A research team representing commerce and academia has made a welcome breakthrough in optical coherence tomography (OCT): meter-scale OCT.1 (For more on OCT, see "Advances in functional OCT.") Not only does the achievement provide dramatically greater capability than before, it also portends high-performance, low-cost systems, with wide-ranging impact for industrial applications as well as biomedical.
It's no surprise that OCT has achieved its greatest success in ophthalmology, and shown the most promise for other uses (such as gastroenterology, cardiology, and dermatology) that benefit from endoscopy-enabled or superficial imaging, as it produces volume images with micron-scale resolution at high speed, at depths from the millimeter-to-centimeter range.
But with the researchers reporting high-speed 3D OCT imaging at cubic-meter volumes, that paradigm will change. According to James G. Fujimoto of the Massachusetts Institute of Technology (MIT; Cambridge, MA), whose lab invented OCT with collaborators in the 1990s, their new "long-range" or "macro-scale" OCT demonstrates "at least an order of magnitude larger depth range and volume compared to previous demonstrations of three-dimensional OCT."
All these years later, Fujimoto and collaborators are measuring objects of various sizes at 15 μm resolution over a 1.5 m region, and making it easy to imagine a host of new OCT applications, from enhanced medical imaging to process monitoring, nondestructive testing, and technical measurements.
Enabling the breakthrough
The new range of operation "requires extremely high-performance light sources, integrated optical receivers, and signal processing," Fujimoto says. The breakthrough comes from a tunable vertical cavity surface-emitting laser (VCSEL) source and a chip-based optical receiver designed for telecommunications applications. The researchers acknowledge that their work builds on impressive progress in photonic integrated circuit (PIC) development, including waveguide integration, photodetectors, swept lasers, and chip-based OCT interferometers.
The VCSEL source, developed by Thorlabs (Newton, NJ) and Praevium Research (Santa Barbara, CA), uses a miniature microelectromechanical systems (MEMS) device to perform swept-source OCT—that is, to quickly change the laser's wavelength over time. Ben Potsaid of MIT and Thorlabs, who coauthored the paper, explained that because their research showed the coherence length of the VCSEL source was orders of magnitude longer than other suitable swept-laser technologies, they saw the possibility of long-range applications.
However, the researchers struggled with difficulties in light detection and data acquisition until they recognized that a silicon photonics coherent optical receiver made by Acacia Communications (Maynard, MA) could overcome them. The discovery also meant the ability to replace several bulky OCT components with integrated optics on a tiny, low-cost, single-chip PIC. Importantly, the PIC receiver supports the very high electrical frequencies and wide range of optical wavelengths required for swept-source OCT while enabling quadrature detection, a technique that doubles the OCT imaging range for a given data-acquisition speed.
The researchers' tests indicated that performance has not reached the fundamental limits for either the VCSEL source or PIC receiver. Now, they are working to increase data acquisition and processing speeds, with a goal of real-time OCT using customized integrated-circuit chips. Paper coauthor Chris Doerr of Acacia Communications says it is realistic to "expect full OCT systems on a single chip within the next five years," and that such systems would be dramatically reduced not only in size, but also in cost.
Experiments and expectations
The researchers used a VCSEL tunable laser with a 1310 nm center wavelength, tuned over an 80 nm full-sweep range at a 100 kHz repetition rate. They showed that meter-range OCT can obtain a strong signal from surfaces of varying geometries and materials.
To demonstrate the new technology's ability to inspect and measure weakly scattering objects with complex shapes and surface profiles, they imaged a bicycle. To show its suitability for noncontact metrology, they quantitatively measured aluminum posts and steel gauge blocks from 1 m to 1 μm with a 65 dB dynamic range.
By imaging a human skull/brain model, they demonstrated meter-range OCT's application to macroscopic anatomical imaging—perhaps for surgical planning or guidance. The technique could also provide 3D measurements in laparoscopy or mapping structures such as the upper airway. (Because its penetration is limited to 1-2 mm in scattering samples, however, the researchers caution that macro-scale OCT will never replace true tomographic modalities such as MRI and CT.) With its ability to provide difficult-to-obtain information on material composition, subsurface structure, coatings, surface roughness and other properties, macro-range OCT is sure to find many new uses in industry, manufacturing, and medicine. Because it can image human-sized objects with high axial resolution, the advance may help solve computer-vision challenges such as 3D object recognition and surveillance.
The technique is analogous to frequency-modulated continuous-wave coherent radar but with orders of magnitude higher speeds. Compared with other 3D ranging technologies, such as lidar, laser trackers, and frequency-comb lasers, OCT is better suited for subsurface evaluation and imaging weakly scattering objects. Compared with other 3D subsurface imaging methods, such as modulated imaging, OCT has a higher axial resolution and is less sensitive to clutter or parasitic reflections—and can potentially provide information about properties such as composition, laminated structures, coatings, and surface roughness.
Yet more applications may emerge from exploration of contrast from polarization, motion, spectroscopy, and elastrography.
1. Z. Wang et al., Optica, 4, 10, 1496 (2016); doi:10.1364/optica.