Daniela Grando

Optical fiber-polymer guide coupling by a tapered graded index glass guide

A new four layer tapered coupler (FLTC) has been studied and tested, able to transfer light efficiently from a monomode fiber to a polymer film guide by means of an auxiliary graded index glass waveguide. A simple theory has been developed to calculate the maximum efficiency of these structures. Experiments have been performed, at /spl lambda/=1.321 /spl mu/m, on several FLTC samples, containing K/sup +/-Na/sup +/ ion-exchanged glass waveguides and spin coated poly-3BCMU films. Insertion losses in the range 3-6.5 dB have been measured, depending on the taper configuration. The agreement between theoretical and experimental data is good, provided that a suitable ripple of the taper profile is assumed. The application of the new coupler to future integrated waveguide systems, including passive glass components and polymer based all optical devices, is assessed. >

Zirconium-doped lithium niobate: photorefractive and electro-optical properties as a function of dopant concentration

Measurements of refractive indices, electro-optic coefficients and photorefractivity are performed for a set of Zirconium-doped congruent lithium niobate (Zr:LN) crystals as functions of the dopant concentration in the range 0.0-3.0 mol%. The photorefractive properties are studied by measuring the green-light induced birefringence change and by direct observation of the transmitted-beam distortion. The index of refraction data show that the threshold concentration, above which there is a change in the Zr incorporation mechanism, is about 2.0 mol%, but photorefractivity results suggest that the concentration of ZrO2 required to strongly reduce the photorefractive effect is somewhat larger than the 2.0 mol% “threshold” concentration derived from index-of-refraction data. The electro-optic coefficients are little influenced by Zr-doping. All the reported results confirm that Zr:LN is a very promising candidate for the realization of efficient electro-optic and all-optical nonlinear devices working at room temperature.

TWO-PHOTON ABSORPTION SPECTRUM OF 3-BUTOXYCARBONYLMETHYLURETHANE POLYDIACETYLENE THIN FILMS

We report the two-photon absorption (TPA) spectrum of a film of poly[4,6-dacadiyne-1,10-diolbis(3-butoxycarbonyl-methyl-urethane)]—commonly known as poly(3-BCMU)—in the wavelength range λ=1.6–0.7 μm. We find that the TPA coefficient of the film, which was prepared by spin coating, increases from ≈2 cm/GW at 1.45 μm to 170 cm/GW at 0.8 μm. The main features of the spectrum can be accounted for by a three-level model with a two-photon state positioned at 2.8 eV.

Efficient coupling between annealed K/sup +/-Na/sup +/ ion-exchanged channel waveguides and 10/125 single-mode fibers at /spl lambda/=1.321 /spl mu/m

The process of thermal annealing of K/sup +/-Na/sup +/ ion-exchanged channel waveguides has been studied with the aim of optimizing their coupling efficiency with commercial single-mode fibers at /spl lambda/=1.321 /spl mu/m. Waveguides obtained in soda-lime glass slides, with mask apertures ranging between 13.4 and 2.6 /spl mu/m, were characterized before the annealing by combining nearfield measurements and an etching procedure. The experimental results were successfully compared with a theoretical model based on the variational principle. The refractive index distribution of K/sup +/-Na/sup +/ ion-exchanged channel waveguides supporting one or a low number of modes was given: compared to the corresponding slab case, the refractive index step /spl Delta/n/sub o/ remained constant, while the waveguide depth was lower. The thermal annealing process of the channels was then performed and modeled by means of the standard diffusion theory. As a result, the channel fabrication parameters for optimum guide-fiber coupling could be predicted: 0.23-dB mode mismatch losses were measured between the optimized channel and a commercial 10/125 single-mode fiber, at /spl lambda/=1.321 /spl mu/m.

Nonlinear optical response of a polycarbazolyldiacetylene film through femtosecond two-photon spectroscopy

Abstract We report the two-photon absorption (TPA) spectrum of a polycarbazolyldiacetylene film in the interval 900–1550 nm. In this range, the TPA coefficient exhibits a broad peak with a maximum of 33 cm/GW around 1125 nm, which indicates a two-photon state positioned at 2.2 eV, slightly below the one-photon state at 2.36 eV. The magnitude of the nonlinear refraction in the infrared is predicted on the base of a Three Level Model, which accounts fairly well for the nonlinear absorption in the explored range.

Photoconductive and Electro-Optical Properties of Hf Doped Lithium Niobate Crystals

The optical and electrical properties of a set of Hf doped Lithium Niobate crystals with doping level from 2 to 8 mol% are reported. The photoconductivity is found to increase remarkably for Hf concentration equal and above 4 mol%. In contrast, the electro-optical coefficients keep almost constant with the crystal structural properties preserved. The conclusion is drawn that the decrease of the photorefractive response in Hf doped Lithium Niobate recently observed by other authors is mainly related to the increased photoconductivity.

An Optical Thermometer Exploiting Periodically Poled Lithium Niobate for Monitoring the Pantographs of High-Speed Trains

Optical thermometers have been widely investigated. Here, the temperature behavior of second harmonic generation (SHG) in periodically poled lithium niobate (PPLN) substrates is analyzed; indeed, the QPM tuning in PPLN devices and the obtained SHG efficiency depend on the crystal thermal expansion and dispersion, particularly in the case of guided propagation. Therefore, such devices are suitable to realize optical thermometers for demanding applications. This investigation originated with the request of a thermometer to be installed on the pantographs of high-speed trains. Therefore, it must be sturdy and reliable, but it has even to work in an EMD environment. The temperature behavior of the SHG was theoretically modeled and experimentally validated at 1550 nm, in both bulk propagation and APE channel waveguides. In the first case, by using a 10-mW source, which was obtained from a laser diode and a fiber amplifier, an accuracy of 0.3 degC was found. The pump power was about three orders of magnitude smaller in guided propagation. In view of testing on the trains, our investigation resulted in the design of a device without mechanical contacts with the input and output fibers. Since it works in free propagation, there are no serious alignment and packaging problems. The performances, which are expected to be the same of our tests, widely satisfy all the requirements for working effectively in a strongly hostile and EMD environment and for giving accurate measurements on a wide range of temperatures

Identification of LiNbO3 compositions with optimized functional properties for advanced electro-optical devices

Various material and functional properties have been measured in lithium niobate crystals (LiNbO3) with different compositions, starting from conventional congruent composition, up to off-congruent and quasi-stoichiometric ones. The UV absorption edge has been measured and correlated with the crystal composition, showing the edge shift towards shorter wavelengths. The ferroelectric transition Curie temperatures have been determined by differential scanning calorimetry, and it increases with Li2O content in the crystal. The surface composition has been checked by micro-Raman spectroscopy. A narrowing of the linewidths has been observed for quasi-stoichiometric crystal, showing an ordered structure, if compared with congruent composition. The coercive field has been measured as a function of temperature for two different crystal compositions, and it has been found lower in the off-congruent substrate. The Ti-indiffusion process has been studied and compared in congruent and off-congruent LiNbO3 substrates by secondary ion mass spectrometry. The main diffusion process parameters have been determined. The Ti diffusion process has been found considerably slower in off-congruent substrates, if compared with conventional congruent LiNbO3, and resulted almost isotropic. The Li-outdiffusion phenomenon has been observed and correlated wit the Ti concentration profile. A careful control on LiNbO3 composition and material properties allows one to find the proper compositional window for the realization of various advanced optical and electro-optical devices.

Electro-optic modulation of laser diode light by mode interference in a multilayer waveguide including a 2-docosylamino-5-nitropyridine Langmuir–Blodgett film

A hybrid four layer guide (FLG) including a glass guide and a 2-docosylamino-5-nitropyridine Langmuir–Blodgett film is described, and some tests are reported on an electro-optic modulator based on this FLG. The modulation of laser diode light was obtained by guided mode interference, without a Mach–Zehnder configuration of the device. From the modulated signal the r22 electro-optic coefficient was calculated as 11 pm/V.

Wavelength shifting of optical pulses in a polydiacetylene waveguide

We characterize the process of wavelength shifting (generation of a pulse at the wavelength λp−Δ from a signal at λp+Δ under the action of a pump at λp) in a planar waveguide of poly (3BCMU) for short. With a propagation length of 4.5 mm, pulses of 20 ps time duration, and λ′ s around 1.3 μm, we observe wavelength conversion with 1% efficiency when the energy of the pump pulse per unit width is about 10−5 J/m.