In the near and middle infrared atmospheric window, infrared stealth material require a low absorptivity (which means a low emissivity according to Kirchhoff’s law of black body), at the same time, it also requires high absorptivity so as to decrease the reflectance at military laser wavelength of 1.06μm. Under this circumstances, compatible stealth of infrared and laser is an urgent demand, but the demand is ambivalent for conventional materials. Photonic crystal (PC), as a new type of artificial periodic structure function material, can realize broadband thermal infrared stealth based on its high-reflection photon forbidden band(also called photonic band gap). The high-reflection photon forbidden band of PC can be adjusted to near and middle infrared wave band through some rational methods. When a defect was added into the periodic structure of PC, a “hole-digging” reflection spectrum, which is high absorption at military laser wavelength of 1.06μm, can be achieved, so compatible stealth of near and middle infrared and military laser wavelength of 1.06μm can be achieved too.
In this paper, we selected near and middle infrared-transparent materials, Te and MgF2 , as high refractive index and low refractive index material respectively, and designed a one-dimensional one-defect-mode PC whose photon forbidden band was broadened to 1-5μm by constructing two photonic crystals into one. The optical property of the PC was calculated by Transfer matrix method(TMM) of thin-film optical theory, and the results shows that the as-designed PC has a high spectral reflectance in the near and middle infrared band, among which the reflectivity in 1.68μm∼5.26μm band reached more than 90%, and the 2.48∼5.07μm band even reached 99.99%. The result also shows that between the band gap of 1-5μm, there are one defect mode locating in the wavelength of 1.06μm, whose reflectance is below 0.70%, which means its spectral absorptivity is greater than 99.30%. All the above we have discussed proved that this “hole-digging spectrum” PC can realize the compatible stealth of near and middle infrared and 1.06μm military laser.
Radar, as the main detection means, plays an extremely important role in modern warfare. However, it is easily attacked
by the electromagnetic pulse (EMP) weapon. In order to defense the threat, a radome formed by plasma is designed. The
plasma radome is used to protect the radar systems against EMP coupling through both ‘front door’ and ‘back door’.
What’s more, the damage effects of EMP to the radar systems are analyzed, and the protection mechanism of plasma to
EMP weapon is discussed. In addition, a simulation experiment is carried out to verify the feasibility of the plasma
radome to defense against EMP. The results indicate that the structure performs well, which makes it a new method
against EMP weapon.
Graphite with good extinction performance can be used as electro-optical passive jamming material for infrared and laser detection. In order to acquire the extinction characteristic of graphite smoke for terahertz wave (THz wave), graphite powder was dispersed in a KBr matrix with concentrations of 0.6 wt% and 1.0 wt% respectively, and those composites were processed in the stoving system and were then pressed into pellets. Meanwhile, the pure KBr powder pellet was prepared with same method under same condition. By utilizing THz-TDS, the THz transmission spectrums of those samples were measured in the frequency range 0.2-1.1 THz. Then, the absorption coefficients of those samples were deduced based on the material parameter estimation method. The experimental results indicate that the absorption coefficients of those samples are enhanced with the increasing THz frequency and that of them are improved with the concentrations of graphite at the same frequency. The results obtained demonstrate that THz wave has strong penetration capacity through graphite smoke and THz radar will be promising for use to make up for the deficiency of the infrared and laser detection system and to detect the targets coated with graphite smoke.
The characteristic matrix method in thin-film optical theory was used to calculate heterogeneous doped one-dimensional photonic crystals (1-D PCs), which were fabricated by alternate deposition of Te, ZnSe, and Si materials on a silicon wafer. The heterogeneous structure was adopted to broaden the photonic band gap, within which the low reflection valley was achieved by doping. Infrared spectrum tests showed that the average emissivities of the 1-D PC were 0.0845 and 0.281, corresponding, respectively, to the bands of 3 to 5 and 8 to 14 μm. Moreover, the emissivity was 0.45 over the 5 to 8 μm nonatmospheric window, and the reflectivity was 0.28 at the wavelength of 10.6 μm. The results indicated that the heterogeneous doped 1-D PC was able to selectively achieve low emissivities over infrared atmospheric windows and a low reflectivity for the CO2 laser, which exhibited remarkable competence in compatible infrared and laser stealth applications.
In order to acquire more information of the scene to improve the veracity of recognition of camouflage targets, an electrically tunable hyper-spectral detection system, which is based on acousto-optic tunable filter (AOTF), was designed. The system includes collimated optical system, AOTF and its controller, imaging lens, CCD sensor and so on. The system has a property of being fast and electronically tunable, so a quick scan of spectrum over the waveband of 550 nm ~ 900nm can be realized. A series of hyper-spectral imaging experiments about a camouflage aluminum plane, coated with three typical camouflage pigments (dark green, light green and khaki) within a complex meadow environment were accomplished at specific wavelengths from 580 nm to 840 nm with 10 nm spectral resolution. The hyper-spectral characteristics of three pigments and various backgrounds were acquired to deduce the intensity contrast information between them. The experimental results demonstrated that the reflex characteristic of three typical camouflage pigments were different from that of natural background. The several wavelengths or wave bands, which were used to detect and recognize the man-made targets placed in typical woodland environment, were obtained by analyzing the experimental data.
On the basis of the principle of polarization detection, a hyper-spectral polarization imaging system, which is based on linear polarizer and acousto-optic tunable filter (AOTF), was designed to detect and recognize camouflage target intelligently and rapidly in this paper. Our design has neither moving parts nor modulation, and has fast and electronically tuning property, so a quick scan of spectrum at 400 nm~1000 nm can be realized electronically. At the same time, it not only could obtain the intensity image, hyper-spectral information, but also could acquire polarization signatures of the scene. Then the spectral polarization experiment about aluminous plane which covered with the bottle green, shallow green and khaki camouflage pigments within meadow were conducted at specifically wavelength by the instrument. Finally, the polarization information of the man-made targets and natural background in the scene, and the fusion image based on HIS color space were deduced through processing the experiment data. The experimental result demonstrates that the polarization characteristics of camouflage pigments were different from that of natural background. As the contrast of target and background could be enhanced by polarization information, the camouflage target could be identified effectively from the image according to polarization information. On the other hand, the camouflage target is more obvious in the fused image. Therefore, the proposed method and the system in this paper are reasonable and effective. Consequently, the hyper-spectral polarization detection technique which relative to the classical intensity detection is of significance to improve the accuracy of recognition of camouflage targets in mixed background under proper detection condition.
Based on the characteristics of terahertz wave, the ability of THz for uncovering target covered by camouflage coating is studied. Firstly, the paper introduces the camouflage coating classification and reflectivity. Secondly, a coating reflection characteristic calculation model is established on conditions of smooth surface with methods of matrix optics. Finally, according to the calculation model, the reflectance of infrared coating and laser coating in different wavebands including 337μm terahertz wave was calculated. According to the calculating result, it’s indicated that THz has better ability to penetrate through coating than IR ; meanwhile, it’s also indicated that THz has the ability for uncovering target on some conditions.