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CW pump creates visible continuum

29 Oct 2008

A visible supercontinuum created by a continuous-wave low-power fibre laser could be ideal for spectroscopy, microscopy and optical coherence tomography.

By modifying the zero-dispersion wavelength (ZDW) along a photonic crystal fibre (PCF), researchers in France have used a continuous-wave (CW) laser to create a supercontinuum that extends into the visible region of the spectrum. The supercontinuum spans from 670 to 1350 nm and has an output power of 9.5 W for a pump of just 20 W (Optics Letters 33 2407).

"We fabricated a 200 m long PCF with longitudinal control of the dispersion in order to enhance the generation of shorter wavelengths," Arnaud Mussot of the Lille University of Science and Technology told optics.org. "To our knowledge, these long tapers have not been reported with lengths of hundreds of metres. Their fabrication is a real technical challenge and requires precise control of drawing parameters."

CW-pumped supercontinuua are characterized by a stronger power density and a flatter spectrum compared with their counterparts that use a pulsed pump. They do not require a synchronization set-up and, as the pump sources are fibre lasers, an all-fibre format is seen to be a practical advantage in many applications.

Mussot and his colleague Alexandre Kudlinski start by drawing a 100 m length of PCF with a constant ZDW of 1053 nm. They gradually increase the drawing speed so that the ZDW in the next 100 m section slowly decreases to 950 nm. The 200 m long PCF is then pumped by a 20 W ytterbium fibre laser operating at 1064 nm. Mussot and Kudlinkski report that 13.5 W of the available 20 W was launched into the PCF.

"The main mechanism responsible for the generation of this supercontinuum is the trapping of dispersive waves associated with soliton self-frequency shift effects," explained Mussot. "One key point is to launch enough power inside the fibre to observe these effects. In order to enhance the efficiency of our supercontinuum, we firstly convert the CW field (low peak power) into a train of solitonic pulses (high peak power) in the first 100 m of fibre by the modulation instability process."

In new work, Mussot and Kudlinski have been able to generate a CW-pumped supercontinuum spanning from 650 to 1380 nm with an output power of 19.5 W for a pump of 50 W.

"We want to generate a spectrum covering more than two octaves (from 0.4 to 2 µm) with a few tens of watts average power at the output," commented Mussot. "We hope to make the more powerful 'white laser' ever fabricated and are looking for partners to develop a prototype with the aim of making commercial products."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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