THz-TDS with gigahertz Yb-based dual-comb lasers: noise analysis and mitigation strategies

• Benjamin Willenberg, Christopher Phillips, Justinas Pupeikis, Sandro Camenzind, Lars Liebermeister, Robert Kohlhaas, Björn Globisch, and Ursula Keller
• received 03/04/2024; accepted 04/26/2024; posted 04/26/2024; Doc. ID 522802
• Abstract: We investigate terahertz time-domain spectroscopy using a low-noise dual frequency comb laser based ona single spatially-multiplexed laser cavity. The laser cavity includes a reflective biprism, which enablesgeneration of a pair of modelocked output pulse trains with slightly different repetition rate and highlycorrelated noise characteristics. These two pulse trains are used, respectively, to generate the THz wavesand detect them by equivalent time sampling. The laser is based on Yb:CALGO, operates at a nominalrepetition rate of 1.18 GHz, and produces 110 mW per comb with 77 fs pulses around 1057 nm. We performTHz measurements with Fe-doped photoconductive antennas, operating these devices with gigahertz 1-µmlasers for the first time and obtain THz signal currents approximately as strong as those from referencemeasurements at 1.55 µm and 80 MHz. We investigate the influence of the laser’s timing noise properties onTHz measurements, showing that the laser’s timing jitter is quantitatively explained by power-dependentshifts in center wavelength. We demonstrate reduction in noise by simple stabilization of the pump power,and show up to 20-dB suppression in noise by the combination of shared pumping and shared cavityarchitecture. The laser’s ultra-low noise properties enable averaging of the THz waveform for repetitionrate differences from 1 kHz to 22 kHz, resulting in a dynamic range of 55-dB when operating at 1 kHz andaveraging for two seconds. We show that the obtained dynamic range is competitive and can be wellexplained by accounting for the measured optical delay range, integration time, as well as the measurementbandwidth dependence of the noise from transimpedance amplification. These results will help enable anew approach to high-resolution THz-TDS enabled by low-noise gigahertz dual-comb lasers.