Martin Bolduc

Noise-equivalent power characterization of an uncooled microbolometer-based THz imaging camera

A THz camera based on an uncooled microbolometer 160X120 pixel array with nominal pitch of 52 μm has been developed at INO and initial transmission and reflection images showed promise. In the present paper, the characterization of both standard infrared and THz-optimized uncooled microbolometer pixel arrays are presented at both infrared and THz wavelengths. Measurements in the THz region has been performed using non-uniform low-power quantum-cascade laser (QCL) and uniform high-power far-infrared laser (FIR laser) beams at 3 THz and 4.25 and 2.54 THz, respectively. A measurement comparison has been achieved in the infrared using a blackbody radiation. Different methods for noise-equivalent power (NEP) measurements have been investigated. These characterization methods are promising especially for non-uniform laser beams irradiated on pixel arrays. The NEP results obtained from the different methods are in good agreement independent of the method used in the experiments. The results show a high sensitivity of the THz-optimized pixel array in the THz region. Large beam area reflection imaging of obscured materials at 2.54 THz have been performed at video rates of 30 frames per second using the THz-optimized pixel array equipped with a semi-custom fast THz objective, proving that the INO THz camera provides a promising solution for stand-alone imaging systems.

A microbolometer-based THz imager

THz imaging is a very promising field rapidly growing in importance. This expanding field is at its early stage of development but already a large number of applications are foreseen. THz imaging promises to be a key technology in various fields, such as defense & security where it can be used to defeat camouflage. Based on its many years of experience in uncooled bolometers technology, INO has developed, assembled and characterized a prototype THz imager. The cameras 160 × 120 pixel array consists of pixels with a 52 μm pitch that have been optimized for the THz region. Custom camera electronics and an F/1 THz lens barrel complete the imager design. Real-time imaging at video rate of 30 frame/sec has been performed with a 3 THz quantum cascade laser set-up. THz images of numerous object-obscurant combinations are presented, proving the feasibility of video imaging in security screening applications.

Resolution capability comparison of infrared and terahertz imagers

Infrared and terahertz are two imaging technologies that differ fundamentally in numerous aspects. Infrared imaging is an efficient passive technology whereas terahertz technology is an active technology requiring some kind of illumination to be efficient. Whats more, the detectors are also different and yield differences in the fundamental physics when integrated in a complete system. One of these differences lies in the size of the detectors. Infrared detectors are typically larger than the infrared wavelengths whereas terahertz detectors are typically smaller than the wavelength of illumination. This results in different constraints when designing these systems, constraints that are imposed by the resolution capabilities of the system. In the past INO has developed an infrared imaging camera core of 1024×768 pixels and tested some microscanning devices to improve its sampling frequency and ultimately its resolution. INO has also engineered detectors and camera cores specifically designed for active terahertz imaging with smaller dimensions (160×120 pixels). In this paper the evaluation of the resolution capabilities of a terahertz imager at the pixel level is performed. The resolution capabilities for the THz are evaluated in the sub-wavelength range, which is not actually possible in the infrared wavebands. Based on this evaluation, the comparison between the resolution limits of infrared detectors and the terahertz detectors at the pixel level is performed highlighting the differences between the wavebands and their impact on system design.

Video-rate THz imaging using a microbolometer-based camera

A THz 160×120 pixel array camera has been developed at INO. Real-time transmission and reflectance imaging at video rates of 30 frames/s were performed with a low-power 3 THz quantum cascade laser. Various hidden objects were imaged, proving feasibility of real-time THz imaging for security screening applications.

Catadioptric optics for high-resolution terahertz imager

INO has developed infrared camera systems with microscanning capabilities in order to increase image resolution. It has been shown in previous works that the image quality may be improved even if the pixel pitch is smaller than the point spread function. This paper introduces a catadioptric optics system with fully integrated microscan for improved resolution in the THz band. The design, inspired by the INOs HRXCAM infrared camera core and adapted for terahertz wavelengths, includes two mirrors and one refractive element. It has a 11.9 degree full field of view and an effective F-number of 1.07 over a wide spectral range, from 100 μm to 1.5 mm wavelength. This diffraction limited optics is used to provide video rate high quality THz images. A THz camera, with 160 x 120 pixel and 52 μm pitch detector, is combined with the microscan objective to provide a 320 x 240 pixel image with a 26 μm sampling step. Preliminary imaging results using a THz illumination source at 118 μm wavelength are presented. A comparison between standard and microscanned images is also presented.

Introducing sub-wavelength pixel THz camera for the understanding of close pixel-to-wavelength imaging challenges

Conventional guidelines and approximations useful in macro-scale system design can become invalidated when applied to the smaller scales. An illustration of this is when camera pixel size becomes smaller than the diffraction-limited resolution of the incident light. It is sometimes believed that there is no benefit in having a pixel width smaller than the resolving limit defined by the Raleigh criterion, 1.22 λ F/#. Though this rarely occurs in todays imaging technology, terahertz (THz) imaging is one emerging area where the pixel dimensions can be made smaller than the imaging wavelength. With terahertz camera technology, we are able to achieve sub-wavelength pixel sampling pitch, and therefore capable of directly measuring if there are image quality benefits to be derived from sub-wavelength sampling. Interest in terahertz imaging is high due to potential uses in security applications because of the greater penetration depth of terahertz radiation compared to the infrared and the visible. This paper discusses the modification by INO of its infrared MEMS microbolometer detector technology toward a THz imaging platform yielding a sub-wavelength pixel THz camera. Images obtained with this camera are reviewed in this paper. Measurements were also obtained using microscanning to increase sampling resolution. Parameters such as imaging resolution and sampling are addressed. A comparison is also made with results obtained with an 8-12 μm band camera having a pixel pitch close to the diffractionlimit.

Development of MEMS microbolometer detector for THz applications

INO has been actively working on extending its microbolometer technology to THz applications. Several techniques have been developed recently to improve the performance of the microbolometer. This article will present these techniques and discuss some potential applications of INO THz microbolometer.

Case study of concealed weapons detection at stand-off distances using a compact, large field-of-view THz camera

The detection of concealed weapons in crowd situations is a critical need and solutions are being sought after by security agencies at the federal, state and municipal levels. Millimeter waves have been evaluated for these kinds of applications, but the currently available technologies are typically too large and bulky to allow for widespread deployment. Alternatively soft X-rays have been considered but safety issues hinder their acceptance. Terahertz technology is ideally suited for such an application as it has the ability to see through clothing, and offers higher resolution than in the millimeter band, also being more compact. THz photons have lower energy than infrared and do not show the ionizing properties of X-ray radiation. The longer Terahertz waves penetrate deeper into various materials then their visible and infrared counterparts. Though the wavelength is longer it has been shown that high resolution in a small form factor can be obtained in the THz wavebands thanks to the use of small pixel pitch detectors. In this paper, a case study for the use of a compact THz camera for active see-through imaging at stand-off distances is presented. More specifically, the cases of seeing through packages and clothing are analyzed in the perspective of concealed weapons detection. The paper starts with a review of the characteristics of a high resolution THz camera exhibiting small pixel size and large field-of-view. Some laboratory results of concealed object imaging along with details of a concept for live surveillance using a compact see-through imaging system are reviewed.

Thermal Dynamics Effects using Pulse-Shaping Laser Sintering of Printed Silver Inks

In recent years, additive manufacturing has been evolving towards flexible substrates for the fabrication of printable electronic devices and circuits. Generally polymer-based, these emerging substrates suffer from their heat sensitivity and low glass-transition temperatures. As such they require new highly-localized sintering processes to treat the electronic inks without damaging the polymer-based substrate. Laser-based sintering techniques have shown great promises to achieve high-quality sintering locally, while controlling the heat penetration to preserve the polymer substrates integrity. In this report, we explore new optimization pathways for dynamic laser-based sintering of conductive silver inks. Multiple passes of a pulsed laser are first performed while varying pulse train frequencies and pulse energies as an attempt to optimize the properties of the silver inks. Then, time-domain pulse shaping is performed to alter the properties of the conductive inks. Together, these pathways allow for the careful control of the time-domain laser energy distribution in order to achieve the best electronic performances while preserving the substrate’s integrity. Sheet resistance values as low as 0.024Ω/□ are achieved, which is comparable to conventional 1-hour oven annealing, with the processing time dramatically reduced to the milisecond range. These results are supported by finite element modeling of the laser-induced thermal dynamics.

Components, concepts, and technologies for useful video rate THz imaging

The recent arrival of TeraHertz large format focal plane arrays has opened the door for numerous applications not previously available for this exciting technology. Video-rate imaging is now a reality and little or no scanning is required. INO has a proven microbolometer based focal plane array that shows good sensitivity over a large THz bandwidth and videos have been successfully acquired of objects hidden behind barrier materials. While these videos successfully proved the real-time capabilities of the INO imager, the quality of the images required substantial system level improvement to be useful for detection and identification purposes in various applications. To this end, INO has designed and fabricated optical components for its active imaging set-up that allows for superior quality real-time images. This paper discusses the concepts and techniques used to create these components. A comparison of images taken using the same THz camera with and without these innovative techniques is presented. In addition, as stand-off detection and imaging is the ultimate objective, an analysis of the atmospheric attenuation over the THz wavelength region is also discussed.