ABSTRACT
It is known that temperature variations and acoustic noise affect ultrastable frequency dissemination along optical fiber. Active stabilization techniques are adopted to compensate for the fiber-induced phase noise. However, despite this compensation, the ultimate link performances are limited by the delay-unsuppressed noise that is related to the propagation delay of the light in the fiber. We demonstrate a post-processing approach which enables us to overcome this limit. We implement a subtraction algorithm between the optical signal delivered at the remote link end and the round-trip signal. In this way, a 6 dB improvement beyond the delay-unsuppressed noise is obtained. We confirm the prediction with experimental data obtained on a 47 km metropolitan fiber link and propose how to extend this method for frequency dissemination.
ABSTRACT
We performed a two-way remote optical phase comparison on optical fiber. Two optical frequency signals were launched in opposite directions in an optical fiber and their phases were simultaneously measured at the other end. In this technique, the fiber noise is passively canceled, and we compared two optical frequencies at the ultimate 10(-21) stability level. The experiment was performed on a 47 km fiber that is part of the metropolitan network for Internet traffic. The technique relies on the synchronous measurement of the optical phases at the two ends of the link, which is here performed by digital electronics. This scheme offers some advantages with respect to active noise cancellation schemes, as the light travels only once in the fiber.
ABSTRACT
In this paper, we propose a new scheme for operating an atomic fountain frequency standard. A sequence of time-spaced balls of atoms, launched over non-overlapping trajectories, makes possible a reduction of the atomic density and consequently the cold collision frequency shift by one order of magnitude without significant reduction of the useful signal and of the overall stability of the clock. The ultimate accuracy of a Cs fountain could be improved below the 10(-15) level currently achieved in operating clocks.
ABSTRACT
The light shift, as observed in the coherent population trapping (CPT) maser, was investigated theoretically and experimentally. It was found that the light shift originates from the various sidebands that are present in the spectrum of the frequency-modulated laser used to observe the CPT phenomenon.
ABSTRACT
The use of coherent population trapping (CPT) for the realization of a Cs coherent microwave emitter without population inversion is described. Preliminary experimental results are reported regarding the radio frequency spectrum of the emitted microwave radiation, the emission profile width, and the transient behavior of the output power. This new approach, based on the coherence properties of the laser radiation, allows the implementation of a microwave frequency standard where the linear light shift is absent and the thermal noise limit for the frequency instability is below 10(-12) for an integration time of 1 s.
ABSTRACT
We have measured the isotopic shifts of the (3)S(3s6s)-(3)P(3s3p) and (3)S(3s7s)-(3)P(3s3p) transitions of Mg I and the values of the hyperfine-structure constant A((3)S(1)) for the 3s6s and 3s7s terms of (25)Mg. The absolute wavelengths have been measured at an accuracy level of +/-3 x 10(-7), mainly limited by the Doppler broadening of the I(2) transitions used as a reference and by the precision of the knowledge of the related wavelengths. The measurements have been performed in an atomic beam rich in metastable atoms by scanning a ring dye laser and observing simultaneously the fluorescence of the beam.
