| Broadband electro-optic modulators are key components in high-speed optical communication and microwave photonic systems.The characteristics of the modulators,such as the modulation bandwidth,the in-band flatness and the half-wave voltage,are critically important for the system performance.Compared with bulk lithium niobite modulators,indium phosphide modulators,silicon modulators and graphene modulators,lithium niobate on insulator(LNOI)-based electro-optic modulators are featured with broad modulation bandwidth,low half-wave voltage and small size.Especially,the modulation bandwidth of the LNOI-based electro-optic modulator has exceeded 100 GHz,and has the potential to reach 500 GHz.Hence,it is recognized as a promising candidate to achieve ultra-wideband electro-optic modulation.Considering these advantages,it is of great scientific significance and application value to make a profound study of LNOIbased electro-optic modulators.In this dissertation,research work is focused on the characteristic analysis and the application of broadband electro-optic modulators,where special attentions are paid to the frequency response characteristic of broadband intensity and phase modulators,as well as the design,the characterization and the application of the LNOI-based electro-optic modulators.The main contents of this dissertation are listed as follows.(1)A new scheme to measure the relative frequency response of the electro-optic intensity modulators with a high resolution is proposed based on dual-carrier modulation and low-frequency detection.In this scheme,continuous-wave(CW)light from a laser source is split into two branches.The frequency of the CW light in one branch is shifted via an acousto-optic frequency shifter.Then,the frequency-shifted CW light is combined with the unshifted CW light from the other branch to generate a dual-carrier with a frequency interval of 100 MHz.Radio-frequency(RF)signal ranging from 10 MHz to 40 GHz is applied to the electro-optic intensity modulator under test via a microwave network analyzer.High-resolution measurement of the relative frequency response for the intensity modulator within 40 GHz is achieved via low-frequency detection at 100 MHz,which is free of the photodetector frequency response calibration.In addition,numerical simulations are implemented to analyze the frequency response measurement error under different modulation indices and direct-current bias drifts.(2)A new scheme to measure the relative frequency response of the electro-optic phase modulators with a high resolution is proposed based on photonic sampling and lowfrequency detection.In this scheme,a passively mode-locked fiber laser with a low repetition frequency is utilized as the optical source.High-frequency RF signal is applied to the electro-optic phase modulator under test via a microwave source.Phasemodulation-to-intensity-modulation(PM-IM)conversion is achieved by using a Sagnac loop structure.Through low-frequency photoelectric detection and electrical spectrum analysis,high-resolution relative frequency response measurement of the PM is achieved,which is free of photodetector frequency response calibration.In the experiment,the relative frequency response of the PMs with bandwidths of 20 GHz and 40 GHz are obtained through low-frequency detection at 30 MHz.(3)An LNOI-based push-pull MZM chip is designed,fabricated and packaged.The frequency response of the packaged device is characterized,where special attention is paid to analyze the resonance characteristic of the modulator chip with and without cap.Based on the characterization and the analysis,another LNOI-based push-pull MZM chip is designed,fabricated and packaged,together with an LNOI-based dual-output MZM chip.The 3-d B modulation bandwidth of the packaged dual-output MZM exceeds 40 GHz.The packaged push-pull MZM is utilized to realize transmission of a PAM4 signal with a bit rate of 200 Gb/s.(4)An LNOI-based monolithic integrated cascaded electro-optic intensity and phase modulator is designed,fabricated and packaged.The frequency response,the half-wave voltage and the insertion loss of the packaged device are characterized.In addition,the RF crosstalk between the intensity modulator and the phase modulator are measured and analyzed.Based on the fabricated device,a flat optical frequency comb generation scheme is proposed,where the flatness of the generated optical frequency combs is optimized via numerical simulation.In the experiment,the fiber loss of the LNOI modulation chip is measured to be 6.97 d B.An optical frequency comb with thirteen teeth and a tooth spacing of 5 GHz is generated.The flatness of the generated optical frequency combs is measured to be 3.6 dB. |
