Thanks for finding this. The paper mentioned in the Description section can be read here: https://www.academia.edu/52566311/_title_Volume_phase_holographic_gra

Thanks for finding this. The paper mentioned in the Description section can be read here: https://www.academia.edu/52566311/_title_Volume_phase_holographic_gratings_and_their_potential_for_astronomical_applications_title_ https://www.academia.edu/52566311/_title_Volume_phase_hologr... My guess is that each of the items in patent diagram labeled 20 and 22 are gratings of this sort, perhaps with the fringes angled at 45 degrees. Some additional info may be gleaned from the one patent citing this one, invented by one of the co-inventors of the latter: https://patents.google.com/patent/DE102019206354A1/en https://patents.google.com/patent/DE102019206354A1/en