Optical source is an integral part of optical transmitter. An optical source converts the incoming electrical energy into optical energy. The optical energy that is light, emitted by an optical source is launched into an optical fiber for transmission. Fiber optic data link performance depends on the amount of optical power launched into the optical fiber. However, there is a limit on the injected light power posed by the electronics.
Progress and developments in fiber optic telecommunication were possible with the development of efficient semiconductor optical sources along with low loss optical fibers. The simultaneous improvements in , led to substantial improvements in fiber optic communications. Semiconductor optical sources have the physical characteristics and performance characteristics that are necessary for successful operation of the installed fiber optic systems.
Optical sources must be compatible with the optical fibers. Optical fibers are thin, cylindrical rods that allows propagation of light. Light sources shall be suitably designed to be compatible with the reduced size and low loss characteristics of optical fibers. Since the core of the fiber, especially single mode fiber (mode field diameter for single mode fibers) is in the range of 8 to 10 micrometers, the light launching area shall be accurately focused. Hence an optical source’s light emitting area shall be capable to launch maximum light in to the fiber.
The launched power from an optical source shall be sufficient so that the detector at the receiving end can detect incoming optical signals. In other words an optical source shall be enough to counter the linear propagation loss and connector losses. Optical signal will weaken as it travels along the length of an optical fiber due to scattering and absorption losses. An optical source shall be able to generate enough light power considering the intermediate losses that happens as the light travels along the length of optical fiber
Another important property of an optical source is its ability to emit light at wavelengths where the optical communication shows minimum loss and dispersion. Optical transmission is sensitive to the medium. Doped silica glass is the medium in fiber optic communication. As the optical signal interacts with the impurities, external stresses and internal flaws in optical fiber, it experience loss and dispersion. Researchers have identified wavelengths suitable for optical transmission based on the minimum levels of attenuation and dispersion. There is a trade off between these two parameters. While in a conventional silica glass fiber, dispersion is nearly zero at 1310nm, attenuation is around 0.35 dB/km. This level of attenuation is considered higher when compared with the attenuation level offered by the 1550 nm wavelength. The problem with 1550 nm is the higher levels of dispersion. Hence an optical source shall be able to generate optical signals at desired wavelength according to the transmission requirements.
Dispersion is affected by the spectral width of the optical source. By definition, dispersion is the broadening of pulse width. If the pulse width is broaden at the very beginning of transmission, that is from the emitting source, then further broadening will make the signals undetectable at the receiver end. Hence light pulses from an optical source shall have narrow spectral width. Also an optical source shall allow direct modulation of optical output power.
Apart from the above, an optical source shall maintain stable operation in changing environmental conditions such as temperature. An optical source shall cost less and be more reliable than electrical devices, permitting fiber optic communication systems to compete with conventional systems. Semiconductor optical sources suitable for fiber optic systems range from inexpensive light-emitting diodes (LEDs) to more expensive semiconductor lasers. Semiconductor LEDs and laser diodes (LDs) are the principal light sources used in fiber optics.