Patent Publication Number: US-2015063820-A1

Title: Cross-talk reduction in a bidirectional optoelectronic device

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. non-provisional application Ser. No. 13/096,648 filed Apr. 28, 2011 in the names of Joel S. Paslaski, Araceli Ruiz, Peter C. Sercel, and Rolf A. Wyss, which in turn claims benefit of (i) co-pending U.S. provisional App. No. 61/328,675 filed Apr. 28, 2010 in the name of Joel S. Paslaski, (ii) co-pending U.S. provisional App. No. 61/358,877 filed Jun. 25, 2010 in the name of Rolf A. Wyss, and (iii) co-pending U.S. provisional App. No. 61/380,310 filed Sep. 6, 2010 in the names of Peter C. Sercel, Araceli Ruiz, and Joel S. Paslaski, each of said provisional applications being incorporated by reference as if fully set forth herein. 
    
    
     BACKGROUND 
     The field of the present invention relates to bidirectional or multi-channel optoelectronic devices, including bidirectional optoelectronic transceivers. In particular, (i) a light source drive circuit, (ii) multi-function encapsulation, and (iii) a light-trapping structure formed on a waveguide substrate are disclosed herein for reducing cross-talk in a bidirectional optoelectronic device. 
     A bidirectional optoelectronic transceiver is a device that can simultaneously (i) receive one or more input optical signals and generate corresponding output electrical signals and (ii) receive one or more input electrical signals and generate corresponding output optical signals. More generally, a multi-channel optoelectronic device is one that can simultaneously handle such conversion between electrical and optical signals for two or more such pairs of corresponding signals (each pair comprising a “channel”). Such multi-channel devices can be “unidirectional” (i.e., wherein all input signals are optical and all corresponding output signals are electrical, or vice versa) or “bidirectional” (already described above). 
     In general, the input and output signals (optical and electrical) can be transmitted and received in any suitable way, including, e.g., free-space propagation (optical or electrical), electrical conduction by conductive wire, cable, or trace (electrical), or propagation as a guided mode in an optical fiber or waveguide (optical). It is common in telecommunications devices for the optical signals (input and output) to be received from or transmitted into an optical fiber or waveguide, and for the electrical signals to be received from or transmitted to a conductive wire, cable, or trace. 
     In this context, each signal (electrical or optical) typically comprises a carrier wave modulated according to a given scheme to encode digital or analog information (e.g., a digital data stream, an analog or digital video signal, or an analog or digital audio signal). The correspondence referred to above (i) between the input optical signal and the output electrical signal, and (ii) between the input electrical signal and the output optical signal, is a correspondence of the information encoded according to their respective modulation schemes. Many modulation schemes exits for encoding information onto an electrical or optical carrier signal. One common example of an electrical modulation scheme includes baseband digital amplitude modulation; another common example includes amplitude modulation of a radio frequency (RF) electrical carrier wave. One common example of an optical modulation scheme includes amplitude modulation of a visible or near-infrared optical carrier wave. Multiple electrical or optical modulation schemes can in some instances be used together or overlaid on one another. In some examples, by using differing carrier frequencies for input and output signals (electrical or optical), both input and output signals can be carried by a common transmission medium (e.g., input and output optical signals carried by a common optical fiber or waveguide, or input and output electrical signals carried by a common conductive wire, cable, or trace). In other examples, input and output electrical signals can be carried by separate conductive wires or traces, or input and output optical signals can be carried by separate optical fibers or waveguides. 
     Typically, care must be taken to limit the effects of cross-talk in a multi-channel or bidirectional optoelectronic device. Electrical cross-talk refers to an electrical signal (input or output) adversely affecting reception or generation of another electrical signal, and optical cross-talk refers to an optical signal (input or output) interfering with reception or generation of another optical signal. In principle, a cross-talk problem can arise in either or both directions (i.e., input affecting output, output affecting input, or both), and limiting cross-talk in both directions can be advantageous. In practice, in a bidirectional device, an input electrical signal (that drives the light source to generate the output optical signal) typically is larger in absolute magnitude than an output electrical signal (generated by photodetection of a typically weak input optical signal). As a result, the input electrical signal typically affects the output electrical signal (or its generation from the input optical signal) to a greater degree than the output electrical signal affects the input electrical signal (or generation of the output optical signal therefrom). Similarly, in a bidirectional device, the output optical signal typically is larger in absolute magnitude than the input optical signal. As a result, the output optical signal typically affects the input optical signal (or generation of the output electrical signal therefrom) to a greater degree than the input optical signal affects the output optical signal (or its generation from the input electrical signal). 
     Cross-talk in a multi-channel or bidirectional optoelectronic device can manifest itself in a variety of ways. In one example, electrical cross-talk can result in decreased sensitivity, in the presence of an input electrical signal, for reception of an input optical signal and generation of a corresponding output electrical signal by the photodetector. In another example, optical cross-talk can result in decreased sensitivity, in the presence of an output optical signal, for reception of an input optical signal and generation of a corresponding output electrical signal by the photodetector. In those examples and in others, such decreased sensitivity can manifest itself, e.g., as decreased signal-to-noise ratio, increased bit error rate for a digital signal, or increased noise floor. Sensitivity is simply the minimum optical power needed to ensure sufficiently faithful encoding on the output electrical signal of information encoded on the input optical signal (e.g., to guarantee a bit error rate below a specified limit for a digital data signal; various suitable criteria can be established for various types of signals). The sensitivity of the photodetector is typically degraded in the presence of an input electrical signal applied to the light source or the resulting output optical signal, relative to its sensitivity in the absence of an input electrical signal or output optical signal. Such degradation can be referred to or quantified generically as a “cross-talk penalty,” expressed as a ratio of the sensitivity of the photodetector with versus without the input electrical signal applied to the light source (or expressed as a difference between sensitivities given as dBm, for example). Reduction of optical or electrical cross-talk is a way to improve the photodetection performance of the bidirectional optoelectronic device, and can in some instances be imperative for meeting photodetection performance requirements of the device. Analogously, a cross-talk penalty can be quantified for faithful encoding on the output optical signal of information encoded on the input electrical signal in the presence of an input optical signal or an output electrical signal. 
     SUMMARY 
     A bidirectional optoelectronic device comprises a photodetector, a light source, and a drive circuit for the light source. The photodetector is arranged (i) to receive an input optical signal modulated to encode first transmitted information and (ii) to generate in response to the input optical signal an output electrical signal modulated to encode the first transmitted information. The light source is arranged (i) to receive an input electrical signal modulated to encode second transmitted information and (ii) to generate in response to the input electrical signal an output optical signal modulated to encode the second transmitted information. A method employing the bidirectional optoelectronic device comprises: receiving at a photodetector the input optical signal; generating with the photodetector, in response to the input optical signal, the output electrical signal; receiving at the light source an input electrical signal; and generating with the light source, in response to the input electrical signal, the output optical signal. 
     The light source has first and second electrical leads for receiving the input electrical signal, and the drive circuit can be arranged to apply a first portion of the input electrical signal to the first electrical lead of the light source and to apply a second portion of the input electrical signal to the second lead of the light source, wherein the second portion of the input electrical signal is a scaled, inverted substantial replica of the first portion of the input electrical signal. The method can further include performing an optimization procedure to determine a selected scale factor that reduces or minimizes electrical cross-talk in the device. 
     The bidirectional optoelectronic device, or a multi-channel optoelectronic device, can include a protective encapsulant arranged to encapsulate its components. The encapsulant includes hollow dielectric microspheres dispersed within its volume so as to reduce a cross-talk penalty arising from unwanted electrical signals present in the encapsulant to a level below that exhibited by the device without the microspheres in the encapsulant. The encapsulant can further include an optical absorber dispersed within its volume so as to reduce a cross-talk penalty arising from unwanted optical signals present in the encapsulant to a level below that exhibited by the optoelectronic device without the optical absorber in the encapsulant. 
     The bidirectional optoelectronic device, or a multi-channel optoelectronic device, can include one or more light collectors or light traps formed in optical waveguide layers on a waveguide substrate. Each light collector or trap comprises one or more lateral surfaces of the optical waveguide layers and a substantially opaque coating deposited on the lateral surfaces. The lateral surfaces of each light trap are arranged to define a corresponding spiral region of the optical waveguide layers; the region includes an open mouth and closed end of the light trap. The lateral surfaces of each light collector are arranged to redirect an optical signal propagating in the waveguide layers, but not guided by a waveguide, to propagate into the open mouth of a light trap. 
     The bidirectional optoelectronic device can be further arranged (i) so that sensitivity of the photodetector, with the input electrical signal applied to the light source, is within about 3 dBm of the sensitivity of the photodetector with no input electrical signal applied to the light source, (ii) so that the photodetector exhibits a cross-talk penalty less than about 3 dBm, or (iii) so that a selected scale factor of the second portion of the input electrical signal relative to the first portion of the electrical signal results in a minimal cross-talk penalty exhibited by the photodetector. 
     Objects and advantages pertaining to multi-channel or bidirectional optoelectronic devices may become apparent upon referring to the exemplary embodiments illustrated in the drawings and disclosed in the following written description or appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically electrical and optical signals in an exemplary bidirectional optoelectronic device. 
         FIG. 2  illustrates schematically unwanted electrical and optical signals in the exemplary bidirectional optoelectronic device of  FIG. 1 . 
         FIG. 3  illustrates schematically electrical and optical signals in another exemplary bidirectional optoelectronic device. 
         FIG. 4  illustrates schematically unwanted electrical and optical signals in the exemplary bidirectional optoelectronic device of  FIG. 3 . 
         FIG. 5  illustrates schematically electrical and optical signals in another exemplary bidirectional optoelectronic device. 
         FIG. 6  illustrates schematically a conventional light source drive circuit for a bidirectional optoelectronic device. 
         FIG. 7  illustrates schematically an exemplary light source drive circuit for a bidirectional optoelectronic device. 
         FIG. 8  illustrates schematically an input electrical signal and first and second portions of the input electrical signal produced by the drive circuit of  FIG. 7 . 
         FIG. 9  illustrates schematically a portion of another exemplary drive circuit for a bidirectional optoelectronic device. 
         FIG. 10  is a plot of cross-talk penalty versus laser diode cathode voltage amplitude for the drive circuit of  FIG. 9 . 
         FIG. 11  illustrates schematically a portion of another exemplary drive circuit for a bidirectional optoelectronic device. 
         FIG. 12  is a plot of cross-talk penalty versus laser diode cathode voltage amplitude for the drive circuit of  FIG. 11 . 
         FIGS. 13 and 14  illustrate schematically portions of other exemplary drive circuits for a bidirectional optoelectronic device. 
         FIG. 15  is a schematic plan view of a light source, waveguide, and exemplary light-trapping structure on a waveguide substrate. 
         FIG. 16  is a schematic plan view of a light source, waveguide, and exemplary light-trapping structure on a waveguide substrate showing paths of guided and stray optical signals. 
         FIGS. 17A ,  18 A, and  19 A are schematic cross-sectional views of various exemplary lateral surfaces of the optical waveguide layers and substantially opaque coatings formed near an optical waveguide. 
         FIGS. 17B ,  18 B, and  19 B are schematic cross-sectional views of various exemplary lateral surfaces of the optical waveguide layers and substantially opaque coatings formed away from any optical waveguide. 
         FIG. 20  is a schematic plan view of an exemplary bidirectional optoelectronic device including an exemplary light-trapping structure. 
         FIG. 21  illustrates schematically a protective encapsulant on another exemplary bidirectional optoelectronic device. 
         FIG. 22  illustrates schematically particles of an optical absorber dispersed in an encapsulant. 
         FIG. 23  illustrates schematically hollow dielectric microspheres dispersed in an encapsulant. 
         FIG. 24  illustrates schematically particles of an optical absorber and hollow dielectric microspheres dispersed in an encapsulant. 
         FIG. 25  illustrates schematically an optical encapsulant and a protective encapsulant on another exemplary bidirectional optoelectronic device. 
     
    
    
     It should be noted that the embodiments depicted in this disclosure are shown only schematically, and that not all features may be shown in full detail or in proper proportion. Certain features or structures may be exaggerated relative to others for clarity. The Drawings should not be regarded as being to scale. It should be noted further that the embodiments shown are exemplary only, and should not be construed as limiting the scope of the written description or appended claims. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Electrical cross-talk in a multi-channel or bidirectional optoelectronic device can arise through any number or mechanisms or pathways involving a photodetector, a light source, a drive circuit for the light source, or an amplification or filtering circuit for the photodetector. Such electrical cross-talk is often attributed to capacitive or inductive coupling between or among those elements. Optical cross-talk in a multi-channel or bidirectional optoelectronic device can arise through any number or mechanisms or pathways involving the photodetector, the light source, waveguides, filters, optical splitters or combiners, optical taps, or other optical elements. Such optical cross-talk is often attributed to unwanted scattering, reflection, or transmission of light between or among those elements. As the size of the multi-channel or bidirectional optoelectronic is reduced, electrical or optical cross-talk can typically become more severe. Regardless of the particular mechanism or combination of mechanisms that give rise to the cross-talk (optical or electrical), it would be desirable to reduce the cross-talk by suitable arrangements or adaptations of the multi-channel or bidirectional optoelectronic device. 
     An example of a bidirectional optoelectronic device is illustrated schematically in  FIGS. 1 and 2 , and comprises a signal photodetector  114  (typically, but not necessarily, a photodiode) and a light source  116  (typically, but not necessarily, a laser diode). The exemplary device also includes a monitor photodetector  118 , but devices that do not include such a monitor photodetector shall fall within the scope of the present disclosure or appended claims.  FIG. 1  illustrates desired electrical and optical signals, and  FIG. 2  illustrates unwanted electrical and optical signals that can result in cross-talk. In  FIG. 1 , an input optical signal  14  propagates along an optical waveguide  104  and is received by the signal photodetector  114 . The signal photodetector  114  generates an output electrical signal  24  from the input optical signal  14 , and the output electrical signal  24  is transmitted from the signal photodetector  114  by conductive traces  124  and conductive wire leads  134 . Any suitable optical or electrical elements can be employed for transmitting input optical signal  14  or output electrical signal  24 . An input electrical signal  26  is transmitted to light source  116  by conductive wire leads  136  and conductive traces  126 . Light source  116  generates an output optical signal  16  from the input electrical signal  26  that propagates along optical waveguide  106 . Any suitable optical or electrical elements can be employed for transmitting input electrical signal  26  or output optical signal  16 . In devices with a monitor photodetector  118 , a portion of the output optical signal  16  is split off to form a monitor optical signal  18  (that propagates along optical waveguide  108  in the example of  FIGS. 1 and 2 ; other suitable optical element(s) can be employed; suitable optical arrangements for splitting off the monitor optical signal  18  are described below). The monitor optical signal is received by the monitor photodetector  118 , which in turn generates a monitor electrical signal  28  that is transmitted by conductive traces  128  and conductive wire leads  138  (in the example of  FIGS. 1 and 2 ; other suitable conductive elements can be employed). The monitor electrical signal  28  typically serves as an input to a light source control circuit (not shown) that generates, modifies, conditions, or otherwise controls electrical input signal  26 . Typically, the monitor electrical signal  28  is coupled to such a control circuit in a suitable feedback arrangement for maintaining a desired output level for output optical signal  16 . The photodetector(s), light source(s), waveguides, and traces are typically positioned on a substrate  10 . The conductive wire leads can be employed to make electrical connections to additional circuit elements not on the substrate  10 . In some examples, such additional circuit elements can be positioned on a circuit board along with substrate  10 . Many other arrangements can be employed. 
     In  FIG. 2 , an unwanted input electrical signal  46  (i.e., a portion of the input electrical signal  26  that does not reach light source  116  but instead arrives at a different, unwanted location; the term “unwanted” shall refer to any portion of an electrical or optical signal that similarly does not reach its intended destination, but arrives instead at a different, unwanted location) is shown propagating from light source  116  or conductive traces/wires  126 / 136  to the signal photodetector  114  or conductive traces/wires  124 / 134 . Unwanted monitor electrical signal  48  is shown similarly propagating from monitor photodetector  118  (if present) or traces/wires  128 / 138  to signal photodetector  114  or traces/wires  124 / 134 . Either or both of those unwanted signals  46  or  48  can distort the output electrical signal  24 , by interfering with its generation by the signal photodetector  114  or with its transmission by traces/wires  124 / 134 . Unwanted output electrical signals  44  are also shown propagating from signal photodetector  114  or traces/wires  124 / 134  to light source  116 , traces/wires  126 / 136 , monitor photodetector  118 , or traces/wires  128 / 138 . Those unwanted signals  44  can interfere with the transmission of input or monitor electrical signals  26 / 28  by the corresponding traces/wires, with generation of monitor electrical signal  28  by monitor photodetector  118 , or with reception of input electrical signal  26  by light source  116 . Because the signal photodetector  114 , light source  116 , monitor photodetector  118 , and corresponding traces/wires are not intended to be directly connected to one another electrically, the propagation described above is typically radiative in nature and arises due to various capacitive or inductive electrical couplings between or among the signal photodetector  114 , light source  116 , monitor photodetector  118 , the corresponding traces/wires, a drive circuit for the light source  116 , or an amplification or filtering circuit for the photodetector  114 . Propagation of unwanted electrical signals  44 / 46 / 48  therefore can occur above, below, and through substrate  10 .  FIG. 2  is exemplary, and does not necessarily show every possible source of unwanted electrical signals or every possible unwanted arrival location of such signals. 
     Also in  FIG. 2 , unwanted output optical signal  36  is shown propagating from light source  116  toward signal photodetector  114 . Unwanted monitor optical signal  38  is shown similarly propagating from monitor photodetector  118  (if present) to signal photodetector  114 . Either or both of those unwanted signals  36  or  38  can interfere with reception of input optical signal  14  by the signal photodetector  114  (e.g., by themselves being received by the signal photodetector  114  and acting as unwanted background noise). Unwanted input optical signals  34  are also shown propagating from signal photodetector  114  to light source  116  and monitor photodetector  118 . Those unwanted signals  34  can interfere with generation of output optical signal  116  (e.g., by unwanted optical feedback into the light source  116 ) or with reception of monitor optical signal  18  by monitor photodetector  118 . Because the optical waveguides  104  and  106  are separate (i.e., not intended to be optically coupled) in the example of  FIGS. 1 and 2 , the propagation described above typically is not in any guided optical mode, but arises from various scattering or reflective elements, structures, or media near the signal photodetector  114 , the light source  116 , or monitor photodetector  118 , or from imperfect optical coupling between the waveguides  104 ,  106 , or  108  and the signal photodetector  114 , the light source  116 , or monitor photodetector  118 , respectively. Propagation of unwanted optical signals  34 / 36 / 38  therefore can occur above, below, or within substrate  10 .  FIG. 2  is exemplary, and does not necessarily show every possible source of unwanted optical signals or every possible unwanted arrival location of such signals. 
     Another example of a bidirectional optoelectronic device is shown in  FIGS. 3 and 4 , and is substantially similar to that of  FIGS. 1 and 2 , except that the input and output optical signals both propagate along a common optical waveguide  102 . An optical splitter/combiner  110  directs input optical signal  14  from optical waveguide  102  to propagate along optical waveguide  104 , and directs output optical signal  16  from optical waveguide  106  to propagate along optical waveguide  102  (in a direction opposite that of input optical signal  14 ). 
     The optical splitter/combiner  110  can comprise any element or combination of elements suitable for directing the input and output optical signals  14 / 16 . Splitter/combiner  110  can comprise a beamsplitter for free-space optical beams positioned between end faces of optical waveguides  102 ,  104 , and  106  (e.g., as disclosed in co-owned U.S. Pat. No. 7,031,575, U.S. Pat. No. 7,142,772, U.S. Pat. No. 7,366,379, U.S. Pat. No. 7,622,708, or U.S. Pub. No. 2010/0078547, each of which is hereby incorporated by reference), or can be implemented within joined waveguides  102 ,  104 , and  106  that do not provide any interval of free-space propagation of the optical signals (e.g., as disclosed in co-owned U.S. Pat. No. 7,330,619, U.S. Pat. No. 7,813,604, or U.S. Pub. No. 2010/0272395, each of which is hereby incorporated by reference). Similar elements or arrangements can be employed to split off a portion of output optical signal  16  to form monitor optical signal  18 . The splitter/combiner  110  can function on the basis of spectral separation of the optical signals (e.g., a dichroic beamsplitter or other filter, or a grating) or differential polarization of the optical signals, or on any other suitable basis for separating optical signals. 
     In  FIG. 4 , in additional to those unwanted signals already described for  FIG. 2 , several additional unwanted optical signals can result in cross-talk. Additional unwanted optical signals  34  and  36  can emanate from the optical splitter/combiner  110  as unguided signals, and can propagate toward the signal photodiode  114 , light source  116 , or monitor photodiode  118  (directly or as a result of scattering or reflection). In addition, because the optical waveguides  104  and  106  are both optically coupled to optical waveguide  102 , unwanted optical signals can arise that propagate along the waveguides as supported optical modes. Unwanted input optical signal  54  can propagate along waveguide  106  to light source  116 , or along optical waveguide  108  to monitor photodiode  118 . Similarly, unwanted output optical signal  56  can propagate along waveguide  104  to signal photodetector  114 . 
       FIGS. 2 and 4  are exemplary, in that they do not necessarily show every possible source of unwanted optical or electrical signals or every possible unwanted arrival location of such signals. In particular, no unwanted optical or electrical signals are shown propagating between the laser source  116  and the monitor photodetector  118 . Such unwanted signals can and often do occur, but are typically less problematic (in terms of cross-talk) than unwanted optical signals  34 / 36 / 38  or unwanted electrical signals  44 / 46 / 48 , because the light source  116  and the monitor photodetector  118  are intended to be coupled, optically via waveguide  108  and electrically through a light source control circuit (not shown). However, methods and apparatus disclosed below can be employed to mitigate the impact of any unwanted optical or electrical signals that might be present and give rise to cross-talk, not only those explicitly shown or described. 
       FIGS. 1 through 4  are also exemplary in that they only show a single light source  116  and a single signal photodetector  114 , i.e., the exemplary bidirectional devices would function as so-called diplexers. More generally, a bidirectional optoelectronic device can include any desired number of light sources or signal photodetectors, and such devices shall fall within the scope of the present disclosure or appended claims. Unwanted optical or electrical signals can emanate from any of those multiple light sources or photodetectors, and result in cross-talk by arriving at any other of the light sources or photodetectors. For example, a so-called triplexer is illustrated schematically in  FIG. 5  (with much detail and unwanted signals omitted for clarity) in which two independent input optical signals  14   a / 14   b  are received by corresponding signal photodetectors  114   a / 114   b  to generate corresponding output electrical signals  26   a / 26   b . Still more generally, in a general multi-channel optoelectronic device, a cross-talk penalty can arise for any input or output signal (electrical or optical) due to an unwanted portion of any other input or output signal (electrical or optical). 
     Optical cross-talk and electrical cross-talk described above tend to become more pronounced as the overall size of the multi-channel or bidirectional device shrinks. In particular, in a multi-channel or bidirectional device wherein the photodetector(s), light source(s), waveguides, and traces are all positioned on a common substrate that is 10 mm or less in its edge dimensions, the cross-talk can become large enough to substantially degrade performance of the device. For example, in a bidirectional device assembled on a 5 mm substrate  10  (e.g., wherein the light source and the photodetector are within 2 or 3 mm of one another), an electrical cross-talk penalty larger than 3 dB has been observed, and an optical cross-talk penalty larger than 3 dB has been observed. 
     Regardless of the particular mechanism or combination of mechanisms that result in electrical or optical cross-talk, it would be desirable to reduce the electrical or optical cross-talk by suitable arrangements or adaptations of the bidirectional device. 
     Light Source Drive Circuit 
     A conventional bidirectional optoelectronic device is illustrated in the functional block diagram of  FIG. 6  and comprises a photodetector  114 , a light source  116 , and a drive circuit  150  for the light source  116 . The photodetector  114  is arranged to receive an input optical signal  14  modulated to encode first transmitted information and to generate in response an output electrical signal  24  modulated to encode the first transmitted information. The photodetector  114  can comprise a p-i-n photodiode, an avalanche photodiode, or any other suitable photodetector. A p-i-n photodiode  114  is shown in the example of  FIG. 6 , connected to circuit  115  and reverse-biased by voltage V PD . The output electrical signal  24  is shown passing through circuit  115 , which can comprise a single resistor in series with the photodiode  114 , or can comprise any suitable filtering, impedance matching, amplification, or other active or passive circuitry (e.g., a transimpedance amplifier and associated components and voltage supply). 
     The light source  116  of the conventional bidirectional optoelectronic device is arranged to receive an input electrical signal  26  from a unipolar signal source  23  that is modulated to encode second transmitted information and to generate in response an output optical signal  16  modulated to encode the second transmitted information. The light source  116  has first and second electrical input leads  126   a / 126   b , and in the example of  FIG. 6  comprises a laser diode  116 . The light source  116  receives the input electrical signal  26  applied by a drive circuit  150  at electrical input lead  126   a , and is connected to an applied voltage V LS  by drive circuit  150  at the electrical input lead  126   b . The input electrical signal  26  may or may not include a DC offset, and can be AC coupled or DC coupled to the light source  116  via circuits  152  and  154 . The circuits  152  and  154  can each comprise a simple connection or diode arranged for DC coupling the input electrical signal  26 , a single capacitor arranged for AC coupling the input electrical signal  26 , or can comprise any suitable filtering, impedance matching, amplification, or other active or passive circuitry for applying the input electrical signal  26  to the light source  116 . An operational amplifier AMP LS , feedback impedance Z LS , transistor T LS , and resistor R LS  are shown in the example of  FIG. 6  and comprise a control circuit for regulating the average optical power emitted by the laser diode  116 . Control and monitor voltages V con  and V mon , respectively, are applied to operational amplifier AMP LS  to regulate the average current flow through transistor T LS  and hence through laser diode  116 . The control and monitor voltages can be generated in any suitable way (e.g., by monitoring the average power of output optical signal  16 , or by monitoring the average power of the input electrical signal  26 ). Many suitable arrangements are known and can be employed for regulating the average current flow through laser diode  116 , or such regulation can be omitted altogether if desired (and is omitted from subsequent figures). 
     An exemplary bidirectional optoelectronic device according to the present disclosure is illustrated in the functional block diagram of  FIG. 7  and comprises a photodetector  114 , a light source  116 , and a drive circuit  250  for the light source  116 . The photodetector  114  is arranged (i) to receive an input optical signal  14  modulated to encode first transmitted information and (ii) to generate an output electrical signal  24  modulated to encode the first transmitted information. The output electrical signal  24  can comprise a baseband amplitude-modulated digital signal; other suitable modulation schemes or carrier frequencies can be employed. The photodetector  114  can comprise a p-i-n photodiode, an avalanche photodiode, or any other suitable photodetector. A p-i-n photodiode  114  is shown in the example of  FIG. 7 , connected to circuit  115  and reverse-biased by voltage V PD . The output electrical signal  24  is shown passing through circuit  115 , which can comprise a single resistor in series with the photodiode  114 , or can comprise any suitable filtering, impedance matching, amplification, or other active or passive circuitry (e.g., a transimpedance amplifier and associated components and voltage supply). 
     The light source  116  of the bidirectional optoelectronic device is arranged (i) to receive an input electrical signal as inverted replica signals  26   a / 26   b  from a bipolar signal source  25  that is modulated to encode second transmitted information and (ii) to generate in response an output optical signal  16  modulated to encode the second transmitted information. The bipolar input electrical signals  26   a / 26   b  shall be referred to collectively as input electrical signal  26 . The input electrical signal  26  can comprise a baseband amplitude-modulated digital signal; other suitable modulation schemes or carrier frequencies can be employed. The light source  116  has first and second electrical input leads  126   a / 126   b , and in the example of  FIG. 7  comprises a laser diode  116 . The light source  116  receives the input electrical signal  26  as first and second portions  27   a / 27   b  applied by a drive circuit  250  at first and second electrical input leads  126   a  and  126   b , respectively, and a supply voltage V LS  is applied through drive circuit  250  at the second electrical input lead  126   b . The input electrical signal  26  may or may not include a DC offset, and can be AC coupled or DC coupled to the light source  116  via drive circuit  250  (as first and second input electrical signal portions  27   a / 27   b ). The circuit  250  can comprise any of myriad arrangements contrived by those skilled in the electrical or electronic arts for receiving input electrical signal  26  and in response applying first and second portions  27   a / 27   b  to the electrical input leads  126   a / 126   b , respectively, as described below, and the present disclosure and appended claims shall be construed as encompassing any circuit arrangement that operates as disclosed herein. Circuit  250  can include any suitable passive components, active components, voltage or current supplies, filtering circuitry, impedance matching circuitry, amplification circuitry, or other active or passive circuitry for receiving input electrical signal  26  and in response applying first and second portions  27   a / 27   b  to the electrical input leads  126   a / 126   b . Drive circuit  250  can further include additional control or regulating circuitry for controlling the average current flowing through (and hence average optical output power from) light source  116  (as in  FIG. 6 ), however, such circuitry is omitted from the drawings for clarity. 
     The light source  116  has first and second electrical input leads  126   a / 126   b  for receiving respective portions  27   a / 27   b  of the input electrical signal  26 . While the exemplary embodiments of the Figures show the input electrical signal  26  as a bipolar signals  26   a / 26   b , drive circuit  250  can also be implemented to receive a unipolar input electrical signal  26  and generate first and second portions  27   a / 27   b  by methods known in the electrical or electronic arts, and such implementations utilizing a unipolar input signal shall fall within the scope of the present disclosure or appended claims. As described above, the drive circuit  250  is arranged to apply the first portion  27   a  of the input electrical signal  26  to the first electrical input lead  126   a  of the light source  116  and to apply the second portion  27   b  of the input electrical signal  26  to the second electrical input lead  126   b  of the light source  116 . The drive circuit  250  is arranged so that the second portion  27   b  of the input electrical signal is a scaled, inverted substantial replica of the first portion  27   a , as illustrated schematically in  FIG. 8 . The input electrical signal  26  may or may not include a DC offset (none shown in the example of  FIG. 8 ). The portions  27   a / 27   b  of the input electrical signal  26  that are applied to the electrical input leads  126   a / 126   b  can each include a DC offset, which can be derived from the DC offset of the input electrical signal  26   a / 26   b  (if present therein), can be added or altered (at a suitable level) by the drive circuit  250 , or can differ from one another. 
     In an exemplary embodiment, the light source  116  comprises a semiconductor light source, typically a laser diode. In such an embodiment, the first electrical input lead  126   a  comprises a cathode of the laser diode  116 , and the second electrical input lead  126   b  comprises an anode of the laser diode  116 . In a conventional laser drive circuit (such as that shown in  FIG. 6 ), typically the unipolar input electrical signal  26  is applied only to the cathode of the laser diode  116  (via input lead  126   a ). The voltage of the laser diode cathode follows the temporal variations of the input electrical signal, while the voltage at the laser diode anode (input lead  126   b ) typically varies with substantially less amplitude. In many conventional laser drive circuits the laser diode anode is in fact RF-grounded by a relatively large capacitance to ground in parallel with and near to the connection to V LS  (e.g., in  FIG. 6  circuit  154  would comprise a single capacitor). In contrast, the drive circuit  250  (shown in  FIG. 7 ) is arranged to deliver inverted, scaled replicas  27   a / 27   b  of the input electrical signal  26  to the electrical input leads  126   a / 126   b  of the laser diode  116 . In the device of  FIG. 7 , voltages at both the laser diode anode and cathode vary according to the input electrical signal  26 , but with smaller and opposite amplitudes (as signals  27   a / 27   b ); the overall voltage drop across laser diode  116  driven by drive circuit  250  can be similar to that driven by conventional drive circuit  150 . It has been observed that the bidirectional optoelectronic device, arranged according to the present disclosure with drive circuit  250 , exhibits lower electrical cross-talk than the conventional bidirectional device with drive circuit  150  (all other factors being equal, e.g., the amplitude of the electrical input signals, the types and relative positions of the photodetectors and light sources, and so forth; i.e., differing with respect to only the bipolar versus unipolar modulation of laser diode  116 ). 
     The reduction in electrical cross talk noted above is typically observed or operationally significant only under certain conditions, typically when the laser diode  116  and the photodetector  114  are positioned sufficiently close together. For example, in a bidirectional optoelectronic device (arranged as in  FIG. 6 ) in which the photodetector  114  (a p-i-n photodiode) and the light source  116  (a laser diode) were positioned on a common substrate about 4 to 5 mm apart, a cross-talk penalty of only about 0.5 dBm due to operation of the laser diode was observed, which is typically tolerable under many common operational scenarios. However, in another example, a bidirectional optoelectronic device (arranged as in  FIG. 6 ) in which the photodetector  114  (a p-i-n photodiode) and the light source  116  (a laser diode) were positioned on a common substrate about 2 mm apart, more significant degradation of photodetector performance due to operation of the laser diode was observed. Typically, a cross-talk penalty of about 4-5 dBm was observed. 
       FIG. 9  illustrates a simplified portion of an exemplary drive circuit  250 , in which a resistor network comprising R 19 , R 77 , R 78 , and R 88  is employed to yield electrical signal portions  27   a / 27   b  from input electrical signal  26 . R 77  and R 78  form a voltage divider that is grounded between its resistors, and the signal portions  27   a / 27   b  pass through resistors R 19  and R 88 , respectively. As the values of R 19  and R 88  are varied (keeping R 19 +R 88 ≈22Ω), the signal portions  27   a / 27   b  are inverted substantial replicas of one another with a relative absolute scale factor that varies and goes through a minimum scale factor near unity (when R 19 =R 88 =11Ω). That symmetric arrangement yields a cross-talk penalty of less than about 3 dBm when the input electrical signal is applied to the drive circuit  250  and light source  116 , an improvement from the 4-5 dBm cross-talk penalty observed for photodetector  114  during operation of drive circuit  150  and light source  116 . 
     The symmetric arrangement of  FIG. 9  does not, however, yield the greatest improvement over the arrangement of  FIG. 6 . An arrangement of drive circuit  250  with R 19 =12Ω and R 88 =9Ω yields a cross-talk penalty of only about 2.5 dBm, which appears to be the minimal cross-talk penalty achievable for the exemplary arrangement of  FIG. 9 . In that arrangement, the amplitude of voltage modulation of the laser anode (lead  126   b ) appears to be somewhat larger than the voltage modulation of the laser cathode (lead  126   a , modulation inverted relative to that of the anode). For a given laser diode or other laser source  116 , photodiode or other photodetector  114 , spatial arrangement of the light source and photodetector, and particular arrangement of drive circuit  250 , the relative amplitudes (i.e., scale factor) of the signal portions  27   a / 27   b  can be optimized to achieve a minimal cross-talk penalty when the input electrical signal is applied to the light source  116  through drive circuit  250  (illustrated in the plot of  FIG. 10 ). 
       FIG. 11  illustrates a simplified portion of another exemplary drive circuit  250 , similar to that of  FIG. 9  but with additional details shown including RF equivalents of a bias circuit for the laser diode  116  (R 34 / 35 , C 30 / 31 , L 10 / 11 ). As in the example of  FIG. 9 , a resistor network comprising R 13 , R 16 , R 32 , and R 33  is employed to yield electrical signal portions  27   a / 27   b  from input electrical signal  26 . R 32  and R 33  form a voltage divider that is grounded between its resistors, and the signal portions  27   a / 27   b  pass through resistors R 13  and R 16 , respectively. As the values of R 13  and R 16  are varied (keeping R 13 +R 16 ≈18Ω), the signal portions  27   a / 27   b  are inverted substantial replicas of one another with a relative absolute scale factor that varies and goes through a minimum scale factor near unity (when R 13 =R 16 =9Ω). That symmetric arrangement yields a cross-talk penalty of less than about 0.3 dB when the input electrical signal is applied to the drive circuit  250  and light source  116 , an improvement from a cross-talk penalty of about 0.8-1.0 dB observed for photodetector  114  during operation of drive circuit  150  and light source  116  (arranged substantially as in  FIG. 11  except for unipolar driving of the light source  116 ). 
     Again, the symmetric arrangement of  FIG. 11  does not yield the greatest improvement over the arrangement of  FIG. 6 . An arrangement of drive circuit  250  with R 13 =11Ω and R 16 =6.8Ω yields a cross-talk penalty of only about 0.1-0.2 dB, which appears to be the minimum “cross-talk penalty” achievable for the exemplary arrangement of  FIG. 11  (as shown in the plot of  FIG. 12 ). In that arrangement, the amplitude of voltage modulation of the laser anode (lead  126   b ) appears to be somewhat larger than the voltage modulation of the laser cathode (lead  126   a , modulation inverted relative to that of the anode). For a given laser diode or other laser source  116 , photodiode or other photodetector  114 , spatial arrangement of the light source and photodetector, and particular arrangement of drive circuit  250 , the relative amplitudes (i.e., scale factor) of the signal portions  27   a / 27   b  can be optimized to achieve a minimal decrease on photodetector sensitivity when the input electrical signal is applied to the light source  116  through drive circuit  250 . 
       FIGS. 13 and 14  illustrate exemplary drive circuits  250  and laser diode  116  in more detail. The exemplary drive circuit  250  of  FIG. 13  includes AC coupling of the input electrical signal  26  to the laser diode  116 , and can be optimized for minimal cross-talk penalty (i.e., for maximal photodiode sensitivity during application of the input electrical signal  26  to the laser diode  116 ) by varying the values of, e.g., resistors R 3  and R 4 . Alternatively, R 7  and R 8 , R 1  and R 2 , or various combinations of the three resistor pairs can be varied to achieve a minimal cross-talk penalty. If varying R 1 /R 2  or R 7 /R 8 , then the associated reactive elements may need to be varied as well to maintain adequate phase matching of the laser anode and cathode voltages over the relevant frequency range. The exemplary drive circuit  250  of  FIG. 14  includes DC coupling of the input electrical signal  26  to the laser diode  116 , and can be optimized for minimal cross-talk by varying the values of, e.g., resistors R 1  and R 2 . In that case, reactive elements (e.g., C 1 /C 2  or L 1 /L 2 ) may need to be varied as well in order to maintain adequate phase matching of the laser anode and cathode voltages over the relevant frequency range. 
     It is reiterated that the embodiments of  FIGS. 7 ,  9 ,  11 ,  13 , and  14  are exemplary, and many other circuits can be made that have more or fewer elements, or differing arrangements of elements, that nevertheless fall within the scope of the present disclosure or appended claims. In particular, some elements of the exemplary embodiments are optional and their presence is not necessarily required (e.g., diode D 2  or inductors L 4  and L 5  in  FIG. 8 ; diodes D 2  and D 3  in  FIG. 9 ). 
     Light-Trapping Structure 
     A common configuration for an optoelectronic device includes a substrate  10  on which are formed one or more optical waveguides, and at least one light source mounted on the substrate and positioned to launch at least a portion of its optical output signal into an optical waveguide on the substrate. The optical signal thus launched propagates along the optical waveguide in a corresponding guided optical mode that is substantially confined in two transverse dimensions. 
     The optical waveguides typically are formed in one or more layers of suitable core or cladding materials grown, deposited, or otherwise formed on the substrate  10 ; those layers can be referred to collectively as optical waveguide layers  20 . The substrate  10  acts as a structural support for the optical waveguide layers  20 . Spatially selective processing of one or more of the optical waveguide layers  20  (by deposition, removal, or alteration of material) defines the optical waveguides; those processed layers (or processed regions of those layers) often act as waveguide cores having a refractive index somewhat higher than surrounding layers, which act as waveguide cladding. A typical waveguide substrate includes regions having only cladding layers and regions having one or more core layers in addition to cladding layers. In some examples of substrates having multiple-core waveguides, distinct regions can have differing numbers of core layers present, with the waveguide typically being defined by those regions where all core layers are present. Many other core/cladding configurations can be employed within the scope of the present disclosure. 
     It is often the case that optical coupling between the light source  116  and the optical waveguide  106  is imperfect, and that part of the optical signal emitted by the light source does not propagate in a guided optical mode as output optical signal  16 , but instead escapes into the surroundings as unwanted optical signal  36 . A certain fraction of that escaped, stray optical signal propagates in one or more of the optical waveguide layers  20 , but without confinement by any of the optical waveguides in their corresponding optical modes. The stray optical signal that propagates in the optical waveguide layers can potentially interfere with or disrupt the performance of other optical components on the waveguide substrate, including optical detectors or other light sources. In particular, as noted above, in a multi-channel or bidirectional optoelectronic device (e.g., a bidirectional optoelectronic transceiver), the stray optical signal emitted by the light source and propagating in the optical waveguide layers can interfere with reception of an incoming optical signal  14  by a photodetector  114 , decreasing the sensitivity of the photodetector  114  to the incoming optical signal  14  in the presence of the stray optical signal  36  (often described or quantified as the so-called “cross-talk penalty”). 
     One way to decrease the negative impact of stray optical signals on the performance of an optoelectronic device is to provide a light-blocking or light-trapping structure on the waveguide substrate  10  or in the waveguide layers  20 . Some examples of such structures are disclosed in: 
     U.S. Pat. No. 6,418,246 entitled “Lateral trenching for cross coupling suppression in integrated optical chips” issued Jul. 9, 2002 to Gampp; 
     U.S. Pat. No. 6,959,138 entitled “Planar optical waveguide” issued Oct. 25, 2005 to Steenblik et al; 
     U.S. Pat. No. 7,221,845 entitled “Planar optical waveguide” issued May 22, 2007 to Steenblik et al; 
     U.S. Pat. No. 7,276,770 entitled “Fast Si diodes and arrays with high quantum efficiency built on dielectrically isolated wafers” issued Oct. 2, 2007 to Goushcha et al; 
     U.S. Pat. No. 7,530,693 entitled “Single MEMS imager optical engine” issued May 12, 2009 to Mihalakis; 
     U.S. Pat. Pub. No. 2002/0137227 entitled “Chemiluminescent gas analyzer” published Sep. 26, 2002 in the name of Weckstrom; 
     U.S. Pat. Pub. No. 2004/0151460 entitled “Deep trenches for optical and electrical isolation” published Aug. 5, 2004 in the names of Kitcher et al; 
     U.S. Pat. Pub. No. 2005/0105842 entitled “Integrated optical arrangement” published May 19, 2005 in the names of Vonsovici et al; 
     U.S. Pat. Pub. No. 2008/0019652 entitled “Planar optical waveguide” published Jan. 24, 2008 in the names of Steenblik et al; and 
     U.S. Pat. Pub. No. 2009/0080084 entitled “Beam dump for a very-high-intensity laser beam” published Mar. 26, 2009 in the names of Pang et al. 
       FIGS. 15 through 20  illustrate schematically an improved light-trapping structure (i.e., one or more light collectors and one or more light traps) formed on a waveguide substrate  10  or optical waveguide layers  20  thereon. 
     In  FIGS. 15 and 16 , an optical waveguide  106  of any suitable type or configuration is formed in optical waveguide layers  20  on a waveguide substrate  10 . The optical waveguide layers  20  and the waveguide substrate  10  can comprise any of myriad suitable materials while remaining within the scope of the present disclosure or appended claims. In a common implementation, substrate  10  comprises silicon, and the waveguide layers  20  can include one or more of silica, doped silica, silicon nitride, or silicon oxynitride. Some suitable examples of optical waveguides are disclosed in co-owned U.S. Pat. No. 6,975,798; 7,136,564; 7,164,838; 7,184,643; 7,373,067; 7,394,954; 7,397,995; or 7,646,957, or co-owned Pub. No. 2010/0092144, which are hereby incorporated by reference. 
     A light source  116  is positioned on substrate  10  or on one or more of the waveguide layers  20 , and is positioned to launch an optical signal (or at least a first fraction  16  of the optical signal, referred to hereafter as the launched optical signal  16 ) to propagate along optical waveguide  106  as a guided optical mode substantially confined by the waveguide  106  in two transverse dimensions. A second, stray fraction  36  of the optical signal (referred to hereafter as the stray optical signal  36 ) propagates from the light source  116  in the optical waveguide layers  20  without confinement by the waveguide  106  in the guided optical mode. The light source  116  can comprise any source of optical signals  16  or  36 , including but not limited to: a laser diode or light-emitting diode, an optical fiber, another optical waveguide on a separate substrate, or a beamsplitter or tap, any of which can be formed or mounted on the substrate  10  or waveguide layers  20 . 
     Without any light-trapping structure, the stray fraction  36  of the optical signal could propagate through the optical waveguide layers  20  and potentially interfere with or disrupt the performance of other optical components on the substrate  20 .  FIGS. 15 and 16  illustrate schematically a light-trapping structure that includes light collectors  310   a / 310   b / 310   c  (referred to generically as light collector  310   x  or collectively as light collectors  310 ) and a light trap  320 . Although three light collectors  310  and one light trap  320  are shown in the exemplary embodiment of the drawings, any suitable number of one or more light collectors or one or more light traps can be employed within the scope of the present disclosure or appended claims. Each light collector  310   x  comprises one or more lateral surfaces  312  of the optical waveguide layers  20  and a substantially opaque coating  330  on the lateral surfaces  312  (FIGS.  17 A/ 18 A/ 19 A). Each light trap  320  comprises one or more lateral surfaces  322  of the optical waveguide layers  20  and a substantially opaque coating  330  on the lateral surfaces  322  (FIGS.  17 B/ 18 B/ 19 B). The lateral surfaces  312 / 322  typically are substantially perpendicular to the substrate  10  and optical waveguide layers  20 , i.e., they are substantially vertical relative to the horizontal substrate  10 . The designations horizontal and vertical are relative and are not intended to designate absolute spatial orientation. Although FIGS.  17 A/ 18 A/ 19 A show the lateral surface  312  formed near a waveguide  106  (as is the case for light collector  310   a , for example), a light collector  310   x  can be formed in any suitable location on substrate  10 , including locations away from any waveguide (which would therefore resemble FIGS.  17 B/ 18 B/ 19 B). Likewise, although FIGS.  17 B/ 18 B/ 19 B show the lateral surface  322  formed away from any waveguide, a light trap  320  can be formed in any suitable location on substrate  10 , including locations near a waveguide  106  (which would therefore resemble FIGS.  17 A/ 18 A/ 19 A). 
     The stray optical signal  36  propagates from light source  116  within the optical waveguide layers  20 , encounters a lateral surface  312  and its substantially opaque coating  330 , and is prevented from propagating further in that direction. The coating  330  typically absorbs a fraction of the incident light and reflects the rest. The lateral surface  312  of each light collector  310   x  is arranged to direct the reflected portion of the stray optical signal toward the light trap  320  (either directly or after redirection by another light collector  310   x ). 
     The lateral surfaces  322  and substantially opaque coatings  330  of the light trap  320  define a corresponding spiral region of the optical waveguide layers  20 . That spiral region includes an open mouth  324  and a closed end  326 . Portions of the stray optical signal  36  that propagate in the optical waveguide layers  20  into the open mouth  324  are repeatedly reflected from the surface  322  and coating  330  further into the spiral region until reaching the closed end  326  (as shown in  FIG. 16 ). Typically, upon each reflection a portion of the stray optical signal  36  is absorbed and the rest is reflected. The spiral region can be arranged in any suitable way, and typically subtends an arc greater than about 180°. In some embodiments, the spiral region can be a cornuate spiral region (i.e., a tapered, horn-shaped spiral that tapers toward the closed end  326 ). 
     The substantially opaque coating  330  typically is arranged to exhibit optical absorption over the operational wavelength range of the light source  330 , to effect attenuation of the stray optical signal  36  as it is repeatedly reflected from lateral surfaces  312 / 322 . A metal coating can often be employed to provide substantial opacity and a suitable degree of optical absorption. In one example, chromium or titanium can be employed over an operational wavelength range of about 1200-1700 nm; any other suitable metal usable over any other suitable wavelength range shall fall within the scope of the present disclosure or appended claims. A thickness of coating  330  greater than about 150 nm can typically provide a sufficient degree of opacity, and larger thicknesses can be employed to ensure adequate opacity. In one example, wherein a chromium or titanium layer is deposited on the lateral surfaces  312 / 322  of optical waveguide layers  20  that comprise, e.g., silica, silicon nitride, or other dielectric materials of similar refractive index, about 45% of the incident stray optical signal  36  is absorbed and about 55% of the stray optical signal  36  is reflected. Each ray representing the stray optical signal  36  undergoes 4 to 6 or more reflection before reaching the closed end  326  of the light trap  320 , so that only about 3% (after 6 reflections) to about 9% (after 4 reflections) of the original optical power remains in the stray optical signal  36  upon reaching the closed end  326  of the light trap  320 . At that low level the stray optical signal  36  is less likely to interfere with or disrupt operation of other optical devices on the substrate. If upon additional reflections a portion of the stray optical signal  36  reemerges from the light trap through its open mouth  324 , it typically would be attenuated to a substantially negligible level (e.g., less than about 1% or even less than about 0.1%). 
     A reflection suppressing layer (i.e., anti-reflection coating) can be employed as a portion of coating  330 , between the lateral surface  312 / 322  and the metal absorbing layer. Reduction of the amount of light reflected (and concomitant increase in the amount absorbed) upon each encounter with a surface  312 / 322  enhances the attenuation of the stray optical signal  36  as it repeatedly encounters surfaces  312 / 322 . Any suitable reflection suppression layer or anti-reflection coating can be employed. Some examples are disclosed in co-owned Pub. No. US 2006/0251849, which is hereby incorporated by reference. 
     The lateral surfaces  312 / 322  in the examples of FIGS.  17 A/ 18 B,  18 A/ 18 B, and  19 A/ 19 B, are shown extending through the entirety of the optical waveguide layers  20  but not extending into the waveguide substrate  10 . Other suitable depths can be employed within the scope of the present disclosure or appended claims. It is typically preferable for the lateral surface  312 / 322  to extend through the entirety of the optical waveguide layers  20 . The lateral surfaces  312 / 322  can extend into the waveguide substrate  10 . It may often occur that the optical waveguide layers  20 , waveguide  106 , surfaces  312 / 322 , and coating  330  are formed or deposited on a wafer scale to fabricate light collectors and traps on many waveguide substrates simultaneously. The lateral surfaces  312 / 322  can be formed during such wafer-scale fabrication, e.g., by any suitable dry or wet etch process, typically by etching one or more trenches into the optical waveguide layers  20  (and perhaps extending into the substrate  10 , as noted above). 
     As shown in the exemplary arrangements of FIG.  17 A/ 17 B,  18 A/ 18 B, or  19 A/ 19 B, differing arrangements for the layer  330  can be employed. In the arrangement of FIGS.  17 A/ 17 B, the coating  330  overlies only the lateral surface  312 / 322 . Practically, that may be all that is needed, but also practically, that arrangement can be difficult to achieve, particularly using standard lithographic deposition techniques to form light collectors and traps on many waveguide substrates simultaneously. Conformal (i.e., non-directional) deposition techniques are not well-suited for selective coverage of only surfaces of a particular orientation, and directional deposition techniques are not well-suited for selective coverage of only vertical surfaces. The arrangement of FIGS.  18 A/ 18 B can be the easiest to achieve, by simply coating all, or nearly all, of the exposed surface of the waveguide substrate  10  and optical waveguide layers  20 . That approach can be employed if there is no reason to avoid the presence of coating  330  over the waveguide  106  or other portions of the waveguide substrate  10  or optical waveguide layers  20 , and if a deposition can be employed that is at least somewhat conformal. An intermediate approach is illustrated by the exemplary arrangement shown in FIGS.  19 A/ 19 B, in which the coating  330  extends partly across horizontal surfaces of waveguide substrate  10  or optical waveguide layers  20 . Portions of the substrate  10  or waveguide layers  20  can be masked to prevent deposition of the coating  330  onto areas where it would be undesirable. 
     Differing arrangements of the optical waveguide  106  and the optical waveguide layers  20  are shown in the exemplary arrangements of FIG.  17 A/ 17 B,  18 A/ 18 B, or  19 A/ 19 B. Any of the waveguide arrangements in those examples can be employed in any combination with any of the arrangements shown for coating  330  in those examples. In the example shown in FIGS.  17 A/ 17 B, the optical waveguide  106  comprises a single, higher-index core between top and bottom lower-index cladding layers. A lateral surface  312  is shown near the waveguide  106  in  FIG. 17A , while only the two cladding layers are present near the lateral surface  322  shown in  FIG. 17B . In the example shown in FIGS.  18 A/ 18 B, the optical waveguide  106  comprises a pair of higher-index cores and top, middle, and bottom lower-index cladding layers. A lateral surface  312  is shown near the waveguide  106  in  FIG. 18A , while only the three cladding layers are present near the lateral surface  322  shown in  FIG. 18B . In the example shown in FIGS.  19 A/ 19 B, the optical waveguide  106  comprises one higher-index core and two higher-index core layers, and the cladding comprises top, upper middle, lower middle, and bottom lower-index cladding layers. A lateral surface  312  is shown near the waveguide  106  in  FIG. 19A , while the four cladding layers and the two core layers (without the core) are present near the surface lateral  322  shown in  FIG. 19B . Boundaries are shown between the various cladding layers to indicate where cladding deposition is interrupted to allow deposition or patterning of an intervening core or core layer, however, such boundaries may or may not be readily apparent in the finished device, particularly if the same material is used for the different cladding layers. 
     In the exemplary embodiment of  FIGS. 15 and 16 , a first light collector  310   a  is curved so as to reflect and redirect a portion of the stray optical signal  36  that diverges from the light source  116  to converge toward the open mouth  324  of the light trap  320 . The collector  310   a  can, for example, approximate a portion of an ellipse with the light source  116  positioned near one focus of the ellipse and the open mouth  324  of the light trap  320  positioned near the other focus of the ellipse. The arrangement of light collector  310   a  is only exemplary; other arrangements of a curved light collector surfaces can be employed. 
     Also in the exemplary embodiment of  FIGS. 15 and 16 , light collectors  310   b  and  310   c  have one or more flat surfaces  312  that are arranged to redirect the stray optical signal  36  (one such surface  312  for light collector  310   b ; three distinct flat segments for light collector  310   c ). Those various flat surfaces  312  are arranged to redirect, by two or more successive reflections, a portion of the stray optical signal  36  into the mouth  324  of light trap  320 . The arrangement of light collectors  310   b  and  310   c  are only exemplary; other arrangements of flat light collector surfaces can be employed. 
     To further reduce the amount of the stray optical signal  36  that avoids the light collectors and light traps, the optical waveguide  106  can include a curved segment. The optical waveguide can pass, before its curved segment, between the light collectors  310   a  and  310   b . The waveguide can pass, after its curved segment, between the light collector  310   a  and the open mouth  324  of the light trap  320 . The light collector  310   b  is arranged so as to substantially block substantially all straight-line propagation paths from the light source  116  through the optical waveguide layers  20  that lie between the first light collector  310   a  and the mouth  324  of the light trap  320 . 
     Light collectors and light traps disclosed herein can be employed in a wide variety of optoelectronic devices that are realized using an optical waveguide on a waveguide substrate. One such example is illustrated schematically in  FIG. 20 , and includes beamsplitters  110  and  111  and photodetectors  114  and  118 . The light source  116  emits a launched optical signal  16  that propagates along optical waveguide  106 . A portion  18  is split off by beamsplitter  111  and directed to photodetector  118 . The electrical signal from photodetector  118  can be employed, e.g., for feedback control of the light source  116 . The remainder of the launched optical signal  16  propagates along optical waveguide  106  until it leaves the device. An incoming optical signal  14  entering the device propagates along waveguide  106  until it is directed to photodetector  114  by beamsplitter  110 . Performance of either or both of the photodetectors  114 / 118  can be affected by the stray optical signal  36  propagating in the optical waveguide layers  20 ; those effects can be reduced or eliminated by the presence of light collectors  310  and light trap  320 . The beamsplitters  110 / 111  can be implemented in any suitable way while remaining within the scope of the present disclosure or appended claims. A waveguide beamsplitter or tap can be employed (e.g., as disclosed in co-owned patents and publications already incorporated by reference). Alternatively, the optical waveguide  106  can include a gap across which an optical signal  14 ,  16 , or  18  can propagate as a free-space optical beam (i.e., unguided) between segments of the waveguide. A beamsplitter can be inserted between the waveguide segments for directing various free-space optical signals to propagate along other waveguides (e.g., as disclosed in co-owned patents and publications already incorporated by reference). It should be noted that beamsplitters  110 / 111 , however implemented, can themselves act as a light source  116  and as a source of a stray optical signal  36 . It can be desirable to provide one or more light collectors  310  or light traps  320  to reduce propagation of stray optical signals arising from a beamsplitter for an optical waveguide, and such implementations shall fall within the scope of the present disclosure or appended claims. 
     Multi-Function Encapsulation 
     In an exemplary embodiment of a multi-channel or bidirectional optical device according to the present disclosure, a multi-purpose encapsulant  500  is employed to encapsulate the multi-channel or bidirectional optoelectronic device, including one or more signal photodetectors  114 , one or more light sources  116 , one or more monitor photodetectors  118  (if present), optical waveguides  102  (if present),  104 ,  106 , and  108  (if present), conductive traces  124 ,  126 , and  128  (if present), and conductive wire leads  134 ,  136 , and  138  (if present). If other optical or electrical elements are employed instead of or in addition to those listed, those can be encapsulated as well (or instead). 
     One purpose of the encapsulant  500  is to provide chemical and mechanical protection for the photodetector(s), light source(s), waveguides, and electrical connections of the bidirectional device; encapsulant  500  can therefore be referred to as a protective encapsulant. The components of device can be relatively delicate, can be deployed in relatively hostile environments (large temperature swings, high humidity, and so on), or can endure rough handling or treatment during installation or while deployed. For one or more of those reasons, it has been conventional to encapsulate the delicate portions of such devices. Typically, the protective encapsulant can comprise a suitable polymer (e.g., silicone, epoxy, or polyurethane polymer; an optically transparent polymer can be preferred in some instances), which is applied in its uncured form (typically liquid or semi-liquid) onto the substrate  10  on which are positioned the components of the bidirectional device (as in the side elevation view of a bidirectional device illustrated schematically in  FIG. 21 , for example; much structural detail and all signals are omitted for clarity). If the substrate  10  is mounted on another, larger substrate or circuit board, the encapsulant can extend beyond substrate  10  onto that other substrate or board. The encapsulant  500  can be selected on the basis of a variety of its properties, depending on the nature of the bidirectional device and the intended deployment environment. The uncured encapsulant is preferably sufficiently fluid to substantially fill in the topography of the bidirectional device (e.g., to fill spaces between components, to completely flow around wire leads, etc.), sufficiently viscous to remain in place during application and curing, sufficiently hard after curing to provide adequate mechanical support and protection, sufficiently soft after curing so that thermal expansion or contraction does not unduly stress or even break the device or any of its components (including, e.g., interconnections such as wires), and chemically resistant to a suitable array of substances likely to be encountered in the use environment (e.g., water vapor in humid environments). Examples of suitable encapsulants include, but are not limited to, silicone rubbers, gels, epoxies, or polyurethanes. 
     The encapsulant  500  can further comprise an optical absorber. The absorber can be any substance that can be mixed into the encapsulant formulation without substantially disrupting the suitability of its physico-chemical properties (before or after curing). The absorber can be dissolved, suspended, or otherwise dispersed in the uncured encapsulant (and remain there during application and after curing), and absorbs light at one or more wavelengths of optical signals  14  and  16 . As a result of mixing such an absorber into the encapsulant  500 , portions of unwanted optical signals  34 / 36 / 38  that propagate above the substrate  10  (i.e., within the encapsulant  500 ) are attenuated. A protective encapsulant  500  that includes an optical absorber therefore can act to reduce optical cross-talk arising from those unwanted optical signals. 
     A suitably chosen dye can be dissolved in the encapsulant  500  to act as an optical absorber. Instead or in addition, insoluble particles  510  can be suspended in the encapsulant to act as the optical absorber ( FIGS. 22 and 24 ). Examples of suitable particles can include carbon particles (e.g., carbon black, lamp black, or acetylene black), mineral pigments (e.g., black ferrites or hematites, black spinels, cobalt black, manganese black, mineral black, or black earth), metal particles, or semiconductor particles, preferably having a mean particle size between about 0.01 μm and about 50 μm. A preferred example includes carbon black particles having an average particle size between about 20 μm and about 30 μm, and comprising between about 0.1% and about 2% of the encapsulant composition by weight. The absorber (whatever its type or composition) can be present in an amount to yield an extinction coefficient κ (over an operationally relevant wavelength range) between about 1-5 cm −1  and about 200 cm −1  (where the absorption coefficient κ is defined by transmitted optical power divided by incident optical power being equal to e −κL , where L is an optical path length through the encapsulant). Reductions in optical cross-talk penalty between about 1 dB and about 5 dB have been observed when an optical absorber is incorporated into the encapsulant  500  (relative to the observed optical cross-talk penalty of the same device with the same encapsulant but without the optical absorber). 
     By reducing the average or effective dielectric constant of the protective encapsulant  500 , the level of electrical cross-talk in the bidirectional device that arises from the portion of the unwanted electrical signals  44 / 46 / 48  propagating above the substrate  10  can be reduced, relative to the electrical cross-talk that would be present with an encapsulant having a higher dielectric constant. To reduce the average dielectric constant of encapsulant  500 , it can include suspended, hollow, dielectric microspheres  520  ( FIGS. 23 and 24 ). Such microspheres are available commercially in a variety of sizes, and often comprise silica-based glass. In an exemplary embodiment, hollow silica microspheres are employed that have a median diameter of about 60 μm and a range of diameters from about 30 μm to about 105 μm (10 th  percentile to 90 th  percentile) or from about 10 μm to about 120 μm (overall range); other suitable materials or sizes can be employed (e.g., median diameter between about 40 μm and about 70 μm). The microspheres can be suspended in the uncured encapsulant, and remain there during application and after curing. To suitably reduce electrical cross-talk in the bidirectional device, the microspheres can comprise between about 25% and about 75% of the encapsulant composition by volume, corresponding to a reduction of the encapsulant effective dielectric constant between about 25% and about 50% (relative to the encapsulant without the microspheres). A silicone encapsulant  500  with microspheres in that volume-fraction range can exhibit an effective dielectric constant between about 2.5 and about 1.7, respectively, relative to a dielectric constant of about 2.8 without the microspheres. Reductions in electrical cross-talk penalty between about 0.1 dB and about 3 dB have been observed when hollow microspheres are incorporated into the encapsulant  500  (relative to the observed electrical cross-talk penalty of the same device with the same encapsulant but without the microspheres). The amount of electrical cross-talk reduction attributable to the hollow microspheres can vary depending on a variety of factors, e.g., the specific arrangement of optoelectronic components and conductive elements on the substrate; the manner in which electrical signals are coupled to or from the optoelectronic devices, such as unipolar, bipolar, or differential coupling; or other measures in addition the hollow microspheres taken to reduce electrical cross-talk. 
     In many examples of a protective encapsulant  500 , a filler is needed to increase the viscosity of the uncured polymer to facilitate its application to the device. Without sufficient viscosity, during application the uncured polymer tends to flow beyond those areas desired to be encapsulated (as is the case for an uncured encapsulant formulation having viscosity less than about 400-600 cps, for example). A filler is often employed to increase the viscosity of the uncured polymer to a suitable level during application, and that filler remains incorporated within the encapsulant after curing. Solid silica particles are commonly employed as a filler, but tend to exhibit a relatively large dielectric constant (between about 3 and 8, depending on specific composition). Incorporation of such high-dielectric filler particles into the encapsulant  500  would tend to increase its effective dielectric constant, thereby also increasing electrical cross-talk in the encapsulated device (beyond that exhibited if the encapsulant  500  included no filler). However, the hollow microspheres  520  can serve as the filler, increasing the viscosity of the uncured polymer to a desired level for application while also reducing its effective dielectric constant (and electrical cross-talk of the encapsulated device). 
     Volume fractions for the hollow microspheres between about 25% and about 50% have been observed to yield viscosity of uncured silicone encapsulant mixtures in a range suitable for application to device (e.g., from a few thousand centipoise up to a few tens of thousands of centipoise; more viscous formulations can be employed, but may be unwieldy to apply due to slow flow). That range of volume fraction also appears to enable the microspheres to remain dispersed in lower-viscosity encapsulant formulations; at lower volume fractions in lower-viscosity encapsulant formulations, the microspheres can tend to separate from the encapsulant. However, other volume fractions (higher or lower) can be employed with other uncured encapsulant formulations having differing viscosities (higher or lower), if the resulting mixture can be induced to flow where desired with a desired volume fraction of microspheres. A faster-curing encapsulant can be employed to reduce or avoid flow of a lower-viscosity encapsulant onto unwanted areas. 
     The reductions in effective dielectric constant or electrical cross-talk disclosed or claimed herein for an encapsulant that includes hollow dielectric microspheres are expressed relative to that same encapsulant without the microspheres or any other filler. Those reductions in cross-talk can be operationally significant and desirable. However, perhaps a more practical comparison would be between the encapsulant with the microspheres and the same encapsulant with solid filler particles, at respective volume fractions that yield similar viscosities (in a range suitable for application to the device). Viewed in this way, the relative reductions in effective dielectric constant and electrical cross-talk that are achieved using the hollow microspheres are even larger than those disclosed herein. Replacement of the solid filler particles with hollow microspheres eliminates the increase in effective dielectric constant arising from the filler particles, in addition to providing the reduction arising from the presence of the microspheres. 
     A volume fraction of hollow microspheres that is too high can yield an uncured encapsulant mixture that is too viscous to flow properly over the device, limiting degree to which the effective dielectric constant can be reduced by incorporating the microspheres. An uncured polymer having a viscosity between about 400 cps and about 600 cps (without the microspheres) can have a volume fraction of microspheres up to about 50% and remain sufficiently fluid for application to the device. Higher volume fraction or higher initial viscosity tends to yield an encapsulant mixture that does not flow sufficiently well to encapsulate the device. Uncured encapsulant formulations of lower initial viscosity could be employed to accommodate higher microsphere volume fractions (and therefore lower effective dielectric constant) while still flowing sufficiently for application to the device. A range of combinations of uncured encapsulant viscosity and microsphere volume fraction can be employed to yield a desired combination of flow of the encapsulant mixture and reduction of the encapsulant effective dielectric constant. 
     In a preferred embodiment, both optical absorber particles  510  and hollow dielectric microspheres  520  are incorporated into the protective encapsulant  500  ( FIG. 24 ). In this way, a single encapsulant  500  can effect reduction of both electrical and optical cross-talk in the bidirectional optoelectronic device. The presence of the microspheres  520  can in some instances enhance the effect of the optical absorber particles  510 , by acting as light scatterers. The light scattered from the microspheres  520  propagates a greater distance through the encapsulant  500 , increasing the likelihood of an encounter with an absorber particle  510 . If combined with hollow microspheres, it can be desirable to avoid high-dielectric or conductive absorber particles (e.g., metal or carbon particles), because those would tend to increase the encapsulant dielectric constant. On the other hand, the hollow dielectric microspheres can be employed to at least partly offset (or completely offset, or more than offset) an increase in encapsulant effective dielectric constant arising from such absorber particles. If conductive absorber particles are employed, it is preferable to ensure full curing of the encapsulant, to reduce or avoid unwanted aggregation or poling of the conductive particles and concomitant increase of the effective dielectric constant of the encapsulant. 
     An optical encapsulant  600  (also referred to herein as a “first level encapsulant”;  FIG. 25 ) can be employed in addition to the protective encapsulant  500  (which can be referred to as a “second level” encapsulant when used in combination with the optical encapsulant  600 ). If the bidirectional device includes any free space portions of optical paths followed by optical signals  14 ,  16  or  18 , an optical encapsulant  600  typically is needed, at least to exclude the protective encapsulant from those spaces. For example, if an optical splitter/combiner  110  is employed that comprises a beamsplitter positioned between end faces of the waveguide  102 ,  104 , and  106 , then the optical encapsulant can fill the optical path between the waveguides and the beamsplitter. Similarly, any gap between a waveguide end face and a photodetector or a light source can be filled with the optical encapsulant. Use of such an encapsulant  600  can prevent contamination of optical transmission surfaces or blockage of optical paths by the encapsulant  500 , or by foreign substances. An optical encapsulant  600  can also be selected to provide index matching with the waveguides, photodetectors, light source, or other elements, to reduce unwanted reflections in the device. Such reflections can act as sources of optical loss for desired optical signals, and also as sources of unwanted optical signals that can result in additional optical cross-talk. The first level encapsulant  600  can also provide protection from environmental degradation of the device, e.g., by corrosion in the presence of moisture. Examples of suitable materials can include, but are not limited to, silicone or epoxy polymers. 
     When an optical encapsulant  600  is employed, it can be applied to the bidirectional device and cured prior to application and curing of the encapsulant  500 . Alternatively, the encapsulant  500  can be applied after application but before curing of optical encapsulant  600 , and both encapsulants can be cured together in a common curing process, assuming the encapsulants  500  and  600  can remain in place and substantially unmixed prior to curing. 
     Combinations 
     Three techniques are disclosed herein for reducing cross-talk in multi-channel or bidirectional optoelectronic devices: a drive circuit for bipolar driving of the light source, light collectors and light traps on a waveguide substrates, and an encapsulant having a reduced dielectric constant or acting an optical absorber. Each of those techniques can be used alone. However, use in a single optoelectronic device of any two or all three techniques in combination, in any of their disclosed variations, shall also be regarded as falling within the scope of the present disclosure or appended claims. In one example, use of light collectors and light traps combined with use of a optically absorbing encapsulant can reduce optical cross-talk to a greater degree than either of those techniques used alone. In another example, use of a bipolar laser drive circuit and an encapsulant with hollow microspheres can reduce electrical cross-talk to a greater degree than either of those techniques used alone. Use of light collectors and traps, a bipolar laser drive circuit, and an encapsulant incorporating both an optical absorber and hollow microspheres can in some examples exhibit still lower levels of electrical or optical cross-talk. 
     It is intended that equivalents of the disclosed exemplary embodiments and methods shall fall within the scope of the present disclosure or appended claims. It is intended that the disclosed exemplary embodiments and methods, and equivalents thereof, may be modified while remaining within the scope of the present disclosure or appended claims. 
     In the foregoing Detailed Description, various features may be grouped together in several exemplary embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that any claimed embodiment requires more features than are expressly recited in the corresponding claim. Rather, as the appended claims reflect, inventive subject matter may lie in less than all features of a single disclosed exemplary embodiment. Thus, the appended claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate disclosed embodiment. However, the present disclosure shall also be construed as implicitly disclosing any embodiment having any suitable set of one or more disclosed or claimed features (i.e., sets of features that are not incompatible or mutually exclusive) that appear in the present disclosure or the appended claims, including those sets of one or more features that may not be explicitly disclosed herein. In addition to methods explicitly disclosed or claimed herein: (i) for using any explicitly or implicitly disclosed devices or apparatus; or (ii) for making any explicitly or implicitly disclosed devices or apparatus, the present disclosure shall also be construed as implicitly disclosing generic methods for using or making any explicitly or implicitly disclosed devices or apparatus. It should be further noted that the scope of the appended claims do not necessarily encompass the whole of the subject matter disclosed herein. 
     For purposes of the present disclosure and appended claims, the conjunction “or” is to be construed inclusively (e.g., “a dog or a cat” would be interpreted as “a dog, or a cat, or both”; e.g., “a dog, a cat, or a mouse” would be interpreted as “a dog, or a cat, or a mouse, or any two, or all three”), unless: (i) it is explicitly stated otherwise, e.g., by use of “either . . . or,” “only one of,” or similar language; or (ii) two or more of the listed alternatives are mutually exclusive within the particular context, in which case “or” would encompass only those combinations involving non-mutually-exclusive alternatives. For purposes of the present disclosure or appended claims, the words “comprising,” “including,” “having,” and variants thereof, wherever they appear, shall be construed as open ended terminology, with the same meaning as if the phrase “at least” were appended after each instance thereof. 
     In the appended claims, if the provisions of 35 USC §112 ¶6 are desired to be invoked in an apparatus claim, then the word “means” will appear in that apparatus claim. If those provisions are desired to be invoked in a method claim, the words “a step for” will appear in that method claim. Conversely, if the words “means” or “a step for” do not appear in a claim, then the provisions of 35 USC §112 ¶6 are not intended to be invoked for that claim. 
     The Abstract is provided as required as an aid to those searching for specific subject matter within the patent literature. However, the Abstract is not intended to imply that any elements, features, or limitations recited therein are necessarily encompassed by any particular claim. The scope of subject matter encompassed by each claim shall be determined by the recitation of only that claim.