From Understanding Lasers

9.11.6 InGaAsP Lasers for Fiber-Optic Systems

The quaternary (four-element) semiconductor InGaAsP is a versatile laser material grown on substrates of InP. The sum of the number of indium and gallium atoms equals the total number of arsenic and phosphorous atoms. The chemical formula can be written as InxGa1 xAsyP1 y ,with the values of x and y varying independently. Adding either element changes the lattice constant, so successfully growing the compound on an InP substrate requires balancing the composition to match the lattice constant to the substrate.

InGaAsP active layers can emit at 1100 to 1650 nm when lattice- matched structures are grown on InP substrates. The major applications of InGaAsP lasers are in fiber-optic communications, so most lasers are manufactured for the fiber-optic bands around 1310 nm and from about 1480 to 1600 nm.

The type of InGaAsP lasers used depends on the number of wavelengths being transmitted through the fiber-optic system. Wavelength tolerances are 20 nm or so for systems transmitting only a single wavelength, which can use uncooled Fabry Perot lasers. Systems transmitting several wavelengths about 20 nm apart (coarse wavelength-division multiplexing) also use Fabry Perot lasers, but more care must be taken to see that their output falls into the proper wavelength channel.

High-speed networks pack wavelengths much closer together to increase capacity; this is called dense wavelength-division multiplexing. These wavelength slots are only a fraction of a nanometer wide, typically 0.8 nm, so they require narrow-line DFB or DBG lasers, which must be cooled to keep them at a stable temperature because laser wavelength varies slightly with temperature.

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Products & Services
Optical Spectrum Analyzers
Optical spectrum analyzers (OSA) can divide a lightwave signal into its constituent wavelengths. This means that it is possible to see the spectral profile of the signal over a certain wavelength range.
Fiber Lasers
Fiber lasers use optical fibers doped with low levels of rare-earth halides as the lasing medium to amplify light.
Wavelength Division Multiplexers (WDM)
Wavelength division multiplexers (WDM) are devices that combine light signals with different wavelengths, coming from different fibers, onto a single fiber. They include dense wavelength division multiplexers (DWDM), devices that use optical (analog) multiplexing techniques to increase the carrying capacity of fiber networks beyond levels that can be accomplished via time division multiplexing (TDM).
Fiber Optic Amplifiers
Fiber optic amplifiers re-amplify an attenuated signal without converting the signal into electrical form.

Topics of Interest

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