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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Laser intensity noise transfer for pulsed vapor-cell clocks with optical detection

Michele Gozzelino, Salvatore Micalizio, Filippo Levi, Aldo Godone, Claudio Eligio Calosso

Published Jul 1, 2017• 1 citation
Publisher

Abstract

We propose a signal-theory approach to estimate the contribution from laser intensity noise to the short-term stability of compact atomic clocks based on optical detection. We remove the approximation of white intensity noise, which is usually found in literature, and propose a method for a more faithful estimate, given an arbitrary noise spectrum at the input. This is of fundamental importance, since laser intensity noise is currently one of the main limiting factors of vapor-cell clocks.

DOI
10.1109/fcs.2017.8088985

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In this paper we discuss how to improve both short and medium-long term frequency stability of a vapor cell frequency standard working in pulsed regime. We propose in particular a multi-step pumping technique that is able to put almost all the atoms in one of the two clock levels, increasing in this way the signal-to-noise ratio with respect to the usual optical pumping made with a single laser pulse. Moreover, we discuss the phenomena affecting the medium-long term frequency stability and how it is possible to control them, making the overall clock stability compatible with sigmay(tau) =3x10-13tau-frac12for integration times up to tau = 105s.
In today's state of the art compact vapour‐cell atomic clocks relying on the pulsed Ramsey‐type interrogation, optical detection noise is a major limitation to the achievable short‐term stability. In this communication, the influence of the optical detection time on the clock's short‐term stability is investigated and a new analytical expression is developed to precisely predict the stability performance, taking into account the details of the optical detection phase of a Ramsey‐type atomic clock. The theory is in good agreement with the experimental results. It is applied for evaluating the clock's shot‐noise limit.
The optically thick vapors employed in gas-cell atomic clocks provide for very efficient conversion of laser phase noise (PM) to amplitude noise (AM). Here, we discuss the role of PM to AM conversion in determining gas-cell clock performance. First, a brief overview of the PM to AM conversion process will be given, highlighting the fact that laser phase noise creates fluctuations in the atom's optical absorption cross-section which have an exponential influence on the laser's transmitted intensity. Experimental results will then be presented showing that the PM to AM conversion process can increase the relative intensity noise of transmitted laser light by orders of magnitude, and that this can, under certain conditions, cause laser-pumped clock performance to be poorer than lamp-pumped clock performance.
We evaluate the influence of 2 types of laser sources with different spectral profiles on the performance of vapor cell atomic clocks based on lin||lin coherent population trapping (CPT) resonances. We show that a short-term stability of 1-2 x 10(-11) tau(-1/2) may be reached in a compact system using a modulated vertical cavity surface-emitting laser. Here the stability is limited by the detection noise level and can be improved up to a factor of 4 by increasing the lock-in detection frequency to several tens of kilohertz, which is not possible in standard double resonance atomic clocks. We compare these results with CPT prepared under the same experimental conditions, using 2 phase-locked extended cavity diode lasers, with which we predict a challenging short-term stability of 1-3 x 10(-13) tau(-1/2), comparable to the state-of-the-art laser-pumped Rb-clocks.