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Optical Sources: Laser Diodes over LEDs

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We now discuss the next type of the most commonly used transmitters- the laser diode. The advantages of LEDs include their compactness, low cost and the ability to modulate directly by modulating the forward drive current. However, LEDs do have some limitations, which are discussed below.
The key limitation of LEDs is their large spectral widths. We have proved earlier that, irrespective of the emission wavelength, the spectral width of LEDs is ~11 THz. So, the different colours representing different spectral components of LED/laser diode, when propagating through
an optical fiber would travel with different speeds and this leads to chromatic dispersion. The effect of chromatic dispersion can be explained as follows. (This topic will be discussed in detail later). When information is encoded and transmitted using LED, all the colours/spectral
components appear at different times due to chromatic dispersion, with the result that the information will get spread out in time. The amount of spread increases linearly with the length of the fiber, and hence for long-distance communication links, LED is not a suitable choice to
design transmitters. Spreading of information bits leads to inter-symbol interference (discussed later in detail) and limits the speed with which the bits can be transported. This issue can be overcome with the use of light sources – such as lasers - with narrow spectral width.

The prime advantage of optical communication is that the carrier frequency is several terahertz, which can be potentially modulated at even up to several gigahertz or up to even hundreds of GHz. However, as described earlier, the modulation bandwidth of LEDs is only a few MHz typically, and hence, the benefit of using THz carrier frequency cannot be utilized with low-
bandwidth LEDs. So, the second limitation is the limited modulation bandwidth and that is fundamentally limited by τ௖
. So, for a communication system which will work at gigabits per
second with bandwidths of several gigahertz, LEDs can not be used as optical sources. However, LEDs will definitely be useful for those applications, which require only MHz or several MHz of
bandwidth for short-distance links, where the chromatic dispersion is not a limitation. For megabits per second data rates, LEDs can be used, because they are cheaper than laser diodes. Laser diodes have a smaller spectral width when compared to LEDs. They have
fundamentally larger modulation bandwidth capability. However, the efficiency of the laser diode is poor as compared to LEDs. The reason behind the smaller spectral width of laser diode is that
emission process (stimulated emission) is fundamentally different compared to LEDs(spontaneous emission). The dominant emission process in laser diodes is stimulated emission, whose characteristic time scales are much smaller compared to spontaneous emission in LEDs.
This process will be discussed in detail in the next few lectures. An intuitive description is given below. In stimulated emission, instead of allowing the electron-hole recombination to happen on its own (spontaneously), photons are injected into the system which stimulates the recombination process. Now, the electron-hole combinations occur at much faster rates which not limited by τ௖ but instead by the rate of injected photons. The corresponding stimulated lifetime is much faster, and hence the laser diode can be modulated much faster compared to LEDs.