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Simple Raman source accesses yellow

04 Feb 2009

A Raman laser that emits picosecond pulses at 559 nm is said to be ideal for two-photon fluorescence microscopy.

Scientists looking for a simple and reliable source of short-pulse yellow light will be interested to hear of a 559 nm picosecond-pulsed source being developed in Australia. The laser emits 3.2 ps pulses and will allow researchers to use a wider range of biological tags in their two-photon fluorescence experiments. (Optics Express 17 569)

"Our system is the first to generate picosecond yellow pulses at 80 MHz continuously starting from a Nd oscillator," Eduardo Granados of Macquarie University told optics.org. "Previous studies have used Q-switched modelocked pump lasers resulting in repetition rates of between 10 and 1000 Hz."

Raman shifting of conventional lasers to access new wavelengths is a well established technique. While it is possible to access yellow wavelengths using a Ti:sapphire laser (by focusing its output into a hollow waveguide for example), this tends to be a complex and expensive process.

"The main advantages of our approach are its simplicity and stability," said Granados. "It also offers better beam quality and lower costs."

To keep things simple, the team started with a commercially available modelocked Nd:YVO4 laser, frequency doubled to emit 10 ps pulses at 532 nm. This pumps a z-fold cavity containing a KGW crystal, another common component that has been used extensively in previous Raman laser studies.

By translating one of the z-fold cavity end mirrors no more than 80 microns, Granados and his colleagues were able to compress the 10 ps pulses down to a duration of just 3.2 ps and shift their wavelength to 559 nm.

"The key result is the very sensitive conditions required for pulse shortening," said Granados. "The pulse shortening is important for biological studies as it increases the peak pulse power to enhance the fluorescence. We also maintain a low average power continuous wave (CW) train of pulses to prevent damage to the biological sample. Our pulses suit applications requiring nJ pulses energies and CW pulse trains."

The team is now trying to optimise the pulse shortening. "By cascading the Raman process, we can design a laser that can be instantly switched between a series of visible wavelengths from yellow to red," predicted Granados. "Utilising different Raman materials could allow us to achieve greater compression factors, generating pulses with durations down to few hundreds of femtoseconds and wavelength flexibility through cascading."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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