25 Aug 2004
An Australian team produce a laser by exposing holey PMMA fiber to a solution of Rhodamine 6G dye.
Scientists from Australia have made a pulsed red laser by doping a microstructured polymer fiber with the dye Rhodamine 6G. The prototype device developed by the University of Sydney and Macquarie University emits up to 16 µJ/pulse at 631.9 nm and has a lifetime of 130 000 shots at a 10 Hz pulse rate. (Optics Letters, 29, 1882)
Rather than adding the laser dye to the initial monomer, the team uses a simple way of doping the fiber pioneered by colleagues at the National Chemical Laboratory, India and the University of Sydney. "The breakthrough came with the solution doping technique which allowed us to dope the polymer very easily at the preform stage, simply by passing a solution of the dye through the holes," Sydney University researcher Alex Argyros told Optics.org. "It only adds about five minutes to the usual fiber fabrication time."
The team found that by exposing the microstructured fiber to a saturated solution of Rhodamine 6G dye in acetone for 30 seconds, the larger dye molecules were able to move into the polymer (PMMA) matrix. Heating the fiber at 90 degC for 16 hours helped the dye diffuse evenly throughout the core region, giving a final dye concentration of around 1 mmol/l in the core.
When pumped by green (532 nm) 10 ns pulses from a frequency-doubled Nd:YAG laser the fiber started to lase. The raw cleaved ends of the fiber acted as cavity mirrors. The team says that the laser's linewidth of about 0.5 nm is much narrower than values reported for other PMMA-R6G systems.
Analyzing a 1.5 m length of fiber, the group reports a threshold of 20 µJ and a slope efficiency of 18%. The maximum output of 16 µJ/pulse, corresponding to a peak power of 2 kW, is limited by the estimated 13 GW/cm2 damage threshold of the polymer.
The researchers are now keen to produce a tunable version of the laser by adding gratings to the fiber. "The material properties of polymers have allowed highly tunable fibre gratings to be demonstrated by other groups," explained Argyros. "We want to combine that with the wide fluorescence bands of organic laser dyes, in the hope that we can make a tunable fibre laser with a very wide tuning range."