Several different fiber Bragg gratings (FBGs) were exposed to accumulated high doses of gamma rays (up to 80 MGy) and neutrons (5*1018/cm3) in a research grade nuclear reactor. The FBG peak wavelengths were measured continuously in order to monitor radiation induced shifts. Gratings inscribed with IR femtosecond pulses through a phase mask showed the smallest shifts (around 20 pm), while under identical conditions point-wise inscribed femtosecond gratings and a UV inscribed grating showed shifts of around 100 pm and 400 pm respectively. The different responses to irradiation are attributed to the various inscription techniques inducing gratings whose refractive index modulation is derived from different physical modifications of the fiber material.
We have integrated tissue quantitative phase imaging with Raman spectroscopy, and used it for analyzing benign and malignant cancer tissue samples without staining. We used the acquired stain-free OPD-based texture features of the tissues and deep learning to segment the urothelium layers, where cancer starts from, with the stained tissue as a ground-truth for training. Then, location-guided Raman spectroscopy measurements were acquired from the urothelium according to the segmentation results. We then classified the tissue type based on the location-guided Raman spectroscopy measurement with higher accuracy compared to classifying them without urothelium segmentation.
In this work, we demonstrate optomechanical measurements of radiation induced alterations of the acoustic velocity in a fluoroacrylate polymer coating of a silica optical fiber. The optomechanical measurement is based on forward Brillouin scattering initiated in the fiber core which stimulates acoustic waves that reach the fiber coating. The measurement may serve as an additional metric to quantify the dose of ionizing radiation to which the fiber was exposed. We have demonstrated that the stiffness of the coating increases following gamma irradiation, as measured by the time of flight of radial acoustic waves through the coating. The measurement was performed on few meters long fiber, but can be extended to a spatially distributed analysis in longer fibers. The tests showed a linear dependence of the acoustic time-of flight on the overall dosage of gamma irradiation. The time of flight decreased by as much as 15% following exposure to 180 Mrad from a 60Co source. In a follow-up study, we found that the stiffness of the previously exposed fiber coatings continued to slowly increase over months, after extraction from the radiation field. These results reveal the vulnerability of the specific coating to ionizing radiation and the potential complexities involved with dosimetry.
We propose a multimodal quantitative, label-free and nondestructive diagnostic metrology technique by integrating off-axis interferometric phase microscopy (IPM) and Raman spectroscopy (RS), for analyzing normal and malignant bladder tissue samples. We built a Mach–Zehnder interferometer connected to a commercial confocal microscope for imaging a large area of tissue slices, up to a few millimeters, by semi-automatic scanning of the tissue. Bright-field image of hematoxylin and eosin stained tissue slice of the same area was also acquired. Measurements of Raman spectra were acquired using our RS system with excitation wavelength of 561 nm. Using the quantitative phase information, we obtained various spatial and morphological parameters of the tissues such as the anisotropy factor, which demonstrated their direct correlation with tumor presence. This method is expected to be useful for stain-free cancer diagnosis, while obtaining both quantitative information about tissue morphological modifications and changes in tissue Raman scattering properties induced by cancer.
In order to test and compare the suitability of various radiation hardened optical fibers, we have established a test site in the 5MW research grade open pool reactor located at Soreq Nuclear Research Center, Israel. Several commercial fiber samples were coiled inside a specially designed apparatus which was lowered into the reactor core. The input and output legs of each fiber coil were prepared sufficiently long to extend outside the reactor pool, and were attached to a source (input) and detector (output). The transmission/attenuation could then be measured during, and in between, operations of the reactor. Since the reactor does not operate continuously, we were able to monitor in real time both transmission changes under very high radiation rates (approx. 0.5 MGy/hr) and doses (about 20 MGy) of gamma rays and neutrons, as well as recovery processes after each reactor shut down. Results are presented comparing the RIA and recovery kinetics of different commercial radiation hardened fibers under identical exposure/relaxation cycles. This study also examined effects on fibers with different coating materials and a fiber with inscribed Bragg gratings.
A novel sensitive and fast technique is described for monitoring pm scale shifts of narrow Bragg grating features. The technique is based on a grating pair and heterodyne measurement, and offers inherent compensation of temperature variations under strain measurement.
A concept for fiber optic measurement of strain in rotating structures where the fiber cannot access the central rotation axis is described. Various interrogation techniques are considered, and the use of a fast spectrometer-based interrogator is preferred. An automated algorithm for optical alignment while the structure is rotating is described.
Fiber Bragg Grating (FBG) sensors may probe ultrafast changes in pressure caused by shock waves propagating in solid and liquid media impacted by high velocity projectiles. The FBG spectra are measured using an optical system comprising economically priced electro-optical components offering 5 nsec temporal resolution and 0.8 – 1.6 nm spectral resolution. We present results showing evolution of 5 kBar shock wave pressure in approx. 100 nsec, as well as the dependence of the FBG response on the physical length of the sensor (1mm and 0.1mm), and on the relative orientation between the FBG axis and the shock wavefront.
FBGs respond to external pressures in ways that reflect both the strain-optic effect and the geometrical variations, both induced by the applied pressure. While the response to static isotropic pressure is quite straight forward and intuitive, the response to anisotropic shock waves is much more complex and depends also on the relative orientation between the fiber and the shock propagation direction. We describe and explain experimental results for both cases.
A multi-wavelength fiber-optic confocal position sensor, employing a diffractive optical element (DOE), is described.
The DOE was designed with the aim of enhancing the chromatic dispersion of the optics, and thus improving the
measurement range of the technique. A proof-of-principle experiment is presented, yielding a five-fold enhancement in
the dispersion and thus in measurement range in excellent agreement with design simulation.
A white light interferometric system for remote monitoring of the expansion of a pressured foil membrane is described. The membrane will serve as a transmission "window" in liquid and gaseous target chambers for the production of radio-isotopes in an accelerator. Alternative commercial solutions are unsuitable in this environment. We describe some feasibility experiments which have been performed on a model cell pressurized to expand the foil. Pressures of up to 25 bar induce expansions of up to 1.5 mm in the center of the foil, which the optical probe, positioned 100 mm away, detects with a precision of 20 μm. The results fit well to an accepted model, yielding also the Young's modulus of the foil and its transition from elastic to plastic behavior.
Imaging a single optical fiber onto a remote, reflective target using several wavelengths - while exploiting the chromatic dispersion of the imaging optics - can be used to sense displacements of the target. Light of each wavelength is back-reflected into the fiber at a unique target position. The basic principles of the method and the optical considerations to optimize its accuracy are described. Experimental demonstrations are presented, showing precisions of around 10-20 micrometers.
To date, a full-scale solar sail has never flown in space. Furthermore, solar sail technology development represents a field that only recently has enjoyed significant support. The goal of this work is to contribute to the development of a low-mass ODS for solar sails that would include research and development in the areas of photogrammetry and thermography. The focus of this work was on the development of the thermography system. A measurement protocol was designed for obtaining accurate temperature measurements using thermal imaging when heat was applied to the membrane surface. Two main limitations were considered during the experimental process. The first is that conventional infrared detector arrays must be kept cool. To minimize the effect that an imager's operating temperature would have on the ODS, a miniature, un-cooled microbolometer was used to acquire temperature measurements from the membrane surface. A second limitation is that a detector array cannot distinguish between emitted and reflected photons, thus presenting a significant problem if one cannot predict the reflected component or if the reflected component is significantly greater than the emitted. To address this limitation, spectral properties of the membrane, including reflectance and transmission, were analyzed using a Hemispherical Directional Reflectometer (HDR) to predict the effects that optical properties would have on sail membrane temperatures. A thermal modeling strategy was also developed. The results of this investigation are presented.
Multi-Photon Laser Scanning Microscopy (MPLSM) requires efficient two-photon absorbing fluorescent (TPF) probes. In particular, probes exhibiting bio-functionality are very attractive for MPLSM studies of biological samples. We have synthesized and studied a new class of TPF probes capable of caging metal ions, such as Ca+2 and Na+, which play an important role in neuronal mechanisms. The TPF probes are based on a tetraketo derivative with a symmetric Donor-Acceptor-Donor (D-A-D) structure. The donor is an azacrown moiety, which also serves as a metal ion-caging unit. We studied the linear and the non-linear spectroscopic properties of these TPF probes as a function of conjugation length and the size of the crown ring. We find that this new class of TPF probes possesses very large two-photon excitation cross-section coefficients (~1000GM) at near IR wavelengths as well as affinity to metal ions. In the presence of changing sodium ion concentration the dye spectra reveals four distinguishable forms and the TPF efficiency changes strongly. We therefore conclude that the dye can perform as a sensitive metal ion TPF probe.
An important ingredient in improving Multi Photon Laser Scanning Microscopy, MPLSM, is the development of efficient two-photon fluorescent (TPF) probes. We previously reported on a new class of TPF probes, specifically designed in order to maximize their efficiency in potential MPLSM applications. The fluorophores are based on a tetraketo derivative (TK) with a symmetric structure Donor-Acceptor-Donor (D-A-D). Those fluorophores have the following properties: a) Very large two-photon absorption coefficients (δ ~ 1000GM); b) Two-photon excitation (TPE) peak wavelength strongly shifted to the red (λ ~ 1µm); c) High fluorescence quantum efficiency; d) Large Stokes shifts of the fluorescence bands. We extended our work to a new fluorophore from this class that is more suitable for biological settings. This new fluorophore has a structure of crown-TK-crown that incorporates the ability to trap metal ions such as calcium. The TPE wavelength dependence of the TK-crown derivative is very similar to its analogous linear derivative with enhancement in the value of the cross-section, due to the stronger donor moieties. The TPE cross-section for the TK-crown derivative was about δ = 950 GM at λmax = 980 nm.
Hanna Schwartz, P. Krief, J. Becker, Lev Shapiro, Vladimir Khodorkovsky, Jacob Klug, E. Kovalev, Guilia Meshulam, Garry Berkovic, Zvi Kotler, Schlomo Efrima
Langmuir-Blodgett films have been prepared from amphiphilc molecules containing an indandione-based nonlinear chromophore. Study of the pressure-area (π-A) isotherm enabled us to find optimal conditions for monolayer transfer to a glass substrate. The multilayer films thus formed exhibited strong optical second harmonic generation with a bulk nonlinear co-efficient equal to the ideal value predicted by the product of the chromophore density and its known molecular hyperpolarizability.
One limitation of using electric field induced second harmonic (EFISH) to determine the molecular first hyperpolarizability (beta) of nonlinear optical molecules lies in the fact that part of the second harmonic signal comes from the second hyperpolarizability (gamma) produced by mixing two optical fields with the DC field. In analyzing EFISH results, the second hyperpolarizability contribution of the studied molecules is generally neglected. We present a modified time resolved EFISH technique that allows us, in a single experiment, to determine separately the beta and the gamma contributions. We study para-nitro aniline dissolved in Glycerol, a highly viscous solvent, and apply the DC field via a high voltage pulse with a fast rise time of approximately 40 nsec. As a result, the orientation of the molecules under the applied electric field is slow relative to the build-up of the field, enabling us to directly measure only the DC induced second harmonic (gamma contribution), at the beginning of the HV pulse. The pure beta contribution is determined from the difference between this signal and the conventional EFISH signal at the plateau of the HV pulse. Our result confirm that the gamma contribution is indeed less than 10% of the total.
An important ingredient in improving Multi Photon Laser Scanning Microscopy, MPLSM, is the development of efficient exogenous two-photon fluorescent (TPF) probes. Here we report on a new class of two-photon fluorophores, specifically designed in order to maximize their efficiency in potential MPLSM applications. The fluorophores possess a symmetric Donor-Acceptor-Donor (D-n-A-n-D) structure with varying conjugation length and have strong donors and acceptors. We have studied the two-photon excitation (TPE) properties of these fluorophores and found the following properties: (1) Very large two-photon absorption coefficients (6 > 1000 GM); (2) Peak TP excitation wavelength which are strongly shifted to the red ((lambda) 1 micrometer); (3) Large fluorescence quantum efficiency; (4) Large Stokes shifts of the fluorescence bands. These properties make them particularly suitable for imaging thicker samples, relying on the large improvement in TPE cross-sections and the reduced attenuation at both the excitation and emission wavelengths. We also describe TPE fluorescence anisotropy experiments revealing the tensorial shape of the fluorophores.
We describe a fiberoptic sensor that uses a frequency modulated laser to simultaneously measure both the position and velocity of a reflecting target. We also discuss the effects ofresidual amplitude modulation and multiple reflections.
The second order nonlinearity of conjugated organic molecules involving, 1,3 indandione derivatives as an acceptor moiety has been studied. Varying the donor from dialkylamino to the chemically similar substituent, N- carbazolyl resulted in a drastic reduction of electric field induced second harmonic (beta) values. For some molecules, even a small negative value of (beta) was received. Quantum chemical calculations indicate that the decrease occurs as a result of two overlapping transitions, which contribute to (beta) with opposite signs. The charge transfer band gives a positive (beta) zzz along the molecular long axis, while a transition essentially within the carbazole moiety provides a negative (betazzz contribution to (betaEFISH. Thus, these molecules must be described with a 2D model as opposed to the 'classical' model of 1D nonlinear optical chromophores. The prediction of the 2D model was verified experimentally by using a combination of two methods, EFISH and Hyper-Rayleigh Scattering, which probe different combination of the (beta) tensor elements.
We report on a new unique photochromic material which is based on a reversible formation - cleavage of a C-C bond. The bicyclic bindon derivative, 2 undergoes a photochemical and/or thermally induced ring opening to form the isomer 3. The form 3 presents a conjugated donor-acceptor system and exhibits a considerable second-order optical nonlinearity as found by the field induced SHG measurements. The photochromic conversion is also observed in the crystalline form indicated visually by a crystal red-to-green color change. We have studied the reversible ring opening - closure process in liquid and polymeric solutions. Optical and thermal switching and the NLO efficiency of these guest- host polymer are reported.
We review recent research showing how significant asymmetry and optical nonlinearity (second harmonic generation) can be produced by injection of electric charge into dye/polymer blends. The mechanism for producing the nonlinearity is different from the usual dipolar alignment of the dye in an electrostatic field. Charge injection has been achieved by purely electrical means using `in-plane' electrodes, or photochemically, by irradiating a bilayer structure comprising a photoconductive layer and the polymer/dye layer.
Significant asymmetry and optical nonlinearity (second harmonic generation) can be produced by injection of electric charge into dye/polymer blends. The mechanism for producing the nonlinearity is different from the usual dipolar alignment of the dye in an electrostatic field. Charge injection has been achieved by purely electrical means using `in-plane' electrodes, or photochemically, by irradiating a bilayer structure comprising a photoconductive layer and the polymer/dye layer. This latter technique allows spatial modulation by light of nonlinear optical coefficients.
Significant asymmetry and optical nonlinearity (second harmonic generation) can be produced by injection of electric charge into dye/polymer blends. The strength of the nonlinearity and its temporal stability depend on the electrical properties of the polymer and the polymer interface. It is shown conclusively that the mechanism for producing the nonlinearity does not arise from dipolar alignment of the dye in an electrostatic field; we even demonstrate production of nonlinearity under conditions of alternating current injection i.e. zero electrostatic field.
We examine how two nonlinear optical signals generated by a multimode laser will interfere. This interference is shown to be analogous to linear interference with partially coherent light. Consequently, we demonstrate an interferometric method for determining the temporal and spatial coherence of a laser, which features no moving parts and does not require precise ultrashort mechanical translations.
We report second harmonic generation studies of an amorphous polymer (polymethylmethacrylate) containing hyperpolarizable dye species poled in an in-plane geometry where the electrodes are thinner than the polymer film. It is found that polymers poled in this geometry exhibit optical nonlinearity both parallel and perpendicular to the direction of the poling field. It is determined that charge injection into the film is responsible for the nonlinearity in the perpendicular direction. This nonlinearity is much stronger than that predicted solely on the basis of dipole moment -- poling field interactions.
A series of side chain liquid crystal co-polymers is prepared, starting from
monomers containing both mesogenic and hyperpolarizable groups. When thin films
of these polymers are poled by an electric field in the plane of the film, they
exhibit optical nonlinearity both parallel and perpendicular to the direction of
the poling field.
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