Patent Publication Number: US-2021181311-A1

Title: VCSEL Array LIDAR Transmitter with Small Angular Divergence

Description:
RELATED APPLICATION SECTION 
     The present application is a non-provisional of copending U.S. Provisional Patent Application Ser. No. 62/538,149, filed Jul. 28, 2017, and entitled “VCSEL Array LIDAR Transmitter with Small Angular Divergence.” The entire content of U.S. Patent Application Ser. No. 62/538,149 is incorporated herein by reference. 
    
    
     The section headings used herein are for organizational purposes only and should not to be construed as limiting the subject matter described in the present application in any way. 
     INTRODUCTION 
     Autonomous, self-driving, and semi-autonomous automobiles use a combination of different sensors and technologies such as radar, image-recognition cameras, and sonar for detection and location of surrounding objects. These sensors enable a host of improvements in driver safety including collision warning, automatic-emergency braking, lane-departure warning, lane-keeping assistance, adaptive cruise control, and piloted driving. Among these sensor technologies, light detection and ranging (LIDAR) systems take a critical role, enabling real-time, high resolution 3D mapping of the surrounding environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present teaching, in accordance with preferred and exemplary embodiments, together with further advantages thereof, is more particularly described in the following detailed description, taken in conjunction with the accompanying drawings. The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating principles of the teaching. The drawings are not intended to limit the scope of the Applicant&#39;s teaching in any way. 
         FIG. 1  illustrates the operation of a LIDAR system according to the present teaching. 
         FIG. 2  illustrates an embodiment of a LIDAR system using two lasers according to the present teaching. 
         FIG. 3  illustrates an embodiment of a multi-emitter laser source for a LIDAR transmitter according to the present teaching. 
         FIG. 4  illustrates a diagram of the cross-section of an embodiment of an illuminator for a LIDAR transmitter of the present teaching. 
         FIG. 5A  illustrates a measurement point cloud for an embodiment of a single-wavelength 2D multi-emitter LIDAR transmitter of the present teaching. 
         FIG. 5B  illustrates a measurement point cloud for an embodiment of a two-wavelength 2D multi-emitter LIDAR transmitter according to the present teaching. 
         FIG. 6  illustrates a perspective view of a schematic diagram of the structure of a prior art bottom-emitting VCSEL laser used in some embodiments the LIDAR transmitter of the present teaching. 
         FIG. 7  illustrates a schematic diagram of an embodiment of a 2D monolithic VCSEL array with twenty-five separate laser emitters according to the present teaching. 
         FIG. 8  illustrates a schematic diagram of an embodiment of a 2D monolithic VCSEL array with twenty-five separate laser emitters comprising sub-apertures according to the present teaching. 
         FIG. 9  illustrates a schematic diagram illustrating light projected by a single emitter in a single lens LIDAR transmitter system according to the present teaching. 
         FIG. 10  illustrates a schematic diagram of light projected by multiple emitters in a single lens LIDAR transmitter system according to the present teaching. 
         FIG. 11A  illustrates a schematic diagram showing an expanded view of light projected by multiple emitters in a single lens LIDAR system with high angular divergence emitters. 
         FIG. 11B  illustrates a schematic diagram illustrating a condensed-view of light projected by the system of  FIG. 11A  that shows the far field. 
         FIG. 12  illustrates a schematic diagram of an embodiment of a two-lens illuminator that projects light in a small angular divergence LIDAR transmitter according to the present teaching. 
         FIG. 13  illustrates a schematic diagram of an embodiment of a multiple-transmitter-array small angular divergence LIDAR transmitter according to the present teaching. 
         FIG. 14A  illustrates a schematic diagram of an expanded view of a small angular divergence LIDAR transmitter illuminator utilizing multiple transmitter arrays on a single substrate according to the present teaching. 
         FIG. 14B  illustrates a schematic diagram of a condensed-view of the small angular divergence LIDAR transmitter of  FIG. 14A . 
         FIG. 15  illustrates schematic diagrams of scaled condensed views of embodiments of the small angular divergence LIDAR transmitters of the present teaching that indicate relative independence of the separation between transmitter arrays. 
         FIG. 16  illustrates an embodiment of an illuminator for a multi-wavelength LIDAR system using multiple transmitter arrays according to the present teaching. 
         FIG. 17  illustrates a measurement point cloud that can be generated with the illuminator described in connection with  FIG. 16 . 
     
    
    
     DESCRIPTION OF VARIOUS EMBODIMENTS 
     The present teaching will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present teaching is described in conjunction with various embodiments and examples, it is not intended that the present teaching be limited to such embodiments. On the contrary, the present teaching encompasses various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. Those of ordinary skill in the art having access to the teaching herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the teaching. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
     It should be understood that the individual steps of the methods of the present teaching can be performed in any order and/or simultaneously as long as the teaching remains operable. Furthermore, it should be understood that the apparatus and methods of the present teaching can include any number or all of the described embodiments as long as the teaching remains operable. 
     The present teaching relates to Light Detection and Ranging Systems (LIDAR) that measure distances to various objects or targets that reflect and/or scatter light. It is desirable that LIDAR systems have a small footprint, high measurement resolution, and high detection sensitivity. VCSEL-array-based transmitters offer the promise of providing these benefits. Improvements are needed in the optical design of these VCSEL-array-based LIDAR transmitters in order to increase performance and to reduce size, weight, power, cost and complexity of LIDAR systems. As one example, LIDAR transmitter designs are needed that produce a small angular divergence in a compact package. 
       FIG. 1  illustrates the operation of a LIDAR system  100  according to the present teaching that is implemented in a vehicle. The LIDAR system  100  includes a laser projector, also referred to as an illuminator, that projects light beams  102  generated by a light source toward a target scene and a receiver that receives the light  104  that reflects off an object, shown as a person  106 , in that target scene. LIDAR systems typically also include a controller that computes the distance information about the object  106  from the reflected light, and an element that can scan or provide a particular pattern of the light that may be a static pattern across a desired range and field-of-view (FOV). The receiver and controller are used to convert the received signal light into measurements that represent a pointwise 3D map of the surrounding environment that falls within the LIDAR system range and FOV. In various embodiments, the controller can be a simple electrical circuit or can be a more complicated processor, depending on the particular application. 
     The laser source and optical beam projection means that form the illuminator and the receiver may be located on the front side of a vehicle  108 . A person  106 , and/or another object, such as a car or light pole, will provide light reflected from the source back to the receiver, and a range, or distance, to that object is determined. As is known in the art, a LIDAR receiver calculates range information based on time-of-flight measurements of light pulses emitted from the light source. In addition, known information about the optical beam profile that illuminates the scene in a target plane associated with a particular range and based on the particular design of the source and projector system is used to determine location information about the reflecting surface, thereby generating a complete x,y,z, or three-dimensional picture of the scene. In other words, the pointwise 3D map of the surrounding environment represents a collection of measurement data that indicates position information from all the surfaces that reflect the illumination from the source to the receiver within the field-of-view of the LIDAR system. In this way, a 3D representation of objects in the field-of-view of the LIDAR system is obtained. The pointwise 3D data map may also be referred to as a measurement point cloud. 
     One feature of the present teaching is that the illuminator may include lasers that emit optical beams with individual, distinct wavelengths. Typically, LIDAR systems do not utilize different laser wavelengths to enable improvements in the angular resolution of the LIDAR system. In particular, one feature of LIDAR systems of some embodiments of the present teaching is that they use multiple laser wavelengths to enable finer angular resolution and performance in a low-cost, compact optical design. Furthermore, multi-wavelength LIDAR systems of the present teaching can provide a simple path to improved security and parallelization. See, for example, U.S. patent application Ser. No. 15/456,789, entitled “Multi-Wavelength LIDAR System” filed on Mar. 3, 2017 and U.S. Patent Application Ser. No. 62/396,295, entitled “WDM Lidar System” filed on Sep. 19, 2016. U.S. patent application Ser. No. 15/456,789 and 62/396,295 are both assigned to the present assignee and are herein incorporated by reference. 
       FIG. 2  illustrates an embodiment of a multi-wavelength LIDAR system  200  using two lasers according to the present teaching. The first laser  202  operates at a first wavelength, and the second laser  204  operates at a second wavelength. The lasers may include integrated or separate collimation optics that form part of an optical projection element that is used to form a beam profile at various target planes across the FOV and range of the LIDAR system. The illuminator  206  can also include an optical device  208  that further shapes and projects the optical beams to form particular beam profiles at target plane  210 . In various embodiments, different types of optical devices can be used to form the optical projection element including, for example, one or more of lenses, diffractive optics, prisms, thin-film wavelength sensitive devices, and partially reflecting mirrors. Single lens or multi-lens optical elements may be used. 
     A receiver  212  receives light reflected off the surface of objects at various target planes  210  in the FOV and range of the LIDAR system. If different wavelengths are used, the receiver  212  may be able to distinguish light from the two wavelengths emitted by the sources  202 ,  204 . In this case, reflected illumination from each wavelength is processed separately. A controller  214  is used to process the received light. The controller  214  provides LIDAR data at an output  216 . The complexity of the controller  214  depends on the particular configuration of the LIDAR system. The controller  214  may be used to control the laser sources  202 ,  204 . In various embodiments, the controller  214  may comprise various electrical circuits, integrated circuits, microprocessors, or computers. It is relatively straightforward to add N lasers with the same or different wavelengths to the LIDAR system shown in  FIG. 2 . In some embodiments, there are additional optical elements that collimate the light and provide the desired FOV. 
     A projection element as described herein is an element that collimates or otherwise shapes and/or projects a laser beam or multiple laser beams in a particular direction. A projection element can comprise one or more optical devices positioned in the path of the optical beams. These devices and their positions, together with the initial shape and paths of the beam or beams emitted from the laser source, produce the desired beam profile, which is a combination of beam shape and/or beam position at a particular point in space. Upon receipt of light reflected off objects at a receiver of the LIDAR system, the system generates a measurement point cloud, or pointwise data map, that is based on the pattern of light projected by the illuminator, and its performance parameters including angular resolution and field-of-view. 
     One feature of the present teaching is the ability to use different wavelengths to produce different LIDAR FOV, range, and/or resolution in a compact system. The light beams at the two or more wavelengths may be able to share at least some of the same optical devices that form the projection element, and yet still realize different beam profiles that result in a measurement point cloud that represents different range and/or FOV and/or resolution at each wavelength. For example, one problem with prior art LIDAR systems that use a signal wavelength is that the launch power required to reach 100-meter range is so high that for close proximity reflections (e.g. a few meters) the receiver saturates. Consequently, these prior art LIDAR systems are blind to near objects. This problem can be solved with a two-wavelength system, where the first wavelength is used for a 100-meter range, but the second wavelength has a low power only meant for near-proximity measurements. Multi-wavelength measurements can be performed simultaneous using a controller with parallel computing capability. The extension of this configuration to more than two wavelengths is relatively straightforward. 
     Another feature of the present teaching is that lasers with additional wavelengths can be added to perform functions other than LIDAR ranging. For example, additional lasers can be added to provide measurements of the orientation of optical devices within the LIDAR system. The light from these sources at additional wavelength may serve a sole purpose to provide angular measurement of the elements that project the optical beams and/or replicate or scan the optical beams. In some embodiments, MEMS devices are used to project beams, and it can be important to have direct feedback of the mirror position. Another laser combined with an appropriate receiver measurement system could provide direct angle measurement of mirror position. A natural extension of the above embodiments would be to use a plurality of lasers of the same wavelength in each case, that is, either a 1D or 2D array of lasers of each wavelength instead of a single laser of each wavelength. 
     In some systems, a single large lens is used to both collimate as well as set the projection angle of each VCSEL device. It should be noted that instead of a single lens, two or more lenses could be used as part of the shared lens configuration. One aspect of using a shared optic for both collimation and projection angle is that there is a direct mapping between the lateral position of the VCSEL device relative to the central axis of the lens and the pointing angle of the projected laser beam. The lateral distance between two VCSEL lasers of the same, or similar, wavelength will correspond to the difference in projection angles created by the shared lens system. 
     Furthermore, since the VCSEL device is not an ideal point source, but instead has a finite lateral size, there will be an additional divergence that cannot be reduced by the optics without also shrinking the FOV of the overall optic system. Also, the shared-optic approach using lasers with the same or similar wavelength may lead to beam overlap or gaps in the 3D measurement span depending on the finite size of the VCSEL, the divergence of the collimated beams, the number of VCSEL devices, and the FOV, among other parameters. 
     One feature of LIDAR systems of present teaching is the use of VCSEL chips with clusters of emitting apertures to take advantage of the higher optical power and large diameter cluster provided by these devices. As described herein, a VCSEL device is not an ideal point source, but rather has a finite lateral dimension. Furthermore, high-power top-emitting VCSEL lasers used for LIDAR illumination typically use multiple light emitting sub-apertures to reach the required high-power output. These multiple sub-apertures form a cluster or group, and ideally are located as close as physically possible, while still maintaining the required electro-optic efficiency. 
     In some embodiments, the VCSEL array is a two-dimensional array. In some embodiments, the VCSEL array is monolithic and the lasers all share a common substrate. A variety of common substrate types can be used. For example, the common substrate may be a semiconductor material. The common substrate may also include a ceramic material. 
     In some embodiments, the VCSELs are top-emitting VCSELS. In other embodiments, the VCSELs are bottom-emitting VCSELS. The individual VCSELs may have either a single large emission aperture, or the individual VCSELs may be formed from two or more sub-apertures within a larger effective emission diameter. A group of sub-apertures forming a larger effective emission region is sometimes referred to as a cluster. 
       FIG. 3  illustrates a multi-element emitter laser source  300  with two different VCSEL wavelengths interleaved uniformly in the vertical direction. The embodiment shown in  FIG. 3  illustrates a single common substrate  304 , but it will be clear to those skilled in the art that multiple substrates could also be used. There are six VCSEL bars  306 ,  308 . The cluster VCSEL devices  302  of the bars  306  emit at one common wavelength. These are the bars  306  labeled “VCSEL λ 1 ” in the figure. The cluster VCSEL devices  302  of the dark bars  308  emit at a different wavelength. The bars  308  are labeled “VCSEL λ 2 ” in the figure. A total of thirty cluster VCSEL devices  302  are shown. 
     The illuminator used in connection with the multi-element emitter laser source  300  of  FIG. 3  can use a shared lens system for both collimation and projection of the beams over the desired FOV.  FIG. 4  illustrates a diagram of a cross-section of an embodiment of an illuminator  400  for a multi-wavelength LIDAR system of the present teaching that uses a multi-wavelength laser source shown in  FIG. 3 . The illuminator  400  includes the multi-emitter laser source  402 , and a projection element  404  comprising a wavelength multiplexer  406  and a first lens  408 . The projection element  404  is used to project the laser beams  410 ,  412  emitted from the laser source  402 . The emitters for the multi-emitter laser source  402  are located on a VCSEL substrate  414 . 
     The projection element  404  in  FIG. 4  comprises two optical devices  406 ,  408 . The first optical device is a wavelength multiplexer  406  that is a wavelength sensitive optic that acts to combine the laser beam  410  at one of the two wavelengths and from one optical path with the laser beam  412  at the other of the two wavelengths that is on another optical path onto a common optical path. In some embodiments, wavelength multiplexers comprise a diffractive optic that is configured to substantially shift the optical path of one wavelength while letting the second wavelength pass through undisturbed. Diffractive optic elements are well known in the art and can be used to provide precise beam steering and beam shaping of lasers. In some embodiments, a wavelength-sensitive diffractive optical element is used. In other embodiments, an array of refractive optics, such as prisms, is used. The second device is a lens  408  that is used to further project and shape the laser beams  410 ,  412  to form a desired pattern of beam shapes and beam positions at the target plane of the LIDAR system. 
       FIG. 4  illustrates how the light from two VCSEL bars of different wavelength travels through the optical system. For clarity, only the laser beams  410 ,  412  from two VCSEL emitters are shown ray traced.  FIG. 4  illustrates only a single lens  408 , however, it will be clear to those with skill in the art how to implement a two-lens, or multi-lens system as described, for example, in connection with  FIG. 12  below with the multi-wavelength system illustrated in  FIG. 4 . 
     In operation, light from the beam profiles formed at a target plane by the illuminator is reflected from the surface of objects in that target plane. A target plane in a LIDAR system is a virtual reference point that operates over a complete range and field-of-view. There are many different target planes at various distances from the LIDAR module such that the system can generate three-dimensional representation of the objects in the field-of-view and range being probed by the LIDAR system. A portion of the light reflected off the surfaces of objects illuminated by the optical beam profiles in the target plane is directed to receivers. The receivers detect the light and then convert the received optical signal to an electrical signal. A controller electrically connected to the light sources and to the receiver converts the received signal into a measurement point cloud. The angular resolution of points in the measurement point cloud depends of the relative position of the beam profiles at a target plane, as described further below. It will be clear to those skilled in the art that many other variations of the embodiment of the illuminator  400  illustrated in  FIG. 4  are within the scope of the present teaching. For example, VCSEL lasers may be located in a common surface, either planar or curved. It will also be clear to those skilled in the art that some deviation of the VCSEL away from a central surface, curved or flat, can be tolerated. 
       FIG. 5A  illustrates a measurement point cloud  500  for an embodiment of a single-wavelength 2D laser source illumination according to the present teaching. The distance between the vertical spacing  502  of the measurement points  504  determines the vertical angular resolution. The horizontal spacing  506  of the points on the point cloud determines the horizontal angular resolution of the point cloud. 
       FIG. 5B  illustrates a measurement point cloud  550  for an embodiment of a two-wavelength 2D laser source illumination of the present teaching. A measurement point corresponding to a VCSEL with λ 1  is shown as a circle  552 , a measurement point with a VCSEL with λ 2  is shown as a triangle  554 . A composite point cloud includes a point cloud derived from reflections received at λ 1  and a point cloud derived from reflections received at λ 2 . 
     For example, the measurement point cloud  550  illustrated in  FIG. 5B  can be realized using the multi-emitter laser source with the pattern of VCSEL emitters of different wavelengths illustrated in  FIG. 3  together with the illuminator configuration of  FIG. 4 . A portion of the light from optical beams generated by the illuminator at the target plane is reflected by the surface of an object and incident on one or more optical receivers that are capable of detecting light at particular wavelengths. The resulting measurement point cloud  550  includes points representing light from the different beam profiles that are at different wavelengths. 
     Referring to  FIGS. 3-5 , different wavelength VCSEL bars  306 ,  308  occupy different rows of the laser source in the vertical direction, and individual VCSEL devices  302  in different rows have their centers offset in the horizontal direction. The optical beams from emitters in different wavelength bars are projected by the projection element  404  so that the optical beam positions are slightly offset at the target plane in the vertical direction. This causes the offset  556  in the measurement point cloud. The offset in the center position of the VCSELs in adjacent bars, together with the design of the projection element causes the measurement points representing each wavelength to be interleaved horizontally along the offset vertical lines. The angular resolution of the measurements in a given dimension is directly related to the offset of the points in that dimension, which is related directly to the positions of the optical beams in that dimension at the target plane. 
     Referring to both  FIG. 5A-B , a performance tradeoff associated with using a two-wavelength solution is clear. In the embodiment shown in  FIG. 5 , the optical beams at one wavelength travel substantially uninterrupted, but the optical beams at the second wavelength are intentionally shifted in position to substantially overlap in one direction with the optical beams at the first wavelength. The offset  556  in position of the optical beams at each wavelength indicated in the drawing in  FIG. 5B  can be adjusted based on the design of the wavelength multiplexer. For example, in some embodiments, the wavelength multiplexer  406  in  FIG. 4  is specifically designed to provide a particular offset  556  in position of the optical beams at each wavelength. In various embodiments, various devices in the projection element are used to position the beams of the lasers at the two wavelengths. These same devices, or other devices, may alter the beam shapes as well their positions at the target plane. 
     As compared to the single wavelength embodiment of  FIG. 5A , the embodiment described in connection with  FIG. 5B  doubles the angular resolution in a preferred direction, which in this case is the horizontal direction, at the expense of halving the angular resolution in the perpendicular direction. This is accomplished while keeping the overall physical size of the system relatively constant. In some applications, finer resolution in one direction may be preferred or necessary, for instance if instead of a pedestrian, the system needs to distinguish a pole or tree with a cross-section of only 100 mm. At 30 m, we would need an angular resolution that is less than 0.15 degrees. For automotive LIDAR systems, it could be highly desired to have a very small angular resolution in the horizontal direction, at the expense of wider angular resolution in the vertical since common objects that need to be recognized, such as humans, poles, and trees are tall but narrow. In some embodiments, the angular resolution of the measurement point cloud is less than 0.4 degree at a predetermined distance from the target plane to the optical projection element. 
     One feature of the present teaching is that single element emitters and multi-element emitters light sources operating at different wavelengths do not need to be located on the same surface. Another feature of the present teaching is that the surfaces may be oriented along different spatial planes in three-dimensional space. For example, the planes may be on two orthogonal planes. In some embodiments, we use a plurality of surface emitting lasers made up of at least two groups of lasers with different wavelengths. We also make use of three-dimensional space and each group of lasers are oriented in two or more surfaces, planar or curved, that are not necessarily orthogonal. In these embodiments, the packaging and optical alignment complexity increases relative to embodiments in which the lasers are co-located on a common surface, but we are able to increase the resolution angle across the full field-of-view in both orthogonal directions, without any compromise. This provides both higher precision as well as full access to all the capabilities associated with more than one wavelength. That is, it is possible to realize simultaneous operation, redundancy, security and other features of multi-wavelength operation. 
     One feature of the present teaching is that the small angular divergence LIDAR transmitter of the present teaching provides a compact LIDAR module especially suited to LIDARs that operate with a range requirement of about 100 meters. Another feature of the small angular divergence LIDAR transmitter of the present teaching is that it may utilize a solid-state optical emitter. Consequently, the LIDAR transmitter of the present teaching can be built utilizing no moving parts. In addition, multiple lasers, which may emit at the same or different wavelengths, can be used to establish a one-to-one mapping between each laser and a 3D measurement point cloud. 
       FIG. 6  illustrates a schematic diagram of a perspective view of a structure of a known bottom-emitting VCSEL laser  600  that can be used in the small angular divergence LIDAR transmitter of the present teaching. Note that the emission area of the VCSEL laser  600  typically ranges from a few microns in diameter for mW power operation, up to 100 microns diameter or more for 100 mW and greater CW power operation. Various embodiments of the present teaching use a variety of known VCSEL laser devices, including top-emitting VCSELs, bottom-emitting VCSELS, and various types of high-power VCSELs. The VCSELs is fabricated on a substrate  602  that can be GaAs, or other semiconductor material. An n-type distributed Bragg reflector (DBR)  604  is positioned on the substrate. An active region  606  is constructed on the n-type DBR  604 , followed by an aperture  608  that can be made from an oxide material. A p-type Distributed Bragg Grating DBR  610  is then grown on the active region  606 . Typically, the p-type DBR is highly reflecting, and the n-type DBR is partially reflecting, resulting in light output  612  from the bottom, substrate-side of the layer structure. The active region  606 , oxide aperture  608  and p-type DBR  610  are formed in a mesa structure  614 . A top contact  616  and a bottom contact  618  are used to provide an electrical current to the active region  606  to generate the output light  612 . An oxide aperture  608  provides current confinement to the active region  606 . The top contact  616  is p-type, and the bottom contact  618  is n-type. Emission apertures  620  are formed in the bottom contact  618  to allow the output light  612  to emerge from the bottom, substrate side of the bottom-emitting VCSEL  600 . This type of VCSEL may be a single element, or multiple VCSELS can be fabricated as one- or two-dimensional arrays on the substrate  602 . 
       FIG. 7  illustrates a schematic diagram of an embodiment of a 2D monolithic VCSEL array  700  with twenty-five separate laser emitters  702  of the present teaching. Each laser emitter  702  has an emission aperture  704  of diameter, a. Emission from each single laser emitter  702  substantially fills the full emission aperture  704 . Each laser emitter  702 , therefore, generates a laser beam with diameter, a, equal to the diameter of the emission aperture  704 . The laser emitters  702  are spaced uniformly in the horizontal direction with a spacing dx  706 . The laser emitters  702  are spaced uniformly in the vertical direction with a spacing dy  708 . The overall size of the array, measured from the centers of the outermost lasers is distance Dx  710  in the horizontal direction and distance Dy  712  in the vertical direction. The actual chip size will be slightly larger than the distance Dx  710  and distance Dy  712 . 
       FIG. 8  illustrates a schematic diagram of an embodiment of a 2D monolithic VCSEL array  800  with twenty-five separate laser emitters  802  of the present teaching. Each laser emitter  802  has an emission aperture  804  of diameter, a. Each laser emitter  802  is formed from multiple sub-apertures  803  that are connected electrically to act as one, and whose combined emission is contained within an emission aperture  804  of dimension, a. Thus, each laser emitter  802  generates a laser beam with diameter, a, equal to the diameter of the emission aperture  804 . The laser emitters  802  are spaced uniformly in the horizontal direction with a spacing  806 , dx. The laser emitters  802  are spaced uniformly in the vertical direction with a spacing  808 , dy. The overall size of the array measured from the centers of the outermost lasers is distance  810 , Dx, in the horizontal direction and distance  812 , Dy, in the vertical direction. The actual chip size will be slightly larger than the distance  810 , Dx, and distance  812 , Dy.  FIG. 8  illustrates laser emitters with emission shapes that are circular. In various embodiments, the emitters may produce beams with various shapes. For example, oval, square, rectangular and various odd shapes may be realized. This emission width is the width of the shape in a particular direction and will determine the angular divergence of the LIDAR transmitter in that particular direction, as described further below. 
     Some embodiments of the present teaching utilize bottom-emitting high-power arrays of VCSELs with a single large aperture per laser, such as the configuration shown in  FIG. 7 . Other embodiments of the present teaching utilize top-emitting high-power arrays of VCSELs with a large aperture comprising sub-apertures as shown in  FIG. 8 . However, one skilled in the art will appreciate that the present teaching is not limited to these configurations of top- and bottom-emitting VCSELs and associated emission apertures. 
     Prior art systems generate a one-to-one mapping between each laser and a specific measurement point, and/or projected angle, using a single shared lens for all the transmitting elements. See, for example, U.S. Pat. No. 7,544,945 that describes utilizing five lasers together with a single projection lens to form five separate projected beams with distinct angular spacing. The single projection lens provides two functions. The first function is to collimate the laser beam to determine the spot size of the beam in the far field. The spot size is set by the requirements of the LIDAR system at the required range. For example, a typical requirement for LIDAR system is that the laser spot at 100 m should be smaller than 0.5 m in diameter. This is equivalent to a full angle divergence of 5 mrad. The second function of the optical lens is to determine the full field-of-view for the projected laser beams which is set by the position of the two outermost beams in the far field at the range of the LIDAR system. The angular resolution between each measurement beam is then determined by the taking the full field-of-view divided by the N−1 the number of lasers in each direction. 
     One disadvantage of prior art single projection lens LIDAR systems is that they fail to account for the finite size of the emission aperture of the laser emitter. In systems that utilize a single projection lens, the lasers are placed at the focal point of the single projection lens in order to collimate the laser beams. 
       FIG. 9  illustrates a schematic diagram of light projected by a single lens LIDAR transmitter system  900 . A lens  902  is placed at the focal length  904 , f, relative to a plane  906  of a laser emitter. The distance  908 , y1, represents the half of the emission width, the radius of the emitted laser beam. The angle  910 ,  01  is the divergence of the emitted laser beam. From classical optics, the optical invariant rule tells us that the product of the beam radius and the beam divergence will be a constant. Therefore, the laser beam radius  912  after the lens is now y2, and the divergence  914  is θ2, where: θ1*y1=θ2*y2. And from geometry, we see that the focal length  904 , f, is related to y2, by the equation: y2=f*θ1. Combining these two equations, we can then obtain the relationship θ2=y1/f. This relationship indicates that the angular divergence of the collimated beam is directly related to the focal length of the lens in a single lens LIDAR transmitter. The relationship also shows how the angular divergence depends on the width of the emitter size and associated radius for a circular shaped aperture. This relationship sets a minimum size constraint on the lens focal length required for a particular size of the emitting laser beam. For example, a typical high-power VCSEL has a round shape with an effective emission diameter of 100 microns. Therefore, y1 equals 50 microns. To meet the criteria described herein for a 100 m LIDAR system, there should be less than 5-mrad divergence (full angle). The minimum focal length of the projection lens system is then 20 mm. 
     The examples described here generally assume a circularly symmetric system with a round emission shape and spherical lenses. However, it will be clear to those skilled in the art that the present teachings apply to emission shapes and lens shapes having other shapes and geometries. The width and focal length relationships described then apply in a particular direction. For example, rectangular emitters and/or lens systems that comprise cylindrical and/or spherical lenses can be used. The choices will depend on the desired beam patterns at the target range. For example, a system may be constructed with a different field-of-view and angular resolution in the horizontal and vertical directions. 
     In the above analysis, we have used classical optics formulations that assume small angles and thin lenses. If dimensions or angles are large, which is common in compact transmitter designs, the classical optics formulations would not necessarily provide sufficient accuracy of the predicted angular divergence and field-of-views obtained. In these cases, full three-dimensional electromagnetic models are preferred. 
       FIG. 10  illustrates a schematic diagram  1000  of light projected by multiple emitters in a single lens LIDAR transmitter system. A single projection lens  1002  is situated a distance from an array of VCSEL laser emitters  1004  on a substrate  1006 .  FIG. 10  illustrates the light projected by an arrayed laser transmitter system fully representing the VCSEL laser emitters  1004  as being finite sources with emission apertures of width, a. In  FIG. 10 , the center-to-center distance  1008 , D, of the outermost lasers, maps to an angular field-of-view  1010 , β, which is the center-to-center angular offset between the two outermost projected beams  1012 ,  1014 . Each vertical position in the focal plane of the lens, where the lasers are located, maps to a unique angular projection angle. As such, for a source of finite emission aperture of dimension, a, a beam divergence will result approximately equal to a*(β/D). 
     Referring to  FIG. 10 , a typical spacing, d, for a VCSEL array would be 250 microns, so the center-to-center distance  1008 , D, of the outermost emitters would be 1.25 mm. Assuming an emission aperture diameter, a, of 100 microns and a maximum divergence full angle of 5 mrad, we can calculate that the full field-of-view  1010  of the lens, β, cannot be greater than 62.5 mrad (3.58 degrees). If a field-of-view larger than 3.58 degrees is generated by the lens, then the divergence of the laser beam due to the finite size of the laser emission area will exceed 5 mrad full angle. 
     The inherent divergence of the VCSEL laser emitters of  FIG. 10  is approximately two degrees. This low divergence keeps the laser beams largely separated as they emerge from the plane of the array substrate  1006  and proceed through the lens toward the far field, except at the point  1016  on the right side of the lens where they all cross over to form a beam waist. 
       FIGS. 11A-B  show a more typical, larger, beam divergence configuration.  FIG. 11A  illustrates a schematic diagram  1100  of an expanded view of light projected by multiple emitters in a single lens LIDAR system with high angular divergence emitters.  FIG. 11B  illustrates a schematic diagram  1150  of a condensed-view of light projected by the system of  FIG. 11A  to illustrate the far field. In both figures, an emitter array  1102 ,  1152  illuminates a respective lens  1104 ,  1154 . Each emitter  1106 ,  1156  produces a laser beam  1108 ,  1158  with a relatively broad divergence as compared to the example shown in  FIG. 10 . The lenses  1104 ,  1154  converge the respective emitted beams  1108 ,  1158  to produce a beam waist  1110 ,  1162 .  FIGS. 11A-B  show how the laser beams would look for a more typical VCSEL divergence of ˜20 degrees. In this configuration, the beams  1108 ,  1158  overlap significantly at the lens  1104 ,  1154  as shown. And, the position  1160  at which the beams fully separate is much further from the lens  1154  as compared to the example of  FIG. 10 , as is shown in  FIG. 11B . We can see that the inherent divergence of the VCSEL, along with the size of the array, will determine the minimum lens aperture for this single lens system. The larger the divergence, and the larger the size of the array, the larger the minimum aperture required to truncate the laser beams emitted by the VCSEL array. 
     Thus, for the LIDAR transmitter, we then have two main causes of divergence in the transmit beam at the target range in the far field that comprises the series of laser beams emitted by the apertures and/or sub-apertures of the VCSEL array. One source of final beam divergence is a function of the size of the laser emission area and the focal length of the lens system. The second source of final beam divergence is a function of the laser emission size and the projected field-of-view of the lens. One feature of the present teaching is the recognition that different lens systems can be designed with identical focal lengths but with different projected field-of-views. 
     A method of light detection and ranging according to the present teaching includes generating a plurality of optical beams that can be multiwavelength optical beams. A first lens is positioned in an optical path of the plurality of optical beams at a distance from at least one of the plurality of light emitters that is less than a focal length of the first lens. The first lens converges the plurality of optical beams to a converged optical beam having a beam waist. A second lens is positioned in the optical path of the converged optical beam so that it projects the converged optical beam to a target range. The position of the second lens can be further selected to decrease an angular resolution of the LIDAR transmitter at the target range. Also in some methods, a size of an aperture of the second lens is chosen to be equal to a size of the beam waist of the converged optical beam. The position of the second lens and an emission width of at least one of the plurality of light emitters are selected to provide a desired field-of-view of the LIDAR transmitter at the target range. 
       FIG. 12  illustrates a schematic diagram of an embodiment of a lens system that projects light in a small angular divergence LIDAR transmitter  1200  of the present teaching. A first lens  1202  is placed in very close proximity to the VCSEL laser array  1204  that comprises multiple individual laser emitters  1206 . The first lens  1202  may be placed closer to the emitters than a distance equal to the focal length of the first lens. The figure illustrates laser beams  1202 ,  1210  that are emitted by the individual laser emitters  1206 . Only the center laser beam  1208  and an outer laser beam  1210  are shown. But all the laser emitters  1206  in the array may produce laser beams, as desired. 
     A second lens  1212  is positioned after the first lens  1202  and projects the laser beams  1208 ,  1210  to a far field position  1214  where the laser beams from individual transmitters are nominally separated. The first lens  1202  is placed at a particular distance  1216  from the array that produces a desired convergence of the laser beams  1208 ,  1210  at the second lens  1212 . The second lens  1212  is placed at a particular distance  1218  from the first lens  1202  to produce a desired field-of-view at a desired far field position  1214 , the target range. 
     As shown in  FIG. 12 , the first lens  1202  acts to converge the beams, turning the laser beam  1210  inward from the outermost VCSEL emitter  1220 , which produces a smaller minimum lens aperture for the second lens  1212  as compared to a single lens system. This means the physical transmitter can be smaller in size because the minimum lens aperture of the second lens  1212  determines the size of the largest lens in the system. In this example, the beam waist produced by the converging first lens  1202  is formed at the position of the second lens  1212 , and the aperture of the second lens is equal to the size of the beam waist. Each of the beams produced by the emitters fills the aperture of the second lens. 
     This two-lens configuration, when compared to a single lens system, with identical focal lengths, has significantly improved performance while maintaining the required low divergence of the output beams. This two-lens configuration also advantageously minimizes the size of the LIDAR transmitter. In addition, the projection angle of the two-lens system can be varied without changing the overall focal length of the lens system. The curvatures of the lens surfaces in this two-lens system can be adjusted to provide an additional degree of freedom that maintains focal length, while enabling different field-of-view. This additional degree of freedom also allows the adjustment of the field-of-view to minimize as necessary the divergence of the individual emitted laser beams, while maintaining an overall compact size for the transmitter. 
     As described herein, the divergence of the transmitted laser beams of the individual laser emitters is a critical factor in determining the resulting field-of-view of the lens system. Once the maximum field-of-view is determined based on the maximum divergence, the angular resolution of the system is then determined by the spacing between individual lasers in the array, which cannot be smaller than the individual emission width. 
     The small angular divergence LIDAR transmitter of the present teaching can be configured in a variety of designs that realize particular field-of-views and angular resolutions that are required to address the needs of the particular sensor application. For example, some automotive applications require a maximum divergence that is five milliradians half angle. Field-of-view, divergence and aperture size are all related. Furthermore, the emitter emission width, emitter pitch, and array size are critical. In one particular embodiment, a sixteen-by-sixteen-element two-dimensional array with an emission width of 125 micron and laser pitch of 250 micron, and a maximum half-angle divergence of 5 mrad is configured for a field-of-view of 18.3 degrees by using a two lens system in which the first lens is placed at a position less than the focal length of the first lens from the array. A larger field-of-view using the present teachings is possible, for example using a smaller emission width. 
     The angular resolution is determined by dividing the field-of-view by the array size. Using the smallest possible emission-width emitters means more elements in the array, and hence the smallest possible angular resolution. One particular embodiment with a sixteen-element array in one dimension, an emission width of 50 micron, a laser pitch of 250 micron, and a maximum half-angle divergence of 5 mrad is configured for a field-of-view of 45.8 degrees. The angular resolution for this configuration is 2.8 degrees. A smaller resolution is possible, for example, using more array elements with a smaller pitch. 
     In various embodiments, the field-of-view and the angular resolution may be different in different directions, for example, in some systems, the horizontal field-of-view and angular resolution are different from the vertical field-of-view and angular resolution. 
     One feature of the present teaching is that the compact size of the two-lens projection system, and other design features, allow for multiple transmitter arrays to be combined into a single transmitter system.  FIG. 13  illustrates a schematic diagram of an embodiment of a lens system that projects light in a small angular divergence LIDAR transmitter  1300  utilizing multiple transmitter arrays  1302 ,  1304 ,  1306  according to the present teaching. In  FIG. 13 , several transmitter arrays  1302 ,  1304 ,  1306  are used to cover a combined wider field-of-view  1308 . The individual VCSEL emitters each use a transmitter arrays  1302 ,  1304 ,  1306 , and the placement of the transmitter arrays  1302 ,  1304 ,  1306  in three dimensional space (x, y, z) allows the beams to be combined in a desired pattern, such as an overall uniform pattern in the far field or a pattern with desired overlap or gaps. 
     For example, the positions of the transmitter arrays  1302 ,  1304 ,  1306  may be chosen so that the pattern of beams at the target range are arranged so the beams from the first transmitter array  1302  and the beams from the second transmitter array  1304  form a gap at the target plane. Alternatively, the positions of the transmitter arrays  1302 ,  1304 ,  1306  may be chosen so that the pattern of beams at the target range are arranged so the beams from the first transmitter array  1302  and the beams from the second transmitter array  1304  overlap at the target plane. In addition, the positions of the transmitter arrays  1302 ,  1304 ,  1306  may be chosen so that the pattern of beams at the target range are arranged so the beams from the first transmitter array  1302  and the beams from the second transmitter array  1304  form a uniform pattern of light at the target plane. It will be clear to those with skill in the art that the relative positions of the transmitter arrays, and associated optical lenses that converge and project the multiple beams that emerge from the transmitter arrays allow a variety of patterns to be projected at the target plane. 
     In some embodiments, the multiple transmitter arrays are used to cover a wider, or narrower, field-of-view by physically adjusting the position (x, y, z) and pointing angle of the individual transmitters, using all six dimensions to produce a desired pattern of laser beams at the target range. The various patterns of laser beams at the target range produce associated desired measurement point clouds when utilized in a LIDAR system. 
     In one embodiment, at least two transmitter arrays are positioned so that the beams substantially overlap at the target range. In this embodiment, the field-of-view of a two-transmitter system is the same as the field-of-view of each transmitter array and lens system. Systems with more than two arrays can also be configured with substantially completely overlapping patterns at the target range. Such an arrangement leads to improved angular resolutions. Embodiments of the present teaching using multiple emitter arrays are capable of achieving angular resolution of less than 0.25 degrees using state-of-the-art emitter array technology. 
     One feature of the present teaching is that multiple transmitter arrays can be placed on a single substrate. Each transmitter array can have a different shape and spacing, and the spacing between each transmitter array can also be varied on the substrate.  FIG. 14A  illustrates a schematic diagram of an expanded view an embodiment of a lens system that projects light in a small angular divergence LIDAR transmitter utilizing multiple transmitter arrays on a single substrate of the present teaching. 
       FIG. 14B  illustrates a schematic diagram of a condensed-view of the embodiment of a lens system that projects light in a small angular divergence LIDAR transmitter of  FIG. 14A . Multiple transmitter arrays  1402 ,  1404 ,  1406 ,  1408  are positioned on a common substrate  1410 . Each transmitter array  1402 ,  1404 ,  1406 ,  1408  has an associated first lens  1412 ,  1414 ,  1416 ,  1418 , and second lens  1422 ,  1424 ,  1426 ,  1428 . For clarity,  FIGS. 14A-B  do not show the full diverging laser beams emitted by each VCSEL emitter, instead a single ray from the center of three VCSEL emitters are shown to illustrate the concept of the present teaching. For each transmitter array  1402 ,  1404 ,  1406 ,  1408 , a single ray from a top outer emitter  1432 ,  1434 ,  1436 ,  1438  is shown. Also, for each transmitter array  1402 ,  1404 ,  1406 ,  1408 , a single ray from a bottom outer emitter  1442 ,  1444 ,  1446 ,  1448  is shown. For each transmitter array  1402 ,  1404 ,  1406 ,  1408 , a single ray from a central outer emitter  1452 ,  1454 ,  1456 ,  1458  is shown. The embodiment of  FIG. 14A-B  uses multiple array transmitters  1402 ,  1404 ,  1406  to increase the angular resolution. For a single transmitter array that uses a single VCSEL array on a substrate, the angular resolution is set by the field-of-view and the number of individual lasers emitters in the array. If denser angular resolution is desired, then multiple transmitters can be combined by overlapping the output beams in the far field at the desired range. 
       FIG. 14A  illustrates the four transmitter arrays  1402 ,  1404 ,  1406 ,  1408  are attached to a common substrate  1410 . While a common substrate is not necessary, it may be desired from an ease-of-assembly point-of-view. A variety of known common substrates may be used including those formed of a semiconductor material or a ceramic material. The common substrate may be a printed circuit board (PCB). In some embodiments, some of the transmitter arrays share one common substrate, for example a single semiconductor or ceramic carrier substrate. In other embodiments, the transmitter arrays are on a different common substrate, and then these two common substrates with arrays are placed on a third common substrate, for example a PC board. 
     Each of the four transmitter arrays  1402 ,  1404 ,  1406 ,  1408  has its own corresponding lens system comprising a first lens  1412 ,  1414 ,  1416 ,  1418  and a second lens  1422 ,  1424 ,  1426 ,  1428  corresponding to each transmitter array  1402 ,  1404 ,  1406 ,  1408 , as shown. The laser beams emanating from the four transmitter arrays  1402 ,  1404 ,  1406 ,  1408  are overlapped and combined by adjusting the position of each of the four lens systems, comprising a first lens  1412 ,  1414 ,  1416 ,  1418  and a second lens  1422 ,  1424 ,  1426 ,  1428 , relative to the position of their corresponding transmitter array  1402 ,  1404 ,  1406 ,  1408 . The position of the second lens  1422 ,  1424 ,  1426 ,  1428  in each transmitter is shown at different radial offsets  1452 ,  1454 ,  1456 ,  1458  of P1, P2, P3, and P4 from the center of each corresponding transmitter array  1402 ,  1404 ,  1406 ,  1408 . The first lens  1412 ,  1414 ,  1416 ,  1418  for each transmitter array  1402 ,  1404 ,  1406 ,  1408 , can also be radially offset from the center of its associated transmitter array  1402 ,  1404 ,  1406 ,  1408 . These offsets will allow each transmitter array  1402 ,  1404 ,  1406 ,  1408  to achieve a desired beam pattern at the target range. The exact radial offset values are chosen as needed to create a specific angular field pattern required by the LIDAR system at range. These radial offset values are not typically equal, but can be equal in particular embodiments. 
     One feature of the present teaching is that the lateral offset between the separate VCSEL arrays is not critical to determining the combined beam pattern at the target range.  FIG. 14A  illustrates lateral offsets  1460 ,  1462 ,  1464  with values of S1, S2, and S3. The exact values of S1, S2, and S3 for a LIDAR system with typical range of 100 m are not critical as the offset is not magnified with distance. In many embodiments, after only a few meters, the initial offset between arrays, represented by lateral offsets  1460 ,  1462 ,  1464  in the figure, is no longer significant compared to the offset due to the difference in projection angles that are set by the lens system for each array. The condensed-view of  FIG. 14B  shows a representation of how the beams combine in the far field. 
     The relative independence of the projected field pattern for lens systems of the present teaching on the lateral offset of the arrays is illustrated by the configuration shown in  FIG. 15 .  FIG. 15  illustrates schematic diagrams of scaled condensed and expanded views of embodiments of the small angular divergence LIDAR transmitters of the present teaching to show relative independence of the separation between transmitter arrays.  FIG. 15  illustrates scale examples of laser beams being combined. An expanded-view and a condensed-view are shown. In one configuration, the initial lateral offset is equal to 10 mm in the expanded-view  1502  and in the condensed-view  1504 . In another configuration  1506 , the initial lateral offset is equal to 20 mm in the expanded view  1506  and in the condensed-view  1508 . 
     In one particular embodiment, the laser beams for each VCSEL array are offset 2-degree, and there is a 1-degree offset between the transmitters such that the laser beams in the final beam pattern are uniformly offset by 1 degree in the far field. It can be seen at ˜5 m that there is no substantial difference in the far field patterns. 
     One feature of the present teaching is that the lens systems are capable of controlling both the divergence and the step size or position of the beams in the far field. Known LIDAR projection systems typically control step size only, and do not have the ability to control both the divergence and the step size or position of the beams in the far field independently. Furthermore, LIDAR systems of the present teaching can introduce additional beam control of the far field pattern by using additional optical elements. In particular, by positioning the first lens in close proximity to the transmitter array, the lens system of the present teaching provides a different step size independent of the focal of the optical system. 
     One feature of the LIDAR systems of the present teaching is the ability to use wavelength to provide control over the laser beam pattern generated in the far field at the target range, and associated measurement point cloud of the LIDAR system.  FIG. 16  illustrates an embodiment of an illuminator  1600  for a multi-wavelength LIDAR system using multiple transmitter arrays of the present teaching. In this embodiment, a plurality of surface emitting laser arrays comprising at least two groups of lasers with different wavelengths, VCSEL λ 1   1602 , and VCSEL λ 2   1604  are used. 
     Another feature of the LIDAR systems of the present teaching is use of three-dimensional space. VCSEL λ 1   1602  and VCSEL λ 2   1604  are oriented in two surfaces that are orthogonal to each other. One skilled in the art will appreciate that a variety of three-dimensional (X, Y, and Z) degrees of freedom, and/or six-dimensional degrees of freedom (X, Y, Z, pitch, yaw, and roll) that include the angles of the VCSELs  1602 ,  1604  can be used in the LIDAR system of the present teaching. The beams are combined by use of a wavelength multiplexer  1806  that passes one wavelength, while reflecting the second wavelength. 
     The wavelength multiplexer  1606  can be realized, for example, by use of a thin film filter that allows the first wavelength to pass through undeflected, while the second wavelength is deflected at 45 degrees, and the output beams are combined. For simplicity, we have shown the multiplexer  1606  in the shape of cube formed by two equal prisms of triangular cross-section, where the thin film filter that reflects or passes the wavelengths is located at the central plane of the cube where the two triangular prisms touch. 
     The positions of the two substrates of VCSEL λ 1   1602  and VCSEL λ 2   1604  can be shifted laterally, relative to the wavelength multiplexer  1606 , to create the desired overlap or interleaving of the two beams.  FIG. 17  illustrates a measurement point cloud  1700  that can be generated with the illuminator embodiment of  FIG. 16 . It will be clear to those skilled in the art that the use of laser emitters with different wavelengths is not limited to the configurations illustrated here. Different wavelengths may be associated with each array, or different wavelengths may be emitted from a single array in various patterns. In addition, the arrays may be configured in a variety of 3D spatial patterns including different emission angles for each array. 
     One feature of the present teaching is that the optical projection system accounts for the finite emission area of the VCSEL lasers that form the emitter array. Known LIDAR projection optics model the laser source as a point source. In some embodiments of the LIDAR system of the present teaching, each of the adjacent VCSELs within the array have a separation pitch greater than the individual diameter of the emission area of each laser. The array of VCSEL lasers all share a common optical lens system with a clear aperture that is smaller than the projected combined diameter of the array of VCSEL lasers in free space. This is accomplished by a first lens that converges at least the outermost beams. In some embodiments, the first lens, which is the converging lens, is positioned adjacent to the VCSEL array at a distance that is shorter than the focal length of the first lens. The maximum angular field-of-view of the lens system is defined by the emission width of the laser, the optical system defining a divergence of the laser beam, and the separation pitch of the lasers in the array. 
     In some embodiments, multiple transmitter arrays are overlapped in free space to create a denser angular resolution than can be created with a single transmitter with the same VCSEL array dimensions. Each transmitter array has an associated first lens. The first lenses of each individual transmitter array are radially offset to produce a different angular pattern. The radial offsets of each lens system do not need to be the same. The transmitter arrays are located on a common substrate and the distances between the various transmitter arrays do not need to be the same. In embodiments with multiple transmitters, the wavelengths of the individual VCSELS that comprise the transmitter array and/or each transmitter array may be the same or different. In embodiments of the LIDAR system of the present teaching that use different wavelengths, a wavelength sensitive element may be used to further converge or diverge the beams in the far field based on their wavelength. 
     A method of light detection and ranging according to the present teaching includes providing a first transmitter array comprising a first plurality of light emitters that generate a first plurality of optical beams that can be multiwavelength optical beams. A first lens is positioned in an optical path of the first plurality of optical beams at a distance from at least one of the plurality of light emitters that is less than a focal length of the first lens so that the first lens converges the first plurality of optical beams to form a first converged optical beam with a beam waist. A second lens is positioned in the optical path of the first converged optical beam so that it projects the first converged optical beam to a target range. The position of the second lens and an emission width of at least one of the first plurality of light emitters are selected to provide a desired field-of-view of the projected first converged optical beam at the target range. 
     The method also includes providing a second transmitter array comprising a second plurality of light emitters that generate a second plurality of optical beams that can be multiwavelength optical beams. A third lens is positioned in an optical path of the second plurality of optical beams at a distance from at least one of the second plurality of light emitters that is less than a focal length of the third lens so that the third lens converges the second plurality of optical beams to form a second converged optical beam with a beam waist. A fourth lens is positioned in the optical path of the second converged optical beam so that the fourth lens projects the second converged optical beam to a target range. The position of the fourth lens and an emission width of at least one of the second plurality of light emitters are selected to provide a desired field-of-view of the projected second converged optical beam at the target range. A position of the first transmitter array and a position of the second transmitter array are chosen to provide a desired field-of-view of the LIDAR transmitter at the target range. 
     EQUIVALENTS 
     While the Applicant&#39;s teaching is described in conjunction with various embodiments, it is not intended that the Applicant&#39;s teaching be limited to such embodiments. On the contrary, the Applicant&#39;s teaching encompasses various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art, which may be made therein without departing from the spirit and scope of the teaching.