Integrated quantum clocks exemplify ultracold-atom-based quantum sensors that rely on lasers as a crucial component. Precise control over the quantum states of ions and atoms used in such devices necessitates lasers with narrow linewidths, high spectral stability, and minimal phase noise. To transfer the absolute spectral characteristics to the cooling and trapping lasers, frequency combs come into play. A reduction of the intrinsic linewidths of frequency combs below a few kHz without need of locking to an optical stabilization cavity would simplify quantum clock experiments significantly. Frequency combs based on mode-locked Er:fiber oscillators are state-of-the-art systems exhibiting several advantages over solid-state lasers like compactness, alignment-free operation and robustness against environmental influences. By employing supercontinuum generation, amplification stages and nonlinear conversion processes, the wavelength range of fiber frequency combs can be extended from 420 to more than 2000 nm. Fiber frequency combs typically have comb lines with an intrinsic optical linewidth in the range of several tens of kilohertz. The broadening of the linewidth is attributed to factors such as pump-induced noise, sensitivity on environmental effects as well as on quantum noise effects. In our recent work we have demonstrated frequency combs exhibiting exceptionally low phase noise resulting in comb linewidths as low as 700 Hz. In this work we employ this technique aiming intrinsically narrow linewidths at wavelengths used in an integrated quantum clock experiment based on Strontium atoms (813 nm, 689 nm).
Our research team has achieved a significant milestone by generating pulses with sub-20-attosecond (as) timing jitter from a 200-MHz all-Polarization-Maintaining (PM) erbium-doped (Er:) Nonlinear Amplifying Loop Mirror (NALM) fiber laser. Accurate measurement of these temporal fluctuations was conducted using the Balanced Optical cross-Correlation (BOC) technique. Through comprehensive investigation, we identified the critical parameters responsible for timing jitter, including dispersion and pump power, and validated their impact. The fine-tuning of the contributing factors allowed us to demonstrate an exceptionally low integrated timing jitter of only 15.59 attoseconds, integrated from 10 kHz to 10 MHz. This accomplishment stands as the lowest value ever documented for any free-running mode-locked fiber lasers that are erbium-doped.
Sources for high frequencies in the THz range are of interest in both precision measurements in spectroscopy as well as high carrier frequencies and bandwidth in next-generation communication. Optically generated THz frequencies allow for broad tunability based on telecom technologies. Optical frequency division via an optical frequency comb has the potential to generate ultralow phase-noise THz sources. We present a system that is capable of generating tunable THz frequencies of up to 10 THz at 0.5 THz/s scan rates, phase-locked to a common comb spectrum providing absolute frequency calibration. We show results of combining the photonic source with an electronic receiving system in the waveguide band WR 2.2 (330-500 GHz).
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