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chapter 11 - THE FIBER FORUM Fiber Optic PRESENTED BY Click...

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Click to edit Master subtitle style 5/13/10 Dr. Joseph C. Palais 11.1 11 THE FIBER FORUM Fiber Optic Dr. JOSEPH C. PALAIS PRESENTED BY
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5/13/10 Dr. Joseph C. Palais 11.1 22 Chapter 11 Noise and Detection
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5/13/10 Dr. Joseph C. Palais 11.1 33 Noise degrades signals. Without noise, it would not matter how little optical power arrived at the receiver. Signal quality is measured in several ways. Analog systems: The signal-to-noise ratio (SNR) is the measure. Digital systems: The bit-error-rate (BER) is the measure. It this chapter we determine how to evaluate the SNR and BER.
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5/13/10 Dr. Joseph C. Palais 11.1 44 Section 11.1 Thermal and Shot Noise
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5/13/10 Dr. Joseph C. Palais 11.1 55 11.1.1 Thermal Noise Recall the simple receiver circuit: RL v P - + i Vb
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5/13/10 Dr. Joseph C. Palais 11.1 66 Even if P = 0 (and the photodiode dark current is zero), a current i = iNT will exist in resistor RL. It has zero average value, but it is random, like: iN T t
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5/13/10 Dr. Joseph C. Palais 11.1 77 The current arises from the random thermal motion of the electrons. The instantaneous noise power is RL i2NT The average thermal noise power is RL i2NT RLi2NT RLi2NT t
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5/13/10 Dr. Joseph C. Palais 11.1 88 i2NT = mean square thermal noise current. It is given by : i2NT = 4kT f /RL k = 1.38 x 10-23 J /K, Boltzmann constant T = temperature, K f = receiver’s bandwidth. Usually f is a bit larger than the information bandwidth.
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5/13/10 Dr. Joseph C. Palais 11.1 99 RL v i2NT = 4kT f /RL The load resistor’s equivalent circuit looks like: where RL is an ideal (noiseless) resistor.
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5/13/10 Dr. Joseph C. Palais 11.1 1010 11.1.2 Shot Noise It is caused by the discrete nature of charge carriers (electrons and holes). Consider a single emitted photoelectron, shown for a vacuum phototube: Vb - + RL i - electron Anode Cathode hf + -
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5/13/10 Dr. Joseph C. Palais 11.1 1111 A current exists in the circuit during the transit time ( τ ) of the emitted electron. τ = time for travel from cathode to anode. The electron recombines at the anode with a positive ion. The current caused by a single electron looks something like : h(t) τ t
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5/13/10 Dr. Joseph C. Palais 11.1 1212 Every electron produces the same current pulse shape. Consider constant optical power P incident on the detector. The expected current is : t i = ( η e/hf)P This current is made up of numerous pulses of the type shown by h(t). i
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5/13/10 Dr. Joseph C. Palais 11.1 1313 Example : t The pulses start at random times, tN. The total current is the sum of these pulses. i = Σ N h(t - tN) tN
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5/13/10 Dr. Joseph C. Palais 11.1 1414 i ( η e /hf)P t The average current is still: i = ( η e /hf)P but noise is superimposed onto this current. This is shot noise . Total current:
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5/13/10 Dr. Joseph C. Palais 11.1 1515 The shot noise current is: iNS = Σ N h(t - tN) - ( η e /hf)P i2NS = 2eI f (11-2) f = receiver’s bandwidth I = average current I = iS + ID where iS = average of the signal current ID = average dark current
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5/13/10 Dr. Joseph C.
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