Optical transport system employing direct-detection self-coherent receivers and compatible transmitters

Abstract

An optical WDM system configured to use direct detection of communication signals that is compatible with electronic CD compensation on a per-channel basis. In an example embodiment, to enable full (e.g., amplitude and phase) electric-field reconstruction at the receiver, the optical WDM system uses a carrier-frequency plan according to which the carrier-frequency comb used at one end of the WDM link and the carrier-frequency comb used at the other end of the WDM link are offset with respect to one another by one half of the bandwidth of an individual WDM component transmitted therethrough. This frequency offset places each local carrier frequency at a roll-off edge of the corresponding incoming data-modulated signal. As a result, the corresponding combined optical signal beneficially lends itself to direct detection that can be followed by full electric-field reconstruction using a known self-coherent Kramers-Kronig method and then by conventional electronic CD compensation.

Claims

1. An apparatus, comprising: a WDM transceiver having a plurality of individual-channel transceivers; and a DEMUX filter having a plurality of pass bands, each of the pass bands configured to feed light to a respective one of the individual-channel transceivers; wherein a first individual-channel transceiver of the WDM transceiver comprises: an optical port connectable to receive a first carrier frequency; an optical modulator connected to the optical port and configured to modulate the first carrier frequency with first data to generate a modulated output signal; an optical coupler connected to the optical port and configured to combine the first carrier frequency with a modulated input signal to generate a combined optical signal, the modulated input signal having a second carrier frequency modulated with second data; a photodetector configured to convert the combined optical signal into a corresponding electrical signal proportional to an optical power of the combined optical signal; and a signal processor configured to process the corresponding electrical signal to recover the second data; wherein a center frequency of at least one of the pass bands is offset with respect to a center of a frequency slot allocated to a corresponding WDM channel; and wherein the second carrier frequency is offset with respect to the first carrier frequency by a fixed nonzero frequency offset.

2. The apparatus of claim 1, wherein each other individual-channel transceiver of the WDM transceiver comprises a nominal copy of the first individual-channel transceiver, the nominal copy being configured to operate using a different respective first carrier frequency and a different respective second carrier frequency.

3. The apparatus of claim 2, wherein respective optical ports of the individual-channel transceivers are connectable to receive the different respective first carrier frequencies generated such that a spacing between two adjacent first carrier frequencies alternates between a first value and a different second value.

4. The apparatus of claim 3, wherein a difference between the first value and the second value is a bandwidth of the modulated output signal.

5. The apparatus of claim 3, wherein the plurality of individual-channel transceivers are configured to operate using a frequency grid that has a constant channel spacing.

6. The apparatus of claim 2, wherein a center frequency of at least one of the pass bands is offset with respect to the respective first carrier frequency.

7. The apparatus of claim 2, wherein a center frequency of at least one of the pass bands is offset with respect to the respective second carrier frequency.

8. The apparatus of claim 1, wherein each of the plurality of individual-channel transceivers is configured to handle optical signals corresponding to a different respective WDM channel of the WDM transceiver.

9. The apparatus of claim 1, wherein the DMUX filter has an operating stability that allows the center frequency of the pass band to drift over time within a fixed frequency interval that has a spectral width that is greater than 20% of a bandwidth of the modulated input signal.

10. The apparatus of claim 1, wherein the fixed frequency offset is approximately one half of a data bandwidth of the modulated input signal.

11. The apparatus of claim 1, wherein the modulated input signal is a PDM signal.

12. The apparatus of claim 1, wherein the optical modulator is further configured to modulate the first carrier frequency to cause the modulated output signal to carry a pilot tone; and wherein the apparatus is configured to lock the second carrier frequency with respect to the pilot tone.

13. The apparatus of claim 1, further comprising a laser connected to the optical port and configured to apply thereto the first carrier frequency; wherein the modulated input signal is configured to carry a pilot tone; and wherein the laser is configured to lock the first carrier frequency with respect to the pilot tone.

14. The apparatus of claim 1, wherein the optical coupler comprises an asymmetric 22 coupler.

15. An apparatus, comprising: a WDM transceiver having a plurality of individual-channel transceivers; and a DEMUX filter having a plurality of pass bands, each of the pass bands configured to feed light to a respective one of the individual-channel transceivers; wherein a first individual-channel transceiver of the WDM transceiver comprises: an optical port connectable to receive a first carrier frequency; an optical modulator connected to the optical port and configured to modulate the first carrier frequency with first data to generate a modulated output signal; an optical coupler connected to the optical port and configured to combine the first carrier frequency with a modulated input signal to generate a combined optical signal, the modulated input signal having a second carrier frequency modulated with second data; a photodetector configured to convert the combined optical signal into a corresponding electrical signal proportional to an optical power of the combined optical signal; and a signal processor configured to process the corresponding electrical signal to recover the second data; wherein the second carrier frequency is offset with respect to the first carrier frequency by a fixed nonzero frequency offset; wherein each other individual-channel transceiver of the WDM transceiver comprises a nominal copy of the first individual-channel transceiver, the nominal copy being configured to operate using a different respective first carrier frequency and a different respective second carrier frequency; and wherein a center frequency of at least one of the pass bands is offset with respect to the respective first carrier frequency.

16. An apparatus, comprising: a WDM transceiver having a plurality of individual-channel transceivers; and a DEMUX filter having a plurality of pass bands, each of the pass bands configured to feed light to a respective one of the individual-channel transceivers; wherein a first individual-channel transceiver of the WDM transceiver comprises: an optical port connectable to receive a first carrier frequency; an optical modulator connected to the optical port and configured to modulate the first carrier frequency with first data to generate a modulated output signal; an optical coupler connected to the optical port and configured to combine the first carrier frequency with a modulated input signal to generate a combined optical signal, the modulated input signal having a second carrier frequency modulated with second data; a photodetector configured to convert the combined optical signal into a corresponding electrical signal proportional to an optical power of the combined optical signal; and a signal processor configured to process the corresponding electrical signal to recover the second data; wherein the second carrier frequency is offset with respect to the first carrier frequency by a fixed nonzero frequency offset; wherein each other individual-channel transceiver of the WDM transceiver comprises a nominal copy of the first individual-channel transceiver, the nominal copy being configured to operate using a different respective first carrier frequency and a different respective second carrier frequency; and wherein a center frequency of at least one of the pass bands is offset with respect to the respective second carrier frequency.

17. An apparatus, comprising: a laser connected to an optical port and configured to apply thereto a first carrier frequency; an optical modulator connected to the optical port and configured to modulate the first carrier frequency with first data to generate a modulated output signal; an optical coupler connected to the optical port and configured to combine the first carrier frequency with a modulated input signal to generate a combined optical signal, the modulated input signal having a second carrier frequency modulated with second data; a photodetector configured to convert the combined optical signal into a corresponding electrical signal proportional to an optical power of the combined optical signal; and a signal processor configured to process the corresponding electrical signal to recover the second data; wherein the second carrier frequency is offset with respect to the first carrier frequency by a fixed nonzero frequency offset; wherein the modulated input signal is configured to carry a pilot tone; and wherein the laser is configured to lock the first carrier frequency with respect to the pilot tone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other aspects, features, and benefits of various disclosed embodiments will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which:

(2) FIG. 1 shows a block diagram of an optical transport system according to an embodiment;

(3) FIG. 2 graphically illustrates example spectral and polarization characteristics of a WDM signal used in the optical transport system of FIG. 1 according to an embodiment;

(4) FIGS. 3A-3C show block diagrams of an individual channel transmitter that can be used in the optical transport system of FIG. 1 according to some embodiments;

(5) FIG. 4 shows a block diagram of an individual channel receiver that can be used in the optical transport system of FIG. 1 according to an embodiment;

(6) FIG. 5 shows a block diagram of an optical transport system according to an alternative embodiment;

(7) FIG. 6 shows a block diagram of an optical transceiver that can be used in the optical transport system of FIG. 5 according to an embodiment;

(8) FIG. 7 shows a block diagram of an optical transceiver that can be used in the optical transport system of FIG. 5 according to an alternative embodiment;

(9) FIG. 8 graphically shows a WDM-channel configuration that can be used in the optical transport system of FIG. 5 according to an embodiment;

(10) FIG. 9 graphically illustrates drift tolerances for de-multiplexers that can be used in the optical transport system of FIG. 5 according to an embodiment; and

(11) FIGS. 10A-10B graphically illustrate a laser-frequency-locking scheme that can be used in the optical transport system of FIG. 5 according to some embodiments.

DETAILED DESCRIPTION

(12) Some embodiments can be used in an optical transport system, e.g., having a relatively large (e.g., massive or >100) number of WDM channels and/or hardware that enables the system to be implemented in a relatively cost-effective manner.

(13) Some embodiments may benefit from the use of certain features of a direct-detection scheme disclosed in A. Mecozzi, et al., Kramers-Kronig Coherent Receiver, Optica 3, pp. 1218-1227 (2016), which is incorporated herein by reference in its entirety.

(14) FIG. 1 shows a block diagram of an optical transport system 100 according to an embodiment. System 100 comprises a WDM transmitter 102 and a WDM receiver 104 connected to receive a WDM signal 152 from the transmitter, by way of a fiber-optic link 150. In an example embodiment, WDM signal 152 can be generated using a frequency (wavelength) grid, such as a frequency grid that complies with the ITU-T G.694.1 Recommendation, which is incorporated herein by reference in its entirety. Example spectral and polarization characteristics of WDM signal 152 are described in more detail below in reference to FIG. 2.

(15) The frequency grid used in system 100 can be defined, e.g., in the frequency range from about 186 THz to about 201 THz, with a 100, 50, 25, or 12.5-GHz spacing of the channels therein. While typically defined in frequency units, the parameters of the grid can equivalently be expressed in wavelength units. For example, in the wavelength range from about 1528.8 nm to about 1563.9 nm, the 100-GHz spacing between the centers of neighboring WDM channels is equivalent to approximately 0.8 nm spacing. In alternative embodiments, other frequency grids can be used as well.

(16) The following terms are used herein to refer to certain characteristics of a frequency grid.

(17) Frequency grid: A reference set of frequencies used to denote nominal central frequencies that may be used for defining specifications and applications.

(18) Frequency slot: The frequency range allocated to a slot and unavailable to other slots within a frequency grid. A frequency slot is defined by its nominal central frequency and its slot width. A frequency slot may also be referred to as a WDM channel.

(19) Slot width: The full width of a frequency slot in a frequency grid.

(20) Channel spacing: The frequency difference between the nominal central frequencies of the pertinent frequency slots (WDM channels).

(21) WDM transmitter 102 comprises individual-channel transmitters 110.sub.1-110.sub.N, each configured to generate a corresponding WDM component of WDM signal 152 using a different respective carrier frequency (wavelength) spectrally arranged in accordance with the operative frequency grid. Transmitters 110.sub.n corresponding to odd WDM channels (e.g., n=1, 3, 5, . . . ) operate to generate the corresponding WDM components of WDM signal 152 such that each of those WDM components has a first (e.g., X) polarization. Transmitters 110.sub.n corresponding to even WDM channels (e.g., n=2, 4, 6, . . . ) similarly operate to generate the corresponding WDM components of WDM signal 152 such that each of those WDM components has a second (e.g., Y) polarization orthogonal to the first polarization. Herein, the index n is an integer from the interval [1, N]. Several example embodiments of transmitter 110.sub.n are described in more detail below in reference to FIGS. 3A-3C.

(22) WDM transmitter 102 further comprises multiplexers 120.sub.1 and 120.sub.2, two optional optical amplifiers 130, and a polarization beam combiner 140. Multiplexer (MUX) 120.sub.1 operates to combine the odd WDM components in a polarization-maintaining manner. A resulting multiplexed signal 122.sub.1 can optionally be amplified in the corresponding one of optical amplifiers 130. MUX 120.sub.2 similarly operates to combine the even WDM components in a polarization-maintaining manner. A resulting multiplexed signal 122.sub.2 can be optionally amplified in the other one of optical amplifiers 130. Polarization beam combiner 140 operates to polarization-interleave (the optionally amplified) multiplexed signals 122.sub.1 and 122.sub.2, thereby generating WDM signal 152.

(23) In an example embodiment, an optical amplifier 130 can be implemented using a polarization-maintaining (PM) erbium-doped fiber amplifier (EDFA).

(24) WDM receiver 104 comprises a 12 coupler 160 that can be implemented, e.g., using a regular polarization-insensitive 3-dB power splitter. The two attenuated copies of the received WDM signal 152 generated by 12 coupler 160 can be optionally amplified by EDFAs 170. In an alternative embodiment, the two EDFAs 170 may be replaced by a single EDFA located upstream from coupler 160.

(25) One copy of the (optionally amplified) received WDM signal 152 produced by coupler 160 is de-multiplexed by a demultiplexer (DEMUX) 180.sub.1 whose pass-bands are configured to allow the odd WDM components of WDM signal 152 to pass through, while significantly attenuating the even WDM components thereof (also see FIG. 2). The other copy of the (optionally amplified) received WDM signal 152 produced by coupler 160 is de-multiplexed by DEMUX 180.sub.2 whose pass-bands are configured to allow the even WDM components of WDM signal 152 to pass through, while significantly attenuating the odd WDM components thereof. Each of the resulting optical signals generated by DEMUXes 180.sub.1 and 180.sub.2 is then applied to and detected using a respective one of individual-channel receivers 190.sub.1-190.sub.N. An example embodiment of receiver 190.sub.n is described in more detail below in reference to FIG. 4.

(26) Polarization interleaving is used in system 100 to reduce (e.g., minimize) inter-channel interference between neighboring channels at the point of detection, thereby allowing for MUXes 120 and DEMUXes 180 to be implemented using optical filters with relaxed frequency tolerances. Besides polarization interleaving, the pass-bands of MUXes 120 and DEMUXes 180 may be configured such that the center frequency of a pass-band has an offset relative to the center frequency of the signal spectrum. The latter feature can be used to reduce (e.g., minimize) the linear beat product(s) from the neighboring channel that has its CW (continuous-wave) component closer to the edge of the channel of interest (also see FIG. 2).

(27) FIG. 2 graphically illustrates example spectral and polarization characteristics of WDM signal 152 according to an embodiment. For illustration purposes and without any implied limitation, FIG. 2 shows only three WDM channels, which are labeled CH1, CH2, and CH3, respectively. A person of ordinary skill in the art will understand that WDM signal 152 can have a different number of WDM channels arranged in a similar manner.

(28) Also schematically shown in FIG. 2 is an example spectral shape of a pass band of DEMUX 180.sub.2 corresponding to channel CH2. A person of ordinary skill in the art will understand that the pass bands of DEMUX 180.sub.1 corresponding to channels CH1 and CH3 may have similar spectral shapes, appropriately shifted in frequency.

(29) As indicated in FIG. 2, each of WDM channels CH1, CH2, and CH3 carries (i) a respective data-modulated signal represented by a rectangle 202.sub.n and (ii) a respective CW component represented by an arrow 204.sub.n. CW component 204.sub.n is approximately spectrally aligned with a roll-off edge of the data-modulated signal 202.sub.n and can be used as a local-oscillator signal at the corresponding individual-channel receiver 190.sub.n to recover the data encoded in the data-modulated signal 202.sub.n, e.g., as explained in the above-cited paper by A. Mecozzi, et al. In the shown embodiment, CW component 204.sub.n is imprinted onto data-modulated signal 202.sub.n at transmitter 110.sub.n. However, in some alternative embodiments, CW component 204.sub.n can be added to the data-modulated signal 202.sub.n at receiver 190.sub.n, e.g., as described in reference to FIGS. 6-7.

(30) The substantially flat portion B.sub.pass of pass band 210 is approximately aligned with the frequency slot allocated to channel CH2 on the frequency grid. B.sub.edge denotes the width of the roll-off edge of pass band 210, e.g., down to 20-dB rejection level from the level of B.sub.pass. The above-mentioned relaxed frequency tolerances of DEMUXes 180 are evident in FIG. 2 from the position of pass band 210 with respect to the frequency slots allocated to channels CH1 and CH3.

(31) FIGS. 3A-3C show block diagrams of an individual channel transmitter 110 (FIG. 1) according to some example embodiments. Each of the shown transmitters 110 is capable of generating a signal for populating a corresponding WDM channel and having the characteristics described above in reference to FIG. 2.

(32) Referring to FIG. 3A, in this particular embodiment, transmitter 110 comprises a laser 302, an I/Q modulator 306, two digital-to-analog converters (DACs) 320, two RF drivers 350, two RF power combiners 330, and an RF synthesizer 340. DACs 320 receive digital signals 318.sub.1 and 318.sub.2 that can be generated by the transmitter's digital signal processor (DSP, not explicitly shown in FIG. 3A) in a conventional manner to cause I/Q modulator 306 to generate the corresponding data-modulated signal 202.sub.n (see FIG. 2) at its optical output 308. RF synthesizer 340 and power combiners 330 operate to insert into optical output 308 a corresponding CW component 204.sub.n at the desired roll-off edge of the data-modulated signal 202.sub.n (also see FIG. 2). For example, the relative phase of the two RF-signal paths corresponding to the two power combiners 330 can be engineered such that the RF-signal components added by RF synthesizer 340 to the data-encoded portion of the RF drive signals applied to I/Q modulator 306 by RF drivers 350 cause the I/Q modulator to generate a corresponding single sideband in the optical spectrum, thereby inserting only a single CW component 204.sub.n at the desired side of the optical spectrum of data-modulated signal 202.sub.n.

(33) In the embodiment shown in FIG. 3B, DACs 320 receive digital signals 316.sub.1 and 316.sub.2 that can be generated by the transmitter's DSP (not explicitly shown in FIG. 3B) in a conventional manner to cause I/Q modulator 306 to generate, at optical output 308, both the corresponding data-modulated signal 202.sub.n and CW component 204.sub.n. For example, conventional digital single-sideband modulation techniques can be used, as known in the pertinent art, to cause RF drivers 350 to apply to I/Q modulator 306 appropriate drive-signal components suitable for this purpose.

(34) In the embodiment of FIG. 3C, transmitter 110 has two I/Q modulators 306 (labeled 306.sub.1 and 306.sub.2, respectively), which are connected in parallel as indicated in FIG. 3C. I/Q modulator 306.sub.1 is driven using digital signals 318.sub.1 and 318.sub.2, DACs 320, and RF drivers 350 (also see FIG. 3A) to generate, at its optical output 308.sub.1, the corresponding data-modulated signal 202.sub.n. I/Q modulator 306.sub.2 is configured to operate as a frequency-shifting modulator to shift a tapped-off portion of the carrier wave generated by laser 302 by an appropriate frequency amount to generate, at its optical output 308.sub.2, the corresponding CW component 204.sub.n. An optical coupler 310 is then used to optically combine the data-modulated signal 202.sub.n and the CW component 204.sub.n received by way of optical outputs 308.sub.1 and 308.sub.2, respectively, to generate a combined optical signal 312 for the corresponding channel, the combined optical signal 312 having the characteristics indicated in FIG. 2.

(35) In the embodiment shown in FIG. 3C, I/Q modulator 306.sub.2 is configured to operate as a frequency-shifting modulator using a drive circuit comprising an RF synthesizer 342 and a phase shifter 344, connected as known in the pertinent art to drive different arms of that I/Q modulator. A person of ordinary skill in the art will understand that alternative drive circuits for causing I/Q modulator 306.sub.2 to operate as a frequency-shifting modulator can similarly be used.

(36) FIG. 4 shows a block diagram of an individual channel receiver 190 (FIG. 1) according to an embodiment. Receiver 190 comprises a photodiode 410, an analog-to-digital converter (ADC) 420, and a DSP 430. Receiver 190 is a direct-detection receiver because photodiode 410 is configured to convert a received optical signal 402 into a corresponding electrical signal that is proportional to the optical power (electric field squared) thereof, and apply that electrical signal to ADC 420. In a typical embodiment, optical signal 402 represents a corresponding WDM component of WDM signal 152 and is generated by passing the latter through a respective pass band 210 (see FIG. 2) of the corresponding DEMUX 180 (FIG. 1). Photodiode 410 can be the only photodiode used in receiver 190.

(37) Due to the presence of both data-modulated signal 202.sub.n and CW component 204.sub.n (see FIG. 2), the digital electrical signal generated by ADC 420 in response to the electrical signal applied thereto by photodiode 410 enables DSP 430 to fully reconstruct the electric field (e.g., both amplitude and phase) of the corresponding WDM component of WDM signal 152. Example signal processing algorithms that can be used by DSP 430 for this purpose are disclosed, e.g., in the above-cited paper by A. Mecozzi, et al.

(38) Using the digital signals that represent the reconstructed electric field of the corresponding WDM component of WDM signal 152, DSP 430 can be configured to perform dispersion compensation and other signal-equalization processing, in a conventional manner. As already indicated above, the latter feature can advantageously be used, e.g., to extend the maximum transmission distance over which WDM signal 152 can be transmitted in system 100 with an acceptable bit-error rate (BER).

(39) FIG. 5 shows a block diagram of an optical transport system 500 according to an alternative embodiment. System 500 is designed and configured for bidirectional data transport between WDM transceivers 502.sub.W and 502.sub.E. For illustration purposes and to simplify the description, WDM transceivers 502.sub.W and 502.sub.E are referred-to herein as being located at the West and East ends, respectively, of an optical link 550. In an example embodiment, optical link 550 can be implemented using one or more optical fibers (e.g., one per propagation direction) or a fiber-optic cable. Optical link 550 may include other optical elements, such as splitters, combiners, couplers, amplifiers, switches, etc., as known in the pertinent art.

(40) System 500 can be used to transport polarization-division-multiplexed (PDM) signals, wherein each of the two orthogonal polarizations of each WDM channel can be used to carry a different respective data stream. In an example embodiment, system 500 can be designed to operate using direct-detection and electric-field-reconstruction principles that are similar to those of system 100 (FIG. 1). An added capability in system 500 is the electronic polarization demultiplexing that can be applied on a per-channel basis in a conventional manner.

(41) System 500 differs from system 100 (FIG. 1) in that, instead of receiving a corresponding CW component 204.sub.n from the remote transmitter (as in system 100), each individual channel receiver 590.sub.n in system 500 is configured to use the carrier wave generated in the corresponding co-located individual channel transmitter 510.sub.n as the aforementioned CW component to enable direct-detection-based electric-field reconstruction by the receiver's DSP. In other words, a single respective laser can be shared by each individual channel transmitter/receiver pair at each of WDM transceivers 502.sub.W and 502.sub.E.

(42) WDM transceiver 502.sub.W comprises individual-channel transmitters 510.sub.1W-510.sub.NW, each configured to generate a corresponding WDM component of a WDM signal 552.sub.W using a different respective carrier frequency spectrally arranged in accordance with an operative frequency grid. A multiplexer (MUX) 520.sub.W operates to combine the WDM components, thereby generating WDM signal 552.sub.W that is applied to link 550 for transmission to WDM transceiver 502.sub.E. Along the propagation path, WDM signal 552.sub.W can be optionally amplified in one or more optical amplifiers 530.sub.W.

(43) WDM transceiver 502.sub.W further comprises individual-channel receivers 590.sub.1W-590.sub.NW, each configured to detect and decode a corresponding WDM component 582.sub.n of a WDM signal 552.sub.E received by way of link 550 from WDM transceiver 502.sub.E. Along the propagation path, WDM signal 552.sub.E can be optionally amplified in one or more optical amplifiers 530.sub.E. A demultiplexer (DEMUX) 580.sub.W separates the WDM components of WDM signal 552.sub.E using a plurality of pass bands, each configured to pass only one corresponding WDM component 582.sub.n and significantly attenuate or reject the others.

(44) In an example embodiment, each WDM component 582.sub.n may have spectral characteristics that are similar to those of data-modulated signal 202.sub.n (see FIG. 2). Prior to detection in the corresponding individual-channel receiver 590.sub.nW, WDM component 582.sub.n is combined (mixed) with the carrier frequency generated by the laser used in individual-channel transmitter 510.sub.nW. This mixing serves to provide a CW component that acts as CW component 204.sub.n (also see FIG. 2). The resulting combined optical signal has both the respective data-modulated signal and the respective CW component, which enables receiver 590.sub.nW to (i) convert the combined optical signal into a corresponding electrical signal using direct detection and (ii) reconstruct the amplitude and phase of the electric field of the corresponding WDM component 582.sub.n using the signal processing algorithms similar to those used in receiver 190.sub.n (FIG. 1).

(45) WDM transceiver 502.sub.E can be constructed using components similar to those of WDM transceiver 502.sub.W and configured to operate in a similar manner. A description of WDM transceiver 502.sub.E can therefore be obtained from the above description of WDM transceiver 502.sub.W, e.g., by interchanging the subscripts E and W. It should be noted however that the set of carrier frequencies generated in individual-channel transmitters 510.sub.1W-510.sub.NW differs from the set of carrier frequencies generated in individual-channel transmitters 510.sub.1E-510.sub.NE, e.g., as further explained below in reference to FIGS. 6-8. Accordingly, MUX 520.sub.W and MUX 520.sub.E may have different pass-band configurations, and DEMUX 580.sub.W and DEMUX 580.sub.E may similarly have different pass-band configurations.

(46) FIG. 6 shows a block diagram of an optical transceiver 600 that can be used in system 500 (FIG. 5) according to an embodiment. More specifically, optical transceiver 600 can be used to implement any transmitter/receiver pair comprising (i) an individual-channel transmitter 510.sub.n and (ii) an individual-channel receiver 590.sub.n at either of WDM transceivers 502.sub.W and 502.sub.E.

(47) In an example embodiment, optical transceiver 600 comprises a light source 601, a transmitter portion 610, and a receiver portion 690. An optical output 618 of transmitter portion 610 is connected to a corresponding optical input of MUX 520.sub.E or 520.sub.W (FIG. 5). An optical input 682 of receiver portion 690 is connected to a corresponding optical output of DEMUX 580.sub.E or 580.sub.W (FIG. 5).

(48) Light source 601 has three optical outputs, which are labeled 608.sub.1, 608.sub.2, and 608.sub.3, respectively. Optical output 608.sub.1 is connected to transmitter portion 610 to provide light of the corresponding carrier frequency to be modulated therein. Optical outputs 608.sub.2 and 608.sub.3 are connected to receiver portion 690 to provide thereto the corresponding CW component 204.sub.n (also see FIG. 2). The optical connections running from optical outputs 608.sub.1, 608.sub.2, and 608.sub.3 to the downstream circuits can be implemented using polarization-maintaining fibers and/or waveguides, as known in the pertinent art.

(49) In an example embodiment, light source 601 comprises a laser 602, an (optional) optical amplifier 604, and a 13 optical coupler 606. Light source 601 operates to generate a light beam 603 having the carrier frequency. Optical amplifier 604 is configured to boost the optical power of light beam 603 such that coupler 606 can transfer sufficient carrier-frequency power to each of optical outputs 608.sub.1, 608.sub.2, and 608.sub.3. The light beam applied to optical output 608.sub.1 typically has both X- and Y-polarization components to enable transmitter portion 610 to generate a PDM signal at optical output 618. The light beams applied to optical outputs 608.sub.2 and 608.sub.3 can be Y- and X-polarized, respectively, to appropriately support the dual-polarization functionality of receiver portion 690.

(50) Transmitter portion 610 comprises a dual-polarization I/Q modulator 616, four DACs 622, and four RF drivers 626. DACs 622 are configured to receive digital signals 620.sub.1-620.sub.4 that can be generated by the transmitter's DSP (not explicitly shown in FIG. 6) in a conventional manner to cause I/Q modulator 616 to generate the corresponding data-modulated signals 202.sub.n (see FIG. 2) for each of the X- and Y-polarizations of the optical output beam generated at optical output 618.

(51) Receiver portion 690 comprises a polarization beam splitter (PBS) 640, optical couplers 644.sub.1 and 644.sub.2, photodiodes 650.sub.1 and 650.sub.2, ADCs 654.sub.1 and 652.sub.2, and a DSP 660. PBS 640 operates to split an optical input signal received at optical input 682 into X- and Y-polarization components thereof, with the X-polarization component being directed to optical coupler 644.sub.1, and the Y-polarization component being directed to optical coupler 644.sub.2. Optical coupler 644.sub.1 operates to mix the received X-polarization component of the optical input signal with the X-polarized carrier wave received from optical output 608.sub.3. Optical coupler 644.sub.2 similarly operates to mix the received Y-polarization component of the optical input signal with the Y-polarized carrier wave received from optical output 608.sub.2.

(52) In an example embodiment, each of optical couplers 644.sub.1 and 644.sub.2 can have a splitting ratio that deviates from 50:50, such as a 33:67 or 10:90 ratio. The 10:90 asymmetric couplers can be specifically used in some embodiments as couplers 644.sub.1 and 644.sub.2, e.g., to reduce the attenuation of the communication signal while avoiding the complexity of a balanced heterodyne setup typically used for similar purposes in conventional receiver front ends. For example, the communication signal can be applied to the 90% coupler port, e.g., to reduce the corresponding insertion loss for that signal. On the other hand, the relatively high insertion losses corresponding to the 10% coupler port can be easily compensated, e.g., by appropriately setting the optical gain of amplifier 604.

(53) Photodiode 650.sub.1 operates to detect the X-polarized mixed optical signal applied thereto by optical coupler 644.sub.1, and ADC 654.sub.1 then appropriately samples the resulting electrical signal. Photodiode 650.sub.2 similarly operates to detect the Y-polarized mixed optical signal applied thereto by optical coupler 644.sub.2, and ADC 654.sub.2 then appropriately samples the resulting electrical signal. DSP 660 operates to jointly process both digital signals generated in this manner by ADCs 654.sub.1 and 652.sub.2 to first reconstruct the optical field individually for each of the X and Y polarizations, and then performs 22 MIMO equalization to, inter alia, reconstruct the electric fields of the two original PDM signal components generated at the remote transmitter, e.g., as known in the art of digital coherent detection.

(54) FIG. 7 shows a block diagram of an optical transceiver 700 that can be used in system 500 according to an alternative embodiment.

(55) Transceiver 700 differs from transceiver 600 (FIG. 6) in that I/Q modulator 616 of transmitter portion 610 is connected to receive the carrier frequency for modulation therein from the previously unused second output port (labeled 746.sub.2 in FIG. 7) of optical coupler 644.sub.2, which is connected to optical output 608.sub.1 of the modified light source 601. The corresponding modification of light source 601 includes replacing the previously used 13 optical coupler 606 by a 12 optical coupler 706.

(56) When optical coupler 644.sub.2 is a 10:90 asymmetric coupler, output port 746.sub.2 outputs 10% of modulated signal power and 90% of unmodulated laser power. Provided that the receive/transmit inter-channel crosstalk caused by the residual (10%) modulated signal applied to optical output 608.sub.1 in transceiver 700 is acceptable for the particular embodiment of optical transport system 500, the shown transceiver can improve the optical power budget by, in effect, recycling the previously lost portion of the output power of laser 602.

(57) In some embodiments, optical output 608.sub.1 can be further connected, e.g., by way of a polarization beam combiner (not explicitly shown in FIG. 7), to also receive light from the second output port (labeled 746.sub.1 in FIG. 7) of optical coupler 644.sub.1. The latter feature can be used, e.g., to further improve the light-recycling efficiency in transceiver 700.

(58) FIG. 8 graphically shows a WDM-channel configuration that can be used in system 500 according to an embodiment. Only one polarization is illustrated in FIG. 8 for clarity. A person of ordinary skill in the art will understand that the second polarization can be transmitted using the same set of carrier frequencies and the same WDM-channel configuration. The top three frequency strips shown in FIG. 8 illustrate the WDM configuration of transceiver 502.sub.W. The bottom three frequency strips shown in FIG. 8 similarly illustrate the WDM configuration of transceiver 502.sub.E.

(59) WDM signal 552.sub.W generated by transceiver 502.sub.W carries a plurality of data-modulated signals 202.sub.nW, where n is the index that points to channel CHn. The topmost frequency strip in FIG. 8 shows an example of the plurality of data-modulated signals 202.sub.nW corresponding to channels CH1-CH9 of WDM signal 552.sub.W. The laser lines generated by the corresponding plurality of lasers 602 (FIG. 6) that are used for generating the shown data-modulated signals 202.sub.nW are shown in FIG. 8 in the second-from-the-top frequency strip. These laser lines are labeled in FIG. 8 as 204.sub.nW because they are also used, on a per-channel basis, for direct-detecting the incoming data-modulated signals 202.sub.nE carried by WDM signal 552.sub.E, which is generated at transceiver 502.sub.E. The spectral shape of a data-modulated signal 202.sub.nW is typically symmetric with respect to the corresponding laser line 204.sub.nW.

(60) The fourth-from-the-top frequency strip in FIG. 8 shows an example of the plurality of data-modulated signals 202.sub.nE corresponding to channels CH1-CH9 of WDM signal 552.sub.E. The laser lines generated by the corresponding plurality of lasers 602 (FIG. 6) that are used for generating the shown data-modulated signals 202.sub.nE are shown in FIG. 8 in the second-from-the-bottom frequency strip. These laser lines are labeled in FIG. 8 as 204.sub.nE because they are also used, on a per-channel basis, for direct-detecting the incoming data-modulated signals 202.sub.nW carried by WDM signal 552.sub.W. The spectral shape of a data-modulated signal 202.sub.nE is typically symmetric with respect to the corresponding laser line 204.sub.nE.

(61) Note that the plurality of laser lines (laser comb) {204.sub.nW} generated at transceiver 502.sub.W do(es) not line up with the plurality of laser lines (laser comb) {204.sub.nE} generated at transceiver 502.sub.E. More specifically, for channel CHn, the frequency offset between laser line 204.sub.nW and laser line 204.sub.nE is approximately B/2, where B is the bandwidth of the corresponding data-modulated signal. For some n (e.g., n=2, 4, 6, 8 in FIG. 8), laser line 204.sub.nW is located at a lower frequency than laser line 204.sub.nE. For some other n (e.g., n=1, 3, 5, 7, 9 in FIG. 8), laser line 204.sub.nW is located at a higher frequency than laser line 204.sub.nE.

(62) As already indicated above, at each of transceivers 502.sub.W and 502.sub.E, the same laser line, 204.sub.nW or 204.sub.nE, is simultaneously used as (i) the carrier wavelength for generating the outgoing data-modulated signal 202.sub.n and (ii) the CW component 204.sub.n for direct-detecting the incoming data-modulated signal 202.sub.n, with the use being on a per-channel basis. As a result, at the n-th receiver portion (e.g., 690, FIG. 6) of transceiver 502.sub.W, which is configured to direct-detect the WDM component of signal 552.sub.E corresponding to CHn, the optical signal applied to the corresponding photodiode (e.g., 650, FIG. 6) includes: (i) data-modulated signal 202.sub.nE generated by the n-th transmitter portion (e.g., 610, FIG. 6) of transceiver 502.sub.E, and (ii) CW component 204.sub.nW generated by the corresponding local laser (e.g., 602, FIG. 6). The spectra of the optical signals applied to the corresponding photodiodes in the receiver portions of transceiver 502.sub.W are shown in FIG. 8 in the third-from-the-top frequency strip. Similarly, at the n-th receiver portion of transceiver 502.sub.E, which is configured to direct-detect the WDM component of signal 552.sub.W corresponding to CHn, the optical signal applied to the corresponding photodiode includes: (i) data-modulated signal 202.sub.nW generated by the n-th transmitter portion of transceiver 502.sub.W, and (ii) CW component 204.sub.nE generated by the corresponding local laser. The spectra of the optical signals applied to the corresponding photodiodes in the receiver portions of transceiver 502.sub.E are shown in FIG. 8 in bottommost frequency strip.

(63) The following nomenclature is used in FIG. 8 to indicate certain features of the shown WDM configuration(s).

(64) As already mentioned above, B denotes the effective bandwidth of a data-modulated signal 202.sub.n (also see FIG. 2). In an example embodiment, each WDM component of signals 552.sub.E and 552.sub.W transmitted in system 500 can have the same effective bandwidth B.

(65) B.sub.CH denotes the slot width in the used frequency grid.

(66) B.sub.1 and B.sub.2 denote the frequency gaps between the roll-off edges of adjacent data-modulated signals, such as between signals 202.sub.n1 and 202.sub.n and between signals 202.sub.n and 202.sub.n+1, wherein B.sub.1B.sub.2. In an example embodiment, B.sub.1B.sub.2. As a result, the gap width alternates between the smaller value and the larger value, with the frequency gap being narrower at one side of data-modulated signal 202.sub.n and wider at the other side thereof. For some signals 202.sub.n, the wider gap B.sub.2 is located at the lower-frequency side of the signal. For other signals 202.sub.n, the wider gap B.sub.2 is located at the higher-frequency side of the signal.

(67) B.sub.REJ denotes a rejection bandwidth of a pass band of DEMUX 580 (FIG. 5). B.sub.REJ can be defined, e.g., as the spectral width between two 20 dB attenuation points, which are measured with respect to the level of the relatively flat portion of the pass band spectrally located in the middle of the spectral range thereof. In an example embodiment, each pass band of DEMUX 580 may have the same spectral shape, albeit shifted in frequency to be aligned with the corresponding WDM channel. Several examples of such spectral shapes (labeled 810.sub.n) are shown in FIG. 8 in the third-from-the-top frequency strip and in the bottommost frequency strip. In the shown example, the pass-band configurations of DEMUX 580.sub.E and DEMUX 580.sub.W are nominally identical. In an alternative embodiment, the pass-band configurations of DEMUX 580.sub.E and DEMUX 580.sub.W may differ from one another.

(68) In some embodiments, one or more of the following features can be implemented: 1. Rejection bandwidth B.sub.REJ can be significantly larger than slot width B.sub.CH. For example, B.sub.REJ can be close to or on the order of 2B.sub.CH; 2. B.sub.2=B+B.sub.1; 3. B.sub.1=B.sub.CH1.5 B; 4. B.sub.2=B.sub.CH0.5 B (follows from features 2 and 3); 5. Laser-line spacing is alternating between (B.sub.CH0.5 B) and (B.sub.CH+0.5 B); 6. Laser combs {204.sub.nW} and {204.sub.nE} are offset relative to each other by approximately B/2 such that the narrower spacing of one laser comb falls within the wider spacing of the other laser comb; and 7. DMUX 580 is implemented using filters that have a very loose stability with respect to the laser combs (e.g., as graphically shown FIG. 9). An approximate drift range of the pass bands can be about B.sub.2/2.

(69) In an example embodiment, the following parameters can be used: B=32 GHz; B.sub.CH=50 GHz; B.sub.1=2 GHz; B.sub.2=34 GHz; and B.sub.REJ=100 GHz.

(70) FIG. 9 graphically illustrates drift tolerances of optical filters that can be used to implement DMUXes 580 according to an embodiment. As an example, spectral positions of pass band 810.sub.3 of DEMUX 580.sub.W at two different times, t.sub.1 and t.sub.2, are shown on the operative frequency grid (also see FIG. 8). As already indicated above, pass band 810.sub.3 corresponds to channel CH3. As explained in reference to FIG. 8, in channel CH3 of WDM transceiver 502.sub.W, the optical signal applied to the corresponding photodiode 650 comprises data-modulated signal 202.sub.3E and laser line 204.sub.3W, both of which are also shown in FIG. 9.

(71) An estimate of the drift range for pass band 810.sub.3, within which the performance characteristics corresponding to channel CH3 are not unacceptably affected by the drift of the pass band can be obtained, e.g., by determining the frequency range within which both data-modulated signal 202.sub.3E and laser line 204.sub.3W remain within a relatively flat portion 902 of the pass band. As shown in FIG. 9, at time t.sub.1, the spectral position of pass band 810.sub.3 is such that laser line 204.sub.3W is located at the high-frequency end of portion 902. At time t.sub.2, the spectral position of pass band 810.sub.3 is such that the low-frequency roll-off edge of data-modulated signal 202.sub.3E is located at the low-frequency end of portion 902. An estimated difference between these two spectral positions of pass band 810.sub.3 is B.sub.2/2. The latter value provides an estimate for the filter drift tolerance.

(72) FIGS. 10A-10B graphically illustrate a laser-frequency-locking scheme that can be used in system 500 (FIG. 5) according to some embodiments. The scheme(s) illustrated by FIGS. 10A-10B can be used on a per-channel basis.

(73) To implement some embodiments of the WDM configuration described above in reference to FIG. 8, in each channel, the local laser line 204.sub.n may need to be spectrally aligned with the received data-modulated signal 202.sub.n in a fairly accurate and stable manner. For example, at WDM transceiver 502.sub.W, each laser line 204.sub.nW may need to be spectrally aligned with the corresponding received data-modulated signal 202.sub.nE to appropriately place the laser line at the intended roll-off edge of the data-modulated signal. Similarly, at WDM transceiver 502.sub.E, each laser line 204.sub.nE may need to be spectrally aligned with the corresponding received data-modulated signal 202.sub.nW to appropriately place the laser line at the intended roll-off edge of the data-modulated signal.

(74) In an example embodiment, the above-indicated spectral alignment can be achieved by frequency-locking laser combs {204.sub.nW} and {204.sub.nE} to one another, within an acceptable tolerance. In some embodiments, this frequency locking can be imposed on all lines of laser combs {204.sub.nW} and {204.sub.nW} at the same time, e.g., as described in U.S. Pat. No. 7,123,402, which is incorporated herein by reference in its entirety. In some other embodiments, this frequency locking can be implemented, e.g., on a per-channel basis. For example, one side of the channel, e.g., transceiver 600 at WDM transceiver 502.sub.W, can declare itself the master by way of a service channel. The other side, e.g., a corresponding transceiver 600 at WDM transceiver 502.sub.E, then responds by declaring itself a slave to the master and causes the laser line of its laser 602 to follow the laser line of the master's laser 602 at a fixed frequency offset, e.g., +B/2 or B/2.

(75) FIG. 10A graphically shows an example spectrum 1002 of a WDM component of WDM signal 552 that can be used to implement the above-indicated frequency locking at a slave transceiver 600 of WDM transceiver 502 according to an embodiment. Spectrum 1002 comprises data-modulated signal 202.sub.n and an RF pilot tone 1004. RF pilot tone 1004 is generated at the corresponding master transceiver 600 at the remote end of link 550. In an example embodiment, RF pilot tone 1004 can be added to the transmitted signal as known in the pertinent art, e.g., using appropriately generated digital signals 620.sub.1-620.sub.4 (see FIG. 6). The frequency f.sub.p of RF pilot tone 1004 in spectrum 1002 can be selected to be outside the bandwidth B of data-modulated signal 202.sub.n, which is spectrally symmetric with respect to the carrier frequency f.sub.c. The frequency f.sub.p can also be selected to be different from the frequency of local laser line 204.sub.n used for direct-detecting data-modulated signal 202.sub.n at the slave transceiver 600. Depending on the embodiment and/or the channel number (n), the frequency f.sub.p may have a slightly higher frequency than (f.sub.c+B/2) or a slightly lower frequency than (f.sub.cB/2). It may also be beneficial for RF pilot tone 1004 to have a higher spectral-power density than that of data-modulated signal 202.sub.n, as indicated in FIG. 10A.

(76) Slave transceiver 600 can be configured, e.g., to appropriately filter RF pilot tone 1004 to determine its frequency f.sub.p. The local laser 602 can then be configured, as known in the pertinent art, to lock the carrier frequency of its light beam 603 to the frequency f.sub.p at a proper fixed frequency offset. Since frequency f.sub.p has a fixed frequency offset with respect to the carrier frequency f.sub.c of the master laser 602, the carrier frequencies of the two lasers 602 located at the opposite ends of link 550 become locked to one another.

(77) FIG. 10B graphically shows an example spectrum 1012 of a WDM component of WDM signal 552 that can be used to implement frequency locking at a slave transceiver 600 of WDM transceiver 502 according to an alternative embodiment. Spectrum 1012 differs from spectrum 1002 (FIG. 10A) in that it has an RF pilot tone 1014 that is located in-band with respect to data-modulated signal 202.sub.n. A person of ordinary skill in the art will understand how to modify the above-described frequency-locking scheme to be compatible with the in-band location of RF pilot tone 1014.

(78) According to an example embodiment disclosed above in reference to FIGS. 1-10, provided is an apparatus (e.g., 500, FIG. 5), comprising: an optical port (e.g., 608, FIG. 6) connectable to receive a first carrier frequency (e.g., 204.sub.nW, FIG. 8); an optical modulator (e.g., 616, FIG. 6) configured to modulate the first carrier frequency with first data (e.g., 620.sub.1-620.sub.2, FIG. 6) to generate a modulated output signal (e.g., 202.sub.nW, FIG. 8); an optical coupler (e.g., 644, FIG. 6) configured to combine the first carrier frequency with a modulated input signal (e.g., 202.sub.nE, FIG. 8) to generate a combined optical signal, the modulated input signal having a second carrier frequency (e.g., 204.sub.nE, FIG. 8) modulated with second data (e.g., 620.sub.1-620.sub.2 of remote 600, FIG. 6); a photodetector (e.g., 650.sub.1, FIG. 6) configured to convert the combined optical signal into a corresponding electrical signal proportional to an optical power of the combined optical signal; and a signal processor (e.g., 654/660, FIG. 6) configured to process the corresponding electrical signal to recover the second data; and wherein the second carrier frequency is offset with respect to the first carrier frequency by a fixed nonzero frequency offset (e.g., B/2, FIG. 8).

(79) In some embodiments of the above apparatus, the apparatus further comprises a WDM transceiver (e.g., 502, FIG. 5) having a plurality of individual-channel transceivers, wherein the optical port, the optical coupler, and the photodetector are parts of a first individual-channel transceiver (e.g., 600, FIG. 6) of the plurality.

(80) In some embodiments of any of the above apparatus, respective optical ports (e.g., 608, FIG. 6) of the individual-channel transceivers are connectable to receive the different respective first carrier frequencies generated such that a spacing between two adjacent first carrier frequencies alternates between a first value (e.g., B.sub.CH0.5B, FIG. 8) and a different second value (e.g., B.sub.CH+0.5B, FIG. 8).

(81) In some embodiments of any of the above apparatus, the apparatus further comprises a laser (e.g., 602, FIG. 6) connected to the optical port and configured to apply thereto the first carrier frequency.

(82) In some embodiments of any of the above apparatus, the modulated input signal is configured to carry a pilot tone (e.g., 1004, FIG. 10A; 1014, FIG. 10B).

(83) In some embodiments of any of the above apparatus, the laser is configured to lock the first carrier frequency with respect to the pilot tone.

(84) According to another example embodiment disclosed above in reference to FIGS. 1-10, provided is an apparatus (e.g., 500, FIG. 5), comprising: a laser (e.g., 602, FIG. 6) configured to generate a first carrier frequency (e.g., 204.sub.nW, FIG. 8); an optical modulator (e.g., 616, FIG. 6) configured to modulate the first carrier frequency with first data (e.g., 620.sub.1-620.sub.2, FIG. 6) to generate a modulated output signal (e.g., 202.sub.nW, FIG. 8); an optical coupler (e.g., 644, FIG. 6) configured to combine the first carrier frequency with a modulated input signal (e.g., 202.sub.nE, FIG. 8) to generate a combined optical signal, the modulated input signal having a second carrier frequency (e.g., 204.sub.nE, FIG. 8) modulated with second data (e.g., 620.sub.1-620.sub.2 of remote 600, FIG. 6); a photodetector (e.g., 650.sub.1, FIG. 6) configured to convert the combined optical signal into a corresponding electrical signal proportional to an optical power of the combined optical signal; and a signal processor (e.g., 654/660, FIG. 6) configured to process the corresponding electrical signal to recover the second data; and wherein the second carrier frequency is offset with respect to the first carrier frequency by a fixed nonzero frequency offset (e.g., B/2, FIG. 8).

(85) In some embodiments of the above apparatus, the apparatus further comprises a WDM transceiver (e.g., 502, FIG. 5) having a plurality of individual-channel transceivers, wherein the laser, the optical coupler, and the photodetector are parts of a first individual-channel transceiver (e.g., 600, FIG. 6) of the plurality.

(86) In some embodiments of any of the above apparatus, each other individual-channel transceiver of the WDM transceiver comprises a nominal copy of the first individual-channel transceiver, the nominal copy being configured to operate using a different respective first carrier frequency (e.g., selected from laser comb {204.sub.nW}, FIG. 8) and a different respective second carrier frequency (e.g., selected from laser comb {204.sub.nE}, FIG. 8).

(87) In some embodiments of any of the above apparatus, respective lasers (e.g., 602, FIG. 6) of the individual-channel transceivers are configured to generate the different respective first carrier frequencies such that a spacing between two adjacent first carrier frequencies alternates between a first value (e.g., B.sub.CH0.5B, FIG. 8) and a different second value (e.g., B.sub.CH+0.5B, FIG. 8).

(88) In some embodiments of any of the above apparatus, a difference between the first value and the second value is a bandwidth (e.g., B, FIG. 8) of the modulated output signal.

(89) In some embodiments of any of the above apparatus, the plurality of individual-channel transceivers are configured to operate using a frequency grid that has a constant channel spacing (e.g., B.sub.CH, FIG. 8).

(90) In some embodiments of any of the above apparatus, the apparatus further comprises a DEMUX filter (e.g., 580, FIG. 5) having a plurality of pass bands (e.g., 810.sub.n, FIG. 8), each of the pass bands configured to feed light to a respective one of the individual-channel transceivers, wherein a center frequency of at least one of the pass bands is offset with respect to the respective first carrier frequency.

(91) In some embodiments of any of the above apparatus, the apparatus further comprises a DEMUX filter (e.g., 580, FIG. 5) having a plurality of pass bands (e.g., 810.sub.n, FIG. 8), each of the pass bands configured to feed light to a respective one of the individual-channel transceivers, wherein a center frequency of at least one of the pass bands is offset with respect to the respective second carrier frequency.

(92) In some embodiments of any of the above apparatus, each of the plurality of individual-channel transceivers is configured to handle optical signals corresponding to a different respective WDM channel (e.g., CHn, FIG. 8) of the WDM transceiver.

(93) In some embodiments of any of the above apparatus, the apparatus further comprises a DEMUX filter (e.g., 580, FIG. 5) having a plurality of pass bands (e.g., 810.sub.n, FIG. 8), each of the pass bands configured to feed light to a respective one of the individual-channel transceivers, wherein a center frequency of at least one of the pass bands is offset with respect to a center of a frequency slot allocated to a corresponding WDM channel.

(94) In some embodiments of any of the above apparatus, the DMUX filter has an operating stability that allows the center frequency of the pass band to drift over time within a fixed frequency interval (e.g., B.sub.2/2, FIG. 9) that has a spectral width that is greater than 20% of a bandwidth (e.g., B, FIG. 8) of the modulated input signal.

(95) In some embodiments of any of the above apparatus, the fixed frequency offset is approximately (e.g., to within 10%) one half of a data bandwidth of the modulated input signal (e.g., B/2, FIG. 8).

(96) In some embodiments of any of the above apparatus, the modulated input signal is a PDM signal (e.g., 552, FIG. 5).

(97) In some embodiments of any of the above apparatus, the optical modulator is further configured to modulate the first carrier frequency to cause the modulated output signal to carry a pilot tone (e.g., 1004, FIG. 10A; 1014, FIG. 10B).

(98) In some embodiments of any of the above apparatus, the apparatus is configured to lock the second carrier frequency with respect to the pilot tone.

(99) In some embodiments of any of the above apparatus, the modulated input signal is configured to carry a pilot tone (e.g., 1004, FIG. 10A; 1014, FIG. 10B).

(100) In some embodiments of any of the above apparatus, the laser is configured to lock the first carrier frequency with respect to the pilot tone.

(101) In some embodiments of any of the above apparatus, the optical coupler comprises an asymmetric 22 coupler (e.g., 644.sub.2, FIG. 7).

(102) According to yet another example embodiment disclosed above in reference to FIGS. 1-10, provided is an apparatus (e.g., 500, FIG. 5) comprising a first WDM transceiver (e.g., 502.sub.W, FIG. 5) and a second WDM transceiver (e.g., 502.sub.E, FIG. 5) optically coupled to one another; wherein the first WDM transceiver has a first plurality of individual-channel transceivers (e.g., 600, FIG. 6), each of the individual-channel transceivers of the first plurality having allocated thereto a respective frequency slot (e.g., CHn, FIG. 8) on a frequency grid and configured to transmit data using a respective first carrier frequency, the respective first carrier frequencies being arranged to form a first frequency comb (e.g., {204.sub.nW}, FIG. 8); wherein the second WDM transceiver has a second plurality of individual-channel transceivers (e.g., 600, FIG. 6), each of the individual-channel transceivers of the second plurality having allocated thereto a respective frequency slot on the frequency grid and configured to transmit data using a respective second carrier frequency, the respective second carrier frequencies being arranged to form a second frequency comb (e.g., {204.sub.nE}, FIG. 8); and wherein the first frequency comb and the second frequency comb are offset with respect to one another by a fixed nonzero frequency offset (e.g., B/2, FIG. 8).

(103) In some embodiments of the above apparatus, the individual-channel transceivers are configured to generate the respective carrier frequencies such that, in each of the first and second frequency combs, a spacing between two adjacent carrier frequencies alternates between a first value (e.g., B.sub.CH0.5B, FIG. 8) and a different second value (e.g., B.sub.CH+0.5B, FIG. 8).

(104) In some embodiments of any of the above apparatus, the fixed frequency offset is approximately (e.g., to within 10%) one half of a data bandwidth (e.g., B/2, FIG. 8) of a modulated signal corresponding to a WDM channel.

(105) In some embodiments of any of the above apparatus, an individual-channel transceiver of the first plurality and a corresponding individual-channel transceiver of the second plurality are configured to operate in a master-slave arrangement to cause the respective first and second carrier frequencies thereof to be frequency locked to one another.

(106) While this disclosure includes references to illustrative embodiments, this specification is not intended to be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments within the scope of the disclosure, which are apparent to persons skilled in the art to which the disclosure pertains are deemed to lie within the principle and scope of the disclosure, e.g., as expressed in the following claims.

(107) Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word about or approximately preceded the value or range.

(108) It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this disclosure may be made by those skilled in the art without departing from the scope of the disclosure, e.g., as expressed in the following claims.

(109) Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

(110) Reference herein to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the disclosure. The appearances of the phrase in one embodiment in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term implementation.

(111) Also for purposes of this description, the terms couple, coupling, coupled, connect, connecting, or connected refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms directly coupled, directly connected, etc., imply the absence of such additional elements.

(112) The description and drawings merely illustrate the principles of the disclosure. It will thus be appreciated that those of ordinary skill in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass equivalents thereof.

(113) The functions of the various elements shown in the figures, including any functional blocks labeled as processors and/or controllers, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term processor or controller should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.