I am working with a He-Ne laser. What I have noticed, is that when resonator cavity length is increased (by moving on of the mirrors), output power goes down. What could be an explanation to that? Additional question: as I use different mirrors, optical power seems to go down as well. Do we have an output power dependency on how stable resonator is? Please explain anyone, thanks

  • $\begingroup$ Just a thought; I'm not sure how much this would affect things, but lengthening the cavity will lead to larger diffraction losses. $\endgroup$
    – march
    Mar 19, 2023 at 16:00

1 Answer 1


You've left out a lot of details, like the overall cavity design, how the beam is confined, etc.

But think of it this way: Assuming the pump power stays constant, it must contribute to either laser output or be lost. Loss mechanisms include absorption by materials in the cavity, scattering, and diffraction loss. By diffraction loss, I mean the power lost because the beam spread is larger than the mirror, so only part of the beam can be reflected. I'm betting on this last one being the cause of your observation.

  • 2
    $\begingroup$ I'm with you on this, as the optimal length of a cavity and the curvature of its mirrors are related. But will add another possible cause: misalignment when the mirror is moved. $\endgroup$
    – garyp
    Mar 19, 2023 at 16:53
  • $\begingroup$ I am thinking about greater range of allowed longitudinal modes and thus maybe waves cancelling each other? Does it make any sense? $\endgroup$ Mar 19, 2023 at 17:22
  • $\begingroup$ @user3600124 Start with the basics. If you've controlled for those and demonstrated they don't cause your loss, then look for exotic explanations. How much did you move the mirror (10 um or 10 mm)? What is the beam divergence in the area of the mirror? Flat or concave mirrors? How much did the output power decrease (0.1 dB or 10 dB)? You haven't shared any of this information so there's very little we can say specifically to help you. $\endgroup$
    – The Photon
    Mar 19, 2023 at 20:15
  • $\begingroup$ Mirror moves in range of ~700mm roughly. Used two concave mirrors. Output power decreased from 1.8mW to 0.7mW (roughly by half) $\endgroup$ Mar 20, 2023 at 0:34
  • $\begingroup$ 700 mm is a heck of a lot. Do you know the beam diameter at the two locations (also with consideration that moving one of the focal elements will change the beam diameter)? $\endgroup$
    – The Photon
    Mar 20, 2023 at 3:06

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