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1 May 2007 LiNbO3 optical modulator for MMW sensing and imaging
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We present several novel technologies for sensing millimeter-wave (mmW) radiation for imaging and spectroscopy based on photonic devices. Along these lines, in our high-sensitivity millimeter-wave (mmW) imaging system, which is based on optical upconversion, the power of mmW radiation is transferred to the sidebands on an optical carrier via an electro-optic (EO) modulator fed by a broadband horn antenna. The detection is realized by measuring the transferred optical power of the sidebands. The sensitivity of this detection system is primarily controlled by the conversion efficiency of the EO modulator at the desired mmW frequency (e.g. 95GHz). Thus, modulators are required that exhibit an ultra-broad bandwidth and small drive voltage. In this paper, we present the design, fabrication, and characteristics of LiNbO3 traveling-wave modulator for the mmW detection system. In a traveling-wave modulator, the bandwidth is limited by the mismatch between electrical and optical propagation constants. We have developed several techniques to finely tune the propagation constant of the mmWs in the modulator and have thereby eliminated this mismatch. Further bandwidth limitations for the modulator arise from losses in the electrode conductor, the substrate and buffer layer dielectrics, and coupling between the traveling-wave mode and the substrate modes. Modulator structures are described to reduce those losses without increasing the device driving voltage. The bandwidth and conversion limits of these structures are also discussed. The mmW detection pixels using the fabricated modulators were assembled, characterized, and analyzed. A high-sensitivity W-band detection system with a low noise-equivalent temperature difference (NETD) has been demonstrated. In addition, we present ongoing work to improve coupling millimeter-wave energy to the modulator at the W-band using techniques viable for packaged devices.
© (2007) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
C. J. Huang, C. A. Schuetz, R. Shireen, S. Shi, and D. W. Prather "LiNbO3 optical modulator for MMW sensing and imaging", Proc. SPIE 6548, Passive Millimeter-Wave Imaging Technology X, 65480I (1 May 2007);

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