PMD Compensation Field Trial Enables 40 Gb/s
The second installment of this two-part article explores PMD mitigation in high bit rate transmission systems.
By Bill Vislosky, Alcatel USA
Contents
Technology challenge
What is PMD?
Compensation methods
Results
Commercialization
Even with advanced modulation techniques for 40 Gb/s, polarization mode dispersion (PMD) can limit transmission distance on installed fiber infrastructures. To make 40 Gb/s and even 10 Gb/s transmission a cost-effective option for existing fiber, PMD compensation is essential. A recent six-month field trial in East Texas conducted by Alcatel demonstrates commercially viable second-generation PMD compensation technology.
Technology challenge
Dense wavelength division multiplexing (DWDM) has enabled carriers to increase the bandwidth of their fiber by up to 8000% over existing OC-48 infrastructures. To keep pace with demand in a more cost-effective way, carriers are deploying higher bit rate signals in the form of 10 Gb/s OC-192 channels.
For many carriers, the cost of deploying OC-192 can be much higher than OC-48 because of signal degeneration imposed by PMD. PMD limits transmission distances proportionally to the bit rate squared, so the transmission length of OC-192 is limited to 1/16 the distance of OC-48, and OC-768 is limited to 1/256 the distance of OC-48.
No commercial technology currently exists to compensate for PMD, so network operators must deploy regenerators whenever the signal reaches its PMD limit. Since the limit for existing routes can be as short as 80 km, the cost of deploying a regenerator for every channel on a DWDM system can quickly make OC-192 a very expensive option.
What is PMD?
Polarization mode dispersion is a time distortion introduced in an optical signal by the fiber material itself. Bifringence in the material produces a "fast" axis and a "slow" axis in the fiber. Pulses sent into the fiber can be viewed as two orthogonally polarized pulses, or principal states of polarizations (PSPs). As these pulses travel down the fiber, the two PSPs experience different fast and slow sections of fiber. The orientation of the PSPs changes along the length of the fiber, so the pulse arriving at the receiver consists of two PSPs, separated by a time delay. This delay, called differential group delay (DGD), changes in time; PMD is the time average of the DGD.
Because PMD is a complex statistical system that varies with time, it is difficult to compensate for the phenomenon. Network designers cannot simply put in a passive amount of fiber to correct for the effect, as they can with chromatic dispersion. A PMD compensator must be an active component that can track changes in the DGD thousands of times per second (see Figure 1).
Compensation methods
Both optical and electrical PMD compensation techniques show promise for commercial viability. Alcatel recently concluded a field trial of an all-optical PMD compensator, the first long-term optical PMD compensation field trial conducted on an OC-192 signal over existing fiber -- 240 km worth, in this case.
Two methods exist for all-optical PMD compensation. The first technique mitigates DGD by delaying each PSP enough to bring the two back together in time. The receiver pulse is split into two polarizations. By inserting a tunable amount of fiber through a series of optical switches, designers can induce a time delay in the signal. Placing a tunable fiber delay on each polarization path imparts the correct amount of time delay in each PSP required to bring the two together.
The second method of all-optical PMD compensation complements the direct compensation of DGD by lining up the input state of polarization with the total PSP. This method promises to be a more elegant and low cost solution than the tunable delay method. The technique takes advantage of a fixed delay, and therefore requires no optical switches. The device can potentially operate much faster than a tunable delay device (see Table 1).
The bit error rate (BER) depends on both the amount of noise in the optical signal (the OSNR) and PMD (see Figure 2). To more easily measure the dependence of the PMD compensator on the dispersion itself, the OSNR was reduced significantly below normal operating conditions. The compensator considerably improves the transmission performance, with BER for some occurrences more than 10,000 times better than the uncompensated performance.
Commercialization
The field trial demonstrated that PMD Compensation is ready for commercial development, though long-term stability required to provide carrier class performance. The performance was also at a level that makes it extremely economical at the network level. By improving the PMD tolerance from 10 ps to 30 ps, the Alcatel PMD Compensator essentially allows a 900% distance increase of 10 Gb/s signals across a fiber that was previously limited by PMD.
PMD compensators will be critical to make networks cost competitive as carriers deploy OC-192 and OC-768. For carriers without the luxury of a new fiber plant where PMD is not an issue, a variety of techniques to compensate PMD are available and will soon appear as commercially available products. Before extremely high bit rates can become cost effective on a large scale for existing fiber networks, PMD compensators are required; luckily, they are almost here.
References:
[1] Chbat, M. et. Al., "Long Term Field Demonstration of Optical PMD Compensation on an Installed OC-192 Link," OFC99 Post Deadline Paper.
About the author…
Bill Vislosky is an optical backbone networks product specialist at Alcatel USA, 1201 E. Campbell Rd., Richardson, TX 75082. Phone: 972-996-6011; fax: 561-325-8461; e-mail: bill.vislosky@usa.alcatel.com.