Patent Publication Number: US-2022239378-A1

Title: Apparatus comprising serially connected electro-absorption modulators

Description:
BACKGROUND 
     Field 
     Various example embodiments relate to optical communication equipment and, more specifically but not exclusively, to optical modulators. 
     Description of the Related Art 
     This section introduces aspects that may 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. 
     An optical modulator is a device that can be used to manipulate a property of light, e.g., of an optical beam. Depending on which property of the optical beam is controlled, the optical modulator can be referred to as an intensity modulator, an amplitude modulator, a phase modulator, a polarization modulator, a spatial-mode modulator, etc. The telecom industry and its partners and suppliers develop, make, sell, and use a large variety of optical modulators, e.g., suitable for optical data transmitters. 
     SUMMARY OF SOME SPECIFIC EMBODIMENTS 
     Disclosed herein are various embodiments of an optical data transmitter having M serially optically connected electro-absorption modulators (EAMs) individually driven using ON-OFF-keying (OOK) electronics, where M is an integer greater than one. In an example embodiment, the optical data transmitter can be used to generate an intensity-modulated optical signal carrying symbols of a 2 M -level unipolar pulse-amplitude-modulation (PAM) constellation. The OOK electronics enables the DC bias voltages and/or AC amplitudes of the two-level drive signals applied to different EAMs to be individually controllable to achieve desired uniform or non-uniform separation between the constellation levels. In at least some embodiments, temperatures of different EAMs may also be individually controllable. In different embodiments, the EAMs may operate in reflection or in transmission. 
     According to an example embodiment, provided is an apparatus, comprising: first and second electro-absorption modulators serially optically connected to generate a modulated optical signal; and an electrical drive circuit connected to electrically drive the first electro-absorption modulator using a first two-level drive signal and to electrically drive the second electro-absorption modulator using a second two-level drive signal, the first and second two-level drive signals being generated to cause the modulated optical signal to carry a sequence of data symbols, each of the data symbols encoding at least two bits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  shows a block diagram of an optical data transmitter according to an embodiment; 
         FIGS. 2A-2C  illustrate certain characteristics of the optical data transmitter of  FIG. 1  according to an embodiment; 
         FIGS. 3A-3C  graphically illustrate example optical waveforms that can be generated in the optical data transmitter of  FIG. 1  according to an embodiment; 
         FIGS. 4A-4D  show parameters of several example configurations of the optical data transmitter of  FIG. 1  according to an embodiment; 
         FIG. 5  shows a block diagram of an electro-optical assembly according to an embodiment; and 
         FIG. 6  shows a block diagram of an electro-optical assembly according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Certain embodiments may benefit from the use of some features, apparatus, and/or methods disclosed in U.S. Pat. Nos. 10,411,807 and 10,727,948, both of which are incorporated herein by reference in their entirety. 
     An example electro-absorption modulator (EAM) is a semiconductor device capable of modulating the intensity of a light beam in response to a varying voltage applied thereto. The EAM&#39;s principle of operation is based on a change in the absorption spectrum caused by an electric field, which changes the semiconductor&#39;s bandgap energy but usually does not involve an excitation of the carriers by the electric field. Some EAMs may rely on the Franz-Keldysh effect. Some other EAMs may rely on the quantum-confined Stark effect (QCSE). An EAM may be designed to operate in transmission or in reflection. 
     Some EAMs may be referred to as being “surface-coupled.” Such an EAM is surface-coupled in the sense that, in operation, this device receives an input light beam and emits a modulated light beam in a direction that is substantially orthogonal to a main plane of the device. Such a main plane typically corresponds to a major surface of the planar substrate on which the layered structures of the surface-coupled EAM (SCEAM) are formed during fabrication. Due to this geometry, a large number of SCEAMs can be manufactured on a single substrate (such as a semiconductor wafer), e.g., as described in the above-cited U.S. Pat. Nos. 10,411,807 and 10,727,948. A major surface of the substrate is typically characterized by two relatively large dimensions, e.g., width and length, and should be contrasted with an edge of the substrate, which may have one relatively large dimension (e.g., length) and one relatively small dimension (e.g., height). Such height may represent the thickness of the substrate. 
     In an example embodiment, a SCEAM may comprise a semiconductor-junction diode (e.g., a p-n or p-i-n diode) that can be appropriately electrically biased. Some of such semiconductor-junction diodes may include multiple-quantum-well (MQW) structures. In operation, the semiconductor diode may receive a combination of an appropriate reverse-bias voltage and a driving electrical radio-frequency (RF) signal, e.g., as described below in reference to  FIGS. 2A-2C . 
     As used herein, the term “reverse bias” refers to an electrical configuration of a semiconductor-junction diode in which the n-type material is at a high electrical potential, and the p-type material is at a low electrical potential. The reverse bias typically causes the depletion layer to grow wider due to a lack of electrons and/or holes, which presents a high impedance path across the junction and substantially prevents a current flow therethrough. However, a very small reverse leakage current can still flow through the junction in the reverse-bias configuration. 
     Similarly, the term “forward bias” refers to an electrical configuration of a semiconductor-junction diode in which the n-type material is at a low potential, and the p-type material is at a high potential. If the forward bias is greater than the intrinsic voltage drop V pn  across the corresponding p-n or p-i-n junction, then the corresponding potential barrier can be overcome by the charge carriers, and a relatively large forward current can flow through the junction. For example, for silicon-based diodes the value of V pn  is approximately 0.7 V. For germanium-based diodes, the value of V pn  is approximately 0.3 V, etc. 
     At a fixed reverse-bias voltage and fixed temperature, the optical transfer function of a SCEAM is typically wavelength-dependent (see, e.g.,  FIG. 2A ). A temperature change typically causes the optical transfer function of the SCEAM to shift in wavelength. 
     The driving electrical RF signal is typically an alternating-current (AC) signal superimposed on a direct-current (DC) reverse bias offset, such that the effective bias remains reverse at any point in the driving cycle. During the positive swing of the driving electrical RF signal, the electric-field strength increases relative to that at the DC bias point, thereby red-shifting the band edge. During the negative swing of the driving RF signal, the electric-field strength decreases relative to that at the DC bias point, thereby blue-shifting the band edge. These band-edge shifts change the light transmittance and/or reflectance in the SCEAM, thereby modulating the intensity of the corresponding output light beam. 
       FIG. 1  shows a block diagram of an optical data transmitter  100  according to an embodiment. In operation, optical data transmitter  100  generates a modulated optical signal  122  in response to an electrical bitstream  148 . In an example embodiment, bitstream  148  may be implemented using a non-return-to-zero (NRZ) electrical binary waveform. Modulated optical signal  122  may have a unipolar 4-PAM format, where PAM stands for pulse-amplitude modulation. A person of ordinary skill in the art will understand that unipolar PAM is a form of intensity modulation. 
     As shown in  FIG. 1 , optical data transmitter  100  comprises a laser  110 , SCEAMs  130   1  and  130   2 , and a planar lightwave circuit (PLC)  120 . SCEAMs  130   1  and  130   2  are mounted on a major surface  138  of a device carrier  140 . In an example embodiment, surface  138  is approximately orthogonal to PLC  120  such that SCEAMs  130   1  and  130   2  can be optically coupled to on-chip waveguides through the ends thereof located at an edge  124  of the PLC. 
     In an example embodiment, PLC  120  comprises optical waveguides  104 ,  114 , and  118 . A portion  116  of waveguide  114  and a portion  102  of waveguide  104  are oriented with respect to the edge  124  at an angle that is slightly (e.g., within ±15 degrees) different from 90 degrees. The ends of waveguide portions  116  and  102  located at the edge  124  are separated from one another by a relatively small distance selected such that both of the waveguide ends are in the field of view of a coupling lens  128   1  located between the edge  124  and SCEAM  130   1 . A portion  106  of waveguide  104  and a portion  108  of waveguide  118  are similarly oriented with respect to the edge  124  at an angle that is slightly different from 90 degrees. The ends of waveguide portions  106  and  108  located at the edge  124  are separated from one another by a relatively small distance selected such that both of the waveguide ends are in the field of view of a coupling lens  128   2  located between the edge  124  and SCEAM  130   2 . This geometric arrangement of PLC  120  and coupling lenses  128   1  and  128   2  serially optically connects laser  110  and SCEAMs  130   1  and  130   2 . Lenses  128   1  and  128   2  are used to appropriately shape and redirect light and to provide relatively high optical-coupling efficiency between PLC  120  and SCEAMs  130   1  and  130   2 . 
     In some embodiments, lenses  128   1  and  128   2  may be absent. In such embodiments, SCEAMs  130   1  and  130   2  are directly optically coupled (e.g., butt-coupled) to the edge  124  of PLC  120 . 
     In operation, laser  110  may apply a continuous-wave (CW) light beam  112  to optical waveguide  114 , e.g., as indicated in  FIG. 1 . Waveguide  114  directs light beam  112 , through portion  116  and lens  128   1 , toward SCEAM  130   1 . Portion  102  of waveguide  104  then receives, through lens  128   1 , a modulated light beam  117  emitted by SCEAM  130   1  in response to light beam  112 . Waveguide  104  directs light beam  117 , from portion  102  thereof, through portion  106  and lens  128   2 , toward SCEAM  130   2 . Portion  108  of waveguide  118  then receives, through lens  128   2 , a modulated light beam  122 , which is emitted by SCEAM  130   2  in response to modulated light beam  117 . Waveguide  118  then directs modulated light beam  122  to an external circuit or optical fiber as indicated in  FIG. 1 . 
     Optical data transmitter  100  further comprises an electrical drive circuit  150  and an electronic controller  180 . Electrical drive circuit  150  operates to generate electrical drive signals  132   1  and  132   2  in response to bitstream  148 . Electrical drive signals  132   1  and  132   2  are then applied to SCEAMs  130   1  and  130   2 , respectively. Controller  180  operates to control the configuration of electrical drive circuit  150 , e.g., as described in more detail below. In some embodiments, controller  180  may further operate to control the temperatures of SCEAMs  130   1  and  130   2  by individually configuring the corresponding temperature-control units  134   1  and  134   2  thereof. In an example embodiment, a temperature control unit  134   i  (where i=1, 2) may include a corresponding electrical heater and/or a corresponding thermoelectric cooler thermally coupled to SCEAM  130   i  as known in the pertinent art. In some embodiments, temperature-control units  134   1  and  134   2  may be absent. 
     Electrical drive circuit  150  comprises a demultiplexer (DMUX)  152 , electrical RF amplifiers  156   1  and  156   2 , and bias tees  162   1  and  162   2 . DMUX  152  operates to demultiplex bitstream  148  into bitstreams  154   1  and  154   2 . Amplifiers  156   1  and  156   2  then amplify the NRZ waveforms of bitstreams  154   1  and  154   2 , thereby generating electrical RF signals  158   1  and  158   2 , respectively. Bias tees  162   1  and  162   2  operate to add respective DC bias voltages to electrical RF signals  158   1  and  158   2 , thereby generating electrical drive signals  132   1  and  132   2 , respectively. 
     In some embodiments, DMUX  152  may be absent. In such embodiments, bitstreams  154   1  and  154   2  may be received from external circuits. Typically, bitstreams  154   1  and  154   2  are uncorrelated. 
     In an example embodiment, each of electrical drive signals  132   1  and  132   2  is a two-level signal that alternates between a respective low level and a respective high level in response to the corresponding one of bitstreams  154   1  and  154   2 . Several examples of low and high levels of electrical drive signals  132   1  and  132   2  are given in the tables of  FIGS. 2C and 4A-4D . 
     Controller  180  is configured to: (i) control the electrical gains of amplifiers  156   1  and  156   2  using control signals G 1  and G 2 , respectively; (ii) apply DC bias voltages V 1  and V 2  to the low-frequency ports of bias tees  162   1  and  162   2 , respectively; and (iii) optionally, control the temperatures of SCEAMs  130   1  and  130   2  using control signals T 1  and T 2  applied to temperature-control units  134   1  and  134   2 , respectively. In an example embodiment, controller  180  is capable of individually controlling the signals G 1 , G 2 , V 1 , V 2 , T 1 , and T 2 . Using this capability and depending on the desired configuration of transmitter  100 , controller  180  may be used to implement some or all of the following features: (i) cause the gains of amplifiers  156   1  and  156   2  to be the same or different; (ii) cause DC bias voltages V 1  and V 2  to be the same or different; and (iii) cause the temperatures of SCEAMs  130   1  and  130   2  to be the same or different. 
       FIGS. 2A-2C  illustrate certain characteristics of optical data transmitter  100  according to an embodiment. More specifically,  FIG. 2A  graphically shows optical transfer functions of optical data transmitter  100  at several different combinations of voltages of electrical drive signals  132   1  and  132   2 .  FIG. 2B  graphically shows an example 4-level unipolar PAM constellation  210  that can be produced using the transfer functions of  FIG. 2A .  FIG. 2C  shows a table that illustrates the configuration of electrical drive circuit  150  corresponding to  FIGS. 2A-2B . In this particular embodiment, controller  180  operates to maintain the same fixed temperature (=20° C.) of SCEAMs  130   1  and  130   2 . All voltages shown in the table of  FIG. 2C  are reverse voltages. 
     Referring to  FIGS. 2A and 2C , the optical transfer functions  202 ,  204 ,  206 , and  208  shown in  FIG. 2A  correspond to the levels of electrical drive circuit  150  indicated in the fifth, fourth, third, and second rows, respectively, of the table of  FIG. 2C . More specifically, in response to a binary “zero” of bitstream  154   1 , electrical drive circuit  150  causes electrical drive signal  132   1  to be at 2 V, which is the high level of that signal. In response to a binary “one” of bitstream  154   1 , electrical drive circuit  150  causes electrical drive signal  132   1  to be at 0.5 V, which is the low level of that signal. In response to a binary “zero” of bitstream  154   2 , electrical drive circuit  150  causes electrical drive signal  132   2  to be at 3 V, which is the high level of that signal. In response to a binary “one” of bitstream  154   2 , electrical drive circuit  150  causes electrical drive signal  132   2  to be at 0.5 V, which is the low level of that signal. When each of bitstreams  148 ,  154   1 , and  154   2  has a voltage swing of 1 V between the two binary levels thereof, the above-indicated levels of electrical drive signals  132   1  and  132   2  can be achieved, e.g., by setting the electrical gains G 1  and G 2  at 1.5 and 2.5, respectively. Referring to  FIG. 2C , constellation  210  can be obtained, e.g., using the optical transfer functions  202 ,  204 ,  206 , and  208  of  FIG. 2A  and by tuning the output wavelength of laser  110  to 1570 nm. Relationship between data symbols of constellation  210  and optical transfer functions  202 ,  204 ,  206 , and  208  of  FIG. 2A  is indicated by the corresponding arrows, which connect  FIGS. 2A and 2B . Each of the data symbols of constellation  210  encodes (exactly) two bits. One of the two bits is provided by bitstream  154   1 , and the other of the two bits is provided by bitstream  154   2 . Constellation  210  is one-dimensional and unipolar in the sense that all data symbols thereof are located on the same side of the constellation axis with respect to the origin. 
     Note that constellation  210  has a non-uniform spacing between its four data symbols, i.e., the distance between neighboring data symbols increases as the symbol power increases. Such a constellation may be useful, e.g., for nonlinearly impaired optical channels, wherein the optical noise increases nonlinearly with an increase of the optical power. An alternative configuration in which the data symbols of the corresponding constellation are substantially uniformly spaced (e.g., are equidistant) is also possible (see, e.g.,  FIGS. 4A-4D ). In general, the four intensity levels corresponding to the data symbols of constellation  210  can be moved with respect to one another and/or with respect to the origin of the constellation using one or more of the following circuit-configuration adjustments: (i) changing the electrical gains of amplifiers  156   1  and  156   2 ; (ii) changing DC bias voltages V 1  and V 2 ; (iii) changing the temperatures of SCEAMs  130   1  and  130   2 ; and (iv) changing the wavelength of light beam  112 . 
       FIGS. 3A-3C  graphically illustrate example optical waveforms that can be generated in optical data transmitter  100  according to an embodiment. More specifically, the shown waveforms correspond to the configuration in which optical data transmitter  100  generates a sequence of data symbols of constellation  210  ( FIG. 2B ) using drive signals  132   1  and  132   2  generated as indicated in  FIG. 2C  when the output wavelength of laser  110  is 1570 nm. The temperature of SCEAMs  130   1  and  130   2  is T=20° C. In this particular example, the bit rate of bitstream  148  is 25 Gbit/s. 
       FIG. 3A  graphically shows an optical waveform  310  generated by SCEAM  130   1  in the above configuration. Optical waveform  310  represents the intensity ratio of optical signals  117  and  112  at SCEAM  130   1 . The extinction ratio (ER) corresponding to the two binary levels of optical waveform  310  is about 1.8 dB. 
       FIG. 3B  graphically shows an optical waveform  320  generated by SCEAM  130   2  in the above configuration. Optical waveform  320  represents the intensity ratio of optical signals  122  and  117  at SCEAM  130   2 . The extinction ratio corresponding to the two binary levels of optical waveform  320  is about 3.4 dB. 
       FIG. 3C  graphically shows an optical waveform  330  generated by the serially optically connected SCEAMs  130   1  and  130   2  driven in the above configuration. Optical waveform  330  represents the intensity ratio of optical signals  122  and  112 . A person of ordinary skill in the art will understand that optical waveform  330  is proportional to the product of optical waveforms  310  and  320 . The four distinct intensity levels corresponding to constellation  210  ( FIG. 2B ) are clearly visible in optical waveform  330 . 
       FIGS. 4A-4D  show parameters of several alternative example configurations of optical data transmitter  100  according to an embodiment. Each of these configurations cause optical data transmitter  100  to generate data symbols of a respective unipolar 4-PAM constellation, in which the intensity levels are substantially equidistant. The last column in each of the tables shown in  FIGS. 4A-4D  explicitly shows those (substantially equidistant) intensity levels. For example, the distance between the neighboring intensity levels in the unipolar 4-PAM constellation corresponding to  FIG. 4A  is approximately 0.06 relative units. The distance between the neighboring intensity levels in the unipolar 4-PAM constellation corresponding to  FIG. 4B  is approximately 0.04 relative units, etc. 
     The circuit configuration corresponding to  FIG. 4A  is an example configuration in which: (i) the DC bias voltages V 1  and V 2  are the same; (ii) the electrical gains G 1  and G 2  of amplifiers  156   1  and  156   2  are different; and (iii) SCEAMs  130   1  and  130   2  are at the same temperature. The circuit configuration corresponding to  FIG. 4B  is an example configuration in which: (i) the DC bias voltages V 1  and V 2  are different; (ii) the electrical gains G 1  and G 2  of amplifiers  156   1  and  156   2  are the same; and (iii) SCEAMs  130   1  and  130   2  are at the same temperature. The circuit configuration corresponding to  FIG. 4C  is an example configuration in which: (i) the DC bias voltages V 1  and V 2  are different; (ii) the electrical gains G 1  and G 2  of amplifiers  156   1  and  156   2  are different; and (iii) SCEAMs  130   1  and  130   2  are at the same temperature. The circuit configuration corresponding to  FIG. 4D  is an example configuration in which: (i) the DC bias voltages V 1  and V 2  are the same; (ii) the electrical gains G 1  and G 2  of amplifiers  156   1  and  156   2  are the same; and (iii) SCEAMs  130   1  and  130   2  are at different respective temperatures. 
       FIG. 5  shows a block diagram of an electro-optical assembly  500  according to an embodiment. Assembly  500  comprises laser  110  and SCEAMs  130   1 - 130   M , which are serially optically connected using a PLC  520 , where M is an integer greater than two. SCEAMs  130   1 - 130   M  are arranged in a linear array on a device carrier or substrate  540  and are optically coupled, using an array of coupling lenses  128   1 - 128   M , to a plurality of waveguide ends located at an edge  524  of PLC  520 , e.g., as indicated in  FIG. 5 . PLC  520  is generally similar to PLC  120 , except that PLC  520  has additional waveguides, labeled  504   2 - 504   M-1 , that are serially connected between waveguide  104  and waveguide  118  (also see  FIG. 1 ). In an example embodiment, each of waveguides  504   2 - 504   M-1  can be similar to waveguide  104 . 
     In some embodiments, lenses  128   1 - 128   M  may be absent. In such embodiments, SCEAMs  130   1 - 130   M  are directly optically coupled to the edge  524  of PLC  520 . 
     In operation, SCEAMs  130   1 - 130   M  can be driven using electrical drive signals  532   1 - 532   M , each of which is a two-level signal that may be generated in a manner similar to that used for generating electrical drive signals  132   1  and  132   2 . More specifically, each of the electrical drive signals  532   1 - 532   M  may be generated using a respective bias tee as a combination of a respective a fixed reverse-bias voltage and a respective electrical RF signal. The fixed reverse-bias voltages may be individually set, e.g., using an electronic controller analogous to controller  180  ( FIG. 1 ). The electrical RF signals may be individually generated in response to the corresponding individual bitstreams, e.g., as explained above in the description of electrical drive circuit  150 . The individual bitstreams may be generated by M-way demultiplexing the corresponding common source bitstream, such as bitstream  148  ( FIG. 1 ). 
     Although individual temperature-control units are not explicitly shown in  FIG. 5 , such temperature-control units, e.g., similar to temperature-control units  134   i , may be present in at least some embodiments of assembly  500 . When present, the temperature-control units may be used to individually control the temperatures of SCEAMs  130   1 - 130   M . 
     Assembly  500  can be used, e.g., to implement an alternative embodiment of optical data transmitter  100 . A person of ordinary skill in the art will understand that such an alternative embodiment is capable of generating, e.g., symbols of a unipolar 2 M -PAM constellation. Each data symbol of this constellation encodes M bits. Constellation  210  ( FIG. 2B ) may be considered as an example corresponding to M=2. In different embodiments, the intensity levels of the unipolar 2 M -PAM constellation enabled by the use of assembly  500  may be equidistant or non-equidistant. 
       FIG. 6  shows a block diagram of an electro-optical assembly  600  according to an embodiment. Assembly  600  is a modification of assembly  500 , according to which PLC  520  is replaced by an array of optical circulators  620   1 - 200   M . Similar to assembly  500 , assembly  600  can be used to implement an alternative embodiment of optical data transmitter  100  capable of generating data symbols of a unipolar 2 M -PAM constellation. 
     According to an example embodiment disclosed above, e.g., in the summary section and/or in reference to any one or any combination of some or all of  FIGS. 1-6 , provided is an apparatus comprising: first and second electro-absorption modulators (e.g.,  130   1 ,  130   2 ,  FIGS. 1, 5, 6 ) serially optically connected to generate a modulated optical signal (e.g.,  122 ,  FIG. 1 ;  522 ,  FIGS. 5, 6 ); and an electrical drive circuit (e.g.,  150 ,  FIG. 1 ) connected to electrically drive the first electro-absorption modulator using a first two-level drive signal (e.g.,  132   1 ,  FIG. 1 ;  532   1 ,  FIGS. 5, 6 ) and to electrically drive the second electro-absorption modulator using a second two-level drive signal (e.g.,  132   2 ,  FIG. 1 ;  532   2 ,  FIGS. 5, 6 ), the first and second two-level drive signals being generated to cause the modulated optical signal to carry a sequence of data symbols, each of the data symbols encoding at least two bits. 
     In some embodiments of the above apparatus, the apparatus is configured to cause the data symbols to be symbols of a unipolar 2 M -pulse-amplitude-modulation constellation (e.g.,  210 ,  FIG. 2B ), where M is an integer greater than one. 
     In some embodiments of any of the above apparatus, intensity levels corresponding to the data symbols are non-equidistant (e.g., as in  210 ,  FIG. 2B ). 
     In some embodiments of any of the above apparatus, the electrical drive circuit comprises a demultiplexer (e.g.,  152 ,  FIG. 1 ) configured to generate first and second bitstreams (e.g.,  154   1 ,  154   2 ,  FIG. 1 ) by demultiplexing a third bitstream (e.g.,  148 ,  FIG. 1 ); and wherein the electrical drive circuit is configured to generate the first two-level drive signal in response to the first bitstream and to generate the second two-level drive signal in response to the second bitstream. 
     In some embodiments of any of the above apparatus, each of the first and second two-level drive signals has a respective low level and a respective high level (e.g., indicated in  FIGS. 2C, 4A-4D ), said respective low and high levels being selected to apply reverse biases to the first and second electro-absorption modulators. 
     In some embodiments of any of the above apparatus, the electrical drive circuit is configured to cause the respective low levels of the first and second two-level drive signals to be different (e.g., as in  FIG. 4A ). 
     In some embodiments of any of the above apparatus, the electrical drive circuit is configured to cause the respective high levels of the first and second two-level drive signals to be different (e.g., as in  FIG. 4A ). 
     In some embodiments of any of the above apparatus, the electrical drive circuit is configured to cause a difference between the low and high levels of the first two-level drive signal to be different from a difference between the low and high levels of the second two-level drive signal (e.g., as in  FIG. 4A ). 
     In some embodiments of any of the above apparatus, the electrical drive circuit comprises first and second electrical radio-frequency amplifiers (e.g.,  156   1 ,  156   2 ,  FIG. 1 ) and first and second bias tees (e.g.,  162   1 ,  162   2 ,  FIG. 1 ), the first electrical radio-frequency amplifier and the first bias tee being serially connected to generate the first two-level drive signal, the second electrical radio-frequency amplifier and the second bias tee being serially connected to generate the second two-level drive signal. 
     In some embodiments of any of the above apparatus, DC bias voltages applied to the first and second bias tees are individually controllable (e.g., V 1 , V 2 ,  FIG. 1 ). 
     In some embodiments of any of the above apparatus, gains of the first and second electrical radio-frequency amplifiers are individually controllable (e.g., via G 1 , G 2 ,  FIG. 1 ). 
     In some embodiments of any of the above apparatus, the apparatus further comprises first and second temperature-control units (e.g.,  134   1 ,  134   2 ,  FIG. 1 ) configured to individually control temperatures of the first and second electro-absorption modulators. 
     In some embodiments of any of the above apparatus, DC bias voltages applied to the first and second bias tees are individually controllable (e.g., V 1 , V 2 ,  FIG. 1 ); and wherein electrical gains of the first and second electrical radio-frequency amplifiers are individually controllable (e.g., via G 1 , G 2 ,  FIG. 1 ). 
     In some embodiments of any of the above apparatus, the apparatus further comprises first and second temperature-control units (e.g.,  134   1 ,  134   2 ,  FIG. 1 ) configured to individually control temperatures of the first and second electro-absorption modulators. 
     In some embodiments of any of the above apparatus, the apparatus comprises M electro-absorption modulators (e.g.,  130   1 - 130   M ,  FIGS. 5, 6 ) serially optically connected to generate the modulated optical signal, said M electro-absorption modulators including the first and second electro-absorption modulators, where M is an integer greater than two. 
     In some embodiments of any of the above apparatus, the apparatus further comprises a planar lightwave circuit (e.g.,  120 ,  FIG. 1 ;  520 ,  FIG. 5 ) including optical waveguides arranged to serially optically connect the first and second electro-absorption modulators. 
     In some embodiments of any of the above apparatus, the apparatus further comprises first and second optical circulators (e.g.,  620   1 ,  620   1 ,  FIG. 6 ) arranged to serially optically connect the first and second electro-absorption modulators. 
     In some embodiments of any of the above apparatus, each of the first and second electro-absorption modulators is configured to operate in reflection. 
     While this disclosure includes references to illustrative embodiments, this specification is not intended to be construed in a limiting sense. 
     Although example embodiments are described above in reference to EAMs designed to operate in reflection, a person of ordinary skill in the art will be able, based on the provided description, to make and use alternative embodiments employing EAMs designed to operate in transmission. 
     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. 
     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. 
     It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this disclosure may be made by those skilled in the art without departing from the scope of the disclosure, e.g., as expressed in the following claims. 
     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. 
     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. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the disclosure. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
     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. 
     Unless otherwise specified herein, in addition to its plain meaning, the conjunction “if” may also or alternatively be construed to mean “when” or “upon” or “in response to determining” or “in response to detecting,” which construal may depend on the corresponding specific context. For example, the phrase “if it is determined” or “if [a stated condition] is detected” may be construed to mean “upon determining” or “in response to determining” or “upon detecting [the stated condition or event]” or “in response to detecting [the stated condition or event].” 
     Throughout the detailed description, the drawings, which are not to scale, are illustrative only and are used in order to explain, rather than limit the disclosure. The use of terms such as height, length, width, top, bottom, is strictly to facilitate the description of the embodiments and is not intended to limit the embodiments to a specific orientation. For example, height does not imply only a vertical rise limitation, but is used to identify one of the three dimensions of a three dimensional structure as shown in the figures. Such “height” would be vertical where the substrate is horizontal but would be horizontal where the substrate is vertical, and so on. 
     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. The same type of distinction applies to the use of terms “attached” and “directly attached,” as applied to a description of a physical structure. For example, a relatively thin layer of adhesive or other suitable binder can be used to implement such “direct attachment” of the two corresponding components in such physical structure. 
     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. 
     The functions of the various elements shown in the figures, including any functional blocks labeled as “processors” and/or “controllers,” may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context. 
     As used in this application, the term “circuitry” may 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. 
     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. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     “SUMMARY OF SOME SPECIFIC EMBODIMENTS” in this specification is intended to introduce some example embodiments, with additional embodiments being described in “DETAILED DESCRIPTION” and/or in reference to one or more drawings. “SUMMARY OF SOME SPECIFIC EMBODIMENTS” is not intended to identify essential elements or features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter.