Brillouin scattering (SBS) [1] in cells with a simple focusing geometry. The relatively high SBS threshold of several 10 kW in such cells, however, limits applications e.g. in continuously pumped laser systems. The SBS-materials used up to now are toxic liquids or gases under high pressure.

Industrial applications require lasers with high beam quality combined with kHz repetition rates. The average output power peak power and the repetition rate should be variable. Using phase conjugation based on stimulated Brillouin scattering (SBS) the beam quality of such high power solid state lasers can be improved significantly. High average power MOPA systems in double-pass configurations with SBS-min or and high repetition rates were developed. To simplify multi amplifier systems, Nd:YALO as active material was used to avoid depolarization of the amplified beams. With two amplifiers, average output powers of more than 200 Watts with high beam quality were realized. The system works with an average repetition rate up to 3 kHz and pulse widths of up to 200 ns. Peak and average output power can be varied over a wide range. Second harmonic generation with such a system leads to an average output power of 50 W at 540 nm wavelength with a conversion efficiency of 50%. Using SBS in glass fibers the power threshold for phase conjugation can be reduced and therefore first experiments with a continuously pumped Q-switched MOPA-system with SBS-mirror were realized.

Using stimulated Brillouin scattering (SBS) in glass fibers as phase-conjugating process, the field of applications of SBS phase-conjugators can be expanded, because fibers offer a low power threshold and a stable behavior even at high peak and average input powers. We have developed a new kind of phase-conjugator consisting of multimode quartz fibers with core diameters between 50 and 400 micrometers . In contrast to commonly used toxic liquids or gases under high pressure as SBS-materials, fibers are harmless to the environment and can be handled easily. Such fiber SBS-mirrors were characterized with respect to the phase conjugating parameters like threshold, reflectivity and fidelity. We achieved reflectivities of more than 50 percent, 93 percent fidelity, and less than 2 kW power threshold at 1 micrometers wavelength. A depolarization of the light could be reduced to less than 0.5 percent by proper configuration of the fiber. High power densities of more than 50 MW/cm2 do not damage the phase conjugator. These new phase conjugators were applied in high-power Nd:YALO-MOPA-systems for the first time. Good beam quality at an average output power of more than 100 W was achieved at 1.08 micrometers wavelength.

The beam quality of conventional flash lamp pumped laser rod systems is improved by using phase conjugating mirrors based on stimulated Brillouin scattering (SBS). Pulsed multi amplifier set-ups with SBS were investigated. Nd:YALO as active medium was used to avoid stress birefringence. In case of serial arrangements an optimised optical system between the individual rods is used to achieve high efficiency independent of thermal lensing. With a two amplifier system an average output power of 1 Watt up to 210 Watt in a near diffraction limited beam was realised. In case of parallel arrangements phase coupling of the individual beams was achieved using a common SBS-cell after the first amplifier pass. Instead of a Brillouin-cell a fibre phase-conjugator can be used to reduce the power threshold down to 6 kW. An average output power of 35 Watt from a pulsed MOPA-system with such a phase-conjugator could be obtained. As applications high power second harmonic generation and applications in materials processing were realised.

Phase conjugation by SBS (stimulated Brillouin scattering) is a powerful and simple tool to increase the beam quality of high power lasers up to the diffraction limit. Master oscillator and power amplifier systems with Nd:YAlO as active medium were realized with an average output power up to 210 watts at 1.08 micrometer in a near diffraction limited beam. Also Nd:YAG-oscillators with phase conjugating mirrors were realized with an average output power up to 17 watt and high beam quality. In the UV range, excimer oscillators with SBS mirrors deliver the same or better beam quality than conventional plane-plane resonators in 3 times shorter pulses. Excimer oscillator-amplifier arrangements have been realized with SBS mirrors. SBS mirrors for the visible and ultraviolet spectral range are well characterized. First experiments for an ordered passive fiber bundle with an SBS mirror show the possibilities and limitations to improve the beam quality of fiber bundle amplifies or even oscillators.

Beam quality improvement of high power solid state lasers for industry and science is a crucial point. A simple way to increase the beam quality of conventional flash lamp pumped rod systems is to use a phase conjugating mirror based on stimulated Brillouin scattering (SBS). Pulsed multi amplifier setups with SBS were investigated. Nd:YALO as active material was used to avoid stress birefringence and decrease of beam quality. In case of serial arrangements on optimized optical system between the individual rods is used to achieve high efficiency independent of thermal lensing. With a two amplifier system an average output power of 1 Watt up to 210 Watt in a near diffraction limited beam was realized. In case of parallel arrangements phase coupling of the individual beams was achieved using a common SBS-cell after the first amplifier pass. First experiments show an average output power of 50 W. CW-pumped MOPA-systems with SBS and repetition rates above 10 kHz are under development. A fiber phase-conjugator is used to reduce the power threshold down to 6 kW. An average output power of 14 Watt from a pulsed MOPA-system was achieved with a fiber phase- conjugator. As an application high power second harmonic generation was realized.

To increase the beam quality of existing lasers we developed flash lamp pumped master oscillator power amplifier systems (MOPA) with phase conjugating SBS-mirrors. The used Nd:YALO as active material shows no remarkable stress birefringence resulting in a simple optical arrangement. With a single amplifier rod average output powers of 4 watt up to 140 watt were realized with a beam quality of 1.1 times the diffraction limit. Serial arrangements of amplifiers were investigated to increase the average output power. By using an optical system between the individual rods the pump power can be tuned over a wide range. The parameters of the used two lens system can be optimized to avoid damage of optical components and secure a high efficiency independent of thermal lensing. With a two amplifier system an average output power of 1 watt up to 210 watts in a near diffraction limited beam could be realized. The system works with a repetition rate of 100 Hz and emits per flash a train of 13 Q-switched pulses with an FWHM of about 90 ns. Calculations for a serial arrangement of six amplifiers were made and show the possibility to achieve an average output power of 1 kW.

Solid-state lasers for high average output power suffer from thermal lensing. stress birefringence and other phase distortions which reduce the beam quality significantly 1 . Using phase conjugation based on stimulated Brillouin scattering (SBS) the beam parameter product of conventional laser systems can be decreased down to the diffraction limit. SBS cannot compensate stress birefringence and therefore different compensation schemes for Nd:YAG were developed 2,3 resulting in complex setups. Therefore in contrast to multi-amplifier YAG systems with SBS 4,5 we use Nd:YALO as active material to avoid the depolarization effect resulting in simple optical arrangements.

For pumping XUV plasma light sources with solid-state lasers and also for micromaterial processing, high beam quality and output power as well as simplicity of the pump lasers are crucial. The beam quality of high-power solid-state lasers can be significantly improved using phase-conjugating mirrors based on stimulated Brillouin scattering (SBS). Hence, SBS phase conjugation may prove one of the key technologies in this field. We describe a single-rod neodymium amplifier with 140 W average output power and nearly diffraction-limited beam. Because of automatic compensation of the thermal lensing by SBS phase conjugation, the output power can be tuned without changing the beam profile.

Summary form only given. In summary, the developed a master oscillator power amplifier (MOPA) system with two Nd:YAlO/sub 3/ laser amplifiers in a serial arrangement consisting of a small number of optical elements to guarantee high stability and reliability at an average output power of more than 200 Watt. To expand this system calculations for serial arrangements of four and six amplifiers were made and show the possibility to achieve an average output power of 1 kW. Theoretical beam propagation for a four-amplifier system is given.