Optical microscopy suffers from a loss of resolving power when imaging targets are embedded in thick scattering media because of the dominance of strong multiple-scattered waves over waves scattered only a single time by the targets. We developed an approach that maintains full optical resolution when imaging deep within scattering media. We use both time-gated detection and spatial input–output correlation to identify those reflected waves that conserve in-plane momentum, which is a property of single-scattered waves. By implementing a superradiance-like collective accumulation of the single scattering (CASS) waves, we enhance the ratio of the single scattering signal to the multiple scattering background by more than three orders of magnitude. An imaging depth of 11.5 times the scattering mean free path is achieved with a near-diffraction-limited resolution of 1.5 μm. Our method of distinguishing single- from multiple-scattered waves will open new routes to deep-tissue imaging and studying the physics of the interaction of light with complex media.
Experimental schematic diagram of the CASS microscope. SLD, diode laser; OL, objective lens; BS1, BS2 and BS3, beamsplitters; SLM, spatial light modulator (working in reflection mode, but indicated here as a transmission mode for simplicity); DG, diffraction grating (an aperture was used to select the first-order diffracted wave); SM, path length scanning mirror; CCD, charge-coupled device camera. For clarity, red, green and dark gold are used to indicate incident, reflected and reference waves, respectively, although their wavelengths are the same.
Sungsam Kang, Seungwon Jeong, et al., "Imaging deep within a scattering medium using collective accumulation of single-scattered waves," Nature Photonics 9, 253-258 (9 Mar 2015)
Thick biological tissues give rise to not only the multiple scattering of incoming light waves, but also the aberrations of remaining signal waves. The challenge for existing optical microscopy methods to overcome both problems simultaneously has limited sub-micron spatial resolution imaging to shallow depths. Here we present an optical coherence imaging method that can identify aberrations of waves incident to and reflected from the samples separately, and eliminate such aberrations even in the presence of multiple light scattering. The proposed method records the time-gated complex-feld maps of backscattered waves over various illumination channels, and performs a closed-loop optimization of signal waves for both forward and phase-conjugation processes. We demonstrated the enhancement of the Strehl ratio by more than 500 times, an order of magnitude or more improvement over conventional adaptive optics, and achieved a spatial resolution of 600 nm up to an imaging depth of seven scattering mean free paths.
- Angle-dependent phase retardation of single-scattered waves give rise to the image distortion and reduction in signal to noise ratio
- Input and output aberrations are hard to distinguish in the case of elastic scattering
S. Kang et al., High-resolution adaptive optical imaging within thick scattering media using closed-loop accumulation of single scattering, Nature Communications 8, 2157 (2017)
C. Choi et al., Optical imaging featuring both long working distance and high spatial resolution by correcting the aberration of a large aperture lens, Scientific Reports 8, 9165 (2018)
Lensless and scanner-free endomicroscope
Recent trends in developing endoscopes is to gain the microscopic resolution and to reduce the diameter of the probes below a millimeter or so. The so-called endomicroscopes satisfying these two requirements provide a minimally invasive way of investigating the fine details of the microenvironments within the target organs. Typically, graded-index (GRIN) lens or image fiber bundles are widely used as imaging probes. In our studies, we used multimode fibers as imaging probes for further reducing the diameter of the unit. Since multimode fibers distort image information due to mode dispersion, bending and twist, we measured the transmission matrix of the fiber to recover the original image. In fact, our method enables us to use any light guiding media as an endoscopic probe. The examples of our investigation are given below.
Schematic layout of single-fiber microendoscope
Examples of endoscopic imaging of rat villi. (a) Conventional transmission imaging. (b) Endoscopic imaging. (c) Numerical propagation of the image in (c)
Youngwoon Choi, et al., "Scanner-free and wide-field endoscopic imaging by using a single multimode optical fiber," Physical Review Letters 109, 203901 (2012), Research Highlights in Nature 491, 641 (2012).
Changhyeong Yoon, et al., "Experimental measurement of the number of modes for a multimode optical fiber," Optics Letters 37, 4558 (2012)
Donggyu Kim, et al., Toward a miniature endomicroscope: pixelation-free and diffraction-limited imaging through a fiber bundle, Optics Letters 39, 1921 (2014)