An optical device (1) includes a cavity (2) for which at least part is ellipsoidal with a first focus (F1) and a second focus (F2). The device includes means of presenting a medium containing components to be analyzed in the cavity at the first focus, and the device also includes means of transporting an excitation light beam in the cavity along a path also passing through the first focus (F1) to illuminate the medium at the first focus. The ellipsoidal part of the cavity includes a wall reflecting light emitted by the medium in response to the excitation light beam, and the device includes means (5) of collecting said emitted light at the second focus (F2).