Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-07-06
2002-11-12
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341300
Reexamination Certificate
active
06480326
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to optical fiber Raman amplifiers and to optical fiber communication systems comprising such amplifiers and more specifically to a system and method for pumping the transmission fiber of an optical fiber telecommunication span to produce distributed Raman gain in the fiber for amplifying the signal(s) being transmitted along the fiber span.
BACKGROUND OF THE INVENTION
Until relatively recently, the amplification of optical signals in fiber-optic telecommunication systems has been achieved primarily through the use of discrete optical amplifiers, mainly erbium-doped fiber amplifiers (EDFAs). The explosive growth in the demand for increased capacity in fiber-optic communication systems has resulted in renewed interest in using distributed Raman amplification. See for instance, P. B. Hansen et al., IEEE Photonics Technology Letters, Vol. 9 (2), p. 262, (February 1997). In this approach, the transmission fiber itself is used as an amplifying medium for signals as they travel towards a repeater or receiving terminal, and the resulting gain is distributed over a length (typically tens of kilometers) of the fiber. Distributed amplification has an important advantage over discrete amplification. The effective noise figure of a distributed amplifier is significantly lower than that of a discrete amplifier having the same gain. See for instance, P. B. Hansen et al., Optical Fiber Technology, Vol. 3, p.221, (1997). This is a direct result of the gain occurring back in the span rather than at the end. The resulting improvement in noise performance not only allows increased capacity and/or span length in unrepeatered systems, but also allows for an increase in the number of spans between costly signal regenerators in multi-span repeatered systems. In addition, Raman amplification offers the possibility of ultra-broadband amplification, since the Raman gain spectrum in silica fiber, even for a single pump wavelength, is relatively broad and can be broadened further by using multiple pump wavelengths. See for instance, K. Rottwitt et al., Proceedings Optical Fiber Communication Conference, Paper PD-6, (February 1998). This is an important consideration for high-capacity wavelength division multiplexed (WDM) systems.
To produce Raman gain in the transmission fiber for signals in a particular wavelength band requires that the fiber be pumped at a relatively high-power level (hundreds of mW) at a wavelength, or wavelengths, shifted down from the signal wavelength(s) by an amount corresponding to the characteristic Raman shift of the fiber. For typical silica fiber, the Raman gain spectrum consists of a relatively broad band centered at a shift of ~440 cm
−1
. Therefore, to provide gain for signals in the C-band (1530 to 1565 nm) for example, requires pump energy in the 1455-nm region.
In typical prior-art distributed Raman amplification embodiments, the output of a high-power laser source (e.g. a Raman fiber laser with a center wavelength of ~1455 nm) or a group of multiplexed laser diodes with wavelengths in the 1455-nm region is launched from a receiving or repeater terminal to pump the fiber and provide gain for the incoming C-band signals. To extend the amplification bandwidth for high-capacity WDM systems, the launched pump spectrum is broadened by using multiple Raman lasers (each with a predetermined power and wavelength) or by multiplexing additional laser diodes of specific wavelength and power.
A characteristic set of power vs. distance curves for the pump, the signals and the noise generated by the amplification process are shown in
FIG. 1
(in this graph, distance is referenced from the receiving or repeater terminal). As can be seen in
FIG. 1
, the gain region is distributed over a length of the transmission fiber extending ~70 km back into the span. However, the bulk of the gain occurs in the last ~15 km of the span. To further increase the noise performance advantage of distributed Raman amplification, it is desirable to pump the transmission fiber in a manner which “pushes” the gain region further back in the span.
In K. Rottwitt et al., Proceedings European Conference Optical Communication, Vol. II, p.144, (September 1999), the authors report a pumping scheme which involves launching a high-power (800 mW) source at a wavelength of 1366 nm from the transmitter terminal, to provide Raman gain along the transmission fiber for 1455-nm energy launched from the receiving terminal. Thus, the power at 1455 nm, which provides the Raman gain for the signals at 1550 nm, is amplified along the fiber according to the local value of the power at 1366 nm. As a result, for the particular case they considered (an 80-km long span with 200 mW at 1455 nm launched from the receiving terminal), a substantial amount of signal gain occurs near both the transmitter and receiver ends of the span and the gain, on average, occurs further back in the span. The authors measured a 3-dB improvement in noise figure and a 1-dB improvement in receiver sensitivity (or link margin) as compared to conventional backward pumping. However, this pumping scheme, particularly if it is to be applied to longer spans (e.g. 125 km), requires two relatively expensive, relatively high-power (in the many hundreds of mW) sources. This disadvantage is exacerbated for the case of high-capacity WDM systems where broadening the gain bandwidth would require even more such sources. In addition, in links where the launch power of the signals is at or very near the limit set by considerations of adverse nonlinear effects in the fiber, the addition of substantial amplification immediately after signal launch could lead to link performance impairments due to these nonlinear effects.
Despite the already-demonstrated potential of distributed Raman amplification for providing low-noise, broadband amplification, there is an ever-present need for further performance improvements and cost reductions in optical communication systems. Thus, a distributed Raman amplifier pumping scheme, such as that disclosed in this application, which results in still lower noise and an increased flexibility and cost effectiveness in broadening and dynamically controlling the gain spectrum is highly desirable.
SUMMARY OF THE INVENTION
In a broad aspect, the invention provides a pumping scheme for producing distributed Raman amplification in the transmission fiber of an optical fiber communication system, according to which, the high pump power at the wavelength(s) required for amplification of the transmitted signals is developed within the transmission fiber itself, rather than being launched directly into the fiber. This pumping method can result in significantly lower amplifier noise and increased flexibility and cost effectiveness in broadening and dynamically controlling the gain spectrum compared to prior-art pumping methods.
More specifically, in a typical exemplary embodiment, a ‘primary’ pump source at a predetermined wavelength &lgr;
p
, shorter than the ultimately desired pump wavelength(s) &lgr;
f
. . . &lgr;
fk
, is launched into the transmission fiber along with one or more lower-power, low-cost secondary ‘seed’ sources at wavelength(s) &lgr;
s1
. . . &lgr;
sn
, where n≧1 and &lgr;
p
<&lgr;
sn
≦&lgr;
fk
. The wavelength and power of the secondary seed source(s) are specifically chosen such that, in the presence of the pump power at &lgr;
p
, a series of n stimulated Raman conversions ultimately lead to high power at the final desired pump wavelength(s) &lgr;
f1
. . . &lgr;
fk
, where k≦n, being present in the transmission fiber.
In a particular exemplary embodiment, a primary pump source at a wavelength of 1276 nm is launched together with two lower-power secondary sources having wavelengths of 1355 and 1455 nm. Energy at the primary pump wavelength of 1276 nm first undergoes a stimulated Raman conversion to 1355 nm and then, in the second step of a Raman cascade, the resulting high power at 1355 nm is converted to yield high power at 1455 nm, the pump wavelength required to prod
Clements Wallace
Karpov Vladimir
Papernyi Serguei
(Ogilvy Renault)
Anglehart James
Hellner Mark
MPB Technologies Inc.
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