Optical sources for atomic sensors

Meiklejohn, James 2025. Optical sources for atomic sensors. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis presents work on novel VCSEL devices intended to better meet the application requirements of miniature atomic sensors. Atomic sensors require a laser light source that meets a number of specifications that are difficult to achieve with conventional VCSEL designs, requiring particular optical output power, polarisation stability, and SMSR. Increased optical power can be obtained by using arrays of multiple individual emitters. Optically coupling multiple single-mode cavities together is an approach that can scale the total single-mode output power, and results are presented from designs intended to achieve lateral optical coupling. Attempts to use angled ends to contain light in an in-plane mode between the emitters with a total internal reflection condition resulted in a small power change (less than 1% of the total optical power) indicating that there is a weak interaction between some pairs of emitters with a centre-centre distance of 12 um. Material and fabrication variations lead to differences between the emission wavelengths of closely-spaced emitters, which makes fabricating laterally coupled emitters challenging, and the effects of these variations are quantified. Conventional VCSELs use an oxide aperture to laterally define the optical cavity, but a defect in a photonic crystal structure can also be used to define the cavity, providing a different method for optical confinement that potentially supports a larger aperture size and so higher single-mode optical output power. Measurements from photonic crystal devices are presented, showing an increase in the single-mode aperture size to 4.25 um, compared to 3 um for an oxide-confined device. Multiphysics simulation of a VCSEL gives insight into the internal mechanisms that determine the operation and performance of a device. Simulations are used as a design tool to determine the effects of design changes, and to develop an understanding of the dominant mechanisms. Using the Harold VCSEL software package a simulation of a VCSEL is set up and validated by comparison with experimental results. It was found that modifications are required to the model to accurately represent the thermal resistance of VCSELs, as bulk material thermal conductivity values result in an underestimate of the thermal resistance by an order of magnitude in the vertical direction due to the effect of phonon scattering at material interfaces. The high-temperature performance of VCSELs is shown to depend on the aperture size. Smaller emitters are shown to have better power conversion efficiency at elevated temperatures and higher turn-off temperatures than larger ones. This motivates the use of small apertures in arrays of VCSELs, and arrays of small emitters are shown to have favourable power scaling properties. The thermal conductivity per emitter goes from 0.21 to 0.17 mW/K when scaling from 1 to 4 emitters, suggesting that arrays of emitters defined in this way represent a viable route for scaling to higher optical output power.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	VCSEL, atomic sensor, temperature, simulation, array, modelling
Funders:	EPSRC
Date of First Compliant Deposit:	15 January 2026
Last Modified:	15 Jan 2026 16:20
URI:	https://orca.cardiff.ac.uk/id/eprint/183916

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