3D astrophotonic tricouplers for nulling interferometry
Nulling interferometry was first proposed back in the 1970’s and has since proven to be a promising technique for imaging exoplanets, overcoming the contrast and angular resolution limitations of single telescopes. This technique, implemented inside an integrated optical circuit, is used by the Guided Light Interferometric Nulling Technology (GLINT) instrument on the Subaru Coronagraphic Extreme Adaptive Optics system (SCExAO) at the Subaru Telescope in Hawaii. An upgrade to GLINT will use 3D waveguide tricouplers to simultaneously achieve nulling and perform fringe tracking for providing the required high contrast at small angular resolution to detect exoplanets. A tricoupler is a waveguide device with 3 inputs and 3 outputs, that unlike a 2D directional coupler, can provide achromatic interferometric suppression of the star light. This thesis investigates the fabrication of perfectly symmetric tricouplers for both, the astronomical J- and H-band, using the femtosecond laser direct-write technique. A symmetric tricoupler exhibits equal splitting ratios between all 3 waveguides. The coupled-mode equations for the tricouplers are numerically solved and fitted to experimental results to determine coupling coefficients and dephasing parameters. Starting from a directional coupler with 2 inputs and 2 outputs, the tricouplers are sequentially optimised. Using the coupled-mode equation results, the device geometry and fabrication parameters are adapted, yielding nearly perfectly symmetric tricouplers. Furthermore, stellar light is unpolarised, thus the polarisation dependent characteristics of the laser-written tricouplers are presented.