TOKYO — NEC Corp. and Sony Corp. have announced violet laser technologies that have reached the commercial-product level. The new entrants, unveiled at the autumn meeting of the Japan Society of Applied Physics (JSAP), challenge market leader Nichia Corp., which introduced the first violet laser products. Nichia's laser is based on a sapphire substrate, on which an active gallium nitride (GaN) layer is grown to produce high-frequency laser diodes.
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.
