"How do these lasers work?"

Excited Dimer = Excimer

The excimer laser is an ultraviolet Argon-Fluoride laser at 193 nano metres (nm) It is a gas laser and requires refilling on a regular basis. The argon is an inert gas whereas the fluorine is highly excitable.

The excimer laser has a photon energy of 6.4 electron volts. The carbon-carbon links of the corneal structures are 3.5 electron volts. Each laser pulse lasts 18 nano seconds and takes off about 0.25 microns of corneal tissue with each pulse.. Hence, when the laser hits the cornea, it "blows apart" the carbon-carbon links and the noise that one hears is the particles from the cornea breaking the sound barrier as they come off the cornea in a mushroom cloud shape similar to an atomic bomb.

For a -3 Dioptre laser correction, the laser will take off about 40 microns. (there are 1000 microns in 1 mm and the cornea is about 550 microns thick)

 Different lasers have different repetition rates. Hence the VISX laser, made in the USA and popular there, fires a broad beam at 10 Hz (10 times a second). The Autonomous laser uses a small 1mm diameter spot and fires at 200 Hz. There is no evidence that smaller spot lasers give better results, although for custom ablations all the manufacturers add in a small spot for fine sculpting. However, small spotted lasers take longer to work and this can have problems in that the cornea may dry out in this time and ablate in an irregular way.

The fact that all the energy of the ablation is taken off the cornea in this supersonic plume means that the temperature of the cornea hardly rises at all and this means that there is virtually no "collateral damage" due to thermal effects on adjacent tissue. Hence the excimer laser gives a very keen cut. Below is a lasered hair of Professor Marshall from St Thomas's Hospital in London, who did some of the pioneer work with Professor Steven Trokel, the inventor of PRK.

This is a graph of corneal temperature change during a 20 second ablation of a human cornea. Normal corneal temperature is about 30-33 C and it rises to about 37 C, which is blood heat.

Finally, a cartoon sent by an optometrist friend of mine!