基于SOI非对称马赫曾德尔结构的集成矢量和微波光子移相器

肖永川，瞿鹏飞，周敬然，刘彩霞，董 玮，陈维友

（吉林大学电子科学与工程学院集成光电子学国家重点联合实验室，吉林长春130012）

基于SOI非对称马赫曾德尔结构的集成矢量和微波光子移相器

肖永川，瞿鹏飞，周敬然，刘彩霞，董 玮，陈维友

（吉林大学电子科学与工程学院集成光电子学国家重点联合实验室，吉林长春130012）

设计和分析了一种基于SOI（绝缘体上的硅）脊型波导非对称马赫曾德尔结构的集成矢量和微波光子移相器。对于10 Gh x的微波信号，设定非对称两臂的长度差为3 983μm时，其相应的时间延迟约为47 ps。分别在两臂上集成了一个热光可调谐可变光衰减器用于光学调谐，当衰减单元的折射率在0～6×10-3变化时，实现了10 Gh x微波信号在0～180°的相位调谐。该器件尺寸小、结构紧凑，易于实现片上集成，在光控相控阵雷达中很有应用前景。

移相器；微波光子学；绝缘体上的硅

1 Introduction

Microwave Photonic Phase Shifter（MWPPS）is a photonic devicewhich is used to process the phase of amicrowave signal in optical domains.It has drawn much attention in both military and satellite communications due to the advantages of compact sixe，lightweight，high operating frequency and large simultaneous band.It plays an important role in Optically Controlled Phased Array Radars（OCPARs）for Optical Beam Forming Networks（OBFNs）.And it can overcome the electronic-bottleneck and tune the phase of high frequency signal in the millimeterwave band even in the Th x domain by taking advantages of microwave and photonics.Various techniques for realixing microwave photonic phase shifters have been reported，including using Optical True-time Delay（OTTD） units［1-3］， wavelength conversion in a Distributed Feedback Laser（DFB）［4］，and heterodyne mixing method［5，6］，vector-sum principle［7，8］.h owever，a practical implementation of phase arrayswith thousands of elements is limited by the sixe and complexity of the conventional phase-shifting schemes.The use of miniaturixed and integrated on-chip devices to perform this function is thus ofmuch interest.

In thiswork，we provide a vector sum MWPPS based on MZ structure in SOIwaveguideswhich can used to achieve the phase-shift from 0°to 180°for a 10 Gh x signal.It features broadband operation，flexibly tunable phase-shifting range，reduced complexity，compact footprint sixe， easy integration.And the doubled phase shift can be achieved by adding another branch.

2 Principle and design

In the vector-sum technique，two cosine signals that have the same frequency but different amplitudes（A1and A2）and phases are summed.We can control the phase of the resultant signal by changing the amplitudes of two signals：

Where

As can be seen from Eq.（3），one can easily control the phase of the resultant signal by changing the amplitude ratio（A1/A2）of the two input signals at fixedΔφ.

Based on above vector-sum technique，a compact and easily implemented on-chip asymmetric Mach-Zendner structure in SOI rib waveguides is proposed，and the schematic of integrated VSMMWPPS is shown in Fig.1.Fig.1（a）the is phase shifter structure and Fig.1（b）is the cross section of the SOIwaveguide.

Fig.1 （a）Basic structure of integrated VSM-MWPPS，（b）waveguide cross section

There are two important aspects needing to be considered.One is singlemode transmission and the another is bend loss.From Fig.2（a），for H= 1.3μm top Si，waveguide rib width of 1.1μm and external rib heightof0.6μm is considered to ensure singlemode transmission，and from Fig.2（b）and（c），when R is bigger than 180μm，less than 1 dB bend loss can be ensured.In this work R is set as 1 000μm.

Fig.2 （a）Determination of single mode condition，（b）and（c）the relationship between bend loss and bend radius when h is 500 nm，600 nm and 700 nm

The maximum fixed phase shift can be expressed as the following equation：

Where R is bend radius，Vgis group velocity，ωsis angular frequency ofmicrowave signal.Andφis the maximum phase difference between two branches，which is determined byωs，R，L0and Vg.

In this design，φis 175°.According to Eq.（4），when a 10 Gh xmicrowave signal ismodulated on optical carrier，the fixed length difference L0is calculated to be 3 983μm.Simultaneously，to ensure single mode transmission and ignorable bend loss，the specific parameters of phase shifter are shown in Tab.1.

Tab.1 Device parameters

As can be seen from Eq.（3），by changing the amplitude ratio（A1/A2），φwill range from 0°to 175°.Theoretically，we can achieve any needed phase shifts by adjusting L0and R.

To avoid degradation and instability of the transfer function of vector sum phase shifter due to coherent interference，thewideband optical source is considered to be the carrier.A schematic diagram of the VOA which is independent to wavelength is shown in Fig.3（a）［9］.Two gradual change tapers，designed to ensure single mode characteristic and low loss less than 1°tilting angle and 1 500μm waveguide length，are designed to bridge the single mode and the multi-mode waveguide.A refractive index variation ofΔwill be achieved by heating the thermo-opticalmodulation section.The smaller the tilting angleαis，the larger attenuation can be obtained with the same variation ofΔ.But the desired large resolution will be unachievable.A compromisedαof 3°is considered，which can result in 20 dB optical power attenuation whenΔincreases over 6×10-3.The relationships between attenuation and refractive index variation is shown in Fig.3（b）and the optical distribution at 20 dB attenuation is shown in Fig.3（c）.

Fig.3 （a）VOA structure，（b）stimulated normalixed output optical power changingwith refractive index variation，（c）stimulated optical field distribution whenΔis greater than 6×10-3

Theoretically analyxed phase shift result is shown in Fig.4.As can be seen，phase shift varies with optical power ratio.

Fig.4 Theoretically analyxed phase shift changes with optical power ratio

3 Conclusions

The designedmicrowave photonic phase shifter based on the asymmetric Mach-Zendner structure in SOI rib waveguides features broadband operation，flexibly tunable phase-shifting range，reduced complexity，compact footprint sixe，easy integration.We control the phase shift by tuning thermo-optic VOAs.Although our demonstration has no limitation in operation frequency，linear phase tuning is not readily achieved.

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TN256

A

2011-04-11；

2011-07-13

国家自然科学基金资助项目（No.61077046），吉林省科技发展计划资助项目（No.20090105），吉林大学基本科研业务费资助项目（No.200903084）

肖永川（1987—），男，重庆梁平人，硕士研究生，主要从事新型光电子器件的研究。

E-mail：xiaoyc10@mails.jlu.edu.cn