Advanced analysis concepts for terahertz time domain imaging B. Pradarutti a, * , G. Mattha ¨us b , S. Riehemann a , G. Notni b , S. Nolte a , A. Tu ¨ nnermann a,b a Fraunhofer Institute for Applied Optics and Precision Engineering (IOF), Albert-Einstein-Strasse 7, 07745 Jena, Germany b Institute of Applied Physics, Friedrich Schiller University, Max-Wien-Platz 1, 07743 Jena, Germany Received 8 February 2007; received in revised form 21 May 2007; accepted 11 July 2007 Abstract Imaging based on ultrashort terahertz (THz) pulses (100–3000 lm) is investigated. The measured pulses are analyzed and the resulting amplitude and time delay information are compared. An algorithm for discrimination of multiple pulses is presented, which can distin- guish several layers inside an object. A new measurement concept is presented, which accelerates the measurement of samples with small optical path differences about two orders of magnitudes. Exemplarily different applications from the field of quality management are shown. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.30.Àd; 07.05.Pj Keywords: THz; Imaging; Time domain analysis; Fs-lasers 1. Introduction THz radiation (30–3000 lm, 0.1–10 THz) has been an unknown gap in the electromagnetic spectrum for a long time due to difficulties in generation and detection. For high-frequency and radio engineering the frequencies have been too high and for optics the wavelengths have been too long. Recent advances in electronics, optoelectronics and optics have made the use of this frequency range applica- ble. While electronics comes from the lower part of the fre- quency spectrum, optics approaches from the high frequency part [1]. The optoelectronics is placed in between and covers typically a range from 0.1 up to 3 THz [2]. On the basis of fs-lasers in optoelectronics it is possible to generate and detect not only continuous wave (cw) but also single cycle ultrashort THz pulses covering more than one order of magnitude in frequency range. Instead of intensity the electric field can be detected time resolved, which makes it possible to determine the amplitude and time delay of measured pulses directly. The development of modern fs-lasers like high power fiber laser amplifiers [3] gives also the possibility for small, compact and indus- trial ready THz table top devices [4–6]. THz radiation has unique features making it interesting for both fundamental research [7,8] and applications e.g. in quality management, medicine, and security. While water is strongly absorptive for THz radiation, plastics and foam are transparent and show distinctive contrast. From the microwave regime it inherits that metal is nearly a perfect reflector. Also many organic substances, explosives, and drugs have characteristic absorption spectra in this fre- quency range [9,10]. For applications in quality management it is often important to monitor the interior of already closed and sealed devices. Because plastic, foam, and paper are trans- parent for THz radiation, this technique is an ideal tool for inspection of devices sealed in those materials. With the help of the ultrashort pulse (and ultrabroadband) THz technique it is even possible to combine imaging with spec- troscopy for example to image drugs in closed envelopes [11]. 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.07.007 * Corresponding author. Tel.: +49 3641 807 237. E-mail address: boris.pradarutti@iof.fraunhofer.de (B. Pradarutti). www.elsevier.com/locate/optcom Optics Communications 279 (2007) 248–254