Optical Performance of Bragg Gratings Fabricated in Ti:LiNbO$_3$ Waveguides by Focused Ion Beam Milling

Abstract

In this paper we propose an efficient technique for micromachining lithium niobate that is used in Ti-diffused waveguides. The use of Focused Ion Beam (FIB) etching allows obtaining homogeneous periodic microstructures. Bragg gratings with a period of 1.05 m and an aspect ratio of 6:1 (depth-to-half period ratio) have been achieved. A reflectivity greater than 95% associated with a bandwidth at half maximum of about 100 nm within a window centered at 1550 nm, is demonstrated for a Bragg grating of period 3=18 m and a length of 144 m, in good agreement with theoretical predictions.

Analysis of optical filtering in waveguides with a high index modulation using the extended coupled mode theory by hybridization of a matrix method

“…Then, we proceeded to the experimental measurements (see setup depicted in section III), in order to compare the theoretical results and measurements of B.G obtained by Focused Ion Beam (FIB) etching of a Ti:LiNbO 3 waveguide. The obtained experimental results are in good accordance with those theoretically predicted [14]. Figure 1 shows a representation of the studied model where an index corrugation is engraved inside a waveguide whose index is n 2 and width is d. Index modulation is characterized by a depth l, a period Λ and a duty cycle r = W Λ .…”

“…This technique of engraving is of an ionic type, it is realized step by step, period by period. The B.G obtained by FIB technique is presented in [14,19]. This B.G.…”

“…Λ = 20 × 1.75 = 35 µm. The shape of the holes obtained by FIB engraving is illustrated in [14,28]. We have to pay attention to the side wall profiles (left and right) obtained have an inclination in respect to the vertical.…”

“…The experimental setup that allows us to measure the reflectivity in terms of the wavelength is described in reference [14,18]. It is composed by a continuous spectrum source [850 − 1750] nm, a set of focusing components to inject light in waveguide, a polarizer to select the electric-field orientation, a beam splitter to separate the input light and the reflected light and an Optical Spectrum Analyzer (OSA).…”

“…The most difficult step of this theory is that it is conditioned by a precise domain where approximations are validated [13]. The various conditions that are required to efficiently satisfy this technique are: (i) index modulation, which in fact is the variation of the refraction index between the guiding layer of the waveguide and its neighbors, must be weak; (ii) The product between the coupling coefficient κ and the perturbation length L, must be in the order of few unities [14,15]. In this paper, we present a modified model which is based on the solving of Maxwell's equations associated to the transfer matrix approach applied to high amplitude of index modulation in optical waveguides.…”