Why is it that equivalent length is so important for laser beam power output (Length = Power/60) - http://www.laserk.com/newsletters/whiteCO.html. I was reading that in order to get multiple kilowatts of power out of a CO2 laser beam you have to fold the beam a great number of times. Why is this the case? Can you instead make the rear reflector a concave mirror and then put a focusing lense in the apex of the concave mirror in order to boost the beam strength? I am trying to build a 10kw CO2 laser and want to make sure I am taking everything into account and that I understand why I am doing what I am doing. In order to get 10kw of power based on the equation above the total equivalent length would be 166 meters and I have a 1 meter long glass tube about 3" in diameter. I suppose I could try to design a single piece multi reflector inside the cavity but 166 times bouncing around would be tough. Is this how they are making 10kw lasers for DOD, etc? I have gone through the following books - Svelto O. Principles of Lasers (5ed., Springer, 2009)(ISBN 1441913017) Milonni P.W., Eberly J.H. Laser Physics (Wiley, 2010)(ISBN 9780470387719) I am looking for a copy of the following to review soon - Lasers - Anthony E. Siegman It seems these texts like to cover a bunch of material but kind of dance around the details required to actually build a high power laser (minimum of 10kw IR beam power).