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Ch6Sec2 - THE FIBER FORUM Fiber Optic Communications...

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THE FIBER FORUM Fiber Optic Communications JOSEPH C. PALAIS PRESENTED BY
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Joseph C. Palais 6.2 2 Section 6.2 LED Operating Characteristics 0 5 Output Power (mW) Input Current (mA) Linear Region Nonlinear Region There is no output power for reverse bias (negative current). 100
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Joseph C. Palais 6.2 3 Section 6.2 LED Operating Characteristics Ideally, optical power increases linearly with input current. This is approximately true over what is labeled the “linear region” on the preceding graph.
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Joseph C. Palais 6.2 4 Proof: Let i = current (coulombs/second) N = number of charges per second entering the junction. Then ( / ) i N electron s e = LED OPERATING CHARACTERISTICS ( / ) ( / ) i Coulombs s N e Coulombs electron =
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Joseph C. Palais 6.2 5 If a fraction η of these charges recombine to emit photons, the output power is = e i W P g η LED OPERATING CHARACTERISTICS = = ion recombinat energy s ions recombinat W N P g ) ( η s energy NW P g / η = (6.3)
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Joseph C. Palais 6.2 6 If W g is in eV, this simplifies to i W g η = P where W g is in eV and P is in Watts. Thus, the linear dependence is proven. η decreases at high currents, creating the nonlinear saturation region. LED OPERATING CHARACTERISTICS e i eV J e eV W P g = ) ( η Finally (6.4)
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Joseph C. Palais 6.2 7 Now % 6 . 3 036 . 0 ) 1 . 0 )( 4 . 1 ( 005 . 0 = = = η η η i W P g LED OPERATING CHARACTERISTICS Example: GaAs LED, W g = 1.4 eV. Find the conversion efficiency η if a current i = 100 mA produces 5 mW.
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Joseph C. Palais 6.2 8 Typical Parameters Operating voltages: 1.2 - 2 volts. Operating currents: 50 - 100 mA. Example: V = 2 volts, i = 100 ma. Compute the electrical power input. Electric power: P = Vi = 2(0.1) = 0.2 W P = 200 mW. LED OPERATING CHARACTERISTICS
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Joseph C. Palais 6.2 9 Only part of the emitted light is measured and useful for fiber applications because photons coming out all sides cannot be collected. + - Lost Photons LED LED OPERATING CHARACTERISTICS
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Joseph C. Palais 6.2 10 Modulating an LED (Digital) Optical Power Optical Power Output 1 1 1 1 0 0 Input Current (Signal) t t LED OPERATING CHARACTERISTICS Signal LED i
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Joseph C. Palais 6.2 11 Modulating an LED (Analog) i
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