Experimental proof of concept of a mid-infrared heterodyne astronomical interferometer using high bandwidth photonic correlation and quantum well detectors

PhD defense of Tituan Allain - Thursday, December 12th, 2024 at 2.00 pm - Manuel Forestini room IPAG

The mid-infrared emission from protoplanetary disks and stellar envelopes carries precious information about their dust and gas composition and the phenomena occurring in the vicinity of young stars, like the formation of terrestrial planets. To study the underlying physics of such dynamical systems, astronomical instruments require a resolution below the astronomical unit on objects a hundred parsecs away. This resolution can only be achieved with long-baseline interferometry because it corresponds to telescope diameters of a few kilometres. The Planet Formation Imager initiative has identified the creation of a mid-infrared interferometric array with a large number of telescopes as the next major step to constrain the theoretical models that describe planetary formation.

Currently, the most sensitive infrared interferometers, like the MATISSE and GRAVITY instruments at VLTI, rely on the direct recombination of light from several telescopes. This method is hardly scalable to many telescopes on kilometric baselines because of transmission losses and the bulkiness of the infrastructures. Heterodyne interferometry, which is widely used in radio-interferometry, has been identified as a possible alternative to direct interferometry in the mid-infrared because it does not require recombining the light from all telescopes physically. Instead, it relies on the heterodyne detection of the astronomical electric field with a phase-referenced local oscillator (a laser) and detecting the intermediate frequency with high-bandwidth detectors. The resulting signals are transmitted to a correlator whose role is to retrieve the interferometric observables from them. However, mid-infrared heterodyne interferometry suffers from reduced sensitivity because of the inherent quantum noise in heterodyne detection. Therefore, to detect weak astronomical objects, considerable efforts must be put to solve the technical and technological challenges that further limit the sensitivity of an heterodyne system.

My PhD thesis concentrates on the correlation and detection aspects of mid-infrared heterodyne interferometry. The correlation aspect consists of setting up, operating, and characterising the HIKE (Heterodyne Interferometry Kilometric Experiment) demonstration bench at IPAG, Grenoble. The bench uses an analogue photonic correlator built with commercial telecom components at 1.5 micrometres wavelength to correlate mid-infrared signals at 10 micrometres wavelength with gigahertz bandwidths. Such a set-up is a world premiere. I have developed a methodology to characterise the noise levels inside the system to identify the top offenders that hamper the measurement of interferometric visibility, and implement solutions to improve the sensitivity of the bench. My results show that the noise level associated with the photonic correlator is sufficiently low not to deteriorate the signal-to-noise ratio of the system. Hence, photonic correlation is sensitive enough to be used by heterodyne interferometry as an alternative to the computationally heavy digital correlation that is often used for radio heterodyne interferometry.

The current top offenders of our system are the commercial mid-infrared detectors that are used for heterodyne detection. Therefore, to improve the sensitivity of the bench, I have studied the possibility of replacing our detectors with high-bandwidth mid-infrared quantum well detectors. This work has been done in collaboration with the QUAD team at LPENS, Paris, where I have characterised metamaterial enhanced Quantum Well Infrared Photodetectors (QWIP) and Quantum Cascade Detectors (QCD). The high bandwidths of these detectors would represent a significant advantage to detect a larger chunk of the astronomical signal. However, despite recent progress, their quantum efficiencies currently remain too low to obtain a game-changing sensitivity improvement in heterodyne interferometry. Further improvement in the technology is required.

Thesis Direction

• Jean-Philippe Berger

The Jury will consist of

• Ludovic Grossard, Associate Professor, Xlim U. Limoges, Referee
• Nicolas Forget, Senior Researcher, INPHYNI CNRS, Referee
• Karine Perraut, Astronomer, IPAG UGA, Examiner
• Aline Dinkelaker, Researcher, AIP Leibniz, Examiner
• Benoît Darquie, Researcher, LPL CNRS, Examiner
• Lucas Labadie, Professor, I. Physics Institute U. Köln, Examiner