Research On Integrated Optical Waveguide Switches And Tunable Delay Lines

Optical delay lines play a very important role in the fields of optical signal buffering and synchronization,optical coherence tomography,optical information processing,and microwave photonic system,especially,in the fields of microwave photonic filtering and optical beamforming of microwave photonics.Optical delay lines can be realized based on optical fibers and optical waveguides technology.With the increase of radar operating frequency,the required delay step becomes smaller and smaller,and the precision cutting of the optical fiber is difficult to meet the requirements.In addition,the large volume of the optical fiber delay lines makes it impossible to achieve on-chip integration so as to meet the requirements of miniaturization and integration of the delay system.The integrated waveguide delay line is defined by lithography technology,making precise delay with sub-picosecond order of resolution that is required by the increasingly high operating frequency of the radar possible.In recent years,with the rapid development of optical waveguide technology,the fabrication technologies of integrated optical waveguide are more and more advanced and mature,and the fabrication materials are also more and more diversified,the integrated optical waveguide delay lines have received more and more attention.The tunability of the delay line provies flexibility to the system.The tunable optical waveguide delay line can be realized by the tunable resonant structure,such as microring,Mach-Zehnder interferometer(MZI),etc.,or by the combination of the optical waveguide switch and the optical waveguide pairs with different lengths to change the length of the optical signal paths.Although the former is usually compact and capable of continuous tuning,its bandwidth is usually smaller.The latter is usually larger in footprint and can only achieve discrete tuning,but the bandwidth is usually larger.Considering the application requirements of delay system,this dissertation focuses on the research of tunable delay lines with low loss,low power consumption,and broadband,as well as the optical switch used in delay lines.The major contents are summarized as follows.1.Research on scalable and discrete tunable optical waveguide delay line with ultralow-loss silica waveguide platform.A spiral structure of delay line is proposed,which can be easily scaled to M bit delay line with a slight increase in dimensions.As verification,1 bit and 4 bit delay line are designed,fabricated and measured.The footprint of our fabricated 1 bit delay line is 33 mm × 13 mm(length × width)and can provide a delay of 6.0 ps with an insertion loss of 1.2 d B at the operating wavelength of 1550 nm,while the footprint of 4 bit delay line is 43 mm × 14 mm and can provide a discrete delay tuning from 6.0 ps to 90.2 ps with a delay deviation lower than 0.2 ps and a tuning response time of 840 ?s.The insertion losses are lower than 2.34 d B and the extinction ratios are greater than 20.42 d B.The average switching power is ?132.6 m W.2.A polymer optical waveguide switch,featuring broadband,low power,and weak polarization dependence,driven by parallel graphene heaters is proposed and experimentally verified.In this work,a specially designed asymmetrical directional coupler is used as 3 d B beam splitter to greatly increase the operating bandwidth of the switch.Secondly,the graphene electrode is in direct contact with the waveguide core,which improves the heat utilization efficiency and reduces the switching power consumption.At the same time,a parallel graphene electrode is employed to achieve a low driving voltage.Finally,the large waveguide core is used to acheve negligible light absorption induced by the graphene heater for the quasi-TE and quasi-TM fundamental modes,low polarization-dependent performances of the switch is realized.The fabricated switch has a length of 9.5 mm,an extinction ratio(ER)of greater than 13.5 d B over 1530-1605 nm,and a maximum ER of 26 d B at 1561 nm.The power consumptions for the two polarizations are ?3.30 m W and 3.12 m W,respectively.The insertion losses are less than10.5 d B,and the rising and falling times are 0.98 ms and 0.52 ms,respectively.3.Three-dimensional(3-D)polymer optical switch driven by a graphene electrode and a tunable delay line based on the proposed 3-D switch are proposed.The switch is based on a vertical directional coupler(VDC)and has the characteristics of low crosstalk,low loss,and compact structure.The use of the graphene electrode by placing it in direct contact with the upper waveguide core not only improves the thermal efficiency and reduces the power consumption,but also reduces the thermal effect on the lower waveguide.The thermal efficiency was further improved by introducing air slots on both sides of the graphene heater.The simulation results show that the switching power is 3.6m W,the crosstalk of the whole C-band is less than-21.6 d B,and is less than-45.2 d B at1550 nm at bar state.The delay line based on the optical switch constructed with VDC can provide continuous tuning functionality by tuning the power coupling ratio of the VDC.As an example,a continuously tunable delay line with a tuning range of 0-200 ps and a minimum 3 d B bandwidth of >10 GHz is contructed based on three VDCs and the corresponding delay waveguides cascade with asymmetric MZI.Compared with a single typical asymmetric MZI structure,the delay bandwidth product of our proposed device is improved by four times.4.A SiON/Polymer hybrid integrated total internal reflection(TIR)thermo-optic(TO)switch and a delay line with mesh structure based on the proposed TIR switch are proposed.The optical switch utilizes the opposite thermo-optic coefficient of SiON and polymer to achieve low power consumption.The fabricated optical switch shows that the extinction ratio is larger than 15.34 d B,the switching power is ?59.6 m W,the rising time and falling time are 1.8 ms and 1.1 ms,respectively,the insertion loss is less than 10.6d B,and the polarization loss is ?0.3 d B.Furthermore,a compact and programmable mesh delay line is designed based on the proposed compact TIR switch.The grid delay lines can form a single microring structure,side-coupled integrated spaced sequence of resonator,coupled resonator optical waveguide,asymmetric MZI,and a series of optical waveguide switches together with pairs of optical waveguides with different lengths,forming continuous tunable delay lines or discrete tunable delay lines with high fabrication tolerance and programmable delay lines.