17 Jun 2002
Scientists in Scotland develop a highly efficient femtosecond source that fits on a sheet of A4 paper.
A portable diode-pumped femtosecond laser developed by a research team at the University of St Andrews, UK, has an electrical-optical efficiency of 4 % (Optics Communications 205 207).
This is the highest reported efficiency for any femtosecond laser, says the team.
Two narrow-stripe diode lasers, one emitting at 660 nm and the other at 685 nm, pump the 3 mm crystal in a Z-cavity arrangement.
The result is transform-limited pulses of 136 fs centered at 859 nm. Average power output is 20 mW for 95 mW incident pump power, and the laser is said to be very stable.
According to Ben Agate, a member of the St Andrews team, the system's big advantage over similar femtosecond lasers is the use of narrow-stripe pump diodes. "What sets our work apart is the use of narrow-stripe diodes, which are very cheap and simplify the laser design," he told Optics.org.
The cavity configuration has also been adapted to produce either gigahertz repetition rates or kilowatt peak powers. Increasing the power level was simply achieved by pumping with two pairs of diode lasers, giving 180 mW. This arrangement produced 122 fs pulses with 1.2 kW peak power and 35 mW average power.
Using an output coupler with 0.07 % transmission produced a 3 mW output at a 1 GHz repetition rate.
The researchers say that their system has the potential to open up femtosecond laser applications that have so far been hindered by the large size and high cost of its predecessors. It could be used as a compact source for generating broad continuum light in photonic crystal fibres, and as an efficient source of ultrashort pulses in the blue region.
The team has now built a system operating in the blue, and is also working on ways to improve wavelength-tunability.
Michael Hatcher is technology editor of Opto & Laser Europe magazine.