Transmission Performances Of Alumina And Germanium Dioxide Mid-Infrared Hollow Waveguides

The CO₂ laser device has high energy conversion efficiency in which the output beam power is able to reach thousands of watts.Metal materials,ceramic materials,organic materials,and human tissues have strong absorption of CO₂ laser,which make it possess many potential applications in the fields of material processing,laser-induced nuclear fusion and laser surgery.In the study of mid-infrared optical fibers for CO₂ laser radiation transmissions,the hollow waveguides have attracted much attention due to their characteristics such as no end reflection,small coupling loss,good heat dissipation,and high damage threshold.At present,it is still very challenging for the development and production of low-cost,stable transmission of high-power CO₂ laser of mid-infrared hollow waveguide.In addition,previous researches focused on the transmission behavior of optical fibers under ambient temperature conditions,which limited the applications of CO₂ lasers in the low-temperature scientific experiments and other fields.In this thesis,we explored the potential of high power CO₂ laser beam transmissions via low-cost polycrystalline alumina capillary tube hollow waveguide.Moreover,the low-temperature transmission characteristics of germanium dioxide?GeO₂?and alumina attenuated total reflection?ATR?mid-IR hollow waveguides were also investigated.The measured and calculated loss spectra of the tube samples exhibit a low-loss window around the wavelength of CO₂ laser?10.6?m?.The transmission losses of the tube samples with the inner diameter size of 1-2 mm were measured experimentally.Among of them,the transmission loss of the 1.5-mm-bore tube is as low as 0.12 dB/m.The energy distribution and divergence angle of the output beam were analyzed.The1.5-mm-bore tube has a better spot quality and the divergence angle is as low as 13.6mrad.Furthermore,we analyzed the transmission loss,output laser beam profile and divergence angle of the 1-mm-bore hollow waveguide sample under bent condition.The waveguide sample has a transmission loss less than 1.79 dB/m at an offset distance of 0¹0 cm,and a divergence angle less than 33 mrad.The temperature distribution of the polycrystalline alumina capillary ATR hollow waveguide was calculated and measured by modifying the theoretical model of the temperature distribution of the leaky hollow fiber.The overall temperature distribution of the fiber decreases in the manner of periodic oscillation.The maximum temperature of the fiber coupling end increases as the inner diameter of the fiber decreases,which is a key factor affecting the maximum transmission power of the fiber.Combined with the thermal performance parameters of the fiber material,the simulation results indicate that the polycrystalline alumina hollow waveguide with an inner diameter of 2 mm has the potential to stably transmit up to 1265 W continuous CO₂ laser.We explored the CO₂ laser transmission performances of polycrystalline alumina and GeO₂ metal capillary tube mid-infrared hollow waveguides at liquid nitrogen temperature.It was found that the laser power transmission efficiencies drop sharply at low temperature,which deviated from the normal transmission behavior in room-temperature environment.Among them,the output laser power of GeO₂ metal capillary hollow fiber decreased from 2.90 W to 1.3 W,and the alumina fiber sample decreased from 3.06 W to 0.93 W.In order to investigate this anomaly,we analyzed the optical constants of GeO₂ and alumina materials at low temperatures.Besides,by comparing the laser transmission performances of different hollow-core gas media and core-diameter fibers at low temperatures,it was found that water vapor condensation and air liquefaction in the core at low temperatures were the main reasons for the reduction of fiber output power.Herein,we proposed a new method for long-term stable operation of hollow waveguide under low temperature condition.By blowing a continuous N₂ gas flow into the 2-mm-bore GeO₂ metal capillary and2-mm-bore alumina hollow waveguide at 7.5 mL/min and 3 mL/min,respectively,the hollow waveguides can transmit laser beams stably at low temperature,in which the output powers can be close to the normal transmission levels at room temperature.