Analysis of High-Index Contrast Lithium Niobate Waveguides Fabricated by High Vacuum Proton Exchange

Abstract

Abstract-High-index contrast waveguides fabricated with precise control and reproducibility are of high interest for nonlinear and/or electro-optical highly efficient and compact devices for quantum and classical optical data processing. Here we present a new process to fabricate planar and channel optical waveguides on lithium niobate substrates that we called High Vacuum Proton Exchange (HiVacPE). The main purpose was to improve the reproducibility and the quality of the produced waveguides by limiting and controlling the water traces in the melt, which is used for the ionic exchange. Moreover, we discovered that, when the acidity of the bath is increased, depending on substrate orientation (Z-cut or X-cut) the waveguides are completely different in term of crystallographic properties, index profiles and nonlinearity. The best-obtained channel waveguides exhibit a refractive index contrast as high as 0.04 without any degradation of the crystal nonlinearity and state of the art propagation losses (0.16dB/cm). We have also demonstrated that the HiVacPE process allows fabricating waveguides on Z-cut substrate with high-index contrast up to 0.11 without degrading the crystal nonlinearity but high strain induced propagation losses. On top of that, we proposed an original and very useful method of analyzing waveguides with complex index profiles. This method can be used for the analysis of any waveguides whose core contains several layers.

Lithium niobate waveguides with high-index contrast and preserved nonlinearity fabricated by a high vacuum vapor-phase proton exchange

“…On the Table 1 is presented the index contrast ∆n e of a given waveguide calculated as the difference between its corrected surface index and the value of the extraordinary index of the substrate (n e =2.2028 at 632.8 nm and room temperature). In the next section a thorough analyze of index profiles will be done by using the mathematical method fully described in previously reported work [15].…”

“…In this section, by using the method fully described in [15], we thoroughly analyze the corrected index profiles for 1h-5h exchange durations and IWKB index profile for 24h respectively. Thus, it one determines the best fit for the experimental points depicted in Fig.…”

“…(1). As it was presented in [15], useful information can be obtained from derivatives of index profiles curves. As we clearly can see in the bottom part on Fig.…”

“…It is important to note that the maximum index contrast allowed by the proton exchange technique is approximately ∆n e =0.12 at λ=633 nm [12], but most of waveguides exhibiting such a high-index contrast present a nonlinear coefficient χ (2) totally degraded [13]. Two other techniques namely High Index Soft Proton Exchange (HISoPE) and High Vacuum Proton Exchange (HiVacPE) allowing the fabrication of channel waveguides that exhibit ∆n e =0.1 with almost unmodified nonlinearities have been recently reported [14,15]. These waveguides are most attractive for applications because they exhibit a high-index contrast which is adequate for tight light confinement allowing strong electro-optical and nonlinear optical effects.…”

“…We previously demonstrated that the exchange process (in liquid bath) performed in a hermetically sealed hourglass tube at very low pressure, in order to diminish as much as possible any traces of water from the acid powders, leads to a new and very reproducible behavior in terms of index contrast, shape of index profile, nonlinearity, waveguide depth etc. [15,16].…”