Enhancement cavity creates green light simply from IR fiber laser

Concocting a green-light-emitting fiber laser has meant taking the output of an IR-emitting fiber laser (for example, around 1060 nm) and frequency doubling it. Researchers at the University of Southampton (Southampton, England) have inserted an “enhancement resonator” within the fiber-laser cavity to get the job done, which does not require active cavity-length stabilization.

A laser-pumped length of gain fiber has, at one end, bulk optics and a diffraction grating that narrows the wavelength band enough that it is smaller than the phase-matching bandwidth for frequency doubling in a chosen nonlinear optical crystal. At the other end, an enhancement cavity contains partially transmitting input- and output-coupler mirrors and the intracavity nonlinear lithium triborate crystal for frequency doubling. With a singlemode 25-µm-diameter core, the polarization-maintaining fiber is pumped at 975 nm and emits at 1080 nm. The crystal is held at 130ºC and positioned between curved dichroic mirrors that transmit green (540 nm) light. Output power in the green is 15 W for a 975 nm pump power of 90 W. Accounting for reflections, the conversion efficiency from 975 to 540 nm is >21%.

Contact Rafal Cieslak at [email protected].