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URSI Asia-Pacific Radio Science Conference

Atomic Frequency Standard based on Coherent Population Trapping in Rb-87 Atoms for Space Application

Pragya Tiwari, Manjula Ramachandra, Madan Mohan Kandpal, Nikhil Thakur, Bijoy Raha, Madan Mohan Mehra, P. Selvaraj, M V Hanumantha Rao, Umesh S.B., Kalpana Arvind, K. V. Sriram, Swarupananda Pradhan, A. S. Laxmiprasad, Prashanth C. Upadhya

Published Mar 1, 2019• 0 citations

Abstract

Space-borne atomic frequency standards demand a high level of performance and stability throughout the mission life in the harsh environment of space. Recently, atomic clocks employing Coherent Population Trapping (CPT), a quantum interference phenomenon in alkali atoms, have shown much promise for space application. This paper presents the development and realization of compact and reliable CPT atomic clock for ISRO's navigational satellites. In our approach, quantum interference in Rb-87 atoms are pursued for obtaining the clock resonance, owing to their larger ground state hyperfine transition energy (higher Q-factor). Quantum interference is realized by generating two resonant optical laser fields from a VCSEL diode by modulating its drive current at RF frequency which corresponds to the ground state hyperfine separation in Rb-87 atoms. The emission frequency of the laser diode is stabilized by locking it with respect to the D1 hyperfine transition with the help of a servo control loop, thereby the instabilities associated with laser frequency fluctuations are minimized. The resulting CPT resonance (<;kHz) as shown in Figure 1 is used as reference to stabilize the crystal oscillator frequency (10 MHz) that serves both as clock output and seed for generating the RF field.

DOI: 10.23919/ursiap-rasc.2019.8738229

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Space-borne atomic frequency standards demand a high level of performance and stability throughout the mission life in the harsh environment of space. Recently, atomic clocks employing Coherent Population Trapping (CPT), a quantum interference phenomenon in alkali atoms, have shown much promise for space application. This paper presents the development and realization of compact and reliable CPT atomic clock for ISRO's navigational satellites. In our approach, quantum interference in Rb-87 atoms are pursued for obtaining the clock resonance, owing to their larger ground state hyperfine transition energy (higher Q-factor). Quantum interference is realized by generating two resonant optical laser fields from a VCSEL diode by modulating its drive current at RF frequency which corresponds to the ground state hyperfine separation in Rb-87 atoms. The emission frequency of the laser diode is stabilized by locking it with respect to the D1 hyperfine transition with the help of a servo control loop, thereby the instabilities associated with laser frequency fluctuations are minimized. The resulting CPT resonance (<;kHz) as shown in Figure 1 is used as reference to stabilize the crystal oscillator frequency (10 MHz) that serves both as clock output and seed for generating the RF field.

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Abstract The coherent population trapping (CPT) atomic clock is very promising for use in next-generation spaceborne applications owing to its compactness and high performance. In this paper, we propose and implement a CPT atomic clock based on the direct modulation of a large-modulation-bandwidth and narrow-linewidth distributed Bragg reflector laser, which replaces the usually used external bulk modulator in high-performance CPT clocks. Our method retains the high performance while significantly reducing the clock size. Using this highly compact bichromatic light source and simplest CPT configuration, in which a circularly polarized bichromatic laser interrogates the 87 Rb atom system, a CPT signal of clock transition with a narrow linewidth and high contrast is observed. We then lock the local oscillator frequency to the CPT resonance and demonstrate an encouraging short-term frequency stability of 3.6 × 10 −13 τ −1/2 (4 s ⩽ τ ⩽ 200 s). We attribute it to the ultralow laser frequency and intensity noise as well as to the high-quality-factor CPT signal. This study can pave the way for the development of compact high-performance CPT clocks based on our scheme.

Coherent population trapping (CPT) has been demonstrated as an interesting technique for miniature atomic frequency references [1,2] and quantum information. It is based on the coupling of the two hyperfine ground states of an alkali atom – namely cesium (133Cs) for atomic clocks – through excitation to a common atomic level by two phase-coherent laser fields nearly resonant with the atomic transitions. The frequency difference between the two laser fields is tuned at the atomic frequency splitting in the microwave range, equal to 9.192 GHz for 133Cs atoms. Outputs powers in the mW range and narrow-linewidth emission (<500 kHz) are required for the two laser beams.

An atomic clock system based on coherent population trapping (CPT) resonance in 85 Rb is reported, while most past works about the CPT clock are in 87 Rb. A new modulation method (full-hyperfine-frequency-splitting modulation) is presented to reduce the effect of light shift to improve the frequency stability of the CPT clock in 85 Rb. The experimental results show that the short-term frequency stability of the CPT clock in 85 Rb is in the order of 10 −10 /s and the long-term frequency stability can achieve 1.5 × 10 −11 /80000 s, which performs as well as 87 Rb in CPT resonance. This very good frequency stability performance associated with the low-cost and low-power properties of 85 Rb indicates that an atomic clock based on CPT in 85 Rb should be a promising candidate for making the chip scale 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.