Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-01-19
2003-07-29
Moskowitz, Nelson (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341310, C359S341200, C372S006000
Reexamination Certificate
active
06600592
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to nonlinear polarization amplifiers, and more particularly to nonlinear polarization amplifiers used to amplify signals in the S
+
band, approximately ranging from 1430-1480 nm.
BACKGROUND OF THE INVENTION
Because of the increase in data intensive applications, the demand for bandwidth in communications has been growing tremendously. In response, the installed capacity of telecommunication systems has been increasing by an order of magnitude every three to four years since the mid 1970s. Much of this capacity increase has been supplied by optical fibers that provide a four-order-of-magnitude bandwidth enhancement over twisted-pair copper wires.
To exploit the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunication industry: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM enables different wavelengths of light to carry different signals parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth, it would not be cost-effective without optical amplifiers. In particular, a broadband optical amplifier that permits simultaneous amplification of many WDM channels is a key enabler for utilizing the full fiber bandwidth.
Silica-based optical fiber has its lowest loss window around 1550 nm with approximately 25 THz of bandwidth between 1430 and 1620 nm. For example,
FIG. 1
illustrates the loss profile of a 50 km optical fiber. In this wavelength region, erbium-doped fiber amplifiers (EDFAs) are widely used. However, as indicated in
FIG. 2
, the absorption band of a EDFA nearly overlaps its the emission band. For wavelengths shorter than about 1525 nm, erbium-atoms in typical glasses will absorb more than amplify. To broaden the gain spectra of EDFAs, various dopings have been added. For example, as shown in
FIG. 3
a
, codoping of the silica core with aluminum or phosphorus broadens the emission spectrum considerably. Nevertheless, as depicted in
FIG. 3
b
, the absorption peak for the various glasses is still around 1530 nm.
Hence, broadening the bandwidth of EDFAs to accommodate a larger number of WDM channels has become a subject of intense research. As an example of the state-of-the-art, a two-band architecture for an ultra-wideband EDFA with a record optical bandwidth of 80 nm has been demonstrated. To obtain a low noise figure and high output power, the two bands share a common first gain section and have distinct second gain sections. The 80 nm bandwidth comes from one amplifier (so-called conventional band or C-band) from 1525.6 to 1562.5 nm and another amplifier (so-called long band or L-band) from 1569.4 to 1612.8 nm. As other examples, a 54 nm gain bandwidth achieved with two EDFAs in a parallel configuration, i.e., one optimized for 1530-1560 nm and the other optimized for 1576-1600 nm, and a 52 nm EDFA that used two-stage EDFAs with an intermediate equalizer have been demonstrated.
These recent developments illustrate several points in the search for broader bandwidth amplifiers for the low-loss window in optical fibers. First, bandwidth in excess of 40-50 nm require the use of parallel combination of amplifiers even with EDFAs. Second, the 80 nm bandwidth may be very close to the theoretical maximum. The short wavelength side at about 1525 nm is limited by the inherent absorption in erbium, and long wavelength side is limited by bend-induced losses in standard fibers at above 1620 nm. Therefore, even with these recent advances, half of the bandwidth of the low-loss window, i.e., 1430-1530 nm, remains without an optical amplifier.
There is a need for low noise Raman amplifiers and broadband transmission systems. There is a further need for distributed, discrete and hybrid amplifiers with improved noise figures. Another need exists for optical amplifiers suitable for wavelengths of 1480 nm or less, where the S+ band is located.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide nonlinear polarization amplifiers.
Another object of the present invention is to provide a broadband fiber transmission system with at least one nonlinear polarization amplifier.
Yet another object of the present invention is to provide a broadband fiber transmission system with reduced fiber non-linear impairments.
A further another object of the present invention is to provide a broadband fiber transmission system that operates over the full low loss window of available and optical fibers.
Another object of the present invention is to provide a broadband fiber transmission system that uses distributed Raman amplification to lower signal power requirements.
One or more of the objects of the present invention are achieved in a broadband amplifier. The broadband amplifier includes a transmission fiber, a splitter, an S+ band distributed amplifier, a second optical amplifier, a combiner, and an output fiber. The splitter can be coupled to the transmission fiber. The splitter can split an optical signal into at least a first wavelength and a second wavelength. The S+ band distributed Raman amplifier can be coupled to the splitter that can operate in the range less than 1480 nm. A pump power of the S+ band distributed Raman amplifier can extend into the transmission fiber. The second optical amplifier can be coupled to the splitter. The combiner can be coupled to the S+ band distributed Raman amplifier and the second optical amplifier. The combiner can combine an optical signal into at least a first wavelength and a second wavelength. The output fiber can be coupled to the combiner.
In another embodiment of the invention, a method produces an amplified broadband optical signal in a transmission system. An optical signal is divided at a predetermined wavelength into a first beam having a wavelength less than the predetermined wavelength and a second beam having a wavelength greater than the predetermined wavelength. The first beam is directed to a transmission link in the transmission system that includes a distributed Raman amplifier operating in the wavelength range less than 1480 nm. The second beam is directed to a second amplifier. The first and second beams are combined to produce an amplified broadband optical signal.
In another embodiment of the invention, an S+ band amplifier includes a distributed Raman amplifier, a WDM, a discrete amplifier, and a pump source. The distributed Raman amplifier can include a signal transmission line with at least a portion of the signal transmission line incorporating a distributed gain medium. The WDM can be coupled to the signal transmission line. The discrete amplifier canbe coupled to the WDM. The pump source can be coupled to the WDM. The pump source can produce a pump beam &lgr;
p
at wavelengths less than 1400 nm.
In another embodiment of the invention, an S+ band amplifier includes a distributed Raman amplifier, a discrete amplifier, a WDM, and a pump source. The distributed Raman amplifier can include a signal transmission fiber with at least a portion of the signal transmission line incorporating a distributed gain medium. The discrete amplifier can be coupled to the transmission line. Additional gain can be generated from the distributed Raman amplifier to compensate for a higher loss in the fiber when the fiber experiences a transmission loss of 0.03 dB/km greater than the transmission loss in the fiber at 1550 nm. The WDM can be coupled to the signal transmission line. The WDM can be positioned between the distributed Raman amplifier and the discrete amplifier. The pump source can be coupled to the WDM. The pump source can produce a pump beam &lgr;
p
.
In another embodiment of the invention, a method produces an amplified broadband optical signal. At least one fiber is provided that has a low loss window of 1430 to 1620 nm and a distributed Rama
Moskowitz Nelson
Xtera Communications Inc.
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