Complex sentence: The optical resonator is sometimes referred to as an "optical cavity", but this is a misnomer: lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a maser.
Split into simpler sentences: The optical resonator is sometimes referred to as an "optical cavity," but this is a misnomer. Lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a maser.