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Advancing atomic sensors: a breakthrough in chip-scale Rb vapour cell spectroscopy
Researchers from the Indian Space Research Organization in Bengaluru, India, used VCSEL technology to achieve an exciting development in the field of atomic sensors
Q
uantum sensors have come to the forefront of sensing technologies,
relying on the interrogation of hot atomic vapours within specialised vapour cells. By examining the states of atoms through optical interrogation, absorption spectra are generated, allowing researchers to monitor and analyse target objects or phenomena. The miniaturisation trend in quantum sensors has paved the way for the creation of smaller hot vapour cells, opening up a multitude of applications. These sensors have found
applications in a variety of fields, including atomic clocks, gyroscopes, magnetometers, frequency-stabilised lasers, frequency standards, gas sensors, as well as GHz and THz imaging and detectors. The key to their adaptability lies in the manipulation of several parameters, including the choice of metallic alkali vapour (commonly Rubidium or Cesium), the incorporation
of inert buffer gases and spin polarisable atomic species like Xenon, and, most crucially, the configuration of chip-scale vapour cells.
Challenges and objectives While the potential for atomic sensors is immense, developing and harnessing their capabilities present challenges. The integration of components in a compact package, precise alignment for repeatable and accurate absorption spectra measurements, and the need for thermal control are critical. Packaging materials must be carefully chosen to ensure longevity and reliability, especially in the face of vibrations during handling and testing. Thermal isolation and control are paramount, given the requirement to heat the vapour cell, often to temperatures exceeding 70ºC. Achieving stable and repeatable absorption spectra is crucial in demonstrating the viability of
atomic sensors for space-borne applications and payloads.
Innovations in the magneto-optic package Researchers from the Laboratory for Electro-Optics Systems in Bengaluru, India, have taken a leap forward by developing and demonstrating a 3D-printed magneto-optic package housing a chip-scale Rubidium atomic vapour cell. This compact design integrates key components such as the vapour cell, VCSEL light source, magnets, quarter wave plates, filters, and photodiode for light sensing.
3D printing eliminates cost
constraints, allowing for custom design and precision down to 0.1mm. The housing has two sections: the holder section containing optical elements and the vapour cell, and the cap section, press-fitted and sealed with epoxy to create a vibration- immune package for testing and handling. The outer housing houses electronic components, including the VCSEL laser, collimating lens, and electrical connector. To ensure thermal isolation for the chip-scale vapour cell, the package is constructed from a polylactic acid material, effectively reducing steady-state power consumption. Jeremiah Hashley, a technical
1.The various components of the slotted MO housing: 1 – photodetector on an acrylic holder, 2 – annular magnets, 3 – heater integrated Rb vapour chip, 4 – ND filter. (b) the closed slotted housing placed inside the outer housing
34 Electro Optics October 2023
writer with Wavelength Electronics, whose WTC3243 Temperature Controller was integral to the project, says: “This research demonstrates the repeatability and reliability of the developed MO package required for atomic sensor design.
This package can be used for future applications in space-borne sensors and payloads.”
Heating the vapour cell To transition the Rb metal within the vapour cell from a liquid to a vapour phase, effective heating is essential. The team developed and demonstrated two approaches: Nichrome (NiCr) and Indium Tin Oxide (ITO). The NiCr method uses a patterned heating element, maintaining a steady state temperature of 60ºC. In contrast, the ITO approach employs sputter coating on the cell, reaching a steady state temperature of 56ºC. Both methods successfully elevate the vapour cell temperature to around 70ºC, creating an ideal environment for hot atomic vapour absorption spectroscopy studies.
The VCSEL light source For absorption spectroscopy, the choice of light source is crucial. The researchers opted for a VCSEL due to its single-mode output and narrow spectral linewidth. The VCSEL source features a built-in thermoelectric heater and thermistor, providing precise control over the laser output. To achieve this level of temperature control, researchers employed the WTC3243 Temperature Controller from Wavelength Electronics. Hashley said: “The organisation was looking for stable temperature control for the VCSEL light source. Changing temperature of the VCSEL can affect and shift the centre frequency of the output. A changing centre frequency can produce errors in the sensor measurements, making the absorption spectra unreliable. With high temperature stability from our WTC3243 Temperature Controller, the output of the VCSEL can be more reliable and stable in terms of centre frequency providing consistent sensor readings. Constant temperature is critical with close proximity to the hot vapour cell in the package. Although the packaging is made to be thermally isolating, better stability of the VCSEL temperature is achieved with the temperature controller from Wavelength.” The WTC3243 Temperature
Controller can provide ±2.2 A of output current to the thermoelectric cooler with both
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