Sony's violet laser operates at the same power level as the 30-milliwatt output power offered by Nichia. Sony already had achieved a lifetime of about 3,000 hours in April, when the company showed a prototype of its laser at its corporate show. Sony engineers subsequently improved the technology to realize a 15,000-hour lifetime with 30-mW output power at 60°C, a Sony spokesman said.

At the JSAP meeting, Sony engineers explained that to employ lasers for a mass-market application such as optical drives, lasers with smaller aspect ratios are desirable. The aspect ratio describes how close the output is to a perfect circle. For example, an aspect ratio of 2.8 means that the ratio of the horizontal axis to the vertical axis of an elliptical spot is 2.8. And the closer a laser beam spot is to a circle, the higher the performance at the disk surface without the need for extra power.

Structure changes

Sony engineers introduced a structure consisting of silicon (Si) and thin silicon dioxide (SiO₂) layers along the ridge stripe to guide the light. The structure realized stable transverse-mode emission and expanded the horizontal beam divergence angle to 8.9°. To narrow the vertical divergence angle, the thickness of the light guide layer was decreased to restrict optical confinement.

The resultant laser operates at a threshold current and voltage of 37.4 milliamps and 4.2 volts. The laser diode has realized the same 15,000-hour life with output power of 30 mW that was realized by the previous prototype with an aspect ratio at 3.2.

For its part, NEC has developed a 410-nanometer wavelength violet laser with a structure that the company claimed is well-suited to volume production compared with current violet lasers. The precise control of width and depth of the optical guiding ridge is a crucial factor for a better yield rate in ridge-type laser production. NEC engineers developed a technology called Rips (ridge by selective regrowth), which features a two-step layer-growing process.

In the first step, layers for the n-cladding layer, the n-optical guiding layer, the multiple-quantum-well layer and the p-optical guiding layer are grown on the GaN substrate. After the first growth, an SiO₂ layer is deposited and a window strip is patterned using standard photolithography techniques. In the second growth step, a remaining p-cladding layer and a p-GaN contact layer are epitaxially regrown on the openings of the SiO₂ layer using epitaxial lateral overgrowth. Then, p- and n-electrodes are evaporated on the top and bottom of the device.

The process does not need dry etching, which causes defects in the active layer. As a result, the laser diode characteristics are improved, an NEC engineer said.