Patent Publication Number: US-2022236384-A1

Title: Focal plane optical conditioning for integrated photonics

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to integrated photonic devices, and specifically to devices and methods for generating an array of optical beams. 
     BACKGROUND 
     Integrated photonic devices are devices that combine optoelectronic, optical, and electronic components, typically on a semiconductor substrate. (The terms “optical” and “light,” as used in the present description and in the claims, refer to electromagnetic radiation in any of the visible, infrared and ultraviolet ranges of the spectrum.) 
     SUMMARY 
     Embodiments of the present invention that are described herein provide integrated photonic devices with improved optical qualities. 
     There is therefore provided, in accordance with an embodiment of the invention, an optical device, including a first array of emitters disposed on a semiconductor substrate and configured to emit respective beams of optical radiation. A second array of micro-optics is positioned in alignment with the respective beams of the optical emitters and arranged to condition phases of the beams so that different ones of the beams are transmitted with different phase qualities. 
     In some embodiments, the emitters and micro-optics are configured so that the different ones of the beams are focused at different, respective distances from an edge of the semiconductor substrate. In some of these embodiments, the beams are focused to respective points along a locus having a predefined curvature. In a disclosed embodiment, the device includes collimation optics having a curved object plane, which coincides with the locus. 
     Additionally or alternatively, the emitters in the first array include respective spot-size converters. In some embodiments, the spot-size converters include tapered waveguides. In a disclosed embodiment, different ones of the spot-size converters have different, respective tapers selected so as to form spots of different, respective spot sizes. Additionally or alternatively, the tapered waveguides have respective output ends, which are offset relative to an output facet of the semiconductor substrate selected such that the output ends are disposed along a curve. In one embodiment, the spot-size converters are mutually non-parallel, so as to emit the respective beams in respective, non-parallel directions. 
     In some embodiments, the micro-optics in the second array include respective microlenses. In a disclosed embodiment, at least some of the microlenses are offset with respect to the respective beams so as to direct the beams at different, respective angles. Additionally or alternatively, at least some of the microlenses have different, respective focal lengths. 
     In a disclosed embodiment, the device includes one or more folding mirrors, which are disposed on the semiconductor substrate so as to direct the beams away from a plane of the semiconductor substrate. 
     There is also provided, in accordance with an embodiment of the invention, a light detection and ranging (LiDAR) system, including a transmitter, which includes a first array of emitters disposed on a semiconductor substrate and configured to emit respective beams of optical radiation and a second array of micro-optics positioned in alignment with the respective beams of the optical emitters and arranged to condition phases of the beams so that different ones of the beams are transmitted with different phase qualities. Collimating optics are configured to direct the beams transmitted by the micro-optics toward different, respective parts of a field of view of the LiDAR system in different, respective directions. A receiver is configured to receive reflections of the beams from the different parts of the field of view and to process the reflections in order to find distances to objects in the field of view. 
     In a disclosed embodiment, the emitters in the first array are configured to output the respective beams with different, respective spot sizes, which are selected so that the beams that are directed toward the different parts of the field of view have different, respective divergences. 
     There is additionally provided, in accordance with an embodiment of the invention, a method for producing an optical device. The method includes arranging a first array of emitters on a semiconductor substrate so as to emit respective beams of optical radiation. A second array of micro-optics is positioned in alignment with the respective beams of the optical emitters so as to condition phases of the beams so that different ones of the beams are transmitted with different phase qualities. 
     In some embodiments, positioning the second array includes arranging the micro-optics so as to focus the different one of the beams at different, respective distances from an edge of the semiconductor substrate, wherein arranging the micro-optics includes focusing the beams to respective points along a locus having a predefined curvature. Collimation optics are provided having a curved object plane, which coincides with the locus, and the method includes jointly optimizing an optical design of the collimation optics with optical properties of the emitters and the micro-optics. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are respective schematic top views of edge-coupled photonic integrated circuits and respective microlens arrays, in accordance with two embodiments of the invention; 
         FIG. 2  is a schematic top view of a photonic integrated circuit with edge coupling, in accordance with another embodiment of the invention; 
         FIG. 3A  is a schematic top view of a photonic integrated circuit with vertical coupling, in accordance with another embodiment of the invention; 
         FIGS. 3B and 3C  are schematic sectional views of the photonic integrated circuit of  FIG. 3A ; 
         FIGS. 4A, 4B and 4C  are schematic side views of collimation optics, in accordance with embodiments of the invention; 
         FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, 5H and 5I  are schematic plots of far-field angular profiles for infrared optical radiation emitted by spot-size converters of various designs, in accordance with embodiments of the invention; 
         FIG. 6  is a schematic top view of a photonic integrated circuit and an associated microlens array, in accordance with an embodiment of the invention; 
         FIGS. 7A and 7B  are graphical representations of the radiant emittances of an output of a beam before and after imaging with a microlens, respectively, in accordance with an embodiment of the invention; 
         FIG. 8  is a schematic side view of a LiDAR system installed in a vehicle, in accordance with an embodiment of the invention; and 
         FIG. 9  is a schematic top view of a photonic integrated circuit and a microlens array for use in the system of  FIG. 8 , in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Typical arrays of laser emitters on a semiconductor substrate, such as one- or two-dimensional VCSEL arrays, emit beams in a direction normal to the substrate surface of the array. Design of beam conditioning or imaging lenses, for example to serve as collimation optics, to achieve the desired far-field illumination pattern must capture the angular emission of each array element (the NA, numerical aperture) emitted by all array elements, while also performing the imaging function for far-field spot position. Such designs typically require beams emitted from sources arranged in a telecentric configuration in object space. In this case, the lenses are designed for array element emission normal to the surface, followed either by focal or afocal image space, with the lenses designed to collimate each laser source to infinity (afocal), or to a finite working distance. Costly and complex lens systems may be required in order to achieve the desired beam quality. Similar limitations are encountered by linear (1D) arrays of edge-emitting lasers or photonic integrated circuit structures. 
     In embodiments of the present invention, integrated photonic devices provide the optical designer with additional optimizations through co-design of the integrated photonics elements and the collimation optics. These embodiments provide an optical device comprising an array of emitters disposed on a semiconductor substrate and configured to emit respective beams of optical radiation. An array of micro-optics, such as microlenses, is positioned in alignment with the respective beams of the optical emitters. The micro-optics condition the phases of the beams so that different beams are transmitted with different phase qualities. The term “phase qualities,” as used in the context of the present description and in the claims, refers to focal and directional qualities of the beams, such as the spot size, divergence, numerical aperture, and chief ray direction. 
     In the embodiments described below, a photonic integrated circuit includes an array of emission apertures formed on a semiconductor substrate, with an array of waveguides on the substrate to guide light to or from other photonic circuit elements. The emission apertures are formed at the input/output surface of the photonic integrated circuit, and may be bidirectional, for example using polarization to split, transmit and receive light. The spot size of these output beams may be controlled by various classes of micro-optic and photonic components, such as spot-size converters on the photonic integrated circuit or a microlens array positioned in alignment with the respective output beams, or a combination of spot-size converters and a microlens array. In systems that are known in the art, however, when good beam quality is required, the design and production of the optics that follow the spot-size converters and/or microlens array are often complex and costly. 
     Embodiments of the present invention that are described herein address these problems by using an array of micro-optics, positioned in alignment with the respective beams of the optical emitters, so as to focus the beams so that different beams are transmitted with different phase quantities. The arrays of emitters and micro-optics may be either one- or two-dimensional arrays, depending on system requirements. The photonic integrated circuit containing the emitters and micro-optics may also contain other components, such as optical receivers, for example in a monostatic configuration together with the emitters. 
     In some embodiments, the emitters and microlenses are configured so that the beams are focused at different, respective distances from an edge of the semiconductor substrate, for example focused to respective points along a locus in space with a predefined radius of curvature. The curved object plane thus created can be co-optimized with the imaging or collimation optics in order to reduce the complexity of the optical design. 
     In some embodiments, micro-optics are arranged to provide vertical emission (normal or near-normal to the plane of the photonic waveguides). 
     Additionally or alternatively, the emitters and micro-optics are configured so that the different beams are focused to form spots of different, respective spot sizes. This approach is useful, for example, in a LiDAR system in which different beams are optimized for detection of objects at different distance ranges within the field of view of the system. 
     System Description 
       FIGS. 1A and 1B  are respective schematic top views of edge-coupled photonic integrated circuits  20 A and  20 B with respective microlens arrays  36 A and  36 B, in accordance with two embodiments of the invention. The spatial mode of each output beam from photonic integrated circuits  20 A and  20 B (i.e., beam dimensions, divergence and direction) is set using micro-optics including an integrated spot-size converter  24 A,  24 B, realized within the photonic integrated circuit, and microlens array  36 A,  36 B, aligned externally with the photonic integrated circuit. 
     Photonic integrated circuits  20 A and  20 B comprise respective arrayed waveguides  22 A and  22 B, wherein guided optical signals propagate from left to right, as shown by respective arrows  23 A and  23 B. For the sake of simplicity, the circuitries of photonic integrated circuits  20 A and  20 B leading to arrayed waveguides  22 A and  22 B, respectively, are omitted from the description. These circuitries may include, for example, semiconductor lasers, which emit respective beams into the waveguides. 
     In the embodiment shown in  FIG. 1A , photonic integrated circuit  20 A comprises spot-size converters  24 A, wherein each spot-size converter is optically coupled by its input end  26 A to a respective waveguide  22 A and receives optical radiation from the waveguide. Spot-size converters  24 A are, through a lithographic fabrication process, configured to guide the optical radiation to their respective output ends  28 A and to emit the optical radiation with a predefined spatial and angular distribution. Thus, each output end  28 A may be regarded as an emitter of optical radiation or as an emission aperture from photonic integrated circuit  20 A. (Similar considerations may be applied to the photonic integrated circuits in the embodiments below.) An example embodiment of spot-size converter  24 A is a tapered coupler, similar to a spot-size coupler  106  with a tapered waveguide  122  in  FIG. 2 , below. 
     Furthermore, spot-size converters  24 A are mutually non-parallel, so as to emit their respective beams in non-parallel directions. In the pictured example, converters  24 A are angled so that the directions of “chief rays”  30 A (the peak emission angle) for the optical radiation emitted by each spot-size converter can be controlled by lithographic definition of the waveguide. Specifically, angled design of spot-size converters  24 A allows selection of the beam parameters, while the angles are not constrained to be normal to an output facet  32 A of photonic integrated circuit  20 A. 
     The radiation emitted from each output end  28 A is received by a respective microlens  34 A of microlens array  36 A, wherein the microlenses are positioned so as not to cause any additional deviation of chief rays  30 A. Alternatively, each microlens  34 A may cause an additional angular deviation of respective chief ray  30 A. Each microlens  34 A is further positioned so that it images one of output ends  28 A onto a respective point  38 A on a curved locus  40 A in space, wherein the locus may comprise a part of a circle, a part of a parabola, or a part of a more general curve. Microlenses  34 A have different, respective effective focal lengths and thus re-image the flat output facet  32 A of photonic integrated circuit  20 A to a curved focal plane at locus  40 A. 
     The radiation emitted from points  38 A on locus  40 A will be received by collimation optics to create a desired far-field illumination pattern (collimation optics are not shown in  FIG. 1A ). The angular orientations of chief rays  30 A, as well as the curved shape of locus  40 A, are utilized to simplify the design and construction of the collimation optics, as a telecentric and flat-field optical design is not required. For example, the collimation optics may be co-designed together with the design of photonic integrated circuit  20 A for an optimal angular orientation of chief rays  30 A and the shape of locus  40 A. 
     In the embodiment shown in  FIG. 1B , photonic integrated circuit  20 B comprises spot-size converters  24 B, which, similarly to spot-size converters  24 A in photonic integrated circuit  24 A, are coupled by their input ends  26 B to respective waveguides  22 B and receive optical radiation from the waveguides. Further similarly to spot-size converters  24 A, spot-size converters  24 B guide the optical radiation to their respective output ends  28 B and emit the optical radiation with a predefined spatial and angular distribution. However, spot-size converters  24 B are not angled with respect to waveguides  22 B, but are rather aligned coaxially with the waveguides. Consequently, the direction of peak emission from each output end  28 B is normal (perpendicular) to an output facet  32 B. 
     The radiation emitted from each output end  28 B is received by a respective microlens  34 B of microlens array  36 B. The centers of microlenses  34 B for at least some of output ends  28 B are offset (translated) relative to the center of the output end, so that each chief ray  30 B exiting from respective microlens  34 B is angled due to the specific offset of the microlens (rather than by the direction of spot size converters, as in photonic integrated circuit  20 A). Similarly to photonic integrated circuit  20 A, output ends  28 B are focused onto points  38 B on a curved locus  40 B. Microlenses  34 B have different, respective effective focal lengths and thus re-image the flat output facet  32 B of photonic integrated circuit  20 B to a curved focal plane at locus  40 B. As in photonic integrated circuit  20 A, the angular orientation of chief rays  30 B, as well as the curved shape of locus  40 B, may be utilized to simplify the optical design and the construction of the collimation optics. 
       FIG. 2  is a schematic top view of a photonic integrated circuit  100  with edge coupling, in accordance with another embodiment of the invention. 
     Photonic integrated circuit  100  comprises, similarly to photonic integrated circuits  20 A and  20 B, arrayed waveguides  102 , with guided optical signals propagating from left to right, as shown by an arrow  104 . Photonic integrated circuit  100  further comprises, similarly to photonic integrated circuit  20 A, angled spot-size converters  106 , which are optically coupled to arrayed waveguides  102  by their input ends  108 . Spot-size converters  106  guide the optical radiation to their respective output ends  110 , and emit the optical radiation with a predefined spatial and angular distribution. As shown in detail in an inset  112 , an offset  114  with respect to an output facet  116  is intentionally introduced in the fabrication of photonic integrated circuit  100  so that output ends  110  of spot-size converters  106  are disposed along a curve  118 . An optical sag  120 , as defined below, is realized, depending upon the material platform, by co-design of spot-size converters  106  and respective offsets  114  of their output ends  110  relative to facet  116 . 
     In the pictured embodiment, output ends  110  comprise suitably tapered waveguides  122 , which emit radiation into a cone described by optical rays  124  around a chief ray  126 . Other shapes of output ends  110  may be used in alternative embodiments. The direction of each chief ray  126  is defined by the angled direction of the respective spot-size converter  106 , and the angular extent of rays  124  is determined by the geometry of tapered waveguide  122 . Rays  124  and chief ray  126  are refracted at facet  116  into rays  128  and into chief ray  130 , respectively. A subtended half-angle  131  of the emitted radiation is defined by one of rays  128  and chief ray  130 . 
     The extensions of rays  128  back into photonic integrated circuit  100 , shown by dotted lines  129 , define an apparent source point  132 , whose distance from facet  116  defines optical sag  120 . Apparent source points  132  formed by all spot-size converters  106  form a curved locus  134  (so close to curve  118  that it is not seen as a separate curve in  FIG. 2 ). Similarly to photonic integrated circuits  20 A and  20 B, apparent source points  132  on locus  134 , with their radiation propagating around refracted chief rays  130 , form the sources for the subsequent collimation optics, with the curved shape of locus  134  and the angled directions of chief rays  130  relaxing the optical design considerations for the optics. 
       FIGS. 3A-3C  schematically illustrate a photonic integrated circuit  200  with vertical coupling, in accordance with a further embodiment of the invention. For the sake of clarity, a Cartesian coordinate system  202  is shown next to each of  FIGS. 3A-3C  with an orientation appropriate to each figure.  FIG. 3A  is a top view, as viewed from the Y-direction down towards the XZ-plane,  FIG. 3B  is a sectional view taken along an XY-plane through a microlens array  224 , and  FIG. 3C  is a sectional view taken along a YZ-plane through a waveguide  204  and a spot-size converter  208 . By “vertical coupling” we mean that the direction of propagation of the radiation coupled out from photonic integrated circuit  200  forms a normal or near-normal angle with a top surface  201  of the photonic integrated circuit. 
     Photonic integrated circuit  200  comprises, similarly to photonic integrated circuits  20 A and  20 B, arrayed waveguides  204 , with guided optical signals propagating from left to right, as shown by an arrow  206 . Photonic integrated circuit  200  further comprises, similarly to photonic integrated circuit  20 B, spot-size converters  208  collinear with arrayed waveguides  204 . The spot-size converters are optically coupled to arrayed waveguides  204  by their input ends  210 , guide the optical radiation to their respective output ends  212 , and emit the optical radiation with a predefined spatial and angular distribution. Further similarly to photonic integrated circuit  20 B, output ends  212  are located at an output facet  214 . 
     In order to configure photonic integrated circuit  200  for vertical emission, a folding mirror  216  is disposed on a substrate  218  of the circuit opposite output ends  212  and direct the beams from output ends  212  away from a plane of the substrate. Folding mirrors  216  are formed on substrate  218  for example by cleaving or etching a silicon waveguide layer  220  and then coating the resulting diagonal surface with a reflective layer  222 , as shown in  FIG. 3C . 
     Array  224  of microlenses  226  is positioned above folding mirrors  216 , with one microlens for each output end  212 . Microlenses  226  are offset with respect to output ends  212 , similarly to microlenses  34 B in photonics integrated circuit  20 B, so as to angle chief rays  228  exiting from each of the microlenses, and focus output ends  212  onto respective points  232  on a curved locus  230 . Similarly to photonic integrated circuits  20 A,  20 B, and  100 , apparent source points  232  on locus  230 , with their radiation propagating around chief rays  228 , form the sources for the subsequent collimation optics, with the curved shape of locus  230  and the angled directions of chief rays  228  relaxing the design considerations for the optics. 
     The configuration of folding mirrors  216  may be used in a similar fashion together with either of the architectures of photonic integrated circuits  20 A and  100  for accomplishing vertical emission from the circuits. 
     Optical Design 
     Co-design of the optics of the photonic integrated circuits and the collimation optics makes it possible to reduce the lens count and complexity of the collimation optics. The designer has a large number of degrees of freedom at his disposal that can be used for designing the photonic integrated circuits and microlenses. For the design of the photonic integrated circuits, the degrees of freedom comprise, for example:
         Spot size and numerical aperture (in two dimensions).   Position of the output ends of the spot-size converters, relative to array center, to compensate for optics distortion.   Emission angle.   Sag.
 
For the design of microlenses, the degrees of freedom comprise, for example:
   Offset from aperture centroid or chief-ray.   Focal length and shape (spherical, aspheric or anamorphic).   Focus (re-image aperture vs. collimate).       

       FIGS. 4A-4C  are schematic side views showing three optical designs of collimation optics  300 ,  320 ,  340 , respectively, in accordance with embodiments of the invention. The three optical designs take advantage of a curved source plane (curved locus of source spot images) and of angled chief rays provided by the design of the photonic integrated circuits and microlenses in the preceding embodiments. The curved source plane and the angled chief rays enable the design of collimation optics with fewer elements while maintaining near diffraction limited collimation of the beam sources (M 2 ˜1). 
     Optics  300 , in  FIG. 4A , comprise three lenses  302 ,  304 , and  306 . A photonic integrated circuit (not shown in  FIG. 4A ) forms source points  308  of optical radiation on a curved locus  310 , propagating along angled chief rays  312 , as described above in reference to  FIGS. 1-3 . (In  FIGS. 4A-4C  only chief rays are shown. Other rays, such as marginal rays, are omitted for the sake of clarity.) The above-mentioned characteristics of the optical radiation received by optics  300  enable the optical design to achieve a nearly diffraction-limited performance, M 2 ˜1.0, for all source points  308  on locus  310 . 
     Optics  320 , in  FIG. 4B , comprise two lenses  322  and  324 . A photonic integrated circuit (not shown in  FIG. 4B ) forms source points  326  of optical radiation on a curved locus  328 , propagating along angled chief rays  330 . These characteristics of the optical radiation received by optics  320  enable the optical design to achieve a nearly diffraction-limited performance, with M 2 ˜1.2 for source points  326  at the edge of locus  328 . 
     Optics  340 , shown in  FIG. 4C , comprise only a single lens  342 . A photonic integrated circuit (not shown in  FIG. 4C ) forms source points  344  of optical radiation on a curved locus  346 , propagating along angled chief rays  348 . An aperture stop  350  is located at a center of curvature  352  of a second optical surface  354  of lens  342 , thus minimizing the amount of coma introduced by the lens. Some residual astigmatism remains. The characteristics of the optical radiation received by optics  340 , as well as the location of stop  350 , enable the optical design to achieve a nearly diffraction-limited performance, with M 2 ˜1.4 for source points  344  at the edge of locus  346 . 
     The three optical designs  300 ,  320 , and  340  may each be optimized to accommodate either a quadratic locus  230 ,  328 , or  346 , respectively, or a locus of higher order, such as 4 th  degree, 6 th  degree, or even higher. 
       FIGS. 5A-5I  contain plots  410 - 492  of calculated far-field angular profiles for infrared optical radiation emitted by spot-size converters of various designs, in accordance with embodiments of the invention. 
     Using spot-size converter  106  of  FIG. 2  as an example, the output spot size can be controlled by setting the width of the end of taper  122  to an appropriate value. The beam divergence typically decreases with increasing output spot size. For example, assuming spot-size converter  106  to be based on a silicon waveguide layer that is 220 nm thick, the spot size of an infrared beam guided through and output from the spot-size converter will increase as the width of the taper decreases below 160 nm, while the beam divergence angle decreases with increasing spot size. 
     Plots  410 - 492  for the far-field angular profiles are calculated as a function of taper width, from a minimum of 60 nm in  FIG. 5A  to a maximum of 200 nm in  FIG. 5I , for a wavelength of 1.55 μm. The plots show the far-field angular profiles in the vertical direction as solid lines and in the horizontal direction as dotted lines. The horizontal axis is the angular extent of the beam in degrees, and the vertical axis is the intensity of the beam in arbitrary units. The numbering of the graphs together with the taper widths are shown in Table 1, below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Plots of far-field angular profiles 
               
            
           
           
               
               
               
               
            
               
                   
                 Taper width 
                 Graph for far-field angular profile 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 FIG. 
                 (nm) 
                 Horizontal 
                 Vertical 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 5A 
                 60 
                 410 
                 412 
               
               
                   
                 5B 
                 80 
                 420 
                 422 
               
               
                   
                 5C 
                 100 
                 430 
                 432 
               
               
                   
                 5D 
                 120 
                 440 
                 442 
               
               
                   
                 5E 
                 140 
                 450 
                 452 
               
               
                   
                 5F 
                 160 
                 460 
                 462 
               
               
                   
                 5G 
                 170 
                 470 
                 472 
               
               
                   
                 5H 
                 180 
                 480 
                 482 
               
               
                   
                 5I 
                 200 
                 490 
                 492 
               
               
                   
                   
               
            
           
         
       
     
     As may be seen from the plots, the horizontal angular extent varies from about ±5 degrees (FWHM) for a taper width of 60 nm to about ±40 degrees for a taper width of 200 nm, and the vertical angular extent varies from about ±20 degrees for the taper width of 60 nm to about ±40 degrees for the taper width of 200 nm. 
       FIG. 6  is a schematic top view of output beams  504  from a photonic integrated circuit and an associated microlens array  502  for modifying the numerical aperture (NA) of the output beams, in accordance with an embodiment of the invention. 
     Microlenses  500  in microlens array  502  receive output beams  504 , emitted by outputs  506  of a photonic integrated circuit (not shown in this figure), similarly to microlens array  36 B in  FIG. 1B  (with the exception that the centers of the microlenses are aligned with the outputs.) As shown in more detail in an inset  508  and in  FIGS. 7A-7B , microlens  500  reimages output  506  to form an output image  512  with a larger effective spot size and reduced NA (reduced from NA=0.4 for output beams  504  to NA=0.25 for imaged output beams  510 ). When used in conjunction with a collimating lens having an effective focal length of 20 mm, this design achieves low far-field divergence, with diffraction-limited performance (M 2 =1.015). 
       FIGS. 7A and 7B  are graphical representations  610  and  620  of the radiant emittances of output  506  (before imaging with microlens  500 ) and output image  512  (after imaging with the microlens), respectively, in accordance with an embodiment of the invention. The x- and y-coordinates in representations  610  and  620  are shown in millimeters. The width of output  506  at 10% of maximum radiant emittance is approximately 2.7 μm, whereas the width of output image  512  at 10% of maximum radiant emittance is approximately 5.7 μm. 
     Lidar Applications 
     As noted earlier, the principles of the present invention may be used in optimizing characteristics of the laser beams used in LiDAR applications, and particularly in coherent LiDAR. For example, in a monostatic coherent LiDAR device, it may be necessary to trade off the beam size against the signal/noise ratio (SNR) as a function of range. The maximum range using a given set of beam parameters is the range at which an object can be detected reliably. The SNR is approximately the number of photons received from the target (for a coherent LiDAR). The maximum range can then be quantified as the farthest object distance at which the SNR is greater than a minimum threshold value for reliable detection, SNR min . Assuming near diffraction-limited performance, the beam size and wavelength determine the angular far-field resolution. For a given transmit power and a Gaussian beam, a smaller beam diameter will result in higher SNR at short range, but faster degradation at longer ranges. Degraded beam parameters (M 2 &gt;1) reduce both range and resolution. 
     On this basis, it is possible to define an optimal beam size for any desired range. For example, a LiDAR channel with a required range of about 40 m will achieve the best SNR using a 10 mm beam (M 2 =1) at the required range, whereas a channel with 200 m required range needs a 20 mm beam diameter (M 2 ˜1) for optimal SNR. 
       FIG. 8  is a schematic side view of a LiDAR system  700  installed in a vehicle  702 , in accordance with an embodiment of the invention. The beam parameters of LiDAR system  700  are set, using the principles of the previous embodiments, to provide a LiDAR system in which resolution and range vary over the field of view of the system. Automotive LiDAR applications, for example, can have variable range and resolution requirements across the vertical field of view: 
     a) Object detection on a road  704  in a range  706  can be optimized for short range and low resolution, favoring small beam diameter at the short range. 
     b) Detection of vehicles and other objects on the freeway in a range  708  should be optimized for long range and high resolution, calling for a larger beam diameter at the long range. 
     c) Detection of objects well above the horizon in a range  710  is of lower importance, and consequently only a reduced resolution and range are required. 
     These objectives can be realized using a multi-channel (array) implementation based on integrated photonics, with per-output optimization. This sort of multi-channel array can be integrated, for example, in the sorts of multi-beam LiDAR systems that are described in U.S. Provisional Patent Application 62/843,464, filed May 5, 2019, which is incorporated herein by reference. Multi-channel implementations are shown specifically in  FIGS. 2-4  of this provisional application. 
       FIG. 9  is a schematic top view of a photonic integrated circuit  800  and a microlens array  808  that are designed to provide beams of different diameters to meet the range requirements of LiDAR system  700  ( FIG. 8 ), in accordance with an embodiment of the invention. Photonic integrated circuit  800  and microlens array  808  serve as the transmitter for LiDAR system  700 , emitting multiple beams of different diameters. A receiver (not shown) receives reflections of the beams from the different parts of the field of view that are shown in  FIG. 8 , and processes the reflections in order to detect the objects and find their respective distances from vehicle  702 . 
     Photonic integrated circuit  800  comprises, similarly to, for example, photonic integrated circuit  20 B ( FIG. 1B ), an array of waveguides  804  and a spot-size converter  806  for each waveguide. Microlenses  802  in array  808  are positioned with their centers aligned with respective output ends of spot-size converters  806 . The optical radiation emitted by spot-size converters  806  is re-focused by respective microlenses  802  to form spots  810 A,  810 B,  810 C,  810 D,  810 E,  810 F,  810 G, and  810 H. The diameter of each of spots  810 A- 810 H may be configured by an appropriate choice of the design parameters of spot-size converter  806  and microlens  802  (or other sorts of micro-optical component) forming the spot, as explained above. 
     The beam diameter and numerical aperture of each spot-size converter  806  can thus be optimized together with the corresponding microlens  802  to give beams that will have different spot sizes, even when used in conjunction with collimation optics having a fixed focal length for all beams. The collimation lens transforms the near-field spot size, i.e., the diameter of each of spots  810 A- 810 H, following microlens  802  into a corresponding far-field beam divergence:
         A small spot after microlens is transformed into a beam with a small divergence and a high intensity in the far field.   A large spot after microlens is transformed into a beam with a large divergence and a low intensity in the far field.   A medium spot after microlens is transformed into a beam with a medium divergence and a medium intensity in the far field.       

     Assuming the optics are designed to maintain separation between the channels in the vertical dimension, each channel can be optimized in this manner for short, medium or long range. In the present example, spots  810 D,  810 E, and  810 F are small spots, projected by collimation optics into beams with a small divergence and a high intensity in the far field, and thus suitable for illuminating range  708  in  FIG. 8 . Spot  810 H is a large spot, projected by collimation optics into a beam with a large divergence and low intensity in the far field, and thus suitable for illuminating range  706 . Spots  810 A,  810 B,  810 C, and  810 G are medium-sized spots, and thus the beams projected by collimation optics from these spots satisfy the illumination requirements for range  710 . 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.