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\bibitem{Huebers_etal2005} H{\"u}bers HW, Pavlov SG, Semenov AD, Tredicucci A, K{\"o}hler R, Mahler L, et al. Investigation of a 2.5 THz quantum cascade laser as local oscillator. In: Proc. 16$^{th}$ Int. Symp. Space Terahertz Technol. G{\"o}teborg, Sweden; 2005. 18.

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