Lidar’s Fourth Generation: No More Compromises

The 4 Generations of Lidar: Rotating, MEMS, Electronically Scanning, Global Shutter Flash

Sense Photonics’s direct Time-of-Flight platform delivers a unique value proposition to the automotive industry:  a lidar platform which does not compromise performance for price. To make sense of the wide array of lidar technologies in the market today, it helps to divide the market into 4 generations of lidar systems:

1st Generation: Mechanically-Rotating Linear Scanning Systems

Mechanically-rotating lidars integrate multiple discrete lasers and detectors in a single enclosure. Vertical resolution is defined by the number of lasers which are packed in the enclosure while a 360 degree field-of-view is attained by fast mechanical rotation of the whole system. 

While this was an exciting technology at the outset of lidar, and helped launch the technology during the DARPA Grand Challenge competition in the 2000’s,  rotating systems offer limited resolution for a high price, because their resolution is defined by the number of discrete lasers which can be assembled into a small enclosure, and by the fine alignment between those laser and the discrete detectors which collect their signal. 

Moreover, these systems suffer from several additional deficiencies common to scanning systems, such as motion blur, rolling shutter, distortion, and reliability issues caused by vibration or shock.   These systems were designed to optimize system performance but neglected SWAP-C3 considerations (size, weight, power, cost, cooling, and compliance). Specifically, they tend to be large and produce 360 degree fields of view, regardless of whether those are necessary. As such, they typically protrude from the car’s roof and do not lend themselves for aesthetic and aerodynamic integration into the vehicle.

2nd Generation: Micro-Actuated Point Scanning Mirrors

In response to the deficiencies of rotating mirror lidar systems, several companies developed micro-actuated scanning systems. These systems operate by deflecting one or more collimated laser beams as well as the received signal from one or more mirrors, thus scanning the field-of-view and directing target echos to a detector. Beam steering is achieved either using discrete microactuators or using an integrated MEMS chip. 

Micro-actuated scanning systems suffer from a number of critical deficiencies. Most notably, these systems trade-off resolution for field-of-view when operating at a given frame rate.

For example, take such a point-scanning system operating at 25 frames-per-second (or 40 msec frame time). Since light takes 1.63 μsec to travel to a 250 m target and back, a singlebeam can only capture 40 msec/1.63 μsec = 24,500 target-directions per frame. If the system’s angular resolution is 0.05° x 0.05°, then the total field of view would be limited to about 60 square degrees, for example 7.7° x 7.7°. Such a narrow field of view is typically insufficient for long-range applications, forcing such lidar systems to operate at variable resolutions across the field of view. Variable resolution runs the risk of missing unexpected objects and creates a computational burden on downstream data fusion engines. 

Similar to Gen 1 systems, Gen 2 systems also suffer from motion artifacts. For example, since Field-of-View acquisition is sequential, an object may be imaged for a first time when the vehicle isin a first position and then the same object may be scanned again when it appears at a different direction due to the vehicle’s changing position. Thus, it is not uncommon to see a street sign appear twice in a point cloud with such systems.

Micro-actuated systems also suffer from mechanical deficiencies. Due to the tight tolerancing required to achieve their angular resolution, these systems are more susceptible to vibration, shock and temperature effects. They also need to be carefully aligned, which, among other factors, contributes to their high cost.

Whether utilizing direct Time-of-Flight signal acquisition or FMCW (Doppler-shift), current Gen 2 lidar systems continue to struggle with complexity, reliability as well as motion artifacts.

3rd Generation: Electronically-Scanning Lidar

The quest for more reliable and cost-effective automotive lidar led some companies to develop electronically scanning lidars. These systems utilize electronics to steer their laser beam without mechanical motion. These methods include Optical Phase Array (OPA) as well as utilizing VCSEL arrays with integrated microlenses which direct the light output of portions of the array to various subregions of the field-of-view. While Gen 3 lidars offer higher reliability than mechanically scanning lidars, they still suffer from motion artifacts because of their sequential acquisition of the complete point cloud. Moreover, a number of the scanning technologies have proven to lack maturity or to be sensitive to temperature or to device-to-device variabilities.   

4th Generation: Solid-State Global Flash Lidar

The holy grail of automotive lidar is a mass-manufacturable, cost-effective and high-resolution lidar, which acquires point clouds without any scanning.  Numerous technological challenges have prevented developers from creating such non-scanning flash automotive lidar systems. High peak optical power, uniformly illuminating a large field of view, has traditionally translated to large power and expensive systems with eye safety limitations. On the receiver end, non-scanning systems require high-resolution focal plane arrays with simultaneous parallel signal processing.

Sense Photonics has developed the only flash lidar system meeting market requirements. The company builds its custom emitter by printing a large number of Vertical-Cavity Surface-Emitting Lasers (VCSELs) using its proprietary Micro Transfer Printing process. Eye safety and reliability are assured by diffusing the outputs of the VCSELs to illuminate the full field of view. For its receiver, Sense selected CMOS Single-Photon Avalanche Diodes (SPADs) integrated in an internally-designed smart imager sensor chip, which processes more than 100,000 pixels’ histograms simultaneously. The integrated system is now shipping to select customers, delivering unprecedented point cloud resolutions across wide fields of view.

Sense’s 4th Generation lidar utilizes mass-manufacturable and automotive-qualified technologies, achieving unprecedented cost efficiencies. Its unique architecture results in true global shutter acquisition, thus removing motion artifacts which are prevalent in legacy lidar generations. Since no scanning is required, the system is robust to vibration and shock, and does not require fine alignment or calibration. With its camera-like architecture, these systems can be efficiently integrated into vehicles in an aesthetic and aerodynamic way.

Conclusion

The arrival of true solid-state global flash lidar will allow, for the first time in the industry, lidar systems which do not compromise cost for performance.  Sense is the only flash lidar company in the world which has achieved high-precision, 200m range using high-volume-manufacturable components. This advancement will allow OEMs to optimize SWAP-C3 parameters while still delivering exponential improvements in Points-per-Second over the three legacy lidar generations.  Combining the scalability and manufacturability of VCSELs with solid-state silicon CMOS SPAD technology , Gen 4 lidars deliver a true paradigm shift in the industry enabling the mass adoption of autonomous and assisted driving systems.

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