Laser Materials Processing: Pulsed CO2 laser increases ablation rates for improved glass processing

Ablation of fused silica and other glass materials can be accomplished using CO₂ or ultrafast lasers. However, the low peak power of typical Q-switched CO₂ lasers (<50 W) had previously limited ablation rates to allow only minor form correction and surface treatment of optical materials. But a more-powerful Q-switched CO₂ laser process being developed by researchers at the Fraunhofer Institute for Laser Technology (Fraunhofer ILT; Aachen, Germany) is changing this picture, enabling bulk glass to be converted to sophisticated, complex, and even freeform shapes in a three-step process of ablation, surface polishing, and—finally—selective laser-based form correction.¹

Pulsed and powerful

Compared to conventional CO₂ lasers, Fraunhofer ILT uses a 200 W average power, 40 kW peak power, m² = 1.33, Q-switched pulsed CO₂ laser with a pulse duration ≥250 ns and maximum repetition rate of 150 kHz. Because of the use of two acousto-optic modulators, laser radiation can be delivered in two modes: a Q-switched pulse (mode 1) for ablation and a modulated pulse (mode 2) for final surface correction (see figure).

Using a scanning strategy that delivers pulse power P_avg with duration t_pulse and repetition rate f_rep with focus diameter d_s across the bulk glass material at speed v_s and pulse distance d_x, an ablation depth of z_abis achieved using mode 1 operation. Experimental calculations demonstrate material removal rates of 2.35 mm³/s, allowing fabrication of rough glass optics such as a honeycomb structure (for weight reduction) on fused silica or even freeform optics. White-light interferometry measurements of the initially ablated optical surfaces allow optimization of repetition rates and fluence values of the laser to maximize material removal.

The high roughness after the ablation process is reduced in a second laser-based process step. Using continuous-wave CO₂ laser radiation, the surface is smoothed by a remelting process and the micro roughness can be reduced to <0.1 nm (see http://dx.doi.org/10.2351/1.4974905).

For the reduction of residual waviness, a third process for correction polishing is necessary using rectangular pulses and only 50 W power levels. This process limits ablation to lower material removal rates, allowing selective figuring of the glass surface in a final error-correction step called Laser Beam Figuring. By controlling the pulse duration of each laser pulse, glass material can be ablated selectively with ablation depths down to 3 nm. The lateral resolution of the ablation process is 100 μm and the vertical resolution is approximately 3 nm.

"The laser-based process chain enables the fabrication of complex-shaped optics made of fused silica with only one laser source," says Christian Weingarten at Fraunhofer ILT. "The surface quality of the fabricated optics is already sufficient for illuminating optics, but with the development of the laser-based correction polishing process, we will fabricate even higher quality illumination optics in the future."

REFERENCE

1. C. Weingarten et al., Appl. Opt., 56, 4, 777–783 (2017).