Optical coherence tomography (OCT)

Topic

Optical coherence tomography (OCT) was originally developed for in-depth visualization of biological materials. However, thanks to intensive research, it has gained traction in other areas of application outside of medicine.

At Fraunhofer IKTS, OCT is used for the three-dimensional imaging of the most varied material structures, such as ceramics, (glass-fiber-reinforced) plastic, glass or biological materials. OCT enables real-time testing without direct contact with the sample.

 

The working principle of optical coherence tomography (OCT)

 

OCT, as a non-invasive, tomographic method, makes it possible to visualize the topography of surfaces and internal structures in scattering media. For this purpose, the studied object is irradiated with wide-band, near-infrared light and the scattered light is processed through a spectrometer. The potential resolution achieved by OCT is between two and 20 µm. OCT is also characterized by its high measuring speed. Volumetric studied areas can be scanned within a few seconds.

 

Fields of application of optical coherence tomography (OCT)

 

The Characterization Technologies workgroup offers application-specific OCT studies for the packaging, plastics, electronics, solar and food industries. Furthermore, the researchers develop client-specific OCT test systems for lab operation, product testing and process monitoring.

In the medical context, pharmacological products and various biomaterials are characterized, e.g. from the fields of dermatology, or ear, nose and throat medicine.

 

© Fraunhofer IKTS
Visualization of a weld seam cross-section using OCT.
© Fraunhofer IKTS
Detection of defects – OCT image of a crack in laser-cut ceramics.
© Fraunhofer IKTS
Cross-section of an inhomogeneous ceramic component with a filament-like entrapment.

Optical coherence tomography (OCT) in additive manufacturing (AM)

© Fraunhofer IKTS
Querschnittsbild eines additiv gefertigten Bauteils.

Additive manufacturing (AM) makes it possible to design components with geometric degrees of freedom that would be impossible to achieve with conventional manufacturing methods. This means AM enables the production of individually shaped components with a production batch size of one, and for special applications. At the same time, the growing number of components produced through additive manufacturing fuels the demand for technologies that can deliver in-line process monitoring and validation of such components. OCT is an excellent candidate for these tasks.

When ceramics, plastics or metals are added layer by layer, defects can occur not only on the surface, but even deep inside the printed structures. OCT enables analyzing the surface of the added materials with high resolution. Additionally, it is possible to monitor the homogeneity and thickness of the printed plastic and ceramic layers. Furthermore, the internal structure of the components produced through additive manufacturing can be visualized in the early stages of production. Any inclusions or inhomogeneities occurring during the printing process can be detected and used for process qualification.

The early detection of defects allows to optimize processes and thus save cost. If critical defects occur during the component production, its manufacture is halted. This aspect is of special interest when it comes to the additive manufacturing of ceramic components, as the final processing steps for these materials (drying and sintering) and any rework required are very costly from a financial and technological standpoint.

Fraunhofer IKTS specializes in the development of client-specific OCT systems. The institute has the complete development chain covered – from system design (resolutions, speeds, measuring ranges, illuminating wavelength used), to the development of highly optimized probes, to the development of control, visualization and data processing software. It is also possible to integrate OCT systems developed by IKTS into existing AM printing equipment.

Advantages of optical coherence tomography (OCT)

 

  • Non-invasive, non-contact measurement method
  • Resolution in the sub-micrometer range (with light sources of very large spectral width)
  • Fast method: more than 30 B-mode images per second
  • No sample preparation needed
  • Non-ionizing radiation
  • Variable measuring range

Services offered for OCT

 

  • Three-dimensional visualization of surfaces and inner structures including analysis
  • Static and dynamic coat thickness measurement (coat thicknesses in the µm to mm range)
  • Process-oriented feasibility studies (also comparative with other methods)
  • Development of universally usable laboratory measuring stations and application-specific test stands
  • Spatially resolved speed measurement using Doppler OCT (example: microfluidics)
  • On-site demonstrations at the client site

 

Cooperative research

 

Cooperative research project with IMM electronics GmbH on the integration of OCT technology in additive manufacturing equipment.

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A tour through the optical coherence tomography (OCT) lab at Fraunhofer IKTS.