Sony Tests Dual-Wavelength Laser
KANAGAWA, Japan -- If your kids are whiling away their summer break playing games on a Sony PlayStation 2, then you have seen the company's latest laser diode in action. Sony Corp.'s Semiconductor Laser Div. had developed a monolithic dual-wavelength laser, which it fast-tracked into production for the release of the game systems in March 2000. Now the company has reported that the devices perform as well as or better than their single-chip counterparts.
Sony Corp.'s monolithic integrated dual-wavelength laser diodes produce 780- and 650-nm light for reading both CDs and DVDs. Company researchers have reported that the chips perform as well as or better than single laser diodes at the wavelengths.
The laser solves a long-standing problem for CD and DVD playback. For a single unit to play all CDs and DVDs, the machine needs two lasers to read the different size pits on the discs and to compensate for their difference in reflectivity. One must operate at the 650-nm wavelength for DVD playback, and the other must produce 780-nm light for CDs.
The most common approach has been to use two separately manufactured laser diodes, sometimes joining them to one silicon wafer and using a prism to combine their output on one optical path, but this increases the cost of the pickup. Monolithic integrated lasers, which are fabricated on the same substrate, are less expensive and more robust, but Kazuhiko Nemoto of Sony's Semiconductor Laser Div. explained that they are difficult to fabricate and that the manufacturing yield is low. The company has solved these problems and has produced a dual-wavelength laser that costs roughly the same as one 650-nm laser diode.
Making the lasers involves depositing the AlGaAs and InGaAlP active layers on a GaAs substrate and then using wet etching to shape and separate the laser islands for the two wavelengths. Metallorganic chemical vapor deposition and photolithography tailor the islands for each wavelength.
According to test results published in the April 16 issue of Applied Physics Letters, the research group, led by Shoji Hirata, achieved up to 12 mW from the 650-nm side and 15 mW from the 780-nm side. The mean times to failure for the 650- and 780-nm elements in continuous-wave operation at 70 °C were greater than 10,000 and 100,000 hours, respectively, Nemoto said. For the 650-nm side, this is roughly the same as single 650-nm laser diodes, he said. The 780-nm element actually performs better than a conventional chip.
The relatively simple manufacturing process makes the chips ideal for mass production, as evidenced by Sony's quick trip to the manufacturing floor. The company has said that it will ramp up production of the PlayStation 2, which sold more than 10 million units in its first year, to 2 million units per month this fall. Nemoto expects variations of the laser diodes to be available for use in near-term applications such as CD read/write.
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