Multi-core fiber-fed integral field spectrograph (MCIFU)-III: An ultrafast laser inscribed photonic reformatter and mask

We report on the conception and the fabrication of a 3D photonic reformatter of 73 waveguides and its associated opaque mask in a wide collaboration to develop a multi-core fiber-fed integral field spectrograph (MCIFU) centered on the Jband. The reformatter is a 3D structure that light from the input quasi-hexagonal multicore fiber is spread out by rearrangement to avoid individual core spectra overlapping when the light is dispersed. The reformatter is fabricated using ultrafast laser inscription (ULI) in a borosilicate glass of 20 mm length. Using a similar ULI process, a 73-hole mask was fabricated in silica glass that precisely matched the waveguides at the output of the reformatter. The output surface of the mask was coated with a 120 nm layer of chromium to block scattered light generated in the bulk material and enhance the signal-To-noise. All inscribed waveguides, characterized using a stable laser centered at 1310 nm from the multicore fiber to the output mask, present consistent single-mode output behavior with a maximum throughput exceeding 60%. Over the 73 cores, the average throughput was measured at 40%. First observations of the full MCIFU device during on-sky measurements have shown promising results to the potential of this novel fiber integral field unit.