Method and system for monitoring optical channels

In accordance with one embodiment of the present disclosure, a system for optical signal dispersion and parameter monitoring comprises a tunable filter configured to filter a portion of one channel of an optical signal. The system comprises a polarization beam splitter configured to split the portion into first and second polarization beams and further comprises first and second photodetectors configured to respectively convert the first and second polarization beams into first and second electrical signals. Also, the system comprises a control unit configured to determine optical dispersion in the portion based on the first and second electrical signals when the portion includes a test signal. The control unit is configured to determine optical signal parameters of the portion such as channel power, channel wavelength, optical spectrum analysis (OSA) and optical signal-to-noise ratio (OSNR) based on the first and second electrical signals when the portion does not include the test signal.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates generally to optical networks and, more particularly, to a method and system for monitoring optical channels.

BACKGROUND

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 comprise thin strands of glass capable of communicating the signals over long distances with very low loss. Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths, thereby increasing network capacity.

An optical signal comprised of disparate wavelengths and modes may experience optical dispersion. Optical dispersion is an often undesirable phenomenon that causes the separation of an optical wave into spectral components with different frequencies. The separation of waves into spectral components with different frequencies may be referred to as chromatic dispersion (CD). Additionally, optical dispersion causes the separation of different modes (e.g., polarization states) of each frequency. The separation of waves according to the different modes of a frequency may be referred to as polarization mode dispersion (PMD). Optical dispersion may refer to both CD and PMD and occurs because the differing wavelengths and modes of wavelengths may propagate at differing speeds. The separation of an optical wave into its respective channels and modes due to optical dispersion may require optical dispersion compensation for the particular optical signal.

Additionally, optical signal parameters such as channel power, channel wavelength, and optical signal-to-noise ratio (OSNR) may vary among each channel due to the wavelengths of the channels being affected differently within an optical network. Accordingly, these parameters may also require monitoring for proper network operation.

SUMMARY

In accordance with one embodiment of the present disclosure, a system for optical signal dispersion and parameter monitoring comprises a tunable filter configured to receive an optical signal comprising a plurality of channels. The filter is further configured to filter a portion of one channel from the plurality of channels. During dispersion monitoring, the portion includes a test signal. The system further comprises a polarization beam splitter (PBS) coupled to the tunable filter and configured to receive the portion from the tunable filter and split the portion into a first polarization beam and a second polarization beam. The system also comprises a first photodetector coupled to the PBS and configured to receive the first polarization beam from the PBS and convert the first polarization beam into a first electrical signal. The system additionally comprises a second photodetector coupled to the PBS and configured to receive the second polarization beam from the PBS and convert the second polarization beam into a second electrical signal. Also, the system comprises a control unit coupled to the first photodetector and the second photodetector. The control unit is configured to receive the first and second electrical signals and determine optical dispersion in the portion based on the first and second electrical signals when the portion includes the test signal. The control unit is additionally configured to determine an optical signal parameter of the portion selected from the group consisting of channel power, channel wavelength, optical spectrum analysis (OSA) and optical signal-to-noise ratio (OSNR) based on the first and second electrical signals when the portion does not include the test signal.

DETAILED DESCRIPTION

FIG. 1is a block diagram illustrating one embodiment of an optical network10including components for monitoring optical dispersion and optical signal parameters of optical channels. Optical network10may include one or more optical fibers28operable to transport one or more optical signals communicated by components of the optical network10. The components of optical network10, coupled together by optical fiber28, include nodes12aand12b. Although the optical network10is shown as a point-to-point optical network with terminal nodes, the optical network10may also be configured as a ring optical network, a mesh optical network, or any other suitable optical network or combination of optical networks, and may include any number of nodes intermediate to nodes12aand12b. The optical network10may be used in a short-haul metropolitan network, a long-haul inter-city network, or any other suitable network or combination of networks.

Node12amay include transmitters14a,14band14c, a multiplexer18, an amplifier26a, a tunable transmitter32, a polarization controller33a filter controller42and a tunable filter34a. Transmitters14a,14band14cmay include any transmitter or other suitable device operable to transmit optical signals. Each transmitter14a,14bor14cmay be configured to receive information and transmit a modulated optical signal that includes the information at a certain range of wavelengths. In optical networking, each range of wavelengths may also be referred to as a channel. Each transmitter14a,14bor14cmay also be configured to transmit this optically encoded information on the associated wavelength. In the present embodiment and as discussed further below, transmitters14a,14band14cmay be configured to transmit information at different bit rates (e.g., ten gigabits per second (Gb/s) for transmitter14a, forty Gb/s for transmitter14band one hundred Gb/s for transmitter14c). The information transmitted by transmitters14a,14band14cmay be referred to as “traffic.”

The multiplexer18may include any multiplexer or combination of multiplexers or other devices operable to combine different channels. Multiplexer18may be configured to receive and combine the disparate channels transmitted by transmitters14a,14band14cinto an optical signal for communication along fibers28.

Amplifier26amay be used to amplify the multi-channeled signal. Amplifier26amay be positioned before and/or after certain lengths of fiber28. Amplifier26amay comprise an optical repeater that amplifies the optical signal. This amplification may be performed without opto-electrical or electro-optical conversion. In particular embodiments, amplifier26amay comprise an optical fiber doped with a rare-earth element. When a signal passes through the fiber, external energy may be applied to excite the atoms of the doped portion of the optical fiber, which increases the intensity of the optical signal. As an example, amplifier26amay comprise an erbium-doped fiber amplifier (EDFA). However, any other suitable amplifier26amay be used.

Tunable transmitter32may comprise any transmitter or other suitable device configured to transmit one or more test signals at one or more selected wavelengths that may be combined with the multi-channel signal amplified by amplifier26aof node12a. In the same or alternative embodiments, the optical signals transmitted by tunable transmitter32may be varied among one or more selected wavelengths. Polarization controller33may comprise any suitable system, apparatus or device configured to change the polarization of signals leaving tunable transmitter32. In some embodiments, polarization controller33may be configured to change the polarization of the test signal, such that the polarization of the test signal cycles through a large number of polarization states. This changing and cycling through the various polarization states may be referred to as “scrambling” the polarization of the test signal.

Tunable filter34amay include any filter or other suitable device configured to receive optical signals forwarded by polarization controller33and amplifier26aof node12a, remove or filter out one or more portions of a channel of the multi-channel signal from amplifier26a, and combine the remaining multi-channel signal with the optical test signal from tunable filter32. In some embodiments, the channels filtered from the multi-channel signal from amplifier26amay be of the same wavelength as the one or more channels communicated from tunable transmitter32. In the same or alternative embodiments, tunable filter34amay be a Fabry-Perot etalon filter.

Filter controller42may be any device configured to track the wavelength or wavelengths of the test signal of tunable transmitter32and communicate control signals to tunable filter34abased on the wavelength of the test signal. In turn, tunable filter34amay be configured to remove or filter out a portion of a channel of the multi-channel signal received by tunable filter34afrom amplifier26abased on the control signals communicated from filter controller42, such that the filtered-out portion of the channel is of the wavelengths of the test signal generated by tunable transmitter32. In some embodiments, filter controller42may receive a test signal from tunable transmitter32of node12a, and based on the received test signal, may communicate control signals to tunable filter34a.

The process of communicating information at multiple channels of a single optical signal is referred to in optics as wavelength division multiplexing (WDM). Dense wavelength division multiplexing (DWDM) refers to the multiplexing of a larger (denser) number of wavelengths, usually greater than forty, into a fiber. WDM, DWDM, or other multi-wavelength transmission techniques are employed in optical networks to increase the aggregate bandwidth per optical fiber. Without WDM or DWDM, the bandwidth in networks would be limited to the bit rate of solely one wavelength. With more bandwidth, optical networks are capable of transmitting greater amounts of information. Referring back toFIG. 1, node12ain optical network10may be configured to transmit and multiplex disparate channels using WDM, DWDM, or some other suitable multi-channel multiplexing technique, and to amplify the multi-channel signal.

As discussed above, the amount of information that can be transmitted over an optical network varies directly with the number of optical channels coded with information and multiplexed into one signal. Therefore, an optical signal employing WDM may carry more information than an optical signal carrying information over solely one channel. An optical signal employing DWDM may carry even more information. Besides the number of channels carried, another factor that affects how much information can be transmitted over an optical network is the bit rate of transmission. The greater the bit rate, the more information may be transmitted.

Improvements and upgrades in optical network capacity generally involve either increasing the number of wavelengths multiplexed into one optical signal or increasing bit rates of information traveling on each wavelength. In either case, it is usually more cost-efficient to use, modify, or add to existing network components than to replace the entire optical system. For reasons relating to the cost of upgrading an optical system, upgrades sometimes occur in stages in which the network must support both new technologies that provide greater bandwidth and old technologies that provide less bandwidth.

Today, many existing networks transmit information at ten GB/s and modulate the optical signal using, for example, a non-return-to-zero (NRZ) modulation technique. Signal transmission upgrades include, for example, transmitting information at forty Gb/s using differential phase shift keying (DPSK) or differential quadrature phase shift keying (DQPSK) to modulate the optical signal. Since upgrading the entire optical network's transmitters would be cost-prohibitive for most optical network operators, many such operators have instead desired to upgrade their networks by incrementally replacing existing ten Gb/s NRZ transmitters with forty Gb/s DPSK or DQPSK transmitters (these types of transmitters being used only as examples). Recently, and in increasing numbers, optical systems are also including DQPSK transmitters capable of transmitting information at one hundred Gb/s.

Another challenge faced in upgrading optical networks to support higher bit rates is that of signal spectrum broadening. Signal spectrum broadening is a phenomenon by which the frequency spectrum of a signal becomes wider as the signal rate increases. For example, in some optical systems, an unmodulated signal may have a narrow signal spectrum of approximately 50 MHz, a ten Gb/s signal may have a signal spectrum of several GHz, a forty Gb/s signal may have a signal spectrum of tens of GHz and a one hundred Gb/s signal may have an even larger signal spectrum.

One challenge faced by those wishing to implement the cost-efficient strategy of integrating upgraded transmitters with existing transmitters is the challenge of optical dispersion monitoring and compensation. Even in existing WDM and DWDM networks, optical signals comprised of disparate wavelengths experience optical dispersion. Optical dispersion refers to the separation of an optical signal into its spectral components with different frequencies (e.g., chromatic dispersion (CD)), and also separation of a signal into its different modes (e.g., polarization mode dispersion (PMD)). Optical dispersion occurs because the differing wavelengths and modes propagate at differing velocities. As optical signals travel across existing optical networks and experience optical dispersion, they may receive appropriate optical dispersion compensation to achieve at least adequate performance. Specially designed dispersion compensation fibers have been developed to compensate for dispersion in an optical signal comprised of channels modulated using the same modulation technique. Additionally it is advantageous to monitor and measure parameters of optical signals (e.g., channel power, channel wavelength, optical signal-to-noise ratio (OSNR) and optical spectrum analysis (OSA)).

However, traditional approaches to optical dispersion monitoring and compensation, and optical signal parameter measuring may have many disadvantages. For example, dispersion compensation may be accomplished using fixed compensation approaches (e.g., dispersion compensating fibers) or “set and forget” approaches which may involve once-per-lifetime manual tuning These approaches are often undesirable as dispersion in an optical network may change due to temperature variations, mechanical vibrations, or other environmental conditions. Traditional dispersion monitors have not proven practicable because they require a dedicated light source and typically do not permit in-service monitoring (e.g., it is often required to take a network offline or out of service for monitoring). Additionally, dispersion monitoring and signal parameter monitoring are traditionally performed by separate components, which increases the number of components in the optical network and thus potentially increases the cost of the system.

As shown inFIG. 1, the WDM signal generated by node12amay include sets of channels using different modulation formats. In particular, the WDM signal may comprise a set of channels communicating information at ten Gb/s using NRZ modulation and transmitted by transmitters14a. The WDM signal may also include a set of channels communicating information at forty Gb/s using nPSK modulation and transmitted by transmitters14b. The WDM signal may further include a set of channels communicating information at one hundred Gb/s using DQPSK modulation and transmitted by transmitters14c.

However, the sets of disparate channels may communicate information at any suitable bit rate and/or using any suitable modulation technique. For example, one or more of the channels may communicate information at a rate of ten, twenty, forty, eighty, one hundred Gb/s, or any other suitable bit rate. One or more of the channels may additionally communicate information using a modulation technique such as return-to-zero (RZ), carrier suppressed return-to-zero (CS-RZ), NRZ, DPSK, DQPSK, or any other suitable modulation technique. As used herein, a “set” of channels may include one or more channels and does not imply any spatial or any other unspecified relationship among the channels (for example, the channels in a set need not be contiguous). In addition, as used herein, “information” may include any information communicated, stored, or sorted in the network. This information may have at least one characteristic modulated to encode audio, video, textual, real-time, non-real-time and/or other suitable data. Additionally, information communicated in optical network10may be structured in any appropriate manner including, but not limited to, being structured as frames, packets, or an unstructured bit stream.

The signal transmitted from node12amay travel over one or more optical fibers28to node12b. An optical fiber28may include, as appropriate, a single, unidirectional fiber; a single, bi-directional fiber; or a plurality of uni- or bi-directional fibers. Although this description focuses, for the sake of simplicity, on an embodiment of the optical network10that supports unidirectional traffic, the present disclosure further contemplates a bi-directional system that includes appropriately modified embodiments of the components described below to support the transmission of information in opposite directions along the optical network10. Furthermore, as is discussed in more detail below, the fibers28may be high chromatic dispersion fibers (as an example only, standard single mode fiber (SSMF) or non-dispersion shifted fiber (NDSF)), low chromatic dispersion fibers (as an example only, non zero-dispersion-shifted fiber (NZ-DSF), such as E-LEAF fiber), or any other suitable fiber types. According to particular embodiments, different types of fiber28create the need for different dispersion compensation schemes to be applied to the signals, as discussed in further detail below.

Node12bmay be configured to receive signals transmitted over optical network10. Node12bmay include an amplifier26band an associated tunable dispersion compensation monitor (TDCM)30, a filter controller62, a tunable filter34b, a monitor36, a demultiplexer20, and receivers22a,22band22c. As described above, amplifier26bmay be used to amplify the WDM signal as it travels through the optical network10.

TDCM30may include any tunable dispersion compensator (TDC), variable dispersion compensator (VDC), other dispersion compensating device configured to perform optical dispersion compensation on a signal or set of channels comprising a signal using one or more modulation techniques, or any combination of the foregoing. Although the optical network10shows TDCM30coupled to a respective amplifier26b, TDCM30may also be positioned separately from amplifier26b.

Tunable filter34bmay be an in-line filter and may include any filter or other suitable device configured to receive optical signals via fiber28, remove or filter out one or more channels of the multi-channel signal received via fiber28, and split the one or more filtered channels from the remaining multi-channel signal. In some embodiments, the channels filtered and split from the multi-channel signal received may be of the same wavelength as the one or more channels communicated from tunable transmitter32. In the same or alternative embodiments, tunable filter34bof node12bmay be a Fabry-Perot etalon filter.

Filter controller62may be any device configured to receive an optical signal originating from tunable transmitter32of node12a, and based on the received optical signal, communicate control signals to tunable filter34b. In some embodiments, filter controller62may be configured to track the wavelength of the test signal, wherein filter controller62may communicate control signals to tunable filter34bsuch that tunable filter34bis able to track the wavelength of the test signal communicated by tunable transmitter32. In the same or alternative embodiments, filter controller62may communicate control signals to tunable filter34bbased on the signal originating from tunable transmitter32. For example, tunable transmitter32may communicate an unmodulated signal or a modulated signal modulated differently than the multi-channel signal of node12aand transmitted via fiber28. Thus, filter controller62may be able to determine the signal originating from tunable transmitter32based on the modulation (or lack of modulation) of such signal, and accordingly communicate control signals to tunable filter34b.

Monitor36may be configured to determine optical dispersion of optical signals in optical network10, including both chromatic dispersion (CD) and polarization mode dispersion (PMD). In some embodiments, monitor36may communicate control signals to TDCM30in order to control the optical dispersion compensation that TDCM30performs on signals. Monitor36may also be configured to determine one or more parameters of optical signals (e.g., channel power, channel wavelength, OSNR and OSA for each channel). Accordingly, monitor36may reduce costs over traditional systems by performing dispersion monitoring and parameter monitoring whereas traditional systems typically include a separate dispersion monitoring component and a separate parameter monitoring component.

Demultiplexer20may include any demultiplexer or other device configured to separate the disparate channels multiplexed using WDM, DWDM, or other suitable multi-channel multiplexing technique. Demultiplexer20may be configured to receive an optical signal carrying a plurality of multiplexed channels, demultiplex the disparate channels in the optical signal, and pass the disparate channels to different receivers22a,22b, and22c.

Receivers22a,22b, and22cmay include any receiver or other suitable device operable to receive an optical signal. Each receiver22a,22b, and22cmay be configured to receive a channel of an optical signal carrying encoded information and demodulate the information into an electrical signal. Additionally, each receive22a,22band22cmay be configured to receive a signal transmitted at a particular bit rate by a corresponding transmitter14a,14bor14c. In the present embodiment, each receiver22amay be configured to receive a ten Gb/s signal transmitted by a corresponding transmitter14a, each receiver22bmay be configured to receive a forty Gb/s signal transmitted by a corresponding transmitter14band each receiver22cmay be configured to receive a one hundred Gb/s signal transmitted by a corresponding transmitter14c.

In operation, transmitters14a,14band14cof node12amay transmit information at different bit rates and/or using different modulation techniques over different channels (e.g., ten Gb/s for transmitter14a, forty Gb/s for transmitter14band one hundred GB/s for transmitter14c). The multiplexer18may combine these different channels into an optical signal and communicate the signal over an optical fiber. An amplifier26may receive the optical signal, amplify the signal, and pass the signal over an optical fiber to tunable filter34a.

During dispersion monitoring, tunable transmitter32of node12amay transmit a test signal at a particular wavelength to tunable filter34a, via polarization controller33and also to filter controller42. The test signal may be transmitted at a wavelength to be monitored for dispersion within optical network10. Tunable filter34amay also receive the multi-channel signal from amplifier26aof node12a. Filter controller42may communicate control signals to tunable filter34ato remove or filter out at least a portion of a channel of the multi-channel signal that is of the same wavelength of the test signal communicated from tunable transmitter32. Additionally, filter controller42may communicate control signals to tunable filter34ato replace the filtered portion of the multi-channel signal with the test signal from tunable transmitter32. Accordingly, tunable filter34amay substitute the traffic of a portion of a channel of the multi-channel signal with a test signal such that the portion includes the test signal. Additionally, tunable filter34amay transmit the combined optical signal, including the test signal, via optical fiber28. Optical fiber28may transport the signal to node12b.

For PMD monitoring, tunable transmitter32may be configured to cycle through each channel of the optical signal to create test signals associated with each channel. The test signals may include information to aid in PMD monitoring such as expected signal wavelength shape and size. Additionally, filter controller42may be configured to communicate control signals to tunable filter34ato also cycle through each channel at the same rate and channel as tunable transmitter32. In some embodiments, tunable filter34amay be configured to filter only a portion of a channel, and tunable transmitter32may transmit a test signal at a wavelength corresponding to the filtered portion of the channel. In such instances, tunable filter34aand tunable transmitter32may cycle through a plurality of portions of a channel before cycling from one channel to the next. Further, polarization controller33may be configured to cycle through or scramble the polarization of each channel received from tunable transmitter32such that a test signal for each channel at various polarization states is generated. Polarization controller33, tunable transmitter32, filter controller42and tunable filter34amay be configured such that polarization controller33cycles through the various polarization states multiple times (e.g., one hundred times) for any particular channel, before tunable transmitter32and tunable filter34amove to another channel. Accordingly, an accurate measurement of PMD for each channel may be obtained based on the scrambled test signal for each channel.

For CD monitoring, tunable transmitter32may also transmit a test signal that contains information that aids in determining CD. Additionally, tunable transmitter32, filter controller42and tunable filter34amay be configured such that tunable transmitter32and tunable filter34acycle through each channel similarly to the cycling done for PMD monitoring. During CD monitoring, test signals corresponding to different channels are compared to determine CD for the WDM signal, which is in contrast to PMD monitoring where the polarization modes of test signals associated with each channel are compared to determine PMD for each channel. Accordingly, tunable transmitter32may cycle through the channels to create test signals that may be compared to obtain a CD measurement, whereas in PMD measuring tunable transmitter32may cycle through channels to determine a PMD measurement for each channel. However, because CD monitoring may not require comparing different polarization states within each channel, like PMD monitoring, the polarization scrambling may not be necessary to measure CD, whereas it may be needed to measure PMD. Therefore, polarization controller33may be configured such that the polarization of the channels is not scrambled during CD monitoring, as is done in PMD monitoring.

During dispersion monitoring (e.g., CD and PMD monitoring), filter controller62may be configured to track the wavelength of the test signal originating from tunable transmitter32and communicate control signals to tunable filter34bto split a portion of the channel that includes the test signal from the remainder of the WDM signal. Accordingly, tunable filter34bmay transmit the test signal, originating from tunable transmitter32, to monitor36, and transmit the remainder of the multi-channel WDM signal to amplifier26bof node12b.

During monitoring of parameters other than dispersion (e.g., power, wavelength, OSA, OSNR, etc.), tunable transmitter32may be deactivated such that a test signal is not transmitted to allow for monitoring of the actual signal instead of a test signal. Further, filter controller42may direct tunable filter34ato allow all channels of the WDM signal to pass through it without filtering a channel associated with a test signal. Therefore, while monitoring parameters other than dispersion, filter34bmay receive the entire multi-channel signal. Additionally, filter controller62may direct tunable filter34bto scan through the various channels transmitted by transmitters14a,14band14c. As tunable filter34bscans through the channels, tunable filter34bmay filter out and split a portion of a channel from the remainder of the multi-channel signal. Tunable filter34bmay transmit the portion of the channel to monitor36and transmit the remainder of the multi-channel signal to amplifier26b.

During dispersion monitoring, monitor36may analyze the test signal originating from tunable transmitter32to measure the chromatic dispersion, polarization mode dispersion, and/or other dispersion experienced by the portion of the test signal. Based on the measured dispersion, monitor36may communicate control signals to TDCM30. Based on the control signals communicated to TDCM30from monitor36, TDCM30may perform optical dispersion compensation on the signal communicated to amplifier26bfrom tunable filter34b. During the monitoring of parameters other than dispersion, monitor36may analyze the signal to determine the power, wavelength, OSNR, OSA, or other parameters associated with one or more channels of the signal.

Amplifier26bmay amplify the communicated signal that is passed through tunable filter34b. Demultiplexer20of node12bmay receive the signal, demultiplex the signal into the signal's constituent channels, and pass the signal's constituent channels. Each channel may be received by an associated receiver22a,22bor22cof node12band be forwarded.

Advantageously, optical system10ofFIG. 1, may overcome traditional approaches to optical dispersion monitoring and compensation, as it permits in-service, per-channel dispersion monitoring and compensation. For example, to monitor dispersion on a particular channel, a test signal having a wavelength of a particular channel may be communicated from tunable transmitter32, and at least a portion of the channel, of the same wavelength as the test signal, may be filtered from the multi-channel signal communicated from amplifier26aof node12a.

In some embodiments, such filtering may be performed by filtering a narrow portion of the channel (e.g., the filter bandwidth may be more narrow than the modulated signal spectrum width). For example, a channel may have a width of 0.4 nanometers (nm) and the tunable filter may filter a 0.06 nm wide portion of the channel, thus leaving the channel largely undisturbed. In other embodiments, the portion may have a width of 0.01 nm, leaving even more of the channel undisturbed. The narrow filtering of a channel may be advantageous by filtering a small enough portion of the channel to allow the receiver associated with the channel to receive and interpret the information transmitted within the channel. Additionally, the test signal transmitted by tunable transmitter32may replace the filtered portion of the channel such that the remaining portion of the multi-channel signal may then be combined with the test signal and communicated to a node12bcomprising dispersion monitoring and compensation devices.

As mentioned above, filter controller62may be configured to track the test signal. Accordingly, filter controller62may be configured to direct tunable filter34bto filter the portion of the channel that includes the test signal such that the test signal may then be filtered by tunable filter34bof node12b, and analyzed for optical dispersion for the particular channel. Therefore, all the other channels and the portion of the channel under consideration not filtered by tunable filter34bmay pass through to their respective receivers22a,22band22c. Accordingly, optical system10requires that only a portion of a single channel of a multi-channel signal be directed to an optical monitor for monitoring and compensation while the multi-channel signal remains in-service, including the channel with the portion that includes the test signal. Because only a portion of one channel is redirected to a monitor, the approaches set forth in this disclosure may lead to a largely negligible or minimal effect on network communication throughput.

In addition, when performing dispersion monitoring and compensation, the test signal of tunable transmitter32may be swept through numerous wavelengths, and the dispersion for each such channel may be monitored and compensated, allowing for the dispersion monitoring and compensation of multiple channels in the optical network, all the while requiring only a portion of one channel to be out-of-service at a time, again leading to a largely negligible or minimal effect on network communication throughput.

Also, during measurement of parameters other than dispersion, the narrow filtering of a portion of a channel may allow parameters such as OSNR, OSA, power and wavelength to be monitored with little to no effect on network communication. Further, by performing dispersion monitoring and other parameter monitoring with the same device, the cost of implementing monitoring systems may also be decreased.

FIG. 2is a block diagram illustrating an embodiment of an optical network100including components for monitoring and compensation for optical dispersion. Optical network100is similar to that of optical network10ofFIG. 1, with modifications. Accordingly, similar elements will not be described again with respect toFIG. 2.

Optical network100may include one or more optical fibers28operable to transport one or more optical signals communicated by components of the optical network100. The components of optical network100, coupled together by optical fiber28, may include nodes112aand112b. Node112amay be substantially similar to node12aofFIG. 1. Node112bmay be similar to node12b, except that node112bmay include a tap (not shown) configured to tap a percentage, such as two percent, of the WDM signal received from amplifier26b. Node112bmay be configured such that tunable filter34breceives the tapped portion of the WDM signal, while the untapped portion may bypass tunable filter34b. Therefore, instead of being an in-line filter like tunable filter34bofFIG. 1, tunable filter34bofFIG. 2may be an out-of-line filter.

Similar to filter controller62of node12bdescribed with respect toFIG. 1, filter controller62of node112binFIG. 2may direct filter34bof node112bto filter a portion of the test signal originating from tunable transmitter32. Monitor36may be configured to receive the filtered portion of the signal from tunable filter34bthat originated from tunable transmitter32. Monitor36may analyze the portion of the test signal originating from tunable transmitter32to measure the chromatic dispersion, polarization mode dispersion, and/or other dispersion experienced by the portion of the signal. Based on the measured dispersion, monitor36may communicate control signals to TDCM30. Based on the control signals communicated to TDCM30from monitor36, TDCM30may perform optical dispersion compensation on the signal communicated to amplifier26bfrom tunable filter34.

In addition, filter controller62of node112bmay be configured to direct tunable filter34bto filter a portion of the WDM signal originating from transmitters14a,14band14c. Monitor36may analyze the portion of the signal originating from the transmitters14a,14band14cto determine the parameters of the optical signal such as power, wavelength, OSNR, OSA, or other parameters associated with the signal.

As noted above, although optical networks10and100are shown as a point-to-point optical network with terminal nodes, one or more of optical networks10and100may also be configured as a ring optical network, a mesh optical network, or any other suitable optical network or combination of optical networks, and may include any suitable number of intermediate nodes interfaced between the terminal nodes.

It should be noted that although particular components have been shown, modifications, additions, or omissions may be made to the optical networks10and100without departing from the scope of the disclosure. The components of the optical networks10and100may be integrated or separated according to particular needs. Moreover, the operations of the optical networks10and100may be performed by more, fewer, or other components.

FIG. 3is a diagram illustrating an embodiment of an optical signal monitoring system300, configured to monitor optical dispersion and optical signal parameters using the same elements. System300may include a tunable filter301that may be configured to perform the operations of tunable filters34bofFIGS. 1 and 2. System300may also include a monitor302configured to perform the operations of monitor36ofFIGS. 1 and 2.

Tunable filter301may comprise an etalon filter that includes an etalon310, a temperature regulator312, a voltage source314and a thermistor316. Etalon310may be configured to filter a narrow portion (e.g., less than 0.06 nanometers) of an optical channel associated with an optical signal according to the temperature of etalon310. Accordingly, etalon310may filter one portion of a channel at one temperature and may filter another portion of the same or a different channel at another temperature.

Temperature regulator312may comprise any suitable apparatus, system or device configured to control the temperature of etalon310and, thus, control which portion of a channel may be filtered by etalon310. In the present embodiment, temperature regulator312may comprise a thermoelectric cooler (TEC) coupled to etalon310to change the temperature of etalon310.

Temperature regulator312may be coupled to voltage source314and may be configured to change temperature according to the amount of voltage driving it. Therefore, voltage source314may be configured to control the temperature of temperature regulator312, which in turn may control the temperature of etalon310. Accordingly, because etalon310filters different portions of the channels of the WDM signal based on temperature, voltage source314may be configured to vary its voltage such that etalon310filters a desired portion of a particular channel. Voltage source314may be controlled by a controller such as filter controller62ofFIGS. 1 and 2or any other suitable device. Accordingly, voltage source314may be configured to vary its voltage such that the temperature of temperature regulator312varies and etalon310cycles through filtering portions of each channel to allow for dispersion monitoring and parameter monitoring for one or more channels of the WDM signal.

Tunable filter301may also include a thermistor316coupled to temperature regulator312. Thermistor316may be configured to adjust its resistance based on its temperature. Accordingly, a correlation between the resistance of thermistor316and the filtering properties of etalon310may be determined such that the channel corresponding to the portion being filtered by etalon310may be determined based on the resistance of thermistor316because the temperature of thermistor316and the filtering properties of etalon310are both directly related to the temperature of temperature regulator312. Thermistor316may be coupled to control unit308to allow control unit308to perform this determination as discussed further.

Monitor302may include a polarization beam splitter (PBS)304, photodetectors306aand306band a control unit308. PBS304may be coupled to etalon310and may be configured to receive the filtered portion from etalon310. PBS304may comprise any suitable apparatus, system or device configured to split an optical beam into two beams where each of the two beams has a single polarization state that is perpendicular to the polarization state of the other beam. These polarization states may be referred to as “horizontal” and “vertical” polarization to convey the perpendicular nature of the two with respect to each other. However, the terms “horizontal” and “vertical” may not refer to specific polarization state orientations and are merely to provide a frame of reference. In some embodiments, PBS304may comprise a Wollaston prism.

Photodetectors306aand306bmay be coupled to PBS304such that photodetector306areceives the horizontally polarized beam from PBS304and photodetector306breceives the vertically polarized beam from PBS304. Photodetectors306aand306bmay comprise any suitable system, apparatus or device configured to convert the optical beams received from PBS304into electrical signals. In some embodiments photodetectors306aand306bmay comprise photodiodes such as avalanche photodiodes.

Control unit308may be coupled to photodetectors306aand306band may be configured to receive the electrical signals from photodetectors306aand306b. Control unit308may be configured to determine chromatic dispersion, PMD and the signal parameters (e.g., OSNR, power, wavelength, OSA, etc.) of the portion of the signal based on the received electrical signals.

For example, during PMD monitoring, control unit308may determine the PMD by comparing the horizontally polarized beams and the vertically polarized beams of a scrambled test signal transmitted at a particular channel. During CD monitoring, control unit308may determine the chromatic dispersion of a system by comparing test signals transmitted at various wavelengths. Further, as more optical systems modulate information onto a horizontal and a vertical polarization state of a channel, control unit308may advantageously determine the chromatic dispersion between the same polarization states of different channels (e.g., the chromatic dispersion of the horizontal polarization of the channels). Also, the narrow filtering done by tunable filter301may also contribute to dispersion of the signal, however the amount of dispersion caused by tunable filter301may be known and control unit308may be configured to compensate for this while performing the dispersion calculations.

Control unit308may also determine one or more signal parameters (e.g., OSNR, OSA, power) of each polarization state for each channel during parameter monitoring. In the same or alternative embodiments, control unit308may determine a parameter for a channel representing both horizontal and vertical polarizations of the channel by averaging the parameters for the horizontal and vertical polarizations, by determining the best parameter (e.g., lowest power) between the horizontal and vertical polarizations or by determining the worst parameter (e.g., highest power)

Control unit308may be coupled to thermistor316to determine which channel is currently being filtered by etalon310for dispersion and parameter monitoring. Control unit308may make this determination based on the resistance of thermistor316and the correlation between the resistance of thermistor316and the filtering characteristics of etalon310because of the temperature dependency of both thermistor316and etalon310. Accordingly, control unit308may determine the wavelength of the portion being filtered by etalon310and thus, also determine which channel is being monitored. Therefore, control unit308may communicate information indicating the dispersion, OSNR, OSA, wavelength, power etc. of the channel. For example, control unit308may be coupled to TDCM30and, based on the dispersion measurements determined for a particular channel, control unit308may communicate control signals to TDCM30. TDCM30may perform optical dispersion compensation on the channel based on the control signals received from control unit308.

Modifications, additions or omissions may be made to system300without departing from the scope of the disclosure. For example, the functionality of control unit308may be performed by components not included in monitor302, such as filter controller62. Additionally, in some embodiments and some applications, control unit308may be configured to determine the particular channel being filtered by etalon310based on the voltage of voltage source314instead of the resistance of thermistor316.

Although the present disclosure 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 disclosure encompass such changes and modifications as fall within the scope of the appended claims.