Nikkei Electronics Asia -- January 2009
Features
Sanyo Develops 450mW Blue-Violet Laser Diode

Sanyo Electric Co Ltd of Japan has developed a blue-violet laser diode with a pulse oscillation output power of 450mW (Fig 1). The emission wavelength is 405nm, designed to make it possible for Blu-ray Disc drives to record 4-layer discs at 12x speed. A 4-layer disc would have a storage capacity of 100 Gbytes, enough to hold about eight hours of high-definition television (HDTV) imagery. With a 12x write speed, it would be possible to dub a two-hour program in only ten minutes (Fig 2).

Blu-ray Disc drives currently use 2-layer discs at 6x speed, but increases are expected in the number of recording layers, rotation speed and other characteristics. Key causes are increased exposure to HDTV programming, and the fact that more consumer digital camcorders can also shoot digital imagery. It is likely that more users in the future will choose Blu-ray Disc to store their imagery, and that expectation is driving manufacturers to increase recording time through multi-layering, accelerate recording by boosting rotation speed and otherwise enhance products.

Sanyo Electric hopes to commercialize a 4-layer, 12x drive in two or three years, and launch volume production of the new laser at that time.

Tackling Key Issues

Manufacturers of laser diodes have been developing 300mW devices to handle demand from 2-layer, 12x drives, but there are some major technical obstacles to jumping from there to lasers for 4-layer, 12x drives. In developing the new 450mW laser diode, Sanyo Electric cites three key issues, namely 1) laser diode device life reduction caused by higher output, 2) reduced conversion efficiency caused by light absorption in the light guide, and 3) kinking (loss of optical output and current linearity). These problems are encountered every time engineers try to boost laser diode output power, but apparently they were especially thorny with the 450mW diode.

The biggest hurdle was 1), shortened service life. A 450mW laser diode has an optical density of several MW/cm2, quite high. This means the light absorption by the end face protection film is severe, causing a correspondingly severe reduction in optical output, with adverse effects on life, reliability and other characteristics.

The end face protection film is a film-type dielectric. In the new diode, its structure has been tweaked in terms of film composition, number of layers and layer thickness so that reflection is only a few percent and optical output 95% or better. Engineers have verified pulsed oscillation for 1,000 hours in an +80*C environment. With the conventional end face protection layer, the diode was destroyed after 300 to 500 hours at 450mW output.

For point 2), reduced conversion efficiency caused by light absorption in the light guide, the key point was absorption of light from the emitting layer by the cladding layer. Impurities in the AlGaN crystal cladding layer cause light absorption. Engineers made improvements to the metal organic chemical vapor deposition (MOCVD) process used in crystal growth to reduce unwanted impurities in the growth process to half or less. As a result, light absorption has been reduced, improving optical output by about 10% for the same drive current.

Higher Beam Stability

Issue 3), kinking, is due to instability in laser light emission site. If drive current is increased in an effort to boost output, the laser light beam position begins to wobble within the emitting layer in the horizontal plane. The higher the drive current the more electron-hole pairs are formed in the active layer, creating regions in the active layer where the supply of carriers is insufficient. When this type of region exists, carrier density within the active layer is uneven, and the optical refraction index drops where carrier density is low, because the two are in a proportional relationship to each other. Worse, the regions with low carrier density change over time, so that carrier density distribution in the plane is not stable. The laser light beam concentrates in regions of the active layer when the refraction index is high, so that when a change in carrier density distribution changes the refraction index, the beam output position moves.

In laser diodes, the beam is concentrated in the light guide, in the middle of the device. The p-type cladding layer, which has a higher refraction index, is ridged, and the region between the electrode and the p-type cladding is filled with a low-refractivity dielectric. The refraction index is high only at the ridge structure, which concentrates the beam and forms the light guide. Even if there are no problems at 350mW output, though, an output of 450mW raises the drive current too high, causing a large fluctuation in carrier density and leading to kinks.

Sanyo Electric handled the problem by using simulation to design a ridge structure that maintains a more stable carrier density distribution even at high drive currents, and improved fabrication precision to make it possible to form those ridges. The fabrication precision of the dry etching process used to form the ridges is about double that of the prior process, with the result that the new laser device shows no kinks up through 600mW to 700mW optical output.

The next step will probably be an increase in Blu-ray Disc layers to eight, which would boost capacity to 200 Gbytes.

Sanyo Electric already predicts demand for a 900mW laser diode, offering about double the output power of the current 450mW model, and plans to continue development toward even higher-output designs.

by Mayuko Uno

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