Proton Exchange Waveguides And Photonic Devices On Single-Crystal Lithium Niobate Thin Films

Lithium niobate (LN) is a remarkable optical material due to its excellent electro-optical (E-O), acousto-optical, nonlinear optical, piezoelectric properties and broad transparent window in visible and infrared range. In recent years, single-crystal LN thin film on insulator (LNOI), produced by crystal ion-slicing and wafer bonding techniques, offers an ideal platform in modern integrated optics. Due to its high refractive index contrast and micron sized film thickness, the integration density of the integrated photonic circuits and the performance of the photonic devices based on LNOI have been significantly improved. Optical waveguides are the fundamental building block in integrated optics, which can confine the light into a micron sized region in the form of guided modes. Many complex photonic devices, such as Mach-Zehnder modulators, nonlinear wavelength convertors and wavelength division multiplexing, are fabricated on the basis of optical waveguides.Proton exchange is an important method to fabricate waveguides in bulk LN, having the advantage of low propagation loss and mature fabrication process. LN is a birefringence crystal. After proton exchange, the extraordinary refractive index (ne) increases while the ordinary refractive index (no) decreases. Therefore, for ne, the exchanged region has higher refractive index than surrounding thus forming waveguide structure. The proton exchange process changes the crystal structure and so the electro-optical and second order nonlinear optical coefficients dramatically reduce. Such coefficients can be restored or preserved by following anneal process or "soft proton exchange" method, along with decreasing propagation loss. Depending on different fabrication conditions, such as the proton source, exchange and anneal temperature/time, the crystal structure after proton exchange can be classified into seven lattice phases:β1、β2、β3、β4、κ1、κ2 and α phase. Since the α phase has the most similar crystal structure to that of the original LN, its optical coefficients are well preserved. There have been some reports about the integrated photonic devices on lithium niobate thin film produced by etching and depositing load strips, in which the loss is relatively high. The main purpose of this dissertation is to reduce the loss of the integrated photonic devices by proton exchange technique, improving their performance. By anneal proton exchange and short time proton exchange, low loss channel waveguides are fabricated, and the electric-optical coefficient is well preserved, which is significant for the development of the LNOI based integrated optics.Photonic crystal (PC) is an artificial material in which the refractive index is periodically distributed. Coherently scattering happens at the interfaces between different media when light propagates in such structure. Some certain ranges of frequency and propagation direction will be forbidden for photons, forming the photonic band gap (PBG). Using PBG, one can control the propagation of light in very small dimension. Although three dimensions PC can control the light in all the three directions, it is difficult to fabricate. An alternated method is making two dimensions PC on a plane waveguide, controlling the propagation of light by PBG and total reflection in the plane and in the direction vertical to the plane, respectively. In this dissertation, high extinction ratio and wide PBG photonic crystal slabs are fabricated on LNOI.The material used in this thesis is LNOI. LNOI is a structure with three layers. The bottom is LN substrate. The upper layer is a 2 μm thick SiO2 film, forming a high index contrast to the surface LN film thus enabling high confinement of light in LN film. The most upper layer is the single-crystal LN film with micron or sub-micron thickness. This film was transferred from another LN substrate by crystal ion-slicing and bonded to the SiO2 layer. Therefore, by using this method, the obtained LN film has better single-crystal structure than that obtained by other methods like sputtering.The work in this thesis mainly contains two parts:1. PE process in LNOI and the fabrication of the related photonic devices.2. Study of PC slab and PC cavity in LNOI. The main results are as follows:1. PE high index contrast waveguide structure in LNOIPE is a mature method to fabricate waveguide in bulk LN. We explore the possibility of producing PE waveguide in LNOI. The results show that the two techniques are compatible. Assuming the step-like refractive index profile, the evolution of the guided modes as the dimension of the waveguide varies is studied. Simulations indicate that such "strip loaded waveguide" always supports fundamental mode whatever the width of the PE region is. The mode size can reach as small as 0.6 μm2. To get high index contrast waveguide, pyrophosphoric acid is used as proton source, and PE is performed at 200℃ for 80 minutes, insuring the whole LN film being exchanged by protons. The refractive index change is measured to be 0.149 (at 633 nm) by a prism coupler. X ray diffraction shows a new phase after PE process. The channel waveguides are fabricated under such condition. The near-field intensity distributions indicate that the mode in PE LNOI waveguide is much smaller than the conventional Ti-diffused LN waveguide. The propagation loss decreases with the increasing of the width of the waveguide, and the lowest one is 11 dB/cm.2. Low loss anneal proton exchange (APE) waveguides in LNOITo get low loss waveguide, we explore the conditions to get a phase waveguide. The a phase can be got by APE method, which lead to Gaussian-like refractive index distribution. Simulations show that the minimal mode size can reach 1.2 μm2. High resolution x ray diffraction (HRXRD) rocking curve show that, for a PE process at 200℃ for 15 minutes, the a phase is difficult to achieve if the anneal is not sufficient, and too strong anneal can destroy the crystal structure of LN film, resulting in high loss in waveguides. Decreasing the PE time can preserve the crystal structure in LN film. In experiment, low loss channel waveguide in z-cut LNOI is fabricated by PE at 200℃ for 5 minutes, followed by anneal at 350℃ for 3 hours. FDTD simulations show that the end-face reflectivity has a strong dependence on the angle between the channel and end-face. Among all the waveguides, the waveguide with 4μm initial mask width exhibits the smallest mode size with a loss of 0.6 dB/cm. The channel waveguides in x-cut LNOI are fabricated by the same methods as z-cut case. The condition is PE at 200℃ for 5 minutes, followed by anneal at 350℃ for 1 hour. The lowest loss among all the waveguides is 1.1 dB/cm.3. Waveguides by short time PE in LNOIDue to the different type of the waveguide in bulk LN and LNOI, guided mode can be formed in LNOI by a very short time PE, allowing most energy of light to guide outside the PE region to decrease the loss. Simulations show that the mode size has a minimal value when the exchange depth and width of the PE region varies. The energy confined in the PE region increases with the exchange depth and width of the PE region, and decreases with the increasing LN film thickness. In experiment, by PE in benzoic acid at 200℃ for 3 minutes, waveguide with a loss as low as 0.2 dB/cm is fabricated. The loss increases with the PE time.4. Y-branch in LNOIY-branch is fabricated by short time proton exchange. A Y-branch with radius of 8000 μm and 3 μm waveguide width exhibits 85%～90% transmission around 1.55 μm, and shows stable splitting ratio 1:1 under different input coupling conditions.5. PE phase modulator in LNOIPE always leads to dramatically reduced E-O coefficient, thus having detrimental influence on the applications in the field of E-O modulations. We introduce a shallow PE layer by a very short time PE process to make most of the light energy guide outside the PE region to avoid its influence. By simulations, an ideal condition to fabricate PE phase modulator in LNOI is found. The 3 μm wide waveguide is first fabricated by photolithography. A second photolithography process is performed on the remaining Cr mask and 10 μm wide electrodes are got. By comparing the shift of the FP resonant peak before and after applying voltage on the electrodes, the voltage-length product (Vπ·L) is evaluated to be 6.5 V-cm and the effective E-O coefficient is 29.5 pm/V.6. PC slabs in LNOIThe photonic band structure and transmission of the PC slabs are simulated by planar wave expansion (PWE) and finite difference time domain (FDTD) methods, showing roughly consistent results. In experiment, the ridge waveguide and the PC slab pattern with lattice constant= 520 nm and radius=130 nm are fabricated by focused ion beam (FIB). Using the tunable laser and end-face coupling method, the transmission of the PC slab is measured, basically consistent with the simulation. The measured extinction ratio exceeds 20 dB.7. One dimensional PC micro-cavity on a single-mode (SM) LNOI photonic wireRidge waveguides in LNOI lead to compact photonic devices due to the high index contrast between the waveguide and surrounding. Single-mode waveguide is often desirable because of the elimination of modal dispersion. We study the properties of the guided modes in ridge waveguides in z-cut and x-cut LNOI by finite difference method, finding the SM condition. Simulations indicate that the mode coupling happens between different modes under some specific conditions. The PC micro-cavity is formed if a row of air holes are etched on the SM photonic wire and one hole is removed. The optimal condition is found by FDTD simulations when this micro-cavity is regarded as an optical filter. The photonic wire and PC micro-cavity are fabricated by FIB. The transmission is measured by end-face coupling method, and a resonant peak occurs at 1400 nm. The maximal transmission, Q factor and extinction ratio are 0.34,156 and 12.5 dB.