An LED is a semiconductor device that emits light when the forward-directed current passes the p-n junction. LEDs are the light sources with a wide range of selected wavelengths from UV to IR. A typical wavelength bandwidth is 20-30 nm for LEDs working in the visible range. An LED’s light power ranges from a few milliwatts to a few watts. Their
light beam divergence is of a few tenths of a degree. They can be used as prospective light sources for many applications because of their high efficiency (conversion of electrical to light energy), long life time (more than 105 hours), ability to emit many different wavelengths (colors), and high brightness.
A superluminescent diode (SLD) is a very bright diode light source with a broad bandwidth. It is usually manufactured using a laser diode technology (heterostructure, waveguide, etc.), but without reflecting mirrors (there is an antireflection coating at the diode faces, or their out-of-parallelism is provided). Its main difference from a LED is that it has a uniform wavefront of the output radiation which allows one to couple its radiation into a single mode fiber. The SLDs are used in different medical OCT systems.
New semiconductor technologies that are based on heterostructure concept are important for fabrication of short-wavelength LEDs, SLDs, and diode lasers. Heterostructure is a semiconductor junction which is composed of layers of dissimilar semiconductor materials with nonequal band gaps. The quantum heterostructure has a size that restricts the movements of the charge carriers and forces them into a quantum confinement that leads to the formation of a set of discrete energy levels with sharper density, than that for structures of more conventional sizes.