Research On Fabrication And Image-transmission Performance Of Tapered Fiber Inverter

Fiber optic inverter is a rigid optical fiber image transmission element developed on the technology of fiber optic plate,which can rotate the input image around its axis 180 degrees.This product is one of the key components of military low-light-level night vision instrument,and belongs to the photoelectric information functional glass component in the high-tech field supported by the state.Traditional methods to improve the performance of optical inverters include improving material composition,reducing fiber diameter,increasing the proportion of fiber cladding,optimizing the preparation process,etc.However,these methods will bring secondary problems,there are technical contradictions that are difficult to reconcile,and they have met technical bottlenecks.It is urgent to find new technical solutions to break through.The tapered fiber inverter is a new type of optical fiber image transmission element.Because the first taper section of the triple tapered optical fiber structure has the effect of light convergence,it is helpful to reduce the optical loss in the torsion region.In theory,compared with the traditional optical fiber inverters,it should have higher resolution,transmittance and contrast.However,due to the complexity of the process and the interaction between the process and material properties,the performance of tapered fiber inverters is affected.In this paper,starting from the fundamental causes of its performance degradation,a new solution for the development of high-performance optical fiber inverters is provided by optimizing the material composition,optical fiber structure and process technology.The main research contents are as follows:(1)In order to solve the problem of high noise rate of fixed pattern in making tapered fiber inverters using the materials of traditional optical fiber inverters,which results in high manufacturing cost and restricts the advantages of image transmission performance,the core glass with good crystallization resistance and chemical stability was optimized.On this basis,the composition of three glass materials,namely,core glass,cladding glass and optical absorption glass,was optimized.By adjusting the glass transition temperature,thermal expansion coefficient,refractive index and transmittance,the matching degree between the three materials was improved,and the yield of fixed pattern noise in tapered optical fiber inverters was increased from 54.3%to 72.5%.(2)According to the theory of wave optics,a reasonable core geometry ratio and refractive index were designed to reduce evanescent wave leakage and improve the resolution of inverters.According to the principle of light transmission,a design scheme of tapered optical fiber inverters with high transmittance and high resolution was proposed by combining theoretical analysis with experimental method.The relationships among effective numerical aperture,torsion width H,effective light-passing area and minimum optical fiber diameter DC were analyzed.The axial structure of the inner optical fiber of the tapered optical fiber inverter was optimized.Six kinds of configuration and four corresponding application structures of the tapered optical fiber inverter were designed.According to the experimental analysis,when the diameter of input fiber D_A?D_C,the incident light with less than the aperture angle of the original fiber can satisfy the condition of total reflection and light transmission,which is independent of the size of the diameter D_B in the transition region.This conclusion provides a theoretical basis for the development of tapered optical fiber inverters with shorter height.(3)In view of the contradiction that the increase of twist zone width of optical fiber inverters is beneficial to improving resolution and transmittance,but bad for gross distortion.Optimizing the structure design of optical fiber blank and temperature distribution of twisting process,and replacing single-cone process with double-cone process,the processing efficiency was increased by 30%while improving edge resolution and contrast performance of products and reducing snake distortion probability.Utilization ratio was increased by 29%~32%,and production cost was reduced.The experimental samples with magnification of1.2,1.5 and 2.0 times were successfully fabricated by the optimized manufacturing process.The highest resolution of the samples was 171 lp/mm,and the gross distortion was less than40?m.(4)The evaluation system of image transmission performance of tapered fiber inverters was studied.Three new evaluation indexes were proposed,which are the percentage of difference between center and edge performance,the brightness of face and the modulation system under the same gray background.The performance comparison between tapered fiber inverters and traditional fiber inverters and between tapered fiber Inverters with different magnifications was realized.(5)According to the new index of image transmission performance evaluation,the corresponding test system was built,and the influence of magnification,taper and taper location parameters on the technical index was analyzed,which provides a reference for the optimization of image transmission performance of tapered fiber inverters.Samples with 1.5times magnification were fabricated by optimum combination of parameters.The average brightness of the center and edge of the sample was 2.1 times that of the traditional 4-micron fiber inverters.The average modulation degree of the center and edge was increased by 14.55%,and the resolution uniformity was improved by 4.95%.The reliability of the sample meets the requirements of military products.