Stefan Seidel

Optimized resonators for high-average-power high-brightness Nd:YAG lasers with birefringence compensation

Transversely cooled, cylindrical Nd:YAG laser rods exhibit a strong thermally induced birefringence when they are optically pumped. The birefringence leads to different refractive indices for radially and tangentially polarized light which affects the beam-quality and the degree of polarization achieved with conventional optical resonators. In spite of this drawbacks the traditional rod-geometry for Nd:YAG lasers is still of great interest because of its technological simplicity, comparatively low costs and acceptable efficiency. In the present study the limitations for the output power of single rod Nd:YAG lasers in TEM00- operation is investigated theoretically and experimentally. Furthermore the possibility of improving the brightness of Nd:YAG rod lasers beyond the traditional limits using dual rod systems with birefringence compensation is demonstrated. Birefringence compensation is established by image-relaying between two identical Nd:YAG laser rods and an additional 90 degree(s) polarization rotation. This compensation method requires specific dynamically stable optical resonators in order to achieve optimal compensation and high efficiencies. With a dual rod Nd:YAG-system up to 80 Watt TEM00 output power is achieved with a beam quality of M2 equals 2.8. The output power is approximately 8 times higher than using a single rod-system separately.

Numerical modeling of thermal effects in nonlinear crystals for high-average-power second harmonic generation

The influence of thermally induced phase-mismatch in nonlinear crystals for frequency doubling caused by absorption of laser power is described. A numerical model is developed, which considers the spatial temperature distribution in the crystal and the corresponding wave-vector mismatch. For the temperature profile an approximate analytical expression is derived from the the heat-transfer equation in cylindrical symmetry. The conversion efficiency is calculated by solving the basic differential equations for frequency doubling with a spatial dependent wave-vector mismatch. Because the absorption coefficients are rather different at the fundamental and second harmonic wavelength, the heat-density in the crystal depends on the conversion efficiency and vice versa. Therefore an iteration method has to be used to calculate self-consistent solutions. Experimentally a Q- switched oscillator-amplifier-system with an average power of 200 W at 1064 nm is frequency doubled to an output of 103 W at 532 nm using a KTP crystal. The numerical calculations are in good agreement with the experimental results.

High-power Nd:YAG laser with birefringence compensation and adaptive HR mirror

In this article we describe an efficient method of the compensation of thermally induced birefringence in high power Nd:YAG rod lasers. Losses at a resonator internal polarizer due to depolarization in the rods are reduced, and a polarized cw laser with an output power of 300 W is realized. Resonators with a small stability range and therefore beam quality near the diffraction limit are introduced. To improve the variability and stability of such resonators an adaptive high reflecting mirror is used as resonator mirror. The maximum average output power of the resonator with adaptive mirror is 145 W at a beam quality of M2 equalsV 2.

Influence of mechanical stress on the conversion efficiency of KTP and LBO

The efficiency of high-average-power frequency doubling is always limited by thermal effects inside the nonlinear crystal. Usually it is assumed that the limitation is caused by a change of the index of refraction due to the temperature variation inside the crystal. This effect is characterized by the temperature acceptance of the nonlinear crystal. Additionally, the temperature distribution inside the crystal produces thermally induced mechanical stress that leads to phase mismatch due to the photoelastic effect. Depending on the sign, this effect will decrease or increase the conversion efficiency. In order to investigate the influence of stress- induced phase mismatch independently of phase mismatch caused by a temperature variation on the conversion efficiency, KTP (Type II) and LBO (Type I) crystals for second-harmonic generation (SHG) of 1064 nm radiation were temperature- stabilized and mechanical stress was applied along different crystal axes. The conversion efficiency of a weak probe beam was measured as a function of the stress.

High-average-power frequency conversion with phase-conjugated laser systems

High average powers at 532 nm are of great interest for material processing applications because the absorption of metals and other materials increases with shorter wavelength. As the fundamental laser source we developed a Q-switched Nd:YAG oscillator-amplifier system with an average output power of 215 watts and a beam parameter product of less than 1 mm*mrad. The system operates with 100 Hz repetition rate and emits a burst of twenty Q-switch pulses during each flash-lamp pulse. The pulse duration of a single pulse increases from 100 to 200 nanoseconds with increasing pumping power. The high beam quality of the amplified beam is realized by a double-pass configuration of the amplifier with a phase conjugating SBS-cell. Additionally the thermally induced birefringence of the amplifiers is compensated for by a 90 degree-rotator between two identical amplifier rods. An output power of 164 W is frequency doubled in a KTP-crystal to 82 Watt average output power in the green which corresponds to 50% of doubling efficiency.

Measurement of the absorption of nonlinear crystals used for high-average-power frequency doubling

The absorption coefficients of nonlinear crystals for fundamental and second harmonic wave are of great importance for high average power second harmonic generation. A practical method to measure low absorption coefficients for high average power second harmonic generation. A practical method to measure low absorption coefficients is to use an interferometric laser calorimeter with high power lasers. Therefore Q-switched Nd:YAG laser systems with intracavity second harmonic generation are used. The measurements are made with optical powers up to 300 W and 45 W, respectively. Because of the high power, the resolution limit for the absorption coefficients is 0.001 percent/cm. The absorption coefficients of KTP and LBO crystals of different manufacturers are determined. The results are used for a numerical model which takes into account the decrease of conversion efficiency due to thermal effects caused by the absorption of laser power in the nonlinear crystal. This model describes saturation effects which appear in the range of 100 W in the green using a KTP crystal. A new idea for compensation of thermal effects will be presented.

High-power blue laser source based on frequency-converted 1.3-μm Nd-lasers

Up to 0.75 Watt of blue laser light have been generated by second harmonic generation and additional sum frequency mixing of a 1.3 micrometer actively cw-modelocked Nd:YAG laser oscillator. Special attention has been paid to the design of the fundamental mode resonators, delivering output powers of up to 8 W at the fundamental wavelength with nearly diffraction limited beam quality. To overcome the thermally induced depolarizing birefringence a compensation scheme has been applied for further enhancement of the laser power.