A method for controlling the frequency dependence of insertion loss in an etalon-lens-fiber (ELF) optical assembly comprises defining target frequencies and insertion loss objectives therefor and adjusting the optical path length between pairs of the etalon, lens and fiber components until insertion loss objectives are achieved. Insertion loss objectives include insertion loss and insertion loss ripple objectives. The method allows for control of the frequency dependence of insertion loss and insertion loss ripple without introducing additional components, such as spectral filters, into the system.