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Holey fiber shows continuum promise

25 May 2004

Pumping a holey fiber with two wavelengths generates a singlemode supercontinuum spanning 400 nanometers.

Researchers in France have produced a singlemode white-light supercontinuum from a normally-dispersive holey optical fiber. The team says the source delivers up to 10 mW and that its output spectrum contains no discontinuities between 350 nm in the ultraviolet right through to 750 nm in the near infrared.

The approach relies on pumping a 4 m length of holey fiber with two wavelengths. Pierre-Alain Champert and colleagues from IRCOM in Limoges say this suppresses the cascaded Raman effect, which usually occurs in normally dispersive fiber. The dual-wavelength scheme also has the advantage that compact low-peak power sources can be used to pump the fiber instead of bulky femtosecond solid-state lasers.

The microstructured fiber is grown in-house at IRCOM. Champert says it has a pitch of 2 microns and a core diameter of around 2.5 microns. The team also measured the fiber’s dispersion profile and found that it was anomalous above 800 nm.

To generate the supercontinuum, the team couples both the fundamental (1064 nm) and second harmonic (532 nm) from a passively Q-switched Nd:YAG microchip laser into the fiber.

“A dual wavelength pumping scheme allows simultaneous excitation of the fiber in its anomalous and normal dispersion regime,” explained Champert. “The presence of the 1064 nm pump signal leads to the first demonstration of the suppression of the cascaded Raman effect, which is induced by the visible pump signal.”

Champert and colleagues now plan to scale-up the system to get more power into the supercontinuum. “The power [in the supercontinuum] will be increased by using a different pump source,” he told Optics.org. “The launched average power in the current system is 30 mW. We will upgrade our system to a 500 mW passively Q-switched laser and then a 1 W system within a few months.”

As for applications, the IRCOM scientists have been working with French firm ABX diagnostics and plan to use the source in machines that look at blood cells.

The researchers unveiled their work at the OSA’s Nonlinear Guided Waves meeting, which was held in March in Toronto, Canada.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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