Per-Wavelength Transmission of 40 Gb/s on Standard Fiber, Part I

Two field trials suggest that 40 Gb/s per channel transmission is possible with existing DWDM filter technology on standard singlemode fiber. The second installment of this two-part article explores the mitigation of the most significant performance-limiting factor: PMD.

By: Paul Harrison, Alcatel USA

Technology aimed at major bandwidth increases in the optical backbone is advancing simultaneously on two fronts: The development of dense wavelength division multiplexing (DWDM) technology to transmit more wavelengths on a single fiber, and the increase in bit rate per channel to transmit more data across a given wavelength. Together, these two approaches will enable multi-terabit transmission on fiber network backbones.

Bit rate increases alone can lead to significant jumps in capacity. During recent field trials in Germany and Portugal, Alcatel (Paris) transmitted a signal at 40 Gb/s on a single fiber (see Figure 1). In a noteworthy advance, the tests involved an installed base of conventional fiber, suggesting that the technology is compatible with existing networks

Barriers to 40 Gb/s
The highest-capacity transport products available today operate at 10 Gb/s per wavelength, equivalent to an OC-192 signal. Achieving a rate four times faster has been a challenge on several technological fronts.

One major barrier to high-bit-rate transmission is the spectral content of the higher rate signal. With the new ITU standards reducing DWDM channel spacing from 100 GHz to 50 GHz, the spectral content of a 10-Gb/s signal has appeared to reach the practical maximum, given the present state of the filter art. Shrinking the 10-Gb/s signal spectrum would make it easier for filter designers to meet channel separation requirements with narrow channel spacing.

Another major obstacle to high-bit-rate transmission is fiber-induced dispersion. Both chromatic dispersion and polarization mode dispersion (PMD) significantly increase the error rates of high-speed signals. In anticipation of these higher bit rates, manufacturers have designed new fibers to minimize optical dispersion. Laying new fiber on existing routes nullifies much of the cost advantage of higher speeds, however, which motivates researchers to find new techniques to correct dispersion.

Results
During the summer of 1998, Alcatel's research laboratories conducted field trials of 40-Gb/s transmission in Stuttgart, Germany and Lisbon, Portugal. These tests were conducted in cooperation with Deutsche Telekom and CPR-Marconi, respectively. Both were "real-world" environments, in that they used ordinary single-mode buried fiber that was taken out of regular commercial service for the trials.

The two trials used a spectrally-efficient modulation technique that combined reduced bandwidth requirements with considerable dispersion tolerance to provide error-free transmission. The modulation technique yielded a bandwidth of approximately 20 GHz at the 3-dB point, suggesting that it would work with current-generation 50-GHz DWDM channel spacing and not generate the expense of pushing the state of filter art.

At the same time, the test technique tolerated uncompensated chromatic dispersion of up to 570 ps/nm—approximately 20 times better than conventional electronic time-division multiplexing (ETDM). Even so, the Stuttgart trial, with overall chromatic dispersion of 1880 ps/nm, required some compensation.

The limiting factor in transmission distance was PMD. Rather than rising linearly with bit rate, PMD is proportional to the square of the bit rate. Most older fiber requires PMD mitigation for bit rates of 10 Gb/s; even with newer fiber, PMD compensation is necessary for significant route distances at a bit rate of 40 Gb/s. We'll explore PMD compensation in Part II of this article.

Product Plans
The proof-of-concept trials in Germany and Portugal were not tied to a specific product platform. Even before the success of the trials, however, plans were in place to apply the new technology to an existing product. The backplane and other key components of Alcatel's 1680 OGM (optical gateway multiplexer) were designed from the start to be upgraded for 40-Gb/s signals. New 1680 OGMs or those that are already in service with 10-Gb/s transmitter/receivers (T/Rs) will be able to handle 40 Gb/s when new T/Rs become available for the higher rate.

Plans are in place to trial the 40-Gb/s version of the 1680 OGM with US carriers during 1999. General availability of both the SONET and SDH versions is expected during the second half of 2000.

About the author…
Paul Harrison, vice president and general manager, optical backbone networks, Alcatel USA, and he can be reached at Paul.A.Harrison@usa.alcatel.com.