Electromagnetic Shielding Methods For Optical Windows Based On Ring And Double-Layer Metallic Meshes

Optical windows are communication channels used for optical detection in precise optoelectronic instruments. Microwave and radio waves can pass through them and decrease their anti-electromagnetic interference capability. It is therefore of great significance to improve the shielding performance of an optical window while its good transmittance is maintained at the same time. Metallic mesh frequency filtering technique can be used to improve the electromagnetic shielding capability of an optical window with a typical single-layer square mesh structure. With the fast development of radar searching techniques and precise optoelectronic detection techniques, the electromagnetic shielding requirement for optical windows is getting more and more stringent, especially when strong electromagnetic shielding effectiveness, high transmissivity and low effect on imaging quality are required at the same time. However, a traditional single-layer square mesh exhibits an inherent conflict between good transmissivity and strong electromagnetic shielding. It degrades imaging quality for the concentration of high order diffraction energy. The solution of above mentioned problems involves a series of important scientific problems and key techniques, and this is why it becomes a cutting-edge subject in this field and is not solved so far.Many kinds of cells with novel shape have been used in recent years in frequency selective surfaces (FFSs) at several wavebands to produce excellent band-pass or band-stop filtering performance. Multi-layer periodic perforated metal films have attracted much attention from the research community for their extraordinary performances such as extraordinary optical transmission, negative index metamaterials, and wide or narrow band filtering with incident angle independence. These will cause new research trends for metallic mesh frequency filtering technique, for example, to improve optoelectronic performances of traditional single-layer square metallic meshes with double or multi-layer metallic mesh structures with new mesh cells. Most of the researches on FFSs and multi-layer periodic perforated metal films are focused on the band-pass or band-stop filtering performance at the wavelengths near the periodic scale of FFSs or perforated holes. However, the metallic mesh frequency filtering technique is essentially a high-pass frequency filtering technique. This means the mesh with submillimeter period allows the transmission of waves at a wavelength far less than the mesh period at optical frequency and the shielding of those waves at a wavelength much longer than the mesh period at microwave and radio frequencies. It is therefore of great significance to use new theoretical methods and necessary experiments to identify the electromagnetic transmitting mechanisms and optoelectronic performances at high and low frequencies for double or multi-layer metallic meshes with new mesh cells. However, to the best of our knowledge, not much work has been done on this particular aspect so far.By finding the reason for the inherent conflict between transmissivity and electromagnetic shielding and the cause for the concentration of high order diffraction energy of a square metallic mesh, electromagnetic shielding methods for optical windows based on ring and double-layer metallic meshes are proposed, and the methods for accurate analysis on the shielding effectiveness and the optical transmitting performance of a tilted square metallic mesh are studied as well. Theoretical and experimental studies were proceeded on above aspects in this doctoral thesis, and the main investigation work and achievements are described as follows:1. In order to solve the problem of concentration of high order diffraction energy for traditional square metallic meshes, an electromagnetic shielding method for optical windows based on ring metallic mesh is proposed. By taking contiguous metallic rings as the mesh structure, the uniform distribution of high order diffraction energy is obtained due to the homogenization of diffraction coefficients. At the same time, the cutoff frequency of microwave is increased because of the reduction of the maximum aperture of a mesh cell, and the porosity of the mesh is increased because of the reduction of the total metal area in the mesh cell, which improve the shielding effectiveness and the optical transmissivity respectively. Experimental results show that, the ring metallic mesh has uniform distribution of high order diffractions and obtains the shielding effectiveness improvement of 2dB comparing with the square mesh with the same period and at the same transmissivity of 97%.2. In order to overcome the inherent conflict between electromagnetic shielding effectiveness and optical transmissivity for traditional square metallic meshes, an electromagnetic shielding method for optical windows based on the electromagnetic coupling difference at high and low frequencies of double-layer metallic meshes is proposed. When the mesh period is far larger than the wavelengths at optical band, the electromagnetic coupling is attenuated and the transmissivity is little changed at different layer spacings. However, when the mesh period is far less than the wavelengths at microwave band, the electromagnetic shielding effectiveness increases quickly with the increasing layer spacings because of the strong electromagnetic coupling, while the increasing tendency is rapidly slowed down when the spacing reaches three times of the mesh period. Thus the conflict mentioned above can be overcome by determining a proper layer spacing. Experimental results show that, the double-layer ring metallic mesh improves the shielding effectiveness of 12dB and has uniform distribution of high order diffractions comparing with the single-layer square metallic mesh at the same transmissivity of 94%.3. In order to analyze the effects of mesh structural parameters and tilted angle on the shape and distribution of diffraction spots at far field, an optical intensity distribution model of Fraunhofer diffraction is established for a tilted square metallic mesh using Huygens-Fresnel diffraction theory. Analysis shows that, when a mesh is tilted, the location of zero order diffraction centre does not change because it is independent of mesh structural parameters and tilted angle. But the shape function of diffraction spots is asymmetrical and is stretched, which causes the stretching and asymmetrical distribution of the shape for zero order diffraction spot, and both the shape and location for high order diffraction spots. Experimental results show that, the model established can accurately analyze the change of diffraction characteristics in far field for a tilted square metallic mesh.4. A novel equivalent refractive index model of metallic mesh with high transparency is proposed to calculate the shielding effectiveness more accurately. Based on Ulrich's empirical method, LZ's equivalent reactance model and Kohin's equivalent film method, the proposed model accurately establishes the relationship among mesh equivalent refractive index, structure parameters and dielectric boundary refractive indexes. And the coefficient of this model is adjusted to reflect the shielding characteristic of the metallic mesh with high transparency. Combining with the classical film theory, the model can be used easily to calculate the mesh shielding effectiveness at various incident angles and further analyze the influence of substrate on the shielding effectiveness. Experimental results show that, the obtained calculation accuracy of the model established is 2dB, more accurate than that of traditional models of 4dB.Based on the above study, corresponding optical windows were fabricated successfully. Experimental results show that, they exhibit the uniform distribution of stray light, and achieve the electromagnetic shielding effectiveness of more than 35dB at 18GHz at the transmissivity of 94%, which exceeds the best performances reported so far. It can be therefore concluded that the study contributes to the solution of the inherent conflict between high transmissivity and strong electromagnetic shielding effectiveness and the problem of concentration of high order diffraction energy for traditional single-layer square metallic meshes. And the proposed methods for accurate analysis on optoelectronic performances of tilted square metallic mesh provide a key theoretical basis for electromagnetic shielding of curved optical windows with high performance.