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Conference on Precision Electromagnetic Measurements

Analysis of the light shift effect in the /sup 87/Rb frequency standard

F. Levi, Carlo Novero, A. Godone

Published Dec 24, 2002• 0 citations
Publisher

Abstract

The /sup 87/Rb cell frequency standard, optically pumped by a laser diode, is of great interest in the development of high short-term stability oscillators. We report experiments performed on a rubidium cell prototype aiming at evaluate the transfer of the laser noise to the hyperfine clock transition via the light shift effect, which appears to be the main contribution to the observed short term frequency stability in this kind of standard. Pulsed optical pumping will also be examined in order to strongly reduce the light shift effect and to achieve the theoretical limit never approached up to now despite the strong detected microwave signals.

DOI
10.1109/cpem.1996.547073

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The /sup 87/Rb cell frequency standard, optically pumped by a laser diode, is of great interest in the development of high short-term stability oscillators. We report experiments performed on a rubidium cell prototype aiming at evaluate the transfer of the laser noise to the hyperfine clock transition via the light shift effect, which appears to be the main contribution to the observed short term frequency stability in this kind of standard. Pulsed optical pumping will also be examined in order to strongly reduce the light shift effect and to achieve the theoretical limit never approached up to now despite the strong detected microwave signals.
We present theoretical and experimental studies concerning the realization of a rubidium vapor cell frequency standard based on the pulsed optical pumping (POP) technique. This technique is extremely powerful since it avoids in principle the light shift effect and the noise conversions from the laser to the clock signal. The detection of the clock transition in the microwave domain (the so called POP maser) has been extensively investigated at INRIM and a frequency stability of 1.1 × 10-12τ-1/2 has been measured until integration times of 105 s, reaching the level of 5 × 10-15 (drift removed). The optical detection is still under evaluation but this technique may lead in effect to some advantages with respect to the microwave detection, such as a lower or negligible cavity pulling, a lower operation temperature (which implies a lower spin exchange contribution) and a higher signal-to-noise ratio. All these features may lead to a better frequency stability performance than that previously reached. Read More: http://www.worldscientific.com/doi/abs/10.1142/9789812838223_0040
It is experimentally demonstrated that, using diode lasers for optical pumping and RF /spl pi/-pulses for population transfer, nearly all of the ground state Rb atoms in a vapor cell can be concentrated into one of the two M/sub F/ = 0 sublevels. The fractional population difference /spl Delta/ between the two M/sub F/ = 0 levels can be made as high as 1. This should result in a very significant improvement (about a factor of 100) in the S/N over the conventional discharge-lamp-pumped rubidium frequency standards (RFSs). This would lead to a substantial improvement in the short-term stability of the RFS. This is a pulsed population transfer scheme. The microwave interrogation takes place in the absence of strong pumping light. This has the potential for eliminating the light shift and thereby improving the long-term frequency stability of a an RFS.>
The paper presents the realisation of a high-performance vapour-cell atomic frequency standard demonstrator based on laser optical pumping. The development of such devices aims to fully exploit the advantages of using a narrow-band diode laser pump light source instead of the discharge lamps commonly used in Rb gas-cell clocks. These advantages include improved control over the pump light spectrum, higher selectivity of the optical pumping, and increased signal contrast, and thus show the potential for the realisation of Rb atomic clocks with improved short-term frequency stability.
One impediment to achieving the promise of enhanced laser-pumped gas cell clock performance resides with the light-shift effect, a phenomenon whereby diode laser optical frequency fluctuations are mapped onto the atomic clock's output frequency. The alternative to tightly stabilizing the diode laser in order to overcome this problem is to develop means of reducing the actual magnitude of the light-shift. Here, we briefly review two strategies for accomplishing this, and introduce a new one based on optical pumping with laser-induced-fluorescence (LIF). The advantage of LIF optical pumping is that the clock's output frequency is insensitive to laser frequency fluctuations. Though LIF optical pumping creates a dependence of clock frequency on laser intensity variations, it is to be noted that diode lasers have very low intensity noise. Consequently, with regards to the light-shift effect the LIF strategy takes advantage of an inherent diode laser strength.