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Study On Silicon-based Single-drive Pushpull Carrier-depletion Electro-optical Modulator Chips

Posted on:2018-09-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y Y ZhouFull Text:PDF
GTID:1368330590455298Subject:Electronic Science and Technology
Abstract/Summary:PDF Full Text Request
Optical modulators are the key devices to convert high-speed large-bandwidth electrical signals from the electrical domain to the optical domain.They are widely used in large-volume optical communication systems and microwave photonics?MWP?systems and have attracted a lot of research in academia and industry worldwide.In recent years,the rapid growth of transmission capacity in fiber-optic networks has required optical modulators to possess a large electro-optic?EO?bandwidth,low power consumption and high modulation linearity to transmit and process optical signals at high bit rates.Compared with the modulators based on the lithium niobate?LiNbO3?or III–V semiconductor materials,the optical modulators made on the silicon platform surpass them as they are compatible with the complementary metal oxide semiconductor?CMOS?techniques for low-cost fabrication and monolithic integration with microelectronics on one single chip.The carrier-depletion-type silicon Mach-Zehnder modulator?MZM?exhibits high performances and has become a hot research topic in both academia and industry.To overcome the issues of the microwave attenuation and modulation nonlinearity in the previous dual-drive push-pull silicon optical modulators,this dissertation presents a silicon single-drive push-pull carrier-depletion-based MZM and focuses on the establishment of equivalent circuit model to facilitate modulator design and optimization.This model is used to improve the modulation speed,modulation linearity and modulation efficiency.This dissertation firstly establishs the equivalent circuit model of dual-drive push-pull carrier-depletion-type silicon modulator.According to this model,three design principles for the traveling wave electrode?TWE?in the silicon modulator are introduced:impedance match,velocity match and low microwave attenuation.The dissertation analyzes the effects of the microwave attenuation and characteristic impedance in order to optimize the silicon modulator.Based on the analysis,a single-drive push-pull carrier-depletion-type silicon MZM is proposed which exhibits low microwave loss,low chirp and high modulation linearity.After that,by using the partial capacitance concept and the conformal mapping,we deduce the formulas to calculate the capacitance,the inductor and the resistance for the equivalent circuit model of this modulator,giving clear directions for the modulator design and optimization.This model is validated by comparing the calculation results with the simulation ones.The equivalent electrical circuit model reaveals that microwave attenuation is decreased because the two PN junctions are connected in series,compared with that of the dual-drive electrode.Based on the equivalent circuit model,this dissertation futher optimizes the design of the single-drive push-pull modulator.The parameters that characterize the modulation performance are firstly introduced.In order to achieve a high modulation performance,the structure and doping concentrations of the PN junction are optimized to achieve a high modulation efficiency and a low optical loss.Then,the segmented PN junction is introduced to decrease the microwave attenuation.The impedance match,velocity match and microwave attenuation of the traveling wave electrode are also studied along with the related factors,including width and gap of the TWE.Based on the equivalent circuit model,we optimize the TWE to achieve a high EO bandwidth.We measure the optical spectrum of a single-drive push-pull carrier-depletion-type silicon modulator,the chip insertion loss is 79 dB and the half wave voltage V?is about5 V.The small signal microwave transmission measurement reveals that the 6.4 dB bandwidth of the electrical-electrical?EE?S21 response is 18.3 GHz.The return loss S11 is below-20 dB when the bias voltage Vb is 0 V,which indicates the characteristic impedance is close to 50?.The measured 3 dB electrical-optical?EO?response is above 29 GHz when the bias is Vb=4V.We establish high-speed modulation setups for the dual-drive and the single-drive silicon modulators.The measurement results show that the single-drive silicon modulator can achieve on-off keying?OOK?modulation with much higher extinction ratio,higher signal to noise ratio and higher modulation speed under a smaller drive voltage and a bias voltage.When the microwave drive signal amplitude is amplified to 7.8 V peak-to-peak voltage,this modulator can achieve 56 Gb/s OOK modulation and 40 Gb/s binary phase-shift keying?BPSK?modulation with the bit error rate?BER?below the forward error correction level.Next we theoretically study the linearity of the carrier-depletion-type silicon modulator based on a couple of transfer functions.In optical communication systems that employ high order modulation formats,the modulation linearity is a pre-requisite which is characterized by the spurious-free dynamic range?SFDR?.Taylor and Jacobi–Anger expansions of the transfer function reveal that the source for the modulation nolinearity stems from three parts:optical loss in the active arms,nonlinearity of the phase modulation,and MZI sinusoidal transfer function.The linearity can be improved by using a push-pull drive scheme,a differential detection and a proper choice of the operation point.Compared with conventional dual-electrode differential drive,the single-drive scheme can more effectively reduce the second-order harmonic distortion?SHD?due to the two auto-aligned push–pull signals from one RF feed.When the modulator is operated at the quadrature point,the SFDRSHD is measured to be85.9 dB·Hz1/2 with 3.9 dB improvement over the previous best result and the SFDRIMD is measured to be 97.7 dB·Hz2/3.Besides,multi-level pulse ampltitude modulation?PAM?is achieved by using this high-linearity modulator with PAM-2,3,4,5 demonstrated at a symbol rate of 40 Gbaud/s and PAM-8 at asymbol rate of 25 Gbaud/s.In order to overcome the inherent nonlinearity of the carrier-depeletion modulator,we present a dual-parallel silicon MZM.In the dual-parallel MZM,the linearity is improved when the two secondary MZMs are driven with RF signals with unequal amplitudes and fed with differnet optical powers so that the distortions induced by the two secondary MZMs counteract each other.A theoretical model has been established,revealing that the linearity can be improved by properly adjusting the RF power,the bias voltage and the operation point of the primary MZM and the secondary MZM.In order to test the dual-parrallel MZM,we perform the electrical package of the modulator chip.The measurement results show that the third-order harmonics and the third-order intermodulation distortions?IMD?can be effectively suppressed with an improved modulation linearity compared to a single MZM.The measured SFDRIMD is 96.5 dB·Hz2/3 and 101.4 dB·Hz2/3/3 at 1GHz and at 10 GHz frequencies,respectively.In the last chapter,we summarize the research work.The research and technology foresight for silicon modulators is pointed out.
Keywords/Search Tags:optical communication, silicon optical modulator, Mach-Zehnder interferometer modulator, traveling wave electrode, PN diodes, equivalent circuit model
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