8 April 2004 Terahertz heterodyne imager for biomedical applications
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Abstract
Terahertz heterodyne imaging is an established technique that offers the potential for extremely large dynamic range and high signal-to-noise ratio while maintaining fast data acquisition, stable magnitude and phase measurements, reasonable frequency flexibility and mm level penetration in tissue and other materials. The authors have set up an imaging system based around a custom fabricated 2.5 THz planar Schottky diode mixer pair and two optically pumped far IR lasers. One laser is used for the signal beam and supplies as much as 70mW at 2.5 THz. The other laser acts as a local oscillator (LO) source for the two mixers. Line pairs very close to each other (CH3OH and CH2F2) are chosen to provide a workable intermediate frequency output (IF=24 GHz). Broader RF bandwidth is possible with tunable signal sources and wider IF band amplifiers. A novel frequency stabilization scheme has been implemented to track and calibrate the laser power (magnitude and phase) over a sample run. The system uses the second THz mixer, a low frequency (GHz) reference oscillator and a lock-in amplifier to monitor and normalize the two lasers (LO and Signal). Stability of ~0.1 dB and <5 degrees have been achieved with a dynamic range of more than 90dB. The present system scans the sample through the focused beam and measures transmission or reflection at a fixed RF frequency. Applications to date include establishing contrast mechanisms in a range of test and biological materials. The measurement system is described and the merits and demerits discussed. Early results on a variety of samples are presented as well as plans to enhance the performance in the near future.
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Peter H. Siegel, Peter H. Siegel, Robert J. Dengler, Robert J. Dengler, } "Terahertz heterodyne imager for biomedical applications", Proc. SPIE 5354, Terahertz and Gigahertz Electronics and Photonics III, (8 April 2004); doi: 10.1117/12.540027; https://doi.org/10.1117/12.540027
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