Abstract: The range of air conversion optical path length L of all optical components in the forward passage of an optical system between a laser emission point and the surface of a disc-shaped recording medium is determined by: i(n0×d)+?L/2<L<(i+1)(n0×d)??L/2 (i is an arbitrary integer) where d is resonator length of a laser diode, n0 is active layer refractive index of the laser diode, and ?L is acceptable surface blurring in the specification of the disc-shaped recording medium, thereby reducing laser noise (optical feedback noise) that is generated when emitted laser beam from a laser diode is interfered with the optical feedback, which is a part of laser beam that has been focused on the recording surface of the disc and has moved back through the forward passage of the optical system to return to the laser diode.

Abstract: A magnetic element is provided on the outer periphery of a ring member which is operatively connected with diaphragm blades. This magnetic element is either shaped or constructed so that it exhibits a magnetic gradient (viz., produces non-uniform magnetic leakage flux properties). A Hall sensor is mounted stationarily with respect to the ring member so as to respond to the change in magnetic leakage flux and indicate the amount of rotation of the ring.