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Optical Networks - _5_4 Receiver_59

Optical Networks - _5_4 Receiver_59 - 294 Transmission...

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294 Transmission System Engineering Note that on the one hand this penalty represents the decrease in signal-to-noise ratio performance of a system with a nonideal extinction ratio relative to a system with infinite extinction ratio, assuming the same average transmitted power for both systems. On the other hand, if we assume that the two systems have the same peak transmit power, that is, the same power for a 1 bit, then the penalty can be calculated to be PP sig - indep = − 10 log r 1 r . Lasers tend to be physically limited by peak transmit power. Most nonlinear effects also set a limit on the peak transmit power. However, eye safety regulation limits (see Section 8.7.1), are stated in terms of average power. The formula to be used depends on which factor actually limits the power for a particular system. The penalty is higher when the system is limited by signal-dependent noise, which is typically the case in amplified systems (Section 4.4.5)—see Problem 5.10. This is due to the increased amount of noise present at the 0 level. Other forms of signal-
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