how to make a laser?

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Post by pixie » Fri Aug 08, 2008 10:57 am

@v_mulligan:
:D My 'laser' shares the same principle

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Post by CTZn » Fri Aug 08, 2008 1:18 pm

It would be a little problematic, though, since I only know how to calculate the focus for a spherical lens at the moment...
Do you think you could use the <sphere> primitive, say 2 spheres: one made of glass and the other having the <null> mat ? you could draw perfect lenses that way ?
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Post by v_mulligan » Fri Aug 08, 2008 1:27 pm

CTZn wrote:
It would be a little problematic, though, since I only know how to calculate the focus for a spherical lens at the moment...
Do you think you could use the <sphere> primitive, say 2 spheres: one made of glass and the other having the <null> mat ? you could draw perfect lenses that way ?
By "perfect" I'm not talking about the imperfections from a polygonal mesh. I'm talking about the fact that a spherical surface results in spherical aberration -- misdirection of light that hits the surface at a glancing angle, so that even if you have a tiny emitter at the focus of the lens, not all of the rays coming out the other side are parallel.

But yes, the sphere primitive probably could be used to make lenses with an arc-shaped cross-section.

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Post by CTZn » Fri Aug 08, 2008 1:36 pm

Ah yes, you where talking about spherical aberration three times already, I understood now :roll:
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Post by Lord of the Rings Junkie » Tue Jan 27, 2009 4:19 am

I haven't read this entire thread, but I think that I should point out that if we are aiming for realism it does not mean a beam with perfectly parallel convergence. All lasers in real life spread out, although the angle can be made to be very small with proper collimation optics.

In fact, a typical laser beam diameter grows by about 1mm per meter of length.

Also, a laser cannot be simulated by any light calculator such as Indigo by building a diode since a laser beam is made through the excitation of electrons, which is not an optical process.

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Post by v_mulligan » Tue Jan 27, 2009 4:28 am

You're quite right, and in that case, you just need to move the emitter slightly closer to the parabolic mirror. Alternatively, you can make the emitter a little bit larger. As it stands, this beam will spread out a little bit because the emitter isn't a perfect point.

The defining feature of a laser isn't parallel rays, but coherent light (i.e. the waves are all in phase). Still, when someone thinks of a laser, the usual thing they think of is a beam that doesn't spread out much at all.

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Post by Lord of the Rings Junkie » Tue Jan 27, 2009 4:32 am

I think the best approximation would be an RGB emitter inside a tube housing with collimating optics. The beam width at the aperture should be around 1mm and spread at the rate that I mentioned in my last post.

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Post by v_mulligan » Tue Jan 27, 2009 5:19 am

You could do that. I suspect it'd be more computationally wasteful though, just as in real life you'd waste more photons. With a collimator, you're basically throwing away any photon that's not going in the right direction to begin with. The advantage of the parabolic mirror is that it redirects the photons that are going in the wrong direction.

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Post by Lord of the Rings Junkie » Tue Jan 27, 2009 5:25 am

With a parabolic mirror alone I suspect that you're approximating a flashlight more than a laser, though. It could be used in combination with collimation to make it less wasteful though.

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Post by v_mulligan » Tue Jan 27, 2009 5:30 am

Well, the major difference between a flashlight and a laser is the coherence of the laser beam, but of course Indigo doesn't simulate that. The other difference is that the light source in a flashlight is not at the focus of the mirror, so there's spread in the beam.

A collimator would make it MORE wasteful, not LESS. It absorbs misdirected photons instead of redirecting them.

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Post by Lord of the Rings Junkie » Tue Jan 27, 2009 5:32 am

Would it be possible to write an IES file with the desired divergence? Then all that would be needed is a tiny triangle.

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Post by CTZn » Tue Jan 27, 2009 6:57 am

Hi LotRJ (I hope you don't mind ;))

A few facts:
Lord of the Rings Junkie wrote:All lasers in real life spread out, although the angle can be made to be very small with proper collimation optics.

In fact, a typical laser beam diameter grows by about 1mm per meter of length.
That's true in the air I guess, due to dispersion. Feel free to use a dispersive medium in your render !
Also, a laser cannot be simulated by any light calculator such as Indigo by building a diode since a laser beam is made through the excitation of electrons, which is not an optical process.
The response of an excited electron is a photon :)

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Post by Lord of the Rings Junkie » Tue Jan 27, 2009 7:21 am

Yes, photons are the end product, but Indigo cannot simulate the cascading electron effect that produces those photons. If it cannot simulate the light amplification, then it cannot simulate a laser. Sorry if I'm not explaining this very well, I'm trying my best.

Also, I believe that beam divergence is an inherent optical effect, not caused by atmospheric scattering.

http://en.wikipedia.org/wiki/Beam_divergence

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Post by CTZn » Tue Jan 27, 2009 7:41 am

This concerns Indigo. I'm aware that all aspects of a laser can't be reproduced by Indigo, but since the beginning it was about approximation; I'm no scientist myself :)

But from this page you can see that the model takes a circle (aperture) with an orthogonal power emission. This could fall into the field of Indigo; see this paragraph of the same page, we know those words don't we ;) ?

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Post by v_mulligan » Tue Jan 27, 2009 7:41 am

Well, yes -- Indigo can't simulate the quantum effects that give rise to a laser, and it can't tell the difference between a coherent beam and an incoherent beam. It only keeps track of the intensity and wavelength of the light it's simulating, not the phase.

If you were to make a lasing cavity in Indigo, you wouldn't get a laser out. There's no way for Indigo to simulate stimulated emission from a medium. Everything suggested in this thread is a way to fake the observable properties typically associated with a laser -- notably its tendency not to spread out much.

Of the techniques suggested, each has strengths and weaknesses:

Collimator -- Results in tight beam with little spread; spread can be controlled by length and diameter of the collimator tubes; BUT absorbs most of the light (computationally wasteful).
Lens -- Results in fairly tight beam; efficient since it uses most of the photons emitted; BUT spherical aberration causes some difficult-to-control dispersion in the beam.
Parabolic mirror -- Results in tight beam with little spread; efficient since it uses most of the photons emitted; a paraboloid is the "perfect" shape so there's no unusual spread caused by spherical aberration; desired spread may be introduced by using a non-point source or by moving the source away from the focus of the paraboloid.

Note that in all of the above, spread is caused by the optics, not by atmospheric scattering. With a real-world laser, one cause of divergence would be diffraction, which Indigo can't simulate.

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