Do You See What Eye See?

It’s been hard to miss the publicity for LASIK, the laser surgery that reshapes the cornea to improve the eye’s ability to focus. Actually, both the cornea and the lens focus light, as shown in the diagram. But the lens, itself mostly water, is bathed in watery fluid on both sides, so upon entering and leaving the lens, light bends, or refracts, relatively little. It refracts far more when passing from air into the cornea.

The structure of the human eye. Note that the lens is bathed in watery fluid on both sides, so its refracting power is much less than that of the cornea. (image courtesy of HyperPhysics, by Rod Nave, Georgia State University).

To demonstrate the precision of ablation of human tissue, IBM scientists cut these slots in a human hair with the excimer laser (image courtesy of IBM research).

Reshaping the cornea can make a big difference. Six months after LASIK surgery, about 95% of patients have uncorrected vision of at least 20/40, the minimum for driving, and about 50% have uncorrected vision of at least 20/20, considered to be ideal normal vision. On the downside, about 5% of patients experience side-effects and 1% suffer serious, vision-threatening problems.

LASIK is performed with an ultraviolet (UV) excimer laser. Excimer stands for excited dimer, an excited, unstable molecule of an “inert” gas and a halogen—argon and fluorine. This short-lived molecule dissociates promptly with the emission of a UV photon of a particular frequency. In an alternate process, if a photon of this frequency hits an as-yet-undissociated dimer, the dimer emits a second photon, in step with the first—a process called “stimulated emission,” the basis of laser action. With the argon and fluorine confined in a tube capped with mirrors, one of which allows some light to escape (see diagram), the result is an intense UV laser beam.

Excimer lasers, unlike the familiar ones in bar-code readers, are pulsed—they pack their output into short bursts about 10 nanoseconds long (10-8 sec). This pulsing makes it ideal for eye surgery, because the intense pulses vaporize tissues without heating the rest of the eye. The UV light is absorbed in a very thin layer of tissue, decomposing that tissue into a vapor of small molecules, which fly away from the surface in a tiny plume. This happens so fast that nearly all of the deposited heat energy is carried away in the plume, leaving too little energy behind to damage the adjacent tissue. The process is called ablation, and its application to surgery was an invention of IBM physical scientists. To see its precision, look at the image of the slots cut by an excimer laser in a human hair. Subsequently, the IBM scientists collaborated with ophthalmologists, giving birth to laser refractive surgery.

Ablation of the outermost layers of the cornea and its covering can produce a number of visual problems after the surgery. To avoid this, the ophthalmologist first shaves a thin slice of the outer corneal tissue, folds it back (see photo), and then with the laser ablates the underlying cornea to produce the required shape. When the flap is folded back in place—no sutures are necessary—the two corneal surfaces grow together and the eye usually heals within a week or less. The drawback of this procedure is that cutting the flap is responsible for most of the side-effects.

To reduce these problems, an interdisclipinary team at the University of Michigan is working with the femtosecond laser, whose pulses last only 10^-13 seconds.

Schematic diagram of laser action. Each wiggly line represents a photon. Note how the number of photons increases through stimulated emission along the path of the original photon. (image courtesy of HyperPhysics, by Rod Nave, Georgia State University).