Calibration of a High-Resolution Slow-Light Fiber-Bragg-Grating Sensor for Temperature Measurements of Laser-Cooled Fibers

Anti-Stokes fluorescence is emerging as an important new technique to eliminate the internal heat generated in rare-earth-doped fiber lasers and amplifiers. The efficiency of cooling is quantified by measuring the fiber temperature as a function of pump power launched in the fiber. Because the temperature changes induced in small-core fibers placed in air are small, these measurements require a sensor that can resolve millikelvin (mK) temperature changes. The accuracy is critical because they provide invaluable information about the degree of quenching and residual absorption in the fiber, which gauge the absolute merit of different core-glass compositions. This function has been well served by a closed-loop fiber sensor with mK resolution utilizing a slow-light fiber Bragg grating (FBG) placed parallel to and in contact with a stripped section of the cooled fiber. In this work, we report several improvements in the operation and calibration of this sensor. First, to obtain a more accurate and reproducible temperature reading, the contact between the two fibers is improved by twisting one fiber around the other. Second, careful calibration shows that the sensor response is independent of the slow-light resonance, of the FBG bandgap, and of the settings of the electronics, in agreement with a model. The response needs to be measured only once, then used for any FBG fabricated in the same type of fiber. Third, measurements are reported to quantify the temperature difference between the cooled fiber and the FBG, and how much the presence of the FBG affects the temperature of the cooled fiber, to determine the correction factor that must be applied to the sensor output to obtain accurate absolute readings. The sensor performance is illustrated with a measurement of cooling in a new Yb-doped aluminophosphosilicate fiber.