Note: Descriptions are shown in the official language in which they were submitted.
CA 02344750 2001-04-18
FULL COMBINED C AND L BANDS OPTICAL AMPLIFIER AND METHOD
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to optical amplifiers in general and in
particular to wideband
amplifiers suitable for long haul and ultra long haul optical transmission
networks. More
particularly, it relates to optical amplifiers for amplifying the full
spectrum of the C and L
bands combined from 1525 nm to 1610 nm without interruption (middle dead-
band).
Prior Art of the Invention
Optical amplifiers are used in long haul transmission networks in order to
reduce the
number of regenerators necessary along transmission lines.. The regeneration
of optical
signals requires their conversion to electronic signals, electronic
regeneration, then
conversion back to optical signals before retransmission. On the other hand, a
number of
data channels are transmitted over the optical fiber. As a result at each
regeneration station,
these channels have to be demultiplexed and go through the regeneration stage
one by one.
As the number of channels increases, new regeneration modules must be added,
if the
architecture is scaleable. Efficient optical amplification reduces the need
for signal
regeneration, which is expensive and a potential bottleneck in optical
networking.
Erbium Doped Fiber Amplifiers (EDFA) are the most common type of optical
amplifiers
used in the long haul WDM (Wavelength Division Multiplexing) and submarine
systems.
In the first EDFA designs, the wavelength range of 1525 nm to 1565 nm, which
is called
the "Conventional" or C band and is the low-attenuation band of optical
fibers, was
exploited. However, the need for more bandwidth directed designers' attention
to the
longer wavelength range of 1570 to 1610 nm, or the L band. This in turn needed
a new
type of doped fiber amplifier with a better gain performance in the L band
rather than C
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band. L band fiber amplifiers emerged, which used basically
the same technique of the C band amplifiers with specialized
doped fibers as the gain medium.
In a typical prior art combined C and L band
amplifier, the WDM signal that consists of C band and L band
channels is split into C band and L band channels by a C/L
band splitter. Each band is then directed to the
corresponding fiber amplifier that forms one arm of the
combined amplifier. Finally the amplified optical signals
for the two paths are combined into the output fiber by a
C/L band combiner.
While this type of C and L bands amplifier
provides the requisite gain for each of the two bands, when
examining the gain across the entire bandwidth, which is
from the bottom of the C band to the top of the L band,
there is a dip in gain in the gap between the two bands.
The middle wavelength range of 1560 to 1570 (also called the
dead-band), cannot be used. This is mainly because of the
fact that the C/L band splitter filters each band and the
middle band of 1560-1570 is in the stop-band of the filter.
This wavelength band can accommodate 12 channels for 100 GHz
channel spacing or 24 wavelength channels for 50 GHz channel
spacing. If each channel carries data at 10 Gbits per
second, a total of 120 Gbits per second to 240 Gbits per
second data can be transferred on these channels.
SUMMARY OF THE INVENTION
According to one aspect of the present invention,
there is provided an optical signal amplifier for amplifying
at least first and second bands of optical data channels
separated by a band gap having spectral width substantially
less than each band's spectral width, comprising: (a)
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optical signal splitter means at an input of said amplifier
for splitting in its totality a signal in said at least
first and second bands and said band gap into first and
second paths; b) first optical signal gain means in said
first path having a first gain profile having gain focussed
in the first band; (c) second optical signal gain means in
said second path having a second gain profile having gain
focussed in the second band; (d) optical signal combiner
means for combining signals amplified by said first and
second optical signal gain means onto an output of said
amplifier; wherein the first gain profile and the second
gain profile together provide amplification of the band gap;
wherein an optical length of the first path and an optical
path length of the second path are substantially equal such
that amplified signals combine constructively in said
optical signal combiner means, where said optical signal
splitter means is a polarization beam splitter means, and
said optical signal combiner means is a polarization beam
combiner means.
According to another aspect of the present
invention, there is provided an optical signal amplifier for
amplifying at least first and second bands of optical data
channels separated by a band gap having spectral width
substantially less than each band's spectral width,
comprising: (a) optical signal splitter means at an input of
said amplifier for splitting according to polarization a
signal in said at least first and second bands and said band
gap into first and second paths; (b) first optical signal
gain means in said first path having a first gain profile
having gain focussed in the first band; (c) second signal
gain means in said second path having a second gain profile
having gain focussed in the second band; (d) optical signal
combiner means for combining signals amplified by said first
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and second optical signal gain means onto an output of said
amplifier; wherein the first gain profile and the second
gain profile together provide amplification of the band gap;
wherein an optical path length of the first path and an
optical path length of the second path are substantially
equal such that amplified signals combine constructively in
said optical signal combiner means, wherein said optical
signal splitter means is a polarization beam splitter means,
and said optical signal combiner means is a polarization
bean combiner means.
According to still another aspect of the present
invention, there is provided a method for amplifying at
least first and second bands of optical data channels
separated by a band gap having spectral width substantially
less than each band's spectral width, comprising: (a)
splitting in its totality a signal in said at least first
and second bands and said band gap into first and second
paths; (b) fully amplifying in the first path said first
band and partially amplifying said band gap; (c) fully
amplifying in the second path said second band and partially
amplifying said band gap; (d) combining amplified signals
into an output signal, wherein an optical path length of the
first path and an optical path length of the second path are
substantially equal such that amplified signals combine
constructively.
Embodiments of the present invention may provide
improved combined C and L bands optical amplifiers, which
include the optical signals in the middle region (dead-band)
between the C band and L bands. In the present amplifiers
the C band and L band channels are not separated. The two
arms of the optical amplifier have different gain profiles,
one for the C band and the other for the L band, and amplify
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the combined optical signal. In some embodiments, the total
combined gain profile is designed to be flat. Moreover, the
optical path lengths of the two amplifier arms are made
equal preferably by means of a fine tuned delay line.
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BRIEF DESCRIPTION OF THE DRAWINGS
The preferred exemplary embodiments of the present invention will now be
described in
detail in conjunction with the annexed drawings, in which:
Figure 1 shows a prior art C and L bands amplifier;
Figure 2 shows the transmission gain versus frequency of the amplifier shown
in figure 1;
Figure 3 shows a full C and L bands amplifier according to the present
invention;
Figure 4 shows the transmission gain of the amplifier shown in Figure 3;
Figure 5 shows the alternative amplifiers embodiment to that shown in Figure
3; and
Figure 6 shows a further alternative amplifier embodiment to that shown in
Figure 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1, the prior art C and L band amplifier is shown. In this
design, a band
splitter 10 splits the incoming WDM optical signal into its two components,
i.e. C and L
bands, and directs the C band to C band amplifier 11 and the L band to L band
amplifier
12, the outputs of which are combined by a band combiner 13 into the outgoing
WDM,
now amplified, optical signal.
Figure 2 shows the resultant transmission gain versus wavelength (or
equivalently
frequency). As may be seen, each of the C and L bands is amplified, but the
overall
transmission gain (the dotted curve) has a dip in-between the two bands.
Accordingly, the
band spectrum from approx. 1560 nm to approx. 1570 is not utilized whereas
both the C
and L band amplifiers show gain in the dead band region. As mentioned above,
this is
equivalent to wasting 12 to 24 channels of some 10 Gbits per second each.
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The optical amplifier of the present invention as shown in Figure 3 does not
split the C and
L bands (or any other bands), but splits the optical signal in its totality.
Thus, an incoming
WDM optical signal is split into two equal paths (-3dB each) by signal
splitter 14, one path
(having one-half of each of the C and L band signals) is applied to a C band
optical
amplifier 15 and the other path to L band optical amplifier 16. In the path of
the C band
amplifier branch it is desirable to have a delay line (DL) 17 to ensure equal
optical path
lengths to the required precision between the signal splitter 14 and a signal
combiner 18
which combines the L at C paths at its output.
It should be noted that application of a non-filtered amplifier combination is
not the
sufficient condition for a full C and L band amplifier. Since the optical
signal travels the
different optical paths of the two arms, the corresponding amplified optical
signals from the
two paths cannot be combined properly. In a proper configuration, the
corresponding
optical pulses amplified in the two branches must add together. If the optical
paths of the
two arms are not the same, distortion or dispersion of the optical pulses, as
well as crosstalk
and filtering effects, may result.
The length of the fiber delay line in the architecture shown in Figure 3 must
be measured
and selected precisely to equalize the two optical path lengths. Different
methods may be
used to measure a precise length for the delay line. A very efficient method
could be
optical length matching by using a precision reflectometer. In this technique,
we first
splice a piece of single mode fiber to the shorter arm to match the geometric
lengths. Then,
using a precision reflectometer, we cut and/or polish the fiber delay line to
match the two
optical path lengths to very low values in the range of micrometers, i.e. the
precision of the
reflectometer.
In another method, a short optical pulse generator can be used to adjust the
length of the
fiber delay line. In this method, an optical source that generates short
optical pulses is
connected to the input of the optical amplifier. The signal at the output of
the optical
amplifier is then monitored on a high-speed optical/ electronic oscilloscope,
or an
autocorrelator. Once the length of the delay line is optimum, the output pulse
train is seen
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with the same repetition as that of the input with a the maximum gain. In a
simpler way, the
output signal of the two arms can be monitored on a two-channel oscilloscope
with respect
to a common time reference. At the optimum delay line length, the two pulses
amplified at
the two arms must exactly coincide at the same time coordinates. This
technique makes the
length adjustment (cutting and polishing) much easier.
The transmission gain of the amplifier of Figure 3 is shown. in Figure 4. As
may be seen,
the overall gain (the dotted curve) does not exhibit the dip in the gap
between C and L
bands as in Figure 2.
In this architecture, the combined C and L signal splits into two branches for
C-band-focus
and L-band-focus amplification. As mentioned before, the output signal is the
result of the
combined amplification in both branches. This is mostly applicable to the
optical signals
close to and in the middle band between C and L bands, since this effect is
very minimal
for the outer edges of the bands. A typical gain profile is shown in Figure 4,
which clearly
illustrates the effect of the combined amplification.
The drawback of the architecture shown in Figure 3 is the loss introduced at
the input port
by the optical splitter. In order to obtain a better power performance, the
architecture
shown in Figure 5 may be used. In this architecture, a Polarization Beam
Splitter (PBS) 19
replaces the beam splitter of the design of Figure 3. This enables
polarization dependent
amplification in each arm of the optical amplifier. For this purpose, it will
be desirable to
use a Polarization Maintaining Fiber (PMF) Amplifiers 20 and 21. The same
techniques
discussed above may be used to match the optical paths. At the output, a
Polarization
Beam Combiner (PBC) 23 is used to combine the two amplified signals into one
output.
The architecture of Figure 5 is potentially capable of achieving a very good
noise
performance as well. To provide this functionality, Linear Polarizers (LP) 24
and 25 are
inserted in the two arms of the amplifiers as shown in Figure 6. The role of
the LPs is to
filter out the unpolarized noise. This results in very good noise reduction,
which in turn
enables a very low noise-figure optical amplifier. Overall, this design
provides a low
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noise, high output power, full C and L band optical amplifier. It should,
however, be
mentioned that linear polarizers to reduce noise are known from United States
patent
5,790,721 granted August 4, 1998 to Lee.
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