NARROW-LINEWIDTH LASERS: Small resonator-stabilized laser has 13 kHz linewidth

摘要

Lasers with linewidths in the low kilohertz range are useful for spectroscopy as well as metrology (for example, length and frequency standards). Researchers at the Max-Planck-Institute for the Science of Light (MPL; Erlangen, Germany) have come up with a simple and compact form of this type of laser that has a linewidth of only 13 kHz. The laser is built around a small whispering-gallery-mode (WGM) resonator made of crystalline calcium fluoride (CaF_(2)). When formed into a disk, a CaF_(2) WGM can have a very high Q factor (the Q factor is a measure of a resonator's damping; a higher Q indicates less damping). The laser assembly is made of a ring of optical fiber with all the relevant optical components inserted into the ring (see figure). A semiconductor optical amplifier (SOA), which provides a spectrally broad emission with a peak around 1560 nm, provides the laser gain; the SOA is placed between two polarization-control (PC) components, and an optical isolator ensures that laser light is produced in only one direction. A fiber coupler directs 1percent of the circulating light into an output fiber.