How is laser penetration controlled?
May 16, 2012 2:27 AM   Subscribe

How is a laser beam controlled so that it only penetrates a fixed distance into a soft material that it could normally pierce right through?

In laser eye surgery, one of the ways to correct the lens shape is to first drill a series of holes a few microns deep in a circle at the front of the eye and lift the circle of perforated tissue (like separating a postage stamp from a sheet) and then using a laser to reshape the exposed surface.

After that, the circle is replaced and being very thin, it sticks to the newly formed shape beneath.

My question is, how is the laser controlled so that it only penetrates a few microns deep as it 'cuts out' the circle rather than the beam just penetrating further into the eye?
posted by Quillcards to Science & Nature (7 answers total)
The laser beam cuts the tissue by essentially depositing enough heat energy on a tightly focused spot to burn it off. The amount of energy delivered depends on power output and time applied, so they may be controlling either of the two factors.
posted by Dr Dracator at 2:49 AM on May 16, 2012 [1 favorite]

The one I saw in use was controlled with a trigger. Each pull delivered only 2-3 subsecond pulses of power; every single "shot" was computer controlled to deliver an exact amount of energy. The doctor inspected the results for a few seconds between each fire. Typically he'd hit the same spot at least a half dozen times before it was at the depth he wanted.
posted by ceribus peribus at 2:56 AM on May 16, 2012

The tissue removal is achieved by ablation. This is vaporisation of the tissue with a short (~10nanosecond), high-power pulse. The brief duration of the pulse means that the total energy of the pulse is low, which allows a small volume of tissue to be removed without heating and damaging the surrounding tissue. See the description here at physics central.
posted by Jakey at 5:33 AM on May 16, 2012

I believe the focal point of the beam has something to do with it too. Sort of like burning holes in leaves with a magnifying glass. The energy of the laser is only concentrated enough at the focal point to do any damage.
posted by gjc at 6:18 AM on May 16, 2012

@gjc and @Dr Dracator

I hadn't thought about the possibility of lasers being able to be focused. I imagined they were just beams like the one that ran up between James Bond's legs.

So focusing sounds like it might be the answer.
posted by Quillcards at 6:28 AM on May 16, 2012

The key is the use of an excimer laser:
The ultraviolet light from an excimer laser is well absorbed by biological matter and organic compounds. Rather than burning or cutting material, the excimer laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning. Thus excimer lasers have the useful property that they can remove exceptionally fine layers of surface material with almost no heating or change to the remainder of the material which is left intact.
posted by exogenous at 6:30 AM on May 16, 2012

@exogenous Thank you. I read further in the Wikipedia entry:

"The high-power ultraviolet output of excimer lasers also makes them useful for surgery (particularly eye surgery) and for dermatological treatment.

Excimer laser light is typically absorbed within the first billionth of a meter (nanometer) of tissue."
posted by Quillcards at 6:46 AM on May 16, 2012

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