Integrating confocal laser and detector increases pickup

March 1, 1999
YOKOHAMA--Researchers at Sony Corp. have developed a novel monolithic confocal optical pickup for future mass production. The device is termed a confocal laser coupler (CLC) because the photodiode and the laser are nearly in the confocal position, and the built-in micromirror acts as a photocoupler in the same way that a beamsplitter prism acts in a conventional optical pickup. The CLC has three discrete components: a 780-nm aluminum gallium arsenide (AlGaAs) laser diode in a stripe geometry, a

Integrating confocal laser and detector increases pickup

Paul Mortensen

YOKOHAMA--Researchers at Sony Corp. have developed a novel monolithic confocal optical pickup for future mass production. The device is termed a confocal laser coupler (CLC) because the photodiode and the laser are nearly in the confocal position, and the built-in micromirror acts as a photocoupler in the same way that a beamsplitter prism acts in a conventional optical pickup. The CLC has three discrete components: a 780-nm aluminum gallium arsenide (AlGaAs) laser diode in a stripe geometry, a 3-µm micromirror, and a p-n photodiode near the emission facet of the laser.

The light emitted from the stripe laser diode is reflected by the built-in micromirror and collimated through the lens to the disk surface (see figure). After reading the pit signal on the disk, the beam returns via the same path. The returned light is spread by diffraction to a spot size with a diameter of 12 µm at the photodiode surface. This beam size is much larger than the initial near-field spot of the laser diode. The photodiode, which lies within the focusing depth of the optics and in the confocal position, receives about 20% of the returned intensity.

The laser diode is grown on a 100-oriented n-type GaAs substrate using metal-organic chemical-vapor deposition. Laser cavities are formed by reactive ion etching. Nondoped GaAs and p-type GaAs layers are selectively grown forming a crystalline 111-B facet as a built-in micromirror and a photo diode beside the laser stripe. The angle between the built-in micromirror and the substrate is 55° and the laser beam is emitted at 70°. Sony is confident that the confocal laser coupler will be applicable to future optical-disk systems as well as to precise sensing systems such as a confocal microscope.

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