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Large-area fibre targets high-power applications
In Thursday’s post-deadline session, Liang Dong of fibre laser specialist IMRA showcased a new class of fibre that supports singlemode operation for fiber laser applications requiring high peak powers. According to Dong, the new all-glass fibre will extend the reach of practical ultrafast amplifiers to millijoule pulse energies, and could also lead to continuous-wave fibre lasers and amplifiers in the 10 kW range.
Dong explained that high peak powers requires singlemode operation to be achieved in a fibre with a large effective area. He pointed out that today’s large-mode area fibres are limited to a core diameter of 30 µm, while photonic crystal fibres have been demonstrated with diameters of up to 100 µm.
Those figures make it even more impressive that IMRA has achieved singlemode operation in fibres with core diameters of up to 170 µm. The fibres exploit the company’s “leakage channel fibre” (LCF) design, in which the core of the fibre is formed by six low-index features arranged in a hexagonal grid. Unlike photonic crystal fibre, in LCF the air holes are filled with flourine-doped silica glass, and so it can be cleaved, spliced and handled in the same way as normal optical fibre.
Versions of the fibre with core diameters of 100 µm and 170 µm were designed as passive LCFs. The 100 µm version achieves a pump absorption of more than 25 dB/m &ndash which Dong says would mean that a typical amplifier could be just tens of centimetres long – while LCFs with larger diameters could yield even shorter amplifiers.
A ytterbium-doped LCF was also fabricated with a core diameter of 50 µm. A single-stage amplifier constructed from this fibre achieved pulse energies of 600 µJ for a pulse length of 600 ps and a repetition rate of 25 kHz, which Dong says shows that “the design is a practical choice for extending the peak power in fibre lasers”.
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