Optical waveguide writing inside Foturan glass with femtosecond laser pulses

We investigated the femtosecond laser writing of optical waveguides inside Foturan glass at various pulse energies and focusing depths. An optimal waveguide fabricated solely by femtosecond laser irradiation showed a refractive index modulation of ∼1.7×10^-3 and a minimum transmission loss of ∼0.80 dB/cm. This type of waveguide had lower transmission loss than those fabricated by a hybrid process of femtosecond laser exposure and following thermal treatment. An optical splitter was also fabricated at high pulse energy. PACS 42.65.Re; 42.82.Et; 42.70.Gi     

The optical properties of planar waveguides in LiB3O5 crystals formed by Cu implantation

A planar optical waveguide has been formed in a LiB₃O₅ crystal using 6.0 MeV Cu⁺-ions with a dose of 1 × 10¹⁵ ions/cm² at room temperature. Possible propagating modes were measured at a wavelength of 633 nm using the prism-coupling method. The refractive index profiles of the waveguide were reconstructed by an effective refractive index method and the beam propagation method was used to investigate the properties of the propagation modes in the formed waveguide. The results suggest that the fundamental TE₀ and TM₀ modes may be well-confined and propagate a longer distance inside the waveguide. The implantation process was also simulated using the transport of ions in matter code (TRIM), which indicates that the nuclear energy deposition may be the main factor for the refractive index change.     

Room-temperature continuous wave laser oscillations in Nd:YAG ceramic waveguides produced by carbon ion implantation

We report on the generation of continuous wave lasers at a wavelength of ∼1064 nm in a Nd:YAG ceramic waveguide at room temperature. The waveguide was fabricated by using 6 MeV carbon ion implantation at a fluence of 3×10¹⁴ ions/cm². Laser operation has been realized with a slope efficiency as high as ∼11%. The pump threshold of an 808-nm laser beam for the waveguide laser oscillation is 19.5 mW.     

Secondary-ion-mass spectrometry study on near-stoichiometric LiNbO3 strip waveguide fabricated by vapour transport equilibration and Ti co-diffusion

Secondary-ion-mass spectrometry was applied to study Ti-concentration profiles in the depth direction and on the surface of near-stoichiometric (NS) Ti:LiNbO₃ strip waveguides fabricated using vapour transport equilibration (VTE) and co-diffusion of Ti-metal strips. Results show that the profile of Ti concentration along the width direction on the waveguide surface can be well fitted by a sum of two error functions, while, in the depth direction, the Ti concentration follows either a complementary error function or a generalized Gaussian function. Surface Ti concentration, 1/e width and depth, mean diffusivities along the width and depth directions of the guide are, respectively, 1.04?×?10²¹?cm^–3, 8.5?µm, 6.3?µm, 0.18 and 0.1?µm²/h. Two-dimensional refractive index profile in the NS waveguides is indirectly constructed by assuming linearity between Ti-induced index change and Ti concentration. The surface refractive index increments at 1545?nm, Δn_o and Δn_e , are evaluated to be 3.132?×?10^–3 and 1.186?×?10^–2, respectively. All of the above-mentioned diffusion and optical parameters are compared with the corresponding data of the common congruent Ti:LiNbO₃ waveguide or NS waveguide fabricated starting from a NS substrate. The rationality of the assumed linear relationship between the index change and the Ti concentration is discussed. The results show that the assumed linearity remains controversial, and all expressions and data with regard to the refractive index are the approximate results and need to be verified by direct measurement on refractive index.     

Sensitivity enhanced all-optical switching using prism-grating coupled surface plasmon modes

A sensitivity enhanced all-optical-switch design using prism-sinusoidal grating coupled surface plasmon modes is demonstrated numerically. Owing to the simultaneous excitation of surface plasmons for the pump and signal at the interface between a nonlinear waveguide layer and a metal layer, a sensitivity enhanced bistability effect in the structure can be obtained. The nonlinear refractive index of the waveguide layer experiences an abrupt change of the local pump field owing to this bistability effect. Such a characteristic greatly improves the performance of optical switch by reducing the threshold pump intensity. An all-optical-switch design with a pump threshold as low as 0.9 GW/cm² is presented.     

Comparison of gold and silver dispersion laws suitable for FDTD simulations

We report optical planar waveguide formation and modal characterization in β-BaB₂O₄ crystals by Cu²⁺-ion implantation at an energy of 3.0 MeV and doses of ∼ 10¹⁴ ions/cm². The prism-coupling method was used to investigate the dark-mode property at wavelengths of 633 nm and 1539 nm. The refractive-index profile of the waveguide was reconstructed by an effective refractive index method. The modal analysis indicates that the fields of TM modes can be well restricted in the guiding region, which means the formation of a non-leaky waveguide in the crystal. The results show that the β-BaB₂O₄ waveguides may be used in the application of high efficiency frequency conversion. PACS 61.80.Jh; 42.70.Mp; 42.65.Wi     

Proton-exchange influence on the polar phonons of LiTaO3 optical waveguides

Refractive index change and Raman scattering of Z-cut proton-exchanged LiTaO₃ optical waveguides with different composition have been studied. Probing of the waveguide depth was carried out by using micro-Raman spectroscopy in order to distinguish layers with different phase states. Varying the proton-exchange and post-exchange annealing regimes, two phases –α and β– in addition to the pure LiTaO₃, were clearly observed. The behaviour of the A ₁ vibrations of the TaO₆ groups was found to be very sensitive to the phase changes. Using data from waveguide Raman spectra, normal and oblique E(TO, LO) phonons were sorted and their properties were followed in the α and β phases. The OH-stretchings are considered to correspond to dipoles turned out of the oxygen planes. Received: 9 July 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

The fabrication of polymer-based evanescent optical waveguide for biosensing

A polymer waveguide was fabricated to amplify the evanescent optical field for biosensing. The structure of waveguide was designed to propagate a normal single mode at the input and output regions for low loss beam coupling and propagation. A sensing region was formed in the middle of the waveguide to activate the evanescent mode and to induce high birefringence by depositing a thin dielectric film with a high refractive index on a single mode waveguide. A polymer waveguide with the dimensions of 7 μm-width and 2.5 μm-thickness was fabricated by photolithography and dry-etching. The active region of the TiO₂ thin film was fabricated with the dimensions of 20 mm-length, 20 nm-thickness and 2 mm-tapered tail. A polarimetric interference technique was used to evaluate the evanescent waveguide biosensor, and biomaterial such as glycerol was tested. The sensitivity of the sensor increased with increasing TiO₂ film thickness. For the fabricated waveguide with a 20 nm-thick TiO₂ film, the measured index change to the lead phase variation of 2π was 1.8 × 10⁻⁴.     

Nd:YAG waveguide laser with 1.3 W output power, fabricated by direct femtosecond laser writing

Tracks of modified material were written with femtosecond-laser pulses in neodymium-doped YAG crystals. Due to a stress-induced change of the refractive index, waveguiding beside the tracks and between two adjacent tracks with a distance of approximately 25 μm was observed. Loss measurements resulted in guiding losses of about 1.6 dB/cm for the double track waveguide. Spectroscopic investigations of the ⁴ F _3/2→⁴ F _11/2 transmission lines of the neodymium ions, which are close to the modified region, revealed a small stress-induced red shift of the lines. Laser oscillation of single-track waveguides and double-track waveguides was demonstrated with Ti:Sapphire laser pumping at a wavelength of 808 nm. Best laser performance with about 1.3 W output power at 2.25 W launched pump power was achieved using a double-track waveguide with a separation of 27 μm at an outcoupling transmission of 95%.     

Laser induced memory bits in photorefractive LiNbO3 and LiTaO3

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO₃ and LiTaO₃ crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10⁻⁴ was recorded in undoped LiNbO₃ at an average light intensity of ∼TW/cm² that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed. M. Sūdžius leaves from: the Institute of Materials Science and Applied Research of Vilnius University, Lithuania.     

Channeling,superfocusing, and nuclear reactions

A highly collimated beam of protons (≈1 MeV) entering the channel of a monocrystal film forms at a certain depth an extremely sharp (<0.005 nm) and relatively long (some monolayers of the crystal) focusing area where the increase of the flux density can reach hundreds times. Impinging atoms in this focusing area can undergo nuclear or atomic reactions with proper foreign dopants which disappear if the crystal is tilted from this position by only 10⁻³ rad. This effect can be called channeling superfocusing, in contrast to the ordinary flux peaking where the increase of flux density reaches only few times. Results are predicted by the exactly solvable quantum mechanical model calculations and confirmed by channeling Monte Carlo simulations accounting for several properties of the real lattice. Unfortunately, the mosaic structure of the film and statistical spread of the film and statistical spread of the axes of channels prevent the observation. Special technologies and materials choice are needed to minimize this effect. The text was submitted by the author in English.     

Characterization of optical waveguide in Nd: LuVO4 crystals by triple-energy oxygen ion implantation

We report on the optical planar waveguide formation and modal characterization in Nd:LuVO₄ crystals by triple-energy O³⁺-ion implantation at energies of 2.4, 3.0, and 3.6 MeV and doses of 1.4, 1.4, and 3.1×10¹⁴ ions/cm², respectively. The prism-coupling method is used to investigate the dark-mode property at a wavelength of 633 nm. The refractive index profiles of the waveguide are reconstructed by the reflectivity calculation method (RCM). The modal analysis shows that the fields of TE modes are well restricted in the guiding region, which indicates the formation of non-leaky waveguide in the crystal.     

Transient characteristics of green upconversion emission of Er3+ in MgO:LiNbO3 crystal: Mg threshold concentration effect

Transient characteristics of upconverted emission (560 nm) of Er³⁺ in LiNbO₃ crystals codoped with 0–7.4 mol% MgO were studied under pulse excitation at 800-nm wavelength. The results show that the transients display considerable Mg-doping-level-dependent nonexponential behavior and a clear Mg optical-damage-resistance threshold concentration effect. Below the Mg threshold concentration, the lifetime increases slightly with the increased Mg concentration. Above the threshold, however, the lifetime drops abruptly by 4–7 times and the nonexponential feature becomes more evident. It is found that each transient can be fitted by a double-exponential function contributed from isolated and clustered Er³⁺ sites. The fit parameters show that doping of MgO above the threshold concentration increases the clustered Er site concentration and the nonradiative cross relaxation probability. The Mg threshold concentration effect derived from the transients is in qualitative agreement with that from the fluorescence spectrum measured as a function of the Mg concentration. The effect of the Mg threshold concentration on the clustered Er site concentration is qualitatively explained on the basis of the microscopic defect model of MgO:LiNbO₃ and is conducted with the Mg site change around the threshold concentration.     

Research on the influence of fabrication parameters on the performance of buried ion-exchanged glass waveguides using the variational method

The variational method is proposed to analyze the influence of the fabrication parameters on the performance of buried K⁺–Na⁺ ion-exchanged Er³⁺–Yb³⁺ ions co-doped glass waveguide. The unknown parameters of the Hermite–Gaussian functions as the trial field distribution are determined based on the scalar variational principle. It is demonstrated that the results calculated in this paper agree with those measured in the experiment. The mode dimensions, the effective refractive index, and the overlap factor as the functions of the fabrication parameters are investigated. These results of the variational analysis are useful for the design and optimization of Er³⁺–Yb³⁺ ions co-doped waveguides.     

Well-confined Yb:GdVO4 laser waveguide formed by MeV C ion implantation

The planar waveguide in x-cut Yb:GdVO₄ crystal has been fabricated by 6.0 MeV carbon ion implantation with the fluence of 1 × 10¹⁴ ions/cm² at room temperature. The modes of the waveguide were measured by the prism-coupling method with the wavelength of 633 nm and 1539 nm, respectively. An enhanced ordinary refractive index region was formed with a width of about 4.0 μm beneath the sample surface to act as a waveguide structure. By performing a modal analysis on the observed transverse magnetic polarized modes, it was found that all the transverse magnetic polarized modes can be well-confined inside the waveguide. Strong Yb-related photoluminescence in Yb:GdVO₄ waveguide has been observed at room temperature, which reveals that it exhibits possible applications for integrated active photonic devices.     

Characterization of laser waveguides in Nd:CNGG crystals formed by low fluence carbon ion implantation

We report on Nd:CNGG active planar waveguides produced by 6.0 MeV carbon ion implantation at fluence from 1 × 10¹⁴ ions/cm² to 8 × 10¹⁴ ions/cm². The refractive index profiles, which were reconstructed according to the measured dark mode spectroscopy, showed that the refractive indices had negative changes in the surface region, forming typical barrier waveguide. The width of waveguide structure induced by carbon ion implantation is ∼3.8 μm. The typical barrier-shaped distribution may be mainly due to the nuclear energy deposition of the incident ions into the substrate. By performing a modal analysis on the observed TE modes, it was found that the TE₀ and TE₁ modes can be well-confined inside the waveguide.     

Characterization of optical planar waveguide in Ce:KNSBN crystal formed by triple-energy helium ion implantation

We report on the optical planar waveguide formation and modal characterization in a Ce:KNSBN crystal by triple helium ion implantation at energies of (2.0, 2.2 and 2.4 MeV) and fluences of (1.5, 1.65 and 2.25) × 10¹⁵ cm⁻². The prism-coupling method is used to investigate the dark-line spectroscopy at wavelength of 632.8 and 1539 nm, respectively. The refractive index profiles of the waveguide are reconstructed by an effective refractive index method. It is found that the ion-beam irradiation creates slight increase of extraordinary index whilst decreases ordinary one in the guide region. The modal analysis shows, at wavelength of 632.8 nm, the fields of one TE and three TM modes are well restricted in the guiding region, which means the formation of non-leaky waveguide in the crystal. The damping coefficients of the waveguide are 0.6 and 1.6 cm⁻¹ for ordinary and extraordinary polarized light at 632.8 nm, respectively.     

Vertical input optical ring resonator with differential operation for optical sensor

A novel waveguide ring resonator optical sensor with two resonant wavelength channels is proposed for a refractive index measurement of a test sample placed on the sensor substrate and its performance characteristics are investigated analytically and numerically. The waveguide device consists of a ring resonator, a split-ring-shaped loop waveguide, and a vertical input/output grating coupler, in which the loop waveguide acts as an additional resonator and provides another output wavelength channel of the sensor. The differential detection between the two wavelength channels enables the highly sensitive detection with temperature compensation. A numerical simulation based on a finite difference time domain (FDTD) method shows that a precise index change detection with a resolution of 10⁻⁶ can be achieved using of the proposed device.     

离子注入结合离子交换技术制备KTiOPO4光波导特性研究

用离子注入结合离子交换技术形成了 KTiOPO₄ 平面光波导,研究了离子注入对离子交换波导结构的影响.使用棱镜耦合法测量了波导特性,结果显示形成了表面折射率升高的多模波导,通过背散射技术研究了离子交换后的 Rb 离子分布.实验表明,注入离子导致样品晶格损伤,在 2.8 μm处对离子交换形成了阻挡层,阻止了交换向KTP晶体的更深处进行.     

基于耦合介质纳米线的深亚波长局域波导

提出了一种基于耦合介质纳米线的深亚波长局域波导,通过两根紧邻的高折射率介质纳米线的耦合,该波导可以将光场有效束缚在纳米线之间的低折射率纳米缝隙中. 计算模拟的结果表明,该波导的有效模场面积达到Λ²₀/200,比单根纳米线波导小一个数量级,这种深亚波长的模场束缚能力可以与表面等离激元混合波导相比拟. 计算模拟的结果还表明,纳米线可能带有的低折射率氧化膜、低折射率衬底的存在、以及纳米线间尺寸存在的一定差异对于该波导结构的实际应用都不会产生很大 关键词： 介质波导 亚波长局域 表面等离激元波导 纳米线