Noob Question About Phase...

[Stiffler]

Lasers are perfectly coherent, perfectly in phase light sources...So, if I combine 2 lasers that are slightly different wavelengths (say, 445 & 447 for argument), wouldn't I get reinforcement & cancellation along the beam?? Even if they are the exact same wavelength, what guarantees that when combining them they don't just cancel each other out?

[-bart-]

Yes, if you would combine two single-longitudinal-mode lasers, 445nm and 447nm, which phases are coupled, you would have constructive and destructive interference along the beam path.

Lasers are perfectly coherent, perfectly in phase light sources

No they are not, in secondary-school-theory they are !

Even a simple 445 diode may run at hunderds or more wavelenghts at the same time, and they don't cancel out either.
There may actually be interference along the beampath, but not a a stationary spot in space, where you could sit and observe it.

It's not a noob question, I think the majority of this community has little real knowledge of the character of light.
And what the heck do I know about path-integrals, or probability-amplitudes ?

[Stiffler]

About a year ago I heard of a laser show where the beam actually changed color along it's length. I did not see that laser show, but it got me thinking about phase & how it might affect the beam, which is why I posted this question.

[mixedgas]

About a year ago I heard of a laser show where the beam actually changed color along it's length. I did not see that laser show, but it got me thinking about phase & how it might affect the beam, which is why I posted this question.

What you saw a picture of, was a pulsed lab grade femtosecond tundable laser doing a "FM Chirp". The trick is to have it chirp through all the tunable spectrum during the time the camera shutter is open. So the camera sees RYGBIV from one side of the exposure to the other. The Human eye does not have a fast shutter and can NOT see that.

Cost to shoot the photo from scratch ~80,000$ worth of lab lasers + 7 years of college for at least two staff members + one expensive camera. Plus one very expensive optic to form the optical vortex rings. Which are incidentally the cross beam version of your phase taken to extremes.

The justification of a photo like this is what it can teach in a single shot, is amazing.

Here is the shot:

http://www.physics.sdsu.edu/~anderson/

The laser is pulsing mode locked (phase again) at somewhere around 80 Megahertz for that shot. Up until last week I operated a similar system, except for the vortex rings..

Dr Anderson does like laser shows of a sort. Watch how the modes interact as they overlap. This is a Grand Prize winner teaching video on transverse modes, phase, and interference:

http://www.youtube.com/watch?v=GrbHG...layer_embedded


Steve

[pschlosser]

Is it not the cancellation of two wave forms of light that attribute to its change in color?

[swamidog]

No. This was an artifact of the camera shutter

About a year ago I heard of a laser show where the beam actually changed color along it's length. I did not see that laser show, but it got me thinking about phase & how it might affect the beam, which is why I posted this question.

[andythemechanic]

There is a calcualtion about moving interference fringes from different frequency lasers in the "Handbook of Optical Systems Vol. 2". I Quote the result:

"It follows that, for an average wavelength of 500 nm, the moving interference
fringes cannot be observed if the wavelengths of two light sources differ by more
than ~10 pm."

If you have the two lasers perfectly at the same wavelength and phase, how will you combine them? Usually it involves a beamsplitter, and than you can indeed have a situation where the beam in one path is completely canceled out.

[mixedgas]

No. This was an artifact of the camera shutter

Mea Culpa,

Yeah, there was that one too.

Steve

[mixedgas]

Is it not the cancellation of two wave forms of light that attribute to its change in color?

When you have wave interference, energy is neither created nor destroyed. Its path is redirected.


What your thinking of occurs in a non-linear medium such as a KTP crystal. That condition in a non-linear material, and only at huge amounts of power per unit area. Its not really a cancellation as much as a vector ADD or SUBTRACT in that case. Energy is still conserved. So for example with a YAG laser, 1064 +1064 = 532 nm green + heating of the crystal.

The actual math uses sums of square roots, but that is a lesson for another day.


Steve