Sanyo Electric Co Ltd developed a blue-violet laser diode whose optical output is 450mW under pulsed operation.
The laser has an emission wavelength of 405nm, which corresponds to 12x recording on a four-layered Blu-ray Disc medium. A standard for systems capable of such operations is now under development, according to Sanyo. The company expects that systems supporting the standard will be commercially available within the next two to three years and plans to start mass-producing the new semiconductor laser diode in time for the commercialization.
The company made three major improvements to increase the output of the semiconductor laser. First, an end face protective film, which absorbs less light, is adopted to ensure a long-term stability. The protective film serves as a mirror, and approximately 95% of the light is passed through the front end face having a multilayer structure. The company improved the material and the structure, such as the number of layers, to reduce the amount of light absorbed by the film.
As a result, the company confirmed operation for "1,000 hours in an environment at +80°C," which is a criterion for practical use. With the existing end face structure, the device would break in about 500 hours, according to the company.
Second, to increase the optical conversion efficiency, Sanyo reduced the loss in the optical waveguide through which the light passes. This time, the company reduced the amount of light absorbed by the cladding layer by reducing the amount of unnecessary impurity, which is incorporated during the crystal growth of the cladding layer, to less than half.
When supplied with the same amount of current, the optical output is about 10% higher than that achieved by the existing method, the company said.
Third, the beam position in the optical waveguide is stabilized to ensure a more stable optical output. The company reportedly optimized the shape of the waveguide based on simulations and improved the precision of dry etching technique used to form the structure so that the beam position can be more stabilized compared with that of the existing products.
In the optical waveguide with the existing structure, two to three laser beam spots are generated when the optical output is high. This poses a problem of unstable optical output.
The existing laser devices have a protrusion called "ridge" that has a higher refraction index in the cladding layer so that laser beam can be concentrated on the ridge. But as the optical output increases, the refraction index in the center portion of the ridge becomes lower because a number of holes and electrons disappear in the emission layer.
Thus, the refraction index of both ends of the ridge becomes relatively higher, increasing the area with a high refraction index and creating two to three laser beam spots.