2.5 光模數轉換
隨著數字信號處理技術的飛速發展，雷達回波的信息提取基本上都在數字域完成。作為連接模擬域回波和數字信號間的橋梁，ADC在雷達接收機中發揮著重要的作用。由于ADC孔徑抖動等原因，大的模擬帶寬和高的有效位數在完全基于電子技術的ADC中難以兼得。因此，電ADC的性能往往成為限制寬帶雷達發展的瓶頸。為突破電ADC的帶寬瓶頸，具有大帶寬、抗電磁干擾能力強等諸多優點的光子技術被引入到ADC系統中，構成了光子輔助ADC，使ADC發展到新的階段。光子輔助ADC最早出現于20世紀70年代。經過40余年的發展，國內外學者提出了多種光子輔助ADC，將光子技術應用到了信號模擬預處理、采樣保持、高速實時量化等多個方面。
光域信號預處理，是指將待轉換的模擬電信號調制到光載波上，利用光器件的超大帶寬實現對模擬信號的處理，以降低信號模數變換的難度，目前主要有信號時域拉伸[88-89]和信號復制[90-91]2種形式。時域拉伸型光子輔助ADC首先利用光脈沖在色散介質中的展寬來拉伸待轉換的模擬信號，這等效為降低信號的瞬時帶寬，因而采用低速電ADC即可完成信號的采樣和量化。而信號復制型光子輔助ADC可在光域對待轉換信號或其片段進行高質量復制，再將復制所得的多個相同信號在時域或頻域展開，然后通過錯位采樣即可獲得等效采樣率的成倍提升。常用的光域信號復制方式包括時域上的多級間插[90]和復制緩存環[92]，以及頻域上的基于四波混頻效應的多波長參量廣播等。
光采樣型光子輔助ADC利用激光脈沖對輸入的電信號進行采樣[93]，基本結構如圖21所示。鎖模激光器輸出光脈沖經復用送入電光調制器，其強度被待轉換電信號所調制，光電探測器將光脈沖序列攜帶的電信號提取出來并送入電ADC進行量化。電ADC的高穩定度時鐘信號由鎖模激光器提供。由于電ADC的采樣速率一般較低，可以在光電探測之前對光脈沖序列進行串并變換（即解復用）。這種光采樣ADC利用了鎖模激光器輸出激光脈寬極窄，脈沖間隔時間抖動極小等特性，使傳統電ADC因孔徑抖動導致的噪聲和失真大大降低。由于電光調制器具有幾十GHz的調制帶寬，光采樣模數轉換系統只需選用市場上ENOB高但模擬帶寬較小的電ADC，便可實現高精度的射頻帶通采樣。
圖21、光采樣型光子輔助ADC 的基本結構
Fig. 21 Schematic diagram of the photonic sampled ADC
光子技術同樣可應用于模擬信號的實時量化。信號量化的本質是將待轉換信號的瞬時幅度映射成多路可供比較器進行門限判決的強度脈沖，映射所得的并行支路越多，則量化位數越高。光量化方案中的這種映射主要由并行多路電光強度調制或光孤子自頻移效應實現。在并行多路電光調制結構中，各支路具有不同的強度調制特性：不同的半波電壓[94]、有相移的相同半波電壓[95]以及二者的混合[96]。當調制端口輸入的模擬電信號變化時，各調制支路輸出的光強按不同的規律改變，經后續處理即可組合出不同的編碼。而基于光孤子自頻移效應的方案[97-98]先用待轉換電信號調制光脈沖串的幅度，再利用頻移與光脈沖幅度的關系將幅度信息映射到光波長域，最后通過光色散器件將不同波長的光分開。這種方案已經實現了6位的量化分辨率[99]。
3、結論與展望
雷達是現代戰爭中極為重要的軍事裝備，是海、陸、空、天各兵種的“眼睛”。為了擦亮這只“眼睛”，下一代雷達向著高頻率、超寬帶、多功能一體化方向發展，以期在提高距離分辨率、改善目標識別成像等諸多性能的同時，又能提高雷達的隱蔽性與抗干擾性能。微波光子技術憑借其寬帶、抗電磁干擾等特性，將逐步取代部分傳統電技術在雷達系統中發揮作用。當前該領域的研究，已經從單元研究向系統研究轉變，全面進入了雷達樣機研制和功能演示階段。但是微波光子雷達各關鍵技術的融合，系統指標的提升，轉換能效，動態范圍，可靠性等方面還需進一步提高以滿足實戰系統的需求。尤其是光電集成技術相對于純電集成技術還較初步，這必將限制微波光子雷達系統的應用范圍。但是科技因未知而美妙，因探索而精彩。通過研究人員在超低相噪光電振蕩器、超寬帶波形產生、多功能信號處理、光控真延時波束形成網絡以及各技術之間融合的探索，一定能推動微波光子雷達系統的大發展。
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作者：潘時龍，張亞梅。南京航空航天大學電子信息工程學院，雷達成像與微波光子技術教育部重點實驗室