CA2385452A1 - Method and system for tuning an optical signal based on transmission conditions - Google Patents
Method and system for tuning an optical signal based on transmission conditions Download PDFInfo
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- CA2385452A1 CA2385452A1 CA002385452A CA2385452A CA2385452A1 CA 2385452 A1 CA2385452 A1 CA 2385452A1 CA 002385452 A CA002385452 A CA 002385452A CA 2385452 A CA2385452 A CA 2385452A CA 2385452 A1 CA2385452 A1 CA 2385452A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/506—Multiwavelength transmitters
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/58—Compensation for non-linear transmitter output
- H04B10/588—Compensation for non-linear transmitter output in external modulation systems
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Abstract
A method and system for tuning an optical signal based on transmission conditions includes receiving information indicative of transmission conditions of an optical link. A modulation characteristic of traffic transmitted over the transmission link is adjusted based on the information.
Description
METHOD AND SYSTEM FOR TUNING AN OPTICAL
SIGNAL BASED ON 'TRANSMISSION CONDITIONS
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to optical communication systems, and more particularly to a method and system for tuning an optical signal based on transmission conditions.
BACKGROUND OF THE INVENTION
Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss.
Optical networks often employ wavelength division multiplexing (WDM) t.o increase transmission capacity. In a WDM network, a number of optical channels are carried in each fiber at di~~parate wavelengths. Network capacity is increased as a multiple of the number of wavelengths, or channels, in each fiber.
The maximum distance that a signal can be transmitted in a WIDM or other optical network without amplification is limited by absorption, scattering and other loss associated with the optical fiber. To transmit signals over long distances, optical networks typically include a number of amplifiers spaced along each fiber route. 'The amplifiers boost received signals to compensate for transmission losses in the fiber.
SIGNAL BASED ON 'TRANSMISSION CONDITIONS
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to optical communication systems, and more particularly to a method and system for tuning an optical signal based on transmission conditions.
BACKGROUND OF THE INVENTION
Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss.
Optical networks often employ wavelength division multiplexing (WDM) t.o increase transmission capacity. In a WDM network, a number of optical channels are carried in each fiber at di~~parate wavelengths. Network capacity is increased as a multiple of the number of wavelengths, or channels, in each fiber.
The maximum distance that a signal can be transmitted in a WIDM or other optical network without amplification is limited by absorption, scattering and other loss associated with the optical fiber. To transmit signals over long distances, optical networks typically include a number of amplifiers spaced along each fiber route. 'The amplifiers boost received signals to compensate for transmission losses in the fiber.
A problem with optical amplifiers, however, is that signals accumulate a number of impairments along the length of the fiber. Such impairments include chromatic dispersion and non--linear effects.
SUMMARY OF THE INVEnfTION
The present invention provides a method and system for tuning an optical signal based on transmission conditions that substantially eliminate or reduce problems and disadvantages associated with previous methods and systems. In a particular embodiment, signal modulation at the transmitter is fine-tuned according to receiver side feedb<~ck to enhance system performance and minimize unpredictable effects.
In accordance with one embodiment of the present invention, a method and system for tuning an optical signal based on transmission conditions includes receiving information indicative of transmission conditions of an optical link. A modulation characteristic of traffic transmitted over the transmission link is adjusted based on the information.
More specifica7Lly, in accordance with a particular embodiment of the pz-esent invention, the modulation depth of the traffic is adjusted. The modulation depth may be the phase modulation depth, frequency modulation depth, intensity modulatlOIl depth, or depth of other suitable modulation characteristic. In addition, a plurality of modulation depths of the traffic may be adjusted based on the information.
Technical advantages of the present invention include providing a method system f.or tuning an optical signal based on transmission conditions. In a particular embodiment, signal parameters are optimized for current transmission conditions by fine-tuning signal modulation based on receiver ~;ide feed back. Accordingly, systems performance is enhanced and unpredictable effects minimized. In addition., signals may be transmitted longer distances without regeneration which improves transmission efficiency arid reduces transmission cost.
Another technical advantage of one or more embodiments of the present: invention include providing an improved transmitter and receiver pair for optical networks. In particular, the receiver provides feed back to the transmitter based on received signal quality in real time. The transmitter adjusts modulation depth or other suitable parameters of transmitted signals based on the receiver feed back to minimize signal degradation during transmission.
Still another technical advantage of the present invention includes providing an improved optical information signal for transmission over an optical link.
In particular, the modulation depth of the signal is configured to account for current transmission conditions of the link. Accordingly, degradation of the signal is minimized during transmission and the signal may be transmitted over longer distances without regeneration.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like numerals represent like parts, in which:
SUMMARY OF THE INVEnfTION
The present invention provides a method and system for tuning an optical signal based on transmission conditions that substantially eliminate or reduce problems and disadvantages associated with previous methods and systems. In a particular embodiment, signal modulation at the transmitter is fine-tuned according to receiver side feedb<~ck to enhance system performance and minimize unpredictable effects.
In accordance with one embodiment of the present invention, a method and system for tuning an optical signal based on transmission conditions includes receiving information indicative of transmission conditions of an optical link. A modulation characteristic of traffic transmitted over the transmission link is adjusted based on the information.
More specifica7Lly, in accordance with a particular embodiment of the pz-esent invention, the modulation depth of the traffic is adjusted. The modulation depth may be the phase modulation depth, frequency modulation depth, intensity modulatlOIl depth, or depth of other suitable modulation characteristic. In addition, a plurality of modulation depths of the traffic may be adjusted based on the information.
Technical advantages of the present invention include providing a method system f.or tuning an optical signal based on transmission conditions. In a particular embodiment, signal parameters are optimized for current transmission conditions by fine-tuning signal modulation based on receiver ~;ide feed back. Accordingly, systems performance is enhanced and unpredictable effects minimized. In addition., signals may be transmitted longer distances without regeneration which improves transmission efficiency arid reduces transmission cost.
Another technical advantage of one or more embodiments of the present: invention include providing an improved transmitter and receiver pair for optical networks. In particular, the receiver provides feed back to the transmitter based on received signal quality in real time. The transmitter adjusts modulation depth or other suitable parameters of transmitted signals based on the receiver feed back to minimize signal degradation during transmission.
Still another technical advantage of the present invention includes providing an improved optical information signal for transmission over an optical link.
In particular, the modulation depth of the signal is configured to account for current transmission conditions of the link. Accordingly, degradation of the signal is minimized during transmission and the signal may be transmitted over longer distances without regeneration.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like numerals represent like parts, in which:
FIGURE 1 is a block diagram illustrating an optical communication system using distributed amplification in accordance with one embodiment of the present invention;
FIGURE 2 is a block diagram illustrating the optical sender of FIGURE 1 in accordance with one embodiment of the present invention;
FIGURES 3A-C are diagrams illustrating non-intensity modulated signals for transmission in the optical communication system of FIGURE 1 in accordance with several embodiments of the present invention;
FIGURE 4 is a block diagram illustrating the optical sender of FIGURE 1 in accordance with another embodiment of the present invention;
FIGURE 5 is a diagram illustrating the optical waveform generated by the optical sender of FIGURE 4 in accordance with one embodiment of the present invention;
FIGURE 6 is a block diagram illustrating the optical receiver of FIGURE 1. in accordance with one embodiment of the present invention;
FIGURE 7 is a diagram illustrating the frequency response of the asymmetric Mach-Zender interferometer of FIGURE 6 in accordance with one embodiment of the present invention;
FIGURES 8A-C are block diagrams illustrating the demultiplexer of F:LGURE 1 in accordance with several embodiments of the present invention;
FIGURE 9 is a flow diagram illustrating a method for communicating data over an optical communication system using distributed amplification in accordance with one embodiment of the present invention;
FIGURE 10 is a block diagram illustrating a bi-directional optical communication system using distributed amplificatic>n in accordance with one embodiment of the present invention;
FIGURE 11 is a block diagram illustrating the optical sender and receiver of FIGURE 1 in accordance 5 with another embodiment of the present invention;
FIGURE 12 is a block diagram illustrating the modulator of FIGURE 11 in accordance with one embodiment of the present invention;
FIGURE 13 is a flow diagram illustrating a method for tuning the modulation depth of an optical signal based on receiver side information in accordance with one embodiment of the present invention;
FIGURE 14 is a block diagram illustrating an optical communication system distributing a clock signal in an information channel in accordance with one embodiment of the present invention; and FIGURE 15 is a block diagram illustrating an optical receiver for extracting a clock signal from a multimodulated signal in accordance with one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIGURE 1 illustrates an optical communication system 10 in accordance with one embodiment of the present invention. In this embodiment, the optical communication system 10 is a wavelength division multiplexed (WDM) system in which a number of optical channels are carried over a common path at disparate wavelengths . It will be understood that the optical communication system 10 may comprise other suitable single channel, multichannel or bi-directional transmission systems.
Referring to FIGURE 1, the WDM system 10 includes a WDM transmitter 12 at a source end point and a WDM
FIGURE 2 is a block diagram illustrating the optical sender of FIGURE 1 in accordance with one embodiment of the present invention;
FIGURES 3A-C are diagrams illustrating non-intensity modulated signals for transmission in the optical communication system of FIGURE 1 in accordance with several embodiments of the present invention;
FIGURE 4 is a block diagram illustrating the optical sender of FIGURE 1 in accordance with another embodiment of the present invention;
FIGURE 5 is a diagram illustrating the optical waveform generated by the optical sender of FIGURE 4 in accordance with one embodiment of the present invention;
FIGURE 6 is a block diagram illustrating the optical receiver of FIGURE 1. in accordance with one embodiment of the present invention;
FIGURE 7 is a diagram illustrating the frequency response of the asymmetric Mach-Zender interferometer of FIGURE 6 in accordance with one embodiment of the present invention;
FIGURES 8A-C are block diagrams illustrating the demultiplexer of F:LGURE 1 in accordance with several embodiments of the present invention;
FIGURE 9 is a flow diagram illustrating a method for communicating data over an optical communication system using distributed amplification in accordance with one embodiment of the present invention;
FIGURE 10 is a block diagram illustrating a bi-directional optical communication system using distributed amplificatic>n in accordance with one embodiment of the present invention;
FIGURE 11 is a block diagram illustrating the optical sender and receiver of FIGURE 1 in accordance 5 with another embodiment of the present invention;
FIGURE 12 is a block diagram illustrating the modulator of FIGURE 11 in accordance with one embodiment of the present invention;
FIGURE 13 is a flow diagram illustrating a method for tuning the modulation depth of an optical signal based on receiver side information in accordance with one embodiment of the present invention;
FIGURE 14 is a block diagram illustrating an optical communication system distributing a clock signal in an information channel in accordance with one embodiment of the present invention; and FIGURE 15 is a block diagram illustrating an optical receiver for extracting a clock signal from a multimodulated signal in accordance with one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIGURE 1 illustrates an optical communication system 10 in accordance with one embodiment of the present invention. In this embodiment, the optical communication system 10 is a wavelength division multiplexed (WDM) system in which a number of optical channels are carried over a common path at disparate wavelengths . It will be understood that the optical communication system 10 may comprise other suitable single channel, multichannel or bi-directional transmission systems.
Referring to FIGURE 1, the WDM system 10 includes a WDM transmitter 12 at a source end point and a WDM
receiver 14 at a destination end point coupled together by an optical link. 16. The WDM transmitter 12 transmits data in a plurality of optical signals, or channels, over the optical link 16 to the remotely located WDM receiver 14. Spacing between the channels is selected to avoid or minimize cross talk between adjacent channels. In one embodiment, as described in more detail below, minimum channel spacing (df) c:omprises a multiple of the transmission symbol and/or bit rate (B) within 0.4 to 0.6 of an integer (N).. Expressed mathematically:
(N+0.4)B<df<(N+0.6)H. This suppresses neighboring channel cross talk.. It will be understood that channel spacing may be suitably varied without departing from the scope of the present invention.
The WDM transmitter 12 includes a plurality of optical senders 20 and a WDM multiplexer 22. Each optical sender 20 generates an optical information signal 24 on one of a set of distinct wavelengths ~
at the channel spacing. The optical information signals 24 comprise optical signals with at least one characteristic modulated t:o encode audio, video, textual, real-time, non-real-time or other suitable data. The optical information signals 24 are multiplexed into a single WDM signal 26 by the WDM multiplexer 22 for transmission on th.e optical link 16. It will be understood that the optical information signals 24 may be otherwise suitably combined into the WDM signal 26. The WDM signal is transmitted in the synchronous optical network (SONET) or other suitable format.
The WDM receiver 14 receives, separates and decodes the optical information signals 24 to recover the included data. In one embodiment, the WDM receiver 14 includes a WDM de~multiplexer 30 and a plurality of optical receivers 32. The WDM demultiplexer 30 demultiplexes the optical information signals 24 from the single WDM signal 26 and sends each optical information signal 24 to a corresponding optical receiver 32. Each optical receiver 32 optically or electrically recovers the encoded data from the corresponding signal 24. As used herein, the team each means every one of at least a subset of the identified items.
The optical link 16 comprises aptical fiber or other suitable medium i.n which optical signals may be transmitted with low loss. Interposed along the optical link 16 are one or more optical amplifiers 40. The optical amplifiers 40 increase the strength, or boost, one or more of the optical information signals 24, and thus the WDM signal 26, without the need for optical-to-electrical conversion.
In one embodiment, the optical amplifiers 40 comprise discrete amp,~ifiers 42 and distributed amplifiers 44. The discrete amplifiers 42 comprise rare earth doped fiber amplifiers, such as erbium doped fiber amplifiers (EDFAs), and other suitable amplifiers operable to amplify the WDM signal 26 at a point in the optical link 16.
The distributed. amplifiers 44 amplify the WDM signal 26 along an extended length of the optical link 16. In one embodiment, they distributed amplifiers 44 comprise bi-directional distributed Raman amplifiers (DRA). Each bi-directional DRA 44 includes a. forward, or co-pumping source laser 50 coupled to the optical link 16 at a beginning of the amplifier 44 and a backward, or counter-pumping source laser' 52 coupled to the optical link 16 at an end of the amplifier 9:4. It will be understood that the co-pumping and counter-pumping source lasers 50 and 52 may amplify disparate or only partially overlapping lengths of the optical link 16.
The Raman pump sources 50 and 52 comprise semiconductor or other suitable lasers capable of generating a pump light, or amplification signal, capable of amplifying the WDM signal 26 including one, more or all of the included optical information signals 24. The pump sources 50 anon 52 may be depolarized, polarization scrambled or pol<~rization multiplexed to minimize polarization sensitivity of Raman gain.
The amplification signal from the co-pumping laser 52 is launched in the direction of travel of the WDM
signal 26 and thus co-propagated with the WDM signal 26 at substantially the same speed and/or a slight or other suitable velocity mismat~~h. The amplification signal from the counter-pumping laser 52 is launched in a direction of travel opposite that of the WDM signal 26 and thus is counter-propagated with respect to the WDM
signal 26. The amplification signals may travel in opposite directions simultaneously at the same or other suitable speed.
The amplification signals comprise one or more high power lights or waves at a lower wavelength than the signal or signals to be amplified. As the amplification signal travels in the optical link 16, it scatters off atoms in the link 16, loses some energy to the atoms and continues with the same wavelength as the amplified signal or signals. In this way, the amplified signal acquires energy over many miles or kilometers in that it is represented by more photons. For the WDM signal 26, the co-pumping and counter-pumping lasers 50 and 52 may each comprise several different pump wavelengths that are used together to amplify .=ach of the wavelength distincts optical information signals 24.
In one embodiment, as described in more detail below, a non-intensity characteristic of a carrier signal is modulated with t:he data signal at each optical sender 20. The non-intensity characteristic comprises phase, frequency or other suitable characteristic with no or limited susceptibility to cross talk due to cross-gain modulation (XGM) from a forward pumping distributed amplifier or a bi-dir_ectional pumping distributed amplifier. The non-intensity modulated optical information signal may be further and/or remodulated with a clock or other non-data signal using an intensity modulator. Thus, the non-intensity modulated optical information signal may comprise intensity modulation of a non-data signal.
In a particular embodiment, as described in more detail below, the WDM signal 26 comprises phase or frequency modulated optic;~l information signals 24 which are amplified using the bi-directional DRAB 44 with no cross talk between the channels 24 due to XGM. In this embodiment, the bi-directional DRAB 44 provide amplification at a :superior optical signal-to-noise ratio and thus enable longer transmission distances and improved transmission performance.
FIGURE 2 illustrates details of the optical sender 20 in accordance with one embodiment of the present invention. In this embodiment, the optical sender 20 comprises a laser 70, a modulator 72 and a data signal 74. The laser 70 generates a carrier signal at a prescribed frequency with good wavelength control.
Typically, the wavelengths emitted by the laser 70 are selected to be within the 1500 nanometer (nm) range, the range at which the minimum signal attenuation occurs for silica-based optir_a.l fibers. More particularly, the wavelengths are generally selected to be in the range from 1310 to 1650 nm but may be suitably varied.
5 The modulator 72 modulates the carrier signal with the data signal 74 to generate the optical information signal 24. The modulator 72 may employ amplitude modulation, frequency modulation, phase modulation, intensity modulatlOIl, amplitude-shift keying, frequency-10 shift keying, pha~>e-shift keying and other suitable techniques for encoding the data signal 74 onto the carrier signal. In addition, it will be understood that different modulators 72 may employ more than one modulation system in combination.
In accordance with one embodiment, modulator 74 modulates the phrase, frequency or other suitable non-intensity characteristic of the carrier signal with the data signal 74. As previously described, this generates a non-intensity optical information signal 24 with poor susceptibility to cross talk due to XGM in long-haul and other transmission systems using bi-directional DRA or other distributed arnplification. Details of the carrier wave, frequency modulation of the carrier wave and phase modulation of the carrier wave are illustrated in FIGURES
3A-C.
Referring to FIGURE 3A, the carrier signal 76 is a completely periodic signal at the specified wavelength.
The carrier signal 76 has at least one characteristic that may be varied by modulation and is capable of carrying information via modulation.
Referring to FIGURE ..B, the frequency of the carrier signal 76 is modulated with a data signal 74 to generate a frequency modulated optical information signal 78. In frequency modulation, the frequency of the carrier signal 76 is shifted as a function of the data signal 74.
Frequency shift keying may be used in which the frequency of the carrier signal shifts between discrete states.
Referring to FIGURE 3C, the phase of the carrier signal 76 is modulated with a data signal 80 to generate a phase modulated optical information signal 82. In phase modulation, tl'ne phase of the carrier signal 76 is shifted as a function of the data signal 80. Phase shift keying may be used in which the phase of the carrier signal shifts between discrete states.
FIGURE 4 illustrates an optical sender 80 in accordance with another embodiment of the present invention. In this embodiment, data is phase or frequency modulated onto the carrier signal and then remodulated with in.tensit.y modulation synchronized with the signal clock to provide superior power tolerance in the transmission system.
Referring to FIGURE 4, the optical sender 80 includes a laser 82, a non-intensity modulator 84 and data signal 86. The non-intensity modulator 84 modulates the phase or frequency of the carrier signal from the laser 82 with the data signal 86. The resulting data modulated signal is passed to the intensity modulator 88 for remodulation with the clock frequency 90 to generate a dual or otherwise mult~imodulated optical information signal 92. Because the intensity modulation based on the clock is a non-random, completely periodic pattern, little or no cross talk due to XGM is generated by the DRAB 44 so long as there is a slight velocity mismatch in the forward pumping direction. FIGURE 5 illustrates the waveform of the dual. modulated optical information signal 92.
(N+0.4)B<df<(N+0.6)H. This suppresses neighboring channel cross talk.. It will be understood that channel spacing may be suitably varied without departing from the scope of the present invention.
The WDM transmitter 12 includes a plurality of optical senders 20 and a WDM multiplexer 22. Each optical sender 20 generates an optical information signal 24 on one of a set of distinct wavelengths ~
at the channel spacing. The optical information signals 24 comprise optical signals with at least one characteristic modulated t:o encode audio, video, textual, real-time, non-real-time or other suitable data. The optical information signals 24 are multiplexed into a single WDM signal 26 by the WDM multiplexer 22 for transmission on th.e optical link 16. It will be understood that the optical information signals 24 may be otherwise suitably combined into the WDM signal 26. The WDM signal is transmitted in the synchronous optical network (SONET) or other suitable format.
The WDM receiver 14 receives, separates and decodes the optical information signals 24 to recover the included data. In one embodiment, the WDM receiver 14 includes a WDM de~multiplexer 30 and a plurality of optical receivers 32. The WDM demultiplexer 30 demultiplexes the optical information signals 24 from the single WDM signal 26 and sends each optical information signal 24 to a corresponding optical receiver 32. Each optical receiver 32 optically or electrically recovers the encoded data from the corresponding signal 24. As used herein, the team each means every one of at least a subset of the identified items.
The optical link 16 comprises aptical fiber or other suitable medium i.n which optical signals may be transmitted with low loss. Interposed along the optical link 16 are one or more optical amplifiers 40. The optical amplifiers 40 increase the strength, or boost, one or more of the optical information signals 24, and thus the WDM signal 26, without the need for optical-to-electrical conversion.
In one embodiment, the optical amplifiers 40 comprise discrete amp,~ifiers 42 and distributed amplifiers 44. The discrete amplifiers 42 comprise rare earth doped fiber amplifiers, such as erbium doped fiber amplifiers (EDFAs), and other suitable amplifiers operable to amplify the WDM signal 26 at a point in the optical link 16.
The distributed. amplifiers 44 amplify the WDM signal 26 along an extended length of the optical link 16. In one embodiment, they distributed amplifiers 44 comprise bi-directional distributed Raman amplifiers (DRA). Each bi-directional DRA 44 includes a. forward, or co-pumping source laser 50 coupled to the optical link 16 at a beginning of the amplifier 44 and a backward, or counter-pumping source laser' 52 coupled to the optical link 16 at an end of the amplifier 9:4. It will be understood that the co-pumping and counter-pumping source lasers 50 and 52 may amplify disparate or only partially overlapping lengths of the optical link 16.
The Raman pump sources 50 and 52 comprise semiconductor or other suitable lasers capable of generating a pump light, or amplification signal, capable of amplifying the WDM signal 26 including one, more or all of the included optical information signals 24. The pump sources 50 anon 52 may be depolarized, polarization scrambled or pol<~rization multiplexed to minimize polarization sensitivity of Raman gain.
The amplification signal from the co-pumping laser 52 is launched in the direction of travel of the WDM
signal 26 and thus co-propagated with the WDM signal 26 at substantially the same speed and/or a slight or other suitable velocity mismat~~h. The amplification signal from the counter-pumping laser 52 is launched in a direction of travel opposite that of the WDM signal 26 and thus is counter-propagated with respect to the WDM
signal 26. The amplification signals may travel in opposite directions simultaneously at the same or other suitable speed.
The amplification signals comprise one or more high power lights or waves at a lower wavelength than the signal or signals to be amplified. As the amplification signal travels in the optical link 16, it scatters off atoms in the link 16, loses some energy to the atoms and continues with the same wavelength as the amplified signal or signals. In this way, the amplified signal acquires energy over many miles or kilometers in that it is represented by more photons. For the WDM signal 26, the co-pumping and counter-pumping lasers 50 and 52 may each comprise several different pump wavelengths that are used together to amplify .=ach of the wavelength distincts optical information signals 24.
In one embodiment, as described in more detail below, a non-intensity characteristic of a carrier signal is modulated with t:he data signal at each optical sender 20. The non-intensity characteristic comprises phase, frequency or other suitable characteristic with no or limited susceptibility to cross talk due to cross-gain modulation (XGM) from a forward pumping distributed amplifier or a bi-dir_ectional pumping distributed amplifier. The non-intensity modulated optical information signal may be further and/or remodulated with a clock or other non-data signal using an intensity modulator. Thus, the non-intensity modulated optical information signal may comprise intensity modulation of a non-data signal.
In a particular embodiment, as described in more detail below, the WDM signal 26 comprises phase or frequency modulated optic;~l information signals 24 which are amplified using the bi-directional DRAB 44 with no cross talk between the channels 24 due to XGM. In this embodiment, the bi-directional DRAB 44 provide amplification at a :superior optical signal-to-noise ratio and thus enable longer transmission distances and improved transmission performance.
FIGURE 2 illustrates details of the optical sender 20 in accordance with one embodiment of the present invention. In this embodiment, the optical sender 20 comprises a laser 70, a modulator 72 and a data signal 74. The laser 70 generates a carrier signal at a prescribed frequency with good wavelength control.
Typically, the wavelengths emitted by the laser 70 are selected to be within the 1500 nanometer (nm) range, the range at which the minimum signal attenuation occurs for silica-based optir_a.l fibers. More particularly, the wavelengths are generally selected to be in the range from 1310 to 1650 nm but may be suitably varied.
5 The modulator 72 modulates the carrier signal with the data signal 74 to generate the optical information signal 24. The modulator 72 may employ amplitude modulation, frequency modulation, phase modulation, intensity modulatlOIl, amplitude-shift keying, frequency-10 shift keying, pha~>e-shift keying and other suitable techniques for encoding the data signal 74 onto the carrier signal. In addition, it will be understood that different modulators 72 may employ more than one modulation system in combination.
In accordance with one embodiment, modulator 74 modulates the phrase, frequency or other suitable non-intensity characteristic of the carrier signal with the data signal 74. As previously described, this generates a non-intensity optical information signal 24 with poor susceptibility to cross talk due to XGM in long-haul and other transmission systems using bi-directional DRA or other distributed arnplification. Details of the carrier wave, frequency modulation of the carrier wave and phase modulation of the carrier wave are illustrated in FIGURES
3A-C.
Referring to FIGURE 3A, the carrier signal 76 is a completely periodic signal at the specified wavelength.
The carrier signal 76 has at least one characteristic that may be varied by modulation and is capable of carrying information via modulation.
Referring to FIGURE ..B, the frequency of the carrier signal 76 is modulated with a data signal 74 to generate a frequency modulated optical information signal 78. In frequency modulation, the frequency of the carrier signal 76 is shifted as a function of the data signal 74.
Frequency shift keying may be used in which the frequency of the carrier signal shifts between discrete states.
Referring to FIGURE 3C, the phase of the carrier signal 76 is modulated with a data signal 80 to generate a phase modulated optical information signal 82. In phase modulation, tl'ne phase of the carrier signal 76 is shifted as a function of the data signal 80. Phase shift keying may be used in which the phase of the carrier signal shifts between discrete states.
FIGURE 4 illustrates an optical sender 80 in accordance with another embodiment of the present invention. In this embodiment, data is phase or frequency modulated onto the carrier signal and then remodulated with in.tensit.y modulation synchronized with the signal clock to provide superior power tolerance in the transmission system.
Referring to FIGURE 4, the optical sender 80 includes a laser 82, a non-intensity modulator 84 and data signal 86. The non-intensity modulator 84 modulates the phase or frequency of the carrier signal from the laser 82 with the data signal 86. The resulting data modulated signal is passed to the intensity modulator 88 for remodulation with the clock frequency 90 to generate a dual or otherwise mult~imodulated optical information signal 92. Because the intensity modulation based on the clock is a non-random, completely periodic pattern, little or no cross talk due to XGM is generated by the DRAB 44 so long as there is a slight velocity mismatch in the forward pumping direction. FIGURE 5 illustrates the waveform of the dual. modulated optical information signal 92.
FIGURE 6 illust:rates details of the optical receiver 32 in accordance with one embodiment of the present invention. In thi~~ embodiment, the optical receiver 32 receives a demultiplexed optica:L information signal 24 with the data modulated on the phase of the carrier signal with phase ~~hift keying. It will be understood that the optical receiver 32 may be otherwise suitably configured to receive and detect data otherwise encoded in an optical information signal 24 without departing from the scope of th.e present invention.
Referring to FIGURE 6, the optical receiver 32 includes an asymmetric interferometer 100 and a detector 102. The interferometer 100 is a.n asymmetric Mach-Zender or other suitable :interferometer operable to convert a non-intensity modulated optical information signal 24 into an intensity modulated optical information signal for detection of data by the detector 102. Preferably, the Mach-Zender interferometer 100 with wavelength dependent loss and good rejection characteristics for the channel spacing.
The Mach-Zender interferometer 100 splits the received optical s:igmal into two interferometer paths 110 and 112 of different lengths and then combines the two paths 110 and 112 interferometrically to generate two complimentary output signals 114 and 116. In particular, the optical path difference (L) is equal to the symbol rate (B) multiplied by the speed of light (c) and divided by the optical index of the paths (n). Expressed mathematically: L=Bc/n.
In a particular embodiment, the two path lengths 110 and 112 are sized based on the symbol, or bit rate to provide a one symbol period, or bit shift. In this embodiment, the Mach-Zender interferometer 100 has a wavelength dependent: loss that increases the rejection of neighboring channels when channel spacing comprises the symbol transmission rate multiple within 0.4 to 0.6 of an integer as previously described.
The detector 102 is a dual. or other suitable detector. In one embodiment, the dual detector 102 includes photodiode~; 120 and 122 connected in series in a balanced configuration and a limiting amplifier 124. In this embodiment, the two complimentary optical outputs 114 and 116 from the Mach-Zender interferometer 100 are applied to the photodiodes 120 and 122 for conversion of the optical signal t:o an electrical signal. The limiting electronic amplifier 124 converts the electrical signal to a digital signs:L (0 or 1) depending on the optical intensity deliveref~ by the interferometer 100. In another embodiment, the detector 102 is a single detector with one photodiode 122 coupled to output 116. In this embodiment, output 114 is not uti:Lized.
FIGURE 7 illustrates the frequency response of the asymmetric Mach-Zender interferometer 100 in accordance with one embodiment of the present invention. In this embodiment, channel spacing comprises the symbol transmission rate multiple within 0.4 to 0.6 of an integer as previously described. As can be seen, optical frequency of neighboring channels is automatically rejected by the asymmetric: Mach-Zender interferometer 100 to aid channel rejection of the demultiplexer 30. It will be understood that the asymmetric Mach-Zender interferometer may be used in connection with other suitable channel spacings.
FIGURES 8A-C illustrate details of the demultiplexer 30 in accordance with one embodiment of the present invention. In this embodiment, phase or frequency modulated optical information signals 24 are converted to intensity modulate optical information signals within the demultiplexer 30 of- the WDM receiver 14 and/or before demultiplexing or bf=_tween demultiplexing steps. It will be understood that the demultiplexer 30 may otherwise suitably demultiplex a.nd/or separate the optical information signals 24 from the WDM signal 26 without departing from the scope of the present invention.
Referring to FIGURE 8A, the demultiplexer 30 comprises a plurality of demultiplex elements 130 and a multi-channel format converter 131. Each demultiplex element 130 separates a received set of channels 132 into two discrete sets of channels 134. Final channel separation is performed by dielectric filters 136 which each filter a specific channel wavelength 138.
The multichannel format converter 131 converts phase modulation to intensity modulation and may be an asymmetric Mach-Zen.der interferometer with a one-bit shift to convert non-intensity modulated signals to intensity modulated signals as previously described in connection with interferometer 7.00 or suitable optical device having a periodical optical frequency response that converts at least two phase or frequency modulated channels into intensity modulated WDM signal channels.
The intensity-conversion interferometer may be prior to the first stage demultiplex element 130, between the first and second stages or. between other suitable stages.
The other demultip:lex elements 130 may comprise filters or non-conversion Mach-Zender interferometers operable to filter the incoming set of channels 132 into the two sets of output channels 134.
In a particular embodiment, the multichannel format converter 131 is an asymmetric Mach-Zender interferometer with a free spectral range coinciding with the WDM
channel spacing or its integer sub-multiple. This allows all the WDM channels to be converted within the Mach-Zender interferometer simultaneously. In this 5 embodiment, a channel spacing may be configured based on the channel bit rate which defines the free spectral range. Placement oi_ the intensity-conversion Mach-Zender interferometer in the demultiplexer 30 eliminates the need for the interferometer 100 at each optical receiver 10 32 which can be bu=Lky and expensive. In addition, the demultiplexer 30 including the Mach-Zender and other demultiplexer elements 130 may be fabricated on a same chip which reduces l:he size and cost of the WDM receiver 14.
Referring to FIGURE 6, the optical receiver 32 includes an asymmetric interferometer 100 and a detector 102. The interferometer 100 is a.n asymmetric Mach-Zender or other suitable :interferometer operable to convert a non-intensity modulated optical information signal 24 into an intensity modulated optical information signal for detection of data by the detector 102. Preferably, the Mach-Zender interferometer 100 with wavelength dependent loss and good rejection characteristics for the channel spacing.
The Mach-Zender interferometer 100 splits the received optical s:igmal into two interferometer paths 110 and 112 of different lengths and then combines the two paths 110 and 112 interferometrically to generate two complimentary output signals 114 and 116. In particular, the optical path difference (L) is equal to the symbol rate (B) multiplied by the speed of light (c) and divided by the optical index of the paths (n). Expressed mathematically: L=Bc/n.
In a particular embodiment, the two path lengths 110 and 112 are sized based on the symbol, or bit rate to provide a one symbol period, or bit shift. In this embodiment, the Mach-Zender interferometer 100 has a wavelength dependent: loss that increases the rejection of neighboring channels when channel spacing comprises the symbol transmission rate multiple within 0.4 to 0.6 of an integer as previously described.
The detector 102 is a dual. or other suitable detector. In one embodiment, the dual detector 102 includes photodiode~; 120 and 122 connected in series in a balanced configuration and a limiting amplifier 124. In this embodiment, the two complimentary optical outputs 114 and 116 from the Mach-Zender interferometer 100 are applied to the photodiodes 120 and 122 for conversion of the optical signal t:o an electrical signal. The limiting electronic amplifier 124 converts the electrical signal to a digital signs:L (0 or 1) depending on the optical intensity deliveref~ by the interferometer 100. In another embodiment, the detector 102 is a single detector with one photodiode 122 coupled to output 116. In this embodiment, output 114 is not uti:Lized.
FIGURE 7 illustrates the frequency response of the asymmetric Mach-Zender interferometer 100 in accordance with one embodiment of the present invention. In this embodiment, channel spacing comprises the symbol transmission rate multiple within 0.4 to 0.6 of an integer as previously described. As can be seen, optical frequency of neighboring channels is automatically rejected by the asymmetric: Mach-Zender interferometer 100 to aid channel rejection of the demultiplexer 30. It will be understood that the asymmetric Mach-Zender interferometer may be used in connection with other suitable channel spacings.
FIGURES 8A-C illustrate details of the demultiplexer 30 in accordance with one embodiment of the present invention. In this embodiment, phase or frequency modulated optical information signals 24 are converted to intensity modulate optical information signals within the demultiplexer 30 of- the WDM receiver 14 and/or before demultiplexing or bf=_tween demultiplexing steps. It will be understood that the demultiplexer 30 may otherwise suitably demultiplex a.nd/or separate the optical information signals 24 from the WDM signal 26 without departing from the scope of the present invention.
Referring to FIGURE 8A, the demultiplexer 30 comprises a plurality of demultiplex elements 130 and a multi-channel format converter 131. Each demultiplex element 130 separates a received set of channels 132 into two discrete sets of channels 134. Final channel separation is performed by dielectric filters 136 which each filter a specific channel wavelength 138.
The multichannel format converter 131 converts phase modulation to intensity modulation and may be an asymmetric Mach-Zen.der interferometer with a one-bit shift to convert non-intensity modulated signals to intensity modulated signals as previously described in connection with interferometer 7.00 or suitable optical device having a periodical optical frequency response that converts at least two phase or frequency modulated channels into intensity modulated WDM signal channels.
The intensity-conversion interferometer may be prior to the first stage demultiplex element 130, between the first and second stages or. between other suitable stages.
The other demultip:lex elements 130 may comprise filters or non-conversion Mach-Zender interferometers operable to filter the incoming set of channels 132 into the two sets of output channels 134.
In a particular embodiment, the multichannel format converter 131 is an asymmetric Mach-Zender interferometer with a free spectral range coinciding with the WDM
channel spacing or its integer sub-multiple. This allows all the WDM channels to be converted within the Mach-Zender interferometer simultaneously. In this 5 embodiment, a channel spacing may be configured based on the channel bit rate which defines the free spectral range. Placement oi_ the intensity-conversion Mach-Zender interferometer in the demultiplexer 30 eliminates the need for the interferometer 100 at each optical receiver 10 32 which can be bu=Lky and expensive. In addition, the demultiplexer 30 including the Mach-Zender and other demultiplexer elements 130 may be fabricated on a same chip which reduces l:he size and cost of the WDM receiver 14.
15 Referring to FIGURE 8B, the demultiplexer 30 comprises a plurality of wavelength interleavers 133 and a multichannel format converter 135 for each set of interleaved optical information signals output by the last stage wavelength interleavers 133. Each wavelength interleaver 133 separates a received set of channels into two discrete sets of interleaved channels. The multichannel format converters 135 may be asymmetric Mach-Zender interferometers with a one-bit shift to convert non-intensity modulated signals to intensity modulated signals as previously described in connection with interferometer 100 or other suitable optical device.
Use of the wavelenc3th in.terleavers as part of the WDM
demultiplexing in front of the format converters allow several WDM channels to be converted simultaneously in one Mach-Zender interferometer even if the free spectral range of the inter:Eerometer does not coincide with an integer multiple of the WDM channel spacing. FIGURE 8C
illustrates transmissions of four Mach-Zender interferometers for a particular embodiment of the demultiplexer 30 using wavelength interleavers 133 in which the free spectral range is three quarters of the channel spacing. In this embodiment, the four Mach-Zender interferometers may be used to convert all of the WDM channels.
FIGURE 9 illustrates a method for transmitting information in an optical communication system using distributed amplification in accordance with one embodiment of the present invention. In this embodiment, data signals are phase-shift keyed onto the carrier signal and the signal i~~ amplif:ied during transmission using discrete and distributed amplification.
Referring to FIGURE 9, the method begins at step 140 in which the phase of each disparate wavelength optical carrier signal is modulated with a data signal 74 to generate the optical information signals 24. At step 142, the optical information signals 24 are multiplexed into the WDM signal 26. At step 143, the WDM signal 26 is transmitted in th.e optical link 16.
Proceeding to step 144, the WDM signal 26 is amplified along the optical link 16 utilizing discrete and distributed amp:Lification. As previously described, the WDM signal 26 may amplified at discrete points using EDFAs 42 and distributively amplified using bi-directional DRAB 44. Because the data signals are modulated onto the phase of the carrier signal, cross talk between channels from XGM due to forward pumping amplification is eliminated. Accordingly, the signal-to-noise ratio can be maximized and the signals may be transmitted over longer distances without regeneration.
Next, at step :L45, the WDM signal 26 is received by the WDM receiver 14. At step 146, the WDM signal 26 is demultiplexed by the demultiplexer 30 to separate out the optical information signals 24. At step 147, the phase modulated optical information signals 24 are converted to intensity modulated signals for recovery of the data signal 74 at step 148. In this way, data signals 74 are transmitted over long distances using forward or bi-directional pumping distributed <amplification with a low bit-to-noise ratio.
FIGURE 10 illustrates a bi-directional optical communication system 150 in accordance with one embodiment of the present invention. In this embodiment, the bi-directional communication system 150 includes WDM
transmitters 152 and WDM receiver. s 154 at each end of an optical link 156. The WDM transmitters 152 comprise optical senders and a multiplexer as previously described in connection with the WDM transmitter 12. Similarly, the WDM receivers 154 comprise demultiplexers and optical receivers as previously described in connection with the WDM receiver 14.
At each end point, the WDM transmitter and receiver set is connected to the optical link 156 by a routing device 158. The routing device 158 may be an optical circulator, optical filter., or optical interleaver filter capable of allowing egress traffic to pass onto the link 156 from WDM transmitter 152 and to route ingress traffic from the link 156 to WDM receiver 154.
The optical link 156 comprises bi-directional discrete amplifiers 160 and bi~-directional distributed amplifiers 162 spaced periodically along the link. The bi-directional discrete amplifiers 160 may comprise EDFA
amplifiers as previously described in connection with amplifiers 42. Similarly, the distributed amplifiers 162 may comprise DRA amplifiers including co-pumping and counter-pumping lasers 164 and 166 as previously described in connection with DRA amplifiers 44.
In operation, a WDM signal is generated and transmitted from Each end point to the other end point and a WDM signal is received from the other end point.
Along the length of the optical link 156, the 4VDM signals are amplified using bi-direct:ional-pumped DRA 162.
Because data is not carried in the form of optical intensity, cross talk due to XGM is eliminated. Thus, DRA and other suitable distributed amplification may be used in long-haul and other suitable bi-directional optical transmission. systems.
FIGURE 11 illustrates an optical sender 200 and an optical receiver 202 in accordance with another embodiment of the pz-esent invention. In this embodiment, the optical sender 200 and the optical receiver 204 communicate to fine-tune modulation for improved transmission performance of the optical information signals 24. It will be understood that modulation of the optical information signals 24 may be otherwise fine-tuned using downstream feedback without departing from the scope of the present invention.
Referring to FIGURE 11, the optical sender 200 comprises a laser 21.0, a modulator 212, and a data signal 214 which operate a~s previously described in connection with the laser 70, the modulator 72 and the data signal 74. A controller :?16 receives bit error rate or other indication of transmission errors from the downstream optical receiver 20:2 and adjust the modulation depth of modulator 212 based on the indication to reduce and/or minimize transmission errors. The controller 216 may adjust the amplitudE=_, intensity, phase, frequency and/or other suitable modulation depth of modulator 212 and may use any suitable control loop or other algorithm that adjusts modulation alone or in connection with other characteristics toward a minimized or reduced transmission error rate. Thus, for example, the controller 216 may adjust a non-intensity modulation depth and a depth of. the periodic intensity modulation in the optical sender 80 to generate and optimize multimodulated signals.
The optical receiver 202 comprises an interferometer 220 and a detector 222 which operate as previously described in connection with interferometer 100 and detector 102. A forward error correction (FEC) decoder 224 uses header, redundant, symptom or other suitable bits in the header or other section of a SONET or other frame or other transmission protocol data to determine bit errors. The FIEC decoder 224 corrects for detected bit errors and forwards the bit error rate or other indicator of transm_Lssion errors to a controller 226 for the optical receiver' 202.
The controller 226 communicates the bit error rate or other indicator to the controller 216 in the optical sender 200 over an optical supervisory channel (OSC) 230.
The controllers 21E~ and 226 may communicate with each other to fine-tune modulation depth during initiation or setup of the tran:~mission system, periodically during operation of the transmission system, continuously during operation of the transmission system or in response to predefined trigger events. In this way, modulation depth is adjusted based on received signal quality measured at the receiver to minimize chromatic dispersion, non-linear effects, receiver characteristics and other unpredictable and/or predictable characteristics of the system.
FIGURE 12 illustrates details of the modulator 212 in accordance with one embodiment of the present invention. In this embodiment, the modulator 212 employs phase and intensity modulation to generate a bi-modulated 5 optical information signal. The phase and intensity modulation depth is adjusted based on receiver-side feedback to minimise tran~;mission errors.
Referring to FIGURE 12, the modulator 212 includes for phase modulation such as phase shift keying a bias 10 circuit 230 coupled. to an electrical driver 232. The bias circuit 230 may be a power supply and the electrical driver 232 a broadband amplifier. The bias circuit 230 is controlled by the controller 216 to output a bias signal to the electrical driver 232. The bias signal 15 provides an index for phase modulation. The electrical driver 232 amplifies the data signal 214 based on the bias signal and outputs the resulting signal to phase modulator 234. Pha~~e modulator 234 modulates the receive bias-adjusted data signal onto the phase of the carrier 20 signal output by i~he laser 210 to generate a phase modulated optical information signal 236.
For intensity modulation such as intensity shift keying, the modulator 212 includes a bias circuit 240 coupled to an electrical driver 242. The bias circuit 240 is controlled by the controller 216 to output a bias signal to the elect=rical driver 242. The bias signal acts as an intensity modulation index. The electrical driver 242 amplifies a network, system or other suitable clock signal 244 ba~;ed on the bias signal and outputs the resulting signal tc> the intensity modulator 246. The intensity modulator 246 is coupled to the phase modulator 234 and modulates t:he receive bias-adjusted clock signal onto the phase modulated optical information signal 236 to generate the bi-modulated optical information signal for transmission to a receiver. It will be understood that phase and intensity modulation at the transmitter may be otherwise suitably contralled based on receiver-s side feedback to minimize transmission errors of data over the optical link.
FIGURE 13 illustrates a method for fine tuning modulation depth of an optical information signal using receiver side information in accordance with one embodiment of the present. invention. The method begins at step 250 in which an optical carrier is modulated with a data signal 214 at the optical sender 200. Next, at step 252, the resulting optical information signal 24 is transmitted to the optical receiver 202 in a WDM signal 26.
Proceeding to step 254, the data signal 214 is recovered at the opi:ical receiver 204. At step 256, the FEC decoder 224 determines a bit error rate for the data based on bits in the SONET overhead. At step 258, the bit error rate is reported by the controller 226 of the optical receiver 202 to tree controller 216 of the optical sender 200 over the OSC 230.
Next, at deci~~ional step 260, the controller 216 determines whether modulation is optimized. In one embodiment, modulation is optimized when the bit error rate is minimized. If the modulation is not optimized, the No branch of decisional step 260 leads to step 262 in which the modulation depth is adjusted. Step 262 returns to step 250 in which the data signal 214 is modulated with the new modulation depth and transmitted to the optical receiver 202. After the modulation depth is optimized from repetitive trails and measurements or other suitable mechanisms, the Yes branch of decisional step 260 leads to the end of the process. In this way, transmission performance is improved and transmission errors minimized.
FIGURE 14 illustrates an optical communication system 275 distributing a clock signal in an information channel in accordance with one embodiment of the present invention. In this embodiment, pure clock is transmitted in channels to one, more or all nodes in the optical system 275.
Referring to F:CGURE 14, optical system 275 includes a WDM transmitter 280 coupled to a WDM receiver 282 over an optical link 284. The WDM transmitter 280 includes a plurality of optical senders 290 and a WDM multiplexer 292. Each optical sender 29U generates an optical information signal. 294 on one of a set of discrete wavelengths at the channel spacing. In the clock channel 296, the optical sender 290 generates an optical information signal 294 with at .Least one characteristic modulated to encode the clock signal. In the data channels 297, the optical sender 290 generates an optical information signal 294 with at least one characteristic modulated to encode a corresponding data signal.
The optical signals 294 from the clock and data channels 296 and 257 are multip:Lexed into a signal WDM
signal 298 by the WDM mult:iplexer 292 for transmission on the optical link 284. Along the optical link 284, the signal may be amplified by discrete and/or distributed amplifiers as previously described.
The WDM receiver 282 receives, separates and decodes the optical information signals 294 to recover the included data and clock signals. In one embodiment, the WDM receiver 282 includes a WDM demultiplexer 310 and a plurality of optical receivers 312. The WDM
Use of the wavelenc3th in.terleavers as part of the WDM
demultiplexing in front of the format converters allow several WDM channels to be converted simultaneously in one Mach-Zender interferometer even if the free spectral range of the inter:Eerometer does not coincide with an integer multiple of the WDM channel spacing. FIGURE 8C
illustrates transmissions of four Mach-Zender interferometers for a particular embodiment of the demultiplexer 30 using wavelength interleavers 133 in which the free spectral range is three quarters of the channel spacing. In this embodiment, the four Mach-Zender interferometers may be used to convert all of the WDM channels.
FIGURE 9 illustrates a method for transmitting information in an optical communication system using distributed amplification in accordance with one embodiment of the present invention. In this embodiment, data signals are phase-shift keyed onto the carrier signal and the signal i~~ amplif:ied during transmission using discrete and distributed amplification.
Referring to FIGURE 9, the method begins at step 140 in which the phase of each disparate wavelength optical carrier signal is modulated with a data signal 74 to generate the optical information signals 24. At step 142, the optical information signals 24 are multiplexed into the WDM signal 26. At step 143, the WDM signal 26 is transmitted in th.e optical link 16.
Proceeding to step 144, the WDM signal 26 is amplified along the optical link 16 utilizing discrete and distributed amp:Lification. As previously described, the WDM signal 26 may amplified at discrete points using EDFAs 42 and distributively amplified using bi-directional DRAB 44. Because the data signals are modulated onto the phase of the carrier signal, cross talk between channels from XGM due to forward pumping amplification is eliminated. Accordingly, the signal-to-noise ratio can be maximized and the signals may be transmitted over longer distances without regeneration.
Next, at step :L45, the WDM signal 26 is received by the WDM receiver 14. At step 146, the WDM signal 26 is demultiplexed by the demultiplexer 30 to separate out the optical information signals 24. At step 147, the phase modulated optical information signals 24 are converted to intensity modulated signals for recovery of the data signal 74 at step 148. In this way, data signals 74 are transmitted over long distances using forward or bi-directional pumping distributed <amplification with a low bit-to-noise ratio.
FIGURE 10 illustrates a bi-directional optical communication system 150 in accordance with one embodiment of the present invention. In this embodiment, the bi-directional communication system 150 includes WDM
transmitters 152 and WDM receiver. s 154 at each end of an optical link 156. The WDM transmitters 152 comprise optical senders and a multiplexer as previously described in connection with the WDM transmitter 12. Similarly, the WDM receivers 154 comprise demultiplexers and optical receivers as previously described in connection with the WDM receiver 14.
At each end point, the WDM transmitter and receiver set is connected to the optical link 156 by a routing device 158. The routing device 158 may be an optical circulator, optical filter., or optical interleaver filter capable of allowing egress traffic to pass onto the link 156 from WDM transmitter 152 and to route ingress traffic from the link 156 to WDM receiver 154.
The optical link 156 comprises bi-directional discrete amplifiers 160 and bi~-directional distributed amplifiers 162 spaced periodically along the link. The bi-directional discrete amplifiers 160 may comprise EDFA
amplifiers as previously described in connection with amplifiers 42. Similarly, the distributed amplifiers 162 may comprise DRA amplifiers including co-pumping and counter-pumping lasers 164 and 166 as previously described in connection with DRA amplifiers 44.
In operation, a WDM signal is generated and transmitted from Each end point to the other end point and a WDM signal is received from the other end point.
Along the length of the optical link 156, the 4VDM signals are amplified using bi-direct:ional-pumped DRA 162.
Because data is not carried in the form of optical intensity, cross talk due to XGM is eliminated. Thus, DRA and other suitable distributed amplification may be used in long-haul and other suitable bi-directional optical transmission. systems.
FIGURE 11 illustrates an optical sender 200 and an optical receiver 202 in accordance with another embodiment of the pz-esent invention. In this embodiment, the optical sender 200 and the optical receiver 204 communicate to fine-tune modulation for improved transmission performance of the optical information signals 24. It will be understood that modulation of the optical information signals 24 may be otherwise fine-tuned using downstream feedback without departing from the scope of the present invention.
Referring to FIGURE 11, the optical sender 200 comprises a laser 21.0, a modulator 212, and a data signal 214 which operate a~s previously described in connection with the laser 70, the modulator 72 and the data signal 74. A controller :?16 receives bit error rate or other indication of transmission errors from the downstream optical receiver 20:2 and adjust the modulation depth of modulator 212 based on the indication to reduce and/or minimize transmission errors. The controller 216 may adjust the amplitudE=_, intensity, phase, frequency and/or other suitable modulation depth of modulator 212 and may use any suitable control loop or other algorithm that adjusts modulation alone or in connection with other characteristics toward a minimized or reduced transmission error rate. Thus, for example, the controller 216 may adjust a non-intensity modulation depth and a depth of. the periodic intensity modulation in the optical sender 80 to generate and optimize multimodulated signals.
The optical receiver 202 comprises an interferometer 220 and a detector 222 which operate as previously described in connection with interferometer 100 and detector 102. A forward error correction (FEC) decoder 224 uses header, redundant, symptom or other suitable bits in the header or other section of a SONET or other frame or other transmission protocol data to determine bit errors. The FIEC decoder 224 corrects for detected bit errors and forwards the bit error rate or other indicator of transm_Lssion errors to a controller 226 for the optical receiver' 202.
The controller 226 communicates the bit error rate or other indicator to the controller 216 in the optical sender 200 over an optical supervisory channel (OSC) 230.
The controllers 21E~ and 226 may communicate with each other to fine-tune modulation depth during initiation or setup of the tran:~mission system, periodically during operation of the transmission system, continuously during operation of the transmission system or in response to predefined trigger events. In this way, modulation depth is adjusted based on received signal quality measured at the receiver to minimize chromatic dispersion, non-linear effects, receiver characteristics and other unpredictable and/or predictable characteristics of the system.
FIGURE 12 illustrates details of the modulator 212 in accordance with one embodiment of the present invention. In this embodiment, the modulator 212 employs phase and intensity modulation to generate a bi-modulated 5 optical information signal. The phase and intensity modulation depth is adjusted based on receiver-side feedback to minimise tran~;mission errors.
Referring to FIGURE 12, the modulator 212 includes for phase modulation such as phase shift keying a bias 10 circuit 230 coupled. to an electrical driver 232. The bias circuit 230 may be a power supply and the electrical driver 232 a broadband amplifier. The bias circuit 230 is controlled by the controller 216 to output a bias signal to the electrical driver 232. The bias signal 15 provides an index for phase modulation. The electrical driver 232 amplifies the data signal 214 based on the bias signal and outputs the resulting signal to phase modulator 234. Pha~~e modulator 234 modulates the receive bias-adjusted data signal onto the phase of the carrier 20 signal output by i~he laser 210 to generate a phase modulated optical information signal 236.
For intensity modulation such as intensity shift keying, the modulator 212 includes a bias circuit 240 coupled to an electrical driver 242. The bias circuit 240 is controlled by the controller 216 to output a bias signal to the elect=rical driver 242. The bias signal acts as an intensity modulation index. The electrical driver 242 amplifies a network, system or other suitable clock signal 244 ba~;ed on the bias signal and outputs the resulting signal tc> the intensity modulator 246. The intensity modulator 246 is coupled to the phase modulator 234 and modulates t:he receive bias-adjusted clock signal onto the phase modulated optical information signal 236 to generate the bi-modulated optical information signal for transmission to a receiver. It will be understood that phase and intensity modulation at the transmitter may be otherwise suitably contralled based on receiver-s side feedback to minimize transmission errors of data over the optical link.
FIGURE 13 illustrates a method for fine tuning modulation depth of an optical information signal using receiver side information in accordance with one embodiment of the present. invention. The method begins at step 250 in which an optical carrier is modulated with a data signal 214 at the optical sender 200. Next, at step 252, the resulting optical information signal 24 is transmitted to the optical receiver 202 in a WDM signal 26.
Proceeding to step 254, the data signal 214 is recovered at the opi:ical receiver 204. At step 256, the FEC decoder 224 determines a bit error rate for the data based on bits in the SONET overhead. At step 258, the bit error rate is reported by the controller 226 of the optical receiver 202 to tree controller 216 of the optical sender 200 over the OSC 230.
Next, at deci~~ional step 260, the controller 216 determines whether modulation is optimized. In one embodiment, modulation is optimized when the bit error rate is minimized. If the modulation is not optimized, the No branch of decisional step 260 leads to step 262 in which the modulation depth is adjusted. Step 262 returns to step 250 in which the data signal 214 is modulated with the new modulation depth and transmitted to the optical receiver 202. After the modulation depth is optimized from repetitive trails and measurements or other suitable mechanisms, the Yes branch of decisional step 260 leads to the end of the process. In this way, transmission performance is improved and transmission errors minimized.
FIGURE 14 illustrates an optical communication system 275 distributing a clock signal in an information channel in accordance with one embodiment of the present invention. In this embodiment, pure clock is transmitted in channels to one, more or all nodes in the optical system 275.
Referring to F:CGURE 14, optical system 275 includes a WDM transmitter 280 coupled to a WDM receiver 282 over an optical link 284. The WDM transmitter 280 includes a plurality of optical senders 290 and a WDM multiplexer 292. Each optical sender 29U generates an optical information signal. 294 on one of a set of discrete wavelengths at the channel spacing. In the clock channel 296, the optical sender 290 generates an optical information signal 294 with at .Least one characteristic modulated to encode the clock signal. In the data channels 297, the optical sender 290 generates an optical information signal 294 with at least one characteristic modulated to encode a corresponding data signal.
The optical signals 294 from the clock and data channels 296 and 257 are multip:Lexed into a signal WDM
signal 298 by the WDM mult:iplexer 292 for transmission on the optical link 284. Along the optical link 284, the signal may be amplified by discrete and/or distributed amplifiers as previously described.
The WDM receiver 282 receives, separates and decodes the optical information signals 294 to recover the included data and clock signals. In one embodiment, the WDM receiver 282 includes a WDM demultiplexer 310 and a plurality of optical receivers 312. The WDM
demultiplexer 310 clemulti.plexes the optical information signals 294 from the single WDM signal 298 and sends each optical information signal 294 to a corresponding optical receiver 312.
Each optical re=ceiver 312 optically or electrically recovers the encoded data or clock signal from the corresponding signal. 294. In the clock channel 296, the clock signal is recovered and forwarded to the optical receivers 312 in the data channels 297 for use in data extraction and forward error correction. The transmission of pure clock in an information channel allows a more stable clock recovery with less fitter.
The stable clock may be used by forward error correction to improve the bit error rate even in the presence of fitter and poor optical signal quality.
FIGURE 15 illustrates an optical receiver 320 for extracting a clock :>ignal from a multimodulated signal in accordance with one embodiment of the present invention.
In this embodiment, the optical receiver 320 receives a demultiplexed optical information signal with data phase modulated onto a carrier signal that is then remodulated with intensity modulation synchronized with the network, system or other suitable clock as described in connection with the optical sender 80. The optical receiver 320 extracts the clock information from the optical signal and uses the stable clock to recover data from the phase modulated signal of the channel. Thus, each channel can recover its own clock.
Referring to FIGURE 15, the optical receiver 320 includes an interfE:rometer 322 and a detector 324 as previously described in connection with the optical receiver 32. The interferometer 322 receives the miltimodulated signal and converts the phase modulation into intensity modulation for recovery of the data signal 330 by the detector 324.
A clock recovery element 326 comprises a photodiode and/or other suitable components to recover the clock signal before phase-to-intensity conversion of the data signal. The clock recovery element 326 may comprise a phase lock loop, a tank circuit, a high quality filter and the like. The clock recovery element 326 receives the multimodulated signal and recovers the clock signal 332 from the intensity modulation.
The data signal 330 and the recovered clock signal 332 are output to a. digital flip flop or other suitable data recovery circuit 334. In this way, the optical receiver 320 extracts the clock information from the optical signal before the phase-to-intensity conversion of the data signal and provides a stable clock recovery with less fitter even with poor optical signal quality corresponding to a bit error rate in the range of 1e-2.
Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art.
It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.
Each optical re=ceiver 312 optically or electrically recovers the encoded data or clock signal from the corresponding signal. 294. In the clock channel 296, the clock signal is recovered and forwarded to the optical receivers 312 in the data channels 297 for use in data extraction and forward error correction. The transmission of pure clock in an information channel allows a more stable clock recovery with less fitter.
The stable clock may be used by forward error correction to improve the bit error rate even in the presence of fitter and poor optical signal quality.
FIGURE 15 illustrates an optical receiver 320 for extracting a clock :>ignal from a multimodulated signal in accordance with one embodiment of the present invention.
In this embodiment, the optical receiver 320 receives a demultiplexed optical information signal with data phase modulated onto a carrier signal that is then remodulated with intensity modulation synchronized with the network, system or other suitable clock as described in connection with the optical sender 80. The optical receiver 320 extracts the clock information from the optical signal and uses the stable clock to recover data from the phase modulated signal of the channel. Thus, each channel can recover its own clock.
Referring to FIGURE 15, the optical receiver 320 includes an interfE:rometer 322 and a detector 324 as previously described in connection with the optical receiver 32. The interferometer 322 receives the miltimodulated signal and converts the phase modulation into intensity modulation for recovery of the data signal 330 by the detector 324.
A clock recovery element 326 comprises a photodiode and/or other suitable components to recover the clock signal before phase-to-intensity conversion of the data signal. The clock recovery element 326 may comprise a phase lock loop, a tank circuit, a high quality filter and the like. The clock recovery element 326 receives the multimodulated signal and recovers the clock signal 332 from the intensity modulation.
The data signal 330 and the recovered clock signal 332 are output to a. digital flip flop or other suitable data recovery circuit 334. In this way, the optical receiver 320 extracts the clock information from the optical signal before the phase-to-intensity conversion of the data signal and provides a stable clock recovery with less fitter even with poor optical signal quality corresponding to a bit error rate in the range of 1e-2.
Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art.
It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.
Claims (22)
1. A method for tuning an optical signal based on transmission conditions, comprising:
receiving information indicative of transmission conditions of an optical link; and adjusting a modulation characteristic of traffic transmitted over the transmission link based on the information.
receiving information indicative of transmission conditions of an optical link; and adjusting a modulation characteristic of traffic transmitted over the transmission link based on the information.
2. The method of Claim 1, further comprising adjusting a modulation depth of traffic transmitted over the transmission link based on the information.
3. The method of Claim 1, further comprising adjusting a phase modulation depth of traffic transmitted over the transmission link based on the information.
4. The method of Claim 1, further comprising adjusting a frequency modulation depth of traffic transmitted over the transmission link based on the information.
5. The method of Claim 1, further comprising adjusting a intensity modulation depth of traffic transmitted over the transmission link based on the information.
6. The method of Claim 1, wherein the traffic comprises multi-modulated signals, further comprising adjusting a plurality of modulation depths of the signals based on the information.
7. The method of Claim 6, wherein the multi-modulated signals comprise phase and intensity modulated signals.
8. The method of Claim 1, further comprising adjusting the modulation depth dynamically based on the information.
9. The method of Claim 1, wherein the information comprises real-time bit error rate information.
10. The method of Claim 1, wherein the information is received from a receiver to which the traffic is transmitted over the transmission link.
11. A system for tuning an optical signal based on transmission conditions, comprising:
means for receiving information indicative of transmission conditions of an optical link; and means for adjusting a modulation characteristic of traffic transmitted over the transmission link based on the information.
means for receiving information indicative of transmission conditions of an optical link; and means for adjusting a modulation characteristic of traffic transmitted over the transmission link based on the information.
12. The method of Claim 11, further comprising means for adjusting a modulation depth of traffic transmitted over the transmission link based on the information.
13. The method of Claim 11, further comprising means for adjusting a phase modulation depth of traffic transmitted over the transmission link based on the information.
14. The method of Claim 11, further comprising means for adjusting a frequency modulation depth of traffic transmitted over the transmission link based on the information.
15. The method of Claim 11, further comprising means for adjusting an intensity modulation depth of traffic transmitted over the transmission link based on the information.
16. The method of Claim 11, wherein the traffic comprises multi-modulated signals, further comprising means for adjusting a plurality of modulation depths of the signals based on the information.
17. The method of Claim 16, wherein the multi-modulated signals comprise phase and intensity modulated signals.
18. The method of Claim 11, further comprising means for adjusting the modulation depth dynamically based on the information.
19. The method of Claim 11, wherein the information comprises real-time bit error rate information.
20. The method of Claim 11, wherein the information is received from a receiver to which the traffic is transmitted over the transmission link.
21. An optical. transmission system, comprising:
a receiver operable to receive an optical information signal over a transmission link, to determine a bit error rate of the optical information signal and to transmit the bit error rate to a transmitter transmitting the optical information signal over the transmission link; and the transmitter operable to receive the bit error rate from the receiver, adjust a modulation depth of traffic transmitted over the link to the receiver and transmit the traffic to the receiver.
a receiver operable to receive an optical information signal over a transmission link, to determine a bit error rate of the optical information signal and to transmit the bit error rate to a transmitter transmitting the optical information signal over the transmission link; and the transmitter operable to receive the bit error rate from the receiver, adjust a modulation depth of traffic transmitted over the link to the receiver and transmit the traffic to the receiver.
22. The system of Claim 21, wherein the optical information signal is a wavelength division multiplex (WDM) signal.
Applications Claiming Priority (2)
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|---|---|---|---|
| US85334001A | 2001-05-10 | 2001-05-10 | |
| US09/853,340 | 2001-05-10 |
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| CA2385452A1 true CA2385452A1 (en) | 2002-11-10 |
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|---|---|---|---|
| CA002385452A Abandoned CA2385452A1 (en) | 2001-05-10 | 2002-05-08 | Method and system for tuning an optical signal based on transmission conditions |
Country Status (5)
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| EP (1) | EP1393474A2 (en) |
| JP (1) | JP3940083B2 (en) |
| CN (1) | CN1526212A (en) |
| CA (1) | CA2385452A1 (en) |
| WO (1) | WO2002091634A2 (en) |
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|---|---|---|---|---|
| US7603045B2 (en) * | 2003-08-28 | 2009-10-13 | Fujitsu Limited | Method and system for automatic feedback control for fine tuning a delay interferometer |
| GB2424134B (en) * | 2005-03-07 | 2007-01-10 | Azea Networks Ltd | Adaptive pulse shape control |
| EP2110970B1 (en) * | 2008-04-15 | 2013-06-12 | Alcatel Lucent | Method for monitoring an optical data transmission line and optical data transmitter |
| JP2011232553A (en) * | 2010-04-28 | 2011-11-17 | Mitsubishi Electric Corp | Optical transmitter, optical communication system and modulation method |
| JP5824912B2 (en) | 2011-06-29 | 2015-12-02 | 富士通株式会社 | Optical transmission apparatus and optical interleave control method |
| CN104243048B (en) * | 2014-08-28 | 2016-09-07 | 北京邮电大学 | Transmission performance optimization method and system based on simplex in coherent optical communication system |
| WO2023019424A1 (en) * | 2021-08-17 | 2023-02-23 | 华为技术有限公司 | Optical network device and communication method therefor |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH06224852A (en) * | 1993-01-25 | 1994-08-12 | Matsushita Electric Ind Co Ltd | Optical transmission system |
| US5526162A (en) * | 1994-09-27 | 1996-06-11 | At&T Corp. | Synchronous polarization and phase modulation for improved performance of optical transmission systems |
| US5946119A (en) * | 1997-02-12 | 1999-08-31 | Tyco Submarine Systems Ltd. | Wavelength division multiplexed system employing optimal channel modulation |
| US6433904B1 (en) * | 1999-07-27 | 2002-08-13 | Sycamore Networks, Inc. | Method and apparatus for improving transmission performance over wavelength division multiplexed optical communication links using forward error correction coding |
-
2002
- 2002-05-08 CA CA002385452A patent/CA2385452A1/en not_active Abandoned
- 2002-05-09 CN CNA028137795A patent/CN1526212A/en active Pending
- 2002-05-09 EP EP02727857A patent/EP1393474A2/en not_active Withdrawn
- 2002-05-09 WO PCT/IB2002/001590 patent/WO2002091634A2/en not_active Application Discontinuation
- 2002-05-09 JP JP2002587979A patent/JP3940083B2/en not_active Expired - Lifetime
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| WO2002091634A2 (en) | 2002-11-14 |
| CN1526212A (en) | 2004-09-01 |
| JP2004530373A (en) | 2004-09-30 |
| EP1393474A2 (en) | 2004-03-03 |
| JP3940083B2 (en) | 2007-07-04 |
| WO2002091634A3 (en) | 2003-10-09 |
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