The influence of laser frequency noise on the stability of a compact cold-atom microwave clock was investigated theoretically in this study. The phase-noisy laser was characterized by a phase-diffusion field. Given the laser linewidth, the analytical expressions of the atomic covariance function and the noise power spectral density of the relative intensity noise transferred from the frequency noise were derived based on stochastic differential equations. Our results provide an effective method to estimate the impact of laser frequency noise on the stability of an integrating sphere cold atomic clock. This method can be extended to other cold atom detection cases.
Optical lattice clocks demonstrate advantages in metrology and frontier physics because of their high stability.Here, we present approaches to enhancing the stability by decoupling the noise related to the short-term and long-term stability.For the short-term stability, we optimize the clock laser by decoupling the frequency noise, and optimize each noise contribution individually until it is below the thermal noise limit.For the long-term stability, we introduce a method to decouple the instability caused by systematic effects.Having identified that the collision frequency shift was the main limiting factor in our systems, we thus optimized the atom number fluctuations in optical lattices.Through targeted optimization, we achieve a synchronous comparison of two clocks with an average stability of 3.2 × 10 -16 = τ p and a long-term stability of 2.4 × 10 -18 at 8000 s.This work provides an analytical framework for enhancing optical clock stability.
We discuss the characterization and specifications of a home-built external cavity diode laser system that is suitable for frequency-domain cold-atom velocimetry and measurements of gravitational acceleration. The laser system is frequency stabilized using a modulation-free technique that lowers the temperature of a magneto-optical trap (MOT) in comparison with a MOT realized using a laser that relies on lock-in spectroscopy. The laser system also incorporates a feedback loop involving an acousto-optic modulator for intensity stabilization, which reduces the effect of low frequency vibrations. By integrating this laser with a home-built, dual-output radio frequency synthesizer, we are able to derive two laser beams with a precisely controlled relative detuning for frequency domain interferometry using laser-cooled atoms. The detuning can range from the mHz to the MHz level with a stability of 10 μHz.
A clock laser based on a 30-cm-long ultrahigh finesse optical cavity was developed to improve the frequency stability of the Sr optical lattice clock at the National Institute of Metrology. Using this clock laser to probe the spin-polarized Sr87 atoms, a Rabi transition linewidth of 1.8 Hz was obtained with 500 ms interrogation time. Two independent digital servos are used to alternatively lock the clock laser to the S01 (mF=+9/2)→P03 (mF=+9/2) transition. The Allan deviation shows that the short-term frequency stability is better than 3.2×10−16 and averages down followed by 1.8×10−15/τ.
Using modulation transfer spectroscopy, we achieve the frequency stabilization of a high-power 3-W blue laser at the wavelength of 420 nm to the
Rb 5S1/2--
6P3/2 transition. The in-loop frequency stability of this laser is $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}/\sqrt{\ensuremath{\tau}}$, reaching $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at 100 s and $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ at 1000 s. An external power feedback loop is established using an acousto-optic modulator, employing the zeroth-order diffracted light for power stabilization, achieving an in-loop power stability of $1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ at 1 s. Moreover, the continuous mode-hop free interval of this high-power laser can simultaneously cover the
5S1/2--
6P3/2 transitions of
85Rb and
87Rb, with successful locking achieved for both isotopes, providing a comprehensive analysis of the
Rb atomic transitions in the blue spectral region. As an application, this 3-W 420-nm laser with excellent power and frequency stabilities is used as a repumping source for diffuse laser cooling of
87Rb atoms, realizing a one-meter-long cold-atom cloud. This paves the way for using blue-light cooling to realize a cold-
Rb-atom active optical clock. In addition, the ultrahigh-stability 420-nm laser also finds applications in various fields such as the Rydberg atom experiment, Bose-Einstein condensation, ultranarrow-bandwidth Faraday atomic filter, and so on.
We present a digital differential detection scheme using a single photodiode to suppress the diode laser frequency noise and improve the short-term stability in atomic clocks based on coherent population trapping (CPT) resonance. One laser light is divided into two beams via modulation of an acousto-optic modulator. One beam induces CPT resonance with circularly polarized light, and the other beam induces only absorption with linearly polarized light. A differential algorithm counteracts common-mode noise from the two beams. A stability of 6.86×10−12 at 100 s and 2.26×10−12 at 1000 s has been demonstrated. The short-term stability is improved by a factor of 2.4 at 100 s. The scheme promises to eliminate laser-related noise and explore potential applications in chip-scale atomic clocks.