Terahertz Quantum Cascade Laser Based 3D Imaging

The NASA Constellation program has a need to non-destructively test (NDT) non-metallic materials (foams, Shuttle Tile, Avcoat, etc) for defects such as delaminations and voids. While imaging systems at terahertz frequencies (0.3 to 3 THz) have been demonstrated for 2D imaging of similar materials, they have not yet demonstrate full 3D volumetric imaging. To meet this need, LongWave Photonics proposes to use high-power, low-frequency terahertz quantum cascade lasers (QCLs) developed at MIT, to demonstrate 3D imaging using Laser Triangulation. By using high-power QCL sources, large signal to noise ratios (SNRs) are attainable, resulting in resolution of subtle defects at fast scan speeds. The shorter wavelengths emitted by QCLs, 60 to 250 µm, allow high lateral and depth resolution. The feasibility of a second system based on Swept-Source Optical Coherence Tomography will also be explored using a recently developed tunable THz QCL from MIT. In addition to the benefits of high SNR, this technique allows sub-wavelength depth resolution. The current generation of QCLs are compatible with a cooling package that is <1 Kg, with <100 W power consumption. Phase II work will package a second generation of QCLs in a compact system to meet NASA's portable 3D NDT needs.

Pharmaceutical applications for this technology include in-process monitoring of thicknesses of polymer coatings in controlled release tablets. Numerous defects in the thin coatings can occur during processing affecting the performance of the tablet, leading drug complications and drug recalls. The use of QCL based 3D imaging technology could improve the uniformity in a batch tablet coating process. In the automotive and aerospace industry spray application of paint is both inefficient and environmentally unfriendly. In situ monitoring of sprayed paint thicknesses would allow reduced paint usage and reduced emissions of volatile organic compounds (VOCs).

The proposed Terahertz QCL based 3D Imaging system will be valuable in characterizing the voids and delaminations in materials used in the Constellation program (e.g. urethane based foams, silica based composites, etc). The high depth resolution enabled by this system will also allow measurement of thin non-conductive polymer layers, such as paints and compositions to verify thicknesses and integrity. Further application include inspection for corrosion damage under paint layers or foams.

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