Standards bodies explore fiber dispersion compensators
WILLIAM B. GARDNER
International standards bodies are seeking solutions to the problem of dispersion management in fiber-optic networks that will not impose constraints on the direction of developing passive dispersion compensators (pdcs).
Higher bit rates and longer regenerator spacings are increasingly forcing network planners and providers to pay closer attention to dispersion management in fiber-optic cable paths (see Lightwave, October 1994, page 14 and November 1995, page 40). A large portion of the worldwide installed fiber plant consists of dispersion-unshifted singlemode fibers whose chromatic dispersion at 1550 nm is about +17 psec/nm-km. Fortunately, fiber cable end-to-end dispersion can be reduced by adding negative dispersion compensators into the path to counterbalance the fiber`s positive dispersion. Because such dispersion compensators are now commercially available, standards bodies are addressing this issue.
Dispersion-compensation techniques can be either passive (for example, using specially designed fibers or fiber gratings) or active (for example, prechirping the transmitter). Ranges of values for the span dispersion and the insertion loss of passive dispersion compensators are given in Section 6.8 of International Telecommunication Union (itu) Draft Recommendation G.671, "Transmission Characteristics of Passive Optical Components." Some guidelines for the use of pdcs are contained in Section 8.3 of itu Draft Recommendation G.691, "Optical Interfaces for Single Channel sdh Systems with Optical Amplifiers, and stm-64 Systems."
Another standards body, the International Electrotechnical Commission Subcommittee 86C Working Group 1, is now considering a 15-page draft, "Guidelines to Accommodate and Compensate for Dispersion in Fiber Optic Systems," authored by Felix Kapron, principal engineer in fiber media and components at Bellcore, Morristown, NJ. The draft`s discussion of active and passive dispersion-compensation schemes is similar to that found in Bellcore`s GR-2854-core, Issue 1 (September 1995), "Generic Requirements for Fiber Optic Dispersion Compensators," and GR-2854-ilr, Issue 1A (December 1996), "Issues List Report for Fiber Optic Dispersion Compensators."
Presently, the most common passive dispersion compensator is a fiber that exhibits a large negative dispersion at 1550 nm, typically on the order of -100 psec/nm-km. By having a large dispersion value, the passive dispersion compensator can be shorter in length than the fiber whose dispersion is being counterbalanced. This characteristic proves important because pdc attenuation is proportional to length.
A negative dispersion even greater than -100 psec/nm-km can exist near the cutoff of a fiber`s higher order LP11 mode. However, passive dispersion compensators that make use of the LP11 mode require mode converters and are also sensitive to the state of polarization.
Chirped or "aperiodic" Bragg gratings written into fibers can also be used for passive dispersion compensation (see Lightwave, February 1996, page 48). Because of their spectral transmission properties, Bragg gratings are more likely to supplement (rather than compete with) fiber-passive dispersion compensators in broadband systems.
To specify passive dispersion compensators, standards committees are investigating the ratio of a large dispersion value to a small attenuation coefficient in a fiber pdc as a proposed figure-of-merit (fom). High fom values are preferred, and fom values of 150 psec/nm-dB are common. For a more complete characterization, the definition of fom can be expanded to include nonlinear effects. q
William B. Gardner represents Lucent Technologies, Norcross, GA, on several fiber standards committees. He can be contacted at tel: (770) 798-2674; fax: (770) 798-4654; e-mail: wbgardner@ lucent.com.