Centre for Lasers and Applications

Mode locking is a method of persuading a laser source to generate a train of short pulses rather than giving a smooth "continuous wave" (CW) output. A train of pulses in the time domain means that the laser output contains a set of evenly-spaced frequencies in the frequency domain - these frequencies are the different longitudinal cavity modes of the laser oscillator, all oscillating with their phases locked together. The laser then outputs a train of short pulses, with the period between pulses close to the round-trip time of the laser cavity. We can see then that the inside the laser cavity there is a single pulse rattling back and forth, of which a little leaks out of the cavity each time it reflects from the output coupler.

There are lots of ways of doing this, but they fall into two types; either active ways where we generate the mode locking forces (e.g. AO modelocking, synchronous pumping), or passive ways where the pulse itself generates the locking force (e.g. Kerr lens, saturable mirrors, colliding pulse). Passive locking generates far shorter pulses since as the pulse gets shorter the mode locking forces it is itself creating get stronger and stronger.

Mode locked lasers are important for generating picosecond and femtosecond pulses for a huge range of applications, including generating the highest possible laser powers (petawatts), measuring fast chemical and physical events, and industrial applications such as material processing.

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At Macquarie University, we have recently described a new type of mode locking that can occur in laser that contain Raman-active materials. This is a long-standing research area of the CLA. We have discovered a spontaneous passive mode locking that leads to a pulsing at the fundamental laser wavelength as well as at the "Stokes-shifted" wavelength generated by the Raman medium. This effect was first observed by Zhao and Jackson at the OFTC, and we have recently explained their observations with a numerical model. The Quicktime movie on the left shows how the fundamental and Stokes pulses develop as they travel back and forth in the laser cavity.

This is an exciting new way to create a mode locked laser. Many promising avenues of research are underway, including more numerical modelling, making picosecond mode locked lasers in crystalline Raman lasers, and understanding and optimising the mode locking in fibre lasers. We are looking for a research student to lead this work.

We are also developing the first femtosecond ultra-violet laser source, based on the laser material Ce:LiCAF. We are working to develop a mode locked laser based on this material - this has the potential to create pulses as short as 3 femtosecond long! This technology will be important for chemical and physical sensing of fast events, and could lead to a new generation of UV Terawatt laser sources.