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Infrared heterodyne detection technique for aperture synthesis imaging in astronomy

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2 Heterodyne detection for aperture synthesis imaging

The principle of heterodyne interferometry aperture synthesis is to encode the incoming celestial signals falling onto each telescope into a radio frequency RF signal by interfering it with a local oscillator synchronized with other telescopes. The resulting signal is then correlated with other RF signals coming from the other telescopes, in order to measure the spatial coherence of the object and allow the image reconstruction to be carried out. While this technique is widely used in the radio and mm regime, its development has been frozen in the infrared domain by lack of sensitivity. During the past few years, the breakthroughs in mid-IR detectors, laser synchronization, microwave photonics, quantum optics and frequency combs, have permanently pushed away the foremost limitations considered for heterodyne interferometry. In the light of these developments, and in the context of the rising of the mid-IR technologies, it appears that the previous limitations in terms of integration bandwidth and sensitivity, signal correlation and local oscillator synchronization, are now considerably modified. This is the objective of this collaboration between LIPhy, and IPAG, in the frame of Guillaume Bourdarot’s PhD.

  • • The first innovative approach is a parallelized heterodyne detection by means of an optical frequency comb. To extend the effective bandwidth of the observation signal and to increase the total flux of the object, we plan to make use of a multi-frequency local oscillator (LO). Instead of a single frequency laser, a comb of optical frequencies is distributed to the telescopes. The light from the telescope is mixed with the LO-comb, and then de-multiplexed to separate the different spectral channels. Each spectral channel (comprising a comb tooth -– the LO-, and the light of interest in the spectral channel) is then detected by a fast photo-detector. The frequency comb will be generated by electro-optic (EO) phase and amplitude modulation of a single frequency laser at 1.55 µm. Each spectral channel is then detected by a fast photo-detector. Another advantage of parallelized heterodyne detection is the possibility of frequency-resolved measurements for spectroscopic applications.
    sketch of a FSL
    Heterodyne interferometer. A comb of optical frequencies (created by EO modulation of a CW laser), is used as a multiple local oscillator, and delivered to both telescopes. The heterodyne signals are recombined on detectors, and cross-correlated.

  • • The second innovative approach is related to the correlation of the signals. Correlating the signals coming from a high number of telescopes is extremely computer-intensive and requires dedicated expensive calculators’ architectures. We propose a radical new approach by transforming the RF signal from each telescope into a phase modulated coherent optical signal. The conversion from the RF world to the telecom wavelength laser regime allows the extraordinary richness of telecommunication photonics such as wavelength multiplexing and fiber time delay optical components to be exploited. The need for costly free-space delay-lines, lossy multiple mirror reflection such as in classical optical interferometry disappears. We have demonstrated in the laboratory in the near infrared (1.55 µm) the heterodyne correlation of two beams using off-the- shelf components.
    See : Bourdarot AA (2020).
    sketch of a FSL
    Example of photonic correlator based on phase modulators.