Confocal THz Laser Microscope
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Confocal THz Laser Microscope Mohammed Adnan Salhi & Ioachim Pupeza & Martin Koch
Received: 22 April 2009 / Accepted: 19 October 2009 / Published online: 17 November 2009 # Springer Science + Business Media, LLC 2009
Abstract We transfer the principle of the optical confocal microscope to a far-field THz imaging system based on an optically pumped gas laser emitting radiation at 2.52 THz. This results in a contrast enhancement. To illustrate the image quality improvement, we show THz images of different objects taken with the suggested scheme and compare them to images taken with other systems. Keywords Terahertz . Far-infrared . Imaging . Confocal microscopy
1 Introduction The recent development of terahertz (THz) technology has been accompanied by a growing interest in THz imaging techniques and their applications. In 1995 Hu and Nuss have demonstrated the first THz imaging system based on optoelectronic time-domain spectroscopy (TDS) [1]. Note that THz TDS is inherently a confocal method, as the photoconductive dipole antennas represent a point source and a point detector, respectively. Many variations of this scheme have been presented since then, including near-field THz imaging [2–4], continuous-wave (CW) THz imaging [5–8], dark-field imaging [9], reciprocal imaging [10, 11], real-time imaging [12–14], polarization imaging [15–17] and M. A. Salhi (*) : M. Koch Institut für Hochfrequenztechnik, Technische-Universität Braunschweig, Schleinitzstraße 22, 38106 Braunschweig, Germany e-mail: [email protected] I. Pupeza Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany I. Pupeza Department für Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany M. Koch Fachbereich Physik, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany
J Infrared Milli Terahz Waves (2010) 31:358–366
359
THz emission imaging [18]. The different imaging schemes are reviewed in [19]. This interest is triggered by numerous applications which are foreseen for THz imaging systems in security [20, 21], pharmacy [22], non-destructive testing [23–26], food inspection [27] and the polymer industry [16, 17, 28]. In this paper we present an improvement of the existing conventional THz far-field imaging systems by applying the principle of the optical confocal microscope (OCM) to a common THz laser-based imaging setup. The employed optically pumped THz gas laser emits radiation at a frequency of 2.52 THz. The OCM, also known as confocal scanning microscope, has been suggested by Marvin Minsky in 1955 and is described in [29]. The idea behind the OCM is to focus the image of a bright point light source to a single sharp point of the sample. The image of this point is subsequently focused again on the detector. In Minsky’s original setup, the point light source is realized as a small pinhole, emitting a circular diffraction pattern with an Airy disk in the center. In the present work we use the coherent properties of the Gaussian THz laser beam and implement
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