World's Brightest

Figure 5: Relationship between wavelength and luminosity

One way to increase the brightness of a laser display is to raise the optical output power. Another method is to use a wavelength of light to which the human eye is extremely sensitive. In the red wavelength band, luminosity increases as the wavelength is shortened (Figure 5), resulting in higher brightness.

The wavelength of the emitted light can be controlled by changing the percentages of gallium (Ga) and Indium (In) in the GaInP light-emitting layer. Sony's challenge was to reduce the wavelength of its 645nm laser by 10nm to 635mm. However, when the wavelength is reduced below 640nm, heat output increases causing a rapid decline in the luminance efficiency of the GaInP light-emitting layer. This frustrated efforts to achieve laser oscillation at a high output level. Sony began developing new technologies to limit reductions in luminance efficiency. Now Sony has succeeded in oscillating a laser to produce an optical output of 7.2W at a temperature of 25°C and a wavelength of 635nm (Figure 6).

Figure 6: Characteristics of the 635nm semiconductor laser array

Future Challenges

Before semiconductor lasers can be used as light sources for display devices, they must meet extremely demanding requirements, including not only high output, but also high brightness, operating temperatures, power conversion efficiency and reliability. Further advances in technology are needed, and Sony will continue its efforts to develop lasers to overcome all of these challenges.