Carruthers J R, Kaminow I P, Stulz L W
Appl Opt. 1974 Oct 1;13(10):2333-42. doi: 10.1364/AO.13.002333.
Increases in the extraordinary refractive index sufficient to produce single and multimode optical waveguides have been produced by the outdiffusion of lithium oxide from the surfaces of lithium niobate and lithium tantalate crystals. The outdiffusion kinetics have been studied in detail by optical interferometry. The data fit a diffusion model for which the vaporizing surface flux is constant with time. For lithium niobate, the activation energy for diffusion is 68 +/- 1.2 kcal/mol and does not vary with orientation. However the gradient of refractive index change at the surface is larger for diffusion normal to the c-axis than parallel to the c-axis. Activation energies for vaporization of 71 kcal/mol and 59 kcal/mol were calculated from the model for diffusion perpendicular and parallel to the c-axis, respectively. The evaporation coefficient, alpha, was estimated to be less than 10(-4) with alpha perpendicular/alpha || approximately 3 so vaporization is surface reaction limited. For lithium tantalate, the activation energy for diffusion is 51 +/- 6 kcal/mol and also does not vary with orientation. The activation energy for vaporization, Q(upsilon), is approximately 63 kcal/mol and, within experimental error, is quite similar to that for lithium niobate. These values of Q(upsilon) indicate that the major vaporization species are probably lithium ions and oxygen ions. The characteristics of graded index waveguides are discussed and compared with those of slab guides. It is shown that in many respects the behavior of the two types is equivalent. We have produced a single-mode guide in lithium niobate with an effective thickness of 12 microm. Effective thicknesses as small as 6 microm are possible in lithium tantalate.
通过氧化锂从铌酸锂和钽酸锂晶体表面向外扩散，已产生了足以制造单模和多模光波导的异常折射率增加。通过光学干涉测量法详细研究了向外扩散动力学。数据符合蒸发表面通量随时间恒定的扩散模型。对于铌酸锂，扩散的活化能为68±1.2千卡/摩尔，且不随取向变化。然而，垂直于c轴扩散时表面折射率变化的梯度大于平行于c轴扩散时的梯度。根据垂直于和平行于c轴扩散的模型分别计算出蒸发活化能为71千卡/摩尔和59千卡/摩尔。蒸发系数α估计小于10^(-4)，α垂直/α平行约为3，因此蒸发受表面反应限制。对于钽酸锂，扩散的活化能为51±6千卡/摩尔，也不随取向变化。蒸发活化能Q(υ)约为63千卡/摩尔，在实验误差范围内，与铌酸锂的相当相似。这些Q(υ)值表明主要的蒸发物种可能是锂离子和氧离子。讨论了渐变折射率光波导的特性，并与平板光波导的特性进行了比较。结果表明，在许多方面这两种类型的行为是等效的。我们已在铌酸锂中制造出有效厚度为12微米的单模光波导。在钽酸锂中有效厚度小至6微米也是可能的。
