Douglas L. Richards

Subcarrier multiplexing for high-speed optical transmission

The performance of high-speed digital fiber-optic transmission using subcarrier multiplexing (SCM) is investigated both analytically and numerically. In order to reduce the impact of fiber chromatic dispersion and increase bandwidth efficiency, optical single-sideband (OSSB) modulation was used. Because frequency spacing between adjacent subcarriers can be much narrower than in a conventional DWDM system, nonlinear crosstalk must be considered. Although chromatic dispersion is not a limiting factor in SCM systems because the data rate at each subcarrier is low, polarization mode dispersion (PMD) has a big impact on the system performance if radiofrequency (RE) phase detection is used in the receiver. In order to optimize the system performance, tradeoffs must be made between data rate per subcarrier, levels of modulation, channel spacing between subcarriers, optical power, and modulation indexes. A 10-Gb/s SCM test bed has been set up in which 4 /spl times/ 2.5 Gb/s data streams are combined into one wavelength that occupies a 20-GHz optical bandwidth. OSSB modulation is used in the experiment. The measured results agree well with the analytical prediction.

Measured temporal and spectral PMD characteristics and their implications for network-level mitigation approaches

Signal degradation due to polarization-mode dispersion (PMD) effects may become significant for signaling rates of 10 Gb/s, 40 Gb/s, and beyond. To assess the utility of various PMD mitigation schemes, temporal and spectral measurements of differential group delay (DGD) were made on 95 km of buried standard single-mode fiber over an 86-d period to determine the distribution and rate of change of high-DGD events. As expected, statistical analysis of variations in DGD indicate that excursions from the mean DGD by factors of 3.7 or higher have very low probability. For this link, the DGD varied slowly with time (having a drift time of about 3.4 d) and rapidly with wavelength. The DGD data agree well with results of similar experiments reported in the literature. Statistical analysis of the measured DGD data shows that high-DGD episodes will be exceedingly rare and short-lived. The impact of PMD on network operations is explored and approaches to ensure network reliability are reviewed for network operators given the task of transporting high-bit-rate channels over fiber links with known PMD characteristics.

10 Gb/s SCM system using optical single side-band modulation

A 10 Gb/s subcarrier multiplexing (SCM) long-haul optical system is reported. 4/spl times/2.5 Gb/s data streams are combined into one wavelength, which occupies a 20 GHz optical bandwidth. Optical single sideband modulation (SSB) is used to increase bandwidth efficiency and reduce dispersion penalty.

10-Gb/s SCM fiber system using optical SSB modulation

A 10-Gb/s subcarrier multiplexing long-haul optical system is reported. 4/spl times/2.5 Gb/s data streams are combined into one wavelength, which occupies a 20-GHz optical bandwidth. Optical single sideband is used to increase bandwidth efficiency and reduce dispersion penalty. The receiver sensitivity is calculated using a simplified receiver model with an optical preamplifier. The measured results agree well with the analytical prediction.

Variation of PMD-induced outage rates and durations with link length on buried standard single-mode fibers

From first-order polarization-mode dispersion (PMD) outage analysis using measured differential group delay (DGD) data on buried standard single-mode fibers, we observed that the outage rates increase monotonically with link length, although not linearly.

Optical channel performance monitoring using coherent detection

We demonstrate a multi-functional optical system performance monitor using coherent heterodyne detection. In addition to performing high-resolution optical spectrum analyzing, the system is capable of monitoring chromatic dispersion and PMD at each wavelength channel.

Study of variation of the laplacian parameter of DGD time derivative with fiber length using measured DGD data

Temporal and spectral DGD measurements were made on different combinations of three 95-km fibers within a slotted-core, direct-buried, standard single-mode fiber-optic cable over long time periods to characterize DGD variability. From this data we observed that DGD varies slowly over time but rapidly over wavelength. We also observed that the DGD variation with time becomes more rapid for longer length fibers. The DGD time derivative data obtained from the measured DGD data showed good agreement with a Laplacian distribution. We observed that the Laplacian parameter, which dictates the Laplacian pdf of DGD time derivative, seems to converge to a lower value as the fiber length is increased. This observation, which needs further verification, if true, would greatly simplify the system outage analysis due to first-order PMD on long-haul optical fiber links.