Optimization of a 3.6-THz Quantum Cascade Laser for Real-Time Imaging with a Microbolometer Focal Plane Array B. N. Behnken, G. Karunasiri Department of Physics, Naval Postgraduate School, CA (USA) D. R. Chamberlin, P. R. Robrish Agilent Laboratories, CA (USA) J. Faist Institute of Physics, University of Neuchâtel (Switzerland)
Work funded by the US Air Force Office of Scientific Research (AFOSR)
Overview Research goals Sensor: 160×120 pixel microbolometer camera Source: 3.6-THz quantum cascade laser Operating conditions / issues Results Summary
Research Goals To identify/develop a THz imaging system that is:
Portable and uncooled Capable of real-time operation under low-power (< 1 mW) THz illumination
Sources (L→R): OE Magazine 2/2005; Luukanen et al (IEEE MWCL 2006 ); Sheffield Hallam University
Detection System Infrared Solutions™ 160×120 pixel IR-160 Microbolometer Camera (τ=10 ms, 50×50 µm2 pixel area)
Quantum Cascade Laser (QCL) Coherent, narrowband, mW-scale THz radiation Fabricated via MBE for emission at 3.6 THz
14-µm MQW active region consisting of 120 GaAs / AlGaAs periods N η h ν ( I − I th ) P= 2e
Optical Configuration • Mounted on copper carrier for improved thermal conduction • Cooled using closed-cycle refrigeration
• QCL mounted in 10 K cold head • 2 gold-plated parabolic reflectors • #1: f/1 w/ f = 51 mm • #2: f/2 w/ f = 102 mm • Imaged objects placed between OAPs
I-V / Power Characteristics (T=10K, D=15%, 300 kHz PRF)
Single Frame Imaging – Steel/plastic
Digital photo: utility blade & plastic tape
50-frame composite image
Single-frame THz image (300 KHz PRF)
50-frame image + MATLAB® processing
Extended Real-Time Imaging (D=20%, I=1.9A)
Utility blade & opaque plastic tape
Mylar® wrapped in plastic tape
Utility blade & common bond paper
Hex wrench & tape in plastic glove
Extended Imaging (GBP security features)
£5 watermark under fluorescent light
£5 watermark w/ THz imaging
£5 security thread & hologram
£5 thread & hologram w/ THz imaging
Temperature Effects QCL suffers from rapid Joule heating at higher bias and/or higher duty cycles
Thermal population of quantized energy levels in QCL Lasing ceases for D>20%
Summary Successfully achieved imaging of metallic and nonmetallic objects using 3.6-THz radiation and an uncooled microbolometer camera Many technical issues would be eliminated with next-generation THz source Future Work
Investigate THz imaging capabilities using more powerful sources NPS bringing FEL on-line mid-2008
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