The upper esophageal sphincter is a ring of muscle fibres.

Alternatively, the lasing cavity may be a fiber ring cavity, and the reflector assembly may include a bendable wavelength-selective fiber coupler.

The fibers include a central fiber (14) and at least one ring of closely packed surrounding fibers with at least some of the fibers being optical fibers that have a core (16, 18) and a surrounding cladding (20, 22).

Employment of the present invention allows for implementation of automatic compensation for line asymmetry of same-cable optical fiber ring networks.

A nonreciprocal polarization conversion optical system in which a test sample is held is inserted into an optical fiber ring interferometer.