lidar/ladar systems ,
optical science & engineering ,
synthetic/robotic vision systems ,
terahertz imaging ,
space systems engineering ,
systems engineering, integration & test (SEIT)
We report the use of a 160×120 pixel microbolometer camera, under illumination by a milliwatt-scale 3.6 THz quantum
cascade laser, for real-time imaging of materials which are exclusively nonmetallic in character. By minimizing
diffraction effects suffered by the camera system and operating the laser at bias currents approaching saturation values,
an imaging scheme was developed in which overlapping samples of nonmetallic materials can be imaged with high
fidelity and long persistence times. Furthermore, an examination of various security features embedded within domestic
and foreign currency notes suggests that this imaging scheme could serve a future role in detection of assorted
counterfeiting practices.
Real-time imaging in the terahertz (THz) spectral range was achieved using a 3.6-THz quantum cascade laser (QCL) and
an uncooled, 160×120 pixel microbolometer camera fitted with a picarin lens. Noise equivalent temperature difference
of the camera in the 1-5 THz frequency range was calculated to be at least 3 K, confirming the need for external THz
illumination when imaging in this frequency regime. After evaluating the effects of various operating parameters on
laser performance, the QCL found to perform optimally at 1.9 A in pulsed mode with a 300 kHz repetition rate and
10-20% duty cycle; average output power was approximately 1 mW. Under this scheme, a series of metallic objects
were imaged while wrapped in various obscurants. Single-frame and extended video recordings demonstrate strong
contrast between metallic materials and those of plastic, cloth, and paper - supporting the viability of this imaging
technology in security screening applications. Thermal effects arising from Joule heating of the laser were found to be
the dominant issue affecting output power and image quality; these effects were mitigated by limiting laser pulse widths
to 670 ns and operating the system under closed-cycle refrigeration at a temperature of 10 K.
The THz wavelengths cover the frequency range of 0.1-10 THz or 30-3000 &mgr;m wavelength band. Currently,
detection of THz radiation is carried out using either antenna-coupled semiconductor detectors or superconducting
bolometers. Imaging of objects using these detection schemes requires complex scanning mechanisms which limits
the applications involving real time imaging. For imaging applications it is desirable to employ focal plane arrays
(FPAs) which leads to more compact systems. The FPAs based on photon detectors commonly used in infrared
require cooling which becomes stringent as the detection extends to THz wavelengths. On the other hand,
microbolometer FPAs using thermal detectors based on temperature change due to infrared absorption have a broad
wavelength response and can be operated at room temperature. The advances of microbolometer technology allow
real time imaging in the 7-13 &mgr;m wavelength range with relatively high sensitivity. However, their ability to detect
THz radiation is relatively unknown. In this paper, imaging of a 3.4 THz (88 &mgr;m) laser beam using an uncooled
microbolometer camera is described.
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