In this letter, the authors achieve for the first time the coherent synthesis of few cycle pulses generated from two independent lasers into a nearly single optical cycle pulse with long-term stability. A brief description of the authors' method is as follows: The authors employ two few-cycle laser sources. One is a Kerr lens mode-locked Ti:sapphire laser with a wavelength range of 700-1000 nm. The other is a fiber supercontinuum laser source driven by a stretched pulse, passively mode-locked, erbium fiber laser with a wavelength range of 950-1400 nm, where the fiber laser wavelength below 1400 nm is filtered with a knife edge. To coherently synthesize these two few-cycle pulses, the authors synchronize their sources with attosecond precision. This requires accurate control of timing jitter and carrier envelope phase of the fiber laser by using two novel feedback mechanisms given by a waveguide lithium niobate electro-optic timing shifter and an acousto-optic frequency shifter, in addition to feedback through an intracavity piezo-actuated mirror. All three feedback signals are obtained from either the balanced optical cross-correlator or balanced homodyne phase detector.
Through this attosecond synchronization of two few cycle sources, the authors clearly show the generation of a pulse with 1.1 optical cycles in duration with a wavelength range of 700-1400 nm by retrieving the envelope of the synthesized pulse retrieved using two-dimensional spectral shearing interferometry. The authors also claim that this pulse synchronization can be achieved for at least several minutes even in an uncontrolled laboratory environment. Therefore, as the authors note, the generation of a single-cycle of the carrier wave through coherent synthesis of individual laser sources can possibly open up a new path of studies such as phase sensitive nonlinear optics including optically driven field emission, carrier wave Rabi flopping, and high harmonic generation.
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