White light continuum
This project investigates the nonlinear phenomenon of supercontinuum generation and filament formation in condensed media, the aim being to assess the technique for novel-ultra-wide band LIDAR and remote sensing applications.
The supercontinuum was first discovered in the 1970 by focusing intense picosecond laser pulses through transparent liquids and solids. This effect can be described by considering the intensity dependence of the medium's refractive index, n=n0+n2I. This intensity dependence means that the phase front at the centre of the pulse advances through a higher refractive index than the periphery, leading whole-beam self-focusing. As the pulse begins to self-focus, the intensity of the beam drastically increases to induce another nonlinear effect; self-phase modulation (SPM).
In the process of SPM, the leading edge of the leading edge of the pulse propagates through a higher index of refraction than the trailing edge, due to the intensity dependent refractive index. Therefore, this part of the medium will appear optically thicker for this part of the pulse. This results in a decrease in the group velocity of the beam and the arrival of this part of the pulse at the output of the medium will be delayed, therefore the frequency is decreased and this part is red shifted. For ultra-short laser pulses with picosecond or femtosecond duration, this spectral broadening can cover several hundred nanometres across the visible spectrum, this is called a supercontinuum (SC).
Supercontinuum spectrum generated in water
Our work employs a Ti:Sapphire laser system with a regenerative amplifier to generate 130fs pulses with peak powers of up to 7000MW. At these powers any aberrations on the spatial profile of the beam will start to self-focus and generate a supercontinuum, these individual sources of SC are called filaments.
Supercontinuum filaments in glass
Our aim is to develop a detailed physical understanding of the processes involved in SC filament generation, assessing its viability as a source in light detection and ranging (LIDAR) experiments. Conventional LIDAR techniques are limited by spectral resolution and tunability of the source laser. The SC, however, offers a broad spectral range allowing the simultaneous excitation of several absorption bands in the target medium. Use of the SC in this type of application requires accurate prediction of continua spectra resulting from self phase modulation, Raman excitation, multi-photon excitation and beam self-focussing. To date, experimental and theoretical studies have looked at SC spectra, filament dimensions, phase relationships and the effects of diffraction on filament formation. More recently, we have demonstrated SC generation in photonic crystal fibres and plan to exploit the coherence property of the SC in fibre sensing applications.