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信道化合成系统的设置如图1所示。系统中,两个自由频谱范围(Free Spectrum Range,FSR)不同的光频梳是由上下两组光频梳生成模块产生的。其中,位于本振支路(上支路)的本振光频梳的自由频谱范围为ωFSRLO,信号支路(下支路)产生的信号光频梳的自由频谱范围为 ωFSRsig(假设ωFSRLO<ωFSRsig,当ωFSRLO>ωFSRsig时情况相似)。由于ωFSRLO和ωFSRsig在数值上不同,定义两个光频梳的自由频谱范围的差值为ωΔFSR,即ωΔFSR=|ωFSRsig−ωFSRLO|。从图2(a)中所示的原理图中可以看到,由实线箭头表示的信号频梳与虚线箭头表示的本振频梳在第一根梳齿对齐。然而,由于自由频谱范围差值ωΔFSR的存在,在其他信道的信号频梳梳齿和本振频梳梳齿的频率差呈现有规律的变化,表现为在第k个信道,梳齿的频率差为k·ωΔFSR。不同信道中双频梳梳齿的不同频率差使得对不同输入信号的阵列化变频成为了可能。值得注意的是,为了实现输出信号中心频率的可重构性,本振支路被移频模块下移频ωshift,因此得到的本振频梳被整体下移频ωshift。移频后的本振频梳和信号频梳被一个偏振合束器合路以相互正交的偏振态合成成为一路光波并输入波分复用器。
图 2 信道化合成原理图。(a)上下支路原理图;(b)合成信号频谱拼接原理图
Figure 2. Principle of the channelized synthesis system. (a) Principle of the two branches; (b) Principle of the spectrum stitching of the synthesized signal
一个波分复用器(Wavelength Division Multiplexer,WDM)将光频梳的每个梳齿划分到不同的信道之后,输入的窄带信号在各个信道完成了对信号光频梳的调制。依据波长,波分复用器将频率为ωLO,k和ωsig,k的本振频梳梳齿和信号频梳梳齿划分在同一个信道,此时的同一信道内的本振频梳梳齿和信号频梳梳齿处于正交的偏振态。假设有N个窄带信号输入N个不同的信道,那么每个信道中的偏振复用调制器依据偏振态的不同能够将窄带信号调制到信号频梳的梳齿上,而本振频梳梳齿保持未调制的状态。之后,另一个波分复用器将各个信道的梳齿合路并输入偏振分束器。由于只有N个信道参与合成,此时合路后的两个双频梳梳齿都只有N个。除此之外,由于信道中的偏振复用调制器实现的是窄带信号的相位调制,因此信号支路还需要FP腔的滤波以完成各个窄带信号的抑制载波单边带调制。经过滤波后,各窄带信号(图2(a)中不同颜色的形状)只保留了+1阶边带。
假设输入窄带信号的表达式为:uk(t)=ak(t) cos(ωIFt + θk(t)),其中k表示第k个信道,ak(t)表示窄带信号的幅度,θk(t)表示窄带信号的相位,ωIF表示窄带信号的中频。经过FP腔滤波后,进入光学混波器前得信号支路表达式可由下式给出:
$${E_{{\rm{sig}}}}(t) \propto \sum\nolimits_{k{\rm{ = }}1}^N \begin{array}{l} \left\langle {{a_k}(t)\exp \left\{ {j\left[ {{\omega _{{\rm{IF}}}}t + {\theta _k}(t)} \right]} \right\}} \right\rangle \\ \exp \left\{ {j\left[ {{\omega _{\rm{c}}}t + (k{\rm{ - }}1){\omega _{{\rm{FSRsig}}}}t} \right]} \right\} \\ \end{array} $$ (1) 式中:ωc表示连续光激光器的种子光的频率。而本振频梳在进入光学混波器前的表达式为:
$${E_{{\rm{LO - shift}}}}(t) \propto \sum\nolimits_{k{\rm{ = }}1}^N {\exp \left\{ {j\left[ {{\omega _{\rm{c}}}t{\rm{ + }}(k{\rm{ - }}1){\omega _{{\rm{FSRLO}}}}t{\rm{ - }}{\omega _{{\rm{shift}}}}t} \right]} \right\}} $$ (2) 由公式(2),由于本振频梳有N个梳齿作为各个信道的本振信号参与拍频,所以在多外差检测时,其他信道的本振频梳梳齿也可能参与某一信道的拍频并产生干扰。以第四信道为例,图3(a)的示意图是将图2(a)的代表第四信道的灰色虚线框区域放大后得到的。显然,在图3(a)中的虚线区域,第四信道的窄带信号u4(t)与同信道的本振梳齿ωLO,4拍频可得到目标信号,目标信号对应于图3(b)中虚线区域内的窄带信号。同样的,第四信道的窄带信号与第五信道的本振梳齿ωLO,5也会拍频,如图3(a)中点状线所示,并且会产生如图3(b)中点状线区域中的干扰信号。由于更高频信道或更低频信道的本振频梳与第四信道窄带信号的拍频超出了合成信号的频率范围,因此可以忽略不予考虑。在通常的方案中,为了避免这种干扰带来的频谱重叠,通常会将输出合成信号的最高频率设置为本振梳齿的FSR的一半,即ωmax ≤ ωFSRLO/2。但在文中,使用如图1所示的带有多频带干扰抑制的多外差探测结构就可以抑制这种干扰,从而将信号的最高频率提高一倍,达到ωFSRLO。干扰抑制的原理如下:
图 3 目标信号与干扰示意图。(a)目标信号与干扰的产生;(b)目标信号和干扰信号在输出频谱的位置
Figure 3. Schematic diagram of the target signal and the interference. (a) Generation of the target signal and the interference; (b) Position of target signal and interference in the output spectrum
知第四信道滤波后的窄带信号表示为:
$$ \begin{split} &{E_{{\rm{sig}},{\rm{4}}}}(t) \propto \left\langle {{a_4}(t)\exp \left\{ {j\left[ {{\omega _{{\rm{IF}}}}t + {\theta _4}(t)} \right]} \right\}} \right\rangle\times \\ & \exp \left\{ {j\left[ {({\omega _c}{\rm{ + }}3{\omega _{{\rm{FSRsig}}}}t)} \right]} \right\} \\ \end{split} $$ (3) 而本振路的第四和第五信道的本振梳齿表达式为:
$$ \begin{split} & {E_{{\rm{LO - shift}}}}(t) \propto \exp \left\{ {j\left[ {({\omega _{\rm{c}}} + 3{\omega _{{\rm{FSRLO}}}} - {\omega _{{\rm{shift}}}})t} \right]} \right\} {\rm{ + }} \\ & \exp \left\{ {j\left[ {({\omega _{\rm{c}}} + 4{\omega _{{\rm{FSRLO}}}} - {\omega _{{\rm{shift}}}})t} \right]} \right\} \\ \end{split} $$ (4) 式中:公式(4)第一项为第四信道本振信号;第二项为第五信道本振信号。
经过光学混波器与平衡探测器(Balance Photodetector,BPD)后,两路BPD的输出分别为:
$$\begin{array}{l} {i_1}(t) \propto {a_4}(t)\cos \left[ {{\omega _{{\rm{IF}}}}t + {\theta _4}(t) + (3{\omega _{\Delta {\rm{FSR}}}} + {\omega _{{\rm{shift}}}})t} \right]{\rm{ + }} \\ {a_4}(t)\cos \left[ {{\omega _{{\rm{IF}}}}t + {\theta _4}(t) + ( - {\omega _{{\rm{FSRLO}}}} + 3{\omega _{\Delta {\rm{FSR}}}} + {\omega _{{\rm{shift}}}})t} \right] \\ \end{array} $$ (5) $$\begin{array}{l} {i_2}(t) \propto - {a_4}(t)\sin \left[ {{\omega _{{\rm{IF}}}}t + {\theta _4}(t) + 3{\omega _{\Delta {\rm{FSR}}}}t + {\omega _{{\rm{shift}}}}t} \right]{\rm{ + }} \\ {a_4}(t)\sin \left[ { - {\omega _{{\rm{IF}}}}t - {\theta _4}(t){\rm{ + }}({\omega _{{\rm{FSRLO}}}} - 3{\omega _{\Delta {\rm{FSR}}}} - {\omega _{{\rm{shift}}}})t} \right] \\ \end{array} $$ (6) 式中:i1(t)和i2(t)中的第一项都表示为第四信道窄带信号u4(t)与第四信道的本振梳齿的拍频产生的频率分量;i1(t)和i2(t)中的第二项都表示第四信道窄带信号u4(t)与第五信道的本振梳齿的拍频频率分量。
之后,90°电桥将i2(t)相移−π/2后与i1(t)相加,可得此时的表达式为:
$${i_{{\rm{channel}}4}}(t) \propto {a_4}(t)\cos \left\{ {\left[ {{\omega _{{\rm{IF}}}}t{\rm{ + }}{\theta _4}(t)} \right]{\rm{ + }}\left[ {3{\omega _{{\rm{\Delta FSR}}}} + {\omega _{{\rm{shift}}}}} \right]t} \right\}$$ (7) 显然,第四信道的窄带信号与更高频的第五信道LO梳齿的拍频产生的干扰分量被抑制,而同信道窄带信号拍频产生的目标信号被保留。
由此,经过多外差探测之后,得到的输出为:
$$i(t) \propto \sum\nolimits_{k = 1}^N {{a_k}(t)\cos \left\{ \! \!\!\!\begin{array}{l} \left[{{\omega _{{\rm{IF}}}}t{\rm{ + }}{\theta _k}(t)} \! \right] {\rm{ + }}\left[\! {(k{\rm{ - }}1){\omega _{{\rm{\Delta FSR}}}} + {\omega _{{\rm{shift}}}}} \right]t \\ \end{array} \!\! \! \right\}} $$ (8) 从公式(8)可以看出,信道化系统输出的信号是输入的N个窄带信号的变频之后的组合。从频域进行分析,如图2(b)所示,当每个窄带信号的带宽恰好等于FSR之差ωΔFSR时,各个窄带信号在频谱上被首尾相连地拼接到一起,组成了一个带宽为N·ωΔFSR的大带宽的合成信号,实现了信道化的合成。
Research on channelized synthesis of ultra-wideband radio frequency signal based on dual optical frequency combs (Invited)
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摘要:
随着现代通信系统的发展,宽带和高频微波射频信号在雷达,通信和信号处理等领域的应用越来越广泛。基于微波光子信道化技术,文中通过两个自由频谱范围不同的光学频梳,实现了超宽带射频信号的信道化合成。在信道化合成系统中,多个独立的窄带信号输入各个信道进行上变频,并在多外差探测中被重组成为一个具有连续频谱的宽带射频信号。在多外差探测中,干扰抑制技术的使用提高了合成射频信号可达到的最高频率。在实验中,合成了一个覆盖频率范围8.4~12.4 GHz,瞬时带宽为4 GHz的宽带射频信号。实验结果显示,干扰的抑制率达到了21 dB,表明干扰抑制技术的使用提高了输出信号的最高频率的同时有效地提高了频谱利用率。
Abstract:With the development of the modern communication system, broadband and high-frequency microwave radio frequency (RF) signals have been widely applied in the fields of radar, communication and signal processing. Based on the microwave photonic channelization, ultra-wideband RF signals were generated through dual optical frequency combs (OFCs) with different free spectrum ranges (FSRs). In the channelized synthesis system, multiple independent narrowband signal was input in each channel for up-conversion and detected by multi-heterodyne detection to reconstruct a wideband RF signal with continuous spectrum. In multi-heterodyne detection, interference suppression technique increased the highest frequency that the synthesized RF signal could reach. In the experiment, a wideband RF signal was synthesized with an instantaneous bandwidth of 4 GHz, covering a frequency range of 8.4-12.4 GHz. The experiment demonstrates an interference suppression ratio of 21 dB, indicating that the interference suppression technique increases the highest frequency of the output signal and effectively improves the spectrum utilization.
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