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Results in Optics

Sub-megahertz laser stabilization by harnessing orthogonality between the acousto-optic modulator dither and atomic spectroscopy

Phatwarach Siriworakoonchai, Parinya Udommai, Nuttanan Tanasanchai, Waranont Anukool

Published Oct 28, 2025• 3 citations

Abstract

A sub-MHz frequency stability of the laser is crucial in the quantum-atom experiment where precise atomic transition is required for control and manipulation of atom like in the atom quantum computing. In this work, we have built the frequency stabilization system for the lasers used in our cold Rubidium atom experiment in the magneto-optical trap (MOT) and the optical tweezers. We utilized a Rubidium vapour glass cell as a frequency reference, which provides atomically absolute reference point with low complexity setup. By mean of the absorption of the laser beam by the Rubidium vapour, the Doppler-free saturated absorption (DFSA) experiment was setup to produce the D2 line spectra. The acousto-optic modulator (AOM) was used to sinusoidally dither the laser frequency and consequently the resonance peaks in the spectra. Detailing in this work, the AOM sinusoidal dithering and the dithered DFSA signal had orthogonality and were processed together to produce the error signal used to stabilize the laser frequency to the resonant peak. This frequency stabilization system achieved the frequency standard deviation below 0.75 MHz over 200 minutes period. The system also allowed for stabilized frequency tunability over a range of 55-60 MHz about the target resonance peak. This performance is significant for stabilizing the lasers in the MOT experiment and related quantum-atom research.

DOI: 10.1016/j.rio.2025.100920

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In this study, a miniature laser frequency stabilization module for cold atom sensing applications was designed and realized.The module can lock the laser frequency to the saturated absorption spectrum of the D2 line of rubidium (Rb) atoms using the frequency modulation spectroscopy method.The module core is a tiny Rb cell, co-packaged with the saturation absorption optical setup, temperature control circuit, and detection circuit through a joint optical-mechanical-electrical design for a total volume of only 40 mm × 15 mm × 30 mm.The frequency fluctuation of the 780 nm laser after frequency stabilization by the module was within 1 MHz over 1000 s, which is adequate for magneto-optical trap experiments.The results verify the feasibility of the module as a frequency reference and provide a light source for portable cold atom sensing devices.

Abstract In optical systems, acousto-optic modulators (AOMs) and electro-optic modulators (EOMs) are commonly utilized for laser frequency manipulation. However, these devices come with drawbacks such as high cost and the requirement of optical adjustments to change the frequency shift direction. In this study, we propose an alternative method that harnesses the Zeeman effect in the saturated-absorption spectroscopy to shift the energy levels in atom. With this technique, the laser frequency can be shifted without the need for AOMs or EOMs. The stability of laser locking was investigated for any case of polarization of probe and pump beam. We found when the polarization of the probe and pump beams is linear and perpendicular, it results in the highest level of stability in laser locking compared to other polarization configurations.

We describe an inexpensive lock-in amplifier that can be built using discrete off-the-shelf RF components and home-built analog circuits. This lock-in has been used in a feedback loop to lock the frequency of a laser to an atomic transition. The frequency stability was tested by trapping rubidium atoms. The feedback loop involves obtaining a modulated saturated absorption signal from a vapor cell using an acousto-optic modulator to modulate the laser frequency. The absorption signal is sent to the lock-in to generate an error signal, proportional to the first derivative of the absorption, which is fed back to the laser to complete the feedback loop. We also demonstrate a simplified optical setup for viewing the saturated absorption spectrum. In this case, the signal consists of narrow saturated absorption spectra riding on top of a Doppler-broadened pedestal. We show that it is possible to greatly reduce the effect of this background and lock the laser to the atomic resonance by modifying the home-built lock-in to generate an error signal proportional to the third derivative of the absorption. The results of this work can be adapted for advanced undergraduate laboratory work.PACS Nos.: 01.50Pa, 07.50.e, 42.62Fi, 42.60Fc

We discuss the characterization and specifications of a home-built external cavity diode laser system that is suitable for frequency-domain cold-atom velocimetry and measurements of gravitational acceleration. The laser system is frequency stabilized using a modulation-free technique that lowers the temperature of a magneto-optical trap (MOT) in comparison with a MOT realized using a laser that relies on lock-in spectroscopy. The laser system also incorporates a feedback loop involving an acousto-optic modulator for intensity stabilization, which reduces the effect of low frequency vibrations. By integrating this laser with a home-built, dual-output radio frequency synthesizer, we are able to derive two laser beams with a precisely controlled relative detuning for frequency domain interferometry using laser-cooled atoms. The detuning can range from the mHz to the MHz level with a stability of 10 μHz.

Vapor-cell spectroscopy is an essential technique in many fields and is particularly relevant for quantum technologies. Most atom- and ion-trapping experiments rely on simultaneous spectroscopy of two atomic transitions, employing a separate apparatus for each transition. We demonstrate simultaneous spectroscopy on two atomic transitions, within a single apparatus, using spatially overlapped beams from two independent lasers. This method enables compact setups and offers superior spectroscopic performance, leading to sharper spectroscopy peaks, stronger absorption signals, and superior laser stability. Doppler-free locking features become accessible over a frequency range several hundred megahertz wider than for standard saturated absorption spectroscopy. Exploring the full parameter space associated with dual-frequency spectroscopy reveals a latticelike structure of sharp resonance features, which enhances experimental versatility by allowing laser frequency stabilization within a wide manifold of locations in two-dimensional frequency space. We present data for

^{133}\mathrm{Cs}

and

^{85}\mathrm{Rb}

and compare our results to a theoretical model. Employing the technique for frequency stabilization close to the

{D}_{2}

line of

^{85}\mathrm{Rb}

results in an improvement in frequency stability by a factor typically between 2 and 3 for averaging times of up to 1 s. The technique will advance portable quantum technologies and facilitate high-precision measurements.

We present the development and complete spectral characterization of our compact and frequency-stabilized laser heads, to be used for rubidium atomic clocks and basic spectroscopy. The light source is a Distributed Feed-Back (DFB) laser diode emitting at 780 nm or 795 nm. The laser frequency is stabilized on a sub-Doppler absorption peak of the 87Rb atom, obtained from an evacuated rubidium cell. These laser heads, including the electronics for the light signals detection, have an overall volume of 0.63 liters. We also present a variant of the laser head into which is integrated an Acousto-Optical Modulator (AOM) that precisely detunes the laser frequency in order to minimize the AC Stark shift in Rb atomic clocks.