A novel approach to the implementation of a fiber optic thermometer with high temperature capability is described. It utilizes a phosphor in the form of a microsphere. With this design, a fiber optic thermal probe having a useful range of 100-1,100 °C has been demonstrated using an LED as the excitation source. From previous work, we believe that it should be possible to extend the maximum operating temperature of such a device to at least 1,400 °C if a laser diode is employed as the excitation source.
Fiber optic temperature sensors based on fluorescence decay and using a monolithic crystalline construction are described. Results obtained with YAG probes terminating in Er:YAG and Yb:YAG tips are reported. To date temperature as high as 1,900 K has been reached with the Yb:YAG sensor. Issues to be addressed in the future development of this type of temperature sensor are discussed.
We report the influence of growth atmosphere and purity of starting material on the uv-visible transmission characteristics of sapphire fibers grown by the laser-heated pedestal growth method. Improved performance in the visible has been achieved by annealing the fibers to remove color center absorption associated with an impurity, likely a transition metal.
Sapphire fibers grown at 20 mm/min in helium, using an improved laser-heated pedestal growth apparatus, have been shown to exhibit very low losses. The laser damage threshold and bending loss of these fibers have been evaluated. The best fiber forms for energy delivery may be a 100-micron diameter fiber with flared ends to increase the power handling capability while retaining the flexibility. Such a fiber has been successfully grown in our laboratory.
Recent progress in rapid growth of high optical quality laser crystals is reported. Small rods of 1 to 3 mm diameter of singly or multiply doped garnets and fluorides can be grown at low cost for material evaluation and for use in diode pumping.