They "want" to oscillate at a given frequency in the same way that a violin string does — a small amount of energy fed into the system, in almost any form, creates the oscillation.

The regularity of the change of the unstable oscillation wave form and the structure of the power spectrum were analysed.

Based on the character of off-axis and multi-pass amplification systems, the possibilities are analyzed that some part of the line could form optical oscillator and occur self-excitation oscillation.