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07 May 2010
A higher order mode fibre delivers 125 fs pulses over a distance of 3.6 m without any pre-chirping of the input pulse.
Researchers in Denmark and Austria have created a higher order mode fibre that allows them to deliver 125 fs pulses directly from a Ti:sapphire laser without any additional optics. Removing the need for pre-chirping means that fibre delivery systems could easily be bolted-on to many existing Ti:sapphire lasers, a move that could aid applications such as terahertz imaging, confocal microscopy and endoscopic multiphoton imaging (Optics Express 18 7798).
"We can deliver ultrashort optical pulses directly from the laser to the location where they are required," Kim Jespersen from OFS Fitel in Denmark told optics.org. "There is no need for additional bulk optical components to pre-compensate for the transmission properties of the fibre. The development of a fibre with anomalous dispersion at 800 nm is the key element."
The major challenge facing researchers developing any fibre-based pulse delivery system is the dispersive and nonlinear properties inherent to fibre technology. Although there have been previous demonstrations of fibre delivery, almost all have had to rely on optical grating or prism configurations that pre-chirp the pulse prior to fibre launch in order to compensate for the dispersion of the fibre.
To overcome these problems, Jespersen and colleagues used a combination of solid silica fibres with different dispersion characteristics. Their femtosecond pulse delivery system comprises a section of higher order mode fibre placed between two sections of singlemode fibre. Each section of fibre offers some dispersion compensation, which leads to a fibre link with a neutral dispersion profile.
"We have made similar fibre assemblies operating at 1 µm and they have been demonstrated as dispersion managing building blocks inside laser cavities and as extra-cavity pulse compressors," commented Jespersen. "In order to shift the operational wavelength down to 800 nm and still obtain anomalous dispersion, we had to increase the waveguide dispersion even more to compensate for the high material dispersion at 800 nm."
In its best result to date, the OFS Fitel team collaborated with Femtolasers of Austria and was able to provide 125 fs pulses from a Ti:sapphire at the end of a 3.6 m fibre. In a preliminary test, the team was also able to deliver 216 fs pulses over a 55 m long fibre based on the same principle.
The collaboration is keen to improve the power performance and is currently looking for industrial partners that are interested in this technology. "Our feedback from the industry is that the power performance is critical and that it needs to be 3–10 times higher than in our demonstration," commented Jespersen. "We know that there is room for improvement in terms of effective area (and also higher order dispersion) and we have a new preliminary fibre design ready. We will also try to reduce the accumulated higher order dispersion, since this is limiting the pulse duration that can be achieved."
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