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文中所研究的微波光子混频器原理框图如图1(a)所示。该微波光子混频器由光生本振单元和波长分离调制单元两部分组成。光生本振单元用于产生波长不同、幅度相同的本振光波。这些本振光波既可以是光域二倍频或高阶倍频的两个边带,也可以是光频梳中的多波长梳齿。波长分离调制单元通过两个光滤波器分别选取两个不同频率ω1和ω2的本振光波,即
图 1 (a) 文中所提微波光子混频器原理框图; (b) 系统中主要节点光信号示意图; (c) 文中实验采用的光生本振单元结构; (d) 波长分离调制芯片照片
Figure 1. (a) Schematic diagram of the proposed microwave photonic frequency mixer; (b) Optical signal at the main point of the system; (c) Optically-carried local oscillator used in the experiment; (d) Photographs of the wavelength-division modulator chip
$$\begin{array}{*{20}{l}} {{E_1}(t) = \exp ({\rm{j}}{\omega _1}t)}\\ {{E_2}(t) = \exp ({\rm{j}}{\omega _2}t)} \end{array} $$ (1) 两个本振光波在光电探测器中拍频产生频率为
${\omega _{{\rm{LO}}}}{\rm{ = }}\left| {{\omega _2}{\rm{ - }}{\omega _1}} \right|$ 的光生本振信号。其中一个本振光波(文中选取频率ω2的分量)通过片上相位调制器调制上频率为ωRF的RF信号。假设采用小信号调制,相位调制器输出的光信号可以表示为:$$ \begin{split} {E_{{\rm{PM - out}}}}(t) =& {{{{J}}_0}(\gamma )\exp ({\rm{j}}{\omega _2}t)} + {{{J}}_1}(\gamma )\cdot\exp [{\rm{j}}({\omega _2}t +{\omega _{{\rm{RF}}}}t +\\ & \pi /2)]{ - {{{J}}_1}(\gamma )\exp [{\rm{j}}({\omega _2}t - {\omega _{{\rm{RF}}}}t - \pi /2)]} \end{split} $$ (2) 式中:Ji(γ)为第i阶贝塞耳函数。该路光信号经光耦合器与另一路本振光波合路后送入光电探测器,此时光信号可以表示为:
$$ \begin{split} {E_{{\rm{PD - in}}}}(t) =& {\exp ({\rm{j}}{\omega _1}t){\rm{ + }}{{{J}}_0}(\gamma )\exp ({\rm{j}}{\omega _2}t)} + {{{J}}_1}(\gamma )\exp [{\rm{j}}({\omega _2}t + \\ & {\omega _{{\rm{RF}}}}t +\pi /2)]{ - {{{J}}_1}(\gamma )\exp [{\rm{j}}({\omega _2}t - {\omega _{{\rm{RF}}}}t - \pi /2)]} \end{split} $$ (3) 经光电探测器光电转换后的电流信号可以表示为:
$$ \begin{split} &s(t)\propto {E}_{\rm{PD-in}}(t)\cdot {E}_{\rm{PD-in}}^{*}(t)={{J}}_{0}(\gamma )\cos[({\omega }_{2}-{\omega }_{1})t]-{{J}}_{1}(\gamma )\cdot\\ &\sin[({\omega }_{2}-{\omega }_{1}+{\omega }_{\rm{RF}})t]-{{J}}_{1}(\gamma )\sin[({\omega }_{2}-{\omega }_{1}-{\omega }_{\rm{RF}})t+\\ &{{J}}_{1}^{2}(\gamma )\cos[2{\omega }_{\rm{RF}}t]={{J}}_{0}(\gamma )\cos({\omega }_{{\rm{LO}}{\text{光}}}t)-{\rm{J}}_{1}(\gamma )\sin[({\omega }_{{\rm{LO}}{\text{光}}}+\\ &{\omega }_{\rm{RF}})t]-{{J}}_{1}(\gamma )\sin[({\omega }_{{\rm{LO}}{\text{光}}}-{\omega }_{\rm{RF}})t+{{J}}_{1}^{2}(\gamma )\cos(2{\omega }_{\rm{RF}}t) \end{split} $$ (4) 从公式(4)可以看出,输出的电信号中包含了上变频信号ωLO光+ωRF和下变频信号ωLO光−ωRF。由于采用了相位调制,理论上不会存在RF分量ωRF。然而,拍频产生的电信号中还会存在光生本振分量ωLO和RF的2次谐波分量2ωRF。在小信号调制下,1阶贝塞耳函数远小于1。因此,RF的2次谐波分量2ωRF强度很弱,可以忽略。在实际系统中,由于光生本振的频率是已知的,可以通过电滤波器予以滤除。
Chip-based microwave photonic frequency mixer (Invited)
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摘要:
提出了一种由光生本振单元和波长分离调制单元组成的微波光子混频方法,并在绝缘体上硅材料上设计实现了上述波长分离调制芯片。该芯片集成了硅基相位调制器、微环滤波器、光电探测器、光耦合器和光栅耦合器。实验搭建了基于该波长分离调制芯片的微波光子次谐波混频系统,结果表明,该微波光子混频器可以将6~16 GHz的RF信号变频到33~23 GHz。此外,针对实验系统中残留的混频杂散,分别提出了增加微环滤波器抑制比降低泄露光生本振强度和引入光移相器修正泄漏光生本振相位两种解决方案。通过仿真验证可知,引入光移相器的方法更为简单,更适合于光子集成芯片。
Abstract:A microwave photonic frequency mixer constituted of an optically-carried local oscillator (LO) and a wavelength-division modulator was proposed. The wavelength-division modulator chip, which was consisted of a silicon phase modulator, two micro-ring filters, a photodetector, two optical couplers, and two grating couplers, was designed and fabricated. Based on the chip, a microwave photonic harmonic frequency mixer was implemented. In the experiment, an optically-carried LO was generated by double-sideband suppressed-carrier modulation at a Mach-Zehnder modulator. An RF signal from 6 to 16 GHz was successfully converted into a signal with a frequency of 33 to 23 GHz. In order to suppress the remaining mixing spurs, two solutions, i.e., increasing the rejection ratio of the micro-ring filter to decrease the intensity of the leaked optically-carried LO and introducing an optical phase shifter to correct the phase of the leaked optically-carried LO, were proposed and verified by simulation. It should be noted that the latter is simpler and more suitable for photonic integration.
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图 1 (a) 文中所提微波光子混频器原理框图; (b) 系统中主要节点光信号示意图; (c) 文中实验采用的光生本振单元结构; (d) 波长分离调制芯片照片
Figure 1. (a) Schematic diagram of the proposed microwave photonic frequency mixer; (b) Optical signal at the main point of the system; (c) Optically-carried local oscillator used in the experiment; (d) Photographs of the wavelength-division modulator chip
图 5 仿真得到的频谱图。微环滤波器抑制比(a) 10 dB和(b) 40 dB;(c) 不同微环抑制比对应的次谐波混频2ωLO−ωRF功率、RF杂散分量ωRF功率和两者间杂散抑制比
Figure 5. Simulated frequency spectra when the rejection ratio of the micro-ring filter is (a) 10 dB and (b) 40 dB; (c) Power of 2ωLO−ωRF and ωRF, and the spurs suppression ratio for the different rejection ratio of the micro-ring filter
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