Quantum Key Distribution (QKD) represents an advanced method for achieving secure communication, leveraging principles rooted in quantum mechanics. Unlike traditional encryption methods reliant on computational complexity, QKD utilizes intrinsic quantum mechanical properties to establish cryptographic keys between remote parties. QKD systems operate using flying-qubit states, generated by photon sources at a single-photon level and employing physical degrees of freedom such as polarization, phase, orbital-angular momentum, etc., for quantum information processing. There are various approaches employed to generate photons for secure communication, including on-demand generation of single photons triggered by deterministic events, as well as the use of non-deterministic generation of photon-pair and weakcoherent states, each suitable for specific scenarios. Each type of photon source offers distinct advantages and disadvantages tailored to applications within QKD systems, considering various protocols and transmission channels such as optical fiber, free-space, and underwater. These factors influence the efficiency, range, and security of QKD implementations, guiding the selection of photon sources to optimize performance in diverse operational conditions. In the scope of thesis, B92-protocol based on polarization encoding of nonorthogonal states is performed over various free-space distances and repetition rates utilizing weak-coherent states.