Study On Narrow-pulse Laser Wavelength Measurement Technology

With the rapid development of optoelectronics and information technology,optoelectronic countermeasures have been improved tremendously. They have become onecompetition focus of the world giant military countries for their precision ranging, precisionguidance and strike and been widely used in the modern wars such as Gulf War, Kosovo Warand Iraq War, etc. To confront these threats, the timely and exact detection of attacking laser'sorientation, wavelength and threatening degree are indispensable. According to these obtainedinformation we can take effective measures to interfere the optoelectronic weapons andequipments of the enemy, causing them malfunctioned, weakened, suppressed and evendestroyed. The present study is so meaningful that it is highly supported by National FundCommittee, Study-abroad Fund Committee of Shanxi Province, the Education Department ofShanxi Province, and Chang Cheng Night-vision Devices Equipment Company Limited ofShanxi Province. It is hoped that this study will be applied to the weapons and militaryequipments and raise the whole fighting level of our army, thus realizing a stride developmentof "strengthening the army with science and technology".After analyzing the parameters of laser technology in military laser rangefinder, laserdirector and tunable laser, as well as the pulse laser wavelength measurement technology, anew method based on fiber optical spectrometer is put forward.First, the optical structure of fiber-optical spectrometer design is given. Then, theprinciple of diffractive grating and detectors, especially the development, principle andparameters of CMOS imaging devices, are discussed in detail. Finally, fiber-optical bundle issimply introduced.After analyzing the transmission attenuation of narrow-pulse Nd-YAG laser beam in theatmosphere, the power density of reached laser beam and sensitivity of detector are analyzedand calculated. The method for choosing scanning frequency and for increasing SNR underhigh sunlight background noise is discussed. The equation for calculating the spectrumresolution is indicated, too.To verify whether fiber-optical spectrometer can detect narrow-pulse laser in arbitrarydirection and at any time or not, indoor and outdoor experiments of narrow-pulse laserwavelength dispersion measurements, outdoor direct incidence and skewed incidenceexperiments at near distance and those at far distance based on an optical telescopic systemand fiber-optical spectrometer to measure the narrow-pulse laser beam from a laserrangefinder are made. These experiments indicate that: 1) The fiber-optical spectrometer canexactly measure the narrow-pulse laser wavelength under the high sunlight background;2)The signal amplitude has a close relation with the incidence direction of narrow-pulse laserbeam;3) The background noise has a direct relation with integral time of fiber-opticalspectrometer, especially for the visible light;4) With an optical telescopic system, theattacking short-pulse laser can be acquired precisely;5) The signal amplitude is increasedgreatly and the further signal can be detected. However, the limited receiving scope of theoptical telescopic system makes it impossible to detect the wavelength in wide view field.To increase the acceptance angle and incidence area, acquire the wavelength of thebroaden view field and detect the narrow-pulse laser beam at anytime, after the acceptanceangle of one fiber bundle is measured, the principle of reflective taper and optic fiber taper isanalyzed. The measurement method and the results by means of reflective taper and opticfiber taper coupled to the detector with fiber-optical bundle respectively, is presented. Thetransmittance rate of optic fiber taper is tested and analyzed, too. These results show that, ifthe decrease to half of the maximum power value is used as the judging standard, with theoptical taper antenna, the acceptance angle is 16° and optical gain is 10;acceptance angle ofoptic fiber taper is 32° and optical gain is 2. It is shown to be simpler and more effective totest the transmittance of optic fiber taper and fiber plate with the single integrating sphere thanthe double one.The acceptance angle of the fiber-optical spectrometer with a reflective taper at the frontis small, so several reflective tapers are put together to form a combined detection area ofnearly 180°. There are a variety of interferences such as outdoor sunlight, battlefieldinterference light source, electromagnetism interference and cosmic rays. With time durationor spectrum width discrimination, the above firelight interferences can be eliminated. Theapplication of optical instruments and CCD or CMOS linear array imaging sensors willenhance the ability of counteracting electromagnetism, cosmic rays and nuclear radiationinterference.The innovations of the present study are as follows:1 The measurement method of narrow-pulse laser wavelength with fiber-opticalspectrometer is put forward and the technology feasibility is validated through a series ofexperiments.2 The method, in which the taper devices are used to increase the luminous flux,enhance the signal amplitude, broaden detection distance and improve SNR, is given. Thistechnology will lay a solid foundation for detecting the narrow-pulse laser wavelength infuture battlefield. The technical project of fiber-optical spectrometer with a reflective taper atthe front is given.3 To improve the optical gain of optic fiber taper, the method for measuring thetransmittance rate of optic fiber taper with single integrating sphere is presented, and therational testing results are indicated.Optoelectronic countermeasures are one of the most active research fields in electroniccountermeasures. This research makes an exploratory study on the measuring principle of thewavelength of narrow-pulse laser beam in the battlefield. Some basic experiments areperformed. The method of fiber-optical spectrometer with reflective taper is put forward andits feasibility is testified by the experiments. To confront the complicated internationalsituation, it is so urgent to perfect this method and put it into practice.