Abstract:
A waveguide pathway and a phased-array device utilizing the waveguide pathway are provided. The waveguide pathway comprises a two-dimensional array of homogeneous unit cells. Each unit cell includes a branch point leading to two waveguide branches. Each waveguide branch passes through a positive phase shift element and a negative phase shift element, in series, and with each waveguide branch passing through the two phase shift elements in opposite order relative to the other waveguide branch. Each unit cell additionally includes a convergence point where the two waveguide branches converge. The wavepath grid and the phased-array device are capable of producing output channels with linear, asymmetrical phase distribution but with symmetrical power distribution without employing amplifiers. The phased-array device can be tuned with single-channel or dual-channel control.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates in general to a design for a linear phased-array device and in particular to a waveguide architecture to be employed on such a device and capable creating symmetrical power output across the array. 
       BACKGROUND 
       [0002]    Phased-array devices operate by splitting a coherent wave source (e.g. electromagnetic or acoustic) into multiple individual sub-beams, shifting the wave phase of the individual sub-beams, and then emitting the sub-beams in close, wavelength-scale, physical proximity relative to one another. This arrangement allows, through control of the relative phases of the individual sub-beams, the generation of engineered wavefronts. Such spatial phase control enables attributes such as solid-state beam steering and engineered depth projection. 
         [0003]    Electromagnetic phased-array devices are well known for microwave systems and are becoming increasingly so for optical (infrared, visible, and ultraviolet) systems. A difficulty with conventional phased-array design is that there is typically an asymmetrical (e.g. linear) output power distribution proportional to the linear phase distribution of the output channels. 
         [0004]    This asymmetrical power distribution can interfere with device functions, such as beam steering, which require accurate control of both phase and amplitude. In microwave systems, asymmetric power output can be mitigated by assigning a separate amplifier to each microwave channel. Such an approach can be difficult to unfeasible for optical systems, however. 
       SUMMARY 
       [0005]    The present disclosure presents a wavepath grid comprising an array of regularly repeating unit cells. The present disclosure additionally presents a phased-array device employing such a wavepath grid. The phased-array device is operable to receive a coherent wave source, such as a laser, to split the source into multiple individual sub-beams and to shift the phases of the multiple individual sub-beams to a linear distribution, and to outcouple the phase-shifted sub-beams with symmetrical power distribution. 
         [0006]    In one aspect, a linear phased-array device is disclosed. The device includes a chip, a wavepath grid including a two-dimensional array of wavepath unit cells, and a plurality of terminal outlets disposed within the wavepath grid and operable to outcouple individual sub-beams of a wave from the chip. Each unit cell includes a waveguide, a branch point in the waveguide, the branch point leading to first and second waveguide branches, and a convergence point of the first and second waveguide branches. The first waveguide branch passes first through a positive phase shift element and subsequently through a negative phase shift element and the second waveguide branch passes first through a negative phase shift element and subsequently through a positive phase shift element. 
         [0007]    In another aspect, a wavepath grid is disclosed. The wavepath grid is operable to receive a coherent wave source, to split the coherence wave source into multiple individual sub-beams, and to differentially shift the phase of each of the multiple individual sub-beams. The wavepath grid includes a two-dimensional array of wavepath unit cells, each unit cell comprising a waveguide, a branch point in the waveguide, the branch point leading to first and second waveguide branches, and a convergence point of the first and second waveguide branches. The first waveguide branch passes first through a positive phase shift element and subsequently through a negative phase shift element and the second waveguide branch passes first through a negative phase shift element and subsequently through a positive phase shift element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Various aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the various aspects taken in conjunction with the accompanying drawings, of which: 
           [0009]      FIG. 1  is a schematic plan view of a wavepath unit cell; 
           [0010]      FIG. 2A  is a schematic plan view of a wavepath grid comprising a plurality of wavepath unit cells of the type illustrated in  FIG. 1 ; 
           [0011]      FIG. 2B  is a calculated output power distribution curve for a wavepath grid of the type shown in  FIG. 2A ; 
           [0012]      FIG. 2C  is a plan view of the wavepath grid of  FIG. 2A  overlaying a size-descriptive grid; 
           [0013]      FIG. 3A  is a schematic plan view of a wavepath grid of the type shown in  FIG. 2A  and having a power splitting region; 
           [0014]      FIG. 3B  is a schematic plan view of a wavepath grid of the type shown in  FIG. 3A  and having a power splitting region and a power mixing region; 
           [0015]      FIG. 4A  is a two-dimensional graph of calculated output power distributions of wavepath grids of the type shown in  FIG. 3B  with power splitting regions of varying length; 
           [0016]      FIG. 4B  is a three-dimensional graph illustrating the data of  FIG. 4A ; and 
           [0017]      FIG. 5  is a perspective view of a phased-array device having a wavepath grid of the type illustrated in  FIG. 2A  but with slightly different unit cell geometry. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present disclosure describes a lightpath architecture for optical phased-array systems. An existing phased-array system splits a light source into multiple parallel channels, passes each channel through a variable phase shifter such as a medium of different refractive index, and then emits the various channels in close proximity to one another (on the order of 1 μm). Such a system allows the creation of an engineered wavefront, controlled by the pattern of interferences and coherences between the various, emitted, phase-shifted channels. This wavefront engineering can enable beam steering without moving parts, useful for example in a Lidar system, or tunable depth perception. 
         [0019]    Previous phased-array systems may use a graded architecture wherein separate wavepath channels are equipped with different phase shift elements. For example, a linear distribution of phase shift element lengths can produce a linear distribution of output phases. Unfortunately, it can also produce an asymmetrical power distribution. 
         [0020]    By contrast, phased array devices of the present disclosure utilize a wavepath grid with unit cell architecture to produce linear phase distribution with symmetrical power distribution. The disclosed architecture features a criss-crossing waveguide network with interspersed with phase shift elements of equal magnitude but opposite sign. The phase at any output point is controlled by the ratio of positive to negative phase shift elements encountered while the power is dictated by the ratio of convergence points to splitting points in the grid, as explained further below. 
         [0021]    Referring now to  FIG. 1 , a wavepath unit cell  100  includes a waveguide  102  operable to direct the propagation of a wave. In some variations, waveguide  102  will be configured to direct the propagation of an electromagnetic wave. In some particular variations, waveguide  102  will be configured to direct the propagation of any or all of an infrared, visible, or ultraviolet electromagnetic wave. Waveguide  102  will typically comprise a rib waveguide or similar structure operable to direct propagation of a wave substantially in one direction at any point in the waveguide  102 . Waveguide  102  can comprise any material suitable to guide propagation of the wave to be propagated. In many instances, waveguide  102  will comprise an optically active material, such as silicon. 
         [0022]    Wavepath unit cell  100  can optionally include a waveguide inlet  104 , operable to receive a coherent wave from a source and couple said coherent wave into waveguide  102 . In some variations, a source of a coherent wave will be a laser. In some particular variations, a source of a coherent wave will be an infrared, visible, or ultraviolet laser. In general, the composition of waveguide  102  will be selected from suitable materials based on the nature and/or wavelength of the wave to be guided. 
         [0023]    Wavepath unit cell  100  additionally includes a branch point  104  where waveguide  102  splits into two waveguide branches  106 A and  106 B. The wave directed into waverguide branch  106 A and the wave directed into waveguide branch  106 B will typically be of approximately equal amplitude, each approximately half that of the amplitude of the wave incident upon branch point  104 . 
         [0024]    Waveguide branch  106 A directs the branched wave through two phase shift elements  108 A and  108 B in series. Phase shift elements  108 A and  108 B will each shift the phase of the coherent guided wave passing through them. The phase shift applied by phase shift elements  108 A and  108 B is substantially equivalent in magnitude, but opposite in sign relative to one another. Waveguide branch  106 B also directs the branched wave through the two phase shift elements  108 B and  108 A, but in the opposite order relative to branch  106 A. For ease of use, a phase shift element  108 A may be referred to as a “positive phase shift element” and a phase shift element  108 B may be referred to as a “negative phase shift element”, although the specific designation of “positive” and “negative” as used here is arbitrary. 
         [0025]    Phase shift elements  108 A,  108 B can be prepared by any suitable means such as temperature modulation of waveguide  102  at the appropriate locations or by inclusion of materials having different inherent refractive index at the appropriate locations. 
         [0026]    Wavepath unit cell  100  additionally includes a convergence point  110  where waveguide branches  106 A and  106 B meet and converge. A unit cell inlet portion  112  a unit cell outlet portion  114  are shown in the exemplary wavepath unit cell  100  of  FIG. 1 . In the example of  FIG. 1 , considered in isolation (i.e. not in combination with other unit cells) an entering wave such as a coherent infrared light wave enters waveguide  102  at waveguide inlet  104  and propagates substantially linearly toward branch point  104 . At branch point  104 , the input wave is split into two branch waves, one propagating through waveguide branch  106 A and one propagating through waveguide branch  106 B. The wave branch propagating through waveguide branch  106 A first interacts with phase shift element  108 A and experiences a phase shift of +θthen subsequently interacts with phase shift element  108 B and experiences a phase shift of −θ. The wave branch propagating through waveguide branch  106 B first interacts with phase shift element  108 B and experiences a phase shift of −θ then subsequently interacts with phase shift element  108 A and experiences a phase shift of +θ. The two branch waves are recombined at convergence point  110  to produce a coherent output wave propagating through waveguide continuation substantially identical to the input wave, aside from incidental power loss. 
         [0027]    While the wavepath unit cell  100  as illustrated in  FIG. 1  is hexagonal, this is but one example, and the shape or structure of the unit cell can be varied in any way which includes the aforementioned elements. It will generally be desirable however that the length of waveguide branches  106 A and  106 B between branch point  104  and convergence point  110  be the same. 
         [0028]    Referring now to  FIG. 2A , a wavepath grid  200  comprises a two-dimensional or three-dimensional array of unit cells of the type described above and illustrated in  FIG. 1 . As an example, the wavepath grid  200  of  FIG. 2A  comprises six complete unit cells  100 A,  100 B,  100 C,  100 D,  100 E, and  100 F, each of generally the same type as wavepath unit cell  100  shown in  FIG. 1 . 
         [0029]    Individual unit cells  100 A,  100 B,  100 C,  100 D,  100 E, and  100 F generally have shared cell boundaries such that discrete portions of waveguide  102  simultaneously serve as different segments of adjacent unit cells. For example, and referring again to  FIG. 2A , a single portion of waveguide  102  serves as the continuation branch  112  of unit cell  100 A, as the inlet of unit cell  100 E, as a section of waveguide branch  106 B of unit cell  100 C, and as a section of waveguide branch  106 A of unit cell  100 C. 
         [0030]    Of importance, the various individual unit cells  100 A,  100 B,  100 C,  100 D,  100 E, and  100 F generally have the same orientation as one another. In particular, positive phase shift elements  108 A are all in the same orientation relative to the immediately preceding branch point  104 , as all negative phase shift elements  108 B are uniformly in the opposite orientation. 
         [0031]    Wavepath grid  200  can be characterized as having a primary inlet  204  and five terminal outlets  206 A,  206 B,  206 C,  206 D, and  206 E which can be operable to emit the wave from the wavepath grid  200 . Terminal outlets  206 A,  206 B,  206 C,  206 D, and  206 E may also be referred to as output channels, and may include various structures such as a diffraction grating, which facilitate wave outcoupling from wavepath grid  200 . As shown, the wave phases across terminal outlets  206 A,  206 B,  206 C,  206 D, and  206 E differ according to a linear distribution. While the example of  FIG. 2A  has five terminal outlets, the wavepath grid  200  can, depending on its size, have any plurality of terminal outlets which can be operable to outcouple the wave. 
         [0032]    As shown, the phases of waves arriving at terminal outlets  206 A,  206 B,  206 C,  206 D, or  206 E have a linear distribution ranging uniformly from +4θ to −4θ. Notably, a wavepacket arriving at any one of terminal outlets  206 A,  206 B,  206 C,  206 D, or  206 E must have passed through the same number of phase shift elements  108 A,  108 B. In the case of the wavepath grid  200  of  FIG. 2A , any wave traveling from primary inlet  204  to any terminal outlet  206 A,  206 B,  206 C,  206 D, or  206 E must pass through four phase shift elements  108 A,  108 B regardless of path taken. The ratio of positive phase shift elements  108 A to negative phase shift elements  108 B encountered differs for wave packets arriving at different terminal outlets, however. For example, a wave packet arriving at terminal outlet  206 A will have passed through four positive phase shift elements  108 A while a wave packet arriving at terminal outlet  206 C will have passed through two positive phase shift elements  108 A and two negative phase shift elements  108 B. The differing ratios of positive phase shift elements  108 A to negative phase shift elements  108 B encountered on the paths to the different terminal outlets creates the linear phase distribution. 
         [0033]    The power distribution of wavepath grid  200  does not correspond directly to the phase distribution. While, as mentioned, phase distribution arises from the ratio of positive to negative phase shift elements encountered on the paths to the various terminal outlets  206 A,  206 B,  206 C,  206 D, and  206 E, the power distribution arises from the ratio of branch points  104  to convergence points  110  encountered on the paths to the various terminal outlets  206 A,  206 B,  206 C,  206 D, and  206 E.  FIG. 2B  shows a calculated power distribution for the wavepath grid  200  of  FIG. 2A . Terminal outlets  206 A,  206 B,  206 C,  206 D, and  206 E correspond to output channels  1 ,  2 ,  3 ,  4 , and  5 , respectively. As shown, the power distribution somewhat approximates a normal distribution, with output power significantly concentrated toward the middle and away from the edges of the grid. As discussed below, the shape of the power distribution can be substantially modified by changing the size and shape of wavepath grid  200 , but first a system for describing the size and shape of a wavepath grid  200  must be defined. 
         [0034]    The wavepath grid  200  of the type described above and exemplified in  FIG. 2  can include any number of unit cells arrayed in a wide variety of arrangements. Such variations of wavepath grid  200  can be described with reference to a cartesian coordinate system, one example of which is shown in  FIG. 2C . In  FIG. 2C , the wavepath grid of  FIG. 2A  is shown overlaying a two-dimensional cartesian coordinate system having (n) and (m) axes. 
         [0035]    In the (n,m) coordinate system as shown, the (n) and (m) axes intersect adjacent to primary inlet point  204 . Repeating structures of wavepath grid  200  appear at regular intervals on the (n, m) coordinate system. The wavepath grid  200  of  FIGS. 2A and 2C  can be described as a 2×2 grid. This is because, in this example, wavepath grid  200  crosses a maximum number of coordinate lines in the (n), exclusive of n=0, and crosses a maximum of two coordinate lines in the (m) direction, on either side of, and exclusive of, m=0. While a variety of different coordinate systems could be used to describe wavepath grid  200  size and shape, the present disclosure employs the (n, m) coordinate system as illustrated to describe the size and shape of different variations of wavepath grid  200 . For ease of use, size of a wavepath grid  200  in the (n) dimension can at times be referred to as “length” while size in the (m) dimension can at times be referred to as “width”. 
         [0036]    Additional variations in the number, and particularly the arrangement, of unit cells  100  employed in a wavepath grid  200  are illustrated in  FIGS. 3A and 3B .  FIG. 3A  shows a wavepath grid  200  of dimension 4×4, while  FIG. 3B  shows a wavepath grid of dimension 8×4. The 4×4 grid phase shifting region  250 . A phase shifting region  250  is a portion of a wavepath grid  200  is one in which higher numbered coordinate positions in the (n) direction correspond to a greater number of coordinate positions occupied in the (m) direction. Stated in general terms, length and width of the wavepath grid  250  simultaneously and proportionally increase in the phase shifting region  250 . 
         [0037]    The wavepath grid  200  of  FIG. 3B  includes the power splitting region  250  of  FIG. 3A  and then proceeds into a power mixing region  252 , where higher numbered coordinate positions in the (n) direction do not correspond to a greater number of coordinate positions occupied in the (m) direction. In general terms, length of wavepath grid  200  increases in the power mixing region, but width does not. Such structural variations, such as inclusion of a power mixing region or varying the length of a power mixing region can have significant effects on the functional properties of a wavepath grid  200 , such as on the output power distribution. In general functional terms, the phase shifting region  250  transforms a single input channel into a plurality of channels having linear phase distribution and power concentrated in the middle channels. The power mixing region  252  maintains the phase distribution created by the phase shifting region  250 , while redistributing a portion of the power away from the middle channels. 
         [0038]      FIG. 4A  shows the calculated power distributions of wavepath grids  200  of varying arrangement. Each wavepath grid  200  assessed in  FIG. 4A  has an equivalent 16×16 power splitting region which then proceeds into a power mixing region of varying (n) dimension. For example, the 32×16 wavepath grid has a 16×16 power splitting region  250  proceeding into a power mixing region  252  increasing the length of the array by an additional  16  units in the (n) direction with no increase in the (m) direction. Each wavepath grid  200  has thirty-three output channels numbered sequentially similar to the five output channels of  FIG. 2B . It is to be noted that the power mixing regions are greater in length than the power splitting regions for the 32×16, 48×16, 64×16, 128×16, and 256×16 wavepath grids. 
         [0039]    As the calculated data from  FIG. 4A  show, the 16×16 wavepath grid has a power distribution heavily concentrated toward the middle output channels, i.e. those corresponding to lower values of |m|. However, as power mixing regions of greater length are included into the arrays, the power distribution becomes substantially broader. This is because the power mixing region  252  allows amplitude mixing across the width of the grid without the increasing channel dilution toward the edges that is created by the phase shifting region  250 . The data from  FIG. 4A  is alternatively displayed in a three-dimensional graph in  FIG. 4B , to greater illustrate the relationship between power mixing region length and power distribution broadening. 
         [0040]    Referring now to  FIG. 5 , a phased-array device  300  includes a wavepath grid  200 , of the type disclosed, arrayed on a chip  302 . While phased-array device  300  can be of any type known in the art, it will typically be an optical phased-array device. While chip  302  can be of any suitable construction, it can in some cases of be of silicon-on-chip construction. 
         [0041]      FIG. 5  shows one example of phased-array device  300 , in which the chip  302  is of silicon-on-insulator construction. Phased-array device  300  includes wavepath grid  200 , wherein waveguide  102  comprises a suitable material such as silicon contactingly overlaying an insulator layer  304 . The wavepath grid  200  includes an array of wavepath unit cells as described above. Insulator layer  304  can comprise an oxide such as silicon oxide or other suitable oxide. Insulator  304  can in some instances contactingly overlay a substrate layer  306 , such as may also comprise silicon. Wavepath grid  200  as employed in phased-array device  300  includes a plurality of terminal outlets such as are created by diffraction grating  308  that is etched into the phased-array device  300  to a depth sufficient to create wave outcoupling from waveguide  102 . As above, the plurality of terminal outlets can be operable to outcouple individual sub-beams of the input wave from the chip. 
         [0042]    The phased-array device  300  can additionally include at least one control channel operable to modulate the refractive index of positive phase shift elements  108 A, negative phase shift elements  108 B, or both. For example, the control channel could comprise a plurality of thermoelectric devices arrayed to simultaneously heat waveguide  102  at locations corresponding to positive phase shift elements  108 A and to cool waveguide  102  at locations corresponding to negative phase shift elements  108 B. 
         [0043]    Alternatively, the control channel could comprise a plurality of heating elements arrayed to heat waveguide  102  at locations corresponding to positive and negative phase shift elements  108 A and  108 B. In such a scenario, negative phase shift elements  108 B may comprise a material which has a higher index of refraction as compared to the material of which waveguide  102  and/or positive phase shift elements  108 A are comprised. 
         [0044]    In an alternative control system, the waveguide can be subject to a DC offset. In such a system, the entire waveguide including positive and negative phase-shift elements is comprised of a single material, such as silicon. The portions of waveguide  112  outside the positive and negative phase-shift elements has an applied voltage such as +0.5 V. The positive phase shift elements have an applied voltage of +1.0 V and the negative phase shift elements have no applied voltage. Thus the preponderance of the waveguide  112  is warmed somewhat, the positive phase-shift elements are warmed to a greater extent, and the negative phase-shift elements are not warmed. 
         [0045]    A control channel of these types, or any other, can be used in conjunction with phased array device  300  to produce a beam steering device with no moving parts. For example, by modulating the magnitude of θ in positive phase shift elements  108 A, in negative phase shift elements  108 B, or both, the relative phases at the terminal outlets are altered. Consequently, the direction of propagation of the emitted beam is controllably steered. 
         [0046]    A control channel of any of the types described, or another type, could be distributed within or upon a thermal layer contactingly overlaying and surrounding wavepath grid  200 . For example, a control channel consisting substantially of a titanium or other conductive electrode could be sputtered on top of a layer of benzylcyclobutane (BCB), the BCB layer contactingly overlaying the waveguide and oxide layer. 
         [0047]    The foregoing description relates to what are presently considered to be the most practical embodiments. It is to be understood, however, that the disclosure is not to be limited to these embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.