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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

High precision 6.8GHz phase locking of coherent laser beams for optical lattice experiment

Xun Ding, Linlin Sang, Chen Zhang, Ge Jin, Xiao Qiang Jiang

Published Dec 31, 2013• 2 citations
Publisher

Abstract

With the optical phase lock loop (OPLL) we made, we can achieve phase locking at frequency differences ranging from 0.5GHz to 7.5 GHz. This OPLL is fully applicable in atomic physics experiments, mostly in coherent lasers frequency locking. Two kinds of modulation modes were brought to ensure the frequency range and precision: the fast feedback current as the injection current and the slow feedback current to adjust the piezo-electric transducer. This device has been put into an optical lattice platform to lock a laser used for cooling and trapping atoms. The beat signal has a -3dB band width of 1Hz at 6.834GHz, corresponding to the hyperfine splitting of the ground state 87Rb atom.

DOI
10.1117/12.2037782

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physics.atom-phCross-listedarXiv:2603.22080•Mar 24, 2026
With the optical phase lock loop (OPLL) we made, we can achieve phase locking at frequency differences ranging from 0.5GHz to 7.5 GHz. This OPLL is fully applicable in atomic physics experiments, mostly in coherent lasers frequency locking. Two kinds of modulation modes were brought to ensure the frequency range and precision: the fast feedback current as the injection current and the slow feedback current to adjust the piezo-electric transducer. This device has been put into an optical lattice platform to lock a laser used for cooling and trapping atoms. The beat signal has a -3dB band width of 1Hz at 6.834GHz, corresponding to the hyperfine splitting of the ground state 87Rb atom.
Abstract Among the existing techniques for Raman laser preparation, optical phase-locked loops (OPLLs) are widely used in quantum precision measurements due to their high regulation accuracy and low phase noise. Aiming at the problems of limited adjustment range and insufficient stability of the current OPLL, this paper designs an OPLL by optimizing the operating points with a bias voltage. Performance tests show that, the tuning range of the optimized OPLL is 2 GHz to 9 GHz. This wide frequency cover can produce Raman laser for both 87Rb and 85Rb atomic gravimeters. It can also be used to simplify the optical path in experiments to test the equivalence principle. The optimized OPLL has a long-term phase-locking capability. For integration times of 1 s and 1000 s, the relative Allan’s variance is 1.6 × 10−11 and 5.2 × 10−13, respectively.
The authors have used an electromagnetically induced transparency resonance in rubidium as a dispersive reference to lock the relative frequency of two lasers to the atomic ground-state hyperfine splitting. The beat frequency between the two lasers directly generates a microwave signal at 3.036GHz (Rb85) or 6.835GHz (Rb87). High bandwidth (600kHz) feedback was achieved with only low-frequency (10MHz) electronics using the frequency modulation sideband method. The spectral width of the microwave beat frequency was reduced to less than 1kHz. The technique offers a convenient and low-cost method suitable for many topical two-frequency experiments, including coherent population trapping, slow light, lasing without inversion, and Raman sideband cooling.
A highly digital optical phase-locked loop (OPLL) is designed and implemented to generate phase-coherent lasers in quantum optics. All the controlling circuit in this OPLL are implemented in a single field programmable gate array (FPGA) chip. Pipeline and parallelism technology have been adapted in PID controller. Especially, an equivalent 3.5 GHz high speed clock and twisted ring counter have been integrated in the most crucial part. This OPLL has the narrow beat note line width below 1 Hz, residual mean-square phase error of 0.05 rad2and transition time of 100 μs under 10 MHz frequency step.
We produced two Raman-laser beams with a frequency offset of 6.834GHz by injection-locking of a master diode-laser to a slave diode-laser. The master laser was phase-modulated at 6.834 GHz with an Electro-Optic Modulator and then injected into the slave laser that was oscillating around one of the side-bands. The relative linewidth of the two lasers was less than 1 Hz. Utilizing these laser beams, we realized the coherent manipulation of atomic wave packets in a Mach-Zehnder type atom interferometry.
An optical lattice is a periodic light crystal constructed from the standing-wave interference patterns of laser beams. It can be used to store and manipulate quantum degenerate atoms and is an ideal platform for the quantum simulation of many-body physics. A principal feature is that optical lattices are flexible and possess a variety of multidimensional geometries with modifiable band-structure. An even richer landscape emerges when control functions can be applied to the lattice by modulating the position or amplitude with Floquet driving. However, the desire for realizing high-modulation bandwidths while preserving extreme lattice stability has been difficult to achieve. In this paper, we demonstrate an effective solution that consists of overlapping two counterpropagating lattice beams and controlling the phase and intensity of each with independent acousto-optic modulators. Our phase controller mixes sampled light from both lattice beams with a common optical reference. This dual heterodyne locking method allows exquisite determination of the lattice position, while also removing parasitic phase noise accrued as the beams travel along separate paths. We report up to 50dB suppression in lattice phase noise in the 0.1Hz - 1Hz band along with significant suppression spanning more than four decades of frequency. When integrated, the absolute phase diffusion of the lattice position is only 10Å over 10 s. This method permits precise, high-bandwidth modulation (above 50kHz) of the optical lattice intensity and phase. We demonstrate the efficacy of this approach by executing intricate time-varying phase profiles for atom interferometry.
We present a standalone frequency-offset locking system for controlling narrow-linewidth lasers using off-the-shelf electronic components. We lock two frequency-doubled 1560 nm lasers to a stable primary laser operating at 780 nm via their optical beat note. This radio-frequency beat note is fed through a broadband variable divider, a frequency-to-voltage converter, and a proportional-integrator controller to lock each follower laser to a tunable offset frequency relative to the primary. This architecture provides a large capture range (>1> 1>1 GHz), fast response times (<1< 1<1 ms), and high linearity. We achieve a frequency resolution of 1.9 kHz and a short-term fractional frequency instability 10−11/τ(s)10^{-11}/\sqrt{τ\rm (s)}10−11/τ(s)​ at 780 nm without the need for a dedicated, precise clock reference. We perform high-resolution spectroscopy of cold 87^{87}87Rb atoms to demonstrate the tunability and precision of our locking system. We designed the system to be modular and extensible, making it applicable to a wide variety of atomic physics experiments, including laser cooling, spectroscopy, and quantum sensing with atoms, ions, and molecules.