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Optical and Quantum Sensing and Precision Metrology II

System-level evaluation of 795nm Q-VCSELs in 87Rb qSensors

Amirhossein Ghods, Newton Rai, G A Garcia, Gabriela Martinez, Olawale Lawal, Sristy Agrawal

Published Mar 5, 2026• 0 citations
Publisher

Abstract

We report system-level evaluation of manufacturable, quantum-optimized 795-nm vertical-cavity surface-emitting lasers (Q-VCSELs) for rubidium (Rb) atomic sensors, with emphasis on integration into an 87Rb coherent population trapping (CPT) clock prototype. Operation at 795 nm (Rb D1) is constrained by increased internal loss/absorption, polarization instability, and stringent requirements on spectral purity. At the component level, we employ epitaxial and layout engineering to improve mode discrimination, polarization robustness, and high-temperature efficiency, achieving stable single-transverse-mode emission and linear polarization from 20–80 °C with side-mode suppression ratio (SMSR) >30 dB and polarization extinction ratio (PER) >20 dB. The devices exhibit low threshold current (<400 µA at 80 °C) and slope efficiency >0.5 mW/mA, enabling >1 mW single-mode, single-polarization output power in chip-scale form factors. Quantum-relevant characterization shows linewidth on the order of ~30–35 MHz and relative intensity noise below −130 dB/Hz at frequencies greater than 1KHz. We then integrate the Q-VCSEL into an 87Rb CPT clock setup and demonstrate a clear CPT resonance, achieving ~2.5% contrast and ~3 kHz CPT linewidth (FWHM), corresponding to a measured figure of merit of ~8.2 ppm. We further map key operating tradeoffs relevant to clock optimization, including optical power broadening (contrast peaking near ~50–60 µW while CPT linewidth increases with optical power) and RF-drive dependence across representative modulation powers. These results connect device-level laser characteristics to clock-level observables and provide a manufacturable component pathway for scalable Rb-based timing modules.

DOI
10.1117/12.3078525

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•Dec 1, 2004•6 citations
We report system-level evaluation of manufacturable, quantum-optimized 795-nm vertical-cavity surface-emitting lasers (Q-VCSELs) for rubidium (Rb) atomic sensors, with emphasis on integration into an 87Rb coherent population trapping (CPT) clock prototype. Operation at 795 nm (Rb D1) is constrained by increased internal loss/absorption, polarization instability, and stringent requirements on spectral purity. At the component level, we employ epitaxial and layout engineering to improve mode discrimination, polarization robustness, and high-temperature efficiency, achieving stable single-transverse-mode emission and linear polarization from 20–80 °C with side-mode suppression ratio (SMSR) >30 dB and polarization extinction ratio (PER) >20 dB. The devices exhibit low threshold current (<400 µA at 80 °C) and slope efficiency >0.5 mW/mA, enabling >1 mW single-mode, single-polarization output power in chip-scale form factors. Quantum-relevant characterization shows linewidth on the order of ~30–35 MHz and relative intensity noise below −130 dB/Hz at frequencies greater than 1KHz. We then integrate the Q-VCSEL into an 87Rb CPT clock setup and demonstrate a clear CPT resonance, achieving ~2.5% contrast and ~3 kHz CPT linewidth (FWHM), corresponding to a measured figure of merit of ~8.2 ppm. We further map key operating tradeoffs relevant to clock optimization, including optical power broadening (contrast peaking near ~50–60 µW while CPT linewidth increases with optical power) and RF-drive dependence across representative modulation powers. These results connect device-level laser characteristics to clock-level observables and provide a manufacturable component pathway for scalable Rb-based timing modules.
The spectroscopic technique of coherent population trapping (CPT) enables an all-optical interrogation of the groundstate hyperfine splitting of cesium (or rubidium), compared to the optical-microwave double resonance technique conventionally employed in atomic frequency standards. All-optical interrogation enables the reduction of the size and power consumption of an atomic clock by two orders of magnitude, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources due to their low power consumption and circular output beam. Several research teams are currently using VCSELs for DARPA's chip-scale atomic clock (CSAC) program with the goal of producing an atomic clock having a volume < 1 cm^3, a power consumption < 30 mW, and an instability (Allan deviation) < 1x10^-11 during a 1-hour averaging interval. This paper describes the VCSEL requirements for CPT-based atomic clocks, which include single mode operation, single polarization operation, modulation bandwidth > 4 GHz, low power consumption (for the CSAC), narrow linewidth, and low relative intensity noise (RIN). A significant manufacturing challenge is to reproducibly obtain the required wavelength at the specified VCSEL operating temperature and drive current. Data are presented that show the advantage of operating at the D1 (rather than D2) resonance of the alkali atoms. Measurements of VCSEL linewidth will be discussed in particular, since atomic clock performance is especially sensitive to this parameter.
We report on the performance of the first table-top prototype atomic clock based on coherent population trapping (CPT) resonances with parallel linearly polarized optical fields (lin‖lin configuration). Our apparatus uses a vertical-cavity surface-emitting laser (VCSEL) tuned to the D1 line of 87Rb with the current modulation at the 87Rb hyperfine frequency. We demonstrate cancellation of the first-order light shift by the proper choice of rf modulation power and further improve our prototype clock stability by optimizing the parameters of the microwave lock loop. Operating in these optimal conditions, we measured a short-term fractional frequency stability (Allan deviation) 2×10−11τ−1/2 for observation times 1 s ≤ τ ≤ 20 s. This value is limited by large VCSEL phase noise and environmental temperature fluctuation. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated lin‖lin CPT resonance clock with environmental impacts minimized.
A new generation of small low-power atomic sensors, including clocks, magnetometers, and gyroscopes, is being developed based on recently available MEMS and VCSEL technologies.These sensors rely on spectroscopic interrogation of alkali atoms, typically rubidium or cesium, contained in small vapor cells.The relevant spectroscopic wavelengths (in vacuum) are 894.6 nm (D1) and 852.3 nm (D2) for cesium, and 795.0 nm (D1) and 780.2 nm (D2) for rubidium.The D1 wavelengths are either preferred or required, depending on the application, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources because of their low power consumption and circular output beam.This paper describes the required VCSEL characteristics for atomic clocks and magnetometers.The fundamental VCSEL requirement is single-frequency output with tunability to the particular spectroscopic line of interest.Singlepolarization and single-transverse-mode operation are implicit requirements.VCSEL amplitude noise and frequency noise are also important because they contribute significantly to the sensor signal-to-noise ratio.Additional desired VCSEL attributes are low cost, low power consumption, and several years of continuous operating lifetime.This paper also describes the 894-nm VCSELs that we have developed for cesium-based atomic sensors.In particular, we discuss VCSEL noise measurements and accelerated lifetime testing.Finally, we report the performance of prototype atomic clocks employing VCSELs.
Chip-scale atomic clocks are critical for applications requiring compact, energy-efficient, and highly stable timing, such as GPS receivers and communication systems. Coherent population trapping (CPT)-based atomic clocks, which commonly use vertical-cavity surface-emitting lasers (VCSELs), have enabled significant miniaturization. However, their precision is limited by the broad linewidth of VCSELs. Alternative laser technologies, such as distributed Bragg reflector lasers, distributed feedback lasers, and external cavity diode lasers (ECDLs), offer potential improvements but face challenges in achieving compact size and adequate modulation bandwidth. This study presents a 795 nm chip-ECDL with a direct modulation bandwidth up to 4 GHz, specifically designed for CPT chip-scale atomic clocks. Based on the chip-ECDL, a CPT atomic clock with a compact volume of 19.4 ml and power consumption of 1.5 W has been developed. Under identical operating conditions, the CPT signal produced by the chip-ECDL exhibits a 34% reduction in linewidth and a 1.14-fold increase in contrast compared to a VCSEL-based clock. Demonstrating short-term stability better than 5×10−11τ−1/2(1 s<τ<2000 s) and reaching a best stability of 6.6×10−13 at 2000 s, this work represents the application of an ECDL in a CPT chip-scale clock, advancing the development of next-generation, high-performance chip-scale atomic clocks.
Previously, we have proposed a method to control the emission spectrum of the vertical-cavity surface-emitting laser (VCSEL) with the synchronized modulation of the injection current at single and doubled frequencies. In this work, the above method is used to improve the metrological characteristics of the coherent population trapping (CPT) resonance in 87 Rb. The dual-frequency (DF) modulation reduces the carrier power and suppresses the light shift of the resonance frequency, if it is unattainable with the single-frequency modulation. In addition, a higher resonance contrast is achieved by equalizing the powers of the resonant components of the spectrum.
We describe chip-scale batch-fabricated cesium cells utilizing semiconductor wafer processing, pin transfer of cesium, and silicon/Pyrex anodic bonding for cell sealing. Highspeed, single-mode linearly polarized VCSELs emitting at the 133Cs D1 line were fabricated, optimized for low threshold and high-speed operation with overall small power dissipation. Coherent population trapping (CPT) end-resonance signals with a contrast and linewidth of 5.4% and 7.1 kHz respectively are achieved using 1 mm optical path length cells and VCSEL driven with 1.25 mW RF modulation and 2.76 mW DC power dissipation. We demonstrate short-term stability with an atomic clock built with these components operating on the CPT end-resonance. The magnetic field is stabilized with a novel magnetic field locking scheme tapping the Zeeman resonance.