Fabrication techniques of lithium niobate waveguides

The role of ion implantation in the fabrication of optical waveguides in lithium niobate is describedtogether with a review of work carried out in this field. In addition the waveguide structure and the process parameters which determine its characteristics are outlined. Finally, the implications of this technology forthe fabrication of more complex integrated optical devices are discussed.

We experimentally investigate some features of the self- defocusing of light beams as well as the relaxation of waveguide channels induced by dark spatial solitons at wavelength of 632.8 nm within slab optical waveguides formed in lithium niobate by iron diffusion or combined diffusion of titanium, iron and copper. We also study an influence of the externally applied electric field on both, spatial self- action of single beam within such waveguides and external focused beam which propagates in direction normal to the waveguide surface. Experiments show that for lowest TE modes the photovoltaic nonlinearity results in the fast but not very strong beam defocusing and for modes of higher order this effect is slower but much stronger. It is also demonstrated that at suitable light intensity the screening of the external electric field significantly contributes to the beam self- action at its initial stage even for mediate value of applied electric field.

Importance of crystal structure of the substrate for diffusion technologies of waveguides fabrication

The authors report a novel process for the fabrication of optical waveguides in lithium niobate using single-step proton-exchange from vapour-phase benzoic acid. Planar and channel waveguides were fabricated.

We report on the fabrication of planar and ridge waveguides in lithium niobate by proton exchange combined with oxygen ion implantation. The implanted energy ranges from 600 to 1400 keV with a dose of 1 x 10(15) ions/cm(2). The modes in proton exchanged waveguide can be modulated by O ion implantation. There are different damage profiles in proton-exchanged and ion-implanted waveguides in Rutherford backscattering/channeling spectra. The refractive index profile in single-mode waveguide in lithium niobate has been obtained based on Intensity Calculation Method. Also ridge waveguide was fabricated on the basis of planar waveguide by Ar ion beam etching. The measured near-field intensity distributions of the ridge waveguide modes show a reasonable agreement with the simulated ones. The estimated propagation loss was approximately 2.2 dB/cm.

The growing interest for Integrated Optics for sensing, telecommunications and even electronics is driving research to find solutions to the new challenges issued by a more and more fast, connected and smart world. This thesis deals with the design, the fabrication and the characterisation of the first prototypes of Microring Resonators realised using ion implanted Lithium Niobate (LiNbO3) ridge waveguides. Optical Resonator is one among the most important devices for all tasks described above. LiNbO3 is the substrate commonly used to fabricate optical modulators thanks to its electro-optic characteristics. Since it is produced in high quantity, good quality and large wafers its price is low compared to other electro-optic substrate. We propose to use ion implantation as fabrication technology because in the other way standard optical waveguides realised in LiNbO3 by Proton Exchange (PE) or metal diffusion do not allow small bending radii, which are necessary to keep the circuit footprint small. We will show in fact that this approach allows to fabricate waveguides on Lithium Niobate that are better than PE or metal diffused waveguides as it allows smaller size devices and tailoring of the refractive index profile controlling the implantation parameters. Moreover, we will show that the ridge technology based on enhanced etching rate via ion implantation produces a waveguide with roughness lower than a dry etched one. Finally it has been assessed a complete technological process for fabrication of Microring Resonator devices in Lithium Niobate by ion implantation and the first prototypes have been produced.

Fabrication and characterization of Titanium Indiffused Proton Exchanged (TIPE) waveguides in lithium niobate

Proton exchange (PE) is an attractive method for the fabrication of optical and acoustic waveguides in lithium niobate (LiNbO3). LiNbO3 substrate has large electromechanical coupling coefficient (K2) and moderate SAW velocity, but its temperature coefficient of frequency (TCF) is as large as -73 ppm/°C. Silicon dioxide (SiO2) is a well-known non-piezoelectric material having the positive TCF for SAW. The SiO2 thin films are deposited on the PE-LiNbO3 substrates to improve the TCF of the SAW devices. The results show that the temperature stability of SAW on SiO2/PE-LiNbO3 substrate is improved to be about -55 ppm/°C.

Photorefractive waveguides that are optically modifiable can be used as the adaptive interconnection channels for optical neural net- works. We investigate experimentally a refined fabrication method for the photorefractive waveguides in a lithium niobate crystal. The experimental results show that nonuniform illumination by a discontinuous movement of the focus can fabricate a better optical waveguide than uniform illumi- nation by a continuous one. The discontinuous movement of the focus reduces the spatial overlap between the successive exposures, and avoids the redistribution of the space-charge density formed previously. This refinement results in a narrower and clearer index distribution of the waveguide. We further show that good waveguides are fabricated if the focus is scanned in a particular scanning direction that makes a small angle with the plane perpendicular to the c axis of the crystal. © 1996 Society of Photo-Optical Instrumentation Engineers.

The nonlinear optical waveguide is one of the essential components of modern photonic integrated circuits. A high-quality lithium niobate (LN) waveguide has recently become available due to advances in thin-film LN materials and the associated fabrication techniques. In the past few years, LN waveguide-based nonlinear photonic devices have been intensively investigated due to their ultra-low loss and large index-contrast features. Here we review the recent progress in LN waveguide-based nonlinear photonics devices, including both passive and active components. We believe that LN-based nonlinear photonic devices will make a profound impact on modern photonic society.

Stimulated Brillouin scattering (SBS) is a significant nonlinear optical phenomenon utilized across various fields. Its applications span optical sensing, microwave photonics, and all-optical signal processing. In the current information-driven era, the laser industry is imposing increasingly stringent demands on laser technologies, thus propelling integrable on-chip SBS devices into a pivotal research trajectory. Simultaneously, the quest for materials manifesting heightened SBS effects has emerged as a primary focal point in optical fiber transmission endeavors. Lithium niobate (LN) stands out as a promising nonlinear optical material endowed with numerous exemplary attributes. These encompass a high refractive index, fostering potent photon-phonon interactions, low loss characteristics, and high integration potential. Consequently, employing LN as a material for Brillouin lasers promises to enhance certain aspects of laser performance and enable more comprehensive research into Brillouin lasers. In this paper, a micro-ring waveguide structure with LN as the main material is constructed, and the forward gain of stimulated Brillouin scattering, the distribution of electric field in multiple modes and the maximum tunable range are further analyzed. The simulation results exhibit that the tunable forward SBS effect can be achieved in the LN optical waveguides, the maximum forward Brillouin gain can reach 1.34W−1 m−1. Additionally, the tunable phonon frequency can range from 1 GHz to 31 GHz, effectively doubling that of silicon ring waveguides and thus extending the range of phonon tunability. Therefore, the LN micro-ring waveguide structure provides a new reference direction for the construction of a new tunable laser and a new idea for the generation of lasers to find a Brillouin medium with high SBS effect.

Proton-exchanged optical waveguides fabricated by glutaric acid

The authors report on the fabrication of buried waveguides in both lithium niobate and periodically poled lithium niobate. First a low insertion loss waveguide is fabricated in z-cut lithium niobate using femtosecond laser waveguide inscription. To fabricate a waveguide exhibiting both low propagation and coupling losses, we used the multiscan fabrication technique to control the size of the waveguide cross section. We measured coupling losses of 1.1 dB/facet and propagation losses as low as 0.6 dBcm -1 . Optical waveguides have been also inscribed in periodically poled lithium niobate by femtosecond laser pulses with the same multiscan technique. Second harmonic generation experiments from a fundamental wavelength of 1567 nm demonstrate that the nonlinear optical coefficient in the waveguides is preserved, yielding a conversion efficiency of 18 % W -1 .

Full optical characterization of planar nonlinear waveguides is of major importance for the assessment of reliable fabrication processes of integrated all-optical devices. A high accuracy in the design of the waveguide parameters fixing the refractive index profile (typically optical depth and index change) and in the knowledge of their wavelength dependence are mandatory for the realization of nonlinear integrated optical devices that meet phase matching conditions and achieve highly efficient interactions. Conventional characterization techniques, such as m-lines spectroscopy, do not always fulfil the accuracy requirements especially in the case of very thin waveguides. Moreover additional preliminary determination of the refractive index of the substrate is often required. In our laboratory we developed different non-conventional optical characterization techniques to overcome the main problems related to nonlinear waveguides and, in particular, to proton-exchanged (PE) waveguides in lithium niobate or lithium tantalate that are the most commonly used waveguides for all-optical devices. Different methods will be presented. In most cases, both radiation and guided modes are exploited in the characterization process. The new techniques proposed allowed a full optical characterization of the ordinary and extraordinary refractive indices of the substrates and index profiles of the exchanged layers for single-mode and multimode PE waveguides and the determination of the Sellmeier dispersion curves. Experimental results on typical waveguides will be presented. A new method, based on prism coupling, for the measurement of waveguide losses will also be discussed.

Inplane scattering levels in titanium-diffused lithium-niobate optical waveguides show two distinct stages as a function of diffusion time at fixed temperature: at first decreasing and then, eventually, increasing. The initially high level of inplane scattering is mainly due to the large number of near-surface defects associated with inhomogeneities occurring at early stages of the titanium indiffusion process. The subsequent decrease in inplane scattering with diffusion time is associated with spreading out of the optical energy and the smoothing out of the defective surface region. For longer annealing times the increasing density of misfit dislocations at the interface between the indiffused layer and substrate becomes the main source for light scattering.