We present the evolution of SCE associated with filaments due to the tilt of focusing lens under tight focusing
geometries. Transform limited femtosecond (fs) pulses (800 nm, 45 fs, 1 kHz repetition rate) were focused in ambient air
using three different focusing geometries f/#6, f/#7.5, and f/#12 corresponding to numerical apertures (NA) of 0.08, 0.06,
and 0.04, respectively. The focusing lens was tilted from zero up to 20 degrees. The filaments decayed into two shorter
parts through tilting of the lens and the separation between shorter filaments increased with increasing lens tilt, in tune
with earlier reports [Kamali et al., Opt. Commun. 282, 950-954 (2009)]. The separation between the filaments matched
well with the predicted distances due to astigmatism induced in loose focusing geometries. However the deviation
increased as we moved to the tighter focusing geometries. The SCE spectrum demonstrated an anomalous behaviour.
The SCE spectrum was suppressed at larger tilt angles of 12 - 20°. However at lower tilt angles, up to 8°, the SCE was
observed to be same to that measured without any tilt of the focusing lens. This behaviour is predominant with tighter
focusing geometries of f/#6 and f/#7.5, wherein the SCE was observed to be higher at 4° and 8° in comparison with that
observed at an angle of 0°. Systematic study of the focusing lens tilt on anomalous SCE spectra and filament
characteristics in the tight focusing geometry are presented.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.