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Microstructured fiber boosts ultrafast photonics

28 Nov 2007

The photonic crystal fiber produces ultrafast pulses at low powers and across a broad spectral range, as well as breaking new conceptual ground in photonics.

A new design of hollow-core photonic crystal fiber (HC PCF) has been developed by an international team led by Fetah Benabid of Bath University in the UK. One immediate result has been a method to produce attosecond laser pulses more efficiently than previous techniques.

The fiber developed by the team has a core based on a Kagome lattice, a geometric structure formed by interlaced triangles. This gives the fiber the counter-intuitive ability to carry core-guided modes and cladding continuum modes at the same time, unlike conventional PCFs where light is transmission is confined to the core. The result is transmission bandwidths that are claimed to be several times wider than conventional photonic crystal fibers.

"This is a new theory of photonic guidance in HC PCFs," Benabid explained to optics.org. "The ability of a fiber to have a core-guided mode and a cladding continuum that shares a strong longitudinal phase match is novel. It's the first experimentally observed instance of the so-called Von Neumann-Wigan quasi-bound state in a continuum, which was first predicted in the 1930s."

The team's discovery could have wide-ranging implications. "This new photonic guidance theory could create new designs for next-generation photonic materials such as broadband HC PCFs and photonic crystals," said Benabid. "The similarity between this guidance and the Von Neumann-Wigner states would also constitute a new conceptual bridge between photonics and quantum mechanics."

“This constitutes a new conceptual bridge between photonics and quantum mechanics.”

The fiber's unique properties have led directly to a second breakthrough, the efficient generation of a broad spectrum of ultrafast pulses from a hydrogen-filled PCF through stimulated Raman scattering.

The conventional technique to create attosecond pulses is high-harmonic generation (HHG), which produces central wavelengths in the XUV or soft X-ray region through the firing of a very intense laser pump pulse into a gas. Benabid's fiber was able to produce ultrashort pulses more simply using through stimulated Raman scattering.

"In our fiber the pump excites the hydrogen to create a very broad and coherent comb-like spectrum spanning from the UV to the mid-IR," said Benabid. "This type of excitation is also new. Creating a coherent spectrum usually requires a special type of excitation, necessitating extremely short pump pulses or finely tuned multi-pump excitation - and in both cases a huge amount of peak power."

Benabid's fiber is claimed to require a pump pulse with power levels six orders of magnitude lower and five orders of magnitude longer than those previously needed for HHG.

"Our new techniques could enable low-cost compact systems for sub-femtosecond optical pulse generation at wavelengths spanning the UV-IR," commented Benabid. "This would have huge impact in areas such as laser science, materials science and biological research."

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