Measurement of energy transfer upconversion in Nd:YAG via the z-scan technique

Yan, R., Yoon, S.J., Beecher, S.J. and Mackenzie, J.I. (2014) Measurement of energy transfer upconversion in Nd:YAG via the z-scan technique. SPIE Photonics Europe '14, , Brussels, Belgium. 14 - 17 Apr 2014. (doi:10.1117/12.2054307).

Abstract

Nd:YAG is one of the workhorse gain materials for many industrial, medical and scientific laser systems. Despite a relatively high quantum defect the spectroscopic properties of the 1064 nm transition enable quite efficient operation, while the lower gain transition around 946 nm, which potentially could be more efficient due to a lower quantum defect, is additionally severely affected by detrimental thermal effects in the host material.

One potentially significant parameter that leads to a non-radiative decay channel during laser operation is energy transfer upconversion (ETU). ETU can have a detrimental effect on the laser performance as an additional source of heat, furthermore reducing the population inversion and lowering the potential gain. Whilst there are many papers studying the influence of ETU on the quasi-four-level laser performance, both experimental and in simulation, the magnitude of the ETU coefficient reportedly ranges from the upper laser level has a significant range of reported values from 5x10^-17 to 3x10^-16 cm³/s [1, 2], as such its actual impact on performance can be difficult to ascertain with certainty. In this work, we investigate 1at.% Nd:YAG using the z-scan technique to obtain a very sensitive measure of the ETU coefficient, determined to be 4.2 ± 0.4 x 10^-17 cm³/s with excellent agreement between simulation and experimental data.

The Z-scan technique is a simple method, in which the change in transmission of the sample is measured as it is moved through the focus of a pump laser beam, and correlated to the saturation irradiance. Our experiment used a Ti:sapphire laser tuned to absorption peaks of Nd³⁺ and focussed to a beam waist radius of 20.6 ± 0.2 µm, providing a "uniform" beam throughout our 3.25 mm long 1at.% Nd:YAG sample (deviating <2% from the front to rear of the crystal). When the pump laser was tuned to 808 nm the peak (on axis) irradiance available was 54 kWcm^-2, nominally four times the saturation irradiance for this absorption line. A simple spatially dependent steady state model, involving just the ground and upper laser level rate equations has been used to interpret the transmission variation as a function of pump beam irradiance, i.e. the sample position. While ground state bleaching increases the pump transmission with higher irradiance values, ETU has the opposite effect, reducing the amplitude of the highest transmission point at the highest pump irradiance, i.e. at focus. This process provides a surprisingly sensitive measure of the magnitude of the ETU coefficient. We will present a comparison of the value determined in our work and that reported in the literature and discuss the implications for the operation of the weaker laser transitions of Nd:YAG.

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Contributors

Author: J.I. Mackenzie

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