Generation of Entangled Photons by A Short Coherence Length Violet Diode Laser

Abstract

Pumping a BBO crystal by a violet diode laser with a wavelength of (405  nm) output power of (24 mW) and a line width of (3nm) was employed to generate entangled  photons with a wavelength of 810 nm by achieving type II phase matching conditions.
The coincidence count rate obtained in this experiment was in the range of (18000) counts/s. Two BBO crystals with different thicknesses of (4 mm and 2 mm) were tested, where maximum count rates of about (18000) counts/s was obtained with a (5*5*2) mm BBO crystal where the short coherence time for the pumping source was tolerated by using shorter  BBO crystals. Also, the effect of compensating crystal on the walk-off effect was studied. The coincidence count rates were increased by using these crystals when their optic axes were fixed in the opposite direction relative to the optic axis of the original BBO crystal