(Invited) Photonic Chip for Laser Stabilization to a Rubidium Atomic Vapor
Authors: Hummon Matthew Taylor , Dellis Argyris , Kang Songbai , Kitching John E , Aksyuk Vladimir A , Srinivasan Kartik , Westly Daron A , Li Qing , Roxworthy Brian J 
Published: February 2020

Abstract— We report on recent progress on stabilization of an optical laser to a photonic chip based MEMS fabricated rubidium vapor cell. The interrogation light is delivered to the cell from the edge of the photonic chip via a fiber-pigtailed SiN waveguide. The light is directed into the vapor cell from the waveguide using a diffractive grating coupler. We will discuss the stability and environmental sensitivity of a laser locked to the photonic chip device. Additionally, observation of sub-Doppler features in the atomic absorption spectrum have been observed using a counter propagating pump beam generated from a partial reflector placed above the photonic chip.
The development of high precision, ultraminiature, quantum based measurement devices has the potential to transform measurement science, enabling precision metrology outside the laboratory [1]. To this end, we are developing a photonic chip based platform for precision optical spectroscopy of alkali vapors. A laser stabilized to an optical transition in an alkali vapor can provide an accurate wavelength standard.
The photonic chip is based on a silicon nitride/silicon dioxide on silicon structure. Fig. 1 shows an optical image and schematic of the photonic chip. Rubidium is introduced into the evacuated MEMS cell via optical heating of an alkali metal dispenser pill located inside the MEMS cell [2]. Waveguides direct probe light from the edge of the photonic chip to the Rb vapor cell. Grating output couplers direct the probe light out of the waveguide up through the Rb vapor cell and onto a silicon photodetector placed on top of the chip. The lower right panel of Fig. 1 shows an absorption spectrum of the Rb D1 line at 795nm taken using the photonic cell. We will report on the stability of a laser locked to this transition using the photonic chip.
Retro-reflection of the probe beam by a partial reflector allows for the observation of sub-Doppler features. These sub-Doppler features allow for higher stability laser stabilization. Additionally, we will compare the performance of photonic chips with grating couplers that provide different output laser beam diameters.
Figure 1. Top Left: Image of photonic chip. The chip contains two wells of dimension 1mm x 1mm. The left well contains a Rb alkali metal dispenser. The right well contains the photonic structures. Upper Right: Close up of photonic waveguides and grating output couplers. Lower Left: Cross section of the photonic chip describing the planar structure. PD: photodetector. Lower Right: Rubidium D1 line absorption spectrum at 795 nm. The photonic cell is heated to a temperature of 90 °C.
Acknowledgment
This work is a contribution of NIST and is not subject to copyright.
References
[1] J. Kitching, et al., “NIST on a Chip: Realizing SI units with microfabricated alkali vapour cells,” J. Phys.: Conf. Ser., in press.
[2] A. Douahi, et al., “Vapor microcell for chip scale atomic frequency standard,” Electron. Lett. vol. 43, pp. 279–280, March 2007.

Figure 1