Ch8Sec5 - THE FIBER FORUM Fiber Optic Communications...

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Dr. Joseph C. Palais 8.5 1 THE FIBER FORUM Fiber Optic Communications PRESENTED BY Dr. JOSEPH C. PALAIS
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Dr. Joseph C. Palais 8.5 2 Section 8.5 Source Coupling
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Dr. Joseph C. Palais 8.5 3 Source Coupling In this section we want to determine the efficiency of coupling between the light source and the fiber. The coupling efficiency is: f s P P η= P f is the power in the fiber and P s is the power emitted by the light source. In dB the loss is: L = -10 log η SOURCE FIBER
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Dr. Joseph C. Palais 8.5 4 8.5.1 Reflection Loss If there is an air gap between the source and the fiber there is a Fresnel reflection loss of given by ( 29 ( 29 2 1 2 2 1 2 n n R n n - = + If n 1 = 1 and n 2 = 1.5 as appropriate for an air- to-glass interface, the reflectance is:
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Dr. Joseph C. Palais 8.5 5 ( 29 ( 29 2 2 1 1.5 0.04 1 1.5 R - = = + There is 4% reflected power and 96% transmitted power. The transmission loss is then: 10log0.96 0.177 L dB = - = We normally round this off and use 0.2 dB as the reflection loss. If there is no gap this loss disappears.
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Dr. Joseph C. Palais 8.5 6 8.5.2 Area-Mismatch Loss If the source is larger than the fiber core, some of the power is lost. The loss is given in terms of the core area A c and source emission area A s . The coupling efficiency is reduced by an amount: c s A A η=
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Dr. Joseph C. Palais 8.5 7 In decibels, the added loss is: 10log c s A L A η = - = - This applies if the light source is larger than the fiber core. The dB loss due to mismatch is clearly zero if the core is larger than the source.
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Dr. Joseph C. Palais 8.5 8 8.5.3 Packing-Fraction Loss
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Ch8Sec5 - THE FIBER FORUM Fiber Optic Communications...

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