| The requirements of Mid-infrared transmission system have been greatly stimulated by the advance in the field of infrared lasers for surgery,cutting,welding,heat treatment,etc. But efficient transmission instruments are still unavailable until now. Available band of traditional fused silica optical fibers usually does not exceed 2um because of molecule vibrancy absorption,though the preparation methods are very mature. Generally speaking,there are two types of optical fibers for Mid-infrared laser transmission. One is solid-core optical fiber and the other is hollow waveguide. For solid-core optical fibers,great endeavor has been made and several kinds of fibers have been obtained. But the following research indicates that traditional solid-core structure has some deadly disadvantages:one is large reflective loss in input ends because of large reflective index of the solid core;the other is low transmission threshold because of the structure disfigurements of solid-core. So their performances are far from the applied requirements. Accordingly,hollow waveguides are more attractive transmission mediums for their great advantages.The emphasis of this paper is the delivery of CO2 laser radiation,including hollow waveguides and coupling devices for input ends,and both of them are based on special theoretic design. According to the mode structure and transmission characteristics of general hollow infrared waveguides,we discuss the transmission mechanism of dielectric-coated metallic hollow waveguides,analyze the low-loss transmission conditions of the HEn mode. Based on these,the most advisable structure of dielectric-coated metallic hollow waveguides is designed and specific guidance for the selection of materials and inner multi-layers are provided.The optimum refractive index of the coated dielectric is,and the optimum thickness is The best choice of themetallic film is silver,with the thickness beyond 0.1 jam. And the relatively appropriate radius of the waveguides should range between 0.3-1.0mm.Because of the restriction of the performances of the infrared materials,we decide that the hollow waveguide should have the structure of single layer dielectric coated metallic film according to the general analysis of process controllability and transmission capability. The dielectric we choose is silver iodide,made from silver and iodine,and the optimum thickness is about 0.77um. The thickness of the silver iodide film is controlled by the concentration of the iodine and the reaction time. While the concentration of the iodine is O.OIM and the reaction time is 60s,the thickness of silver iodide film is about 0.7um. In thiscondition,the silver iodide has two crystal types,the main type and few ytype,and the transmission efficiency reaches the maximum. The transmission loss at 10.6um band is 0.5dB/m,tested by Cut-off method. The accessional loss due to bend is also very small. While the bend angle is 90 degree,the accessional loss is less than IdB/m. The tested hollow waveguides have the radius of l.Oum and the length of l-2m.As far as input coupling devices concerned,we design a feasible taper coupler by means of the theory of waveguide coupling and outside cavity waveguide laser and make decision of the sizes of the lens and the taper coupler,and the relative distance of them. The focus of the lens is 65mm and the radius is 5mm. The length of the taper coupler is 15mm,and the radiuses of the input and output ends separately are 17mm and 0.65mm. The design distance between the lens and the coupler is 44mm.The coupling system has a fiber insert-pullout structure,avoiding the trouble of adjusting the fiber each time. The tested results indicate that transmission rate of the taper coupler reaches 90%. But because of the higher modules loss caused by the coupling condition,the total transmission rate of the system can only reaches 65%. The output energy can be stable at 13W,which can satisfy the requirements of laser survey by and large.
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