A novel technique has been developed to passively phase-lock heterodyne-detected three-pulse photon echo experiments for two-dimensional optical spectroscopy. By using a diffractive optic to generate the pulses required, and with careful introduction of the time delays between the pulses, we achieve excellent passive phase-locking, approaching lambda/100 at a wavelength of 540 nm. The ability to generate phase-locked pulse pairs with independent time delays solves a long standing impediment for stable phase sensitive detection in true optical analogues of multi-dimensional nuclear magnetic resonance (NMR) techniques, and should be equally valuable in executing related multi-dimensional spectroscopies in the infrared. (C) 2004 Elsevier B.V. All rights reserved.