We present an imaging system designed for use in the terahertz range. As the radiation source a backward-wave oscillator was chosen for its special features such as high output power, good wave-front quality, good stability, and wavelength tunability from 520 to 710 GHz. Detection is achieved with a pyroelectric sensor operated at room temperature. The alignment procedure for the optical elements is described, and several methods to reduce the etalon effect that are inherent in monochromatic sources are discussed. The terahertz spot size in the sample plane is 550 µm (nearly the diffraction limit), and the signal-to-noise ratio is 10, 000:1; other characteristics were also measured and are presented in detail. A number of preliminary applications are also shown that cover various areas: nondestructive real-time testing for plastic tubes and packaging seals; biological terahertz imaging of fresh, frozen, or freeze-dried samples; paraffin-embedded specimens of cancer tissue; and measurement of the absorption coefficient of water by use of a wedge-shaped cell.

© 2004 Optical Society of America

1. See, for example, Eleventh International Conference on Terahertz Electronics, J. Nishizawa, K. Hiromasa, and H. Ito, eds. (IEEE, Piscataway, N.J., 2003).
2. Y. Watanabe, K. Kawase, T. Ikari, H. Ito, Y. Ishikawa, and H. Minamide, “Component spatial pattern analysis of chemicals using terahertz spectroscopic imaging,” Appl. Phys. Lett. 83, 800–802 (2003).
3. K. Kawase, Y. Ogawa, and Y. Watanabe, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549–2554 (2003), http://www.opticsexpress.org.
4. X.-C. Zhang, “Terahertz wave imaging: horizons and hurdles,” Phys. Med. Biol. 47, 3667–3677 (2002).
5. T. S. Hartwick, D. T. Hodges, D. H. Barker, and F. B. Foote, “Far infrared imagery,” Appl. Opt. 15, 1919–1922 (1976).
6. B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20, 1716–1718 (1995).
7. K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3005 (2002).
8. D. M. Mittelman, M. Gupta, R. Neelamani, R. G. Baraniuk, J. V. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B 68, 1085–1094 (1999).
9. T. Bauer, J. S. Kolb, T. Löffler, E. Mohler, H. G. Roskos, and U. C. Pernisz, “Indium-tin-oxide-coated glass as a dichroic mirror for far-infrared electromagnetic radiation,” J. Appl. Phys. 92, 2210–2212 (2002).
10. P. H. Siegel, Submillimeter Wave Advanced Technology, Jet Propulsion Laboratory, California Institute of Technology, Mail Code 168–314, 4800 Oak Grove Drive, Pasadena, Calif. 91109 (personal communication, December 2003).
11. K. Kawase, J. Shikata, and H. Ito, “Terahertz wave parametric source,” J. Phys. D 34, R1–R14 (2001).
12. O. A. Simpson, B. L. Bean, and S. Perkowitz, “Far infrared optical constants of liquid water measured with an optically pumped laser,” J. Opt. Soc. Am. 69, 1723–1726 (1979).
13. D. M. Wieliczka, S. Weng, M. R. Querry, “Wedge shaped cell for highly absorbent liquids: infrared optical constants of water,” Appl. Opt. 28, 1714–1719 (1989).
14. M. R. Querry, D. M. Wieliczka, and D. J. Segelstein, “Water (H₂O),” in Handbook of Optical Constants of Solids II, E. D. Palik, ed. (Academic, San Diego, Calif., 1991), pp. 1059–1077.
15. S. Hadjiloucas, L. S. Karatzas, and J. W. Bowen, “Measurement of leaf water content using terahertz radiation,” IEEE Trans. Microwave Theory Tech. 47, 142–149 (1999).
16. K. J. Siebert, T. Löffler, H. Quast, M. Thomson, T. Bauer, R. Leonhardt, S. Czasch, and H. G. Roskos, “All-optoelectronic continuous wave THz imaging for biomedical applications,” Phys. Med. Biol. 47, 3743–3748 (2002).
17. T. Löffler, T. Bauer, K. J. Siebert, H. G. Roskos, A. Fitzgerald, and S. Czasch, “Terahertz dark-field imaging of biomedical tissue,” Opt. Express 9, 616–621 (2001), http://www.opticsexpress.org.
18. J. Nishizawa, “Exploring the terahertz range,” J. Acoust. Soc. Jpn. 57, 163–169 (2001).
19. P. Knobloch, C. Schildknecht, T. Kleine-Ostmann, M. Koch, S. Hoffmann, M. Hofmann, E. Rehberg, M. Sperling, K. Donhuijsen, G. Hein, and K. Pierz, “Medical THz imaging: an investigation of histo-pathological samples,” Phys. Med. Biol. 47, 3875–3884 (2002).
20. T. Löffler, K. Siebert, S. Czasch, T. Bauer, and H. G. Roskos, “Visualization and classification in biomedical terahertz pulsed imaging,” Phys. Med. Biol. 47, 3847–3852 (2002).

Appl. Opt.

• T. S. Hartwick, D. T. Hodges, D. H. Barker, and F. B. Foote, “Far infrared imagery,” Appl. Opt. 15, 1919–1922 (1976).
• D. M. Wieliczka, S. Weng, M. R. Querry, “Wedge shaped cell for highly absorbent liquids: infrared optical constants of water,” Appl. Opt. 28, 1714–1719 (1989).

Appl. Phys. B

• D. M. Mittelman, M. Gupta, R. Neelamani, R. G. Baraniuk, J. V. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B 68, 1085–1094 (1999).

Appl. Phys. Lett.

• Y. Watanabe, K. Kawase, T. Ikari, H. Ito, Y. Ishikawa, and H. Minamide, “Component spatial pattern analysis of chemicals using terahertz spectroscopic imaging,” Appl. Phys. Lett. 83, 800–802 (2003).
• K. J. Siebert, H. Quast, R. Leonhardt, T. Löffler, M. Thomson, T. Bauer, H. G. Roskos, and S. Czasch, “Continuous-wave all-optoelectronic terahertz imaging,” Appl. Phys. Lett. 80, 3003–3005 (2002).

IEEE Trans. Microwave Theory Tech.

• S. Hadjiloucas, L. S. Karatzas, and J. W. Bowen, “Measurement of leaf water content using terahertz radiation,” IEEE Trans. Microwave Theory Tech. 47, 142–149 (1999).

J. Acoust. Soc. Jpn.

• J. Nishizawa, “Exploring the terahertz range,” J. Acoust. Soc. Jpn. 57, 163–169 (2001).

J. Appl. Phys.

• T. Bauer, J. S. Kolb, T. Löffler, E. Mohler, H. G. Roskos, and U. C. Pernisz, “Indium-tin-oxide-coated glass as a dichroic mirror for far-infrared electromagnetic radiation,” J. Appl. Phys. 92, 2210–2212 (2002).

J. Opt. Soc. Am.

• O. A. Simpson, B. L. Bean, and S. Perkowitz, “Far infrared optical constants of liquid water measured with an optically pumped laser,” J. Opt. Soc. Am. 69, 1723–1726 (1979).

J. Phys. D

• K. Kawase, J. Shikata, and H. Ito, “Terahertz wave parametric source,” J. Phys. D 34, R1–R14 (2001).

Opt. Lett.

• B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett. 20, 1716–1718 (1995).

Phys. Med. Biol.

• X.-C. Zhang, “Terahertz wave imaging: horizons and hurdles,” Phys. Med. Biol. 47, 3667–3677 (2002).
• P. Knobloch, C. Schildknecht, T. Kleine-Ostmann, M. Koch, S. Hoffmann, M. Hofmann, E. Rehberg, M. Sperling, K. Donhuijsen, G. Hein, and K. Pierz, “Medical THz imaging: an investigation of histo-pathological samples,” Phys. Med. Biol. 47, 3875–3884 (2002).
• T. Löffler, K. Siebert, S. Czasch, T. Bauer, and H. G. Roskos, “Visualization and classification in biomedical terahertz pulsed imaging,” Phys. Med. Biol. 47, 3847–3852 (2002).
• K. J. Siebert, T. Löffler, H. Quast, M. Thomson, T. Bauer, R. Leonhardt, S. Czasch, and H. G. Roskos, “All-optoelectronic continuous wave THz imaging for biomedical applications,” Phys. Med. Biol. 47, 3743–3748 (2002).

Other

• T. Löffler, T. Bauer, K. J. Siebert, H. G. Roskos, A. Fitzgerald, and S. Czasch, “Terahertz dark-field imaging of biomedical tissue,” Opt. Express 9, 616–621 (2001), http://www.opticsexpress.org.
• M. R. Querry, D. M. Wieliczka, and D. J. Segelstein, “Water (H₂O),” in Handbook of Optical Constants of Solids II, E. D. Palik, ed. (Academic, San Diego, Calif., 1991), pp. 1059–1077.
• See, for example, Eleventh International Conference on Terahertz Electronics, J. Nishizawa, K. Hiromasa, and H. Ito, eds. (IEEE, Piscataway, N.J., 2003).
• K. Kawase, Y. Ogawa, and Y. Watanabe, “Non-destructive terahertz imaging of illicit drugs using spectral fingerprints,” Opt. Express 11, 2549–2554 (2003), http://www.opticsexpress.org.
• P. H. Siegel, Submillimeter Wave Advanced Technology, Jet Propulsion Laboratory, California Institute of Technology, Mail Code 168–314, 4800 Oak Grove Drive, Pasadena, Calif. 91109 (personal communication, December 2003).

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