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瞬时频率测量(instantaneous frequency measure-ment,IFM)是雷达警报接收机、防静电防御和电子智能系统中的重要任务。快速的测量速度、精确的测量和大的频率测量范围对于这些应用至关重要。传统的电域IFM系统易受电磁干扰的影响,由于电子部件的带宽有限,通常测量范围限制在18GHz以内。微波光子技术可用于将IFM接收机的带宽扩展到数十甚至数百吉赫兹,并不受电磁干扰的影响。
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微波功率信息可以瞬时测量,通过测量微波功率并根据频率与功率的对应关系[4-7],从而可以实现IFM。该方法的基本原理是将未知的微波信号经电光调制转换光信号,已调信号经过两个具有不同频率相关微波功率衰耗的光学通道,经光电探测器检测出相应通道的微波信号功率,实现频率到微波功率的映射,从而建立微波功率的幅度比较函数(amplitude comparison function,ACF)。若ACF与特定频率范围内的微波信号频率具有单调关系,则可以实现IFM。以参考文献[7]为例,工作原理如图 1所示。待测微波信号经相位调制被分成两路。在上通道中,由色散引起固定的功率衰减函数。通过调节下通道中的偏振控制器来产生与上支路变化相反的功率衰减函数。图 1中上通道与下通道的功率比函数f(ωs)可表示为:
$ f\left( {{w_{\rm{s}}}} \right) = \gamma \frac{{{{\sin }^2}\left( {z{\beta _2}w_{\rm{s}}^2/2} \right)}}{{{{\sin }^2}\left( {z{\beta _2}w_{\rm{s}}^2/2 + \psi } \right)}} $
(1) 图 1 基于相位调制的瞬时频率测量系统[7]
式中, γ为常数,由系统结构决定;z为色散链路的长度;β2为单模光纤的色散系数;ωs为微波信号的频率;ψ为两条链路的相位差。
由于两条支路所实现的功率衰减函数变化趋势相反,功率比函数在宽频带上呈准线性变化。实验中得到的测频范围为1.6GHz~24.6GHz,测量误差在±0.3GHz以内。该方法的优点只需要无偏置漂移的相位调制器和单个激光源,系统结构简单。
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通过监测微波功率进行IFM的主要问题是需要高频光电探测器(photodetector,PD)和微波设备。为了避免使用这些昂贵的装置,降低测频系统的成本,可以利用光滤波器[8-10]使得已调光信号边带的幅度随不同输入信号频率发生改变,因此通过检测光功率,便可以进行IFM。以西南交通大学研究小组提出的实现IFM方案为例[10],该方法使用了单个可调光源和两个正交光滤波器。正交滤波器的滤波响应如图 2所示。调节可调激光源的波长使已调光信号的边带位于同相滤波响应的峰值点,正交响应的-3dB点。经正交滤波器对的两个信号功率与参考信号的功率相对比建立两个正交功率比函数。测频系统结构如图 3所示。实验中得到的测频范围为20GHz~36GHz,测量误差在±0.4GHz以内。该方法理论上可以实现的测频范围为整个光滤波器的自由频程(free spectral range,FSR),优于参考文献[8]和参考文献[9]中FSR/2的测量范围,并且该系统构架比参考文献[8]简单,避免使用了双光源以及波分复用器。
图 2 正交滤波器对的滤波响应[10]
图 3 采用两个正交光滤波器和单光源实现IFM[10]
另一种基于频率-光功率映射的实现方法是采用微波鉴频器的原理,工作原理如图 4所示。图中V0为输入信号幅值,Ω为输入信号的频率,τ为延迟的时间。在电域利用该方法实现测频的问题是电延迟线损耗大,所使用的混频器工作带宽有限。利用光子学的方法对信号进行延迟混频可以克服在电域遇到的问题。乘法器可以使用两个级联马赫-曾德尔调制器(Mach-Zehnder modulator,MZM)[11]或者光学非线性效应[12-13]来实现,平方律检波器可以用PD实现。2008年,墨尔本皇家理工大学研究小组利用两个级联MZM法,实现了2.2GHz~3GHz频率测量范围。测频范围由两路信号的时间延迟差τ决定,τ越小测频范围越大。系统的敏感度可以利用锁定放大技术改善,结合高非线性光纤产生的四波混频效应可以实现0.04GHz~ 40GHz的测量带宽、小于100MHz的测量误差以及51dB的动态范围[13]。
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随着数字信号处理和模数转换技术的发展,数字化技术被广泛应用于电子战的宽带接收机中。光子模数转换技术能充分利用数字化技术的优势,实现宽带数字化微波频率测量。西南交通大学研究小组提出利用相移光滤波器阵列实现微波信号频率的数字化输出[28],实验实现了5位二进制编码的数字化频率输出,测频范围为10GHz~40GHz,频率分辨率为3.9GHz。2012年,浙江大学研究人员采用双边带信号在色散介质中传播后引起微波功率衰落原理,设置不同长度光纤引入不同色散量,对微波功率进行量化编码[29],实验中实现了4位二进制编码的数字化频率输出,测频范围为0GHz~17.5GHz。相比于参考文献[28],在相同测量精度条件下,该方案所需的通道数大大减少,简化了系统结构。
基于光子辅助型模数转换实现测频的方案有利用高频激光脉冲作为电子模数转换器的采样时钟,PD端的输出信号经低速电子模数转换器采样,最后通过频率恢复算法得出待测微波信号的频率[30],实验中实现了0GHz~20GHz的频率测量范围以及±8kHz的测量误差。光脉冲作为采样时钟能克服电域中时间抖动的问题,从而提高测频的精度。另一种测频原理是采用光子时间拉伸法[31],工作原理如图 8所示。输入信号的时域波形被放大,从而可以使用采样速率较低的电子模数转换器对信号进行采样,最后经电信号处理获得待测信号频率。时间拉伸倍数M为两次拉伸后的时域脉冲宽度与调制时的时域脉冲宽度的比值,假设两段光纤的色散系数D相同,L1为色散光纤1的长度,L2为色散光纤2的长度,则拉伸倍数M可以表示为:
$ M = \frac{{D\left( {{L_1} + {L_2}} \right)}}{{D{L_1}}} = 1 + \frac{{{L_2}}}{{{L_1}}} $
(2)
基于光子学的微波信号频率测量研究进展
Research progress of frequency measurement of microwave signal based on photonics
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摘要: 微波频率测量是电子侦察中的重要内容,随着雷达电子战的发展,微波工作频率不断攀升,电域的测频方案由于测量带宽的限制,无法满足电子侦察的发展需求。利用微波光子技术实现频率测量的系统具有瞬时带宽大、低损耗、抗电磁干扰等特点,能克服电子领域在微波频率测量中所面临的瓶颈问题。根据目前基于光子学的微波信号频率测量方案,从瞬时频率测量、光子辅助微波信道化、多频测量、基于光子模数转换技术、光子压缩感知技术5种不同类型的测频原理展开了介绍和分析,并对基于集成光学的微波信号频率测量技术进行了探讨。在微波信号频率测量技术的发展中,基于光子学的测量方法具有广阔的应用前景。Abstract: Microwave frequency measurement is an important part of electronic reconnaissance. With the development of radar electronic warfare, the operating frequency of microwave increases rapidly. Conventional electronic frequency measurement schemes cannot meet the development of electronic reconnaissance due to their limitation in measurement bandwidth. Approaches of microwave frequency measurement based on photonics have the characteristics of large instantaneous bandwidth, low loss and immunity to electro-magnetic interference. According to the current frequency measurement schemes of microwave signal based on photonics, five technical approaches are introduced and discussed, including instantaneous frequency measurement, photonic-assisted microwave channelization, multi-frequency measurement, microwave frequency measurement based on photonic analog-to-digital conversion, photonic compressive sensing. Moreover, the potential of integrated optics for photonics-based microwave frequency measurement is briefly discussed. In the development of microwave frequency measurement, the photonics-based method has a broad prospect of application.
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图 1 基于相位调制的瞬时频率测量系统[7]
图 2 正交滤波器对的滤波响应[10]
图 3 采用两个正交光滤波器和单光源实现IFM[10]
图 6 基于傅里叶余弦变换测频系统结构图[19]
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