Asynchronous optical sampling with GHz repetition rate femtosecond lasers for high precision terahertz spectroscopy

We report a terahertz time-domain spectrometer with 6 THz spectral coverage and 1 GHz resolution which is based on high-speed asynchronous optical sampling. High-speed asynchronous optical sampling is based on two femtosecond lasers with approximately 1 GHz repetition rate. The two lasers are stabilized in their repetition rate to an off-set frequency of approximately 2 kHz. The time delay between a pump pulse exciting a photoconductive THz emitter and the probe pulses is hence scanned at 2 kHz scan rate without employing a mechanical delay stage. The timing jitter between the pump and the probe pulses is reduced to 40 fs over the full scan range giving a high time-resolution. We demonstrate the capabilities of this system for gas spectroscopy and the characterization of frequency selective surfaces with characteristic frequencies in the THz range.