The invention relates an optical probe (1) suitable for non-linear optics such as two-photon imaging for medical purposes. The probe has an optical guide (2) and a lens system (6) positioned rigidly at an end portion (2a) of the optical guide. Additionally, a housing (3) with a cavity for the optical guide (2) and the lens system (6), the housing having at its distal end a transparent window (4), is comprised in the probe. The optical guide (2) with the lens system (6) is displaceably mounted within the housing, preferably in a transverse direction. Also, the housing (3) has an auxiliary, peripheral optical guide (5) optically connected to the transparent window (4). The invention is advantageous for obtaining an optical probe with a significantly larger collection efficiency. The optical probe may advantageous be applied in connection with two-photon spectroscopy where both ballisitic photons and diffusing fluorescence photons can be used in the detection of an event.