| Laser interferometry for measuring velocity is a technology of measurement whichconverted velocity of DUT into corresponding light intensity based on the opticaldoppler effect, and has the advantages of high temporal resolution, high dynamic rangein velocity measuring and excellent detecting precision. This technology has beenwidely used in the field of modern transient photoelectric measurement, detonationphysics, new materials, strain characteristic research, medical testing and so on. As akey module, the high-speed photodetector directly affects the sensitivity, velocity rangeand accuracy of the laser interferometry for measuring velocity system by theconversion gain, bandwidth, noise figure and other performances. However the researchon the filed of photodetector which devoted to laser interferometry in china is yetdeficient. Study on the treatment of the conversion gain, bandwidth, noise figure andother performances of this kind of high-speed photodetector is an unsolved topic.Furthermore, application research in this field is mainly relying on import productswith high price. Based on AFDISR platform and both domestic and foreign experiences,this paper discusses the study design on how to achieve a special ultra-widebandphotodetector for laser interferometry to measure velocity.Through the research on the principle of AFDISAR system, we designed aultra-wideband photodetector based on hybrid microwave integrated circuits andbroadband photoelectric detection cricuit. The photocurrent of both cathode and anodeof Photodiode Simultaneously were collected, and then the photocurrent was amplifiedby two groups of amplifier with different bandwidth, finally two sets of channels signalwere merged by continuous band pass diplexers, in which way,we obtained an ultrabroadband photodetector with bandwidth over DC-12GHz, conversion gain of400V/W.Ditails includes:1. Studying on the bandwidth and noise figure of photodiode, a new design schemeof how to isolate the output common-mode voltage was put forward, we provided thephotodiode`s bias voltage by using amplifier input, and then the output of commonmode voltage was isolated via the high common-mode rejection of the differential amplifier.In this way, a photodetector is obtained with the badwidth of0.1GHz-12GHz.2. Studying on theory of the multi-octave microwave amplifier circuit, a distributedamplifier strategy using HMIC with p-HEMT was designed, and a new method wasproposed to get the same length of gate line and drain line by adding compensatingelements to the gate and the drain. In this way,the parastic effect of circuit is shrinkedsignificantly, and finally a photodetector is obtained with the badwidth of0.1GHz-12GHz.3. The principle of microwave diplexer is on research in this thesis, we designed aplanar microstrip low-pass filter with wide stop band (DC-2.5GHz,stopband over fiveoctave) and a suspended microstrip high pass filter (2.5GHz-12GHz,passband over5octave), then the reduction of double terminal contiguous-band diplexer (with frequencyband over DC-12GHz) by optimal design was obtained, so as to combine both the lowfrequency band and high frequency band signal together.4. After repeated experiments, the actual circuit was designed and the simulationwas carried out to test and verify the feasibility and reliability of the system.an ultrawideband photodetector with bandwith of DC-11.4GHz and conversion gain of433.85V/W. |
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