MOPA with kW average power and multi MW peak power: experimental results, theoretical modeling and scaling limits

A Master-Oscillator-Power-Amplifier (MOPA) design combining rod and slab laser technology for high pulse energy, high average power and near diffraction limited beam quality for industrial use has been developed. To achieve the good beam quality at high average and high pulse power, an advanced birefringence compensation scheme, which ensures a high mode overlap while simultaneously minimizing the power densities on optical surfaces, has been developed and applied. The prototypes deliver an average power of up to 860 W with M-2 < 2 or 1.3 M with M-2 < 12 at 10 kHz repetition rate and 5-16 ns pulse duration. At 1 kHz up to 420 mJ pulse energy can be achieved. The prototypes are fully computer controlled and can be operated from 0 to 100% output power and from single shot to 10 kHz. They are currently operated for plasma generation in a laboratory surrounding and have run for more than one thousand hours without failure up to now. An analytical solution of the thermally induced refractive index profile in dependency of a radially symmetric pump light distribution including the effect of thermally induced birefringence, temperature dependency of the thermal conductivity and the second derivative of the refractive index with the temperature (d(2)n/dT(2)) has been derived. This allows a fast calculation of thermally induced aberrations without the use of FEA. Experimental results are compared to predictions from analytical and FEA modelling. Based on experimental and theoretical results, scaling limits of rod based MOPAs are predicted.