Both Semiconductor Light Emitting Diodes (LEDs) and Laser Diodes (LDs) are used in optical transmission as light sources. Semiconductor LEDs emit incoherent light, meaning the output light is not consistent. The output light from an incoherent light source is more wider (in comparison with the light from a coherent source). Spontaneous emission of light in semiconductor LEDs produces light waves that lack a fixed-phase relationship. In other words the change of phase within the beam does not occur for all the photons at the same time. Light waves that lack a fixed-phase relationship are referred to as incoherent light. In an incoherent light source, phase change is abrupt.
The use of LEDs in single mode systems is severely limited because they emit unfocused incoherent light. Even LEDs developed for single mode systems are unable to launch sufficient optical power into single mode fibers for many applications. LEDs are the preferred optical source for multimode systems because they can launch sufficient power at a lower cost than semiconductor LDs.
Laser Diodes produce light waves with a fixed-phase relationship (both spatial and temporal) between points on the electromagnetic wave. In other words the change of phase with in the bea occurs for all the photons at the same time. Light waves having a fixed-phase relationship are referred to as coherent light. In a coherent light source, phase change happens at the same time.
Since semiconductor Laser Diodes (LDs) emit more focused light than Light Emitting Diodes (LEDs), they are able to launch optical power into both single mode and multimode optical fibers. LDs are usually used only in single mode fiber systems because they require more complex driver circuitry. The cost of Laser diodes are higher than the cost of Light Emitting Diodes.
Optical power produced by optical sources can range from microwatts (mW) for LEDs to tens of milliwatts (mW) for semiconductor LDs. However, effective coupling of all the available optical power into an optical fiber is not possible in actual transmission. The amount of optical power coupled into the fiber is what matters in telecommunication. It depends on the factors such as;
– The angle of light emission
– Size of Light emitting area vs the fiber core dimension
– Alignment of light source and fiber
– Numerical Aperture of fiber
– Refractive index of fiber
Light from a semiconductor laser spreads out over an angle of 10 to 15 degrees generally. Semiconductor LEDs emit light spread out larger than 15 degrees typically. Coupling loss, that is the loss occurs when ligth is coupled to a fiber is also higher for Light emitting diodes. Source to fiber coupling efficiency is a measure of the effective optical power. of several decibels can easily occur when coupling light from an optical source to a fiber, especially with LEDs. The coupling efficiency depends on the type of fiber that is attached to the optical source. Coupling efficiency also depends on the coupling technique.
Source-to-fiber coupling involves centering a flat fiber end face over the emitting region of the light source. If the fiber end face is directly placed over the source emitting region, it is referred to as butt coupling. If the output light pattern of source is larger than the acceptance pattern of the optical fiber, source-to-fiber coupling efficiency can be improved by placing a small lens between the source and fiber. Lensing schemes improve coupling efficiency when coupling both LEDs and LDs to optical fibers.