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

像在近红外波段工作的超大型望远镜ELT这样的地面望远镜是天文学各种科学案例的支柱之一，例如，寻找系外行星，基础物理学（暗物质，暗能量，宇宙时间物理的普遍有效性），黑洞，恒星中的核合成和物质循环，星系的形成和演化。虽然大气干扰导致的波前变形可以通过自适应光学来抵消，但这些望远镜的灵敏度受到强大气OH发射线的严重阻碍，用于近红外低至中分辨率光谱。这些谱线的强度比微弱恒星或星系的地外背景大几个数量级，因此它对摄谱仪内部的杂散光信号有很大的贡献，这使得测量微弱的地外信号非常具有挑战性。然而，由于这些发射线的光谱宽度只有几十皮米，因此可以使用窄陷波滤波器来抑制它们并减少仪器内的杂散光。特别是光纤布拉格光栅（FBG），由光纤芯中的周期性折射率调制组成，具有滤除OH发射线的优异性能，这在之前已经得到了证明。然而，为了滤除大量的OH发射线，必须串联实现许多单独的光栅，造成很大的传输损失。非周期fbg （afbg）是一系列简单fbg的替代方案。AFBGs的思想是在光纤芯内以最小的叠加折射率调制重叠几个不同共振波长的光栅。因此，单个AFBG可以同时反射多个波长，从而滤除大量OH发射线。OH- super项目希望通过加入两个研究机构的互补专业知识来开发高度复杂的非周期光栅，从而实现近红外OH抑制的颠覆性创新。在联合空中演示器中使用两种类型的光栅（由UV或fs-IR激光结构编写）的竞争性实现将说服ESO等基础设施运营商在望远镜上使用性能最佳的解决方案来实现后续设备。OH- super的两个主要目标是，首先，开发基于UV和fs-NIR激光的刻字技术，以便能够始终提供适合原型水平OH抑制的afbg。其次，将得到的滤波器作为前端子系统集成到西班牙Calar Alto天文台3.5m望远镜上的CARMENES演示器中，以便进行验证和比较性能研究。

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.