Bidirectional optical communication with minimal guard band

A module, system and method adjusts a tunable filter to have an adjustable frequency response based on one of an outbound optical signal on a transmit channel and an inbound optical signal on a receive channel. The tunable filter is in an optical path of the outbound optical signal and in an optical path of the inbound optical signal. The transmit and the receive channels are configured as part of a channel plan of a bidirectional (bi-di) dense wavelength division multiplexing (DWDM) system.

FIELD

The embodiments discussed herein are related to bidirectional (bi-di) communication modules.

BACKGROUND

In a bidirectional (bi-di) dense wavelength division multiplexing (DWDM) system, two different sets of wavelength channels are used to carry upstream and downstream data traffic in opposite directions in a single fiber. One common example is to use C-band wavelengths for upstream data traffic and L-band wavelengths for downstream data traffic. A second example uses a lower half of C-band (“C−” band) wavelengths for upstream and higher half of C-band (“C+” band) wavelengths for downstream. An edge filter is needed to separate the two sets of wavelengths and a guard-band, for example of approximately 12 nm, is needed to allow the edge filter to separate the two sets of wavelengths. Accordingly, the guard-band takes up a significant portion of the available band and reduces available wavelengths channels in a single fiber.

DETAILED DESCRIPTION

FIG. 1illustrates an example optical communication system configured as a bidirectional (bi-di) dense wavelength division multiplexing (DWDM) system100(hereinafter “system100”) that implements wavelength division multiplexing (WDM) to communicate multiple optical signals bidirectionally across an optical fiber between multiple communication modules. InFIG. 1and other figures herein, a direction from left to right through the optical fiber is referred to as east, while a direction from right to left through the optical fiber is referred to as west.

The system100may include, for example, forty communication modules at each of two ends of an optical fiber110. At a first end, a first and last of the communication modules (e.g., communication module120-1and communication module120-40) are depicted and respectively labeled “Bi-Di TRX01” and “Bi-Di TRX40”. At a second end, a first and last of the communication modules (e.g., communication module130-1and communication module130-40) are depicted and respectively labeled “Bi-Di TRX01” and “Bi-Di TRX40”. Due to space constraints in the drawings, communication modules 2-39 are not illustrated at either end of the optical fiber110. Each communication module includes a transmitter configured to emit an optical signal that is representative of an electrical signal received from a host device at a designated frequency (and wavelength) that is different than a designated frequency (and wavelength) of other optical signals emitted by transmitters of other communication modules in the system100. The various designated frequencies (and corresponding wavelengths) may be referred to as channels. Each communication module additionally includes a receiver configured to receive an optical signal in a particular one of the channels.

The system100additionally may include, for example, a 100 gigahertz (GHz) optical multiplexer/demultiplexer (Mux/Demux)140at the first end of the optical fiber110between the optical fiber110and the communication modules120. The system100additionally may include, for example, a 100 gigahertz (GHz) optical multiplexer/demultiplexer (Mux/Demux)150at the second end of the optical fiber110between the optical fiber110and the communication modules130. Each Mux/Demux140/150may include, for example, forth module-side ports and a single fiber-side port. In operation, Mux/Demux140is configured to receive, for example, forty eastbound optical signals at the forty module-side ports from the forty communication modules120-1through120-40and to spatially combine (e.g., multiplex) the forty eastbound optical signals for output through the fiber-side port to optical fiber110. The forty spatially combined eastbound optical signals are transmitted eastward through optical fiber110to Mux/Demux150. Mux/Demux150is configured to receive, for example, the forty spatially combined eastbound optical signals from optical fiber110through the fiber-side port and to spatially separate (e.g., demultiplex) out the individual forty eastbound optical signals. The forty eastbound optical signals are output through the forty module side ports of Mux/Demux150such that each of the forty eastbound optical signals is provided to a different one of the forty communication modules130-1through130-40.

Analogously, Mux/Demux150is configured to receive forty westbound optical signals on the forty module-side ports from the forty communication modules130-1through130-40and to spatially combine (e.g., multiplex) the forty westbound optical signals for output through the fiber-side port to the optical fiber110. The forty spatially combined westbound optical signals are transmitted westward through optical fiber110to Mux/Demux140. Mux/Demux140is configured to receive the forty spatially combined westbound optical signals from optical fiber110through the fiber-side port and to spatially separate (e.g., demultiplex) out the individual forty westbound optical signals. The forty westbound optical signals are output through the forty module-side ports of Mux/Demux140such that each of the forty westbound optical signals is provided to a different one of the forty communication modules120-1through120-40.

FIG. 2illustrates a channel plan200of a bidirectional (bi-di) dense wavelength division multiplexing (DWDM) system100of Mux/Demux140/150along with port and channel assignments for system100. Each eastbound channel is assigned to a different port of each Mux/Demux140/150and exhibits a different transmission peak in the channel plan200than other eastbound channels. For instance, eastbound channels220-1through220-40are respectively assigned to ports142-1through142-40and to different transmission peaks of the channel plan200where the transmission peaks, for example, have a center-to-center spacing of 100 GHz. Similarly, westbound channels230-1through230-40are respectively assigned to ports152-1through152-40and to different transmission peaks of the transmission spectrum200where the transmission peaks, for example, have a center-to-center spacing of 100 GHz.

FromFIG. 2and the foregoing description, it can be seen that two channels are assigned to each port, where one, for example channel220-1, of the two channels assigned to each port is an eastbound channel and the other is a westbound channel, for example channel230-1. It can also be seen fromFIG. 2that a guard band250of unused channels is provided between eastbound channels220and westbound channels230.

Thus, Mux/Demux140is configured to receive an eastbound optical signal n (where n is an index from 1-40) emitted by a transmitter122-1through122-40on channel220-1through220-40(e.g., Ch01A to Ch40A which correspond to 01East to 40East) of communication modules120-1through120-40on module-side of ports142-1through142-40and to spatially combine all eastbound optical signals for output at combined port144to optical fiber110for eastward transmission to combined port154of Mux/Demux150. Mux/Demux150receives and spatially separates the eastbound optical signals and outputs each eastbound optical signal on module-side ports152-1through152-40to be received by communication modules130-1through130-40.

Similarly, Mux/Demux150is configured to receive a westbound optical signal emitted by a transmitter132-1through132-40on channel230-1through230-40(e.g., Ch01B to Ch40B which correspond to 01West to 40West) of communication modules130-1through130-40on module-side ports152-1through152-40and to spatially combine all westbound optical signals for output at combined port154to optical fiber110for westward transmission to Mux/Demux140. Mux/Demux140receives and spatially separates the westbound optical signals and outputs each westbound optical signal on module-side ports142-1through142-40to be received by communication modules120-1through120-40.

Each communication module120/130in the system100may include a single input/output port124/134through which an outbound optical signal generated by the transmitter of the communication module120/130is output, and also through which an inbound optical signal received from the corresponding Mux/Demux140/150may be received. In these and other embodiments, each communication module120/130may include a wideband filter126/136configured to pass the outbound signal and reflect the inbound signal, or vice versa.

In an example embodiment, each wideband filter126in communication modules120may have a filter response260. The filter response260is designed to be aligned to the transmission spectrum of the channel plan200of each Mux/Demux140/150inFIG. 1. Further, each wideband filter136in communication modules130may have a filter response270. The filter response270is also designed to be aligned to the transmission spectrum of the channel plan200of each Mux/Demux140/150inFIG. 1.

As illustrated by the channel plan200and filter response260, each wideband filter126in communication modules120may include a lowpass filter configured to pass all the eastbound signals on eastbound channels220-1through220-40(e.g., 01East to 40East) and to reflect all the westbound channels on westbound channels230-1through230-40(e.g., 01West to 40West). For instance, wideband filter126-1in communication module120-1may be configured to pass the optical signal emitted by the transmitter122-1(TX Ch01A) on eastbound channel220-1(01East) so that it may be input to Mux/Demux140through the module-side port142-1and to reflect the optical signal received from Mux/Demux140through the module-side port142-1on westbound channel230-1(01West) to be received by receiver128-1(RX Ch01B).

As further illustrated by the channel plan200and filter response270, each wideband filter136in communication modules130may include a highpass filter configured to pass all the westbound signals on westbound channels230-1through230-40(e.g., 01West to 40West) and to reflect all eastbound channels on eastbound channels220-1through220-40(e.g., 01East to 40East). For instance, wideband filter136-1in communication module130-1may be designed to pass the optical signal emitted by transmitter132-1(TX Ch01B) on westbound channel230-1(01West) to input to Mux/Demux150through the module-side port152-1and to reflect the optical signal received from Mux/Demux150through the module-side port152-1on eastbound channel220-1(01East) to be received by receiver138-1(RX Ch01A).

InFIG. 2, the eastbound channels220may include forty channels with 100 GHz spacing in the ITU-T C-band (e.g., 1530-1565 nanometers (nm)). Additionally, the westbound channels230may include forty channels with 100 GHz spacing in the ITU-T L-band (e.g., 1568-1610 nm). Further, the guard band250occupies not an insignificant portion of the channel plan200. The width of guard band250is based at least in part on the slope262/272of the filter responses. For more relaxed slopes262/272of the filter responses, the guard band250becomes wider. Slopes262/272of the filter responses may be shortened by using more complex filters126/136, however, shortened filter response filters126/136require tightened filter specifications and, therefore, more filter components and a more complex filter design resulting in increased size and expense.

FIG. 3illustrates another example optical communication system configured as a bidirectional (bi-di) dense wavelength division multiplexing (DWDM) system300(hereinafter “system300”), arranged in accordance with at least one embodiment described herein. Similar to the system100, the system300may include a left and right 100 GHz Mux/Demux140/150communicatively coupled by an optical fiber110, with forty left communication modules320-1through320-40and forty right communication modules330-1through330-40. Each 100 GHz Mux/Demux140/150inFIG. 3may be the same as inFIG. 1.

In the system100ofFIG. 1, each of the eighty total communication modules120/130includes a common fixed-response wideband filter126/136to pass a corresponding outbound optical signal emitted by a corresponding transmitter and reflect a corresponding inbound optical signal toward a corresponding receiver. In comparison, in the system300ofFIG. 3, each communication module320/330may include a tunable filter326/336to pass a corresponding outbound optical signal emitted by a corresponding transmitter122/132and reflect a corresponding inbound optical signal toward a corresponding receiver128/138. The configuration ofFIG. 3may accommodate a higher channel density by allowing the guard band to be tunable or adjustable for each port or groups of ports in system300. As used herein, “tunable filter” may include a filter that is adjustable based on a control signal such as a programmable control signal described with respect to the embodiment ofFIG. 6or may include a filter that is configurable to a single band (i.e., fixed) either by a fixed programmable control signal as described with respect to the embodiment ofFIG. 6or by a fixed signal such as by an electric circuit or electrical component.

FIG. 4illustrates one embodiment of a channel plan for system300. In more detail,FIG. 4additionally includes the channel plan400of each Mux/Demux140/150along with port and channel assignments in the system300. Similar toFIG. 1, inFIG. 3, each eastbound channel is assigned to a different port of each Mux/Demux140/150and different transmission peak of the channel plan400than other eastbound channels, while each westbound channel is assigned to a different port of each Mux/Demux140/150and different transmission peak of the channel plan400than other westbound channels. However, inFIG. 3, the guard band450for each east/west pair is adjustable due to tunable filter326/336. Alternatively, the guard band450for groups of east/west pairs may also be adjustable due to tunable filters326/336.

In one example embodiment, each wideband filter326in communication modules320may have a filter response460. The filter response460is designed to be adjustable to the channel plan400of each Mux/Demux140/150inFIG. 3. Further, each wideband filter336in communication modules330may have a filter response470. The filter response470is also designed to be adjustable to the channel plan400of each Mux/Demux140/150inFIG. 3.

As illustrated by the channel plan400and filter response460, each wideband filter326in communication modules320may include a tunable lowpass filter configured to pass all the eastbound signals on eastbound channels420-1through420-40(e.g., 01East to 40East) and to reflect all the westbound channels on westbound channels430-1through430-40(e.g., 01West to 40West). For instance, tunable filter326-1in communication module320-1may be configured to pass the optical signal emitted by the transmitter122-1(TX Ch01A) on eastbound channel420-1(01East) so that it may be input to Mux/Demux140through the module-side port142-1and to reflect the optical signal received from Mux/Demux140through the module-side port142-1on westbound channel430-1(01West) to be received by receiver128-1(RX Ch01B).

As further illustrated by the channel plan400and filter response470, each tunable filter336in communication modules330may include a tunable highpass filter configured to pass all the westbound signals on westbound channels430-1through430-40(e.g., 01West to 40West) and to reflect all eastbound channels on eastbound channels420-1through420-40(e.g., 01East to 40East). For instance, tunable filter336-1in communication module330-1may be designed to pass the optical signal emitted by transmitter132-1(TX Ch01B) on westbound channel430-1(01West) to input to Mux/Demux150through the module-side port152-1and to reflect the optical signal received from Mux/Demux150through the module-side port152-1on eastbound channel420-1(01East) to be received by receiver138-1(RX Ch01A).

InFIG. 4, the eastbound channels420may include forty channels with 50 GHz spacing in, for example, the ITU-T C-band. Additionally, the westbound channels430may include forty channels with 50 GHz spacing in the ITU-T C-band. Further, the width of guard band450is based at least in part on the slopes462/472of the filter responses. Yet further, the location in channel plan400of guard band450is adjustable or tunable, in the present embodiment, for each pair of channels420/430. Specifically, the tunable filter326/336for each communication module320/330may be individually incrementally tuned along channel plan400to provide a guard band450-1through450-40where each guard band450may be incrementally individually positioned along channel plan400. The tunable filter326/336may be incrementally tuned by controller610as illustrated below with respect toFIG. 6or tunable filter326/336may be incrementally tuned by differently configured fixed filters as illustrated below with respect toFIG. 7.

Channel plan400A illustrates a first pair of channels420-1/430-1traversing over port1of system300. Guard band450-1may be skewed away from the channel430-1to accommodate relaxed slopes462-1/472-1of the filter responses. Similarly, channel plan400B illustrates a fortieth pair of channels420-40/430-40traversing over port40of system300. Guard band450-40may be skewed away from the channel420-40to accommodate relaxed slopes462-40/472-40of the filter responses.

In the embodiment ofFIG. 3, the slopes462/472of the filter responses may be relaxed or lengthened because the guard band for each channel pair on each port may be nearly as wide as the band spacing between the pair of channels. By relaxing or lengthening the guard band450, less expensive, physically smaller, and more reliable tunable filters326/336may be used. Further, the adjustability of guard band450due to tunable filters326/336, allows the reuse of some or all of the channels occurring in guard band250ofFIG. 2.

FIG. 5illustrates another embodiment of a channel plan for system300. InFIG. 4, the channel plan400illustrates bandwidth450having group-wise tunable filters326/336resulting in guard band450for each group of ports being group-wise offset from the guard band of a neighboring group of ports. While fine granularity in tuning the guard band may be desirable as illustrated in the embodiment ofFIG. 4, such fine tunable granularity could require eighty (forty for eastbound transmit signals and 40 for westbound transmit signals) differently configured communications modules320/330. While flexibility in tuning the guard band may be desirable, such tuning capability will require additional control circuitry (as in the tunable filter embodiment ofFIG. 6) or a large quantity of uniquely fixed value tunable filters (as in the tunable filter embodiment ofFIG. 7).

In the embodiment ofFIG. 5, a fewer quantity of differently configured communication modules320/330are disclosed. Specifically, tunable filters326/336may be configured to provide a common frequency response for a group of pairs of channels. It is noted that “tunable filter” as used herein may also include filters that are fixed in value, either by a control signal as inFIG. 6described below, or through the use of an electric circuit as inFIG. 7also described below. Further, the groups of tunable filters326/336may be tuned by controller610as illustrated below with respect toFIG. 6or tunable filter326/336may be incrementally tuned by differently configured fixed filters as illustrated below with respect toFIG. 7.

Accordingly, the tunable filter326/336for each communication module320/330may be step-wise tuned. For example, a plurality of channels560/570may be tuned using one of four (two for eastbound transmit signals and two for westbound transmit signals) different step-wise fixed filter values for groups of channels along channel plan500to provide step-wise guard bands550-1through550-2where each guard band550may be flexibly step-wise positioned along channel plan500. For example, a first group of eastbound transmit channels520-1through520-20may have a first common fixed filter value for tunable filters326-1through326-20. A second group of pairs of eastbound transmit channels560-21through560-40may have a second common fixed filter value for tunable filters326-21through326-40. Further, a third group of westbound transmit channels570-1through570-20may have a third common fixed filter value for tunable filters336-1through336-20. Finally, a fourth group of westbound transmit channels570-21through570-40may have a fourth common fixed filter value for tunable filters336-21through336-40.

For example in the present embodiment, each of communication modules320may be configured between two different values for each of tunable filters326. Channel plan500A illustrates, for example, a first filter response560-C−L for tunable filters326-1through326-20(not separately illustrated) corresponding to eastbound transmit channels 1 through 20 and respectively coupled to ports 1 through 20 coupled to communication modules320-1through320-20. Channel plan500B illustrates, for example, a second filter response560-C−H for tunable filters326-21(not separately illustrated) through326-40corresponding to eastbound transmit channels 21 through 40 and respectively coupled to ports 21 through 40 coupled to communication modules320-21through320-40. Similarly, channel plan500C illustrates, for example, a third filter response570-C+L for tunable filters336-1through336-20(not separately illustrated) corresponding to westbound transmit channels 1 through 20 and respectively coupled to ports 1 through 20 coupled to communication modules330-1through330-20. Channel plan500D illustrates, for example, a fourth filter response570-C+H for tunable filters336-21(not separately illustrated) through336-40corresponding to westbound transmit channels 21 through 40 and respectively coupled to ports 21 through 40 coupled to communication modules330-21through330-40. The forty channels correspond to the forty channel pairs described above.

The embodiment ofFIG. 5, illustrates four separate filters, two for eastbound transmit channels and two for westbound transmit channels. Such an arrangement allows for a smaller quantity (e.g., four) of differently configured communication modules320/330. It should be noted that the quantity of four differently grouped filter responses is only illustrative and other numbers of different grouped filters responses, including a lesser and greater quantity are also contemplated.

FIG. 6illustrates a transceiver300′ with tunable filter and programmable control, arranged in accordance with the various embodiments described herein. Transceiver320′,330′ may include receiver128,138and transmitter122,132as described above. Transceiver320′,330′ may further include a tunable filter326′,336′ which may be tunable (i.e., configurable) by way of a control signal616. Tunable filter326′,336′ may be implemented in a communication module320′,330′ with a controller610configured according to a processor612and memory614to tune each of tunable filters326′,336′ according to one of the embodiments ofFIG. 4(i.e., where each channel pair is uniquely configured) orFIG. 5(i.e., where a plurality of channel pairs are group-wise similarly configured). Further, controller610may be configured as active or passive circuitry to provide a fixedly tuned tunable filter326′,336′.

Tunable filter326′,336′ in communication module320′,330′ may be configured according to one of channel plan400or channel plan500.

FIG. 7illustrates a transceiver300″ with tunable filter and fixed control, arranged in accordance with the various embodiments described herein. Transceiver320″,330″ may include receiver128,138and transmitter122,132as described above. Transceiver320″,330″ may further include a tunable filter326″,336″ which may be tunable (i.e., configurable) by way of a fixed signal716such as an electric circuit or electrical component. Tunable filter326″,336″ may be implemented in a communication module320″,330″ with an electrical circuit710configured, in one example, according to a jumper or switch712and one or more resistors714. While the electrical circuit710is illustrated as containing multiple circuit elements, a single element such as a resistor or capacitor is also contemplated.

Electrical circuit710illustrates a plurality of resistors714-1,714-2,714-3, and714-4which may be separately selected to differently configure the tunable filter326″,336″ according to one of the embodiments ofFIG. 4(i.e., where each channel pair is uniquely configured) orFIG. 5(i.e., where a plurality of channel pairs are group-wise similarly configured).

Tunable filter326,336in communication module320″,330″ may be configured according to one of channel plan400or channel plan500.

In yet a further embodiment illustrated with respect toFIG. 5andFIG. 7, the transceiver300″ may include transceiver320″,330″ which respectively include receiver128,138and transmitter122,132as described above. Transceiver320″,330″ may further include a tunable filter326″,336″ which may be fixed to two or more fixed filter values. Tunable filter326″,336″ may be implemented in a communication module320″,330″ with an electrical circuit710configured, in another example, according to a fixed connection (e.g., jumper712may be configured as a conductive trace rather than as a “switch”) and one or more resistors714. In such a fixed tunable filter configuration, a control signal is not necessary since the values of the tunable filters326″,336″ are fixed. While the electrical circuit710is illustrated as containing multiple circuit elements, a single element such as a resistor or capacitor is also contemplated.

In the further embodiment, the electrical circuit710may include only a single resistor714which may be fixed to configure the tunable filter326″,336″ according to a specific frequency response. For example and with respect to the channel plan500of FIG.5, the transceiver300″ may include a first fixed lower frequency tunable filter326″ for communication modules320″ coupled to channels Ch01-Ch20. Further, the transceiver300″ may include a second fixed higher frequency tunable filter326″ for communication modules320″ coupled to channels Ch21-Ch40. Further, the transceiver300″ may include a third fixed lower frequency tunable filter336″ for communication modules330″ coupled to channels Ch41-Ch60. Yet further, the transceiver300″ may include a fourth fixed higher frequency tunable filter336″ for communication modules330″ coupled to channels Ch61-Ch80. In the present embodiment, the transceiver300″ may be configured with only four different tunable filter values that are fixed to one of four different frequency responses. Accordingly, the tunable filters in the present embodiment may be fixed to one or four different values depending on the channels of the channel plan. While the present embodiment has been described with respect to four different fixed filter values for the tunable filters326″,336″ of transceiver300″, a greater number of fixed filter values is also contemplated.

FIG. 8is a flowchart of a method800for adjusting a tunable filter to have an adjustable frequency response as part of a channel plan of a bidirectional (bi-di) dense wavelength division multiplexing (DWDM) system.

At block802, the tunable filter is adjusted to have an adjustable frequency response based on one of an outbound optical signal on a transmit channel and an inbound optical signal on a receive channel. The tunable filter is in an optical path of the outbound optical signal and in an optical path of the inbound optical signal. The transmit and the receive channels are configured as part of a channel plan of a bidirectional (bi-di) dense wavelength division multiplexing (DWDM) system.

At block804, the outbound optical signal is emitted on the transmit channel.

At block806, the inbound optical signal is received on the receive channel.

The embodiments described herein include a tunable filter that is transmissive to an outbound optical signal emitted by a transmitter and that reflects an inbound optical signal toward a receiver. In other embodiments, the tunable filter may be transmissive to the inbound optical signal and may reflect the outbound optical signal, in which case the positions of the transmitter and the receiver may be switched as compared to the embodiments illustrated in the figures.