Optical Coherence Tomography, OCT

Ultra-high resolution Optical Coherence Tomography (UHR-OCT) is an invaluable tool enabling micron scale cross sectional or 3D imaging of the subject under investigation. 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.

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 in contrast to alternative microscopy techniques where only the surface or shallow layers of the subject can be investigated. 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 1991 by Huang et al. [1] and a very thorough description of principles and applications has been provided by Drexler and Fujimoto in Optical Coherence Tomography: Technology and Applications [2]. 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 ophthalmology field.

The SuperK supercontinuum lasers offer extreme optical bandwidths, excellent spatial coherence and high optical power density that are key parameters for UHR-OCT. The number of OCT applications using SuperK sources is increasing rapidly.

OCT basics

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, and hence 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.

OCT_basic_principle

The sensitivity of OCT can be very high, which enables even the weak signals originating from sub-surface reflections to be detected. In this way, cross sectional imaging of the subject under test can be realized in a way similar to ultrasound, but with a much better resolution, and 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 these realizations, even for SS-OCT when utilizing a rapidly scanning bandpass filter to effectively sweep the center wavelength, but have mostly been applied to SD-OCT based on spectrometer detection (Sp-OCT).

How others have used supercontinuum laser sources for OCT

Papers describing OCT using the SuperK supercontinuum source:

 

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