基于级联微环的微波光子滤波器带宽压缩
Microwave photonic technology can process radio frequency (RF) signals in the optical domain. Compared with the traditional electrical processing methods, it has the advantages of low loss, broadband, good tunability, and sound anti-electromagnetic interference. As an important component for various applications such as radar, communications, and radio astronomy, microwave photonic filter (MPF) has become a research hotspot in microwave photonics in recent years. With the development of photonic integration technology, integrated MPFs have attracted research attention. Recently, microring resonators (MRRs) have been widely employed in MPFs thanks to their compact sizes and good adjustability. The MPF should have a narrow RF bandwidth to achieve precise RF resolution. As known, typically the RF bandwidth of the MPF based on MRR is the same as the optical bandwidth of the MRR when crosstalk is ignored. Therefore, reducing the optical bandwidth of the MRR by improving its quality factor (Q factor) is the most direct and effective way to reduce the MPF bandwidth. However, the MRR loss should be reduced to increase the Q factor, which is difficult to achieve since the scattering loss caused by the waveguide sidewall roughness is usually unavoidable. Under typical silicon-on-insulator (SOI) fabrication processes, optical bandwidth of about GHz for MRR can be obtained, which cannot meet the requirements of high-precision MPF with sub-GHz frequency resolving capability. We propose and demonstrate an MPF based on three cascaded MRRs and phase modulation. With this configuration, the 3-dB RF bandwidth of the MPF can be well compressed compared with the 3-dB optical bandwidth of the MRR, and flexible tunability of the MPF is achieved.
We put forward an MPF based on cascaded three MRRs and phase modulation. By introducing two more MRRs, the phase differences between the optical carrier and the ±1 order optical sidebands can be changed much steeper from 0-π compared with the MPF constructed by a single MRR. As a result, the photocurrent obtained by beating the optical carrier and the ±1 order optical sidebands changes abruptly from constructive interference to destructive interference. Thus the slopes on both sides of the filter peak of the MPF response can be increased to achieve RF bandwidth compressing compared with that of the MPF based on a single MRR. Simulation and experimental results show that the MPF based on cascaded three MRRs and phase modulation can compress the RF bandwidth.
We simulate the phase spectra of the optical carrier and the ±1 order optical sidebands of the MPF based on cascaded three MRRs and the MPF based on single MRR. The results show that the phase difference between 8.9-9.5 GHz for the MPF based on cascaded three MRRs is 1.12π, while the phase difference for the MPF based on single MRR is only 0.83π, which means much steeper phase changing from 0-π is achieved by the MPF based on three MRRs compared with the MPF based on single MRR [Fig. 4(b)]. Additionally, the simulation results show that compared with the MPF based on single MRR, the RF bandwidth of the MPF based on cascaded three MRRs is compressed by about 52%, and the 3-dB attenuation slope is increased about 1.1 times than that of the MPF based on single MRR [Fig. 4(d)] without enhancing the Q factor . The experimental results show that the MPF based on cascaded three MRRs can compress the RF bandwidth by about 69%, and the 3-dB attenuation slope is increased about 3.6 times than that of the MPF based on single MRR (Fig. 9). Meanwhile, continuous frequency tuning in the range of 11.5-20.3 GHz [Fig. 10(b)] and RF bandwidth tuning in the range of 187.1-1597.0 MHz [Fig. 10(a)] are achieved.
We propose and demonstrate a bandwidth compressing method for the MPF based on cascaded three MRRs and phase modulation. By adopting this method, the phase differences between the optical carrier and the ±1 order optical sidebands can be changed much steeper from 0-π than that of the MPF based on single MRR to compress the RF bandwidth of the MPF. Compared with the MPF based on single MRR, the RF bandwidth of the MPF based on cascaded three MRRs is compressed by about 69% without increasing the Q factor. Additionally, the 3-dB attenuation slope is increased about 3.6 times than that of the MPF based on single MRR. Continuous frequency tuning in the range of 11.5-20.3 GHz and RF bandwidth tuning in the range of 187.1-1597.0 MHz are achieved. Furthermore, the proposed method can achieve an even narrower RF bandwidth if an MRR with a higher Q factor is adopted. Meanwhile, the proposed MPF has the potential to be fully integrated into a chip and could find extensive utilization in microwave photonic signal processing systems.
1 引言
微波光子技术可以实现在光域上处理微波信号,和传统的电处理方式相比,微波光子技术具有可调谐性好和抗电磁干扰能力强等优点[1]。微波光子滤波器(MPF)作为微波光子技术的一项重要组成部分,在雷达、通信和射电天文领域有广泛的应用价值[2],是近年来研究的热点之一。目前,基于离散光学器件的MPF主要有基于受激布里渊散射的MPF[3]、基于光子晶体的MPF[4]、基于光纤光栅的MPF[5]、基于光纤环的MPF[6]等,这些采用分立原件的滤波器都有体积大、不易集成、缺乏可调节性等缺点。随着集成工艺的发展,MPF也逐渐集成化,目前集成的MPF中的核心光学滤波器主要包括马赫-曾德尔干涉仪(MZI)[7]、微盘谐振腔(MDR)[8]、微环谐振腔(MRR)等[9-14]。其中,MRR因为具有尺寸小和可调性好等优点被广泛应用于集成光学滤波器领域,基于MRR的MPF也成为研究者们关注的对象。为了实现更精细的滤波,要求MPF有更窄的滤波带宽,通常在忽略相位串扰的条件下,MPF的滤波带宽与其使用的光滤波器带宽相同。因此,提升光滤波器的品质因子Q值成为减小MPF滤波带宽最直接有效的方法。但是,要提升MRR的Q值,需要减小MRR的损耗,而由于微纳加工工艺所导致的波导侧壁粗糙度引起的散射损耗通常无法避免。因此,通常绝缘体上的硅(SOI)MRR滤波器的带宽都在GHz量级,无法满足高精度的微波光子滤波。2018年,Qiu等[15]提出了使用多模跑道型微环代替单模微环,减少了侧壁损耗,提升了微环的Q值。2020年,Zhang等[16]使用多模欧拉弯曲跑道型微环,进一步减少了微环的弯曲损耗,提升了微环的Q值,但是该微环的耦合区需要进行特殊的设计,对工艺误差的容忍度较小。2022年,Ji等[17]对MRR的结构进行特殊设计,提升了微环的Q值。上述三个方案都是从减少侧壁损耗出发提升了微环的Q值。就目前已有的报道,多微环级联MPF主要应用于增大带宽,例如在2019年,Xu等[18]提出了基于多微环级联的MPF,通过多个微环级联且优化每个微环的中心滤波频率,可以实现宽带宽的微波光子滤波。2022年,Liu等[1]提出了基于多模微环级联的MPF,使用多模微环提升单个微环的Q值,通过级联且优化每个微环的中心频率,可以实现更宽范围的带宽调谐。但是,对于使用级联微环进行MPF带宽压缩的应用,据本文作者所知还没有报道。
基于此,本文提出了一种基于级联三微环的MPF,有效提升了光载波与±1阶边带拍频所得光电流信号的相位差从0~π变化的陡峭度,进而增大了微波光子滤波中心频率两侧的斜率,实现了MPF带宽压缩。理论仿真结果表明,在不提升微环本身Q值的前提下,相比单微环构建的MPF,基于级联三微环的MPF将带宽压缩了约52%,3 dB衰减斜率提高了约1.1倍。实验结果表明,相比单微环MPF,基于级联三微环的MPF将带宽压缩了约69%,3 dB衰减斜率提高了约3.6倍。另外,该MPF还实现了11.5~20.3 GHz的频率连续调谐和187.1~1597.0 MHz的带宽连续调谐。
2 工作原理
基于单微环与级联三微环的MPF链路如
图 1. MPF原理图。(a)基于单微环的MPF;(b)基于级联三微环的MPF
Fig. 1. Schematic diagram of microwave photonic filter (MPF). (a) MPF based on single microring; (b) MPF based on cascaded three microrings
对于单微环MPF而言,光载波与±1阶边带拍频所得电流信号在滤波频率处满足相长干涉条件,而在远离滤波中心频率处由于MRR残余相位的原因,近似满足相消干涉条件,基于此可以获得带通型滤波响应。为了获得更窄带宽的滤波频谱,本文通过使光载波与±1阶边带拍频所得电流信号的相位差在0~π变化更陡峭,进而增大微波光子滤波中心频率两侧的斜率,如
图 2. MPF滤波中心频率调谐和MPF带宽调谐。(a)频率调谐原理图;(b)带宽调谐原理图
Fig. 2. Frequency tuning of MPF filtering center and bandwidth tuning of MPF. (a) Schematic diagram of frequency tuning; (b) schematic diagram of bandwidth tuning
3 数值仿真与分析
根据后面的实验结果,在仿真中,设置MRR环长L=3.5 mm,采用双条形氮化硅波导,其截面图如
图 3. 波导及MRR的结构。(a)双条形氮化硅波导截面图;(b)全通型MRR示意图;(c)级联三微环滤波器示意图
Fig. 3. Structures of waveguide and microring resonator (MRR). (a) Cross section of double stripe silicon nitride waveguide; (b) schematic diagram of all-pass MRR; (c) schematic diagram of cascaded three microring filter
采用传输矩阵法可以得到如
式中:a为光绕环一周的振幅传输因子;t为直波导的振幅透射系数,在不考虑耦合损耗的前提下,满足t2+k2=1,其中k为直波导与微环的振幅耦合系数;Δφ为通过加热电极(heater1)引入的相位改变;φ为光绕微环一周的相位,可以表示为
式中:L为微环环长;neff为波导模式有效折射率;λ为入射光波长。为了避免工艺误差对微环耦合系数的影响,所有微环的耦合区都是由两个耦合器构成热调谐MZI,如
后文实验中使用的级联三微环MRRs结构如
式中,dφ1、dφ2和dφ3分别表示通过heater2、heater3和heater4加热引入的相位改变量。
在MPF链路中,光载波经相位调制后的光场可以表示为
式中:Ec表示入射光载波的振幅;ωc表示光载波的角频率;φc表示光载波的初相位;ωf表示微波信号的角频率;Ji(m)表示第i阶贝塞尔函数,其中,m为调制系数,表示为
式中:VRF表示微波信号的电压;Vπ表示相位调制器的半波电压。由
经相位调制后的光载微波经级联微环后,可以得到光载波、±1阶边带信号分别为
光载波与±1阶边带经光电探测器拍频后的光电流有以下关系:
其相位可以表示为
式中,φa、φb分别为光载波与±1阶边带拍频所得光电流信号的相位。最后,输出的光电流信号可以表示为
根据后面实验中级联三微环光子滤波器透射光谱,结合
图 4. 基于级联三微环的MPF仿真结果。(a)射频响应图:① 低频带(3.0~8.3 GHz);② 中频带(8.3~9.5 GHz);③ 高频带(9.5~20.0 GHz);(b)光载波与±1阶边带拍频所得光电流相位差图;(c)光载波与±1阶边带拍频所得光电流幅度差图;(d)单微环MPF与级联三微环MPF的射频响应对比图
Fig. 4. Simulation results of MPF based on cascaded three microrings. (a) Radio frequency (RF) response image: ① Low frequency band (3.0~8.3 GHz); ② intermediate frequency band (8.3~9.5 GHz); ③ high frequency band (9.5~20.0 GHz); (b) phase differences of photocurrents from beating between optical carrier and ±1 order sidebands; (c) amplitude differences of photocurrents from beating between optical carrier and ±1 order sidebands; (d) comparison between RF responses of MPF based on single microring and cascaded three microrings
由
另外对于级联三微环MPF,光载波与±1阶边带拍频所得的光电流幅度如
为验证级联三微环MPF的带宽压缩不受微环本身Q值影响,取振幅透射系数t=0.9819的单微环MPF,其射频响应谱如
为了衡量性能提升的效率,引入衰减斜率(slope)用来反应MPF射频响应曲线的下降速率,单位为dB/oct[19]。具体为曲线在某频率区间内的下降斜率,即衰减幅度与频率变化之间的比例。本文中采用3 dB衰减斜率来衡量MPF射频响应曲线的下降斜率,即计算振幅响应衰减一半对应的衰减斜率,其表达式为
式中:fc表示滤波中心频率;f3dB表示射频响应从最高点下降到一半时对应的频率,且满足f3dB>fc。由
4 实验结果
本文基于LioniX公司的低损耗双条形氮化硅光波导流片平台,制备了级联可调微环滤波器芯片,如
搭建如
单个MRR的透射光谱如
图 7. 测试得到的透射光谱。(a)单微环透射光谱;(b)三微环透射光谱;(c)合并后的三微环透射光谱
Fig. 7. Measured transmission spectra. (a) Transmission spectra of single microring; (b) transmission spectra of cascaded three microrings; (c) transmission spectra of combined three microrings
之后,搭建如
图 8. 基于级联三微环的MPF测试链路图
Fig. 8. Experimental setup for characterizing MPF based on cascaded three microrings
基于该MPF测试链路,首先测试了上述基于单微环微的MPF的射频响应,如
图 9. 单微环MPF与级联三微环MPF的RF响应对比图
Fig. 9. Comparison of RF responses of MPFs based on single microring and cascaded three microrings
实验得到的射频响应与理论仿真结果趋势符合较好,但由于制备的微环自由光谱范围(FSR)较小,且实验中的单微环是将三级联微环中的两个微环的耦合系数调谐到1(即接近不谐振)来实现的。因此,
为了表征本文方案提出的MPF的可调谐性,
图 10. 级联三微环MPF调谐特性测试结果。(a)带宽调谐;(b)频率调谐
Fig. 10. Measurement results of MPF based on cascaded three microrings. (a) Bandwidth tuning; (b) frequency tuning
5 结论
本文提出并验证了基于级联氮化硅三微环的MPF,通过多微环级联的方式,使光载波与±1阶光边带拍频后的相位差谱从0~π变得更陡峭,从而实现了对MPF的带宽压缩。相较于单微环MPF,本文提出的MPF在不提升微环本身Q值的前提下,将滤波带宽压缩了约69%,3 dB衰减斜率提高了约3.6倍。另外,该MPF还实现了11.5~20.3 GHz的滤波频率连续调谐和187.1~1597.0 MHz的滤波带宽连续调谐。若使用Q值更高的MRR,采用本文提出的方案可以得到更窄的滤波带宽,具有很好的应用前景。
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