Novel Chromatic Dispersion Compensation

Breck Hitz

A collaboration between researchers at OFS and Bell labs of Murray Hill and Holmdel, N.J., respectively, has produced a novel system for tunable fiber optic dispersion compensation that is effective with any wavelength division multiplexing (WDM) signal, independent of channel count, data rate or channel spacing. The all-fiber apparatus has a lower insertion loss than any previously reported tunable compensator.

Chromatic dispersion degrades the digital signal traveling through an optical fiber because the individual pulses are lengthened and start overlapping. The deleterious effect, which scales as the square of the data rate, occurs because the high-frequency Fourier components of the pulses travel faster than the low-frequency components in the dispersive fiber. Chromatic dispersion can be compensated using a number of techniques that allow the low-frequency components to catch up with their speedier companions.

There are two approaches to this: channelized and continuous solutions. In the former, the channels of the WDM signal are separated, and each channel is compensated separately. Chirped Bragg gratings are an example of this approach, where each channel has its own grating.

Continuous solutions, on the other hand, often rely on a length of dispersion-compensating fiber whose dispersion has the opposite sign to that of the normal fiber. The Fourier components that are faster in the normal fiber are slower in the dispersion-compensating fiber, so that all components take the same time to travel through both fibers. This approach is continuous because it works for all WDM channels, and the channels need not be separated.

In many cases, the dispersion compensation must be tunable -- that is, the amount of catching up by the low-frequency components must be adjustable. Environmental variations necessitate short-term changes in compensation, and link-design modifications necessitate long-term changes. Until recently, tunable compensators have been channelized, a method that lacks the flexibility of the continuous approach. The scheme demonstrated by the team is the first continuous, tunable dispersion compensator that operates over a broad band of frequencies.

Figure 1. The new tunable dispersion compensator consists of five lengths of high-order-mode fiber and five switchable, long-period fiber gratings (SLPG). Each grating can be switched to determine which of two polarization modes propagates in the fiber following the grating.

The technique relies on adjustable propagation through high-order mode fiber (Figure 1). The optical signal can propagate through the fiber in either the LP₀₁ or LP₀₂ mode, and the switchable, long-period fiber grating in front of each length of fiber determines which mode propagates in that length. Because the two modes have different dispersion values, the total dispersion can be adjusted by switching the gratings. And because there are five gratings, 32 (2⁵) different values of total dispersion are possible. Samples of these values are plotted in Figure 2. (For clarity, only half of the 32 values are shown.)

Figure 2. The dispersion is continuous over the wavelength region. Each line in this plot represents one particular setting of the switches (gratings) in Figure 1.

The bandwidth of the approach -- 30 nm in Figure 2 -- is limited by the bandwidth of the long-period gratings. Long-period gratings with greater bandwidth have been demonstrated elsewhere, so the researchers anticipate that the compensator's bandwidth can be expanded to cover the C-band. The insertion loss, at 3.7 dB lower than has been known before, is primarily attributable to the fiber splices, and the researchers believe that even better results could be easily obtained by improving the quality of the splices.