Re: realistic minimum spot sizes for lasers
- From: LukeCampbell <lwcampbe@xxxxxxxxxxxxxxxx>
- Date: Mon, 31 Oct 2005 11:22:01 -0500
Werevole wrote:
I've been playing around with a spreadsheet today and noticed thatYes, it can. It is not difficult to make lasers that can vaporize small quantities of any material at close ranges. Modern research lasers easily produce beams that can instantly flash small quantities of any matter into a plasma hotter than the core of our sun (I am thinking of femptosecond duration pulsed titanium sapphire lasers, emitting pulses between 1 milijoule and 1 microjoule energies and with repitition rates on the order of kilohertz. These things are available for a few thousand dolars and easily fit on a tabletop). Note, however, that the maximum amount of material you can vaporize has an upper limit set by the total amount of energy delivered, not the energy per area (I assume you are using this laser for military purposes, from the rest of your post). Drilling a very narrow hole through objects in a short amount of time is difficult, if that is what you are thinking of, since the vaporized stuff gets in the way of the beam, the light-matter interaction tends to form a plasma which absorbs the beam, and non-cooperative targets have an annoying tendancy to move around, throwing your aim off of the hole you are drilling it in. My best guess is that there is a way to do this with a rapidly pulsed beam delivering hundereds or thousands of about 1 joule pulses in less than a millisecond, but this has never been demonstrated so my guess could well be way off.
when a lasers beam diameter is sub-millimeter (usually within a few
meters of the lens) the energy per area can be disturbingly intense.
Is there a realistic minimum (due to material heat limits,With good optics at close focus, you can get spot sizes approaching the wavelength of the light in the beam. This is with today's technology - spot sizes of around a micron for visible light. Of course, this is only for focusing at a range of around one lens diameter or so.
technological capability, etc...) a laser spot size has to adhere to?
You can start getting into limits if the intensity gets too high. Once the air starts to ionize it acts as a lens to defocus the beam. On the other hand, at intensities close to but not quite at the ionization limit you can get non-linear effects acting to focus the beam more rapidly. The combination of these two effects can lead to "self focused" or "self trapped" beams - fillaments of light and plasma that propagate long distances through an atmosphere with, if I recall correctly, beam diameters of about a mm per fillament. Intense beams tend to break up into several co-propagating fillaments, however.
Otherwise, at long ranges, you can get close to the diffraction limit with good optics. The diffraction limited spot size is about 1.3 x (l/D) x R where l is the wavelength of the light, D is the diameter of the final focusing optics, and R is the range from the focusing optics to the point you are focusing the laser at. Real life mirrors never actually get exactly this good, but it is close enough for estimating the spot size. For practical purposes you can multiply by another factor of 1.2 to 2 depending on how good your mirror is. This is assuming good adaptive optics, otherwise non-uniformities in the atmosphere can cause the beam to spread more.
Modern military lasers tend to have wavelengths of 1 micron (or about 4 microns for older laser technologies). A focal mirror of roughly a meter is typical of the MTHEL is roughly a meter in diameter, so it could acheive 5mm at 4 km and 0.5mm at 400 m - reasonable engagement distances (the MTHEL actually used a 4 micron deuterium fluoride laser, so that would be 5mm at 1 km and 0.5mm at 100 meters). For practical purposes, these are the sorts of spot sizes you will see unless you start using lasers for close range combat.After a bit of Googling I found a range of theories, some saying a laser (in atmosphere, and for military purposes) is usually not less than 5mm, others say 0.5mm, 1mm or no minimum size (other than what is dictated by wavelength and lens radius).
Luke .
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