Characterization of annealed, proton-exchanged optical waveguides in ZnO:LiNbO(3) crystal

We report a systematic study of the annealing process in proton-exchanged ZnO:LiNbO(3) optical waveguides. A z-cut multimode waveguide was subjected to annealing for different durations of time. A two-stage change in index profiles with annealing time was observed, which was consistent with the change in the m-line spectrum. A power-law relationship was established to correlate the optical parameters with annealing time. Annealed Li(+) concentration in the waveguide was solved based on the thermal diffusion equation. An analytical function was used to model the annealed-index profile of single-mode, proton-exchanged waveguides. Good agreement between the theoretical analysis and the experimental result was obtained.     

Accurate method for determining the refractive-index profiles of planar waveguides in uniaxial media with the optical axis normal to the surface

A new method for characterization of uniaxial planar waveguides from their measured effective mode indices is presented. The theory is outlined and expressions for efficient computer analysis are given. Uniaxial waveguide samples have been made in c-cut LiNbO(3) by proton exchange with and without post annealing in order to test the method on both steplike and graded-index profiles. The resulting characterization of the samples is discussed in relation to the inverse WKB method. Finally, the importance of incorporating the effects of material birefringence in the characterization of these kinds of waveguides is investigated.     

Planar optical waveguides in Bi4Ge3O12 crystal fabricated by swift heavy-ion irradiation

We report on the fabrication of the planar waveguides in Bi4Ge3O12 crystal by using 17 MeV C5+ or O5+ ions at a fluence of 2×10(14) ions/cm2. The reconstructed refractive index profiles of the waveguides produced by either C5+ or O5+ irradiation are the "well" + "barrier" pattern distribution. The two-dimensional modal profiles of the planar waveguides, measured by using the end-coupling arrangement, are in good agreement with the simulated modal distributions. After thermal annealing treatment at 260 °C for 30 min, the propagation loss for C5+ and O5+ irradiated waveguides could be reduced down to ~1.1 and ~4.8 dB/cm, respectively, which exhibit acceptable guiding qualities for potential applications in integrated optics.     

Surface periodic domain structures for waveguide applications

We report the results of fabrication and investigations of surface periodic domain structures created by a set of quasi-point e-beam irradiations both on the Y- and X-cuts of LiNbO(3), and on Ti:LiNbO(3) and Zn:LiNbO(3) planar waveguides. Domain gratings with spatial periods from 4.75 to 7.25 μm were formed by a 25-keV e-beam. Doses from 500 to 2000 μC/cm(2) were used for different structures to estimate optimal fabrication conditions. The investigations allowed the visualization of the formed surface domain structures, estimation of their uniformity, and determination of waveguide generation of the second optical harmonic. The surface structures can be used in optical devices for the realization of quasi-phase-matched frequency conversion, which includes the creation of compact radiation sources based on waveguides.     

Planar LiTaO(3) waveguides fabricated by proton exchange in concentrated and diluted pyrophosphoric acid with annealing

Annealed proton-exchanged z-cut LiTaO(3) planar waveguides fabricated with pyrophosphoric acid have been characterized. For proton exchange, the extraordinary index increase Δ n(e) ranged from 0.0119 to 0.0141, depending on the exchange temperature. The effective diffusion coefficient D(e) ranged from 0.1325 μm(2)/h at 240°C to 0.545 μm(2)/h at 280 °C. Single-mode propagation losses were α = 0.7 dB/cm. Compared with benzoic acid, pyrophosphoric acid produces waveguides with a higher Δ n(e) and a lower propagation loss. For proton exchange in lithium phosphate-diluted pyrophosphoric acid, a lower Δ n(e) was obtained, but D(e) and propagation losses were not reduced. After proton exchange, the waveguides were annealed. The surface index initially increased, peaked, and then decreased. The waveguide depth d and the surface index n(s) were measured at regular intervals. Figures and empirical formulas relating the waveguide depth d and the surface index increase Δ n(s) to the anneal time, anneal temperatures, and the waveguide depth after proton exchange are given. An example is given in which, the desired waveguide parameters d and Δ n(s), the fabrication conditions could be calculated with the previously derived formulas. Propagation losses decreased to 0.4 dB/cm after prolonged annealing.     

Dilute-melt, proton-exchange slab waveguides in LiNbO 3 : a new fabrication and characterization method

A method of dilute-melt proton exchange employing a mixture of glycerol and KHSO4 with lithium benzoate added is used to fabricate planar waveguides in c -cut LiNbO3 . With this exchange melt system the waveguide refractive-index profiles can be fabricated with a high degree of reproducibility. In contrast with the traditional dilute-melt method, which uses benzoic acid and lithium benzoate, the need for sealed containers during the exchange process can be avoided when the glycerol, KHSO4 , and lithium benzoate system is used for the exchange process. A new characterization method for determining the waveguide refractive-index profile from the measured-mode indexes is introduced. The main advantage of this characterization method compared with other methods is that it also applies to single-mode waveguides. Using the new characterization method, we investigate in detail the relation between waveguide refractive-index change and composition of this glycerol, KHSO4 and lithium benzoate exchange melt system.     

Proton-exchanged 36 degrees Y-X LiTaO3 waveguides for surface acoustic wave

A nontoxic proton source, octanoic acid, was adopted to fabricate proton-exchanged (PE) waveguides in 36 degrees Y-X lithium tantalate (LiTaO3) substrates. The PE ability of octanoic acid on LiTaO3, the penetration depth, was investigated by secondary-ion mass spectrometry (SIMS). The penetration depth of hydrogen ion exhibited an obviously step-like profile, which will be excellent for waveguide application. The relationship between waveguide depth (d) and exchanging time (t) was represented by d = 0.0653 X square root of t at T = 200 degrees C. To deserve to be mentioned, the octanoic acid has a slight dissociation coefficient and low activation energy, thus the accurate waveguide depth control can be obtained. For the application of acoustic wave guided acousto-optic devices, the leaky surface acoustic wave (LSAW) properties of PE 36 degrees Y-X LiTaO3 waveguides were investigated. The phase velocity slightly decreased with the increase of kd, where k was wavenumber. An indispensable parameter of acoustic wave device, the temperature coefficient of frequency (TCF), calculated from the frequency change of the output of LSAW delay line showed an increase with increased kd.     

K(+)-Na+ ion-exchanged waveguides in Er(3+)-Yb3+ codoped phosphate glasses using field-assisted annealing

Buried channel waveguides were fabricated in Er(3+)-Yb3+ codoped phosphate glasses with use of the thermal two-step ion-exhange method. The K(+)-Na+ ion-exchange process was first carried out in pure KNO3 molten bath, and then field-assisted annealing (FAA) was used to make the buried waveguides. The fluorescence properties of bulk glasses and channel waveguides were characterized, and the waveguide properties were studied. The formation mechanism of buried waveguides was analyzed, and the improved gain characteristics of the waveguide amplifiers with use of different FAA temperatures were measured. The maximum net gain of the waveguide amplifiers at 1.534-microns wavelength was measured to be approximately 3.4 +/- 0.1 dB/cm with 120-mW pump power at 975-nm wavelength.     

Side radiation of Rayleigh waves from synchronous SAW resonators

Surface acoustic wave (SAW) resonators on lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are investigated. The amplitude of the acoustic fields in the resonators are measured using a scanning laser interferometer. The amplitude profiles of the surface vibrations reveal the presence of distinct acoustic beams radiated from the transducer region of the SAW resonators and propagating with low attenuation. We suggest that this radiation is generated by the charges accumulating at the tips of the finger electrodes. The periodic system of sources, namely oscillating charges at the fingertips, generates Rayleigh-wave beams in the perpendicular and oblique directions. Green's function theory is used to calculate the coupling strength and slowness of the Rayleigh waves on 42 degrees Y-cut LiTaO3 and Y-cut LiNbO3 substrates as a function of the propagation direction. Furthermore, the propagation angles of the Rayleigh-wave beams as a function of frequency are calculated. The computed angles are compared with the measured ones for both the LiTaO3 and LiNbO3 substrates.     

Relationships between structural and optical properties of proton-exchanged waveguides on Z-cut lithium niobate

By using x-ray diffraction and optical waveguide characterization, it has been shown that proton-exchanged layers on Z-cut LiNbO(3) can have as many as seven different crystallographic phases that are stable at room temperature. This study allows us to establish some correlation between fabrication parameters and the crystalline structure, the index profiles, and the propagation losses of waveguides prepared by this process.     

Spectral Measurement of the Film-Substrate Index Difference in Proton-Exchanged LiNbO(3) Waveguides

We report the spectral characterization of proton-exchanged lithium niobate (PE:LiNbO(3)) waveguides in terms of the variation of the refractive-index difference between the waveguiding layer and the substrate. The dispersion of the extraordinary refractive-index increase (deltan(e)) is measured from 405 to 1319 nm with several light sources. Two types of proton-exchanged waveguide, prepared under different conditions, are studied. These measurements should be of use in the optimization of PE:LiNbO(3) waveguides for nonlinear optical applications, particularly in second-harmonic generation in the blue-green wavelength region.     

Photonic crystal waveguide-mode orthogonality conditions and computation of intrinsic waveguide losses

We simulate the propagation of light in a W1 planar photonic crystal waveguide with the three-dimensional finite-difference time-domain method and apply an inner product against previously calculated mode profiles to the simulated field cross sections. We show that this inner product satisfies mode orthogonality for both photonic crystal waveguides and segmented waveguides and use the obtained data to evaluate waveguide losses.     

Evaluation and selection of LiNbO(3) and LiTaO(3) substrates for SAW devices by the LFB ultrasonic material characterization system

This paper demonstrates the evaluation and selection of commercially available LiNbO(3) and LiTaO(3) single crystals and wafers for surface acoustic wave (SAW) devices using the line-focus-beam ultrasonic material characterization (LFB-UMC) system. This system enables measuring leaky-SAW (LSAW) propagation characteristics precisely and efficiently for a number of specimens. The wafers are prepared from the top, middle, and bottom parts of four 128 degrees YX LiNbO(3) and seven X-112 degrees Y LiTaO(3) single crystals. For both series of crystals, the measured LSAW velocities increase from top to bottom in the crystals and with the increasing crystal growth number. The velocity changes for all wafers are 0.036% for 128 degrees YX LiNbO(3) and 0.035% for X-112 degrees Y LiTaO(3), corresponding to changes of 0.038 mol% and 0.075 mol% in Li(2)O concentration, respectively. Moreover, the inhomogeneity in the first X-112 degrees Y LiTaO(3) single crystal caused by some undesirable wafer fabrication processes can be detected precisely, although it is difficult for the conventional methods to obtain such information.     

RBS/channelling study of ion-implanted and proton-exchanged LiNbO3:Nd:MgO planar waveguides

We present a structural study of two Nd³⁺/MgO codoped LiNbO₃ waveguides fabricated by two different methods: ion-implantation and proton-exchange. Rutherford backscattering spectrometry in combination with channelling was used to study the influence of the waveguide fabrication method on the possible modification of the LiNbO₃ crystal structure within the waveguiding layer. The results show that the ion-implanted waveguide mainly maintains the same properties as the virgin crystal, apart from a deterioration of a thin surface layer, whilst the proton-exchanged waveguide exhibits a quite different behaviour, although its surface is less damaged.     

Fabrication of channel waveguides from sol-gel-processed polyvinylpyrrolidone/ SiO(2) composite materials

Sol-gel-processed composite materials of polyvinylpyrrolidone (PVP) and SiO(2) were studied for optical waveguide applications. PVP is a polymer that can be crosslinked, so it is expected to have high thermal stability after crosslinking. However, thermal crosslinking and thermal decomposition of pure PVP take place around the same temperature, 200 °C, therefore pure PVP had a high optical propagation loss as a result of the absorption of the decomposed molecules after crosslinking. The incorporation of sol-gel-processed SiO(2) prevented the thermal decomposition of PVP and provided remarkably low optical propagation losses. The PVP/SiO(2)composite material also produced thick (>2-μm) crack-free films when the PVP concentration was 50% or higher. An optical propagation loss of 0.2 dB/cm was achieved at 633 nm in the 50% PVP/SiO(2) composite planar waveguide. Several aspects of the thermal stability of the waveguides were evaluated. The slab waveguide was then used for fabrication of channel waveguides with a selective laser-densification technique. This technique used metal lines fabricated with photolithography on the slab waveguide as a light absorbent, and these metal lines were heated by an Ar laser. The resultant channel waveguide had an optical propagation loss of 0.9 dB/ cm at 633 nm. This technique provides lower absorption loss and scattering loss compared with the direct laser-densification technique, which uses UV lasers, and produces narrow waveguides that are difficult to fabricate with a CO(2) laser.     

Accurate measurements of the acoustical physical constants of LiNbO (3) and LiTaO(3) single crystals

The acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of commercial surface acoustic wave (SAW)-grade LiNbO(3) and LiTaO(3) single crystals were determined by measuring the bulk acoustic wave velocities, dielectric constants, and densities of many plate specimens prepared from the ingots. The maximum probable error in each constant was examined by considering the dependence of each constant on the measured acoustic velocities. By comparing the measured values of longitudinal velocities that were not used to determine the constants with the calculated values using the previously mentioned constants, we found that the differences between the measured and calculated values were 1 m/s or less for both LiNbO(3) and LiTaO(3) crystals. These results suggest that the acoustical physical constants determined in this paper can give the values of bulk acoustic wave velocities with four significant digits.     

Optical branching effect in Ti:LiNbO3 waveguides: near-field pattern studies

The paper presents a detailed study of a single optical beam splitting into several beams (the branching effect) in photorefractive sensitive Ti:LiNbO3 optical slab waveguides. The near-field patterns of the multibeam structures are presented for different values of optical power coupled into TE guided modes of different orders. The process of partial recovery of the optically damaged waveguide (the partial shrinking of the multibeam bundle created) is also described.     

Surface acoustic wave properties of proton-exchanged LiNbO3 waveguides with SiO2 film

Surface acoustic wave (SAW) properties of proton-exchanged (PE) z-cut lithium niobate (LiNbO3) waveguides with silicon dioxide (SiO2) film layers were investigated using octanoic acid. The distribution of hydrogen measured by secondary ion mass spectrometry (SIMS) showed a step-like profile, which was assumed to be equal to the waveguide depth (d). The SiO2 film was deposited on z-cut LiNbO3 waveguide by radio frequency (rf) magnetron sputtering. We investigated the important parameters for the design of SAW devices such as phase velocity (Vp), insertion loss (IL) and temperature coefficient of frequency (TCF) by a network analyzer using thin-film aluminum interdigital transducer electrodes on the upper SiO2 film surface. The experimental results showed that the Vp of SAW decreased slightly with the increase of h/lambda, where h was the thickness of SiO2 films and lambda was the wavelength. The IL of SAW increased with increased h/lambda. The TCF of SAW calculated from the frequency change of the output of SAW delay line showed an evident decrease with the increase of h/lambda. The TCF for PE z-cut LiNbO3 was measured to be about -54.72 ppm/degreees C at h/lambda = 0.08. It revealed that the SiO2 films could compensate and improve the temperature stability as compared with the TCF of SAW on PE samples without SiO2 film.     

Intensity and phase mapping of guided light in LiNbO3 waveguides with an interferometric near-field scanning optical microscope

The design and implementation of a phase-sensitive near-field scanning optical microscope incorporating both heterodyne interferometric detection and a phase feedback mechanism are described. Using this microscope we demonstrate a new method for measuring the effective index of the guided mode of a waveguide from the phase images. Two types of LiNbO3 waveguide, defined by titanium diffusion or annealed proton exchange, were studied. Both the profile and the effective index of the mode were measured experimentally. For titanium-diffused waveguides, both agree well with values determined from numerical simulation. In annealed proton-exchanged waveguides that contain periodically poled domains, we find evidence for backreflection from the boundaries between neighboring regions of opposite pole directions, which could result in transmission loss in this type of waveguide.     

Synthesis of symmetric and asymmetric planar optical waveguides

A novel and accurate refractive index profile synthesis method for planar optical waveguides is presented and demonstrated using the transmitted near-electric-field-data. This method is based on the inverse transmission-line (TL) technique. From Maxwell's equations, a TL equivalent circuit (electric T-circuit) for the refractive index profile of a planar optical waveguide is derived. The authors demonstrate how to use this model to carry out the inverse problem and synthesise the exact refractive index profile numerically from near-field-data. The TL method can reconstruct arbitrary refractive index profiles for planar optical waveguides that support singlemode or multi- modes. The cases of both symmetric and asymmetric arbitrary refractive index profile planar waveguides are discussed. The accuracy of the reconstructed waveguides is examined numerically.