Optical-pumping noise in laser-pumped, all-optical microwave frequency references

Abstract

We demonstrate that optical pumping plays a significant role in determining the noise in certain types of laserpumped vapor-cell microwave frequency standards by changing the way in which the laser's FM noise is converted to AM noise by the optical-absorption profile. When this FM-AM conversion is the dominant noise source, the noise spectrum of the transmitted intensity can be dramatically altered by the optical-pumping process. FM noise at Fourier frequencies larger than the optical-pumping time is converted to AM noise differently from noise at lower Fourier frequencies. This effect can modify the optimum design of vapor-cell frequency references and adds an additional FM-AM-related noise source that cannot be eliminated with laser tuning.

Chip-scale atomic devices

“…For example, the average phase shift for the hyperfine transition of 87 Rb on Paraflint TM is 23 $0.08 rad, from which the wall shift and relaxation linewidth can be calculated from the above equations.…”

“…Vapor cell atomic clocks 6,[26][27][28] are based on the excitation of hyperfine transitions in alkali atoms (usually 87 Rb) confined in a vapor cell; a typical configuration is shown in Fig. 3.…”

“…Each of these states has hyperfine structure with the energy splitting in the MHz-GHz range. It is the ground state hyperfine splitting (6.8 GHz for 87 Rb) that is used in most microwave atomic clocks. Finally, each hyperfine level is split again in the presence of a magnetic field.…”

Electromagnetically induced transparency with noisy lasers

“…For Electromagnetically Induced Transparency (EIT) in atomic and other systems [2], such deleterious noise conversion is of great interest because of the wide application of EIT in atomic clocks [3,4], magnetometers [5], quantum optics [6,7] and quantum communications [8][9][10]. Laser phase-noise-to-intensitynoise conversion in the absence of atomic ground state coherence and its role in atomic-clock instability [1,[11][12][13] and the effect of laser phase-noise on photon-photon correlations in atomic vapor cells [6,7] have been studied previously. However, the role of EIT coherence in noise processes remains to be understood.…”

“…To observe EIT, we employed a Vertical Cavity Surface Emitting Laser (VCSEL) operating on the 87 Rb D 1 line (795 nm) with a linewidth of 100 MHz, chosen for compatibility with real-world CPT atomic clocks [12] and magnetometers. The large phasenoise bandwidth allows us to neglect other noise sources such as laser intensity noise and polarization noise.…”