Juan Hernández-Cordero

Power spectral distributions of pseudo-turbulent bubbly flows

An experimental study was carried out to determinate the power spectral density (PSD) of mono-dispersed bubbly flows in a vertical channel using flying hot-film anemometry. To improve bubble detection, optical fibers were installed in close proximity to the anemometer sensing element; in this way, the collisions of bubbles with the probe can be detected and removed from the signal. Measurements were performed with gas fractions up to 6%. The PSD distributions were found to decay with a power of −3, in agreement with previous studies, but for a much wider range of Reynolds and Weber numbers. Our measurements indicate that the power decay does not depend strongly on the nature of hydrodynamic interactions among bubbles.

Microbubble generation using fiber optic tips coated with nanoparticles

We show that fiber optic tips can be used as microbubble generators in liquid media. Using standard single-mode silica fibers incorporating nanoparticles (carbon nanoparticles and metallic powders), bubbles can be generated with low optical powers owing to the enhanced photothermal effects of the coating materials. We provide details about the hydrodynamic effects generated in the vicinity of the fiber tip during the coating process, bubble generation and growth. Flow visualization techniques show that thermal effects lead to bubble formation on the tip of the fibers, and coating optimization is crucial for optimal performance of the probes.

Technique for referencing of fiber-optic intensity-modulated sensors by use of counterpropagating signals

We present a new all-optical fiber-referencing scheme for intensity-modulated sensors. It consists of a closed loop traversed by sensing and reference optical signals in opposite directions. With the proposed scheme the noise induced by power fluctuations of the optical source and mechanical perturbations can be greatly reduced. We experimentally demonstrate the efficiency of the scheme and discuss its use in a sensor array.

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.

Multiwavelength and Tunable Self-Pulsating Fiber Cavity Based on Regenerative SPM Spectral Broadening and Filtering

We experimentally demonstrate the operation of a pulsed source based on self-phase modulation followed by offset spectral filtering. This source has three operation regimes: a continuous-wave regime, a self-pulsating (SP) regime where the source self ignites and produces pulses, and a pulse-buffering (PB) regime where no new pulse is formed from spontaneous noise but only pulses already propagating, or potentially pulses injected in the cavity, can be sustained. In the SP and PB regimes, the pulsed source is multiwavelength and continuously tunable over the entire gain band of the amplifiers. The output pulsewidth is quasi-transform-limited with respect to the spectral-width of the filters used in the cavity. Overall, this device is extremely simple to implement and is a strong candidate as a pulsed source and for signal buffering.

Single Polarization-Mode-Beating Frequency Fiber Laser

We demonstrate a single-frequency fiber laser cavity that allows for measuring changes in the relative phase of two independent orthogonal polarization modes. Longitudinal-mode selection is carried out by a matched-pair of fiber Bragg gratings in a Fabry-Perot configuration and an intracavity polarizing beam splitter allows for adjustments on each polarization independently. The polarization-mode-beating signals generated with this laser are stable over long periods of time, owing to the noise cancellation effect achieved by the common cavity shared by both polarizations. This heterodyne detection scheme allows for measuring changes in frequency as small as 5 kHz, or equivalently, wavelength changes of 40 am (attometers) which are practically impossible to resolve in the wavelength domain. The proposed configuration has possible significance as an ultrasensitive and stable polarimetric optical fiber sensor.

Intra-cavity fiber laser technique for high accuracy birefringence measurement

When a device under test (DUT) with birefringence is placed within a laser cavity two distinct sets of orthogonally polarized longitudinal modes will result. If the output of the laser is sent through a 45(o) linear polarizer, polarization mode beating (PMB) between these two sets of longitudinal modes can be detected. We demonstrate the relation between PMB and the birefringence of the DUT and show that by tracking the PMB it provides a sensitive measurement of the birefringence of the device. We first examined the birefringence of a Newport PM fiber and then measured the birefringence of a 3M (Austin, TX) Chirped grating 1.0 m in length. For comparison, birefringence measurements were performed using a Hewlett-Packard Polarization Analyzer (HP 8509B).

Liquids analysis using back reflection single-mode fiber sensors

We demonstrate a new and simple configuration for measuring physical properties of liquids. With proper data analysis, the proposed setup can potentially extend its application to chemical analysis and characterization of two-phase fluids. The fiber sensor is based on the characteristic reflection spectrum obtained from different samples allocated on the tip of a cleaved single-mode optical fiber. Different tests fluids provide a distinctive reflection spectrum and the physical properties of the sample can thus be inferred through spectral analysis. Our experiments are based on a single mode optical fiber dipped into the liquid of interest and a fiber Bragg grating (FBG) interrogator for registering the backreflected light. We analyze how the pending drops formed on the tip of the fiber generate an interference pattern whose features depend on the physical properties of the test liquid. The experimental setup is controlled through a virtual instrument to perform the tests automatically. Each experiment involves data acquisition from the FBG interrogator and recording images by means of a CCD camera. Image analysis provides information about the geometry of the drop and fiber positioning is carefully controlled to provide consistent reflection patterns for different experiments with the same sample. Preliminary results show the correlation between the spectral response and the drops geometry, which in turn is directly related to surface tension of the liquid sample. The performance of the proposed configuration is evaluated with two liquids (glycerin and PDMS polymer) showing the feasibility of this approach for developing a simple fiber optic liquid analyzer.

Laser induced deformation in polydimethylsiloxane membranes with embedded carbon nanopowder

We demonstrate optically induced micron-range deformation of polydimethylsiloxane (PDMS) membranes with embedded carbon nanopowder. The mechanical deformation can be controlled by low power laser irradiation of the samples and the resulting surface modifications are analyzed via dynamic speckle measurements. Photothermal effects due to optical absorption by the nanopowder are shown to deform the polymer samples leading to localized mechanical stresses induced via thermal expansion of the PDMS.

Photothermal Effects and Applications of Polydimethylsiloxane Membranes with Carbon Nanoparticles

The advent of nanotechnology has triggered novel developments and applications for polymer-based membranes with embedded or coated nanoparticles. As an example, interaction of laser radiation with metallic and carbon nanoparticles has shown to provide optically triggered responses in otherwise transparent media. Incorporation of these materials inside polymers has led to generation of plasmonic and photothermal effects through the enhanced optical absorption of these polymer composites. In this work, we focus on the photothermal effects produced in polydimethylsiloxane (PDMS) membranes with embedded carbon nanoparticles via light absorption. Relevant physical parameters of these composites, such as nanoparticle concentration, density, geometry and dimensions, are used to analyze the photothermal features of the membranes. In particular, we analyze the heat generation and conduction in the membranes, showing that different effects can be achieved and controlled depending on the physical and thermal properties of the composite material. Several novel applications of these light responsive membranes are also demonstrated, including low-power laser-assisted micro-patterning and optomechanical deformation. Furthermore, we show that these polymer-nanoparticle composites can also be used as coatings in photonic and microfluidic applications, thereby offering an attractive platform for developing light-activated photonic and optofluidic devices.