The fluorescence intensity ratio technique for optical fiber-based point temperature sensing is reviewed, including the materials suitable for this technique. The temperature dependence of the fluorescence intensity ratio has been studied using thermally coupled energy levels in seven different rare earth ions doped into a variety of glasses and crystals. Sensor prototypes developed using Pr3+:ZBLANP, Nd3+-doped silica fiber and Yb3+-doped silica fiber as the sensing material have been used to measure temperatures covering the range of approximately −50 to 600 °C with a resolution of the order of 1 °C.

Articles you may be interested inCharacteristics of doped optical fiber for fluorescence-based fiber optic temperature systems Rev. Sci. Instrum. 74, 5212 (2003); 10.1063/1.1623624 Analysis of dopant concentration effects in praseodymium-based fluorescent fiber optic temperature sensors Rev. Sci. Instrum. 71, 100 (2000)Nd 3+ -doped optical fiber temperature sensor using the fluorescence intensity ratio technique Rev. Sci. Instrum. 70, 4279 (1999); 10.1063/1.1150067 Determination of local high temperature excursion in an intrinsic doped fiber fluorescence-based sensor Rev. Sci. Instrum. 69, 2930 (1998); 10.1063/1.1149036Quasidistributed fluorescence-based optical fiber temperature sensor system Rev.The performance of the two most promising fluorescence-based temperature sensing techniques, namely the fluorescence intensity ratio ͑FIR͒ and fluorescence lifetime ͑FL͒ schemes, have been compared. Theoretical calibration graphs for the two methods illustrate the useful monotonic change of the response with temperature variation. Comparison of the responses and the sensitivities of the two schemes show that at very low temperatures the FIR method exhibits a significant variation with temperature, while the response of the FL method becomes constant with its sensitivity approaching zero. With increasing temperature, the FIR and the FL methods ͑with short relaxation times and shorter intrinsic lifetimes of the upper energy levels͒ share a similar sensitivity over a wide temperature range. The presence of a long relaxation time or a longer intrinsic lifetime of the upper level in the use of the FL method gives a less satisfactory response. Experimental data obtained for a range of dopant ions in various host materials are found to be consistent with the theoretical expectation, with each material having a specific energy gap difference. The sensitivities of each material are compared graphically which would allow the most appropriate sensor for an intended application to be selected.

The application of quantitative phase microscopy to refractive-index profiling of optical fibers is demonstrated. Phase images of axially symmetric optical fibers immersed in index-matching fluid are obtained, and the inverse Abel transform is used to obtain the radial refractive-index profile. This technique is straightforward, nondestructive, repeatable, and accurate. Excellent agreement, to within approximately 0.0005, between this method and the index profile obtained with a commercial profiler is obtained.

The variation in the green intensity ratio ((2)H(11/2) and (4)S(3/2) energy levels to the ground state) of Er ions in silica fibers has been studied as a function of temperature. The different processes that are used to determine the population of these levels are investigated, in particular 800-nm excited-state absorption in Er-doped fibers and 980-nm energy transfer, in Yb-Er-codoped fibers. The invariance of the intensity ratio at a fixed temperature with respect to power, wavelength, and doped fiber length has been investigated and shown to permit the realization of a high-dynamic-range (greater than 600 °C), autocalibrated fiber-optic temperature sensor.

Nondestructive images of refractive-index variation within a type I fiber Bragg grating have been recorded by the differential interference contrast imaging technique. The images reveal detailed structure within the fiber core that is consistent with the formation of Talbot planes in the diffraction pattern behind the phase mask that had been used to fabricate the grating.