Proton-exchanged Waveguides Research Articles

Correction of Y-Branches on Proton-exchanged Waveguides in Lithium Niobate By Femtosecond Writing Technology

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Femtosecond supercontinuum generation and Čerenkov conical emission in periodically poled LiTaO3

We investigated the femtosecond supercontinuun generation and Čerenkov conical emission in periodically poled LiTaO3. A broadband spectrum was achieved from 380nm to 440nm and 490nm to 970nm. We have also demonstrated a novel way of calculating the nonlinear refractive index in the filamentation using Čerenkov radiation method. The nonlinear refractive index was about 0.024, it has the same order of magnitude with the refractive index change in proton-exchanged waveguide.

Transmission Spectral Characteristics of Photonic Crystals Milled in Annealed Proton-Exchange LiNbO3 Waveguide**Supported by the National Natural Science Foundation of China under Grant Nos 50872089, 61077039 and 61377060, the Research Grants Council of the Hong Kong Special Administrative Region of China under Grant No 11211014, the Key Program for Research on Fundamental to Application and Leading Technology of Tianjin Science and

Transmission spectra of triangular lattice photonic crystals milled in the top surface of an annealed proton-exchange waveguide are numerically simulated. The effects of the finite depth, conical shape, trapezoidal shape and hybrid shape of holes are theoretically analyzed. Due to the difficulty of milling high aspect-ratio cylindrical holes in lithium niobate (LiNbO3), a compromised solution is proposed to improve the overlap between shallow holes and the waveguide mode, and useful transmission spectra with strong contrast and sharp band edges are achieved.

Structural phase transformations of proton-exchanged layers of lithium niobate during annealing

ABSTRACTKinetics of phase transitions in the surface layer of proton-exchanged planar waveguides on X-cut of lithium niobate crystal during annealing at 330, 350, and 370°C was investigated by optical methods (dark field microscopy, crossed-polarizers bright field microscopy, and mode spectroscopy). Nucleation and growth stages of phase transformations were identified. Based on mode spectroscopy data and analysis of previous works, we assume that transformation of κ2-phase into κ1-phase and further transformation into α-phase were observed during experiment.

ОЦЕНКА ПРИМЕНИМОСТИ КРИСТАЛЛОВ НИОБАТА ЛИТИЯ Z-СРЕЗА С ЗАДАННЫМ РАСПРЕДЕЛЕНИЕМ ПРИМЕСЕЙ ДЛЯ ИЗГОТОВЛЕНИЯ ПРОТОНООБМЕННЫХ ВОЛНОВОДОВ

Different methods (optical, laser, and electron microscopy) used to compare of the structural features of the near-surface layers of single-crystal lithium niobate plates with a predetermined distribution of impurities along its length and lithium niobate from Crystal Technology (USA). Investigation of optical characteristics (refractive index and depth of planar waveguide) was performed using the method of mode spectroscopy, and also the obtained characteristics were compared with the parameters sample of Crystal Technology. It is shown that the characteristics of planar proton-exchange waveguides are very similar in their values to each other. However, the state of the near-surface layers of the investigated single-crystal lithium niobate plates with a predetermined distribution of impurities along its length is significantly different from the quality of the plates produced by Crystal Technology. The investigated samples of lithium niobate with a predetermined distribution of impurities along its length contain many scratches and points defects on the surface, as well as defective near-surface layer with a depth of 15 μm, which is significantly worse than sample produced by Crystal Technology. It is impossible to use such lithium niobate plates in the manufacture of integral optics elements (channel waveguides, electrodes). Multiple surface defects will significantly reduce the quality of structures, up to their complete rejection.

Electro-optic beam deflection based on a lithium niobate waveguide with microstructured serrated electrodes.

We report on electro-optic beam deflection using an annealed proton exchange waveguide in lithium niobate (LN) with microstructured serrated array electrodes. Due to the electro-optic effect of the LN material, the experimental results show that the beam deflection and modulation of the LN waveguide can be realized with relatively low voltages. The total length of the serrated prism array electrodes structure is ∼5 mm. With 20 V applied to the electrodes of 50, 100, and 150 μm wide waveguides, ∼1.28, ∼0.96, and ∼0.64 μm beam deflections were obtained, respectively, which are in accordance with theoretical simulation. This configuration can be potentially applied in optical beam scanning, high-speed switches, and optical beam smoothing technology.

Frequency conversion between UV and telecom wavelengths in a lithium niobate waveguide for quantum communication with Yb+ trapped ions

We study and demonstrate the frequency conversion of UV radiation, resonant with 369.5 nm transition in Yb+ ions to the C-band wavelength 1580.3 nm and vice-versa using a reverse proton-exchanged waveguide in periodically poled lithium niobate. Our integrated device can interface trapped Yb+ ions with a telecom infrastructure for the realization of an Yb+ based quantum repeater protocol and to efficiently distribute entanglement over long distances. We analyse the single photon frequency conversion efficiency from the 369.525 nm to the telecom wavelength and its dependence on pump power, device length and temperature. The single-photon noise generated by the spontaneous Raman scattering of the pump is also measured. From this analysis we estimate a single photon conversion efficiency of ∼9% is achievable with our technology with almost complete suppression of the Raman noise.

Leaky-mode waveguide modulators with high deflection angle for use in holographic video displays

Film display holograms typically diffract light over a wide enough view-angle to be viewed, directly, without intervening optics. However, all holographic video displays (with the exception of eye-tracked systems) must use optics beyond the hologram surface to overcome the challenges of small display extent and low diffraction angle by using some form of demagnification and derotation (i.e. angle magnification and optical multiplexing). We report a leaky mode waveguide spatial light modulator with sufficiently high angular diffraction to obviate the need for demagnification in scanned aperture systems. This high angle was achieved by performing a number of experiments to determine the depth of the annealed, proton-exchanged waveguide which corresponded to a maximized diffracted angle. Diffraction sweeps were recorded in excess of 19.5° (corresponding to only 70 MHz of input bandwidth) for 632.8 nm light which is above the 15° required for direct view display. Device geometries are proposed which might achieve greater than 20° of total angular sweep for red, green, and blue light.

Nonlinear diffusion model for annealed proton-exchanged waveguides in zirconium-doped lithium niobate.

Photorefractive-damage- (PRD) resistant zirconium-oxide-doped lithium niobate is investigated as a substrate for the realization of annealed proton-exchanged (APE) waveguides. Its advantages are a favorable distribution coefficient, PRD resistance comparable to magnesium-oxide-doped lithium niobate, and a proton-diffusion behavior resembling congruent lithium niobate. A 1D model for APE waveguides was developed based on a previous model for congruently melting lithium niobate. Evidence for a nonlinear index dependence on concentration was found.

Theoretical analysis of optical mode deflection in lithium niobate waveguide with serrated array electrodes

In this paper, we establish a lithium niobate (LN) thin film planar waveguide and an annealed proton exchange (APE) waveguide model with serrated array electrodes configuration to deflect and reshape the optical mode. The refractive index profile of the waveguide can be electro-optically manipulated by applying an external voltage into the serrated array electrodes. Through the simulation results, it is observed that different refractive index manipulation of LN waveguide can make the optical beam have different deflection and reshaping. The comparison between the thin film and APE bulk planar waveguide is investigated. When the beam deflection is modulated via the high-speed electro-optic effect of LN, the temporal and spatial beam smoothing can be achieved for the application of high power laser system and other optical signal processing.

Microstructure and defects probed by Raman spectroscopy in lithium niobate crystals and devices

Raman microprobe applied on LiNbO3 (LN) crystals and derived materials or devices is shown to be a tool to detect either local variations or changes of the whole structure. Position, width, or intensity of one Raman line can be used as markers of a structural change. Indeed, each Raman line can be assigned to a peculiar ionic motion and is differently sensitive to application of strain, temperature change, and electric field. Some vibrational modes are especially associated to the site of Li ion, or Nb ion, or still oxygen octahedron, so that they can be affected by the introduction of dopant ion on one or another site. Therefore, Raman Spectroscopy (RS) can be used as a site spectroscopy to describe the mechanism of doping incorporation in the LN lattice, allowing the optimization of some linear and non-linear optical properties according to the dopant concentration and substitution site. The composition or the content of non-stoichiometry related defects could be derived from the width of some lines. Any damage or local disorder can be detected by a line broadening. The quality or preservation of the structure after chemical treatment, or laser pulses, can be thus checked. The structure of ion-implanted or proton-exchanged wave-guides and periodically poled lithium niobate as well can be imaged from frequency shift or intensity change of some lines. RS is thus a useful way to control the structure of LN and/or to optimize the preparation parameters and its properties.

Annealed proton-exchanged waveguide quasi-phase-matched second-harmonic generation devices in 8 mol % MgO-doped congruent LiTaO3 crystal

Heavily (8 mol %) MgO-doped congruent LiTaO3 (MgO:cLT) crystal has wider transparency range and higher photorefractive damage resistance than LiNbO3 or non-doped LiTaO3 crystal. We examined the possibility of forming annealed proton-exchanged (APE) waveguides with various conditions. We obtained good quality APE waveguides with the propagation loss 0.7 dB/cm for 1.064 µm wavelength and demonstrated a prototype waveguide quasi-phase-matched second-harmonic generation device of 56%/W normalized efficiency for the first time in MgO:cLT crystal.

Development of photon pair sources using periodically poled lithium niobate waveguide technology and fiber optic components

To support quantum technologies that require entangled photon pairs and/or heralded photons for operation, a photon pair source was developed that uses periodically poled lithium niobate (PPLN) waveguides that are coupled to optical fibers. Both Ti-indiffused and annealed proton-exchanged (APE) waveguide technologies were studied, and waveguide/fiber interfaces were designed to increase the coupling efficiency of the photon pairs into optical fiber. PPLN waveguide devices were fabricated and the optical loss, wavelength conversion efficiency, and heralding efficiency were measured. The maximum heralding efficiencies achieved were 75 and 68% for Ti-indiffused and APE waveguides, respectively. A compact photon pair source based on a packaged PPLN waveguide device and commercially available fiber optic components is presented.

Phase Composition and Electro-Optic Properties of Proton-Exchanged Waveguides in Lithium Niobate Crystals*

Based on experimental data on the shift of the fundamental absorption edge, we have quantitatively estimated the electro-optic coefficient r 13 in proton-exchanged waveguides containing different H x Li1–x NbO3 phases. The phase composition of the waveguides was determined based on IR reflectance and micro-Raman spectroscopy data. We established that the electro-optic properties of the waveguides depend on their phase composition. The results obtained allowed us to optimize the process for preparation of phase modulators based on channel waveguides in LiNbO3 crystals.

Anisotropic model for the fabrication of annealed and reverse proton exchanged waveguides in congruent lithium niobate.

An anisotropic model for the fabrication of annealed and reverse proton exchange waveguides in lithium niobate is presented. We characterized the anisotropic diffusion properties of proton exchange, annealing and reverse proton exchange in Z-cut and X-cut substrates using planar waveguides. Using this model we fabricated high quality channel waveguides with propagation losses as low as 0.086 dB/cm and a coupling efficiency with optical fiber of 90% at 1550 nm. The splitting ratio of a set of directional couplers is predicted with an accuracy of ± 0.06.

Mode switch based on electro-optic long-period waveguide grating in lithium niobate.

We propose a mode switch based on an electro-optic long-period grating formed in a lithium-niobate (LN) two-mode waveguide. Our fabricated device consists of an 8mm long grating in a proton-exchanged z-cut LN waveguide. At a driving voltage of 35V, it can switch between the fundamental mode and the higher-order mode with a mode extinction ratio of -18 dB and a 3-dB bandwidth of ∼25 nm at the wavelength 1544nm. The performance of the device is insensitive to temperature variations. The experimental results agree well with the simulation. This mode switch could find applications in reconfigurable mode-division-multiplexing systems.

Acousto-Optic Modulator Driven by Surface Acoustic Waves

In this paper, an introduction to the design, fabrication, evaluation, and application of acousto-optic modulators using the Bragg di raction of an optical guided wave in a channel optical waveguide due to a surface acoustic wave is provided. First, the surface acoustic wave mode dependence in the acousto-optic Bragg di raction was discussed by considering the surface acoustic wave power required for the maximum di raction P100 calculated using coupled mode theory and the measured P100 in Ti-di used planar optical waveguides fabricated on LiNbO3 substrates. Next, performances of several devices that the author and the coworkers developed are reviewed. These include a waveguide-type acousto-optic modulator using a tapered crossed-channel proton-exchanged optical waveguide on a 128◦-rotated Y -cut LiNbO3 substrate for an optical wavelength of 1.55 μm, a monolithically integrated tandem acousto-optic modulator in which several waveguide-type acousto-optic modulators are connected in tandem on the same substrate, and a waveguide-type acousto-optic modulator in which laser lights of the three primary colors, red, green, and blue, can be modulated by the same modulator at the same driving frequency.

A Novel Coupled Resonator Photonic Crystal Design in Lithium Niobate for Electrooptic Applications

High-aspect-ratio photonic crystal air-hole fabrication on bulk Lithium Niobate (LN) substrates is extremely difficult due to its inherent resistance to etching, resulting in conical structures and high insertion losses. Here, we propose a novel coupled resonator photonic crystal (CRPC) design, combining a coupled resonator approach with that of Bragg gratings. CRPC design parameters were optimized by analytical calculations and FDTD simulations. CRPC structures with optimized parameters were fabricated and electrooptically tested on bulk LN annealed proton exchange waveguides. Low insertion loss and large electrooptic effect were observed with the fabricated devices, making the CRPC design a promising structure for electrooptic device applications.

Subsurface disorder and electro-optical properties of proton-exchanged LiNbO3 waveguides produced by different techniques

It has been established, that proton-exchanged LiNbO3 waveguides have a marked subsurface layer with structural disorder inducing degradation of electro-optical properties of these waveguides. At the same time, such a subsurface disorder is found to be less pronounced in soft proton-exchanged (SPE) waveguides in comparison with annealed proton-exchanged (APE) ones. The experimental samples of phase modulators fabricated by SPE technique exhibit a better electro-optical efficiency compared to the LiNbO3 modulators produced by the standard and improved APE techniques.

Phase composition and stress in LiTaO3 proton-exchanged optical waveguides

The phase composition of the top layer of Li1−xHxTaO3 waveguide layers produced at different modifications of the proton exchange (PE) technology has been analyzed based on their IR reflection spectra. These spectra contain new bands within the range , each phase having its own reflection spectrum. Since the top layer is actually the strongest proton-exchanged one of all sublayers building the waveguide layer, the recognition of the top sublayer's phase in many cases could be used to make conclusions about the phases building the rest of the entire PE layer. The intrinsic stress caused by crystal lattice deformation due to the PE was calculated by an optical integral method. An attempt to explain the level of stress is made based on the phase composition of the studied samples.