Research On Microwave Photonics Measurement Based On Optical Frequency Comb

High-precision microwave photonics measurement plays a critical role in the construction,evaluation and optimization of microwave photonic systems and optical fiber communication systems.High-speed microwave photonic devices are the basis for microwave photonic systems to achieve microwave signal transmission,processing and demodulation in the optical domain.Their frequency response characteristics will directly affect the bandwidth and transmission rate of the entire system,so it is necessary to measure the frequency response characteristics,especially for high linearity,large bandwidth and hyperfine applications.In this dissertation,we investigated and demonstrated the frequency response measurement methods of microwave photonic devices based on optical frequency comb(OFC)for Mach-Zehnder modulators(MZMs),photodetectors(PDs)and electro-absorption modulated lasers(EMLs),with the capability of ultra-wideband,hyperfine and self-calibration operation.The relevant theoretical models have been established and experimentally verified.The main research works of the dissertation are summarized as follows:(1)For the ultra-wideband measurement of MZMs,the envelope modulation method is proposed to extract the frequency responses of MZMs over the frequency range of 40GHz by detecting the low-frequency signal with a fixed frequency of 10 k Hz,avoiding the extra optical-to-electrical calibration.The two-tone photonic sampling method based on OFC is then proposed to obtain the modulation index and half-wave voltage of MZMs by utilizing the detection bandwidth of f_r/2(f_r is the repetition frequency of OFC),while eliminating the effects of the OFC and PD.Finally,the envelope photonic sampling method is proposed to achieve the self-calibrated measurement of modulation index and half-wave voltage of MZMs with a single microwave source(MS).The above methods can further reduce the detection bandwidth to the MHz or even k Hz level by reducing the repetition frequency of OFC.(2)For the ultra-wideband measurement of PDs,the single-tone photonic sampling method based on OFC is proposed.The method employs the segmented electro-optical up-conversion and symmetric frequency modulation to eliminate the effects of the OFC and the MZM.Finally,through intra-segment measurement and inter-segment stitching,the frequency response of PD can be extracted within the ultra-wideband and scalable frequency range.In the typical experiment,the frequency response measurement of PDs within the frequency range of 49.419 GHz can be achieved by using an OFC with the repetition frequency of 96.9 MHz and electro-optical modulation at 2.475 GHz,extending the twenty-fold frequency range.(3)For the hyperfine measurement of PDs,the two-tone photonic sampling method is proposed,in which the self-calibrated frequency response measurement of PDs at any frequency within the frequency range of 5f_r is achieved by varying the frequency of two-tone microwave signal from DC to f_r/2.The envelope photonic sampling method is then proposed,which enables the frequency response measurement of PDs at any frequency by employing a single MS with the bandwidth of f_r/2.In the experiment,the frequency response of PD in the frequency range of 49.77 GHz is obtained with the help of an OFC with the repetition frequency of 9.954 GHz and an MS with the bandwidth of 4.977 GHz,extending the ten-fold frequency range and reaching the frequency resolution of 10 Hz.(4)For the on-line measurement of optical transceiver links,the single-tone photonic sampling method based on OFC is proposed.The method utilizes the down-conversion sampling and the comb lines of the low-repetition frequency OFC to achieve the ultra-wideband and on-line measurement of the MZM and the PD in the optical transceiver link,which overcomes the problem of the frequency response coupling between transceivers.In the experiment,the frequency responses of MZM and PD within the frequency range of 50 GHz are measured simultaneously by employing an OFC with the repetition frequency of 96.9 MHz.Through extracting the difference-and sum-frequency components of the microwave subcarrier,the envelope modulation method is proposed to realize the self-calibrated and on-line frequency response measurement of EML and PD in the electro-absorption modulation transceiver link.Meanwhile,the measurement of these two devices does not affect each other,and the frequency response measurement is experimentally verified within the frequency range of 40 GHz.