微波光子雷达及关键技术（六）

2.5 光模数转换

随着数字信号处理技术的飞速发展，雷达回波的信息提取基本上都在数字域完成。作为连接模拟域回波和数字信号间的桥梁，ADC在雷达接收机中发挥着重要的作用。由于ADC孔径抖动等原因，大的模拟带宽和高的有效位数在完全基于电子技术的ADC中难以兼得。因此，电ADC的性能往往成为限制宽带雷达发展的瓶颈。为突破电ADC的带宽瓶颈，具有大带宽、抗电磁干扰能力强等诸多优点的光子技术被引入到ADC系统中，构成了光子辅助ADC，使ADC发展到新的阶段。光子辅助ADC最早出现于20世纪70年代。经过40余年的发展，国内外学者提出了多种光子辅助ADC，将光子技术应用到了信号模拟预处理、采样保持、高速实时量化等多个方面。

光域信号预处理，是指将待转换的模拟电信号调制到光载波上，利用光器件的超大带宽实现对模拟信号的处理，以降低信号模数变换的难度，目前主要有信号时域拉伸^[88-89]和信号复制^[90-91]2种形式。时域拉伸型光子辅助ADC首先利用光脉冲在色散介质中的展宽来拉伸待转换的模拟信号，这等效为降低信号的瞬时带宽，因而采用低速电ADC即可完成信号的采样和量化。而信号复制型光子辅助ADC可在光域对待转换信号或其片段进行高质量复制，再将复制所得的多个相同信号在时域或频域展开，然后通过错位采样即可获得等效采样率的成倍提升。常用的光域信号复制方式包括时域上的多级间插^[90]和复制缓存环^[92]，以及频域上的基于四波混频效应的多波长参量广播等。

光采样型光子辅助ADC利用激光脉冲对输入的电信号进行采样^[93]，基本结构如图21所示。锁模激光器输出光脉冲经复用送入电光调制器，其强度被待转换电信号所调制，光电探测器将光脉冲序列携带的电信号提取出来并送入电ADC进行量化。电ADC的高稳定度时钟信号由锁模激光器提供。由于电ADC的采样速率一般较低，可以在光电探测之前对光脉冲序列进行串并变换（即解复用）。这种光采样ADC利用了锁模激光器输出激光脉宽极窄，脉冲间隔时间抖动极小等特性，使传统电ADC因孔径抖动导致的噪声和失真大大降低。由于电光调制器具有几十GHz的调制带宽，光采样模数转换系统只需选用市场上ENOB高但模拟带宽较小的电ADC，便可实现高精度的射频带通采样。

图21、光采样型光子辅助ADC 的基本结构
Fig. 21 Schematic diagram of the photonic sampled ADC

光子技术同样可应用于模拟信号的实时量化。信号量化的本质是将待转换信号的瞬时幅度映射成多路可供比较器进行门限判决的强度脉冲，映射所得的并行支路越多，则量化位数越高。光量化方案中的这种映射主要由并行多路电光强度调制或光孤子自频移效应实现。在并行多路电光调制结构中，各支路具有不同的强度调制特性：不同的半波电压^[94]、有相移的相同半波电压^[95]以及二者的混合^[96]。当调制端口输入的模拟电信号变化时，各调制支路输出的光强按不同的规律改变，经后续处理即可组合出不同的编码。而基于光孤子自频移效应的方案^[97-98]先用待转换电信号调制光脉冲串的幅度，再利用频移与光脉冲幅度的关系将幅度信息映射到光波长域，最后通过光色散器件将不同波长的光分开。这种方案已经实现了6位的量化分辨率^[99]。

3、结论与展望

雷达是现代战争中极为重要的军事装备，是海、陆、空、天各兵种的“眼睛”。为了擦亮这只“眼睛”，下一代雷达向着高频率、超宽带、多功能一体化方向发展，以期在提高距离分辨率、改善目标识别成像等诸多性能的同时，又能提高雷达的隐蔽性与抗干扰性能。微波光子技术凭借其宽带、抗电磁干扰等特性，将逐步取代部分传统电技术在雷达系统中发挥作用。当前该领域的研究，已经从单元研究向系统研究转变，全面进入了雷达样机研制和功能演示阶段。但是微波光子雷达各关键技术的融合，系统指标的提升，转换能效，动态范围，可靠性等方面还需进一步提高以满足实战系统的需求。尤其是光电集成技术相对于纯电集成技术还较初步，这必将限制微波光子雷达系统的应用范围。但是科技因未知而美妙，因探索而精彩。通过研究人员在超低相噪光电振荡器、超宽带波形产生、多功能信号处理、光控真延时波束形成网络以及各技术之间融合的探索，一定能推动微波光子雷达系统的大发展。

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作者：潘时龙，张亚梅。南京航空航天大学电子信息工程学院，雷达成像与微波光子技术教育部重点实验室