CPT-based Atomic Clocks Research Articles

Dual microwave frequency modulation of the VCSEL injection current for a CPT-based atomic clock.

Previously, we have proposed a method to control the emission spectrum of the vertical-cavity surface-emitting laser (VCSEL) with the synchronized modulation of the injection current at single and doubled frequencies. In this work, the above method is used to improve the metrological characteristics of the coherent population trapping (CPT) resonance in 87Rb. The dual-frequency (DF) modulation reduces the carrier power and suppresses the light shift of the resonance frequency, if it is unattainable with the single-frequency modulation. In addition, a higher resonance contrast is achieved by equalizing the powers of the resonant components of the spectrum.

Protocol for Light-Shift Compensation in a Continuous-Wave Microcell Atomic Clock

Light-shifts are known to be an important limitation to the mid- and long-term fractional frequency stability of different types of atomic clocks. In this article, we demonstrate the experimental implementation of an advanced anti-light shift interrogation protocol onto a continuous-wave (CW) microcell atomic clock based on coherent population trapping (CPT). The method, inspired by the Auto-Balanced Ramsey (ABR) spectroscopy technique demonstrated in pulsed atomic clocks, consists in the extraction of atomic-based information from two successive light-shifted clock frequencies obtained at two different laser power values. Two error signals, computed from the linear combination of signals acquired along a symmetric sequence, are managed in a dual-loop configuration to generate a clock frequency free from light-shift. Using this method, the sensitivity of the clock frequency to both laser power and microwave power variations can be reduced by more than an order of magnitude compared to normal operation. In the present experiment, the consideration of the non-linear light-shift dependence allowed to enhance light-shift mitigation. The implemented technique allows a clear improvement of the clock Allan deviation for time scales higher than 1000 s. This method could be applied in various kinds of atomic clocks such as CPT-based atomic clocks, double-resonance Rb clocks, or cell-stabilized lasers.

Noise suppression in coherent population-trapping atomic clock by differential magneto-optic rotation detection.

We propose and investigate a scheme for differential detection of the magneto-optic rotation (MOR) effect, where a linearly polarized bichromatic laser field is coherent population-trapping (CPT)-resonant with alkali atoms, and discuss the application of this effect to CPT-based atomic clocks. The results of our study indicate that laser noise in a vertical cavity surface-emitting laser-based CPT atomic clock can be effectively suppressed by the proposed scheme. The proposed scheme promises to realize a packaged MOR-CPT atomic clock that has significantly better frequency stability coupled with similar power consumption, volume, and cost when compared with currently available packaged CPT atomic clocks.

Ramsey-CPT spectrum with the Faraday effect and its application to atomic clocks**Project supported by the National Natural Science Foundation of China (Grant Nos. 11304362 and 11204351).

A method that obtains the Ramsey-coherent population trapping (CPT) spectrum with the Faraday effect is investigated. An experiment is implemented to detect the light polarization components generated from the Faraday effect. The experimental results agree with the theoretical calculations based on the Liouville equation. By comparing with the method without using the Faraday effect, the potential of this method for a CPT-based atomic clock is assessed. The results indicate that this method should improve the short-term frequency stability by several times.

Oscillator-free atomic clock using a multimode laser

We developed an atomic clock using two modes from a single extended-cavity diode laser in multimode operation. The two modes are phase locked with reference to a dispersion signal from a coherent population trapping (CPT) resonance of R85b at 3.036 GHz. The design is in principle free from an oscillator and a modulator and it is a significant simplification over a conventional CPT-based atomic clock. Allan deviation of the beat frequency is 1×10−10 at 200 s integration time.

Modulation-induced frequency shifts in a coherent-population-trapping-based atomic clock

We investigate systematic errors associated with a common modulation technique used for phase sensitive detection of a coherent population trapping (CPT) resonance. In particular, we show that modification of the CPT resonance lineshape due to the presence of off-resonant fields leads to frequency shifts which may limit the stability of CPT-based atomic clocks. We also demonstrate that an alternative demodulation technique greatly reduces these effects.