A laser operates on the fundamental principle that a pumped medium amplifies a travelling light beam as it propagates through the medium. Without mirrors a laser becomes a single-pass travelling wave amplifier (Figure E.26.1).
This is a system without feedback. If we now place mirrors at each end of the pumped medium, we introduce feedback into the system.
When the gain of the medium becomes large enough the system oscillates creating a coherent output light beam. That is laser operation. If the gain of the medium is less that that required to sustain oscillation, the system is known as a regenerative travelling-wave amplifier (RTWA).
Let the electric field of a light beam incident on the RTWA from the left be the excitation of the system, Einc(s), and let the electric fields of the reflected light, Erefl (s), and the transmitted light,
Etrans (s), be the responses of the system (Figure E.26.3).
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