New 3-D OCT technique allows imaging of large objects such as this life-size mannequin and chessboard. [Image: James G. Fujimoto/Massachusetts Institute of Technology]
A U.S. industry-academic collaboration, led by OSA Fellow James Fujimoto, Massachusetts Institute of Technology (MIT; USA), reports a proof-of-principle demonstration of cubic-meter volume optical coherence tomography (OCT). The researchers used advanced photonics integrated circuits (PICs) and MEMS technology to boost OCT performance on multiple scales over long ranges. Specifically, the team demonstrated 3-D subsurface tomographic imaging on a meter scale at a 100-kHz axial scan rate with 15-µm depth resolution (Optica, doi: 10.1364/OPTICA.3.001496).
OCT is a 3-D imaging technology commonly used in ophthalmology to get micron-scale images of small structures in the human eye, at a depth of up to a few centimeters. Now, Fujimoto’s team is a step closer to expanding the use of swept source OCT (SS-OCT) imaging to larger objects—such as entire organ systems or the brain—with micrometer-scale axial resolution. According to the authors, long-range SS-OCT could also find applications in industry, to evaluate materials in a manufacturing setting nondestructively.
MEMS boosts laser-coherence length
Thorlabs Inc. and Praevium Research played a key role in improving the laser-coherence length needed for macro-scale SS-OCT, by combining a tunable vertical cavity surface-emitting laser (VCSEL) with MEMS technology—thus creating an optically pumped MEMS-tunable VCSEL centered at 1310 nm. “The coherence length of this VCSEL source was orders of magnitude longer than other swept-laser technologies suitable for OCT,” says co-author Ben Potsaid.
With the laser-coherence challenge resolved, the researchers looked to Acacia Communications’ telecom technology—in the form of PICs—to provide the high-detection bandwidths and enhanced signal-processing needed for macro-scale SS-OCT to function.
OCT-on-a-chip
Acacia Communications created a new, silicon photonics coherent optical receiver. This receiver supports the high electrical frequencies and wide range of optical wavelengths necessary to enable light detection and data acquisition from the optically pumped MEMS-tunable VCSEL swept-source laser.
The receiver—a single-chip PIC in-phase and quadrature receiver—integrates waveguides, polarization splitting elements, 90-degree phase shifters and waveguide couplers, to increase the imaging range by two orders of magnitude at a given acquisition bandwidth. The single chip replaces a large subsection of the equivalent fiber-optic OCT interferometer with integrated optics.
SS-OCT demonstration
The researchers demonstrated their cubic-meter volume SS-OCT technique on different surfaces and materials—including a bicycle, human mannequin, models of a human skull and brain—as well as aluminum posts and steel gauge blocks. To test the precision of their new SS-OCT technique, the team performed 100 repeated measurements of a mirror at a long delay. They found no variation of the measured mirror position within an axial resolution of 15 µm.
The researchers are working on new PIC components that will speed up image acquisition and data processing times. The authors say this could someday enable real-time SS-OCT macro-scale imaging with small, customized integrated circuit chips.