The optical power spectrum of a signal externally modulated using a Mach-Zehnder modulator is calculated analytically. Optical power spectra are calculated for binary signals for which the drive signal has either raised-cosine or Bessel-filtered pulse shape as well as for duobinary signals created by Bessel filtering. While the optical power spectrum is often approximated by the electrical power spectrum of the drive signal, this approximation usually underestimates the spectral spreading of the optical signal. Differences between the optical spectrum and the drive-signal electrical spectrum are most significant for drive signals having longer rise and fall times. Modulator chirp also broadens the optical spectrum. Chirp-induced spectral broadening is more significant for drive signals having longer rise and fall times.
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