Fiber Optic Light Source for Optical Communication Systems

>> Light Source Requirement in a Fiber Optic Communication System
Light source plays a significant part in a fiber optic communication system. The basic optical fiber system consists of a transmitter, an optical fiber, and a receiver. The transmitter has a light source which is modulated by a suitable drive circuit in accordance with the signal to be transmitted.
The choice of an optical source is determined by the particular application. For high speed fiber optic communication systems, which operate at speed higher than 1 Gbit/s, the selection of light source is even more critical. The source should meet several basic requirements. 
The first requirement is that it needs to emit a wavelength which corresponds to low loss window of fused silica, the most common optical fiber material, namely 1.3um and 1.5um windows. This is very critical since fiber links often operate at several tens of kilometers span without repeater. For a given optical power at the wavelength, lower fiber losses would lead to larger repeater spacings.
The second requirement is high speed digital modulation. Current generation of fiber optic communication systems have reached speed to 40Gb/s and 100Gb/s. This requires the light source to be modulated at speeds in excess of 2.5Gb/s. To meet this requirement, two types of modulation methods have been developed. The first type is to directly modulate the light source at the speed desired. The second modulation type is to use a LiNbO3 external modulator. For the second type, the light source is required to give steady power output.
The next very important characteristic of this light source is small spectral linewidth of the source. This significantly affects the magnitude of dispersion which is directly proportional to the linewidth of the source. Dispersion in fiber causes signal overlap and significantly reduces the system's bandwidth capacity.
Although there are many different types of light sources, fiber optic communication systems usually just use either LED(light emitting diodes) or laser diodes (LD) because of the requirements as stated above. LEDs and LDs feature small size, high power efficiency and many other beneficial features.
>> Laser Diodes (LD)
LASER stands for Light Amplification by Stimulated Emission of Radiation. Laser is highly monochromatic, it is similar to an electronic oscillator in concept. A laser consists of an active medium that is capable of providing optical amplification and an optical resonator that provides the necessary optical feedback.
The most common laser diode is formed from a p-n junction and powered by injected electric current. It is formed by doping a very thin layer on the surface of a crystal wafer. The crystal is doped to produce an n-type and a p-type region, one above the other, resulting in a p-n junction.
Laser diodes are available as laser diode modules. Some manufacturers provide a large selection of laser diode modules ranging from continuous wave, line generator, modulatable, NIR and more. 
Diode lasers use microscopic chips of Gallium-Arsenide or other exotic semiconductor to generate coherent light in a very small package. The energy level differences between the conduction and valence band electrons are what provide the mechanism for laser action. 
High power diode lasers are the most efficient light emitter. They can also be used for laser diode instrumentation which gives the user the ability to precisely control the laser diode current and temperature. They can be operated in continuous wave mode by selecting a laser drive current or modulated by using a modulation feature on most drivers. The laser temperature can be fixed for precise wavelength stabilization.
The active element is a solid state device not all that different from an LED. LD do have some disadvantages in addition to critical drive requirement. Optical performance is usually not equal to that of other laser types. In particular, the coherence length and monochromicity of some types are likely to be inferior.

>> Light Emitting Diodes (LED)
An LED is a forward biased p-n junction in which e-h recombination leads to the generation of optical radiation through the process of spontaneous emission. The structure of LED is similar to that of a laser diode except that there is no cavity for feedback. The emission from an LED is due to spontaneous recombinations and the output from an LED differs significantly from that of a LD (laser diode).
LEDs have many advantages such as lower energy consumption, longer lifetime, improved robustness, smaller size, and greater reliability. Unlike the laser diode, there is no threshold and the output power increases smoothly as a function of current. At large currents the output power saturates. The total power output from LEDs can be a few milliwatts.
Because spontaneous emission is random and appears along all directions, the output from an LED is not directional. Output beam angles may be typically in the range of 30° perpendicular to the junction, to about 120° parallel to the junction.
LEDs are also used in many other applications except fiber optic communication, such as aviation lighting, automotive lighting, and traffic signals, etc.