Fourier domain mode-locked lasers could achieve femtosecond pulses

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Fourier domain mode-locked (FDML) lasers could produce pulses shorter than 30 femtoseconds, according to researchers at the Electrical Engineering and Information Technology Department at the Technische Universitaet Muenchen (TUM) and the Physics Department of Ludwig-Maximilians-Universitaet (LMU) Munich.

The researchers have concluded that based on their models and simulations they can achieve 30 femtosecond pulses with FDML lasers, which are considered cheap, robust semiconductor sources. This conclusion was made after successfully completing experiments that produced 60 picosecond pulses, with the FDML.

‘Our models and simulations actually let us identify changes in the experimental setup that could yield a further thousand-fold improvement in performance potentially producing pulses shorter than 30 femtoseconds,’ said Dr Christian Jirauschek. He is a researcher at the Technische Universitaet Muenchen’s Institute for Nanoelectronics.

Pulses as short as 30 femtoseconds are expected to be possible because the researchers’ experiments showed that the continuous wave output from the FDML laser could be reshaped to short intense pulses. The FDML laser, typically, sweeps through a range of wavelengths.

It works by storing the entire energy of each 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 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 the colours exit the second fibre at the same time, forming a short laser pulse. This preserves the high output energy while the pulse time is shortened.

This technique avoids the need for increasing the power or using a larger device. This process is also thought to be more efficient than storing the energy in the semiconductor structure of the laser source.

Dr Robert Huber, a leader of the LMU research group that was involved in the work, is also the co-inventor of the FDML laser. The femtosecond FDML research was supported by the German Research Foundation and the European Union. The two German universities work was published in the peer reviewed journal Nature Communications.