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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

New method for enhancement of contrast of coherent population trapping resonance in Rb vapour

Sergey Khripunov, Sergey M Kobtsev, Daba A. Radnatarov, Ivan Popkov, Valeriy Andryushkov, Tatiana Steschenko, Vladimir Lunin, Yurii Zarudnev

Published Feb 20, 2017• 1 citation
Publisher

Abstract

The present work presents a new method for enhancement of contrast of coherent population trapping resonance in Rb vapour based on feedback and fast digital processing of the error signal in the feedback loop. In the proposed method, when the frequency difference between the pump field components is detuned from the resonance of coherent population trapping, a linear combination of two measured values, — pump field power prior and after passing through the cell, — is stabilised. This parameter combination is stabilised through adjustment of the pump radiation power with an electrooptical amplitude modulator. The studied method is shown to improve CPT resonance contrast by more than two orders of magnitude, while also improving the signal-to-noise ratio more than two-fold. The possibilities and limitations of the proposed method for enhancement of CPT resonance contrast are analysed.

DOI
10.1117/12.2251522

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The present work presents a new method for enhancement of contrast of coherent population trapping resonance in Rb vapour based on feedback and fast digital processing of the error signal in the feedback loop. In the proposed method, when the frequency difference between the pump field components is detuned from the resonance of coherent population trapping, a linear combination of two measured values, — pump field power prior and after passing through the cell, — is stabilised. This parameter combination is stabilised through adjustment of the pump radiation power with an electrooptical amplitude modulator. The studied method is shown to improve CPT resonance contrast by more than two orders of magnitude, while also improving the signal-to-noise ratio more than two-fold. The possibilities and limitations of the proposed method for enhancement of CPT resonance contrast are analysed.
This work reports on the study of a new approach to achievement of high-contrast resonant signals from coherent population trapping (CPT) resonances in 87 Rb vapour based on feedback control and real-time digital processing of several measured parameters. This method consists in stabilisation of the value of a function depending on several system parameters measured as the frequency difference of the bichromatic pump radiation is scanned through adjustment of the pumping radiation power with a feedback loop. The present work made use of two such parameters: the pumping radiation power incident on and exiting from the optical cell. Exploration of the proposed method has shown that stabilisation of a linear combination of these two parameters results in a resonant peak whose contrast exceeds that of regular CPT resonance by more than two orders of magnitude at relatively slow CPT resonance scan rates, (scanning frequency of the frequency difference of the bichromatic field ∼1 Hz). When dynamically exciting the CPT resonance (the scan frequency of the frequency difference of the bichromatic field equal to 2 kHz), the resonant peak contrast was enhanced by over an order of magnitude.
This work reports on possibilities of contrast enhancement of dynamically excited coherent population trapping (CPT) resonance in 87Rb vapour arising from application of feedback methods. Controlling the bichromatic pump radiation power through a feedback loop that stabilises Rb atom luminescence when scanning the frequency difference of the bichromatic pump radiation resulted in a more than an order-or-magnitude improvement in the amplitude of the CPT resonance at scanning frequencies over 100 Hz. It is established that the excursion of the pump radiation power controlled by the feedback loop under dynamic excitation is by an order of magnitude smaller than that under quasistationary excitation at scan frequencies < 1 Hz.
We present the results of an experimental study of Coherent Population Trapping (CPT) in potassium, obtained by means of modulation of laser light amplitude with kHz frequency. The radiation from an external cavity diode laser, matching the D1 line of K, is modulated by an acousto-optical modulator. In the cell containing buffer gas, the CPT resonance width is reduced more than three orders of magnitude as compared to the cell containing pure potassium vapor. In K this resonance narrowing occurs with high resonance contrast; such behavior is not observed in buffered cells containing Rb or Cs, where the optical pumping to the non-interacting with the light ground level is very effective and depletes the population of the working ground Zeeman sublevels. The narrow CPT resonance of reduced fluorescence transforms to the one of enhanced fluorescence with the cell temperature rising. The transformed resonance exhibits higher contrast and lower width than those of the reduced fluorescence resonance. Hence, beside its scientific importance the resonance sign reversal can be used for the improvement of the CPT resonance parameters.
We present the first experimental observation of Coherent Population Trapping (CPT) in Potassium, obtained with kHz-frequency modulation of the laser light amplitude. It is performed by acousto-optical amplitude modulation of the radiation from an external cavity diode laser, matching the D1 line of K. The CPT resonances are detected both through K absorption and fluorescence. The resonances are studied in three kinds of K cells: i) pure-evacuated, ii) polydimethylsiloxane (PDMS)-coated-evacuated and iii) Ne-gas buffered. In all cases CPT-resonance narrowing with cell temperature is observed. In the pure-evacuated cell we registered the lowest contrast and the highest width of the resonance, while in buffered/coated cells a strong enhancement of the CPT resonance contrast up to 15% is observed. This behavior is the opposite to the one exhibited by Cs and Rb. The observed contrast enhancement in K is accompanied by more than two orders of magnitude reduction of the resonance width. The results here presented prove the advantage of using Potassium in CPT-based applications.
We report the results of experimental investigation of the Raman – Ramsey method of pulsed excitation of coherent population trapping (CPT) resonances in a rubidium vapour cell filled with a mixture of Ar and Ne buffer gases. For the development of compact quantum microwave frequency standards, this method is an alternative to the pulsed optical pumping method recently implemented using the same rubidium cell. Both methods have advantages over the traditional double radio-optical resonance technique, the main of which is the substantial suppression of the light shift of the clock transition frequency, since the evolution of the coherence of atomic states forming the clock transition occurs in the absence of laser radiation in the cell. The narrow (110 – 240 Hz) Raman – Ramsey resonances are obtained using the scheme of pulsed excitation of CPT resonances on the D 1 lines of the 87 Rb atom with the same linear (lin||lin) polarisations of the bichromatic laser radiation components. The process of optimising the central fringe linewidth, its contrast, and the linewidth-to-contrast ratio is described. The magnetic dependence of the Raman – Ramsey line central fringe linewidth is experimentally investigated.
We propose a novel way of radiation wavelength stabilisation in atomic clocks based on coherent population trapping (CPT). It uses quadrature-phase component of the CPT signal within the CPT resonance feedback loop and automatic wavelength locking of radiation from a VCSEL to the atomic absorption line. We demonstrate advantages and limitations of the new method applied to vapour cells with buffer gas or anti-relaxation coating. Also provided are measurements of short- and long-term stability of a CPT-based atomic clock using the proposed method with various optical cells. Discussed are the prospects of this method in chip-scale atomic clocks.