基于微波光子的通信技术，凭借其多频段，大带宽，重量轻，体积小，抗电磁干扰等众多优势，有望应用在雷达、侦查、卫星通讯、宽带无线通信等众多领域。本文针对无线通信系统的重要环节——接收前端部分展开了深入研究。传统电子接收前端由于电子瓶颈的限制，高频段、大带宽的信号接收往往需要极为复杂的接收结构，体积重量功耗大、且系统可重构性差。基于微波光子的接收前端虽然针对上述因素有着先天的优势，然而目前所报道的大多数方案也都存在明显的缺点。功能方面：多数混频方案功能单一，无法适应复杂的接收环境，例如需要波束赋形的雷达、侦察接收机；性能方面，需要进行多维度的考虑，例如系统增益、噪声系数、非线性优化等。本文重点针对这些问题，研究接收前端所涉及混频、移相、线性优化和色散补偿等技术要点，取得以下创新成果：

针对微波光子接收前端混频技术，提出了微波光子混频和移相的新方案。该方案利用一个集成的双偏振正交相移键控调制器对射频实现抑制载波的双边带调制方式，对本振信号实现抑制载波的单边带调制方式，并对两者实现偏振复用，利用偏振控制器对正交偏振态引入相位差的原理，对混频后的信号在0到360度范围内引入可调相移。实现了微波光子混频与移相的有机结合，在波束成型网络，矢量信号正交（IQ）解调等方向具有潜在的应用价值。由于本振信号的单边带调制方式，光纤色散引入的相位无法通过干涉作用影响信号功率，使该系统与光纤传输有更好的兼容性。同时还实验验证了该方案在矢量信号IQ解调上的有效性，对四进制相移键控、16进制正交幅度调制信号实现了IQ解调处理，得到了清晰的解调星座图。

针对微波光子接收前端及传输链路优化技术，提出了基于偏振调制器的同时线性优化和光纤色散补偿的方案，利用偏振调制器实现调制边带的奇偶阶偏振分离，通过分析非线性光边带的产生机理以及光纤色散对不同边带引入得相位变化，利用偏振控制器对正交偏振态引入相差的原理，同时对非线性和色散进行优化，系统无杂散动态范围至少提高10dB；提出了基于双偏振马赫-曾德尔调制器的系统综合优化方案，通过对调制器直流偏压，正交偏振态信号相位差以及偏分复用信号的偏振方向的控制，同时对色散、非线性以及载波边带比进行优化，显著优化了系统增益，无杂散动态范围以及噪声系数等；提出了基于双偏振正交相移键控调制器的混频与链路综合优化方案，将系统优化应用至接收前端，在下变频的同时，利用拉格朗日极值法求得射频衰减、偏振角度的最优值，以完成对非线性的优化。通过调节主调制器直流偏压，完成了光纤色散的补偿，通过调节子调制偏压，提高了系统增益。

针对传统信道化接收机通道间隔离度、通道一致性差，解调效率低等缺点，提出了基于信号偏振复用的超高效信道化接收机。所提方案利用4线射频光梳和3线本振光梳就实现了12个信道的解调，相对与传统方案，光梳利用率提升至2-4倍；由于信号的偏振复用，所占用光谱范围也降低至传统方案的一半以上。采用双输出的镜像抑制解调结构，并利用集成的双偏振相干接收板，不仅减少了器件的使用，降低系统成本和复杂度，同时还使得系统更加稳定。最后本方案利用简单的数字信号处理算法，有效的解决了宽带信号IQ解调接收情况下，IQ幅度相位不平衡的难点，提升了通道间隔离度、杂散抑制等指标。对于该方案偏振隔离度差导致的通道间隔离度变差的问题，提出了利用双芯光子晶体光纤的偏振分束器（偏振消光比为50 dB，带宽为60 nm）或硅偏振分束器（偏振消光比高达40 dB）替代原系统中偏振消光比较差的偏振分束器的解决思路，以提升隔离度的指标。系统结构简单，可扩展性强，为未来微波光子信道化接收提供了理论基础和关键技术支撑。

The communication technology based on microwave photonics, with its advantages of multi-frequency band, large bandwidth, light weight, small size, and anti-electromagnetic interference, is expected to be applied in many fields such as radar, reconnaissance, satellite communication, broadband wireless communication and so on. This article has launched an in-depth study on the important part of the wireless communication system-the receiving front-end part. Due to the limitation of the electronic bottleneck of the traditional electronic receiving front-end, the signal reception of high frequency band and large bandwidth often requires a very complicated receiving structure, large volume and weight, high power consumption, and poor system reconfigurability. Although the reception front-end based on microwave photons has inherent advantages for the above factors, most of the currently reported solutions also have obvious shortcomings. In terms of function: most mixing schemes have a single function and cannot adapt to complex receiving environments, such as radars and reconnaissance receivers that require beamforming; in terms of performance, multi-dimensional considerations are required, such as system gain, noise figure, nonlinear optimization, etc. This article focuses on these issues and studies the technical points involved in the reception front-end such as mixing, phase shifting, linear optimization and dispersion compensation, and has achieved the following innovative results:

 

Aiming at the front-end mixing technology of microwave photon receiver, a new scheme of microwave photon mixing and phase-shifting is proposed. In this scheme, an integrated dual polarization quadrature phase shift keying modulator is used to realize the double sideband modulation for raido frequency signals and the single sideband modulation for local oscillator signals, and the polarization multiplexing is realized for both. Based on the principle of introducing phase difference to the orthogonal polarization state by the polarization controller, a tunable phase shift is introduced to the mixed signal in the range of 0 to 360 degrees. It realizes the organic combination of microwave photon mixing and phase shifting, and has potential application value in beamforming network, vector signal in-phase (I) and quadrature (Q) demodulation and other directions. Because of the single sideband modulation of local oscillator signal, the system overcomes the problem of power periodic fading caused by fiber dispersion, and makes the system more compatible with fiber transmission. At the same time, the effectiveness of the scheme in the vector signal IQ demodulation is verified by experiments. The quaternary phase shift keying and hexadecimal quadrature amplitude modulation signals are demodulated, and the demodulation constellation is obtained.

 

Aiming at the optimization technology of microwave photonic receiving front-end and transmission link, a scheme of simultaneous linear optimization and fiber dispersion compensation based on polarization modulator is proposed. The odd and even order polarization separation of modulation sideband is realized by using polarization modulator. The generation mechanism of nonlinear optical sideband and the phase change caused by fiber dispersion to different sidebands are analyzed, and the phase difference of orthogonal polarization state is introduced by using polarization controller By optimizing the nonlinearity and dispersion, the spurious free dynamic range of the system can be improved by at least 10dB; a comprehensive optimization scheme based on dual polarization Mach-Zehnder modulator is proposed, which significantly optimizes the gain and noise of the system by controlling the direct current bias voltage of the modulator, the phase difference of the orthogonal polarization state and the polarization direction of the polarization-multiplexed signal, as well as the dispersion, nonlinearity and carrier sideband ratio The system optimization is applied to the front end of the receiver, and the optimal values of radio frequency attenuation and polarization angle are obtained by using Lagrange extremum method at the same time of down conversion, so as to complete the optimization of nonlinearity. By adjusting the direct current bias voltage of the main modulator, the fiber dispersion is compensated. By adjusting the sub modulation bias voltage, the system gain is improved.

 

Aiming at the disadvantages of traditional channelized receiver, such as poor channel isolation, poor channel consistency and low demodulation efficiency, a super-efficient channelized receiver based on signal polarization multiplexing is proposed. The proposed scheme uses 4-wire radio frequency optical comb and 3-wire local oscillator optical comb to realize 12 channels demodulation. Compared with the traditional scheme, the utilization ratio of optical comb is increased to 2-4 times; due to the polarization multiplexing of signal, the occupied spectral range is reduced to more than half of the traditional scheme. Using dual output image suppression demodulation structure and integrated dual polarization coherent receiving board, not only reduces the use of devices, reduces the cost and complexity of the system, but also makes the system more stable. Finally, the scheme uses a simple digital signal processing algorithm to effectively solve the difficulty of IQ amplitude and phase imbalance in the case of broadband signal IQ demodulation reception, and improves the channel isolation, spurious suppression and other indicators. In order to solve the problem of poor isolation between channels caused by poor polarization isolation, the polarization beam splitter (polarization extinction ratio is 50 dB, bandwidth is 60 nm) or silicon polarization beam splitter (polarization extinction ratio is up to 40 dB) of dual core photonic crystal fiber is proposed to replace the polarization beam splitter with poor polarization extinction ratio in the original system, so as to improve the isolation index. The system has simple structure and strong scalability, which provides a theoretical basis and key technical support for the future microwave photonic channelization reception.

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