Number of Photons inside a Laser Cavity

  • #1
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<< Mentor Note -- thread moved from the technical forums, so no Homework Help Template is shown >>

Let's say you have a laser cavity with two mirrors at either end, one is considered 100% reflective, the other 99.9%, so that a wave beam is emitted through this lower reflectivity mirror.
I know:
-The length of the laser cavity
-The reflectivity of the mirrors
-The power and wavelength of the emitted wave beam

I am asked to determine the number of photons present in the cavity (I presume average considering they are continually being absorbed and re-emitted). I am having a hard time understanding how to even go about this with the, at least what seems to me, limited information.

I thought about working backwards, if let's say the output mirror has a reflectivity of 99.9%, then only 0.1% of the photons inside have been emitted so N(out)/0.001 would give the average number of photons inside the cavity? I feel like I'm missing something. I know that the photons interfere to create standing waves some integer multiple fitting the cavity length.
 

Answers and Replies

  • #3
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Asked by whom?
By my teacher.
 
  • #4
Nugatory
Mentor
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By my teacher.
In that case, we're going to have to ask you to post this question to the "Introductory Physics Homework" subforum instead.

[Edit: looks like Berkeman beat me to it]
 
  • #5
jbriggs444
Science Advisor
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I thought about working backwards, if let's say the output mirror has a reflectivity of 99.9%, then only 0.1% of the photons inside have been emitted so N(out)/0.001 would give the average number of photons inside the cavity?
N(out) is what, exactly?
 
  • #6
tnich
Homework Helper
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When the cavity reaches a state of equilibrium, what could you say about it?
 

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