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arXiv:2205.04875 · Optics Express

Robust single-sideband-modulated Raman light generation for atom interferometry by FBG-based optical rectangular filtration

Guochao Wang, YaNing Wang, Ying Bo Kang, Huankai Zhang, Xu Zhang, Qixue Li, Xuan Li, Enlong Wang, Xiaoyan Yu, Aiai Jia, Shuhua Yan, Jun Jie Yang, Lingxiao Zhu

Published Jul 15, 2022• 17 citations

Abstract

Low-phase-noise and pure-spectrum Raman light is vital for high-precision atom interferometry by two-photon Raman transition. A preferred and prevalent solution for Raman light generation is electro-optic phase modulation. However, phase modulation inherently brings in double sidebands, resulting in residual sideband effects of multiple laser pairs beside Raman light in atom interferometry. Based on a well-designed rectangular fiber Bragg grating and a plain electro-optic modulator, optical single-sideband modulation has been realized at 1560 nm with a stable suppression ratio better than -25 dB despite of intense temperature variations. After optical filtration and frequency doubling, a robust phase-coherent Raman light at 780 nm is generated with a stable SNR of better than -19 dB and facilitates measuring the local gravity successfully. This FBG-based all-fiber single-sideband-modulated Raman light source, proposed for the first time and characterized as robust, compact and low-priced, is practical and potential for field applications of portable atom interferometry.

DOI: 10.1364/oe.463405

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A method for the agile generation of the optical frequencies required for laser cooling and atom interferometry of rubidium is demonstrated. It relies on fiber Bragg grating technology to filter the output of an electro-optic modulator and was demonstrated in a robust, alignment-free, single-seed, frequency-doubled, telecom fiber laser system. The system was capable of 50 ns frequency switching over a ~40 GHz range, ~0.5 W output power and amplitude modulation with a ~15 ns rise/fall time and an extinction ratio of 120

\pm

2 dB. The technology is ideal for enabling high-bandwidth, mobile industrial and space applications of quantum technologies.

A compact and robust laser system is essential for mobile atom interferometers. Phase modulation can provide the two necessary phase-coherent frequencies without sophisticated phase-locking between two different lasers. However, the additional laser frequencies generated can perturb the atom interferometer. In this article, we report on a novel method to produce a single high-power laser beam composed of two phase-coherent sidebands without the perturbing carrier mode. Light from a diode laser is phase-modulated by using a fiber-coupled electro-optic modulator driven at 3.4 GHz and passes through a Fabry-Perot cavity with a 6.8 GHz free spectral range. The cavity filters the carrier mode to leave the two first-order sidebands for the two-photon Raman transition between the two hyperfine ground states of 87 Rb. The laser beam is then fed to a single tapered amplifier, and the two sidebands are both amplified without mode competition. The phase noise is lower than that of a state-of-the-art optically phase-locked external-cavity diode laser (−135 dBrad 2 /Hz at 10 kHz) at frequencies above 10 Hz. This technique can be used in all-fiber-based laser systems for future mobile atom interferometers.

A method for the agile generation of the optical frequencies required for laser cooling and atom interferometry of rubidium is demonstrated. It relies on fiber Bragg grating technology to filter the output of an electro-optic modulator and was demonstrated in an alignment-free, single-seed, frequency-doubled fiber laser system. The system was capable of frequency switching over a 30 GHz range in less than 40 ns, with ∼ 0.5 W output power and amplitude modulation with a ∼ 15 n s rise/fall time and an extinction ratio exceeding 80 dB. The technology is ideal for enabling high-bandwidth, mobile industrial, and space applications of quantum technologies.

A compact laser system for a portable 87Rb atom interferometry gravimeter has been demonstrated in this work. This laser system is based on frequency doubling of a single seed laser at the wavelength of 1560 nm. The frequency of the seed laser is controlled by a digital unit with an analog feedback circuit. By using this frequency control method, the frequency of the laser system can be shifted over 1 GHz. Based on this method, the Raman frequency can be locked on the F = 3 → F′ = 4 transition of 85Rb atoms. Moreover, the Raman sideband and the repumping laser are generated by a phase modulator, and it can generate different laser frequencies to meet the requirements of a typical atom interferometer. Additional sidebands in the Raman beam produced from the phase modulator are optimized and reduced, allowing us to observe atom interference with a free evolution time of 320 ms. The control unit including the laser system has been integrated into a box with a volume of 1.5 m × 0.6 m × 0.6 m, and the weight of which is only 150 kg. Using this compact optical scheme, our atomic gravimeter has achieved a sensitivity of 53 μGal/Hz1/2 and a resolution of better than 1 μGal (1 μGal = 1 × 10−8 m/s2) in an integration time of 3000 s.