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An injection-seeded THz-wave parametric generator (is-TPG) uses a near-infrared (NIR) pump beam and seed beam input to the MgO:LiNbO3 crystal. In contrast, the THz parametric detector uses a THz wave as the seed beam instead of a NIR beam. In the detection configuration, when the pump beam and THz wave are input into the crystal, the THz wave is upconverted to the NIR idler beam by parametric processes in the crystal that are measured using a NIR detector. This THz generation and detection scheme has allowed us to develop a high dynamic range THz wave spectroscopic system that can be used for spectroscopic imaging of saccharides hidden in thick envelopes. Recently, we have improved the sensitivity of the THz parametric detector drastically using a multistage configuration in order to suppress the spontaneous THz emission and enhance the gain. In our new system, the THz parametric detectors were divided into two parts, i.e., for the up-conversion (pre-amplifier) and for the main amplifier. THz waves were upconverted to a NIR idler beam in the first part of the set-up. An iris positioned behind the upconverted region passed the idler beam and blocked the broadband spontaneous THz emission. Thus, in the amplification region, only the desired idler beam was clearly amplified. The amplified detection idler beam was then measured by a NIR beam profiler.
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