Although titanium-doped sapphire is a versatile laser gain material and the main ingredient in widely tunable and ultrafast Ti:sapphire lasers used across a broad spectrum of photonic applications, it requires a high-brightness (bulky and expensive) pump source—typically, a multiwatt argon-ion or frequency-doubled neodymium laser. Now, increased power levels for gallium nitride (GaN)-based blue and green laser diodes have enabled researchers at the University of Strathclyde's Institute of Photonics (Glasgow, Scotland) to be the first to demonstrate a modelocked Ti:sapphire laser directly pumped by a laser diode.1
An unlikely result
Because the optical power of blue and green laser diodes is generally low and their wavelength is poorly matched to the broad, but weak absorption spectrum of the Ti:sapphire gain material, the laser industry has always felt that diode-pumped Ti:sapphire was an unlikely achievement. Nonetheless, a compact 1 W, 452 nm GaN laser diode from Nichia (Tokushima, Japan) is sufficient for a Ti:sapphire laser with a continuous-wave output of 19 mW at 800 nm in a standard cavity (see figure).
To achieve lasing, an aspherical collimating lens, a two-element cylindrical-lens telescope, and a spherical focusing lens were used to concentrate the laser-diode output onto a Ti:sapphire crystal within a four-mirror cavity. The calculated cavity-waist radius was 25 × 15 μm in the crystal. For an output coupling of 0.5%, a modelocking threshold of 750 mW and a maximum average output power of 9 mW were obtained with 870 mW incident on the crystal. Using interferometric autocorrelation, the transform-limited output had a measured full-width half-maximum pulsewidth of 116 fs.
The measured output power was lower than predicted by modeling. The researchers say this is due to pump-induced losses at the lasing wavelength; however, the losses are not observed at wavelengths above 477 nm. Because progress has been made in longer-wavelength GaN laser diodes, the team is confident that future experiments will soon produce a higher-output-power direct-diode-pumped Ti:sapphire laser. Even using the current laser diode at 452 nm along with double-sided pumping or polarization-combining techniques, the research team is confident that output powers around 50 mW could be achieved.
"Diode-laser-pumping of Ti:sapphire enables drastic reductions in complexity over current systems," says PhD student Peter Roth. "As a result, some of the unrivaled performance of today's high-cost tabletop Ti:sapphire lasers may soon be available at a fraction of the current cost and footprint. With currently available GaN diode lasers [approximately 1 W per device around 450 nm], a tunable femtosecond Ti:sapphire laser with an average output power of roughly 50 mW should be possible by multiplexing two diodes... Such a laser would find numerous applications from imaging to spectroscopy–for example, as a bolt-on accessory to a fluorescence microscope."
- P.W. Roth et al., "Modelocking of a diode-laser-pumped Ti:sapphire laser," CLEO 2010, paper CMNN1, San Jose, CA.