U.S. patent application number 09/745157 was filed with the patent office on 2006-09-21 for optical band scanning monitor system and method.
Invention is credited to Walid A. Atia, Jeffrey A. Korn.
Application Number | 20060210276 09/745157 |
Document ID | / |
Family ID | 36939558 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060210276 |
Kind Code |
A1 |
Korn; Jeffrey A. ; et
al. |
September 21, 2006 |
OPTICAL BAND SCANNING MONITOR SYSTEM AND METHOD
Abstract
A scanning optical monitoring system and method are appropriate
for high speed scanning of a WDM signal band. The system and method
are able to identify dropped channels or, more generally,
discrepancies between the determined or detected channel inventory
and a perpetual inventory for the WDM signal, which perpetual
inventory specifies the channels that should be present in the WDM
signal assuming proper operation of the network. The system
includes a tunable optical filter that scans a pass band across a
signal band of a WDM signal to generate a filtered signal. A
photodetector then generates an electrical signal in response to
this filtered signal. A decision circuit compares the electrical
signal to a threshold and a controller, which is responsive to the
decision circuit, inventories the channels in the WDM signal.
Inventors: |
Korn; Jeffrey A.;
(Lexington, MA) ; Atia; Walid A.; (Lexington,
MA) |
Correspondence
Address: |
J GRANT HOUSTON;AXSUN TECHNOLOGIES INC
1 FORTUNE DRIVE
BILLERICA
MA
01821
US
|
Family ID: |
36939558 |
Appl. No.: |
09/745157 |
Filed: |
December 20, 2000 |
Current U.S.
Class: |
398/85 |
Current CPC
Class: |
H04J 14/0227 20130101;
H04J 14/0246 20130101; H04B 10/0795 20130101; H04B 10/07957
20130101 |
Class at
Publication: |
398/085 |
International
Class: |
H04J 14/02 20060101
H04J014/02 |
Claims
1. A scanning optical monitoring system, comprising: a tunable
optical filter that scans a pass band across a signal band of an
input signal to generate a filtered signal corresponding to a
spectrum of the input signal; a photo detector that generates an
electrical signal in response to the filtered signal; a decision
circuit that compares the electrical signal to a threshold; and a
controller that is responsive to the decision circuit to identify
spectral features in the input signal by comparing a spectral
position of an instantaneous pass band of the tunable filter to a
response of the decision circuit to determine the spectral features
of the input signal, the instantaneous pass band of the filter
being determined by reference to a delay from a generation of a
trigger signal starting the scan.
2. A scanning optical monitor system as claimed in claim 1, wherein
the tunable optical filter tunes across the signal band in less
than 1 millisecond.
3. A scanning optical monitor system as claimed in claim 1, wherein
the tunable optical filter begins and ends a complete scan in less
than 1 millisecond.
4. A scanning optical monitor system as claimed in claim 1, wherein
the tunable optical filter tunes across one half of the signal band
in less than 1 millisecond.
5. A scanning optical monitor system as claimed in claim 1, wherein
the tunable optical filter is a Fabry-Perot filter.
6. A scanning optical monitor system as claimed in claim 1, further
comprising an electronic filter that low pass filters the
electronic signal from the photo detector prior to reception by the
decision circuit.
7. (canceled)
8. A scanning optical monitor system as claimed in claim 1, wherein
the controller compares the spectral features to expected signal
information to assess a validity of the input signal.
9. A scanning optical monitor system as claimed in claim 1, wherein
the tunable filter comprises an electrostatic drive cavity in which
an electrostatic field is generated to displace a flexible membrane
of the tunable filter.
10. A scanning optical monitor system as claimed in claim 1,
wherein a free spectral range of the tunable filter is greater than
a bandwidth of the signal band of the input signal.
11. A scanning optical monitor system as claimed in claim 1,
wherein a free spectral range of the tunable filter is less than a
bandwidth of the signal band.
12. A scanning optical monitor system as claimed in claim 1,
wherein a flee spectral range of the tunable filter is less than a
bandwidth of the signal band but greater than one-half of the
bandwidth of the signal band.
13. A scanning optical monitor system as claimed in claim 12,
further comprising: an input filter for separating the filtered
signal into a first sub-band and a second sub-band; and a first
sub-band detector and a second sub-band detector.
14. A scanning optical monitor system as claimed in claim 1,
further comprising a timing recovery circuit that controls sampling
of the decision circuit by the controller.
15. A scanning optical monitor system as claimed in claim 1,
wherein the controller generates a threshold set signal the
specifies a level of the threshold applied by the decision
circuit.
16. A scanning optical monitor system as claimed in claim 1,
further comprising a filter tuning voltage generator that generates
a tuning voltage to the optical tunable filter.
17. A scanning optical monitor system as claimed in claim 1,
further comprising a filter tuning voltage generator that generates
a tuning voltage to the optical tunable filter that improves a
linearization of the tuning of the passband as a function of time
over at least a portion of the scan of the signal band.
18. A scanning optical monitor system as claimed in claim 1,
further comprising a filter tuning voltage generator that generates
a tuning voltage to the optical tunable filter that linearizes the
tuning of the passband as a function of time over at least a
portion of the scan of the signal band.
19. A scanning optical monitor system as claimed in claim 18,
wherein the filter tuning voltage generator maps an inverse of a
tuning function of the optical tunable filter.
20. A scanning optical monitor system as claimed in claim 18,
wherein the filter tuning voltage generator comprises a look-up
table.
21. A scanning optical monitoring system, comprising: a tunable
optical filter that scans a pass band across a signal band of an
input signal to generate a filtered signal corresponding to a
spectrum of the input signal; a photo detector that generates an
electrical signal in response to the filtered signal; a variable
decision circuit that compares the electrical signal to a variable
threshold; and a controller that sets a level of the variable
threshold and is responsive to the decision circuit to analyze
power in the input signal based on the level of the variable
threshold by comparing a spectral position of an instantaneous pass
band of the tunable filter to a response of the decision circuit to
determine spectral features of the input signal, the instantaneous
pass band of the filter being determined by reference to a delay
from a generation of a trigger signal starting the scan.
22. A method for analyzing an input signal comprising: tuning a
pass band of a filter across a signal band of the input signal to
generate a filtered signal corresponding to the spectrum of the
input signal; detecting the filtered signal; comparing a level of
the detection signal to a threshold; comparing an instantaneous
pass band spectral position of the filter to a level of the
detection signal relative to the threshold to analyze spectral
features in the input signal; and determining the instantaneous
pass band of the filter by reference to a delay from a generation
of a trigger signal starting the scan.
23. A method as claimed in claim 22, further comprising tuning the
filter across the signal band in less than 1 millisecond.
24. A method as claimed in claim 22, further comprising tuning the
filter across one-half of the signal band in less than 1
millisecond.
25. A method as claimed in claim 22, further comprising low pass
filtering a detection signal prior to the step of comparing the
detection signal to the threshold.
26. A method as claimed in claim 22, further comprising comparing
the spectral feature to perpetual inventory information.
27. A method as claimed in claim 22, further comprising tuning
multiple modes of the filter across the signal band
simultaneously.
28. A method as claimed in claim 22, further comprising changing
the threshold between scans to determined channel powers in the
input signal.
29. A method as claimed in claim 22, further comprising driving the
filter with a tuning function that is non-linear with response to
time across the scan and improving a linearization of the tuning of
the passband as a function of time over at least a portion of the
scan of the signal band.
30. (canceled)
31. A method for analyzing a WDM signal comprising: tuning a pass
band of a filter across a signal band of the WDM signal to generate
a filtered signal in a first scan of the WDM signal; detecting the
filtered signal; comparing a level of a detection signal to a first
threshold; comparing an instantaneous pass band of the filter to a
level of the detection signal relative to the first threshold;
tuning the passband of the filter across the signal band in a
second scan of the WDM signal; comparing the level of the detection
signal to a second threshold; comparing an instantaneous pass band
of the filter to a level of the detection signal relative to the
second threshold; comparing the first scan and the second scan to
determined channel power; and determining the instantaneous pass
band of the filter by reference to a delay from a generation of a
trigger signal starting the scan.
32. A scanning optical monitor system as claimed in claim 1,
wherein the input signal is a wavelength division multiplexed
signal and the spectral features are an inventory of WDM channels.
Description
BACKGROUND OF THE INVENTION
[0001] Wavelength division multiplexing (WDM) systems typically
comprise multiple separately modulated laser diodes at the
transmitter. These laser diodes are tuned to operate at different
wavelengths. When combined in an optical fiber, the WDM optical
signal comprises a corresponding number of spectrally separated
channels. Along the transmission link, the channels are typically
collectively amplified in gain fiber, such as erbium-doped fiber
and/or regular fiber, in a Raman pumping scheme. At the receiving
end, the channels are usually separated from each other using thin
film filter systems, to thereby enable detection by separate
photodiodes.
[0002] The advantage of WDM systems is that the transmission
capacity of a single fiber can be increased. Historically, only a
single channel was transmitted in each optical fiber. In contrast,
modern WDM systems contemplate hundreds or thousands of spectrally
separated channels per fiber. Such configurations yield concomitant
increases in the data rate capabilities of each fiber. Moreover,
the cost per bit of data for WDM systems is typically less than
comparative non-multiplexed systems. This is because any
amplification system required along the link can essentially be
shared by all of the separate channels transmitted in a single
fiber link. With non-multiplexed systems, each channel/fiber would
require its own amplification system.
[0003] The economics pulling for WDM in the context of long-haul
optical links is only one factor suggesting the long-term
applicability of the technology. Another application concerns the
dynamic routing of individual wavelength slots or channels in
optical WDM networks with multiple network access nodes. Such
network functionality requires devices that can add and drop
specific channels in an optical link.
SUMMARY OF THE INVENTION
[0004] Major engineering challenges exist today in the design of
add/drop devices that have robust operation. One such challenge
concerns the monitoring of the operation of such devices by
reference to the channel signals populating the input and output
optical links to the devices. At least three factors define the
scope of this challenge. First, in dense WDM (DWDM) systems, the
channel spacings can be tight, 100 GigaHertz (GHz) to as tight as
50 GHz in some currently proposed systems. Further, the number of
potential channels on a link can be large. Observation of the ITU
Grid suggests 100's of channels on a link in the L.sub..alpha.,
C.sub..alpha., and S.sub..alpha. bands, even if the 50 GHz offset
of the L.sub..beta., C.sub..beta., and S.sub..beta. band is
ignored. Finally, improperly routed or dropped channel signal
should be detected quickly. SONET specifications dictate that
problems must be detected within 1 millisecond (msec).
[0005] The present invention is directed to a scanning optical
monitoring system and method. Specifically, a system and method are
appropriate for high speed scanning of a WDM signal band. The
system and method are able to quickly identify dropped channel
signals or, more generally, discrepancies between the determined or
detected channel inventory and a perpetual inventory for the WDM
signal, which perpetual inventory specifies the channel signals
that should be present in the WDM signal assuming proper operation
of the network.
[0006] In general, according to one aspect, the invention features
a scanning optical monitoring system. The system comprises a
tunable optical filter that scans a pass band across a signal band
of a WDM signal to generate a filtered signal. A photodetector then
generates an electrical signal in response to this filtered signal.
A decision circuit compares the electrical signal to a threshold,
and a controller, which is responsive to the decision circuit,
inventories the channels in the WDM signal based on the information
from the scan.
[0007] In a preferred embodiment, the tunable optical filter tunes
across a signal band of the WDM signal in less than one
millisecond. Specifically, the tunable optical filter begins and
ends a complete scan in less than a millisecond allowing it to then
be reset to immediately perform a subsequent scan.
[0008] However, in an alternative embodiment, more than one mode of
the optical filter is used to scan the entire signal band of the
WDM signal. In this case, one of the modes is tuned only across
one-half, for example, of the signal band, with another mode
handling the other half of the signal band.
[0009] In a current implementation, a low pass filter is used to
filter the electronic signal from the photodetector prior to
reception by the decision circuit. This removes any high frequency
interference, allowing the controller to determine the existence,
or not, of specific channels during the scan.
[0010] The controller determines the channel inventory by comparing
an instantaneous power in the pass band of the tunable filter,
typically during the scan, to a threshold of the decision circuit
to determine whether or not a channel exists at the current pass
band or channel slot. An inventory determined from the scan by the
controller is then compared to perpetual inventory information that
can be accessed by the controller. In this way, the controller can
identify whether or not the WDM signal contains the expected
channels.
[0011] According to the preferred embodiment, the tunable filter is
a Fabry-Perot device, having an electrostatic drive cavity in which
an electrostatic field is used to displace a flexible membrane. In
one implementation, this cavity is sized such that a free spectral
range of the tunable filter is greater than a bandwidth of a signal
band of the WDM signal. In an alternative embodiment, the free
spectral range is less than a bandwidth of the signal band. In this
implementation, multiple modes of the tunable filter are used to
simultaneously scan the entire signal band.
[0012] In one embodiment, a timing recovery circuit is used to
control the sampling of the decision circuit by the controller.
Such timing recovery circuits function as a software/hardware phase
locked loop that locks onto the time series representing the
channels in the WDM signal. Therefore, the location, or not, of a
channel at a specific point in the scan can be determined.
[0013] According to a further embodiment of the invention, a filter
tuning voltage generator generates a tuning voltage to the optical
tunable filter that improves the linearity of the tuning of the
pass band as a function of time over at least a portion of the scan
of the signal band. Specifically, in one implementation, the tuning
generator is triggered by the controller to in effect generate an
arbitrary waveform. The waveform, however, is selected so that the
pass band center wavelength changes linearly with time over as
least a portion of the scan. Such filter tuning voltage generators
map an inverse of a wavelength tuning function of the optical
tunable filter. For example, in one implementation, the generator
is a look-up table. Alternatively, the filter tuning voltage
generators map an inverse of a frequency tuning function of the
optical tunable filter. For example, in one implementation, the
generator is a look-up table.
[0014] In general, according to another aspect, the invention can
also be characterized in the context of a method for analyzing a
WDM signal. Such method comprises tuning a pass band of a filter
across a signal band of the WDM signal to generate a filtered
signal. The filtered signal is then detected and a level of the
detection signal is compared to a threshold. Finally, an
instantaneous pass band of the filter is compared to a level of the
detection signal relative to the threshold to analyze a channel
inventory in the WDM signal.
[0015] The above and other features of the invention including
various novel details of construction and combinations of parts,
and other advantages, will now be more particularly described with
reference to the accompanying drawings and pointed out in the
claims. It will be understood that the articular method and device
embodying the invention are shown by way of illustration and not as
a limitation of the invention. The principles and features of this
invention may be employed in various and numerous embodiments
without departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] In the accompanying drawings, reference characters refer to
the same parts throughout the different views. The drawings are not
necessarily to scale; emphasis has instead been placed upon
illustrating the principles of the invention. Of the drawings:
[0017] FIG. 1 is a block diagram showing a scanning optical
monitoring system of the present invention;
[0018] FIG. 2A is a plot of the pass band center wavelength in
nanometers as a function of tuning voltage in Volts for the
Fabry-Perot tunable filter;
[0019] FIG. 2B is a plot of tuning voltage in Volts as a function
of time in milliseconds to yield the linearization of the present
invention; and
[0020] FIG. 2C is a plot of pass band in nanometers as a function
of time in milliseconds showing the resultant linearized pass band
tuning of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] FIG. 1 is a schematic block diagram illustrating an optical
band scanning monitoring system 100, which has been constructed
according to the principles of the present invention.
[0022] Specifically, a WDM signal 10 is received by a tunable
optical filter 110. In preferred embodiment, this filter is an
electrostatically deflected Fabry-Perot device and is preferably a
high finesse device that has a tunable pass band that is narrow to
resolve the individual channels in the WDM signal. In one
implementation, the WDM signal is a DWDM signal that has multiple
channel slots across a signal band of the WDM system. In the
illustration of FIG. 1, the WDM signal has 100 GHz channel spacings
according to the ITU grid or 50 GHz spacing according to the 50 GHz
offset. Specifically, in such an implementation, the pass band of
the tunable filter 110 is less than 25 GHz, and preferably less
then 5 GHz.
[0023] In one embodiment, the free spectral range of the filter 110
is greater than the signal band of the WDM signal. Thus, it is
preferably greater than 100 nanometers (nm), preferably about 120
nm. In another embodiment, the free spectral range is set to be
about one half of the signal band, with two modes being used to
scan the signal band. In this second implementation, a WDM filter
is used along with two electronic channels to allow for
simultaneous scanning. This general configuration is illustrated in
U.S. patent application Ser. No. 09/648,263, filed on Aug. 25,
2000, entitled Optical Channel Monitoring System with Simultaneous
C-Band and L-Band Detection, by Flanders, et al., the teachings of
which are incorporated herein in their entirety by this
reference.
[0024] The tunable filter 110 applies the graphically illustrated
pass band 112 to yield a filtered optical signal 114. This is
detected by photodetector 116. This photodetector or transducer
converts the optical signal to an electrical signal 118.
[0025] The electrical signal that is generated in response to the
filtered optical signal is received by a low pass filter 120.
Specifically, this is an integrate-and-dump receiver, which has a
time constant that is a function of the scan speed of the pass band
and the bandwidth of the scanned channels.
[0026] The output from the low pass filter 120 is received by a
decision circuit 130. This decision circuit preferably has a
variable threshold. In its simplest implementation, this is simply
an operational amplifier that is logic high or logic low depending
on whether or not the signal from the low pass filter is above or
below a threshold voltage that has been set by a digital to analog
converter, for example. In the preferred embodiment of the system,
the controller sets the level of the threshold. Further, by varying
the threshold between scans, the controller can determine the power
of the signals in the channel slots.
[0027] One advantage of the present invention is that it avoids the
need for an analog to digital converter between the photodiode 116
and the controller. While providing for fast sampling, scan-to-scan
delay can be large in systems using A/D converters because of the
latency associated with dumping the acquired data to the
controller. This can be avoided with the present invention.
Specifically, in one embodiment, the scans are performed in less
than millisecond. The threshold is changed between scans to thereby
assess the power of individual channels by reference to other
scans.
[0028] The decision circuit 130 produces a quasi-digital signal
132. It is a digital signal in the sense that it is either a logic
high or logic low state. These state changes, however, may not be
synchronized to any system clock of a controller. In one
embodiment, signal 132 from the decision circuit 130 is received by
a channel detect-latch 134. This latches the logic high or logic
low signal 132 from the decision circuit 130 based upon a channel
trigger from timing recovery circuit 136. This allows the
controller 150 to sample the latched signal.
[0029] The controller 150 and recovery circuit 136 function as a
phase locked loop that triggers the latch 134 when the pass band of
the tunable filter is coincident with a channel slot in the signal
band of the WDM signal 10. The timing recovery circuit phase locks
on the time series generated by the scanning across the channel
slots in the WDM signal 10.
[0030] The controller 150 by sampling the state of the channel
detect latch as the pass band passes over channels in signal 10 is
able to inventory the populated wavelength slots. As a result, it
is able to determine which channels in the ITU grid contain actual
optical carrier signals by reference to whether or not the power is
above or below the applied threshold. This actual accumulated
inventory information of the WDM signal 10 is then compared to
perpetual inventory information 20 received from a system
controller. This perpetual inventory information is the inventory
that the controller 150 should have found in the DWDM signal,
assuming the proper-operation of upstream transmitting devices. In
this way, the controller can identify faults when channels are
present that should not be present by reference to the perpetual
inventory or contrastingly, when channels are not present but are
indicated as being present by the perpetual inventory
information.
[0031] According to another aspect of the invention, the system 100
further comprises a filter tuning voltage generator 160. This
generator 160 receives a scanstart trigger and/or voltage per
second selection signal from the controller 150. Specifically, it
generates through, preferably, a digital-to-analog converter 162
the tuning voltage to the tunable filter 110. Specifically, this
tuning voltage is used to generate an electrostatic drive voltage
in the tunable filter that causes the deflection of an optical
membrane to yield the Fabry-Perot tunable cavity filter
functionality.
[0032] In a preferred embodiment, the voltage generator 160
generates a tuning voltage to improve a linearization of the tuning
of the pass band as a function of time over at least a portion of a
scan of the signal band. This functionality is illustrated by
reference to FIGS. 2A-2C.
[0033] By reference to FIG. 2A, typically, there is a non-linear
relationship between the tuning voltage on the horizontal axis and
the pass band center frequency of the tunable filter 110. This is
due to the electrostatic characteristics of the drive cavity of
these tunable filters and the membrane's mechanical
characteristics. As a result, incremental changes in the tuning
voltage in the early part of the scans, such as around 1550 to 1560
nanometers (nm) yield small shifts in the pass band center. In
contrast, near the end of the scan as, for example, between 1620
and 1630 nm, relatively small changes in voltage yield large jumps
in the pass band center.
[0034] According to one aspect, the tuning voltage generator 160
stores an inverse tuning characteristic as illustrated in FIG. 2B.
Specifically, this function has the effect of yielding large
changes in voltage as a function of time an early part of the scan
and relatively small changes in the voltage as a function of time
during the later part of the scan. As a result, when a trigger
signal is applied to the tuning voltage generator, the stored
function, as illustrated in FIG. 2B is applied to the filter
110.
[0035] As shown by FIG. 2C, when the inverse tuning characteristic
is applied to the tunable filter, a linear frequency tuning
relationship results, i.e., the tuning of the pass band is linear
with time. In one implementation, the tuning is linear with
wavelength in time, as illustrated. Alternatively, the tuning is
linear with frequency in time. As a result, this allows the time
recovery circuit 136 to expect a consistent or near consistent
channel-to-channel delay across all or part of the scan allowing it
to phase lock onto the power peak series as if it were a clock
series to thereby control the latch 134. Further, the controller
and recovery circuit are able to identify whether or not a slot is
populated by reference to the delay from the generation of the
start trigger to the inverse function generator 160.
[0036] Generally, whether the filter is linearized in frequency or
wavelength depends on the application. Modern WDM systems specify
channel spacings in frequency, whereas spectral analysis typically
bases analysis in wavelength.
[0037] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *