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Case Study CS-TC08 Rev. A

VCSEL Absorption Spectroscopy

of Chip-scale Rubidium Atomic

Vapor

August, 2023

Page 1

ABSTRACT

Researchers from the Indian Space Research Organization in Bengaluru, India have developed and

demonstrated the absorption spectroscopic capabilities of a chip-scale Rubidium (Rb) atomic vapor cell

using a thermoelectric cooler integrated VCSEL light source in a magneto-optic package. The custom

three-dimensionally printed design provides real time analysis of spectral data by laser interrogation of

the D1 hyperfine transition states of the Rb vapor in the chip-scale cell. With over 600 hours of data,

absorption resonance lines were recorded and analyzed for transitions 85Rb and 87Rb with absorption

amplitude and FWHM data agreeing with known literature values. This magneto-optic Rb atomic cell

package proves potential for atomic sensors, particularly in space borne applications or payloads.

HOT ATOMIC VAPORS

Recent developments of quantum sensors rely on

sensing hot atomic vapors in a specific type of vapor

cell.1 By optically interrogating the atoms and their states,

an absorption spectra can be obtained that can lead to

changes or monitoring of the sensor's object of interest.

The miniaturization of these quantum sensors also requires

increasingly smaller-sized hot vapor cells. Fabrication on

a single wafer with hundreds of vapor cells or chip-scale

vapor cells enable quantum sensors to be utilized in a

wide range of applications. Quantum sensing applications

include: atomic clocks, gyroscopes, magnetometers,

frequency-stabilized lasers, frequency standards, gas

sensors, as well as GHz and THz imaging and detectors.1

There are multiple parameters that are changed and

manipulated for different experiments. First, the vapor cell

is filled with the appropriate metallic alkali vapor. Common

vapors are Rubidium (Rb) or Cesium (Cs). Secondly, some

applications require the addition of inert buffer gases and

spin polarizable atomic species such as Xenon (Xe).1

Lastly, and most importantly, the configuration of the chip-

scale cells determine how the atomic vapors are spectrally

observed. Vapor cells can be constructed with planar,

spherical, and cubical shapes using MEMS fabrication

technology.

Typically, the alkali vapor cell is the first priority of the

system, as a miniature chip-scale cell and absorption

spectroscopy based on that vapor cell can be a tedious

task to setup and calibrate. The other component that is

crucial for this type of sensor is the light source for optical

interrogation of the atoms inside the vapor cell. A laser

diode, Light Emitting Diode (LED), or a Vertical Cavity

Surface Emitting Laser (VCSEL) are common choices for

absorption spectroscopy in a small package.

PROBLEMS AND GOALS

The packaging containing the light source, vapor cell,

heating element, and optics requires precise alignment

to allow absorption spectra measurements to become

repeatable and accurate. Compactness and error-free

alignment can expand the range of applications for quantum

sensors, especially in space applications.

Vibrations in handling and testing also can affect the

alignment of components inside the packaging containing

the vapor cell and optics. Careful consideration of the

packaging materials and design must be taken to ensure

longevity and repeatability. Because the cell needs to be

heated, ex. 70ºC, thermal isolation and control is required

for the light source. The element used in the vapor cell

needs to have a boiling point that is reasonable to achieve

and maintain for a constant vapor phase to observe atomic

absorption resonances. Higher temperatures can also

create stronger absorption resonances for observation.

The cell heating design and method can effect temperature

stability, power consumption, and number density of vapor

required for absorption spectroscopy experiments. In this

study, two approaches are developed for integrated thin film

heaters.

When the packaging, optics, and alignment have shown to

be reliable for absorption studies, obtaining spectra is one

of the final steps for developing an atomic sensor using

chip-scale vapor cells. Achieving stable and repeatable

absorption spectra is critical for demonstrating a viable

atomic sensor that can be implemented into a wide range of

applications in space borne sensors and payloads.1