S. A. Payne, J. A. Caird, L. L. Chase, L. K. Smith, N. D. Nielsen, and W. F. Krupke, "Spectroscopy and Laser Properties of Nd3+ -Doped CaF2, SrF2 and BaF2,"
in Advanced Solid State Lasers, G. Dube, ed.,
Vol. 6 of OSA Proceedings Series
(Optical Society of America, 1990), paper NL6.