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Technological Training


Chromatic Dispersion and Dispersion Compensation in Long Distance High Capacity Optical Transmission Systems


The growth of transmitted data and the appearance of new telecommunication services has been demanding ever higher transmission capacity in fiber optic links. Fiber chromatic dispersion and loss are the key obstacles to obtaining this. The emergence of erbium-doped fiber amplifiers operating in the 1550 nm region has eliminated fiber loss as a fundamental limit to achievable transmission distance and made it possible to transmit optical data over thousands of kilometers of single-mode fibers without electronic regeneration. However, the lack of intermediate regenerative repeaters causes the chromatic dispersion to build up along the entire system length, rather than just in between repeaters. The use of dispersion-shifted (DS) fiber with zero dispersion wavelength near 1550 nm is one obvious solution. However, this is not cost effective as, first, DS fiber exhibits higher loss, second, it is more expensive and, third, because the embedded fiber plant throughout the world largely contains standard single-mode fiber. Consequently, alternative means for compensating for the chromatic dispersion must be sought if standard fiber is to remain in use for building the future high-speed amplified systems.

Several strategies for dispersion compensation (DC) have been explored and can be categorized into three main areas: pre-distortion, post-detection and all-optical techniques. Pre-chirping of laser pulses and frequency-to-amplitude conversion are examples of pre-distortion schemes. Post-detection makes use of frequency-dependent delay lines in conjunction with coherent detection or special signal processing at the receiver if direct detection is used. All-optical techniques include the use of dispersion-compensating fibers, the highly dispersive higher-order mode in a two-mode fiber, optical phase-conjugation (or midpoint spectral inversion), and optical filters (such as birefringent crystals, Gires-Tournois interferometer, cascaded Mach-Zehnder interferometers, fiber grating-assisted intermodal coupling, periodic fiber gratings, and chirped fiber Bragg gratings). By far, R&D activities on dispersion-compensated optical transmission systems have been mostly concentrated on pre-chirping, dispersion-compensating fibers and chirped fiber Bragg gratings, which is probably because of their simplicity and practicality.

In this course, we initially describe the principles of Chromatic Dispersion and the limitations it imposes in the transmission capacity of analog and digital optical systems. Next, we discuss the existing fiber dispersion measurement techniques and present a survey of commercially available dispersion measurement instruments. Then, we discuss the use of dispersion shifted fibers (DSF and NZD) and present the main techniques proposed so far for dispersion compensation in monocanal and WDM optical fiber links. "Pre-Chirping", "Spectral Inversion", "Dispersion Compensating Fiber" and "Chirped Fiber Grating" techniques are discussed in details. The use of Chirped Fiber Gratings is specially highlighted for being the most practical and economic solution. Its operation principles, fabrication techniques, theoretical modeling, applications to telecommunications, etc, are introduced. Finally, chromatic dispersion mapping and management along the fiber link are discussed.

Duration: 4 or 8 Hours

Instructor: Dr. Sérgio Barcelos