Amplified spontaneous emission in Cassegrainian amplifiers

We consider all possible paths for amplified spontaneous emission (ASE) in multipass laser amplifiers using a Cassegrainian telescope geometry. In particular, we study ASE which is reflected back into the medium off the telescope mirrors themselves. These ASE components are unavoidable in this amplifier geometry. We show that there is a component of the ASE which makes approximately double the number of passes through the amplifier as the laser signal makes. We also show that these high order ASE components are also present in amplifiers which are almost Cassegrainian. They cannot be eliminated by changing the ratio of the scraper and hole radii or the separation of the mirrors. It is likely that these ASE components will be more significant in pulsed lasers than cw lasers.