13 Nov 2008
Biophotonic applications look set to be the first beneficiary of a microstructured-fibre-based OPO that produces sub-100 femtosecond pulses.
A fibre optical parametric oscillator (FOPO) based on a 4.2 cm length of microstructured fibre that emits 70 fs pulses has been unveiled by researchers at the US universities of Cornell and California Merced. The system is said to deliver the shortest optical pulses reported for any FOPO and is a significant step towards making the technology commercially viable. (Optics Express 16 18050)
"The majority of OPOs are not portable and occupy a large footprint on an optical table," Jay Sharping of Merced's School of Natural Sciences told optics.org. "My motivation is to generate tunable pulsed light of sufficient output power in a portable fibre platform. This result is a step in that direction as it explores the generation of ultrafast laser light with tens of mW of average power."
Parametric laser systems have been used for many years to generate new wavelengths from a fixed wavelength source. FOPOs are a relatively new technology and use the nonlinear optical response of the glass fibre to convert the output of a laser from one wavelength range to another.
"There are several reasons why our result has not been achieved before," explained Sharping. "We have discovered the novelty of using very short optical fibres and there are a variety of microstructured fibres commercially available today. There are now also robust fibre-based pump lasers, so it is a combination of factors."
Sharping and colleagues start with a commercially available microstructured fibre that has been drawn down to a reduced core size in order to modify the fibre's dispersion profile. They place the 4.2 cm length of fibre in a Fabry-Perot cavity and pump it using a ytterbium-doped fibre laser emitting at 1032 nm. The end result is 70 fs, 0.4 nJ pulses at 880 nm with an output peak power for 5kW for a pump peak power of 22 kW.
"The system operates for hours without adjustment or automatic control and we have not had problems with fibre damage," commented Sharping. "It turns out that because of the high gain of the fibre parametric process, we can use a wide range of end mirrors in the cavity and the best choice is still under investigation."
Sharping says that although the group is studying the existing system to optimise its performance, fibre integration and making the system portable are the ultimate goals.
"FOPOs are not commercially available, but there are companies that have expressed an interest," he said. "I am convinced that one more technical achievement will get those companies off the bench. Fibre-based systems are more robust and affordable compared with traditional lasers, and these new sources will enable the routine use of ultrafast laser technology in a wide variety of settings."
Jacqueline Hewett is editor of Optics & Laser Europe magazine.