Cost estimates suggest that a 40-Gbit/s transponder will be significantly cheaper than four 10-Gbit/s transponders. But the transponder cost represents only part of the total cost picture: The economical measure that matters for service providers is the cost of transmitting bits over a given distance, i.e., the total dollars/bit/km. Consequently, the supported fiber transmission distance, without costly electronic regeneration, is an important figure of merit for a 40-Gbit/s DWDM system. That distance is determined in turn by several imperfections in the transmission path.

The transmission challenges at 40 Gbits are similar to what is experienced in 10-Gbit/s systems today. Although tolerances at 40 Gbits are tighter than at 10 Gbits, clever design choices in the end terminals (transponders) will typically allow a smooth transition without forklift upgrades.

Longer distances of fiber expose the signal more to deteriorating effects inherent to the fiber such as nonlinearities, polarization-mode dispersion (PMD) and chromatic dispersion. In particular, the two dispersion effects are often exaggerated when debating 40-Gbit transmission issues.

PMD is caused by the refractive index's not exhibiting perfect rotational symmetry around the fiber axis. As a result, the two possible polarization states of the fiber propagate light at slightly different speeds. The difference in propagation speed between the slow and fast fiber axis — called differential group delay (DGD) — leads to a broadening of the transmitted bits.

Today, optical fibers are fabricated with very low PMD (below 0.05 picosecond/km), and around 90 percent of post-1994-deployed fiber has sufficiently low PMD to allow 40 Gbits/s to be transmitted over several thousands kilometers of fiber. At 40 Gbit/s, the tolerance to DGD is some 10 picoseconds. However, on routes with older fibers or on very long routes, PMD compensation or mitigation techniques may be required. Note that one can regard RZ modulation as a simple mitigation method since it renders the signal more robust to PMD compared with conventional NRZ signals.

The chromatic dispersion in optical fibers leads to pulse broadening of the transmitted signal. When increasing the bit rate, the negative impact on signal quality rises significantly. That has led many to sanction the prevailing myth surrounding 40-Gbit/s transmission: that it will never be cost-effective, because of the detrimental impact of chromatic dispersion. The reality is very different: Chromatic dispersion reduces nonlinear effects in the fiber and as such enables 40-Gbit DWDM over long distances when combined with well-known in-line dispersion compensating fiber (DCF).

Today, DCFs are widely used in 10-Gbit DWDM systems. They consist simply of a single-mode dispersion-compensating fiber with a dispersion that is of opposite sign relative to the transmission fiber. These conventional compensating fibers can compensate both the dispersion and the dispersion slope (i.e., the wavelength dependency of the dispersion) of some types of transmission fiber (SSMF and TWRS). For fiber types like LEAF and TW Classic, new types of DCFs are emerging, consisting of either dispersion-compensating fiber with higher relative dispersion slope or so-called higher-order mode fiber.

The true challenge with chromatic dispersion at 40 Gbits is the dependency on temperature variations. The accumulated dispersion in long-haul systems might exceed the dispersion tolerances of the receiver as the environmental temperature of the transmission fiber and DCF changes over time. Adaptive dispersion compensators are a practical solution, both for this problem and for the issue of insufficient dispersion slope compensation.

An attractive technique based on a tunable fiber Bragg grating can significantly increase the tolerance. Fully packaged, field-deployable units are now routinely fabricated with more than 200 ps/nm of tuning interval. That is sufficient to accommodate for realistic temperature variations as well as incomplete slope compensation over distances of more than 1,000 km of fiber. Another parameter of importance to tunable dispersion compensators is the tuning speed. System experiments where the dispersion is tuned over 200 ps/nm to the optimum value show tuning times on the order of a few seconds, i.e., more than enough to continuously track the relatively slow environmental temperature variations.

An added advantage of such a tunable dispersion compensator scheme is that it relaxes the requirement to the design accuracy of the DCFs elsewhere in the system, essentially allowing the length of dispersion-compensating fiber to be fabricated with practical and economical granularity. In combination with intelligent software, the introduction of the tunable dispersion compensators will enable plug-and-play optimization when the transponder is installed in the network.

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