SOLID-STATE LASERS: Ytterbium-doped laser emits CW at 1.047 microns

Researchers from the Laser Photonics Group at the University of Manchester (Manchester, UK) have demonstrated CW output power of more than 30 mW at 1.047 µm from a diode-pumped ytterbium-doped strontium fluorapatite (Yb³⁺:S-FAP) laser. The device is pumped by a single 899-nm CW InGaAs laser diode and operates with slope efficiencies greater than 17% at a dopant level of 0.5%.

Ytterbium-doped crystals have previously been successfully demonstrated in the 1-µm region and operate at room temperature with host crystals of YAG and fluorapatite (Ca₅(PO₄)₃F). Replacing the calcium in the host crystal with strontium (Sr₅(PO₄)₃F) provides a better laser material with a longer storage lifetime (1.26 ms), a small quantum defect, and narrow spectral linewidth. These characteristics make Yb³⁺:S-FAP—which has already been successfully pumped with both a pulsed diode laser and a Ti:sapphire laser—an ideal candidate for CW diode-laser pumping.

The demonstration crystal measures 4 × 4 × 7 mm³ and was coated on one face to produce an input mirror. The diode-pump-laser output is collimated and focused into the crystal and the hemispherical cavity of the device is completed with a selection of output couplers. Input pump power to the crystal was 1.7 W after the collimating optics; 30% of this power was absorbed by the crystal, which was enough to bleach the crystal because of its low saturation intensity. Two cavity configurations were tested; a 12-cm hemispherical cavity exhibited a threshold of 300 mW absorbed power with a slope efficiency of just over 16% and 98% coupling. A 25-cm hemispherical cavity with 95% coupling showed a threshold of 320 mW with a slope efficiency of 17.5%. In both cavity configurations the laser operated in a single transverse mode.

The research group expects to improve the laser`s performance in future. One problem is the poor overlap of the diode-laser emission and ytterbium absorption spectra. In addition there is only a 10% overlap between the pump and laser modes. Improvements using selected narrow-bandwidth and highly polarized laser diodes together with higher doping levels in the crystal are currently underway, with an indication of significantly higher output powers. Intracavity frequency doubling is also being assessed using KTP as the doubling crystal. Efficient doubling might produce a laser system that can compete successfully with diode-pumped Nd:YAG lasers in a range of applications.

The crystals used by the Manchester group were grown in the USA at the University of Central Florida (Orlando). Although Yb³⁺:S-FAP is not yet a commercial material, it is likely to be available from US and other sources in the future. Financial backing for the project is provided by the UK Department of Trade and Industry together with Laser Quantum (Manchester, UK) whose managing director, Gianpaolo Paiutta, is investigating the commercial potential of the system.