08 Lights Sources for Fiber Optics

08 Lights Sources for Fiber Optics - Light Sources for...

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Light Sources for Fiber Optic Systems The minimum of loss and dispersion in optical fibers occurs in the range 1.3 < λ < 1.6 μ m. Light sources on which to impress information are needed in this range. Semiconductor LED’s and lasers are the most desirable because they are compact, efficient and robust. LED’s – Light Emitting Diodes LED’s are incoherent sources of light that results from the direct recombination of electrons and holes in a p-n junction. + p n - I V E c E v E F E k Emission at 1.3 microns requires materials with E g 0.95 eV; at 1.55 microns E g 0.8 eV. Electrons recombine with holes so that energy and momentum are conserved. i.e. h ν = E e -E h . h ν = E c (k) – E v (k) |hk photon | = |hk electron | hk hole |
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P N Junction + p n - I V E c E v E F
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Momentum Conservation and Spectral Width hole electron electron photon electron photon hole electron photon photon p p p p a p p a p h h k p << = = = - - 10 x 5 ~ cm 10 x 1 cm 10 x 5 ~ ~ ~ 2 2 4 4 - 8 , λ π ρ (E) E E c E v
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Consequences of Spectral Width At room temperature, kT = 0.26 eV. Thus the spectral width of an LED is E ~ 0.05 eV. E = hc/ λ . Thus E = hc/ λ ( ∆λ/λ29 and ∆λ ~ 94 nm. With this large of a spectral width the maximum bit rate that can be sustained for ONLY 1Km of fiber is B ~ ( τ ) -1 = (D ) -1 = (100 psec/Km/nm x 1 Km x 94 nm) -1 B ~ (9.4 x 10 3 psec) -1 ~ 100 Mb/s. For 10 Km , B ~ 10 Mb/s. These are too slow for practical optical communication systems. Thus LEDs are not suitable for long distance communications systems. A laser must be used. Lasers for Optical Communications Systems Requirements: 1. λ = 1.55 μ m to minimize loss 2. Laser must be capable of high speed modulation > 10 9 b/s 3. Laser must be able to emit modest powers ~ 3-10 dBm. 4. Laser must have narrow linewidth, Efficient light emission requires direct band gap.
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Energy Gaps of Important Semiconductors Other Semiconductor E g ’s HgTe Direct ~0.0 eV CdTe Direct ~1.5 eV ZnTe Direct ~2.2 eV CdS Direct ~2.4 eV ZnSe Direct ~2.6 eV ZnS Direct ~3.7 eV Note: Although GaSb has a direct energy gap near 1.55 microns, it is not suitable as a material to make efficient lasers.
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Alloy Compound Semiconductors There are no elemental or binary compound semiconductors that have the appropriate materials properties for 1.3 – 1.55 micron lasers. Furthermore, the elemental semiconductors, Si and Ge, are indirect bandgap materials – they are not efficient light emitters. To construct materials with the appropriate energy gaps with direct band gaps we must consider alloys of the binary compound semiconductors. It is possible to form alloy compound semiconductors whose properties vary continuously from the end point compositions. For example the binary compound semiconductors GaAs and InAs can be mixed to form a ternary alloy compound, Ga x In 1-x As where x varies from 0 to 1. The properties of this alloy vary continuously with x. For example, the energy gap varies from E
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08 Lights Sources for Fiber Optics - Light Sources for...

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