Determining the dependence of phase difference modulations between light pulses in a modified Mach-Zehnder interferometer was used to develop an optical system coding the information and working as an eavesdropping sensor for an optical fibre information exchange system. The basic challenge in the system development is to maintain stable operation in changing environmental conditions, as well as to ensure optimal parameters of the phase modulator. The system was tested for various many-kilometer long transmission lines of single-mode fibres. The research was focused on achieving the normative Bit Error Rate for the system in the 100 Mbit/s range (STM-1). Such a system can be used in commercial applications for the code key secure transmission in the physical layer of the link.