Excimer laser induced shock wave and its dependence on atmospheric environment

High speed shadow photography is performed on excimer laser ablated porcine corneas and rubber stoppers to capture the excimer laser induced shock waves at various time delays between 40 and 320 nanoseconds. The shock waves in air, nitrogen and helium are recorded by tangentially illuminating the ablated surface with a tunable dye laser the XeCl excimer laser pulse. The excimer laser ablates the specimen and excites the dye laser, which is then passed through an optical delay line before illuminating the specimen. The shadow of the shock wave produced during ablation is then cast on a screen and photographed with a CCD video camera. The system is pulsed at 30 times per second to allow a video recording of the shock wave at a fixed time delay. The velocity of the shock wave in air when ablating a rubber stopper at 1000 mJ/cm² reaches 3.3 km/s at 40 nsec and decreases to 1.4 knVs at 320 nsec. When helium is blown on the target, the shock wave velocity increases by a factor of approximately two, to 5.9 km/s at 40 nsec and 2.7 km/s at 320 nsec. The increase in shock wave velocity in helium during corneal ablation is similar. We conclude that high energy acoustic waves and gaseous particles are liberated during excimer laser corneal ablation, and dissipate on a submicrosecond time scale. The velocity of their dissipation is dependent on the atmospheric environment and can be increased two-fold when the ablation is performed in a helium atmosphere. Therefore, local temperature increases due to the liberation of high energy gases may be reduced by using helium during corneal photoablation.