Optical Waveguides Obtained Via Proton Exchange Technology in LiNbO3 and LiTaO3 - A Short Review

Lithium niobate (LN) and lithium tantalate (LT) are nonlinear, birefringent crystals with an important role in optoelectronics. Beside their excellent optical properties, their most prominent feature is the high electro-optical coefficient (r33»30.5 pm/V) which allows the light field to be easily controlled by electric signals. Proton exchange (PE) is a method of obtaining layers of LiM1-xHxO3 (M = Nb, Ta) on LiMO3 substrates. The PE layers have a significantly higher extraordinary refractive index (ne) than the non-protonated crystal; the change is Dne = 0.120-0.150 for LN and Dne = 0.015-0.020 for LT at wavelength 0.633 mm providing a strong waveguide and polarizing effect. The value of x determines the phase composition of the waveguide layer; this value lies within the concentration limits of the different phases of the PE layers (there are 7 possible phases for LN and 5 for LT). In the most general case, the PE layers may be considered as phase multilayers, which makes their characterization and optical performance somewhat complicated. Since the emergence of the technology, the main efforts have been directed to finding ways to control the phase composition and methods for characterization of the waveguides with regard to their phase composition. The present paper aims to analyze and summarize the most significant technological modifications of the method of PE, its advantages and disadvantages. Discussed are the methods of characterization of waveguide layers and the most important waveguide structures of LiM1-xHxO3 for different modulators used in modern optoelectronic devices (navigation equipment, communication systems, biosensors, etc.).