Effect of the Structure of the Lithium Niobate Surface Layer on the Characteristics of Optical Waveguides
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Effect of the Structure of the Lithium Niobate Surface Layer on the Characteristics of Optical Waveguides A. V. Sosunova,*, R. S. Ponomareva, b, S. S. Mushinskya, A. B. Volyntseva, A. A. Mololkinc, d, and V. Maléjacqe a Perm
b
State University, Perm, 614990 Russia Perm Federal Research Center, Ural Branch, Russian Academy of Sciences, Perm, 614990 Russia c OAO Fomos Materials, Moscow, 107023 Russia d National Research Technological University “MISiS,” Moscow, 119049 Russia e University of Nantes, Nantes, 44035 France *e-mail: [email protected] Received September 23, 2019; revised October 25, 2019; accepted November 11, 2019
Abstract—The surface and surface layer of X-cut plates of congruent lithium niobate (produced by OAO Fomos Materials) have been investigated by structural and optical methods. It is shown that the plate surface is of high optical quality, comparable with that of foreign analogs. The existence of an oxygen-saturated (and, correspondingly, niobium-deficient) damaged surface layer with a depth of up to 20 μm is demonstrated. Ooptical waveguides have been designed based on the investigated crystal using the proton exchange method. The optical characteristics of these waveguides are comparable with those for Crystal Technology lithium niobate crystals; however, the proton exchange and annealing processes should be corrected. A comprehensive analysis of the Fomos Materials samples showed that they can be used to design integrated optical circuit elements for various instrumentation tasks. DOI: 10.1134/S1063774520050223
INTRODUCTION Integrated optical circuits (IOS) are optical analogs of electronic integrated circuits. They have a number of advantages: complete spark, fire, and explosion safety, complete isolation of elements, possibility of using passive circuits and sensors in sinsulation design without batteries, and the electromagnetic interference protection of signals transmitted through optic fibers [1]. Integrated optical circuits are widely used in communication systems to modulate the intensity of optical signals transmitted through fiber lines and in navigation, making it possible to combine a beam splitter, a phase modulator, and a radiation polarizer in fiber-optic gyroscopes into a single device [2–4]. One of the main IOS elements is a channel waveguide formed directly under the surface of a lithium niobate (LN) crystal substrate. Owing to its high electrooptical coefficients and low optical loss, an LN single crystal is a material that is most widely used in IOS channel waveguides [5, 6]. The waveguides are formed by ion exchange, during which a part of lattice ions is replaced by impurity ions. In this case, due to the opto-elastic effect, the local enhancement of the refractive index occurs, which is sufficient to satisfy the total internal reflection conditions [7].
Optical circuits are rejected according to their optical loss, a numerical aperture of the channel waveguide, a number of light modes propagating at a specific wavelength, etc. In the approved
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