Optical Coherence Tomography, OCT

Ultra-high resolution Optical Coherence Tomography (UHR-OCT) is an invaluable tool that enables micron-scale cross-sectional or 3D imaging of a subject. It is relevant for various applications, from analysis of tissue in medical applications to visualization of sub-micron structures in manufacturing.

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 the 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 Professor Huang et al.. A very thorough description of principles and applications has been provided by Professor Drexler from the Medical University Vienna and Professor Fujimoto from MIT in “Optical Coherence Tomography: Technology and Applications”.

Over the last 20 years, OCT has become an essential imaging tool in ophthalmology with particular emphasis on detailed analysis of the retina and the surrounding tissue.

OCT applications are, however, not limited to ophthalmology. 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. Light from one arm is reflected or scattered off the subject under investigation and interferes with light from a reference arm.

The light from both arms originates from the same light source. The two light beams will interfere if the difference in the path lengths is within the coherence length of the optical signal.

This coherence gating lets the detection system to discriminate between reflections from closely spaced reflectors enabling high-resolution imaging.

The sensitivity of OCT is so high it can detect even the weak signals from sub-surface reflections. In this way, cross-sectional imaging can be realized – much like ultrasound – but with a much higher resolution. Image depths of several mm into the tissue can be achieved.

Practical realizations of OCT

There are different practical realizations of OCT:

  • Time-domain OCT (TD-OCT): The reference mirror is moving hence enabling coherence gating at different depths in the sample arm. This was the very first realization of OTC 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.
  • Spectrometer based OCT (Sp-OCT): A broadband source (such as a SuperK source) is used to generate the interference spectrum which is detected with a high-speed spectrometer, typically with several thousand pixels and sub-nm optical resolution.
  • Swept-source OCT (SS-OCT): A tunable source is rapidly scanning the relevant spectral range. The spectral response of the interferometer is detected by a single or a balanced detector.

Each of these techniques has different advantages and disadvantages, which make them more or less relevant for certain 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).

Low noise ensures high-contrast images

The SuperK FIANIUM supercontinuum laser series has the lowest noise on the market. Optimized for low-noise performance, it gives high-contrast low-noise images in OCT systems. Its performance matches that of bulky Ti:Sapphire lasers.

Below are shown two OCT images of a human eye. The image on the right is recorded using a SuperK EXTREME OCT source, while the image on the left is obtained using a Ti:Sapphire laser. Both images are recorded by Angelika Unterhuber from Prof. Dr. Wolfgang Drexlers group at Medical University Vienna.

How others have used supercontinuum white light lasers for OCT

Papers describing OCT using a SuperK supercontinuum source: