Terahertz frequency standard based on the photomixing of two lasers stabilized to a common acetylene reference cell

We have developed a terahertz (THz) frequency standard based on photomixing of two 1.5 μm telecom lasers. Both lasers were simultaneously stabilized to two distinct rotational-vibrational transitions of acetylene (¹³C₂H₂) molecules contained in a single gas cell, exploiting common-mode rejection of the long-term laser-frequency fluctuations. The THz frequency standard achieved a fractional frequency instability of 4.5 × 10^-10 at 100 s averaging time, and its absolute frequency was determined to be 288 028 072.4 (0.4) kHz through repeated operations spanning more than a month. As a practical application, the system was employed for the frequency calibration of a commercial Fourier transform infrared (FT-IR) spectrometer. This THz frequency standard exhibited high precision and good reproducibility sufficient for serving as a useful wavelength reference for a wide range of THz applications.