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Journal of Raman Spectroscopy

Bichromatic spectroscopy of coherent population trapping resonances with phase‐locked fields

Alexey V. Akimov, Arthur Matveev, А. В. Соколов, В. Н. Сорокин, S. I. Kanorsky, N. Kolachevsky

Published Feb 1, 2005• 6 citations
Publisher

Abstract

Abstract We propose a new approach to the study of coherent population trapping (CPT) resonances using a phase‐coherent bichromatic light source based on a mode‐locked femtosecond laser as an intermediate frequency coupler. The source consists of two diode lasers tuned to the rubidium D2 line (780 nm) which are phase locked to different longitudinal modes of a femtosecond laser MIRA‐900F with 75 MHz repetition rate. Using this bichromatic source with a stable differential frequency, we excited and investigated the CPT resonances in the three‐level system 5s S1/2 (F = 2) ↔ 5s P3/2 (F = 2) ↔ 5s S1/2 (F = 3) of the 85Rb atom. Potentially, this method opens up the possibility of coherently exciting Raman transitions with differential frequencies up to hundreds of terahertz. Copyright © 2005 John Wiley & Sons, Ltd.

DOI
10.1002/jrs.1290

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Journal of the Optical Society of America B•Nov 1, 2006•23 citations
Abstract We propose a new approach to the study of coherent population trapping (CPT) resonances using a phase‐coherent bichromatic light source based on a mode‐locked femtosecond laser as an intermediate frequency coupler. The source consists of two diode lasers tuned to the rubidium D2 line (780 nm) which are phase locked to different longitudinal modes of a femtosecond laser MIRA‐900F with 75 MHz repetition rate. Using this bichromatic source with a stable differential frequency, we excited and investigated the CPT resonances in the three‐level system 5s S1/2 (F = 2) ↔ 5s P3/2 (F = 2) ↔ 5s S1/2 (F = 3) of the 85Rb atom. Potentially, this method opens up the possibility of coherently exciting Raman transitions with differential frequencies up to hundreds of terahertz. Copyright © 2005 John Wiley & Sons, Ltd.
Abstract We experimentally studied the narrow resonances of coherent population trapping (CPT) in a rubidium vapor cell with the help of a phase‐coherent bichromatic light source based on a mode‐locked femtosecond laser. We investigated the dependence of the contrast and the width of CPT resonance on the light power, as well as the sensitivity of the shape of the resonance to a magnetic field. Prospects of using the novel bichromatic source for phase‐coherent Raman spectroscopy of CPT resonances in atomic systems with a large energy separation of the lower levels are discussed. Copyright © 2006 John Wiley & Sons, Ltd.
A new source of the bichromatic laser field with a stable mean value of the difference frequency is used to study coherent population trapping (CPT). A high stability of the difference phase of the fields of the radiation source is achieved by phase locking the radiation frequency of two semiconductor lasers to longitudinal modes of a femtosecond laser. The first experimental results on the spectroscopy of CPT resonances in rubidium are presented. The possibility of using the new method for spectroscopy of CPT resonances in rare-earth atom vapours is discussed.
A new source of two diode laser beams, spatially separated but optically phase-locked with each other, is used to study the modulation transfer spectroscopy of coherent population trapping resonance (CPT). The spectrum for the (87)Rb D2 line is obtained with narrow linewidth and high signal-to-noise ratio, and analyzed with different experimental parameters. A theoretical analysis of the CPT modulation transfer spectra is deduced from the density matrix equation of motion, and found to be in good agreement with the experimental results.
We are studying a new spectroscopic method using laser-cooled rubidium atoms that allows us to lock the carrier frequency of modulated laser in the coherent population trapping (CPT) clock experiment. This method uses counter-propagation of the bichromatic laser field produced by the modulated laser. The counter-propagation cancels the CPT effect near the optical resonance and gives rise to enhanced Doppler-free absorptive resonances with high-contrast away from the transition. We demonstrate the CPT clock operation by locking the modulated laser using this resonance. We also describe a theoretical model that we have developed based on density-matrix formalism to simulate this spectroscopic scheme for accurately predicting the lineshape and amplitude of resonances under different excitation conditions. Our study suggests that this spectroscopic method is also quite suiatble for developing cold-atom based optical frequency standard.
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 present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.