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Illustrating the principal components of the probing and read-out. The red probe beam sequence forms the Ramsey–Bordé interferometer. A derivative of the fringe signal is produced at the output of the lock-in amplifier, which is then employed in a feedback network tying the 657 nm laser to the clock transition. ECDL, extended cavity diode laser; M, mirror; L, lens; PD photodetector; PDH, Pound–Drever–Hall; PMT, photomultiplier tube; and D, beam separation that affects the fringe resolution.
A coarse representation of the physical overlap between the 423 nm read-out laser, the atomic beam, and the atoms involved in producing the saturation absorption (and Ramsey–Bordé fringes). The relevant energy levels are shown alongside.
Ramsey–Bordé fringe patterns produced by the frequency reference. (a) The vertical scale is proportional to the ground state population and is measured by detecting fluorescence on the de-excitation. First harmonic detection with a lock-in amplifier is used and the signal then integrated. The averaging time on the lock-in amplifier is 300 ms and the scan time across the complete central fringe is 2 s. (b) The fringe signal generated by third harmonic detection.
(a) Schematic of the clock vs CSO comparison. PD, photodetector; , repetition rate; MSF, microstructured fiber; and BS, beam splitter (fiber).
Frequency instability measurements. Diamonds: Ca optical frequency reference vs a CSO. Hollow circles: Ca reference vs a hydrogen maser. Triangles: synthesis chain for the CSO signal. Asterisks: the predicted when the Ca reference is limited by the shot-noise of the 423 nm fluorescence.
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