Characterization of photorefractive LiNbO3 waveguides fabricated by combined proton and copper exchange

Abstract

Photorefractive planar waveguides in LiNbO3 are fabricated by a combined proton and copper exchange. The dependence of refractive index profiles, optical absorption, and electrooptic coefficients on different fabrication steps is investigated. With holographic methods dark and photoconductivity, holographic sensitivity, and light‐induced refractive index change in the waveguides are measured. By the additional copper exchange the steady state diffraction efficiency of holographic gratings in our proton‐exchanged waveguides is increased from 0.01 to 65%.

Holographic Recording in Planar Cu:H:LiTaO3 Waveguides

“…Therefore, in the last few years the investigation of new fabrication methods of photorefractive waveguides has been considerably intensified. In the case of LiTaO 3 crystals the combined proton and copper exchange is a new method, which has clear advantages compared to other techniques [1,4,5]. Moreover, further improvement of Cu :H : LiTaO 3 waveguides should be possible by optimizing the fabrication conditions, because the quality of the waveguiding layer may be further improved by annealing treatments.…”

Photorefractive Waveguides

“…Therefore techniques have been developed to measure electro-optic coefficients in both planar and channel waveguides. In planar waveguides, attenuated total reflection (ATR) spectroscopy may be used, a method that is well known from the investigation of electro-optic polymer layers [15,16]. Alternatively, interferometric methods can be applied [17,18].…”

“…This effect has been used to fabricate nonleaky waveguides [100], where light is confined without the possibility of barrier tunneling. Higher doses of 10 16 cm À2 and more have resulted in a decrease of both refractive indices. Furthermore, the implantation through an appropriate mask on the substrate surface has enabled the fabrication of both single and multimode channel waveguides in various materials [99,101].…”

“…By using proton exchange in LiNbO 3 and LiTaO 3 , waveguides with strongly enhanced photorefractive sensitivity can be fabricated when the exchange of protons is combined with a successive copper exchange from melts containing Cu þ or Cu 2þ ions [16,53]. As this technique of copper doping is a lowtemperature process well below the Curie temperature of, e.g., LiTaO 3 , it is of particular interest for the fabrication of photorefractive waveguides in this material.…”

“…These so-called annealed proton-exchanged (APE) waveguides have a graded refractive index profile. The waveguiding layer is completely converted to the a-phase [44] with x < 0:12 and an index change of dn e < 0:03 [16]. Furthermore, for APE waveguides, very small loss coefficients of about 0:03 cm À1 have been measured [46].…”