Andreas Haase

Cr4+:YAG as passive Q-switch for a Nd:YALO oscillator with an average repetition rate of 2.7 kHz, TEM00 mode and 13 W output

A conventional pulsed Nd:YALO oscillator was passively Q-switched with a Cr4+:YAG crystal. During each of the flashes, with a repetition rate of 100 Hz, a burst of 27 Q-switch pulses with a half width (FWHM) of 140 ns was generated. The minimal pulse-to-pulse time interval within the burst was about 5 µs. The average repetition rate of these Q-switch pulses was 27 × 100 Hz. Nd: YALO as active material does not show any thermally induced birefringence and therefore a good TEM00 mode was realized despite the high thermal load of the crystal. The 4 × 79 mm laser rod produced 13 W average output power at 1080 nm with an efficiency of 0.5%. In contrast to earlier used LiF:F2− absorbers as passive Q-switch the Cr4+:YAG-crystals did not bleach and therefore the system operated very stable and reliable.

High beam quality by SBS phase conjugation of a single rod Nd-amplifier up to 140 W average output power

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.

Fiber phase conjugator as reflecting mirror in an MOPA arrangement

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.

HIGH POWER SOLID STATE LASERS WITH PHASE CONJUGATION FOR APPLICATIONS WITH HIGH BEAM QUALITY

A lot of important laser applications in industry and science require high average output power in conjunction with high beam quality. To increase the beam quality of high power solid state lasers up to the diffraction limit phase conjugation by SBS (stimulated Brillouin scattering) is promising. Oscillators and different master oscillator power amplifier systems (MOPA) with Nd:YALO as active medium and phaseconjugating mirrors were realised. Using simple SBS-cells as phase conjugating mirrors results in an average output power up to 210 Watts at 1.08 gm in a near diffraction limited beam. Applications in material processing were tested with these systems.

High-repetition-rate continuously pumped solid state lasers with phase conjugation

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.

Absorption bistability and nonlinearity in evaporated thin films

Abstract Nonlinear optical absorption of evaporated, polycrystalline CdSSe thin films is studied experimentally. A cw Rh6G-dye laser and a frequency doubled alexandrite laser with 5 ns pulse duration are used for excitation. Thermal and electronic nonlinearities are studied under steady state conditions. The thermal effect is used to realize absorption bistability and to determine thermal relaxation times. Absorption bleaching is observed with ns excitation and the maximum bleaching coefficient is 1.4 cm/kW. The low quantum efficiency of -2 for fluorescence indicates that the charge carrier lifetime is dominated by nonradiative processes. The relaxation time for bleaching determined by a ps-pump-probe technique is about 50 ps.

Fiber phase conjugators for high-power lasers

Using stimulated Brillouin-scattering (SBS) in glass fibers as phase-conjugating process, the field of application of SBS phase-conjugators can be expanded, because fibers offer a decreased power threshold and a stable behavior even at high input powers. We have developed phase conjugators consisting of multimode fused silica fibers in standard or tapered geometry with core diameters between 25 and 400 μm. Such fiber phase conjugators were characterized for different lengths in respect of the phase conjugating parameters like threshold, reflectivity, and fidelity. Dependencies of all SBS parameters on the fiber length and on the coherence length of the input beams were determined. Such optimized fiber phase conjugators operate with more than 50% reflectivity, 93% fidelity, and less than 200 W threshold in the spectral range from IR down to the UV. A depolarization of SBS pulses could be reduced to less than 0.3% by proper configuration of the fiber. High power densities of more than 1 GW/cm2 do not damage our fiber phase conjugators, because the absorption of the fiber materials used is very low. Furthermore, fiber phase conjugators are harmless to the environment and beat the performance of dangerous liquid or high pressure gas SBS- cells used in high power lasers up to now in all concerns. High economic benefits can be expected due to the popular fiber materials used from the glass fiber technology.

Phase conjugation for high-power solid state lasers with repetition rates in the kHz-range

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-mirror 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 percent. 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.

Phase conjugation for realizing lasers with diffraction-limited beam quality and high average power

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.

Phase conjugation for improvement of solid state and excimer lasers

To improve the beam quality of industrial solid state and excimer lasers, self-pumped phase conjugating mirrors based on stimulated Brillouin scattering (SBS) were applied in different oscillator and master oscillator power amplifier systems. 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. 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. The beam profile could be converted from rectangular to near gaussian and the brightness could be improved. As an alternative to widely used SBS-materials like liquids and gases we developed a new kinds 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.