  Single-cycle optical pulses offer a strong carrier-envelope-offset (CEO)
dependent electric field and the highest peak intensity for a given pulse
energy. Absence of demonstrated GHz single-cycle lasers constrains exploration
of single/sub-cycle dynamics at this repetition rate. By leveraging fiber
soliton effects and suppressing higher-order dispersion, we achieve
single-cycle pulse generation at a 1 GHz repetition rate in an all-fiber
format. The laser produces 7.1 fs (1.1-cycle) pulses with 1.8 W average power,
centered around 1970 nm. Temporal characterization shows 60% of the pulse
energy is concentrated in the pulse center, yielding a peak power of 110 kW.
The seed laser demonstrates a 43 dB signal-to-noise ratio for the CEO
frequency, facilitating comb stabilization and CEO control. Our model, which
matches experimental observations, identifies conditions for achieving
single-cycle duration and predicts scalability to a 2 GHz repetition rate. This
work presents the first GHz single-cycle source. We envision these advances
will drive studies in single/sub-cycle light-matter interaction, spectroscopy,
microscopy, and CEO-sensitive nonlinear optics.
