A narrow-linewidth Brillouin laser for a two-photon rubidium frequency standard

TL;DR

This work tackles the challenge of achieving high short-term stability in a deployable optical frequency standard based on a two-photon rubidium transition. By using a photonic integrated circuit SBS laser with a loaded Q of ~130 million and instantaneous linewidth $<10$ Hz to drive high optical intensities, the authors suppress both photon shot-noise and intermodulation noise, attaining a short-term instability of $\sim 2\times 10^{-14}$ at 1 s. The measured intermodulation and shot-noise limits for the SBS-stabilized system are $\sigma^{(IM)}_y(1\text{s}) = 3.46(20)\times 10^{-15}$ and $\sigma^{(SN)}_y(1\text{s}) = 4.2(3)\times 10^{-15}$, respectively, representing a significant improvement over prior reports. RAM and ac-Stark shifts remain dominant sources of instability, highlighting the need for RAM suppression and ac-Stark mitigation to realize robust field-deployable optical clocks with high stability across timescales.

Abstract

High precision portable and deployable frequency standards are required for modern navigation and communication technologies. Optical frequency standards are attractive for their improved stability over their microwave counterparts; however, increased complexities have anchored them in the laboratory. Sacrificing sensitivity of the most stable optical clocks has led to the recent development of deployable and portable optical frequency standards, leveraging hot atomic or molecular vapor. The short term limit for a majority of previous reports on two-photon rubidium standards is either the shot-noise or intermodulation limit hindering the one second fractional frequency stability to around $1\times10^{-13}/\sqrtτ$. The answer for the shot-noise limit is to increase optical power and collected fluorescence, while the intermodulation limit solution requires improvements in laser linewidth, stimulated Brillouin scattering (SBS) lasers are known to reduce frequency noise, suppressing noise of the pump laser at high ofset frequencies. We investigate an optical frequency standard based on the two-photon transition in $^{87}$Rb probed with a narrow linewidth photonic integrated circuit SBS laser with a quality factor over 130 million and instantaneous linewidth $<$ 10 Hz. The use of a narrow linewidth clock laser coupled with operating at higher optical intensities yields clock instabilities of $2\times10^{-14}$ at one second, currently the best reported short-term stability for a two-photon rubidium optical frequency standard.

Figures (5)

• Figure 1: (a) System diagram of the ECDL locked to an integrated silicon nitride resonator, as described in the text. The stimulated Brillouin scattering output (labeled out) is used to probe the $5S_{1/2}\rightarrow 5D_{5/2}$ two-photon transition. (b) The decrease in measured frequency noise of the SBS laser over the ECDL as measured against a stable cavity locked laser (fractional frequency $<3\times 10^{-15}$ at one second). Horizontal lines indicating intermodulation limited clock performance are also shown. PD-photodiode, EDFA- erbium doped fiber amplifier, EOM- electro-optic modulator, ECDL- external cavity diode laser, LO-local oscillator, SBS-stimulated Brillouin scattering.
• Figure 2: System diagram of the SBS laser locked to the two-photon transition in Rb as described in the text. The clock laser is directly compared via a heterodyne with both a cavity-stabilized laser and an optical frequency comb stabilized to a molecular iodine standard labeled Evergreen in the figure. PD-photodiode, EDFA- erbium doped fiber amplifier, EOM- electro-optic modulator, LO-local oscillator, SBS-stimulated Brillouin scattering, VCXO-voltage controlled crystal oscillator, AOM -Acoustic optic modulator, SHG-second harmonic generation, PMT-Photomultiplier Tube.
• Figure 3: Alongside the measured and expected clock performance are the measured environmental driven instabilities: Rb-Rb collisional, RAM, and ac-Stark all properly scaled to account for the impact on clock stability.
• Figure 4: Solid blue line shows the measured heterodyne between the iodine comb and the two-photon clock, while the dashed blue line show expected performance of this heterodyne including expected reference instabilities and drifts. Solid red line shows the measured heterodyne between the cavity laser and the two-photon clock, while the dashed red line show expected performance of this heterodyne including expected reference instabilities and drifts. Alongside the measured and expected clock performance are the measured environmental driven instabilities: Rb-Rb collisional, RAM, and ac-Stark properly scaled to account for their impact on clock shift
• Figure 5: The top graph shows the measured vertical (blue) and horizontal (red) alignment changes as the base plate temperature is varied as well as the frequency shifts (green). The middle graph shows the PMT temperature change (red) and frequency change (green) and the bottom graph shows the baseplate temperature change (blue) along with frequency shifts (green).