This dissertation presents the development of temperature sensors which employ a fiber-optic probe consisting of single crystal YB3BAlB5BOB12B (YAG) fiber with a phosphor of short length grown directly onto one end using the laser heated pedestal growth method. The response of all the crystalline temperature sensors derives from the temperature-dependent decay time of fluorescence. Yb3+P ions served as the fluorescer, while the addition of various rare-earth codopants (i.e., NdP3+ and ErP3+) with YbP3+ provided an additional path in the form of phonon assisted energy transfer. With the additional nonradiative decay path, the temperature sensors exhibited a more desirable response. A thermally compensated fluorescence decay rate fiber optic temperature sensor was demonstrated for the first time experimentally to the best of our knowledge to make accurate surface temperature measurements. Overall, this novel technique is envisioned to aid in the perpetual challenge of precise surface temperature measurements in comparison to current methods, with the emphasis in the area of rapid thermal processing of semiconductors.