Spectroscopic features of Er:YAG at cryogenic temperatures are studied in detail. We report that the major absorption line at ~1532.3 nm is much narrower and much stronger than previously measured. Spectroscopic analysis suggests that its impact on the laser performance is limited because of the strong pump saturation effect. It is shown that the high efficiency of the laser is, in fact, achieved due to absorption in the wing of this line, which is comprised of a few other transitions. It is also shown that pumping into the relatively weak 1534 nm absorption line provides practically the same laser efficiency (~75%) as pumping into the major 1532 nm absorption line. This is explained by a numerical laser model which takes into account saturation effects of pump and laser intensities. The model is validated by experimental data.
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