Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1996-07-30
2001-01-16
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S337000, C359S349000
Reexamination Certificate
active
06175436
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to optical fibers and optical amplifiers. In particular, it relates to gain stabilization in erbium-doped fiber amplifiers for multi-wavelength signals.
BACKGROUND ART
Modern communication networks are increasingly being implemented with optical fiber being used as the transmission medium. Fiber's early usage was concentrated on long haul transmission in which its huge bandwidth, measured in hundreds of terahertz, could be more immediately exploited. However, well made single-mode optical fiber exhibits relatively low but still finite absorption. Its useful transmission length is measured in tens to hundreds of kilometers at which point the signal amplitude has decreased so much that the signal is no longer readily detected.
In older systems, optical regenerators were placed periodically along a fiber transmission path. A regenerator detects the optical signal and converts it to electrical form, that is for most systems, detects the intensity of the optical envelope which corresponds to the electrical signal used to modulate the laser at the transmitting end. The regenerator then processes the electrical signal to regenerate a copy of the original modulating signal, and the copy is used to modulate another laser impressing its modulated optical signal on the next fiber link. Regenerators, however, tend to be expensive since they need to incorporate complex high-speed electronic circuitry. Furthermore, the design, construction, and operation of regenerators depend strongly on the data rate and the data format of the impressed electrical signal. If the data rate or format changes, the regenerators need to be replaced even though the fiber needs no adjustment for the changes and upgrades.
Erbium-doped fiber amplifiers (EDFAs) were discovered around 1987 to amplify an optical signal near the important fiber band around 1550 nm, at which wavelength the silica fiber absorption is a minimum, and they have gained nearly immediate acceptance in the telecommunications industry as a replacement for optical regenerators. In an EDFA, a silica-based fiber is doped with erbium, which forms an optically active ion having a number of excitable metastable states. A pump laser having a wavelength of, for example, 980 nm or 1490 nm, optically pumps the erbium ions until their state population is inverted. An optical signal traversing the region of inverted population will absorb energy from the excited ions and will thus grow in intensity.
Wavelength-division multiplexing (WDM) was being developed contemporaneously with erbium-doped fiber amplifiers. The data capacity of an optical-fiber transmission link is generally limited not by the fiber but by the electronics and opto-electronics at its two ends. In WDM, separate electrical circuits create separately modulated optical signals having different optical carrier wavelengths, and these multi-wavelength signals are combined (multiplexed) and carried on one optical fiber. At the receiving end of the fiber, the multi-wavelength signals are optically demultiplexed and thereafter separately detected. Thereby, the capacity of the fiber is increased by the number of wavelength channels.
Erbium-doped fiber amplifiers offer distinct advantages for WDM systems. A representative gain spectrum
10
for an EDFA is shown in
FIG. 1
although the details depend upon many factors not to be discussed here. The illustrated spectrum
10
has been optimized for a relatively wide flat-gain band
12
between about 1540 and 1560 nm, but a relatively intense gain peak
14
exists around 1532 nm, corresponding to a peak in the amplified stimulated emission (ASE) at the same wavelength.
It is important that the different WDM channels experience approximately equal gain at each amplifier in a multi-amplifier chain. Otherwise, the differences exponentially increase along the chain to the point that the different WDM signals cannot be treated equivalently. Placing all the WDM channels in the flat-gain band
12
reduces the differential gain problem. Eight WDM channels with equal 2 nm spacings can be placed in this band
12
, a demanding but not impossible design. This even arrangement of WDM channels is referred to as a wavelength comb. The wavelength channels, however, can be dispersed within the flat-gain band
12
with unequal spacings. Such uneven spacings affect neither the problems discussed below nor the solutions provided by the invention.
A further problem with the operation of EDFAs in a WDM system arises from the operational fact that not all the WDM channels are necessarily being used at the same time. Some channels may be dark. The variability of channel number is an especial problem in all-optical WDM networks in which optical signals are switched between different fibers according to the respective wavelengths of the signals. A simple exemplary WDM network, shown in
FIG. 2
, includes three terminal nodes
20
,
22
,
24
, which are linked by optical fibers through a wavelength-selective switch
26
which directs an optical signal according to its wavelength. The switch
26
can be reconfigured between selected switching states. Importantly for this invention, the fiber link
28
between the switch
26
and the third node
24
is long and contains several amplifiers
30
spaced along its length. At some times, the first node
20
may be sending one channel at &lgr;
1
to the second node
22
and a second channel at &lgr;
2
to the distant third node
24
while at other times, it may be sending both channels &lgr;
1
, &lgr;
2
to the third node
24
. The same situation obtains if the two channels originate from different nodes
20
,
22
and are directed to a common node
24
, but both channels are not always active. In either case, dependent upon traffic, the number of channels on the long fiber
28
will vary.
An erbium-doped fiber amplifier shows a complex relation between the input and output powers. That is, the gain G which is the ratio of the output to input powers P
OUT
/P
IN
depends on the magnitude of power. The linearly scaled graph of
FIG. 3
plots optical output power P
OUT
as a function of optical input power P
IN
at a fixed pump power. In a constant gain region
32
, the two powers are proportionally related, and the gain G is constant. However, an EDFA can provide only a finite amount of optical output power, and in a saturation region
34
further increases in input power produce progressively smaller increases in output power. The graph of
FIG. 3
can be replotted, as shown in
FIG. 4
, for the gain as a function of input power P
IN
. The numerical values of this (graph are measured values but are given only as representative since they depend on many factors, but the general trends are considered to be nearly universal. It is seen that gain is relatively flat at lower input power but falls off for high values of input power. The fall off is even more dramatic when gain is plotted as a function of output power.
It is general practice to operate an erbium-doped fiber amplifier in saturation for at least two reasons. Over some power range, the signal-to-noise ratio is better in saturation. Also, a saturated amplifier produces an output that has a relatively fixed amplitude regardless of the input power. That is, the output level is typically clamped. With clamped outputs, all the transmitters and amplifiers of an entire network do not need to be concurrently tuned to produce acceptable signal levels at the network output if the intermediate amplifiers tend to output signals of a fixed level.
However, the saturation effects shown in
FIGS. 3 and 4
in a homogeneously broadened EDFA depend upon the total optical power, that is, the sum of the optical powers in a multi-wavelength WDM signal. The distinction between homogeneously and inhomogeneously broadened EDFAs will not be discussed, but a silica-based EDFA is homogeneously broadened, at least at room temperature. Ideally, N active WDM channels will have N times the optical power as one active WDM channel however, a saturated amp
Morgan & Lewis & Bockius, LLP
Pascal Leslie
Tellium Inc.
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