Next Generation OH Suppression Fiber Bragg Gratings: towards Operation on Sky

• Applicants: Professor Dr. Stefan Nolte; Dr. Aashia Rahman, since 5/2022
• Subject Area: Astrophysics and Astronomy
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 455425131

Ground-based telescopes like the Extremely Large Telescope ELT operating in the near-infrared are one of the pillars of Astronomy for various science cases such as, e.g., the search for exoplanets, fundamental physics (dark matter, dark energy, universal validity of physics over cosmic time), black holes, nucleosynthesis in stars and the cycle of matter, the formation and evolution of galaxies. While atmospheric disturbances leading to deformations of the wavefront can be counteracted by adaptive optics, the sensitivity of these telescopes is severely hampered by strong atmospheric OH emission lines for low to medium resolution spectroscopy in the NIR. The intensity of these lines is several orders of magnitude larger than the extraterrestrial background of faint stars or galaxies and thus it contributes substantially to the stray light signals inside the spectrograph, making the measurement of the faint extraterrestrial signals very challenging. However, as the spectral width of these emission lines is only a few tens of picometer, narrow notch filters can be used to suppress them and reduce stray light within the instrument. Especially, fiber Bragg gratings (FBG), consisting of a periodic refractive index modulation in the fiber core, offer excellent properties to filter out the OH emission lines, which has been demonstrated before. Yet, in order to filter out a large number of OH emission lines, many individual gratings would have to be realized in series causing significant transmission loss. An alternative to a series of simple FBGs are aperiodic FBGs (AFBGs). The idea of AFBGs is to overlap several gratings with different resonance wavelengths inside the fiber core with a minimized superimposed refractive index modulation. Therefore, a single AFBG can reflect many wavelengths simultaneously to filter out a large number of OH emission lines. The OH-SUPER project aspires to realize a disruptive innovation for OH suppression in the NIR by joining the complementary expertise of the two research institutions involved to develop highly complex aperiodic gratings. A competitive implementation with two types of gratings (written by UV or fs-IR laser structuring) within a joint on-sky demonstrator will convince infrastructure operators such as ESO to implement a follow-up device using the best-performing solution at the telescope. The two major goals of OH-SUPER are, firstly, to develop UV and fs-NIR laser-based inscription techniques to be able to consistently deliver AFBGs suitable for OH suppression at the level of prototypes. Secondly, the resulting filters shall be integrated in an onsky demonstrator as a front-end subsystem to CARMENES at the 3.5m telescope of Calar Alto Observatory, Spain, to allow for a validation and comparative performance study.

• DFG Programme: New Instrumentation for Research
• Major Instrumentation: High power, high coherence UV laser
• Instrumentation Group: 5710 Gas-Laser
• Ehemaliger Antragsteller: Professor Dr. Martin Matthias Roth, until 5/2022