Title: Optical transmission systems, optical receivers and receiving methods
Abstract: Optical systems of the present invention include an electrical signal distortion compensator configured to electrically distort an electrical signal to offset optical distortion imposed by a Fabry-Perot filter on an optical signal corresponding to the electrical signal. The electrical signal distortion compensator can be used in an optical transmitter to distort the electrical signal prior to optical transmission, or in an optical receiver after optical transmission. The distortion compensation can be performed on a baseband signal or a modulated electrical carrier. Likewise, the distortion compensator can be deployed in combination with an optical receiver, which allows the use of the F-P filter-optical receiver combination with transmitters and receivers that do not include F-P filters or distortion compensators. For example, F-P filter/receivers can be used in receiver terminals or regenerators, at various signal monitoring points including amplifier sites, in add/drop devices, and as an optical spectrum analyzer at the monitoring points and optical nodes.
Patent Number: 6,980,741 Issued on 12/27/2005 to Price
| Inventors:
|
Price; Alistair J. (Columbia, MD)
|
| Assignee:
|
Corvis Corporation (Columbia, MD)
|
| Appl. No.:
|
384939 |
| Filed:
|
March 10, 2003 |
| Current U.S. Class: |
398/83; 398/147; 398/148 |
| Intern'l Class: |
H04J 014/02 |
| Field of Search: |
398/83,141,158,171,178,200,147,148
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Tweel; John
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 09/398,540,
filed Sep. 17, 1999, now U.S. Pat. No. 6,577,423.
Claims
1. An optical system comprising:
a tunable transmitter configured to transmit information carried by a first electrical
signal via an optical signal channel;
a receiver including a tunable Fabry-Perot filter for converting the optical
signal to a second electrical signal;
a distortion compensator for electrically distorting one of the first and the
second electrical signals to compensate for optical distortion introduced into
the optical signal by the tunable Fabry-Perot filter; and
a network management system configured to control the tunable transmitter and
the tunable Fabry-Perot filter for remotely provisioning channels for the optical
system.
2. The optical system of claim 1, wherein the network management system is further
for rerouting communication traffic in the optical system to accommodate operating
conditions in the optical system.
3. The system of claim 1, wherein the operating conditions in the optical system
include a changes in an optical traffic pattern in the optical system.
4. The system of claim 1, wherein the operating conditions in the optical system
include a service disruption in the optical system.
5. The system of claim 1, wherein the distortion compensator is distorting the
electrical signal according to a function described by:
6. The system of claim 1, wherein the distortion compensator is in communication
with the tunable transmitter and is for distorting the first electrical signal
to compensate for the optical distortion introduced by the tunable Fabry-Perot filter.
7. The system of claim 1, wherein the distortion compensator is in communication
with the tunable Fabry-Perot filter and is for distorting the second electrical
signal to compensate for the optical distortion introduced by the tunable Fabry-Perot filter.
8. An optical system comprising:
a tunable transmitter configured to transmit information carried by a first electrical
signal via an optical signal channel;
a receiver including a tunable Fabry-Perot filter for converting the optical
signal to a second electrical signal;
a distortion compensator configured to electrically distort one of the first
and the second electrical signal to compensate for optical distortion introduced
into the optical signal by the tunable Fabry-Perot filter; and
a network management system configured to receive signal characteristics provided
by the receiver and configured to control the tunable transmitter and configured
to control the tunable Fabry-Perot filter in the optical system for remotely provisioning
channels for the optical system.
9. The system of claim 8, wherein the tunable Fabry-Perot filter is set at a
fixed wavelength by the network management system.
10. The system of claim 8, wherein the tunable Fabry-Perot filter is further
for scanning the tunable Fabry-Perot filter through a portion of the signal channels.
11. The system of claim 8, wherein the signal characteristics include a bit error
rate of the signal channel.
12. The system of claim 8, wherein the signal characteristics include a signal
to noise ratio of the signal channel.
13. The system of claim 8, wherein the distortion compensator is distorting the
electrical signal according to a function described by:
14. The system of claim 8, wherein the distortion compensator is in communication
with the tunable transmitter and is for distorting the first electrical signal
to compensate for the optical distortion introduced by the tunable Fabry-Perot filter.
15. The system of claim 8, wherein the distortion compensator is in communication
with the tunable Fabry-Perot filter and is for distorting the second electrical
signal to compensate for the optical distortion introduced by the tunable Fabry-Perot filter.
16. An optical system comprising:
a tunable transmitter configured to transmit information carried by a first electrical
signal via an optical signal channel;
a receiver including a tunable Fabry-Perot filter for converting the optical
signal to a second electrical signal, wherein the tunable Fabry-Perot filter includes
an optical spectrum analyzer for providing monitoring data;
a distortion compensator for electrically distorting one of the first and the
second electrical signal to compensate for optical distortion introduced by the
optical signal by the tunable Fabry-Perot filter; and
a network management system configured to receive the monitoring data from the
optical spectrum analyzer and configured to control the tunable transmitter and
configured to control the tunable Fabry-Perot filter in the optical system for
remotely provisioning channels for the optical system.
17. The system of claim 16, wherein the monitoring data is provided for controlling
an optical component disposed along an optical link.
18. The system of claim 17, wherein the optical component is an optical amplifier.
19. The system of claim 16, wherein the monitoring data is provided for controlling
optical system operations.
20. The system of claim 16, wherein the monitoring data is provided to the network
management system via a system supervisory channel through an optical link.
21. The system of claim 16, wherein the monitoring data is provided to the network
management system via an optical node.
22. The system of claim 16, wherein the monitoring data is provided to the network
management system via a direct connection over a communication network.
23. The system of claim 22, wherein the communication network is WAN.
24. The system of claim 16, wherein the distortion compensator is in communication
with the tunable transmitter and is for distorting the first electrical signal
to compensate for the optical distortion introduced by the tunable Fabry-Perot filter.
25. The system of claim 16, wherein the distortion compensator is in communication
with the tunable Fabry-Perot filter and is for distorting the second electrical
signal to compensate for the optical distortion introduced by the tunable Fabry-Perot filter.
Description
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
The present invention is directed generally to communication systems. More particularly,
the invention relates to wavelength selection and receiving techniques for use
in optical receivers and transmission systems.
The continued development of digital technology has provided electronic access
to vast amounts of information. The increased access to information has driven
demand for faster and higher capacity electronic information processing equipment
(computers) and transmission networks and systems linking the processing equipment
(telephone lines, cable television (CATV) systems, local, wide and metropolitan
area networks (LAN, WAN, and MAN)).
In response to this demand, communications companies have turned to optical transmission
systems to provide substantially larger transmission capacities than traditional
electrical communication systems. Early optical transmission systems, known as
space division multiplex (SDM) systems, transmitted one information signal using
a single wavelength in single waveguide, i.e. fiber optic strand. Time division
multiplexing (TDM) multiple low bit rate, information signals onto a single wavelength
in a known sequence that can be separated upon receipt has further increased the
transmission capacity of optical systems.
The continued growth in traditional voice, video, and data communications systems
and the emergence of the Internet as a means for accessing data has further accelerated
the demand for higher capacity transmission systems. Communications service providers,
especially long distance telecommunications companies, have looked to wavelength
division multiplexing (WDM) to further increase the capacity of their existing systems.
Additional transmission capacity is added to WDM systems by increasing
the number of information carrying optical signal wavelengths, or signal channels,
used in the system. Generally, unique optical transmitter/receiver pairs operated
at fixed transmit/receive wavelengths are deployed to provide additional signal
channels in WDM systems. The transmitters and receivers used in the WDM systems
are generally the same in construction, except for the wavelength transmitted or
received. Different wavelength optical sources or selective devices are provided
in the transmitters and corresponding different optical filters or local oscillators
are provided in the optical receivers to provide the different signal channels.
In optical systems, one of the more common techniques for selecting individual
wavelength signal channelsinvolves the use of grating technology, usually fiber
Bragg gratings ("FBG"). Fiber Bragg gratings have proven to be extremely useful
wavelength selective devices, because the fiber gratings can be spliced directly
into a transmission fiber and used to provide nearly distortion free separation
and stabilization of optical signal wavelengths. Also, fiber Bragg gratings can
be produced having well controlled reflectivities and reflective bandwidths. These
attributes make Bragg gratings very well suited for use as optical filters in optical
receivers and wavelength stabilizers in optical transmitters. See U.S. Pat. No. 5,077,816.
A current shortcoming of fiber Bragg gratings is the reflective wavelengths can
only be efficiently tuned over a relatively narrow range, typically around 1 nm.
It is therefore necessary to provide different fiber Bragg gratings for each different
wavelength that must be separated or stabilized in the WDM system.
The need to use different Bragg gratings for each wavelength increases the complexity
of manufacturing and maintenance of WDM systems. Whereas, a broadly tunable filter
would streamline filter manufacturing by allowing the same device to be manufactured
and then tuned to a desired operating wavelength when deployed in WDM systems.
Tunable filters can also provide wavelength agility in optical transmitters and
receivers, which allows flexible wavelength allocation and network planning and
protection in WDM systems.
Another wavelength filtering technique employs Fabry-Perot ("F-P") filters,
which can be used to separate wavelengths from the WDM signal. Unlike Bragg gratings,
Fabry-Perot filters can be tuned over a relatively wide wavelength range. However,
narrow bandwidth F-P filters can introduce unacceptable levels of distortion into
the filtered optical signals. As such, narrow bandwidth F-P filters havegenerally
been limited to use in non-signal processing applications, such as in optical spectrum
analyzers and other power measurement devices, and laboratory apparatuses and test systems.
The lack of a robust tunable wavelength selection technique constrains current
WDM system designs and manufacturing capabilities. It will become increasingly
necessary to provide tunable wavelength selection techniques to facilitate continued
growth in WDM system capacity and capability. In view of these present constraints,
there is a clear need for improved wavelength selection techniques and optical
receivers and systems to facilitate the development of higher capacity, longer
distance optical communication systems.
SUMMARY OF THE INVENTION
The apparatuses and methods of the present invention address the above need for
tunable wavelength selection techniques, optical receivers and optical systems
for use therein. Optical systems of the present invention generally include an
electrical signal distortion compensator configured to electrically distort an
electrical signal to offset optical distortion imposed by a Fabry-Perot filter
on an optical signal corresponding to the electrical signal.
The electrical signal distortion compensator can be used in an optical transmitter
to distort the electrical signal prior to optical transmission, or in an optical
receiver after optical transmission. The distortion compensation can be performed
on a baseband signal or a modulated electrical carrier. Likewise, the distortion
compensator can be deployed in combination with an optical receiver, which allows
the use of the F-P filter-optical receiver combination with transmitters and receivers
that do not include F-P filters or distortion compensators.
The distortion compensator can be embodied as group delay equalizer shaped to
offset group delay response of the Fabry-Perot filter. The distortion compensator
can be used in combination with fixed and tunable F-P filters at various locations
along an optical link. For example, F-P filter/receivers can be used in receiver
terminals, regenerators, and add/drop devices, at various signal monitoring points
including amplifier sites, and as an optical spectrum analyzer at the monitoring
points and optical nodes.
Accordingly, the present invention addresses the aforementioned needs
for improved wavelength selection techniques, optical receivers, and optical systems
to increase the efficiency and capacity of optical components and communication
systems without commensurate increases in the cost of optical components. These
advantages and others will become apparent from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described, by way of
example only, with reference to the accompanying drawings for the purpose of illustrating
embodiments only and not for purposes of limiting the same; wherein like members
bear like reference numerals and:
FIGS. 1(
a&
b) show the distortion compensator and receiver embodiments;
FIGS. 2 and 3 show optical system embodiments;
FIGS. 4(
a&
b) show optical transmitter embodiments;
FIGS. 5(
a&
b) show optical receiver embodiments;
FIGS. 6(
a-c) show response and group delay distortion plots for
an exemplary filter design, a F-P filter, and an electrical distortion compensator;
FIG. 7 is an exemplary circuit embodiment of distortion compensators;
FIGS. 8(
a&
b) are detected optical signal eye patterns
without and with electronic distortion compensation; and,
FIGS. 9(
a-d) show exemplary add and/or drop device embodiments.
DETAILED DESCRIPTION OF THE INVENTION
Optical systems
10 of the present invention include an electrical
distortion compensator
12 and an optical receiver
14 configured to
receive and convert an optical signal into a corresponding electrical signal (FIG.
1(
a)). The receiver
14 generally includes a Fabry-Perot filter
16, which is typically a fiber device, configured to selectively pass one
or more wavelengths to an optical to electrical converter
18, such as a
photodiode (FIG.
1(
b)).
As shown in FIG. 2, the system
10 will also generally include one or more
optical transmitters
20 configured to transmit information via one or more
information carrying optical signal wavelengths, or signal channels, λ
i
through an optical fiber
22 to the receiver
14. One or more optical
amplifiers
24 may be deployed to optically regenerate attenuated optical
signals in optical links
26 between optical nodes
28, which may include
transmitters
20 and/or receivers
14.
Wavelength selective and non-selective optical combiners
30 and
distributors
32, such as multiplexers, couplers, demultiplexers, and splitters,
can be provided to combine signal channels from multiple transmitters
20
or fibers
22 and distribute signal channels among multiple receivers
14
or fibers
22. The optical nodes
28 may also include other optical
components, such as one or more add/drop devices
34 and optical switches
36 interconnecting the transmitters
20 and receivers
14. For
example, broadcast and/or wavelength reusable, add/drop devices, and optical and
electrical/digital cross connect switches and routers can be deployed in the system
10 as necessary. As shown in FIG. 3, the system
10 can be embodied
as a network in which communications traffic is routed in various topologies, i.e.,
rings, mesh, etc., and controlled by a network management system
38. The
transmitters
20 and receivers
14 can interface directly with electrical
transmission systems or via electrical or optical switches or interfaces to other
optical systems that operate using the same or different wavelengths.
In various embodiments, the signal distortion compensator
12 can be provided
before or included in the transmitter
20 and configured to electrically
distort the electrical signal before it is imparted onto an optical signal wavelength,
FIGS.
4(
a) and
4(
b), respectively. Distortion compensation
can be performed when the electrical signal Λ
E is in various baseband
formats (i.e., RZ, NRZ, duobinary, and various shift key formats), as well as after
the electrical signal is modulated onto an electrical carrier (FIG.
4).
The distorted electrical signal Λ
ED can be imparted onto an optical
signal channel λ
i by directly or externally modulating an optical
source
40, such as a laser, to produce a distorted optical signal Λ
OD.
Alternatively, the distorted electrical signal Λ
ED can be upconverted
onto the optical signal channel λ
i using an optical source
40
having a different optical wavelength than the signal channel.
Likewise, the distortion compensator
12 can be used to compensate
for optical distortion after an undistorted optical signal Λ
O
has been filtered using the F-P filter
16. A distorted optical signal Λ
OD
provided by the F-P filter
16 is converted to a distorted electrical
signal Λ
ED by the receiver
14 (FIGS.
5(
a&
b)).
The compensator
12 distorts the distorted electrical signal Λ
ED
to compensate for the optical distortion and produces a distortion compensated
electrical signal Λ
E. The compensator
12 can be configured
to provide distortion compensation for both coherently and directly detected optical
signals, as will be further discussed below.
The distortion compensator
12 can be designed to apply various distortion
patterns to the electrical signal time FIG.
6(
a) shows an example
of a linear phase filter having a response and group delay suitable for use in
optical systems. In contrast, a typical response and group delay of a F-P filter
16 is approximately equal to the first pole of the linear phase filter,
which is shown in FIG.
6(
b). As is shown and observed in practice,
the F-P filter
16 group delay response is not constant resulting in signal
distortion that often renders the F-P filters unusable for many signal processing
applications. However, the distortion compensator
12 of the present invention
are designed to impart a complementary group delay, as shown in FIG.
6(
c),
to offset the distortion from the F-P filter
16, thereby providing a substantially
less distorted and usable signal.
Fabry-Perot filters
16 are generally known in the art, for example,
see U.S. Pat. Nos. 5,212,745, 5,212,746, 5,289,552, 5,375,181, 5,25,039, 5,509,093,
5,563,973, and 5,838,437, which are incorporated herein by reference. Both fixed
and scanning/tunable wavelength F-P filters are commercially available from various
vendors, such as Micron Optics, Inc. of Atlanta, Ga.
Distortion compensators of the present invention can be described by the
general equation:
##EQU1##
Z=impedance
L=inductance
R=resistance
C=capacitance
f0=frequency
Q=Q factor
D(ω)=group delay
H(s)=Transfer function
An exemplary embodiment of the electrical distortion compensator
12 to
compensate for distortion caused by a Fabry-Perot filter is shown in FIG.
7.
Testing was performed to compare the performance of F-P filter
16
in the optical transmission system
10 without and with the distortion compensator
12 of the present invention. An optical signal was generated, transmitted
through a span of optical fiber, and filtered using a GHz fiber F-P filter. A broadband
receiver was used to detect the optical signal and convert it to an electrical
signal. No distortion compensation was performed and the resulting detected signal
eye pattern is shown in FIG.
8(
a).
A pole/zero transfer function analysis was performed to establish the impedance
values (R
1=5Ω, L
1=L
2=7.6 mH, C
1=100
pF, C
2=C
3=2.6 pF) for the circuit shown in FIG. 7 to compensate
for the fiber F-P filter. The resulting distortion compensator was used to distort
an electrical signal, which was used to generate an optical signal that was transmitted
through the optical fiber. The optical signal was filtered using the 2.5 GHz F-P
filter and detected using a conventional photodiode optical receiver, i.e., broadband
and SONET receivers. The detected eye pattern shown in FIG.
8(
b)
shows a substantial decrease in the signal distortion results from the use of the
distortion compensator
12.
In various embodiments, the receivers
14 include tunable F-P filters
16
that are used in combination with tunable transmitters
20 to provide flexibility
in wavelength allocation in the optical system
10. In optical systems
10
employing these embodiments, the network management system
38 can be used
to set the wavelengths of the tunable transmitters
20 and the F-P filters
16 in the receivers
14 to achieve a desired wavelength provisioning
plan for the system
10. The network management system
38 can also
be used to groom or reroute communications traffic by tuning the operating wavelength
of the transmitter
20 and the F-P filters
16 in the receiver
14
to accommodate changes in traffic patterns or service disruptions in the network.
In other embodiments, receivers
14 with F-P filters
16 are provided
at various monitoring points, such as amplifier sites, along the optical fiber
22 path. The F-P filters
16 at the monitoring sites can be set at
a fixed wavelength or scanned through at least a portion of the signal channels
λ
i to provide signal characteristics, i.e., bit error rate, signal
to noise ratio, etc., as the signal channels traverse the optical link
26.
When the receivers
14 include the distortion compensator
12 to
compensate for distortion following reception, the receiver/compensator can be
used in combination with other transmitters and receivers that do not include F-P
filters
16 and compensators
12. In addition, it may be desirable
to provide distortion compensation following a direct detection receiver, which
provides a flexible receiver without the use of a local oscillator. Amplitude compensation
in the direct detection configurations can often be sufficient to provide acceptable
signal quality without having to account for phase distortion introduced by the
F-P filter
16.
It may also be desirable to configure the distortion compensator
12 to
compensate for only a portion of the distortion, as in the direct detection embodiment.
The present invention can also be effectively implemented with other electrical
and optical compensation techniques used in optical systems. For example, the distortion
compensator
12 can be further configured to compensate for at least a portion
of the optical distortion that occurs as a result of chromatic dispersion and non-linear
effects in the fiber.
Receivers
14 with tunable F-P filters
16 can also be deployed
as optical spectrum analyzers
42 to provide monitoring data for controlling
amplifiers
24 and other optical components, as well as system operations.
The monitoring data can also be sent to the network management system
38
via either a system supervisory channel through the optical fiber
22 or
the optical nodes
26 or a direct connection using a WAN or other communication network.
The optical spectrum analyzers
42 of the present invention provide not
only the usual capabilities of power level detection across the spectrum, but further
provides error checking capability at each monitoring point. The ability to detect
errors at the various monitoring points allows specific spans between monitoring
points to be identified as the source of the errors.
The receivers
14 and F-P filters
16 can be used to provide fixed
or tunable add/drop multiplexers
34 as shown in FIG.
9. The F-P filter
16 can be used in combination with one or more directional components, such
as circulators
46 and couplers
48, to add and/or drop signal wavelengths
in the pass band of the filter
16. For example, the embodiment shown in
FIG. 9
a can employ a fiber Fabry-Perot filter
16 to transmit a drop
wavelength λ
i to the receiver
14, while reflecting the
remaining wavelength λ
k. In FIG. 9
b embodiments, the F-P
filter
16 is used to extract the drop wavelength λ
i, as
well as insert add wavelengths at λ
i. FIGS.
9c-d embodiments
employ couplers
48 and isolators
50, and circulators
46, respectively,
with the F-P filter
16 to extract the drop wavelength λ
i and
couplers
48 to add signal wavelengths λ
j.
In various embodiments, the transmitter
20 is configured to encode the
information being transmitted to allow for error correction following reception.
Error correction schemes, such as forward error correction, can be implemented
to increase the transmission error tolerance of the system
10 and to provide
another performance metric for optical spectrum analysis. At the monitoring points,
the error correction schemes can track the introduction of errors into each span
along the fiber
22 and report the error information to the network management
system
38.
The wavelength selection and compensation techniques of the present invention
provide increased flexibility and capability in optical systems. The increased
flexibility can also help ameliorate component supply problems in WDM systems by
providing a common receiver that can be deployed for use over a portion or all
of the wavelength range in the WDM system.
Those of ordinary skill in the art will further appreciate that numerous modifications
and variations that can be made to specific aspects of the present invention without
departing from the scope of the present invention. It is intended that the foregoing
specification and the following claims cover such modifications and variations.
*
