| Polymers Main Page > Research Facilities > Optical Coherence Tomography Facility |
Optical Coherence Tomography Facility |
| Overview |
| Optical coherence tomography (OCT) is a powerful tool for non-destructive, bulk volumetric imaging of synthetic materials. With this technique, information about features with a size of 10 mm and above is obtained. A requirement for this type of tomography is that the material be transparent to near infrared light, making many polymer systems excellent candidates. The imaging is performed by collecting back-reflected light generated from heterogeneities within the sample. The higher the refractive index mismatch at a boundary, the stronger the return reflection at any given depth. Thus, OCT has the ability to distinguish different types of features. OCT is attractive because of its high sensitivity, which enables a typical sample to be imaged at depths anywhere from 0.5 mm to 4 mm. The resolution of the system is 10 mm to 20 mm. The cost of the system ranges anywhere from 20k to 300k, depending upon the type of source and complexity of optics and detection scheme. Recently, resolutions of 3 mm have been reached with state-of-the art lasers, electronics, and optics. Additionally, OCT systems have been built that are polarization sensitive (PS-OCT). These systems gather information about sample birefringence. The NIST OCT system will be upgraded to a PS-OCT system during Fall 2001. |
| Method |
| Basically, OCT is a confocal technique that is enhanced by highly efficient interferometric rejection of out-of-plane image scattering. The NIST OCT uses a low coherence source with a fiber optic based Michelson interferometer. The source is a fiber-optic pumped superlumenescent diode with 13 mW of power at the output. The high power improves imaging depth. In this configuration, the sample is the fixed arm of the interferometer and the fiber optic acts as a confocal aperture. Reflections from heterogeneities within the sample are mapped as a function of thickness by path matching with the moving reference arm to generate interferograms. Each interferogram is then bandpass filtered, rectified and digitized. The reflections are then displayed as peaks as a function of depth at any one position within the sample. Volume information is generated by translating the sample on a motorized stage through perpendicular axes. Information about the location and size of a feature within the sample is obtained. |
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| Applications |
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As stated previously, the NIST OCT is used to study bulk structure on the length scale of 10 mm and above. In our program, this technique has been mainly applied to the areas of tissue engineering and composites. In tissue engineering, porous polymeric scaffolds are synthesized to act as substrates for living cells. The microstructure of the scaffold is one important variable in determining the viability of the seeded cells. The pores must be large enough to accommodate the cells and well connected to promote the migration of the cells throughout the entire scaffold. OCT can give a unique insight into 3D scaffold architecture. Additionally, the scaffold can be imaged in biological media which provides information about its in-situ structure. Two issues in the area of composites were addressed using OCT. Reinforcement microstructure affects the success of a part during the manufacturing process. OCT was used to image the reinforcement microstructure and thereby provide insight into the flow characteristics of the resin inside the mold. Secondly, OCT was used to identify various damage modes within a glass reinforced composite. |
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| Contact Information |
| Joy Dunkers |
| NIST |
| Characterization and Measurement Group |
| 100 Bureau Dr., STOP 8541 |
| Gaithersburg, MD 20899-8541 |
| Voice: (301) 975-6841 |
| Fax: (301) 975-3928 |
| E-mail: joy.dunkers@nist.gov |
| NIST Materials Science & Engineering Laboratory - Polymers Division |
