集成光电子学国家重点实验室知识库

微波光子学是一门涉及微波工程和光电子技术的交叉学科。基于微波光子学产生的光生微波、高频滤波、宽带射频接收识别等技术由于在光域对微波进行了信号处理，因此克服了传统电子信号处理系统中的带宽瓶颈可以工作在更高的带宽并且具有更低的窜扰。偏振复用技术利用光在单模光纤中偏振不相干的特性,将相同波长下的两个正交的偏振方向作为两路独立的传输信道,成倍提高了系统容量、增加了频谱利用率。基于以上原因，偏振复用与微波光子学相结合无疑能够相得益彰，使基于微波光子学的各种技术方案在原有基础上进一步提升系统的指标性能。本文正是在这一背景下进行了基于偏振复用技术的光子微波\毫米波信号的产生、接收与处理方面的研究。

本论文在第二章中设计了一种基于两个级联的偏振调制器的精确π相位微波编码系统。该系统利用偏振调制器的宽带调制特性，可以在无任何光域、电域滤波的条件下实现DC~40GHz带宽内微波/毫米波信号的高速精确π相位翻转，从而在宽谱范围内产生了连续可调的二元相位编码微波/毫米波信号。同时，本文还展示了一种基于级联偏振调制器与边沿滤波器的相位调制微波脉冲发生结构。利用这种结构，我们可以在省去波形“斩断”单元的基础上，直接产生可以用于无线发射的带通性相位编码微波脉冲，在降低了目前已报道系统的复杂度的同时，拓展了工作带宽，提升了系统稳定性。

本文的第三章则利用偏振复用技术与传统的微波光子滤波器相结合，分别设计了一种基于偏振处理结构的单通带微波光子滤波器以及利用SOA慢光效应的双抽头复系数滤波器。第一种方案利用偏振模拟复用器在两个垂直的偏振方向上对宽谱光源进行分束延时并结合偏振调制器产生微波调制，由色散位移光纤延时后经光电探测器拍频，最终在频域产生滤波通带。该方案通过调节延时与光源谱宽，实现了中心频率在DC~20GHz范围内的连续调节以及3-dB带宽的连续变化。由于系统所用器件少，集成度高，为今后的单片集成提供了很好的设计思路。针对复系数微波光子滤波器偏置电压漂移与受相干长度限制的影响，本文利用正交偏振光与SOA慢光效应对双抽头微波光子滤波器进行了设计，从结构上避免了这两个缺点，并且通过切换偏振调制器输出光的偏置角度，实现了2π范围内的移相、陷波点在一个自由光谱范围内可连续移动的效果。

在第四章中，本文针对目前基于光梳的宽谱射频/微波信道化接收机在光梳产生方面成本高、稳定性不足等缺陷进行了改进。利用偏振复用技术结合声光移频器，在不增加新的光梳频率成分的基础上，通过正交分束移频的方法，实现了空间上正交光梳的交织复用。通过模拟仿真，在不降低测量精度的条件下，经改进后，信道化接收机的测量范围在原有基础上提升了一倍。

Microwave photonics is an interdisciplinary subject involving microwave engineering and photonics technology. Since it can process microwave signals directly in the optical domain, plenty of technologies, such as microwave photonic generation, microwave photonic signal processing and broadband RF receiving and recognizing, based on this subject can benefit from its wide operation bandwidth, low insertion loss and high isolation efficiency. Photonic polarization-division multiplexing is a physical layer method for multiplexing signals carried on the orthogonally polarizied light-waves with the same frequency, which can significantly increase the channel capacity and improve the spectrum efficiency. Therefore, there couldn’t be a better way to melt the PDM technology with microwave photonics to generate a brandnew technology featuring wider processing bandwidth and higher operation frequency. Based on the above, this thesis aims to investigate the novel methods to generate, process, receive and recognize microwave signals using microwave photonic technology based on the polarization-division multiplexing.

In the chapter II, we presents a novel approach to generate a binary phase-coded microwave signal with accurate π-phase shift and large continuous operating bandwidth. In the system, the phase-coding modulation with an accurate π-phase shift has been realized by the joint use of two cascaded polarization modulators (PolMs). The generation of phase-coded microwave signals at 10 GHz, 18 GHz, and 28 GHz has been experimentally demonstrated, which verifies the proposed technique positively. Since there is no use of any optical filters and fiber Bragg gratings (FBGs), this system is rather simple and free from the optical bandwidth limitation problem with operating in a continuous microwave bandwidth as large as limited only by the PolMs (from DC to 40 GHz). Further more，we also demonstrate a technique of generating a binary phase coded microwave pulse based on two cascaded polarization modulators (PolMs). The first PolM (PolM1) followed by an optical band-pass filter is used to generate two phase-locked and polarization orthogonal optical frequencies. The second PolM (PolM2) aims to change their polarization states. A polarizer attached to the output of PolM2 allows only one of the two optical frequencies passing, or combines them with positive/negative phase difference. By changing the voltage level of the electrical modulation signal applied to PolM2, series of binary phase coded microwave pulses are directly generated from a continuous wave microwave signal in the optical domain. In the proposed system, the precise amplitude control or amplification of the modulation signal are avoided. The waveform of the generated pulse is very stable. For a proof-of-concept experiment, a series of 25-GHz pulses with∼2.08-ns pulse duration and∼10.24-ns repetition time is generated. The pulses are phase coded by a 13-bit Barker code.

In chapter III, it focuces on the signal processing unit of the microwave photonic technology. Based on the PDM technology, a widely tunable microwave photonic filter based on polarization processing of non-sliced broadband optical source has been firstly demonstrated, which features single-bandpass response and wide span of operation bandwidth. The BOS is orthogonally polarized by a polarization division multiplexing emulator (PDME) with a tunable time delay between the two polarization states and incident at ±π/4 to one principle axis of a polarization modulator (PolM). The PDME cascaded a PolM and a polarizer realizes a microwave modulation making the phase of the carrier able to be tuned while ±1^st sidebands unchanged, which after propagating in a dispersive medium results in a tunable single-bandpass response in the RF domain. We experimentally verified the MPF. By adjusting time delay amount and the optical spectrum bandwidth, the pass-band center frequency wascontinuously tuned from DC to 20GHz and the 3-dB pass-band bandwidth changed while the optical spectrum bandwidth ranges from 1nm to 4nm. Besides, with the combination of the slow light effect in SOA, we present another two-tap complex-coefficient tunable microwave photonic notch filter based on the polarization modulation assisted by a tunable band-pass filter (TBPF). In the proposed filter, the light wave is modulated by the microwave driven signal through the polarization intensity convertor (PIC) that generates two complementary microwave signals with identical amplitudes carried by two orthogonal polarized optical carriers with the same frequency, the complex coefficient is generated from the phase-shifted light wave passing through the SOA in the upper arm. The measured experimental results demonstrated that the notch position of the filter could be tuned in the whole range of the free spectral range (FSR) without changing the shape of the amplitude-frequency response.