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Optics Continuum

Integrated optical system for free-fall cold atomic clocks

Qiang Hao, Shaojie Yang, Hui Zhang, Sichen Fan, Shuai Nie, Yang Bai, Xinliang Wang, Jun Ruan, Shougang Zhang

Published Feb 9, 2025• 2 citations
Publisher

Abstract

The free-fall atomic clock has shown considerable potential for long-term stability and mobility, yet reports on its design and implementation remain limited. Herein, we introduce a simple integrated optical system for free-fall cold Rb atomic clocks. A distributed feedback (DFB), frequency-doubled fiber laser, and a 780 nm DFB laser diode are utilized as the cooling and repumping light source, respectively. Both lasers feature a broad mode-hop-free tuning range and reduced sensitivity to acoustic noise. Benefiting from a high signal-to-noise ratio of modulation transfer spectroscopy (MTS), the linewidth of the fiber laser is measured to be 42 kHz. The power and frequency management of the cooling beams is implemented by two 110-MHz acousto-optic modulators (AOMs), enabling a frequency detuning as large as −92 MHz. The power-frequency management module is integrated into a single side bench of 316 mm × 270 mm × 52 mm. The power fluctuation of the probe beam is measured to be 0.8% (RMS) over 24 hours measurement. Using an ultra-high vacuum system, the functionality of the optical system is preliminarily verified by the time-of-flight (TOF) signal of the cold atom cloud.

DOI
10.1364/optcon.543623

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The free-fall atomic clock has shown considerable potential for long-term stability and mobility, yet reports on its design and implementation remain limited. Herein, we introduce a simple integrated optical system for free-fall cold Rb atomic clocks. A distributed feedback (DFB), frequency-doubled fiber laser, and a 780 nm DFB laser diode are utilized as the cooling and repumping light source, respectively. Both lasers feature a broad mode-hop-free tuning range and reduced sensitivity to acoustic noise. Benefiting from a high signal-to-noise ratio of modulation transfer spectroscopy (MTS), the linewidth of the fiber laser is measured to be 42 kHz. The power and frequency management of the cooling beams is implemented by two 110-MHz acousto-optic modulators (AOMs), enabling a frequency detuning as large as −92 MHz. The power-frequency management module is integrated into a single side bench of 316 mm × 270 mm × 52 mm. The power fluctuation of the probe beam is measured to be 0.8% (RMS) over 24 hours measurement. Using an ultra-high vacuum system, the functionality of the optical system is preliminarily verified by the time-of-flight (TOF) signal of the cold atom cloud.
The paper presents the results of the development of a laser-optical system for atomic interferometer based on rubidium atoms cooled to sub-Doppler temperatures. The laser-optical scheme was created to ensure the processes of cooling, pumping, and detecting atoms involved in interferometric measurements of free-fall acceleration. The system utilizes frequency doubled fiber lasers and broadband fiber electro-optical modulators. The choice of fiber lasers is driven by their high efficiency, narrow spectral line, low phase noise levels, as well as ease of operation and reliability. Laser frequency stabilization was carried out using modulation transfer spectroscopy and optical phase-locking methods. Experimental setups were described that allow the generation of multichromatic radiation and minimize spontaneous scattering by detuning from excited levels. It was noted that the design provides a full set of optical frequencies necessary for Raman spectroscopy. The amplitudes of frequency fluctuations of the cooling and pumping lasers were studied. It was shown that the implemented laser-optical system can ensure continuous operation of an atomic interferometer based on clouds of cold rubidium atoms, and an assessment of the fundamental sensitivity limit was conducted.
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.
Here, we demonstrate a homemade compact portable 795nm DFB laser system for atom interferometer. The whole laser system including the control unit is integrated in a module with a dimension of 345 mm×275 mm×70 mm . The laser frequency is respect to 87Rb D1 line F=2→F’=2 transition by modulation transfer spectroscopy (MTS) technique. The short-term linewidth is less than 700kHz@1.68ms, and the short-term stability of frequency is less than 157.4 kHz@1.5min (standard deviation). The output power after a single mode polarization maintaining fiber is about 30 mW, and the laser power instability is better than 0.23%@11 h (RMS) and 2.4%@11 h (peak-to-peak value). The polarization extinction ratio (PER) for output fiber is specified to be greater than 25.9 dB, and the mode-hop-free tuning range is greater than15 GHz. The compact laser system has been applied in the state selection process of our atom interferometer and achieves about 90% state preparation efficiency.
We demonstrate an optical frequency standard by locking the output frequency of an external cavity diode laser to the 780 nm D2 transition of diffuse laser cooled 87Rb atoms using frequency modulation spectroscopy (FMS). A 50-cm-long cold atomic cloud is obtained within a ϕ 2 cm ×l 50 cm vacuum glass tube, featuring greater simplicity and reliability. By analyzing the impact of power and detuning of cooling and repumping lasers, and power of frequency-modulated laser on FMS based on cold atoms, we choose the optimal parameter and lock the laser frequency to the 87Rb 52S1/2 (F = 2) → 52P3/2 (F′ = 3) cycling transition. The cold-atom-based optical frequency standard achieves an in-loop frequency stability of 3.3 × 10−15 at 1 s, with peak-to-peak frequency fluctuations being less than 50 Hz within 6000 s. This represents the integration of cold atoms into an optical frequency standard utilizing FMS for frequency stabilization. In contrast to the common cold-atom optical frequency standards, our approach requires fewer lasers and eliminates the need for a complex and expensive Pound–Drever–Hall (PDH) system. Its low complexity makes it a promising candidate for development into a space cold-atom clock, promoting advancements in space science and navigational positioning. Additionally, it possesses great potential to serve as an absolute wavelength standard.
A new epitaxial layer design with a double mode expander layer, high refractive index claddings, and an aluminum-free active area has been used to demonstrate distributed feedback lasers operating at 778.1 nm wavelength with reduced Lorentzian linewidth aimed at miniature atomic clock applications. The design also reduces the vertical beam divergence to improve the modal matching with optical fibers as well as maintain the high power output and reduce the emission linewidth. The lasers demonstrate single-mode operation with an over 35 dB side-mode suppression ratio, a power output ≤58 mW, a coupling efficiency to tapered fibers ≤40%, and a Lorentzian linewidth of 3.7 kHz. The performance allowed the free-running distributed feedback lasers to demonstrate spectroscopy of Rb vapor, which resolved the 85Rb and 87Rb two-photon transitions.
We demonstrate an integrated laser system for the mobile integrating sphere cold atom clock. Three distributed Bragg reflector diode lasers (780 nm) with custom drive circuits are used for the cooling, repumping, pumping, and probe lights. Automatic frequency-locking and relocking of the laser are presented. All of the optical elements are integrated on two sides of an aluminum base plate. The mechanical structure is simulated and optimized to minimize the deformation of the base plate. We optimize, measure, and discuss the frequency and intensity noises of the laser system. The techniques and designs used in this laser system can also be used in other mobile platforms for quantum sensing experiments.