In this work, we report the homogeneous spectral spanning of terahertz QCL frequency comb characterized by a kHz level frequency stability. With microwave modulation, the terahertz comb can span over 300 GHz continuously which is 8% of the central frequency of the laser. Employing the homogeneous spectra, we successfully demonstrate the spectroscopic application in GaAs etalon transmission measurement and ammonia gas identification. For the fast detection, we show that the terahertz intersubband quantum well photodetectors are able to respond fast modulated terahertz light by optimizing the chip size and microwave transmission line. The terahertz quant well photodetector mixer can be also used for imaging by converting the terahertz frequency into microwave regime.
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