Study On Wavelength-Mode Pulse Interleaver Based On Silicon Optical Delay Lines

Silicon photonic chips have the advantages of compact size and high integration.In recent years,they have received extensive attention from academia.As the performance of various discrete silicon photonic devices improves,people are increasingly dissatisfied with the function of a single device.Multiple discrete electronic and photonic components are integrated on the same chip to achieve more complex functions.Silicon photonics technology plays an important role in the field of data communication with its complementary metal-oxidesemiconductor(CMOS)compatibility.Various types of lasers,modulators,detectors and optical switches based on silicon photonics technology have been continuously verified and developed.An optical delay line has a very high application prospect in the field of optical communication.Optical pulses multiplexing based on optical delay lines improves the repetition rate.High-frequency pulses can find many applications in data communication,photonic signal processing,optical analog-to-digital conversion and other fields.This dissertation studies a large-scale optical pulse interleaver chip.The use of multiplexing technologies improves the pulse repetition rate,allowing high-speed optical sampling.First of all,this dissertation introduces the concept and metics of the optical delay line and discusses its implementation architectures.From the perspective of delay tuning range,resolution,transmission loss,power consumption,and chip size,the structural characteristics of the delay line are clarified,which provides a theoretical basis for the realization of various optical pulse interleavers.After that,the dissertation presents several key components for wavelength-division multiplexing(WDM)and mode-division multiplexing(MDM)and introduces the working principle and design of various WDM and MDM structures.For the MDM device,multiple modes supported in the waveguide are analyzed.High-order modes generated by waveguide coupling are simulated.Discrete devices and the overall system of the wavelength-mode interleaver are simulated,proving the feasibility of the wavelength-mode pulse interleaving scheme.After that,in order to realize pulse time-division multiplexing,a continuously tunable delay line is implemented,which combines ring resonators and series-connected switch array.The switch array provides large-range coarse delay tuning,while the ring resonators provide short-range fine-tuning.The dual-stage Mach-Zehnder interferometer(MZI)switch elements are utilized to improve the switching extinction ratio.The delay line chip is implemented on a60-nm-thick silicon photonics platform,with an average waveguide propagation loss of 0.35 d B/cm.The maximum delay tuning range is 1.28 ns.The on-chip insertion loss is 12.4 d B,including loss caused by test ports.A 30 Gbps on-off keying(OOK)signal is successfully transmitted through the delay line chip with a high signal fidelity under various delays.The optical delay line chip can be used to tailor the pulse train in a reconfigurable manner.Optical pulse multiplexing is realized based on the delay line chip,where the switches are configured to give an even splitting ratio.Such an optical time-division multiplexing(OTDM)scheme is useful in the generation of a high repetition rate pulse train for optical sampling.Quasi-arbitrary waveform generation(QAWG)has also been demonstrated when the switch splitting ratio and variable optical attenuator(VOA)attenuation are adjusted to give unequal pulse magnitudes.The realization of OTDM and QAWG based on the delay line chip demonstrates its flexibility and reconfigurability.The chip can be potentially used as a powerful programmable optical signal processor.Then,the dissertation reports an 8-channel wavelength-mode optical pulse interleaver on a silicon photonic chip.Wavelength and mode division multiplexing techniques are combined to increase the repetition rate of the pulses.The interleaver uses a cascaded Mach-Zehnder interferometer structure as a wavelength division(de)multiplexer,an asymmetric directional coupler as a mode(de)multiplexer,and various lengths of silicon waveguides as delay lines.The influence of various parameters,such as waveguide loss,delay error and channel bandwidth,on the pulse interleaver performances,such as loss,delay interval,peak power,pulse width and crosstalk,is studied,laying the foundation for experimental implementation.The experiment verifies that a pulse sequence with a time interval of 125 ps is implemented with a delay error of 3.2%.Subsequently,based on the wavelength-mode interleaver,an integrated optical sampling chip is demonstrated.High-repetition-rate optical sampling pulses are obtained by interleaving low-repetition-rate input optical pulses through WDM and MDM techniques.The WDM pulse interleaver is based on a loopback arrayed waveguide grating(AWG)with linearly increasing delays.It has self-aligned wavelength channels,compact structure,large dispersion,and low loss.The amplitude of the interleaved pulses can be adjusted by the attenuators integrated in the loopback waveguides.Two high-order modes in a multimode waveguide are utilized to further increase the pulse repetition rate.The optical sampling pulses are modulated by a multimode Mach-Zehnder modulator with the active arms integrated with L-shaped PN junctions.The multimode modulator can perform 30 Gb/s OOK modulation.The sampled pulses are separated by mode and wavelength demultiplexers to further parallel processing.Multi-wavelength pulses are separated using dual-ring assisted Mach-Zehnder interferometer(DR-MZI)based WDM filters with a high extinction ratio.Mode-division pulse interleaving and separation are enabled by adiabatic directional coupler-based mode(de)MUXer.As both wavelength and mode multiplexing techniques are used,the pulse repetition rate can be greatly increased.The increment factor is 8,only limited by the number of WDM filters at the back end.All building blocks including high-speed modulator,polarization splitter rotator,delay lines,and WDM and MDM devices are all integrated into a single silicon chip,fully utilizing the integration capability of silicon photonics.The successful implementation of a high-speed sampling chip marks a significant step forward in the full integration of analog-to-digital converters on a single chip and opens up avenues toward real applications of microwave photonics.Finally,the dissertation summarizes the research work.The technology foresight for the integrated pulse interleaving chips is pointed out.