Blue Diodes Get Ultrashort Pulses

Michael D. Wheeler

PicoQuant GmbH has introduced a blue diode laser system with a picosecond pulse width and high repetition rate that may prove useful as an excitation source in time-resolved fluorescence research.

The German company has taken the Japan-based Nichia Chemical Industries Ltd.'s blue diode laser and applied its own proprietary technique to create ultrashort pulses, resulting in a commercially available laser with a 50-ps pulse, a repetition rate up to 40 MHz and a center wavelength at 400 nm. The average power is 1 mW, with a peak power of up to 400 mW.

Biochemical analysis

The commercial availability of these diode lasers could mean good news for those performing time-correlated fluorescence spectroscopy, a powerful tool used in biochemical analysis, such as identifying a single antigen or strand of DNA. The technique derives its sensitivity from measuring the short time it takes for a given fluorophore to decay. The problem arises in attempting to record the time-decay profile of the signal from a single excitation-emission cycle. Typical fluorescence from organic fluorophores lasts only a few hundred picoseconds to a few nanoseconds, placing a stringent demand on the excitation source and detector.

Finding a short-wavelength excitation source with ultrashort pulses and enough power was a particularly thorny problem. In the past, the common sources were nanosecond flashlamps and Ti:sapphire or argon-ion lasers. Although inexpensive, flashlamps had low repetition rates and low power. Frequency-doubled Ti:sapphire or argon-ion dye lasers were an improvement, but they were bulky and prohibitively expensive.

"These systems have been around for a while and are intended for people well-trained in optics and electronics," said Steve Soper, an assistant professor at Louisiana State University in Baton Rouge, who installed a Ti:sapphire for time-correlated single-photon counting several years ago.

"Between the optical table, laser, optics and electronics, the cost was about $160,000. Now with a pulsed diode, you're looking at a much lower cost," he said.

Although PicoQuant's blue diode lasers have been available for only weeks, Soper said they could have a large impact on areas like DNA sequencing and other bioanalytical research.

"One of the big points with our system is it's cheap and easy to handle," said Michael Wahl, a senior scientist at PicoQuant. The company has already received the first orders for the lasers, which will initially cost less than $20,000.