Significance Homodyne coherent wireless optical communication enables the direct recovery of baseband signals by interfering a local oscillator with the received signal light under identical frequency and stable phase conditions. This approach provides outstanding detection sensitivity, strong resilience against interference, and high spectral efficiency, making it a promising candidate for next generation high capacity and long reach optical links. However, its practical realization depends critically on maintaining precise and stable phase alignment between the signal and the local oscillator. This requirement imposes rigorous demands on laser source coherence, phase locking accuracy, and adaptive control algorithms. Addressing these challenges is fundamental to harnessing the full potential of homodyne detection in real world dynamic environments such as free space optical channels, which are often affected by atmospheric turbulence, polarization fluctuations, and other time varying impairments.

Progress Significant progress has been made in the key technologies of the robust homodyne coherent receiver, including wavefront distortion correction technology, local oscillator noise suppression technology, phase locking technology, polarization control technology, mixing technology and dual balanced detection technology. Wavefront distortion correction, primarily implemented through adaptive optical systems, compensates for phase errors induced by atmospheric turbulence and optical imperfections. Early conceptual work proposed using deformable mirrors, and subsequent development of wavefront sensors enabled closed loop operation. Modern adaptive optical systems employ multi actuator deformable mirrors, liquid crystal spatial light modulators for static or quasi static correction, and hybrid schemes that combine fast steering mirrors with high order deformable mirrors. These improvements enhance correction bandwidth, spatial resolution, and suitability for real time operation in turbulent channels. Local oscillator noise suppression targets phase, frequency, intensity, and quantum noise sources. Optical injection locking and self injection locking techniques effectively reduce phase noise. Electronic feedback and servo control, often using PID algorithms, provide precise frequency stabilization. Intensity noise is mitigated through balanced detection architectures and by operating the local oscillator at an optimal power level that maximizes the signal to noise ratio. Quantum noise, a fundamental limit, is addressed through techniques such as squeezed state generation and nonlinear optical processing, though these remain areas of active research. Phase locking technology ensures the local oscillator accurately tracks the signal phase. Optical phase locked loops form the core of this subsystem, evolving from discrete component based designs to architectures incorporating digital signal processing. Key developments include decision driven optical phase locked loops, Costas loop based designs for carrier recovery, and hybrid optical phase locked loops that combine optical and electronic feedback. Phase recovery algorithms, adapted from digital coherent communications, play a complementary role. The Viterbi Viterbi algorithm, blind phase search, and Kalman filtering are widely used to estimate and compensate residual phase noise, especially for high order modulation formats like 16 QAM and 64 QAM. Polarization control ensures the polarization states of the signal and local oscillator are matched at the mixer. Early systems relied on mechanical polarization controllers with slow feedback. The field progressed towards integrated electro optic devices based on lithium niobate, offering faster response. Recent trends focus on system level integration, combining polarization tracking with phase recovery algorithms, and on novel devices like metasurface based polarization controllers that offer compact size and programmable functionality. Polarization diversity receivers and polarization insensitive mixer designs also help alleviate the control burden. Mixing techniques generate the in phase (I) and quadrature (Q) components required for complex signal recovery. Fiber based mixers using 3 dB couplers were common but suffer from polarization sensitivity. Integrated optical hybrids on platforms like silica or indium phosphide offer improved stability and compactness. Crystal based mixers, particularly those utilizing the electro optic effect in periodically poled lithium niobate, provide inherent phase accuracy and polarization insensitive operation. Recent advances in thin film lithium niobate enable broadband, low loss mixers suitable for high speed applications. Dual balanced detection converts the optical interference signals into differential electrical outputs, effectively rejecting common mode noise and amplifying the weak signal component. The core of this technology is a pair of matched photodiodes in a differential configuration. Developments have focused on improving photodetector bandwidth, responsivity matching, and integration with planar lightwave circuits. Systems achieving common mode rejection ratios exceeding 40 dB have been demonstrated, directly translating to higher receiver sensitivity and dynamic range.

Conclusions and Prospects Homodyne coherent systems exhibit superior sensitivity and spectral efficiency compared to conventional heterodyne and direct detection schemes. The continuous refinement of wavefront correction, noise suppression, phase locking, polarization control, mixing, and balanced detection has progressively lowered implementation barriers, enabling laboratory demonstrations and early field trials. Current performance benchmarks show clear advantages in receiver sensitivity, tolerance to optical signal to noise ratio, and resilience to certain channel impairments. Nevertheless, challenges remain for widespread deployment. Further reduction of phase noise is critical for higher order modulation formats. Managing the coupled dynamics of phase, polarization, and frequency under rapidly varying atmospheric conditions requires more intelligent, adaptive control systems. System complexity and cost also need reduction for practical applications. Future research will likely focus on several key directions: 1) Deep photonic integration, combining lasers, modulators, mixers, and detectors on a single chip using platforms like silicon photonics or thin film lithium niobate to reduce size, power consumption, and cost. 2) Advanced digital signal processing and machine learning algorithms for joint estimation and compensation of multiple impairments in real time. 3) Exploration of quantum inspired techniques, such as phase squeezed states, to surpass classical sensitivity limits. 4) Development of robust and adaptive adaptive optical systems tailored for specific turbulent channel characteristics. Progress along these paths will enhance the robustness, scalability, and energy efficiency of homodyne coherent receivers, solidifying their role in future high performance terrestrial and satellite based wireless optical communication networks.