Future Optics: 3D imaging advances could enable important practical applications - an interview with Byoungho Lee
Dramatic advances in 3D imaging require the invention of new imaging devices.
OSA: What got you interested in optics and photonics?
Byoungho Lee: I started in electrical engineering at Seoul National University in Korea, and did a master's degree in integrated circuit design. But for my PhD, I wanted a deeper academic discipline and was fortunate to get into the optics lab at the University of California at Berkeley to study under Professor T. Kenneth Gustafson. I did my dissertation on rewritable holography in photorefractive materials, and became very interested in holography. Then, Seoul National University hired me as a professor, with a large budget to study holographic data storage. Later, Korea's strong display industry funded my group to study three-dimensional displays using holography and other technologies.
OSA: Imaging has been a central application for optics since the time of Galileo. How have optical advances changed imaging?
BL: Classical imaging lets us observe on scales from the microscopic to the galactic, but with limited resolution and image quality. New techniques have overcome those classical limits. Three developers of super-resolution microscopy shared the 2014 Nobel Prize in chemistry. Near-infrared optical tomography (NIROT) has the amazing ability to profile physiological characteristics of the living brain by detecting how tissue changes the path of laser light transmitted through the scalp. Adaptive optics enhances astronomical resolution by using deformable mirrors or liquid-crystal phase modulators to compensate for atmospheric distortion, and can also improve resolution for microscopy or imaging of the retina.
The concept of light-field imaging has become a powerful tool for image display. Based on a set of rays that contain information on direction and position, it can generate the depth information needed to produce 3D displays.
OSA: Three-dimensional laser holography promised a revolution in imaging when Leith and Upatnieks introduced it half a century ago. How has holography evolved since then?
BL: Their work stimulated widespread interest in holography for imaging, encryption, neural networks, pattern recognition, biosensing, and data storage. Early holography relied on analog recording on silver halides, photorefractive materials, or photopolymers, which offers quite good resolution, but it records images slowly and cannot generate dynamically changing images.
Digital holography began slowly in the late 1960s. Recently, it has advanced dramatically, thanks to remarkable advances in electronics and computers. Charge-coupled devices (CCDs) and complementary metal-oxide semiconductor (CMOS) cameras can record holograms. Computers can produce holograms purely by numerical analysis, or can digitally refocus 3D holographic images by digitally manipulating recorded phase data. Synthetic aperture imaging can produce super-resolution holographic images. Digital holography even can record wavefront data on incoherent light, potentially opening the way to holographic imaging without a coherent light source. Now, the combination of powerful computation and holography has become hot.
OSA: How has the public perception of 3D imaging as entertainment affected development of 3D technology?
BL: The 3D movie Avatar attracted huge public interest and led to large investment in 3D animation. However, viewers were disappointed because seeing 3D images on current 2D devices requires special glasses, degrades resolution, and limits viewing angles and positions. So, 3D film and television development has slowed.
Holography would offer the ultimate 3D display because it preserves the amplitude and phase data needed to reconstruct the wavefront to show depth. That would avoid the sensory conflicts and eye fatigue caused by other 3D displays that don't preserve that data, such as polarizing glasses or lenticular lenses. Currently, holographic displays are limited by the low resolution of spatial light modulators, but rapid improvement in that resolution should lead to better holographic displays.
3D imaging is not just for entertainment. Techniques such as electronic speckle pattern interferometry can monitor the surface quality or structural strain on the bodies of automobiles or audio speakers, the surfaces of huge mirrors, or on highway bridges. 3D medical imaging is an important research frontier that could improve our health and quality of life in the near future.
OSA: What's new and exciting in 3D imaging, and where is the technology going?
BL: Virtual reality (VR) is among today's hottest display topics. Typical VR consoles use stereoscopic head-mounted displays (HMDs), with each eye seeing a different 2D field. Vendors have pushed the technology to increase frame rates above 60 Hz, improve resolution, and reduce sensory conflicts to make the 3D headsets comfortable for gaming and other applications.
The next step in 3D visualization will be the realization of augmented reality (AR), which blurs the distinction between our real world and the virtual world generated by a computer. The popular Pokémon GO game is a simple example, with players seeking virtual creatures superimposed on 2D smartphone screens. More advanced AR systems will overlay virtual 3D images on real-world scenes. The challenge now is to support the growth of AR ecosystems and develop applications to take advantage of their revolutionary capabilities.
Another important field now emerging is high-resolution 3D imaging microscopy. Exciting medical applications include 3D imaging inside brains, blood vessels, and retinas.
OSA: What new imaging developments are needed over the next few decades?
BL: Dramatic advances in 3D imaging require the invention of new imaging devices. Just as rapid advances in 2D imaging came after the invention of the CCD, the invention of new types of high-resolution image sensors could stimulate the growth of new 3D imaging technologies. Spatial light modulators that record sub-micrometer pixels at high speeds could bring us a new generation of much more realistic 3D displays and holography for AR. Combining that new technology with AR could lead to important practical applications.
OSA: You also study plasmonics. How does that research connect to your interests in 3D imaging and digital holography? What type of devices can be developed with plasmonics?
BL: I started plasmonics research in 2007 with a nine-year project intended to develop active plasmonics for applications that included 3D imaging systems. Although our plasmonics research has shifted away from 3D imaging systems, nanostructured plasmonic devices have been studied for generating high-intensity subwavelength light spots and use in super-resolution microscopes. We also invented subwavelength aperture arrays that can be combined with nanocavities to make high-resolution color filters for displays. More recently, planar metamaterials called metasurfaces have attracted much attention as a novel platform for powerful, ultrathin diffractive optical elements and holography, and we are studying their potential for thin lenses and thin holograms.
BYOUNGHO LEE is professor and chair of the school of electrical and computer engineering at Seoul National University, where he also heads the Optical Engineering and Quantum Electronics Laboratory. He is a fellow of The Optical Society, SPIE, and IEEE as well as a member of the Korean Academy of Science and Technology, and has served as a director at large of OSA.