A method and system for multi-channel optical XPM compensation may include a DCM to improve performance of a feed-forward control loop in an optical path in an optical network. Additionally, various spectral overlap schemes may be used with multi-channel WDM optical signals using XPM compensators in parallel, such as at a ROADM node. Polarization diversity may also be supported for XPM compensation including a DCM.

BACKGROUND

Field of the Disclosure

The present disclosure relates generally to optical communication networks and, more particularly, to a multi-channel optical cross-phase modulation (XPM) compensator.

Description of the Related Art

Telecommunication, cable television and data communication systems 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 may comprise thin strands of glass capable of communicating the signals over long distances. Optical networks often employ modulation schemes to convey information in the optical signals over the optical fibers. Such modulation schemes may include phase-shift keying (PSK), frequency-shift keying (FSK), amplitude-shift keying (ASK), and quadrature amplitude modulation (QAM).

As data rates for optical networks continue to increase, reaching up to 1 terabit/s (1T) and beyond, the demands on optical signal-to-noise ratios (OSNR) also increase, for example, due to the use of advanced modulation formats, such as QAM and PSK with dual polarization. In addition, phase shifts of optical signals transmitted over optical networks may be observed. The phase shift may be self-phase modulation (SPM) in which light interacts with an optical fiber during transmission. Additionally, XPM may occur in which one wavelength of light can alter the phase of another wavelength of light.

SUMMARY

In one aspect, a disclosed reconfigurable optical add-drop multiplexer (ROADM) may include a first wavelength selective switch (WSS) to switch groups of adjacent channels included in a wavelength division multiplexed (WDM) optical signal provided as input to the first WSS. In the ROADM, a group of adjacent channels may represent an optical band transmitted by the WDM optical signal. The ROADM may further include a first cross-phase modulation (XPM) compensator to receive a first group of the groups of adjacent channels from the first WSS. In the ROADM, the first XPM compensator may further include a feed-forward XPM regulation loop to generate an XPM control signal, the feed-forward XPM regulation loop including a dispersion compensation module (DCM) to add dispersion corresponding to a fraction of an effective length of a fiber optic span carrying the WDM optical signal subsequent to the ROADM. The first XPM compensator may still further include a phase modulator to receive the first group and to receive the XPM control signal, and to output an XPM compensated first group, and a second WSS to receive the XPM compensated first group.

In any of the disclosed embodiments, the ROADM may further include a plurality of XPM compensators in addition to the first XPM compensator to respectively receive additional groups of adjacent channels from the first WSS and to output XPM compensated groups to the second WSS.

In any of the disclosed embodiments of the ROADM, the second WSS may receive the XPM compensated groups and may switch channels corresponding to the WDM optical signal for transmission.

In any of the disclosed embodiments of the ROADM, the first XPM compensator may exclusively compensate a first subgroup for XPM, where the first group includes the first subgroup and at least one additional adjacent channel switched to the first XPM compensator by the first WSS. In the ROADM, the second WSS may drop the at least one adjacent channel received by the first XPM compensator.

In any of the disclosed embodiments of the ROADM, the first XPM compensator may further include a second input to the feed-forward XPM regulation loop to receive the WDM optical signal, and an optical bandpass filter applied to the second input to pass selected groups of adjacent channels from the WDM optical signal in the feed-forward XPM regulation loop.

In any of the disclosed embodiments of the ROADM, the first XPM compensator may be enabled to compensate XPM with polarization diversity for an X-polarization component and a Y-polarization component, while the first XPM compensator further includes a first phase modulator for compensating a first phase corresponding to the X-polarization component, and a second phase modulator for compensating a second phase corresponding to the Y-polarization component.

In another aspect, a disclosed optical system may include a first WSS to switch groups of adjacent channels included in a WDM optical signal provided as input to the first WSS. In the optical system, a group of adjacent channels may represent an optical band transmitted by the WDM optical signal. The optical system may further include a first XPM compensator to receive a first group of the groups of adjacent channels from the first WSS. In the optical system, the first XPM compensator may further include a feed-forward XPM regulation loop to generate an XPM control signal, the feed-forward XPM regulation loop including a DCM to add dispersion corresponding to a fraction of an effective length of a fiber optic span carrying the WDM optical signal subsequent to the optical system. The first XPM compensator may still further include a phase modulator to receive the first group and to receive the XPM control signal, and to output an XPM compensated first group, and a second WSS to receive the XPM compensated first group.

In any of the disclosed embodiments, the optical system may further include a plurality of XPM compensators in addition to the first XPM compensator to respectively receive additional groups of adjacent channels from the first WSS and to output XPM compensated groups to the second WSS.

In any of the disclosed embodiments of the optical system, the second WSS may receive the XPM compensated groups and may switch channels corresponding to the WDM optical signal for transmission.

In any of the disclosed embodiments of the optical system, the first XPM compensator may exclusively compensate a first subgroup for XPM, where the first group includes the first subgroup and at least one additional adjacent channel switched to the first XPM compensator by the first WSS. In the optical system, the second WSS may drop the at least one adjacent channel received by the first XPM compensator.

In any of the disclosed embodiments of the optical system, the first XPM compensator may further include a second input to the feed-forward XPM regulation loop to receive the WDM optical signal, and an optical bandpass filter applied to the second input to pass selected groups of adjacent channels from the WDM optical signal in the feed-forward XPM regulation loop.

In any of the disclosed embodiments of the optical system, the first XPM compensator may be enabled to compensate XPM with polarization diversity for an X-polarization component and a Y-polarization component, while the first XPM compensator further includes a first phase modulator for compensating a first phase corresponding to the X-polarization component, and a second phase modulator for compensating a second phase corresponding to the Y-polarization component.

In yet a further aspect, a disclosed method for XPM compensation of optical signals may include switching groups of adjacent channels included in a WDM optical signal provided as input to a first WSS. In the method, a group of adjacent channels may represent an optical band transmitted by the WDM optical signal. The method may include receiving a first group of the groups of adjacent channels from the first WSS at a first XPM compensator. In the method, the first XPM compensator may be enabled for generating an XPM control signal using a feed-forward XPM regulation loop, the feed-forward XPM regulation loop including a DCM to add dispersion corresponding to a fraction of an effective length of a fiber optic span carrying the WDM optical signal subsequent to a second WSS. In the method, the first XPM compensator may further be enabled for sending the first group and the XPM control signal to a phase modulator to output an XPM compensated first group, and receiving the XPM compensated first group at the second WSS.

In any of the disclosed embodiments, the method may further include receiving additional groups of adjacent channels from the first WSS, respectively sending the additional groups to corresponding plurality of XPM compensators in addition to the first XPM compensator, and outputting XPM compensated groups to the second WSS from the XPM compensators. In the method, the second WSS may receive the XPM compensated groups and may select channels corresponding to the WDM optical signal for transmission.

In any of the disclosed embodiments of the method, the first XPM compensator may exclusively compensates a first subgroup for XPM, where the first group includes the first subgroup and at least one additional adjacent channel switched to the first XPM compensator by the first WSS.

In any of the disclosed embodiments of the method, the second WSS may drop the at least one adjacent channel received by the first XPM compensator.

In any of the disclosed embodiments, the method may further include receiving the WDM optical signal at a second input to the feed-forward XPM regulation loop, and passing selected groups of adjacent channels from the WDM optical signal by an optical bandpass filter applied to the second input in the feed-forward XPM regulation loop.

In any of the disclosed embodiments, the method may further include compensating XPM with polarization diversity for an X-polarization component and a Y-polarization component using the first XPM compensator, including compensating a first phase corresponding to the X-polarization component using a first phase modulator, and compensating a second phase corresponding to the Y-polarization component using a second phase modulator.

DESCRIPTION OF PARTICULAR EMBODIMENT(S)

Throughout this disclosure, a hyphenated form of a reference numeral refers to a specific instance of an element and the un-hyphenated form of the reference numeral refers to the element generically or collectively. Thus, as an example (not shown in the drawings), device “12-1” refers to an instance of a device class, which may be referred to collectively as devices “12” and any one of which may be referred to generically as a device “12”. In the figures and the description, like numerals are intended to represent like elements.

Referring now to the drawings,FIG. 1illustrates an example embodiment of optical network101, which may represent an optical communication system. Optical network101may include one or more optical fibers106to transport one or more optical signals communicated by components of optical network101. The network elements of optical network101, coupled together by fibers106, may comprise one or more transmitters102, one or more multiplexers (MUX)104, one or more optical amplifiers108, one or more optical add/drop multiplexers (OADM)110, one or more demultiplexers (DEMUX)105, and one or more receivers112.

Optical network101may comprise a point-to-point optical network with terminal nodes, a ring optical network, a mesh optical network, or any other suitable optical network or combination of optical networks. Optical network101may be used in a short-haul metropolitan network, a long-haul inter-city network, or any other suitable network or combination of networks. The capacity of optical network101may include, for example, 100 Gbit/s, 400 Gbit/s, or 1 Tbit/s. Optical fibers106comprise thin strands of glass capable of communicating the signals over long distances with very low loss. Optical fibers106may comprise a suitable type of fiber selected from a variety of different fibers for optical transmission. Optical fibers106may include any suitable type of fiber, such as a Single-Mode Fiber (SMF), Enhanced Large Effective Area Fiber (E-LEAF), or TrueWave® Reduced Slope (TW-RS) fiber.

Optical network101may include devices to transmit optical signals over optical fibers106. Information may be transmitted and received through optical network101by modulation of one or more wavelengths of light to encode the information on the wavelength. In optical networking, a wavelength of light may also be referred to as a channel that is included in an optical signal (also referred to herein as a “wavelength channel”). Each channel may carry a certain amount of information through optical network101.

To increase the information capacity and transport capabilities of optical network101, multiple signals transmitted at multiple channels may be combined into a single wideband optical signal. The process of communicating information at multiple channels is referred to in optics as wavelength division multiplexing (WDM). Coarse wavelength division multiplexing (CWDM) refers to the multiplexing of wavelengths that are widely spaced having low number of channels, usually greater than 20 nm and less than sixteen wavelengths, and dense wavelength division multiplexing (DWDM) refers to the multiplexing of wavelengths that are closely spaced having large number of channels, usually less than 0.8 nm spacing and greater than forty wavelengths, into a fiber. WDM or other multi-wavelength multiplexing transmission techniques are employed in optical networks to increase the aggregate bandwidth per optical fiber. Without WDM, the bandwidth in optical networks may be limited to the bit-rate of solely one wavelength. With more bandwidth, optical networks are capable of transmitting greater amounts of information. Optical network101may transmit disparate channels using WDM or some other suitable multi-channel multiplexing technique, and to amplify the multi-channel signal.

Optical network101may include one or more optical transmitters (Tx)102to transmit optical signals through optical network101in specific wavelengths or channels. Transmitters102may comprise a system, apparatus or device to convert an electrical signal into an optical signal and transmit the optical signal. For example, transmitters102may each comprise a laser and a modulator to receive electrical signals and modulate the information contained in the electrical signals onto a beam of light produced by the laser at a particular wavelength, and transmit the beam for carrying the signal throughout optical network101.

Multiplexer104may be coupled to transmitters102and may be a system, apparatus or device to combine the signals transmitted by transmitters102, e.g., at respective individual wavelengths, into a WDM signal.

Optical amplifiers108may amplify the multi-channeled signals within optical network101. Optical amplifiers108may be positioned before or after certain lengths of fiber106. Optical amplifiers108may comprise a system, apparatus, or device to amplify optical signals. For example, optical amplifiers108may comprise an optical repeater that amplifies the optical signal. This amplification may be performed with opto-electrical or electro-optical conversion. In some embodiments, optical amplifiers108may comprise an optical fiber doped with a rare-earth element to form a doped fiber amplification element. When a signal passes through the fiber, external energy may be applied in the form of an optical pump to excite the atoms of the doped portion of the optical fiber, which increases the intensity of the optical signal. As an example, optical amplifiers108may comprise an erbium-doped fiber amplifier (EDFA).

OADMs110may be coupled to optical network101via fibers106. OADMs110comprise an add/drop module, which may include a system, apparatus or device to add and drop optical signals (for example at individual wavelengths) from fibers106. After passing through an OADM110, an optical signal may travel along fibers106directly to a destination, or the signal may be passed through one or more additional OADMs110and optical amplifiers108before reaching a destination.

In certain embodiments of optical network101, OADM110may represent a reconfigurable OADM (ROADM) that is capable of adding or dropping individual or multiple wavelengths of a WDM signal. The individual or multiple wavelengths may be added or dropped in the optical domain, for example, using a wavelength selective switch (WSS) (not shown) that may be included in a ROADM.

As shown inFIG. 1, optical network101may also include one or more demultiplexers105at one or more destinations of network101. Demultiplexer105may comprise a system apparatus or device that acts as a demultiplexer by splitting a single composite WDM signal into individual channels at respective wavelengths. For example, optical network101may transmit and carry a forty (40) channel DWDM signal. Demultiplexer105may divide the single, forty channel DWDM signal into forty separate signals according to the forty different channels.

InFIG. 1, optical network101may also include receivers112coupled to demultiplexer105. Each receiver112may receive optical signals transmitted at a particular wavelength or channel, and may process the optical signals to obtain (e.g., demodulate) the information (i.e., data) that the optical signals contain. Accordingly, network101may include at least one receiver112for every channel of the network.

Optical networks, such as optical network101inFIG. 1, may employ modulation techniques to convey information in the optical signals over the optical fibers. Such modulation schemes may include phase-shift keying (PSK), frequency-shift keying (FSK), amplitude-shift keying (ASK), and quadrature amplitude modulation (QAM), among other examples of modulation techniques. In PSK, the information carried by the optical signal may be conveyed by modulating the phase of a reference signal, also known as a carrier wave, or simply, a carrier. The information may be conveyed by modulating the phase of the signal itself using two-level or binary phase-shift keying (BPSK), four-level or quadrature phase-shift keying (QPSK), multi-level phase-shift keying (M-PSK) and differential phase-shift keying (DPSK). In QAM, the information carried by the optical signal may be conveyed by modulating both the amplitude and phase of the carrier wave. PSK may be considered a subset of QAM, wherein the amplitude of the carrier waves is maintained as a constant.

Additionally, polarization division multiplexing (PDM) technology may enable achieving a greater bit rate for information transmission. PDM transmission comprises independently modulating information onto different polarization components of an optical signal associated with a channel. In this manner, each polarization component may carry a separate signal with other polarization components, thereby enabling the bit rate to be increased according to the number of individual polarization components. The polarization of an optical signal may refer to the direction of the oscillations of the optical signal. The term “polarization” may generally refer to the path traced out by the tip of the electric field vector at a point in space, which is perpendicular to the propagation direction of the optical signal.

In an optical network, such as optical network101inFIG. 1, it is typical to refer to a management plane, a control plane, and a transport plane (sometimes called the physical layer). A central management host (not shown) may reside in the management plane and may configure and supervise the components of the control plane. The management plane includes ultimate control over all transport plane and control plane entities (e.g., network elements). As an example, the management plane may consist of a central processing center (e.g., the central management host), including one or more processing resources, data storage components, etc. The management plane may be in electrical communication with the elements of the control plane and may also be in electrical communication with one or more network elements of the transport plane. The management plane may perform management functions for an overall system and provide coordination between network elements, the control plane, and the transport plane. As examples, the management plane may include an element management system (EMS) which handles one or more network elements from the perspective of the elements, a network management system (NMS) which handles many devices from the perspective of the network, and an operational support system (OSS) which handles network-wide operations.

Modifications, additions or omissions may be made to optical network101without departing from the scope of the disclosure. For example, optical network101may include more or fewer elements than those depicted inFIG. 1. Also, as mentioned above, although depicted as a point-to-point network, optical network101may comprise any suitable network topology for transmitting optical signals such as a ring, a mesh, and a hierarchical network topology.

As discussed above, XPM may occur in which one wavelength of light can alter the phase of another wavelength of light, such as among the channels of a WDM optical signal. Phase modulation from one WDM channel to another WDM channel may be apparent as a power variation that occurs due to dispersion of the optical signal. Therefore, XPM compensators are known that modulate an entire optical path or optical span between two nodes. While some XPM compensation systems may be effective in improving signal quality when relatively few channels are present (less than about 15 channels), certain XPM compensation systems may actually have a negative effect on optical signal-to-noise ratio (OSNR) as the number of channels increases (greater than about 15 channels).

As will be described in further detail, methods and systems are disclosed herein for implementing a multi-channel optical XPM compensator. The multi-channel optical XPM compensator disclosed herein may enable XPM to be compensated for all channels in a multi-channel WDM optical signal, even for large numbers of channels greater than 15 channels. The multi-channel optical XPM compensator disclosed herein may provide a feed-forward XPM compensation loop with a dispersion compensation module (DCM) to simulate dispersion along an effective length of a subsequent fiber optic span. The multi-channel optical XPM compensator disclosed herein may further be used in configurations that enable simultaneous XPM compensation for all WDM channels, without having to introduce a delay in the propagation of individual WDM channels. The multi-channel optical XPM compensator disclosed herein may be implemented using various spectral overlap schemes to optimize XPM compensation.

In operation of optical network101, for example, ROADM nodes included in optical network101may be equipped with the multi-channel optical XPM compensator disclosed herein.

Referring now toFIG. 2A, a block diagram of selected elements of an example embodiment of an XPM compensator200-1is depicted. InFIG. 2A, XPM compensator200-1is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensator200-1may be operated with additional or fewer elements.

InFIG. 2A, XPM compensator200-1includes a feed-forward control loop that extends from optical tap202to phase modulator204, which are placed along a WDM optical path having input WDM optical signal210and output WDM optical signal220. It is noted that different arrangements of components in the feed-forward loop in both the optical and the electrical domain may be implemented in different embodiments. At optical tap202(also referred to as an optical splitter), a portion of input WDM optical signal210is diverted to the feed-forward control loop. Specifically, DCM206receives the optical signal from optical tap202and is enabled to add a certain amount of dispersion into the feed-forward control loop in order to enable XPM compensation of the optical signal in an effective length of the optical fiber subsequent to XPM compensator200-1. Because chromatic dispersion (CD) results in pulse spreading and inter-symbol interference (ISI), the addition of dispersion at DCM206may result in improved XPM compensation in the feed-forward loop by simulating XPM that is caused by a power variation of the optical signal along the effective length. Specifically, the dispersion may correspond to a calculated fraction of the effective length, where the fraction is between 0 and 1. After DCM206, photodiode212(or another type of photosensor) receives the optical signal in the feed-forward loop and generates a corresponding electrical signal. As shown in XPM compensator200-1, an RF amplifier208may then amplify the electrical signal received from photodiode212. Then, a low pass filter (LPF)213may be applied to the electrical signal output by RF amplifier208. After LPF213, a variable delay214applies a time delay to compensate for path length variations before outputting the electrical signal to phase modulator204. In the exemplary configuration ofFIG. 2A, the optical path between tap202and phase modulator204is assumed to be long enough such that variable delay214is capable of tuning or matching the delay between the optical signal arriving at phase modulator214and the feed-forward signal at variable delay214. Phase modulator204may operate to modulate the phase of WDM input optical signal210, based on a received portion of WDM input optical signal210from optical tap202, to generate output WDM optical signal220, which is XPM compensated.

Referring now toFIG. 2B, a block diagram of selected elements of an example embodiment of an XPM compensator200-2is depicted. InFIG. 2B, XPM compensator200-2is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensator200-2may be operated with additional or fewer elements.

InFIG. 2B, XPM compensator200-2includes all the same elements depicted with regard to XPM compensator200-1inFIG. 2A. Additionally, XPM compensator200-2includes an optical bandpass filter (OBPF)216, which may be used to select an optical band from input WDM optical signal210, such as an optical band including a discrete number of optical channels. When OBPF216is used to isolate center wavelength (non-edge wavelength) channels, some improvement in XPM for center wavelength channels may be observed. However, because OBPF216uses a more narrowband for the feed-forward loop than input WDM optical signal210, XPM compensation for edge wavelength channels may suffer, because signal intensity from neighboring channels outside the bandpass of OBPF216is not detected for feed-forward compensation and does not contribute to XPM compensation in XPM compensator200-2.

Referring now toFIG. 2C, a block diagram of selected elements of an example embodiment of an XPM compensator200-3is depicted. InFIG. 2C, XPM compensator200-3is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensator200-3may be operated with additional or fewer elements.

InFIG. 2C, XPM compensator200-3includes a feed-forward control loop that receives an external input222and does not rely on an optical tap202from input WDM optical signal210. In this manner, XPM compensator200-3may be integrated into various ROADM environments that use a WSS (see alsoFIG. 6). After receiving external input222, the feed-forward loop in XPM compensator200-3may include the same elements as described above with respect to XPM compensator200-2inFIG. 2B.

Referring now toFIG. 3, selected elements of an embodiment of an XPM compensation example300are depicted.FIG. 3, XPM compensation example300is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensation example300may include additional or fewer elements.

In XPM compensation example300shown inFIG. 3, it is assumed that an input WDM optical signal310consists of nine wavelength channels, shown successively as λ1through λ9. It is noted that in various embodiments, different numbers of channels may be included in input WDM optical signal310and different numbers of XPM compensators200may be used in a variety of different spectral allocation schemes, as desired. XPM compensation example300illustrates a spectral allocation scheme in which three instances of XPM compensator200-1are used in parallel to compensate XPM on subbands of input WDM optical signal310. At splitter304, input WDM optical signal310may be split into three separate fibers to OBPF308-1,308-2,308-3in parallel. Each OBPF308may be programmed to pass a certain subband of input WDM optical signal310. In the example embodiment shown inFIG. 3, each OBPF308passes a subband including 3 wavelength channels. Accordingly, OBPF308-1passes wavelengths λ1, λ2, λ3; OBPF308-2passes wavelengths λ4, λ5, λ6; and OBPF308-3passes wavelengths λ7, λ8, λ9. At combiner306, the XPM compensated subbands are combined to form output WDM optical signal320.

Referring now toFIG. 4, selected elements of an embodiment of an XPM compensation example400are depicted.FIG. 4, XPM compensation example400is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensation example400may include additional or fewer elements.

In XPM compensation example400shown inFIG. 4, an arrangement using WSS404,406instead of splitter304and combiner306fromFIG. 3is shown. In comparison to splitter304and combiner406, the use of two WSS provides the ability to select individual channels to add and drop from a subband. In one exemplary embodiment, the same spectral allocation scheme described above with respect toFIG. 3may be implemented using XPM compensation example400, in which WSS404passes each subband in parallel from input WDM optical signal310to a respective XPM compensator200-1, while WSS406is used to recombine the subbands into output WDM optical signal320, which is XPM compensated. It is noted that in various embodiments, different numbers of channels may be included in input WDM optical signal310and different numbers of XPM compensators200may be used in a variety of different spectral allocation schemes, as desired.

It is further noted that XPM compensation example400inFIG. 4may be used to implement various different spectral allocation schemes, such as described below with respect toFIG. 5.

Referring now toFIG. 5, selected elements of an embodiment of an XPM compensation example500are depicted.FIG. 5, XPM compensation example500is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensation example500may include additional or fewer elements.

In XPM compensation example500shown inFIG. 5, it is assumed that an input WDM optical signal310consists of nine wavelength channels, as shown inFIG. 3. It is noted that in various embodiments, different numbers of channels may be included in input WDM optical signal310and different numbers of XPM compensators200may be used in a variety of different spectral allocation schemes, as desired. XPM compensation example500illustrates a spectral allocation scheme in which three instances of XPM compensator200-1are used in parallel to compensate XPM on subbands of input WDM optical signal310. At splitter304, input WDM optical signal310may be split into three separate fibers to OBPF308-1,308-2,308-3in parallel. Each OBPF308may be programmed to pass a certain subband of input WDM optical signal310. In the example embodiment shown inFIG. 5, each OBPF308passes a subband including a different number of wavelength channels. As shown, OBPF308-1passes wavelengths λ1, λ2, λ3, λ4; OBPF308-2passes wavelengths λ2, λ3, λ4, λ5, λ6, λ7; and OBPF308-3passes wavelengths λ5, λ6, λ7, λ8, λ9. Then, in XPM compensation example500, a second OBPF516is used to remove the overlapped wavelength channels. Accordingly, OBPF516-1passes wavelengths λ1, λ2, λ3; OBPF516-2passes wavelengths λ4, λ5, λ6; and OBPF516-3passes wavelengths λ7, λ8, λ9. The use of overlapped spectra in XPM compensation example500may improve XPM compensation in the respective feed-forward loops of XPM compensator200-1, while channels with poorer XPM compensation may be dropped. It is noted that gain equalization (not shown) may be applied in XPM compensation example500after OBPF516, depending on the actual spectrum overlap scheme used. Then, the XPM compensated subbands are combined at combiner306to form output WDM optical signal320.

It is noted that the spectral allocation described above may be implemented using XPM compensation example400shown inFIG. 4. For example, first WSS404may switch the spectral subbands of wavelength channels to respective XPM compensator200-1, while second WSS406may drop the overlapped wavelength channels.

Referring now toFIG. 6, selected elements of an embodiment of an XPM compensation example600are depicted.FIG. 6, XPM compensation example600is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensation example600may include additional or fewer elements.

In XPM compensation example600shown inFIG. 6, it is assumed that an input WDM optical signal310consists of nine wavelength channels, as shown inFIG. 3. It is noted that in various embodiments, different numbers of channels may be included in input WDM optical signal310and different numbers of XPM compensators200may be used in a variety of different spectral allocation schemes, as desired. XPM compensation example600illustrates a spectral allocation scheme in which three instances of XPM compensator200-3are used in parallel to compensate XPM on subbands of input WDM optical signal310. At splitter304, input WDM optical signal310may be split into four separate fibers in parallel: one fiber may be used as an input degree602for WSS404, while the other three fibers may be used as external inputs222for each respective XPM compensator200-3. Each OBPF216in XPM compensator200-3(seeFIG. 2C) may be programmed to pass a certain subband of external input222, which carries input WDM optical signal310. In the example embodiment shown inFIG. 6, output degree610-1from WSS404may pass wavelengths λ1, λ2, λ3; output degree610-2from WSS404may pass wavelengths λ4, λ5, λ6; and output degree610-3from WSS404may pass wavelengths λ7, λ8, λ9. Concurrently, each external input signal222may be subject to OBPF216in respective XPM compensator200-3, such that external input signal222-1is spectrally narrowed to a passed subband having wavelengths λ1, λ2, λ3, λ4; external input signal222-2is spectrally narrowed to a passed subband having wavelengths λ2, λ3, λ4, λ5, λ6, λ7; and external input signal222-2is spectrally narrowed to a passed subband having wavelengths λ5, λ6, λ7, λ8, λ9. It is noted that in some embodiments, external input signal222-2may be narrowed using an OBPF that is external to XPM compensator200-3. Then, XPM compensated subband620-1includes wavelengths λ1, λ2, λ3; XPM compensated subband620-2includes wavelengths λ4, λ5, λ6; and XPM compensated subband620-3includes wavelengths λ7, λ8, λ9. The use of overlapped spectra in XPM compensation example600may improve XPM compensation in the respective feed-forward loops of XPM compensator200-3, while channels with poorer XPM compensation may be dropped. It is noted that gain equalization (not shown) may be applied in XPM compensation example600, depending on the actual spectrum overlap scheme used. Then, the XPM compensated subbands620are combined at WSS406to form output WDM optical signal320.

Referring now toFIG. 7A, a block diagram of selected elements of an example embodiment of an XPM compensator700-1with polarization diversity is depicted. InFIG. 7A, XPM compensator700-1is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensator700-1may be operated with additional or fewer elements.

InFIG. 7A, it is assumed that input WDM optical signal710has polarization diversity, such that an X-polarized component and a Y-polarized component of the optical signal are present. XPM compensator700-1includes a feed-forward control loop that extends from optical tap202to phase modulators704, which are placed along a WDM optical path having input WDM optical signal710and output WDM optical signal720. At optical tap202(also referred to as an optical splitter), a portion of input WDM optical signal710is diverted to the feed-forward control loop. Specifically, DCM206receives the optical signal from optical tap202and is enabled to add a certain amount of dispersion into the feed-forward control loop, as described above with respect toFIG. 2A. After DCM206, a polarization beam splitter (PBS)702further splits the optical signal into the X-polarized component and the Y-polarized component. The X-polarized component is fed from PBS702to photodiode212-X, which generates an electrical signal that is amplified by RF amplifier208-X and filtered using LPF213-X. The Y-polarized component is fed from PBS702to photodiode212-Y, which generates an electrical signal that is amplified by RF amplifier208-Y and filtered using LPF213-Y. Then, an variable combiner722may be applied to the electrical signals from LPF213-X,213-Y, using inputs p, q and output r, s, such that: r=h11p+h12q, s=h21p+h22q, where h is a weighting factor. In one example, h11=h12=h21=h22=0.5, although different values may be used in different embodiments. Furthermore, variable delays714-X and718-X are used before and after variable combiner722for the X-polarized component signal, while variable delays714-Y and718-Y are used before and after variable combiner722for the Y-polarized component signal. Then, variable delay718-X outputs a control signal for the X-polarized component to X-phase modulator704-X, while variable delay718-Y outputs a control signal for the Y-polarized component to Y-phase modulator704-Y, to generate output WDM optical signal720, which is XPM compensated with polarization diversity.

Referring now toFIG. 7B, a block diagram of selected elements of an example embodiment of an XPM compensator700-2with polarization diversity is depicted. InFIG. 7B, XPM compensator700-2is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensator700-2may be operated with additional or fewer elements.

InFIG. 7B, it is assumed that input WDM optical signal710has polarization diversity, such that an X-polarized component and a Y-polarized component of the optical signal are present. XPM compensator700-2includes a feed-forward control loop that extends from optical tap202to phase modulators204, which are placed along a WDM optical path having input WDM optical signal710and output WDM optical signal720. Input WDM optical signal710is received at PBS702, which separates the X-polarized component and the Y-polarized component along different optical fibers. At optical tap202-X, a portion of the X-polarized component is diverted to an X-polarization feed-forward control loop, while at optical tap202-Y, a portion of the Y-polarized component is diverted to a Y-polarization feed-forward control loop. Specifically, in the X-polarization feed forward control loop, DCM206-X receives the optical signal from optical tap202-X and is enabled to add a certain amount of dispersion into the X feed-forward control loop, as described above with respect toFIG. 2A, while in the Y-polarization feed forward control loop, DCM206-Y receives the optical signal from optical tap202-Y and is enabled to add a certain amount of dispersion into the Y feed-forward control loop. After DCM206-X, the X-polarized component is fed to photodiode212-X, which generates an electrical signal that is amplified by RF amplifier208-X and filtered using LPF213-X. After DCM206-Y, the Y-polarized component is fed to photodiode212-Y, which generates an electrical signal that is amplified by RF amplifier208-Y and filtered using LPF213-Y. Then, variable combiner722may be applied to the electrical signals from LPF213-X,213-Y, as described above with respect toFIG. 7A, including variable delays714-X and718-X that are used before and after variable combiner722for the X-polarized component signal, and variable delays714-Y and718-Y that are used before and after variable combiner722for the Y-polarized component signal. Then, variable delay718-X outputs a control signal for the X-polarized component to a first phase modulator204-1, while variable delay718-Y outputs a control signal for the Y-polarized component to a second phase modulator204-2. The output signals from phase modulators204are combined at polarization beam combiner724to generate output WDM optical signal720, which is XPM compensated with polarization diversity.

Referring now toFIG. 7C, a block diagram of selected elements of an example embodiment of an XPM compensator700-3with polarization diversity is depicted. InFIG. 7C, XPM compensator700-3is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensator700-3may be operated with additional or fewer elements.

InFIG. 7C, it is assumed that input WDM optical signal710has polarization diversity, such that an X-polarized component and a Y-polarized component of the optical signal are present. XPM compensator700-3includes a feed-forward control loop that extends from optical tap202to phase modulators704, which are placed along a WDM optical path having input WDM optical signal710and output WDM optical signal720. At optical tap202, a portion of input WDM optical signal710is diverted to the feed-forward control loop. Specifically, DCM206receives the optical signal from optical tap202and is enabled to add a certain amount of dispersion into the feed-forward control loop, as described above with respect toFIG. 2A. Photodiode212, RF amplifier208, and LPF213operate in a substantially similar manner as described with respect toFIG. 2A. After LPF213, the electrical signal is fed to variable delay714-X and variable delay714-Y in order to adjust for the X-polarized component and the Y-polarized component. The arrangement shown inFIG. 7Cmay be substantially equivalent to the use of an variable combiner with h11=h12=h21=h22=0.5, as shown in FIG.7A. Then, variable delay714-X outputs a control signal for the X-polarized component to X-phase modulator704-X, while variable delay714-Y outputs a control signal for the Y-polarized component to Y-phase modulator704-Y, to generate output WDM optical signal720, which is XPM compensated with polarization diversity.

Referring now toFIG. 7D, a block diagram of selected elements of an example embodiment of an XPM compensator700-4with polarization diversity is depicted. InFIG. 7D, XPM compensator700-4is shown in a schematic representation and is not drawn to scale. It is noted that, in different embodiments, XPM compensator700-4may be operated with additional or fewer elements.

InFIG. 7D, it is assumed that input WDM optical signal710has polarization diversity, such that an X-polarized component and a Y-polarized component of the optical signal are present. XPM compensator700-4includes a feed-forward control loop that extends from optical tap202to phase modulators204, which are placed along a WDM optical path having input WDM optical signal710and output WDM optical signal720. At optical tap202, a portion of input WDM optical signal710is diverted to the feed-forward control loop, while the remaining portion is diverted to PBS702. Specifically, DCM206receives the optical signal from optical tap202and is enabled to add a certain amount of dispersion into the feed-forward control loop, as described above with respect toFIG. 2A. Photodiode212, RF amplifier208, and LPF213operate in a substantially similar manner as described with respect toFIG. 2A. After LPF213, the electrical signal is fed to variable delay714-X and variable delay714-Y in order to adjust for the X-polarized component and the Y-polarized component. The arrangement shown inFIG. 7Dmay be substantially equivalent to the use of an variable combiner with h11=h12=h21=h22=0.5, as shown inFIG. 7A. Then, variable delay714-X outputs a control signal for the X-polarized component to a first phase modulator204-1, while variable delay714-Y outputs a control signal for the Y-polarized component to a second phase modulator204-2. The first phase modulator204-1receives the X-polarized component from PBS702, while the second phase modulator204-2receives the Y-polarized component from PBS702. The outputs from the first and second phase modulators204, corresponding to the X-polarized component and the Y-polarized component, are combined at PBC724to generate output WDM optical signal720, which is XPM compensated with polarization diversity.

Referring now toFIG. 8, a flowchart of selected elements of an embodiment of a method800for XPM compensation, as described herein, is depicted. In various embodiments, method800may be performed using XPM compensators200,700in a ROADM node in an optical network, for example, corresponding to XPM compensation examples400,600. It is noted that certain operations described in method800may be optional or may be rearranged in different embodiments.

Method800may begin at step802by switching groups of adjacent channels included in a WDM optical signal provided as input to a first WSS. At step804, a first group of the groups of adjacent channels is received from the first WSS at a first XPM compensator including a DCM. At step806, an XPM compensated first group is received at a second WSS.

Referring now toFIG. 9, a flowchart of selected elements of an embodiment of method900for XPM compensation, as described herein, is depicted. In various embodiments, method900may be performed by XPM compensators200,700in a ROADM node in an optical network, for example, in XPM compensation examples400,600. It is noted that certain operations described in method900may be optional or may be rearranged in different embodiments.

Method900may begin at step902by generating an XPM control signal using a feed-forward XPM regulation loop, the feed-forward XPM regulation loop including the DCM to add dispersion corresponding to a fraction of an effective length of a fiber optic span carrying the WDM optical signal subsequent to the second WSS. At step902, the first group and the XPM control signal are sent to a phase modulator to output the XPM compensated first group.

As disclosed herein, method and system for multi-channel optical XPM compensation may include a DCM to improve performance of a feed-forward control loop in an optical path in an optical network. Additionally, various spectral overlap schemes may be used with multi-channel WDM optical signals using XPM compensators in parallel, such as at a ROADM node. Polarization diversity may also be supported for XPM compensation including a DCM.