The study of laser-neuron interaction has gained interest over the last few years not only for understanding of
fundamental mechanisms but also for medical applications such as prosthesis because of the non-invasive characteristic
of the laser stimulation. Several authors have shown that near infrared lasers are able to stimulate neurons. It is suggested
that a thermal gradient induced by the absorption of the laser radiation on cells is the primary effect but the exact
mechanism remains unclear. We show in this work that infrared laser radiations provide a possible way for stimulating
retinal and vestibular ganglion cells. We describe relevant physical characteristics allowing safe and reproducible neuron
stimulations by single infrared pulses. Calcium fluorescence imaging and electrophysiological recordings have been used
to measure ionic exchanges at the neuron membrane. The stimulation system is based on a pulsed laser diode beam of a
few mW. Effects of three different wavelengths (from 1470 to 1875 nm) and stimulation durations have been
investigated. Variations of the stimulation energy thresholds suggest that the main physical parameter is the water optical
absorption. Measurements of the temperature at the cell membrane show that a constant temperature rise is required to
stimulate neurons, suggesting a photothermal process.
We report the realization of a surface plasmon resonance imaging biosensor potentially capable of dynamically
characterizing optical anisotropy by means of polarimetric measurements. Our approach relies on a light beam
propagating through a high refractive index glass-prism (Kretschmann-Raether configuration) in order to excite a surface
plasmon wave along a metal-dielectric interface. This evanescent wave probes the metal-dielectric vicinities with subnanometer
sensitivity, thus resolving optical characteristics of adsorbed biomolecular targets. Fixing wavelength and
angle of incidence of the beam enables real-time monitoring of adsorptions and desorptions of targets onto the whole
surface of the chip, allowing for example characterization of DNA:DNA interaction kinetics with applications to genetic
diagnosis. The polarimetric surface plasmon resonance imaging device uses a pyramid of high index glass and two
orthogonal SPR imaging sensor arms. The interface is probed along two orthogonal directions. A signal difference in
reflection between the two arms should allow us to resolve local optical anisotropy of the dielectric medium, keeping the
parallel and real-time capabilities of the system. Additional information can be obtained by varying the angle of
incidence of the light beam or tuning its wavelength. We believe that this type of sensor will be useful for studying
collective biomolecular assemblies' conformational changes.
Surface plasmon resonance (SPR) technique is an optical method that allows the real time detection of small
changes in the physical properties (in particular the refractive index) of a dielectric medium near a metallic surface. This
technique is today applied to the realization of dynamic optical biochips where multiple interactions can be monitored in
parallel and in real time. One of the main advantage compared to other techniques as fluorescence detection is that it
does not require the presence of labels, which could influence the kinetics or the equilibrium of the biomolecular
interactions. However, as the SPR signal amplitude depends on the refractive index shift of the dielectric medium in the
contact with the metallic layer, one way to increase the SPR signal shift is to incorporate a substance possessing a strong
dispersive refractive index. We present the influence of organic chromophores incorporated in the DNA target molecules
on the spectral SPR response of a SPR sensor. Theoretical and experimental results are presented, showing that the DNA
target molecules labeled with chromophores presenting strong spectral refractive index variation in the spectral range of
the SPR spectrum induce significant spectral SPR response changes. The use of specific chromophores provides a
potential way of SPR response enhancement and initial results suggest that this phenomenon can also be used in realtime
SPR imaging detection.
Surface plasmon resonance (SPR) technique has become over the last ten years a powerful tool allowing the analysis and
the detection of biomolecular surface interactions in real time without any use of labels. The highest sensitivity is
currently obtained using a mono-spot sensor through the measurement of the complete surface plasmon resonance curve
(in angular or spectral configuration), since it is inherently more robust than a single intensity variation measurement.
However, this last approach is used to perform SPR imaging, allowing parallel monitoring of hundreds of sensing spots
onto a camera. We present in this work a SPR spectro-imaging system including dynamical multi-spectral capabilities.
The system is based on the illumination over a vertical slit of the biochip (y-dimension) by a white light source. The
spectrum of the reflected light obtained through a grating is then imaged on the x dimension of the camera. The complete
spectral resonance curve of a full column of sensing spots can be monitored in parallel and in real-time by this simple
apparatus. The influence of the main instrumental parameters and of different data processing are investigated. Clear
improvements of the sensitivity have been obtained on refractometric tests and preliminary results on DNA:DNA
interactions are finally presented.
Laser joining is a promising technique for wafer-level bonding. It avoids subjecting the complete MEMS package to a high temperature and/or the high electric field associated with conventional wafer-level bonding processes, using the laser to provide only localized heating. We demonstrate that a benzo-cyclo-butene (BCB) polymer, used as an intermediate bonding layer in packaging of MEMS devices, can be satisfactorily cured with a substantial reduction of curing time compared with an oven-based process by using laser heating. A glass-on-silicon cavity bonded with a BCB ring can be produced in few seconds at typical laser intensity of 1 W/mm2 resulting in a local temperature of ~ 300°C. Hermeticity and bond strength tests show that such cavities have similar or better performance than cavities sealed by a commercial substrate bonders which require a minimum curing time of 10 minutes. The influence of exposure time, laser power and pressure on degree of cure, bond strength and hermeticity is investigated. The concept of using a large area, uniform laser beam together with a simple mirror mask is tested, demonstrating that such a mask is capable of protecting the centre of the cavity from the laser beam; however to prevent lateral heating via conduction through the silicon a high conductivity heat sink is required to be in good thermal contact with the rear of the silicon.
KEYWORDS: Laser welding, Control systems, Signal processing, Aerospace engineering, Optical fibers, Manufacturing, Control systems design, Optical amplifiers, Chromatic aberrations, Lasers
Laser welding is being introduced in the aerospace industry due to its many advantages over traditional techniques. However, welding of parts with complex shapes requires precise control of the focal point of the laser in order to achieve full penetration over the entire seam. In this paper, we present an improved control system for real-time adjustment of the correct focal position, which is based on the monitoring of the light emitted by the process in two different spectral bands. The reported system has been optimized for use in a real environment: it is robust, compact, easy to operate, able to adjust itself to different welding conditions, materials and laser setups, and includes a direct connection to an external PC. Results from recent field trials on complex aerospace structures are provided.
The application of a novel sensing fiber with an inverted parabolic-index profile to the detection of refractive-index changes of a medium surrounding the fiber core is investigated. The detection experiments were made with a set- up consisting of a fiber connector-based coupler allowing the excitation of the sensing fiber by a light source of a relatively small size placed on the fiber axis in a preset distance from the fiber end face, and an optical cell containing a declad segment of the sensing fiber in an immersion whose refractive index could be varied in a known way by heating. Various light sources, such as outputs of multimode and single-mode fibers were tested as more or less accurate approximations to the ideal point light source. For overcoming problems with the profile imperfections in the central core region appearing in some of the produced fibers and originating from the fiber manufacturing technology, the excitation of the fiber by an inclined beam from a bevelled single-mode fiber was studied as well. For comparison, PCS fibers were used in the experiments, too. The results have shown that in an index range slightly below the index of silica, higher sensitivity can be achieved with the novel fiber than with PCS fibers. The possibility of shifting the measuring range down to a more practical lower index values by using inclined-beam fiber excitation and confinement of the index profile to the outer region at the core will be shown.
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