Terahertz Quantum Cascade Laser Based 3D Imaging

LongWave Photonics proposes a terahertz quantum-cascade laser based swept-source optical coherence tomography (THz SS-OCT) system for single-sided, 3D, nondestructive evaluation (NDE) of non-conductive materials. The THz SS-OCT system uses a frequency tunable QCL array to generate an interferometric signal between a reference mirror, and a sample. An algorithm is used to transform this signal into depth information of the interfaces within the sample. Phase I demonstrated the feasibility of measuring the interfaces of a dielectric on metal sample. In Phase II, we propose to demonstrate a complete scanning system for 3D imaging by upgrading the optics and mechanics. Improvements in the power levels and frequency bandwidth of the QCL source are expected to greatly improve the depth resolution and signal to noise ratio of the system. The milliwatt power levels of the QCL are expected to result in fast scan speeds. Operation of the SS-OCT system is expected to be relatively simple as the QCL is an electrically pumped, solid state source of terahertz radiation, capable of operation in a compact, high reliability crycooler as demonstrated in Phase I.

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 THz SS-OCT system will be useful for characterizing the voids and delaminations in materials used in space flight (e.g. urethane based foams, silica based materials, 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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