Unchirped Pulse Research Articles

Evolution of chirped pulses in nonlinear single-mode fibers

Computer studies of the evolution of pulses in nonlinear single-mode fibers are presented. In the case of anomalous dispersion, the time-bandwidth product of the initial pulse is varied by imposing a linear frequency chirp. The pulse broadening resulting from chirp greatly exceeds the broadening caused by the loss. In the case of normal dispersion the evolution of an initially unchirped pulse is shown to depend critically on the loss. This fact is explained by the position of the chirp's maxima and minima relative to the pulse-amplitude distribution.

Pulse distortion in single-mode fibers 3: Chirped pulses

The theory of pulse distortion in single-mode fibers is extended to include laser sources that suffer a linear wavelength sweep (chirp) during the duration of the pulse. The transmitted pulse is expressed as a Fourier integral whose spectral function is given by an analytical expression in closed form. The rms width of the transmitted pulse is also expressed in closed form. Numerical examples illustrate the influence of the chirp on the shape and rms width of the pulse. A somewhat paradoxical situation exists. A given input pulse can be made arbitrarily short by a sufficiently large amount of chirping, and, after a given fiber length, this chirped pulse returns to its original width. But at this particular distance an unchirped pulse would be only [equiation] times longer. Thus chirping can improve the rate of data transmission by only 40%.

Ionospheric dispersion of electromagnetic pulses

When an electromagnetic pulse having a Gaussian envelope is passed through the ionosphere it remains a Gaussian pulse; however, its amplitude is reduced and its pulsewidth is increased with respect to what these parameters would be for free space propagation. It is pointed out that if this Gaussian pulse is chirped (i.e., if it is given a linear FM), then after passing through the ionosphere and following the pulse compression in the receiver, the signals envelope is still Gaussian. Furthermore, when the bandwidths for both signals are the same (or equivalently, when the pulsewidth of the compressed transmitted chirped pulse is the same as that of the transmitted unchirped Gaussian pulse) the amplitude degradation and pulsewidth spreading are identical for both signals. This not too surprising result applies as long as the time-bandwidth product is large enough for the chirped signal. The results obtained for chirped Gaussian pulse are compared, as to the pulse distortion introduced by the ionosphere and available bandwidth of the ionosphere, with the results obtained in the literature for other waveforms. It is pointed out that the amplitude degradation and pulsewidth spreading for a chirped signal having a 40 dB Taylor or Hamming weighting is about the same as obtained with the chirped pulse having a Gaussian envelope as long as the compressed pulsewidths of the transmitted signals are identical. Finally, the numerical results obtained previously in the literature are presented in a more convenient form.