Ch12Sec1_1

Ch12Sec1_1 - THE FIBER FORUM Fiber Optic Communications...

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Dr. Joseph C. Palais 12.1 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY
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Dr. Joseph C. Palais 12.1 2 Chapter 12 System Design
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Dr. Joseph C. Palais 12.1 3 Section 12.1 Analog System Design
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Dr. Joseph C. Palais 12.1 4 We will start with an example design of an analog system. The system requirements will be quite modest: 12.1.1 System Specification Closed circuit video security system Bandwidth = 6 MHz SNR = 10 5 , SNR dB = 50 dB Design constraint: The system must be as low cost as possible.
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Dr. Joseph C. Palais 12.1 5 We will try the following components: LED Wavelength: λ = 0.85 μ m LED power output: P = 1 mW (average power) Rise time: t LS = 12 ns Spectral width: ∆λ = 35 nm Emitting surface diameter: D < 50 μ m
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Dr. Joseph C. Palais 12.1 6 Multimode SI Fiber NA = 0.24 Optical modal distortion bandwidth-length product: f 3-dB x L = 33 MHz x km Core diameter = d = 50 μ m Fiber loss = 5 dB/km
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Dr. Joseph C. Palais 12.1 7 Multimode GRIN Fiber NA = 0.24 (axial NA) Optical modal distortion bandwidth-length product: f 3-dB x L = 500 MHz x km (modal spread only) Core diameter: d = 50 μ m Fiber loss: 5 dB/km PIN Photodiode Responsivity: ρ = 0.5 A/W Diode capacitance: C d = 5 pF Diode dark current: I D = 2 nA
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Dr. Joseph C. Palais 12.1 8 We will compute the SNR at 100% modulation. Modulation is illustrated on the next page.
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Dr. Joseph C. Palais 12.1 9 t 50 Signal current i (mA) 100 0 1 2 P(mW) Optical power t
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Dr. Joseph C. Palais 12.1 10 R L V B + i - F = 2 (300 K) Electronic Amplifier Receiver (300 K) P
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Dr. Joseph C. Palais 12.1 11 The receiver bandwidth is: f 3-dB = 1/2 π R L C d Set this equal to the video signal bandwidth: f 3-dB = 6 MHz Now solve for the value of load resistance: R L = 1/2 π (6 x 10 6 ) 5 x 10 -12 = 5305 If we choose this as the value of the load resistance, the entire system bandwidth is used up. We need to save some bandwidth for the fiber and for the light source.
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Dr. Joseph C. Palais 12.1 12 Therefore, we must choose R L < 5305. For example, try: R L = 5100 Now set up the power budget. We will next set up the power budget and then return to bandwidth considerations.
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12.1 13 12.1.2 Power Budget From (11-24), with T replaced by FT S / N = [(m 2 / 2) (M η eP/hf) 2 R L ] / {M 2 2e fR L [ I D +( η eP/hf)] + 4kFT f } For 100% modulation, m = 1. If there is no gain in the photodetector, then M = 1 as
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Ch12Sec1_1 - THE FIBER FORUM Fiber Optic Communications...

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