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Name: Eki
Status: other
Grade: other
Location: NJ 
Country: N/A
Date: 3/30/2005


Question:
If you can burn through thick steel with a laser, how come a thin sheet of glass that is actually a mirror can reflect the same laser beam?


Replies:
It is not magic, it is just design considerations you have not heard mentioned yet.

To burn, the subject must get hot. To get hot, the subject must absorb the light, (that happens to be arranged in a beam-like shape, and made by a laser). Perfect ideal mirrors do not absorb any light, so they do not get hot, and they will not burn through.

Many mirrors and shiny metals do not absorb much until they get hot and darker-looking. Then they absorb more and get hotter. Eventually a vapor plume rises, and the laser light shakes electrons loose, and this plasma plume absorbs all the light, keeping the plasma furiously hot. It is a kind of ignition. Even with the powerful laser beam steady on the object, sometimes a source of ignition must be found, or nothing happens.

Real mirrors are never perfect; they always absorb some small percentage of the light. If that is enough to start damaging the mirror, then the mirror will be cracked, melted, vaporized much like any metal. People go to some expense to get very good low-absorption mirrors designed not to be damaged by high-power laser beams. There is always some "damage threshold" specification to such mirrors. If you used a common bathroom mirror, it would probably be no more impervious than an ordinary metal sheet.

For laser beams of visible light, Steel is often only about 70% reflective, even when it is polished to its best shine. As a mirror, it is a slightly darkened, grayish mirror. Aluminum is often closer to 90% reflective. Often it is harder to cut aluminum with a laser than to cut steel, even though aluminum melts at a much lower temperature than steel.

Metal generally absorbs more than 1% of any visible light that falls on it. This is a problem. Dielectric mirrors do not have any metal. They are a multi-layer stack of dielectric substances with alternating high/low index of refraction. If this alternation matches wavelength of the light, it convinces 99% of the photons to be reflected. The remaining 1% mostly go on through in their original direction. A much tinier fraction, less than 0.1%, is absorbed. So dielectric mirrors are often used to redirect intense lasers.

For infrared lasers such as CO2 lasers, it is harder to find non-absorbing materials for dielectric mirrors. So people have used polished Copper with water cooling inside. Copper and Silver are the most conductive substances for electricity, so the infrared waves are more than 99% reflected. They are also among the most conductive substances for heat, and the industrial-strength water cooling is only 1mm away. So those mirrors can be a little tougher to ignite than the toughest metal work piece.

Another important trick is: Focus. If the laser beam is 2" wide when it hits the mirror, it cannot burn much of anything. The mirror is curved (concave), so the beam is then focused to a 1 millimeter pinpoint on the work piece. There it has 2500 times higher power density, so it is much more likely to exceed the damage threshold of any surface.

Jim Swenson



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