Objective  The 8-10 μm far-infrared spectrum is in the infrared radiation band at natural temperatures and covers the characteristic "fingerprint spectrum" of many molecules, so it has important applications in the military, medical and environmental monitoring fields. Infrared coherent fiber bundles which can realize the flexible transmission of infrared image are the basic components for assembling various infrared optical systems, and they can be used in the narrow space, high-intensity electric or magnetic field in particular. The main types of far infrared fibers mainly include crystal fiber, hollow fiber, photonic crystal fiber and Te- based chalcogenide glass fiber. Among them, Te-based fiber is an excellent far-infrared transmission material due to its wide transmission band, stable thermal, chemical properties, which means it is especially suitable for the preparation of coherent optical fiber bundles with large array. Until now, a series of components such as Ge-As-Se-Te, GeTe-AgI, Ga-Ge-Te, Ge-Te-I and As-Se-Te have been studied. However, the optical loss of Te-base fiber is still higher at present, which limits the transmission distance of infrared signal and the resolution of the infrared bundles. Therefore, it is necessary to study the purification technology for optimizing the optical loss.

  Methods   High purity raw materials of As, Se and Te were purified by multi-distillation purification technique and the content of O element was examined by EPMA. As-Se-Te chalcogenide glass was chosen and melted by different preparation process and their infrared transmission spectra were measured by FTIR. The optical fiber was drawn by the rod-in-tube method. The drawing temperature was 240 ℃ with the accuracy of ±0.2 ℃, and the drawing speed was about 10 m/min. The coherent fiber bundle was prepared by ribbon-stacking technique. The end face was observed by microscope. Infrared image was detected by home-made optical system and mercury cadmium telluride detector was used (Fig.2).

  Results and Discussions   The oxygen content of As, Se, Te raw materials decreased from 1.3 at%, 0.46 at% and 0.48 at% in raw materials to 0 at% (undetected), 0.06 at% and 0.15 at% in purified materials respectively, indicating that the distillation process was effective (Tab.1). The transition temperature Tg is 137.5 ℃ for core material and 139.1 ℃ for clad material (Fig.3), which are very close and match well. No obvious crystallization peak was observed in the test temperature range, indicating that the core and clad glass are suitable for fiber drawing. Smooth spectrum was obtained in the sample of aluminum as a deaerator (Fig.4). The optical fiber with an outer diameter of 100 μm was obtained. Its bending radius is less than 5 mm, and the baseline of the optical loss is about 0.2 dB/m in the far infrared range (Fig.5). Finally, the coherent fiber bundle with 22.5 thousand pixels and close-packed arrangement was prepared. The total fracture rate is less than 3‰ and there are none black or dark pixels in the center region of the bundle. The bundle transmits infrared beam uniformly and the image of the infrared target is clear and distortionless, which indicates that the comprehensive properties of the bundle are satisfactory (Fig.6).

  Conclusions  Far-infrared fiber bundles was prepared and measured. In order to eliminate impurities, As-Se-Te chalcogenide glass was chosen and the high purity raw materials of As, Se and Te were purified. As-Se-Te glasses were melting by different preparation process and their infrared transmission spectra were measured and analyzed. The results show that excellent thermal and far-infrared transmitting performance can be obtained in the sample of Al as deoxidizer process. The optical fiber was drawn with an outer diameter of 100 μm, bending radius of less than 5 mm, optical loss of 0.2 dB/m. The coherent fiber bundle was prepared by ribbon-stacking technique. It has 22.5 thousand pixels and the total fracture rate is less than 3‰. The infrared target imaging was distortionless and showed fine temperature resolution, demonstrating that the bundles can be widely used in infrared imaging systems.