Research

Optical Coherence Tomography

Optical coherence tomography (OCT) is a non-invasive label-free imaging technique that utilizes light waves to capture high-resolution cross-sectional and volumetric images of biological tissues. Similar to how B-mode and C-mode ultrasound provide images of the body's internal structures, OCT can rapidly detect optical backscattering in tissues. This allows for the creation of detailed 3D images with microscale resolution, providing valuable insights into the structure and function of living tissues. Invented by Prof. Fujimoto and his colleagues at MIT in 1991, OCT has grown into a standard examination in ophthalmology and an emerging tool for intravascular research in cardiology. 

Imaging human or animal biotissue in vivo is often challenging due to the limited number of backscattered photons resulting from weak scattering in the sample and the need to adhere to maximum permissible exposure (MPE) for laser safety. OCT overcomes this challenge by optically amplifying the backscattered light through interference with a higher-intensity reference plane wave. The backscattered photons interact with a larger number of plane-wave photons, creating an orders-of-magnitude larger cross-correlation term in the resulting intensity. A depth profile along a single axial line, called axial scan or A-scan, can be recorded by measuring this cross-correlation term across a wide wavelength band.

An OCT system can be implemented in various configurations. Early-day OCT systems used a broadband light source and a physically moving reference arm scanning one depth position at a time, thereby receiving the name time-domain OCT (TD-OCT). Current generation of OCT systems are called Fourier-domain OCT (FD-OCT), which involves a wavelength-swept laser (swept-source OCT) or a spectrometer based on a line-scan camera (spectral-domain OCT), to detect the whole interference spectrum almost instantaneously without moving the reference arm. One of the core research topics in our lab is to develop multi-MHz wavelength-swept lasers to enable snapshot-like 3D OCT imaging, as explaned below.

Wavelength-Swept Lasers

Wavelength-swept lasers stand as the powerhouse driving some of the fastest OCT systems to date. These lasers can typically be assembled in standard laboratory environments, utilizing fiber-optic components based on technology originally developed for telecommunications. Having an advantage in high-speed light source technology significantly enhances the generation of new imaging results. In our lab, we are spearheading the development of a groundbreaking wavelength-swept laser technique known as stretched-pulse active mode locking (SPML). This work is being done in collaboration with the Harvard-MGH Wellman Center for Photomedicine and KAIST.

SPML achieves wavelength sweeps at rates of multiple million repeats per second (multi-MHz), or even tens of MHz, enabling snapshot-like 3D or video-rate 4D imaging across wide fields of view or with high transverse pixel resolution. Our current research focus is on refining this technique within the 1060-nm band, where precise compensation for cavity round-trip residual dispersion is crucial. We envision that these research efforts will lead to new imaging capabilities with significant potential for clinical and biological research applications.

One of the unmatched advantages of SPML is its inherent consistency in sweep-to-sweep time, which results from not needing mechanical tuning filters. This consistency allows for the acquisition of 3D complex-valued data that maintains phase consistency, enabling the use of 3D k-space image processing techniques, as explained below.

3D k-Space Image Processing

We develop computational techniques to enhance the quality of images in coherent imaging systems by manipulating data in the 3D Fourier domain (k-space). Drawing inspiration from synthetic aperture radar (SAR) image processing and principles of diffraction tomography, such as the Fourier diffraction theorem, we aim to pioneer the next-generation image processing platform for OCT as a reflective synthetic k-space imaging modality. The platform's potential functionalities encompass defocus correction, aberration correction, angular compounding, and angular aperture synthesis. These methods address physical errors in the imaging environment, enhance visualization of translucent features by emulating incoherent light, and enable synthetic super-resolution, all through post-processing techniques.