Semiconductors have revolutionized the electronics industry. Almost all modern devices and equipments that we enjoy in today’s day to day life make use of semiconductors. Similarly, Fiber optic industry is deeply indebted to Semiconductor industry. In other words, developments in fiber optic industry were all possible only due to the advancements happened in semiconductor industry.
Semiconductor light emitting diodes and laser diodes make the fiber optic telecommunication possible in reality. A basic knowledge of semiconductor material and the device properties are required to understand optical emission in semiconductor lasers and LEDs.
Semiconductor light sources are diodes. They possess all of the characteristics of typical diodes. Construction of semiconductor light source includes a special layer, called the active layer, which emits photons (light particles) when a current passes through the layer. The particular properties of the semiconductor are determined by the materials used and the layering of the materials within the semiconductor.
Silicon (Si) and Gallium Arsenide (GaAs) are the two most common semiconductor materials used in electronics opto-electronic devices. Other elements, such as aluminum (Al), indium (In) and phosphorus (P), are added to the base semiconductor material to modify the semiconductor properties in some semiconductors. The process of adding thiese elements is called doping and doped elements are called dopants.
Electric current flowing through a semiconductor optical source causes it to produce light. Light is produced spontaneously. Spontaneous emission is the random generation of photons within the active layer of the LED. The emitted photons move in random directions. A certain percentage of the photons exit the semiconductor and are coupled into the fiber. Many of the photons are absorbed by the LED materials and the energy is dissipated as heat. The random generation of photons cause the light output from an LED to be incoherent. This kind of emission gives the LEDs to have a broad spectral width and a wide output pattern. Function of an LED is simple.
In contrast, function of a Laser diode is more complex. Laser/LASER stands for Light Amplification by the Stimulated Emission of Radiation. Laser diodes produce light through stimulated emission when an electric current is passed through them. The photons, initially produced by spontaneous emission interact with the laser material to produce additional photons in a laser diode. This process occurs in the laser cavity, which is the active area of the diode. The process does not affect the original photon. The stimulated photon has many of the same properties (wavelength, direction, phase) as the original photon. Hence a Laser diode gives out consistent light which is known as coherent light.
In Laser diodes also, all of the photons produced are not emitted from the laser diode. Some of the photons are absorbed and the energy is dissipated as heat. The emission process and the physical characteristics of the diode cause the light output to be coherent, have a narrow spectral width, and have a narrow output pattern. The common thing with LED and LD is that in both, all of the electrical energy is not converted into optical energy. A significant portion is converted to heat. Different types of LED and laser diodes have differing properties to convert and emit different amounts of electrical energy into optical energy.