Ultrafast Setup Machines and Measures

Hank Hogan

Researchers at the University of Aarhus in Denmark have developed laser techniques for manufacturing and measuring on the microscopic scale. The method uses the same femtosecond laser pulses to ablate material and to profile depths on the fly.

Peter Balling, an associate professor at the university's Institute of Physics and Astronomy and a collaborator on the project, said the technique could have an impact on laser micromachining and surgery. "With the depth-profiling technique, a cross-sectional profile is available online, which can be used as a feedback signal for optimization of the laser drilling and cutting process," he added.

In their setup, the researchers used a CPA-1000 amplified Ti:sapphire laser from Clark-MXR Inc. of Dexter, Mich., to generate a string of 100-fs pulses of 800-nm light at a 1-kHz repetition rate. These ultrafast pulses struck a beamsplitter, with half of the output focused through a lens onto the sample being machined. The other half followed a detour to a nonlinear optical crystal.

The pulses from the first leg vaporized the surface of the sample, and the backscattered light from the resulting plasma intersected with the other beam at the crystal. The interaction of the two with the crystal produced a third stream of photons at half the wavelength of the original.

A CCD camera from Koninklijke Philips Electronics NV collected this light, and a frame grabber made by Data Translation Inc. of Marlborough, Mass., digitized the signal, which revealed the depth profile of the sample.

Because the measurements are performed with short-duration pulses, the resolution of the depth profile is quite good. The resolution ultimately depends on the material, with stainless steel yielding 1-µm resolution and biological materials approximately 5 µm.

The technique is being used for ablation studies of various materials, and Balling has formed a company, MicMacMo, to commercialize the setup. The resulting device may be a stand-alone unit, he said, but it ideally will be integrated into a femtosecond-laser-based micromachining workstation.