Probe crystal as temperature sensor for piezoelectric resonance laser calorimetry

• Published in: 2016 Progress in Electromagnetic Research Symposium (PIERS) p. 1439
• Main Authors: Korolkov, A. E., Ryabushkin, O. A., Konyashkin, A. V.
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01-08-2016
• Subjects: Crystals; Laser theory; Measurement by laser beam; Power lasers; Probes; Temperature measurement; Temperature sensors
• DOI: 10.1109/PIERS.2016.7734674

Development of high power laser systems is limited by the problem of optical materials overheating by laser radiation. For optical absorptance testing laser calorimetry [1] is widely used. Its main part is temperature kinetics measurement of the sample exposed to laser radiation. Temperature is measured mostly by indirect methods with sensors adjusted to sample surface. However, absorption of scattered radiation can induce additional heating of the sensor. It was shown that piezoelectric resonance impedance spectroscopy (PRIS) can be applied for precise noncontact temperature measurement of nonlinear optical crystals [1]. Nevertheless this method is inappropriate for examination of nonpiezoelectric materials. For precise temperature measurement of any sample interacting with laser radiation we propose to use tiny probe piezoelectric crystals placed at certain points in thermal contact with the sample. Probe crystals temperature is determined noncontactly by measuring its piezoelectric resonance (PR) frequency shift induced by heating. To prove this concept we used piezoelectric quartz crystal as a sample to be able to measure its own temperature via PRIS. Block scheme of experimental setup is shown in Figure (a). AC voltage is applied to capacitor electrodes with probe crystal in between. Resonance frequency Rf of certain PR depends on crystal temperature T. Figure (b) shows R voltage drop phase response near PR. Calibration at uniform heating reveals linear relation Rf(T) − Rf(T 0 ) = K prt (T − T 0 ). When sample crystal is heated by laser radiation of power P, its nonuniform temperature distribution can be replaced by equivalent temperature derived from Rf value of PR: Θ eq (P) = T 0 + [Rf(P) − Rf(0)]/K prt [2]. T 0 is crystal temperature at P = 0. Dependences of equivalent temperature of the probe crystal (lithium niobate) and sample crystal on laser power transmitted through the sample are shown in Figure (c). Discrepancy beetween its Θ eq (P) values is below measurement error. Thus temperature of any dielectric sample can be determined via noncontact PRIS of allocated piezoelectric probe crystals. Main advantage of present approach is that crystal sensors are transparent and its heating by scattered radiation is eliminated. Two ways of sample temperature measurement were used to measure optical absorptance of the sample crystal via piezoelectric resonance laser calorimetry technique. Obtained values of absorptance coefficient of the sample quartz crystal at 1064nm wavelength are (3.7±0.3) su· 10 −4 cm −1 and (3.9±0.3)· 10 −4 cm −1 based on measurement of sample crystal and probe crystal temperature respectively.