The split-step method is a numerical method that can be used to solve a nonlinear PDE (https://en.wikipedia.org/wiki/Split-step_method). Even Wikipedia does not refer to the original authors (F.D. Tappert and R.H. Hardin in 1974).
The oldest record that I have found (1982) is an old military thesis (Naval Postgraduate School Monterey, CA) that also refers to a paper by Tappert and Hardin on the split-step method. (Link: https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=57e1a171c96fc086d2c4e0c05471a3967df7f67e)
Their paper was published in the 8th International Conference on Acoustics (Goldcrest, London, 1974). I have resorted to this link (https://www.icacommission.org/ica-congress/past-ica-proceedings/) but could not find it there.