Design methodology of annealed H+ waveguides in ferroelectric LiNbO3

Abstract

Relations between fabrication conditions and optical characteristics of planar waveguides made by proton exchange in benzoic acid are documented in the literature, but reports on the characterization of waveguide fabrication processes, performed in a systematic way, could not be found, resulting in the need to combine data from several authors. Discrepancies among results from different researches are evident, resulting from different experimental methodologies and calibration of equipment. Therefore, aiming at extracting a consistent data set, optical characterization of the refractive index profile was employed to study series of samples. The objective was to develop a methodology for fabrication of proton-exchanged channel waveguides in LiNbO 3 operating in the single-mode regime at several wavelengths, with specific characteristics required to optimize integrated devices. To achieve this, it is necessary to obtain the relations between the optical characteristics of the waveguides and their fabrication conditions, and to introduce models of the waveguide formation process. Proton ExchangeThe technique of proton exchange is interesting due to its easy implementation, its capability of producing large refractive index variations at low temperature ͑150 to 300°C͒, and the low susceptibility of its products to photorefractive damage. 6,7 Recently, several interesting integrated devices, with optimized characteristics, were produced by this means, providing, e.g., high-efficiency second-harmonic generation 8 and optical parametric generation 9 in periodically poled lithium niobate.

Phase Composition of HiVac-VPE Lithium Niobate Optical Waveguides Identified by Spectroscopic Investigations

“…We assume that it can be explained by the fact that in the case of the APE technique, after proton exchange and before annealing, the waveguides exhibit a step-like index profile. This shape of the index profile is the initial condition found in any modeling and experimental study of annealing in the case of the APE technique, which is not the case in our investigation [4][5][6][7][8]. Our initial conditions are quite different: the index profile is expressed as the sum of two generalized exponential functions, as presented in Figure 1, and in agreement with our previous work [1].…”

Room temperature direct bonding of LiNbO3crystal layers and its application to high-voltage optical sensing

“…An APE [27] y-propagating waveguide in z-cut LiNbO 3 is designed to be near cutoff at the working wavelength (λ = 1550 nm) and centered in a domain inverted region. The waveguide has a width of 3.5 μm and a depth of 4 μm, where the vertical profile is assumed to be half-Gaussian with an index step of around 0.02 [28]. Another lithium niobate substrate is afterward bonded on top of the waveguide.…”

Low-loss waveguides in a single-crystal lithium niobate thin film

“…According to a previous study about the proton diffusion mechanism in amorphous SiO 2 [14], the activation energy for protons in SiO 2 (below 1 eV, depending on the different mechanisms and experiments) was smaller than that in LN (1.21 eV) [15]. So the amorphous SiO 2 layer probably could not prevent the protons from diffusing to a deeper place.…”

“…The top 560 nm thin film was untouched, while the bottom part was replaced by a SiO 2 layer. The parameters related to the APE process, such as the diffusion coefficients, were determined according to the work in [15]. The PE time and anneal time were 5 min and 3 h, respectively (this condition was used to fabricate the channel waveguides, as discussed below).…”