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A Cold-Atom Laser with <1 Intracavity Photon

James K. Thompson

Published Jan 1, 2013• 0 citations
Publisher

Abstract

We demonstrate a cold-atom Raman laser that operates quasi-continuously with as few as 0.2 photons on average inside the cavity and with a single-atom Raman decay rate below 1 Hz. We demonstrate that the laser’s coherence is primarily stored within the atoms, and consider the relationship between the Schawlow-Townes linewidth and the standard quantum limit on phase estimation using unentangled atoms. This proof-of-principle experiment may lead to novel hybrid sensors and guide the development of future ultra-stable lasers with intrinsic insensitivity to currently-limiting thermal and technical sources of cavity mirror vibration.

DOI
10.1364/cleo_qels.2013.qth4c.1

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We demonstrate a cold-atom Raman laser that operates quasi-continuously with as few as 0.2 photons on average inside the cavity and with a single-atom Raman decay rate below 1 Hz. We demonstrate that the laser’s coherence is primarily stored within the atoms, and consider the relationship between the Schawlow-Townes linewidth and the standard quantum limit on phase estimation using unentangled atoms. This proof-of-principle experiment may lead to novel hybrid sensors and guide the development of future ultra-stable lasers with intrinsic insensitivity to currently-limiting thermal and technical sources of cavity mirror vibration.
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We demonstrate a Raman laser using cold (87)Rb atoms as the gain medium in a high-finesse optical cavity. We observe robust continuous wave lasing in the atypical regime where single atoms can considerably affect the cavity field. Consequently, we discover unusual lasing threshold behavior in the system causing jumps in lasing power, and propose a model to explain the effect. We also measure the intermode laser linewidth, and observe values as low as 80 Hz. The tunable gain properties of this laser suggest multiple directions for future research.
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The technology of generating Raman laser is not only an important research content in the field of quantum precision measurement, but also a core technology of quantum inertial sensors such as cold atom gravimeter, gyroscope. For 87Rb atoms, two 780-nm lasers with a frequency difference of 6.834 GHz and a stable phase are needed to generate Raman light. Raman lasers can be generated by optical phase-locked loops of two 780-nm narrow linewidth external cavity tunable semiconductor lasers (ECDL). But the system thus developed is complicated in structure and very poor in environmental adaptability. The other method to generate Raman laser is based on intracavity 1560-nm laser with frequency doubling and electro-optic modulation technology. This system is simple in structure and strong in environmental adaptability, but it will introduce sideband effects and cannot achieve phase lock due to the limit by the linewidth and feedback bandwidth performance of the laser. In view of this, based on two new 1560-nm external cavity lasers and a home-made phase-locked circuit, in this paper the phase lock of the laser is achieved, and a Raman laser with low phase noise is obtained. The phase noise of beat note signal is as low as –95 dBc/Hz at the Fourier frequency in a range from 1 kHz to 10 kHz. A comparison of this system with the phase-locked performance of the 780-nm dual laser and the hybrid dual laser shows that this scheme has a slight advantage. In addition, the effect of the phase-locking performance on the phase noise of the cold atom interferometer through the method of piecewise integration is analyzed in this work. The experimental results given in this work provide a scheme for developing a miniaturized Raman optical system suitable for external fields.