Hybrid electro-optic visible multi-telescope beam combiner for next generation FIRST/SUBARU instruments

Abstract

FIRST (Fibered Imager foR a Single Telescope instrument) is an instrument that enables high contrast imaging and spectroscopy, thanks to a unique combination of sparse aperture masking, spatial filtering by single-mode waveguides and cross-dispersion in the visible. In order to increase the instrument’s stability and sensitivity, we propose an active hybrid photonic beam combiner. The device consists on a 5T integrated optics beam combiner. The idea is to split the architecture in two parts: A first part, concerning input beam splitting and active phase modulation, requiring relatively simple optical circuits (Y junctions and straight waveguides) is obtained in an electro-optic crystal (Lithium Niobate). A second part, where the complex beam recombination of all the split inputs is achieved (for N inputs, N(N-1)/2 recombinations). This stage implies many waveguide crossings, bendings and lengthy waveguides. Therefore, a high transmission, high confining glass is used. In both cases, classical lithography and ion in-diffusion techniques are used to fabricate the waveguides. Both stages have been optimized in terms of mode matching and single mode spectral bandwidth. They have been assembled together and with input/output fibered V-grooves. The work presented here consists on the characterization of the hybrid 5T beam combiner on the optical bench simulator of the FIRST/SUBARU instrument that is developed at LESIA. We will present results in terms of transmission, polarization and active phase modulation, showing that with relative low voltages, active fringe scan is achieved directly on-chip, at frequencies only limited by the readout time of the camera.