Description:
TECHNOLOGY AREA(S): Materials
OBJECTIVE: Design, construct, and deliver an imager operating in the 1-5 THz region with a frequency tunable source, a high dynamic range heterodyne receiver, and wavelength-scale spatial resolution.
DESCRIPTION: The Army has a need for high spatial resolution non-destructive evaluation (NDE) of non-conductive materials that cannot be effectively imaged with ultrasound or x-ray technology [1-3]. The use of terahertz frequencies (0.3 THz to 10 THz) for NDE is desirable because it allows non-contact, operator-safe, high-resolution imaging of materials that would otherwise be opaque to visible and infrared frequencies: polymers, ceramics, semiconductors and electrical insulators. While there are many suppliers of time domain terahertz NDE imagers, these systems are relatively complex due to the optical down conversion from infrared to terahertz frequencies. The inefficient down conversion process is ameliorated by coherent detection resulting in peak signal to noise ratios of 60 dB. While these systems produce pulses with frequency content from 50 GHz to 3 THz, the lossy samples act as low pass filters effectively limiting pulses to < 500 GHz of frequency content, which reduces spatial resolution. As an alternative, high-power far-infrared gas lasers, which produce ~50 mW of average power at 2.5 THz, have been demonstrated in heterodyne imaging using Schottky diode detectors [4]. Using a source laser and a second, local-oscillator laser resulted in signal to noise ratios of 110 dB. The drawbacks to this system are the cost, the complexity of the optical alignment, and the constraint to operate at discrete frequencies of the lasing gas. A promising alternative approach to terahertz imaging involves the use of terahertz Quantum-Cascade Lasers (QCL), which may be combined with a Schottky diode detector for heterodyne imaging. For heterodyne imaging, two semiconductor QCLs, which have demonstrated power levels of 10's of mW [5-7], are required to emit at slightly offset frequencies, with one serving as local oscillator (LO) and the other as the Signal. The Signal and LO are combined in a reference detector and offset frequency locked. In a separate beam path, the Signal is passed through an object, and then is combined with the LO on a second Schottky detector. Further down-conversion of the intermediate frequency (IF) signal allows lock-in detection, amplification, and recovery of the phase and magnitude of the reference and transmission/reflection through the object. Because of the dual requirements for high dynamic range and wavelength-scale spatial resolution, the focused Signal may be raster scanned through the object quickly, with the objective of rendering a near video frame rate scene (30 frames per sec (fps)) that captures the imagery of the target object in real time. Cryogenic operation of the QCLs is acceptable, preferably if cooled by a closed cycle system not requiring the supply of external cryogens.
PHASE I: Design a heterodyne terahertz imager with high dynamic range (> 90 dB) frequency tunable in the 1-5 THz region with wavelength-scale spatial resolution and capable of near video frame rate operation (30 fps). The source need not span the entire spectral region, but it must be frequency tunable. The design must specify the source, detector, and image acquisition technologies, the spectral tuning range, the anticipated dynamic range, the imager's field of view, the spatial resolution, and the expected frame rate. The ideal imager will operate in both transmission and reflection modes.
PHASE II: Construct, characterize, and optimize the performance of the heterodyne terahertz imager designed in Phase I, exhibiting high dynamic range (> 90 dB) frequency tunability in the 1-5 THz region with wavelength-scale spatial resolution and capable of near video frame rate operation (30 fps). The complete, proof-of-concept imager will be delivered at the end of Phase II along with a working graphical user interface for displaying, manipulating, and enhancing the image.
PHASE III: Advance the technology readiness level of the proof-of-concept delivered in Phase II to an affordable, packaged, marketable, high resolution imager that may be used by a broad commercial market for non-destructive testing of non-conducting objects. In addition, frequency tunability and a sensitive heterodyne receiver will allow the development of depth-resolving three-dimensional imagers using frequency modulation continuous wave (FMCW) radar techniques.
REFERENCES:
"Advanced Photonix Awarded $1.4 Million Contract for Handheld Terahertz Scanner," (Advanced Photonix, 2015), http://www.prnewswire.com/news-releases/advanced-photonix-awarded-14-million-contract-for-handheld-terahertz-scanner-300021296.html.
N. Palka, and D. Miedzinska, "Detailed non-destructive evaluation of UHMWPE composites in the terahertz range," Optical and Quantum Electronics 46, 515-525 (2014).
C.-P. T. Chiou, F. J. Margetan, D. J. Barnard, D. K. Hsu, T. C. Jensen, and D. J. Eisenmann, "Nondestructive characterization of UHMWPE armor materials," (2011).
P. Siegel, and R. Dengler, "Terahertz Heterodyne Imaging Part II: Instruments," International Journal of Infrared and Millimeter Waves 27, 631-655 (2006).
B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno, "High-power terahertz quantum-cascade lasers," Electronics Letters 42, 89 - 91 (2006)
A. W. M. Lee, Q. Qin, S. Kumar, B. S. Williams, Q. Hu, and J. L. Reno, "High-power and high-temperature THz quantum-cascade lasers based on lens-coupled metal-metal waveguides," Optics Letters 32, 2840 - 2842 (2007).
M. Wienold et al., Real-time terahertz imaging through self-mixing in a quantum-cascade laser. Appl. Phys. Lett. 109, 011102 (2016
KEYWORDS: Terahertz Imaging, Heterodyne Receiver, Quantum Cascade Laser
