Intensity Of Optical Signal Research Articles

Theory of the nonlinear optical frequency mixing effect in graphene

A quantum theory of the frequency mixing effect in graphene is developed. A graphene layer is assumed to be irradiated by two monochromatic waves with the frequencies ω1 and ω2 and the nonlinear third order electromagnetic response at the mixed frequency ωe=2ω1−ω2 is calculated. The corresponding third order nonlinear conductivity of graphene is calculated as a function of frequency, charge carrier density, as well as the intensity and the polarization of the incident waves. The intensity of the nonlinear optical signal is also calculated as a function of the number N of graphene layers in a multi-layered system and it is shown that the best performance of the graphene/graphite based nonlinear optical mixer or frequency multiplier can be achieved in thin graphite layers with N≃25−30.

Measurement of intensity of the second optical harmonic in heteroepitaxial cadmium-mercury telluride structures

Features of the generation of the second optical harmonic in the heterostructures HgCdTe/GaAs and CdTe/HgCdTe/GaAs manufactured by means of molecular-beam epitaxy are experimentally investigated for the first time. The dependences of the intensity of a nonlinearly optical signal on the direction of the pumping radiation polarization and the position of the laser beam on the study surfaces are measured for a pure monocrystalline substrate and for a sample with a deposited functional polycrystalline CdTe layer. Substantial differences between these dependences are discovered.

Development of functional nanoprobes for optical near-field characterization

Subwavelength optical imaging can be accomplished by scanning a nanoscale aperture or ananoprobe containing a locally defined nanoscale optical source. Currently, most suchmethods, including various implementations of near-field scanning optical microscopy(NSOM), form near-field images by measuring the intensity of optical signals generated byoptical transmission, scattering or fluorescence. Here we report the development of ananoprobe that can extend NSOM functionalities by focusing on the dynamical aspects oflight emission (such as fluorescence lifetime measurement) and nonlinear optical processes(such as second harmonic generation). Our nanoprobes consist of a silica fiber taper, asingle nanowire or a nanotube, and appropriate functional nano-optical structures. Thefabrication, characterization and potential applications of such nanoprobes are discussed.

Cerenkov Luminescence Imaging of Medical Isotopes

The development of novel multimodality imaging agents and techniques represents the current frontier of research in the field of medical imaging science. However, the combination of nuclear tomography with optical techniques has yet to be established. Here, we report the use of the inherent optical emissions from the decay of radiopharmaceuticals for Cerenkov luminescence imaging (CLI) of tumors in vivo and correlate the results with those obtained from concordant immuno-PET studies. In vitro phantom studies were used to validate the visible light emission observed from a range of radionuclides including the positron emitters (18)F, (64)Cu, (89)Zr, and (124)I; beta-emitter (131)I; and alpha-particle emitter (225)Ac for potential use in CLI. The novel radiolabeled monoclonal antibody (89)Zr-desferrioxamine B [DFO]-J591 for immuno-PET of prostate-specific membrane antigen (PSMA) expression was used to coregister and correlate the CLI signal observed with the immuno-PET images and biodistribution studies. Phantom studies confirmed that Cerenkov radiation can be observed from a range of positron-, beta-, and alpha-emitting radionuclides using standard optical imaging devices. The change in light emission intensity versus time was concordant with radionuclide decay and was also found to correlate linearly with both the activity concentration and the measured PET signal (percentage injected dose per gram). In vivo studies conducted in male severe combined immune deficient mice bearing PSMA-positive, subcutaneous LNCaP tumors demonstrated that tumor-specific uptake of (89)Zr-DFO-J591 could be visualized by both immuno-PET and CLI. Optical and immuno-PET signal intensities were found to increase over time from 24 to 96 h, and biodistribution studies were found to correlate well with both imaging modalities. These studies represent the first, to our knowledge, quantitative assessment of CLI for measuring radiotracer uptake in vivo. Many radionuclides common to both nuclear tomographic imaging and radiotherapy have the potential to be used in CLI. The value of CLI lies in its ability to image radionuclides that do not emit either positrons or gamma-rays and are, thus, unsuitable for use with current nuclear imaging modalities. Optical imaging of Cerenkov radiation emission shows excellent promise as a potential new imaging modality for the rapid, high-throughput screening of radiopharmaceuticals.

Measurement of Superconducting Sphere Spin-Axis Position Using Fiber Optical Sensor

A practical method to measure spin axis position of superconducting sphere rotor in cryogenic instrument using a reflective intensity modulated optical fiber sensor is presented. The deviation amount and direction of the spin axis are obtained by calculating the amplitude and phase of the reflective optical intensity signal. The experiments are carried out for the verification of the theoretical model and the performance evaluation of the sensor system. The experimental results show that the resolution, repeatability and linearity of the sensor system can achieve 10 ?m, 2% and 0.9988 respectively.

Mathematical Modeling of Intensity-Modulated Bent-Tip Optical Fiber Displacement Sensors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Intensity-modulated optical fiber displacement sensors have a potential to be used in a number of applications, including those in industry, military, aerospace, and medicine. Compared with other types of optical fiber sensors, intensity-modulated sensors offer distinctive advantages in that they are usually less complex, inexpensive, and less sensitive to thermal-induced strain. They are able to perform accurate contactless sensing while being of a small size and having a wide dynamic range. A common form of the intensity-modulated optical fiber sensor performs its measurement by making use of a pair of straight parallel optical fibers integrated with a moving reflector modulating the reflected optical signal intensity. Although such an optical modulation configuration exhibits good sensing ability, improvement on its performance could still be made to widen the extent of its application areas. This leads to the development of more effective intensity modulation mechanisms utilizing bent-tip optical fibers and a reflector that can either laterally slide or longitudinally move with reference to the central axis of the fibers. This paper describes such alternative sensing structures and demonstrates the derivations of mathematical models proposed for analyzing their sensing characteristics. Based on experimental studies, the models are verified and validated for the analysis of sensitivity and linearity. </para>

Simultaneous measurement of optical and dynamic mechanical properties of plastic optical fibers

Dynamic Mechanical Analysis (DMA) is one of the most powerful tools to study the behavior of plastic and polymer composite materials and it is potentially very useful tool to simulate behavior of plastic optic fibers (POF) in real applications. Possibility of simultaneous measurements of some optical properties during DMA would significantly upgrade investigations of POF alone or embedded in some materials. In this work, single cantilever DMA of the POFs that was done simultaneously with measuring the transmitted optical signal intensity is described and discussed. In order to compare mechanical results of the same material for cylindrical and rectangular specimens, rectangular plates were prepared by melting POFs and the same kind of tests were performed. It is shown that changing the optical signal intensity corresponds to the changes of storage modulus of the POF during DMA, and the maximums in optical signals intensity indicate the beginning of glass transition processes in the POF material.

Ischemia-induced brain damage is enhanced in human renin and angiotensinogen double-transgenic mice

To investigate the role of brain angiotensin II (ANG II) in the pathogenesis of injury following ischemic stroke, mice overexpressing renin and angiotensinogen (R+A+) and their wild-type control animals (R-A-) were used for experimental ischemia studies. Focal brain ischemia was induced by middle cerebral artery occlusion (MCAO). The severity of ischemic injury was determined by measuring neurological deficits and histological damage at 24 and 48 h after MCAO, respectively. To exclude the influence of blood pressure and local collateral blood flow, brain slices were used for oxygen and glucose deprivation (OGD) studies. The severity of OGD-induced damage was determined by measuring indicators of tissue swelling and cell death, the intensity of the intrinsic optical signal (IOS), and the number of propidium iodide (PI) staining cells, respectively. Results showed 1) R+A+ mice showed higher neurological deficit score (3.8 +/- 0.5 and 2.5 +/- 0.3 for R+A+ and R-A-, respectively, P < 0.01) and larger infarct volume (22.2 +/- 1.6% and 14.1 +/- 1.2% for R+A+ and R-A-, respectively, P < 0.01); 2) The R+A+ brain slices showed more severe tissue swelling and cell death in the cortex (IOS: 140 +/- 6% and 114 +/- 10%; PI: 139 +/- 20 cells/field and 39 +/- 9 cells/field for R+A+ and R-A-, respectively, P < 0.01); 3) treatment with losartan (20 micromol/l) abolished OGD-induced exaggeration of cell injury seen in R+A+ mice. The data indicate that activation of ANG II/AT(1) signaling is harmful to brain exposed to ischemia.

Effect of the optical excitation signal intensity on the impulse response of a metal-semiconductor-metal photodiode

The effect of the optical excitation signal intensity on the impulse response of a photodetector based on a set of metal-semiconductor-metal (MSM) rectifier contacts is studied. The response of the detector is better at a low optical excitation signal. When the energy of an optical excitation pulse is high, the response can be improved by increasing the bias voltage. The advantages of a GaN-based MSM diode in detecting high- energy radiation pulses are established. It is shown that the speed of the GaN-based MSM detector may reach 25 ps for a 60-pJ optical excitation pulse at a wavelength of 290 nm.

Remote sensing of wave roller lengths in the laboratory

The optical intensity signals from surf zone waves in a laboratory flume are analyzed using several different phase‐averaging techniques, and a methodology is developed for estimating wave roller lengths and local wave dissipation. The intensity signals (i.e., phase‐averaged intensity profiles) of individual breaking waves are compared with the wave profiles measured by in situ wave gauges, and the optical signal of the wave roller is shown to ramp up from the toe of the wave roller on the front face of the wave to a maximum intensity at the wave crest. The remote sensing observations capture the growth, equilibrium, and decay phases of the roller as it propagates over a fixed bed arranged in a bar/trough morphology. Next, for the regular wave conditions considered here, the local maxima of the phase‐averaged intensities are shown to better indicate the initial onset of wave breaking and the occurrence of wave breaking in the bar trough, as compared to the more commonly used time‐averaged mean intensity. In addition, the phase‐averaged profiles are used to measure the size of the roller, and these measurements are compared to previous observations of smaller‐scale rollers in equilibrium. The observed roller lengths are shown to agree with predictions from a wave roller model and to provide a new physical link between the remotely sensed signal and roller dissipation. Finally, as an example application of these new data, a simple wave height inversion model is presented that allows an estimation of surf zone wave heights from the remotely sensed roller lengths.

Real-time monitoring of cell membrane modification during supercritical CO 2 pasteurization

This study addresses cell membrane permeabilization phenomena over microorganisms induced by high pressure CO 2: a real-time measurement has been set-up and performed by an innovative cell fluorescent staining technique. A common yeast, S. cerevisiae was used as test-microorganism. Different sets of experiments were carried out in batch mode at 100 bar, 36 °C and different treating time (10–30 min). The change of optical signal intensity collected by an optical spectrometer indicates a progressive permeabilization of the cell envelopes during the treatment. This new analytical method provides a real-time indication of cell membrane integrity during the pasteurization process. Both the permeabilization and inactivation kinetic rates were calculated and compared; the reported data evidence a correlation between the cellular death and the CO 2 permeabilization inside the cell, which, in turn, can be considered the key factor of microbial inactivation.

High-Speed All-Optical First- and Second-Order Differentiators Based on Cross-Phase Modulation in Fibers

An ultrafast all-optical differentiator generating the first- and the second-order temporal derivative of the intensity of optical signals is presented in this paper. Differentiation is obtained via an optical fiber that plays the role of an optical phase modulator, an optical bandpass filter and a photodetector. The operation of the proposed device is theoretically studied in order to highlight significant parameters that affect the performance of the differentiator, namely the filter transfer function, the power of the propagating waves and the fiber characteristics (length and nonlinear coefficient). The comparison between the numerically calculated derivatives and the theoretically expected ones is performed by estimating the correlation coefficient between them. According to the numerical analysis, high correlation coefficients can be achieved in certain operating regimes. The same device can be utilized in order to produce ultrawideband (UWB) impulse signals. Electrical monocycle or doublet pulses can be obtained at the output of the photodetector (PD) using the proper tunable optical filter. Experimental verification of the theoretically predicted and numerically calculated results is finally presented for high bit-rate signals.

An all-optical limiter with high-accuracy thresholding based on self-phase modulation assisted by preparatory waveform conversion

To achieve high reliability in nonlinear optical signal processing in optical networks, we propose an all-optical limiter with high-accuracy thresholding based on self-phase modulation (SPM) assisted by preparatory waveform conversion. By converting an input pulse into a super-Gaussian one to be fed into an optical fibre for SPM, we can achieve the function of an optical limiter with high-accuracy thresholding which enables a precise stabilization of the intensities of optical signals. The simulation results show that the proposed optical limiter realizes a very accurate thresholding by which the intensity fluctuation is stabilized to less than 0.06 dB, for a relatively wide input range of 5.4 dB.

Periocular Triamcinolone Enhances Intraocular Gene Expression after Delivery by Adenovirus

A noninvasive imaging technique was used for serial assessment of gene expression after intraocular gene transfer. Bioluminescence after intravitreous administration of an adenovirus vector containing the firefly luciferase gene was measured serially and noninvasively. The optical signal was then used as a bioassay to determine whether periocular immune modulation affects intraocular transgene expression. Sixty-two, 8-week-old, male BALB/c mice were used. The correlation of optical signal intensity was determined by tissue luciferase level after injecting 30 mice with one of three intravitreous doses of Ad-Luc-GFP (10(8), 5 x 10(8), or 10(9) particles in 1 microL). Ocular bioluminescence was measured at days 2, 5, 8, and 14. The bioluminescence was then directly compared with measured tissue luciferase levels. The remaining 32 mice were divided into two groups. One group (n = 16), was injected with periocular corticosteroid (400 mug in 10 microL). Two days later, Ad-Luc-GFP was administered by intravitreous injection (10(9) particles in 1 microL). The remaining mice (n = 16) were injected with the same dose of intravitreous Ad-Luc-GFP without corticosteroid pretreatment. Ocular bioluminescence was then assessed longitudinally on days 2, 4, 6, 8, 11, 14, 22, and 30 after intravitreous injection in n = 10 mice per group. The optical signal intensity in each group was compared over the study period. The remaining 12 mice (n = 6, each group) were used to assess histologic differences between the two groups. In vivo measurement of ocular bioluminescence was well correlated with tissue luciferase levels (Spearman's correlation, r = 0.969, P < 0.001). Periocular TA injection markedly decreased the acute inflammatory reaction associated with intravitreous Ad-Luc-GFP and was associated with a significant increase in the duration of peak luciferase expression as well as the total period of luciferase expression. A significant enhancement of intraocular transgene expression is associated with periocular pretreatment with corticosteroid. Histologic evidence of immune cell reduction in ocular tissues in corticosteroid-treated eyes implies a local immune response. Periocular treatment with corticosteroids may enhance adenovirus-mediated gene expression in the eye.

Multi-way principal component analysis for the endpoint detection of the metal etch process using the whole optical emission spectra

An endpoint detection algorithm based on multi-way principal component analysis (MPCA) is developed for plasma etching processes. Because many endpoint detection techniques use a few manually selected wavelengths, noise renders them ineffective and it is hard to select important wavelengths. Furthermore, process drift and faulty condition should be considered for more robust endpoint detection at the same time. In this paper, MPCA with the whole optical emission spectra is used for effective endpoint detection using a large set of data. And the fault detection was achieved by concept of ‘product’ and ‘mean deviation value’ chart with the result of each wafer’s endpoint detection. The product was defined by the multiples of OES data with loading vector and mean deviation chart was defined by a chart of the difference between the product value of the target wafer and mean value of previous wafers. Therefore, a robust model for endpoint detection can be developed by excluding faulty wafers. This approach is successfully applied to the metal etch process of TiN/Al-0.5%Cu/TiN/Oxide stack in an inductively coupled BCl3/Cl2 plasma. The optical emission signal intensities of the 129 wavelengths were measured and saved in a four-dimensional (wavelengths, time, intensity, and wafers) matrix for the subsequent data processing. With this approach the endpoint signal was improved with the whole emission spectra and the process drift was considered by MPCA after information of faulty wafers was discarded.

Signal Intensity Estimators for Free-Space Optical Communication With Array Detectors

Estimators motivated by the minimum mean-square estimation (MMSE) criterion are presented for the estimation of the optical signal intensity in free-space optical (FSO) communication systems impaired by atmospheric-induced scintillation. Joint parameter estimators for the estimation of signal intensities across a photodetector array are investigated. Three scenarios are considered. First, it is assumed that the received signal is detected using a p-i-n photodiode array and that the receiver operates under a shot-noise-limited condition. In the second scenario, the received signal is detected using an APD array and the receiver operates under a shot-noise-limited condition. In the third scenario, the received signal is detected using an APD array, but the receiver is impaired by thermal noise as well as shot noise. The mean primary electron count received by the APD is assumed to be large enough to justify the Gaussian approximation. The optical channel is modeled using a log-normal distribution with correlated random signal intensities across the array. With the aid of simulation, it is shown that the proposed estimators outperform the maximum-likelihood estimators for small scintillation indices while providing a performance similar to the maximum a posteriori estimator.

Signal Intensity Estimators for Free-Space Optical Communication With Array Detectors

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Estimators motivated by the minimum mean square error estimation criterion are presented for the estimation of the optical signal intensity in free space optical communication systems impaired by atmospheric-induced scintillation. Joint parameter estimators for the estimation of signal intensities across a photodetector array are investigated. Three scenarios are considered. First, it is assumed that the received signal is detected by using a p-i-n photodiode array and that the receiver operates under a shot-noise-limited condition. In the second scenario, the received signal is detected by using an avalanche photodiode (APD) array and the receiver operates under a shot-noise-limited condition. In the third scenario, the received signal is detected by using an APD array, but the receiver is impaired by thermal noise as well as shot noise. The mean primary electron count received by the APD is assumed to be large enough to justify the Gaussian approximation. The optical channel is modeled using a lognormal distribution with correlated random signal intensities across the array. With the aid of simulation, it is shown that the proposed estimators outperform maximum likelihood estimators for small scintillation indices while providing a performance similar to the maximum <emphasis emphasistype="italic">a posteriori</emphasis> estimator. </para>

Second harmonic generation response by gold nanoparticles at the polarized water/2-octanoneinterface: from dispersed to aggregated particles

Gold nanoparticles with a diameter of approximately 20 nm have been observed at thepolarized water/2-octanone interface by the nonlinear optical technique of second harmonicgeneration. Electric field induced adsorption of the gold particles at this liquid/liquidinterface is clearly observed and confirms that these are negatively charged. The process isquasi-reversible at high potential sweep rates, but aggregation at the interface is observedat slower sweep rates through the loss of the nonlinear optical signal. The timeevolution of the second harmonic signal is also reported during potential stepexperiments. After a rapid increase due to adsorption, a continuous decreasein the nonlinear optical signal intensity is observed due to aggregation of theparticles into large islands at the interface. Diffusion of these large islands at theinterface was observed for a longer timescale through large signal fluctuations.

Porous Silicon One-Dimensional Photonic Crystals for Optical Signal Modulation

Porous silicon one-dimensional (1-D) photonic crystal microcavities are fabricated as building blocks for optical modulators. The microcavities consist of porous silicon layers with alternating low and high porosity. Since the pore size is much smaller than the incident wavelength of light, each porous silicon layer is treated as an effective medium with a specific effective refractive index that corresponds to the given porosity. Active tuning of the microcavity resonance is accomplished through electrical actuation of liquid crystals infiltrated into the pores. As the refractive index of the liquid crystals change in response to the external stimulus, the effective refractive index of the porous silicon layers changes and the resonance wavelength shifts accordingly. The resonance wavelength shift creates an optical signal intensity modulation at a particular wavelength. Extinction ratios greater than 10 dB are achievable for these devices. The performance characteristics of the porous silicon photonic crystal microcavities can be tailored to specific applications based on the choice of infiltrated optically active material and the quality factor of the device

System performance using different types of in-line optical regenerators

We analytically compare the system performance of various types of in-line optical regenerators in long distance amplified links. The evolution of the probability density function (pdf) of the optical signal intensity is evaluated, and bit error ratio (BER) is estimated numerically. Critical issues of common solutions are found.