The broad spectral content of the white-light supercontinuum means that it has great potential for a wide-range of applications such as remote sensing, light detection and ranging experiments. In order to realise these applications it is essential to gain an understanding of the various physical properties which affect the filamentation process. This paper firstly presents accurate measurements of the filament length and width by various experimental techniques. A Ti:Sapphire laser was combined with a regenerative amplifier system to provide up to ImJ pulses centred around 800nm. Use of a positive lens to focus these pulses through condensed media such as glass and water led to the generation of a randomly-distributed cluster of filaments. These filaments were directly imaged onto a CCD array parallel to the axis of propagation (z axis), subsequent analysis of the captured images allowed the filament width to be. deduced and was found to range from 15 to 26µm. Through translating the positive lens in the z direction it was possible map-out the filament profile and obtain a measurement for its length and also its longitudinal intensity profile. The length of the filaments is found to be between 1 and 2mm. The location of each filament is random and originates from aberrations on the wave front of the beam. However, with the use of a cylindrical lens to focus the beam a line focus is produced resulting in a one dimensional array of filaments which is much easier to study. It is shown that at an energy of 780µJ only two filaments are generated in water and, in the absence of any imaging optics, diverge to form remarkably stable interference fringes . A theoretical analysis is performed and treats each filament as a circular aperture, this reveals that for a fringe spacing equal to measured spacing the circular aperture i.e. filament, must be 13µm in width, this is in good agreement with the measured filament width of 15.6µm This coherence property is also demonstrated by using an array of diffractive microlenses to focus the beam through a sample of B 270 crown glass with the use of a diffractive microlens array. In this experiment each lenslet of the array forms between 1 and 6 filaments, all regularly arranged in a square or rectangular pattern. When only two neighbouring lenslets are illuminated the filaments diverge to form stable interference fringes. Furthermore, the introduction of a p phase delay between neighbouring filaments results in a shift of the interference pattern i.e. bright fringes change to dark . Finally we present a simple technique for inducing filamentation without the aid of a focussing lens and controlling the location of the filament on the axis of propagation. A circular aperture with 1.7mm diameter is used to induce Fresnel diffraction on the incident beam, this causes the central portion of the transmitted beam to oscillate through a series of maxima and minima. By translating a 5cm-long sample of glass in the z direction it is shown that filamentation is induced only when the input surface of the glass is in the region of an axial intensity maximum. © 2005 IEEE.
|Title of host publication
|2005 European Quantum Electronics Conference, EQEC '05
|Published - 2005
|2005 Conference on Lasers and Elctro-Optics Europe - Munich, Germany
Duration: 12 Jun 2005 → 17 Jun 2005
|2005 Conference on Lasers and Elctro-Optics Europe
|12/06/05 → 17/06/05