Effect of Gaussian Aperture Width on Visibility for Different Crystal Length

The main goal in Quantum Cryptography is high security and this can be achieved by using single photon sources. The present work is a theoretical analysis devoted to investigate the interference pattern of biphoton amplitude generated by spontaneous parametric down conversion (SPDC) in a nonlinear crystal (BBO) pumped by femtosecond optical pulses. We have studied the visibility as a function of Gaussion aperture width for different parameters, such as the crystal length. The best visibility can be obtained at crystal length of (1.5 mm) . In our results, patterns occur and increase with increase of crystal length and decreasing aperture diameter. When using slit aperture (vertical and horizontal direction), the coincident count is more symmetry in horizontal with dimension (1×7) mm. With double crystal and parallel direction,the visibility decreases dramatically with increase in the Gaussian aperture width, while at a distance between crystals equal to (3mm) the visibility vanishes slowly at crystal thickness (L=0.5mm).