Ch12Sec2_1

# Ch12Sec2_1 - THE FIBER FORUM Fiber Optic Communications...

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Dr. Joseph C. Palais 12.2 1 THE FIBER FORUM Fiber Optic Communications Dr. JOSEPH C. PALAIS PRESENTED BY

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Dr. Joseph C. Palais 12.2 2 Section 12.2 Digital System Design
Dr. Joseph C. Palais 12.2 3 We will illustrate digital design with a specific example. 12.2.1 System Specification: R = 400 Mb/s NRZ code L = 100 km (no repeaters) P e < 10 -9 The required rate length product is then: R x L = 400 x 100 = 40,000 Mb/s x km R x L = 40 Gb/s x km

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Dr. Joseph C. Palais 12.2 4 12.2.2 Rise-Time Budget (Bandwidth Budget) Consider a single pulse: T Input: 10% Output: t t 90% t s
Dr. Joseph C. Palais 12.2 5 For the NRZ code τ = pulse duration T = Repetition period τ = T = 1/R Require the total system rise time to be no more than: t S = 0.7 τ That is, we will allow the total system rise time to be no more than 70% of the pulse duration. This is a reasonable restriction.

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Dr. Joseph C. Palais 12.2 6 In this example the system rise time is limited to: t S = 0.7/R = 0.7/400 x 10 6 t S = 1.75 x 10 -9 = 1.75 ns As before, the total rise time in terms of its components is: = + + 2 2 2 S LS F PD t t t t Light Source Fiber Receiver
Dr. Joseph C. Palais 12.2 7 Consider the fiber’s rise time t F : t F = 0.35/f 3-dB (electrical) From (3-19) f 3-dB (electrical) = 0.35/ ∆τ Thus t F = 0.35/(0.35/ ) t F = (12-7) Result: the fiber’s rise time and pulse spread are approximately equal.

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Dr. Joseph C. Palais 12.2 8 If the entire system rise time were allotted to the fiber, its pulse spread per unit length would have to be no more than: ( τ /L) = 1.75 ns /100 km = 0.0175 ns/km ( τ /L) = 17.5 ps/km For multimode SI fibers, and considering only modal distortion (table 5-5): ( τ /L) ~ 15 ns/km
Dr. Joseph C. Palais 12.2 9 For multimode GRIN fibers, and considering only modal distortion: ( τ /L) ~ 1 ns/km These values are much greater than the 17.5 ps/km allowed in the rise-time budget. We conclude that these multimode fibers cannot be used. We will investigate single-mode fibers as a solution.

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Dr. Joseph C. Palais 12.2 10 For a typical single-mode fiber at 0.8 μ m: ( τ /L) ~ 0.5 ns/km This limit is mostly caused by the high material dispersion in the first window. We will have to operate either at 1.3 μ m or 1.55 μ m to achieve small enough pulse spreading to meet the system specifications. Before continuing with the rise-time (bandwidth) budget, lets look at the power considerations.
Dr. Joseph C. Palais 12.2 11 Compare fiber losses at 1.3 μ m and 1.55 μ m: At λ = 1.3 μ m, we have loss = 0.5 dB/km For L = 100 km, the total fiber loss would be: 0.5(100) = 50 dB (We will see later that our power budget is only 37 dB, so the 1.3 μ m wavelength will not work.) At λ = 1.55 μ m, we have loss = 0.25 dB/km For L = 100 km, the total fiber loss is: 0.25(100) = 25 dB This will leave 37 – 25 = 12 dB for other components.

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Dr. Joseph C. Palais 12.2 12 Now continue with the rise-time budget calculations. Check the pulse spreading at 1.55 μ m.
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• Spring '10
• PALAIS
• Fiber-optic communication, Multi-mode optical fiber, Single-mode optical fiber, JOSEPH C. PALAIS, dr. joseph c., Dr. JOSEPH C. PALAIS

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Ch12Sec2_1 - THE FIBER FORUM Fiber Optic Communications...

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