Acousto-optic interaction in polyimide coated optical fibers with flexural waves.

Acousto-optic coupling in polyimide-coated single-mode optical fibers using flexural elastic waves is demonstrated. The effect of the polyimide coating on the acousto-optic interaction process is analyzed in detailed. Theoretical and experimental results are in good agreement. Although the elastic attenuation is significant, we show that acousto-optic coupling can be produced with a reasonably good efficiency. To our knowledge, it is the first experimental demonstration of acousto-optic coupling in optical fibers with robust protective coating.

Lab-in-a-fiber-based integrated particle separation and counting.

An all-fiber integrated device capable of separating and counting particles is presented. A sequence of silica fiber capillaries with various diameters and longitudinal cavities are used to fabricate the component for size-based elasto-inertial passive separation of particles followed by detection in an uninterrupted continuous flow. Experimentally, fluorescent particles of 1 µm and 10 µm sizes are mixed in a visco-elastic fluid and fed into the all-fiber separation component. The particles are sheathed by an elasticity enhancer (PEO - polyethylene oxide) to the side walls. Larger 10 µm particles migrate to the center of the silica capillary due to the combined inertial lift force and elastic force, while the smaller 1 µm particles are unaffected, and exit from a side capillary. A separation efficiency of 100% for the 10 µm and 97% for the 1 µm particles is achieved at a total flow rate of 50 µL min-1. To the best of our knowledge, this is the first time effective inertial-based separation has been demonstrated in circular cross-section microchannels. In the following step, the separated 10 µm particles are routed through another all-fiber component for counting and a counting throughput of ∼1400 particles per min is demonstrated. We anticipate the ability to combine high throughput separation and precise 3D control of particle position for ease of counting will aid in the development of advanced microflow cytometers capable of particle separation and quantification for various biomedical applications.

Optofluidic magnetometer developed in a microstructured optical fiber.

A directional, in-fiber optofluidic magnetometer based on a microstructured optical fiber (MOF) Bragg-grating infiltrated with a ferrofluidic defect is presented. Upon application of a magnetic field, the ferrofluidic defect moves along the length of the MOF Bragg grating, modifying its reflection spectrum. The magnetometer is capable of measuring magnetic fields from 317 to 2500 G. The operational principle of such in-fiber magnetic field probe allows the elaboration of directional measurements of the magnetic field flux.

Dynamics of long-period gratings tuned with internal fiber electrodes.

The physics of electrically switched long-period grating in a twin-hole fiber with internal electrodes is studied. Dynamic measurements for the two polarizations show how the grating spectra shift in time due to the mechanical stress and heat transfer in the core and the cladding.

Reflectometry, ablation and fluid retrieval with an optical fiber.

A technique combining low-coherence reflectometry, laser ablation and microfluidics in a single microstructured fiber is developed. Experimental results demonstrate the possibility to ablate thin aluminum foil samples with fiber-guided Nd:YAG laser light, to collect liquid in the holes of the fiber and to simultaneously monitor the positioning of fiber for ablation and the fluid collection process with low-coherence reflectometry. Potential applications of the technique include minimally invasive retrieval of liquid samples with low contamination risk.

A spectrally tunable microstructured optical fibre Bragg grating utilizing an infiltrated ferrofluid.

The spectral response of a Bragg grating reflector inscribed in a microstructured optical fibre is tuned by employing an infiltrated ferrofluid, while modifying the overlap of the ferrofluidic medium with the grating length. Significant spectral changes in terms of Bragg grating wavelength shift and extinction ratio were obtained under static magnetic field actuation. Spectral measurements revealed non-bidirectional propagation effects dependent upon the relative position between the ferrofluid and the grating. The actuation speed of the device was measured to be of the order of few seconds.

Actively Q-switched all-fiber laser with an electrically controlled microstructured fiber.

Actively Q-switching of an all-fiber laser system is demonstrated. The active element is a polarization switch with nanosecond risetime based on a microstructured fiber with electrically driven internal electrodes. Optical feedback between two 100% reflectors is inhibited until a nanosecond current pulse Q-switches the laser. After a short optical pulse develops several roundtrips later, the fiber switch is turned off, removing the short optical pulse from the cavity through a polarization splitter. Pulses of 50 W peak power and approximately 12 ns duration are obtained with 400 mW pump power at 100 Hz.

Who needs a cathode? Creating a second-order nonlinearity by charging glass fiber with two anodes.

We report that it is possible to create a fiber electret by having both internal electrodes of a twin-hole fiber at the same anodic potential, i.e., without the use of a contacted cathode electrode. We find that a stronger and more temperature-stable charge distribution results when the fiber core is subjected to an external field near zero. Negative charges from the air surrounding the fiber are sufficient for the recording of an electric field across the core of the fiber that is twice stronger than when one anode and one cathode electrode are used. The enhancement in stability and in the strength of the effective chi((2)) induced are a significant step towards the wider use of fibers with a second order optical nonlinearity.

All-fiber cavity dumping.

Cavity dumping of an all-fiber laser system is demonstrated. The active element is a pulse-picker with nanosecond rise time consisting of a microstructured fiber with electrically driven internal electrodes. The device is used for intracavity polarization rotation and dumping through a polarization splitter. The optical flux is removed from the cavity within one roundtrip and most of the amplified spontaneous emission, spiking and relaxation oscillation that follow during the gain recovery phase of the laser are blocked from the output signal.

Photonic scanning receiver using an electrically tuned fiber Bragg grating.

A 5-cm-long electrically tuned fiber Bragg grating is used to filter a microwave signal on an optical carrier at 1.55 mum. A chirped distributed-feedback structure is employed, with a transmission bandwidth of 54 MHz and relative optical carrier rejection of >30 dB for rf frequencies >2 GHz. The rapid monotonic sweep of the Bragg wavelength is translated into a fast-frequency sweep for rf analysis.

Birefringence switching of Bragg gratings in fibers with internal electrodes.

A fiber Bragg grating was written in a side-hole fiber with internal metal alloy electrodes. The initial geometrical birefringence of this fiber gives rise to two Bragg resonances separated by 43 pm. Nanosecond risetime current pulses of up to 23 A were applied to the metal electrode, which heated and expanded rapidly. This caused mechanical stress in the fiber on a nanosecond scale, resulting in a negative shift of the Bragg wavelength peak for the fast axis mode, and positive but smaller shift for the slow axis mode. The fast change increased the peak separation to approximately 143 pm, corresponding to an increase in birefringence from 4.0 x 10(-5) to 1.3 x 10(-4). Both peaks subsequently experienced a red-shift due to the relaxation of mechanical stress and the increasing core temperature transferred from the metal in many microseconds. Simulations give accurate description of the experimental results.

Nanosecond switching of fiber Bragg gratings.

A FBG was written in a two-hole fiber with internal alloy electrodes. Nanosecond high current pulses cause metal expansion, increase birefringence and tune the gratings with a response time of 29 ns. This short length, low loss, all-spliced high-speed wavelength switching devices described here has potential use in Q-switching fiber laser.

High performance micro-flow cytometer based on optical fibres.

Flow cytometry is currently the gold standard for analysis of cells in the medical laboratory and biomedical research. Fuelled by the need of point-of-care diagnosis, a significant effort has been made to miniaturize and reduce cost of flow cytometers. However, despite recent advances, current microsystems remain less versatile and much slower than their large-scale counterparts. In this work, an all-silica fibre microflow cytometer is presented that measures fluorescence and scattering from particles and cells. It integrates cell transport in circular capillaries and light delivery by optical fibres. Single-stream cell focusing is performed by Elasto-inertial microfluidics to guarantee accurate and sensitive detection. The capability of this technique is extended to high flow rates (up to 800 µl/min), enabling a throughput of 2500 particles/s. The robust, portable and low-cost system described here could be the basis for a point-of-care flow cytometer with a performance comparable to commercial systems.

Evaluation of two imaging techniques: near-infrared transillumination and dental radiographs for the detection of early approximal enamel caries.

OBJECTIVES: The aim of this paper was to evaluate a transillumination (TI) system using near-infrared (NIR) light and bitewing radiographs for the detection of early approximal enamel caries lesions. METHODS: Mesiodistal sections of teeth (n = 14) were cut with various thicknesses from 1.5 mm to 4.75 mm. Both sides of each section were included, 17 approximal surfaces with natural enamel caries and 11 surfaces considered intact. The approximal surfaces were illuminated by NIR light and X-ray. Captured images were analysed by two calibrated specialists in radiology, and re-analysed after 6 months using stereomicroscope images as a gold standard. RESULTS: The interexaminer reliability (Kappa test statistic) for the NIR TI technique showed moderate agreement on first (0.55) and second (0.48) evaluation, and low agreement for bitewing radiographs on first (0.26) and second (0.32) evaluation. In terms of accuracy, the sensitivity for the NIR TI system was 0.88 and the specificity was 0.72. For the bitewing radiographs the sensitivity ranged from 0.35 to 0.53 and the specificity ranged from 0.50 to 0.72. CONCLUSION: In the same samples and conditions tested, NIR TI images showed reliability and the enamel caries surfaces were better identified than on dental radiographs.

Electro-optical fiber modulation in a Sagnac interferometer.

The linear electro-optical effect was induced in optical fibers by thermal poling. A Sagnac all-spliced fiber loop was constructed incorporating an electro-optical fiber used as a phase modulator at 1.5 microm and stable switching was obtained. A rise time of approximately 40 ns was obtained, and signals <2 V generated a measurable optical response.

Experimental studies on efficient frequency doubling in glass optical fibers.

Experiments were carried out in order to investigate efficient second-harmonic generation in phosphor-doped glass fibers. Peak conversion efficiencies > 5% corresponding to peak powers> 1 kW at 0.53 microm have been obtained. By measuring the side light emitted and also by cutting the fibers, both the second-harmonic output power and the generation rate were studied along the fiber. Other measurements, including the polarization and mode structure of the frequency-doubled output, are also described.

Four-photon fiber laser.

Strong infrared optical signals at the wavelengths 0.991 and 1.149 microm were produced as a result of phase-matched four-photon mixing when Nd:YAG laser light was coupled into a <4-m-long piece of single-mode glass fiber. Stimulated gain was observed, and laser action was obtained with the fiber in an external cavity.

Large increase in photosensitivity through massive hydroxyl formation.

We report a large increase in photosensitivity of germanium-doped silicate fibers by rapid heat treatment of hydrogen-loaded fibers at 1000 degrees C before exposure of the fibers to 242-nm radiation. The increase in photosensitivity is compared with thermally induced absorption caused by introduction of massive amounts of hydroxyl species. The absorption loss was measured to be 0.02 dB/cm mol.% OH at 1.55 mum. Strong gratings (Dn > 1 x 10(-4)) in germanium-free phosphorous-doped fibers in the presence of 242-nm radiation have also been manufactured by this technique.

Interferometric study of poled glass under etching.

An interferometric technique was developed to characterize in real time the etching rate of glasses with <100-nm resolution. The interferometer can be used to help reveal the charge distribution in poled glass.

Dye laser pumped by Nd:YAG laser pulses frequency doubled in a glass optical fiber.

Efficient frequency doubling of a cw Q-switched and mode-locked Nd:YAG laser has been observed in commercial single-mode optical glass fibers. Pulses of duration ~55 psec and intensities as high as ~0.55 kW were produced at 0.53 microm. The maximum peak power-conversion efficiency measured was ~3%. The frequency-doubled light generated in the glass fibers was sufficient to pump a commercial Rh6G dye laser with ~19% efficiency at 570 nm.