Temperature measurements inside an Er3+–Yb3+ co-doped fluoride crystal heated by a NIR laser diode and probed by red-to-green upconversion

The temperature of a transparent Cd0.7Sr0.3F2: Er3+(4%)–Yb3+(6%) crystalline plate 0.3 mm thick heated by a near-infrared (974 nm) laser diode and probed by a red (652 nm) laser was accurately evaluated as a function of the infrared power absorbed by the Yb3+ ions. The green emission generated by the Er3+ ions directly excited by the red laser consists of three major lines (coming from three individual Stark levels in thermal equilibrium) whose intensities were measured according to the absorbed infrared power and the distance between the heated and probed volumes, to evaluate the heating induced by the excitation of Yb3+ and Er3+ ions at 974 nm by applying the Boltzmannʹs equation linking the populations of emitting levels to the temperature. In the case where the Yb3+ ions excited by the laser diode are situated at a distance of about 0.5 mm from the edge of the crystal and for an absorbed infrared power of 100 mw, the crystalʹs edge temperature is reaching 80 °C after 20 s of continuous excitation at 974 nm.