Optical: systems and elements – Optical amplifier – Optical fiber
Reexamination Certificate
2000-05-23
2002-10-01
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Optical amplifier
Optical fiber
Reexamination Certificate
active
06459528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical passive components used in conjunction with a bi-directional amplifier system capable of simultaneously and independently providing optical amplification to light-wave data channels propagating in opposite directions through an optical fiber.
2. Description of the Related Art
The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to augment the amount of actual fiber-optic cable as well as to utilize the bandwidth of existing fiber-optic cable more efficiently. The latter practice of increasing the carrying capacity of existing fiber cable is sometimes referred to as the creation of “virtual fiber” and is clearly more cost effective than adding real fiber.
One of the ways in which “virtual” fiber is created is through the practice of Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.” To increase data carrying capacity in a given direction, the number of such channels or bands must be increased.
Additionally, it is desirable to use existing fiber for bi-directional communications. More particularly, through the use of WDM, a single optical fiber may be used to transmit, both simultaneously and independently, both eastbound (northbound) as well as westbound (southbound) data.
This bi-directional data-carrying capability of optical fiber increases the need for additional channels still further. However, since all the channels (wavelength bands) must reside within specific low-loss wavelength regions determined by the properties of existing optical fiber, increased channel capacity requires increased channel density. Thus, as the need for increased data carrying capacity escalates, the demands on WDM optical components-to transmit increasing numbers of more closely spaced channels with no interference or “crosstalk” between them and over long distances-becomes more severe.
Optical amplifiers are important components of fiber-optic communication systems. Traditionally, signal regeneration has been accomplished through the use of repeaters, which are combinations of demultiplexers, receivers, signal recovery electronics, transmitters (light sources together with optical modulators), and multiplexers. In a repeater, the signal for each channel is recovered electronically and transmitted anew. Unfortunately, the complexity and cost of repeater-based systems becomes unwieldy with increase in the number of channels of WDM systems. Optical amplifier systems have therefore become attractive alternatives to repeaters. Erbium-doped fiber amplifier (EDFA) systems have become especially popular owing to their gain characteristics near the 1.5 &mgr;m (micrometer) transmission band of conventional optical fiber.
EDFA systems are generally associated with sets of set of so-called “optical passive components” which perform various signal and laser pump beam combination, separation, and re-direction functions. Also included in the set of optical passive components are optical isolators, generally one disposed to either side of the optical gain element (Er-doped fiber), which guard against amplification and subsequent transmission of backward-propagating signals. An example of a conventional EDFA system
100
including the various optical passive components is shown in FIG.
1
. More particularly,
FIG. 1
shows an amplifier of the prior art, which includes a basic EDFA block diagram. The EDFA system
100
shown in
FIG. 1
is a unidirectional amplifier.
In
FIG. 1
, optical isolators, such as isolator
101
and isolator
102
, are disposed to either side of an Er-doped fiber
103
. At least one pump laser, such as co-pump laser
104
and/or counter-pump laser
105
, are generally required to generate the fluorescence in the Er-doped fiber
103
, which leads to amplification. These pump lasers
104
,
105
generally have wavelengths of 980 or 1480 nm whereas the signal(s) has a wavelength near 1500 nm. Generally, at least two lasers are used such that one laser-the co-pump laser
104
-directs its light along the Er-doped fiber
103
in the same direction as signal propagation and the other laser-the counter-pump laser
105
-directs its light in the direction opposite to signal propagation.
Because of the differences in wavelengths between signal(s) and laser(s), wavelength-division multiplexers/demultiplexers (WDM's) such as input WDM
106
and output WDM
107
are required. Input WDM
106
combines the co-pump laser light together with the signal light such that both propagate in the same direction through the Er-doped fiber
103
. Likewise, output WDM
107
combines the counter-pump laser light together with the signal light such that the two lights propagate in opposite directions through the Er-doped fiber
103
. Furthermore, input WDM
106
and output WDM
107
remove any residual counter-pump light or co-pump light, respectively, from the system.
Other optical passive components, such as bandpass filter or isolator
108
A and bandpass filter or isolator
108
B, may be present to prevent laser light of the opposite laser from entering each respective pump laser. Finally, signal taps, such as input tap
109
and output tap
110
, may be present so as to sample small proportions of the input and output signals, respectively. These samples of input and output signals may be directed to separate respective photo-detectors such as photo-detector
111
A and photo-detector
111
B so as to monitor the amplifier system performance using comparison and control logic
112
.
Bi-directional lightwave communications systems are those in which signal lights are carried in both directions within individual optical fibers. In the current state of the art, separate amplifiers are used for eastbound (northbound) and westbound (southbound) communications channels as shown in the prior art band bi-directional amplifier
200
shown in FIG.
2
. The counter-propagating signals are respectively separated and recombined on either side of the pair of optical amplifiers.
For instance, in
FIG. 2
, if the “blue” or relatively short wavelength band
201
shown as solid lines represents westward propagating signals and the “red” or relatively long wavelength band
202
shown as dash-dot lines represents eastward propagating signals, then these two signals are separated and recombined by WDMs
203
A and
203
B. Between the two WDMs
203
A,
203
B, the blue and red signals propagate on separate physical optical fiber sub-paths
204
and
205
, respectively, but to either side of each WDM, the westbound blue and eastbound red signals co-propagate along the same physical fiber pathways
211
and
212
. Each of the fiber sub-paths
204
and
205
contains its own amplifier system,
206
and
207
, respectively.
Optional second amplifiers
208
and
209
may be placed in each of the fiber sub-paths and the locations between each of the resulting sequential amplifiers
206
and
208
or
207
and
209
corresponds to mid-stage access ports
210
A and
210
B in the blue and red sub-paths, respectively. Generally, each of the optical amplifier systems,
206
and
207
and, optionally,
208
and
209
, shown in
FIG. 2
, comprises all the basic components illustrated in FIG.
1
and possibly others. In particular, the amplifier
206
(and optionally
208
) contains optical isolators that only permit westward light propagation and the amplifier
207
(and optionally
209
) contains optical isolators that only permit eastward light propagation.
One example of the wavelength constitution of co-propagating bi-directional signals is illustrated in
FIG. 3
, which corresponds to a band bi-directional amplifier. In
FIG. 3
, as an example, the “blue” band
301
and the “red” band
Avanex Corporation
Hughes Deandra M.
Staas & Halsey , LLP
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