OPTICAL DEVICE

PROBLEM TO BE SOLVED: To securely suppress noise by reflecting emission light from a laser element in an optical system, by returning a part of the reflected light to the laser element through light path, and by setting the self-excited oscillation frequency of the laser element being determined by the influence of the returned reflected light so that specific conditions are met. SOLUTION: Light emitted from a semiconductor laser element 1 enters a beam splitter 5 via a coupling lens 4, and, furthermore, enters the recording surface of an optical disk 3 via an objective lens 6. Reflected light that picks up information on the recording surface of the optical disk 3 returns to the beam splitter 5 via the objective lens 6 again, and one part of the reflected light enters a photo detector 8 via a sensor lens 7. On the other hand, remaining reflected light returns to a laser element 1 through a light path length, a self-excited oscillation frequency fsp of the laser element 1 being determined by the influence of the returned reflected light is set so that (m-0.25).(c/2 Lex)