Abstract
This thesis brings into close connection two scientific disciplines, Microwave Photonics (MWP) and Quantum Communications (QC). The main objective is the proposal, analysis and experimental validation of quantum key distribution (QKD) systems based on the frequency coded (FC) technique by means of tandem modulator configurations. FC-QKD systems are compatible with the incorporation of multiplexing techniques used in MWP to QKD systems. The thesis presents a successful operation of a wavelength division multiplexing (WDM) optical network based on a subcarrier multiplexing (SCM) QKD system with BB84 protocol. SCM-QKD systems brings several advantages such as high spectral efficiency and the sharing of the optical source by all the multiplexed channels, which reduce the complexity of the system and permit the possibility of upgrading with WDM to increase the number of parallel keys and to coexists with other classical information channels over the same fiber infrastructure. 
A theoretical analysis has been developed in order to understand the functionalities of the QKD systems based on FC technique. Expressions for the quantum key error rate (QBER) and the transmission key rate have been found taking into account several limiting factors of the SCM-QKD systems, including the dispersion of the fiber and intermodulation effects. Complementing this theoretical analysis, different schemes have been experimentally implemented in the laboratory in order to evaluate the experimental viability of this type of structures and technologies for its employment in QKD systems. Finally, a four independent channel QKD system featuring a sifted key rate of 10 kb/s/channel over an 11 km link with a QBER lower than 2 % is demonstrated. These results open the way for multi-quantum key distribution over optical fiber networks.