Fiber gratings for dispersion compensation

Abstract

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...

Optimization of apodized linearly chirped fiber gratings for optical communications

“…In practice, however, the optimum positiveapodization profile can be approximated by a profile with flat center region (60%-80% of total grating length) and -apodized edges (20%-10% of total grating length each). Such approximate profiles have been used successfully for the apodized linearly chirped fiber gratings for the practical implementation of high-performance dispersion compensators [5], [7], [23]. Fig.…”

“…Periodic or aperiodic fiber gratings with constant refractiveindex modulation depth (i.e., unapodized), however, show reflection spectra with large side lobes, as well as highly nonlinear dispersion characteristics which make them unsuitable for high-performance applications. These characteristics are attributed to residual multiple reflections at the grating ends [14] and can be significantly suppressed by a suitable variation (apodization) of the modulation depth along its length [15]- [23].…”

“…As a result, smooth and tight apodization profiles result in enhanced side-lobe suppression and superior grating performance [15], [16]. Various apodization profiles have been considered theoretically and experimentally in order to smoothen the reflection spectrum and linearize the dispersion characteristics of aperiodic (chirped) gratings [17]- [23]. It has been realized that tight apodization profiles, in general, result in smooth features at the expense, however, of grating reflectivity, bandwidth, and dispersion.…”

Recent progress in soliton transmission technology

“…[50][51][52][53][54] An alternative dispersion compensation technique to the DCF is the chirped fiber Bragg grating ͑CFBG͒. 55,56 CFBGs are more compact and less expensive than DCF, and do not suffer from nonlinear effects in the fiber. Recently, the group delay ripples that appear in the dispersion curve of a CFBG have been reduced and we can therefore further improve the transmission characteristics of DM solitons using SMFs and CFBGs.…”