How Do Lasers Ensure Photons Travel in the Same Direction?

[jhirlo]

How is it that all photons emitted by excited atoms or molecules in laser have same direction. Why they all are emitted in one specific direction, and why photon of energy that correspond energy difference of excited and ground state of excited atom provokes and causes relaxation and emitting identical photon (that emission happens in the same direction) ?

What's the explanation

 

[Deeviant]

Originally posted by jhirlo
How is it that all photons emitted by excited atoms or molecules in laser have same direction. Why they all are emitted in one specific direction, and why photon of energy that correspond energy difference of excited and ground state of excited atom provokes and causes relaxation and emitting identical photon (that emission happens in the same direction) ?

What's the explanation

I have no clue what you are trying to ask on the 2nd part of your questions but..

The key to getting coherent light from lazers is the lenses that they use. They allow light to escape the lasing chamber only in a controlled manner(coherentely)

Your second question sounds more like a statement then a question, perhaps you could rephrase it.

 

[Integral]

The answer to much of your question lies in Quantum Mechanics. A laser has an inverted quantum state population, this is an unstable state. The nature of the beast is that when one photon transitions to a more stable state many go with it. This simultaneous transition is where the coherence comes from. The directionality is created by the laser cavity which is carefully tuned to be an integral number of wavelengths of the desired frequency of light. The cavity has parallel mirrors at the ends, one of which is highly reflective the other reflects only a fraction of the incident light. At the partially reflective mirror the light not reflected is transmitted out of the system as the laser beam.

The tuned length of the cavity also helps to diminish any photons of undesired wavelengths from being amplified by the system.

 

[pallidin]

Does that work the same for P-N junction diode lasers? Where are the mirrors? Or is it some other mechanism?

 

[Integral]

Fundamentally I believe solid state diodes are similar. That is they have a quantum state population inversion. What they lack is a laser tube. Thus diodes generally have a comparatively large divergence and a short coherence length.

 

[pallidin]

OK, thanks for the info.

 

[suyver]

Originally posted by Integral
What they lack is a laser tube. Thus diodes generally have a comparatively large divergence and a short coherence length.

While this is true, I thought it would be good to quantify this 'large divergence' a bit: A laserpointer is a solid state laser and its spot is typically about 3 mm when leaving the pointer. At 100 meters, this spot will still be less than 1 cm. So yes, there is divergence, but not a lot.

However, good tube-lasers can project a spot of about 1 m on the moon, when the beam leaving the laser is about 1 mm. So, they are indeed a lot more converged!

 

[Integral]

Originally posted by suyver
While this is true, I thought it would be good to quantify this 'large divergence' a bit: A laserpointer is a solid state laser and its spot is typically about 3 mm when leaving the pointer. At 100 meters, this spot will still be less than 1 cm. So yes, there is divergence, but not a lot.

However, good tube-lasers can project a spot of about 1 m on the moon, when the beam leaving the laser is about 1 mm. So, they are indeed a lot more converged!

That is why that word "comparatively" was in there

 

[Nereid]

Originally posted by suyver
*SNIP

However, good tube-lasers can project a spot of about 1 m on the moon, when the beam leaving the laser is about 1 mm. So, they are indeed a lot more converged!

I thought they had to be sent through a telescope (from the focal plane back through the mirrors and then up to the Moon), if only to ensure that atmospheric 'seeing' didn't play havoc (typical air cell in seeing is >> 1mm, but ~< 1m)

 

[suyver]

Originally posted by Nereid
I thought they had to be sent through a telescope (from the focal plane back through the mirrors and then up to the Moon), if only to ensure that atmospheric 'seeing' didn't play havoc (typical air cell in seeing is >> 1mm, but ~< 1m)

Really? I didn't know that. I thought (as stated) that a 'normal' high-power laser could 'just' be pointed to the right place on the moon (where the mirror is) and that would do it. If I have time, I'll look it up.

I don't think I understand your comment on "atmospheric 'seeing'", would you mind to elaborate? Maybe I am just unfamiliar with the term (do you mean beam-decolimation due to refraction on turbulences in the Earth's atmosphere?).

 

[Nereid]

Parlez-vous français?
http://wwwrc.obs-azur.fr/cerga/laser/laslune/instrum.htm

From a NASA website:
http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html

http://www.mrao.cam.ac.uk/telescopes/coast/theses/rnt/node3.html and his successors if you want to dive deeper).

 

[suyver]

Cool! Thanks for the links.
I learn something new every day.

 

[Raisin-toe]

since lasers are set to a specific frequency, is it possible to cancel out the light wave of a laser by having another laser of the same frequency pointing directly at it?

Just the same way as a car muffler works to cancel out sound waves, isn't it possible to do the same thing with a laser?

 

[Raisin-toe]

oops, I forgot to take a look at the date that this thread was posted