WO2023147402A2 - Optical equalizer with bidirectional reentry for intersymbol interference mitigation - Google Patents

Abstract

An optical signal equalizer for equalizing one or more optical signals, the equalizer comprises a first optical coupler configured to couple one or more first-side ports to two or more second-side ports, each port-coupling of the first optical coupler being characterized by an optical coupling ratio. The optical signal equalizer also comprises a second optical coupler configured to couple two or more first-side ports to two or more second-side ports, each port-coupling of the second optical coupler being characterized by an optical coupling ratio. The optical signal equalizer also comprises two or more optical signal paths extending from the two or more second-side ports of the first optical coupler to the two or more first-side ports of the second optical coupler, one of the optical signal paths having a propagation delay different from another of the optical signal paths. The optical signal equalizer and one or more bidirectional optical elements connecting at least one of the two or more second-side ports of the second optical coupler to another of the two or more second-side ports of the second optical coupler.

Description

OPTICAL EQUALIZER WITH BIDIRECTIONAL REENTRY FOR INTERSYMBOL INTERFERENCE MITIGATION

CROSS-REFERENCE TO RELATED APPLICATIONS

[1] This application claims the benefit of U.S. Provisional Patent Application No. 63/303,597, filed on January 27, 2022, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

[2] This disclosure relates generally to optical equalization arrangements and, more particularly, to optical equalization such as implementing an optical equalizer with bidirectional reentry for intersymbol interference mitigation.

BACKGROUND

[3] This section introduces aspects that can help facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.

[4] Equalizers in the electrical domain can mitigate various impairments in digital optical communication systems. An equalizer can be implemented using a multistage lattice filter with many adjustable phase parameters, and the parameters can be chosen (e.g., by optimization algorithms) to compensate one wavelength channel at a time. Single impairment optical compensators such as chromatic dispersion (CD) compensators and polarization-mode dispersion (PMD) compensators can provide such functionality to address only one source of impairment at a time.

SUMMARY OF THE INVENTION

[5] Disclosed herein are various embodiments of optical signal equalizers that include equalizer units that allow bidirectional reentry of optical signals. [6] According to an example embodiment disclosed above, provided is an optical signal equalizer for equalizing one or more optical signals, the equalizer comprising: a first optical coupler configured to couple one or more first-side ports to two or more second-side ports, each port-coupling of the first optical coupler being characterized by an optical coupling ratio; a second optical coupler configured to couple two or more first-side ports to two or more second-side ports, each port-coupling of the second optical coupler being characterized by an optical coupling ratio; two or more optical signal paths extending from the two or more second-side ports of the first optical coupler to the two or more first-side ports of the second optical coupler, one of the optical signal paths having a propagation delay different from another of the optical signal paths; and one or more bidirectional optical elements connecting at least one of the two or more second-side ports of the second optical coupler to another of the two or more second-side ports of the second optical coupler.

[7] In some embodiments of the above optical signal equalizer, the optical power coupling ratios of the first optical coupler are equal to the optical power coupling ratios of the second optical coupler.

[8] In some embodiments of the above optical signal equalizer, the first optical coupler is configured to bidirectionally couple the one or more first-side ports to the two or more second- side ports.

[9] In some embodiments of the above optical signal equalizer, the second optical coupler is configured to bidirectionally couple the two or more first-side ports to the two or more second-side ports.

[10] In some embodiments of the above optical signal equalizer, at least one path of the optical signal equalizer comprises a controllable unit for adjusting the relative phase of an optical signal passing through the path.

[11] In some embodiments of the above optical signal equalizer, at least one path of the optical signal equalizer comprises a controllable unit for adjusting a magnitude of an optical signal passing through the path.

[12] In some embodiments of the above optical signal equalizer, at least one path of the optical signal equalizer comprises a controllable unit for adjusting the propagation delay of an optical signal passing through the path. [13] In some embodiments of the above optical signal equalizer, at least one of the one or more bidirectional optical elements comprises a waveguide that operates at a frequency of the one or more optical signals.

[14] In some embodiments of the above optical signal equalizer, at least one of the one or more bidirectional optical elements comprises one or more optical fibers that operate at the frequency of the one or more optical signals.

[15] In some embodiments of the above optical signal equalizer, the optical coupling ratio of the first coupler and the second coupler is less than 0.05.

[16] In some embodiments of the above optical signal equalizer, the optical coupling ratio of the first coupler or the second coupler is tunable.

[17] In some embodiments of the above optical signal equalizer, one of the one or more first-side ports of the first optical coupler provides an input for the optical signal equalizer.

[18] In some embodiments of the above optical signal equalizer, one of the one or more first-side ports of the first optical coupler provides an output for the optical signal equalizer.

[19] In one general aspect, a method for equalizing one or more optical signals includes receiving the one or more optical signals at an optical signal equalizer, the optical signal equalizer including a first optical coupler configured to couple one or more first-side ports to two or more second-side ports, each port-coupling of the first optical coupler being characterized by an optical coupling ratio, a second optical coupler configured to couple two or more first-side ports to two or more second-side ports, each port-coupling of the second optical coupler being characterized by an optical coupling ratio, and two or more optical signal paths extending from the two or more second-side ports of the first optical coupler to the two or more first-side ports of the second optical coupler, one of the optical signal paths having a propagation delay different from another of the optical signal paths. The method also includes causing, by one or more bidirectional optical elements, the received one or more optical signals to reenter the two or more second-side ports of the second optical coupler, wherein the one or more bidirectional optical elements connect at least one of the two or more second-side ports of the second optical coupler to another of the two or more second-side ports of the second optical coupler. [20] In some embodiments of the above method, the optical power coupling ratios of the first optical coupler are equal to the optical power coupling ratios of the second optical coupler.

[21] In some embodiments of the above method, the first optical coupler is configured to bidirectionally couple the one or more first-side ports to the two or more second-side ports.

[22] In some embodiments of the above method, the second optical coupler is configured to bidirectionally couple the two or more first-side ports to the two or more second-side ports.

[23] In some embodiments of the above method, at least one path of the optical signal equalizer comprises a controllable unit for adjusting the relative phase of an optical signal passing through the path.

[24] In some embodiments of the above method, at least one path of the optical signal equalizer comprises a controllable unit for adjusting a magnitude of an optical signal passing through the path.

[25] In some embodiments of the above method, at least one path of the optical signal equalizer comprises a controllable unit for adjusting the propagation delay of an optical signal passing through the path.

[26] In some embodiments of the above method, at least one of the one or more bidirectional optical elements comprises a waveguide that operates at a frequency of the one or more received optical signals.

[27] In some embodiments of the above method, at least one of the one or more bidirectional optical elements comprises one or more optical fibers that operate at the frequency of the one or more received optical signals.

[28] In some embodiments of the above method, the optical coupling ratio of the first coupler and the second coupler is less than 0.05.

[29] In some embodiments of the above method, the optical coupling ratio of the first coupler or the second coupler is tunable.

[30] In some embodiments of the above method, one of the one or more first-side ports of the first optical coupler provides an input for the optical signal equalizer.

[31] In some embodiments of the above method, one of the one or more first-side ports of the first optical coupler provides an output for the optical signal equalizer. [32] Particular embodiments of the subject matter described in this specification can be implemented to realize one or more of the following advantages. A bidirectional element included in an optical signal equalizer allows an equalizer unit (included in the optical signal equalizer) to make double use of its optical path structure by allowing optical signals to propagate forward, and reenter the equalizer unit (after forward propagation) and propagate backward through the same optical path structure, thereby allowing a single optical equalizer path structure to operate as two equalizer units (e.g., as two lattice elements). Allowing use of the equalizer unit in both forward and backward directions, a more compact physical structure can be implemented compared to structures, for example, an all-unidirectional structure that concatenates multiple equalizer units that only forward-propagate light signals. In contrast to a unidirectional reentry element, which perfectly replicates the transfer function of an allunidirectional structure, a bidirectional reentry element generates a different transfer function but can generally be more efficiently designed and produced in integrated photonics, so the element is readily manufacturable in a practical manner, thereby reducing the complexity, cost, etc. while providing a desirable optical signal equalizer transfer function (albeit with different settings of the equalizer adjustment elements).

[33] The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the invention will become apparent from the description, the drawings, and the claims.

[34] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict with patent applications or patent application publications incorporated herein by reference, the present specification, including definitions, will control.

BRIEF DESCRIPTION OF THE DRAWINGS

[35] 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: [36] FIG. 1 shows a block diagram of an optical signal equalizer for equalizing one or more received signals modulated at a preselected modulation bit rate;

[37] FIGs. 2A and 2B show block diagrams of optical signal equalizer arrangements;

[38] FIG. 3 shows an effect of an equalizer on received optical signals;

[39] FIG. 4 shows an optical signal equalizer implemented using a first and second equalizer unit;

[40] FIG. 5A shows an implementation of the two equalizer units of FIG. 4;

[41] FIG. 5B shows the transmissivity through one equalizer for different coupler drive voltages and a fixed phase voltage;

[42] FIG. 6 shows another implementation of an optical signal equalizer with unidirectional reentry;

[43] FIG. 7 is a graphical representation of an optical signal equalizer that employs first and second equalizer units;

[44] FIGs. 8A-8B are amplitude and phase responses of the optical signal equalizer of FIG. 7;

[45] FIG. 9 is a graphical representation of an optical signal equalizer with unidirectional signal reentry;

[46] FIGs. 10A-10B are amplitude and phase responses of the optical signal equalizer of FIG. 9;

[47] FIG. 11 is a graphical representation of an optical signal equalizer with bidirectional signal reentry.

[48] FIGs. 12A-12B are amplitude and phase responses of the optical signal equalizer of FIG. 11;

[49] FIGs. 13A-13B are graphical representations of optical equalizers using bidirectional signal reentry to couplers with more than two first-side and second-side ports.

[50] FIG. 14 shows an illustrative optical transmission system including a transmitter, an optical amplifier, an optical equalizer, and a receiver; and

[51] FIG. 15 is a flow chart of operations of an optical signal equalizer with bidirectional signal reentry. DETAILED DESCRIPTION OF SOME EMBODIMENTS

[52] In a digital binary amplitude shift keying (ASK) system, performance can be measured by the "eye" opening at the decision time. Intersymbol interference (ISI) is the spreading of energy from bit slots into other bit slots, causing eye closure. In a digital binary ASK system whose performance is limited by optical amplifier noise, a significant source of eye closure is due to unwanted signal energy in the "0"s or "spaces" data bit (e.g., 331, 333 in FIG. 3). In some arrangements, a system using an optical amplifier, the beating of energy in the “0”s with spontaneous emission can further degrade performance. If the extinction ratio of the "0"s (e.g., 371, 373 of FIG. 3) can be improved, overall performance can dramatically improve. Similar improvements can be achieved for a reduced amplitude of the mark ("1") levels, where the goal of an equalizer is to increase degraded “l”s. In addition, in multi-level signals such as pulse amplitude modulation signals (e.g., PAM-4, PAM-6, PAM-8 signals, etc.) intersymbol interference can degrade the opening of the multiple eyes and an optical equalizer restores the eyes to be closer to their full openings. In many situations, an optical equalizer can be more effective than an electrical equalizer for many kinds of impairments— in the electrical domain at the receiver it can be too late to address the spaces to avoid the signal-spontaneous beat noise.

[53] An equalizer can address (e.g., clean up) the spaces by taking a controllable portion of the energy at each time instant (e.g., 341 of FIG. 3) and adding it +/- 20 ps away, which is close to the bit period for 40 Gb/s signals (e.g., 342 of FIG. 3), with a controllable phase adjustment. For example, a relatively low cost transmitter can have a poor extinction ratio in the solitary spaces (spaces with adjacent marks). The equalizer can add energy from the surrounding marks to these with 180° phase, fixing the extinction ratio. As another example, first-order chromatic dispersion has a symmetric complex impulse response. The equalizer can generate a symmetric complex impulse response that approximately cancels the dispersion.

[54] FIG. 1 illustrates a block diagram of an optical signal equalizer for equalizing one or more received signals 100 modulated at a preselected modulation bit rate in an optical system. The equalizer includes a coupler 101 with a variable coupling ratio for splitting signal light into two portions, arms 102 and 103 of a controllable interferometer 101-105. The first arm 102 of the controllable interferometer receives the first of the two portions, the second arm 103 receives the second of the two portions. The first arm 102 having an additional path length which creates an additional delay. In some arrangements, that delay is equal to the modulation symbol rate, e.g., an equalizer for a 56-GBaud PAM-4 signal can be designed with a delay of 1/56 GHz = 17.9 ps. In some arrangements, the delay is chosen shorter than the modulation symbol rate, such that adjacent peaks of an equalization magnitude transmission function (e.g., chart 800 of FIG. 8) are separated by a frequency larger than the symbol rate. For example, in some arrangements, an equalizer for 56-GBaud PAM4 may be designed with a delay of 15 ps. In yet another arrangement, that delay is chosen longer than the modulation symbol rate, such that adjacent peaks of the equalization magnitude transmission function are separated by a frequency smaller than the symbol rate. For example, in some embodiments, an equalizer for 56-GBaud PAM4 may be designed with a delay of 20 ps. In yet another arrangement, that delay is equal to an integer multiple of 1/Af, where Af is the channel spacing between adjacent wavelengths that may be utilized by the optical system. Thus, when the optical system is a multi -wavelength system Af is the spacing between channels. When the optical system uses only a single wavelength, e.g., an add/drop multiplexer, where Af is the channel spacing between adjacent wavelengths that may be utilized by that add/drop multiplexer. The first arm 102 has a controllable phase delay 104 for adjusting the relative phase of the light passing through. Note that the controllable phase 104 has a range of at least +/- 180 degrees and could also have been located in the second arm 103, or for symmetry reasons could be located in both arms. A coupler 105 then combines the signal portions from the first and second arms to form an equalized output signal 106. By controlling the relative optical signal magnitude (through 101 and 105) and phase (through 104) of the two signal portions in arms 102 and 103, our equalizer can improve the extinction ratio of the "0"s at the decision point. While the coupler 101 is shown as having a variable coupling ratio and the coupler 105 having a fixed coupling ratio, coupler 105 could also have a variable coupling ratio. Moreover in some arrangements, the coupler 105 can have the variable coupling ratio and the coupler 101 have the fixed coupling ratio. The coupler 101, coupler 105, and/or the phase delay 104 could be preset (e.g., by a manufacturer), made adjustable in the specific application, etc.

[55] FIG. 2A illustrates another implementation of an optical signal equalizer. This arrangement, the variable coupling ratio coupler 101 of FIG. 1 is implemented using a fixed ratio coupler 201. The fixed ratio coupler 201 has two output arms and a controllable transmission unit 201 A is in the first arm (but could similarly be located in second arm) for adjusting the ratio of the magnitudes of the two portions. Thus, fixed ratio coupler 201 and controllable transmission unit 201 A can perform the function of variable coupling ratio coupler 101 of FIG. 1. Typically, the arrangement of FIG. 1 can have a lower loss, and thereby can be preferable.

[56] FIG. 2B presents a block diagram of another implementation of an optical signal equalizer. This this arrangement, an interferometer 200 includes three arms, with the time delay differences between any two arms of the three being equal to an integer multiple of the fundamental equalizer unit time delay, which can in various arrangements be chosen equal to the modulation symbol rate, shorter or longer than the modulation symbol rate, or l/(the channel spacing in frequency).

[57] With reference to FIG. 3, optical signals are presented in the time domain for demonstrating the functionality of the equalizer regarding the extinction ratio of the "0"s at the decision point. As shown by chart 310, a non-return-to-zero (NRZ) amplitude-shift keying (ASK) optical data signal, "010110," is illustrated (e.g., being transmitted from a transmitter location). Impairments such as an imperfect transmitter and/or receiver, filter narrowing, and CD can cause some of the energy from each bit (" 1 " bit) to propagate into its neighbors bit ("0" bit). For this example, the spilled energy is out of phase with the original signal. As shown by chart 320, signal impairments appear as represented by portions 321 and 323. Note that bit 312 is defined as a solitary bit, a bit with both neighbors different from it. A solitary bit can add energy to both of its adjacent "0" bits, as shown by the impairments in portions 321 and 323. The additional energy in "0" bit 313 is the added energy from the "1" bit 314. Without equalization, the detector (a square law detector) would be decoding a signal that has a power as shown in chart that 330, with significant energy in the "0" bits at portions 331 and 333. The equalizer of FIG. 1 (or equalizer 400 of FIG. 4) has an impulse response as shown in chart 340 and generates a controllable amount of energy or compensation signal 342 (using variable ratio coupler 101) and adds it to the signal at locations +20 ps (At bit) away, which is close to the bit period for 40 Gb/s signals, with a controllable phase (controllable delay 104). [58] The controllable magnitude 101 and phase 104 of the equalizer of FIG. 1, can be selected so the energy signal 342 is generated to cancel out the added energy (in portion 323) to the neighboring bits caused by transmission impairments, especially concentrating on minimizing the power in the "0"s at portion 333 (decision point). Chart 350 presents the result of the equalization provided by the equalizer of FIG. 1. As shown by portion 356, the equalizer of FIG. 1 compensates for that portion of the impairment in the "0" bit, 326, located +20 ps (At bit) away from the "1" bit of portion 315. The equalizer of FIG. 1 only compensates for that portion of the impairment in the "0" bit, portion 323, located +20 ps (At bit) away, which was caused by solitary bit 312, but does not compensate for the portion of the impairment in the "0" bit, portion 353, caused by the "1" bit 314. Similarly, the impairment in the "0" bit, portion 331, located -20 ps (At bit) away, which was caused by solitary bit (not shown), is not compensated by the equalizer of FIG. 1. In general, a second equalizer unit 420 is needed to compensate for impairments to a "0" bit position which is -20 ps (At bit) away (i.e., the impairment caused in "0" bit portion 323 by "1" bit 314). Such a second equalizer unit (not shown in FIG. 1) could be connected in series, so that its output connects to the input of equalizer of FIG. 1. The second equalizer unit can be implemented and operate in the same manner as the first equalizer 1 of FIG. 1, except that couplers 101 and 105 are adjusted so as to cause a large contribution of the impulse response (e.g., the largest contribution) to come from the longer interferometer arm. The two concatenated equalizer units may be provided in various implementations, for example the first equalizer unit 400 the second equalizer unit 420 may be implemented as shown in FIG. 4.

[59] With joint reference to FIGS. 3 and 4, as shown in chart 360, the second equalizer unit 420 can take a controllable amount of energy (using variable ratio coupler 423) from bit 362 and adds it to the signal at locations -20 ps (At bit) away as represented in portion 361. The operation of the second equalizer unit 420 operates in essentially the same manner as the equalizer of FIG. 1 (or equalizer 400 of FIG. 4) except that the second equalizer unit affects "0" bit locations -20 ps away rather than +20 ps away. Thus, as shown in chart 370, the second equalizer unit 420 compensates for the "0" bit impairments of portions 351 and 353 to produce the equalized "0" bit shown by portions 371 and 373, respectively. [60] As such, the equalizer can provide an especially dramatic improvement in a reduced bit error rate (BER) in the transmission of data signals (due to the more accurate detection of the "0" bits) that utilize a non-return-to-zero (NRZ) amplitude-shift keying (ASK) format. The equalizer can also improve the performance of carrier-suppressed return-to-zero (CSRZ), and possibly other transmission formats impaired by intersymbol interference. The equalizer can also mitigate many impairments simultaneously, including those due to transmitter and/or receiver imperfections, filter narrowing, CD, and PMD.

[61] Equivalent equalization functionality as described above for the “0” bit of ASK or NRZ signals also applies to “1” bits: By adding energy to a “1” whose amplitude is degraded by intersymbol interference, the equalizer can improve the quality of the “1” level.

[62] The functionality also generalizes to multi-level signals featuring multiple sub-eyes formed by adjacent signal levels, where intersymbol interference decreases the respectively higher level of that sub-eye and increases the respectively lower level of that sub-eye, and the equalizer adjusts those levels towards their nominal positions.

[63] By using a first and second equalizer unit (e.g., 400 and 420 of FIG. 4) the "0" bit impairments of portions 321, 323, and 326 can be reduced (e.g., minimized) as shown by portions 371, 373, and 376, respectively.

[64] In general, the above-described equalization operation of the two equalizers units works for a multi-channel optical system if the channel spacing is equal to N (an integer) times the free spectral range (FSR) (i.e., an integer multiple of 1/ Af, where Af is the channel spacing) and the data bit rate is essentially equivalent for all channels.

[65] FIG. 4 shows an arrangement of the optical signal equalizer implemented using a first, 400, and second, 420, equalizer units. The first equalizer unit 400 includes a first variable ratio coupler 411, a controllable interferometer 412, and a second variable coupling ratio coupler 413. The variable ratio coupler 411 includes a fixed ratio (e.g., 50/50) coupler 402 and a controllable variable phase (0 +/- 90 degrees) element 403. Illustratively, each variable ratio coupler 411 and 413 may be implemented, for example, as a small Mach-Zehnder interferometer (MZI) (402, 403, 404) with a controllable thermo-optic phase shifter in one arm and a quarter- wavelength length increase in the other arm. [66] The coupler 402 (of the variable coupler 411) can be a fixed ratio coupler that receives an optical signal at input 401 (note, input 401 A is unconnected). The variable coupler 411 includes a controllable variable phase element 403 to control the relative phase of the optical signals applied to controllable interferometer 412. The controllable interferometer 412 includes two fixed ratio (e.g., 50/50) couplers 404 and 407 interconnected by two waveguide arms 405 and 405A. In some arrangements, the first arm 405 has an additional delay which is equal to an integer multiple of 1/Af, where Af is the channel spacing of the received optical signals of the multi -wavelength system, and the additional delay is approximately equal to one modulation bit period. The first arm 405 also includes a controllable phase shifter 406 (0-360 degrees) for adjusting phase (e.g., of signal 415 with respect to signal 414). The coupler 407 recombines the optical signals from arms 405 and 405 A. The coupler unit 413 includes a controllable variable phase (0 +/- 90 degrees) element 408 and a fixed ratio (e.g., 50/50) coupler 402. The variable ratio coupler 413 receives the optical signal from waveguide arms

405 and 405A of controllable interferometer 412 and controllable transmission unit 408 adjusts the relative phase of the optical signal into coupler 409. The controllable transmission units 403 and 408 together control magnitude adjustments in equalizer 400. Additionally, the controllable transmission units 403 and 408 can both be located in the same or in the opposite arms of coupler units 411 and 413. The coupler 409 combines the optical signals and the output signal on output 410 is applied to a lower input 421 A of the second equalizer unit 420. At the output 410, a signal 414 representing a main "1" bit signal and a signal 415 representing the "0" bit compensation are provided. The second output 410A of coupler 409 is unconnected. Illustratively, controllable interferometer 412 may be implemented as a Mach- Zehnder interferometer (MZI) (404, 406, 407) with a controllable thermo-optic phase shifter

406 in one arm and also has an additional delay which in some arrangements is equal to an integer multiple of 1/Af.

[67] In this arrangement, the implementation of second equalizer unit 420 is identical to the first equalizer unit 400. The equalizer unit 420 operates in a manner similar to the first equalizer unit 400 and includes components 421, 421 A, 422, 423, 424, 425, 425A, 426, 427, 428, 429, 430, 430A and stages 431, 432, 433 that operate similar to corresponding components of the first equalizer unit 400. However, the inputs 421 and 421 A and outputs 430 and 430 A of second equalizer unit 420 are connected in a different manner than the inputs 401 and 401 A and outputs 410 and 410A of the first equalizer unit 400. In the first equalizer unit 400 the upper input port 401 A and upper output port 410 are used while in the second equalizer unit 420 the lower input port 421 A and lower output port 430 A are used. Thus, in the second equalizer 420 the lower input 421 A connects to the upper output 410 of the first equalizer 400. The output 430A of the second equalizer 420 provides the signal 414 representing the main "1" bit and both the signal 415 representing a leading (+At bit) “0” bit compensation (or satellite) signal and a signal 434 representing a trailing (-At bit) "0" bit compensation (or satellite) signal. The use of the upper inputs/outputs in the first equalizer unit and the lower inputs/outputs in the second equalizer unit generates a linear-phase overall equalizer transfer function, which is desirable for equalizer performance.

[68] Control #1 A controls both the controllable phase delay units 403, 408 of the first equalizer unit 400, and Control #1B controls the controllable phase delay units 423 and 428 of the second equalizer unit 420. A Control #2 sets the controllable phase shifter 406 for adjusting the phase of 415. A Control #3, sets the controllable phase shifter 426 for adjusting the phase of the signal 434. In general, the impulse responses of many realistic impairments are symmetric, so Control #1 A and Control #1B can generally be driven with the same value, leaving a total of three controls for the equalizer. The controllable phase shifters 406 and 426 may each be implemented using a thermo-optic phase shifter. Controllable interferometer 412 and 432 can both be implemented as Mach-Zehnder interferometers (MZIs).

[69] As such, the equalizer of FIG. 4 may be implemented essentially using two identical MZIs in series with tunable couplers, each MZI having a free-spectral range of, e.g., 50 GHz. The controllable interferometers 412 and 432 can also be implemented using fixed couplers (e.g., as shown FIG. 2), but with controllable attenuators in the MZI arms. Using electrical equalizer terminology, the equalizer of FIG. 4 can be considered a two-tap linear equalizer and can be designed to simultaneously compensate multiple 40 Gb/s channels on an integer multiple of 50 GHz grid. The equalizer can dramatically mitigate various intersymbol interference impairments, such as transmitter bandwidth and extinction ratio limitations, filter narrowing, chromatic dispersion, and polarization-mode dispersion. [70] Shown in FIG. 5 A is another arrangement of the two equalizer units of FIG. 4 and FIG. 5B shows the measured transmissivity through one equalizer for different coupler drive voltages and a fixed phase voltage. Referring to FIG. 5A, each equalizer unit, e.g., 400, has two tunable couplers, 403 and 408, and a thermo-optic phase shifter, 406, in one arm. The biasing of the tunable coupler MZI's can significantly reduce polarization-dependent loss (PDL). Both couplers in each equalizer units 400, 420 are typically at the same value to reduce (e.g., minimize) insertion loss, so the controls for couplers 403, 408, 423, 428 are connected together as Control #1. More generally, the couplers of each of the equalizer units 400 and 420 can have separate controls, Controls #1 A and #1B. Such an equalizer would then have four control signals, each controlling approximately one of the following: the amplitude and phase of the left and right compensation (satellite) pulses (see 342, 361 of FIG. 3) of the equalizer impulse response. In general, higher order chromatic dispersion significant enough to distort a channel would require an asymmetric equalizer impulse response (which would require a different control signal voltage Control #1 A for couplers 403, 408 of equalizer 400 and a Control #1B for couplers 423, 428 of equalizer 420). The equalizer units 400, 420 of FIG. 4, are considered absent of significant higher order chromatic dispersion and thus all the couplers connect to a single voltage source (e.g., Control #1). As such, only three control signals Controls #1, #2, and #3 are generally needed to obtain the results presented in FIG. 5B. The measured transmissivity through one of the MZIs for different coupler drive voltages and a fixed phase voltage is shown in FIG. 5B. Ripple 501 presented in the figure can generally not be reduced to zero because of imperfect directional coupling ratios. The fiber-to-fiber insertion loss, including one connector, is 2.0 dB, and the PDL is <0.5 dB for one stage.

[71] FIG. 6 shows another arrangement of an optical equalizer. In this case, an interferometer 600 is used twice to provide the functionality. To be employed twice, a signal first passes through the interferometer 600, then passes through a unidirectional element 601 such as an optical isolator or circulator, and reenters for a second pass through the interferometer 600. The first pass through the interferometer 600 creates a satellite in the impulse response on one side (e.g., a represented by the signal 415 in FIG. 4) and the second pass through the interferometer 600 creates a satellite on the other side (e.g., represented by the signal 434 in FIG. 4). While individual control of magnitudes and phases of the impulse response satellites is a challenge for this arrangement (compared to FIG. 4), only a single interferometer is needed when employing this arrangement.

[72] FIG. 7 shows a graphical representation of an optical signal equalizer 700 that employs two equalizer units (e.g., first and second equalizer units 702, 704 connected in series). Similar to the equalizer units 400, 420 shown in FIG. 4, each of the equalizer units 702, 704 may include couplers and interferometers (MZIs) to compensate for intersymbol interference (e.g., leading interference, lagging interference) of signals received at an input 706 of the first equalizer unit 702. While not represented in the graphical representation of the optical signal equalizer 700, one or multiple controls may be included in the optical signal equalizer 700; for example, a magnitude control, a phase control, etc. may be included (e.g., similar to the magnitude and phase controls shown in FIGs. 1 and 2A). Other designs, system, components, etc. may also be employed to implement the equalizer units 400, 420; for example, each unit be modeled as a finite impulse response (FIR) filter, other type of filter, etc. to determine design parameters, etc. To compensate using the illustrated example, the first equalizer unit 702 provides a leading (+At bit) compensation (or satellite) signal at output 708. The second equalizer unit 704, which is connected in series with the first equalizer unit 702 (e.g., an input 710 is connected to the output 708), provides a trailing (-At bit) compensation (or satellite) signal. Both the leading (+At bit) compensation signal and a trailing (-At bit) compensation signal are provided at an output 712 of the second equalizer unit 704.

[73] In this arrangement, the first equalizer unit 702 includes two tunable couplers 714, 716 and the second equalizer unit 704 also includes two tunable couplers 718, 720. In this design, each tunable coupler (714, 716, 718, 720) can be considered as having a first side and a second side, and each side has two ports. Referring briefly to graphical representation of an exemplary tunable coupler 770, the coupler 770 includes a first side 772 and a second side 774. The first side 772 includes two ports 776 and 778 the second side 774 also includes two ports 780 and 782. First-side ports are bidirectionally optically coupled to second-side ports and the coupling is quantifiable by coupler power splitting ratios. For example, by passing along a “through”-path of the coupler (e.g., a signal enters first-side port 776 of the coupler 770 and propagates to second-side port 780 through the coupler), one power splitting ratio (represented by “a”) can be used to quantify the signal output from the coupler, i.e., an optical power P launched into first-side port 776 results in an optical power aP at the coupler’s second-side port 780. Conversely, due to the bidirectional nature of the coupler, an optical power P launched into second-side port 780 results in an optical power aP at the coupler’s first-side port 776. The same power splitting ratio applies to the other “through”-path of coupler 770, i.e., a signal with optical power P launched into first-side port 778 yields an optical signal with power aP at second-side port 782, and conversely a signal with optical power P launched into second-side port 782 yield an optical signal with power aP at first-side port 778. For signals traversing the “cross”-paths of the coupler 770 (e.g., a signal that enters first-side port 776 of coupler 770 and crosses to second-side port 782 of coupler 770), a companion power spitting ratio (represented by “(1-a)”) can be used to quantify the signal output from the coupler, i.e., coupling an optical power P into first-side port 776 results in an optical power (l-a)P on second-side port 782. Conversely, coupling an optical power P into second-side port 782 results in an optical power (l-a)P on first-side port 776. The same applies to the second “cross”-path, i.e., coupling an optical power P into first-side port 778 results in an optical power (l-a)P on second-side port 780, and conversely, coupling an optical power P into second-side port 780 results in an optical power (l-a)P on first-side port 778. [74] Returning to the graphical representation optical signal equalizer 700, a signal present on the input 706 (of the first equalizer unit 702) is provided to first-side port 730 of the coupler 714. The coupler splits the signal into a signal at second-side port 732 and second-side port 734. The signal of second-side port 732 travels through arm 724 (that has additional delay At) and is received by first-side port 740 of the coupler 716. Coupler 716 again splits the signal between its second-side ports 742 and 744. The signal of second-side port 734 travels through arm 725 and is received by first-side port 746 of the coupler 716. Coupler 716 again splits the signal between its second-side ports 742 and 744. With equalizer unit 702 having its input 706 coupled to port 730 and its output 708 coupled to port 742, equalizer unit output 708 thus receives a signal that is the sum of a signal that travels port 730-> port 732-> path 724 - port 740~> port 742 and a signal that travels port 730~> port 734~> path 725 -> port 746~> port 742. This sum-signal is further passed through equalizer unit 704, whose input port 710 is coupled to output port 708 and whose output port is 712. The signal at the equalizer output port 712 is given by the sum of a signal that travels port 756~> port 752~> path 727 -> port 760- port 764 and a signal that travels port 756~> port 754~> path 726 -> port 766 -> port 764. As in equalizer unit 702, path 727 of equalizer unit 704 has an additional delay of At relative to path 726. Either of paths 724 and/or 725 as well as either of paths 726 and/or 727 may also have phase and/or magnitude control elements as shown in FIGs. 1 and 2A. Traveling these paths (through the equalizer units 702 and 704), signals experience a transfer function H730-742 (impulse response I1730-742) of the first equalizer unit 702 and a transfer function H756-764 (impulse response I1756-764) of the second equalizer 704, where H730-742 denotes the transfer function between the port 730 and the port 742 of the equalizer unit 702 and H756-764 denotes the transfer function between the port 756 and the port 764 of the equalizer unit 704. By concatenating the H730-742 and H756-764 transfer functions, an overall equalizer transfer function H730-742H756-764 is obtained, which features a generally flat phase response of the optical signal equalizer 700.

[75] FIG. 8A shows an amplitude response of the optical signal equalizer 700 and FIG. 8B shows a phase response of the optical signal equalizer 700. The amplitude response is presented in a chart 800 that shows the amplitude response of the optical signal equalizer versus frequency (relative to the input signal carrier frequency). In this example, the coupler power splitting ratio (a) is set for the optical signal equalizer 700 to achieve a 3 dB equalization. In this example, a power splitting ratio (“a”) of 0.08 achieves a 3 dB equalization at a frequency l/(2At) away from the optical carrier. Further, as provided by a chart 802 shown in FIG. 8B, a linear phase response is provided by this design for the optical signal equalizer 700.

[76] FIG. 9 shows a graphical representation of an optical signal equalizer 900 that employs one equalizer unit 902 twice (similar to the operations of the optical equalizer of FIG. 6). To operate in this manner, a unidirectional element 904 (e.g., an optical isolator, circulator, etc.) is connected to the equalizer unit 902 to allow signals to reenter the equalizer 900 for a second pass (e.g., second pass through the interferometer comprising couplers 906 and 908 as well as paths 912 and 914). The equalizer unit 902 includes two couplers 906, 908 that operate similar to the couplers of FIG. 7 (e.g., couplers 714, 716) that include first-side ports and second-side ports for inputting and outputting signals. In this example, an input signal 910 enters first-side port 920 of the coupler 906. The signal at second-side port 932 is the sum of the signal that travels through port 920 - port 922 - path 912— > port 930~> port 932 and the signal that travels through port 920~> port 924 -> path 914— > port 936~> port 932. The unidirectional element 904 directs that sum-signal from second-side port 932 into second-side port 934 of the coupler 908 (to re-enter the equalizer unit 902 in backward propagation direction) to generate a signal at output port 916 that is the sum of a signal that travels through port 934~> port 930~> path 912— > port 922 - port 926 and a signal that travel through port 934~> port 936~> path 914— > port 924 -> port 926. Traveling these two passes (i.e., the first pass in forward direction and the second passes in backward direction through the equalizer unit 902), signals experience the transfer function H920-932 (upon forward propagation) that denotes the transfer function between the port 920 and the port 932 of the equalizer unit 902 and the transfer function H934-926 (upon backward propagation) that denotes the transfer function between the port 934 and the port 926. This achieves an overall equalizer transfer function H920-932H934-926, which is equivalent to the transfer function of the equalizer of FIG. 7. Compared to the two-stage design of optical signal equalizer 700 (i.e., having equalizer unit 702 and equalizer unit 704), identical transfer functions (impulse responses) are therefore applied to input signals using the same settings for couplers. As such, the optical signal equalizer 900 similarly employs the H920-932 and H934-926 transfer functions and a generally flat phase response is achieved by the optical signal equalizer 900.

[77] Since identical transfer functions are present in the optical signal equalizer 900 compared to the optical signal equalizer 700, the amplitude and phase responses of the two optical signal equalizers are identical. FIG. 10A shows the amplitude response of the optical signal equalizer 900 and FIG. 10B shows the phase response of the optical signal equalizer. The amplitude response is presented in a chart 1000 that shows the amplitude response of the optical signal equalizer versus frequency (relative to the input signal carrier frequency). Equivalent to the amplitude and phase responses 9A, 9B, the same coupler power splitting ratio (“a”) is set for the optical signal equalizer 900 to achieve a 3 dB equalization. In particular, a power splitting ratio (a) of 0.08 achieves a 3 dB equalization at a frequency l/(2At) away from the optical carrier. Turning to the phase response, a linear phase response is also provided for the optical signal equalizer 900 as presented in chart 1002 show in FIG. 10B. [78] The unidirectional element 904 allows signals to reenter the optical signal equalizer and utilize a single equalizer unit (i.e., equalizer unit 902). However, the unidirectional elements (e.g., optical isolators, circulators, etc.) can be challenging to produce (e.g., manufacture), especially in an integrated-optics context. To improve the design, one or more techniques may be implemented that allow signals to reenter into a single equalizer unit that utilize one or more components that can be less challenging to produce (e.g., readily manufactured). For example, a bidirectional component may be utilized for re-entering signals into an equalizer unit that provides amplitude and phase responses similar to the optical signal equalizers 700 and 900 while the bidirectional component is more readably producible. Various types of bidirectional components can be employed to attain these responses; for example, the bidirectional component may include one or more optical waveguides. In one arrangement, a waveguide can be employed that includes one or more integrated-optical waveguides on a photonic integrated circuit (PIC) built in Silicon Photonics, Indium Phosphide, glass, or polymer material systems. In another arrangement, a waveguide can be employed that includes one or more optical fibers that allow bidirectional signal propagation for re-entry (e.g., into an equalizer unit). Various types of optical fibers may be employed (e.g., fibers with one or multiple refractive indices, coatings, fiber materials, frequency responses, etc.) along with various fiber arrangements (e.g., fiber bundling techniques, connectors, etc.), various fiber parameters (e.g., physical size, geometry, etc.), etc. In some arrangements, one or more mirrors may also be utilized individually, in concert with one or more optical fibers, etc. For example, multiple mirror elements (e.g., double mirrors) may be used for reentering optical signals into an equalizer unit.

[79] FIG. 11 shows a graphical representation of an optical signal equalizer 1100 that twice employs the functionality one equalizer unit 1102 (similar to the operations of the optical equalizer of FIG. 9). A bidirectional element 1104 (e.g., one or more waveguides, etc.) is connected to the equalizer unit 1102 to allow signals to reenter the optical signal equalizer 1100 for a second pass (e.g., second pass through an interferometer in backward direction, etc.). The equalizer unit 1102 includes two couplers 1106, 1108 that operate similar to the couplers of FIG. 7 (e.g., couplers 714, 716) and include first-side and second-side ports that can operate as input and output ports. In this example, an input signal 1110 enters first-side port 1120 of the coupler 1106, where it is split into two signals. Second-side port 1132 of coupler 1108 yields the sum of a signal that travels through port 1120— > port 1122— > path 1 1 12— > port 1130~> port 1132 and a signal that travels through port 1 120— > port 1 124— > path 1114-> port 1136- port 1132 to produce transfer function H1120-1132. At the same time, second-side port 1134 of coupler 1108 yields the sum of a signal that travel through port 1 120— > port 1122- path 1 1 12— > path 1130- path 1134 and a signal that travels through port 1120— > port 1124 - path 11 14~> port 1136- port 1134 to produce transfer function H1120-1134. Both of these signals are then fed back to propagate through the structure in reverse. Bidirectional feedback signals in output 1116 experience an overall transfer function H1120- 1132H1134-1126+H1120-1134H1132-1126, which is different transfer function compared to the transfer functions of the equalizers of FIGs. 7 and 9.

[80] Since the bidirectional element 1104 allows bidirectional signal propagation (as graphically represented by arrows on the bidirectional element 1104), optical signals can reenter the equalizer unit 1102 from multiple points of entry. In the illustrated example, signals can depart second-side port 1132 of the coupler 1108 and reenter at second-side port 1134 of the coupler 1108. In a similar manner, signals can depart second-side port 1134 of the coupler 1108 and reenter at second-side port 1132 of the coupler 1108. Based upon the bidirectional capability, another path can be traveled from the input 1110 of the optical signal equalizer 1100 to the output of the equalizer unit 1102. A signal entering input 1110 is provided to first- side port 1120 of coupler 1106 and can couple across the coupler 1106 and travel along paths 1112 and 1114 and be output at second-side port 1132 of the coupler 1108. The signal then travels through the bidirectional element 1104 and reenters the equalizer unit 1102 at second- side port 1134 of the coupler 1008. The signal then couples across the coupler 1108, travels paths 1112 and 1114, and is provided by first-side port 1126 of coupler 1106 to the output 1116 of the optical signal equalizer 1100. So by allowing reentry from a second direction, input signals can also experience transfer function H1120-1132 (from first-side port 1120 of coupler 1106 to second-side port 1132 of coupler 1108) and the transfer function H1132-1126 (from second-side port 1132 of the coupler 1108 to first-side port 1126 of the coupler 1106). Based upon these additional paths and corresponding transfer functions, the complete transmission spectrum experienced by an input signal would be: H1120-1132 * H1134-1126 + H1120- 1134 * H1132-1126.

[81] FIG. 12A shows the amplitude response of the optical signal equalizer 1100 and FIG. 12B shows the phase response of the optical signal equalizer 1100. The amplitude response is shown in a chart 1200 that presents the amplitude response of the optical signal equalizer versus frequency (relative to the input signal carrier frequency). Due to the bidirectional signal reentry capability provided by the bidirectional element 1104 of the optical signal equalizer 1100, a different coupler power splitting ratio (compared to optical signal equalizers 700 and 900) achieves a 3 dB equalization at a frequency l/(2At) away from the optical carrier. For optical signal equalizer 1100, a coupler power splitting ratio (a) of 0.038 achieves a 3 dB equalization. Referring to chart 1202 of FIG. 12B, a linear phase response is also provided for the optical signal equalizer 1100.

[82] FIGs. 13 A and B generalize the above description of an optical equalizer using bidirectional signal reentry to couplers with more than two first-side and second-side ports. In some arrangements, the equalizer comprises: (i) two optical couplers, the first optical coupler having at least one first-side port and N>1 second-side ports and the second optical coupler having N first-side ports and N second-side ports; (ii) N optical signal paths, wherein at least one path has a propagation delay different from at least one other propagation path; and (iii) at least one bidirectional element connecting at least two second-side ports of the second coupler. Referring to FIG. 13A, one of the first-side ports 1304 of coupler 1302 acts as a signal input to an optical equalizer 1300 and a second first-side port 1306 of coupler 1302 acts as a signal output of the optical equalizer 1300. Paths of the optical equalizer 1300 can include a propagation delay 1308; for example, at least one path includes a propagation delay relative to at least one other path, and the other paths may optionally include a delay (as graphically represented with dashed line boxes). Similarly, each path can optionally include a phase controller 1310 and/or a magnitude controller 1312 (as graphically represented with dashed line boxes). Each signal path connects to a first-side port of a second coupler 1314 that is connected (via a second-side port) to a bidirectional element 1316, and each bidirectional element connects to a pair of the second-side ports of the second coupler 1314. In FIG. 13B, a first-side port 1354 of coupler 1352 acts as a signal input to and as a signal output 1356 of an optical equalizer 1350, whereby input and output signals are separated by a directionally splitting element 1358 such as a circulator or a directional coupler. Similar to the optical equalizer 1350, at least one propagation delay 1360 is present in the paths connected to the second-side ports of the coupler 1352. Each path may also include a phase controller 1310 and/or a magnitude controller 1364 prior to being connected to a first-side port of the coupler 1366. Similar to the coupler 1314, a bidirectional element (e.g., a wave guide) connects pairs of second-side ports of the second coupler 1366.

[83] The optical signal equalizers (e.g., equalizer 1100) can be employed in various types of applications. For one example, FIG. 14 shows a block diagram that illustrates an optical transmission system 1400 that includes an optical transmitter 1402, an optical facility 1404, a semiconductor optical amplifier (SOA) 1406 (which can also be any other suitable optical amplifier, or no optical amplifier may be used), an optical signal equalizer 1408 (e.g., optical signal equalizer 1100), an optical facility 1412, and an optical receiver 1414. In one arrangement the optical facility 1404 and/or the optical facility 1412 includes one or more optical cables or other types of optical components for transmission of optical signals.

Various type of design adjustments can be realized for the optical transmission system 1400. For example, additional systems can be included in the optical transmission system 1400. In one arrangement, an additional optical amplifier 1410 can be positioned between the optical signal equalizer 1408 and the optical facility 1412. The amplifier 1406 and optical equalizer 1408 (and the second amplifier 1410) can be integrated into a common system element and located at an intermediate node of the system. In another example, the amplifier 1406 and the optical signal equalizer 1408 can be co-located with the optical transmitter 1402 at a transmitter node, where the optical transmitter 1402, the amplifier 1406, and the optical signal equalizer 1408 are integrated into a common system element. For example, the optical transmitter 1402 can include an optical modulator, and the optical signal equalizer 1408 can be positioned downstream of the optical modulator and used to equalize the modulated optical signals from the optical modulator. In some arrangements, component positions can be adjusted within the optical transmission system 1400. For example, the position of the amplifier 1406 and the optical signal equalizer 1408 can be reversed. In another arrangement, the amplifier 1406 and the optical signal equalizer 1408 can be co-located with the optical receiver 1414 at a receiver node, in which the amplifier 1406, the optical signal equalizer 1408, and the optical receiver 1414 can be integrated into a common component together. Typically, at the receiver node the amplifier 1406 may be located in front of the optical signal equalizer 1408, however, other arrangements (e.g., reversing the position of the amplifier 1406 and the optical signal equalizer 1408) can be implemented. Various design arrangements, system components, operating parameters (e.g., for system components), etc. are also described in U.S. Patent 6,804,434, the content of which is incorporated by reference in its entirety.

[84] Similar to the optical signal equalizers 700 (of FIG. 7) and 900 (of FIG. 9), the optical signal equalizer 1100 can reduce the overshoots on the ' 1' bits on the rising edges, i.e., the transitions from 'O' to ' 1', or more generally equalizes the intersymbol interference in a multilevel optical modulation system. Such overshoots can lead to an increase in the required average power of a data-stream to close a link, resulting in a power penalty. By employing the optical signal equalizer 1100, optical signal degradations characterized by these overshoots and signal transitions can be reduced along with reducing inter-symbol interference (ISI) in a receiver caused by rising edges of overshoots. As such, the optical signal equalizer 1100 reduces overshoots and the penalty caused by electrical ISI in the receiver.

[85] FIG. 15 shows a flow chart 1500 of operations of an optical signal equalizer with bidirectional signal reentry. At step 1502, one or more optical signals are received at the optical signal equalizer. The optical signal equalizer includes a first optical coupler configured to couple one or more first-side ports to two or more second-side ports. Each port-coupling of the first optical coupler is characterized by an optical coupling ratio. The optical signal equalizer also includes a second optical coupler configured to couple two or more first-side ports to two or more second-side ports. Each port-coupling of the second optical coupler is characterized by an optical coupling ratio. The optical signal equalizer also includes two or more optical signal paths extending from the two or more second-side ports of the first optical coupler to the two or more first-side ports of the second optical coupler. One of the optical signal paths has a propagation delay different from another of the optical signal paths. For example, portions of or all of step 1502 may be executed by an optical signal equalizer such as the optical signal equalizer 1100 graphically represented in FIG. 11 or one or more other implementations.

[86] At step 1504, one or more bidirectional optical elements cause the received one or more optical signals to reenter the two or more second-side ports of the second optical coupler. The one or more bidirectional optical elements connect at least one of the two or more second-side ports of the second optical coupler to another of the two or more second- side ports of the second optical coupler. For example, portions of or all of step 1504 may be executed by an optical signal equalizer such as the optical signal equalizer 1100 graphically represented in FIG. 11 or one or more implementations.

[87] 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.

[88] Some embodiments can be implemented as circuit-based processes, including possible implementation on a single integrated circuit.

[89] 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.

[90] 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 can be made by those skilled in the art without departing from the scope of the disclosure, e.g., as expressed in the following claims.

[91] The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures.

[92] 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.

[93] Reference herein to “one embodiment” or “an embodiment” or “one arrangement” or “an arrangement” 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” on “in one arrangement” in various places in the specification are not necessarily all referring to the same embodiment or arrangement, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.”

[94] Unless otherwise specified herein, the use of the ordinal adjectives “first,” “second,” “third,” etc., to refer to an object of a plurality of like objects merely indicates that different instances of such like objects are being referred to, and is not intended to imply that the like objects so referred-to have to be in a corresponding order or sequence, either temporally, spatially, in ranking, or in any other manner.

[95] 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.

[96] As used herein in reference to an element and a standard, the term compatible means that the element communicates with other elements in a manner wholly or partially specified by the standard, and would be recognized by other elements as sufficiently capable of communicating with the other elements in the manner specified by the standard. The compatible element does not need to operate internally in a manner specified by the standard.

[97] The described embodiments are to be considered in all respects as only illustrative and not restrictive. In particular, the scope of the disclosure is indicated by the appended claims rather than by the description and figures herein. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. [98] 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.

[99] The functions of the various elements shown in the figures, including any functional blocks labeled or referred to as “processors” and/or “controllers,” can 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 can be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which can 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 can 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, can also be included. Similarly, any switches shown in the figures are conceptual only. Their function can 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.

[100] As used in this application, the term “circuitry” can refer to one or more or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry); (b) combinations of hardware circuits and software, such as (as applicable): (i) a combination of analog and/or digital hardware circuit(s) with software/firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions); and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.” This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

[101] Some details of the systems, apparatuses, or modules of the various embodiments described in this document are described in U.S. Patent 6,804,434, filed on December 12, 2003 as U.S. patent application 10/735,176 (and issued on October 12, 2004), and, U.S. Patent 6,785,446, filed on March 20, 2003 as U.S. patent application 10/393,483 (and issued on August 31, 2004). The contents of the above patents and patent applications are incorporated herein by reference in their entirety.

[102] It should be appreciated by those of ordinary skill in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure.

Claims

CLAIMS What is claimed is:

1. An optical signal equalizer for equalizing one or more optical signals, the equalizer comprising: a first optical coupler configured to couple one or more first-side ports to two or more second-side ports, each port-coupling of the first optical coupler being characterized by an optical coupling ratio; a second optical coupler configured to couple two or more first-side ports to two or more second-side ports, each port-coupling of the second optical coupler being characterized by an optical coupling ratio; two or more optical signal paths extending from the two or more second-side ports of the first optical coupler to the two or more first-side ports of the second optical coupler, one of the optical signal paths having a propagation delay different from another of the optical signal paths; and one or more bidirectional optical elements connecting at least one of the two or more second-side ports of the second optical coupler to another of the two or more second-side ports of the second optical coupler.

2. The optical signal equalizer of claim 1, wherein the optical power coupling ratios of the first optical coupler are equal to the optical power coupling ratios of the second optical coupler.

3. The optical signal equalizer of claim 1, wherein the first optical coupler is configured to bidirectionally couple the one or more first-side ports to the two or more second-side ports.

4. The optical signal equalizer of claim 1, wherein the second optical coupler is configured to bidirectionally couple the two or more first-side ports to the two or more second-side ports.

5. The optical signal equalizer of claim 1, wherein at least one path of the optical signal equalizer comprises a controllable unit for adjusting the relative phase of an optical signal passing through the path.

6. The optical signal equalizer of claim 1, wherein at least one path of the optical signal equalizer comprises a controllable unit for adjusting a magnitude of an optical signal passing through the path.

7. The optical signal equalizer of claim 1, wherein at least one path of the optical signal equalizer comprises a controllable unit for adjusting the propagation delay of an optical signal passing through the path.

8. The optical signal equalizer of claim 1, wherein at least one of the one or more bidirectional optical elements comprises a waveguide that operates at a frequency of the one or more optical signals.

9. The optical signal equalizer of claim 1, wherein at least one of the one or more bidirectional optical elements comprises one or more optical fibers that operate at the frequency of the one or more optical signals.

10. The optical signal equalizer of claim 1, wherein the optical coupling ratio of the first coupler and the second coupler is less than 0.05.

11. The optical signal equalizer of claim 1, wherein the optical coupling ratio of the first coupler or the second coupler is tunable.

12. The optical signal equalizer of claim 1, wherein one of the one or more first-side ports of the first optical coupler provides an input for the optical signal equalizer.

13. The optical signal equalizer of claim 1, wherein one of the one or more first-side ports of the first optical coupler provides an output for the optical signal equalizer.

14. A method for equalizing one or more optical signals, the method comprising: receiving the one or more optical signals at an optical signal equalizer, the optical signal equalizer including a first optical coupler configured to couple one or more first-side ports to two or more second-side ports, each port-coupling of the first optical coupler being characterized by an optical coupling ratio, a second optical coupler configured to couple two or more first-side ports to two or more second-side ports, each port-coupling of the second optical coupler being characterized by an optical coupling ratio, and two or more optical signal paths extending from the two or more second-side ports of the first optical coupler to the two or more first-side ports of the second optical coupler, one of the optical signal paths having a propagation delay different from another of the optical signal paths; and causing, by one or more bidirectional optical elements, the received one or more optical signals to reenter the two or more second-side ports of the second optical coupler, wherein the one or more bidirectional optical elements connect at least one of the two or more second-side ports of the second optical coupler to another of the two or more second-side ports of the second optical coupler.

15. The method of claim 14, wherein the optical power coupling ratios of the first optical coupler are equal to the optical power coupling ratios of the second optical coupler.

16. The method of claim 14, wherein the first optical coupler is configured to bidirectionally couple the one or more first-side ports to the two or more second-side ports.

17. The method of claim 14, wherein the second optical coupler is configured to bidirectionally couple the two or more first-side ports to the two or more second-side ports.

18. The method of claim 14, wherein at least one path of the optical signal equalizer comprises a controllable unit for adjusting the relative phase of an optical signal passing through the path.

19. The method of claim 14, wherein at least one path of the optical signal equalizer comprises a controllable unit for adjusting a magnitude of an optical signal passing through the path.

20. The method of claim 14, wherein at least one path of the optical signal equalizer comprises a controllable unit for adjusting the propagation delay of an optical signal passing through the path.

21. The method of claim 14, wherein at least one of the one or more bidirectional optical elements comprises a waveguide that operates at a frequency of the one or more received optical signals.

22. The method of claim 14, wherein at least one of the one or more bidirectional optical elements comprises one or more optical fibers that operate at the frequency of the one or more received optical signals.

23. The method of claim 14, wherein the optical coupling ratio of the first coupler and the second coupler is less than 0.05.

24. The method of claim 14, the optical coupling ratio of the first coupler or the second coupler is tunable.

25. The method of claim 14, wherein one of the one or more first-side ports of the first optical coupler provides an input for the optical signal equalizer.

26. The method of claim 14, wherein one of the one or more first-side ports of the first optical coupler provides an output for the optical signal equalizer.