Simultaneous measurement of the 1st and 2nd harmonics of a phase modulated coherent frequency-domain THz spectrometer

We report on a frequency domain terahertz spectrometer with an integrated, fiber-coupled, custom manufactured lithium-niobate optical phase-modulator. We discuss employing optical phase modulation instead of bias modulation for simultaneously measuring the 1^st and 2^nd harmonics of the phase modulated THz radiation. This technique allows the removal of the photomixing interference pattern and does not suffer from the same issues as our previous technique when the 0° and 90° data was recorded sequentially. The system also enjoys the benefits of common mode phase drift rejection.