Optical mass memory

Claim 1

An optical memory comprising: a rotatable substrate having a memory surface and a reflective edge surface essentially orthogonal to the memory surface, a memory medium attached to the memory surface of the rotatable substrate and capable of having a plurality of tracks of bits of information recorded thereon, a first motor means for rotating the substrate and the memory medium, movable arm means extending over the memory surface, the movable arm means being capable of motion in a direction essentially parallel to the memory surface and essentially orthogonal to the reflective edge surface, light source means for producing a light beam for reading and writing on the memory medium, final lens means for focusing the light beam to a focused light spot on the memory medium, final lens mounting means for mounting the final lens means to the movable arm means, a reflective surface attached to the movable arm means, interferometer means for measuring the relative distance between the reflective edge surface and the reflective surface and producing an analog electrical signal indicative of the relative distance, the interferometer means comprising: interferometer light source means for providing a monochromatic light beam, beam splitter means for splitting the monochromatic light beam into first and second light beams which traverse first and second paths respectively, and then recombining the first and second light beams to form a recombined light beam having an interference fringe pattern therein, the first path terminating with the reflective surface such that the first light beam is reflected back to the beam splitter means over the first path, and the second path terminating with the reflective edge surface such that the second light beam is reflected back to the beam splitter means over the second path, mirror means positioned in one of the first and second paths to cause the first and second paths to be parallel to one another, first lens means mounted on the movable arm means for focusing the first light beam to a first focused light spot at the reflective surface, second lens means in the second path for focusing the second light beam to a second focused light spot at the reflective edge surface, phase splitter means for splitting the recombined light beam into a first and second portion, the first and second portions being separated in phase by 90* in the interference fringe pattern, first detector means for receiving the first portion and producing a first detector signal indicative of the intensity of the first portion, second detector means for receiving the second portion and producing a second detector signal indicative of the intensity of the second portion, steering logic means for producing an electrical pulse for each fringe maximum and minimum from each detector and for directing the electrical pulses to an add or a subtract channel depending upon the sign of the phase difference between the first and second detector signals, the sign of the phase difference being indicative of the direction of relative motion of the reflective surface with respect to the reflective edge surface, bidirectional counter means connected tO the add and subtract channels for receiving the electrical pulses and producing a digital electrical signal indicative of number of interference fringe maxima and minima from a predetermined reference fringe, and first digital-to-analog converter means for converting the digital electrical signal to an analog electrical signal, track selecting means for producing an analog track selection signal indicative of the desired distance between the reflective edge surface and the reflective surface, signal comparing means for receiving the analog electrical signal and the analog track selection signal and for producing a servo control signal indicative of a difference of the analog electrical signal and the analog track selection signal, and second motor means for positioning the movable arm means in response to the servo control signal.