Abstract: A method of creating a local oscillator light beam LO for a phase-resolved spectroscopy measurement comprises the steps of providing a first measuring light beam (1) and a second measuring light beam (2) being aligned to each other, creating the local oscillator light beam LO by an optical non-linear interaction of a first portion (1A) of the first measuring light beam (1) and a first portion (2A) of the second measuring light beam (2) in an optical nonlinear medium (20), and superimposing the local oscillator light beam LO, a second portion (1B) of the first measuring light beam (1) and a second portion (2B) of the second measuring light beam (2) with a predetermined mutual phase relationship, for providing a sample light beam (3) for the phase-resolved spectroscopy measurement. The local oscillator light beam LO and the second portions (1B, 2B) of the first and second measuring light beams (1, 2) are superimposed with a displaced Sagnac interferometer (10).