Thermal oxidation-resistant GeO₂ ATR hollow waveguide based on NiCr capillary tube and its thermal effects

Transmission characteristics of a metallic tube GeO₂ ATR hollow waveguide degenerate due to metal thermal oxidation caused by transmission, coupling and misalignment losses. NiCr has intrinsic resistance to thermal oxidation and is often used in thermocouple heater. Thermogravimetric analysis shows that NiCr substrate tube material suffers no thermal oxidation with temperature up to 1100 °C. Thus, NiCr capillary tube is investigated for construction of a thermal oxidation-resistant GeO₂ ATR hollow fiber. A NiCr capillary tube GeO₂ ATR hollow fiber was fabricated based on liquid phase deposition. The fiber sample has a transmission attenuation of 0.25 dB/m when transmitting a 6-W CO₂ laser. Sample temperature distributions that originated from transmission loss and coupling loss are theoretically simulated and practically measured. Results confirm that the simulation can properly tell the influence of laser power losses on the fiber temperature and is further used to predict the maximum input power of the ATR fibers. To illustrate the thermal oxidation resistance of NiCr ATR hollow fiber, NiCr-based and stainless steel (SUS)-based ATR hollow fibers are directly irradiated by 6 W CO₂ laser. The SUS fiber input end became darkened. No obvious changes happened on NiCr fiber. Output beam profile and divergence angle are also taken to support the analysis. The NiCr capillary tube GeO₂ ATR hollow fiber is a good candidate for construction of durable GeO₂ ATR hollow fiber in high power applications.