Séminaire Temps & Fréquence (SYRTE)
Spectral purity transfer for optical atomic clocks
Hector Alvarez Martinez – LNE-SYRTE
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Optical frequency combs (OFCs) and ultra-stable lasers are key elements opening the way to optical clocks at the 10^-18 level. While the quantum projection noise certainly allows levels below 10^-17/t^1/2 for 10^4 atoms probed simultaneously (t being the integration time), the residual frequency noise of the local oscillator probing the narrow atomic resonance leads to the so-called Dick effect, limiting even the best optical clocks to a few 10^-16/t^1/2
In this context, it is very convenient to build ultra-stable lasers at wavelengths where components and spectrally pre-narrowed lasers are available such as the band 1530 nm – 1565 nm (C-band, used for optical infrared telecommunications). Regardless of the laser stabilization method used to this end, it is necessary to transfer the stability of the best SYRTE optical oscillator (master laser) to others target metrological wavelengths such as 698 nm for Sr, 1062 nm for Hg, and 1160 nm for SHB (slave lasers). This is achieved via an OFC operated, in our case, in the “narrow linewidth regime” and by the so called transfer oscillator technique, applicable even when the wavelengths of the master and slave lasers are far apart.
First, I will briefly introduce the frequency chain architecture together with the OFC, the core of the chain, which allows us to link the microwave and optical domains. I will describe how two optical clocks are compared both locally and internationally, and I will comment the services that we can offer to those who want to compare against our system.
Second, I will show an easy and reliable way to deal with one of the two degrees of freedom of the OFC, its carrier envelope offset (CEO) frequency. Our approach presents several benefits while being still compatible with the most advanced optical clocks accuracy results and spectral purity transfer methods.
Finally, I will present a new OFC based scheme transferring 6x10^-16 at one second from a 1542 nm wavelength laser (reference channel for the European fiber link) to a 1062 nm laser (metrological probing of the 199Hg atoms, after frequency quadrupling), a 698 nm laser (probe laser for the 87Sr atoms) and a 1062 nm laser (laser based on the spectral hole burning technique). With a residual noise due to the transfer in the 10^-18 range, I will show the potential and the main advantages of this new approach with respect to others, already done here at SYRTE and other laboratories, and its possible limitations regarding the final residual noise introduce by the transfer itself.