Frequency noise in Terahertz dual-comb spectroscopy using two repetition-frequency stabilized and synchronized femtosecond lasers

Terahertz (THz) dual-comb spectroscopy (THz-DCSS) is a competitive spectral technique due to its high frequency resolution. However, because the pump and detection femtosecond (fs) pulses are from two stabilized fs lasers, the perfect mutual coherence is difficult to reach, which prohibits long time data average and improving frequency resolution. Here, two frequency noise sources, the residual unlocked and transferred ones are investigated for a THz dual comb spectrometer (THz-DCSM). In time domain, the amplitude noise plays a dominate role on the performance of THz-DCSS; the repetition-frequency noises are negligible within Fourier-transform frequency resolution. In frequency domain, the relative synchronization error originating from the instantaneous frequency differences between the two fs lasers is 4 × 10^-13 $(\Delta \text{f}/\text{f}\text{)}$; the relative frequency resolution is ∼ 4 × 10^-10, limited by the radio frequency reference. Our investigations are helpful for improving the performance of THz-DCSS and developing high frequency resolution THz spectroscopy.