Method of creating a local oscillator light beam and local oscillator source apparatus for phase-resolved spectroscopy

A local oscillator source apparatus, being configured for creating a local oscillator light beam for a phase-resolved spectroscopy measurement, comprising

• an input device being arranged for aligning a first measuring light beam and a second measuring light beam,
• an optical nonlinear medium being arranged for creating the local oscillator light beam by an optical non-linear interaction of a first portion of the first measuring light beam and a first portion of the second measuring light beam, and
• a combining device being arranged for superimposing the local oscillator light beam, a second portion of the first measuring light beam and a second portion of the second measuring light beam with a predetermined mutual phase relationship, for providing a sample light beam for the phase-resolved spectroscopy measurement, wherein
• the combining device comprises a displaced Sagnac interferometer being arranged for creating the local oscillator light beam and for superimposing the local oscillator light beam and the second portions of the first and second measuring light beams, wherein
• the displaced Sagnac interferometer has a first interferometer ring beam path being arranged for guiding the first portions of the first and second measuring light beams to the optical nonlinear medium and for further guiding the local oscillator light beam created in the optical nonlinear medium,
• the displaced Sagnac interferometer has a second interferometer ring beam path having a displacement relative to the first interferometer ring beam path and being arranged for guiding the second portions of the first and second measuring light beams,
• the displaced Sagnac interferometer has in succession a first reflecting element, comprising a transparent plate and being arranged for coupling light into and out of the displaced Sagnac interferometer, a second reflecting element, a third reflecting element and a fourth reflecting element,
• the reflecting elements are arranged for directing the first portions of the first and second measuring light beams from the first reflecting element along the first interferometer ring beam path to the optical nonlinear medium and for directing the local oscillator light beam created in the optical nonlinear medium further along the first interferometer ring beam path to the first reflecting element,
• the reflecting elements are arranged for directing the second portions of the first and second measuring light beams from the first reflecting element along the second interferometer ring beam path back to the first reflecting element, and
• the first reflecting element is arranged for combining the local oscillator light beam and the second portions of the first and second measuring light beams into the sample light beam and for coupling the sample light beam out of the displaced Sagnac interferometer.