An optical microcavity or microresonator is a structure formed by reflecting faces on the two sides of a spacer layer or optical medium, or by wrapping a waveguide in a circular fashion to form a ring. The former type is a standing wave cavity, and the latter is a traveling wave cavity. The name microcavity stems from the fact that it is often only a few micrometers thick, the spacer layer sometimes even in the nanometer range. As with common lasers, this forms an optical cavity or optical resonator, allowing a standing wave to form inside the spacer layer or a traveling wave that goes around in the ring.