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04 Mar 2005
US scientists have taken rejected pulses from a femtosecond laser and turned them into a real-time optical probe.
A workstation that allows real-time measurement of ablation depth while micromachining with femtosecond laser pulses is being developed by researchers in the US. The team says that its system could help avoid the laborious post-process imaging of work-pieces. (OPTICS LETTERS 30 373)
The workstation takes an unused portion of the system's unamplified laser pulse train and converts it via an interferometer arrangement into a real-time optical depth probe.
The Colorado School of Mines set-up is based on a kilohertz Ti:sapphire chirped-pulse amplification (CPA) system that generates 100 fs pulses suitable for laser ablation. A by-product of the process is a rejected high-frequency pulse train that is fed into a Michelson interferometer and recombined with the amplified beam to form an optical depth probe.
"The beauty of this technique is that it utilizes laser light that was traditionally 'thrown away'," principal investigator Jeff Squier told Optics.org. "In a CPA system you are only amplifying a single pulse out of a high rep rate pulse train - the amount of light available for a probe is enormous - and is there for free essentially."
Squier feels that, in principle, the technique would suit almost any materials processing application that uses a femtosecond laser. The team tested its in-situ method on samples of 400 nm thick aluminium film evaporated onto a glass slide substrate and confirmed measurements with an atomic force microscope.
The system works because light at the centre of the ablation beam's focal volume has to travel further into the freshly-machined micro-crater than light at its edge, producing interference fringes with obvious discontinuities. Captured by a CCD camera, these interference patterns can be interpreted to profile the depth of the ablation feature even, as the researchers discovered, in the presence of significant debris.
The group has submitted a patent application for its design and Squier hints that there may be ways of extending the method beyond just depth measurement. "The actual spectral interferogram contains much more information about the machined sample that we have yet to exploit," he said.
Author
James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.
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