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An expander system of high stable laser beam for cesium atomic fountain clocks

Jiang Chen, Rui Lin, Dandan Liu, Hui Zhang, Jun Ruan, Junru Shi, Fengxiang Yu, Shougang Zhang

Published Oct 24, 2017• 0 citations
Publisher

Abstract

The quality of the cold atoms sample plays an important role to improve the performance of cesium atomic fountain clocks. The preparation of cold atoms sample has a strict requirement for cooling lasers characteristics such as power stability, polarization, collimation, uniformity and verticality. This work implements a design of an expander system of high stable laser beam for the cold atom preparation. The cooling laser characteristics can be adjusted by the design, and the use of non-magnetic materials to avoid the magnetic field on the impact of cold atom. The cold atom sample with the number of 2×108 and temperature of 5μk was obtained by expander system. The result meets the requirements of the preparation of cold atoms for cesium atomic fountain clocks.

DOI
10.1117/12.2284668

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•Jan 1, 2006•0 citations
The quality of the cold atoms sample plays an important role to improve the performance of cesium atomic fountain clocks. The preparation of cold atoms sample has a strict requirement for cooling lasers characteristics such as power stability, polarization, collimation, uniformity and verticality. This work implements a design of an expander system of high stable laser beam for the cold atom preparation. The cooling laser characteristics can be adjusted by the design, and the use of non-magnetic materials to avoid the magnetic field on the impact of cold atom. The cold atom sample with the number of 2×108 and temperature of 5μk was obtained by expander system. The result meets the requirements of the preparation of cold atoms for cesium atomic fountain clocks.
Cesium atomic fountain clock is the quantum frequency standard with very high accuracy. Research on laser cooling and trapping of cesium atom at Peking University began in 1993. A cesium atomic fountain frequency standard is being developed. A magneto-optical trap with some 108 trapped atoms and an optical molasses with the cold atoms of the temperature of 10/spl mu/K have been achieved previously. The device has been carefully designed and is under construction. It is composed of two main parts: vacuum system and laser system. The article describes the apparatus and presents a brief introduction to some considerations on the atomic clock.
To improve the signal to noise ratio (SNR) and the short-term stability of cesium atomic fountain clocks, the work of two-laser optical pumping is presented theoretically and experimentally. The short-term stability of the NIM6 fountain clock has been improved by preparing more cold atoms in the | F = 4, mF = 0〉 clock state with a shortened cycle time. Two π-polarized laser beams overlapped in the horizontal plane have been applied after launching, one is resonant with | F = 4〉 → | F′ = 4〉 transition and the other is resonant with | F = 3〉 → | F′ = 4〉 transition. With optical pumping, the population accumulated in the | mF = 0〉 clock state is improved from 11% to 63%, and the detection signal is increased by a factor of 4.2, the SNR of the clock transition probability and the short-term stability are also improved accordingly.
The technique of laser cooling of atoms gives an opportunity to improve the performance of atomic clocks by using laser-cooled atoms. The most successful cold atom clock, called the atomic fountain, is now widely used as the primary frequency standard in many labs. The cold atom clock for satellite applications, however, has not been reported so far due to special requirements of space applications. Here, we report the development of an engineering model of a satellite-borne cold atom clock, which satisfied all requirements of in-orbit operation. The core of the clock’s principle is the laser cooling of atoms by diffuse laser lights inside the microwave cavity. The structure of the physics package is presented, and its main parameters are also given. The principle and design of the optical bench are described. The initial test results are presented, and the possible improvements are also discussed.
The realisation of cold and slow atomic beams has opened the way to a series of precision measurements of high scientific interest, as atom interferometry, Bose-Einstein Condensation and atomic fountain clocks. The latter are used since several years as reference clocks, given the high performance that they can reach both in accuracy and stability. The common philosophy in the construction of atomic fountains has been the pulsed technique, where an atoms sample is launched vertically and then falls down under the effect of the gravity. The Observatoire de Neuchâtel had a different approach and has built a fountain clock (FOCS1) operating in a continuous mode. This technique offers two main advantages: the diminution of the undesirable effects due to the atomic density (e.g. collisions between the atoms and cavity pulling) and to the noise of the local oscillator (intermodulation effect). To take full advantage of the continuous fountain approach however, we need to increase the atomic flux. The techniques chosen to reach this goal are an efficient transverse collimation together with more atoms to begin with. Here we report on a study of different transverse collimation techniques performed in a two-dimensional phase stable optical lattice (namely gray molasses and magnetically induced laser cooling) as well as the development and characterisation of a 2D-magneto-optical trap used to load the fountain. Best performances are reached in a 2D+-MOT configuration of such a pre-source, for which we detect an atomic flux 20 times greater than the one measured when the fountain is loaded by a Cs vapour. This gain could improve the atomic clock stability by a factor 6. An even better stability is expected after introducing a pre-cooling stage before performing the transverse collimation. This new configuration is currently under investigation and will be implemented in a second continuous fountain (FOCS2).
We demonstrate the construction of the optical system in the atomic clock-beyond atomic fountain based on 87Rb atom. The optical system includes a high-stability laser system and an optical lattice. The high-stability laser system with the new scheme of frequency locking and shift is introduced in detail, which is an important laser source for laser cooling. The optimized frequency and intensity stability are achieved to 4 × 10–14 τ−1/2 ( τ is the averaging time) and 4 × 10–5 τ−1/2 , respectively, which are highly stable. On the basis of the conventional atomic fountain clock, the optical lattice is specially investigated along the direction of gravity and its characteristics are studied systematically. For the optimized and novel exploration, we predict the achievable stability of 3.6×10−14τ−1/2 and it has the potential to be improved to 3.6×10−15τ−1/2 . The realizability of the construction due to the stabilized laser and optical lattice makes the beyond fountain promising candidate for the next-generation high performance microwave atomic clock.
The realisation of cold and slow atomic beams has opened the way to a series of precision measurements of high scientific interest, as atom interferometry, Bose-Einstein Condensation and atomic fountain clocks. The latter are used since several years as reference clocks, given the high performance that they can reach both in accuracy and stability. The common philosophy in the construction of atomic fountains has been the pulsed technique, where an atoms sample is launched vertically and then falls down under the effect of the gravity. The Observatoire de Neuchâtel had a different approach and has built a fountain clock (FOCS1) operating in a continuous mode. This technique offers two main advantages: the diminution of the undesirable effects due to the atomic density (e.g. collisions between the atoms and cavity pulling) and to the noise of the local oscillator (intermodulation effect). To take full advantage of the continuous fountain approach however, we need to increase the atomic flux. The techniques chosen to reach this goal are an efficient transverse collimation together with more atoms to begin with. Here we report on a study of different transverse collimation techniques performed in a two-dimensional phase stable optical lattice (namely gray molasses and magnetically induced laser cooling) as well as the development and characterisation of a 2D-magneto-optical trap used to load the fountain. Best performances are reached in a 2D+-MOT configuration of such a pre-source, for which we detect an atomic flux 20 times greater than the one measured when the fountain is loaded by a Cs vapour. This gain could improve the atomic clock stability by a factor 6. An even better stability is expected after introducing a pre-cooling stage before performing the transverse collimation. This new configuration is currently under investigation and will be implemented in a second continuous fountain (FOCS2).