Lithium niobate materials and devices

After returning to Hong Kong in 1988 the applicant started fundamental studies on proton-exchanged LiNbO3 waveguides, and the work have led to significant results: demonstrating that the refractive index profile is a graded function and not a step function, and proton sources having superior characteristics. 

Recently, a novel method to fabricate near-stoichiometric (NS) Ti:LiNbO3 strip waveguides using simultaneous vapour transport equilibrium (VTE) treatment and thermal in-diffusion of Ti-strip delineated photolithographically on the surface of z-cut congruent LiNbO3 substrate has been developed.  Single-mode NS Ti:LiNbO3 strip waveguides with smooth surfaces operating at 1550nm wavelength have been obtained.   The results obtained are promising, and are applicable to Er3+ doped LiNbO3 substrates with further investigations and optimization of the fabrication conditions.

Up to now, active Ti:Er:LiNbO3 waveguide devices reported were realized using congruent LiNbO3 substrates.  Serious photorefractive damage in the visible and near infrared wavelength regions limits the choice of pump wavelength and the applications of these devices in the visible spectrum. The VTE method will be further extended to realize NS MgO-doped Ti:Er:LiNbO3 optical waveguides that exhibit low waveguide loss and low photorefractive damage susceptibility.   Local erbium (Er3+) rare earth ions doping will be carried out in congruent MgO doped LiNbO3 crystals first.  Subsequently, VTE treatment and co-indiffusion of titanium (Ti) metal can be carried out simultaneously, or VTE treatment is carried out first and followed by standard thermal indiffusion of the Ti metal.    The surface morphologies, the structural and crystalline phases, the Li compositions, the photorefractive and the Er3+ spectroscopic properties of the NS waveguides will be characterized.  These parameters will provide useful feedbacks in the optimization of the fabrication of high optical quality NS MgO doped Ti:Er:LiNbO3 waveguides.

                                                                                                 

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a: SEM image of near-stoichiometric Z-cut strip waveguides with initial Ti-strip widths of 6 and 8µm. Insets b and c: Images of the waveguide patterns taken by an optical microscope. The two insets on the top show the near field single-mode patterns of the two waveguides measured at 1550nm.

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