Optical coherence tomography (OCT) is a high-resolution method for the non-invasive examination of tissue or the structural analysis of inorganic materials. A Michelson interferometre is used to determine the backscattering behavior of the sample. From the measured values, a two or three-dimensional representation of the internal microstructure with an optical resolution between 1 μm and 15 μm can then be obtained.

There are two different methods:

• TD-OCT – Time domain measurement: With a broadband light source, an interference pattern is created when the optical path difference in the sample and reference arm is within the coherence length of the light source. With optical delay lines, the path length in the reference arm is repeatedly changed and the backscattering signal is continuously measured. Scanning in the x and y direction results in a three-dimensional depth profile.
• FD-OCT – Frequency domain measurement: With this simple and fast method, the depth profile of the sample is calculated from the backscattering signal and the spectral information using the Fourier transformation. Either a narrow-band tunable laser source (laser-based FD-OCT) or a combination of a broadband light source and a very fast spectrometre (spectrometre-based FD-OCT) is used.

In OCT, it is often desirable to focus on different planes. In ophthalmology, for example, both the depth profile of the retina and the structure of the cornea are determined. Motorized optical delay lines quickly adjust the OCT systems to different depth planes.