Thomas Gretzinger

Mid-infrared astrophotonics: study of ultrafast laser induced index change in compatible materials

The mid-infrared wavelength regime 3.5 – 4.1μm, known as the astronomical L’ band is of special interest for exoplanet hunting. Mid-IR compatible photonic technologies are an enabling platform for a range of critical observational science using compact instruments on the next generation of Extremely Large Telescopes. Pupil remapping interferometry is a technique in which subapertures of the telescope pupil (2D) are reformatted into a 1D linear array. This can be done efficiently using 3D photonics. One of the most important techniques to fabricate 3D photonic devices in glass is ultrafast laser inscription. However, common silicate glasses are opaque above 2–2.2 μm and therefore not useful for the fabrication of waveguides at mid-infrared wavelengths. Here we present a study of mid-infrared transparent materials that are compatible with the ultrafast laser inscription technique. This study will inform the development of mid-infrared photonic devices for future exoplanetary discovery.

Ultrafast laser inscription in chalcogenide glass: Thermal versus athermal fabrication

Chalcogenide glasses are of great interest for a variety of applications, such as nonlinear optics, sensing and astronomy due to their high optical nonlinearity, broad infrared transparency as well as high photosensitivity. We report a detailed comparison of the inscription of single-mode waveguides in gallium lanthanum sulphide chalcogenide glass using 800 nm femtosecond lasers. The athermal and thermal fabrication regimes are explored by using laser repetition rates between 1 kHz and 5.1 MHz. Three different techniques are exploited to create waveguides with circular mode-fields: multiscanning and slit-beam shaping in the athermal regime and cumulative heating in the thermal regime. The fabricated structures are characterized in terms of physical size and shape, refractive index contrast as well as mode-field diameter and propagation loss to provide a roadmap for the inscription of low loss waveguides.

GLINT South: A photonic nulling interferometer pathfinder at the Anglo-Australian Telescope for high contrast imaging of substellar companions

With many thousands of exoplanets discovered one of the important next steps in astronomy is to be able to characterise them. This presents a great challenge and calls for new observational capabilities with both high angular resolution and extreme high contrast in order to efficiently separate the bright light of a host star to that of a faint companion. Glint South is an instrument that uses photonic technology to perform nulling interferometry. The light of a star is cancelled out by means of destructive interference in a photonic chip. One of the challenges is the star light injection into the chip. This is done by a unique active system that optimises the injection and provide low order correction for the atmospheric turbulence. We are reporting on the latest progress following several tests on the Anglo Australian Telescope.

Measuring stellar diameters with a compact integrated photonic nulling interferometer in a 8 meter-class telescope

One of the main challenges of astronomy these days is to directly image exoplanets and protoplanetary disks using diffraction limited optics. Due to the proximity of the planet to the host star and its small size, the discovery of new planets requires high angular resolution and high contrast imaging. Achieving these two requirements is challenging with existing astronomical instrument. However, the emerging field of physics called astrophotonics utilises photonic circuits to help build the core of astronomical instruments to address this problem. In addition, astrophotonics enables compact fully-integrated optical chips such as interferometers that are much easier to stabilise against mechanical and thermal changes in the environment.

Ultrafast laser waveguide inscription in gallium lanthanum sulfide

We report the fabrication of low-loss single-mode waveguides in the near- and mid-infrared in gallium lanthanum sulfide using femtosecond lasers. Three different techniques were explored: cumulative heating, slit-beam shaping and multiscan. Low propagation losses of 1.82 ± 0.07 dB/cm at 1550 nm were achieved.