Patent Publication Number: US-2019170943-A1

Title: Optical switching between waveguides by adjacent resonant structure coupling

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein was developed with government support under Grant Contract Number N66001-12-2-4007 as issued by the United States Defense Advanced Research Projects Agency (DARPA). The government has certain rights in the invention. 
    
    
     BACKGROUND 
     Switching of optical signals may have significant roles to facilitate in network infrastructure, including at datacenter scale and above. Optical or light signals may transmit via waveguides that guide propagation of a signal. Some optical networks may join multiple light signals of different wavelengths for transmission along the same waveguide, such as for example in wavelength-division multiplexing (WDM). Yet the multiple light signals may not share the same network destination. Thus, optical switching may occur when a network switches some selected light signals, such as of particular wavelengths, to another waveguide, such as to a different destination. Optical switching may be performed by conversion of light signals to electrical format for passage through an electronic switch, with later reconversion to optical format. In contrast, some all-optical switch technologies may enable all-optical switching designs, without conversion. An all-optical switch may allow for all data signals to remain purely optical. Nevertheless, such a switch design may allow for electronic controls, such as in its configurations or parameters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain examples are described in the following detailed description with reference to the drawings, of which: 
         FIG. 1  is a block diagram of an optical switching device, according to some examples of the present disclosure. 
         FIG. 2  is a block diagram of a planar cross-section of an optical switching device, according to some examples of the present disclosure. 
         FIG. 3  is a block diagram of an optical switching device that includes a control mechanism, according to some examples of the present disclosure. 
         FIGS. 4A-4C  are block diagrams of a racetrack resonant structure for use with an optical switching device, according to some examples of the present disclosure. 
         FIG. 5  is a flow diagram of a method of performing optical switching with an adjacent racetrack resonant structure, according to some examples of the present disclosure. 
         FIG. 6  is a block diagram of an optical switching device performing a method of optical switching, according to some examples of the present disclosure. 
         FIGS. 7A-7B  are chart diagrams that simulate a signal transfer function for an optical switching device, according to some examples of the present disclosure. 
         FIGS. 8A-8C  are block diagrams of an optical switching device that includes a region of modified waveguide width, according to some examples of the present disclosure. 
         FIGS. 9A-9B  are chart diagrams that simulate a signal transfer function for an optical switching device that includes a region of modified waveguide width, according to some examples of the present disclosure. 
         FIG. 10  is a block diagram of a system for optical switching that includes a set of adjacent racetrack resonant structures, according to some examples of the present disclosure. 
         FIG. 11  is a block diagram of a system for optical switching that includes a set of electronic controls, according to some examples of the present disclosure. 
         FIG. 12  is a flow diagram of a method of performing optical switching with a set of adjacent racetrack resonant structures, according to some examples of the present disclosure. 
         FIGS. 13A-13C  are block diagrams of a system for optical switching that includes a region of modified coupling width, according to some examples of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Examples of the present disclosure relate to a device, system, and method for optical switching by adjacent resonant structure coupling. Such optical switching design may perform all-optical switching with low loss and enable efficient switch topologies for optical switches, including for designs of high-radix switching systems. All-optical switching may refer to optical switching wherein data signals remain in purely optical form. Thus, all-optical switching technologies may allow switching without conversion of optical signals, in contrast for example to optoelectronic switching using an electronic switch. Nevertheless, all-optical switching may still implement or accommodate for suitable control mechanisms or processes. In particular, embodiments of the present disclosure present a design for low-loss all-optical switching, including electronic control mechanisms to enable dynamic parameter controls in operation. Based on such design, some embodiments may implement a switching system that includes a set of racetrack resonant structures for efficient high-radix switching. Some embodiments may include a design with modifications to either of a waveguide width or a coupling gap width, thereby enabling control over switch design characteristics or relevant operating parameters. 
     In general, optical switching may play a significant role in a modern network infrastructure. While optical switching may be implemented with various different electronic switches via conversion, an all-optical switching design may be desirable to avoid significant costs imposed by such conversions, including in energy use and network latency. Similarly, while all-optical switching may be attempted through various different all-optical switching technologies without conversion costs, such respective designs for an all-optical switch may likewise impose various yet significant costs, undesirable constraints, or inefficiencies, for example in slow reconfiguration times, high transmission losses, or high circuit complexities. For example, designs based on micro-electro-mechanical systems (MEMS) may operate with reconfiguration times on the order of hundreds of milliseconds, which may be too slow for various practical uses. Similarly, designs based on signal transmission through one or more microring resonators including electronic controls may suffer signal losses of 2-3 decibels (dB) per microring, resulting overall in either of high losses or high power usage for tuning per microring. Finally, designs based on wavelength redirection such as by arrayed gratings may need laser sources of tuning precisions that remain impractical or otherwise unavailable. Such costs, constraints, or inefficiencies of a particular switch design may grow particularly unmanageable in construction of an expanded or high-radix system for all-optical switching. Thus, it may be desirable to develop a low-loss and expandable all-optical switch design, of a switch topology more suitable to construct an efficient, high-radix optical switching system. 
     In examples of the present disclosure, an optical switching device includes a first waveguide, a second waveguide, and a racetrack resonant structure. A waveguide may refer to any structure suitable to guide or direct a light signal. A racetrack resonant structure may refer to any waveguide arranged in a closed-loop shape, including various microring resonators. Thus, the first waveguide may be positioned to receive an input light signal. The second waveguide may be positioned to enable the input light signal to couple between the first waveguide and the second waveguide through a first coupling gap. The first and second waveguides may thereby design, implement, and/or function as a directional coupler. The first coupling gap may have a particular design, such as in its length and in its width with respect to the first and second waveguides that characterize it. In particular, the first coupling gap may be designed such that any input light signal that is received by the first waveguide tends to exit from the first coupling gap by the second waveguide, e.g. in a “cross” state, as opposed to by the first waveguide, e.g. in a “bar” state. Thus, any light signal input to the first coupling gap may tend to exit in a cross state, based on the particular design as described. 
     In such examples, the racetrack resonant structure may be positioned adjacent to the first coupling gap. The racetrack resonant structure may allow select input light signals to couple between one of the first and second waveguides and the racetrack resonant structure through a second coupling gap. Thus, the racetrack resonant structure may be positioned adjacent to the directional coupler to allow for coupling between the first and second coupling gaps within the device, i.e. prior to exit of the input light signal from the first coupling gap. The second coupling gap may then have a particular design, such as in its length and width, with respect to the first coupling gap. In particular, the second coupling gap may be designed such that the racetrack resonant structure is over-coupled to the respective waveguide, e.g. designed with a width such that losses due to coupling is small or negligible compared to the coupled power. Furthermore, the second coupling gap may be designed such that its length is suitably or substantially shorter than the length of the first coupling gap. In summary, such a particular design of the second coupling gap and its positioning adjacent to the first coupling gap may therein allow any input light signal selected to couple over the second coupling gap to “undo”, “reverse”, or otherwise modify the coupling effect over the first coupling gap. For example, an input light signal that couples over the second coupling gap may then exit the first coupling gap by the first waveguide, e.g. as output in the bar state, not in the cross state by which it might have exited otherwise. The device may thus perform all-optical switching. 
     In some such examples, the racetrack resonant structure may be positioned symmetrically with respect to the first coupling gap, such that the second coupling gap may be positioned symmetrically with respect to the length of the first coupling gap. However, as explained in further detail below, various examples may implement suitable designs with relative flexibility in positioning of the racetrack resonant structure or the second coupling gap, e.g. adjacent to the first coupling gap and without a particular limitation of symmetry. In general, a symmetrical position may facilitate switching for the device, by symmetrically undoing the coupling effect over the length of the first coupling gap with respect to a first half of its length and a second half of its length. Suitable designs may nevertheless enable the second coupling gap to “undo”, “reverse”, or otherwise modify the coupling over the first coupling gap without an exact symmetry in positioning. In general, the racetrack resonant structure may thus determine a “frequency passband” for the device in performing switching of output between the two waveguides, as all input signals may tend to couple over the first coupling gap similarly, such that undoing or modifying the coupling effect switches output. In some such examples, for any input light signal, a first portion of the signal that coincides with the frequency passband may exit by the input waveguide, e.g. output in the bar state, and a second portion of the signal that does not coincide with the frequency passband may exit by the adjacent waveguide, e.g. output in the cross state. In some cases, either of the portions may be null, e.g. an input signal may also be switched in its entirety or in no portion. 
     As used herein, the terms “coupling” or “optical coupling” may be understood to refer more generally to evanescent wave coupling as a phenomena of transfer, division, or distribution of energy or power for a light signal over a coupling gap under certain conditions. An input light signal propagating in a first waveguide may tend to develop an evanescent wave in a second waveguide when the second waveguide is positioned sufficiently nearby or adjacent to the first waveguide, as in a suitable directional coupler. The input light signal in the first waveguide may then “couple over” the first coupling gap or “couple between” the first and second waveguides throughout a length of the first coupling gap. The input light signal may then exit from the length of the first coupling gap with a first portion of power or energy still propagating in the first waveguide and a second portion “coupled into” the second waveguide for propagation therein. As noted above, the first coupling gap may have such a particular design that the input light signals may “couple entirely” into the second waveguide, such that all or relatively all of the energy or power tends to exit by the second waveguide and continues to propagate in the second waveguide, e.g. as output in a cross state. The first coupling gap may be designed, as in its length and width, to couple all input light signals, of any wavelength or frequency. Similarly, it may tend to output all signals in the cross state. 
     With respect to the second coupling gap, however, coupling to or between the racetrack resonant structure may be designed to select for a frequency passband, as noted. In general, a racetrack resonant structure may exhibit a resonance at a resonant wavelength, based upon its closed-loop structure. In particular, an input light signal propagating along the first coupling gap that does not coincide with the resonant wavelength may continue to propagate unimpeded along the length of the first coupling gap, so as to exit by the second waveguide, e.g. in the cross state as noted. In contrast, an input light signal that coincides with the resonant wavelength may develop an evanescent wave in the racetrack resonant structure and couple into the racetrack resonant structure while propagating along the length of the first coupling gap. More generally, the racetrack resonant structure may determine a frequency passband with a passband width that may be based on a central frequency. The central frequency may at be the resonant wavelength. Thus, the racetrack resonant structure may operate as a notch filter with respect to the frequency passband. Likewise for any input light signal, a first portion of the signal may couple to the racetrack resonant structure, while a second portion of the signal may continue propagating unimpeded. In operation as a notch filter over the second coupling gap, the racetrack resonant structure may characterize a signal transfer function for output by the switching device, as further described below. In particular, the racetrack resonant structure may thereby “switch” the output of the frequency passband as between the two waveguides, e.g. from the directional coupler over the first coupling gap. 
     In coupling based on resonance, the racetrack resonant structure may impose a phase shift of pi (π) on the frequency passband as it propagates along the first coupling gap. As noted, the design of the device may position the racetrack resonant structure suitably, for example symmetrically with respect to the first coupling gap, such that the phase shift by the second coupling gap may operate effectively to “undo”, “reverse”, or otherwise modify the coupling over the first coupling gap for that portion of the input light signal. Specifically, the coupling along the prior length of the first coupling gap, e.g. that preceding the second coupling gap, may be modified by the coupling along the later length of the first coupling gap, e.g. that following the second coupling gap. Based on the imposed phase shift, the first portion of the input light signal that coincides with the frequency passband may thereafter continue in propagating along the first coupling gap but then exit instead by the first waveguide (output in bar state), rather than by the second waveguide (output in cross state). In contrast, the second portion of the input light signal that does not coincide with the frequency passband may continue to propagate unimpeded and thus continue to exit the first coupling gap by the second waveguide (output in cross state). Notably, such a design allows input signals to transmit through the two waveguides as in a directional coupler, not directly through the racetrack resonant structure as for example in switch designs that may implement a resonant coupling in series between waveguides for input and output. Rather, the racetrack resonant structure may merely apply a phase shift on a select frequency passband via resonant coupling. Thus, the optical switching device may perform all-optical switching with low insertion loss, based on the signal power remaining within the two waveguides. The racetrack resonant structure may also determine, select, or control the frequency passband dynamically. 
     Examples of the present disclosure may implement suitable control mechanisms, including electronic controls for such a switching device as described in the above. Characteristics or parameters of switching may therefore be rapidly reconfigurable during operation of the device, for example on the order of picoseconds (ps). The resonant wavelength of a racetrack resonant structure may be tuned or modified, for example, by changing an effective refractive index of the racetrack resonant structure. Changing the voltage or the temperature of a portion of the racetrack resonant structure may result in a changing of the effective refractive index. In several examples of the present disclosure, an electronic control mechanism may thus modify a voltage or a temperature of the racetrack resonant structure, to tune the resonant wavelength and/or to control the frequency passband with respect to switching by the device. Additionally, other parameters of the signal transfer function of the switching device, such as edge steepness and tail or dispersive characteristics of the frequency passband, may be adjusted by modifying the racetrack resonant structure. 
     Examples of the present disclosure may expand on designs as described in the above in construction of more efficient high-radix systems that include electronic controls. In particular, switching designs or topologies as described in the above may allow expansions that include additional waveguides and/or racetrack resonant structures arranged similarly or otherwise suitably relative to a first coupling gap as described in the above. Each racetrack resonant structure may thus determine a respective or distinct passband, based on a respective or distinct resonant wavelength. Each racetrack resonant structure may also correspond to an electronic control mechanism to tune or modify its passband, allowing for dynamic switching control during system operation. For example, a 128-radix switch may be constructed with less than about 1000 switching elements. Finally, characteristics of switching in such systems may be modified further with switch designs incorporating one or more modifications in the dimensions of any or all of the waveguides, the second coupling gap, or the first coupling gap. For example, a system may include a region of modified width for the first coupling gap, including to facilitate suitable positioning for additional racetrack resonant structures. For example, a system may include a region of modified width for one of the waveguides, including to characterize the frequency passband of the switching system. Thus such systems as described in the above may perform efficient, high-radix, all-optical switching with rapidly reconfigurable electronic controls, wherein the design or topology may allow for expansions or several variations on the switching designs and/or the characteristics of the switching. In these ways and others, the examples described herein may materially improve performance of a method, device, or system for optical switching, and of all-optical switching in particular. 
     The examples of the present disclosure are hereafter described with reference to the following figures. Unless noted otherwise, the figures and accompanying descriptions are non-limiting, such that no element is either exclusive to or characteristic of any particular example. Accordingly, features from one example may be freely incorporated into any other examples without departing from the spirit and scope of the content in the present disclosure. 
     An optical switching device for use with the present disclosure is described with reference to  FIG. 1 . In that regard,  FIG. 1  is a block diagram of an optical switching device  100 , according to some examples of the present disclosure. The optical switching device  100  includes a first waveguide  110 , a second waveguide  120 , and a racetrack resonant structure  130 . The first waveguide  110  is positioned to receive an input light signal. The second waveguide  120  is positioned to enable the input light signal to couple between the first waveguide  110  and the second waveguide  120  through a first coupling gap  145 . The racetrack resonant structure  130  is positioned adjacent to the first coupling gap  145  to enable the input light signal to couple between one of the first waveguide  110  and the second waveguide  120  and the racetrack resonant structure  130  through a second coupling gap  155 . As illustrated in  FIG. 1 , the racetrack resonant structure  130  is positioned adjacent to the second waveguide  120 , while the device  100  may be understood to allow for the racetrack resonant structure  130  to be similarly positioned adjacent to the input or first waveguide  110 , without limitation. Similarly, while the input light signal is discussed with reference to the first waveguide  110 , the device  100  may be understood to allow for either or both waveguides  110 ,  120  to receive simultaneous, similar, or distinct input light signals, without limitation. 
     The first coupling gap  145  may be understood to be illustrative of the scope of a characteristic coupling region between the first waveguide  110  and the second waveguide  120 , not as limiting design. The designs of the first waveguide  110  and the second waveguide  120  may determine the characteristics of the first coupling gap  145 . In particular, the relative dimensions of the first coupling gap  145  are illustrative, without limiting a design parameter of the device  100 . For purposes of characterization as discussed above, the first coupling gap  145  may be referred to as a continuous region with a particular length and a particular width, for example as represented in device  100  of  FIG. 1  via illustrated rectangle. In several examples of the present disclosure, the first coupling gap  145  is designed to have a width and length such that all signals input to the device  100  tend to be output in a cross state, for example in absence of additional design elements such as racetrack resonant structure  130 . Thus, any signal input by one of the waveguides  110 ,  120  may tend to couple entirely over to the other of the waveguides  120 ,  110  and tend to output by the other waveguide  120 ,  110 . 
     The second coupling gap  155  may similarly be understood to be illustrative of the scope of a characteristic coupling region between one of the waveguides and the racetrack resonant structure  130 , not as limiting design. The designs of the respective waveguide and the racetrack resonant structure  130  may determine the characteristics of the second coupling gap  155 . Also for purposes of characterization, the second coupling gap  155  may be referred to as a continuous region with a particular length and a particular width, for example again as represented in device  100  of  FIG. 1  via illustrated rectangle. The relative dimensions of the second coupling gap  155  are illustrative, again without limiting a particular design parameter of the device  100 . In several examples of the present disclosure, the second coupling gap  155  is designed to have a width suitably narrow such that the racetrack resonant structure  130  is overcoupled to the respective waveguide, such that any coupling losses are insubstantial or negligible relative to the fraction of power coupled into or out of the racetrack resonant structure  130 . In several examples of the present disclosure, the second coupling gap  155  is designed to have a shorter length relative to the length of the first coupling gap  155 , as illustrated in  FIG. 1  via the two relative rectangles. Designs of length and width may impact, vary, or characterize switching in the device  100 . In particular, designs of the length and width for the second coupling gap may undo, reverse, or otherwise modify the coupling effect over the first coupling gap for a portion of an input signal. The portion of the input signal that is thus affected may be referred to as coinciding with a frequency passband. The frequency passband may characterize or result in a switching in the output of the device  100 . 
     In some examples of the present disclosure, the second coupling gap  155  is designed or positioned symmetrically with respect to the first coupling gap  145 , as illustrated in  FIG. 1 . However, the device  100  may also be understood to allow for designs that position the second coupling gap  155  adjacently to the length of the first coupling gap  145  in suitable non-symmetric variations, without limitation as to a particular design parameter of the device  100 . Design variations may include modifications to any of the symmetries, shapes, widths, and/or spacings of the waveguides  110  and  120  and the racetrack resonant structure  130  of the device  100 . Positioning, including of the second coupling gap  155  relative to the first coupling gap  145 , may impact, vary, or characterize switching in the device  100 . 
     In  FIG. 1 , the first waveguide  110  and the second waveguide  120  may thereby design, enable, or function as a directional coupler with respect to the first coupling gap  145 , with input-output ratios of 1×1, 1×2, 2×1, and/or 2×2 with respect to waveguides  110 ,  120 . In general, a directional coupler may refer to any structure suitable to allocate, to divide or otherwise to distribute signal power between input and output based on the properties of the coupler itself and/or the input signal, including for example as applied to light signals in wavelength-division multiplexing (WMD). In particular, either and/or both of waveguides  110 ,  120  may receive an input light signal that is coupled to the other waveguide  120 ,  110  over the first coupling gap  145 . Any input signal may then be output by either and/or both of waveguides  120 ,  110 , including in divided or distributed form, based on properties of the input signal and design of the first coupling gap  145 . The waveguides  110 ,  120  may remain coupled continuously over the length of the first coupling gap  145 . As noted above, such a directional coupler may produce output in a bar state, e.g. by the input waveguide, and/or a cross state, e.g. by the adjacent waveguide. The output may be in a combination with a first portion of the input signal output in bar state and a second portion of the input signal output in cross state. In several examples of the present disclosure, the first coupling gap  145  is designed, e.g. in length and width, such that the directional coupler as designed by the waveguides  110  and  120  tends to output in a cross state any or all signals input to the first coupling gap  145 , for example in the absence of additional design elements of the device  100  such as the racetrack resonant structure  130 . Thus, all signals that are input to the device  100  may undergo coupling over the first coupling gap  145  in a similar or comparable manner. 
     Racetrack resonant structure  130  may refer to a waveguide formed in a closed loop, such that it is suitable for resonance. In several examples of the present disclosure, the racetrack resonant structure  130  is a microring resonator. The racetrack resonant structure  130  may exhibit a resonance at a resonant wavelength. The racetrack resonant structure  130  may be positioned adjacent to the first coupling gap  145 , as adjacent to one of the waveguides  110 ,  120  that constitute the directional coupler, such that a light signal propagating along the first coupling gap  145  may couple into the racetrack resonant structure  130  over the second coupling gap  155 . The coupling over the second coupling gap  155  may occur for particular, selected, or otherwise controlled conditions. An input light signal propagating at a resonant wavelength along the first coupling gap  145  may tend to develop an evanescent wave in the racetrack resonant structure  130  by coupling. However, a light signal propagating at a non-resonant wavelength along the first coupling gap  145  may tend to continue to propagate unimpeded along the first coupling gap  145 , with no coupling into the racetrack resonant structure  130 . Thus, racetrack resonant frequency  130  may select for a particular frequency. 
     The racetrack resonant structure  130  may have a frequency passband that is centered on a central frequency. The central frequency may be at the resonant wavelength. Thus, the racetrack resonant structure  130  may design or operate as a notch filter, by allowing light signals outside of the frequency passband to continue to propagate unimpeded along the first coupling gap  145 , while coupling light signals inside of the frequency passband into the racetrack resonant structure  130 . The notch filter may be characterized by a signal transfer function. Note, however, that light signals inside of the passband that are coupled into the racetrack resonant structure  130  may also couple back over the second coupling gap  155  and continue to propagate along the first coupling gap  145 , due to resonance. Thus, an overall characterization of a signal transfer function may depend upon the relative positioning of the second coupling gap  155  with respect to the first coupling gap  145 . In several examples of the present disclosure, the racetrack resonant structure  130  is positioned to impose a phase shift of pi (π) for the frequency passband. In some such examples, the design positions the racetrack resonant structure  130  symmetrically with respect to the first coupling gap  145 , such that the second coupling gap  155  is positioned symmetrically, as is illustrated in  FIG. 1 . In such designs, coupling of the frequency passband over the second half of the length of the first coupling gap  145  may undo, reverse, or otherwise cancel the coupling over the first half of the length of the first coupling gap  145 , based on the phase shift of pi (π) for the frequency passband. Thus, the device  100  may produce output in a bar state for the frequency passband. In some examples, the phase shift otherwise modifies the output of the device  100 . The signal transfer function may include dispersive or additional characteristics based on the phase shift. 
     The racetrack resonant structure  130  is thereby to determine a frequency passband, such that a first portion of the input light signal that coincides with the frequency passband is output by the first waveguide  110 , and a second portion of the input light signal that does not coincide with the frequency passband is output by the second waveguide  120 . In some examples of the present disclosure, either of the first portion or the second portion of an input light signal is a negligible or null portion, such as when an input light signal either coincides in entirety or else coincides in no portion with the determined frequency passband. The device  100  may thus perform all-optical switching with respect to a frequency passband. 
     While  FIG. 1  illustrates the elements of the optical switching device  100  in an example design, configuration, or arrangement, it is understood that the illustrated elements of  FIG. 1  may also be designed, configured, or arranged in several different variations of the optical switching device  100  without loss of generality according to the present disclosure. For example, while  FIG. 1  illustrates the first waveguide  110  and the second waveguide  120  as similar or symmetric in dimensions, it may be understood that the first waveguide  110  and the second waveguide  120  may likewise be dissimilar, non-symmetric, or otherwise distinct in characteristics so long as the second waveguide  120  is suitably positioned to enable the coupling over the first coupling gap  145 . Similarly, while  FIG. 1  illustrates the racetrack resonant structure  130  as positioned adjacent to the second waveguide  120 , it is understood that the racetrack resonant structure  130  may likewise be suitably positioned adjacent to the first waveguide  110 , so long as the racetrack resonant structure  130  is suitably positioned adjacent to the first coupling gap  145  to enable the coupling over the second coupling gap  155 . Finally, while  FIG. 1  illustrates the first coupling gap  145  and the second coupling gap  155  as similar rectangles, it is understood that the first coupling gap  145  and the second coupling gap  155  may likewise be dissimilar, non-symmetric, or otherwise distinct in dimensionality or characteristics. As noted, in several examples of the present disclosure, the second coupling gap  155  is designed to have shorter length than the first coupling gap  145 . The second coupling gap  155  may also be designed such that the racetrack resonant structure  130  is overcoupled to one of the waveguides  110 ,  120 . Overcoupling may refer to coupling wherein a power loss due to coupling is small relative to a fraction of power coupled, such that any internal losses in the racetrack resonant structure  130  are relatively negligible. The racetrack resonant structure  130  may be overcoupled with respect to the second coupling gap  155  by positioning it sufficiently nearby to the respective adjacent waveguide  110 ,  120 . 
     Further examples are described in detail with reference to  FIG. 2 . In that regard,  FIG. 2  is a block diagram of a planar cross-section of an optical switching device  200 , according to some examples of the present disclosure. The optical switching device  200  may be substantially similar to the optical switching device  100  of  FIG. 1 . In particular, similarly numbered elements in  FIG. 2  may be substantially similar to those of  FIG. 1  as described above, including waveguides  110  and  120 , racetrack resonant structure  130 , the first coupling gap  145 , and the second coupling gap  155 . The elements of the device  200  may be positioned in the same plane and/or designed with a similar height dimensionality, including any or all of the waveguides  110 ,  120  and the racetrack resonant structure  130  of  FIG. 1 , as illustrated in  FIG. 2 . However, the device  200  may be understood to allow for non-planar or multi-planar designs without limitation, so long as the element positioning suitably enables the first coupling gap  145  and the second coupling gap  155 , as described with reference to  FIG. 1 . In cross-section,  FIG. 2  illustrates the second waveguide  120  to be positioned sufficiently nearby and adjacent to the first waveguide  110  to establish the first coupling gap  145  between them. In cross-section,  FIG. 2  illustrates the racetrack resonant structure  130  of  FIG. 1  as a near cross-section  130 -A and far cross-section  130 -B, with a dotted outline merely to denote closed-loop structure. The near cross-section  130 -A is positioned sufficiently nearby to one of the waveguides  110 ,  120  to establish the second coupling gap  155  between them. Consistent with  FIG. 1 ,  FIG. 2  again illustrates the second coupling gap  155  as by the second waveguide  120 , without limiting designs that may use the first waveguide  110  instead. In cross-section,  FIG. 2  illustrates the second coupling gap  155  to be positioned sufficiently adjacent to the first coupling gap  145  to enable coupling between them. Thus, the first coupling gap  145  and the second coupling gap  155  may coincide along portion of the overall length of the device  200 , including the cross-section that is illustrated in  FIG. 2 . However, as illustrated in  FIG. 1 , the second coupling gap  155  may also be designed to have a length that is shorter than the length of the first coupling gap  145 . 
     Further examples are described in detail with reference to  FIG. 3 . In that regard,  FIG. 3  is a block diagram of an optical switching device  300  that includes a control mechanism  360 , according to some examples of the present disclosure. The optical switching device  300  may be substantially similar to the optical switching device  100  of  FIG. 1 . In particular, similarly numbered elements may be substantially similar to those of  FIG. 1 , including the waveguides  110  and  120 , the racetrack resonant structure  130 , the first coupling gap  145 , and the second coupling gap  155 . In several examples of the present disclosure, the control mechanism  360  is an electronic control mechanism. The control mechanism  360  may apply a control process to vary a parameter of the racetrack resonant structure  130 , such as a voltage or a temperature parameter. As noted above, such parameters may tune or vary the resonant wavelength or control the frequency passband of the device  300 , for example by changing the effective refractive index of the racetrack resonant structure  130 . The frequency passband may determine or affect switching characteristics of the device  300 , such that the control mechanism  360  may enable rapid reconfiguration of switching parameters. Thus, switching by the device  300  may be reconfigurable on the order of picoseconds (ps). The control mechanism  360  may use, include, or implement control logic and/or digital logic in any combination of hardware and/or software designs, including complimentary metal-oxide semiconductor (CMOS) technology, integrated circuit (IC) or application-specific integrated circuit (ASIC) designs, a controller or microcontroller, or other suitable control technology that may vary, modify, or otherwise control a parameter of racetrack resonant structure  130 . 
     Further examples are described in detail with reference to  FIGS. 4A-4C . In that regard,  FIGS. 4A-4C  are block diagrams of a racetrack resonant structure  430  for use with an optical switching device, according to some examples of the present disclosure. The racetrack resonant structures  430  may be used with any of the device  100  of  FIG. 1 , the device  200  of  FIG. 2 , the device  300  of  FIG. 3 , or with other suitable designs of comparable devices. The racetrack resonant structures  430  may be substantially similar to the racetrack resonant structure  130  of  FIG. 1 , as described above. As noted, the racetrack resonant structure  130  of  FIG. 1  may refer to any suitable waveguide arranged in a closed loop form or structure, including various types of microring resonators. In that regard,  FIG. 4A  illustrates a racetrack resonant structure  430 -A designed in the shape of an ellipse;  FIG. 4B  illustrates a racetrack resonant structure  430 -B designed in the shape of a circle; and  FIG. 4C  illustrates a racetrack resonant structure  430 -C designed in the shape of a rounded rectangle. Shapes are illustrated without limitation to designs, so long as a form design may enable suitable resonant coupling. 
     Further examples for implementing optical switching in accordance with the present disclosure are discussed in detail with reference to  FIG. 5 . In that regard,  FIG. 5  is a flow diagram of a method  500  of performing optical switching with an adjacent racetrack resonant structure, according to some examples of the present disclosure. The description of method  500  may be understood to be non-limiting. Blocks may be added to or omitted from the method  500  without departing from the disclosure. Unless noted otherwise, blocks of the method  500  may be performed in any order, including concurrently by one or more device elements. In general, the method  500  may be equally suitable for performance by using the device  100  of  FIG. 1 , device  200  of  FIG. 2 , device  300  of  FIG. 3 , or any other suitable device. For purposes of clarity, the following may refer to the device  100  of  FIG. 1  without limitation. 
     The method  500  may start in block  502  for a suitable optical switching device. In block  502 , the device receives an input light signal by a first waveguide of a directional coupler. For an example referencing  FIG. 1 , the device  100  receives an input light signal by the first waveguide  110  of the directional coupler designed by the waveguides  110  and  120 . In some such examples, the first waveguide  110  is positioned to receive the input light signal to device  100 . In some such examples, the second waveguide  120  is positioned relative to the first waveguide  110  to design a directional coupler, including the first coupling gap  145 . The device  100  may also receive similar or distinct input light signals by either of the first waveguide  110  or the second waveguide  120 , including some simultaneously as noted earlier. 
     In block  504 , the device couples the input light signal in the first waveguide to a second waveguide of the directional coupler. For an example referencing  FIG. 1 , the device  100  couples the input light signal in the first waveguide  110  to the second waveguide  120  of the directional wave coupler of waveguides  110  and  120 , as the signal propagates lengthwise along the first coupling gap  145 . In some such examples, the second waveguide  120  is positioned to enable the input light signal to couple between the first waveguide  110  and the second waveguide  120  through the first coupling gap  145 . The device  100  may be designed thereby to couple an input light signal in its entirety, regardless of an eventual output state that may distinguish a first or second portion of signal, as discussed in  FIG. 1 . As noted earlier, the first coupling gap  145  may also be designed so as to tend to output any or all input signals in a cross state, or by the second waveguide  120  for input by the first waveguide  110 , for example in absence of other design elements such as the racetrack resonant structure  130 . 
     In block  506 , the device couples the input light signal in the directional coupler to a resonant structure that has a resonant frequency. For an example referencing  FIG. 1 , the device  100  couples the input light signal in the directional coupler of waveguides  110  and  120  to the racetrack resonant structure  130 , as the signal propagates lengthwise along the first coupling gap  145 . As noted earlier, the racetrack resonant structure  130  may operate with a resonant frequency. The racetrack resonant structure  130  may thereby determine a frequency passband or a characteristic waveguide dispersion based on a central frequency. Thus, a first portion of the input light signal may couple into the racetrack resonant structure  130 , whereas a second portion may continue unimpeded along the first coupling gap  145 . The first portion of the input light signal may thereafter couple back or continue to propagate along the first coupling gap with a characteristic phase shift, including a phase shift of pi (π). The racetrack resonant structure  130  may also be controlled to vary the resonant frequency. The device  100  may thus dynamically control a frequency passband, including electronically. 
     In block  550 , the device outputs, by the directional coupler and based on the resonant frequency, a first portion of the input light signal to the first waveguide and a second portion of the input light signal to the second waveguide. For an example referencing  FIG. 1 , the device  100  outputs, by the directional coupler of waveguides  110  and  120  and based on the resonant frequency of the racetrack resonant structure  130 , a first portion of the input light signal by the first waveguide  110  and a second portion of the input light signal by the second waveguide  120 . In some such examples, a passband portion of the input light signal that corresponds to the frequency passb and is output by the first waveguide  110 , and a remaining portion of the input light signal is output by the second waveguide  120 . As discussed earlier, the racetrack resonant structure  130  may be positioned with respect to the first coupling gap  145  such that the imposed phase shift cancels, reverses, or otherwise modifies the coupling over the first coupling gap  145 . For example, the racetrack resonant structure  130  is positioned symmetrically or at halfway with respect to the first coupling gap  145 , to impose a phase shift of pi (π) that undoes or reverses the coupling with respect to the prior length of the first coupling gap  145  as over the remaining length of the first coupling gap  145 . The device  100  may thus perform switching of the input light signal by coupling to the racetrack resonant structure  130 . In some such examples, the first portion coincides with the frequency passband of the racetrack resonant structure  130 , whereas the second portion does not coincide with the frequency passband. The first portion output by the first waveguide  110  may be referred to as output in a bar state, whereas the second portion as output by the second waveguide  120  may be referred to as output in a cross state. In some such examples, one of the first portion or the second portion may be null, as when the input signal coincides in its entirety or in no portion with the frequency passband. The first portion may thereafter continue to propagate distinctly in the first waveguide  110 , and the second portion may continue to propagate distinctly in the second waveguide  120 . In general, the output of the device  100  may thus be characterized by a signal transfer function. 
     Further examples of optical switching are described with reference to  FIG. 6 . In that regard,  FIG. 6  is a block diagram of an optical switching device  600  performing a method of optical switching, according to some examples of the present disclosure. The device  600  of  FIG. 6  may be substantially similar to the device  100  of  FIG. 1 , as well as to other devices of the present disclosure. In particular, similarly numbered elements may be substantially similar to those of  FIG. 1 , including the waveguides  110  and  120 , the racetrack resonant structure  130 , the first coupling gap  145 , and the second coupling gap  155 . Arrows illustrated in  FIG. 6  may be understood to illustrate a direction of propagation in the device  600 , without limitation to designs of the device  600  or to performance of a suitable method. The method of switching performed by the optical switching device  600  may be substantially similar to the method  500  of  FIG. 5 , as well as to other methods of the present disclosure. 
     In particular, the device  600  may receive an input light signal  601  in the first waveguide  110  of a directional coupler. This may be performed in a manner substantially similar to block  502  of the method  500 . The device  600  may couple the input light signal  601  in the first waveguide  110  to the second waveguide  120  of the directional coupler. This may be performed in a manner substantially similar to block  504  of the method  500 . The device  600  may couple a portion  625  of the input light signal  601  in the directional coupler into the racetrack resonant structure  130 . This may be performed in a manner substantially similar to block  506  of the method  500 . The device  600  may thereby design that a first or passband portion  660  that corresponds to a frequency passband of the racetrack resonant structure  130  is output by the first waveguide  110  and a second or remaining portion  670  of the input light signal  601  is output by the second waveguide  120 . This may be performed in a manner substantially similar to block  550  of the method  500 . The first or passband portion  660  may be substantially similar to the portion  625  coupled into the racetrack resonant structure  130 . Thus, the device  600  may perform optical switching by coupling between the directional coupler and the adjacent racetrack resonant structure  130 . The racetrack resonant structure  130  may determine the frequency passband for switching. The device  600  may also dynamically control or reconfigure the frequency passband, including by electronic controls. As noted earlier, the first or passband portion  660  may coincide with the frequency passband as determined by the racetrack resonant structure  130 ; the second or remaining portion  660  may not coincide with the frequency passband. In several examples of the present disclosure, the device  600  thus performs the method  500  of optical switching for input light signal  601 . 
     Further examples of optical switching according to the present disclosure are described with reference to  FIGS. 7A-7B . In that regard,  FIGS. 7A-7B  are chart diagrams that simulate a signal transfer function for an optical switching device, according to some examples of the present disclosure. In particular,  FIG. 7A  charts the signal transfer function over a larger wavelength scale; and  FIG. 7B  charts the signal transfer function over a smaller wavelength scale.  FIGS. 7A-7B  may be understood to refer to or characterize any or all of the device  100  of  FIG. 1 , device  200  of  FIG. 2 , device  300  of  FIG. 3 , device  600  of  FIG. 6 , or any other suitable optical switching device.  FIGS. 7A-7B  may also be understood with reference to a particular frequency passband, whereas a device may vary frequency passbands as noted. In particular,  FIGS. 7A-7B  display the respective wavelength scale in micrometers (μm) of wavelength λ for the input light signal as referenced above. The charts distinguish output in the through or bar state from output in the cross state respectively, as discussed earlier, wherein the two output portions may account for substantially all of the input signal. Thus the transmission scale of 0 to 1 may be understood to refer to the portion of the input light signal output in each respective state. Transmission values between 0 and 1 may refer to or characterize the dispersive effects of the switching via switching device. For purposes of clarity, the following discussion may refer to the device  100  of  FIG. 1 , without limitation. 
     In  FIG. 7A , the signal transfer function may be understood to display recurring characteristics over a larger wavelength scale. As discussed above, the output in a through or bar state may occur when input wavelength is near or coincides with a resonant frequency. Such resonance may recur over a wavelength interval. For wavelengths far from resonance, including between such intervals, output may be substantially in a cross state. Thus,  FIG. 7A  illustrates the optical switching device performing a selective output switching by resonance. As illustrated, switching to bar state output may be designed to be quite specific to resonance. 
     In  FIG. 7B , the signal transfer function may be understood to have particular dispersive effects for wavelengths relatively near to the frequency passband. In the illustrated example, the frequency passband is centered near to 1.67 μm. The center value may represent the resonant wavelength of the racetrack resonant structure  130 . For this example, the device sensitivity is simulated to be sufficient to vary the output in the bar or through state between substantially 0 and substantially 1 within less than 0.005 μm variation in wavelength. Within less than the 0.005 μm to 1.67 μm, a frequency passband may output mostly in the bar state. The design of a device may affect, modify, or vary this signal transfer function, including the width and symmetry of the frequency passband and other dispersive effects of the switching. 
     Further examples of a device for optical switching are described in detail with reference to  FIGS. 8A-8C . In that regard,  FIGS. 8A-8C  are block diagrams of an optical switching device  800 A-C that includes a region of modified waveguide width  812  or  822 , according to some examples of the present disclosure. In particular,  FIG. 8A  illustrates the device  800 -A with a region of modified waveguide width  812  for the first waveguide  110 ;  FIG. 8B  illustrates the device  800 -B with a region of modified waveguide width  822  for the second waveguide  120 ; and  FIG. 8C  illustrates the device  800 -C with a region of modified width  812  for the first waveguide  110  and a region of modified waveguide width  822  for the second waveguide  120 . Elements of the devices  800 A-C may be substantially similar to those of the device  100  of  FIG. 1 . In particular, similarly numbered elements may be similar or identical to elements of  FIG. 1  as described in the above, including waveguides  110  and  120 , racetrack resonant structure  130 , first coupling gap  145 , and second coupling gap  155 . 
     In several examples of the present disclosure, a region of modified waveguide width  812  or  822  is designed on one of the waveguides  110  or  120  in a position near, adjacent to, or along the length of the second coupling gap  155 . In several such examples, the modified region  812  or  822  is also designed to have shorter length than the first coupling gap  145 , as illustrated in  FIGS. 8A-8C . The relative dimensions of the modified region  812  or  822  are illustrative, without limiting a particular design parameter of the devices  800 A-C. Thus, the designs of the waveguides  110  or  120  may determine the characteristics of the respective modified regions  812  or  822 . For purposes of characterization, the modified regions  812 ,  822  may be referred to as a continuous region with a particular length and a particular width, as represented in the devices  800 A-C via dotted line relative to the respective waveguides  110 ,  120  or to respective waveguide widths of the waveguides  110 ,  120  along the length of the first coupling gap  145 . Finally, while  FIGS. 8A-8C  illustrate the modified region  812  and the modified region  822  as like areas, it may be understood that the modified region  812  and the modified region  822  may likewise be dissimilar, non-symmetric, or otherwise distinct in dimensionality or other characteristics. As illustrated in  FIG. 8C , the modified regions  812  and  822  may be distinct in their length, for example such that modified region  822  is shorter than modified region  812  or vice versa. Designs of the lengths and widths of either of the modified regions  812 ,  822  may impact, vary, or characterize switching in devices  800 A-C. 
     In several examples of the present disclosure, the modified region  812  or  822  includes a slight increase in width for the waveguide  110  or  120  near, along to, or adjacent to the second coupling gap  155 . Such a design may tend to impact, vary, or characterize the output or switching performed by the devices  800 A-C, including dispersive or additional characteristics of the respective signal transfer functions as noted. For example, the modified regions  812 ,  822  may enable more rapid variation in switching output relative to input wavelength. The modified regions  812 ,  822  may also facilitate, enable, or result in a lower electrical or voltage variation to modify, configure, or else control the racetrack resonant structure  130  in operation of devices  800 A-C. Devices  800 A-C may thus operate with less power use or more efficiency. Designs may also select for signal transfer functions. 
     Further examples of optical switching according to the present disclosure are described with reference to  FIGS. 9A-9B . In that regard,  FIGS. 9A-9B  are chart diagrams that simulate a signal transfer function for an optical switching device that includes a region of modified waveguide width, according to some examples of the present disclosure. In particular,  FIG. 9A  charts the signal transfer function over a larger wavelength scale; and  FIG. 9B  charts the signal transfer function over a smaller wavelength scale.  FIGS. 9A-9B  may be understood to refer to or characterize any or all of devices  800 A-C of  FIGS. 8A-8C  or any other suitable optical switching device.  FIGS. 9A-9B  may also be understood with reference to a particular frequency passband, whereas such a device may control or vary the frequency passband as noted. In particular,  FIGS. 9A-9B  display the respective wavelength scale in micrometers (μm) of wavelength λ for the input light signal as referenced above. The charts distinguish output in the through or bar state from output in the cross state respectively, as discussed earlier, wherein the two output portions may account for substantially all of the input signal. Thus the transmission scale of 0 to 1 may be understood to refer to the portion of the input light signal output in each respective state. Transmission values between 0 and 1 may refer to or characterize dispersive effects of switching by the optical switching device. 
     In general,  FIGS. 9A-9B  may display charts similar or comparable to  FIGS. 7A-7B . In particular,  FIG. 9A  may be understood to illustrate recurring characteristics over a larger wavelength scale, as comparable to  FIG. 7A . In contrast to  FIG. 7A ,  FIG. 9A  may be understood to indicate a more rapid variation in switching relative to input wavelength, as indicated by a smaller wavelength interval in micrometers (μm) of wavelength λ for the input light signal as referenced above. Further, the frequency passband may be extended in shape. Thus, designs of a region of modified waveguide width may modify switching characteristics. 
     Likewise,  FIG. 9B  may be understood to illustrate particular dispersive effects for wavelengths relative to the frequency passband. In contrast to  FIG. 7B ,  FIG. 9B  may be understood to indicate that output may substantially remain in the through or bar state due to an extended shape of the frequency passband over the smaller wavelength intervals between resonance. As illustrated, switching to a bar state output may be designed to be less specific to resonance than in  FIGS. 7A-7B . Devices  900 A-C may thus operate with less power usage or may perform relatively more efficient switching by such designs. Designs may also select for a particular signal transfer function, as indicated between  FIGS. 7A-7B  and  FIGS. 9A-9B . 
     Further examples of a device for optical switching are described in detail with reference to  FIG. 10 . In that regard,  FIG. 10  is a block diagram of a system for optical switching  1000  that includes a set of adjacent racetrack resonant structures  130 - 1  through  103 - n,  according to some examples of the present disclosure. The system  1000  of  FIG. 10  may be substantially similar to the device  100  of  FIG. 1 . In particular, similarly numbered elements may be substantially similar to those of  FIG. 1  as described in the above, including the waveguides  110  and  120 , the first coupling gap  145 , the racetrack resonant structures  130 - 1  through  130 - n,  and the second coupling gaps  155 - 1  through  155 - n.  In the example of n=1, the system  1000  of  FIG. 10  has one racetrack resonant structure  130 - 1  and one second coupling gap  155 - 1 , such that it may be substantially equivalent to the device  100  of  FIG. 1 . However,  FIG. 10  may be understood to illustrate the system  1000  for any integer value n of racetrack resonant structures 1 to n and second coupling gaps 1 to n. Furthermore, while racetrack resonant structures  130 - 1  and  130 - n  and second coupling gaps  155 - 1  and  155 - n  are illustrated as similar, device  1000  may be understood to allow likewise for elements to be suitably dissimilar, non-symmetric, or otherwise distinct in dimensionality or characteristic, without limitation as to a particular design parameter of the device  1000 . As discussed with respect to  FIG. 1 , design variations may include modifications to any of the symmetries, shapes, widths, and/or spacings for either the waveguides  110  and  120  or each of the racetrack resonant structures  130 , including non-planar or multi-planar designs without limitation, so long as each racetrack resonant structure  130  of the set is suitably positioned to enable a suitable coupling between the first coupling gap  145  and the respective second coupling gap  155 , as described in prior detail with reference to racetrack resonant structure  130  in  FIG. 1 . 
     Referring to system  1000  of  FIG. 10 , the first waveguide  110  is positioned to enable an input light signal to couple from the first waveguide  110  to the second waveguide  120  through the first coupling gap  145  and over a coupling length between the first waveguide  110  and the second waveguide  120 . Each racetrack resonant structure  130  is positioned along the coupling length of the first coupling gap  145  to enable the input light signal to couple from one of the first waveguide  110  and the second waveguide  120 , such that a second set of coupling gaps  155  corresponds to the first set of racetrack resonant structures  130 . Further, each racetrack resonant structure  130  is positioned adjacent to one of the first waveguide  110  and the second waveguide  120  over the coupling length of the first coupling gap  145 , such that each coupling gap  155  is respectively positioned adjacent to the first coupling gap  145 . 
     In some examples, the set of racetrack resonant structures  130  is substantially shorter in length relative to the first coupling gap  145 . In some examples, the set of racetrack resonant structures  130  is positioned symmetrically or near to midway relative to the length of the first coupling gap  145 , such that the set of coupling gaps  155  is likewise positioned symmetrically. As discussed in prior detail with reference to  FIG. 1 , based on positioning each of the racetrack resonant structures  130  may determine a frequency passband for a corresponding portion of the input light signal, so as to switch the output as between the first waveguide  110  and the second waveguide  120 . Each of the racetrack resonant structures  130  may determine a distinct or separate frequency passband, such that a passband portion of the overall output of the device  1000  may correspond to a set of respective frequency passbands. Thus, the switching system  1000  may be designed to perform high-radix optical switching. As with the device  100  of  FIG. 1 , the system  1000  of  FIG. 10  may be characterized by a signal transfer function. Variations in the design of the system  1000  may affect, modify, or vary characteristics of switching, including dispersive effects of the signal transfer function. 
     Further examples are described in detail with reference to  FIG. 11 . In that regard,  FIG. 11  is a block diagram of an optical switching device  1100  that includes a set of electronic controls  1160 - 1  through  1160 - n,  according to some examples of the present disclosure. The optical switching device  1100  may be substantially similar to the optical switching device  1000  of  FIG. 10 . In particular, similarly numbered elements may be substantially similar to those of  FIG. 10 , including the waveguides  110  and  120 , the first coupling gap  145 , the racetrack resonant structures  130 - 1  through  130 - n,  and the second coupling gaps  155 - 1  through  155 - n.  In several examples of the present disclosure, each electronic control  1160  varies the frequency passband of the respective racetrack resonant structure  130 . The electronic control  1160  may vary a parameter of the respective racetrack resonant structure  130  such as a voltage or a temperature parameter, as discussed earlier with respect to  FIG. 3 . The electronic controls  1160  may enable rapid reconfiguration of switching parameters. Thus, switching by the device  1100  may again be reconfigurable on the order of picoseconds (ps). As with device  300  of  FIG. 3 , the electronic controls  1160  of  FIG. 11  may include control logic and/or digital logic in any combination of hardware and/or software designs, including with complimentary metal-oxide semiconductor (CMOS) technologies, integrated chip (IC) or application-specific integrated circuit (ASIC) designs, a controller or microcontroller, or any other suitable electronic control technology that may be used to vary, modify, or otherwise control a parameter of a racetrack resonant structure  130 . While electronic controls  1160 - 1  and  1160 - n  are illustrated as similar in  FIG. 11 , device  1100  may be understood to allow likewise for any electrical controls  1160  to be suitably dissimilar, non-symmetric, or otherwise distinct in dimensionality, connections, or other characteristics, without limitation as to a particular design parameter or control functionality for device  1100 . 
     Further examples for implementing optical switching in accordance with the present disclosure are discussed in detail with reference to  FIG. 12 . In that regard,  FIG. 12  is a flow diagram of a method  1200  of performing optical switching with a set of adjacent racetrack resonant structures, according to some examples of the present disclosure. As noted with respect to method  500  of  FIG. 5 , the description of method  1200  may be understood to be non-limiting. Blocks may be added to or omitted from the method  1200  without departing from the scope of the present disclosure. Unless otherwise noted, blocks of the method  1200  may be performed in any order, including simultaneously or concurrently by one or more device elements. In general, the method  1200  is equally suitable for performance using the devices  FIG. 1, 2, 3, 6, 8A-8C, 10 , or  11 , as well as any other suitable switching device. For purposes of clarity, the discussion may refer to device  1000  of  FIG. 10 , without limitation. 
     Blocks  1202 ,  1204 ,  1206 , and  1250  of the method  1200  may each correspond with substantial similarity to the blocks  502 ,  504 ,  506 , and  550  respectively of the method  500  as described in the above. In block  1202 , the system receives an input light signal by a first waveguide of a directional coupler. This may be performed substantially as described in block  502  of  FIG. 5  and with reference to the first waveguide  110  of  FIG. 10 . In block  1204 , the system couples the input light signal in the first waveguide to a second waveguide of the directional coupler. This may be performed substantially as described in block  504  of  FIG. 5  and with reference to the second waveguide  120  of  FIG. 10 . 
     In block  1206 , the system couples the input light signal in the directional coupler to each racetrack resonant structure of a set of racetrack resonant structures, wherein each resonant structure has a resonant frequency of a set of resonant frequencies. This may be performed substantially as described in block  506  of  FIG. 5  and with respect to each racetrack resonant structure  130  of  FIG. 10 . In particular, the system  1000  may repeat block  506  of the method  500  with each racetrack resonant structure  130  as the input light signal propagates along the length of the first coupling gap  145  as in  FIG. 10 , from the racetrack resonant structure  130 - 1  and past the racetrack resonant structure  130 - n.  Thus with reference to  FIG. 6 , the system  1000  may couple a respective portion  625  into each racetrack resonant structure  130  of the set, wherein some such portions may be null insofar as the input light signal does not coincide with the respective frequency passband or the resonant frequency. By such repetition, the system  1000  may perform selective switching for a set of frequency passbands that define a passband portion of output with respect to the input light signal. In some examples wherein multiple racetrack resonant structures  130  have similar or identical passband frequencies, a similar or identical portion  625  of the propagating signal may couple into multiple racetrack resonant structures  130  and result in a modified or distinct phase shift. As discussed, a remaining portion of the signal may still continue unimpeded or unaffected. 
     In block  1210 , the system changes at least one parameter for one racetrack resonant structure  130  to change at least one frequency passband. This may be performed substantially as described with respect to the control mechanism  360  of  FIG. 3  or the electronic controls  1160  of  FIG. 11 . The system  1000  may thereby vary, control, or modify one or more of the racetrack resonant structures  130  dynamically for rapid reconfiguration of switching. They system  1000  may independently control each or any of the racetrack resonant structures  130 . Variation in the set of frequency passbands may define the passband portion of output, e.g. that portion output in bar state by the first waveguide instead of by the second waveguide. As noted, such a passband portion may correspond to a set of multiple frequency passbands. Thus, a larger number of racetrack resonant structures  130  may allow controls over relatively more frequency passbands. This may facilitate more control over the signal transfer function. 
     In block  1250 , the system outputs a first or passband portion of the input light signal by the first waveguide and a second or remaining portion by the second waveguide. This may be performed substantially as described in block  550  of  FIG. 5  and with reference to the first portion  660  and the second portion  670  as described above with respect to  FIG. 6 . As noted, the passband portion may be a set of frequency passbands, as defined in the system. Such a system as in  FIG. 10  may thus perform the method  1200  to switch an input light signal. 
     Further examples of a system for optical switching are described in detail with reference to  FIGS. 13A-13C . In that regard,  FIGS. 13A-13C  are block diagrams of an optical switching system  1300 A-C that includes a region of modified coupling width  1312  or  1322 , according to some examples of the present disclosure. In particular,  FIG. 13A  illustrates the system  1300 -A with a region of modified coupling width  1312  in the first waveguide  110 ;  FIG. 13B  illustrates the system  1300 -B with a region of modified coupling width  1322  in the second waveguide  120 ; and  FIG. 8C  illustrates the system  1300 -C with a region of modified width  1312  in the first waveguide  110  as well as a region of modified waveguide width  1322  in the second waveguide  120 . Elements of the systems  1300 A-C may be substantially similar to those of the system  1000  of  FIG. 10 . In particular, similarly numbered elements may be substantially similar or identical to elements of  FIG. 10  as described in the above, including the waveguides  110  and  120 , the first coupling gap  145 , the set of racetrack resonant structures  130 - 1  through  130 - n,  and the set of second coupling gaps  155 - 1  through  155 - n.    
     In several examples of the present disclosure, a region of modified coupling width  1312  or  1322  is designed in one of the waveguides  110  or  120  in a position near, adjacent to, or along the length of the set of second coupling gaps  155 . In several such examples, the modified region  1312  or  1322  is also designed to have a shorter length than the first coupling gap  145 , as illustrated in  FIGS. 13A-13C . The relative dimensions of the modified region  1312  or  1322  are illustrative, without limiting a particular design parameter of the devices  1300 A-C. Thus, the designs of the waveguides  110  or  120  may determine the characteristics of the respective modified regions  1312 ,  1322 . For purposes of characterization, the modified regions  1312 ,  1322  may be referred to as a continuous region with a particular length and a particular width, as represented in the devices  1300 A-C via dotted line relative to the respective waveguides  110 ,  120  or to a respective coupling width of the first coupling gap  145 . Finally, while  FIGS. 13A-13C  illustrate the modified region  1312  and the modified region  1322  as like areas, it may be understood that the modified region  1312  and the modified region  1322  may likewise be dissimilar, non-symmetric, or otherwise distinct in dimensionality or other characteristics. As illustrated in  FIG. 13C , the modified regions  1312  and  1322  may be distinct in their length, for example such that the modified region  1322  is shorter than the modified region  1312  or vice versa. Designs of the lengths and widths of either of the modified regions  1312  or  1322  may impact, vary, or characterize switching parameters in devices  1300 A-C. 
     In several examples of the present disclosure, the modified region  1312  or  1322  includes a slight increase in width for the first coupling gap  145  near, along to, or adjacent to the set of second coupling gaps  155 . For example, the first coupling gap  145  between the waveguides  110 ,  120  may thus be increased to a few micrometers (μm) in its overall width. Notably, such designs maintain the waveguides  110 ,  120  in proximity relative to their coupling distance, as in a directional coupler. For example, common-mode fabrication errors continue to dominate over total fabrication error in the devices  1300 A-C. Thus, the waveguides  110  and  120  do not include additional elements such as phase shifters to correct for phase variations based on the separation. This design is in contrast, for example, to a ring-assisted Mach-Zehnder Interferometer (MZI), where use of notably wider separation between waveguides may introduce design complexities to ensure suitable phase alignments. 
     These designs of  FIGS. 13A-13C  may tend to impact, vary, or characterize the output or switching performed by the devices  1300 A-C, including in dispersive or additional characteristics of the respective signal transfer functions as discussed above. For example, the modified regions  1312 ,  1322  may facilitate, enable, or result in allowing device designs to position the set of racetrack resonant structures  130  with more flexibility relative to the first coupling gap  145 , including use of less central positions, nearer to the edges of its length, or less symmetric placements. This may also allow an increase in the number of racetrack resonant structures  130  relative to a particular or fixed length of the first coupling gap  145 . For example, the modified regions  1312 ,  1322  may also facilitate, enable, or result in designs with improved wavelength-selective switching, by improving upon the independence of each racetrack resonant structure  130  or by lowering interference between frequency passbands or resonant frequencies. This may allow the devices  900 A-C to modify, configure, or control effects of each racetrack resonant structure  130  more independently during device operation. Devices  900 A-C may thus operate with more efficient, reliable, or selective optical switching. Such design variations may also facilitate any particular or desirable signal transfer function. 
     The elements of  FIGS. 13A-13C  may be similarly combined or implemented in other suitable device designs of the present disclosure. In particular, the modified coupling regions  1312 ,  1322  may be suitable for use in the example devices of  FIG. 1, 2, 3, 6, 8A-8C, 10 , or  11 , and may devices  1300 A-C may perform any or all blocks of the method  500  of  FIG. 5  or the method  1200  of  FIG. 12 . Similarly, elements of the  FIGS. 8A-8C  may be combined or implemented in other suitable device designs of the present disclosure, including  FIGS. 13A-13C  and others. Thus, in some example devices of the present disclosure, designs implement both of one or more regions of modified waveguide width  812 ,  822  and one or more regions of modified coupling width  1312 ,  1322 . Such elements may be defined by the waveguide design, as noted. The combined effects of such additional design elements on any such device may further vary, modify, characterize or improve on switching by the device. 
     In the foregoing description, numerous details are set forth to provide an understanding of the subject disclosed herein. However, implementations may be practiced without some or all of these details. Other implementations may include modifications and variations from the details discussed above. It is intended that the appended claims cover such modifications and variations.