mm wave and THz imaging using very inexpensive neon-indicator lamp detector focal-plane arrays

D. Rozban1a, b, A. Levanona, b, A. Akrama, b, A. Abramovicha, N. S. Kopeikab, H. Josepha, b, Y. Yitzthakyb, A. Belenkyb and O. Yadid –Pechtb

aDepartment of Electrical and Electronic Engineering, Ariel University Center of Samaria, Ariel, Israel

bDepartment of Electro-Optical Engineering, Ben Gurion University of the Negev, Beer-Sheva, Israel

ABSTRACT

Development of focal plane arrays (FPA) for mm wavelength and THz radiation is presented in this paper. The FPA is based upon inexpensive neon indicator lamp Glow Discharge Detectors (GDDs) that serve as pixels in the FPA. It was shown in previous investigations that inexpensive neon indicator lamps GDDs are quite sensitive to mm wavelength and THz radiation. The diameter of GDD lamps are typically 3-6 mm and thus the FPA can be diffraction limited. Development of an FPA using such devices as detectors is advantageous since the costs of such a lamp is around 30-50 cents per lamp, and it is a room temperature detector sufficiently fast for video frame rates. Recently a new 8X8 GDD FPA VLSI board was designed, constructed, and experimentally tested. First THz images as well as DSP methods using this GDD FPA are demonstrated. Super resolution was achieved by moving the 8X8 pixel board appropriately in the image plane so that 32X32 pixel images are also obtained and shown here, with much improved image quality because of much reduced pixelization distortion. Keywords: Far infrared; terahertz; Terahertz imaging; Detectors; Plasmas

1. INTRODUCTION Imaging systems in the electromagnetic spectrum between 100 GHz and 10 THz are required for applications in

medicine, communications, homeland security, and space technology. This is because there is no known ionization hazard for biological tissue, and atmospheric attenuation of terahertz (THz) radiation is low compared to that of infrared and optical rays. The lack of inexpensive room temperature detectors and FPAs in this spectral region makes it difficult to develop detection and imaging systems, especially real time ones.

A candidate for FPA pixel is miniature neon indicator lamp called Glow Discharge Detector (GDD) which was tested experimentally and found to be a very good THz detector and pixel in THz FPA [1]. Later a different GDD device was found to be suitable for THz imaging by others [2]. The NEP of the GDD was found to be on the order of 10-9 W/Hz1/2, and rise time is on the order of a microsecond and less. The detection mechanism of the GDD involves both enhanced ionization [3-8] and enhanced diffusion current [3, 4, 7, 9-11] caused by the incident terahertz wave. The former increases lamp current, while the latter decreases it. The dominant detection mechanism in such devices was found to be the enhanced ionization process leading to increase of lamp current. Detection mechanism effects have been studied in such devices and, indeed, incident THz electric field polarity has a noticeable effect on GDD responsivity [10] according to these opposing effects. In general, best response is obtained when the enhanced ionization is dominant. The lamp is ignited when internal dc bias breaks down the gas. As dc bias voltage may be 60-100 V and dc bias current may be several mA, the bias dc electric field in the lamps is quite high, especially since electrode separation is only about a millimeter or less. Light is emitted from the lamp because the high ionization and excitation collision rates between high kinetic energy free electrons and neutral gas atoms give rise to subsequent recombination and de-excitation. The incident THz electric field serves to increase free electron kinetic energy incrementally. However, this small increase in electron energy is sufficient to convert some inelastic collisions into ionization collisions, and some inelastic collisions into excitation collisions. In the latter case, ionization takes place with subsequent collisions with neutral atoms. In both

Terahertz Emitters, Receivers, and Applications II, edited by Manijeh Razeghi, Nicolas Péré-Laperne, Henry O. Everitt, John M. Zavada, Tariq Manzur, Proc. of SPIE Vol. 8119,

81190I · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.892571

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cases, the GDmanner are ththus amplifyiwhich the thibe on the ordpower [1, 8].desirable to mis then used t

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Figure 2: A photo of 8X8 FPA with VLSI board.

2. EXPERIMENTAL SET-UP A 100 GHz source was used to illuminate the object in this experimental set-up. The 100 GHz source is based on

GaAs multipliers of Virginia Diodes, Inc. (Charlottesville, VA, USA) that multiply a low frequency source of 12.5 GHz 8 times to 100 GHz [13-15]. The 12.5 GHz signal was generated by an RF synthesizer and it was modulated by a 100 kHz reference signal from the VLSI board of the FPA. The 100 GHz radiation was coupled out to free space by waveguide horn antenna. The quasi optical design of the experimental set-up is shown in Fig. 3. This experimental set up is different from the one used in our previous work [16]. The difference is the additional diagonal mirror that enables us to locate an object horizontally rather than vertically, thus simplifying the holding of the object. Furthermore, it enables to locate powders and liquids in a very flexible manner. Detailed description of the quasi-optics is given in [16].

Figure 3: Experimental set-up of the THz imaging system including the new quasi optical design.

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100 GHz source

Imaging mirror

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levels values and it is seen especially in

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Figure 7: Depixel image o(C); The sam

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However, when the ruler was merely wet without such a long path through water for the 100 GHz radiation, it was imagable.

4. CONCLUSIONS THz imaging using the 8X8 GDD FPA was demonstrated in this work. This proof of concept suggests that use of very inexpensive miniature gas discharge indicator lamps as pixels can greatly reduce the cost of millimeter wave and THz imaging systems while still obtaining good image quality. The experimental results of the “F” shape images are with excellent agreement to simulation results of Fig. 4. The quasi optical design and components prove themselves and minimize the diffraction effects in the images, as can be viewed in Figs. 6,7 and 8. It was shown in fig. 8 that 5 mm of water blocked the image almost completely. The quality of the 32X32 pixel images obtained using super resolution method improves significantly THz image of relatively small objects as was shown in Fig. 7. Although it has been shown recently that plasmas can be used for mm wave imaging by recording the spatial and temporal variations in glow intensity as in the positive column for example [17], the electronic technique shown here seems to be much more sensitive and simple while still retaining similar speed of response.

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