Swept Heterodyne Correlation for Long-Baseline Stellar Interferometry

Heterodyne detection of thermal light relative to a common phase reference is fundamental to heterodyne stellar interferometry. This technique allows for the reconstruction of information about stellar diameters and circumstellar environments by correlating signals from multiple telescopes without the requirement to stabilise the optical path of the ultra-weak stellar light. Pioneering work by Charles Townes and collaborators in the thermal infrared [1] demonstrated the power of this technique, achieving extreme angular resolutions on the order of a few tens of milliarcseconds <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\simeq 0.2\mu \text{rad})$</tex>) at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10.6 \mu \mathrm{m}$</tex>. However, extending this approach to optical wavelengths faces a significant bottleneck: an inherently low [2] signal-to-noise ratio (SNR), limited by (1) the narrow detection band-width of electronic detectors (<50 GHz) with regards to the bandwidth of thermal sources and (2) the shot-noise fluctuations of the single-frequency reference laser.