Coherent optical communication using quadrature phase-shift keying (QPSK) provides high-speed data transmission using in-phase and quadrature (IQ) modulation formats over long distances. A quadrature optical modulator with two Mach-Zehnder interferometers in a two-way configuration with simultaneous phase shift in its arms significantly improves the technical characteristics and ensures the electromagnetic compatibility of digital signal transmission paths. The two-cycle configuration of an optical modulator is the basis for high-speed and noise-resistant data transmission. The research results prove that, after certain corrections to the known coherent optical transmission structures, the system can be used for key distribution. The structure of a coherent two-band optical key sequence formation system is presented. The model for generating optical radiation with modulation by a phase-shifted radio signal is based on a quadrature optical modulator with two parallel Mach-Zehnder interferometers with constant bias voltages on all control electrode arms. Key sequence formation is implemented by modulating the in-phase and quadrature components of radio signals at the subcarrier frequency. A set of two amplitude multiplication coefficients for each bit is specified by the electronic encoding device. The coefficients represent the signals that modulate the amplitude of the in-phase and quadrature components of the subcarrier frequency, respectively. The optical radiation generated at the modulator output provides key distribution (setting the values of zero "0" and one "1" bits in the binary number system) according to a protocol with four phase states in the rectangular and diagonal bases. A module for controlling the encoding of the in-phase and quadrature components of a radio signal at a subcarrier frequency is proposed. The use of quadrature phase shift keying (QPSK) provides noise immunity to external influences at high data transfer rates
The use of broadband optical amplification, wave division multiplexing, dispersion compensation of optical radiation, and differential phase-shift keying enables data transmission at rates of up to 40 Gbit/s. Prospects for further increasing transmission rates to 100 Gbit/s are associated with the use of multilevel modulation formats for radio signals on multiple subcarrier frequencies, modulation of the radiation from a single optical quantum generator by radio signals on multiple subcarriers, balanced homodyne detection of coherent optical radiation, and digital signal processing. Symbol-based transmission via quadrature phase-shift keying (QPSK) provides high data rates. Prior studies have substantiated the use of a single-sideband coherent optical radiation generation algorithm with subcarrier QPSK modulation. Due to hardware instabilities, amplitude and phase errors may arise, leading to quadrature imbalance. These inaccuracies introduce additional errors during demodulation of the received signal, which can significantly degrade reception interference immunity. The aim of this study is to analyze the process of generating single-sideband optical radiation modulated by a QPSK radio signal on a subcarrier frequency using two parallel Mach–Zehnder interferometers. A distinguishing feature of the proposed approach is that the derived mathematical relationships make it possible to subsequently assess the impact of amplitude and phase errors in quadrature signal generation (quadrature imbalance) on reception quality.
