Beam Profile Characterization of a 4-Parabolic-Mirror THz Time-Domain Transmission Imaging System

Knowing the beam properties in THz spectroscopy and imaging systems is paramount for accurate extraction of material properties and good image qualities. Utilizing knife-edge and pinhole scanning, we beam profile a commercial U-shape 4-parabolic-mirror transmission imaging system based on integrated Si lens photoconductive switches. The beam is minimally asymmetric at the focal plane with corrected beam waist along the direction parallel to the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i>-field and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i>-field ranging from 2.4 mm to 1.1 mm and from 2.3 mm to 1.0 mm, respectively, in the frequency range 0.5 to 1.2 THz. The experimental cross-polar value is consistently -10 dB approximately. The confocal distance, also known as Rayleigh range, that imposes a limitation in the maximum thickness sample for accurate spectroscopy imaging, is experimentally estimated in the range 0.5 to 1.2 THz to be from 11.3 mm to 6.9 mm along the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i>-plane, and from 13 mm to 4.6 mm along the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i>-plane.