Abstract-We use linearization to compute the noise evolution in a 10-Gb/s single-channel chirped return-to-zero (CRZ) system propagating over 6100 km, transmitting 32 bits. Linearization allows us to efficiently and accurately compute eye diagrams and bit error rates (BERs) without the use of Monte Carlo simulations. Phase jitter prevents the successful application of linearization unless it is removed, and we describe a method to separately remove it from each pulse in the signal. We show that the BER in our test system is dominated by the bit pattern that leads to the smallest eye opening in a noise-free simulation.Index Terms-Amplifier noise, linear approximation, Monte Carlo methods, optical fiber communication, optical kerr effect, phase jitter, receivers, spectral analysis.