Note: Descriptions are shown in the official language in which they were submitted.
CA 02638618 2010-03-17
FIBER SPAN LOSS AND DISPERSION MEASUREMENTS
BACKGROUND OF THE INVENTION
Optical fiber communications systems commonly employ dense wavelength
division multiplexing (DWDM) to provide additional capacity by multiplexing a
number of optical carrier channels on a single optical fiber using a range of
optical
wavelengths. Conventional fiber optic systems may transmit optical signals in
a
wavelength range where longer wavelength components are subject to slightly
longer propagation delays than shorter wavelength components. This phenomenon,
known as chromatic dispersion, causes light pulses to spread or widen as they
travel
down an optical fiber. As the pulses widen, they may begin to overlap into
adjacent
bit cells resulting in communication errors such as bit errors thereby
potentially
limiting bandwidth and maximum transmission distance of a fiber span between
network nodes. These errors may become even more pronounced as a transmission
rate increases.
Known compensation techniques are used to reduce the effects of dispersion,
including passive dispersion compensation elements (DCE), such as a spool of
dispersion compensating fiber, and more recently, tunable dispersion
compensation
elements. With the introduction of tunable DCEs, there is a need for
automatically
measuring the amount of dispersion present on a span of fiber prior to setting
the
tunable DCE in order to correctly compensate dispersion. Furthermore, it may
be
necessary to determine fiber and DCE insertion losses in order to properly
program
optical amplifiers within the network.
During installation and deployment of an optical network, testing may be
performed to characterize fiber dispersion and fiber insertion loss. Prior to
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installation of the DCE, fiber dispersion is measured and an appropriate DCE
may
be installed or tuned. After installation is complete and the network begins
carrying
user traffic, dispersion or insertion loss measurements may need to be
performed as
the network is reconfigured as a result of design changes or equipment
failure.
SUMMARY OF THE INVENTION
A method and corresponding apparatus for configuring a network link
according to an example embodiment of the invention may include accessing an
optical signal at an ingress side of a connection point for a dispersion
compensation
element (DCE) coupling an egress side of a fiber span at the ingress side of
the DCE
to an optical amplifier at a connection point for an egress side of the
dispersion
compensation element. The example embodiment may include determining
chromatic dispersion of the fiber span based on the optical signal and
reporting
information associated with chromatic dispersion.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing will be apparent from the following more particular
description of example embodiments of the invention, as illustrated in the
accompanying drawings in which like reference characters refer to the same
parts
throughout the different views. The drawings are not necessarily to scale,
emphasis
instead being placed upon illustrating example embodiments of the present
invention.
FIG. 1 is a network diagram of an optical communications network element
according to an example embodiment of the invention;
FIG. 2 is a block diagram illustrating network elements implementing an
example embodiment of the invention;
FIG. 3 is a block diagram illustrating network elements in additional detail
according to an example embodiment of the invention;
FIG. 4 is a block diagram of a circuit pack and related elements according to
an example embodiment of the invention;
FIG. 5 is a block diagram of a circuit pack and related elements according to
an alternative example embodiment of the invention;
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FIG. 6 is a block diagram of a circuit pack and related elements according to
another alternative example embodiment of the invention;
FIG. 7 is a schematic diagram illustrating in further detail an example
embodiment similar to that described in FIG. 4;
FIG. 8 is a schematic diagram illustrating in further detail an example
embodiment similar to that described in FIG. 5; and
FIG. 9 is a flow diagram of an example procedure performed in accordance
with example embodiments of the invention.
DETAILED DESCRIPTION OF THE INVENTION
A description of example embodiments of the invention follows.
FIG. 1 is a network diagram of an optical communications network element
100 illustrating aspects of an example embodiment of the invention. The
optical
communications network element 100 may include network element components
such as amplifier circuit packs 105, 110 at each end of the network element
coupled
to each other via optical fibers 115, 120. The optical communications network
element 100 may be implemented using dense wavelength division multiplexing
(DWDM) technology whereby multiple optical carrier signals are multiplexed on
a
single optical fiber using multiple wavelengths.
As used herein, a signal may refer to a particular wavelength (e.g., 1510 nm)
or may refer to a wavelength modulated on a carrier wave or more generally as
a
communication signal that includes multiple wavelengths (e.g., 44 different
wavelengths using D)VDM technology).
The amplifier circuit packs 105, 110 may be coupled to filter circuit packs
125, 130 such as a reconfigurable optical add drop multiplexer (ROADM). A
ROADM 125, 130 is an optical add-drop multiplexer that allows the network
element 100 to remotely switch traffic in a DWDM network at the wavelength
layer.
For example, an add filter 135 may be used to add user wavelengths at a
transmitting
node, and a drop filter 140 may be used to drop user wavelengths at a
receiving
node. The ROADM 125, 130 also allows a system operator to remotely
configure/reconfigure the network.
The amplifier circuit packs 105, 110 may include optical amplifiers, such as
an erbium doped fiber amplifier (EDFA) 145, 150 for use in amplifying a
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transmitted optical signal (e.g., via EDFA 150) at a transmitting network node
or a
received optical signal at a receiving network node, as needed (e.g. via EDFA
145).
The amplifier circuit pack 105, 110 may also include a dispersion
measurement element (DME) 155 for use in measuring, for example, chromatic
dispersion of an optical fiber span. Chromatic dispersion describes a
phenomenon
whereby light pulses spread or widen as they travel down an optical fiber. As
the
pulses widen, they may begin to overlap into adjacent bit fields resulting in
communication errors, such as bit errors thereby limiting bandwidth and
maximum
deployable fiber length.
As used herein, chromatic dispersion or material dispersion or simply
dispersion may be used interchangeably. Furthermore, information associated
with
chromatic dispersion of a fiber span may include information in the form of a
metric,
an estimate, a level, a result (e.g., above or below a threshold) and the
like.
Chromatic dispersion effects may be mitigated through the use of a
dispersion compensation element (DCE) 160a-b. The DCE 160a-b may be coupled
to the circuit packs 105, 110 via input and output connectors (not shown) to
compensate for chromatic dispersion of, for example, an input fiber span 165a-
b.
However, there may be situations where a particular fiber span does not need
to be
compensated for, and in these cases, the DCE 160a-b may be replaced with a
simple
optical jumper cable. Note that "dispersion compensation element" and
"dispersion
compensation circuit pack" may be used interchangeably herein and are
collectively
referred to as a "DCE."
FIG. 2 is a block diagram of an optical communications element 200
illustrating an example embodiment of the invention. Optical signals 205 may
be communicated via a fiber span 210 from another network node, such as an
upstream or downstream node (not shown). The optical signal may be
accessed by an accessing unit 215 via, such as a wavelength filter or tap. A
representation of the optical signal 205 may be communicated to a
dispersion/insertion loss determination unit 220. The dispersion/insertion
loss
determination unit 220 may be used to determine the chromatic dispersion of
the fiber span 210, insertion loss of the fiber span 210, and insertion loss
of the
DCE 235 using the techniques discussed below in reference to FIGS 4-6.
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The dispersion/insertion loss determination unit 220 may communicate
determined dispersion results to a reporting unit 225 and compensation
processor
245. The reporting unit 225 may report chromatic dispersion information 230 to
an
upstream (or downstream) node, element management system (EMS), service
provider, server, or the like. A fixed or tunable dispersion compensation
element
235 may be used to compensate for chromatic dispersion. The compensation
processor
245 may be used to configure a tunable DCE 235 based on a metric associated
with
the determined chromatic dispersion, a predetermined value, event, or the
like. An
amplifier (e.g., EDFA) 240 may be used to amplify the optical signal prior to
transmitting the optical signal to a downstream or upstream network node.
FIG. 3 is a block diagram of a circuit pack 305 illustrating components of a
network element 300 according to an example embodiment of the invention and
may
include a circuit pack 305 (e.g., type 2 with an optical amplifier) and a
dispersion
compensation element (fixed or tunable) 330. The circuit pack 305 may include
a
DME 310, compensation processor 315, and input amplifier (e.g., EDFA) 365.
Note
that the DME 310, DCE 330, and compensation processor 315 can all be combined
on a single circuit pack separate from the input optical amplifier, or all
three
elements can reside on the same circuit pack containing the input optical
amplifier
365, or combination thereof. However, keeping the DCE 330 separate from the
input optical amplifier 365 provides additional flexibility by accommodating
use of
either a fixed (typically less expensive) or tunable (typically more
expensive) DCE
330 in a modular fashion. Furthermore, the circuit pack containing the input
optical
amplifier may also include the ROADM component (125) and/or the output optical
amplifier (150).
Unamplified and uncompensated optical signals may be communicated from
other network elements (not shown) to the circuit pack 305 via a fiber span
320
coupled to an input connector 325. The input optical signal flows to the DME
310
where chromatic dispersion of the fiber span 320 may be measured with the DME
310 using the technique described above in reference to FIG. 2, and further
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described below with respect to particular implementations described with
reference
to FIGs. 4-6.
The circuit pack 305 may also contain electrical connections and
corresponding transmission paths (shown as a dotted line) between the
compensation processor 315, DME 310, and DCE 330. The compensation processor
315 may use these connections to send control information to, or receive
status
information from, the dispersion compensation element 330. In this manner, the
compensation processor 315 may be used to control the fiber dispersion
measurement and to set the DCE 330 when a tunable DCE 330 is used. The
compensation processor 315 may initiate dispersion measurements by instructing
the
DME 310 to execute a measurement after which the compensation processor 315
gathers the results of the measurement and transforms the results into a
format that
can be used to correctly set the tunable DCE 330.
Based on the determined dispersion, a fixed DCE 330 may be coupled to the
circuit pack 305 such that the optical signal flows out of the circuit pack
305 via
optical connector 335, into the DCE 330 via optical connector 350, through the
DCE
330, out the DCE's output optical connector 345, back to the circuit pack 305
via
input connector 340 and finally to the EDFA 365. The EDFA 365 may then amplify
and compensate the optical signal as necessary.
In the case of the tunable DCE 330, the compensation processor 315 may be
used to communicate a control signal to the DCE 330 for use in tuning the
amount
of dispersion to be compensated for. The compensation processor may receive an
electrical signal from the DME 310 and based on this signal may also
communicate
an electrical control signal to the DCE 330 via an electrical connection 370
via
connectors 355, 360.
Although FIG. 3 depicts a tunable DCE 330 under direct control of the
circuit pack 305 on which the input optical amplifier 365 resides, the
compensation
processor 315 may be physically separate from the input optical amplifier
circuit
pack 305 and the DCE 330. In this case, the compensation processor 315 may be
used to communicate information to and from the DCE 330 and the circuit pack
305
on which the input optical amplifier 365 resides.
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An example embodiment of a method and corresponding apparatus of
configuring a network link may include accessing an optical signal at an
ingress side
of a connection point for a dispersion compensation element (DCE) coupling an
egress side of a fiber span at the ingress side of the DCE to an optical
amplifier at a
connection point for an egress side of the DCE. The method may further include
determining chromatic dispersion of the fiber span based on the optical
signal, and
reporting information associated with the chromatic dispersion. The optical
signal
may be an optical test signal and accessing the optical test signal may
include
separating at least a portion of the optical test signal from other signals
before
determining chromatic dispersion or insertion of the fiber span or DCE
insertion
loss.
An alternative example embodiment may further include determining a first
power level of the optical signal at the ingress side of the DCE (or DCE
connection
point) and a second power level at a transmitter side of a forward path of the
fiber
span toward the DCE (or DCE connection point) and reporting fiber span
insertion
loss based on a differential of the first and second power levels. Accessing
the
optical signal may further include tapping a percentage of the optical test
signal
along with other optical signals, if present, and separating the optical test
signal from
the other optical signals, if present.
In another example embodiment, the method and corresponding may further
include determining chromatic dispersion by detecting a time difference
between
two optical signals at different wavelengths in a forward direction of the
fiber span,
and may be determined in the presence or absence of the DCE. Determining
chromatic dispersion may also include determining a length of the fiber span
and
calculating the chromatic dispersion based on the length and/or fiber type.
Another example embodiment may further include configuring the DCE
based on the chromatic dispersion, or in addition or alternatively, at least
one of the
following: a predetermined value, stored value, calculated value, instruction,
or
event. Configuring the DCE may also include tuning the DCE.
In yet another example embodiment, the method and corresponding
apparatus may further include accessing the optical signal at the egress side
of the
DCE, determining insertion loss of the DCE or fiber span based on a difference
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between power levels of the optical signal at the ingress and egress sides,
and
reporting the insertion loss and may include adjusting gain of the optical
amplifier as
a function of both insertion losses. The embodiment may further include
accessing
the optical test signal includes separating at least a portion of the optical
test signal
from other signals before determining the insertion loss. Determining
insertion loss
further includes measuring leakage power of the filtered optical test signal
during a
period in which no user signals are on the fiber span.
Other example embodiments may further include adjusting the gain as a
function of the leakage power, and or in addition, measuring the leakage power
at
the egress side of the DCE. The embodiment may also include reporting
insertion
loss based on a differential of power levels on the ingress and egress sides
of the
DCE.
The example embodiments described above and in FIGs. 4-6 illustrate, in
additional detail, alternative example implementations of a network node
employing
a circuit pack "type 2" as described in FIG. 3. In particular, FIG. 4 depicts
a circuit
pack "type 2A," FIG. 5 depicts a circuit pack "type 2B," and FIG. 6 depicts a
circuit
pack "type 2C." Although the implementation details of each type may vary,
each
embodiment, nonetheless, provides the ability to measure fiber dispersion,
fiber
insertion loss, and DCE insertion loss, and may do so in the presence or
absence of
user communication signals.
FIG. 4 is a more detailed block diagram of a network element 400 employing
a circuit pack (type 2A) 405 with an input optical amplifier according to an
example
embodiment of the invention. The circuit pack 405 may include a dispersion
measurement element (DME) 410, taps 415, 425 and optical amplifier, such as an
EDFA 430. The DME 410 may further include an input test signal filter 435, tap
2
420, test signal processor 440, test signal generator 445, and output test
signal filter
450.
Unamplified and uncompensated optical signals arrive at a LINE IN
connector 455 via a fiber span (not shown) and further propagate to the input
test
signal filter 435 at the DME 410. The input test signal filter 435 is chosen
to
separate an input test signal (or signals) of a particular wavelength (or
wavelengths)
from the input optical signal received at the LINE IN connector 455. The
separated.
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input test signal is transmitted to tap 2 420 where the signal is tapped and a
power
reduced portion of the input test signal is transmitted to, for example,
photodiode 2
and the remaining power reduced portion is transmitted to the signal processor
440.
An input optical signal may arrive at the LINE IN connection 455 that
includes an input test signal having a wavelength of, for example, 1510 rim.
The
input optical signal may include only the input test signal (e.g., during
system
installation) or may include the input test signal and user data
communications (e.g.,
44 user wavelengths). The input test signal filter 435 filters or passes
through only
the input test signal, which in this case may be the 1510 nm wavelength, while
blocking any remaining wavelengths, if present, to the test signal processor.
The
1510 nm input test signal is far enough away in frequency from the user data
wavelengths, such as those located in the C-band, that the filter need not
display
perfect transfer characteristics, thereby simplifying the filter's design.
The filtered input test signal flows to the test signal processor 440, where
dispersion may be measured or calculated based upon the received test signal.
The
test. signal processor may transmit electrical signals to the test signal
generator 445
via an electrical transmission path (depicted as dotted line 470). The test
signal
generator 445 generates an "output test signal" that is further transmitted to
the
output test signal filter 450 wherein the signal may be combined with a
"transmit
output data" signal at the output test signal filter 450 and further
transmitted to a
LINE OUTPUT connector 460. The output test signal may then be used to measure
the chromatic dispersion of the fiber span at an upstream node. The output
test
signal filter 450 may be an optical filter of a different optical
wavelength(s) than
those wavelengths contained within the transmit output data signal. As the
input test
signal is the same wavelength as the output test signal, it is suitable for
measuring
dispersion present on the fiber span. In an alternative example embodiment,
chromatic dispersion may also be determined using multiple wavelengths by
launching a pulse train at the transmitting node simultaneously using at least
two
different wavelengths and measuring the phase difference of the multiple
wavelengths upon arrival at the DME 410 of a receiving node.
It should be noted that dispersion measurements can be made regardless of
the presence of the "transmit output data" or "receive input data" signals
because the
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output test signal and the input test signal are used in the dispersion
measurement
procedureand it is assumed that these two signals are always available.
Furthermore, due to the placement of the DCE 465, the dispersion measurements
may be made in the presence or absence of the DCE 465.
Continuing to refer to FIG. 4, the "receive input data" signal flowing out the
upper leg of the input test signal filter 435 includes all wavelengths not
filtered out
by the input test signal filter 435, that is, user data signals (e.g., C-band
wavelengths), if present. The receive input data flows to tap 1 415, wherein a
power
reduced portion is transmitted to photodiode 1 and the remaining power reduced
portion is transmitted to the DC OUT connector 475, through the DCE 465 (or
optical jumper), and back in the circuit pack 405 via a DC IN connector 480.
The
DCE 465 may be used to compensate for the measured chromatic dispersion
associated with a fiber span, and may be a fixed DCE 465 (e.g., a spool of
dispersion
compensation fiber) or a tunable DCE 465 thereby correcting for the effects of
chromatic aspersion associated with the fiber span.
The optical signal then flows from the DC IN connector 480 to tap 3 425
wherein a power reduced portion of the optical signal is transmitted to
photodiode 3
and the remaining power reduced portion of the signal is transmitted to the
input
optical amplifier 430. The signal (now compensated for dispersion) may be
amplified by the input optical amplifier 430 to compensate for amplitude loss,
as a
result of, for example, fiber span insertion loss, prior to transmitting the
optical
signal to other network elements components.
In order to properly set the gain of the input optical amplifier 430, the
insertion loss of the fiber span and the insertion loss of the DCE 465 need to
be
determined. This is often done during network installation, such that once the
network of nodes begin carrying user traffic signals, the gain settings of all
network
amplifiers have been properly programmed. In addition, by setting the gain of
the
input optical amplifiers before the network begins carrying user traffic,
placement of
the correct input amplifiers can be verified before the network goes "live"
(assuming
there are multiple input amplifiers having different gain ranges).
Insertion loss related to the fiber span may be measured using the optical
taps
associated with the DME 410. For example, tap 1 415 and tap 2 420 may be used
to
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siphon off predetermine portions of the optical power of the two outputs of
the input
test signal filter 435. Assuming that the "output test signal" and the
"transmit output
data" signals are launched at known power levels at the previous network node,
the
outputs of tap 1 415 and tap 2 420 may be transmitted to photodiode 1 and
photodiode 2 and used to measure span insertion loss associated with the fiber
between network nodes.
Insertion loss related to the DCE 465 may be measured even though the input
test signal has been filtered before the signal arrives at the DCE 465 (and in
the
absence of user traffic). This may be done by choosing an input test signal
filter 435
such that the input test signal is only partially attenuated through the
filter 435.
Such a filter will allow the test signal to pass to the test signal processor
and would
also allow an attenuated version of the input test signal to pass out the
upper leg of
the input test signal filter 435. For example, the input test signal filter
435 may be
chosen such signals at the upper leg of the filter are attenuated by 15 dB.
Consequently, it is possible to in-system measure the insertion loss of the
DCE 465
by using the photodiodes associated with tap 2 and tap 3 (assuming the input
test
signal is present). The amplifier gain can then be set based upon the
combination of
in-system fiber span insertion loss and DCE insertion loss measurements.
Alternatively, insertion loss of the DCE may be measured "out-of-system," that
is,
prior to connecting the DCE 465 to the circuit pack 405
Thus, the implementation with respect to circuit pack "type 2A" 405
provides the ability to perform fiber dispersion, fiber insertion loss, and
DCE 465
insertion loss measurements, and the measurements may be conducted in the
presence or absence of user communication signals and in the presence or
absence of
a dispersion compensation element.
FIG. 5 is a more detailed block diagram of a network element 500 employing
an example embodiment of an alternative circuit pack "type 2B" 505 according
to
aspects of the invention. This embodiment provides similar capabilities of the
embodiment described with respect to FIG. 4, however, with the addition of a
second input test filter 525, this embodiment provides a simpler method of
determining total insertion loss (i.e., span and DCE) for use in programming,
for
example, the optical amplifier 585.
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In this embodiment, the circuit pack 505 may include a DME 510, tap 3 530,
input optical amplifier 585, DCE connection points 515, 520, and second input
test
signal filter 525, and may optionally include a fixed attenuator 535. However,
in
this embodiment, the optical signal is tapped upon arrival with tap 1 545
wherein a
power reduced portion of the input optical signal flows to the first input
test signal
filter 550 and the remaining power reduced portion flows to the DC OUT
connector
515. Therefore, all wavelengths of the input optical signal (e.g., a 1510 nm
input
test signal and/or C-band user signals) are present at the first input test
signal filter
550 and the DC OUT connector 515. In addition, a second input test signal
filter
525 is placed between the DC IN connector 520 and the input optical amplifier
585.
Unamplified and uncompensated optical communications signals may arrive
from a previous network element (not shown) at a LINE IN connector 540 and
further propagated to the DME 510 and tap 1 545. At tap 1 545, a power reduced
portion of the input optical signal is "tapped" off and flows to the first
input test
signal filter 550 and the remaining power reduced portion flows to the DC OUT
connector 515. The first input test signal filter 550 may be chosen to filter
or
separate out an input test signal transmitted at a wavelength of, for example,
1510
nm similar to that described above reference to FIG. 4 in order to measure the
fiber
span dispersion. The first input test signal filter 550 is required for the
situation
where user wavelengths are present on the input fiber span. Since only a small
portion of the optical power is siphoned off by tap 1 545, the majority of the
optical
power for both the test signal and the user wavelengths is passed through the
DCE
590.
The filtered input test signal may then flow to tap 2 555 and is further
tapped
wherein a power reduced portion of the signal continues to flow to a
photodiode 2
and the remaining power reduced portion flows to a test signal processor 560
where
dispersion may be measured or calculated based upon the received test signal.
The test signal processor 560 may process the input test signal as appropriate
and may be in electrical communication with a test signal generator 565 such
that
electrical signals may be transmitted to the test signal generator 565 via an
electrical
transmission path (depicted as dotted line 580). The test signal generator 565
may
then be instructed to generate an "output test signal" that may be further
transmitted
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to the output test signal filter 570 wherein the signal may be combined with a
"transmit output data" signal at the output test signal filter 570 and further
transmitted to a LINE OUTPUT connector 575. The output test signal, in
conjunction with the input test signal, may then be used to measure the
dispersion of
a fiber span coupling the circuit pack 505 to another network element.
The output test signal filter 570 may be an optical filter of a different
optical
wavelength(s) than those wavelengths contained within the transmit output data
signal. Given that the output test signal may be created such that it is the
same
wavelength as the input test signal, it may be suitable for measuring
dispersion
present on the fiber span. In an alternative example embodiment, chromatic
dispersion may also be determined using multiple wavelengths by launching a
pulse
train at the transmitting node simultaneously using at least two different
wavelengths
and measuring the phase difference of the multiple wavelengths upon arrival at
the
DME 510 of a receiving node.
Continuing to refer to FIG. 5, the power reduced portion of the input optical
signal flows out tap 1 545 and flows to the DC OUT connector 515, through the
DCE 590, back in the circuit pack 505 via DC IN connector 520, and to the
second
input test signal filter 525. As mentioned above, the majority of the optical
power
for both the test signal and user wavelengths is passed through the DCE 590
and
arrives at the second input test signal filter 525, where the optical power
associated
with the test signal can be measured after filtering. Thus, in this
embodiment, the
fiber span insertion loss and the DCE's insertion loss can be directly
determined by
making one measurement at photodiode 4 (assuming the optical power of the test
signal is launched with a known power level at the transmitting optical
network
node).
Thus, the implementation with respect to circuit pack "type 2B" 505
similarly provides the ability to perform fiber dispersion, fiber insertion
loss, and
DCE 590 insertion loss measurements, and these measurements may be conducted
in the presence or absence of user communication signals and in the presence
or
absence of a dispersion compensation element.
FIG. 6 is a more detailed block diagram of a network element 600 employing
an implementation of another alternative circuit pack "type 2C" 605 according
to an
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example embodiment of the invention. In this embodiment, the DME 610 follows
the DCE 625 rather than preceding the DCE 625 as was the case with the
embodiments described above with reference to FIG. 4 (type 2A) and FIG. 5
(type
2B). Here, the full optical power of the test signal is allowed to pass
through the
DCE 625 and can be measured at a single point (photodiode 2), thereby
simplifying
total insertion loss measurements (i.e., the combination of fiber span and
DCE).
However, when the DCE 625 is present, dispersion measurements of the span
itself
may require knowledge of the particular DCE 625 installed in the network,
since the
dispersion measurement is made after the DCE. In this case, if the DCE is a
tunable
DCE, then the DCE may be set to "0 dispersion compensation" prior to making
the
dispersion measurement of the span..
In this embodiment, the input optical signal arrives at a LINE IN connector
640, and flows to tap 3 675 wherein a power reduced portion of the input
optical
signal flows to photodiode 3 and the remaining portion flows out a DC OUT
connector 620, through the DCE 625, and back in the circuit pack 605 via a DC
IN
connector 615. The signal continues to flow to the DME 610, and on to the
input
test signal filter 635 chosen to separate out the input test signal from the
input
optical signal. The input test signal flows to tap 2 640 wherein a power
reduced
portion of the signal flows to photodiode 2 and the remaining power reduced
portion
flows to a test signal processor 645, where dispersion is measured or
calculated
based upon the received test signal.
The test signal processor 645 may process the input test signal as appropriate
and may be in electrical communication with a test signal generator 650 such
that
electrical signals may be transmitted to the test signal generator 650 via an
electrical
transmission path. The test signal generator 650 may be instructed to generate
an
"output test signal" that is further transmitted to the output test signal
filter 655
wherein the signal may be combined with a "transmit output data" signal at the
output test signal filter 655 and further transmitted to a LINE OUTPUT
connector
660.
In the absence of the DCE, the signal arriving at Line in 640 may be routed
to the DME 610 by placing an optical jumper cable between DC in and DC out on
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circuit pack 605. Dispersion can then be measured or calculated based on the
received test signal.
The output test signal filter 655 may be an optical filter of a different
optical
wavelength(s) than those wavelengths contained within the transmit output data
signal. Since the output test signal may be generated such that it is of the
same
wavelength as the input test signal, it may be suitable for measuring
dispersion of
the fiber span. In an alternative embodiment, chromatic dispersion may also be
determined using multiple wavelengths by launching a pulse train at the
transmitting
node simultaneously using at least two different wavelengths and measuring the
phase difference of the multiple wavelengths upon arrival at the DME 610 of a
receiving node.
Thus, in the embodiment illustrated in circuit pack type 2C, insertion loss
related to the span and DCE 625 may be measured at photodiode 2, both in the
presence or absence of user traffic signals. In addition, the insertion loss
of only the
fiber span can be directly measured using photodiode 3 (assuming no user
wavelengths are present).
FIG. 7 is a schematic diagram 700 illustrating in additional detail the
implementation described above in reference to FIG. 4 with respect to circuit
pack
"type 2A" according to an example embodiment of the invention. In this
embodiment, an optical supervisory channel (OSC), commonly available in most
DWDM systems, is used as a test signal for span dispersion and span insertion
loss
measurements. As a result, the OSC signal may be used to measure span
dispersion,
span insertion loss, and DCE 735 insertion loss in a similar manner as that
described
above in reference to FIG. 4.
An input optical signal arriving at a LINE IN connector 705 flows to an OSC
filter 710 where the OSC signal is separated and transmitted to tap 1 715
wherein the
signal is tapped and a power reduced portion of the signal is transmitted to
photodiode 1 which may be used to measure span insertion loss. The remaining
power reduced portion of the OSC signal is transmitted to an optical
transceiver 720.
User data traffic (e.g. the 44 C-band wavelengths) may flow out the upper leg
of the
OSC filter 710 and on to tap 3 725 wherein the signals are tapped and a power
reduced portion of the user traffic flows to photodiode 3 and the remaining
power
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reduced portion of the user data signal flows to a DC OUT connector 730,
through
the DCE 735 and back to the DC IN connector 740. The signal continues to flow
to
tap 2 745 wherein the signal is further tapped and a power reduced portion of
the
signal flows to photodiode 2 and the remaining power reduced portion of the
signal
flows to the input optical amplifier 750.
FIG. 8 is a schematic diagram 800 representing in additional detail the
implementation described above with reference to FIG. 5 with respect to
circuit pack
"type 2B" according to another example embodiment. This embodiment also makes
use of the OSC signal as a test signal for span dispersion and span insertion
loss
measurements. Thus, the OSC signal may be used to measure span dispersion,
span
insertion loss, and DCE 835 insertion loss in a similar manner as that
described
above in reference to FIG. 5.
An input optical signal arrives at a LINE IN connector 805 and flows to a tap
810 wherein the input optical signal is tapped and a power reduced portion of
the
signal flows to an OSC filter 815 and the remaining power reduced portion
flows to
a DC OUT connector 830. For example, the tap 810 may be a 15% tap such that
15% of the input optical signal power arriving at the LINE IN connector 805 is
tapped off and flows to the OSC filter 815, and the remaining 85% of the
signal
power flows to the DC OUT connector 830, through the DCE 835 and back in the
DC IN connector 840 arriving at an OSC filter 845. The lower leg of the OSC
filter
845 separates out the OSC signal and its power may be measured using
photodiode
1. The upper leg of the OSC filter 845 transmits the remaining signals to tap
850
wherein a power reduced portion of the signal flows to photodiode 9 and the
remaining power reduced portion flows to the amplifier 855. The lower leg of
the
other OSC filter 815 separates out the OSC signal where flows to tap 825 and a
power reduced portion of the signal flows to photodiode 3 and the remaining
power
reduced portion flows to the optical transceiver 820. The unfiltered
wavelengths
flows out the upper leg of the OSC filter 815, then to a fixed attenuator 860,
and a
line test connector 865.
FIG. 9 is a flow diagram of a procedure 900 illustrating an example
embodiment of the invention. The procedure 900 begins (905) and may access an
optical signal at an ingress side of a connection point for a DCE coupling an
egress
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side of a fiber span at the ingress side of the DCE to an optical amplifier at
a
connection point for an egress side of the DCE (910). Chromatic dispersion of
a
fiber span may be determined based on the input optical signal (915). The
procedure
900 may then determine if other measurements are to be performed (920), and if
so
the procedure 900 determines if fiber insertion loss measurements are to be
performed (925) or if DCE insertion loss measurements are to be performed
(935),
and if so appropriate power measurements are performed (930, 940). After the
measurements have been performed, the measured results may be reported (945)
to,
for example, a system operator, element management system (EMS), server, or
the
like, and then the procedure ends (950).
It should be understood that the procedure 900 described in FIG. 9 is an
example embodiment used for illustrative purposes only. Other embodiments
within
the context of performing dispersion or insertion loss measurements or similar
network characteristics may be employed. Furthermore, the techniques
illustrated in
FIG. 9 may be performed sequentially, in parallel or in an order other than
that
which is described. It should be appreciated that not all of the techniques
described
are required to be performed, that additional techniques may be added, and
that
some of the illustrated techniques may be substituted with other techniques.
Some or all of the procedure 900 may be implemented in hardware,
firmware, or software. If implemented in software, the software may be (i)
stored
locally with the network node, such as a circuit pack, or some other remote
location,
or (ii) stored remotely and downloaded to the network node when, for example,
the
procedure 900 begins (905). The software may also be updated locally or
remotely.
To begin operations in a software implementation, the network node loads and
executes the software in any manner known in the art. It should be apparent to
those
of ordinary skill in the art that methods involved in the invention may be
embodied
in a computer program product that includes a computer usable medium. For
example, such a computer usable medium may consist of a read-only memory
device, such as a CD-ROM disk or convention ROM devices, or a random access
memory, such as a hard drive device or a computer diskette, having a computer
readable program code stored thereon.
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Furthermore, it is common in the art to speak of software, in one form or
another (e.g., program, procedure, process, application, module, unit, logic,
and so
on) as taking an action or causing a result. Such expressions are merely a
shorthand
way of stating that the execution of the software by a processing system
causes the
processor to perform an action to produce a result.
While this invention has been particularly shown and described with
references to example embodiments thereof, it will be understood by those
skilled in
the art that various changes in form and details may be made therein, for
example in
a computer program product or software, hardware or any combination thereof,
without departing from the scope of the invention encompassed by the appended
claims.
