Sources: Lasers and LEDs
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Output power.
How much optical power does the source couple into the fiber? Both LEDs and lasers used in network/premises cabling applications couple tens of microwatts of optical power into a fiber. The amount coupled from a given device depends on the fiber's core diameter and NA. The light from an LED spreads out in a wide pattern so that only a small portion of the total light emitted is actually coupled into the fiber. A fiber with a large core or a higher NA gathers more of this light. For example, an LED might couple 45 W into a 62.5/125 fiber, but only 35 W into a 50/125 fiber. The difference is due to the larger diameter and NA.
A singlemode fiber generally requires a laser source. Unlike an LED, a laser produces a very narrow beam of light matched to the small core of the fiber. A laser can couple more power into a fiber—over a watt is not untypical in telecommunications applications. While lasers used in premises cabling applications couple roughly the same amount of light into a fiber as an LED, the efficiencies of singlemode fibers permit longer transmission distances. In an FDDI network, for example, the span between stations is 2 km for multimode fiber and 40 km for singlemode fibers.
Spectral width. What is the spread of wavelength in the light? An LED emits a wider range of light, while a laser emits a very narrow range. Different wavelengths travel at different speeds through a fiber, which contributes to dispersion and thus limits the fiber's bandwidth. LEDs have spectral widths of around 25 to 40 nm. For an LED with a nominal wavelength of 850 nm, a 40-nm spectral width means that actual range of wavelengths is from 830 nm to 870 nm.
Laser, on the other hand, have very narrow spectral widths, ranging from around 1 or 2 nm for a VCSEL or as narrow as 0.1 nm for a distributed feedback laser.
Speed.
How fast can the device be turned on and off? The speed at which the source can be modulated determines the data rate it can support. In general, LEDs can be used at speeds under 1 GHz, while lasers can be used for speeds upwards of 50 GHz.
Types of Lasers
There are three main types of lasers of interest in fiber optics.
Fabry-Perot lasers are "general-purpose" lasers having a spectral width of perhaps 2 nm.
Distributed-feedback lasers use an internal grating to suppress wavelengths and achieve an ultranarrow spectral width: only 0.1 nm or so. Such a narrow output is important to today's multiplexed system, where individual optical signals are separated by as little as 0.8 nm. For example, channel 1 is 1550.116 nm, channel 2 is 1550.918 nm, channel 3 is 1551.712 nm, and so forth. DFB lasers are also very fast, capable of achieving speeds of over 10 Gb/s.
Vertical-cavity surface-emitting lasers are a type of low-cost laser well suited to multimode applications. Unlike Fabry-Perot and DFB lasers, which emit light from the edge in an ellipitical pattern, VCSELs emit light from their surface in a circular pattern closely matched to the core of a multimode fiber. Another area of interest of VCSELs is that they can be applied in arrays with multiple outputs. In some ways, you can think of a VCSEL as a high-end LED or a low-end laser.
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