Ch10Sec2

Ch10Sec2 - lead to detection errors. A circuit example is...

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Dr. Joseph C. Palais 10.2 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY
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Dr. Joseph C. Palais 10.2 2 Section 10.2 Laser Diode Modulation and Circuits
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Dr. Joseph C. Palais 10.2 3 LD P-i Characteristics 30 0 C 70 0 C P I TH (30 0 C) I TH (70 0 C) i
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Dr. Joseph C. Palais 10.2 4 LD modulators need temperature compensation. These include : 1. Thermoelectric coolers to maintain fixed temperatures. 2. Monitoring the output power and varying the current to maintain the desired operating point. 10.2.1 Analog Modulation The same analog circuit as used for the LED works. The characteristic is illustrated on the next slide.
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Dr. Joseph C. Palais 10.2 5 t Output Signal P I TH i S Input Signal i I dc Saturation
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Dr. Joseph C. Palais 10.2 6 Typically 5 < I TH < 100 mA I dc = 1.5 I TH
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Dr. Joseph C. Palais 10.2 7 P I TH i S ON OFF i t 10.2.2 Digital Modulation Signal 1 0 0 1 1 1
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Dr. Joseph C. Palais 10.2 8 The LD is set at threshold to decrease the turn-on time. Note that the ON-OFF ratio is not infinite. This may
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Unformatted text preview: lead to detection errors. A circuit example is illustrated on the next slide. Dr. Joseph C. Palais 10.2 9 I v GS V dc v d R 1 D + S -v DS G GaAs MESFET LD +-R C I D High Speed Digital Modulator I 1 Dr. Joseph C. Palais 10.2 10 GaAs MESFETs are faster than Si FETs. I D = drain current I = LD current Operation If v GS = 0 the drain source channel has low resistance. Drain current I D is high. The diode current is I = I 1 - I D = I TH This is the OFF state. Dr. Joseph C. Palais 10.2 11 If v GS &lt;&lt; 0 (a few volts negative) the drain-source channel resistance is high. More of the supply current will now flow through the LD. This is the ON state. Typically I &gt; 1.5 I TH Now the MESFET drain-source voltage is : v DS = IR + v d v d = 2 volts (diode ON voltage drop) The resistor R assures v DS is large enough to operate the MESFET...
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Ch10Sec2 - lead to detection errors. A circuit example is...

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