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Lasers are used in various industrial processes that need intense, localised, heat (3d printers and laser cutters come to mind).

My question is: why use lasers? There are many other (cheaper, brighter) light sources. There are even other monochromatic and coherent light sources (LEDs and mercury vapour lamps respectively), and this video shows someone sintering desert sand using a Fresnel lens and sunlight, which is of course neither monochromatic nor coherent.

So, what is it about lasers that make them so better than a conventional light source combined with appropriate focusing elements?

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You have to look at each, individual use and ask yourself "Which properties are needed here, which are convenient and which superfluous?" and then compare to the things you suggest are alternatives (this is, after all, what the engineers who made those choices did). And recall that "laser" covers a lot of ground ... –  dmckee Apr 24 at 16:06
    
Because lasers are awesome. –  Adam Davis Apr 24 at 19:48
    
@AdamDavis The Sun is awesome too. –  Blackbody Blacklight Apr 25 at 3:50

3 Answers 3

up vote 13 down vote accepted

If you want to make a small very-hot spot, spatial coherence is important. Contrary to what you say, the sun has quite high spatial coherence (not as high as a laser, but higher than most other bright light sources). That's why you can focus sunlight very well. If you focus sunlight perfectly, you get a spot the shape of the sun. That spot would have the same light intensity as if you were standing on the surface of the sun, looking down.

If you perfectly focus the light from an incandescent light bulb, you can get a spot shaped like the tungsten filament. That spot would have the same light intensity as if you were a tiny person standing on the surface of the tungsten filament, looking down. (This intensity is much lower than the the sun's).

An extreme example of low spatial coherence is the blue light from the blue sky. You cannot use a lens to focus that light into a bright blue spot on the ground. Try it! This blue light has almost no spatial coherence, which means you cannot focus it. There is plenty of blue light coming at you, but it already has as much intensity as it is capable of having.

This is quantified by the law of conservation of etendue. Light starts out with a certain radiant intensity, and then it can never be increased, no matter what kind of lenses or mirrors you use.

Lasers can have far higher radiant intensity than any other light source. It's not just how many watts they emit, it's how they emit it -- with high spatial coherence, which means it can be focused very effectively.

Lasers come in all shapes and sizes, and intensities, and wavelengths, and form-factors, and prices. Red diode lasers cost a few cents each. Other lasers cost $100,000 or more. Lasers are basically a generic way to create coherent (and therefore high-radiant-intensity) light, in many different systems. So if you need a high-radiant-intensity light source, the best one for the job is quite likely to be a laser.

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To be an answer, more sources should be considered. Wikipedia says arc flashes can heat a small region of space up to ~20 kK, I'm sure the principle could be harnessed with an appropriate voltage source. Optical elements compatible with such a wavelength are another story. There must be a trade-off happening between power density at the source, temperature, and laser efficiency. –  Blackbody Blacklight Apr 25 at 4:12
    
Thanks! "Etendue" is definitely one of the things I was missing. It still leaves me with some conceptual problems however: consider an observer on the surface of the sun, with some mirrors above them covering the "sky" from their point of view, filling the sky with more sunlight. Would they not be subject to higher radiant intensity at this point? (I have learned to ignore my intuition, because it is too easily mislead). To put it another way, if one light source can't be focused to sufficient intensity, what about combining multiple sources? –  BenRW Apr 25 at 13:41
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@BlackbodyBlacklight -- Yes, various flashtubes and discharge and arc lights have much higher intensity than focused sunlight (although still not nearly as much as a powerful laser). These have plenty of applications. But lasers are much more diverse. For example, for thermal emission, you get higher intensity by increasing temperature, but that also tends to decrease the wavelength towards UV. But lasers can also give you high-intensity infrared light, if that's what you want. And lasers can be solid, liquid, gas, low-voltage, high-voltage, ... –  Steve B Apr 25 at 17:34
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@BenRW -- You're right, I should have said "on the surface of the sun with a mirror overhead", for the maximum theoretically-possible intensity. I was wrong by a factor of two. However, if we're talking about laser cutters, you can't usually heat an object by illuminating it from all directions at once. If it's the surface of an object, you can only access it from one hemisphere obviously. And usually you illuminate it from a much smaller angular range than that. (A high-numerical-aperture lens would be impractical in a laser cutter for many reasons!) –  Steve B Apr 25 at 17:43

The spatial coherence from the source to the needed location allows the use of more compact, lighter weight optics, and makes it relatively easy to take the light from a very bright source and bring it to the target.

If you require 5W of heating at a spot smaller than a mm square, you would need to bring the 5W incoherent light source near the target, and use a series or lenses much larger than the target to bring the light to the target. If you instead need to move 5kW of light that far, you're going to have that much more difficult a problem.

But let's say the target can't be close to the source. Since the light isn't coherent you get to use a series or mirrors and lenses that are very large, or you have to use very, very many of them, in order to contain the light in the desired path.

By comparison, the low natural divergence of the laser is very easy to move and control with relatively small mirrors and optics, and is easy to focus down to an area much smaller than 1mm square if needed.

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I don't follow this answer. Even though it is coherent isn't it typically focused through a lens? And once an incoherent source has been focused and straightened how is it different from a focused laser? I.e., can you explain how coherence distinguishes these two scenarios? –  feetwet Jul 18 at 4:11

There are laser diodes - have a look at your CD player - so, LEDs aren't strictly an alternative to lasers, they are just one type of laser generating device. The main feature of a laser is the high energy density. Also, lasers have a better efficiency than e.g. light bulbs. Sunlight is in fact a good light source, but what do you do on a rainy day?

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This is not quite accurate. LEDs and laser diodes are different devices, because the light of the former has a much lower spatial coherence. Only the latter relies on stimulated emission. –  Emilio Pisanty Apr 24 at 18:19

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