Patent Application
20230375672
The present disclosure relates to a Light Detection And Ranging (LIDAR) device with a dense array of spatial light modulators in the receiving optical path.
Light Detection and Ranging (LIDAR), is a remote sensing method that uses light e.g. in the form of a pulsed laser to measure ranges (variable distances) to one or more object surfaces in a field of view. A LIDAR device is for instance disclosed in EP3460519 A1. In detail, light is transmitted towards the object and single photodetectors or arrays of photodetectors such as PIN photodiodes, avalanche photodiodes (APDs), single-photon avalanche diodes (SPADs), multipixel single photon counter (MPPC), or silicon photo-multipliers (SiPMs) receive reflections from objects illuminated by the light, and the time it takes for the reflections to arrive at various sensors in the photodetector array is determined. This is also referred to as measuring time-of-flight (ToF). The spatial position of a surface point is acquired in each case by the distance to the targeted surface point being measured by the laser and this measurement being linked to items of angle information of the laser emission, e.g. a rapidly settable deflection element, for example, a scanning mirror (sweeping or rotating mirror) or a refracting optical component, according to a defined scanning grid in order to vary the transmission direction of the distance measuring beam, for example, with respect to one or more independent spatial directions, whereby a three-dimensional measuring or scanning region can be acquired. The spatial position of the acquired point can be determined from these items of distance and angle information and, for example, a surface can be surveyed in an ongoing manner.
Well known applications are scanning ranging for mobile entities such as airplanes, drones or cars, for instance use in driver assistance systems, for detecting other objects or measuring air turbulences. In the field of autonomously driving vehicles, the roads to be traveled are typically acquired in advance and imaged in a model. For this purpose, for example, vehicles equipped with scanners are used, which scan and map the relevant region and therewith provide geometric data of the world around the car at a very high resolution.
As a preferred application of LIDAR, a LIDAR is e.g. embodied as or is part of theodolite or total (scan) station or as an airborne LIDAR, can be used to survey many different settings such as construction sites, industrial facilities or any other applicable setting for example in order to sample a cloud of 3D points (so called point cloud) within a coordinate system, representing the object's surface points. Additionally, a camera may be associated with a laser scanner and may be configured to capture images associated with the setting being scanned. Further measuring tasks of scanning measuring devices are, for example, the monitoring of an environment, for example, in the context of a warning or monitoring system for an industrial manufacturing plant.
The required large field of view of the receiver being at least 100-1000 times the size of the laser beam has several drawbacks. For example, the solar background noise is strongly increased and limits the detection threshold for weak return pulse signals. Thus the transmission power needs to be increased for achieving a sufficient signal to noise ratio (SN), whereby however eye-safety limits have to be taken into account. Typically, the readout time of a detector depends on the size of the detector. Thus, the response time of larger detectors is increased, i.e. limiting the overall scanning speed and/or the scanning resolution, and the detector bandwidth and manufacturability is typically reduced. Additionally cost of detectors is increased and availability of large detectors is limited.
As an alternative to a LIDAR with a sweeping deflection element as described above, it is known to use a flash LIDAR system which scans by using multiple detectors. Each detector is aligned so that it only detects light coming from a certain direction. The amount of signal that can be received is determined by the area of the detector and the acceptance angle of the detector. The transmitted light from the LIDAR system illuminates all the objects (points) to be measured, while each detector only receives light from the objects that are in its field of view. One difficultly of the flash LIDAR system is finding a powerful laser source with high enough peak power that can illuminate the whole scene with a very short pulse of preferably less than 1 nanosecond. Due to this, the transmitted power may be limited by the capability of today's lasers, instead of being limited by laser eye safety limits.
Recently, there are LIDAR devices known, e.g. by EP3833999 A1 or U.S. Ser. No. 10/247,811 B2, that comprise in the receiving path an array of spatial light modulators such as a Digital Micromirror Device (DMD). A DMD is a two dimensional array of e.g. 4090×2160 or up to eight million modulator elements, each of which may be referred to as a DMD pixel, and arranged in a generally rectangular or other form. Each of the individual spatial modulator elements sees some small part of the field of view. In case of a DMD, each modulator element is a micromirror that is configured to be activated by a positive electrical signal (first modulation state/ON state) or activated by a negative electrical signal (second modulation state/OFF state) thousands of times per second by receiving electrical signals sent from a controller (e.g. a microcontroller or other processing unit). The electrical signals control a tilting mechanism of a corresponding modulator element such that for example tilting angles of +12° and −12° can be activated. Typically, there is also a third or deactivated state without electrical signal with a flat position of about 0° (undefined tilting position). In the first activated state, a tilt of an individual micromirror is configured to redirect received measurement light towards a detector. In a 2nd activated state, a tilt of the micromirror is configured such that light impinging on the micromirror is deflected away from the detector. In other words, when using the DMD in the receiving optical system, switching on and off different micromirrors corresponds to passing light through the optical system for detection or rejecting it. Hence, received light can be detected pixel-by-pixel, thus scanning the object surface point-by-point.
It is an object of the present disclosure to provide an improved LIDAR device with an array of spatial light modulator elements on the receiving side.
The disclosure relates to a, particularly multichannel, Light Detection And Ranging (LIDAR) device for detection of a portion of a three dimensional environment, preferably within measurement ranges of above 100 m. The LIDAR comprises a transmission unit with a sequential (point or line like) or simultaneous (areal) pulse illumination source or sequential or simultaneous burst illumination source. Preferably, the illumination has a wavelength in the wavelength range between 1000 nm and 2000 nm, in particular has a wavelength of 1064 nm or 1550 nm.
Further, the device comprises a receiver unit with a receiving optics, multiple detectors for detection of received illumination light and feed of respective detection channels. Above that, it comprises a dense array of equal spatial light modulator elements (in the following also abbreviated as “modulator elements”), preferably a digital micromirror device (DMD). The array of spatial light modulator elements (in the following also abbreviated as “modulator array”) is arranged in a focal plane of the receiving optics between the receiving optics and the detectors, whereby the spatial light modulators provide a first spatial modulation state and a second spatial modulation state each, the two states differing in light redirection, and the array of modulator elements and the detectors are arranged such that only in the first modulation state light from the receiving optics is redirected via a respective modulator element in a targeted manner towards a detector. An evaluation electronics evaluates signals of respective detection channels for distance determination based on the principle of time of flight.
The receiver unit comprises a dense array of optical wedges (in the following also abbreviated as “wedge array”) in between the array of modulator elements and the detectors, whereby the wedges are juxtaposed with respect to the focal plane, each wedge covers a different area of the array of modulator elements and the refractive planes of the wedges are differently oriented, such that light coming from the array of modulator elements in the first modulation state is refracted area-wise in different refraction directions by the wedge array. Preferably, the detectors are spatially separated from each other according to a respective refraction direction each such that light of a respective modulator element area is receivable by a respective detector. The different wedges of the wedge array define domains on the array of modulator elements which addresses different regions of the receiving field of view (FOV) of the LiDAR.
In other words, the array of individual spatial modulator elements which in themselves provide equal spatial modulation in their first modulation state, is segmented in areas or domains of different light redirection by the array of different optical wedges. The different refraction resp. effective light direction allows for a spatial separation of the detectors. That is, the detectors need not to be placed densely juxtaposed to cover the dense modulator array/field of view. Though the detectors have gaps between them, the field of view can nevertheless be completely or gaplessly measured. In other words, the at least substantially dense, gapless or continuous modulator array can be mapped without loss to the non-dense, distributed, discontinuous or spaced field of detection provided by the detectors. For example, the receiving optics and the modulator array define a cohesive field of view (FOV) of at most 40°×30°, in particular of at most 20°×15°, which can be fully monitored by the detectors though they are spatially separated.
Preferably, the wedge array is monolithic, e.g. made from a single piece of glass or plastic, and/or the first optical element in the light path between modulator array and detectors. This includes that the wedge array is the only optical element in this part of the beam path. Alternatively, some sort of relay optics follows the wedge array.
As a preferred option, a respective detector is embodied as an avalanche photo diode (APD) and/or has a bandwidth of at least 100 MHz, in particular at least 1 GHz.
As an option, a photo sensitive area of a respective detector is (substantially) covering the same field of view as a respective modulator area or domain. That is, the area of spatial modulator elements allocated to a respective detector is geometrically equal or corresponding to the effective detection area.
As an alternative option, there are lens arrays e.g. as a part of a relays optics in the optical path after or downstream of the wedge array, each sublens of the lens array mapping the respective modulator area versus the center of the detector. Hence, the photosensitive area can be smaller than the respective area of the modulator domain resp. field of view part to be scanned therewith. In particular, a photosensitive area of a respective detector can be at least ten times smaller than the respective modulator area, whereby optionally the photo sensitive area has a diameter of at most 350 μm.
As an option, the LIDAR device comprises a camera with an image sensor for capturing one or more 2D-images, whereby in the second modulation state a respective modulator element directs light from the receiving optics to the image sensor. Hence, in this embodiment, the second spatial modulation state is used for generating e.g. an on-axis Intensity, Gray-Scale or RGB-image of the current field of view. This 2D-imageing can be done in parallel to a 3D-measurement, whereby only the individual pixels (modulator element) currently used for the 3D-measurement are lost in the 2D-image, which however is neglectable.
As another option, the device comprises means for light absorption such as a beam dump, e.g. a black glass plate, for absorbing light redirected by a modulator element in the second modulation state or a third or further modulation state. Hence, light reflected from the modulator elements not used for measurement can be absorbed which lowers the impact of unwanted straylight. Said third modulation state is for example in case of a DMD a parked position as a resting position.
Preferably, measurement channels of the LiDAR can be parallelized and therewith parallel detection for segments of the modulator array and therewith field of view can be enabled. That is, multiple or even all detection channels are optionally connected in parallel and the evaluation electronics is configured for parallel or simultaneous multiple distance determination. For example, in case of a certain number of modulator areas or domains (and according, equal number of wedges) and an according, equal number of allocated detection channels, an equal number of object points can be scanned in parallel as each spatial modulator element can be switched individual or independently and light redirected from a modulator element of each modulator area or segment of the modulator array can be detected independently.
As an option, instead of full illumination of the FOV (flash LIDAR), the transmission unit comprises means for emitting the illumination in form of multiple light fans spaced to each other, e.g. a grating or diffractive optical element, multiple line lasers or linear arrays of VCSEL. The light fans are oriented in accordance to lines of the modulators. That is, the illumination light is shaped to illuminate simultaneously or in direct sequence (sequential flash LIDAR) multiple distinct, spatially separated lines of modulator elements. Then, within every currently illuminated line, one or more of the modulator elements at once (e.g. one element of each currently illuminated modulator domain at a time for parallel detection) can be switched in the first state for light detection.
The present disclosure provides the advantage that the uniform field of light redirection provided by the dense arrangement of equal spatial light modulator elements as a modulator array is broken up by the optical wedge array. Instead, the wedges put out different, non-parallel light propagation directions for different regions or domains of the modulator array and hence regions of the measurement field of view. This allows for a flexible arrangement of detection optical paths and finally for a distributed arrangement of optical detectors.
This enables in particular the usage of photodetectors with limited photosensitive areas. For instance, an APD as a detector well fitted for wavelengths between 1000 and 2000 nm, in particular 1550 nm, has an insensitive border region around the photosensitive inner region. Hence, even if multiple APDs are packed as dense as possible, there are detection gaps inside the package caused by the insensitive border of each APD. In this case, this would mean the great disadvantage that the dense field of view provided by the dense i.e. gapless modulator array could not be covered without gaps.
Now to the contrary, the spreading of the field of view by the wedge array enables that an assembly of APDs can be arranged spatially separated but nevertheless densely covering the field of view. All received light is directed to a photosensitive region and no received light is directed to a dead border region. No “pixel” is lost without the need of relying on rimless detectors which may be not available or applicable in a LIDAR as demanded. An arrangement of multiple detectors is in particular advantageous for parallelization of LIDAR measurements.
In an alternative multichannel LIDAR device with an array of spatial light modulator elements, the LIDAR comprises a transmission unit with a sequential or simultaneous pulse or burst illumination source and a receiver unit with a receiving optics and a multichannel dense detection array, preferably in the form of an application-specific integrated detector array.
The dense detection array provides multiple closely neighbored or juxtaposed, but independent detection areas or regions. Each detection region covers an allocated area of the modulator array whereby the whole of the modulator area or the complete field of view is covered gaplessly by the whole of the detection zones, i.e. by the detection array. The dense detection array is for example preferably embodied as a monolithic structure for example as a dense array of MPPC (Multi Photon Pixel Counter) or SiPM (Silicon Photo Multiplier). Such an array of MPPC is e.g. provided on a single, monolithic and segmented chip. Said otherwise, each segment of the dense detection array is designed for detection of an allocated segment of the spatial modulator array and hence segment of the field of view.
The dense array of equal spatial light modulator elements, preferably embodied as a digital micromirror device, is arranged in a focal or imaging plane of the receiving optics between the receiving optics and the detection array. As in principle already described above, the spatial light modulator elements provide a first spatial modulation state and a second spatial modulation state each, the two states differing in light redirection and the modulator array and the detection array are arranged such that only in the first modulation state light from the receiving optics is redirected via a respective modulator in a targeted manner towards the detection array.
An evaluation electronics evaluates signals of respective detection channels for distance determination based on the principle of time of flight. Each detection area of the array or each segment individually feeds a detection channel. Therewith, multiple or preferably all detection channels can be evaluated simultaneously. Thus, received light from multiple or all segments or areas of the modulator array can be detected in parallel.
As each spatial modulator element can be switched individually or independently, a measurement or scan with each modulator segment or modulator region independent of the other regions is enabled by individually switching a pixel in each region and in sequence for the other pixels in each region until all pixels have been switched (i.e. each sub-field of view has been fully covered by and by). This sequence can be done in parallel for multiple or all regions, hence, multiple or all regions of the field of view as defined by the segmentation of the detection array can be scanned in parallel, therewith multiplying the measurement rate compared to single measurements. The more of said parallel detection regions are available, the higher the multiplication of the measurement can be/the more object points can be scanned in parallel. For example, with a detection array comprising n detection areas and accordingly parallel measurement channels and spatial modulator elements with a switch rate of the modulation state of m Hz, the measurement rate is n*m. A typical switching speed of a modulator element is 100 kHz, a LiDAR with nine parallel electronic detection channels yields thus a measurement rate of 9*100 kHz=0.9 MPoints per second.
As an option, the area of the detection array equals the area of the modulation array. Said otherwise, there is no effective optical magnification or minification but a 1:1 imaging.
In another alternative LIDAR device with an array of spatial light modulator elements, the LIDAR is a single channel LIDAR and comprises a transmission unit with a sequential or simultaneous pulse or burst illumination source and a receiver unit with a receiving optics and a single detector, for example a APD or MPPC, preferably designed for wavelengths between 1000 nm and 2000 nm.
Said dense array of equal spatial light modulator elements, preferably embodied as a digital micromirror device, is arranged in an image plane or a focal plane of the receiving optics between the receiving optics and detector. As in principle already described above, the spatial light modulator elements provide a first spatial modulation state and a second spatial modulation state each, the two states differing in light redirection and the modulator array and the detector are arranged such that only in the first modulation state light from the receiving optics is redirected via a respective modulator in a targeted manner towards the detector.
In between the modulation array and the detector, a relay optics comprising a lens array is arranged. The lens array is designed and arranged in such a way that each lens of the array maps a portion of the area of the spatial light modulator versus the center of the detector. Said otherwise, the light bundles redirected from the spatial modulator array are imaged to a reduced area in the detection plane. The diameter of the light bundle cross section is reduced at the photodetector e.g. by a factor of five or more. Hence, a detector with an accordingly—i.e. 25 times or more-smaller area than the area of the modulation array can be used, e.g. a high-speed small area detector like an APD with a diameter of 350 μm or 1.5 mm for a modulator area of 3 mm×3 mm.
Obviously, it is not required to scan the entire scene with the full resolution defined by the modulator array. Hence, as an option, only a reduced resolution is acquired by using only every n-th pixel of the modulator array to cover the whole field of view at a reduced resolution in order to achieve a higher acquisition speed. This can for example be used to get an overview over the entire scene. Another option is to scan a dedicated region of interest (ROI) of the scene by activating a subset of the modulator array at full resolution. Other constellations are possible as for example multiple spatially separated regions of interest at full resolution.
In the following, the LIDAR devices will be described in detail by referring to exemplary embodiments that are accompanied by figures, in which:
The modulator array (SLM) 1 is an array of, for example between 200 k pixels and 8 M pixels, individual equal spatial light modulator elements M, in the example embodied for spatially modulating by reflection. Using the SLM in reflection mode is favourable because of first its polarisation independent reflection and second its broad wavelength range, e.g. covering the wavelengths between 1000 nm and 2000 nm. The array 1 is dense i.e. it covers the field of the received light 10 in principle or ideally gaplessly, in a real modulator array 1 such as a Digital Micromirror device (DMD) substantially gapless i.e. almost without any gaps as e.g. tiny slits between the micromirrors M are unavoidable. Each micromirror M has for example a size of 10 to 20 μm and can be (actively switched) separately or independently of the other mirrors M in a first modulation state 1a or a second modulation state 1b, e.g. with a switching rate of several thousand per second. In the example, a mirror M in the first modulation state 1a has a first tilt different to a second tilt of the second modulation state 1b. At least in the first modulation state 1a, a respective mirror M has a well-defined tilt angle.
Hence, impinging light 10 is reflected with a first angle in the first state 1a and with a different angle in the second state 1b by a respective micromirror M. This is indicated in the figure by the dashed arrows 11, representing light reflected by mirrors M in the first state 1a, whereas light 13 reflected by mirrors M in the second state 1b is reflected into a different direction. As the micromirrors M are equal and at least the first modulation state 1a is well defined, the first directions 11 are equal compared to each other, indicated in the figure by the parallelism of arrows 11. More generally spoken, at least two different redirections (in the example directions of reflection) for received modulated radiation 10 are provided by each of the individual modulator elements M of the spatial modulator array 1.
The first modulation state 1a is used to selectively redirect light 11 to one or more photo detectors 2, the detectors preferably having Gigahertz high-speed and/or intrinsic amplification (for simplicity, only one shown is in the figure), where it is detected. (Any possible optical elements as relay optics, light homogenizer and optical band pass filter in between prism 22 and detector 2 has been omitted, too, for better clarity of the figure). The detection signal is fed to a respective electronic detection channel 24, for evaluation by an evaluation electronics 23. Based on the detected signal and the principle of time of the flight, the distance to the object surface is determined. Said otherwise, according to the modulation state 1a or 1b a respective modulator element M of the modulator array 1 is, it either redirects received measurement light 10 such that it can be detected or it disables that received light 10 can be detected.
The micromirrors M of array 1 enable a scanning of an object surface on the receiving side in that by sequential switch of mirrors M into the first reflection state 1a and back, received measuring light 10 can be redirected mirror by mirror and so to say pixel by pixel towards the detectors 2. As the position of each mirror M within the array 1 is known, detected light redirected from a mirror M in the first modulation state 1a can be associated to a known measurement position and finally object point position. Hence, by a synchronized sequence of tilting (modulation state switch) of mirrors M, mirror after mirror, at well defined instances of time, measurement light 20 of one modulator/pixel M at a time is transmitted or redirected to a photodetector 2.
An advantage of this proposed solution is the reduction of background sunlight. As the actual reception field of view can be arbitrarily chosen and therefore reduced by the number of simultaneously enabled mirrors, the amount of background sunlight seen by the detectors is substantially reduced. Therefore, with a limited amount of emitting laser power the signal to noise ratio can be improved which leads to a higher precision and longer measurement ranges.
The time measurement unit of the LiDAR measures (pixel by pixel of the DMD array) the time of flight between emission and detection with a typical resolution of 1 picosecond. Thus, the distance of the object points defined by the receiving FOVs of the modulator elements or pixels M is determined with an accuracy of better than 1 mm.
With a DMD, e.g. 4096×2160 pixel or scan points can be selectively covered. Therewith, for instance, a “static” scanning—static in that the light receiving direction needs not to be changed-with a resolution of nearly 9 Mpoints within a field of view (FOV) e.g. of 80°×42°, 40°×21° or 20°×10.5°, defined by the focal length of the receiving optics and the modulator array size is enabled as a so to say “solid-state scan” (the spatial resolution in the object space equals the pixel size at the modulator array 1 divided by the effective focal length of the receiving optics 20; longer focal length provides a higher angular resolution, however, the full FOV becomes smaller). In case of a receiving FOV of 40°×21°, the spatial per pixel becomes 40°/4096=0.6 arcmin. Even at a distance of 100 m, the measurement points are still rather dense with a spacing of only 17 mm. Such a solid-state scan comprises e.g. a synchronized sequence of movement/switch of a line of micromirrors 1a,b such that there are well defined time points at which light of exactly one of the mirrors 1a,b directs light to the detector 2 and therewith covers one surface point of the measured object. In embodiments with a moving mirror in the transmitter as described above, the transmitter mirror and the modulator array 1 are arranged in a defined spatial relationship and operated in a synchronized manner, whereby demands on accuracy of the mirror resp. the transmitting beam shift are low, e.g. in case the illumination beam covers a slightly larger solid angle than the FOV-angle of the active spatial modulator element (pixel), there is even no need of measuring a deflection angle of the mirror/beam alignment.
As depicted, in this embodiment, the device comprises an additional dense array 3 of optical wedges 3i. The wedge array 3 is for example an assembly of transparent prisms which each cause light refraction in a defined and different direction, wherein the beam entrance plane and the exit plane are not parallel but have different angles, whereby the difference is small, e.g. 1°-5° or 10° at most. Preferably, the wedge array 3 is monolithic and for example manufactured from plastic or made of glass and arranged in between the modulator array 1 and the detectors 2i, on top of the modulator array 1, therewith in any case the first optical element after the modulator array 1 and close to the focal plane of the receiving optics (cf.
The dense wedge array 3 covers the modulator array 1 and comprises different wedges 3i, of which in the side view
Thus, light reflected or redirected from the array of the spatial light modulator 1 is refracted area-wise in different directions in space, which is indicated in the figure by arrows 11a-11c pointing in different directions. Hence, in contrast to the embodiment according to
In other words, due to the different wedges 3i of the wedge array 3, subarrays or domains Ai of the modulator array 1 are defined and provided with finally different angles of reflection for the same modulation state for modulator elements M not within the same subarray Ai. For each subarray Ai in this embodiment there is a separate detector 2i (in the example with six wedges, defining six subarrays, there are six detectors) arranged and oriented according to the respective (one of the exemplary six) effective redirection direction or refraction direction 11a-11c and coupled to the LiDAR evaluation unit 23, providing an according number (here: six) of detection channels 24a-24c.
Due to the area-wise spatial separation of the reflected beam-cones of received measurement light or splitting the optical axis of the primary receiver optics 20 into a set of for instance 3×2 different optical axes at the dense modulator array 1 by the dense wedge array 3, the detectors 2i can be arranged spatially separated from each other, with a distance in between them. The wedge array 3 enables more possibilities for detector arrangement and is in particular advantageous in that detectors 2I can be applied that have a non-sensitive edge zone. For example avalanche photo diodes (APDs) show “dead areas” at their border wherefore in case of a dense detector assembly of APDs as would be needed without the wedge array 3, there would be detections gaps and not all “pixels” of the dense array 1 of modulator elements M would be “imaged”. Preferably in an embodiment as shown, an actual detection area or usable photo sensitive zone of a detector 2a-c is substantially equal to a respective modulator area A1-3 (hence, any possible relay optics in between at the end leaves the optical size unchanged resp. has a magnification factor of 1). For example, for a DMD-chip of an overall size of 6 mm×9 mm, each modulator area Ai and accordingly a respective of the six detection areas has a size of 3 mm×3 mm.
A respective detection channel 24i receives at a time signals from one of the modulator elements M in the first modulation state of a respective array area Ai allocated to a respected detector 24i, e.g. domain A1 (provided by wedge 3a)—detector 2a-channel 24a. Thereby, the bandwidth is at least 100 Mhz or at least 1 GHz, thus enabling distance measurement with an accuracy in the millimetre or submillimeter range. The modulator elements M of different areas Ai can be activated independently and simultaneously for different areas Ai, hence, a scanning in parallel for multiple/all areas Ai or multiple parallel detection channels 24i is enabled. In the example, six spatial modulator elements M (one pixel of each region Ai) can be simultaneously be switched into their first modulation state for redirecting measurement light towards a respective detector 2i and therewith six objects points can be measured at once. This parallelisation of modulators and detection channels allows for a multiplication of the measurement rate; for instance with a DMD 1 with a micromirror switch rate of 50 kHz, a six fold measurement rate of 300 kpoints/sec is provided. For example, the detection channels 24i are connected to a thresholding electronics (comparator, Schmitt trigger), FPGA or AD-converters via a multiplexer as a channel selector such that multiple or all channels 24i can be switched on in parallel. For instance, a 6:3 MUX can connect six detectors to three ADCs or FPGA ports. Alternatively, the detectors 2i can be e.g. connected directly to an equivalent number of ADC-channels.
Fast switching DMD yield a pixel or switching speed of 2.5 microseconds, e.g. the time from the 1st modulation state to the 2nd modulation state. The transition from the 2nd to the 1st modulation state is also actively steered by an opposite electronical control signal and achieving the same high switching speed. Because of ringing and settle down time, the feasible ON-time or shutter time is 8 microseconds, this yields a highest achievable point to point measurement is ⅛ μs=125 kPts/s when using a single channel ranging setup.
The array of lenses 6 is designed in such a way that each sublens of the lens array 6 maps the modulator area A1 versus the center of the photodetector 2a, indicated in the figure by converging directions 11a′ in direction of detector 2a. This arrangement images the reflected light bundles 11a of a modulator area A1 to a smaller area in the photodetector plane. As a result, the detection area needed for covering the whole of a modulator area Ai is reduced, for instance the photo sensitive area can be at least ten times smaller than the modulator area Ai to be covered. Thus, relatively small detectors 2a can be used, e.g. an APD with a diameter of below 0.5 mm.
The dense detection array 12, e.g. segmented MMPC, is shown in
Thus, nine spatial modulator elements (one of each region Ri covered by one detection region 12i) can simultaneously be switched into their first modulation state for redirecting measurement light towards detector 12 and therewith nine object points can be measured at once by nine parallel measurement channels 24i. This parallelisation of modulators and detection channels allows for a multiplication of the measurement rate.
That is, all modulator elements M2 in the second state or “non-measurement” state 1b redirect received light to the camera sensor 35. Hence, an image of the field of view of the LIDAR device can be generated by the camera 33 (except for the pixel(s) M1 in the first, measurement state 1a, of course). Therewith, for example a live 2D-image can be recorded or displayed to a user during scanning, in parallel to the distance measurements. Another relevant advantage is the access of a coaxial 2D-image to the recorded 3D-point cloud of the targeted object surface.
The optical absorber 36 is arranged in such a way that light impinging at modulator elements M2 in a second modulation state 1b-or alternatively a third modulation state-is directed towards it (arrows 13) and absorbed from it. Thus, laser light impinging on all other pixels outside of pixel 1a and solar light and secondary produced straylight within the LIDAR is reduced, leading to a reduced noise and crosstalk to the activated channel defined by pixel 1a. Said otherwise, the absorber 36 is positioned such that all light currently not used for measuring is absorbed.
The mentioned further, non-first and non-second, modulation state is for example in case of a DMD as modulator array a so called parked-position or in-plane position. Such spatial modulator elements then have for example two actively controlled well-defined modulation states with well-defined angular orientation-whereby the first state 1a is used for scanning and the second state 1b e.g. can be used for 2D-imaging as described above- and a third state without operational current and without any well-defined orientation. In particular in case of (micro-)mirrors as spatial modulators, leaving any mirror not actively used in such a parked position is advantageously as in this state fewest stray light is generated. The absorber can also be arranged and designed in such a way that not only in the parked position but both in the second and third modulation state light is absorbed. In the same way, also sunlight collected by the receiving lens 20 (cf.
Hence, after reflection at the object surface, the received light fans illuminate not the full FOV of the receiver/the complete modulator array 1, but only a respective line L1-L3 of spatial modulator elements of the modulator array 1. In the example, there are three light fans 10a-c, illuminating an array 1 portioned into nine areas Ai, e.g. assigned to optical wedges as described above, whereby each light fan 10a-c covers three areas Ai “vertically”. Of each modulator area Ai, one modulator element or pixel is in the first or measurement state 1a at a time. In sequence, the further pixels in the second state 1b of the currently illuminated line L1-L3 are activated or switched into the first state 1a and back, in parallel for all currently illuminated lines L1-L3, until all pixels of the current scanning line L1-L3 have been detected. Then, the light fans 10a-c switch spatially to the “horizontally” neighbored modulator lines or next set of modulator lines, e.g. by activating a next line of VCSEL of the array or next line lasers, and the modulator elements of these next, “horizontally” neighbored modulator lines are sequentially used for scanning as described by switching into the first state 1a. This procedure is repeated until all modulator elements or pixels (all modulator lines or the whole of each modulator region Ai) have been illuminated and used for measuring by light redirection to the detectors.
Said otherwise, subsegments of the receiving FOV are illuminated in series or sequentially by activation or shift of illumination synchronized to the activation within lines of spatial modulators L1-L3, whereby multiple spaced subsegments are illuminated and multiple modulator lines L1-L3 are active preferably in parallel or alternatively in sequence by the multiple light fans 10a-c. For example, the illumination can be implemented by eight laser diodes with an optics for shaping a pattern of three horizontal(ly spaced) light fans 10a-c per laser diode. Hence, the emitted light fans 10a-c cover the current viewing or receiving solid angles of multiple modulators in the scanning state 1a. Such a design is in particular advantageous for long measurement ranges of above 100 m, particularly for a measurement wavelength of 1064 nm, 1310 nm or 1550 nm. The fan-like illumination of the environment to be measured is of higher intensity compared to a flash-like illumination of the complete FOV of the LiDAR-receiver, e.g as 80°×42° 40°×21°, or 20°×10.5°.
A skilled person is aware of the fact that details, which are here shown and explained with respect to different embodiments, can also be combined in other permutations if not indicated otherwise.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22174897.3 | May 2022 | EP | regional |
