11 March 2008

Tapered fibre improves laser performance

A tapered fibre that suppresses nonlinear effects as well as offering cost-effective pump light absorption could soon be deployed in high power fibre lasers and amplifiers.

A research team from Finland and Russia has unveiled a tapered double-clad fibre (T-DCF) that it says is an ideal active medium for high-power singlemode fibre lasers pumped by diode bars. Other unique features of the fibre tailor it for use in high-power amplifiers. (Optics Express 16 1929).

"A large input diameter of up to 2 mm and a high clad pump absorption make T-DCF a unique double-clad fibre that can be pumped by sources with a poor beam-parameter product, up to 150-300 mm×mrad, such as diode bars," researcher Valery Filippov from the Optoelectronics Research Centre (ORC) in Tampere, Finland, told

A double-clad amplifying fibre typically comprises a core doped with a rare-earth element while the pump radiation propagates in the cladding.

"In our T-DCF the large-diameter end of the taper serves as an efficient pump combiner/launcher while the narrow/singlemode section acts as a fundamental mode filter which ensures diffracted-limited output beam quality with high power," commented Filippov. "This is an exceptional combination."

The researchers fabricate their T-DCF using a special tower, furnace and advanced pooling procedure. While Filippov says that modern technology is routinely available to make such a development, essential efforts were required to produce high-quality T-DCF.

"The key parameters of the T-DCF that determine the system performance are doping level, clad-pump absorption, tapering ratio, input/output diameter and the axial shape of the taper," explained Filippov. "The shortest length of taper that we can do at the moment is around 1 meter with a tapering ratio of 1:5. We are using silica preforms and plasma chemical vapour to dope the fibre."

Another key advantage of T-DCF is its ability to suppress nonlinear effects, which could be exploited in both pulsed and continuous wave (CW) regimes. "The gradual change of the fibre core with the length should result in strong suppression of stimulated Brillouin scattering (SBS) - a major limiting factor in fibre amplifiers," commented Filippov. "The suppressed SBS in T-DCF would be promising for amplifying quasi-CW signals or hundreds-ns-long pulses as well as narrow-line continuous-wave CW signals."

Filippov and his colleagues from Ultrafast and Intense Optics Laboratory of ORC led by Oleg Okhotnikov are now implementing the T-DCF in various devices. "We have reported an ytterbium-doped singlemode fibre laser based on T-DCF that emits over 200W and a cost-effective 1kW system is the next target," he said. "We are also planning experiments on Q-switched lasers using T-DCF."

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