09 Aug 2002
Australian researchers say that frequency-doubling a femtosecond laser is the way to pain-free dentistry.
Femtosecond pulses hold the key to lasers replacing the dentist's drill, according to a study by scientists in Australia.
Using two different Ti:sapphire sources, Andrei Rode and colleagues found that the 400 nm second harmonic gave the best results (Journal of Applied Physics 92 2153).
Now widely used in many surgical procedures, lasers have long been touted as pain-free replacements for the dreaded dentist's drill. However, the hardness of dental tissue has demanded high-energy pulses for drilling. Previous attempts have resulted in unacceptably slow removal of tooth enamel, and have also damaged teeth.
These previous efforts were made with relatively long pulsed sources, which cause shock waves, vibrations and also heat up the tooth's softer tissue, causing significant pain to the patient.
Because femtosecond pulses are so short, heat conduction effects are virtually eliminated.
The Australian team, which comprised groups at MacQuarie University, the Australian National University and the Swinburne University of Technology, experimented with two sources - a 780 nm Clark-MXR CPA-2001 laser emitting 150 fs pulses, and a Spectra Physics Tsunami system generating 95 fs pulses at 805 nm. The latter was also frequency-doubled to determine any difference in ablation threshold between the two wavelengths.
At the fundamental wavelength, the ablation threshold was 2.2 J/cm2, and half this when frequency-doubled. No collateral damage to the drilled teeth was seen. After three minutes of laser ablation, a rise of 10 ° C in dental tissue was noticed, but air-cooling maintained the temperature below the damage threshold of 5.5 ° C.
With the 150 fs laser, the ablation rate was about 1 µm per pulse. To approach the ablation rate of a mechanical drill would therefore require a pulse repetition rate of 200-500 kHz, whereas the Clark-MXR source operates at only 2 kHz.
Rode and colleagues say that as the dental tissue below the enamel is significantly softer than the hard outer shell, the required repetition rate is more likely to be around 20 kHz.
The Australian team is convinced that the frequency-doubled beam offers the best option: "The improved contrast ratio, together with the reduced [ablation] threshold fluence are clear indications in favour of using the second harmonic," they say.
Michael Hatcher is technology editor of Opto and Laser Europe magazine.