Role of the high vacuum in the precise control of index contrasts and index profiles of LiNbO3 waveguides fabricated by high vacuum proton exchange

Precise control and reproducibility of optical waveguides fabrication are extremely important for efficient and compact devices for a wide range of applications in either classical or quantum photonics. In this way, wave guides fabricated in lithium niobate (LN) crystals offer one of the most established platforms in modem photonics. Here we experimentally validate the excellent control and reproducibility of the index contrast and index profile of optical waveguides fabricated on Z-cut lithium niobate (LN) substrates by the recently proposed High Vacuum Proton Exchange (HiVacPE) technique. Based on the known chemical reactions involved in the refractive index variation mechanism we explain why the high vacuum, having a drying effect, is the key feature to obtain highly reproducible HiVacPE waveguides.