High Performance Proton-Exchange LiTaO 3 Devices For Integrated Optical Sensor Applications

Abstract

ABSTRACT Optical studies of phase modulators, polarizers and directional couplers realized in X -cut LiTaO3 fabricatedby the proton- exchange method (PEM) are described. The ability to temperature adjust the coupling lengths ofoptical directional couplers is discussed in detail. The above guided -wave optical components can be integratedto perform certain optical signal processing functions such as extracting the rotation rate information in fiber -opticgyroscopes.1.INTRODUCTIONIn the mid -70s, optical waveguides and devices have been fabricated in LiTaO3 by high temperature indiffusion of such metals as niobium 1,2 , copper 3-5 , titanium 6,7 , and more recently by vapor indiffusion of zinc8. However, recently optical waveguides and electrooptic devices have been demonstrated in LTa03 by proton -exchange (PE) 9-15. One of the most important advantages in the use of the PEM is that the devices fabricateddid not have to be repoled since the fabrication temperature was around 235 °C, i.e. below the Curie temperatureof LiTa03 (660° C). One of the possible reasons why there was not a widespread use of LTa03 based integratedoptical devices was the non -repeatablewaveguide and electrooptic properties after high temperature indiffusionof the metals and the re- poling that followed afterwards.Table 1 shows a comparison of some important properties of LiNbO3 and LiTaO3. The transmission range(for> 50% transmission), the effective index value and the clamped, constant strain electrooptic coefficient arevery similar in both the above mentioned materials. The two important differences are found in the values of theCurie temperature and the relative dielectric constant under constant strain. The latter difference will haveimportantconsequences in the design of fast electrooptic devices (> 1 GHz). One important advantage in theuse of LiTaO3 over LiNbO3 substrates for the realizations of integrated optical devices is the inherent ability ofLiTaO3 to handle higher optical power densities (two orders of magnitude higher) 16.