Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2000-10-03
2002-07-23
Tarcza, Thomas H. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
Reexamination Certificate
active
06424455
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to Raman amplifiers and, more particularly, to Raman amplifiers having a bandwidth which exceeds the peak Raman Stokes gain shift of the transmission medium with which the Raman amplifier is utilized.
BACKGROUND OF THE INVENTION
Optical fiber technology is currently utilized in communications systems to transfer information, e.g., voice signals and data signals, over long distances as optical signals. Over such long distances, however, the strength and quality of a transmitted optical signal diminishes. Accordingly, techniques have been developed to regenerate or amplify optical signals as they propagate along an optical fiber.
One well-known amplifying technique exploits an effect called Raman scattering to amplify an incoming information-bearing optical signal (referred to herein as a “signal wavelength”). Raman scattering describes the interaction of light with molecular vibrations of the material through which the light propagates (referred to herein as the “transmission medium”). Incident light scattered by molecules experiences a downshift in frequency from the power-bearing optical signal (referred to herein as the “pump wavelength”). This downshift in frequency (or increase in wavelength) from the pump wavelength is referred to as the “Stokes Shift.” The downshift of the peak gain from the pump wavelength is referred to herein as the “peak Stokes shift.” The extent of the downshift and the shape of the Raman gain curve is determined by the molecular-vibrational frequency modes of the transmission medium. In amorphous materials, such as silica, molecular-vibrational frequencies spread into bands which overlap and provide a broad bandwidth gain curve. For example, in silica fibers, the gain curve extends over a bandwidth of about 300 nm from the pump wavelength and has a peak Stokes shift of about 100 nm.
The overall concept of Raman scattering is well known and is described in numerous patents and publications, for example, R. M. Stolen, E. P. Ippen, and A. R. Tynes, “Raman Oscillation in Glass Optical Waveguides,” Appl. Phys. Lett, 1972 v. 20, 2 PP62-64; and R. M. Stolen, E. P. Ippen, “Raman Gain in Glass Optical Waveguides,” Appl. Phys. Lett, 1973 v. 23, 6 pp. 276-278), both of which are incorporated herein by reference. With respect to the present invention, the most relevant aspect of Raman scattering is its effect on signal wavelengths traveling along the transmission medium.
FIG. 1
illustrates prior art optical amplifier which utilizes Raman scattering to amplify a signal wavelength. Referring to
FIG. 1
, a pump wavelength &ohgr;p and a signal wavelength &ohgr;s are co-injected in opposite directions into a Raman-active transmission medium
10
(e.g., fused silicon). Co-propagating pumps may be used, although a counter-propagation pump scheme reduces polarization sensitivity and cross talk between wavelength division multiplexed (WDM) channels. Providing the wavelength of the signal wavelength &ohgr;s is within the Raman gain of power wavelength &ohgr;p (e.g., about 300 nm in silica), the signal wavelength &ohgr;s will experience optical gain generated by, and at the expense of, the pump wavelength &ohgr;p. In other words, the pump wavelength &ohgr;p amplifies the signal wavelength &ohgr;s and, in so doing, it is diminished in strength. This gain process is called stimulated Raman scattering (SRS) and is a well-known technique for amplifying an optical signal. The two wavelengths &ohgr;p and &ohgr;s are referred to as being “SRS coupled” to each other. A filter
16
transmits all signals of the signal wavelength &ohgr;s and blocks signals of the pump wavelength cop thereby filtering out the pump wavelength.
FIG. 1A
illustrates the gain curve for a signal wavelength &ohgr;s amplified using a single pump wavelength &ohgr;p. As shown in
FIG. 1A
, while gain occurs over a broad bandwidth (e.g. 300 nm in silica), only a portion of it (e.g., about 50 nm) is, from a practical standpoint, useable to effectively amplify the signal wavelength &ohgr;s. This useable bandwidth is referred to herein as the“effective Raman gain.” The effective Raman gain is determinable by one skilled in the art and depends on a number of factors including the desired degree of amplification and the desired flatness across the amplification bandwidth. In silica, the effective Raman gain having less than 3 dB gain variation extends about 25 nm on either side of the peak Raman Stokes shift of about 100 nm. Therefore, the bandwidth of the effective Raman gain occurs from about 75 to about 125 nm from the pump wavelength as shown between points A and B on the Raman gain curve in FIG.
1
A.
FIG. 2
is a schematic drawing illustrating the relationship between pump wavelengths and the signal wavelengths of a prior art Raman amplifier. The schematic of
FIG. 2
shows multiple pump wavelengths cop through &ohgr;p+n which are used to amplify signal wavelengths &ohgr;s through &ohgr;s+m. Because the effective Raman gain occurs about 75to about 125 nm from the pump signal, signal wavelengths separated from a pump wavelength within this range will be effectively SRS coupled to the pump wavelength. In
FIG. 2
, pump wavelength &ohgr;p (1370 nm) is separated from signal wavelength &ohgr;s (1470 nm) by approximately 100 nm. Thus, assuming that the transmission medium
10
of
FIG. 1
is silica, pump wavelength &ohgr;p will be SRS coupled to and amplify signal wavelength &ohgr;s.
If only a single pump wavelength &ohgr;p is used, only signals in the bandwidth from &ohgr;s−25 nm to &ohgr;s+25 nm would be within the effective Raman gain. However, the use of multiple pump wavelengths &ohgr;p through &ohgr;p+n as shown in
FIG. 2
allows the gain bandwidth to be expanded to amplify signal wavelengths &ohgr;s through &ohgr;s+m. Furthermore, the use of multiple pump wavelengths serves to reduce gain variation (improve flatness) within this bandwidth due to the cumulative effect of multiple gain curves.
Despite multiple pump configurations, prior art Raman amplifiers are nevertheless limited in bandwidth, which in turn limits the capacity of WDM systems. More specifically, because the effective Raman gain tails off at about 125 nm from the pump wavelength, signal wavelengths beyond this point are not effectively amplified. Furthermore, the applicants have found that in multi-pump systems, where excellent flatness in amplification is achievable through the cumulative effect of multiple gain curves, signal wavelengths preferably should be within the peak Stokes shift of a pump wavelength, e.g., about 100 nm, for optimum flatness. This limitation in SRS coupling limits the bandwidth of signals, e.g. &ohgr;s from &ohgr;s+m as shown in
FIG. 2
, to the peak Stokes shift of a pump wavelength since extending the signal bandwidth beyond &ohgr;s+m would require introducing pump wavelengths into the signal bandwidth, beyond &ohgr;s.
Injecting pump wavelengths into the signal bandwidth, however, has traditionally been avoided due to backward Rayleigh scattering (BRS) resulting from the pump signals. BRS results from random localized variations of the molecular positions in glass that create random inhomogeneities of the reflective index that act as tiny scatter centers. Although the pump and signal wavelengths can be easily separated by filtering in a counter-propagating scheme, the BRS from the pump wavelengths, which propagates in the direction of the signals, is not easily filtered. Furthermore, BRS from longer pump wavelengths falls into the Raman gain generated by shorter pump wavelengths, thereby causing this BRS to be amplified such that it equals or exceeds the intensity of the signal wavelengths. For example, a pump wavelength generated at point A and intended to amplify a signal wavelength at point B would coincide with signal wavelength &ohgr;s+2. The BRS from the pump wavelength at point A is affected by the Raman gain of the lower pump wavelengths, thus introducing undesired noise into the signal wavelengths near point A
Hughes Deandra M.
Tarcza Thomas H.
TyCom (US) Inc.
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