Proceedings Article | 3 July 2003
KEYWORDS: Light emitting diodes, Semiconductor lasers, Collimation, Imaging systems, Visible radiation, Lenses, Indium gallium nitride, Coherence (optics), Mirrors, Diffraction
Laser diodes are used in high radiance applications, requiring spatially and angularly localized optical power, in order to minimize system size, cost, and power requirements. For applications requiring visible wavelengths (400nm to 700nm), AlInGaP laser diodes are utilized for wavelengths between 630nm and 650nm, and recently InGaN-based laser diodes spanning the violet to blue spectral range (400nm to 450nm) have become available, with longer wavelength devices being actively developed. It is not clear, however, when reliable laser diodes emitting in the green to yellow portion of the visible spectrum (>500nm) will be realized. In addition, single mode laser diodes required for high radiance applications typically exhibit speckle effects, are temperature sensitive, require non-linear drive electronics, and can be effected by optical feedback. High radiance LEDs with appropriate emitter dimensions can be used in place of laser diodes provided that moderate coherence and power levels are acceptable. This article examines the general dependencies of beam focusing, collimation, and coherence, on LED emitter size and radiance. Specific model examples are considered, using parameters consistent with typical laser diode collimating lenses. Edge emitting LEDs or EELEDs used for miniature scanned displays are described as high radiance LEDs suitable for other applications requiring collimation, partial coherence, or localized irradiance.
