The dispersion characteristics of apodized, linearly chirped fiber Bragg gratings and their potential as dispersion compensators have been studied systematically. It is shown that the positive hyperbolic-tangent profile results in an overall superior performance, as it provides highly linearized time-delay characteristics with minimum reduction in the linear dispersion. To compensate for the linear dispersion of 100 km of standard telecom fiber over certain bandwidth (in nanometers), the required grating length is 19.24 cm/nm.
In this paper, we present a detailed experimental and theoretical study, showing that a novel nonzero dispersion-shifted fiber with negative dispersion enhances the capabilities of metropolitan area optical systems, while at the same time, reducing the system cost by eliminating the need of dispersion compensation. The performance of this dispersion-optimized fiber was studied using different types of optical transmitters for both 1310-and 1550-nm wavelength windows and for both 2.5and 10-Gb/s bit rates. It is shown that this new fiber extends the nonregenerated distance up to 300 km when directly modulated distributed feedback (DFB) laser transmitters at 2.5 Gb/s are used. The negative dispersion characteristics of the fiber also enhance the transmission performance in metropolitan area networks with transmitters that use electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers, which are biased for positive chirp. In the case of 10 Gb/s, externally modulated signals (using either EA-DFBs or external modulated lasers using Mach-Zehnder modulators), we predict that the maximum reach that can be accomplished without dispersion compensation is more than 200 km for both 100-and 200-GHz channel spacing. To our knowledge, this is the first demonstration of the capabilities of a nonzero dispersion-shifted fiber with negative dispersion for metropolitan applications.
234 0 Thursday Morning OFC '97 Technical Digest fundamental mode further into the cladding to increase the waveguide dispersion. However, while ostensibly providing high dispersions and low intrinsic losses,, such designs lead to fibers with high bend losses in the wavelengths of interest and cannot be packaged for practical use.Dispersion compensators should possess several favorable properties to be considered efficient and practical. First and foremost, they should have negatiive dispersions with large absolute values, leading to short interaction lengths. In addition, the compensators should not be excessively lossy. Low losses enable their insertion in two-stage amplifiers without incurring a penalty in noise figure. (A figure-of-merit (FOM), defined as the absolute ratio of the dispersion to the loss, combines these two properties. Commercially available DCFs have FOMs of 150-250 ps/nm-dB). In addition to having high FOMs, DCFs should also have negative slopes (D'(A) = dD/dA in ps/nm2-km) to provide optimized second-order compensation and allow for broadband operation. This property makes the DCF "future-proof," enabling the addition of extra channels in a subsequent system modification. Low nonlinearities are also desirable and are usually dictated by the effective areas of the modes propagating in the fiber core. Finally, one of the most important practical parameters is that of product realization, a broadly defined term that covers the feasibility of manufacturing and assembling high-performance, high-yield dispersion compensators.We reach several conclusions based on the above criteria. First, dispersion compensating fibers provide the best option for upgrading installed networks for broadband operation over the entire erbium band. This statement is supported by the well-established fiber manufacturing processes and the ease of assembling dispersion compensating modules. Also, once DCF modules are installed in a system, extra channels can be added for further upgrades. The major drawbacks of DCFs continue to be high nonlinearities (due to their small mode sizes) and FOM's <300 pshm-dB.Our second ccinclusion pertains to the emerging field of dispersion compensating gratings. Short interaction lengths and low nonlinearities are the key advantages provided by chirped gratings. However, efficient compensation is possible only in an ultra-narrow wavelength band. In addition, the group-delay curves possess ripples that lead to spiky dispersion curves. As a result, small wavelength changes on the order of angstroms (which may be caused by laser aging) lead to precipitous changes in dispersion and affect system performance. Furthermore, manufacturing reproducible, packaged, temperature-insensitive products is an arduous task and may not be cost-effective. These factors lead us to believe that chirped gratings may find use in a few niche applications (for example, single-wavelength CATV systems) where narrowband performance is desirable. Their future as a competing technology for broadband, future-proof systems...
The optimum design for a 10-Gb / s NRZ chirped fiber grating dispersion-compensated system operating at 1.55 m over standard fiber is investigated. The study considers self-phase modulation, dispersion, modulator chirp, and amplifier noise. Transmission over 1700 km of fiber may be achieved by incorporating gratings every 200 km and optimizing the modulator chirp.ᮊ 1997 Academic Press
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