Case Study CS-TC08 Rev. A

Page 2

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METHOD

Researchers from the Laboratory for Electro-Optics

Systems in Bengaluru, India, have developed and

performed absorption spectroscopic studies of a compact,

three-dimensional (3D) printed, magneto-optic (MO)

package containing a chip-scale Rubidium atomic vapor

cell. The printed MO package allows the integration of key

components including the vapor cell, the VCSEL used as

the light source, magnets for affecting how the light passes

through the polarizing elements, quarter wave plates,

the filters, and photodiode for sensing the light. These

elements and cross-section of the MO package are shown

in

Figure 1.

Figure 1. The schematic cross-section of the MO

package. 1 – chip-scale vapor cell, 2 – VCSEL laser,

3 – annular magnets, 4 – collimating lens, 5 – ND filter, 

6 – photodiode.1

The ability to 3D print the package eliminates high costs,

allows custom design, and ensures accuracy and precision

of up to 0.1 mm with a compact setup. The cross section

in

Figure 1 shows precisely machined slots for the

variety of optical elements and vapor cell. The package

is split into two sections: a slotted housing and the outer

housing. Both of these sections can be seen in

Figure 2.

The slotted housing is comprised of the holder section that

contains the optical elements and vapor cell as well as the

cap section that is press-fitted on the holder section and

sealed with epoxy to create a vibration-immune package

for handling and testing purposes. The outer housing

contains the electronic components of the setup: VCSEL

laser, collimating lens, and a miniature electrical connector.

Other components, like the current source and temperature

controller are outside the MO package and connected to

the appropriate elements.

The MO package is made from a polylactic acid material

creating a relatively low thermal conductivity design. This

ensures good thermal isolation for the chip-scale vapor

cell. Not only does this reduce how the temperature of the

vapor cell affects the package and specifically the VCSEL

temperature and output, but it reduces the steady state

power consumption of the overall package.1

Figure 2. (a) showing the various components of the

slotted MO housing 1 – photodetector on an acrylic

holder, 2 – annular magnets, 3 – heater integrated Rb

vapor chip, 4 – ND filter. (b) the closed slotted housing 

placed inside the outer housing.1

The chip-scale vapor cell's dimensions are 12 mm x 8 mm x

1.5 mm with an optical cavity 3 mm in diameter and an

optical path length of 0.5 mm. The cell is designed with a

glass-silicon-glass stack, and a Rb dispenser pill is used

in the development stage and inserted into the cavity to

be thermally activated and released. The boiling point of

Rb is 39.48ºC. So, at room temperature, the Rb metal is

in liquid phase and is found in condensed droplets on the

windows of the cell. Therefore, the cell needs to be heated

to transition the Rb metal into a vapor phase to observe

atomic absorption resonances.1

To heat the Rb vapor cell to a temperature well above the

boiling point, two approaches are taken and demonstrated:

Nichrome (NiCr) and Indium Tin Oxide (ITO).

Figure 3

shows both methods of heating the vapor cell as well as a

thermal image under test.

Figure 3. Heater integrated chip-scale Rb vapor cells

(a) patterned NiCr heating element. (b) ITO coated vapor

cell with aluminum contact pads. (c) thermal image of

the integrated chip-scale heater under test.1