Fabrication of Bragg Gratings on LiNbO3 optical waveguides

We report on three approaches to achieve 1086 nm period Bragg Gratings structures (BGs), corresponding to the third Bragg order at 1550 nm, on X-cut lithium niobate (LiNbO3) substrates. The first method relies on Reactive Ionic Etching (RIE) with fluorine gases, associated with e-beam lithography and electroplating deposition. BGS with etched angles close to 60°, an aspect ratio AR = 1.4 and a reflectivity of 13% have been fabricated. The second process is based on Focused Ion Beam (FIB) milling. BGs are etched in standard optical waveguides. The process has led to etched angles close to 85° with AR = 6.3 and to a reflectivity of 50%. Finally, we propose an original method based on the use of lateral FIB etching on the edge of deep-etched ridge optical waveguides predefined by “optical grade dicing”. Etched angles close to verticality with AR > 9 have been obtained and optical characterizations are under work.

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► Nanostructuring LiNbO3 substrates with large aspect ratios is very challenging.
► We report on three approaches to achieve 1086 nm period Bragg Gratings structures.
► A reflectivity of 13% at 1550 nm has been measured with the dry etching method.
► Reflection coefficient of 50% is reported for the direct FIB milling method.
► A large improvement of the aspect ratio can be reached with lateral FIB milling.