Intensity Of Coherent Light Research Articles

High-speed feedback based wavefront shaping for spatiotemporal enhancement of incoherent light through dynamic scattering media.

Feedback-based wavefront shaping is a promising and versatile technique for enhancing the contrast of a target signal for both coherent and incoherent light through a highly scattering medium. However, this technique can fail for a dynamic sample with a short correlation time. So far, most proposed methods for high-speed wavefront shaping can only directly enhance the intensity of coherent light but not incoherent light. Here we try to fill this gap to directly enhance incoherent light with high speed, such as fluorescence, which is essential in extending wavefront shaping to biomedical applications. For this purpose, we develop a technique based on a single acousto-optic deflector (AOD) with field-programmable gate array (FPGA) acceleration for spatiotemporal focusing within milliseconds. With the digital time gating of the feedback signal, spatiotemporal focusing of laser light with high contrast can be formed behind dynamic scattering media in milliseconds resulting in fluorescence enhancement. Furthermore, FPGA-based wavefront shaping is shown to effectively enhance fluorescence directly behind dynamic samples with short correlation times.

Synthesis of gold decorated silica nanoparticles and their photothermal properties

Silica-Gold Nanostructures (SGNs), composed of a silica core decorated by gold nanoparticles, have the photothermal capacity to transform near-infrared (NIR) wavelengths into heat. This work presents a simple, efficient, and replicable method of synthesis of SGNs and their characterization by: (1) transmission electron microscopy to obtain micrographs of the particles and their corresponding diameter distribution; (2) diffraction patterns showing the amorphous atomic arraignment of the silica and the crystalline atomic arrangement of the gold nanoparticles; (3) zeta potential confirming the stability of the SGNs in a colloidal solution; and (4) thermal images displaying the capacity of SGNs to convert NIR irradiation into heat and their respective increment in temperature. SGNs were synthesized over silica cores with diameters of 63, 83, and 132 nm and decorated with a partial gold shell. They were heated with a coherent light intensity of 340 mW/cm2 with a wavelength of 852 nm. This wavelength is within the range of the optical window of the human body; therefore, SGNs may be used for the photothermal ablation of tumors with no damage to the tissue. The heating of different dimensions of SGNs took 6–8 min of NIR radiation, and their cooling, once the laser was turned off, was in the order of 2–3 min. It was found that SGNs, with a core diameter of 132 nm, have a notable photothermal capacity. That enables them to increase the temperature of their surroundings by 4.4 ºC. This increment in temperature is sufficient to induce cellular necrosis, which makes SGNs a good option for photothermal treatments.

Design on constant coherent light intensity fringe locking system for scanning beam interference lithography

Design on constant coherent light intensity fringe locking system for scanning beam interference lithography

Highly parallel, interferometric diffusing wave spectroscopy for monitoring cerebral blood flow dynamics.

Light-scattering methods are widely used in soft matter physics and biomedical optics to probe dynamics in turbid media, such as diffusion in colloids or blood flow in biological tissue. These methods typically rely on fluctuations of coherent light intensity, and therefore cannot accommodate more than a few modes per detector. This limitation has hindered efforts to measure deep tissue blood flow with high speed, since weak diffuse light fluxes, together with low single-mode fiber throughput, result in low photon count rates. To solve this, we introduce multimode fiber (MMF) interferometry to the field of diffuse optics. In doing so, we transform a standard complementary metal-oxide-semiconductor (CMOS) camera into a sensitive detector array for weak light fluxes that probe deep in biological tissue. Specifically, we build a novel CMOS-based, multimode interferometric diffusing wave spectroscopy (iDWS) system and show that it can measure ∼20 speckles simultaneously near the shot noise limit, acting essentially as ∼20 independent photon-counting channels. We develop a matrix formalism, based on MMF mode field solutions and detector geometry, to predict both coherence and speckle number in iDWS. After validation in liquid phantoms, we demonstrate iDWS pulsatile blood flow measurements at 2.5 cm source-detector separation in the adult human brain in vivo. By achieving highly sensitive and parallel measurements of coherent light fluctuations with a CMOS camera, this work promises to enhance performance and reduce cost of diffuse optical instruments.

Methodology of filter-type multi-dithering phase control for quasi parallel light interference

The quasi parallel light interference is one kind of basic ways to use the energy of interference light to interact with matter. Because the phase of each parallel light beam needs to meet the coherent condition, it is required that the phase of each light beam be controlled timely. There are some kinds of phase control methods, such as the heterodyne phase-locking method, the stochastic parallel gradient descent algorithm, the self-referred and self-synchronous phase-locking method the multi-dithering phase-locking method, etc. Among them, the multi-dithering method needs not the referenc light, it is to load multi-frequency sinusoid signals to the phase modulator, and realize the recognition of phase difference and the output of feedback voltage by multiplying circuit and integrating circuit. In view of the shortcomings of the existing methods, a scheme of filter-type multi-dithering phase control for quasi parallel light interference is proposed, in which the phase differences are identified and corrected by the modulation signals and filtering signals of different frequencies. Theoretical analysis of coherent light intensity for the scheme is made. The principle of filter-type multi-dithering phase control method is put forward, and the numerical analysis and simulation experiment for filter-type multi-dithering phase control method are carried out. In the simulation experiment, the fiber interference light path is used to simulate the light intensity of quasi parallel light interference at one point in space, and the change of photoelectric signal indicates the change of interference light intensity. The phase control feedback loop is composed of photoelectric signal amplifying circuit, bandpass filtering circuit, amplitude measuring circuit, direct current amplifying circuit and adder circuit. The results have shown that the phase difference among light beams can be recognized by the method, and the direct current voltage signal that is proportional to the phase difference of signal can be fed to control the phase modulator. The phase difference can be corrected. The control bandwidth is 2.5 kHz, and the output voltage range of phase control is 0.034.45 V. Compared with the classical multi-dithering method, the method of filter-type multi-dithering phase control has some advantages. Each multiplying circuit in the classical method needs a very small amplitude reference signal, which causes the reference signal to have a very small range of values, and the relationship between integral time and modulation period needs considering. The integral time is usually ten times longer than the modulation period, which causes the control bandwidth of the system to decrease. However, the feedback loop of the filter-type multi-dithering phase control method does not require any reference signal, so each signal does not affect each other, and the increase in the number of beams does not have a significant influence on the control bandwidth either. Therefore the filter-type multi-dithering phase control method is a useful phase-control method.

Nano Slit And Dot Array Pattern Of Au On SiO2 substrates Using Laser Beam Lithography

Highly monochromatic coherent light intensity of lasers provides the possibility of fabricating periodic nano-patterns which are needed in many applications. Here, we report 2D grating structure and periodic nano hole and dot arrays fabricated using two laser beam interference lithography. Using this method periodic grating as well as dot patterns have been fabricated with structure size of 50 -100 nm and pitch of 200-300 nm on SiO2 substrate. The nano-patterns are formed by inverting an array of photoresist posts by a lift-off process and subsequent reactive-ion etching using chromium as an etch mask. The depth of patterns can be tuned simple by laser dose to obtain high aspect ratio gratings. Transmission spectra measured in Au slits shows strong resonance features at 520 and 640 nm. Finite difference time domain simulations are used to estimate the near-field enhancement at the center of the slit pattern. Copyright © 2013 VBRI press.

Influence of short-wavelength radiation of the visible range on the optical transmission of photorefractive lithium niobate samples

Special features of the optical transmission of lithium niobate crystal samples with surface doping by photorefractive impurities at the wavelength of a helium-neon laser under short-wavelength incoherent background illumination are experimentally investigated and discussed. The possibility of optically controlled thermooptical modulation of the coherent light intensity with characteristic time of a few seconds is demonstrated for samples with surface doping by a combination of iron and copper ions.

Acoustooptic system for fast measurement of coherent light intensity distribution

Spatial distribution of coherent light intensity is measured with an acoustooptic Bragg cell. With step function or short pulse driving signals, the integral function of the spatial intensity distribution or the distribution itself appears at the time dependent output of a single detector. Speed, resolution, and dynamic range are comparable or better than these parameters of detector arrays.

General theory of photochemical reaction kinetics. II. Generalized mass action law and formal photochemical reactions kinetics

On the basis of a consistent set of photochemical kinetic equations previously obtained by the authors the effect of a resonant optical field on chemical equilibrium and on uni- and bimolecular chemical reaction kinetics is studied. We established, in particular, the relationship between steady-state concentrations of reactants in a quasicontinuous light pulse field, that is, the generalized mass action law. Dependences of reaction product yields on the intensity of coherent light were found, also the relationships between effective reaction rare constants and coherent optical field parameters as well as relaxation interactions. The results are compared with those obtained from a phenomenological description widely used in pratical photochemistry, based on elementary kinetics equations of balance type for reactant concentrations. It is shown that photochemical processes is coherent optical fields cannot be described correctly by rate (or balance) equations.

RADIATION DENSITY IN LASER CAVITY CALCULATED BY FIELD STRENGTH SUPERPOSITION

On principle, the coherent light intensity in a laser cavity should be calculated according to field strength superposition of the light waves propagating back and forth in the cavity. This paper compares the calculations using the field strength superposition and that using the usual light intensity superposition. Our work indicates that the results on local saturated gain and light intensity calculations really give considerable discrepancies, but the field distribution and power output calculations agree quite well.

Method of direct registration of the binary function of particles distributed in volume

AbstractA new method of registering the full binary distribution function of light‐scattering particles in volume is suggested. This method is based on a statistic analysis of space distribution for the intensity of coherent light scattering by particles in a volume. Theory and block diagrams illustrating the method are presented.