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Review of Scientific Instruments

Measuring the phase noise of Raman lasers with an atom-based method

Yaning Wang, Shuhua Yan, Dongyang Xu, Mei Hu, Qixue Li, Xu Zhang, Huankai Zhang, Mengjie Lv, Aiai Jia, Guochao Wang, Lingxiao Zhu, Jun Jie Yang

Published Mar 1, 2021• 3 citations
Publisher

Abstract

Phase noise of Raman lasers is a major source of noise for a Raman-type cold atom interferometer, which is traditionally measured using the signal source analyzer. We report here an atom-based method to measure the phase noise performance between two Raman lasers. By analyzing and calibrating the system noise sources, we can characterize the contribution of phase noise from the total deviation of the relative atom population at the middle of the interference fringe. Knowing the transfer function specified by the operation sequence of the interferometer, we can obtain the transfer function and power spectrum density of the phase noise term. By varying the time sequences of the interferometer, we can measure the white phase noise floor and the phase noise performance over a large range of Fourier frequencies from 1 to 100 000 Hz with a minor difference of 1 dB compared with results from the traditional method using a signal analyzer, which proves the validity of the atom-based method. Compared with the traditional measurement method, the atom-based method can have higher accuracy and have the ability of self-calibrating.

DOI
10.1063/5.0037422

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Phase noise of Raman lasers is a major source of noise for a Raman-type cold atom interferometer, which is traditionally measured using the signal source analyzer. We report here an atom-based method to measure the phase noise performance between two Raman lasers. By analyzing and calibrating the system noise sources, we can characterize the contribution of phase noise from the total deviation of the relative atom population at the middle of the interference fringe. Knowing the transfer function specified by the operation sequence of the interferometer, we can obtain the transfer function and power spectrum density of the phase noise term. By varying the time sequences of the interferometer, we can measure the white phase noise floor and the phase noise performance over a large range of Fourier frequencies from 1 to 100 000 Hz with a minor difference of 1 dB compared with results from the traditional method using a signal analyzer, which proves the validity of the atom-based method. Compared with the traditional measurement method, the atom-based method can have higher accuracy and have the ability of self-calibrating.
We have developed a modified Michelson interferometer type Raman laser system to manipulate cold 87 Rb atoms to interfere. A frequency modulated continuous wave technique was introduced to determine the optical path difference, thus compensating it to zero to minimize the effects of common mode noise. The linewidth (full width at half maximum) of the beat signal at 6.834 GHz was measured to 1Hz limited by the resolution bandwidth of the spectral analyzer. The measured rms phase variance of the phase noise at 166 MHz was 0.015 rad2, mainly restricted by our poor performance radio frequency microwave source. With modest improvements, we plan to apply this laser system to form an atom interferometer for acceleration and rotation measurements.
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.
In our high-precision atom interferometer, the measured atomic phase shift is sensitive to rotations and accelerations of the apparatus, and also to phase fluctuations of the Raman lasers. In this paper we study two principal noise sources affecting the atomic phase shift, induced by optical phase noise and vibrations of the setup. Phase noise is reduced by carrying out a phase lock of the Raman lasers after the amplification stages. We also present a new scheme to reduce noise due to accelerations by using a feed-forward on the phase of the Raman beams. With these methods, it should be possible to reach the range of the atomic quantum projection noise limit, which is about 1 mrad rms for our experiment, i.e. 30 nrad s-1 Hz-1/2 for a rotation measurement.
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Lasers are the workhorse of quantum engineering in the atomic-molecular-optic community. However, phase noise of the laser, which can be especially large in popular semiconductor-based lasers, can limit gate fidelity. Here we present a fully fiberized instrument detecting and correcting the fast, submicrosecond, phase fluctuations of lasers. We demonstrate a measurement noise floor of less than 0.1 Hz2{\mathrm{Hz}}^{2}Hz2/Hz and a noise suppression of more than 20 dB for Fourier frequencies in the 1--10 MHz region (reaching up to 30 dB at 3 MHz), where noise is critical for Rydberg-based quantum gates. We observe the improvement offered by this fast-phase-noise eater on a Raman transition driven by two such stabilized lasers. These measurement and correction techniques are important tools for high-fidelity manipulation of the excited electronic states of atoms and molecules.
Atom interferometry-based sensors have demonstrated remarkable precision, making them attractive for diverse applications compared to traditional sensors. However, practical experiments inevitably encounter noise, which limits precision. We investigate the performance of two commonly used methods, ellipse fitting (EF) and fringe fitting (FF), for differential phase estimation in a dual atom interferometer under various noise conditions. We comprehensively analyze the efficacy of these methods for each type of Gaussian noise, including phase noise, amplitude noise, and offset noise. Using a colored noise model, we investigate the Allan deviation results. Our investigations will offer valuable insights for future quantum sensing based on dual interferometers.