微 波 光 子 信 道 模 拟 器 相 位 噪 声 抑 制 分 析

Objective Microwave photonic channel emulators effectively break through the performance constraints of traditional electronic devices in operating frequency range, instantaneous bandwidth and parallel processing capacity, and thus exhibit great potential for constructing ultra^-wideband, high-frequency and high-fidelity electromagnetic simulation environments. Nevertheless, when microwave signals are generated, transmitted and processed via optical carriers, phase noise induced by finite laser linewidth, optical frequency comb disturbance and link jitter seriously deteriorates signal fidelity. For phase^-sensitive modulation schemes including orthogonal frequency division multiplexing (OFDM) and high^-order quadrature amplitude modulation (QAM), such noise impairments mainly lead to constellation rotation, severe inter^-carrier interference and degraded bit error rate (BER) performance, which seriously hinder practical deployment of microwave photonic channel emulators. To solve this problem, this study constructs a full^-link phase noise suppression framework by jointly optimizing optical domain phase locking, transceiver front^-end design and digital domain phase error compensation. Methods This study proposes an integrated full^-link phase noise suppression scheme combining optical phase^-locked loop (OPLL), sideband optical phase^-locked loop (SB-OPLL) and coherent optical orthogonal frequency division multiplexing (CO-OFDM). At the transmitter, a flat optical frequency comb is generated by cascading phase modulator and intensity modulator. By jointly optimizing modulation depth and DC bias of Mach‒Zehnder modulator, uniform power distribution among comb lines is realized, which provides multi^-wavelength optical sources for multi^-band coherent radio frequency (RF) signal generation. On this basis, a heterodyne phase discrimination OPLL is adopted to precisely synchronize signal optical frequency comb and local oscillator optical frequency comb. The beat signal extracted from central comb carriers mixes with ultra^-stable RF reference signal to generate error signal, which is fed back to the acousto-optic frequency shifter in local oscillator branch to achieve dynamic phase calibration. At the receiver, an all^-optical channelized receiving structure based on SB-OPLL is designed. The received RF OFDM signal is loaded onto signal comb lines via polarization division multiplexing dual^-parallel Mach ‒ Zehnder modulator, and realizes accurate phase matching with local oscillator comb under SB-OPLL regulation. A 9:1 optical power splitter divides input optical signals into main down^-conversion branch and phase control branch. The main branch completes signal down^-conversion and coherent detection, while the control branch extracts real^-time phase error information, performs filtering processing and feeds back control signals to realize dynamic local oscillator phase correction. In the digital domain, this paper analyzes the phase noise evolution mechanism inside CO-OFDM systems, and conducts performance comparison among multiple compensation algorithms including LMS, DF, DF-LMS, SC-DF, and SC-DFL via Monte Carlo numerical simulations under different laser linewidths, subcarrier quantities and modulation orders. Results and Discussions Theoretical derivation and simulation results show that the proposed optical frequency comb generation method stably outputs multi^-wavelength optical sources with power flatness superior to 1 dB and more than seven valid comb lines, which lays a solid foundation for multi^-band coherent RF signal synthesis. Benefiting from heterodyne OPLL, precise phase synchronization between signal comb and local oscillator comb is well guaranteed, which effectively suppresses relative phase fluctuation caused by laser linewidth, ambient temperature drift and link interference. Accordingly, multi^-band, frequency-adjustable and highly coherent RF OFDM signals can be steadily generated, offering a feasible solution for high-fidelity broadband microwave photonic signal generation. From the perspective of loop control characteristics, lock^-losing recovery time depends on loop bandwidth, loop filter parameters, actuator response speed and external disturbance intensity, which essentially reflects the inherent trade^-off among phase locking precision, system stability margin and transient response capability. At the receiving end, the designed SB-OPLL^-based all^-optical channelized receiving system realizes coherent phase matching between two sets of optical frequency combs, and supports high-fidelity down^-conversion and large-capacity parallel processing of broadband signals. Combined with 90° optical quadrature mixing and electric I/Q demodulation technology, the system completely retains signal amplitude and phase characteristics, and achieves alias^-free frequency conversion and multi^-channel parallel reception. Relevant analysis also verifies that although fiber chromatic dispersion does not directly produce random phase noise, it introduces obvious group delay difference among comb lines, optical sidebands and OFDM subcarriers. Such delay difference further causes frequency^-selective residual phase deviation and aggravates inter^-carrier interference in CO^-OFDM receiving systems, especially under large bandwidth, numerous subcarriers and high^-order modulation conditions. This conclusion indicates that apart from laser linewidth and link fluctuation, dispersion^-induced phase deviation is also a key factor restricting phase fidelity of microwave photonic receiving systems. In terms of digital domain compensation, residual phase noise in CO-OFDM systems mainly embodies as common phase error (CPE) and inter^-carrier interference (ICI). CPE triggers integral constellation rotation, while ICI causes spectrum energy leakage between adjacent subcarriers. With the increase of laser linewidth, system noise characteristics gradually transform from single CPE dominance to joint dominance of CPE and ICI, and the negative influence of ICI becomes increasingly prominent. Among all tested algorithms, SC^-DFL adopts two-stage hierarchical processing architecture. In the first stage, decision^-directed LMS rapidly tracks and compensates CPE to restrain global constellation deflection. In the second stage, integrated SC^-DF module works: self^-cancellation mechanism pre^-suppresses ICI by utilizing inherent correlation features of adjacent subcarriers, and decision feedback cooperates with frequency domain equalization to further eliminate residual interference. Simulation results demonstrate that under 16QAM modulation and laser linewidth ranging from 200 kHz to 1800 kHz, narrowing laser linewidth always helps improve overall system performance. Compared with LMS, DF, DF-LMS and SC^-DF, SC^-DFL shows weaker performance degradation against increasing laser linewidth and possesses stronger BER anti^-interference capability under moderate and large linewidth conditions. This phenomenon does not mean wide laser linewidth brings positive effects, but proves that SC-DFL can simultaneously suppress CPE and ICI more efficiently under severe phase noise interference. Comparative experiments under diverse modulation orders and subcarrier numbers further reveal that higher modulation order and more subcarriers both strengthen system sensitivity to phase noise and accelerate BER deterioration. Hence, advanced phase noise compensation algorithms are indispensable under wide linewidth scenarios, and properly reducing modulation order is an effective way to guarantee system operating reliability. Conclusions This study establishes a complete full^-link phase noise suppression architecture for microwave photonic channel emulators from three dimensions: optical domain precise phase locking, transceiver front^-end structural optimization and digital domain adaptive noise compensation. At the transmitting terminal, the cooperation of flat optical frequency comb and heterodyne OPLL supports flexible generation of multi^-band, frequency^-tunable and high^-coherence RF OFDM signals. At the receiving terminal, the SB-OPLL^-based all^-optical channelized receiving scheme realizes high-precision alias^-free down^-conversion and efficient parallel processing of broadband signals. In digital signal processing, SC-DFL algorithm presents the optimal BER robustness under moderate and large laser linewidth conditions owing to its dual suppression capability for both CPE and ICI. In general, this research provides solid theoretical basis and practical technical references for the design and engineering implementation of low phase noise, broadband and high-fidelity microwave photonic channel emulators. Future research can further optimize real^-time locking response speed of optical phase^-locked loops and explore lightweight intelligent phase noise compensation algorithms to adapt to complex actual channel environments.