Method and apparatus for full C-band amplifier with high...

Optical: systems and elements – Optical amplifier – Correction of deleterious effects

Reexamination Certificate

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Reexamination Certificate

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06563629

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates generally to optical amplifiers used in fiber optics for telecommunications. More particularly, the invention relates to an optical fiber amplifier and a method and apparatus for spectral equalization of multi-channel amplified output at multiple gain values.
In optical communication systems using wavelength division multiplexing (WDM) several streams of information, each at different wavelength channel are being transmitted. This information is typically transmitted along an optical fiber.
In all fiber optic cables, the cumulative and combined effects of absorption and scattering attenuate the transmitted signals. Information is usually transmitted through fiber optic transmission lines using laser wavelengths of 1530-1565 nm—the so called C-Band wavelength region, and 1570-1620 nm—the so called L-Band wavelength region, where there are low attenuation loss windows. Although the signal attenuation rate in optical fibers is low within these bands, signal reduction with increasing transmission distance requires periodic signal amplification for long distance transmission.
In these systems, an Erbium-doped fiber amplifier (EDFA) is the most commonly used device to amplify all wavelengths simultaneously [“Erbium-doped Fiber Amplifiers”, P. C. Becker, et. al., p. 335-346. Academic Press. 1999]. Due to the atomic properties of the Erbium ions in the silica fiber, the gain obtained by each of the wavelengths is different, and thus signals that enter with the same power into the amplifier can exit with power differences that can reach a few decibels [“Optical Fiber Communication Systems”, L. Kozovsky, et. al., p. 578-584, Artech House, 1996]. Two spectral regimes of amplification within the C-Band wavelength region are known: A) 1530-1540 nm, and B) 1540 nm-1565 nm. The wavelength dependent amplification behavior in regime A is relatively linear, while the amplification in regime B is non-linear, due to the non-uniform spectroscopic behavior of the Erbium ion. The simultaneous existence of linear and non-linear gain regimes in the EDFA render the task of gain equalization over a wide range of gains extremely difficult. An amplifier that is not gain-equalized can have several serious consequences for the communication system, including a non-optimized power budget for a system operating in the entire C-Band Spectral range. Furthermore, in the case of a chain of a few amplifiers, each amplifier can enhance the power non-uniformly, and, in extreme cases, the wavelengths with smallest gain may be undetectable. Other serious consequences include cross-talk that can occur after traversing optical filters in components such as optical multiplexers or optical demultiplexers, and a non-optimized use of the population inversion of the Erbium-doped fiber amplifier because of strong amplified spontaneous emission radiation, or noise, in the strongly amplified wavelengths.
For the reasons mentioned above, a need for a gain equalization filter was identified by A. R. Charplevy (Charplevy et al., U.S. Pat. No. 5,225,922). Gain equalization is usually applied for a limited gain range (1-2 dB) across the full C-Band and is accomplished by means of passive filters with devices such as thin-film filters [“DWDM Technology”, S. V. Kartalopoulos, p. 75-77, SPIE Press, 2000], Bragg gratings [Kartalopoulos, ibid. p. 78], long period gratings, and tapered fibers [“Optical Networks”, R. Ramaswami, et al., p. 101-102, Academic Press, 1998].
In dual-stage amplifiers there are other ways to achieve gain equalization, including choosing different types of Erbium fiber for the two stages (Sugaya et al., U.S. Pat. No. 6,055,092), different lengths of Erbium fiber (Alexander, U.S. Pat. No. 5,696,615), or by inserting devices such as filters, isolators or even attenuators between the two stages (Taylor, et al., U.S. Pat. No. 6,061,171; Alexander, U.S. Pat. No. 5,696,615). Dynamic filters, using acoustically tuned optical amplifiers, have also been suggested (“Fiber Based Acousto-optic Filters”, B. Y. Kim, et al., OFC 99, TuN 4, p. 199-201, Olshansky, U.S. Pat. No. 5,276,543).
In all the cases mentioned above, the gain equalization filter is usually suitable for a predetermined amplifier gain. When there is a need to change the gain, the power equalization of the different wavelengths will degrade and no longer be optimal.
Another conventional way to achieve dynamic gain equalization is by using two amplifier stages with opposite gain tilts [Yadlowsky M. J., U.S. Pat. No. 6,215,581]. Opposite tilt signs are achieved by differentiating the optical pump level for each of the stages. However, it is well known that in this case the dynamic gain equalization range, within a specific flattening tolerance, is limited.
As the equality of the power levels is important, in many applications an attenuator is inserted in front of the amplifier (Sugaya, U.S. Pat. No. 5,812,710) or between the amplifier's stages (Taylor, U.S. Pat. No. 6,049,413), to lower the signal power and accommodate the need for optimized gain for power equalization. However, it is well known to those skilled in the art that this technique wastes energy, and degrades the amplifier's signal to noise characteristics.
Recently, commercial devices for dynamic gain equalization have been developed. These devices utilize dynamic filters based on acousto-optic filters (Pearson, U.S. Pat. No. 5,514,413), liquid crystal filters (Kuang-Yi Wu, U.S. Pat. No. 5,963,291) and Mach-Zehnder filters (Miller, U.S. Pat. No. 5,351,325, Ranalli et. al., “Planar tapped delay line based, actively configurable gain-flattening filter”, ECOC 2000, Vol. 3, p. 21). These devices have been employed in conjunction with optical amplifiers. However, they are cumbersome, suffer high insertion loss, require high power resources, are not “stand-alone”, and necessitate wavelength monitoring of the amplifier output in order to reach gain equalization.
Active gain tilting elements are presently employed for compensating for the linear gain regime B (1540-1564 nm) in EDFAs. Commercial companies such as Chorum and Sumitomo manufacture such components. However, it is important to emphasize that with these elements, high dynamic gain equalization can be achieved only in this linear spectral gain regime. Moreover, these elements require wavelength monitoring means, as well as complicated performance control means.
The use of a Thulium-doped fiber (Tm-fiber) as a passive gain tilting element at the output or inside an optical amplifier operating in the linear gain regime (1540-1564), was suggested by Kitabayashi et. al (U.S. patent application Ser. No. 20010017728A1, “Active gain-tilt compensation of EDFA using Thulium doped fiber as saturable absorber”, and ECOC 2000, Vol. 2, p.177). The Tm-fiber's linear absorption characteristics, described by S. D. Jackson et. al. in “Theoretical modeling of Tm-doped silica fiber lasers”, J. of Lightwave Technology, Vol. 17, no. 5, p.948 (1999), are exploited for compensating for the Erbium fiber's linear wavelength-gain dependence between 1540-1564 nm. The gain tilt of the Er-doped fiber and the loss tilt of the Tm-fiber are both linear with respect to the wavelength at 1540-1564 nm. The slopes of these linear tilts have opposite signs and their absolute values are similar (Kitabayashi et. al, above). However, the Tm-fiber compensates for the EDFA gain tilt only in the linear gain regime between 1540 nm and 1564 nm, where the Erbium wavelength-gain dependence is linear. Thus, this method is not suitable for compensating for the non-linear Erbium fiber regime in the 1530-1540 nm spectral region. Since present day communication systems operate in the full C Band (1530-1565 nm), any partial solution for gain equalization (for example—gain flattening of only part of the spectrum) render such Tm-fiber/EDFA system practically useless.
A method utilizing excited state trapping in Erbium-doped Fluoride fiber has also been repor

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