Laser researchers have managed to get compact, inexpensive, long-lived devices to produce ultra-short, high-energy light pulses.
Ultra-short laser pulses are highly desirable in applications such as biomedical imaging, material processing and communications and the team from Munich, Germany, has now shown that a new approach could deliver the desired capabilities with no problematic trade-offs.
Their latest paper, published in Nature Communications, describes experiments showing that cheap, robust semiconductor lasers can produce high-energy light pulses as short as 60 picoseconds (trillionths of a second) without the drawbacks of previous approaches in terms of power consumption and device size.
And at the same time the paper presents theoretical results predicting that this technique will break the next barrier for such lasers: subpicosecond pulses.
"Our models and simulations actually let us identify changes in the experimental setup that could yield a further thousand-fold improvement in performance," says Dr Christian Jirauschek of Technische Universitaet Muenchen (TUM), "potentially producing pulses shorter than 30 femtoseconds."
The research was the result of collaboration between members of the Electrical Engineering and Information Technology Department at TUM and the Physics Department of Ludwig-Maximilians-Universitaet (LMU) Munich.
The team’s approach employs a relatively new kind of laser - the Fourier domain mode-locked (FDML) laser co-invented by leader of the LMU group Dr Robert Huber – in a novel configuration.
Rather than emitting light cantered on one highly specific "colour," the FDML laser rapidly and repeatedly sweeps through a range of wavelengths. The idea behind the experiment now is to reshape the continuous wave output from the FDML laser to short intense pulses.
"The advantage of this experimental configuration," Huber explains, "comes from storing the entire energy of each FDML laser sweep directly as a light field, spread out like colours of an infrared rainbow, in a kilometre-long optical fibre inside the laser resonator."
The researchers say this is more efficient than storing the energy in the semiconductor structure of the laser source.
The different wavelength components travel at different speeds and enter a second optical fibre, outside the laser, at different times.
This second fibre is laid out so that the different speeds exactly compensate for the different entry times: all colours exit the second fibre at the same time, forming a short laser pulse.
This is the key to preserving high output energy even while shortening the pulse time – without increasing power consumption or requiring the use of a larger device.