Stand-off inspection using THz sources is a promising technology for non-destructive evaluation by means of direct imaging as well as by identifying spectroscopic signatures of the material being inspected. This proposal addresses critical developments for realizing this promise. In particular we address the issue of developing THz sources to the point where stand-off detection at ranges of 3-5 m will be practical. Our approach provides a narrow bandwidth and tunable source based on difference frequency generation capable of operating at any frequencies where water absorption is minimized. Furthermore, we specifically design the THz source to significantly increase the average power to the point where standoff detection will be practical. In addition to these source improvements we will perform fundamental measurements of the THz properties of materials using THz sources in our laboratory. These measurements will help address basic scientific questions related to the signature science associated with corrosion, a key to developing the most robust imaging system for non-destructive aircraft inspection applications.
Benefit: A considerable commercial opportunity exists for the development of THz component technology and a corresponding imaging system. The past decade has seen steady progress in the generation and detection of THz radiation (conventionally 0.3-10 THz). Time-domain systems based on femtosecond (fsec) laser sources are capable of broadband THz generation and detection; indeed, these systems are now commercially available from companies such as Teraview and Picometrix, a testament to the maturing nature of the technology. Measurements performed with these imaging systems have demonstrated the utility of THz systems for a variety of applications including homeland security, spectroscopy, and biomedical imaging and diagnostics. Tunable narrow bandwidth and high average power sources such as that proposed here will facilitate novel imaging modalities, atmospheric trace species detection, and other spectroscopic measurements. Of particular importance to the Air Force is THz imaging beneath paint and other barriers for nondestructive evaluation (NDE). Currently, the inspection of aircraft structures and critical components requires that any coating be removed before a visual inspection or other technological methods can be applied to detect cracks, corrosion, and other defects. A cost-saving approach is to develop a technology to look through the coatings. The technology and methodology developed through this proposal will aid in determining the feasibility of appropriate sources and measurement modalities for these important applications.
Keywords: Terahertz, Difference Frequency Generation, Non Destructive Evaluation, Corrosion, Submillimeter, Microwave, Stand Off Imaging
