An optical device for determining at least one signal light component being characteristic for an optical near-field interaction of a probe with an object to be investigated, wherein the near-field interaction is subjected to a fundamental modulation at a fundamental frequency .OMEGA., comprises an interferometer device with an illumination light path (I) being directed to the probe, a reference light path (II) being directed to a detector device for obtaining detector output signals including signal light components, and a signal light path (III) being directed from the probe to the detector device, wherein the reference and signal light paths (II, III) are superimposed at the detector device, and a demodulation device for determining the signal light components by demodulating the detector output signals, wherein the reference light path (II) does not contain the probe, an interferometer phase modulator is arranged in the reference light path (II) or signal light path (III) for changing an interferometer phase comprising the optical phase difference between the reference light and the signal light, and the demodulation device is adapted for determining the signal light components from the detector output signals obtained at three or more different interferometer states, which represent three or more different interferometer phases or at least two different interferometer phases with at least one state wherein the reference light path (II) is blocked.