Flash LADAR

An ordinary camera gives information about the brightness of each point in the scene, but gives no information about how far away each point is. This depth information is essentially absent because the image is a flat two-dimensional mapping. The goal of three-dimensional (3D) imaging is to measure depth explicitly.

One 3D imaging technique is flash-laser radar. In this technique, the scene is illuminated with a very short (<0.5 ns) flash of light and, as in a conventional camera, imaged with a lens. Instead of measuring the amount of light, however, the imager pixel measures the time of arrival of the light to a pixel, which indicates the round trip time of the light to the corresponding point in the scene, and therefore measures depth.

It is challenging to build compact high-performance LADAR (LAser Detection And
Ranging) systems because most of the light scatters off the scene in random directions, and only a miniscule fraction returns to the collection lens of the camera. Most previous LADAR systems were not able to produce an image from a single flash of light, but used various techniques requiring much longer exposures.

Flash LADAR image of a van.Figure 1. Flash LADAR image of a van. (Click image to view movie of rotated views of the van.)

Lincoln Laboratory has developed imagers specifically for flash LADAR, based on Geiger-mode avalanche photodiodes  (GMAPDs), which are single-photon-sensitive detectors.  An array of GMAPDs is bonded to high-speed digital timing circuits to make the imager. Each pixel can detect a single photon and measure its time of arrival with a precision of a fraction of a nanosecond. The pixel extracts and digitizes the relevant piece of information, relieving the rest of the system of burdens associated with sensing and processing extremely weak pulses of light. This device is the first all-solid-state single-photon-sensitive area array imaging device capable of subnanosecond time resolution of the time of return from a single flash of light.

A GMAPD acts essentially as a digital device; a single photon can cause the diode to discharge to a voltage just below its breakdown voltage in a very short time (tens of psec). This voltage step can be consistent with signals on modern complementary metal-oxide semiconductor (CMOS) digital logic devices. We developed methods to fabricate arrays of GMAPDs and to integrate this diode array with CMOS logic, making one connection to a GMAPD per pixel. The CMOS logic was then designed with control and timing circuitry that was independent for each pixel.

Using this type of array, we have built a number of very compact (since the laser source needs only enough power to return a few photons per pixel) and sensitive LADAR systems. The 3D image in Figure 1 is of a van. The depth information is color coded: red points are points closest to the LADAR system and blue points are the most distant. Now that every point on the image is known in three dimensions, it is possible to view it from different directions on our 2D monitors. The LADAR system was directed at the front of the van to collect the image data, and this view is shown in Figure 1. (To see a movie of other views of the van rotated in different directions, click on Figure 1.)

 

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