Abstract:
Aspects of the present disclosure describe planar lightwave circuit systems, methods and structures including a resonant mirror assembly having cascaded resonators that provide or otherwise facilitate the control of the transmissivity/reflectivity of a planar lightwave circuit (PLC)—or portion thereof—over a range of 0% to substantially 100%.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/245,798 filed Oct. 23, 2015 which is incorporated by reference as if set forth at length herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to photonic integrated circuit (PIC) technologies and more particularly to planar lightwave circuits (PLCs) exhibiting controllable transmissivity/reflectivity. 
       BACKGROUND 
       [0003]    As is known, PLCs have found widespread applicability in a number of technological arts including communications and biomedical instrumentation—among others. In a number of such applications, controlling light that propagates through the PLC (i.e., transmissivity/reflectivity of the PLC) is of critical importance. Given this importance, systems, methods structures that provide or otherwise facilitate control of the transmissivity/reflectivity of a PLC would represent a welcome addition to the art. 
       SUMMARY 
       [0004]    An advance in the art is made according to aspects of the present disclosure which describes systems, methods and structures that include a resonant mirror assembly having a number of cascaded resonators that provide or otherwise facilitate the control of the transmissivity/reflectivity of a planar lightwave circuit (PLC)—or portion thereof—over a range of 0% to substantially 100%. 
         [0005]    Viewed from a first aspect, the present disclosure describes a planar lightwave circuit comprising an input port; an output port; and a resonant mirror assembly optically coupling the input port to the output port, the resonant mirror assembly including a number of cascaded optical resonators, each optical resonator exhibiting a pair of coupling coefficients, wherein one of the coupling coefficients of at least one of the plurality of resonators is different in value from all the other coupling coefficients. Of particular advantage, such difference may be achieved at the time of fabrication and/or during operation of the PLC. 
         [0006]    Viewed from another generalized aspect the present disclosure is directed to a planar lightwave circuit comprising an input waveguide; an output waveguide; n optical resonators where n&gt;=3; n+1 bus waveguides, each of bus waveguides having a first end and a second end; a first coupler optically coupling the input waveguide to the first end of the first one of the n bus waveguides and to the first end of the nth of the n bus waveguides; a second coupler optically coupling the output waveguide to the second end of the first one of the n bus waveguides and to the second end of another one of the n bus waveguides; wherein each of the n optical resonators are optically coupled to at least two of the bus waveguides, each of the resonator bus couplings being defined by a coefficient K; and wherein at least one of the coefficients is different in value from all of the others. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]    A more complete understanding of the present disclosure may be realized by reference to the accompanying drawing in which: 
           [0008]      FIG. 1  depicts a schematic diagram of an illustrative, generalized PLC tunable reflector including n cascaded tunable resonators according to an aspect of the present disclosure; 
           [0009]      FIG. 2  depicts a schematic diagram of an illustrative PLC tunable reflector including two cascaded tunable resonators according to an aspect of the present disclosure; 
           [0010]      FIG. 3  depicts a schematic diagram of another alternative illustrative PLC tunable reflector including three cascaded tunable resonators according to an aspect of the present disclosure; 
           [0011]      FIG. 4  depicts a plot of transmitted and reflected light as a function of coupling coefficients K 1  through K 6  according to an aspect of the present disclosure; 
           [0012]      FIG. 5  depicts an illustrative multi-port tunable reflector according to an aspect of the present disclosure; 
           [0013]      FIG. 6  depicts a schematic of an illustrative tunable waveguide laser according to an aspect of the present disclosure; 
       
    
    
       [0014]    The illustrative embodiments are described more fully by the Figures and detailed description. Inventions according to this disclosure may, however, be embodied in various forms and are not limited to specific or illustrative embodiments described in the Figures and detailed description. 
       DESCRIPTION 
       [0015]    The following merely illustrates the principles of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope. 
         [0016]    Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. 
         [0017]    Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
         [0018]    Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure. 
         [0019]    Unless otherwise explicitly specified herein, the FIGs comprising the drawing are not drawn to scale. 
         [0020]    Turning now to  FIG. 1 , there is shown a schematic diagram depicting a generalized, PLC tunable reflector including cascaded tunable resonators according to an aspect of the present disclosure. As may be observed from that  FIG. 1 , the tunable reflector includes an input waveguide and an output waveguide optically coupled—through the effect of a pair of couplers—to a series of bus waveguides which in turn are coupled to one or more resonator structures. As may be appreciated by those skilled in the art, this “cascaded resonator structure” including bus waveguides and coupled resonators operatively form a resonant mirror assembly which advantageously permits the selective control of the transmissivity/reflectivity of the overall tunable reflector structure with regard to light traversing the tunable reflector structure. 
         [0021]    Of particular significance to those skilled in the art is that the structure(s) shown in  FIG. 1  may advantageously be fabricated from any of a variety of known materials, techniques and/or processes. Note that while the resonators shown in the Figure(s) are schematically shown as ring resonators, those skilled in the art will readily appreciate that any of a variety of known structures providing resonator functionality including ring, racetrack, solid disk, bragg grating, Mach-Zehnder, etc. may be employed—including different individual (mixed) ones in a particular reflector structure. Similarly, while the couplers are shown in the Figure(s) as “Y” couplers, this disclosure is not so limited and therefore alternative coupler structures may likewise be advantageously employed. Additionally, particular waveguide structures may likewise be any of a variety known in the art as suitable for PLC applications including—but not limited to—TRIPLEX waveguides described in U.S. Pat. Nos. 7,146,087 and 7,142,759, each of which is incorporated by reference as if set forth at length herein. Finally—and as we shall discuss in more detail later—we note that controllability of the overall transmissivity/reflectivity of structures according to the present disclosure and as generally shown in  FIG. 1  may advantageously be achieved through the effect of techniques and/or structures that affect the resonant structures characteristics including heat, stress, etc. More specifically, phase shifters shown in the Figure(s) may be constructed from any of a variety of known structures/techniques/materials that are compatible with the particular structure(s) employed and produce the desired amount of heat, stress, etc., that affects characteristics in a desired manner. Note further that the phase shifters shown as part of input waveguides or bus waveguides are optionally added to the structures disclosed herein as desired and/or necessary to achieve desired functionality. 
         [0022]    Turning now to  FIG. 2 , there is shown a schematic diagram depicting an illustrative PLC tunable reflector including two cascaded tunable resonators according to an aspect of the present disclosure. As may be observed from  FIG. 2 , tunable reflector  200  includes input port  215 , output port  225 , input waveguide  210 , output waveguide  220  and resonant mirror assembly  250 . Input waveguide  210  and output waveguide  210  are optically coupled to resonant mirror assembly  250  by couplers  230 - 1  and  230 - 2 , respectively. Individual phase shifters are shown with respect to the input waveguide  210  and bus waveguide  240 . 
         [0023]    As may be readily understood from  FIG. 2 , input waveguide  210  receives input light  291  at input port  215  and provides reflected light  293  to that same port. Output waveguide  220  provides output light  292  at output port  225 . 
         [0024]    Operationally—and as will be appreciated by those skilled in the art—tunable reflector  200  receives input light  291  and controls the wavelength(s) of reflected light signal(s)  293 , as well as distribution of light in transmitted light signal  292  and reflected light signal  293 . 
         [0025]    Resonant mirror structure  250  exhibits a controllable reflectivity and is operatively coupled between input waveguide  210  and output waveguide  220 . Resonant mirror structure  250  is shown including a number of ring resonators namely, R 1 , and R 2  and bus waveguides  240 ,  260  and  280 . As generally depicted in  FIG. 2 —and as will be readily understood by those skilled in the art—a ring resonator (optical ring resonator) is a set of waveguides in which at least one is a closed loop coupled to some light input and output which may be—but are not limited to being—waveguides. Notably, while the ring resonators R 1  and R 2  are shown as “single ring” structures, those skilled in the art will appreciate that the specific structures of individual resonators may differ from those shown, i.e., “double” or “multi-ring” resonator structures, or resonator structures exhibiting different radii from those depicted in the figure or one another. Additionally—and as previously noted generally with respect to  FIG. 1 —specific configurations may include a different number of resonator structures and different number of bus waveguides and other structures (including any optional phase shifter(s) in the bus or input or other waveguides) which are included in a particular, overall resonant mirror structure. 
         [0026]    Operationally, resonant mirror structure  250  reflects wavelengths in input light  291  back to input port  215  as a function of collective resonance(s) of ring resonators R 1  and R 2  that are included in resonant mirror structure  250 . As will be understood by those skilled in the art, resonant mirror structure  250  is analogous to ring resonator-based mirrors described in U.S. Pat. No. 7,835,417 which is incorporated herein by reference as if set forth at length however, resonant mirror structure  250  exhibits additional functionality in that it is operable for controlling the amount of optical power reflected to input port  215  and conveyed to output port  225 . Notably, and as will be further understood by those skilled in the art, resonant mirror structure  250  reflects a plurality of wavelength components which are separated by the free-spectral range (FSR) of the composite resonant structure. Advantageously, the wavelengths reflected may be tuned anywhere within the spectral range represented by this FSR. 
         [0027]    As previously noted, input waveguide  210  and output waveguide  220  are optically coupled to resonant mirror assembly  250  by couplers  230 - 1  and  230 - 2 . As depicted in  FIG. 2 , couplers  220 - 1  and  220 - 2  are a known type of coupler namely, “Y” couplers as they are known in the art and may be advantageously fabricated using conventional, integrated optics techniques and methods. As shown in  FIG. 2 , coupler  230 - 1  optically couples input waveguide  210  equally (50:50) with bus waveguides  240  and  260 . As a result, input light  291  is split substantially equally into the two bus waveguides  240  and  260 . Additionally—and while not specifically shown in  FIG. 2 —it is noted and understood that in certain implementations it may be desirable to optically couple input waveguide  210  to bus waveguides  240  and  260  using other structures such as a directional coupler. 
         [0028]    Similarly, coupler  230 - 2  may be a conventional integrated-optics Y-coupler that optically couples bus waveguides  240  and  280  to output waveguide  220  such that light traversing the bus waveguides is combined to form output light  292 . Again, it is noted that in certain implementations it may be desirable to optically couple output waveguide  220  to bus waveguides  240  and  280  using other structures such as a directional coupler. 
         [0029]    Each of the ring resonators R 1  and R 2  may advantageously be formed as an integrated-optics waveguide ring that is optically coupled to a pair of bus waveguides. The individual resonance(s) of resonators R 1  and R 2  may be advantageously controlled individually by respective phase shifters  270 - 1  and  270 - 2 . Note further that while phase shifters  270 - 1 , and  270 - 2  are depicted in  FIG. 2  as extending completely over respective resonator(s), phase shifters employed in a particular configuration may or may not so extend. 
         [0030]    Notably, while this illustrative embodiment shown in  FIG. 2  employs ring resonators that—in conjunction with waveguides and couplers—collectively define resonant mirror structure  250 , those skilled in the art will readily understand and appreciate that alternative resonant structures may be employed including—but not limited to—cascaded resonant structure(s) having at least one alternative tunable resonant element for example, a tunable optically resonant cavity, a tunable coupled-cavity filter, and the like. 
         [0031]    We additionally note that while coefficient K may be defined over a range of 0≦K≦1, structures according to the present disclosure will preferably exhibit a range of 0.05≦K≦1. With structures exhibiting two individual resonator structures and characterized by coefficients K 1 , K 2 , K 3 , and K 4  such as that shown in  FIG. 2 , it is generally preferable to configure it such that at least three of the coefficients (i.e., K 2 , K 3 , and K 4 ) are substantially equal to one another while the fourth (i.e., K 1 ) is different (greater or lesser) than the other three (i.e. K 2 =K 3 =K 4  and K 1 ≠K 2 ; K 1 ≠K 3 ; and K 1 ≠K 4 ; or K 1 =K 3 =K 4  and K 2 ≠K 1 ; K 2 ≠K 3 ; and K 2 ≠K 4 ) By choosing an appropriate combination of coupling coefficients K 1  through K 4 , the overall reflectance and transmittance of resonant mirror structure  250  may be selectively controlled. As noted previously—according to the present disclosure—at least one of the coefficients is different in value from the others which are all the same value. 
         [0032]    Turning now to  FIG. 3 , there is shown a schematic diagram depicting an illustrative PLC tunable reflector including cascaded three tunable resonators according to an aspect of the present disclosure. As may be observed from  FIG. 3 , tunable reflector  300  includes input port  302 , output port  304 , input waveguide  306 , output waveguide  308  and resonant mirror assembly  310 . Input waveguide  306  and output waveguide  308  are optically coupled to resonant mirror assembly  310  by couplers  320 - 1  and  320 - 2 , respectively. 
         [0033]    As may be readily understood from  FIG. 3  and similar to that previously described—input waveguide  306  receives input light  324  at input port  302  and provides reflected light  328  to that same port. Output waveguide  308  provides output light  326  at output port  304 . 
         [0034]    Operationally—and as will be appreciated by those skilled in the art—tunable reflector  300  receives input light  324  and controls the wavelength(s) of reflected light signal(s)  328 , as well as distribution of light in transmitted light signal  326  and reflected light signal  328 . 
         [0035]    Advantageously, waveguides employed in tunable reflector  300  may be any of a variety of known integrated optical waveguides suitable for use in PLC structures. Of further advantage, waveguides employed in tunable reflector(s) according to the present disclosure such as that shown in  FIG. 3 , as may include multi-core waveguides. 
         [0036]    Resonant mirror structure  310  exhibits a controllable reflectivity and is operatively coupled between input waveguide  306  and output waveguide  308 . Resonant mirror structure  310  is shown as a coupled-cavity resonator that includes a number of ring resonators namely, R 1 , R 2 , and R 3  and bus waveguides  312 ,  314 ,  316 , and  318 . As generally depicted in  FIG. 3 —and as will be readily understood by those skilled in the art—a ring resonator (optical ring resonator) is a set of waveguides in which at least one is a closed loop coupled to some light input and output which may be—but are not limited to being—waveguides. Notably, while the ring resonators R 1 , R 2 , and R 3  are shown as “single ring” structures, those skilled in the art will appreciate that the specific structures of individual resonators may differ from those shown, i.e., “double” or “multi-ring” resonator structures, or resonator structures exhibiting different radii from those depicted in the figure or one another. 
         [0037]    Operationally, resonant mirror structure  310  reflects wavelengths in input signal  324  back to input port  302  as a function of collective resonance of ring resonators R 1 , R 2 , and R 3  that are included in resonant mirror structure  310 . As will be understood by those skilled in the art, resonant mirror structure  310  is analogous to ring resonator-based mirrors while exhibiting additional functionality in that it is operable for controlling the amount of optical power reflected back to input port  302  and transmitted to output port  304 . Resonant mirror structure  310  reflects a plurality of wavelength components which are separated by the free-spectral range (FSR) of the composite resonant structure and may advantageously be tuned anywhere within the spectral range represented by the FSR. 
         [0038]    As previously noted, input waveguide  306  and output waveguide  308  are optically coupled to resonant mirror assembly  310  by couplers  320 - 1  and  320 - 2  that are illustratively depicted as Y-couplers that may be advantageously fabricated using conventional, integrated optics techniques and methods. As shown in that Figure, coupler  320 - 1  optically couples input waveguide  306  equally (50:50) with bus waveguides  312  and  318 . As a result, input light  324  is split substantially equally into the two bus waveguides  312  and  318 . Additionally—and while not specifically shown in  FIG. 3 —it is noted that in certain implementations it may be desirable to optically couple input waveguide  306  to bus waveguides  312  and  318  using other structures such as a directional coupler or other couplers exhibiting different splitting ratios. When such directional coupler(s) are used as replacement for one or both couplers  320 - 1 ,  320 - 2  improved transmittance through overall reflector structure may advantageously be achieved. 
         [0039]    Similarly, coupler  320 - 2  may be a conventional integrated-optics y-coupler that optically couples bus waveguides  312  and  314  to output waveguide  308  such that light traversing the bus waveguides is combined to form output light  326 . Again, it is noted that in certain implementations it may be desirable to optically couple output waveguide  308  to bus waveguides  312  and  314  using other structures such as a directional coupler. 
         [0040]    Each of the ring resonators R 1 , R 2 , and R 3  may advantageously be formed as an integrated-optics waveguide ring that is optically coupled to a pair of bus waveguides. The individual resonance(s) of resonators R 1 , R 2 , and R 3  may be controlled respectively by phase shifters  322 - 1 ,  322 - 2 , and  322 - 3 . Note further that while phase shifters  322 - 1 ,  322 - 2 , and  322 - 3  are depicted in  FIG. 3  as extending completely over respective resonator(s), phase shifters employed in a particular configuration may or may not so extend. 
         [0041]    By way of illustrative example only, phase shifters employed may extend over only a portion of respective ring resonator(s) and advantageously enable separate control of coupling coefficients for a ring and its associated bus waveguides. As a further illustrative example, ring resonator R 1 —shown operatively coupled with phase shifter  322 - 1  in  FIG. 3 —may include independent portions such that independent control of coupling coefficients K 1  (shown between ring R 1  and bus waveguide  312  in  FIGS. 1 ) and K 2  (shown between ring R 1  and bus waveguide  314  in  FIG. 3 ) may be controlled by—for example—phase shifter portion  322 - 1 ( a ) and  322 - 1 ( b ), respectively. Similar independently controllable structures may be employed in any or all of the individual resonators as desired. 
         [0042]    Notably, while this illustrative embodiment shown in  FIG. 3  employs ring resonators that—in conjunction with waveguides and couplers—collectively define resonant mirror structure  310 , those skilled in the art will readily understand and appreciate that alternative resonant structures may be employed including—but not limited to—cascaded resonant structure(s) having at least one alternative tunable resonant element for example, a tunable optically resonant cavity, a tunable coupled-cavity filter, and the like may be employed as well. 
         [0043]    With this illustrative, overall structure described, those skilled the art will understand that each of the ring resonators R 1 , R 2 , and R 3  is characterized by a quality factor (i.e., “Q” factor) that may advantageously be controlled or otherwise influenced by its respective heater(s) or portions. By choosing an appropriate combination of coupling coefficients K 1  through K 6 , the overall reflectance and transmittance of resonant mirror structure  310  may be selectively controlled. 
         [0044]    Continuing with our discussion of  FIG. 3 , it may be observed that phase shifter  322 - 4  is operatively coupled to bus waveguide  312  such that it may operate as a phase shifter for light traversing the bus waveguide resulting in a positive coherent superposition of the light in the bus waveguide. In alternative embodiment(s), phase shifter  322 - 4  may be operatively coupled to bus waveguide  314  or—in a further alternative embodiment—a similar phase shifter(s) may be operatively coupled to both waveguides  312 ,  314  and/or other waveguide(s) as desired and/or necessary such that independent control of the phase of light in individual waveguides is enabled. In still further illustrative embodiments, a different phase shifter/controller may be used to control the phase of light in one or more of the bus waveguides, such as stress-tuning elements described in U.S. patent application Ser. No. 14/580,831, filed Dec. 23, 2015, the entire contents of which is incorporated by reference as if set forth at length herein. 
         [0045]    With reference now to  FIG. 4 , there is shown a plot of transmitted and reflected light for illustrative structures according to the present disclosure such as that depicted in  FIG. 3  as a function of coupling coefficients K 1  through K 6 . More particularly, the plot shown in  FIG. 4  illustrates the effect(s) of tuning coefficient K 1  from 0.0-1.0 while each of K 2  through K 6  is maintained at 0.1. 
         [0046]    As should now be appreciated by those skilled in the art, structures and methods according to the present disclosure may be employed in a variety of useful system configurations. By way of illustrative example,  FIG. 5  depicts an illustrative multi-port tunable reflector according to an aspect of the present disclosure. 
         [0047]    With reference now to  FIG. 5 , it may be observed that the multi-port tunable reflector includes a tunable reflector assembly according to the present disclosure such as that shown and described previously along with a tunable coupler configured as part of a Mach-Zehnder arrangement, an input port and a number of output ports. By including the tunable coupler between the input port and the reflector, the overall reflectivity and transmittance of the multi-port tunable reflector may be tunable from 0-100% and 100-0%, respectively. Inasmuch as the transmittance may be described by T∈[0,1], the reflectivity is described by (1−T) 2  and the second output port will exhibit a lower output power as generally described by (T−T) 2 , neglecting waveguide propagation loss. 
         [0048]    By way of yet another illustrative example,  FIG. 6  depicts a schematic of an illustrative tunable waveguide laser according to an aspect of the present disclosure. Tunable waveguide laser includes a source, a tunable coupler and a tunable reflector according to the present disclosure. 
         [0049]    Advantageously, source may include a semiconductor optical amplifier (SOA) having a gain section. In alternative configurations, source may be a different optical-gain element, for example an erbium-doped fiber amplifier, a semiconductor laser, or other, known, source elements. When configured as shown in  FIG. 6 , two output(s) are provided. 
         [0050]    At this point, while we have presented this disclosure using some specific examples, those skilled in the art will recognize that our teachings are not so limited and that various alternative configurations may be readily devised by those skilled in the art. Accordingly, this disclosure should be only limited by the scope of the claims attached hereto.