Abstract:
An imaging system comprises a matched pathlength combining waveguide array including input optical couplers for receiving light, combining waveguides for combining the light received from different input optical couplers and relaying the light to output optical couplers. A lens system is also provided for imaging the light from the output optical couplers. Compared to imaging systems, this imaging system can be much more compact. A standard imaging system requires a focal length at least equal to the aperture (width) of the lens. Because the aperture size of a lens determines the performance of a system (resolution and collected light) there is a limit to how compact a traditional high performance imaging system can be. In contrast, the present system removes that limitation because the minimum practical focal length is now determined by the size of the aperture of the outputs, which can be significantly smaller (by factors of more than 10×, typically).

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 62/349,856, filed on Jun. 14, 2016, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Phased array transmitters are typically composed of a regular two-dimensional array of radiating or transmitting elements. Each of these elements typically has an associated phase shifter. Beams are formed by shifting the phase of the signal emitted by each of the radiating elements. The result is constructive and destructive interference in the far field that enables the steering of the beam. 
         [0003]    The same principle can be applied to phased array receivers. Similarly, a two-dimensional array of antenna or detection elements receives the incoming radiation. Their corresponding phase shifters shift the relative phase of the signals from each of the detection elements in order to create the constructive interference based on the incoming signal&#39;s angle of incidence on the receiver. 
         [0004]    Traditionally, phased array systems have been common in RADAR systems. These systems operate in the radio frequency regime, in the Megahertz to GigaHertz frequencies. More recently, optical phased array systems are being proposed and built. 
         [0005]    For example, the present inventors have proposed a variant of traditional phased array systems called Zero Optical Path Difference Phased Arrays. These are described in US Pat. Appl. Pub. No. 2016/0245895 A1, which is incorporated herein by this reference in its entirety. This concerns zero-optical-pathlength-difference optical phased arrays built with essentially planar photonic devices. They were proposed to be used to determine a direction to an incoherent optical source, such as a star, in one example. The zero-optical-pathlength-difference phased arrays were optically connected to interferometers. 
         [0006]    At the same time, other have proposed to use systems of microlenses, waveguides and output lenses for imaging applications. U.S. Pat. No. 7,187,815 describes wavefront relay devices that sample an incoming optical wavefront at different locations, optically relay the samples while maintaining the relative position of the samples and the relative phase between the samples. The wavefront is reconstructed due to interference of the samples, but on a smaller scale, i.e., reduced pupil. In one application, the device could function as a telescope but with negligible length. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention builds on a number of aspects of reduced pupil imaging on integrated platforms. It can be used to provide reduced pupil imaging where the incident wavefront is recreated at a smaller scale, and re-imaged. Lens arrays on the input couplers and output couplers can be used to improve light collection and limit background interference. The present system further provides precise pathlength control, which can be achieved with micro-fabrication techniques and trimming and active tuning. Further, the field-of-view can be steered by altering the pathlengths in a controlled manner to produce a desired effect at the output, such as steering and focusing. There is the method for using combining waveguides such as H-trees to combine the light and lower the number of output couplers necessary. The combination of a reduced pupil, H-tree arrays, and various modes of steering enable a very compact optical system with high performance. 
         [0008]    The present invention concerns a system that relays an incoming wavefront using waveguides. This can be used to reduce the effective pupil size for any subsequent imaging optics. It further utilizes matched pathlength optical waveguides to sample the incoming wavefront at multiple locations and then reproduce that wavefront. The relative optical lengths of the waveguides can be tuned, however, in order to steer the field-of-view. Moreover, the present invention uses networks of combining waveguides. This has the effect of combining the samples received at multiple, distributed input couplers into a single sample that is transferred to an output coupler. As a result, the present system is much more optically efficient and able to be more easily integrated, due to a reduced number of output couplers. 
         [0009]    Compared to a standard lens system, this imaging system can be much more compact. A standard imaging system requires a focal length at least equal to the aperture (width) of the lens. Because the aperture size of a lens determines the performance of a system (resolution and collected light) there is a limit to how compact a traditional high performance imaging system can be. In contrast, the present system removes that limitation because the minimum practical focal length is now determined by the size of the aperture of the output optical couplers, which can be significantly smaller (by factors of more than 10×, typically). 
         [0010]    In general, according to one aspect, the invention features an imaging system. The system comprises a matched pathlength combining waveguide array including input optical couplers for receiving light, combining waveguides for combining the light received from different input optical couplers and relaying the light to output optical couplers. A lens system is also provided for imaging the light from the output optical couplers. 
         [0011]    In the illustrated embodiments, the matched pathlength combining waveguide array comprises a series of matched pathlength combining waveguide tiles, each comprising a multiple input couplers feeding an output coupler. Each of the tiles can comprise at least 4 or 8 input optical couplers for every output coupler. There could be at least a 4 by 4 array of the tiles. In other cases, however, there is at least a 64 by 64 array of the tiles. 
         [0012]    In examples, the combining waveguides comprise waveguides for guiding the light to optical combiners that combine the light from multiple waveguides. 
         [0013]    Each of the tiles should include a phase shifting system for shifting the phase of the light to steer a field-of-view. A phase jump tuner is also useful for providing incremental phase shifts to enable steering to higher angles. 
         [0014]    In general, according to another aspect, the invention features an imaging method. This method comprises receiving light at an array input optical couplers, combining the from the input optical couplers in a matched pathlength combining waveguide array, emitting the combined light from output optical couplers, and imaging the light from the output optical couplers. 
         [0015]    In general, according to still another aspect, the invention features an imaging system. It comprises a waveguide array including input optical couplers for receiving light and relaying the light to output optical couplers, a lens system for imaging the light from the output optical couplers, and a phase shifting system for shifting the phase of the light to steer a field-of-view. 
         [0016]    This phase shifting system can include a phase jump tuner for providing incremental phase shifts to enable steering to higher angles. 
         [0017]    In general, according to still another aspect, the invention an imaging method. This method comprises receiving light at an array input optical couplers and coupling the light into a waveguide array, shifting the phase of the light in the waveguide array to steer a field-of-view, emitting the light from output optical couplers, and imaging the light from the output optical couplers. 
         [0018]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
           [0020]      FIG. 1  is a schematic side view of an imaging system according to the present invention; 
           [0021]      FIG. 2  is a schematic plan view of the imaging system showing the arrangement of several matched pathlength combining waveguide tiles; 
           [0022]      FIG. 3  is a schematic diagram showing a plan view in detail of a single matched pathlength combining waveguide tile; 
           [0023]      FIG. 4  is a schematic perspective illustration of a portion of a combining waveguide tile; 
           [0024]      FIGS. 5A and 5B  are schematics of an embodiment of phase shifters effecting steering of the field-of-view; 
           [0025]      FIGS. 6A-6D  show the results of a simulation illustrating how the field-of-view can be steered; 
           [0026]      FIG. 7  is a schematic diagram showing the implementation of the phase shifting system; and 
           [0027]      FIG. 8  is a schematic showing the details of a phase jump tuner. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0029]    As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Further, the singular forms and the articles “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms: includes, comprises, including and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Further, it will be understood that when an element, including component or subsystem, is referred to and/or shown as being connected or coupled to another element, it can be directly connected or coupled to the other element or intervening elements may be present. 
         [0030]      FIG. 1  shows a reduced pupil imaging system  10  which has been constructed according to the principles of the present invention. 
         [0031]    The incoming optical wavefront of light  8  is captured by an input lens array  105 . The input lens array  105  comprises an array of lenslets  106 - 1 ,  106 - 2 ,  106 - 3 , . . . ,  106 - n . These lenslets  106  focus the incoming light onto an corresponding array of optical input couplers  104 - 1 ,  104 - 2 ,  104 - 3 , 104 - 4 , . . . ,  104 - n  that are arranged on a top face of a waveguide chip  202 . 
         [0032]    Although not strictly necessary, the lens array  105  provides increased collection of the desired light while reducing the collection of unwanted light that is outside the field-of-view. 
         [0033]    The input couplers  104  can be implemented a number of ways. Examples include gratings, etched mirrors, and plasmonic antennae. 
         [0034]    These input optical couplers couple the incoming light into a matched pathlength combining waveguide array  50  that is implemented in the waveguide chip  202 . This matched pathlength combining waveguide array  50  transmits the light to an array of output optical couplers  110 - 1 ,  110 - 2 ,  110 - 3 , . . . ,  104 - m.    
         [0035]    Preferably, number (m) of output optical couplers  110  is less than the number (n) of optical input couplers  104 . This is a consequence of the combining waveguide array  50 . This has the advantage of reducing space contention on the waveguide chip  202  as the waveguides converge to the output couplers  110 . In one example, the ratio of input couplers  104  to optical couplers  110  is greater than 4:1, and is usually greater than 16:1 
         [0036]    In addition, for many embodiments, lateral extent (b) (length and width) of the array of output optical couplers  110  is smaller than the lateral extent (a) (length and width) of the array of input couplers  104 . As a result, the waveguide chip  202  has the effect of reducing the pupil size of the imaging system  10  over typical lens-base imaging systems. In a typical example, the lateral extent (a) of the array of input couplers  104  is at least four times larger than the lateral extent (b) of the array of output couplers  110 . 
         [0037]    The light travels across the chip  202  and is then re-emitted using a similar shaped, but more compact array of output couplers  110 . If the waveguide pathlengths are kept exactly equal, this new wavefront is identical (except smaller) then the incident wavefront  8 . This new wavefront can be reimaged using standard optics (which is much smaller than that which would be necessary to image the original wavefront), producing an image that contains all the resolution information in the original wavefront. 
         [0038]    In some embodiments, the waveguide chip  202  is a silicon wafer. The matched pathlength combining waveguide array  50  is fabricated within a thickness of the chip  202 . The optical waveguides may be made of glass or another material that is optically transparent at wavelengths of interest. The optical waveguides may be solid or they may be hollow, such as a hollow defined by a bore in the thickness of the wafer, and partially evacuated or filled with gas, such as air or dry nitrogen. The optical waveguides may be defined by a difference between a refractive index of the optical medium of the waveguides and a refractive index of the substrate or other material surrounding the optical waveguides. The waveguide chip may be fabricated using conventional semiconductor fabrication processes, such as the conventional CMOS process. 
         [0039]    An output lens array  107  comprises output lenslets  108 - 1 ,  108 - 2 , . . . ,  108 - m , corresponding to the array of output couplers  110 . These guide the light exiting from the waveguide chip  202  to imaging optics  210 . The imaging optics  210  then forms an image at an image plane. In the illustrated example, an image detector  212 , such as a CCD array or CMOS detector is located at the image plane in order to detect the image. 
         [0040]    Although resolution is preserved, field-of-view is sacrificed. The field-of-view can be steered, however, by moving the lens array. For this method to work, precise pathlength control is necessary to preserve the wavefront. Although current fabrication methods are extremely precise, some fine tuning, such as active phase shifters are expected to be required to achieve the sub-wavelength control necessary for this application. 
         [0041]    It is also possible to alter the waveguide pathlengths in a controlled manner to produce desired results at the output. For example, a lens can be effectively built into the waveguides by altering the pathlengths to produce the same pathlength delays that an additional lens would. It is also possible to alter the locations of the outputs to produce a similar effect. 
         [0042]      FIG. 2  shows the arrangement of the matched pathlength waveguide tiles in the matched pathlength combining waveguide array  50 . 
         [0043]    In the illustrated example, the waveguides are arranged into several matched pathlength combining waveguide tiles  100 - 1 ,  100 - 2 ,  100 - 3 ,  100 - 4 . Each of these tiles  100  combines the light received at several optical input couplers  104  and combines the feeds from those input couplers, successively, through a branching waveguide network  115  until the light from the optical couplers  104  for a tile  100  is combined onto a final root waveguide  124  that terminates in an output coupler  110 . 
         [0044]    In general, the average phase is preserved through the tiles  100 , and then the output couplers  110  of the tiles  100  are arranged in an array to produce a smaller copy of the incident wavefront. This method can be used to reduce the number of output elements necessary. Although shown in a regular arrangement, other arrangements of the tiles  100  are possible. 
         [0045]    In general, a larger field-of-view (without sacrificing resolution) requires more output elements and therefore more complexity. Together these methods create a means for choosing the size of the field-of-view based on the application, while providing methods of steering the field-of-view for a large field-of-view. 
         [0046]    Specifically, in the illustrated example, the light received by the input optical couplers  104  of the first matched pathlength combining waveguide tile  100 - 1 , for example, is successively combined in the branching waveguide network  115 - 1  until the combined optical signal is provided on its corresponding root waveguide  124 - 1 . 
         [0047]    All together, the root optical waveguides  124 - 1 ,  124 - 2 ,  124 - 3 ,  124 - 4  for the respective tiles  100 - 1 ,  100 - 2 ,  100 - 3 ,  100 - 4  each provide their light to its corresponding optical output coupler  110 - 1 ,  110 - 2 ,  110 - 3 ,  110 - 4  so that it can be subsequently imaged by the imaging optics  210  onto the image detector  212 . 
         [0048]    In some embodiments, phase jump tuners  350 - 1 ,  350 - 2 ,  250 - 3 ,  350 - 4  are further provided on the root waveguides  124 - 1 ,  124 - 2 ,  124 - 3 ,  124 - 4  of each of the tiles  100 - 1 ,  100 - 2 ,  100 - 3 ,  100 - 4 . These phase jump tuners  350 - 1 ,  350 - 2 ,  250 - 3 ,  350 - 4  control the relative phases of the light from each of the tiles. These are used to steer the field of view to higher angles by compensating for a tilted wavefront across the extent of the matched pathlength combining waveguide array  50 . The jump tuners  350 - 1 ,  350 - 2 ,  250 - 3 ,  350 - 4  provide each tile  100  incremental jumps in phase delay. 
         [0049]      FIG. 3  is a schematic diagram showing a plan view of a matched pathlength combining waveguide tile  100 . This illustrated example shows an H-tree arrangement. 
         [0050]    Other arrangements of optical couplers are anticipated, such as arrangements that provide an asymmetric effective field-of-view. Yet other arrangements of optical couplers are also anticipated. For example, X-trees may be used, although X-trees may require crossing optical paths. However, for simplicity, the following examples are described herein using H-trees. 
         [0051]    The illustrated tile  100  shows a 32×32 array of input couplers  104  feeding to a single output computer  110 . The optical input couplers  104  are connected to leaves or first level waveguides  112  of the H-tree branching waveguide network  115 . (For clarify of the figure, only the upper right portion of the branching waveguide network  115  is labeled with reference numerals. Due to the symmetry of the network, this description applies to the other sections as well.) The first level optical waveguides  112  each meet at first level optical combiners  118 . The first level optical combiners  118  in turn feed second level waveguides  116 . The feeds from the second level waveguides  116  are then combined in second level combiners  120 . The second level combiners  120  in turn feed third level combiner  122 . 
         [0052]    Depending on the depth of the branching waveguide network  115 , the optical signals are combined through successive combinations of waveguides and combiners until the root waveguide  124  is reached. In the specific illustrated network, there are fourth level combiners  126 , fifth level combiners  128 , sixth level combiners  130 , and seventh level combiners  132 , until a final eighth level combiner  134  feeds the single root waveguide  124 . 
         [0053]    The optical waveguides of the same level are of matched, specifically equal, lengths. Similarly, other pairs of optical waveguides that meet at common combiners are of equal lengths. 
         [0054]    In the illustrated example, the direction of combination alternates (left-right, up-down) between successive optical combiners  118  to  120  to  124  to  126  to  128  to  130  to  132  to  134  to ensure each signal combination occurs in phase. The resulting branching phased array  115  operates over a broad range of wavelengths. The entire array  115  feeds a root optical waveguide  124 , which is referred to herein as a “root” of the H-tree. 
         [0055]    In some embodiments, the optical couplers  104  are sized and spaced apart by less than one wavelength. If elements are this close then the input lens array  105  and its array of lenslets  106 - 1 ,  106 - 2 ,  106 - 3 , . . . ,  106 - n . are not used. However, in other embodiments, the optical couplers  104  may be spaced apart by more than one wavelength, including tens or hundreds of wavelengths. In these cases, then the input lens array  105  is used. 
         [0056]    The illustrated embodiment includes an array of 32×32 optical couplers  104  with 100 μm spacing. 
         [0057]      FIG. 4  is a schematic perspective illustration of a building bloc portion of a combining waveguide tile  100  of  FIG. 3 . Specifically four optical input couplers  104 - 1 ,  104 - 2 ,  104 - 3 , and  104 - 4  are arranged in an array. These optical couplers  104 - 1 ,  104 - 2 ,  104 - 3 ,  104 - 4  feed the first level waveguides  112 - 1 ,  112 - 2 ,  112 - 3 ,  112 - 4 . The first level waveguides  112  terminate in first level combiners  118 - 1 ,  118 - 2 . These first level couplers feed second level waveguides  116 - 1 ,  116 - 2 , which terminate in a second level combiner  120 . The second level combiner in turn feeds a third level waveguide  121 . 
         [0058]    Design of the optical combiners, such as optical combiners  118 ,  120 ,  122 ,  126 ,  128 ,  132 , and  134 , should be selected for low loss and coherent power combination. For example, multi-mode interferometers (MMI), which are compact and perform over somewhat large bandwidths, may be used as optical combining/splitting elements. Other possible combiner/splitter designs that are possible are adiabatic couplers, resonant couplers, and hybrid-ring combiners. 
         [0059]      FIGS. 5A and 5B  illustrate the operation of a preferred embodiment in which phase shifters are provided for each input optical couplers  104  in order to provide for the steering of the field-of-view. 
         [0060]    In more detail, with respect to  FIG. 5A , consider the downrange view  70  for a matched pathlength combining waveguide array  50 . Light received from the object of interest  72  has a tilted wavefront from the perspective of the matched pathlength combining waveguide array  50 . As a result, light from the object of interest  72  will not be in-phase and thus will destructively interfere in the cascade of combiners  118 ,  122 , etc in each tile  100 . 
         [0061]      FIG. 5B  shows the addition of a phase shifting system  300  within the matched pathlength combining waveguide array  50 . The field-of-view of the waveguide array  50  is steered by adding a phase shift to the feeds from each of the input optical couplers  104 . It is thus possible to steer to the field-of-view. 
         [0062]    The phase shifting system  300  compensates for the tilted wavefront  74  of off-axis light and allows off-axis light to constructively interfere within the cascade of combiners  118 ,  122 . 
         [0063]    As shown, the phase shifting system  300  imparts a different, but predictable phase shift to the feeds from each input optical coupler  104 . 
         [0064]    Of note is the fact that all the tiles  100  on the matched pathlength combining waveguide array  50  require (precisely) the same phase shifts; so it is possible to control all the all the tiles  100  in the array  50  with the same drive electronics. 
         [0065]      FIGS. 6A-6D  show the results of a simulation illustrating how the field-of-view can be steered. 
         [0066]      FIG. 6A  shows the case without steering. Shown is the downrange view  70 . The object of interest  72  is in the lower part of the view. The field-of-view  74  of the imaging system  10  is in the center of the downrange view  70 . 
         [0067]      FIG. 6B  shows the light output from the unsteered array  50 . There is nothing in the image since no light escapes the matched pathlength combining waveguide array  50 . 
         [0068]      FIG. 6C  shows the case with steering. Shown is the downrange view  70 . The object of interest  72  is in the lower part of the view. The field-of-view  74  of the imaging system  10  is steered to be coincident with the object of interest  72 . 
         [0069]      FIG. 6D  shows the light output from the steered array  50 . Now there is an image since light from the object of interest  72  escapes through the matched pathlength combining waveguide array  50 . 
         [0070]      FIG. 7  is a schematic diagram showing the implementation of the phase shifting system  300  into the branching waveguide network  115  of a portion of a tile  100 . 
         [0071]    The optical waveguides  112 ,  116 ,  121  of each of the respective levels include respective exemplary pathlength tuners  312 ,  316 ,  321 . These pathlength tuners  312 ,  316 ,  321  provide dynamically tunable optical delays for the optical signal exiting each level of optical combiners  118 ,  120 ,  126 . 
         [0072]    In one implementation, the pathlength tuners  312 ,  316 ,  321  are heated sections of the waveguides, controlling the index of refraction. 
         [0073]    Heaters are preferably fabricated in the waveguide chip  202 . The amount of heat generated by each heater is controlled by the phase controller  360 , which also controls the phase jump tuner  350 . Thus, each dynamically tunable optical delay line includes a thermally phase-tunable optical delay line. 
         [0074]    In should be noted that in some embodiments, trimming portions are additionally included into the optical waveguides  112 ,  116 ,  121 . Each trimming section is made of a material whose refractive index can be permanently changed, such as by annealing the material in the trimming section. Thus, the waveguides of the array can be adjusted as part of a manufacturing/calibration operation to ensure matching optical delays within the branching waveguide network  115  of each tile  100 . 
         [0075]    In the illustrated example, the feeds from input optical couplers  104 - 1  and  104 - 2  are combined in first level combiner  118 - 1 . The relative delay between these feeds is controlled by pathlength tuners  312 - 1  and  312 - 2  under the control of the phase controller  360 . In a similar way, feeds from input optical couplers  104 - 3  and  104 - 4  are combined in first level combiner  118 - 2 . The relative delay between these feeds is controlled by pathlength tuners  312 - 3  and  312 - 4  also under the control of the phase controller  360 . 
         [0076]    Then delay in the signals at the second level combiners  120  can also be tuned. For example, the delay of the light from combiner  118 - 1  is modulated by the control of the pathlength tuners  316 - 1  and the delay of the light from combiner  118 - 2  is modulated by the control of the pathlength tuners  316 - 2 . In the illustrated embodiment subsequent pathlength tuners  340  and  342  are provided to enable pathlength control deeper in the network  115  to further facility steering control via the phase controller  360 . 
         [0077]      FIG. 8  shows the details of one embodiment of the phase jump tuner  350  that provides additional phase adjustment for each tile  100 . 
         [0078]    It is anticipated that that the pathlength tuners  312 ,  316 ,  321  within the waveguides will be able to compensate for the tilted wavefront across the extent of a tile. However, to compensate for the tilted wavefront across the extent of the matched pathlength combining waveguide array  50 , each tile  100  will need to provide incremental jumps in phase. 
         [0079]    In one embodiment, the phase jump tuner  350  comprises a series of optical switches  352 ,  354 ,  356  that switch the optical signal on the root waveguide  124  from the branching network  115  between respective short paths  361 ,  363 ,  365 , and long paths  362 ,  364 ,  366 . In this way, increments of delay can be added for each tile  100  under the control of the phase controller  360  to facilitate steering to higher angles. 
         [0080]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.