laser Illuminocity changes with cavity length

I got a HeNe tube with a constant size which has a concave mirror at one end.
Im changing the distance of the other mirror from the tube.. now, i know some distances arent stable, but appart from the Heaviside function, is there any other change in the beam's strength?
the only thing that comes to my mind is that as i move the mirror away less light is reflected back, because it catches less of the cone of light made by the concave mirror in the back.... so assuming that the shape of light beam from that mirror is a conus, the leading order is $$\frac{1}{r}$$... am i right?
are there any other factors?
 PhysOrg.com physics news on PhysOrg.com >> Cheap, color, holographic video: Better holographic video displays>> First entanglement between light and optical atomic coherence>> EUROnu project recommends building Neutrino Factory
 Recognitions: Science Advisor I need a little clarfication on some points before I can answer your question. Firstly, what do you mean by illuminocity and beam strength? I have a feeling you are either referring to the lasers output power, or its output irradiance, could you confirm this? Just to clarify, you are asking how the output beam of a laser will change as you vary the distance between the mirrors? One final question, is the mirror you are using at the output end planar or concave? Claude.

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