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New Journal of Physics

Feedback-controlled and digitally processed coherent population trapping resonance conversion in87Rb vapour to high-contrast resonant peak

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

Published Apr 11, 2017• 11 citations
Publisher

Abstract

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.

DOI
10.1088/1367-2630/aa68b9

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Journal of Physics Conference Series•Nov 1, 2016•2 citations
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.
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 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.
This thesis reports the development of a simple-architecture laser system resonant at 895 nm used for the detection of high-contrast coherent population trapping (CPT) resonances in Cs vapor cells. The laser system combines a distributed feedback-diode (DFB) laser, a pigtailed Mach-Zehnder intensity electro-optic modulator (EOM) driven at 4.596 GHz for the generation of optical sidebands frequency-splitted by 9.192 GHz and a Michelson delay-line system to produce a bichromatic optical field that alternates between right and left circular polarization. This polarization pumping scheme, pioneerly proposed by Happer's group in Princeton on K atoms, allows to pump optically a maximum number of Cs atoms into the 0-0 magnetic field insensitive clock transition. Advanced noise reduction techniques were implemented in order to stabilize the laser power, the optical carrier suppression at the output of the EOM and the DFB laser frequency. Using this system, we demonstrated the detection of CPT resonances with a contrast of 80% in cm-scale Cs vapor cells. This contrast was measured to be increased until a saturation effect with the laser power at the expense of the CPT line broandening. To circumvent this issue, we proposed with a simple setup Ramsey spectroscopy of CPT resonances in vapor cells to combine high-contrast and narrow linewidth of the CPT resonances. In this setup, the EOM is used both for optical sidebands generation and light switch to produce Ramsey interaction. Ramsey fringes of 166 Hz linewidth with a contrast better than 30% were detected with this setup. This laser system will be in a near future devoted to be used for the development of a high-performance CPT-based atomic clock.
This letter presents for the first time a systematic study of an atomic frequency standard based on coherent population trapping (CPT) effect in87Rb vapour optically pumped with a multifrequency laser radiation formed with sidebands of a single-frequency laser whose injection current is modulated at a subharmonic frequency (1/2-1/6) relative to the frequency of hyperfine ground state splitting. Measured are the CPT resonance parameters and the atomic clock short-term stability at different subharmonic frequencies of microwave modulation. The possibility of CPT resonance light shift suppression is experimentally demonstrated at modulation frequencies between 1/3 and 1/5 of the resonant frequency for continuous wave optical pumping of the87Rb vapour D1 line. Analyzed are the capabilities and potential of reduced energy consumption of atomic clocks when CPT resonance in rubidium vapour is excited at subharmonic modulation frequencies.
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.
Characteristics of CPT (coherent population trapping) signal are investigated in small-size glass vapor cells containing 87 Rb and Ne buffer gas with narrow linewidth laser pumping on D 2 line. Parameters of CPT absorption signals are measured using small-size vapor cells with Ne buffer gas pressure in range of 200-400 Torr, cell temperature in range of 65-120 °C and values of laser pumping power of 30-400 μW/cm 2 . Optimum conditions, under which the minimal value of short-term instability of resonance line is achieved, are obtained in experiments. CPT signals using vapor cells based on integrated technologies containing 87 Rb in atmosphere of Ne are also investigated. The CPT signals with typical linewidths of 2-3 kHz and signal-to-noise ratio of 1500 in 1 Hz bandwidth are observed, which allows one to provide relative frequency instability of 10 -11 at 100 s.