【作者】 张瑞锋；

【导师】 卢霏；

【作者基本信息】 山东大学 ， 光学工程， 2008， 博士

【摘要】 激光技术的发展离不开各种性能优良的光学材料的支持,其中最令人瞩目的材料之一就是磷酸钛氧钾（KTiOPO₄,KTP）晶体。磷酸钛氧钾晶体是一种性能非常优良的非线性光学材料,具有较高的非线性光学系数、较高的抗光伤阈值、大的线性电光系数、良好的热稳定性等优点,是非线性领域不可或缺的材料。KTP晶体被广泛应用于多个领域,例如激光倍频、参量振荡、电光调制和声光调制等。尤其是准相位匹配技术的出现,使得KTP晶体在非线性光学领域有了更大的应用空间。此外,KTP晶体还被广泛应用于电光波导、倍频波导等集成光学领域,这对于实现器件的小型化具有很大的促进作用,而且光波导结构作为集成光路的最基本的组成单元在现代光通信领域具有十分重要的用途。所谓的波导结构就是指由折射率较低的区域包裹的折射率较高的区域。这种结构可以把光限制在较小的区域传播,从而提高传输光的能量密度,更好地利用晶体的非线性性质。光波导是集成光学的基本单元,是全光网络的基础,在各种光器件的制造中起着重要的作用。人们一直在探索制备性能优良的光波导的有效方法。鉴于KTP晶体结构的特殊性,人们可以利用离子交换技术来制备光波导结构。离子交换可以使样品表面的折射率升高从而形成波导结构。离子注入技术是一种制备光波导的有效方法,由于注入离子在晶体内部的沉积可以产生一个折射率降低的光学位垒,在光学位垒和空气之间的区域就构成了光波导结构。条形光波导是光波耦合器、波导调制器、波导开关以及波导激光器等无源器件和有源器件的基础。探讨条形光波导的制备不但是光波导应用研究的基础,还可以拓展该技术在光电子领域的应用。相比其它的离子交换过程如Rb—K交换,有关Cs—K交换制备光波导的研究报道要少的多,尤其是在制备条形波导方面。由于Cs—K交换要求有较高的交换温度,对光刻掩膜材料的选择和成膜条件提出了很高的要求。本论文主要研究了利用Cs—K离子交换技术在KTP晶体上制备平面和条形光波导,并进一步研究了离子注入技术对离子交换波导结构的调制。主要内容包括:（1）用硝酸铯作为交换源,用离子交换技术在KTP晶体上制备了平面光波导,交换温度从420℃到470℃,交换时间从15分钟到2个小时;用棱镜耦合法研究了所制备的平面波导在633nm和1539nm下的暗模特性。交换波导在633nm下均为多模波导,且模式数随着交换时间的增加而增多。根据测量得到的各阶导模的有效折射率,用iWKB（inverse Wenzel-Kramers-Brillouin）方法拟合了波导层的折射率分布。离子交换波导的折射率分布为指数分布,通过计算可以得到离子交换波导的有效深度和表面折射率增量。研究结果表明,随着交换时间的增加,波导表面折射率增量逐渐增大并趋于饱和,而提高交换的温度则有利于提高离子交换的速度。在1539nm波长下,交换温度为430℃,交换时间为15min的样品呈现单模特性,提高交换温度使得波导承载的TE模式增加而TM模数目不变。用椭圆偏振技术研究了单模波导在1539nm波长下的折射率分布。利用WKB方法优化了椭偏参量的计算过程,计算得到该单模波导的有效深度为2.8μm,三个主轴方向波导表面折射率增量分别为0.0151,0.0127和0.0139。该方法也可以用于研究其他波导的折射率分布。对交换温度为430℃,交换时间分别为60分钟和22分钟的样品进行了退火处理。退火温度从200℃到410℃,退火时间最长达3个小时。退火前后用棱镜耦合仪测量了波导的模式特性,发现随着退火温度的提高和退火时间的延长,波导中基模的有效折射率逐渐降低;模式的有效折射率之间的间隔减小。用iWKB方法拟合了波导层的折射率分布,发现退火后波导表面折射率逐渐降低,同时波导的有效深度逐渐增加,波导层的折射率分布逐渐变得平缓。（2）为了研究离子注入对离子交换的影响,我们分别用H、C和Cu离子注入KTP晶体,注入能量分别为:0.3keV、5MeV和1.5MeV,在对离子注入样品进行退火处理后,再将样品放入熔融的硝酸铯中进行离子交换,交换时间为15分钟到2个小时,交换温度为430℃。用棱镜耦合法测量了注入和交换前后KTP样品的暗模特性,通过对波导导模的变化和对应的有效折射率的大小进行分析,发现离子注入对离子交换有一定的阻挡作用,注入造成的损伤层类似于一个位垒,阻碍离子在晶体内部的扩散;同时,由于交换温度较高,交换的过程对注入后的样品同样相当于一个退火过程,而退火会造成离子注入引起的缺陷的扩散和晶格结构的恢复,对离子交换过程的影响较大。（3）一般的波导都以介质和空气的界面作为波导的一个界面,晶体表面的情况会直接影响波导区光场的分布和传输,而埋层波导可以获得相对对称的折射率分布,并且可以避免由于晶体表面的影响而引起额外的损耗,引起了人们广泛的关注。我们尝试利用Cs-K离子交换和Si离子注入相结合的方法,在KTP晶体上制备芯层折射率增加的埋层波导。Cs离子交换的温度为430℃,时间为60min。离子注入采用550keV的Si离子注入,注入剂量为1×10¹⁵ions/cm²。用端面耦合技术研究了埋层波导的近场光强分布。（4）我们利用离子交换技术在KTP晶体上制备条形波导,利用光刻工艺和溅射法在KTP晶体表面生长了一层Cr作为掩膜。用硝酸铯作为交换源,发现由于交换温度较高,作为掩模的Cr膜在交换过程中会脱落,但是脱落在交换盐中的Cr离子并不参与交换过程。能量色散X射线荧光光谱（EDX）测试发现在覆盖有金属Cr膜的区域,交换过后在晶体表面发现有Cr离子的存在,这说明金属Cr以离子的形式进入了KTP晶体内部,造成了掺杂。用棱镜耦合技术测量了交换之后波导的暗模特性,用iWKB方法拟合了波导的折射率分布。结果发现,在覆盖有Cr膜的地方,晶体表面的折射率也有增加,但只发生在晶体表面很薄的一层。我们用显微镜和扫描电镜观察了交换之后晶体表面的条形结构。利用端面耦合技术测量了1550nm和633nm的光在条形波导中传输的近场光强分布。通过对交换样品进行刻蚀,我们发现在覆盖有Cr膜的地方和没有Cr膜覆盖的地方,晶体的刻蚀速度不同,这种选择性刻蚀可能与交换过程中Cr离子进入KTP晶体内部而导致晶体表面的铁电畴极化方向发生变化有关。这为下一步研究通过离子交换实现晶体周期性极化反转提供了重要的信息。更多还原

【Abstract】 High-performance optical materials are indispensable to the developments of laser technology. Potassium titanyl phosphate （KTiOPO₄, KTP） is one of the most attractive materials. KTP is a nonlinear optical crystal with superior properties, such as high optical damage threshold, large linear electro-optical coefficient, high thermal stability etc. Due to the high non-linear optical coefficients, KTP is an important and necessary material in the non-linear optics field. Moreover KTP has been widely used in many fields, for examples, electro-optical modulations, acoustic-optical modulations, frequency doubling and optical parametric oscillation. Especially the quasi-phase match technology extends the applications of KTP crystals in non-linear optics field. On another hand, KTP crystal can be used as an integrated optical device such as an electro-optical waveguide or a frequency doubling waveguide, which are benefit to achieve the miniaturization of optical devices. As the most fundamental integral unit of the integrated optics circuits, the optical waveguide structure is of great importance in the field of modern optical communication.A waveguide is characterized by a region of high refractive index surrounded by regions of lower index. In this structure the optical energy can be restricted in a small space, thereby enhancing the optical energy density and make better use of the non-linear optical properties of crystal. Optical waveguide is the basic unit of integrated optics and the all-optical network. It also plays an important role in the fabrication of various optical devices.Many researchers apply themselves to explore the effective method to fabricate the waveguide on crystal. Due to the special lattice structure of KTP, the ion exchange method is commonly used to fabricate waveguide on KTP crystal. Ion exchange process results in a refractive index increment at the surface thus forming waveguide on crystal. The ion implantation is an effective method to fabricate optical waveguide. Ion implantation can generate a barrier with a reduced refractive index in the depth region of high nuclear energy deposition and the region between the barrier and the atmosphere composes a waveguide structure.Most optoelectronics devices such as optical coupler, modulator, optical switch and waveguide laser are based on channel waveguide structure. Investigation on the fabrication of channel waveguide is the foundation of application of waveguide device, and is also helpful to the application in optic-electric field.Compared with Rb-K ion exchange, few research reports about fabricating waveguides by Cs-K ion exchange can be found, especially about fabricating channel waveguide. Higher ion exchange temperature is needed for Cs-K exchange, which put very high demands for the selection of lithography mask material and conditions of the film deposition.In this dissertation, we report the formation and the properties of the planar and channel waveguide on KTP crystal fabricated by Cs-K ion exchange. Furthermore, the modulation of ion exchange waveguide structure by ion implantation is also investigated. The main contents include:（1） Planar waveguides are fabricated on KTP crystal by ion exchange in molten CsNO₃. The exchange temperature ranges from 420℃to 470℃and the time from 15 minutes to 2 hours. The prism coupling method is used to measure the dark mode spectra of the waveguide at the wavelength 633 nm and 1539 nm, respectively. The samples exhibit multi-mode waveguide at 633 nm and the number of guide modes increases with the exchange time increasing. Based on the effective index of the guide mode measured, the refractive index profiles are obtained by inverse WKB method. The exponent distribution is used to fit the refractive index profile of the waveguide, and the effective depth and the increment of the refractive at the surface are obtained. The results show that along with the ion exchange increase, the refractive index increment of the waveguide at surface will increase gradually and achieve saturation. But raising the temperature is helpful to enhancing the ion exchange.After exchange at 430℃for 15 min, the sample exhibits a single mode waveguide at 1.54μm. Higher temperature leads a larger number of TE modes. The refractive index profiles of this single mode waveguide are investigated by ellipsometry method. The WKB approximation is used to simplify the calculating process. The results show that the effective depth of the waveguide is 2.8μm and the refractive index increments in three principal directions are 0.0151, 0.0127 and 0.0139, respectively. This method can be also used to reconstruct the profile of other waveguide.The annealing behavior of samples exchanged for 60 min and 22 min are studied. The anneal temperature rises gradually from 200℃to 410℃, and process last for 3 hours. The prism coupling method is used to measure the dark mode spectra of the waveguide after each anneal. The effective index of the first mode decreases with the enhancement of annealing temperature and time, and the interval of the modes also decreases. The index profiles of waveguides are reconstructed by iWKB method. The refractive index increment at surface reduces and the effective depth increases gradually, which make the refractive distribution smooth.（2） The influence of ion implantation on the ion exchange waveguide is also studied. H, C, Cu ions are implanted into KTP crystal with the energy of 0.3keW 5MeV and 1.5MeV, respectively. After annealing, Cs-K ion exchanges are performed on these samples. The dark mode spectra of samples are measured by prism coupling method. Compared with the results of sample without suffering ion irradiation, we can find that the damage layer generated by ion implantation act as a barrier to block the diffusion of the Cs ions. On the other hand, the ion exchange exerts an anneal-like process to the samples. This anneal-like process can induce the diffusion of defects and the repair of the lattice damage, which will affect the ion exchange greatly.（3） The atmosphere usually acts as a part of waveguide to confine the propagation of light. In this case, the propagation of light and its field pattern in the waveguide will be directly affected by the surface situation of the sample. For buried waveguide, no loss from surface is introduced and usually it has relatively symmetrical refractive distribution, which makes them attractive to researchers. We fabricate the buried waveguide on KTP crystal by combining the Cs-K ion exchange and Si ion implantation. The temperature of ion exchange is 430℃and the time is 60 min. The energy and the dose of Si⁺ ion implantation are 550keV and 1×10¹⁵ions/cm² respectively. The end fire method is used to measure the near field pattern of the buried waveguide.（4） The channel waveguide is formed by ion exchange in molten CsNO₃. Cr film is applied to the z face of KTP sample by sputtering and patterned using standard photolithography techniques. During the ion exchange process, the Cr will be dissolved in molten salt because of the high temperature. The results of the energy dispersion X ray fluorescence measurement show that Cr can be detected in the region that covered with Cr film after ion exchange. That is to say, chromium can be doped into KTP in ionic form through exchange. The dark mode spectra are measured by prism coupling method and the refractive index profiles are reconstructed by iWKB method. The refractive index increase also in the region covered by Cr film, but it only exists at a very thin layer.The channel structure of the waveguide is observed by microscope and scanning electron microscope after ion exchange. The end fire method is used to measure the near field pattern of the channel waveguide at 1550 nm and 633 nm. The etching results of an ion exchange KTP channel waveguide show a different etch rate between the region covered by Cr film and that without Cr film. The fact of selective etching indicates that the doping of Cr ions may result in the change of the ferroelectric domain of crystal. The results provide some important information about fabricating periodically domain inverse KTP crystal by ion exchange.更多还原