| At present, the electromagnetic signal environment is becoming increasingly com-plex. It contains a large variety of di?erent types of signals, such as pulse compression,frequency agility and so on, covering a more widened frequency range and with a pulsedensity of millions per second. Thus the receiver is required to detect and identify mul-tiple simultaneous signals in a wider instantaneous bandwidth and a greater dynamicrange. Apparently the traditional receivers can not fully satisfy those requirements. Inthis paper, combining the acousto-optic de?ection technology with the coherent opti-cal detection technology, a method for the frequency measurement is proposed, whichuses optical processing method to achieve a continuous and seamless measurement forthe frequency with the high-speed parallel processing capability, good real-time per-formance, high probability of intercept(POI), high sensitivity, large bandwidth, highaccuracy and simple structure, and moreover can simultaneously obtain all the infor-mation including frequency, magnitude and direction-of-arrival (DOA) and etc. Allthose advantages make it well suited to the application in the complex and dense elec-tromagnetic signal environment to achieve an accurate and real-time measurement formultiple signals arriving at the same time.The system for the frequency measurement is studied by theoretical analysis andexperimental tests. Firstly, the acousto-optic e?ect and its di?raction patterns areintroduced. From the coupled wave equation about light waves and sound waves inthe medium, the law about acousto-optic interaction is analyzled, and then the cou-pled wave equation and di?raction e?ciency about the normal Bragg di?raction areobtained. Meanwhile, the principle, advantages and key techniques of the coherentoptical detection technology are researched as the theoretical basis for the scheme.Secondly, aiming at meeting needs of the real-time detection, parallel processing capa-bility, bandwidth and resolution, and compared with several traditional technologiesfor the frequency measurement such as superheterodyne(SH) receivers, crystal videoreceivers(CVR), instantaneous frequency measurement(IFM) receivers and so on, anovel system for the frequency measurement is designed based on the acousto-optice?ect and coherent optical detection. The innovation of the system is injecting thebeams into PMF to propagate and process, and thus the coherent detection e?ciencyis improved and multiplexing is introduced. Meanwhile, its operating principle andadvantages are generalized. Thirdly, the optical coupling theory about coupling Gaus-sian beams into single-mode ?bers is described, and meanwhile the coupling e?ciencye?ected by the alignment error is analyzed by the computer simulation. Lastly, thein?uence that the laser linewidth and the power of the reference beam have on the per- formance of the coherent optical detection system is simulated. And then based on theabove analysis, a experimental platform for the frequency measurement is establishedwith the purchased or homemade devices. As a result, the experimental veri?cationand performance tests are accomplished.By experiments it has been shown that this coherent optical detection systemin the operational bandwidth can e?ectively realize a continuous and seamless mea-surement for the amplitude and frequency of weak signals. It is with a sensitivity of?68.7dBm and a frequency resolution less than 30KHz. Not only is the output signal-to-noise ratio(SNR) improved by at least 6dB compared with the input, but also thesensitivity is almost increased by 10dB compared with the direct detection system.Furthermore, this system also has good stability and real-time parallel processing ca-pability. Thus its application prospect is very bright. |
PDF Full Text Request