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.
We've done this before-- this is the conservation of energy, and in order to find the period of the simple harmonic oscillation, we take the time derivative.
以前算过-,这是能量守恒，为了算出,简谐振荡的周期，我们求时间导数。
3
Energy and optimal pressure of amplified spontaneous emission and laser oscillation are compared.