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APL Photonics

778.1 nm distributed feedback lasers for Rb two-photon atomic systems with sub-4 kHz linewidths

Eugenio Di Gaetano, Brendan Keliehor, Kevin Francis Gallacher, Paul F. Griffin, Marc Sorel, Erling Riis, Douglas J. Paul

Published May 1, 2024• 12 citations
Publisher

Abstract

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.

DOI
10.1063/5.0191088

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Applied Optics and Photonics China (AOPC)•Oct 28, 2025•0 citations
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.
Distributed feedback (DFB) laser chips have recently become available at wavelengths that match the D1 and D2 resonance transitions of alkaline atoms. We investigated the spectral properties, tuning characteristics and modulation behavior of continuous wave, single-mode DFB diodes at 778-780 nm and performed high-resolution spectroscopy of Rubidium vapor. The mode-hop free tuning range of the DFB diodes was as large as 2.4 nm (1186 GHz). The line width of the laser diodes was examined both with a heterodyne beat experiment and with high-resolution Doppler-free two photon spectroscopy, yielding a half width of 2-2.5 MHz. The saturation spectra of the D2-line of 85Rb and 87Rb were recorded with a resolution close to the natural line width. The emission frequency was actively stabilized to Doppler-free transitions with a relative accuracy of better than 4 parts in 109 using commercially available servo devices only. The output power of 80 mW was sufficient to allow for two photon spectroscopy of the 5S-5D-transition of 87Rb. We conclude that the performance of the DFB laser equals that of grating-stabilized external-cavity diode lasers (ECDLs), without the mechanical complexity of the latter systems. The DFB diode is thus well-suited to high-resolution applications in alkaline spectroscopy, including laser cooling and optical manipulation of ultra-cold atoms.
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.
We have performed high resolution spectroscopy of rubidium with a single mode continuous wave distributed feedback (DFB) laser diode. The saturation spectrum of the D 2 -line of 85 Rb and 87 Rb was recorded with a resolution close to the natural line width. The emission frequency was actively stabilized to Doppler-free transitions with a relative accuracy of better than 7 parts in 10 9 using commercially available servo devices only. An output power of 80 mW was sufficient to allow for two-photon spectroscopy of the 5S-5D-transition of 87 Rb. Further, we report on the spectral properties of the DFB diode, its tuning range and its frequency modulation properties. The line width of the diode laser, determined with high resolution Doppler free two photon spectroscopy, was 4 MHz without applying any active stabilization techniques. For time scales below 5 μs the line width drops below 2 MHz.
Optical wavelength standards, based on the optical transitions of atoms or molecules, provide ultra-high precision frequency references and are the core technology of next-generation optical clocks. Their stability far exceeds that of the microwave frequency range, laying the foundation for precision metrology and cutting-edge scientific research. The wavelength standard device based on the 5S1/2-5D3/2 hyperfine level two-photon transition of 87Rb atoms exhibits significant advantages such as no Doppler broadening, high signal-to-noise ratio (SNR), and potential for miniaturization and chip integration, showing important application potential in precision measurement fields like rubidium atomic clocks and optical communication. In this paper, an experimental setup for two-photon transition was constructed using an external cavity diode laser (ECDL), achieving stable locking of the 778 nm wavelength by modulating the laser current. The effects of the temperature and length of the atomic vapor cell on the blue fluorescence linewidth, SNR, and frequency stability were systematically studied, further optimizing the system. By using an atomic vapor cell designed at Brewster's angle, optical reflection loss was effectively reduced, compensating for the impact of shortened optical path on frequency stability in miniaturized atomic vapor cells. This provides new research directions and performance enhancement solutions for the miniaturization and chip integration of two-photon transition systems. In the improved system, a Brewster-angle glass vapor cell with a length of 30 mm was used and heated to 110°C, achieving a linewidth of 1.69 MHz full width at half maximum (FWHM) and a SNR of 20 dB. The system demonstrated a instability of 9.86×10-11 over a 1-second averaging time and a minimum instability of 5.79×10-11 over a 5.7-second averaging time. This achievement provides key technological support for the practical application of the next generation of miniaturized optical atomic clocks, with the potential for widespread use in portable metrology equipment and space science through integrated design and noise suppression optimization in the future.