Q factor is a measure of the "quality" of a resonant system. Resonant systems respond to frequencies close to the natural frequency much more strongly than they respond to other frequencies. On a graph of response versus frequency, the bandwidth can be defined as the "full width at half maximum" or FWHM. The Q factor is defined as the resonant frequency divided by the bandwidth:

Q = ω0 / Δω

In a TRF:

In optics, the optical Q factor of a resonant cavity is the ratio of energy stored to energy dispated in the cavity. If the Q factor of a laser's cavity is abruptly changed, the laser can be induced to emit a pulse of light; this technique is known as Q-switching.

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