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Positive move for cavity-dumped Ti:sapphire

08 Nov 2006

Spectroscopy, laser-based electron sources and high-order harmonic generation are all set to benefit from a cavity-dumped Ti:sapphire oscillator operating in the positive dispersion regime.

Researchers at the University of Colorado and NIST, US, claim to have demonstrated the first cavity-dumped Ti:sapphire oscillator to work in the positive dispersion regime. According to the team, this source fills the gap between standard Ti:sapphire oscillators and complex oscillator-amplifier systems. (Optics Express 14 9750)

"This mode of operation gives a per-pulse energy that is an order of magnitude greater than a cavity-dumped laser operating in standard negative dispersion mode," Henry Kapteyn of JILA and the Department of Physics told optics.org. "This gives us enough peak power in a pulse to generate white light through filamentation, once we compress the pulse using prisms."

The challenge for researchers to date has been to find a cavity configuration that permits a smooth transition between negative and positive dispersion regimes, without loss of modelocking.

Kapteyn and colleagues Margaret Murnane and Xibin Zhou use a Bragg cell driver and an 18W amplifier to drive a fused silica cavity dumper. All mirrors in the setup are standard dielectric mirrors with low dispersion and the path length in the Ti:sapphire crystal is approximately 5 mm. The entire system has a footprint of just 0.3m2.

"We obtained 450 nJ pulse energy at a repetition rate of 0.8 MHz using 6.5W of pump power at 532 nm," said Kapteyn. "We compressed this pulse to 60 fs duration, which gave a peak power of approximately 7.5MW. Higher pump powers do not give a higher output. We optimised the laser's output coupler to achieve these results."

The team went on to focus the compressed pulse into thin sapphire and fused silica plates using an aspheric lens. This resulted in a white-light continuum spanning the wavelength range 450 nm to 1 micron.

In their paper, the researchers state that they believe this to be the first demonstration of a "white-light continuum from a filament generated by the unamplified output directly from a sub-100fs Ti:sapphire laser oscillator".

According to Kapteyn, the next step is to improve the quality of the pulse compression. "The bandwidth we obtained should allow us to generate a 40-50fs pulse if optimally recompressed," he said. "We are also interested in implementing carrier-envelope stabilization of the output pulses and have some ideas to increase the pulse energy obtained from the laser."

One application that could benefit from this new laser is chemical spectroscopy. As Kapteyn explains, chemists wanting broad bandwidth light for ultrafast spectroscopy generally use a more expensive laser oscillator-amplifier system.

"Our laser occupies the same footprint as a standard modelocked Ti:sapphire laser," he said. "It also allows you to directly generate broad bandwidth light at a repetition rate that can easily be varied to allow the system of interest to "recover" between pulses."

Kapteyn revealed that the group has tentative plans to develop a commercial version of its positive dispersion laser.

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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