Advances in nonlinear photonic devices based on lithium niobate waveguides

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.

We report on the characterization of single-mode and multimode proton-exchanged periodically segmented waveguides in lithium niobate. Variation of effective indices of different modes for different annealing times as a function of duty cycle and period of segmentation is presented. It is shown that the proton-exchanged periodically segmented waveguide can be modeled by an equivalent z-invariant depth-independent graded-index waveguide with a Gaussian index distribution. Although the depth of the equivalent waveguide is independent of duty cycle, the peak refractive-index change increases almost linearly with the duty cycle and saturates at large duty cycles. The results should benefit the design of linear and nonlinear devices that are based on segmented waveguides.

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 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.

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.

We investigate the dependence of photonic waveguide propagation loss on the thickness of the buried oxide layer in Y-cut lithium niobate on insulator substrate to identify trade-offs between optical losses and electromechanical coupling of surface acoustic wave (SAW) devices for acousto-optic applications. Simulations show that a thicker oxide layer reduces the waveguide loss but lowers the electromechanical coupling coefficient of the SAW device. Optical racetrack resonators with different lengths were fabricated by argon plasma etching to experimentally extract waveguide losses. By increasing the oxide layer thickness from 1 µm to 2 µm, we were able to reduce propagation loss of 2 µm (1 µm) wide waveguide from 1.85 dB/cm (3 dB/cm) to as low as 0.37 dB/cm (0.77 dB/cm). Resonators with a quality factor greater than 1 million were demonstrated as well. An oxide thickness of approximately 1.5 µm is sufficient to significantly reduce propagation loss, due to leakage into the substrate and simultaneously attain good electromechanical coupling in acoustic devices. This work not only provides insights on the design and realization of low-loss photonic waveguides in lithium niobate, but also, most importantly, offers experimental evidence of how the oxide thickness directly impacts losses and guides its selection for the synthesis of high-performance acousto-optic devices in Y-cut lithium niobate on insulator.

This study proposes a novel scheme of a crosscorrelation frequency-resolved optical gating (X-FROG) measurement for an optical arbitrary waveform (OAW) based on the sum frequency generation (SFG) effect of a periodically poled lithium niobate (PPLN) waveguide. Based on the SFG effect and combined with the principal component generalized projects algorithm on a matrix, the theory model of the scheme is established. Using Matlab, the proposed OAW measurement X-FROG scheme using the PPLN waveguide is simulated and studied. Simulation results show that a rectangular pulse is a suitable gate pulse because of its low errors. Moreover, the increased complexity of OAW and phase mismatch decrease measurement accuracy.

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.

In the paper, the most commonly used fabrication techniques of optical waveguides in lithium niobate are reviewed. Diffusion processes such as out-diffusion of lithium oxide from the crystal surface, in-diffusion of metals, ion exchange and proton exchange, and ion implantation are analysed. The characteristics of LiNbO3 optical waveguides prepared by the different technological procedures, are reported in relation with the fabrication conditions. High quality optical waveguides on lithium niobate can be produced by Ti in-diffusion in a dry O2 atmosphere at temperatures above 1000°C for long diffusion times (>10 h). However, despite the large amount of research done in this field, correlations between optical properties and structural characteristics have not yet been established.

Monomode channel waveguides for infrared light around 1550 nm are fabricated in lithium niobate by successive indiffusion of titanium stripes and thin evaporated layers of copper. The fabricated waveguides are optically characterized and investigated with an atomic force microscope. To evaluate the photorefractive properties holograms are recorded with green light of an argon ion laser and read in the various channels in reflection geometry with infrared light. The saturation values of the light-induced refractive-index change and the photoconductivity in the channels are measured. The results are consistent with a one-centre model for the charge transport. To demonstrate the multiplexing capabilities of the photorefractive waveguides three reflection holograms with 90% diffraction efficiency and a channel spacing of only 0.8 nm are superimposed in one sample.

Short-period domain patterning by ion beam irradiation in lithium niobate waveguides produced by soft proton exchange

We demonstrate a frequency multiplexed photon pair generation based on a quadratic nonlinear optical waveguide inside a cavity which confines only signal photons without confining idler photons and the pump light. We monolithically constructed the photon pair generator by a periodically poled lithium niobate (PPLN) waveguide with a high reflective coating for the signal photons around 1600nm and with antireflective coatings for the idler photons around 1520nm and the pump light at 780nm at the end faces of the PPLN waveguide. We observed a comblike photon pair generation with a mode spacing of the free spectral range of the cavity. Unlike the conventional multiple resonant photon pair generation experiments, the photon pair generation was incessant within a range of 80nm without missing teeth due to a mismatch of the energy conservation and the cavity resonance condition of the photons, resulting in over 1000-mode frequency multiplexed photon pairs in this range.

Nonlinear frequency conversion in thin-film lithium niobate (TFLN) is a promising method for visible light generation. In particular, efficient green light generation remains challenging due to the lack of direct gain media, making nonlinear frequency conversion a viable alternative. Here, we demonstrate an efficient approach that combines modal phase-matching (MPM) and quasi-phase-matching (QPM) in a single TFLN waveguide with a single poling period, enabling green light generation under continuous-wave (CW) pumping. Second harmonic generation (SHG) occurs between fundamental transverse electric (TE) modes, while third harmonic generation (THG) is phase-matched with the higher-order TE20 mode, verified by mode profile measurements. Our device achieves normalized conversion efficiencies of 795%W-1cm-2 (SHG) and 2,775%W-2cm-4 (THG), with on-chip SHG power reaching the mW level and THG power in the μW range. This compact and scalable design offers an efficient green light source for applications in quantum optics, bio-imaging, and advanced display technology.

In this study, lithium niobate (LN) optical modulator devices were fabricated using cost-effective sputtered LN thin films. We successfully achieved a significantly small product of half-wave voltage and interaction electrode length (VπL) of 1.2, 1.0, and 0.75 V cm using red (λ = 638 nm), green (λ = 520 nm), and blue (λ = 473 nm) laser light, respectively. This small VπL is owing to the use of a low dielectric material of LaAlSiInOx for buffer insulator between LN waveguides and electrodes, and the change of geometrical configuration of the electrode. Thus, fabricating visible light modulators at low cost can be realized, and sputtered LN thin films can be adopted for visible light communications and consumer-based applications.

We propose to integrate the electro-optic (EO) tuning function into entangled photons generation process in a domain-engineered lithium niobate (LN) waveguide. Through suitable domain designs, we realize the generation of electro-optically tunable nonmaximally mode-entangled photon state.