Transmission of subcarriers having different modulation formats

Consistent with the present disclosure, an optical communication system is provided in which data is carried over optical signals including subcarriers. The subcarriers may be modulated with the standard modulation formats noted above, but the modulation formats are selectively assigned to the subcarriers, such that some subcarriers are modulated with different standard modulation formats than others. As used herein, a “standard modulation format” is one of BPSK, and n-QAM, where n is an integer greater than one. Such n-QAM modulation formats include of 3-QAM, 4-QAM (QPSK), 8-QAM, 16-QAM, 64-QAM, 128-QAM, and 256-QAM. By selecting the number of subcarriers and the types of modulation formats employed, an optical signal with an effective SE that is between that of the standard modulation formats can be generated for transmission over a distances that more closely matches the link distance. Such custom or intermediate SE signals can be tailored to a particular optical link SNR to provide data transmission rates that are higher than the low order modulation formats that would otherwise be employed for optical signals carried by such links. As a result, more efficient data transmission can be achieved.

Optical communication systems are known in which data is carried over amplitude/phase modulated optical signals, which are transmitted along an optical fiber link to a receiver node. Such optical signals may be transmitted in accordance with a variety of standard modulation formats using polarization multiplexing (also known as dual polarization), such as binary phase shift keying (BPSK), 3-quadrature amplitude modulation (3-QAM), quadrature phase shift keying (QPSK, or 4-QAM), 8-QAM, 16-QAM, 32-QAM, and 64-QAM, with spectral efficiency (SE) of 2, 3, 4, 6, 8, 10, and 12 b/dual-pol-symbol, respectively. Higher order modulation formats have a high SE, as well as an associated constellation with points that are relatively close to one another. Accordingly, distinguishing such constellation points may be difficult, especially if the high SE signal is transmitted over an optical link that has an associated low signal-to-noise ratio (SNR) or has other impairments. As a result, high SE signals are more susceptible to noise and may have higher bit error rate for a given SNR. On the other hand, low order modulation format signals have a low SE, with associated constellation points that are relatively far apart. Thus, transmission of such low SE (i.e., low order modulation format) signals over a link with an associated SNR will incur fewer errors than if high SE signals were transmitted. Put another way, for a given SNR, high SE signals will incur more errors and have a higher bit error rate (BER) than low SE signals.

Thus, there is a tradeoff between capacity and reach. Lower order modulation formats having a low SE can be transmitted farther because fewer errors are incurred, but such lower order modulation formats have fewer bits per symbol and thus less capacity. Higher order modulation formats, on the other hand, have a higher SE, such that more bits per symbol can be transmitted to provide greater capacity. However, such higher order modulation formats are more susceptible to errors, and thus, for a given power level, cannot be transmitted over longer distances because the farther an optical signal is transmitted the more errors will be incurred.

Thus, a given modulation format can be transmitted a certain distance, which as noted above, is longer for lower order modulation formats, and shorter for higher order modulation formats. If the length of a particular link, however, is between the transmission distances associated with one of the standard modulation formats, the standard modulation format having a transmission distance closest to that of the link, but having an associated BER less than that associated with the link will be selected. However, although relatively few errors will occur because the selected standard modulation format transmission distance is significantly more than the link distance, the capacity of the selected standard modulation format will be less than that which could be realized if an intermediate modulation format having a transmission distance close to that of the link were employed.

Although transmission with intermediate modulation formats, such as 5-QAM (5b/dual-pol-symbol) and 7-QAM (7b/dual-pol-symbol), other than the standard modulation formats, can be used, the circuitry required to generate/decode such modulation formats is complex and requires relatively high gate counts and high power in implementation.

Thus, there is a need for optical communication systems that can generate, without complex circuitry, optical communication signals that have SEs between those of the standard modulation formats, such as BPSK, QPSK, 3-QAM, 8-QAM, 16-QAM, 32-QAM and 64-QAM for example.

SUMMARY

Consistent with an aspect of the present disclosure, an apparatus is provided that comprises a laser that supplies light and a modulator that receives the light. In addition, a transmission circuit is provided that supplies an electrical signal to the modulator, the modulator modulating the light based on the electrical signal to generate a modulated optical signal having first and second pluralities of subcarriers. Each of the first plurality of subcarriers has an associated first modulation format and each of the second plurality of subcarriers has an associated second modulation format, which is different than the first modulation format.

Consistent with a first aspect of the present disclosure, an apparatus is provided that includes a laser that supplies light, and a modulator that receives the light. The apparatus further includes a transmission circuit, such that, based on a plurality of control signals, electrical signals are supplied to the modulator, the modulator modulates the light to supply first and second pluralities of subcarriers based on the electrical signals, each of the first plurality of subcarriers having an associated first modulation format and each of the second plurality of subcarriers having an associated second modulation format, which is different than the first modulation format.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one (several) embodiment(s) of the invention and together with the description, serve to explain the principles of the invention.

DESCRIPTION OF THE EMBODIMENTS

Consistent with the present disclosure, an optical communication system is provided in which data is carried over optical signals including subcarriers. The subcarriers may be modulated with the standard modulation formats noted above, but the modulation formats are selectively assigned to the subcarriers, such that some subcarriers are modulated with different standard modulation formats than others. As used herein, a “standard modulation format” is one of BPSK, and n-QAM, where n is an integer. Such n-QAM modulation formats include of 3-QAM, 4-QAM (QPSK), 8-QAM, 16-QAM, 64-QAM, 128-QAM, and 256-QAM. By selecting the number of subcarriers and the types of modulation formats employed, an optical signal with an effective SE that is between that of the standard modulation formats can be generated for transmission over a distances that more closely matches the link distance. Such custom or intermediate SE signals can be tailored to a particular optical link SNR to provide data transmission rates that are higher than the low order modulation formats that would otherwise be employed for optical signals carried by such links. As a result, more efficient data transmission can be achieved.

Various circuits and techniques for generating and processing optical signals including subcarriers are described in the following: U.S. Patent Application Publication No. 2014/0092924 titled “Channel Carrying Multiplexed Digital Subcarriers”; U.S. Patent Application Publication No. 2015/0280834 titled “Frequency And Phase Compensation For Modulation Formats Using Multiple Sub-Carriers”; U.S. Patent Application Publication No. 2015/0280853, titled “Configurable Frequency Domain Equalizer for Dispersion Compensation of Multiple Sub-Carriers”; U.S. patent application Ser. No. 14/788,564, filed Jun. 30, 2015, and titled “Feedback Carrier Recovery Device”; and U.S. patent application Ser. No. 14/754,521, filed Jun. 29, 2015, and titled “Frequency Domain Coded Modulation With Polarization Interleaving For Fiber Nonlinearity Mitigation In Digital Sub-Carrier Coherent Optical Communication Systems.” The entire contents of each of the foregoing are incorporated herein by reference. Reference will now be made in detail to the present exemplary embodiments of the present disclosure, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIG. 1illustrates an optical link of optical communication system100consistent with an aspect of the present disclosure. Optical communication system100includes a plurality of transmitter blocks (Tx Block)12-1to12-n provided in a transmit node11. Each of transmitter blocks12-1to12-n receives a corresponding one of a plurality of data or information streams Data-1to Data-n, and, in response to a respective one of these data streams, each of transmitter blocks12-1to12-n may output a group of optical signals or channels to a combiner or multiplexer14. Each optical signal carries an information stream or data corresponding to each of data streams Data-1to Data-n. Multiplexer14, which may include one or more optical filters, for example, combines each of group of optical signals onto optical communication path16. Optical communication path16may include one or more segments of optical fiber and optical amplifiers, for example, to optically amplify or boost the power of the transmitted optical signals.

As further shown inFIG. 1, a receive node18is provided that includes an optical combiner or demultiplexer20, which may include one or more optical filters, for example, optical demultiplexer20supplies each group of received optical signals to a corresponding one of receiver blocks (Rx Blocks)22-1to22-n. Each of receiver blocks22-1to22-n, in turn, supplies a corresponding copy of data or information streams Data-1to Data-n in response to the optical signals. It is understood that each of transmitter blocks12-1to12-n has the same or similar structure and each of receiver blocks22-1to22-n has the same or similar structure.

FIG. 2illustrates one of transmitter blocks12-1in greater detail. Transmitter block12-1may include a digital signal processor (DSP)202including circuitry or circuit blocks CB1-1to CB1-10, each of which receiving, for example, a corresponding portion of Data-1and supplying a corresponding one of outputs or electrical signals202-1to202-10to a circuit, such as application specific integrated circuit (ASIC)204. ASIC204include circuit blocks CB2-1to CB2-10, which supply corresponding outputs or electrical signals204-1to204-10to optical sources or transmitters OS-1to OS-2provided on transmit photonic integrated circuit (PIC)205. As further shown inFIG. 2, each of optical sources OS-1to OS-2supplies a corresponding one of modulated optical signals having wavelengths λ1to λ10, respectively. The optical signals are combined by an optical combiner or multiplexer, such as arrayed waveguide grating (AWG)208, for example, and combined into a band or group of optical signals supplied by output206-1. Alternatively, a known optical power multiplexer may be provided to combine the optical signals. Optical sources OS-1to OS-10and multiplexer208may be provided on substrate205, for example. Substrate205may include indium phosphide or other semiconductor materials. AlthoughFIG. 2illustrates ten circuit blocks CB1-1to CB1-10, ten circuit blocks CB2-1to CB2-10, and ten optical sources OS1-1to OS-10, it is understood that any appropriate number of such circuit blocks and optical sources may be provided. Moreover, it is understood, that optical sources OS-1to OS-10, as well as multiplexer208, may be provided as discrete components, as opposed to being integrated onto substrate205as PIC206. Alternatively, selected components may be provided on a first substrate while others may be provided on one or more additional substrates in a hybrid scheme in which the components are neither integrated onto one substrate nor provided as discrete devices.

DSP and ASIC202collectively constitute transmission circuit1that supply drive signals (electrical signals) to the modulators in optical source OS-1as well as the remaining optical sources.

FIG. 3aillustrates a portion of transmission circuit1, namely, circuit block CB1-1of DSP202in greater detail. Circuit block CB-1includes a Forward Error Correction (FEC) encoder302that receives data stream Data-1and encodes the data stream to provide input data Data In at a rate R. The same FEC encoder engine is used to encode data associated with each subcarrier. Data In is supplied to demultiplexer circuit304which includes a plurality of switches that supply portions of Data In at respective rates R0to Rm−1 to a corresponding one of engine circuits306-1to306-m. Typically, the average of R0to Rm−1 (namely, (R0+Rm−1)/m) is equal to rate R.

Each engine circuit (collectively referred to as “306”) supplies a digitized analog signal (SC0to SCm−1) that is representative of a respective one of a plurality of subcarriers of an optical signal which is ultimately output from an optical source, as discussed in greater detail below. Each of the digitized analog signals is next fed to a corresponding one of fast Fourier transform (FFT) circuits308-1to308-m to convert each such signal into a respective frequency domain signal. Each frequency domain signal is then subject to filtering by a corresponding one of filters310-1to310-m. In one example, a power associated with each frequency domain signal is adjusted for optimal performance to provide greater energy or power to high order modulation format subcarriers which may be more prone to errors and less power to low order modulation format subcarriers which are less susceptible to errors. Put another way, particular powers are assigned to each subcarrier, such that the average bit error rate (BER) of all subcarriers is optimized to have a lowest value possible. That is, each higher order subcarrier and each lower order subcarrier, for example, has a respective one of a plurality of bit error rates (BERs). The average BER, i.e., an average of the BERs over all the high and lower order subcarriers, has a lower value than if the powers of the subcarriers were not optimized as described above. Such minimum average BER can be obtained when the BER of each subcarrier is substantially the same or uniform.

In one example, digitized analog signals corresponding to lower order modulation formats, such as BPSK, preferably have an associated power that is less than a power associated with digitized analog signals corresponding to higher order modulation format, such as 8-QAM. The filtered or power adjusted frequency domain signals are next input to a multiplexer312that distributes the power adjusted frequency domain signals over each of four outputs314. These frequency domain signals are then converted back to the time domain by inverse fast Fourier transform circuit (IFFT)316and, the resulting time domain signals are next supplied on outputs318, each of which being coupled or connected to a respective one of digital to analog converters (DACs)410,412,414, and416. Spectrum317is a representation of digitized analog subcarrier signals SC0to SCm−1 in the frequency domain prior to input to IFFT316, and output waveform319form IFFT316is a representation of the subcarrier signals in the time domain.

As further shownFIG. 3a, each of the Tx Engine circuits receives a corresponding one of a plurality of control signals. Selection of a desired modulation format by the Tx Engine circuits will next be described with reference toFIG. 3b, which shows Tx Engine circuit306-1in greater detail. Remaining Tx Engine circuits306-2to306m−1 have the same or similar structure as Tx Engine circuit306-1.

As shown inFIG. 3b, a control signal may be supplied to switch320, which may be implemented in either firmware or software. In response to the control signal one, switch320directs the data to one of modulation format circuit321to327, and the selected modulation format circuit is activated to supply a digitized analog signal associated with a selected modulation format, which in this example, is one of BPSK, 3-QAM, QPSK, 8-QAM, 16-QAM, 32, QAM, and 64-QAM). The control signal may further be employed to control switch328to direct the digitized analog signal to FFT308-1from the selected one of modulation format circuits321to327. Accordingly, for example, if an optical link has a particular SNR which causes a given number of bit errors to occur during propagation along the length of the link, a combination of subcarrier modulation formats can be selected, such that the effective BER associated with an optical signal carrying such subcarriers approximates the BER of the link, and thus the transmission distance of an optical signal including such subcarriers more closely approximates the link distance. As such, an intermediate SE that provides maximum data transmission rate can be obtained for the link. As noted above, in the conventional approach, a standard transmission format would be employed for such link having a low SE that yields a capacity that is less than that associated with the intermediate SE described above. An expression for determining an intermediate SE of an optical signal including m subcarriers will next be presented. If M is the total number of subcarriers, A is the number of subcarriers with modulation format1, SE=X1for modulation format1, B is the number of subcarriers with modulation format2, and SE=X2for modulation format Y, an average SE (SEavg) of the optical signal satisfies: SEavg=(A/M)*X1+(B/M)*X2(Eq. 1), where M is a sum of A+B, A being a number of the first plurality of subcarriers and B being a number of the second plurality of subcarriers. Accordingly, there are M−1 additional SEs between the two available ones (X1and X2). Such additional SEs can be realized without complex hardware requiring high gate counts or a higher power in implementation.

The above expression for SEavg is for when there are two modulation formats. A general expression for the average SE is: SEavg=(A1/M)*X1+(A2/M)*X2+(A3/M)*X3+ . . . (An/M)*Xn, (Eq. 2) where M is the total number of subcarriers, A1is the number of subcarriers with modulation format1, SE=X1for modulation format1, A2is the number of subcarriers with modulation format2, SE=X2for modulation format2, etc., An is the number of subcarriers with the nth modulation format, and SE=Xn for the nth modulation format. Put another way,

If all the subcarriers are passed to one single FEC circuit, as noted above, the equivalent BER is the mean BER of all the different subcarriers.

Turning toFIG. 4, DACs410and412receive a respective one of a pair of the outputs318from IFFT316. DACs410and412, in turn, output corresponding analog signals, which are filtered by low-pass or roofing filters418and420to thereby remove, block or substantially attenuate higher frequency components in these analog signals. Such high frequency components or harmonics are associated with sampling performed by DACs410and412and are attributable to known “aliasing.” The analog signals output from DACs414and416are similarly filtered by roofing filters422and424, respectively. The filtered analog signals output from roofing filters418,420422, and424may next be fed to corresponding driver circuits426,428,430, and432, which supply modulator driver signals that have a desired current and/or voltage for driving modulators present in PIC206to provide optical signals with the desired subcarriers noted above. PIC206is discussed in greater detail below with reference toFIG. 5a.

FIG. 5aillustrates transmitter or optical source OS-1in greater detail. It is understood that remaining optical sources OS-1to OS-10have the same or similar structure as optical source OS-1.

Optical source OS-1may be provided on substrate205and may include a laser508, such as a distributed feedback laser (DFB) that supplies light to at least four (4) modulators506,512,526and530. DFB508may output continuous wave (CW) light at wavelength λ1to a dual output splitter or coupler510(e.g. a 3 db coupler) having an input port and first and second output ports. Typically, the waveguides used to connect the various components of optical source OS-1may be polarization dependent. A first output510aof coupler510supplies the CW light to first branching unit511and the second output510bsupplies the CW light to second branching unit513. A first output511aof branching unit511is coupled to modulator506and a second output511bis coupled to modulator512. Similarly, first output513ais coupled to modulator526and second output513bis coupled to modulator530. Modulators506,512,526and530may be, for example, Mach Zehnder (MZ) modulators. Each of the MZ modulators receives CW light from DFB508and splits the light between two (2) arms or paths. An applied electric field in one or both paths of a MZ modulator creates a change in the refractive index to induce phase and/or amplitude modulation to light passing through the modulator. Each of the MZ modulators506,512,526and530, which collectively can constitute a nested modulator, are driven with data signals or drive signals supplied via driver circuits426,428,430, and432, respectively. The CW light supplied to MZ modulator506via DFB508and branching unit511is modulated in accordance with the drive signal supplied by driver circuit426. The modulated optical signal from MZ modulator506is supplied to first input515aof branching unit515. Similarly, driver circuit328supplies further drive signals for driving MZ modulator512. The CW light supplied to MZ modulator512via DFB508and branching unit511is modulated in accordance with the drive signal supplied by driver circuit428. The modulated optical signal from MZ modulator512is supplied to phase shifter514which shifts the phase of the signal 90° (π/2) to generate one of an in-phase (I) or quadrature (Q) components, which is supplied to second input515bof branching unit515. The modulated data signals from MZ modulator506, which include the remaining one of the I and Q components, and the modulated data signals from MZ modulator512, are supplied to polarization beam combiner (PBC)538via branching unit515.

Modulator driver430supplies a third drive signal for driving MZ modulator526. MZ modulator526, in turn, outputs a modulated optical signal as either the I component or the Q component. A polarization rotator524may optionally be disposed between coupler510and branching unit513. Polarization rotator524may be a two port device that rotates the polarization of light propagating through the device by a particular angle, usually an odd multiple of 90° . The CW light supplied from DFB508is rotated by polarization rotator524and is supplied to MZ modulator526via first output513aof branching unit513. MZ modulator526then modulates the polarization rotated CW light supplied by DFB508, in accordance with drive signals from driver circuit430. The modulated optical signal from MZ modulator526is supplied to first input517aof branching unit517.

A fourth drive signal is supplied by driver432for driving MZ modulator530. The CW light supplied from DFB508is also rotated by polarization rotator524and is supplied to MZ modulator530via second output513bof branching unit513. MZ modulator530then modulates the received optical signal in accordance with the drive signal supplied by driver432. The modulated data signal from MZ modulator530is supplied to phase shifter528which shifts the phase the incoming signal 90° (π/2) and supplies the other of the I and Q components to second input517bof branching unit517. Alternatively, polarization rotator536may be disposed between branching unit517and PBC538and replaces rotator524. In that case, the polarization rotator536rotates both the modulated signals from MZ modulators526and530rather than the CW signal from DFB508before modulation. The modulated data signal from MZ modulator526is supplied to first input port538aof polarization beam combiner (PBC)538. The modulated data signal from MZ modulator530is supplied to second input port538bof polarization beam combiner (PBC)538. PBC538combines the four modulated optical signals from branching units515and517and outputs a multiplexed optical signal having wavelength λ1to output port538c. In this manner, one DFB laser508may provide a CW signal to four separate MZ modulators506,512,526and530for modulating at least four separate optical channels by utilizing phase shifting and polarization rotation of the transmission signals. Although rotator536and PBC538are shown on the PIC, it is understood that these devices may instead be provided off-PIC.

In another example, splitter or coupler510may be omitted and DFB508may be configured as a dual output laser source to provide CW light to each of the MZ modulators506,512,526and530via branching units511and513. In particular, coupler510may be replaced by DFB508configured as a back facet output device. Both outputs of DFB laser508, from respective sides508-1and508-2of DFB508, are used, in this example, to realize a dual output signal source. A first output508aof DFB508supplies CW light to branching unit511connected to MZ modulators506and512. The back facet or second output508bof DFB508supplies CW light to branching unit513connected to MZ modulators526and530via path or waveguide543(represented as a dashed line inFIG. 5a). The dual output configuration provides sufficient power to the respective MZ modulators at a power loss far less than that experienced through 3 dB coupler510. The CW light supplied from second output508bis supplied to waveguide543which is either coupled directly to branching unit513or to polarization rotator524disposed between DFB508and branching unit513. Polarization rotator524rotates the polarization of CW light supplied from second output508bof DFB508and supplies the rotated light to MZ modulator526via first output513aof branching unit513and to MZ modulator530via second output513bof branching unit513. Alternatively, as noted above, polarization rotator524may be replaced by polarization rotator536disposed between branching unit517and PBC538. In that case, polarization rotator536rotates both the modulated signals from MZ modulators526and530rather than the CW signal from back facet output508bof DFB508before modulation.

The polarization multiplexed output from PBC538, may be supplied to multiplexer208inFIG. 2, along with the polarization multiplexed outputs having wavelength λ2to λ10from remaining optical sources OS-2to OS-m. Multiplexer208, which, as noted above, may include an AWG204, supplies a group of optical signals to multiplexer14(seeFIG. 1). It is understood that PICS present in transmitter blocks12-2to12-n operate in a similar fashion and include similar structure as PIC206shown inFIGS. 2 and 5to provide optical signal including subcarriers having different modulation formats to provide a desired SE, as noted above.

Thus, by selecting digitized analog signal corresponding different modulation formats by applying appropriate control signals to Tx Engines306-1to306-m, respective drive signals are applied to the nested MZ modulator shown inFIG. 5a, such that an optical signal including subcarriers modulated in accordance with the modulation formats associated with the selected digitized analog signals is output from the PIC. The optical signal thus generated also has a desired SE, as noted above. As further noted above, in one example, certain subcarriers may have a first modulation format while others have a second modulation format in response to first control signals. Consistent with an aspect of the present invention, the first and second modulation formats are different from one another and are selected from the group of standard modulation formats: BPSK and n-QAM, where n is an integer greater than 1, such that n-QAM modulation formats includes 3QAM, 4QAM (QPSK), 8QAM, 16QAM, 32QAM, 64-QAM, 128-QAM, and 256-QAM. In another aspect of the present disclosure, the first modulation format is an N-QAM modulation format, where N is a first integer, and the second modulation format is an M-QAM modulation format, where M is a second integer that is less than the first integer. The number of subcarriers, such as SC0to SCm−1, having the first modulation format may be the same or different than the number of subcarriers having the second modulation format. Consistent with an additional aspect of the present disclosure, third and fourth control signals may also be applied to the Tx Engines306-1to306-m so that third and fourth modulation formats may be applied to third and fourth groups of the subcarrier. Here also, the third and fourth modulation formats may be selected from the standard modulation formats, and number of subcarriers in the third group may be the same or different than the number of subcarriers in the fourth group. In addition, the third and fourth modulation formats may be different from one another, as well as different from the first and second modulation formats. Further, the subcarriers may include first, second, third and fourth groups of subcarriers having different modulation formats from one another based on appropriate application of control signals. It is understood, that the combinations of modulation formats discussed above is exemplary only, and that any appropriate combination of modulation formats and number of subcarriers can be employed.

FIG. 5billustrates an examples of a spectrum associated with an optical signal output from optical source OS-1. Here, the optical signal includes four subcarriers (SC0-SC3). Subcarriers SC0and SC3may have a first modulation format, such as, QPSK, and second subcarriers, such as subcarriers SC1and SC2, may have a second modulation format, such as 8QAM to provide an effective or average SE that is between that associated with QPSK and 8QAM. As generally understood, subcarriers may be generated by modulating a carrier frequency (e.g., f0inFIG. 5bhaving a zero baseband frequency), which is the frequency of the light output from the laser to the modulator discussed above with reference toFIG. 5a, as opposed to modulating individual carriers supplied from respective lasers. In addition, subcarriers associated with the same carrier frequency may be encoded with a common or shared FEC encoder engine or circuit, as well as decoded with a shared FEC decoder engine or circuit.

FIG. 5cillustrates an example in which power of certain subcarrier has been adjusted to optimize or obtain a lowest average bit error rate (BER) for a particular combination of subcarriers and modulation formats. In this example, inner subcarriers SC1and SC2are modulated in accordance with a 16QAM modulation format, and outer subcarriers SC0and SC3are modulated in accordance with a QPSK modulation format. As shown inFIG. 5c, inner subcarriers having a higher order modulation format have a higher power, in this example, relative to the outer subcarriers. It is understood, however, that various devices described herein may impart excessive loss or noise, for example, even to subcarriers having a lower order modulation format. In such instances, such subcarriers having a lower order modulation format may have a power that exceeds that of subcarriers having a higher order modulation format and transmitted with the lower order modulation format subcarriers.

In another example, the following modulation formats and corresponding SEs are available for polarization multiplexed (PM) subcarriers: PM-BPSK(SE=2), PM-3QAM(SE=3), PM-QPSK(SE=4), PM-8QAM(SE=6), and PM-16QAM(SE =8). Various combinations of subcarriers modulated in accordance with two or more of these modulation formats can yield optical signals that can have one of (12) SEs without extra hardware or power. The optical signal SE (obtained from particular combinations of subcarrier modulation formats) can be selected from one of: 2.25 2.5 2.75 3.25 3.5 3.75 4.5 5 5.5 6.5 7 7.5. These SE values are calculated based on the SE expression noted above.

An example of power optimization of the subcarriers shown inFIG. 5cwill next be described with reference toFIGS. 5dand 5e. Preferably, a ratio of the power of the subcarriers having a first modulation format, such as 8QAM, to the power of the subcarriers having a second modulation format, such as QPSK, is adjusted so that the average BER (BERavg) of the optical signal is at a minimum (seeFIG. 5d, which shows the Optimum Power Ratio and corresponding Minimum BER). It is noted that for a fixed total power for the optical signal, at lower power ratios, such as at 0 dB, the power of 8QAM subcarriers is the same as the QPSK subcarriers, resulting in the 8QAM subcarriers having a higher BER. Accordingly, the average BER is higher for such low power ratios. On the other hand, if the power ratio is high (to the right inFIG. 5d), the 8QAM subcarriers have high power and the QPSK subcarriers have relatively low power. As such, the QPSK subcarriers incur a significant number of bit errors, and the average BER is high under these circumstances as well. In one example, the optimum power ratio may be 3.7 dB and, in another example, the optimum power ratio is 3.3 dB. As noted above, the system SNR may impact the optimum power ratio.

Put another way, the modulator is capable of outputting each of the n pluralities of subcarriers (n being greater than 1) at a plurality of powers, such that the modulator supplies a first one of the plurality of n pluralities of subcarriers at a first one of the plurality of powers and a second one of the n pluralities of subcarriers at a second one of the plurality of powers. A ratio of the first and second powers is one of a plurality of ratios of each of the plurality of powers to one another and is associated with a Q value of the optical signal that is greater than Q values associated with remaining ones of the plurality of ratios, or an average BER that is less than an average BER associated with remaining ones of the plurality of ratios.

Preferably, the modulator is controlled to output an optical signal with subcarriers having power levels and a corresponding power ratio that yields a minimum BERavg.

BERavg=1∑n=1M⁢Xn⁢∑n=1M⁢Xn·BERn(Eq.⁢4)
where BERn is the bit error rate of subcarriers having the nth modulation format, and Xn, as noted above, is the spectral efficiency of such nth modulation format. In a case in which first and second pluralities of subcarriers are provided, each such plurality being modulated in accordance with first and second modulation formats, respectively, Eq. 4 can be written as:
BERavg=[X1*BER1+X2*BER2]/(X1+X2)  (Eq. 5)
where BER1and BER2are the bit error rates of the first and second pluralities of subcarriers, respectively, X1is the spectral efficiency (SE) of the first modulation format, and X2is the SE of the second modulation format. In the example discussed above with respect toFIG. 5cin which the first and second subcarriers are modulated based on 8QAM (e.g., first) and QPSK (e.g., second) modulation formats, respectively,

BER1and BER2are the bit error rates of the polarization multiplexed (PM)-8QAM and PM-QPSK subcarriers, respectively, the spectral efficiency of PM-8QAM is 6, and the spectral efficiency of QPSK or 4. Substituting these values into Equation 5:
BERavg=[6*BER1+4*BER2]/(6+4)  (Eq. 6)
or
BERavg=[3*BER1+2*BER2]/5  (Eq. 7)

As generally understood, BER is inversely related to a Quality (Q) Factor. For example, for binary modulation formats, such as BPSK and QPSK, BER=0.5*erfc(Q/√2), where Q (in dB) is 20*Log 10(Q).

Accordingly, as shown inFIG. 5e, at the optimum power ratio, the Q Factor associated with the optical signal is at a maximum.

In the above examples, a given signal-to-noise ratio (SNR) is assumed. Different SNRs may result in different optimal power ratios and different maximum Q Factors, as well as corresponding minimum values for BERavg.

In another example, minimum BERavg may be obtained when the BER of each individual subcarrier is substantially the same. In addition, the powers of the subcarriers may be set or adjusted so that the BER of each subcarrier is preferably within a range of ±20% of the average BER (BERavg), as calculated in accordance with equations Eq. 4-7. In another example, the powers are selected so that the BER of each subcarrier is more preferably within a range of ±15% of the average BER, and, in another example, the subcarrier powers are such that the BER of each subcarrier is preferably within a range of ±15%. And, in further preferred embodiments, the subcarrier powers are such that the BER of each subcarrier is preferably within a range of ±10% of the average BER and even more preferably within a range of ±5% of the average BER. By adjusting the subcarrier powers to be close to the average BER or close to having the same powers, the average BER is reduced and can approximate the minimum BER. Improved performance can thus be achieved compared to an optical signal in which the BERs fall outside a range of 20% of the average BER, for example. In addition, the powers of the subcarriers discussed above, such as in regard toFIG. 5c, may yield BERs that fall within the above noted 5% to 20% ranges about the average BER.

It is noted that the optical signals disclosed herein are typically not orthogonal frequency division multiplexed (OFDM) optical signals. The spectra of subcarriers in such OFDM optical signals typically overlap with one another (due to modulation based on time-domain rectangular pulses that ensure orthogonality). The subcarriers disclosed herein, however, do not spectrally overlap (due to Nyquist pulse shaping used in generating the subcarriers), as shown inFIGS. 5band 5c, for example.

As noted above, optical signals output from transmitter block12-1are combined with optical signals output from remaining transmitter blocks12-2to12-n onto optical communication path16and transmitted to receive node18(seeFIG. 1). In receive node18, demultiplexer20divides the incomings signal into optical signal groupings, such that each grouping is fed to a corresponding one of receiver blocks22-1to22-n.

One of receiver blocks22-1is shown in greater detail inFIG. 6. It is understood that remaining receiver circuitry or blocks22-2to22-n have the same or similar structure as receiver block22-1.

Receiver block22-1includes a receive PIC602provided on substrate604. PIC602includes an optical power splitter603that receives optical signals having wavelengths λ1to λ10, for example, and supplies a power split portion of each optical signal (each of which itself may be considered an optical signal) to each of optical receivers OR-1to OR-10. Each optical receiver OR-1to OR-10, in turn, supplies a corresponding output to a respective one of circuit blocks CB3-1to CB3-10of ASIC606, and each of circuit blocks CB3-1to CB3-10, supplies a respective output to a corresponding one of circuit blocks CB4-1to CB4-10of DSP608. DSP608, in turn, outputs a copy of data Data-1in response to the input to circuit blocks CB4-1to CB4-10.

Optical receiver OR-1is shown in greater detail inFIG. 7. It is understood that remaining optical receivers OR-2to OR-10have the same or similar structure as optical receiver OR-1. Optical receiver OR-1may include a polarization beam splitter (PBS)702operable to receive polarization multiplexed optical signals λ1to λ10and to separate the signal into X and Y orthogonal polarizations, i.e., vector components of the optical E-field of the incoming optical signals transmitted on optical communication path16, which may include an optical fiber, for example. The orthogonal polarizations are then mixed in 90 degree optical hybrid circuits (“hybrids”)720and724with light from local oscillator (LO) laser701having wavelength λ1. Hybrid circuit720outputs four optical signals O1a, O1b, O2a, O2band hybrid circuit724outputs four optical signals O3a, O3b, O4a, and O4b, each representing the in-phase and quadrature components of the optical E-field on X (TE) and Y (TM) polarizations, and each including light from local oscillator701and light from polarization beam splitter702. Optical signals O1a, O1b, O2a, O2b, O3a, O3b, O4a, and O4bare supplied to a respective one of photodetector circuits709,711,713, and715. Each photodetector circuit includes a pair of photodiodes (such as photodiodes709-1and709-2) configured as a balanced detector, for example, and each photodetector circuit supplies a corresponding one of electrical signals E1, E2, E3, and E4. Alternatively, each photodetector may include one photodiode (such as photodiode709-1) or single-ended photodiode. Electrical signals E1to E4are indicative of data carried by optical signals λ1to λ10input to PBS702demodulated with LO701(λ1). For example, these electrical signals may comprise four base-band analog electrical signals linearly proportional to the in-phase and quadrature components of the optical E-field on X and Y polarizations.

FIG. 8shows circuitry or circuit blocks CB3-1and CB4-1in greater detail. It is understood that remaining circuit blocks CB3-2to CB3-10of ASIC606have a similar structure and operate in a similar manner as circuit block CB3-1. In addition, it is understood that remaining circuit blocks CB4-2to CB4-10of DSP608have a similar structure and operation in a similar manner as circuit block CB4-1.

Circuit block CB3-1includes known transimpedance amplifier and automatic gain control (TIA/AGC802) circuitry802,804,806, and808that receives a corresponding one of electrical signals E1, E2, E3, and E4. Circuitry802,804,806, and808, in turn, supplies corresponding electrical signals or outputs to respective ones of anti-aliasing filters810,812,814, and816, which, constitute low pass filters that further block, suppress, or attenuate high frequency components due to known “aliasing”. The electrical signals or outputs form filters810,812,814, and816are then supplied to corresponding ones of analog-to-digital converters (ADCs)818,820,822, and824.

ADCs818,820,822, and824, may sample at the same or substantially the same sampling rate as DACs310,312,314, and316discussed above. Preferably, however, circuit block CB4-1and DSP608have an associated sampling rate that is less than the DAC sampling rate, as described in greater detail in U.S. patent application Ser. No. 12/791,694 titled “Method, System, And Apparatus For Interpolating An Output Of An Analog-To-Digital Converter”, filed Jun. 1, 2010, the entire contents of which are incorporated herein by reference.

Turning toFIG. 9, outputs from ADCs818,820,822, and824are supplied to FFT902, which converts these outputs to the frequency domain. The frequency domain signals correspond to subcarriers SC0to SCm−1 are input to a demultiplexer, which provides each subcarrier representation to a corresponding one of chromatic dispersion (CD) and polarization mode dispersion (PMD) equalization circuits903-1to903-m. Each output of CD and PM Eq circuits903-1to903-m is fed to a respective one of IFFTs904-1to904-m. Based on the received inputs to the IFFTs, each IFFT supplies a corresponding time domain signal to a respective one of Rx Engine circuits906-1to906-m. Each of the Rx Engine circuits, in turn, may decode the received time domain signal in accordance with the modulation format associated with such signal. Each Rx Engine circuit may also be controlled by a user to accommodate an associated modulation format. The outputs of each Rx Engine circuit is a copy of a portion of Data In, at a respective one of data streams R0to Rm−1. The data portions are supplied to a multiplexer908, which combines the portions to provide a copy of the Data In at rate R (shown as Data Out inFIG. 9). Data Out is supplied to FEC decoder circuit910, which performs error correction on Data Out and supplies a copy of the data stream input to the system inFIG. 1. As noted above, the same FEC engine or a common FEC engine is used to decode data carried by each subcarrier.

FIG. 10illustrates a series of plots1000of BER vs. SNR for various SEs. SEs of 2, 4, and 8 [b/dual-pol-symbol] correspond to standard modulation formats noted above. As further seen inFIG. 10, plots between those labeled SE=2, SE=4, and SE=8 are associated with intermediate modulation formats and SEs that are obtained by combining combinations of the standard modulation format subcarriers, as discussed above. One such plot is associated with an SE of 6, for example.

It is noted that electrical signals associated with the subcarriers may experience more loss at higher frequencies than lower frequencies as such electrical signals propagate in transmission lines and traces in DSPs202and608, ASICs204and606, as well as in electrical connections to various devices on the transmit and receive PICs discussed above. In order to compensate for such losses (i.e., electrical transmission impairments) higher frequency subcarriers may be modulated in accordance with low order modulation formats, which are less susceptible to noise and may incur fewer errors, and lower frequency subcarriers may be modulated in accordance with higher order modulation formats, because the lower frequency subcarriers do not experience as much loss and thus will have less noise. Such lower frequency subcarriers can, therefore, carry data with fewer errors and can be modulated in accordance with a higher order modulation format.

Thus, in the examples shown inFIGS. 5band 5c, higher frequency subcarriers SC0and SC3may be modulated in accordance with a lower order modulation format, such as QPSK, as noted above, or BPSK, whereas lower frequency subcarriers SC1and SC2may be modulated in accordance with a higher order modulation format, such as 8QAM or 16QAM.

Other transmission impairments in the electrical domain as well as those in the optical domain may also be compensated by appropriate choice of subcarrier modulation formats. For example, subcarriers can be modulated at lower order modulation formats at frequencies that are more susceptible to optical loss, polarization mode dispersion (PMD), chromatic dispersion (CD), or other optical transmission impairments. However, those subcarriers that experience fewer optical transmission impairments can be modulated at higher order modulation formats.

FIG. 11illustrates another example of a portion of transmission circuit1similar to discussed above in connection withFIG. 3a. InFIG. 11, however, one Tx Engine circuit, such as Tx Engine1106, receives pairs of Data In portions having rates R0and R1, respectively. This is because certain modulation formats are advantageously encoded across two subcarriers. Here, Tx Engine circuit1106supplies digitized analog signals representative of spectrally adjacent ones of the first plurality of subcarriers, such as SC0and SC1. The digitized analog signals SC0and SC1are further processed by FFT308-1and308-2, respectively, in a manner similar to or the same as that discussed above with reference toFIG. 3a. Other circuits shown inFIG. 11also operate in a manner similar to that described above with reference toFIG. 3a.

As noted above, subcarriers in an optical signal are modulated with different modulation formats to provide a variety of SEs that facilitate efficient data transmission over a variety of optical link distances.