Optical phase modulation

An optical modulator provides for preceding of a high data rate phase modulated optical signal. According to one embodiment, the modulator includes a first phase modulator and a second phase modulator. The first phase modulator modulates an optical carrier signal according to a first data signal to generate a modulated optical signal. The second phase modulator modulates the modulated optical signal according to a time delayed version of its optical output signal. According to another embodiment, the modulator electrically precodes multiple data streams and modulates the precoded data streams using a series of phase modulators to generate a precoded optical data signal.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to optical communication networks and, more particularly, to high data rate optical phase modulation.

BACKGROUND OF THE INVENTION

To meet the ever-increasing demands for bandwidth in communication networks, many service providers choose to increase data transmission rates. This squeezes more capacity out of existing infrastructure, however, broadband signals used in high data rate transmissions can prove difficult to effectively process. Specifically, in optical networks using phase shift keying modulation, broadband signals may disrupt conventional preceding and postcoding of electrical data signals.

SUMMARY OF THE INVENTION

In accordance with the present invention, techniques for high data rate optical phase modulation are provided.

According to one embodiment of the present invention, an optical modulator includes a first phase modulator that phase modulates an optical carrier signal according to a first data stream to generate a modulated optical signal. The optical modulator further includes a second phase modulator that phase modulates the modulated optical signal according to a second data signal to generate an optical data signal, wherein the second data stream comprises a time-delayed version of the optical data signal.

According to another embodiment of the present invention, an optical modulator includes multiple electrical preceding modules. Each of the precoding modules receives a data stream and precodes the data stream. The optical modulator further includes multiple phase modulators linked in series. Each of the phase modulators receives the precoded data stream from a corresponding one of the preceding modules, receives an optical signal from a preceding one of the phase modulators in the series, and modulates the received optical signal according to the precoded data stream from the corresponding preceding module.

Embodiments of the invention provide various technical advantages. The disclosed techniques provide a number of embodiments for precoding of data signals transmitted on an optical line. More specifically, these techniques permit precoding of data transmitted in phase modulated optical data signals. This preceding permits direct detection of optical data signals at optical receivers. This can reduce complexity and cost of optical receivers. Moreover, the precoding of the data signals can reduce the effects of errors in transmission along an optical fiber. For example, the precoding may prevent an error in one transmitted bit from propagating along the bit stream to cause errors in receiving subsequently transmitted bits.

According to particular embodiments, precoding may occur during modulation of optical signals, thus reducing the need for electrical components. This reduction in electrical components can reduce susceptibility to high frequency disruptions, thus permitting higher data rate transmissions compared to many conventional systems. According to other embodiments, a modulator precodes a number of relatively low data rata electrical data streams and combines these precoded electrical data streams using a series of phase modulators to generate a single precoded high data rate optical data stream. These techniques permit precoding of electrical data streams at frequencies tolerable to electrical components.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1illustrates an optical communication system, indicated generally at10, that includes a transmission module12coupled across one or more spans of optical fibers14to a receiver16. Transmission module12includes a light source18for generating an optical carrier and an optical modulator20for modulating data from a data source22to generate a phase modulated optical data signal. In general, transmission module12generates precoded optical data signals for communicating information on optical fiber14by passing an optical carrier generated by light source18through optical modulator20. According to particular embodiments, optical modulator20precodes the optical data signal using feedback from the optical data signal. According to alternative embodiments, optical modulator20electrically precodes multiple data streams and combines the precoded data steams to generate a single higher data rate precoded optical data signal.

Transmission module12represents any suitable collection and arrangement of hardware, including any appropriate logic, for generating and transmitting optical signals on optical fiber14. While illustrated in a relatively simple embodiment, system10contemplates transmission module12having any appropriate elements for communicating information on optical fiber14. For example, the embodiment illustrated includes multiple sets of optical signal generating components (light source18and optical modulator20), with signals from these elements multiplexed together by multiplexer28for transmission of multiple optical signals across optical fiber14at various wavelengths.

To generate optical data signals for communication to receiver16, transmission module12includes light source18, modulator20, and data source22. Light source18represents equipment, such as a laser, that generates an optical carrier signal. Data source22provides a stream of data, encoded in electrical signals, for transmission in optical signals to receiver16. However, while the embodiment illustrated includes data source22within transmission module12, system10contemplates transmission module12receiving one or more data streams from internal and/or external data sources. Thus, for example, transmission module12may receiver four, ten gigabit data streams for combination and transmission as a single forty gigabit optical signal on optical fiber14.

Modulator20phase modulates an optical carrier signal generated by light source18according to the data stream from data source22to generate a phase modulated optical data signal for communication to receiver16. More specifically, modulator20may modulate the optical carrier signal using phase shift keying (PSK) modulation and additionally precode the information communicated in the phase modulated optical signal, thus permitting direct detection of the information in the optical data signal by receiver16.

Receiver16represents any suitable combination and arrangement of hardware, including any appropriate logic, for receiving, separating, and decoding optical signals received on optical fiber14. In the embodiment illustrated, receiver16includes a phase to intensity conversion module24and a photo detector26. Conversion module24receives a phase modulated optical signal on fiber14and converts the phase modulated optical signal into an intensity modulated optical signal. For example, in a phase modulated optical signal encoding binary information, a value of “0” may be represented with a phase shift of 0 degrees while a value of “1” may be represented with a phase shift of 180 degrees. In an intensity modulated optical signal, a value of “0” may be represented with the absence of a light pulse, while a value of “1” may be represented with the presence of a light pulse.

To convert a phase modulated optical signal into an intensity modulated optical signal, conversion module24splits the received optical signal, delays one stream of the split optical signal, and interferes the delayed stream with the non-delayed stream of the received optical signal. According to particular embodiments, conversion module24is implemented using a Mach-Zehnder interferometer having arm lengths appropriately configured. Given an appropriate delay along one path of the split optical signal, the interference when the two streams recombine converts the phase modulated optical signal into an intensity modulated optical signal. Photo detector26may then convert the intensity modulated optical signal into an electrical data signal.

Because conversion module24interferes a time delayed version of the received optical signal with the received optical signal, the output of conversion module24will not provide the straight conversion of the phase modulated signal received into intensity format. For example, given a time delay of one bit period between the two streams of the received optical signal, the output of conversion module24represents the exclusive or (XOR) of consecutive bits in the bit stream. Thus, if the transmitted bit sequence from transmission module12is given as T=[t(0),t(1),t(2) . . . t(n)], then the output R of conversion module24is:⁢r⁡(0)=t⁡(0)⊕0⁢r⁡(1)=t⁡(1)⊕t⁡(0)⁢r⁡(2)=t⁡(2)⊕t⁡(1)⁢⋮⁢r⁡(n)=t⁡(n)⊕t⁡(n-1).

To compensate for the variation between the output of conversion module24and the phase modulated signal communicated by transmission module12, transmission module12precodes the bit stream communicated in the optical data signal. This preceding within transmission module12compensates for the exclusive or (XOR) operation resulting from the design of conversion module24. Given the previous example with a time delay of one bit period, transmission module12may compensate for the exclusive or (XOR) performed upon consecutive bits received by receiver16through preceding. Let B represent the original bit sequence from data source22. The transmitted bit sequence, T, is then determined as:⁢t⁡(0)=b⁡(0)⊕0⁢t⁡(1)=b⁡(1)⊕t⁡(0)⁢t⁡(2)=b⁡(2)⊕t⁡(1)⁢⋮⁢t⁡(n)=b⁡(n)⊕t⁡(n-1).

Given the preceding of data from data source22according to this formula, the output of conversion module24, and subsequently the electrical data stream generated by photo detector26, will equal the original bit sequence, B, from data source22.

Within transmission module12, modulator20performs precoding of the data stream received from data source22. According to particular embodiments, modulator20performs precoding of the data stream using feedback from the optical data signal generated by modulator20. The operation of modulator20according to a particular one of these embodiments is described in greater detail with respect toFIG. 2below. According to other embodiments, modulator20performs precoding by electrically precoding several data streams and then combining these precoded streams into a single higher data rate optical data stream. The operation of modulator20according to a particular one of these embodiments is described in greater detail with respect toFIG. 3below.

FIG. 2is a block diagram of an optical modulator40illustrating a particular embodiment for implementing optical modulator20of transmission module12. In the embodiment illustrated, modulator40includes optical splitters42, phase modulators44, an optical coupler46, a photo detector48, a tunable delay module50, and a driver amplifier52. In general, modulator40precodes information communicated in a phase modulated optical data signal using feedback generated from the optical data signal.

In the embodiment illustrated, light paths are represented using broad lines, while electrical paths are represented using thinner lines. Thus, a light path enter modulator40and is split into two streams56and58by splitter42. One of these streams of light stream56, passes through a first phase modulator44(PM1), through a second phase modulator44(PM2), and into a second splitter42. The second splitter42splits the received light into two streams60and62. Modulator40outputs stream60as the phase modulated optical data signal. The other stream from the second splitter42, stream62is merged with the other stream from the first splitter42, stream58, using coupler46.

Splitters42represents optical elements for splitting light into multiple streams. Similarly, coupler46represents an optical element for combining multiple received light streams into a single light stream. According to particular embodiments, splitters42are configured to deliver streams of light having substantially equivalent power to coupler46. This ensures optimal performance of coupler46, thus delivering a “clean” signal to photo detector48. To deliver substantially equal power on paths58and62, the splitting ratios of splitters42should be appropriately calibrated. For example, assume a splitting ratio of the first splitter42of 1:x. Given this splitting ratio, if a stream of light on path58has a power of 1, the corresponding split stream of light on path56will have a power of x. Now assume a splitting ratio of 1:y for the second splitter42. The values for x and y may be calculated using the formula x=1+y. Configuring the splitting ratios of splitters42according to this formula can ensure that optical signals having substantially equal power will be delivered to coupler46along paths58and62. However, system10contemplates modulator40including any appropriate equipment calibrated and/or configured to split and couple light paths.

Phase modulators44receive optical signals and modulate received optical signals according to electrical data input. For example, PM1receives an optical carrier signal on path56and modulates the received carrier according to an electrical data signal received from data source22. Similarly, PM2receives an optical signal from PM1and modulates the received signal according to an electrical signal received on a feedback line64. According to particular embodiments, modulation of optical signals within phase modulators44takes place using PSK modulation. That is, phase modulators44shift the phase of an optical signal based upon a received electrical signal. Thus, binary electrical data dictates phase shifting of an optical signal within phase modulator44. For example, upon receiving an electrical signal indicating a value of “1”, phase modulator44shifts the phase of an optical signal by 180 degrees. For a received value of “0”, phase modulator44does not shift the phase of the optical signal (a phase shift of 0 degrees).

Modulator40also includes electrical and optoelectrical components, including photo detector48, tunable delay module50, and amplifier52. Photo detector48represents any suitable element or elements for converting a received optical signal into a corresponding electrical signal. For example, upon detecting the presence of light received from coupler46, photo detector48may generate an electrical signal indicating a binary value of “1”. Similarly, given the absence of light received from coupler46, photo detector48may generate an electrical signal indicating a binary value of “0”.

Tunable delay module50represents any suitable elements for selectably delaying an electrical signal from photo detector48. For example, using tunable delay module50, modulator40may configure the time that it takes an electrical signal from photo detector48to reach PM2. This permits tuning of the delay along line64such that the delay equals an integer multiple of bit-periods in the optical communication stream. For example, according to particular embodiments, the delay along line64is substantially equal to one bit period. This results in PM2modulating the received optical signal with its own output delayed by the duration of a single bit. This delay along feedback line64may be set to any appropriate integer multiple of bit-periods. However, at receiver16, converter24should be configured to match the delay used in modulator40. For example, given a delay of three bit periods along feedback line64, converter24in receiver16should use the same delay.

In operation, modulator40receives an optical carrier signal on path54from light source18. The first splitter42splits the optical carrier onto path56and path58according to its configured splitting ratio. PM1receives the optical carrier on path56and modulates the optical carrier according to a data stream received from data source22. PM1communicates this modulated optical signal to PM2. PM2modulates the modulated optical signal according to the data stream received on line64to generate an optical data signal for output by modulator40. As previously discussed, the data stream on feedback line64represents a time delayed version of the optical data signal generated by PM2.

To obtain feedback from the generated optical data signal, modulator40taps the optical data signal using splitter42. Thus, splitter42splits the optical data signal onto path60and path62, with path60used as an output of modulator40and path62used for feedback. The tapped optical data signal on path62and the split optical carrier signal on path58interfere within coupler46to generate an intensity modulated version of the optical data signal. Thus, while the optical data signal generated by PM2is phase modulated, the output resulting from coupler46is intensity modulated. This intensity modulated signal simplifies the operation of photo detector48, since, in an intensity modulated signal, the presence of light indicates a binary on while the absence of light indicates a binary off.

Photo detector48converts the intensity modulated optical signal received from coupler46into an electrical data stream. This electrical data stream is delivered along feedback line64, through tunable delay module50and amplifier52, as the electrical data stream for phase modulation within PM2. This results in precoding of the optical data signal generated by modulator40, thus permitting direct detection of the optical data signal by receiver16.

However, while the embodiment illustrated and the preceding description focus on a particular embodiment of modulator40that includes specific elements, system10contemplates modulator40having any suitable combination and arrangement of elements for preceding an optical data signal using time delayed feedback of the optical data signal. For example, while the embodiment illustrated uses tunable delay module50to moderate the delay along feedback line64, the length of feedback line64may be designed such that inherent delay is equal to an integer multiple of the bit period of bits within the optical data signal, thus obviating the need for tunable delay module50. Moreover, system10contemplates modulator40using any appropriate optical technology, such as planar lightwave circuits, discreet coupled elements, free space optics, and/or other suitable optical technologies.

FIG. 3is a block diagram of an optical modulator80illustrating a particular embodiment for implementing modulator20of transmission module12. In the embodiment illustrated, modulator80includes a deinterlace module82, preceding modules84, a delay module86, and phase modulators88. In general, modulator80generates a phase modulated optical data signal having a precoded data stream. To generate the precoded optical data signal, modulator80electrically precodes multiple data streams and interlaces these precoded data streams using phase modulators88.

As in the illustration of modulator40, the broad lines within modulator80represent light paths, while the thinner lines represent electrical paths. In addition, phase modulators88(PM1and PM2) operate similarly to phase modulators44of modulator40.

The electrical components within modulator80perform precoding of two or more data streams for combination into a single higher data rate optical data signal. In the embodiment illustrated, these components include deinterlace module82, a first precoder84(PC1), a second precoder module84(PC2), and delay module86. Deinterlace module82receives a single high data rate bit stream and converts the bit stream into two or more lower data rate bit streams. In the embodiment illustrated, deinterlace module82converts the bit stream received from data source22into two bit streams. To split the bit stream, deinterlace module82delivers each consecutive bit to a different one of PC1and PC2. Thus, for example, deinterlace module82may deliver the first received bit to PC1, the second received bit to PC2, the third received bit to PC1, etc. This results in two bit streams each having half the data rate of the original bit stream received from data source22. For example, given a bit stream from data source22having a rate of forty gigabits per second, deinterlace module82delivers bit streams having data rates of twenty gigabits per seconds to each of PC1and PC2.

Precoding modules84represent any suitable elements for preceding electrical data signals.FIG. 3expands upon PC2to illustrate a particular embodiment implementing precoding module84using an exclusive or gate90and D flip flops92. This configuration precodes the received electrical signal according to the precoding formula discussed above.

To ensure that the two bit streams from PC1and PC2are appropriately interlaced into an optical data signal, the output of PC2may pass through delay module86. According to the embodiment illustrated, delay module86delays the bit stream received from PC2the duration of the bit period of the combined data streams. For example, if PC1and PC2each operate on twenty gigabit per second data streams, the delay period of delay module86is equal to the bit period of a forty gigabit per second data stream.

Thus, in operation, modulator80performs electrical precoding of multiple data streams and then phase modulates each of these precoded data streams into a single optical data signal. This generates an optical data signal having a data rate equal to the sum of the data rates from the multiple precoded electrical data streams. Using these techniques, electrical preceding of data signals occurs at lower data rates compared to the resulting optical data rate, thus preventing problems that may occur within electrical components operating at high data rates.

While the embodiment illustrated and the preceding description of modulator80focus on a particular embodiment that includes specific elements, system10contemplates modulator80having any suitable combination and arrangement of elements for electrically preceding multiple electrical data streams and phase modulating these streams into a single optical signal. Thus, for example, while modulator80is illustrated as having two precoders84operating on two data streams, modulator80may have any suitable number of precoders84with corresponding phase modulators88to combine any number of data streams. Moreover, system10contemplates modulator80using any appropriate optical technology, such as planar lightwave circuits, discreet coupled elements, free space optics, and/or other suitable optical technologies.

FIG. 4is a flowchart illustrating a method, performed by modulator40, to generate a precoded optical data signal using feedback from the optical data signal. Modulator40receives an optical carrier signal at step100and splits the optical carrier signal at step102. For example, within modulator40, splitter42may split a received optical carrier signal onto two light paths, path56and path58.

Modulator40phase modulates the first optical carrier stream with a data signal at step104. For example, within modulator40, PM1modulates the optical carrier signal received on path56according to an electrical data stream received from data source22. Modulator40then modulates the modulated optical signal again, this time according to the time delayed output signal at step106. For example, PM2receives the modulated optical signal from PM1and phase modulates the received signal according to an electrical signal received along feedback line64. This generates a precoded, phase modulated optical signal. Modulator40splits the generated optical data signal at step108and transmits the first stream of the split optical data signal as output at step110. Modulator40couples the second stream of the optical data signal and the second stream of the optical carrier signal at step112. For example, the tapped optical data signal on path62may interfere with the split optical carrier signal on path58within coupler46. As previously discussed, this produces an intensity modulated optical data signal. Modulator40converts the coupled streams to an electrical output signal at step114and time delays the electrical output signal at step116. As discussed above, this delay ensures that the data stream provided to the second phase modulator is delayed for a duration equal to an integer multiple of bit periods.

FIG. 5is a flowchart illustrating a method performed by modulator80for generating a precoded optical data signal by electrically preceding multiple data streams and combining these data streams to provide a single higher data rate optical data signal. Modulator80receives a high rate electrical data stream at step130and splits the high rate data stream into multiple data streams at step132. For example, as illustrated in modulator80, deinterlace module82may split a received data stream into multiple data streams. Modulator80precodes each lower rate data stream at step134. Modulator80delays one or more of the low rate data streams at step136. As previously discussed, the delay provides for appropriate interlacing of the multiple low rate data streams in the series of phase modulators. The series of phase modulators phase modulate each low rate data stream with an optical signal at step138. This generates a single optical data signal encoding the original high rate data stream in precoded format. Modulator80transmits the generated optical data signal at step140.

The preceding flowcharts illustrate only exemplary methods of operation, and system10contemplates modulators using any suitable techniques and elements for generating precoded optical data signals according to various embodiments. Thus, many of the steps in these flowcharts may take place simultaneously and/or in different orders than as shown. In addition, modulators may use methods with additional steps, fewer steps, and/or different steps, so long as the methods remain appropriate.

Although the present invention has been described in several embodiments, a myriad of changes and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes and modifications as fall within the scope of the present appended claims.