Experimental study of nonlinear switching characteristics of conventional 2×2 fused tapered couplers

The nonlinear switching characteristics of fused fiber directional couplers were studied experimentally. By using femtosecond laser pulses with pulse width of 100 fs and wavelength of about 1550 nm from a system of Ti:sapphire laser and optical parametric amplifier (OPA), the nonlinear switching properties of a null coupler and a 100% coupler were measured. The experimental results were coincident with the simulations based on nonlinear propagation equations in fiber by using super-mode theory. Nonlinear loss in fiber was also measured to get the injected power at the coupler. After deducting the nonlinear loss and input efficiency, the nonlinear switching critical peak powers for a 100% and a null fused couplers were calculated to be 9410 and 9440 W, respectively. The nonlinear loss parameter P_(N) in an expression of α_(NL)=αP/P_(N) was obtained to be P_(N)=0.23 W.

A method for analyzing the coupling of 3-D tapered rib waveguides with fibers

The fields in 3-dimensional tapered waveguides are unstable compared with the fields in the straight waveguides. In the case of waveguide-to-fiber coupling and fiber-to-waveguide coupling, a sequence of short straight waveguides has been modeled to approximate the 3-dimensional tapered waveguide; and the unstable incident and reflected fields, as well as their derivatives, were determined by the beam propagation method(BPM). Then free space radiation mode(FSRM) was employed to calculate the reflected and transmitted powers. Analysis results of the coupling of fiber with silicon-on-insulator(SOI) tapered rib waveguides showed the feasibility of the method.

High performance of 1.55 mu m DFB integrated with vertically tapered self-aligned spot-size converter

A new type of self-aligned spotsize converter (SSC) integrated 1.55 mum DFB lasers had been proposed in this article. The upper optical confinement layer and the butt-coupled tapered thickness waveguide were regrown simultaneously, which not only offered the separate optimization of the active region and the integrated SSC, but also reduced the difficulty of the butt-joint selective regrowth. The vertical and horizontal far field angles were 9degrees and 12degrees respectively, the 1- dB misalignment tolerance were both 3.6 and 3.4 mum. The directed coupling efficiency to tapered single mode fiber was 48%.

Fabrication of 4 x 4 tapered MMI coupler with large cross section

MMI coupler with large cross section has low coupling loss between the device and fiber. However, large chip area is required. Recently proposed N x N tapered MMI coupler shows a substantial reduction in device geometry. No such kind of devices with N > 2 has yet been realized up to now. The authors have demonstrated a 4 x 4 parabolically tapered MMI coupler with large cross section that can match the SM fiber in silicon-on-insulator (SOI) technology. The device exhibits a minimum uniformity of 0.36 dB and excess loss of 3.7 dB, It represents a key component for realization of MMI-based silicon integrated optical circuit technology.

Design of Tapered Rib Spot-Size Converter with Double-Cladding Structure

A novel structure of spot-size converter is designed to allow low loss and large alignment tolerance between single-mode rib waveguide devices and fiber arrays theoretically. The spot-size converter consists of a tapered rib core region and a double-cladding region. Through optimizing parameters,an expanded mode field can be tightly confined in the inner cladding and thus radiation loss be reduced largely at the tapered region. The influence of refractive index and thickness of the inner cladding on coupling loss is analyzed in particular. A novel,easy method of fabricating tapered rib spot-size converter based on silicon-on-insulator material is proposed.

Symmetrically tapered <30-micron-thick quasi-planar Ge waveguides as chemical sensors for microanalysis

Symmetrically tapered planar IR waveguides have been fabricated by starting with a ZnS coated concave piece of single-crystal Ge, embedding it in an epoxide resin as a supporting substrate, and then grinding and polishing a planar surface until the thickness at the taper minimum is <30 μm. Such tapering is expected to enhance a waveguide's sensitivity as an evanescent wave sensor by maximizing the amount of evanescent wave energy present at the thinnest part of the waveguide. As predicted by theory, the surface sensitivity, i.e., the absorbance signal per molecule in contact with the sensing region, increases with decreasing thickness of the tapered region even while the total energy throughput decreases. The signal-to-noise ratio obtained depends very strongly on the quality of the polished surfaces of the waveguides. The surface sensitivity is superior to that obtained with a commercial Ge attenuated total reflection (ATR) accessory for several types of sample, including thin films (<10 ng) and small volumes (<1 μL) of volatile solvents. By using the waveguides, light-induced structural changes in the protein bacteriorhodopsin were observable using samples as small as ∼50 pmol (∼1 μg). In addition, the waveguide sensors can reveal the surface compositions on a single human hair, pointing to their promise as a tool for forensic fiber analysis.

An In Vitro Comparison of Different Cementation Strategies on the Pull-out Strength of a Glass Fiber Post

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Evaluation of the flexural strength of carbon fiber-, quartz fiber-, and glass fiber-based posts

This study investigated the flexural strength of eight fiber posts (one carbon fiber, one carbon/quartz fiber, one opaque quartz fiber, two translucent quartz fiber, and three glass fiber posts). Eighty fiber posts were used and divided into eight groups (n = 10): G1: C-POST (Bisco); G2: ÆSTHETI-POST (Bisco); G3: ÆSTHETI-PLUS (Bisco); G4: LIGHT-POST (Bisco); G5: D.T. LIGHT-POST (Bisco); G6: PARAPOST WHITE (Coltene); G7: FIBERKOR (Pentron); G8: REFORPOST (Angelus). All of the samples were tested using the three-point bending test. The averages obtained were submitted to the ANOVA and to Tukey's test (p < 0.05). The mean values (MPa) of the groups ÆSTHETI-POST - carbon/ quartz fiber post (Bisco) and ÆSTHETI-PLUS - quartz fiber post (Bisco) were statistically similar and higher than the mean values of the other groups. The mean values of the groups C-POST - carbon fiber post (Bisco), LIGHT-POST - translucent quartz fiber post (Bisco), D.T. LIGHT-POST - double tapered translucent quartz fiber post (Bisco), PARAPOST WHITE - glass fiber post (Coltene) and FIBREKOR - glass fiber post (Pentron) were similar and higher than the group REFORPOST - glass fiber post (Angelus). Copyright © 2005 by the American Association of Endodontists.

Fracture resistance of bovine incisors restored with different glass fiber posts: Effect of the diameter of fiber post

Aim: Compare the effect of three post designs on the fracture resistance and failure modes of composite core-fiber post-crownless tooth sets. Materials and Methods: Ninety bovine incisors were selected and divided into nine groups of 10 specimens. The teeth were assigned to three groups based on the post design: Cylindrical, tapered, and double-tapered. Each group was subdivided into three subgroups in accordance with the diameter of the post: Small (No.1), medium (No.2), and large (No.3). The Panavia F system was used for post cementation. The specimens were mounted in acrylic resin blocks with a layer of silicone rubber covering the roots. A universal testing machine compressively loaded the specimens from the palatal side at a crosshead speed of 1 mm/min and at an angle of 135I to the long axis of the teeth, until failure occurred. The failure mode was determined by a stereomicroscope inspection of all the specimens. Data were analyzed by one-way ANOVA and the Tukey test (P < 0.05). Results: The fracture resistance was affected by the type of post (P < 0.0001). A narrower diameter for all of the post systems allowed for higher resistance. The main failure mode in the large cylindrical group was catastrophic fractures, while the main failures in the other eight groups were favorable. Conclusion: Narrower diameter posts showed higher fracture resistance. The dominant failure pattern was repairable fracture, except for those with large cylindrical groups.

Fatigue surviving, fracture resistance, shear stress and finite element analysis of glass fiber posts with different diameters

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Root canal filling: fracture strength of fiber-reinforced composite-restored roots and finite element analysis

The aims of this study were to evaluate the effect of root canal filling techniques on root fracture resistance and to analyze, by finite element analysis (FEA), the expansion of the endodontic sealer in two different root canal techniques. Thirty single-rooted human teeth were instrumented with rotary files to a standardized working length of 14 mm. The specimens were embedded in acrylic resin using plastic cylinders as molds, and allocated into 3 groups (n=10): G(lateral) - lateral condensation; G(single-cone) - single cone; G(tagger) - Tagger's hybrid technique. The root canals were prepared to a length of 11 mm with the #3 preparation bur of a tapered glass fiber-reinforced composite post system. All roots received glass fiber posts, which were adhesively cemented and a composite resin core was built. All groups were subjected to a fracture strength test (1 mm/min, 45°). Data were analyzed statistically by one-way ANOVA with a significance level of 5%. FEA was performed using two models: one simulated lateral condensation and Tagger's hybrid technique, and the other one simulated the single-cone technique. The second model was designed with an amount of gutta-percha two times smaller and a sealer layer two times thicker than the first model. The results were analyzed using von Mises stress criteria. One-way ANOVA indicated that the root canal filling technique affected the fracture strength (p=0.004). The G(lateral) and G(tagger) produced similar fracture strength values, while G(single-cone) showed the lowest values. The FEA showed that the single-cone model generated higher stress in the root canal walls. Sealer thickness seems to influence the fracture strength of restored endodontically treated teeth.

Thermal excitations of optical nanofibers measured with a fiber-integrated Fabry-Pérot cavity

Efficient coupling of light to quantum emitters, such as atoms, molecules or quantum dots, is one of the great challenges in current research. The interaction can be strongly enhanced by coupling the emitter to the eva-nescent field of subwavelength dielectric waveguides that offer strong lateral confinement of the guided light. In this context subwavelength diameter optical nanofibers as part of a tapered optical fiber (TOF) have proven to be powerful tool which also provide an efficient transfer of the light from the interaction region to an optical bus, that is to say, from the nanofiber to an optical fiber. rnAnother approach towards enhancing light–matter interaction is to employ an optical resonator in which the light is circulating and thus passes the emitters many times. Here, both approaches are combined by experi-mentally realizing a microresonator with an integrated nanofiber waist. This is achieved by building a fiber-integrated Fabry-Pérot type resonator from two fiber Bragg grating mirrors with a stop-band near the cesium D2-line wavelength. The characteristics of this resonator fulfill the requirements of nonlinear optics, optical sensing, and cavity quantum electrodynamics in the strong-coupling regime. Together with its advantageous features, such as a constant high coupling strength over a large volume, tunability, high transmission outside the mirror stop band, and a monolithic design, this resonator is a promising tool for experiments with nanofiber-coupled atomic ensembles in the strong-coupling regime. rnThe resonator's high sensitivity to the optical properties of the nanofiber provides a probe for changes of phys-ical parameters that affect the guided optical mode, e.g., the temperature via the thermo-optic effect of silica. Utilizing this detection scheme, the thermalization dynamics due to far-field heat radiation of a nanofiber is studied over a large temperature range. This investigation provides, for the first time, a measurement of the total radiated power of an object with a diameter smaller than all absorption lengths in the thermal spectrum at the level of a single object of deterministic shape and material. The results show excellent agreement with an ab initio thermodynamic model that considers heat radiation as a volumetric effect and that takes the emitter shape and size relative to the emission wavelength into account. Modeling and investigating the thermalization of microscopic objects with arbitrary shape from first principles is of fundamental interest and has important applications, such as heat management in nano-devices or radiative forcing of aerosols in Earth's climate system. rnUsing a similar method, the effect of the TOF's mechanical modes on the polarization and phase of the fiber-guided light is studied. The measurement results show that in typical TOFs these quantities exhibit high-frequency thermal fluctuations. They originate from high-Q torsional oscillations that couple to the nanofiber-guided light via the strain-optic effect. An ab-initio opto-mechanical model of the TOF is developed that provides an accurate quantitative prediction for the mode spectrum and the mechanically induced polarization and phase fluctuations. These high-frequency fluctuations may limit the ultimate ideality of fiber-coupling into photonic structures. Furthermore, first estimations show that they may currently limit the storage time of nanofiber-based atom traps. The model, on the other hand, provides a method to design TOFs with tailored mechanical properties in order to meet experimental requirements. rn

Spectral behavior of a low-cost all-fiber component based on untapered multifiber unions

Untapered multifiber unions are reported to show a spectral behavior similar to the tapered ones. Their oscillatory behavior does not depend on the biconical regions. This suggests a novel way to make low-cost all-fiber devices with applications as passive components such as optical filters and wavelength multiplexers/demultiplexers. Two types of multimode fibers have been studied and information about the index profile influence has been obtained. Polarization insensitivity and temperature stability have been observed.

Spectral characteristics of tapered LPG device as a sensing element for refractive index and temperature

The fabrication and characterization of long-period gratings (LPGs) in fiber tapers is presented alongside supporting theory. The devices possess a high sensitivity to the index of aqueous solutions due to an observed spectral bifurcation effect, yielding a limiting index resolution of ±8.5×10-5 for solutions with an index in the range 1.330-1.335.

Spectral characteristics of tapered LPG device as a sensing element for refractive index and temperature

The fabrication and characterization of long-period gratings (LPGs) in fiber tapers is presented alongside supporting theory. The devices possess a high sensitivity to the index of aqueous solutions due to an observed spectral bifurcation effect, yielding a limiting index resolution of ±8.5 × 10-5 for solutions with an index in the range 1.330-1.335. © 2006 IEEE.