Optical coherent techniques are used to eliminate power fading found in optical subcarrier multiplexed systems. In a double-side band optical subcarrier system with direct detection the signal experiences a periodic power fading that is dependent on the fiber dispersion and subcarrier frequency. This power fading results from interference between the two side-bands following the square-law photodetector. It is shown that the use of an appropriately modulated optical local oscillator to coherently detect the subcarrier channel can eliminate this power fading as well as phase error that gives rise to eye distortion. For homodyne detection an optical local oscillator, centered at the optical carrier, is double-sideband suppressed-carrier (DSB-SC) amplitude modulated by the subcarrier frequency of interest. By independently controlling the phases of the optical local oscillator and the DSB-SC modulation both the phase error and power fading of the detected subcarrier channel can be eliminated. This technique also allows the subcarrier to be selected optically, before the optical-to-electrical conversion.
A bandpass optical channel contains four degrees of freedom (DOF) corresponding to two quadrature phase and two quadrature polarization components. If independent information is sent on each of these four components, the spectral efficiency is nearly four times that achieved when utilizing a single component. For a given spectral efficiency, the minimum number of required photons per bit Np diminishes with increasing number of degrees of freedom (DOF), but the lower limit on Np, obtained when the spectral efficiency goes to zero, is 0.693 independent of DOF. For the case of an incoherent channel with square law detection and without polarization filters an upper bound to spectral efficiency and a lower bound to the number of photons per bit is obtained by assuming parallel coherent channels, but in which the same signal is transmitted in each channel. An even tighter bound on performance of the incoherent channel is obtained by computing the mutual information for a specific choice of input distribution utilizing a Markov chain approach.
Carrier-suppressed return-to-zero modulation (CS-RZ), in which there is a 180-degree phase shift between successive pulses, reduces the effects of intrachannel impairments. However, there are a number of variants of CS-RZ that differ in their method of generation, their bandwidth requirements, and their performance. It is shown that a recently proposed technique, in which the RZ signal is generated by filtering a CW signal that is square wave phase modulated (termed CWSW), performs comparably or better than alternative techniques. Relative to a modified duobinary system that results in alternate mark inversion( AMI), CWSW achieves better performance in systems with small dispersion, but slightly poorer performance in systems with larger dispersion and dispersion compensation. The small improvement in performance of AMI relative to CWSW in this latter case is achieved at the expense of requiring a larger transmission bandwidth and more complex transmitter. The physical basis of the impairments in these systems, a peak intensity enhancement phenomenon in CWSW that counters the effects of dispersion, and factors affecting the bandwidth of alternative techniques is discussed.
We propose and evaluate a new return-to-zero (RZ) transmission format, which simplifies the transmitter and produces significantly improved eye diagrams at the receiver compared with other formats. By applying synchronous square wave phase modulation (PM) to a continuous wave (CW) signal, with a 180 degrees phase shift (phase reversal) between adjacent bit slots, and followed by an optimally tuned optical filter, we generate a train of RZ pulses which is inherently stable and resistant to dispersive and nonlinear effects. Separate amplitude modulation (AM) is unnecessary in this approach to shape the RZ pulses. This format (which we term CWSW) not only suppresses the growth of spurious pulses on adjacent "1" states, but also results in a peak intensity enhancement where waveform distortion caused by fiber dispersion initially improves the eye opening during transmission. Single-channel 40 Gbits/s systems employing dispersion-shifted fibers are investigated by computer simulations. We show that spurious pulse suppression and peak intensity enhancement increase the maximum transmission distance and improve the eye profile at the receiver relative to alternative transmission formats.
The variational method is used to model output pulse widths in concatenated fiber links designed for dispersion
compensation. The expressions for the output widths are given in analytical form with explicit dependence on the
key parameters of initial width and chirp, the dispersion constant ofeach fiber section and the length of each
section. In the ideal linear case, perfect compensation occurs when the sum ofthe dispersion-distance products of
the two links is zero. However, in the presence ofnonlinearities a new relationship is found to govern many
operating systems. Specifically, the dispersion-distance product ofthe second link should equal minus one-half the
dispersion-distance product ofthe first link.
The response of a fiber to a sinusoidally modulated input which models an alternating hit sequence is studied to analyze the
cross-phase modulation penalty in a WDM system. The derived expression shows good agreement with numerical results in
conventional single-mode fiber systems over a wide range of channel spacing. JJ and in dispersion-shifted fiber systenis
when 4f> 100GHz.
In spectrum-sliced WDM systems with an optical preamplifier receiver there is an optimum m equals BoT (Bo equals optical channel bandwidth, T equals bit duration) to minimize the average number of photons-per-bit (Np) required at the receiver for a given error probability (Pe). These results, previously obtained for the case of rectangular filters and no interchannel interference, are extended to the case of practical filters. It is shown that interchannel interference increases the optimum m and the minimum Np. Operating at this optimum, the total system throughput with first-order filters is maximized at a channel spacing-to- bandwidth ratio of 3.3, and this throughput is 31 Gbit/s when the total system bandwidth is 4.4 THz (35 nm).
The variational method is used to establish that the spectrum of a pulse in a nonlinear single-mode fiber asymptotes to a finite limit. The limiting pulse shape and RMS spectral width are calculated in closed form.