We investigate the use of different duobinary modulation implementations in a wavelength-switched network operating at 10 Gb/s. The work specifically focuses on two methods of generating duobinary: phase shaped binary transmission (PSBT) which uses both a phase modulator and a Mach-Zehnder modulator (MZM) and enhanced PSBT (EPSBT) which uses two MZM's. We evaluate the benefit associated with modifying the extinction ratios of both implementations in order to optimise the signal for either short-(low back-to-back penalty) or long-distance transmission (good dispersion tolerance). The tolerance of these implementations to impairments associated with the settling of tunable lasers < 200 ns after switching (such as wavelength error and linewidth broadening) are also investigated and the associated penalties are found numerically.
Abstract-We investigate the use of different direct detection modulation formats in a wavelength switched optical network. We find the minimum time it takes a tunable sampled grating distributed Bragg reflector laser to recover after switching from one wavelength channel to another for different modulation formats. The recovery time is investigated utilizing a field programmable gate array which operates as a time resolved bit error rate detector. The detector offers 93 ps resolution operating at 10.7 Gb/s and allows for all the data received to contribute to the measurement, allowing low bit error rates to be measured at high speed. The recovery times for 10.7 Gb/s non-return-to-zero on-off keyed modulation, 10.7 Gb/s differentially phase shift keyed signal and 21.4 Gb/s differentially quadrature phase shift keyed formats can be as low as 4 ns, 7 ns and 40 ns, respectively. The time resolved phase noise associated with laser settling is simultaneously measured for 21.4 Gb/s differentially quadrature phase shift keyed data and it shows that the phase noise coupled with frequency error is the primary limitation on transmitting immediately after a laser switching event.
An experimental method for characterizing the time-resolved phase noise of a fast switching tunable laser is discussed. The method experimentally determines a complementary cumulative distribution function of the laser's differential phase as a function of time after a switching event. A time resolved bit error rate of differential quadrature phase shift keying formatted data, calculated using the phase noise measurements, was fitted to an experimental time-resolved bit error rate measurement using a field programmable gate array, finding a good agreement between the time-resolved bit error rates.
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