Abstract:
A system for observing a field of view including a light transmitter and a light detector. A path is defined between the transmitter and the detector. The light transmitter generates substantially coherent light onto a transmit portion of a light path and the light detector is positioned in a receive portion of the light path defined by reflected light from some of the target locations. The system includes a component for configuring at least a portion of the light path such that a subset of the plurality of potential target points are included in the light path.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates, in general, to laser radar, and, more particularly, to devices, systems and methods for measuring properties of multiple remote targets. Measured properties include one or more including position, range, velocity, reflectivity, polarization texture, or vibration. 
     2. Relevant Background 
     Most conventional lidar (light detection and ranging) and ladar (laser detection and ranging) sensor systems use a single laser beam to reflect off a remote target. Properties of the reflected light, detected at the sensor location, are used to extract information about the target. In the case of detecting range, time of flight ranging is commonly used, wherein the time taken for a short light pulse to travel to the target and back is measured. By detecting the state of polarization (SOP) of the reflected light compared to the transmitted light it is possible to extract information about man-made versus natural targets. Similar information may be obtained by measuring the reflectivity of the target compared with the surroundings. Velocity and vibration information can be obtained by detecting shifts in the radial (along the laser beam) position of the target with time, or through the use of Doppler frequency measurements. Such measurements are described, for example, in U.S. Pat. No. 5,237,331 to Henderson et al. and U.S. Pat. No. 5,815,250 to Thomson et al. which are incorporated herein by reference. 
     It is frequently desired to interrogate more than one point location in a field of view (FOV). This is useful in order to differentiate between desired target locations and the surrounding scene, as well as to conduct searches over a FOV for targets that have characteristics of interest for identification and/or classification purposes. It is therefore of interest to image a FOV or parts of a FOV by interrogating multiple points. Early generations of imaging systems utilized mechanical scanners to direct a single beam in such a manner that an image could be ‘painted’, for example by moving the beam in a raster pattern across a scene. Disadvantages of this approach include: rapid scanning is difficult to do quickly, especially if the transmitted beams are large, since the required mechanical hardware becomes heavy and large; mechanical scanners are subject to wear and reliability concerns; it is difficult to maneuver scans in arbitrary patterns rapidly; and it may be time consuming to scan a large FOV. The latter is a particular concern if the scene is changing rapidly or if the laser radar platform is moving rapidly, such as is often the case with laser radar systems mounted to aircraft that scan the ground. Moreover, collecting and analyzing the large amounts of data that can result from interrogating multiple points in a large scene is problematic. 
     To remedy this situation a great deal of effort has been directed to two areas. One is the development of non-mechanical scanning (NMS) techniques. These include micro-electro-mechanical (MEMS) devices, liquid crystals, and acousto-optic devices. It is noted that NMS techniques may solve some problems, for example less bulk and higher reliability, but they do not by themselves solve the problem of collecting data from a large scene rapidly and efficiently. 
     The second development area is directed at systems that collect data from numerous regularly spaced points in the FOV simultaneously. These are usually referred to as ‘flash’ imaging systems and operate similar to a conventional camera in that they collect a whole image at one time using detector arrays. Examples of such systems include publications by Marino et al. (pp.1 in Laser Radar Technology and Applications VIII, SPIE Proc. 5086, 2003) and Halmos (ibid. pp.70) which are incorporated herein by reference. Further examples are given by Landon in U.S. Pat. No. 5,353,109 where multiple beams are generated using a static diffractive device (Dammann gratings and holograms noted by the inventor) and also a system described in U.S. Pat. No. 5,610,705 to Brosnan and Komine which are incorporated herein by reference. By combining NMS techniques with flash imaging it is possible to relatively rapidly collect data from a large scene and to also point the ‘camera’ in the desired direction without the use of large and heavy mechanical means. 
     One problem with these prior approaches is that they do not provide sufficient flexibility to always be useful. In considering a general FOV it is often the case that most of the scene contains little or no information of interest. For example, a scene may contain several vehicles against a background filled with natural features. In this case one is typically interested in interrogating details of the vehicles, not the background. Illuminating the whole scene with light can then be extremely wasteful. For example, if the targets of interest only occupy 1% of the FOV then 99% of the illumination may be wasted. Not only is this wasting laser power in illumination, it also means that the electronics signal processor is spending much of its time performing calculations that are of no interest. Compounding the problem is that many lidar systems, especially those on board aircraft, have very limited electrical power and/or computational resources available. It is imperative that power usage be as efficient as possible in order to minimize the amount of illumination light that has to be provided. This in turn minimizes size, weight, cooling requirements, and system cost, as well as maximizing reliability by minimizing the number of system components. 
     A second problem with conventional approaches arises where the system uses coherent (e.g. heterodyne or homodyne) detection. In such cases a local oscillator (LO) laser beam is aligned carefully in position and angle with the return signal. Such alignment requirements normally account for a significant portion of the cost associated with designing coherent laser radar systems, even when only one LO beam has to be aligned properly. Scaling coherent imaging systems from a single pixel to, for example, imaging a FOV comprising 1,000×1,000=10 6  pixels requires providing LO beams for each pixel, which can become extremely complex. Furthermore, requiring that a local oscillator laser&#39;s power be divided to provide power to such large pixel counts can also put extraordinary demands on the LO laser, in order that sufficient power is provided on each pixel to achieve shot-noise limited detection sensitivity. The latter is highly desired to maximize detection of weak return signals from the target. In cases where the FOV contains mostly background information of relatively little interest, the system design would be considerably enhanced if only a selected subset of pixels were addressed. 
     A third problem may arise in the case of coherent lidar and is caused by the time delay between sending light to a target and receiving scattered light back. Since the speed of light in air is approximately 300,000 km/s, the round trip time is 67 μs for every 10 km distance to the target. Systems of this type are frequently operated in pulsed mode where short laser pulses are sent to the target. If the pulse spacing (inverse of the pulse repetition frequency or “PRF”) of the transmitter is shorter than the time taken for light from the previous pulse to return, for sufficiently distant targets a scanner may redirect the system to send a pulse in a different direction before the previous pulse is received back. Unless this is compensated for, the system will not be properly aligned and the detection efficiency will degrade considerably. A very similar problem arises from rapidly moving platforms, where the viewing angle may change rapidly, or for rapidly scanned systems, and is generally referred to as the ‘lag-angle’ problem. Even small angular misalignments between the transmitted and received beam paths, due to time lags between transmission and reception, cause degradation of the detection efficiency. If the scanning motion (or relative angular motion between the target and the system platform) is relatively constant, this can be compensated with through the use of fixed ‘lag-angle compensators’. An alternative method has been disclosed by Welch in U.S. Pat. No. 4,515,472 which are incorporated herein by reference. In the Welch method an array of local oscillator beams is generated to correspond to a variety of anticipated lag angles. In operation the intent is to ensure that even if the lag-angle is a priori unknown, the receive beam will match up with one of the generated LO beams and therefore detection can take place. A similar method has been disclosed by Meyzonnetie in U.S. Pat. No. 5,485,009 which are incorporated herein by reference. These approaches may have some use, but they do not solve the general problem of maximizing efficiency. If a large number of target points is illuminated and a large set of LO beams has to be generated for each pixel, the LO generation problem may become worse, rather than better. 
     A fourth problem that relates to the previously noted problems is that the volume of data may become difficult, impractical, or even impossible to process. For example, if a detector array has even 10,000 pixels (such as a 100×100 element array) and each pixel is sampled at, for example, 1 gigasamples per second (Gs/s), then the total data rate is 10 terasamples per second (Ts/s). This is not only difficult to process, it is also difficult to transfer from a detector chip to the processor and would likely lead to the construction of electronics that are far more complex, expensive, and bulky than desired. In cases where only a small fraction of the pixels carry information of interest, it becomes clear that this approach is inefficient. Much current effort is geared towards incorporation of pre-processing functionality into the detector/receiver arrays to reduce the computational loads on processors. In these implementations each detector pixel is coupled with a small electronics cell that performs desired pre-processing functions, such as amplification, filtering, thresholding, and the like. 
     One approach to the data reduction problem is to incorporate threshold detection into the electronics, such that only pixels that detect signals above some threshold would transfer the data out for further processing. In some circumstances this may work, but in other cases it does not. For example, coherent laser radar systems are typically purposely designed to operate in such a manner that only very weak signals are received, e.g. with receiver carrier-to-noise (CNR) levels near unity or even far less. The same situation is also true in many continuous-wave (CW) modulated cases. Signal recovery in these cases does not rely on a high instantaneous CNR but rather rely on the total signal collected over a predetermined measurement time. Since the CNR is so low, simple instantaneous intensity thresholding does not work and hence the approach of building thresholding circuitry into the receiver at the pixel level fails. 
     It should be noted that the detector arrays do not generally sample continuously at rates on the order of gigasamples/s so total data rates in the Tb/s range generally refers to input burst rates. The average rates may be significantly smaller yet the impact of receiving signals at very high rates nevertheless has a considerable impact on the design of the detector and receiver because the data is collected and stored at high speed even if it is transferred for post-processing at lower rates. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the present invention involves systems for observing a field of view including a transmitter and a detector. A path (e.g., a light path or electromagnetic energy path) is defined between the transmitter and the detector. The transmitter generates electromagnetic energy (e.g., energy ranging in a spectral range from infrared to extreme ultraviolet) onto a transmit portion of the path and the detector is positioned in a receive portion of the path defined by reflected electromagnetic energy from some of the target locations. The system includes a component for configuring at least a portion of the path such that a subset of the plurality of potential target points are included in the path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a lidar architecture according to prior art. 
         FIG. 2  shows an imaging lidar system according to prior art. 
         FIG. 3  shows a lidar transmit beam with a transmit BFE that produces multiple illumination points at a target. 
         FIG. 4  shows a lidar transmit beam with a switchable fixed transmit BFE to illuminate a target with a switchable pattern of target illumination. 
         FIG. 5  shows a lidar system with a transmit BFE to produce a flexible target illumination pattern and a large detector/receiver array. 
         FIG. 6  shows a lidar system incorporating a transmit BFE and a receive BFE for efficient direction of received light to a smaller detector/receiver array. 
         FIG. 7  shows two-dimensional and linear arrays of detectors and receiver electronics. 
         FIG. 8  shows an example of using a fixed receive BFE to map variable target illumination patterns to a fixed small detector array. 
         FIG. 9  shows an example of a configuration where the beam forming elements form a replaceable unit. 
         FIG. 10  shows a laser radar configuration according to an alternative embodiment of the invention. 
         FIG. 11  shows a laser radar configuration according to a second alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is illustrated and described in terms of lidar systems that incorporate a light transmitter such as a laser as the transmitter. However, it is also recognized that non-laser sources may advantageously utilize the described invention with suitable adaptations. As used herein, the term “light” means electromagnetic energy in a spectral range from far infrared (IR) to extreme ultraviolet (UV). Many of the specific examples use coherent electromagnetic energy sources, however, the degree of coherency may be selected to meet the needs of a particular application. Substantially coherent transmitters include single frequency or monochromatic sources as well as narrow bandwidth and broadband lasers, sometimes referred to as “incoherent lasers”. In order to properly understand the invention a brief description of single pixel lidar configurations and conventional imaging lidar systems is provided. 
     1. Single-Pixel Lidar Configurations 
     Lidar systems can be divided into two top-level cases. In the first case (“bistatic”) separate optics are used to transmit light and receive light. The advantage is that very good isolation between the two paths can be achieved so that little transmitted light is inadvertently scattered from e.g. imperfections in optics to the detector. Bistatic configurations, however, place high demands on relative alignment of the two paths and may require that at least one of the two paths can be realigned dynamically in order to compensate for target planes at varying distances (ranges) from the lidar. 
     In the second (“monostatic”) case the two paths are coincident and an optic, such as a beamsplitter or polarizer, is inserted to separate received light from transmitted light. Faraday isolators can also be used and in the case of pulsed transmitters temporal multiplexing to separate the transmit from the received light can be used. For illustration purposes, this is illustrated in  FIG. 1  for the simple case of a single beam and using a polarizing beamsplitter and quarter-wave plate for separation of the transmitted and the received light. Here a transmit beam  101  is sent from a transmitter laser  102  through a beam splitter  103  and through the transmit optics  104  to the target  105  along path  106 . Scattered light  107  is received by the same optic  104  and is reflected from beam splitter  103  along path  108  to detector  109 . In many practical cases polarization techniques are used to achieve a high efficiency in this path separation. It is common that the transmitter laser is linearly polarized and that a quarter-wave plate (QWP)  110  is inserted into the optical path  101 . If the beam splitter  103  is a polarizing beam splitter (PBS), light transmission and reflection efficiencies of the transmitted and received beams of substantially 100% can be achieved assuming that the target reflection does not significantly depolarize the light. For example, when a polarizer is used as shown in the figure, light transmitted through QWP to the target is then circularly polarized. Upon reflection from the target, circularly polarized light with the opposite handedness becomes polarized at 90 degrees to the transmit light upon transmission through the QWP. Because of the polarization sensitive nature of PBS  103  the received light is now reflected to the detector with very high efficiency. 
     The laser radar configuration described is referred to as direct detection (DD) because the detector detects photons scattered from the target onto the detector surface. An alternative system is referred to as coherent detection (CD) lidar, which adds the means to optically mix the return signal with other light which is then incident on the detector. A simple example illustration a configuration that allows this is shown in  FIG. 1  with the added elements shown as dashed lines. In the CD case a master oscillator (MO) laser  111  is typically used to produce a stable frequency reference. One laser beam  112  from MO  111  is split off using beam splitter  113  and is used to set the frequency of transmitter laser  102  with a splitting ratio determined by the specifics design parameters of the system. The second part of the laser beam  114  is reflected from beam splitter  115  in such a manner that the reflected beam and the part of beam  108  that is also transmitted through beam splitter  115  optically mix to form a common beam  116  that is then incident on detector  109 . The splitting ratio of beam splitter  115  is also determined by the specific system but may be designed such that 5-10% of MO beam (referred to at this stage in the system as the local oscillator or LO beam) is reflected and 85-90% of the signal beam  108  is transmitted towards the detector. Care is taken to ensure that the two beams overlap spatially and in propagation direction following mixing, and that they have the substantially the same polarization. This detection configuration is referred to as homodyne. If a frequency shift is introduced between the beams  114  and  108  the configuration is known as heterodyne. 
     Unless explicitly noted the implementations discussed herein refers to either direct detection or coherent detection, the latter case incorporating both homodyne and heterodyne. It is further noted that other detection configurations exists, such as autodyne detection whereby a portion of the received signal itself is used as the local oscillator, that may be incorporated with suitable modifications. Polarization/depolarization measurements can also be carried out by incorporating two or more detectors or detector arrays. Optical quadrature detection as disclosed by for example Hogenboom, D. O. and diMarzio, C. A. (“Quadrature detection of a Doppler signal”, Applied Optics 37, 2569 (1998)) may also be incorporated in the coherent detection case, as can balanced receivers as discussed for example in “Noise in homodyne and heterodyne detection,” Yuen H. P. and V. W. S. Chan, Opt. Lett. 8, pp. 177, 1983. 
     Although the specific examples of the present invention described herein relate to monostatic lidar architectures it is contemplated that the disclosed invention applies equally well to and is readily adapted to bistatic lidar architectures with suitable modifications. 
     2. Imaging Lidar Configurations 
       FIG. 2  shows a typical architecture of an imaging lidar system. The architecture contains essentially the same elements as the single pixel case with some important differences. As in  FIG. 1 , the imaging architecture starts with a transmitter laser  201  that produces a laser beam  202  propagating in the indicated direction. When the system is operated in coherent detection mode there is also normally a master oscillator laser  250 , a part  251  of whose laser power is tapped off and used to set the frequency of transmitter laser  201 . Optical components  223  and  205  are used to form an extended illumination area  206  at the target. The transverse extent of the transmitted beam is indicated by lines  204 . Optical element  223  is shown as a lens whose purpose is to ensure that when used in conjunction with optical system  205  the desired illumination is produced. There is also shown a splitting element  203 , such as a polarizer, that is used to separate transmitted light from received light. In order to effect this polarization separation a quarter-wave plate (not shown) is inserted between components  203  and  205 . This element has been omitted for clarity. 
     Light scattered from illuminated area  206  returns in direction  207  back through optical system  205 , is reflected from element  203 , and propagates as beam  208  to detector array  211 . Also shown is an optical element  209  that may be inserted into the path of beam  208 . The function of this element, in conjunction with optical system  205 , is to form an image of target  206  onto the surface of detector array  211  with a magnification generally such that the image of the fully illuminated target area approximately fills the area of the detector array  211 . Detector array  211  is connected to receiver  212 , which comprises suitable electronics components to amplify and filter the detector signals, and may also incorporate other suitable pre-processing functions. As noted in the introduction the degree of complexity of such electronics is normally very low for high pixel count arrays. One reason for this is that each pixel element may only measure on the order of 100 micrometers squared, or significantly less, so there is generally very limited space available for electronics behind each pixel element. Data from the receiver is sent on a data bus  213  to signal processor  214 , which carries out processing of the signals to extract desired information about the target. Typically such processed data is then passed to a user interface  216  as indicated by line  215 . Most lidar systems would also incorporate a system controller unit  217  that communicates with signal processor  214  and user interface  215 , as indicated by lines  218  and  219 . Additional subsystems are not explicitly shown but are also normally incorporated into lidar systems of this type, including laser drivers, timing circuitry, temperature controllers, and the like. Also shown in  FIG. 2  for completeness is a front view  220  of the detector array  211 . It shows a grid of individual detector pixels  255  with an image  256  of the target  206  superimposed. 
     For conventional intensity imaging the configuration shown in  FIG. 2  can work well, because the technology to make large arrays (for example 1000×1000 pixels) is well established. Digital cameras with &gt;1 million pixels are available today as essentially commodity items. However, such large pixel counts are possible in part because the electronics associated with each pixel is very simple and readout rates are very low. Conventional digital cameras essentially integrate the number of photons received at each pixel location and form an intensity image. A large pixel count is acceptable because the frame rate can be very low, for example 30-60 Hz, or much less, for still cameras. Performing more complex processing of information from each pixel for such large arrays can become extremely difficult as noted above. Since conventional lidar systems process data at far higher rates that conventional imaging systems it is generally difficult and expensive to scale the illustrated architecture to high pixel counts. 
     3. Embodiments of the Invention 
     The present invention encompasses an integration of Beam Forming Elements (BFEs) into the transmit and/or receive paths of an imaging lidar transceiver as well as detector/receiver arrays and data management functionalities that permit the laser radar system to perform multi-pixel measurements in a laser power efficient and computationally efficient manner. Depending upon the desire to carry out specific imaging functions, the BFEs may take several forms, as follows: 
     Fixed passive elements are elements that do not need electrical and other input in order to perform their function, such as generating multiple beams or redirecting beams from one location to another. Examples of passive elements include refractive, diffractive, and reflective devices such as micro-prism arrays, diffractive optical elements (DOE), mirror arrays, holograms, and the like. 
     Switchable passive elements refer to assemblies of passive devices that can be inserted or removed from a system with operator assistance or using e.g. mechanical means. An example would be a set of DOEs used to generate different diffractive beam patterns and mounted to a turret such that a preselected element can be inserted into a transmit or receive path in the lidar by an operator or remotely by a computer. 
     Active elements refer to devices whose operation on a beam is controlled e.g. electrically. Examples of active devices include sets of discrete mirror arrays (for example of the type used in telecommunications cross-bar switches), deformable mirrors, diffractive MEMS devices, liquid crystal arrays, electro-optic devices, and the like. 
     Incorporation of a BFE in a lidar transmit path permits programmable illumination of a target plane in a highly flexible manner. Examples of illumination patterns include: regular lines, grids, or areas of illuminated spots; irregular distributions of illuminated spots; and flood illumination of predetermined areas within the field-of-view (FOV). The chief requirement on the transmit BFE is that it provides the desired illumination pattern without significant degradation of the transmitted beam. It is generally desired that the BFE not alter, except in a desired way, the polarization state of the incident beam, that it does not alter the frequency of the transmitted beam in an uncontrollable manner, that it does not significantly degrade wavefront quality in an uncontrollable manner, and that the efficiency is high (low insertion loss). For example, if it is desired to produce 10 illumination spots at a target, it may be ideally desired that 10% of the incident beam&#39;s power is directed at each of the 10 target points. Highly efficient delivery of light distributions with total efficiencies in the range of 50-100% is generally feasible with a number of technologies, such as micro-mirrors and liquid crystal devices. 
     Incorporation of BFEs in the lidar receiver path permits imaging or mapping selectable portions, points, samples, discrete marker points, or other prescribed features of the illuminated, high resolution target scene onto a fixed and limited size detector array. A key notion is that the target FOV is frequently large in terms of the number of resolvable spots, for example measuring 1000×1000 points (pixels) or more. In a conventional imaging system there is a one-to-one correspondence between target points and detector elements. However, in many situations, including those where heterodyne detection is employed, is it inconvenient and frequently cost-prohibitive, to incorporate such a large number of detector elements. Large detector arrays can be deployed, but it is then advantageous to not process signals received on all elements in order to reduce the bandwidth requirements on the signal processor. 
     The common purpose of all these devices is to alter one or more characteristics of light incident upon the device. Alterable characteristics include direction, phase, frequency, amplitude, and/or polarization. All such elements will, for simplicity, be referred to as Beam Forming Elements (BFE). The invention improves upon the general system architecture shown in  FIG. 2  so it is important to be clear about important features:
         A laser is used in conjunction with optical elements to illuminate an area at a target plane with a predetermined spatial distribution of light.   Using the same, or additional optical elements, light scattered from the target plane is collected and transformed such that an image of the target is formed at a plane, which we refer to as the “image plane” or “target image plane” below. In  FIG. 2  the detector array is placed at the image plane. An important point is that there is a one-to-one relationship between points at the target and in the image plane. Each small area at the image plane can be treated as a small bundle of light rays that have propagated from the corresponding area at the target, through the receive path imaging system and to the image plane.   At the detector plane, detected photons are converted to electrical signals that are transported to a signal processor for extraction of target information.       

     A significant issue resolved with the present invention is enabling simultaneous measurements of M flexibly selected target points without requiring reading out data corresponding to substantially more than M pixels. In accordance with the present invention, this can be done using multiple approaches that simultaneously introduce flexibility into at least two of the three essential features noted above. That is, we may flexibly alter the transmission of illumination patterns, we may flexibly make alterations at the target image plane, and/or we may flexibly alter the operation of the system at the detector plane. These three “degrees of freedom” give rise to a number of possible permutations, or cases, the ones most relevant to this invention being summarized in Table 1 below. 
     Case 1 is the case of most conventional lidar systems. A single beam is transmitted to the target and scattered light is received on a single detector. This case obviously affords no flexibility in interrogating multiple target points except through the introduction of a scanning system that operates identically on both transmit and receive beams. This is commonly done but suffers from a number of limitations, including lag angle effects when the angular alignment between the target and the receiver changes during the transit time of light to the target and back. 
     Case 2 incorporates methods, such as those disclosed by Meyzonnetie, that couple a single transmit beam with N detectors to ensure that at least one detector receives the signal. As noted by Meyzonnetie the intent is to use this configuration with a linear detector array to account for unknown amounts of lag angle that may occur in one linear direction. It is consequently poorly suited for tracking unknowns in two dimensions, where N could easily become very large, resulting in a high degree of inefficiency in the use of computational resources. 
     Case 5 is a conventional imaging case where a fixed set of M illumination beams is generated and imaged back to a set of fixed detectors of equal number. This case makes no provisions for incorporating flexibility. 
     Case 6 is a variation of case 5 wherein M illumination beams are imaged back onto a large detector array and the entire detector array is read out whether data exists on specific pixels or not. As noted, when N becomes large this approach makes very inefficient use of computational resources. 
     All other eight cases noted in Table 1 are variations of the present invention that incorporate flexibility into the system by altering at least two essential aspects of the system design as denoted by bold print in Table 1. In each of these cases a flexibility element in the transmit path is coupled with either a receive BFE element to reduce the number of required detectors, and/or with a larger detector array with individually addressable pixel readout. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Case 
                 Transmit Pattern 
                 Receive BFE 
                 Detector/Receiver Pixels 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Single point static 
                 No 
                 1 
               
               
                 2 
                 Single point static 
                 No 
                 N static 
               
               
                 3 
                 Single point dynamic 
                 No 
                 N addressable 
               
               
                 4 
                 Single point dynamic 
                 Yes 
                 1 
               
               
                 5 
                 Fixed M point 
                 No 
                 M pattern matched to 
               
               
                   
                   
                   
                 transmit 
               
               
                 6 
                 Fixed M point 
                 No 
                 N static 
               
               
                 7 
                 Fixed M point 
                 No 
                 N addressable 
               
               
                 8 
                 Fixed M point 
                 Yes 
                 M 
               
               
                 9 
                 Switchable fixed M 
                 No 
                 N addressable 
               
               
                 10 
                 Switchable fixed M 
                 Yes 
                 M 
               
               
                 11 
                 Dynamic M 
                 No 
                 N addressable 
               
               
                 12 
                 Dynamic M 
                 Yes 
                 M 
               
               
                   
               
             
          
         
       
     
     In operation a programmable (real-time or preprogrammed) BFE in the transmit path (“transmit BFE”) is used to illuminate the scene with a selectable pattern of coherent or incoherent light. The transmit BFE could operate in two fundamentally different manners.  FIG. 3   a ) illustrates a case where the BFE  302  subdivides the transverse extent of the incident laser beam  301 , having a diameter D, into 6×6 sub-areas, and where each sub-area is acted upon by a segment of BFE  302 . In this example, BFE  302  could generate up to 6×6=36 independently pointing beams, each one intersecting target plane  306  at different points. The target plane  306  is shown as being divided into a grid of rectangles for illustration purposes only. The extent of the full grid is referred to as the field-of-view (FOV) and represents the full extent of locations where transmitted beams may be directed. Although the grid is shown as rectangles for clarity, it is clear that pointing of the beams does not have to be done on the basis of points aligned to a grid. If the input beam diameter D is matched to the size of the BFE device, the transverse extent of each sub-beam would be approximately D/6 in this case. For clarity only three such smaller beams  303 - 305  are shown. It is obvious that the numbers 6×6 is used only as an example. The number could range from 1 to a large number, such as 100×100. It is also obvious that this device could be used to redirect the power from all subbeams to a single position at target plane  306 . 
     A number of different devices could be used to implement the BFE, including mirror arrays or diffractive optics, including liquid crystal devices. Although the beam is illustrated as being transmitted through the BFE it is obvious that operation in reflective mode is also possible. This would be the case where one or more sets of mirrors are utilized. 
       FIG. 3   b ) illustrates a case where the BFE acts by diffraction effects on the whole incident laser beam in such a manner that the far-field diffraction pattern provides the desired illumination pattern. Here incident laser beam  307  is incident on BFE  308 , which acts on the whole beam to generate, by diffraction, several sub-beams  309 - 311  that produce a distinct illumination pattern at target plane  312 . Close to the BFE  308  there is substantially a single beam due to overlap of the generated diffraction beams, but as the beams propagate farther from BFE  308  they separate to provide the desired illumination pattern at target plane  312 . Diffractive optical elements of this type can be either static, for example binary optics or gratings, or they could be dynamically addressable devices. Examples of the latter include liquid crystal devices, for example the “512×512 Multi-level/Analog Liquid Crystal Spatial Light Modulator” available form Boulder Nonlinear Systems, or diffractive mirror devices, such as Grating Light Valves from Silicon Light Machines. Both of these specific devices operate in reflection rather than transmission, but implementing a transmissive system is also contemplated. 
     It is clear that transmit BFE devices incorporate cases where a single beam is generated that may or not be offset in angle from the incident beam. This is done through the insertion of a reflective, refractive, or diffractive device that acts upon the incident beam like a mirror, prism, or diffraction grating. 
     Optical devices could be disposed of between the transmit BFE ( 302  or  308 ) and the target ( 306  or  312 ) to improve the functionality of the lidar system. Such devices include lenses, telescopes, or other similar functional elements used for example to produce appropriate magnification of the target illumination pattern. Another class of devices would be scanners that direct the ensemble of transmitted beams generated by the transmit BFE to a target area of interest by changing the propagation angle of the ensemble of beams. A third class of devices may incorporate wavefront sensors and/or compensators that correct the transmitted beams for distortions imposed by atmospheric turbulence, imperfections in optical fabrication, and the like. 
     In cases where the transmit BFE is dynamically programmable, information available from other sensors, or from the laser radar itself, can be used to ensure that the desired target plane illumination is achieved. For example, information about wavefront distortions caused by atmospheric turbulence can be used to correct for those wavefront errors either through a separate correcting element as noted or the function may be incorporated into the programmable transmit BFE itself. 
     An example of incorporation of a switchable fixed transmit BFE is shown in  FIG. 4 . In  FIG. 4  is shown a disk  401  that holds multiple fixed devices  402  each of which is designed to produce a different beam pattern. These devices are exemplified by previously noted elements  302  and  308 . As shown in  FIG. 4  device  403  is located such that an incident laser beam  404  is transmitted through device  403  to produce three beams  405  that are incident on a target  406  illustrated as a grid. Disk  401  is also mounted on an axis  408  such that it can be rotated about that axis as shown by curved arrow  409 . This permits alternative elements  402  to be rotated into the incident beam  404  in order to produce other beam patterns  405  at target plane  406 . Although in this example 6 possible patterns are indicated, there is no specific limitation on the number of elements  402  that are present, nor is there any requirement that they all produce the same number of beams  405 , although it is generally desired that the elements do not produce more than M illumination points where M is equal to the number of active detector elements in the system. It may in some cases, as an example, be desirable that one sample element  402  is completely clear to enable all the laser power or energy to be deposited at one target point. The implementation of mounting multiple elements  402  to a rotatable disk in the manner shown is also clearly just an example. Many other possibilities are obvious to those skilled in the art, such as mounting multiple elements linearly like a film strip and using linear translation to affect a switch between elements. 
     It is also contemplated that a combination of devices can be used to produce desired illumination patterns. For example, it may be desirable to interrogate a 100×100 pixel target plane over a predetermined period of time. One could then, for example, use a fixed diffractive device to generate a set of 10×10 pixels with a sparse pattern where the spacing between the illumination pixels is 10 beam diameters. This diffractive set of beams could then be directed through a dynamic element, for example a liquid crystal device, such that the 10×10 pattern is moved in small increments to fill in the 100×100 pixel field over time. 
     4. Embodiment of a Flexible Lidar System with Addressable Receiver 
     Producing arbitrary illumination patterns as described above is one step in producing a flexible imaging lidar system. Used in conjunction with an addressable receiver array this forms the basis for the first implementation of such a system, which we now describe with reference to  FIG. 5 . The embodiments described in this portion of the specification relate to cases 3, 7, 9, and 11 in Table 1. 
     In  FIG. 5  is shown a transmit laser  501  that produces a laser beam  502  that propagates to a transmit BFE  503 . Operation of BFE  503  is determined by system controller  550  which is used to determine, based on desired use, what illumination pattern should be produced at a target  506 . System controller  550  issues appropriate commands  551  to transmit BFE driver  552 , which in turn produces drive signals  553  to BFE  503  in order to produce the desired angular distribution of beams, illustrated in this example as 3 beams  504 . In the case of a fixed transmit BFE device the BFE driver and associated control signals are absent, whereas in the case of switchable elements (e.g.  FIG. 4 ) the function of the BFE driver and associated control signals are used to control the insertion or removal of BFE elements in the transmit beam. The generated beams  504  propagate through beam splitter or polarizer  505  and are transmitted to illuminate target  506  as beams  507 , using optical system  508 . Optical system  508  may be a simple lens, a more complex telescope, or any other suitable system that forms the desired illumination pattern at target  506  given an input distribution of beams  504 . Optical element  508  may also incorporate beam directing optics for steering of the beam bundle  507 , and may also incorporate wavefront sensors to allow measurement of the wavefront of the transmitted or received signals. The wavefront information can then be used to drive the transmit BFE  503  or other adaptive optical elements that can be incorporated into optical element  508   
     Light scattered from target  506  returns along the same direction, passing again through optical system  508 , and is reflected from optical element  505 . From optical element  505  the beams propagate to detector array  509  where an image of target  506  is formed. Optical system  508  functions to image the target onto the detector plane. In order to meet for example physical constraints such direct imaging may not be feasible in which case additional optical elements may be required to be inserted into the system. The manner in which this is done is not essential to the understanding of the system and is therefore not described in detail, except to note that it is desired that the optical imaging systems used approximately match the size of the target image to the size of the active part of the detector array. Such additional optical elements may take the form of, for example, lenses disposed between elements  505  and  578  to produce the desired effect. 
     A key issue is that in this system only a subset of detector pixels receive light scattered from the target. The system may be designed, as an example, such that the transmit BFE can be used to address N possible locations at the target plane (where N may be for example 1000×1000 pixels), but that only a much smaller number of beams M is generated by BFE  503 , for example M=100. In such a case only a limited number of pixels at detector  509  receive scattered light. Such pixels  507  are indicated by filled rectangles in  FIG. 5 , while unused pixels are indicated by unfilled rectangles  508 . 
     In order to achieve the benefit of efficient use of processing capabilities a further essential element of the embodiment of the invention is the way in which data is selected for transmission to the signal processor. The system controller  550  controls the distribution of illumination at the target  506  as noted. As a result it is also known which pixels in the detector array will receive scattered light. Consequently it is only necessary to read out data from those active pixels and ignore the rest. To meet this requirement the system consequently contains a receiver controller  561  that receives input information on line  560  from system controller  550  as to which pixels are valid at any given time. The receiver controller  561  in turn enables data output from only those pixels in receiver  510  that contain valid data. That data is transported on a data bus  563  to signal processor  564  for extraction of target information. Processed information can then be further output on line  565  to a user interface  563 . In order for the signal processor  564  and/or user interface  566  to reconstruct images properly it is also desired to interface those units to the system controller  550  as shown by lines  567  and  568 . This novel manner of operating an imaging lidar system has the clear benefits stated at the outset. In particular it is noted that the data rate reduction can be extremely high. For example, if the target plane contains N=1000×1000=1 million addressable points the detector array would have a substantially similar number of detector elements. But at any given time there may only be, for example, M=100 actual illuminated points at the target plane, in which this operating mode reduces the amount of data by a factor of 10,000. 
     The agile imaging lidar is useful for direct detection measurements. The system can also be configured for coherent detection measurements by adding the elements shown in dashed lines in a manner similar to what was described in the context of  FIG. 6  above. As previously described a master oscillator  570  is added to provide a frequency reference. The laser beam from MO  570  is split using beam splitter  573  into beam  571  that is used to set the frequency of the transmit laser, and beam  573  that is used to form the LO beam. In contrast to the single pixel case of  FIG. 6 , a local oscillator BFE (LOBFE)  574  is added. This LOBFE receives a drive signal  577  from a driver unit  576 , which in turn receives signals  575  from the system controller  550 . The purpose of the LOBFE  574  is to generate a set of local oscillator beams  575  that, when optically mixed with the return signal beams at beam splitter  578 , create the coherent mixing beams  577  required for homodyne or heterodyne detection at detector  509 . 
     In many cases the angular beam pattern generated by LOBFE  574  may be essentially identical to that generated by transmit BFE  503 , however it need not be. One case where the two patterns may differ is where lag effects, due to angular changes in the line of sight between the lidar and the target as a result of relative motion between the lidar and the target during the time of flight, causes a lateral shift between the received signal pattern and the generated LO illumination in the detector surface. If the relative motion is known the LOBFE can be used in a compensating manner to deliberately shift the LO beam pattern to match the received beam pattern. A second case where the patterns may differ is when there is a sufficient time lag between transmission of target illumination and reception that the transmit BFE has altered the transmit pattern by the time the previously transmitted light is received. A third case occurs when one of the BFEs is additionally used to compensate for optical distortions due to e.g. refractive turbulence or imperfections in optics. In such cases the transmitted beams  504  or the local oscillator beams  575  may be deliberately pre-distorted to compensate for distortions imposed as the light propagates. With liquid crystal arrays, as an example, it is a simple matter to add essentially arbitrary phase variations across the cross-section of a beam. Such phase variations can then be added to the already present phase variations used to generate desired beam patterns. 
     It is further important to note that there need not be an exact one-to-one relationship between the number of beams generated and the number of active pixels in the detector array. In some cases received light from a single illumination beam may image onto several adjacent detector pixels, for example a 2×2 sub-array centered on the desired pixel. In such a case the receiver controller would enable data readout from all such sub-arrays. In the coherent detection case the LOBFE would obviously also be directed to generate local oscillator beams that cover the same pixels that are covered by the received signal beams. A possible need to read out data from, for example, 4 adjacent pixels rather than just one, does not strongly compromise the gains in data reduction afforded by this approach. Using the example above of having 10 6  addressable points but only 100 illuminated spots, if each illuminated point maps to 4 pixels instead of one the data reduction rate is still a factor of 2,500 compared with the case of reading out all pixels. 
     It is also apparent to those skilled in the art that in some coherent detection cases it may be advantageous to simply illuminate the whole array with local oscillator light, particularly if sufficient amounts of LO power is readily available or if the array is not excessively large. The tradeoff is that a greater amount of LO power is required, but uniform illumination also obviates the need for the LOBFE. Detector heating may be a concern with large amounts of LO power, but this problem may be reduced or eliminated by switching the LO illumination on only during those time intervals when signals are anticipated to be present. 
     Although primarily discussed in the context of coupling a pixel addressable detector/receiver array with multiple transmit beams, it is important to note that the invention has very substantial benefits even in the case of using only a single transmitted beam (Case 3 in Table 1). In operational scenarios where, at a given time t, there is a significant angular difference between the laser beam being sent out at time t, compared with light received as a result of light transmission at an earlier time t-t 0 , a single element detector could be looking in the wrong direction when the earlier light returns. By having a large array of pixels available the system controller could compensate for the time lag effect and direct the receiver controller to read out the appropriate pixel. This is fundamentally different from prior solutions where multiple pixels are always read out based on the lack of information about which pixel would receive valid signal light. In the disclosed case the correct pixel is known, which enables the system to conserve computational resources by only processing a single pixel, or perhaps a very small sub-array as noted above. 
     5. Implementation of Receiver BFEs 
     A number of further advances in the construction of lidar systems can be obtained through the incorporation of BFEs in the receive part of the system. These implementations permit covering cases 4, 8, 10, and 12 in Table 1 above. The essential idea is that, as an alternative to building a large detector array with possible preprocessing capability at each pixel location and then not using but a small fraction of the pixels at any given time, it is often more efficient to employ BFEs in the receive path for the purpose of redirecting received beams to predetermined detector pixel positions. The general idea then is to configure a lidar system in such a manner that a transmit BFE is used to illuminate M points at a target plane containing a potentially much larger number N of addressable points, and use one or more BFEs in the receive path to redirect scattered light from those M points to a substantially similar number of detector pixels. In this manner it is possible to construct an imaging lidar that transmits for example M=100 beams to a predetermined 100 points out of a possible, for example, N=10 6  addressable points, and to receive all the light using a detector array containing only M=100 elements. The starting point for implementing receive BFEs is consequently the notion discussed in conjunction with  FIG. 2 , that each point or small area at the target image plane can be viewed as a small bundle of light that has propagated from the target back through the receive system. Instead of placing a large detector array at the image plane we instead place there a BFE element that redirects such bundles of light (beams) towards a smaller detector array. 
     6. Receive BFE Embodiment 
     In the first embodiment of the invention to incorporate a BFE in the receive path, the receive BFE consists of a set of elements that redirect each image point to a separate detector element as shown in  FIG. 6 .  FIG. 6  has many elements in common with  FIG. 5  and where there is a one-to-one correspondence in function the numbering has been retained. It is important is that in the target image plane detector  509  has been replaced with a receive BFE  601 . This element consists of an array of pixels  602  that are capable of redirecting an incident light beam onto a specific element of a detector array  605 . Detector array  605  is connected to receiver  606  and outputs data on data bus  607  to signal processor  564  for extraction of target information and delivery of processed data to user interface  566  via connection  565 . 
     In the embodiment of  FIG. 6 , there is no need to incorporate a large detector array and receiver capable of detecting and processing large numbers of pixels. If the lidar system through transmit BFE  503  is capable of addressing N points at the target but only the smaller number M is used, it is only necessary for the detector array to have M elements (although it may be desirable to increase this number somewhat to allow for single target points to illuminate several adjacent pixels). Also, the pattern of the detector elements can be whatever is convenient (within optical design limitations). This can reduce the complexity of the detector/receiver by large factors, such as 1-3 orders of magnitude depending upon the application. 
     Moreover, in the embodiment of  FIG. 6  there is generally no need to implement receiver controller circuitry that selects which detector array elements should be read out. Also, the lidar system can still address the same N target points as long as the receive BFE  601  contains N array elements. Further, the reduced number of required detector elements has a major advantage in designing detectors and receivers. Detector arrays with large pixel counts are typically laid out with a square or rectangular 2 dimensional pattern of elements. This is illustrated schematically in  FIG. 7   a ) where a detector  701  comprises a number of individual light sensitive pixels  702 . When pre-processing electronics is added it is commonly done by attaching a matching array of electronics  703 , commonly referred to as a ROIC (Read-Out Integrated Circuit) to the detector array in a sandwich configuration. In this case the individual detector pixels  702  are connected to individual electronics sub-areas  704  with electrical connections  705  that may take several forms, bump bonding being one common approach. 
     The limitation with such architectures is that the physical size of the electronics sub-areas must generally on average be no larger than the size of the pixel element that sits in front of it, while noting that some improvement may be obtained by, for example, adding a lens array to permit the focusing of light of a given area to a smaller detector area. High-speed detectors and large pixel counts mean that these individual detector elements may measure in the range of 10-100 micrometers on a side. This severely limits the amount of electronics that can be placed on the same chip with the detector elements, even with state-of-the-art integrated circuit fabrication techniques. Reducing the number of detector elements eases this problem significantly. Not only is the cost associated with fabricating large arrays reduced, the power requirements are also reduced. 
     Even more important, in many cases, with a reduced pixel count there is less restriction on the geometry of the array. Since each image plane pixel can be pointed arbitrarily in 2 dimensions by the receive BFE one can, for example, select to point all pixels into sequential points along a line. When this is done there is generally only a restriction on the center-to-center spacing of the detector array elements, but there is less of a restriction in the second linear dimension. As illustrated in  FIG. 7   b ) a linear array  708  of individual detector elements  706  can be coupled to associated electronics circuits  707  that are far wider perpendicular to the line of detector pixels. This allows for packing greater amounts of electronics into each cell associated with a given detector element and further provides for simple readout lines  709  along array edges. Clearly, this linear approach is only one of many possibilities that are apparent with fewer required pixels. For example, “butterfly” layouts where electronics areas are laid out in a staggered fashion on either side of the detector array are also possible and permits convenient data readout from both edges of the electronics chip. 
     As one further alternative one can produce the same effect of permitting a greater electronics area relative to the detector area by simply increasing the center-to-center spacing of detector elements in a rectangular or square array. High packing density is typically driven by the desire to maximize the fill factor of the array and minimize it&#39;s total size, but with the smaller pixel counts enabled through the implementation of a receive BFE the size issue may be less relevant, and given the flexibility to point image plane light essentially anywhere in a 2-dimensional space there is also no requirement on having a high fill factor. 
     As illustrated in  FIG. 6  the system architecture is clearly applicable to both direct detection and coherent detection scenarios. In the coherent case the master oscillator  570  and associated elements  574 ,  572 ,  576 , and  578  would be added in a manner similar to  FIG. 5 . In the case of coherent detection it is important to note that mixing the local oscillator beams with the receive beams prior to the receive BFE reduces the optical quality requirements on the receive BFE and any other optical elements that are present after the mixing. Optical distortions that occur in one beam (LO or signal) but not the other prior to mixing results in a degraded heterodyne efficiency. Distortions that occur after mixing effectively affect both component the same way and do not degrade the heterodyne efficiency. 
     The receive BFE  601  may be fabricated in a number of different ways and operation of the lidar system in the described manner is not dependent on a specific method of implementation. The type of device may be passive or active and it may also operate in transmission mode (as shown in  FIG. 6 ) or in reflection. In transmission mode each receive BFE element  602  acts as a small prism for redirecting incident light to a predetermined detector pixel. In reflection mode each element acts as a small mirror tilted in a direction appropriate to direct the incident beam to a predetermined detector pixel. In addition to redirecting the individual pixels the each receive BFE may contain focal power to control the divergence of the output from the BFE in a desired manner. 
     One type of active device would be a liquid crystal array comprising N addressable cells. Each cell is then configured such that it acts as a prism (or tilted mirror if used in reflection mode). This is done by creating a linear phase gradient across the cell with a gradient corresponding to the desired deflection angle. To produce this effect it may be desirable that each addressable cell in turn comprises a number of smaller cells. By applying suitable voltage gradients to the cells the required phase gradients can be produced that in turn produce the desired steering of an incident beam. Such liquid crystal arrays can clearly be 1 dimensional or 2 dimensional for steering beams in 1 or 2 angular directions. An advantage of using liquid crystal devices is that they can be reprogrammed dynamically as conditions require. They can also incorporate additional functionality. For example, if required to prevent excessive spreading of the beams between the receive BFE and the detector, non-linear phase gradients may be added to refocus beams. A further advantage of programmable active devices like liquid crystal arrays is that the cell size can be dynamically altered. For example if a 1000×1000 element array is used it may be partitioned into 10000 cells each comprising 10×10 sub-cells or it could be partitioned into 1600 cells each comprising 25×25 sub-cells, or some other partitioning that is desired for a specific application. 
     A second type of device comprises an array of reflective mirrors. MEMS techniques have been developed to produce 2 dimensional arrays of individually addressable mirrors that can be inserted as the receive BFE. Such devices are fabricated for optical telecommunications crossbar switches. 
     Passive devices may also be advantageously used for the receive BFE. Fabrication of micro-prism arrays is relatively straightforward using e.g. lithographic techniques. One advantage of passive prism arrays over active devices is that they can easily be fabricated and replicated with very large pixel counts at low cost. A second advantage is that they require no power thereby generally reducing the cost, weight and complexity of the system. 
     An apparent disadvantage of passive prism (or mirror) arrays is that once the redirection function has been defined and fabricated it cannot easily be altered except by refabrication. For a general pattern of transmitted beams one must avoid situations where more than one imaged target point is redirected to a single detector pixel. With proper design this is not necessarily a significant drawback. It simply means that complete flexibility in generating and imaging beam patterns is not possible with a single passive element. However, since the redirection function of each BFE element can be defined a priori many different transmit beam patterns can be generated that still permit unambiguous pointing of receive BFE elements to detector pixels using a single receive BFE. 
     An example is shown in  FIG. 8  where we assume for simplicity that the transmit BFE can address a 4×4 grid of points at the target but the detector array comprises only 4 elements. In  FIG. 8   a ) is shown such a grid  801  of 16 addressable points. The center grid  802  illustrates the corresponding 16 point grid at the image plane where the receive BFE is placed. The last grid  803  illustrates the 4 detector elements. The idea is now to assign image plane pixels to detector pixels. Once such assignment is illustrated in the central grid of  FIG. 8   a ) where each image plane pixel denoted by “1” is mapped to the detector element also denoted “1” in the detector grid, each image plane element denoted by “2” is mapped to the detector element denoted “2” etc. In practice this means that light reaching any of the image plane elements denoted by a given number is redirected to the detector element with the same number. With this mapping assignment (which is obviously not unique in any way) if we illuminate the target plane with laser light as shown in the left column of  FIG. 8   b ) (illuminated target points  804  indicated with black circles) the result will be illumination of the corresponding points  805  in the image plane as illustrated by the central grid in  FIG. 8   b ). Given the mapping shown in  FIG. 8   a ) this in turn means that each of the illuminated image plane points gets redirected to a separate detector element and in this case all detector elements  806  receive signals.  FIG. 8   c )- f ) show other target illumination patterns (again indicated by black circles) that would also unambiguously map image plane illumination to unique detector elements. It is then clear that even for this simple case, a single fixed mapping as shown in  FIG. 8   a ) will permit the interrogation of targets with a wide variety of patterns, including vertical (a), horizontal (f), diagonal (c and g), sparse rectangular (d), and dense rectangular (e). The last row,  FIG. 8   h ) shows an example of an illumination pattern that would not work. With the illumination shown in the left grid, all points illuminated at the image plane point the beams to the last detector element  808  resulting in an ambiguous detection situation. In some cases this ambiguity can be resolved, e.g., in the case of a pulsed lidar the range to the target maybe different for each of the pixels so the signals will arrive back at the receiver at different times, which will allow them to be separated, even though they fall on the same pixel. 
     As noted there is nothing unique about the exemplary mapping shown in this case. A very large number of mappings is possible, particularly as the number of grid elements (pixels) increases from N=16. It is also clear that if particular mappings cannot be unambiguously represented with a single fixed receive BFE, it is quite possible to incorporate multiple mappings into the lidar system. The technique could be the same as that described in conjunction with  FIG. 4 , e.g. the use of a turret holding multiple receive BFEs for insertion as needed. 
     In all cases discussed it is contemplated that it is part of the function of the system controller to track the mapping functions, and if multiple receive BFEs are incorporated, also control the insertion of such elements. An important difference between the fixed receive BFE and an active receive BFE is that the active device can be reprogrammed without the need for a physical change of the element. 
     7. Replaceable Subsystems 
     A very attractive possibility is apparent when considering system architectures such as the one illustrated in  FIG. 1 , namely that pre-aligned interchangeable subsystems can be created to meet specific imaging scenarios. Lidar systems, in particular coherent lidar systems, require very high degrees of alignment precision in order to properly overlap beams with detectors, and in the coherent case, overlap local oscillator beams with return signals. Performing such alignments in the field can be time consuming and difficult, yet may be required in order to retain system measurement flexibility. For example, if a measurement system uses passive fixed BFE elements to minimize cost, weight, and power, but needs to be switched from interrogating targets using a linear illumination pattern to a 2-dimensional grid pattern then multiple elements must be replaced or the disclosed technique of implementing switchable fixed elements may be used. 
     However, it is quite possible to create field replaceable subsystems that can be readily interchanged.  FIG. 9  illustrates such a case where, what we refer to as the “beam forming subsystem”  908 , defined by a transmit laser input beam  901 , an MO laser input beam  902  (present in the coherent case), a set of target illumination beams  904 , a set of LO beams  907  (present in the coherent case), a set of received signal beams  913 , and a set of detector/receiver output beams  903 . As in previous illustrations, within the beam forming subsystem  908 , there are present a transmit BFE  909  that generates the target illumination beams  904 , a LOBFE  906  to generate a set of matching local oscillator beams  907 , and a receive BFE  905  to redirect received light  913  to the receiver. In this case the alignment issue relates to relative alignment between the transmit BFE, the receive BFE, and the LOBFE (if one is present). 
     With reference to  FIG. 9  it is evident that if these elements are mounted as a subsystem having a common mechanical carrier or housing to form a beam forming subsystem  908  then the components can be aligned relative to one another in the common housing off-line and the entire beam forming subsystem  908  containing pre-aligned elements can be replaced in the lidar system as required. It is then possible to fabricate, align, and store a variety of pre-aligned subsystems for replacement as the needs for the imaging system changes. It is clear that such a pre-aligned subsystem can hold a variety of additional components, such as the polarizer  910  and beam splitter  911  normally deployed, but that the most important parts of such a subsystem are the BFEs. 
     8. Second Embodiment Incorporating a Receive BFE 
     Two additional embodiments of the present invention are of significant value when coherent detection is employed. This second embodiment, along with the third embodiment incorporating a receive BFE as discussed below, are not meant to provide a complete list of all possible configurations, merely to indicate that multiple variations are possible within the spirit of the invention. What is important in any implementation is not the specific architecture or details of the specific devices, but rather the functionality of the incorporated elements. 
       FIG. 10  shows a beam forming subsystem for coherent detection wherein a transmit laser input beam  1001  is incident on a transmit BFE  1009  to produce a set of target illumination beams  1004  that are transmitted through a polarizer  1010  to the target. Received signal beams  1013  are reflected from polarizer  1010  and are transmitted to first receive BFE  1011 . First receive BFE  1011  redirects the incident beams  1013  in the form of a second set of beams  1005  such that each of beams  1013  is incident on a separate element of second receive BFE  1014 . Second receive BFE  1014  in turn redirects beams  1005  in such a manner that resulting set of beams  1015  propagates as a set of parallel beams towards the detector receiver. The spacing of the set of beams  1015  is generally such that each beam in the set is incident on a separate element of the detector array and each beam in the set has a size that substantially matches the size of each detector pixel. 
     Separately master oscillator beam  1002  is incident on MOBFE  1006  to generate a set of local oscillator beams  1007  that are formed in such a manner that upon reflection from beam splitter  1016  each local oscillator beam in the set  1007  is substantially matched in propagation angle, transverse position, and transverse size to a corresponding beam in previously noted set of received beams  1015 . This arrangement causes efficient optical mixing between the local oscillator beams  1007  and received signal beams  1015  to produce a set of coherently mixed beams  1003  that propagate to the detector/receiver. 
     First we note that the combination of two receive BFE elements with some spatial separation between then, each of which can steer an incident beam in two angles, means that each received and redirected beam that emerges from second receive BFE  1014  can be directed at an arbitrary angle at an arbitrary position. This is a frequently used feature in manipulating optical beams. For this reason it may be convenient to use the two receive BFE elements to create a regularly spaced array of beams  1015  that propagate in parallel towards the detector array. Any other distribution could also be created but this arrangement is often convenient in matching the receive beams with a set of regularly spaced local oscillator beams, the latter of which is generally easy to produce. 
     Second, with this arrangement it is always sufficient to generate a fixed set of local oscillator beams regardless of the distribution of target illumination beams. Two receive BFE devices can always be used to match the receive beam pattern to a pre-existing set of local oscillator beams, provided only that the number of transmit beams does not exceed the number of local oscillator beams. 
     Third, it is apparent that the arrangement in  FIG. 10  effectively decouples the LO generation problems from the illumination and reception problem. This means that complete subsystems for specific imaging scenarios may be produced, for example as field replaceable units, by incorporating for example the elements shown outlined with box  1017  as a single unit subassembly. 
     Fourth, a number of relatively simple methods exist to generate the requisite set of local oscillator beams. In the case where a rectangular or linear detector array exists that has a relatively high fill factor, the simplest method would be to simply expand the transverse extent of the master oscillator beam to fill the entire array aperture. In this case there is no actual division of the MO beam into separate LO beams which is very simple to do using simple optics. Generating a regularly spaced one or two dimensional array of LO beams to match a similarly shaped detector array is straightforward through the use of diffraction gratings and lenses. These may be produced such that all individual LO beams are parallel to match the configuration shown in  FIG. 10 . At the same time we stress that that parallelism of beams is not a requirement. Coalignment of the LO beams with the set of received signal beams  1015  to match a set of detectors is the primary objective. It is consequently apparent that the set of beams  1015  generated by receive beam forming elements  1011  and  1014  need not to be parallel but could be dispersed with some angular distribution suitable to match the generated LO beams and the detector array geometry. 
     9. Third Embodiment Incorporating a Receive BFE 
     A third embodiment of the invention to incorporate a receive BFE element is illustrated in  FIG. 11 . The central idea of this third embodiment is that in measurement situations where there is not a need to produce different angular distributions of transmitted and LO light (due to time of flight and lag angle compensation considerations as described above), it may be advantageous to use the same BFE element to produce both light distributions. This may save cost and components and can also be advantageously used reduce the alignment requirements between multiple elements. Numerous system architectures can be used to incorporate this idea, one of which is illustrated in FIG.  11 ,. 
     In  FIG. 11  a transmit input beam  1101  and an MO laser input beam  1102  are combined in an optical element  1103  to produce two co-propagating beams. One convenient method to implement this combination is to use different linear polarization states for the two beams, for example ensuring that transmit beam  1101  is polarized in the plane of the paper and MO beam  1102  is polarized perpendicular to the plane of the paper. In this case optical element  1103  can be a simple polarizing beam splitter with coatings designed to reflect and transmit different polarization states differently. Such elements are commonly used in optical systems and are widely available. Co-propagating beams  1104  are shows with a slight transverse offset for visual clarity, as has been done in previous figures, but in practical cases the two beams may overlap spatially. Beams  1104  are next incident on BFE element  1105  to produce two sets of diffracted beams  1106  with substantially identical angular distributions. These two sets of beams propagate to optical element  1107  that splits the two sets in such a manner that the beam set generated from transmit beam  1101  propagate through element  1107  as beams  1111  and continue to the target. The set of beams generated from MO beam  1102  reflect off element  1107  as a set of beams  1108 . This may again be accomplished by making element  1107  an appropriately coated polarizing beam splitter. As in previous embodiments light scattered from the target returns through the system and is reflected off element  1107 . As noted previously this may be accomplished by also incorporating a quarter-wave plate in the transmit beam portion of the system. At this stage the receive beams reflected off element  1107  have the same polarization as beams  1108 . 
     Beams  1108  in turn reflect from optical element  1110  in such a manner that the incident beams retrace their paths back to optical element  1107 . If element  1107  is a perfect polarizing beam splitter the retracing beams will now reflect from element  1107  rather than be transmitted through the device to mix with the received signal beams. One method to prevent this problem from occurring is to design reflective coating to be slightly imperfect for the appropriate polarization state. For example a coating may be designed to substantially completely transmit the transmit beam polarization to efficiently produce the desired transmit beam pattern. At the same time the coating can be designed to be somewhat “leaky” for the opposite polarization state. It may, for example, reflect 90% of the light and transmit 10%. In that case 90% of the return signal light is reflected from element  1107  and is mixed with the 10% of the MO light reflected from element  1110  to produce optically mixed beams  1112  that co-propagate to receive BFE  1113  where they are redirected to the detector/receiver array as beams  1114  in a manner described above. In the described architecture it is clear that if 90% of the MO beam light from transmit BFE  1103  is reflected towards element  1110  then 10% of the light is also transmitted toward the target. This is normally not problematic since the peak power of this light is substantially lower than the peak power in the transmit beams. To prevent detrimental issues caused by scattering of this light back into the system towards the detector it is also feasible to insert additional elements in the path of beams  1111 , such as additional polarizers to absorb or reflect this stray MO light, possibly used in conjunction with additional baffles or absorbing materials. It is also noted that element  1110  is illustrated as a mirror in  FIG. 11 . This element could also comprise a phase conjugator, a fixed BFE element as described above, or a combination of optical elements that have the effect of retracing the optical beams incident on it. 
     While the present invention has been shown and described in the context of specific examples and embodiments thereof, it will be understood by those skilled in the art that numerous changes in the form and details may be made without departing from the scope and spirit of the invention as encompassed in the appended claims. Examples of such changes include, but are not limited to:
         Alternative implementations of lasers. The invention is not limited to the specific implementation of lasers for transmission of light or their use as master oscillators. As illustrated in the document the master oscillator has been shown as a separate element from the transmitter laser for ease of illustration only. In some circumstances there may be only one laser present and light is split off and used for multiple purposes. For example, there may be one laser whose output is split into two parts, where one part is used for the local oscillator beam, and one part is used for the transmit laser, perhaps with a pulsed or continuous-wave amplifier inserted in the transmit beam. In such a case a suitable frequency shifter may be inserted into one of the beams to produce a desired relative frequency shift. In an alternative implementation the two lasers (LO and transmitter) may be two separate units that are locked together electronically by, for example, beating them together on a detector to determine the frequency difference and using servos and tuning elements to maintain a desired frequency difference (that could include no difference frequency).   Alternative system receiver/signal processing implementations. The invention does not in any manner depend on the specific implementation of means for detecting and processing received light signals, except as specified herein. The invention is therefore equally usable with coherent and incoherent detection, as well as with autodyne detection wherein a part of the received signal is itself used as the local oscillator.