Many THz systems have very limited portability and mobility due to their large size and sensitivity to vibrations, pressures, and torque loadings. Further, much of the demand for THz technology comes from research, industrial, and military applications where the operator is not expected to have experience in advanced optical systems. For most field applications, especially for defense applications, a mobile, robust, turnkey, miniature, or handheld THz time-domain spectrometer is essential. This technology provides methods and apparatus for detecting variations in THz electromagnetic fields that employ polarization detection devices and controllers to maintain or vary the polarization of modulated signals as desired. The technology enables the apparatus and its optical components to be relative compact and modular.