We can select narrow FSR fringes in a multipass cell of the type described by Herriott et al. by setting the mirror spacing to give a particular type of beam path: M = odd integer, N = 4M ± 4.
© 1990 Optical Society of America
J. Barry McManus and Paul L. Kebabian, "Narrow optical interference fringes for certain setup conditions in multipass absorption cells of the Herriott type," Appl. Opt. 29, 898-900 (1990)