Just before the deposition of your Er:Al2O3 active materials, the coupling circuit, i.e., the strip waveguides, the method changeover tapers and the mode converters have been protected which has a 800 nm thick layer of hydrogen silsesquioxane (HSQ, n ~1.45) to forestall their coating with the active materials, as demonstrated https://charlesh207bkr5.blogoscience.com/profile