Abstract:
The present disclosure includes systems and techniques relating to reconfigurable optical transmitters. In some implementations, an apparatus, systems, or methods can include multiple ports to receive independent optical data signals or independent electrical signals that are converted into independent optical data signals, at least one optical pump laser, and one or more nonlinear optics elements configured and arranged to generate a phase conjugate for each of the independent optical data signals at least by combining the respective independent optical data signal with an output of the optical pump, and generate an output optical signal from the independent optical data signals at least by combining each of the independent optical data signals with its corresponding generated phase conjugate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the priority of U.S. Provisional Application Ser. No. 61/643,191, filed May 4, 2012 and entitled “RECONFIGURABLE OPTICAL TRANSMITTER,” the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure describes systems and techniques relating to reconfigurable optical transmitters. 
     Dramatic growth in data capacity demand in optical networks has necessitated a simultaneous increase in the data speeds of terminal transmitters. For example, state-of-the-art transceivers are capable of providing 100-Gbit/s data rates or more per channel, and these channels can employ the use of spectrally efficient higher-level modulation amplitude/phase formats as well as coherent technologies. 
     SUMMARY 
     The present disclosure includes systems and techniques relating to reconfigurable optical transmitters. According to an aspect of the described systems and techniques, a method includes receiving independent optical data signals from different sources, generating a phase conjugate for each of the independent optical data signals, and generating an output optical signal from the independent optical data signals at least by combining each of the independent optical data signals with its corresponding generated phase conjugate to remove phase noise. 
     In some implementations, generating the phase conjugate of each of the independent optical data signals can include combining the respective independent optical data signal with a pump output. In some implementations, the method can include introducing a delay between the independent optical data signal and its phase conjugate prior to combining them together. In some implementations, the independent optical data signals can be received from independent electrical to optical devices, and the independent optical data signals can be lower order data signals than the generated output optical signal. 
     According to another aspect of the described systems and techniques, a reconfigurable optical transmitter includes multiple ports to receive independent optical data signals or independent electrical signals that are converted into independent optical data signals, at least one optical pump laser, and one or more nonlinear optics elements configured and arranged to generate a phase conjugate for each of the independent optical data signals at least by combining the respective independent optical data signal with an output of the optical pump, and generate an output optical signal from the independent optical data signals at least by combining each of the independent optical data signals with its corresponding generated phase conjugate to remove phase noise. 
     In some implementations, the one or more nonlinear optical elements can include two periodically-poled-lithium-niobate waveguides. In some implementations, the reconfigurable optical transmitter can also include a chromatic dispersion element. The chromatic dispersion element can include a dispersion compensating fiber. In some implementations, the reconfigurable optical transmitter can also include a beam-shaping filter. The beam-shaping filter can include a liquid-crystal-on-silicon filter. 
     According to another aspect of the described systems and techniques, a method includes receiving an optical signal from a coherent optical frequency comb, where the optical signal includes multiple elements, generating modulated signals by modulating at least a set of elements of the multiple elements, and generating an output optical signal at least in part by phase coherent addition of the modulated signals. 
     In some implementations, the phase coherent addition of the modulated signals can include combining a pump output with the modulated signals and another set of elements of the multiple elements. In some implementations, the phase coherent addition can be performed by a single periodically-poled-lithium-niobate waveguide. In some implementations, the set of elements can be modulated using a quadrature-phase-shift-keyed format. 
     According to another aspect of the described systems and techniques, a tunable optical transmitter includes one or more ports to receive a coherent optical frequency comb including multiple elements, a modulator configured to modulate a set of elements of the multiple elements, and a nonlinear optics element configured to generate an output optical signal at least in part by phase coherent addition of the modulated set of elements. 
     In some implementations, the tunable optical transmitter can also include an optical pump laser, and the nonlinear optics element can be configured to generate the output signal, at least in part, by combining an output of the pump laser with respective elements of the modulated set of elements and another set of elements of the multiple elements. In some implementations, the modulator can be configured to modulate the set of elements with a quadrature-phase-shift-keyed format. In some implementations, the nonlinear optics element can include a single periodically-poled-lithium-niobate waveguide. In some implementations, the phase coherent addition of the modulated set of elements can be performed through two cascaded second order nonlinear wave mixings. In some implementations, the optical output signal can be a higher-order quadrature-amplitude-modulation signal. In some implementations, the tunable optical transmitter can also include a liquid crystal on silicon (LCoS) filter configured to select the set of elements of the multiple elements. The LCoS filter can be configured to select the set of elements, at least in part, by writing complex weights on the multiple elements and separating the multiple elements into the set of elements and another set of elements. 
     Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. The described methods and techniques can be implemented to generate high order (e.g., 16 QAM, 64-QAM and higher order) modulation formats by using optical approaches to provide higher baud rates and constellation sizes. 
     The described methods and techniques can also be implemented to create a flexible reconfigurable optical transmitter whose capacity can be tailored and shared among different channels since the traffic demands vary in a dynamic heterogeneous network. Flexible, reconfigurable bandwidth allocation can enable optimal use of terminal equipment as well as create new opportunities for scalable and cost-effective network architectures. 
     Details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages may be apparent from the description and drawings, and from the claims. 
    
    
     
       DRAWING DESCRIPTIONS 
         FIG. 1A  shows a diagram of an example of optical multiplexin of M OOK channels into a single M-Level Phase/Amplitude channel. 
         FIG. 1B  shows a diagram of an example of optical multiplexing of four OOK inputs into a 16-QAM and a star 16-QAM signal. 
         FIG. 2  shows an exemplary setup for generating a coherently multiplexed signal. 
         FIGS. 3A-3F  show examples of two incoming OOK signals multiplexed into a single 4-PAM channel. 
         FIGS. 4A-4D  show an example of optical multiplexing of four OOK inputs into a 16 QAM signal with a DC offset and optical multiplexing of 4 OOK channels into a star 16 QAM signal. 
         FIG. 5  shows a block diagram of an example of a flexible capacity QAM transmitter. 
         FIG. 6  shows an example of a setup for a flexible transmitter. 
         FIG. 7A  shows an example of an optical spectrum of a first stage in which phase conjugate copies of original QPSK signals are generated at corresponding wavelengths. 
         FIG. 7B  shows an example of an optical spectrum of an output of a second PPLN as well as a final I-Q constellation. 
         FIG. 7C  shows an example of a 64-QAM with EVM ˜7%. 
         FIGS. 8A-8B  show an example of two QPSK multiplexed to generate a 16-QAM. 
         FIGS. 8C-8D  show an example of a reconfigured transmitter to generate 3 QPSK signals. 
         FIG. 9  shows a diagram of an example of BER (bit error rate). 
         FIG. 10  shows a block diagram of an example of a tunable optical QAM transmitter. 
         FIG. 11  shows an example of a setup for the tunable QAM encoder. 
         FIGS. 12A-12B  show examples for generating QAM and pulse-amplitude-modulation Signals. 
         FIG. 12C  shows another diagram of an example of BER. 
     
    
    
     DETAILED DESCRIPTION 
     Part I: Optical Tunable Phase-Coherent Multiplexing of Four 20-Gbaud OOK Signals into a Single 80-Gbit/s 16-QAM and Star 16-QAM Signal 
     Part I of this document describes systems and techniques for optical tunable phase-coherent multiplexing, for example, of four 20-Gbit/s on-off keying (OOK) channels at different wavelengths into a single 80-Gbit/s 16-QAM channel and also a single star 16-QAM channel using periodically-polled lithium niobate (PPLN) devices. 
     Higher-order modulation formats can provide high spectral efficiency and high tolerance to fiber-based dispersion and nonlinear effects. For example, 16 quadrature-amplitude-modulation (QAM) and star 16-QAM are generally of interest, and data constellations of about 512-QAM have been reported (e.g., S. Okamoto, et al, ECOC 2010, paper PD2.3). 
     Conventionally, 16 QAM can be generated by n-phase/quadrature (IQ) modulators using either: (a) 2 cascaded IQ modulators, with binary electrical driving signals, or (b) generating two separate 4-level amplitude signals to drive each arm of one IQ modulator. These approaches may not scale easily to high baud rates or constellation sizes. However, it may be desirable to generate 16 QAM and higher order modulation formats using optical approaches, such that the base format is simple on-off keying (OOK), the multiplexing is bit-rate tunable, and the potential exists for high baud rates and constellation sizes. 
     In part I of this specification, examples of tunable phase-coherent optical multiplexing are described. For example, tunable phase-coherent optical multiplexing of four 20-Gbaud OOK Signals from different wavelengths into a single 80-Gbit/s 16-QAM channel and also a single star 16-QAM channel based on coherent vector addition of input signals using periodically-polled lithium niobate (PPLN) waveguides, and tunable phase-coherent multiplexing of two 20-Gbaud OOK Signals into a single quadrature-phase-shift-keyed (QPSK) and also a single four level amplitude/phase modulation (PAM) channel respectively, are described. 
       FIG. 1A  shows a diagram of an example of optical multiplexing of m OOK channels into a single M-Level Phase/Amplitude channel (e.g., multiplexing of 4 OOK channels into a single 16 QAM channel). Multiple on-off keying (OOK) input signals, such as OOK input signals  110 - 1 ,  110 - 2 ,  110 -m, can be multiplexed into an M-level phase/amplitude modulated signal  130  based on coherent vector addition by optical multiplexer  120 . The optical multiplexer  120  can include nonlinear optical devices and wave shaper  125 . 
     Each point of the 16-QAM constellation in the I/Q plane can be interpreted as a coherent vector addition of a first constellation point from OOK 1 , a second constellation point from OOK 2 , a third constellation point from OOK 3  and a fourth constellation point from OOK 4 . In some implementations, the vector magnitude of OOK 1  and OOK 4  should be twice (e.g., a 6-dB power difference) that of the vector magnitude of OOK 2  and OOK 3  and also a π/2 phase difference between OOK 1 , OOK 4  and OOK 2 , OOK 3  should be applied in order to get 16-QAM. 
       FIG. 1B  shows a diagram of an example of optical multiplexing of four OOK inputs into a 16-QAM and a star 16-QAM signal. The optical multiplexing can be based on coherent vector addition using two periodically-polled Lithium Niobate (PPLN) waveguides. Phase conjugate copies  158  of incoming signals  152  can be generated using a second harmonic generation (SHG) process  155  in a first PPLN waveguide. 
     After applying an appropriate phase  162  and amplitude weight  164  to each OOK signal by an optical wave shaper, the phase conjugate copies of input signals and the input OOKs are filtered and sent to a second PPLN waveguide with a similar quasi-phase matching (QPM) wavelength. All input signals can become coherent as a result of sum frequency generation (SFG)  174  between the input OOKs and their phase conjugate copies as this process erases the phase differences between input OOK signals. Therefore all incoming signals can add coherently at an identical wavelength based on a difference frequency generation (DFG) process  172  using a continuous wave (CW) pump. If the multiplexed signal has an offset constellation, the DC signal can be removed by a coherent vector addition of a CW pump with an appropriate phase and amplitude and the offset multiplexed signal. 
       FIG. 2  shows an exemplary setup  200  for generating a coherently multiplexed signal. In this example, four continuous-wave (CW) lasers  210 - 1 ,  210 - 2 ,  210 - 3 , and  210 - 4  are modulated using a Mach-Zehnder modulator (MZM)  220  to generate OOK channels each, for example, at 20-Gbit/s. which are then decorrelated using three tunable delay lines (TDL)  230 - 1 ,  230 - 2 , and  230 - 3 , and passed through an erbium-doped fiber amplifier (EDFA) a band-pass filter (BPF). 
     The four independent OOK signals and a CW pump  235  at QPM wavelength are then sent to a first periodically-polled Lithium Niobate (PPLN)  240 , for example with a QPM wavelength of ˜1550.7 nm. In this example, the phase conjugate copies of inputs and incoming signals are then filtered and sent to a second PPLN  260  with the same QPM wavelength after passing through an optical wave shaper  250  to apply appropriate phases and amplitude weights to the original signals. Another CW pump  255  can be used for the DFG process, and the coherently multiplexed signal is filtered out and detected using a coherent receiver  270 . 
     As shown in the example of  FIGS. 3A-3F , two incoming OOK signals  310 - 1 ,  310 - 2  can be multiplexed into a single 4-PAM channel  350  if the OOK signals at the wave shaper output are in phase and with a 6 dB power difference, for example. Two OOK inputs can also be multiplexed into a QPSK channel if the input OOKs have the same power and a π/2 phase difference respectively.  FIGS. 3A-3B  show an example of back-to-back optical eye and constellation diagrams of two 20 Gb/s OOK inputs  310 - 1 ,  310 - 2 .  FIG. 3C  shows an example of an optical spectrum  370  at the output of the first PPLN with two incoming OOK signals, such as OOK inputs  310 - 1  and  310 - 2 , and a CW pump to remove DC from the constellations.  FIG. 3D  shows an example of an optical spectrum  380  at the output of the second PPLN.  FIG. 3E  shows an example of an optical eye and constellation diagram of a multiplexed 4-PAM signal  350 .  FIG. 3F  shows an example of an optical constellation diagram of a multiplexed QPSK signal  360  with EVM of 18.5. 
       FIGS. 4A-4D  show an example of optical multiplexing of four OOK inputs into a 16 QAM signal with a DC offset and optical multiplexing of 4 OOK channels into a star 16 QAM signal.  FIG. 4A  shows an example of an optical spectrum  410  at the output of the first PPLN with four incoming OOK signals.  FIG. 4B  shows an example of an optical spectrum  420  at the output of the second PPLN.  FIG. 4C  shows an example of an optical constellation diagram of a 16-QAM signal  430  with a DC offset generated by multiplexing of four OOKs.  FIG. 4D  shows an example of an optical constellation diagram of a star 16-QAM signal  440  generated by multiplexing of four OOKs and a CW pump as a DC remover. 
     Part II: Flexible, Reconfigurable Capacity Output of a High-Performance 64-QAM Optical Transmitter 
     Part II of this document describes systems and techniques related to a reconfigurable optical flexible transmitter to generate arbitrary optical quadrature-amplitude-modulation (QAM) (e.g., optical 16-QAM and 64-QAM can be generated at EVM 8.5% and 7.2% respectively). For example, transmission through 80-km SMF-28after compensating with 20-km DCF can be achieved with negligible penalty. 
     Although transmitters can provide enormous data pipes, two specific challenges may remain: (1) a large discrepancy between high-rate and low-rate data channels, such that the large capacity of a single data channel from the transceiver may not be required and/or under-utilized, and (2) the large capital investment in terminal transmitters may not be efficiently utilized. A laudable goal would be to have transmitters whose extremely large capacity can be tailored and be shared among many different channels as the traffic demands vary in a dynamic, heterogeneous network. Flexible, reconfigurable bandwidth allocation can enable optimal use of terminal equipment as well as create new opportunities for scalable, cost-effective network architectures. 
     In some implementations, a flexible transmitter can be provided by using optical nonlinearities to perform reconfigurable multiplexing of different data channels, such that the capacity and data constellation can be reapportioned among different output wavelengths, Le., data can emerge as a single 64-QAM (quadrature-amplitude modulation) signal or 3 independent QPSK (quadrature-phase-shift-keyed) signals, for example. Wave-mixing using nonlinearities can have: (a) &gt;THz bandwidths, (b) minimal noise generation, (c) wavelength, conversion of a data channel, and/or (d) transparency to the data bit rate and modulation format. 
     Conventional implementations for a flexible transmitter may include methods in which: (a) IQ modulators on individual frequency combs allocate the bandwidth and (b) elastic optical path network using OFDM and variable-bandwidth wavelength crossconnects. With respect to using nonlinearities, (a) multiplexing of two 10 Gbaud QPSK signals to a 10 Gbaud star 16-QAM, (b) numerical results on various 16-QAM generation using dual-parallel Mach-Zehnder modulator (MZM) and phase modulators, and (c) numerical results on multiplexing one OOK and two binary phase-shift keying (BPSK) signals into 8-amplitude phase-shift keying (APSK) using cross phase modulation (XPM) and four wave mixing (FWM) in highly nonlinear fiber (HNLF) have been implemented. 
     In this specification, a flexible, reconfigurable capacity output of, for example, a high-performance 64-QAM optical transmitter is described. In some implementations, nonlinearity based optical signal processing can be used to achieve reconfigurability and flexibility in allocation of capacity per channel and bandwidth. 
       FIG. 5  shows a block diagram of an example of a flexible capacity QAM transmitter. In this example, multiple (here 6) electrical signals  510  are input to the system and the output capacity can be configured by changing the lasers  515  configuration into the flexible transmitter  520 . In the flexible transmitter  520 , first, many quadrature-amplitude-shift-keyed (QPSK) signals at various wavelengths can be generated by an I/Q modulator  522  in order to convert the electrical signals  510  to the optical domain. The optical QPSK signals can be injected into an optical system that generates various higher order QAM signals at different wavelengths from the QPSK signals. 
     In some implementations, the optical QAM multiplexer/wavelength converter  524  utilizes a series of cascaded second order nonlinear wave mixings (i.e., sum frequency generation (SHG), difference frequency generation (DFG) and second harmonic generation (SHG)) in periodically-poled-lithium-niobate (PPLN) waveguides in conjunction with dispersion compensating fiber (DCF) to create the output. In the optical QAM multiplexer, a continuous wave (CW) pump (E CW ) at the quasi-phase-matching (QPM) wavelength of the PPLN waveguide can be injected to the PPLN waveguide together with the QPSK signals (E Di (t)). This generates copies of the signals at new wavelengths with complex fields |E CW | 2 (E Di (t))*. The original QPSK signals, their copies and the CW pump can be sent into a DCF that induces one symbol delay between the original signal and its copy and then into the second PPLN waveguide for multiplexing. Another QPSK signal (E P (t)) can also be injected into the second PPLN. Because all signals and pumps are on the same optical path, the coherency can be preserved and according to SFG, DFG and SHG rules, the multiplexed signal can become proportional to Σ|E CW | 2 (1+α|E Di (t)| 2 e j(φ(t)−   φ(t−T)) )(E P (t))*. This gives the multiplexing an equation of multiplexing of E Di (t)&#39;s and E P (t). Thus, although the phase of E Di (t) may be lost in the process, because of one symbol delay, the difference of the phase can be preserved and used for multiplexing. Because nonlinear mixings can be controlled by reconfiguring pump lasers, the flexible transmitter  520  can simultaneously allocate capacity on different wavelengths, flexibly and reconfigurably. Three example output configurations, i.e., configuration # 1  (a 64-QAM signal), configuration # 2  (a QPSK and a 16-QAM signal), and configuration # 3  (three QPSK signals), as shown in  FIG. 5 , are described below with respect to  FIGS. 7A-8D . 
       FIG. 6  shows an example of a setup  600  for a flexible transmitter, such as flexible transmitter  520 . A nested Mach-Zehnder modulator  610  can be used to generate the 40-Gbit/s QPSK data (PRBS 2 31 -1), for example at ˜1558.2, 1555 and 1553.4 nm. 
     The resulting signals can be sent to WDM filter  620  to separate and uncorrelate them with respect to each other. The two QPSK signals  622  and  624 , for example at 1553.4 and 1555 nm, can be combined with CW pump laser  628  and sent to a first PPLN  630  (e.g., 5-cm length) after enough amplification to generate a phase conjugate of the original signal. The output of the first PPLN  630  can be passed through a DCF  640  to induce one bit delay between input signals and their corresponding phase conjugate copies. The signals, their copies and CW pump lasers as well as QPSK signal  626 , for example at 1558.2, can be sent to a second PPLN  650  (e.g., 4-cm length) to generate the desired output. The output signal is then sent to a fiber spool (e.g., 80 km SMF-28) and DCF (e.g., 20 km). The resulting signal can be sent to a coherent receiver  660  to obtain I-Q constellation and measure the bit error rate (BER). 
       FIG. 7A  shows an example of an optical spectrum  710  of a first stage in which phase conjugate copies of original QPSK signals are generated at corresponding wavelengths. These copies can be used in a second stage to generate an output signal. 
     In order to implement the first configuration (i.e., configuration # 1  as shown in  FIG. 5 ) an offset 16-QAM signal, which can be generated by multiplexing two QPSK signals and a CW pump laser with appropriate phases and amplitudes, can be mixed with another QPSK signal, for example.  FIG. 7B  shows an example of an optical spectrum  720  of an output of a second PPLN as well as a final I-Q constellation. When using the described flexible transmitter, a 64-QAM  730  with EVM ˜7% as shown in  FIG. 7C  can be generated, for example. 
     As shown in  FIGS. 8A-8B , for the second configuration (i.e., configuration # 2  as shown in  FIG. 5 ), similar to the previous one, two QPSK can be multiplexed to generate a 16-QAM  810  at the first output channel, for example. The second output  815 , on the other hand, is a copy of the third QPSK signals at the output. 
     In some implementations, the transmitter can be reconfigured to generate 3 QPSK signals  820  (i.e., configuration # 3 l as shown in  FIG. 5 ), for example, at 1542.17, 1543.8 and 1545.4 nm, as shown in  FIGS. 8C-8D . In some implementations, an EVM of ˜11% can be achieved for all QPSK signals. 
       FIG. 9  shows a diagram  900  of an example of bit error rates (BER). In this example, various BER scenarios of a 16-QAM that is transmitted through a fiber spool (in this example 80 km SMF-28) and DCF (in this example 20 km) to evaluate transmission capability, are shown. In this example, the OSNR penalty after the transmission (in this example 100 km) is negligible. 
     Part III: Tunable Optical Generation of Modulation Formats Using Nonlinearities and Coherent Frequency Comb 
     Part III of this document describes the generation of optical quadrature-amplitude-modulation (QAM) (e.g., optical 16-QAM and 64-QAM can be generated at EVM 6.8% and. 6.4% respectively) using nonlinearities and coherent frequency comb. For example, transmission through 80-km SMF-28 after compensating with 20-km DCF can be achieved with negligible penalty. 
     Higher-order modulation formats for optical communication systems can provide higher spectral efficiency in terms of bit/s/Hz. Quadrature phase shift keying (QPSK) is an example of 4-ary phase encoding, whereas 16 quadrature amplitude modulation (QAM) is an example of 16-ary amplitude/phase encoding. Moreover, QAM can go to higher orders, such as 64 and beyond. 
     Conventionally, higher-order formats can be generated by using electronic circuits to drive an I/Q modulator. However, key challenges for this approach are: (a) the limited linearity of the electronics at high baud rates, such that the spacing of the data constellation points on the I/Q plot will no longer be uniform, and/or (b) electronic approaches may become difficult at rates exceeding 100-Gbaud. 
     As an alternative, nonlinear optical processes hold the promise of high speed, format and phase transparency, low noise, and high linearity. For example, an NOLM loop can be used to multiplex 4 OOK signals into a 16-QAM. These methods may not be transparent for phase modulation formats. In part II of this document, a method to multiplex QPSK signals into a 64-QAM using 2 cascaded nonlinear stages has been described. In some implementations, the quality of the generated QAM can be improved by reducing the number of nonlinear stages. Fewer nonlinear stages may provide better efficiency and the potential to go to higher-order formats. 
     In this specification, a scheme for tunable optical generation of higher-order modulation formats using nonlinearities and coherent frequency comb is described. In some implementations, because of the coherency of comb fingers, coherent multiplexing process can be done in one nonlinear stage (e.g., optical 16-QAM and 64-QAM can be generated at EVM 6.8% and 6.4% respectively). For example, transmission through 80 km SMF-28 after compensating with 20 km DCF can be achieved with negligible penalty. 
       FIG. 10  shows a block diagram of an example of a tunable optical QAM transmitter  1000 . Multiple elements (also referred to as fingers) from a coherent frequency comb source  1010  can be selected and modulated using an I/Q modulator  1020  with QPSK modulation format. A frequency comb can be a light source whose spectrum consists of a series of discrete, equaly spaced elements. These modulated signals along with another set of coherent comb fingers  1025  with equal frequency spacing and a CW pump laser (E CW ) can be injected into a periodically-poled-lithium-niobate (PPLN) waveguide  1030  to perform phase coherent addition of the modulated QSPK signals. This process can be done through two cascaded second order nonlinear wave mixings (sum frequency generation (SFG), difference frequency generation (DFG)). The multiplexed signal can become proportional to (E CW (t))*ΣE Pi (t)·E Si (t). The amplitude and phase of comb fingers can determine the coefficients of this coherent addition and thus, by varying these parameters, QAM  1040  with different constellation size and/or encoding can be generated. 
       FIG. 11  shows an example of a setup  1100  for the tunable QAM encoder that can generate higher-order QAM. A mode-locked laser (MLL), for example with 10 GHz repetition rate and 2-ps pulse width, can be used to generate a coherent comb (e.g., with 10-GHz frequency spacing). In this example, the optical comb  1110  is passed through a delay-line-interferometer (DLI)  1112  (e.g., with FSR 20-GHz) to increase the frequency spacing. The comb (e.g, 20-GHz) is then passed through a highly nonlinear fiber (HNLF)  1114  to generate a flat and broad spectrum. A Liquid Crystal on Silicon (LCoS) filter  1120  can be used to select and write complex weights on comb fingers and separate them into signal path and pump path. A nested Mach-Zehnder modulator  1130  can be used to generate the 20/25.1-Gbit/s BPSK/QPSK data (PRBS 2 31 -1) on the signal path, for example. These signals  1132  along with the comb fingers selected for pump path  1134  and a CW laser pump  1136  can be sent to a PPLN  1140  (e.g., 5-cm length) after enough amplification to perform coherent multiplexing of the original signals. The multiplexed signal is then filtered and sent to the coherent receiver  1150 . 
       FIG. 12A  shows an example for generating QAM using 20-Gbaud QPSK signals. By coherent multiplexing of two QPSK signals with appropriate weights, a 16-QAM with EVM 6.8% can be generated, for example. A 64-QAM constellation  1210  with EVM 6.4% is also shown in  FIG. 12A , which can be generated from three QPSK signals, for example. In some implementations, if the original signals are two or three BPSK signals, pulse-amplitude-modulation (PAM) signals  1220  with 4 and 8 points respectively can be generated as shown in  FIG. 12B . The performance of the higher-order QAM encoder can be assessed using bit error rate (BER) measurements as shown in  FIG. 12C . In some implementations, a 16-QAM signal can be generated at both 20-Gbaud and 25.1-Gbaud. For example, the 20-Gbaud 16-QAM can be transmitted through 80 km SMF-28 and 20 km DCF fiber with negligible power penalty. 
     While this specification contains many specifics, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments. 
     Other implementations fall within the scope of the following claims.