On the Motion of Carbon Nanotube Clusters near Optical Fiber Tips: Thermophoresis, Radiative Pressure, and Convection Effects

We analyze the motion of multiwalled carbon nanotubes clusters in water or ethanol upon irradiation with a 975 and 1550 nm laser beam guided by an optical fiber. Upon measuring the velocities of the nanotube clusters in and out of the laser beam cone, we were able to identify thermophoresis, convection and radiation pressure as the main driving forces that determine the equilibrium position of the dispersion at low optical powers: while thermophoresis and convection pull the clusters toward the laser beam axis (negative Soret coefficient), radiation pressure pushes the clusters away from the fiber tip. A theoretical solution for the thermophoretic velocity, which considers interfacial motion and a repulsive potential interaction between the nanotubes and the solvent (hydrophobic interaction), shows that the main mechanism implicated in this type of thermophoresis is the thermal expansion of the fluid, and that the clusters migrate to hotter regions with a characteristic thermal diffusion coefficient D(T) of 9 × 10(-7) cm(2) K(-1) s(-1). We further show that the characteristic length associated with thermophoresis is not that of the nanotube clusters size, O(1) μm, but that corresponding to the microstructure of the clusters, O(1) nm. We finally discuss the role of the formation of gas-liquid interfaces (microbubbles) at high optical powers on the deposition of carbon nanotubes on the optical fiber end faces.

Six mode selective fiber optic spatial multiplexer

Low-loss all-fiber photonic lantern (PL) mode multiplexers (MUXs) capable of selectively exciting the first six fiber modes of a multimode fiber (LP₀₁, LP_11a, LP_11b, LP_21a, LP_21b, and LP₀₂) are demonstrated. Fabrication of the spatial mode multiplexers was successfully achieved employing a combination of either six step or six graded index fibers of four different core sizes. Insertion losses of 0.2-0.3 dB and mode purities above 9 dB are achieved. Moreover, it is demonstrated that the use of graded index fibers in a PL eases the length requirements of the adiabatic tapered transition and could enable scaling to large numbers.

Thermocapillary flow in glass tubes coated with photoresponsive layers

Thermocapillary flow has proven to be a good alternative to induce and control the motion of drops and bubbles in microchannels. Temperature gradients are usually established by implanting metallic heaters adjacent to the channel or by including a layer of photosensitive material capable of absorbing radiative energy. In this work we show that single drops can be pumped through capillaries coated with a photoresponsive composite (PDMS + carbon nanopowder) and irradiated with a light source via an optical fiber. Maximum droplet speeds achieved with this approach were found to be ~300 μm/s, and maximum displacements, around 120% of the droplet length. The heat generation capacity of the coatings was proven having either a complete coating over the capillary surface or a periodic array of pearls of the photoresponsive material along the capillary produced by the so-called Rayleigh-Plateau instability. The effect of the photoresponsive layer thickness and contact angle hysteresis of the solid-liquid interface were found to be important parameters in the photoinduced thermocapillary effect. Furthermore, a linear relationship between the optical intensity I(o) and droplet velocity v was found for a wide range of the former, allowing us to analyze the results and estimate response times for heat transfer using heat conduction theory.

New perspectives for direct PDMS microfabrication using a CD-DVD laser

A simple and inexpensive alternative to high-power lasers for the direct fabrication of microchannels and rapid prototyping of poly-dimethylsiloxane (PDMS) is presented. By focusing the infrared laser beam of a commercial, low-power CD-DVD unit on absorbing carbon micro-cluster additives, highly localized PDMS combustion can be used to etch the polymer, which is otherwise transparent at such wavelengths. Thanks to a precise and automated control of laser conditions, laser-induced incandescence is originated at the material surface and produces high-resolution micropatterns that present properties normally induced with lasers of much greater energies in PDMS: formation of in situ nanodomains, local fluorescence and waveguide patterns. An extensive study of the phenomenon and its performance for PDMS microfabrication are presented.

Evaluation of mechanical behavior of soft tissue by means of random laser emission

We demonstrate the use of random laser emission for mechanical testing of bovine pericardium. An apparatus designed for tensile tests of soft and thin materials, incorporating optical and mechanical devices, allows for obtaining the mechanical behavior of the tissue samples. Using both, digital image correlation (DIC) and random laser emission analysis, the apparatus provides information regarding the response of the bovine pericardium under different stress levels. Our results show that changes in the spectral features of the random laser correlate well to the mechanical response obtained with conventional uniaxial tensile analysis coupled with DIC. Furthermore, parameters such as the shear and Young moduli are consistent with values reported previously and obtained with other techniques. Changes at the microstructural level of the tissue may thus be evaluated through spectral analysis of the random laser emission from biological samples.

Device for characterization of thermal effusivity of liquids using photothermal beam deflection

We propose and study a novel optoelectronic device for thermal characterization of materials. It is based on monitoring the photothermal deflection of a laser beam within a slab of a thermo-optic material in thermal contact with the sample under study. An optical angle sensor is used to measure the laser deflection providing a simple and experimental arrangement. We demonstrate its principle and a simple procedure to measure thermal effusivity of liquids. The proposed device could be implemented into a compact sensor head for remote measurements using electrical and fiber optic links.

Polarization effects in a high-birefringence elliptical fiber laser with a Bragg grating in a low-birefringence fiber

The polarization characteristics of a fiber laser cavity that combines a high-birefringence Er(3+)-doped elliptical core fiber with a Bragg grating fabricated in a low-birefringence (Low-Bi) fiber are studied. It is shown that, by use of an adequate length of Low-Bi fiber and an intracavity polarization controller, the dependence of the fiber laser's state of polarization on the pump's state of polarization can be altered. This fiber laser cavity design allows for the realization of a double-frequency, orthogonally polarized single-mode fiber laser source insensitive to the polarization parameters of the pump source.

All-fiber coherent beam combining of fiber lasers

One half of a 2x2 fused taper single-mode fiber coupler is used for the all-fiber spatial and spectral beam combining of two fiber lasers.