Frequency Combs Enable Remote 3-D Mapping

A laser-based imaging system can now create high-definition 3-D maps remotely, with the ranging accuracy made possible by frequency combs.

A team at the National Institute of Standards and Technology demonstrated the imaging system, which can create the 3-D surface maps from as far away as 10.5 m. Using lidar, the new system is also sensitive enough to detect weak reflected light.

The frequency combs were previously demonstrated at NIST, but are now being used to continuously calibrate the laser imaging system. The system operates on a principle similar to optical coherence tomography, but can operate much farther away from its target.

Detailed image of a shoe sole as mapped by NIST’s 3-D frequency-modulated CW laser detection and ranging system from 10.5 m. The colors indicate distance, with blue/purple indicating shorter distances (or higher areas of the shoe sole) and red/brown longer distances. The prominent shoe sole patterns are 1 to 2 mm deep. Courtesy of NIST.

The system uses 9 mW of laser power to perform a point-by-point 3-D scan of a target object, measuring the distance to each point. It uses that distance data to create a 3-D image of about 1 million pixels, and in less than 8.5 minutes at the current scanning rate.

In 2013, NIST reported in Optics Letters that such a system, without 3-D imaging capability, is accurate to within 1 mm.

With the new system, distances to points on a rough surface that reflects light in many directions can be determined quickly and with an accuracy that is traceable to a frequency standard, according to the researchers.

The researchers combined the initial laser output with the reflected light, with the resulting beat signals being converted to voltage and analyzed by digital signal processing to generate time-delay data, which is used to calculate the distance.

Basic lidar techniques are well established, but the new NIST system is able to sweep continuously across a band of frequencies. It can operate more rapidly as well, via the frequency combs that continuously calibrate the swept laser. The system also uses real-time processing to produce the 3-D megapixel images.

The system could be used in forensics to nondestructively and noninvasively collect evidence. It could also be used in a variety of manufacturing applications such as precision machining and assembly.

The work was funded by NIST and DARPA. The research will be published in Optics Express later this month.

For more information, visit www.nist.gov.