Active Balance in Free-Space Electro-Optic Detection of Terahertz Waves

A method for increasing the stability of terahertz (THz) time-domain spectrometers utilizing electro-optic balanced detection is presented in this paper. The stability of these systems is sensitive to optical alignment and polarization quality; small changes in temperature, pressure, stress, and strain can all affect SNR. High stability is achieved by precise rotation of the quarter-wave plate (QWP) balance position during the operation of the THz spectrometer. Providing feedback via QWP rotation optimizes the noise floor with minimal modulation to the THz signal in a nonideal optical system. Applying active balance control eliminates constant tuning by the operator between scans, after purging with dry nitrogen, applying vacuum, and during system setup.