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\bibitem{Rabanus_etal2009} Rabanus, D., Graf, U. U., Philipp, M., Ricken, O., Stutzki, J., Vowinkel, B., et al. (2009). Phase locking of a 1.5 terahertz quantum cascade laser and use as a local oscillator in a heterodyne HEB receiver. \textit{Optics Express}, \textit{17}(3), 1159--1168.

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