Optical EngineeringUncertainty aspects of coherent and direct detection of amplitude shift keying and continuous phase frequency shift keying in optical fiber transmission
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The impulse and step responses as well as the pulse responses of a quadratic phase medium, the single-mode optical fiber, are presented. Analytical expressions of the fiber impulse and step responses are obtained, indicating the change of the phase of the lightwave carrier. Both coherent and incoherent detection schemes under the amplitude shift keying and continuous phase frequency shift keying (CPFSK) incorporating the maximum likelihood sequence estimation are examined with respect to the uncertainty depending on the number of bits of the sequence. The detection uncertainty shows that the coherent detection offers a high degree of discrimination as compared to its incoherent counterpart, especially for the case of CPFSK. The frequency discrimination detection of the CP FSK is correlated with the bandwidth-bit period product, the product between filter bandwidth and the bit period. Resilience to dispersion of ±9520 ps/nm can be achieved due to this minute uncertainty of this detection scheme.