The free electron laser as the ideal stress-wave generator

Laser-induced thermal-elastic stress waves are of importance both in therapeutic applications and in potential morbidity associated with laser surgical procedures. Recent experiments have shown that drug cytotoxicity can be enhanced with stress waves and that direct cell injury correlates with the stress gradient (stress rate of change). To systematically investigate the biological effects of stress waves, it is essential to vary individually the parameters of the stress wave. The Free Electron Laser (FEL) is the ideal laser for generating controllable stress waves. A unipolar stress wave can be characterized by its rise time, duration, peak pressure, and decay time. For short laser pulses, the rise time and decay time are dependent upon the absorption depth of the tissue and can be varied by changing the wavelength of the FEL. The duration of the stress wave can be changed by selecting a different number of micropulses from the FEL macropulse with a Pockels cell. The peak pressure can be altered by varying the laser intensity. Results on water have confirmed that the individual parameters of the stress wave can be varied independently.