High resolution optical microscopy is an invaluable tool for a wide range of applications. Optical Coherence Tomography (OCT) enables cross sectional or 3D imaging of the subject under investigation. This is a huge advantage compared to alternative microscopy techniques where you can only investigate the surface or shallow layers of the subject.
Cross sectional or 3D imaging is relevant for various applications, ranging from analysis of tissue in medical applications to visualization of sub-micron structures in manufacturing.
The OCT principle for imaging was first demonstrated in 1991 by Huang et al. and a very thorough description of principles and applications has been provided by Drexler and Fujimoto in Optical Coherence Tomography: Technology and Applications.
Over the last 20 years, OCT has developed into being an essential imaging tool in ophthalmology with particular emphasis on detailed analysis of the retina and the surrounding tissue.
The applications in OCT are, however, not limited to ophthalmology and an increasing amount of research is made in OCT outside the ophthalmology field.
The SuperK supercontinuum lasers offer several key parameters relevant for UHR-OCT:
- Extreme optical bandwidths
- Excellent spatial coherence
- High optical power density
The number of OCT applications using SuperK sources is increasing rapidly.
OCT in a nutshell
OCT is based on interferometry, where light reflected or scattered off the subject under investigation interfere with light from a reference arm.
The light from both arms originate from the same light source, consequently the two beams will interfere if the path length difference of the two arms in the interferometer is within the coherence length of the optical signal.
This coherence gating allows the detection system to discriminate between reflections from closely spaced reflectors, thus enabling high resolution imaging.
The sensitivity of OCT can be very high, which makes it possible to detect even the weak signals which originate from sub-surface reflections. In this way, cross sectional imaging of the subject under test can be realized in a way similar to ultrasound, but with a much higher resolution. Image depths of several mm into tissue can be achieved.
Practical realizations of OCT
There are different practical realizations of OCT:
- Time-domain OCT (TD-OCT), where the reference mirror is moving, hence enabling coherence gating at different depth positions in the sample arm. This was the first realization and it is still relevant e.g. for Full-Field OCT, where the interference pattern for a full 2-dimensional array is detected simultaneously by a 2D detector array (e.g. CCD or CMOS).
- Spectral-domain OCT (SD-OCT) also known as Fourier-domain OCT (FD-OCT), where the reference mirror is fixed and the interference pattern is detected spectrally and converted to spatial information by the Fourier transformation. SD-OCT can furthermore be divided into
- Spectrometer based OCT (Sp-OCT), where a broadband source (such as a SuperK source) is used to generate the interference spectrum and this is detected with a high-speed spectrometer, typically with several thousand pixels and sub-nm optical resolution
- Swept-source OCT (SS-OCT), where a tunable source is rapidly scanning the relevant spectral range, and the spectral response of the interferometer is detected by a single or a balanced detector.
There are different advantages and disadvantages for each of these techniques, which make them more or less relevant for specific applications.
SuperK sources can be used in all of the above realizations. For SS-OCT a SuperK can be used with a rapidly scanning bandpass filter to effectively sweep the center wavelength. Most SuperK sources, however, have been applied to SD-OCT based on spectrometer detection (Sp-OCT).
How others have used supercontinuum white light lasers for OCT
Papers describing OCT using the SuperK supercontinuum source:
- Simultaneous optical coherence tomography and lipofuscin autofluorescence imaging of the retina with a single broadband light source at 480nm by Minshan Jiang, Tan Liu, Xiaojing Liu, and Shuliang Jiao, published in Biomedical Optics Express, Vol. 5, Issue 12, pp. 4242-4248 (2014)
- All fiber ultra-high resolution Fourier domain optical coherence tomography for old master paintings By Chi Shing Cheung, Marika Spring and Haida Liang
- Feasibility of the Assessment of Cholesterol Crystals in Human Macrophages Using Micro Optical Coherence Tomography By Manabu Kashiwagi, Linbo Liu, Kengyeh K. Chu, Chen-Hsin Sun, Atsushi Tanaka, Joseph A. Gardecki, Guillermo J. Tearney, published in PLoS ONE 9(7) (2014).
- Ultra-high-resolution and ultra-high-sensitive optical micro-angiography based on supercontinuum light source By Zhongwei Zhi, Lin An, Jia Qin and Ruikang K. Wang, published in SPIE proceedings Vol. 7889 (2011).
- Non-invasive imaging and monitoring of rodent retina using simultaneous dual-band optical coherence tomography By Peter Cimalla, Anke Burkhardt, Julia Walther, Aline Hoefer, Dierk Wittig, Richard Funk and Edmund Koch, published in SPIE proceedings Vol. 7889 (2011).
- Simultaneous dual-band optical coherence tomography in the spectral domain for high resolution in vivo imaging By Peter Cimalla, Julia Walther, Mirko Mehner, Maximiliano Cuevas, and Edmund Koch, published in Optics Express, Vol. 17, Issue 22, pp. 19486-19500 (2009)
- Phase-dispersion light scattering for quantitative size-imaging of spherical scatterers By Tasshi Dennis, Shellee D. Dyer, and Andrew Dienstfrey
- Shear ﬂow-induced optical inhomogeneity of blood assessed in vivo and in vitro by spectral domain optical coherence tomography in the 1.3 μm wavelength range By Peter Cimalla, Julia Walther, Matthaeus Mittasch and Edmund Koch
- Gangnus, Sergei V., and Stephen J. Matcher. “Visible-light OCT spectrometer for microvascular oximetry.” Proceedings of SPIE, the International Society for Optical Engineering. Society of Photo-Optical Instrumentation Engineers, 2008.