| The key to optical wave modulation in optical communication and microwave photonics systems is the high-speed electro-optic modulator.Its frequency-domain characterization directly affects the bandwidth and speed of the entire working system.Therefore,the urgency of resolving the issue of how to accurately and precisely characterize the frequency response of high-speed electro-optic modulators is pressing.The traditional optical spectrum analysis method has simple structure and measure process,which can characterize high-speed electro-optical intensity/phase modulator,but the measurement resolution is limited by the optical spectrum analyzer(about GHz),and unable to obtain precision results;The electric spectrum analysis method can achieve ultra-high resolution measurement,while the measurement results include the cascade response of the photodetectors(PDs),and usually only the electro-optic intensity modulator can be characterized.When measuring the electro-optic phase modulators(EOPMs),an additional phase-intensity conversion system needs to be built;The optical heterodyne analysis method can realize the self-calibrated measurement of electro-optic modulator,but the system structure and the cost are so hard,that unable to be popularized.In view of the key problems such as the low measurement resolution of the optical spectrum analysis method,the extra calibration of the electrical spectrum analysis method,and the complex structure of the optical heterodyne analysis method,we propose a frequency response measurement method of the high-speed electro-optic modulator based on the pilot in this paper.The theoretical analysis,numerical simulation and experimental studies are conducted to further research the frequency response characteristics of the Mach-Zehnder Modulator(MZM)and High-Speed Electrooptic Modulator(EOPM).The main research contents are as follows:(1)For the frequency response measurement of MZM: a MZM frequency response measurement method based on dual pilot signals is proposed.The modulation coefficient and half-wave voltage of MZM with 40 GHz operating bandwidth are measured by detecting two low frequency pilot signals about k Hz.The problem of additional opticalelectric calibration is avoided,and achieves a self-calibrated measurement,which largely saves the bandwidth requirements of the device at the detection end.In the case of only measuring the relative frequency response,the pilot signal on the MZM bias electrode can be removed to further simplify the experimental system;A MZM frequency response measurement method based on single pilot signal is proposed.The high frequency characteristics of MZM can be obtained by measuring the amplitude of a fixed lowfrequency component in the photocurrent and fitting its square root with the driving power.Self-calibrated and high-accuracy measuring of MZM are achieved with extremely simple system structure and low measurement cost.(2)Aiming at the frequency response measurement of high-speed EOPM,an EOPM frequency response measurement method based on dual pilot signal is proposed.The phase modulation to intensity modulation conversion is achieved through a MZ interference structure.The pilot signal is loaded into the measurement system through the acousto-optic frequency shifter on the MZ interference arm,which enhances the stability of the MZ interference structure.Using the amplitude ratio of two fixed low-frequency signals,the modulation index and half-wave voltage of the high-speed EOPM are obtained,which eliminates the influence of the uneven response of the PDs and achieves the self-calibrated measurement.At the same time,the response at high frequency of the device under test can be obtained only by detecting two low frequency signals of about70 MHz,which largely cuts down the bandwidth demands of the PDs and the electrical spectrum analyzer. |
