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Acta Physica Sinica

Raman coupling in atomic Bose-Einstein condensed with phase-locked laser system

Zengming Meng, Lianghui Huang, Peng Peng, 陈良超 CHEN Liang-chao, Hao Fan, Pengjun Wang, Jing Zhang

Published Jan 1, 2015• 2 citations
Publisher

Abstract

We present a simple, versatile and reliable phase-locked laser system. The system consists of an external cavity diode laser, Ti: Sapphire laser, fast detector, phase frequency detector (PFD) and loop filters. The beat signal of the laser is detected with a detector. From the PFD, we can obtain an error signal. The loop filter converts the output of the PFD into a control voltage and thus drives piezoelectric ceramic transducer (PZT) and current of diode laser. After locking, the bandwidth of the beat signal is reduced form MHz to Hz. So the line-width of the diode laser is almost close to that of Ti: Sapphire laser. The locking range is from sub-MHz to 10 GHz. So it is used for the ground hyperfine state transition of 87Rb. Through the use of the phase-locked loop system, we can drive the transition of 87Rb atoms between two ground hyperfine states F=2 and 1. The system is used to demonstrate Raman transition between two states through changing the detuning of the beat signal. From this, we can obtain Rabi frequency = 10 kHz. So, this system can be used to induce an effective vector gauge potential for 87Rb Bose-Einstein condensed and realize the spin-orbit coupling.

DOI
10.7498/aps.64.243202

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We present a simple, versatile and reliable phase-locked laser system. The system consists of an external cavity diode laser, Ti: Sapphire laser, fast detector, phase frequency detector (PFD) and loop filters. The beat signal of the laser is detected with a detector. From the PFD, we can obtain an error signal. The loop filter converts the output of the PFD into a control voltage and thus drives piezoelectric ceramic transducer (PZT) and current of diode laser. After locking, the bandwidth of the beat signal is reduced form MHz to Hz. So the line-width of the diode laser is almost close to that of Ti: Sapphire laser. The locking range is from sub-MHz to 10 GHz. So it is used for the ground hyperfine state transition of 87Rb. Through the use of the phase-locked loop system, we can drive the transition of 87Rb atoms between two ground hyperfine states F=2 and 1. The system is used to demonstrate Raman transition between two states through changing the detuning of the beat signal. From this, we can obtain Rabi frequency = 10 kHz. So, this system can be used to induce an effective vector gauge potential for 87Rb Bose-Einstein condensed and realize the spin-orbit coupling.
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.
The advancement of cold atom gravimeters (CAG) is based on the precise control and stabilization of lasers used for the atom interferometry.The 5S → 5P transition in rubidium (Rb) at 780.241 nm is crucial for such quantum technology applications.In this study, the hyperfine structure of 5 2 S1/2 → 5 2 P3/2, D2 transition line of 87 Rb atoms obtained through saturation absorption spectroscopy, are used as a reference for the frequency stabilization of an external cavity diode laser (ECDL) operating at 780 nm.The ECDL is locked to the hyperfine saturated absorption peak of 87 Rb (F = 2 → 3').This laser beam is combined with another 780 nm ECDL laser to obtain the beat frequency spectrum, generating a beat note signal of ~ 6.8 GHz, which is needed for phase stabilization between lasers.These are critical for the Raman beam generation in the CAG, as the 87 Rb atoms ground hyperfine states (F = 1 and F = 2) have separation of ~ 6.834 GHz, and two Raman beams with an 6.8 GHz offset can drive transitions between these hyperfine states of 87 Rb to realize the CAG interferometry scheme.
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.
Two phase-coherent Raman laser beams with a frequency offset of 6.835 GHz were generated by sideband injection-locking technique. A master diode laser was phase-modulated at 6.835 GHz by a fiber electro-optic modulator. A slave diode was injection-locked to the -1 sideband of the phase-modulated beam, and another diode was locked to the master laser carrier. This method produced stable and spatially separated Raman lasers with a large frequency shift range (>180 MHz). The relative linewidth of these two beams was ∼1 Hz, and the unwanted carrier mode was suppressed down to -24 dB. Stimulated Raman transitions and Ramsey fringes were driven by Raman lasers in a cold atomic beam.
An all-optical locking technique without extra electrical feedback control system for a semiconductor laser has been used in stabilizing the laser frequency to a hyperfine crossover transition of 87Rb 5(2)S(1/2), F = 2 → 5(2)P(3/2), F' = 2, 3 with 1 MHz level accuracy. The optical feedback signal is generated from the narrow-band Faraday anomalous dispersion optical filter (FADOF) with nonlinear saturation effect. The peak transmission of the narrow-band FADOF corresponding to 5(2)S(1/2), F = 2 → 5(2)P(3/2), F' = 2, 3 crossover transition is 18.6 %. The bandwidth is as wide as 38.9 MHz as the laser frequency changes. After locking, the laser frequency fluctuation is reduced to 1.7 MHz. The all-optical laser locking technique can be improved to much higher accuracy with increased external cavity length. The laser we have realized can provide light exactly resonant with atomic transitions used for other atom-light interaction experiments.
We describe the development of a phase-locked laser system tailored to an ion-trap-based quantum information processor with (40)Ca(+). Laser outputs from an extended cavity diode laser and a Ti:sapphire laser with output laser wavelengths of approximately 850 and 854 nm, respectively, were phase locked and used to excite a Raman transition between the D(3/2) and D(5/2) metastable states qubit. Development and the performance of the laser system are described. We also compare the characteristics and the benefits of the developed qubit coupling with those in the conventional approaches.