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17 Jun 2002
Danish start-up MicMacMo has developed a method to monitor ablation with ultrafast laser pulses in real time.
Courtesy of Opto & Laser Europe (OLE) magazine
MicMacMo's technique uses the same pulse both to ablate the sample and to measure the ablation depth reached, and is the first to combine time-gated imaging and laser ablation.
Peter Balling of the University of Aarhus set up the firm to commercialize the method. His profiling technique uses time-of-flight optical gating and a non-collinear beam set-up to monitor the depth of ablated holes.
As the hole deepens, the portion of the pulse reflected by the sample takes longer to get to the camera. The optical gate then translates this delay into a measurement of hole depth.
"Instead of relying on subsequent analysis of the structures formed by the ablation process [such as microscopy], this technique makes it possible to do the analysis on-the-fly and to use the data as a feedback signal during machining," said Balling.
Balling and Aarhus colleague Rune Lausten have studied metals ablated using the method and have also applied it to biological materials. "The technique allows surgery of both hard and soft tissues to a predetermined depth," said Balling. The depth resolution in biological samples is 5 µm, compared with 1 µm for steel.
The technique uses an 800 nm Ti:sapphire laser to emit 100 fs pulses. It is then split into two beams, one of which is used to ablate the sample while the other is reflected through a delay line and a nonlinear crystal before reaching a camera. Any incident light that is backscattered during ablation is collected by the optical system and is focused onto the camera in the same way.
Balling said: "Because we perform the image gating with a specific non-collinear geometry, we do not need to scan the delay of the gate beam to get a depth coordinate. In other words, the depth and [size of the hole in] one transverse direction are obtained for each individual laser pulse."
In practice, the repetition rate of the laser is faster than the frame rate of the camera, so that each image captured is actually an average that is "smeared" over several tens of pulses.
"Each laser pulse samples a slightly different geometry, probably because of debris left on the surface from previous pulses. This effect leads to smearing in the depth coordinate - the degree of which depends on the specific material being ablated," explained Balling.
There tends to be less smearing in hard, metallic materials than in soft biological tissue - hence the discrepancy in resolution limits. For medical applications the system could be invaluable - because all tissue specimens are inherently different, a feedback mechanism is essential to monitor treatments that use laser ablation, such as laser eye surgery.
Balling plans to integrate the profiling system, called ProfiMac 400, into ultrafast laser micromachining workstations.
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