Towards high brightness multi-kilowatt solid state lasers

Abstract

HIGH average power (HAP) solid state laser output with improved beam quality has introduced new capabilities in materials processing. At the 500 watt level and with a beam quality of a “few” times the diffraction limit, the General Electric Nd:YAG slab is able to drill 5 cm of stainless steel in a few seconds. We expect that 2-3 kW of near infrared laser output in a low order spatial mode would enable metal working now unknown to industry. The HAP output of slab lasers is limited by the size of the available laser crystals and the pump power. Core free, six cm diameter Nd:YAG boules have been grown by Allied Signal Corp. on an experimental basis. High optical quality Nd:GGG can be obtained up to 10 cm in diameter. We present the results of our modeling based on these crystals pumped by advanced arc-lamps or laser diode arrays. We project HAP laser outputs of 1.6 kW from an existing (but un-tested) Vortek pumped 18 × 7 × 0.5 cm3 Nd:GGG oscillator and about 2 kW from a 20 × 4 ×.7 cm3 diode pumped Nd:YAG device. Several kW of laser output can be expected from two such slabs in a MOPA configuration before optical damage limits are reached. The three dimensional stress-optic code (TECATE) which we used to optimize our designs, was normalized to available experimental data obtained with the above Nd:GGG slab at the 500 Watt level and a 40 Watt diode pumped Nd:YAG test bed. Our calculations indicate the essential parameters for attainment of high beam quality. Cooling uniformity across the pumped faces of the slab is critical and the location of the transition between pumped and un-pumped regions towards the slab tips is very important. A flat pumping profile was found to be desirable and predicted one wave of distortion which should be correctable over about 75% of the aperture however, an even better wavefront was predicted over 90% of the aperture when the regions near the edges of the slab were slightly over-pumped relative to the central regions and the regions near to the ends were tapered to compensate for transition effects. The challenge is now to design reflectors and diode pumping arrays to provide the desired pumping flux profile on the face of the slab with less than 1% uncertainty. Our spectral measurements of the VortekTM lamp show this lamp to be a superior HAP pumping source for slab laser designs above one kW. Our lamps have been optimized for efficient pumping by pulsing them at the best current density. As production costs come down, however, laser diodes will gradually replace lamps in high brightness solid state lasers for industrial materials processing.HIGH average power (HAP) solid state laser output with improved beam quality has introduced new capabilities in materials processing. At the 500 watt level and with a beam quality of a “few” times the diffraction limit, the General Electric Nd:YAG slab is able to drill 5 cm of stainless steel in a few seconds. We expect that 2-3 kW of near infrared laser output in a low order spatial mode would enable metal working now unknown to industry. The HAP output of slab lasers is limited by the size of the available laser crystals and the pump power. Core free, six cm diameter Nd:YAG boules have been grown by Allied Signal Corp. on an experimental basis. High optical quality Nd:GGG can be obtained up to 10 cm in diameter. We present the results of our modeling based on these crystals pumped by advanced arc-lamps or laser diode arrays. We project HAP laser outputs of 1.6 kW from an existing (but un-tested) Vortek pumped 18 × 7 × 0.5 cm3 Nd:GGG oscillator and about 2 kW from a 20 × 4 ×.7 cm3 diode pumped Nd:YAG dev...