We present a novel method of etching lithium niobate during the Ti diffusion process. A hypothesis for this etching process is explained by defining the kinetics of the Ti diffusion process as an electrochemical reaction. The Ti ions diffuse into the X cut LiNbO3 crystal by swapping with Nb ions generating an electric field. Investigations were carried out by placing a bare LiNbO3 wafer on top of the Ti patterned LiNbO3 substrate during the diffusion process in a wet oxygen atmosphere. The built-in electric field during the Ti diffusion process is neutralised with the bare LiNbO3 placed on top and is evident from the material removal that takes place from the top bare substrate and deposited on the bottom substrate were Ti is diffused. Hence the bare substrate is etched in the regions where Ti is present on the bottom substrate. Features can be arbitrarily defined (using Ti etching) and can have dimensions of 1 micron or smaller. Etch depths of the order of 1 micron have been demonstrated while maintaining smooth surfaces. The crystalline nature of the etched surface is analysed using X-ray diffraction techniques. The refractive index measurement and the surface roughness of the etched surface are also presented.
Free-space quantum key exchanges between ground stations and low earth orbiting satellites will be characterized by high link losses, typically of the order of 30 dB or higher. These losses, together with the need to transmit weak Poissonian laser pulses containing on average substantially less than 0.1 photons per transmitted bit to preserve security, will result in exceedingly low channel efficiencies, typically of the order of 10-5. In order to achieve even a relatively modest secure key bit rate of 100 kbps, it will therefore often be necessary to key the transmitter at rates in excess of 10 Gbps. In this paper we outline several different methods of achieving such fast polarization keying including the use of dual drive Mach-Zehnder intensity modulators on lithium niobate in a hybrid fibre-guided modulator structures. We then propose a total integrated polarization keying structure in LiNbO3. We describe the fabrication and testing of such ultra-broadband polarization keyers suitable for use in high loss, short-wave free-space quantum key distribution systems employing silicon photon counters. We also indicate how these devices may be incorporated into quantum key satellite courier payloads and ground station terminals.