焦磷酸质子交换LiTaO3光波导

本文首次报道了在低于300℃的焦磷酸中制备质子交换LiTaO_3光波导的方法。通过测量有效扩散系数D给出D_0=3.91×10~6μm~2/h和激活能E_a=0.74eV;并测量了波导参数。实验结果表明:只有非寻常折射率产生增量△n_6,其中TM单模波导的传输损耗低达0.1dB/cm(633mm)。发现,热退火处理后的波导表面非寻常折射率增大了。     

High energy implantation of buried insulating layers

Single crystals of CaF₂, SrF₂, and BaF₂ were irradiated at room temperature with alpha particles emitted from an effectively semi-infinite ²³⁸PuO₂ source to a cumulative dose of 3 × 10²⁰ alpha particles/m². Although no change in the lattice parameter of CaF₂ was observed, the crystals exhibited increasing coloration with dose. The lattice parameters of both SrF2 and BaF2 increased exponentially with dose, in agreement with a previously developed model, but the crystals developed no apparent coloration. Isochronal and isothermal-step annealing were used to study the recovery behavior of the lattice defects in both the SrF₂ and BaF₂ single crystals. A single recovery stage, tentatively associated with cation defects, was observed in each and the activation energy determined.     

Secondary ion mass spectrometry and optical characterization of Ti:LiNbO3 optical waveguides

Secondary ion mass spectrometry (SIMS) and m-lines spectroscopy have been applied to study Ti:LiNbO₃ slab optical waveguides with high titanium surface concentration. By combining the two techniques, a saturation in the dependence of the refractive index change on the dopant concentration has been found. By the use of SIMS in image mode, the lateral diffusion of titanium in Ti:LiNbO₃ channel waveguides has been observed and analyzed.     

Waveguides in lithium niobate fabricated by focused ultrashort laser pulses

We report on investigations of the bulk microstructuring of lithium niobate crystals with intense femtosecond laser pulses. In different crystal cuts, optical waveguides were produced whose properties depend strongly on the processing parameters. To explore the origin of the refractive index changes, we subjected the crystals to different conditions (like temperature, illumination, etc.) while monitoring the waveguide output. This way several mechanisms for the change in refractive index could be singled out. These include the photorefractive effect, inhomogeneous ion concentrations and stress in the crystalline lattice. As an application, we demonstrate frequency doubling of 1064 nm laser radiation in a microstructured phase-matched waveguide.     

Annealing effect on planar waveguides in LiNbO3 produced by oxygen ion implantation

We reported on planar waveguides in stoichiometric lithium niobate fabricated by 4.5 MeV oxygen ion implantation with a dose of 6 × 10¹⁴ ions/cm² at room temperature. After ion implantation, these samples were annealed at 240 °C, 260 °C, and 300 °C for 30 min. We investigated annealing effect on the guiding modes and near-field images in the waveguides by prism-coupling method and end-face coupling method respectively. We found that for the extraordinary refractive index a positive alternation occurred in the near-surface region while a negative alternation happened at the end of ion track. Moreover, we measured the transmission spectra for the pure sample and implanted samples before and after annealed at different temperatures, and we observed an absorption peak at ∼480 nm (2.6 eV) in all of these SLN samples.     

Confocal micro-luminescence of Zn-diffused LiNbO3:Tm channel waveguides

Zinc-diffused channel waveguides fabricated in thulium-doped LiNbO₃ have been analyzed by means of confocal micro-luminescence techniques. Modifications induced by Zn²⁺ ions in the doped-crystal substrate are characterized following the evolution of the ³H₄→³H₆ luminescence of thulium ions at 795 nm, showing the existence of two different regions in the waveguide. Several Raman modes of LiNbO₃ have been used as well to study the differences between the waveguide and the bulk material.     

Low-loss polymeric optical waveguides with large cores fabricated by hot embossing

A simple low-cost method of fabricating polymeric optical waveguides with large core sizes for plastic optical fibers is presented. The waveguides are fabricated by hot embossing with an ultraviolet-cured epoxy resin stamper. The stamper is fabricated by replication of a rectangular groove mold that is made from silicone rubber replicated from a ridged original silicon master. The master is fabricated by anisotropic etching of (110) single-crystal silicon. Optical waveguides with large core sizes of 100-500 microm have been fabricated, and a low propagation loss of 0.19 dB/cm at 650 nm was achieved.     

Serially grafted polymer optical waveguides fabricated by light-induced self-written waveguide technique

Serially grafted polymer optical waveguides were fabricated by the light-induced self-written (LISW) waveguide technique for the first time to our knowledge. To realize functional waveguide cores by the LISW technique, transparent materials at the writing wavelength were selected. By inserting thin transparent partitions, a serial-graft structure consisting of passive and active waveguides without any misalignment was realized automatically. This technique is advantageous for its extremely easy process over conventional fabrication techniques.     

Photonic band gap grating in He+-implanted lithium niobate waveguides

In this work, we report the investigation of 2D photonic crystals in lithium niobate associated with waveguiding structures fabricated by He⁺ implantation. From the material point of view, the investigation of PBG structures in lithium niobate is of great interest for optoelectronics technology since this crystal is one of the most important materials widely used in integrated and non linear optics. The choice of the implantation technique to produce the waveguide is motivated by the possibility of having both Transverse Electric (TE) and Transverse Magnetic (TM) guided modes in order to obtain a total photonic band gap (PBG).     

Laser-assisted fabrication of new slab-coupled lithium niobate optical waveguide

Recently a new type of lithium niobate waveguide was suggested for potential nonlinear optic applications. The waveguide consists of a uniform large core and a leaky coupled slab for realizing a lateral optical confinement to support the fundamental spatial mode propagation. Inside the waveguide, the slab layer is required to have a refractive index slightly lower than that of the core, but higher than that of the substrate. Lithium niobate doped with magnesium oxides shows an increased refractive index that is dependent on the dopant's concentration. Therefore, in order to fabricate such waveguides, the pulsed laser deposition approach was used to study the growth of such composition-modified lithium niobate as the slab layer. The as-grown films were characterized on its expitaxy, structure, and optical performance, via X-ray diffraction analysis, optical guiding experiment, etc.     

Single-mode optical waveguides fabricated in LiNbO3 by multienergy C implantations

Single-mode waveguides in LiNbO₃ are demonstrated by use of prism coupling method. The waveguides are fabricated by three different energies and single energy C²⁺ implantations at the equal total doses, respectively. Dark modes and propagation loss are measured by use of prism coupling and moving fiber methods, respectively. Damages produced by implantation are measured by RBS/channeling technique. The waveguide structures are investigated in two different implantation cases. The results of analysis indicate that the heavy ion-implanted waveguides are still defined by synergetic characteristics from both the raised-index region and the low-index barrier. The broadened barrier from multienergy implantation is demonstrated to play a significant role in reducing propagation loss.     

Modelling of optical amplification in Er/Yb co-doped LiNbO3 waveguides

The optical amplification of LiNbO₃:Er³⁺/Yb³⁺ channel waveguides has been modelled in the small signal regime using the overlapping integrals method and the rate-equation formalism. It has been found that Yb³⁺ -sensitisation improves the pump efficiency at 980 nm and a higher gain is achievable in the high power-limit compared to singly-doped LiNbO₃:Er³⁺ amplifiers.     

Determination of refractive index profiles of gradient optical waveguides by ellipsometry

Ag⁺-Na⁺ and K⁺-Na⁺ ion-exchanged optical waveguides in soda-lime glass are characterised by ellipsometry. Refractive index profiles of the waveguides are calculated from ellipsometric multiple angle of incidence data using the Newton-Kantorovitch type iterative procedure and compared with those reconstructed by inverse WKB method. It is demonstrated that such continuous profiles with relatively small index gradient (of the order of 0.1 and 0.01), extending to few micrometers in depth, can be determined by ellipsometric measurements. A good agreement is found between results obtained by ellipsometry and by the inverse WKB method at depths above 500–600 nm, while there is a difference in the subsurface region, where ellipsometry is more sensitive to the quality of the surface. The profiles obtained by the two methods are consistent if the surface thin layer is etched.     

Formation and characteristics of graded-index optical waveguides buried in glass

Low-loss optical waveguides have been prepared through thermally induced ion exchange, where sodium ions in glass are replaced by silver ions from an unstirred silver nitrate melt. The index distributions of the inhomogeneous guides produced in the glass are deduced by analysing the measured mode spectra with the WKB-method. The increase in the refractive index caused by the presence of the silver is found to have a nearly Gaussian form with a peak of about 0.082. The index profile is correlated to the two parameters of the preparation process, the diffusion time and the operating temperature. In a typical case (9 hrs and 221°C) the index takes on its maximum value 2.6 μm below the surface of the glass and one obtains an 8-mode guide with five modes buried within the glass and three modes extending to the air-glass interface.A single-mode guide is formed in less than 5 min. The measurements yield an activation energy for the diffusion of Ag⁺ in glass of 8.4·10⁴J/mole and a diffusion coefficient at 221°C of 0.67·10⁻¹² cm²/s, in good agreement with the experimental results reported by other authors. The present observations suggest that a potential gradient is set up in the glass when silver diffuses into it so that the silver ions drift into the glass with a velocity of about 0.3μm/hr.     

Self-ordered sub-micron structures in Fe-doped LiNbO3 formed by light-induced frustration of etching

We have produced millimetre-sized areas of self-ordered, sub-micron scale, complex structures in iron-doped single crystal lithium niobate, using the combined interaction of laser light and etching with acid. We report here the factors that influence the formation of the structures, and illustrate the types of structure that may be formed, using images taken by a scanning electron microscope (SEM) and subsequent Fourier Transform analysis of those images. We also consider the physical processes which initiate and control the growth of the structures.     

Nonlinear optical waveguides fabricated in Mg-doped LiNbO3 by swift heavy ion irradiation: anomalous photorefractive damage behavior

Using swift heavy fluorine ion irradiation, we have successfully fabricated optical waveguides in Mg-doped LiNbO₃ substrates. A systematic characterization of these structures has been carried out including refractive index profiles, propagation losses, nonlinear coefficients, and, specially, photorefractive optical damage. Step-like refractive index profiles with Δn _e ≈ 0.1 and Δn _o ≈ 0.2, propagation losses lower than 0.5 dB/cm and high nonlinear optical coefficients similar to those of the substrate have been obtained. Unexpectedly, the photorefractive damage is only moderately reduced with regard to the one presented in congruent LiNbO₃ waveguides. Specifically, light intensity damage thresholds I _th are only a factor 2 higher at RT and a factor 4 at 90 °C with regard to undoped waveguides. At this latter temperature, a remarkably high I _th = 30.000 W/cm² is reached. A final discussion on the observed anomalous optical damage behavior induced by swift heavy ion irradiation is also included.     

Damage profiles in LiNbO3 by low-energy H+ implantation

Low-energy 120 and 150 keV H⁺ was implanted in z-cut LiNbO₃ at room temperature. The fluence of H⁺ is 5?×?10¹⁶ ions/cm². The damage profiles in LiNbO₃ induced by implantation were investigated using Rutherford backscattering/channelling. The damage profiles were extracted using the channelling results. The experimental damage profiles in LiNbO₃ were analyzed and compared to the simulated results from TRIM. The results show a good consistency between experimental and simulated results. The present results are useful for the fabrication of H-implanted waveguides of oxide crystals, especially LiNbO₃.     

EPR and optical measurements of weakly doped LiNbO3: Er

Optical spectroscopy measurements of the congruent LiNbO₃ (LN) single crystals, weakly doped with Er (0.1–0.3 wt%) and Er (0.3 wt%) and Yb (0.5 wt%), have been carried out. The shape of the optical absorption and additional absorption bands registered after γ-irradiation suggests the presence of Er³⁺ ion pairs. EPR investigations were performed for LN single crystal doped with Er (0.1 wt%). Unusual behaviour of the temperature dependence of the intensity and linewidth of the main EPR line—corresponding to the fine transition of ^evenEr³⁺—ions, is reported. The main EPR line appears to be a superposition of several paramagnetic centres originating from isolated ^evenEr³⁺ ions and ^evenEr³⁺–^evenEr³⁺ pairs of ions. In low temperature region (below 20 K), the main EPR signal is dominated by signals arising from ^evenEr³⁺–^evenEr³⁺ pairs of ions. The inverse intensity of the EPR line at low temperature region fulfils the Curie–Weiss law and enabled to determine the Curie–Weiss constant Θ=1.5±0.5 K. The positive sign of Θ suggests that the ferromagnetic interactions arise in the system of ^evenEr³⁺–^evenEr³⁺ ion pairs in LN. Our results suggest that the distribution of Er ions in congruent LN is not homogeneous and Er impurity ions can form clusters in host lattice even in the case of weak erbium doping.     

EPR study of radiation-induced radicals in glutaric and amino acid derivatives in solid state

Gamma radiation-induced radicals of 2-methylglutaric acid (2MG), diethyl amino malonate hydrochloride (DEAMHCl), ethyl malonate monoamide have been investigated at room temperature by the electron paramagnetic resonance technique. The type of radicals formed and their room temperature stability were evaluated. Three different radicals have been detected. The free radicals formed in compounds were attributed to the HOOCCH₃?CH₂CH₂ COOH, CH₃?HCO₂CHNH₂COCH₂CH₃ HCl and NH₂COCH₂COO?HCH₃ radicals, respectively. The results were found to be in good agreement with the existing literature data and theoretical predictions conformation.     

Mode-selective coupler for wavelength multiplexing using LiNbO<Subscript>3</Subscript>:Ti optical waveguides

A mode-selective directional coupler based on titanium in-diffused channel waveguides in lithium niobate is investigated. This coupler may be utilized as a key part of an add-drop multiplexer for dense wavelength division multiplexing in optical network nodes. The proposed coupler is based on evanescent coupling of the fundamental mode of a single-mode channel to the first higher mode of a parallel bi-modal waveguide. Our experimental results show that a compact directional coupler with coupling efficiencies larger than 90%, large bandwidth around 1550 nm, and with negligible polarization dependence can be realized using electro-optic lithium niobate substrates. Presented at 9-th International Workshop on Nonlinear Optics Applications, NOA 2007, May 17–20, 2007, Świnoujście, Poland