Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-02-26
2002-08-13
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S337000
Reexamination Certificate
active
06433922
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical amplifiers used in fiber optics for telecommunications. More particularly, the invention relates to a Raman optical fiber amplifier and method and apparatus for enabling dynamic self adjusting gain optimization and equalizing amplified optical output.
2. Background Art
In optical fiber communication systems, communication channels can be provided by transmitting signals impressed on laser beams having different wavelengths (WDM). Although optical fiber communication systems utilizing wavelength-distinct modulated channels may carry information over long distances, signals transmitted through optical fibers are attenuated mainly by the cumulative and combined effects of absorption and scattering. While the signal attenuation per kilometer in optical fibers used for communications is typically low, signals transmitted over increasing transmission distances require periodic amplification over long distances. Amplification in fiber optic communication systems is performed mainly by electronic repeaters, Erbium doped fiber amplifiers (EDFA's), semiconductor optical amplifiers, waveguide amplifiers and Raman amplifiers.
While amplification using electronic repeaters involves optical to electrical to optical conversions, amplification using EDFA's, semiconductor optical amplifiers, waveguide amplifiers and Raman amplifiers is performed directly on optical signals, involving no optical to electrical to optical conversions. The Raman amplification process significantly differs from other amplification methods mentioned, as the transmission line itself can serve as the gain medium, whereas a module or component dedicated for the amplification process is used in other amplification methods. The Raman amplification process is based on the Raman effect, which describes conversion or scattering of a fraction of the optical power from an incident optical beam having a higher optical frequency to an optical beam having a lower optical frequency. The optical frequency shift between the incident beam and the scattered beam is determined by the vibrational states of the medium through which both beams are propagating. The Raman effect in silica-based fibers is described by quantum mechanics as scattering of an incident photon by a molecule to a photon with a lower optical frequency, while the molecule makes a transition between two vibrational states of the medium. Raman amplification involves Stimulated Raman scattering, where the incident beam, having the higher optical frequency, often referred to as the pump beam, is used to amplify the lower optical frequency beam, often referred to as the Stokes beam or the signal beam, through the Raman effect. The pump beam pumps the molecules of medium to an excited vibrational state, while the photons of the signal beam propagating through the excited molecules stimulate the emission of photons at the signal frequency, thereby amplifying the signal while the excited molecules return to their lower vibrational states [See for example “Nonlinear fiber optics” by G. P. Agrawal, pp. 316-369, Academic Press, 2nd edition, 1995]. Stimulated Raman scattering may involve a multiplicity of pumps at different frequencies and a multiplicity of signals at different frequencies. A Raman amplifier, which is based on the Stimulated Raman scattering effect, may amplify a single optical channel, as well as collectively amplify a series of optical signals, each carried on a wavelength corresponding to a distinct channel. A Raman amplifier with a single pump source, where a fiber optic is used as the gain medium can amplify signals extending over a wide frequency range, referred to as the Raman gain spectrum or the Raman gain band. The Raman gain spectrum in silica optical fibers extends over a wide frequency range, with a broad peak downshifted by about 13 THz from the pump frequency. The Raman gain spectrum in optical fibers is not associated with fixed energy levels of the gain medium, as with rare earth element dopants in glass based fibers such as Erbium. Consequently, Raman amplification can be achieved practically at any wavelength in the near infra-red spectrum, as long as the appropriate pumping light source is available. This advantage allows Raman amplification to be applied for optical communications across the entire optical communication transmission window of silica optical fibers.
Raman amplification in optical fibers was thoroughly investigated in the seventies [R. G. Smith, “Optical power handling capacity of low loss optical fibers as determined by Stimulated Raman and Brillouin scattering”, Applied Optics, Vol. 11 No. 11, p. 2489, 1972, R.H. Stolen et al., “Raman gain in glass optical waveguides”, Applied Physics Letters, Vol. 22 No.6, p. 276, 1973, and J. Auyeung et al., “Spontaneous and stimulated Raman scattering in long low loss fibers”, Journal of Quantum Electronics, Vol. QE-14 No. 5, p. 347, 1978]. By the early eighties, the use of Raman amplifiers in optical communication systems had been proposed for multi-wavelength transmission [Mochizuki et al., “Optical repeater system for optical communication”, U.S. Pat. No. 4,401,364; Hicks, Jr. et al., “Optical communication system using Raman repeaters and components therefor”, U.S. Pat. No. 4,616,898; and Mollenauer et al., “Optical communications system comprising Raman amplification means”, U.S. Pat. No. 4,699,452]. However, reliable commercial and affordable high power means for Raman pumping of single mode fibers did not exist in the 1980s, and Raman amplification was usually considered for highly special uses such as Soliton transmission [Mollenauer et al., “Optical communications system comprising Raman amplification means”, U.S. Pat. No. 4,699,452].
In the late 90's, as high power EDFA's became common, reliable high power pump laser diodes at the 1480 nm wavelength range were commercially available. As this wavelength range is also suitable for pumping of silica fibers Raman amplifiers [See for example “Erbium-Doped Fiber Amplifiers—Fundamentals and Technology”, by P. C. Becker et al., pp. 346-351, Academic Press, 1999], Raman amplifiers received renewed attention [Grubb et al., “Article comprising a counter-pumped optical fiber Raman amplifier”, U.S. Pat. No. 5,623,508; Grubb et al., “Article comprising low noise optical fiber Raman amplifier”, U.S. Pat. No. 5,673,280; Kerfoot et al., “Lightwave transmission system employing Raman and rare-earth doped fiber amplification”, U.S. Pat. No. 6,038,356; Kidorf et al., “Wide bandwidth Raman amplifier capable of employing pump energy spectrally overlapping the signal”, U.S. Pat. No. 6,052,219; Kidorf et al., “Optical fiber communication system with a distributed Raman amplifier and a remotely pumped Er-doped fiber amplifier”, U.S. Pat. No. 6,081,366; Y. Akasaka et al., “Raman amplifier optical repeater and Raman amplification method”, WO005622A1; T. Terahara et al., “128×10 Gbits/s transmission over 840-km standard SMF with 140-km optical repeater spacing (30.4-dB loss) employing dual-band distributed Raman amplification”, paper PD28 Optical Fiber Communication Conference 2000, Baltimore, Md., USA, March 7-10, 2000]. Although Raman amplifiers are generally more complex and costly than EDFA's, they allow amplification of a wide optical spectrum, with typically lower optical noise and over longer distances.
In contrast to EDFAs, where amplification properties are dependent only on the EDFA module, the transmission line itself can be used as the gain medium of a Raman amplifier, and thus, amplification properties such as gain and gain equalization are closely related to the type, properties and characteristics of the fiber used and the fiber condition (e.g. the distribution of losses along the fiber, the fiber effective area, Raman gain coefficient of the fiber and fiber length) [C. Fludger et. al., “An analysis of the improvements in OSNR from distribute
Eyal Ophir
Ghera Uri
Meshulach Doron
Friedman Mark M.
Hellner Mark
REDC Optical Networks Ltd.
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