Shifted Excitation Raman Difference Spectroscopy (SERDS) has proven an effective method for performing Raman
analysis of fluorescent samples. This technique allows achieving excellent signal to noise performance with shorter
excitation wavelengths, thus taking full advantage of the superior signal strength afforded by shorter excitation
wavelengths and the superior performance, also combined with lower cost, delivered by silicon CCDs. The technique is
enabled by use of two closely space fixed-wavelength laser diode sources stabilized with the Volume Bragg gratings
(VBGs). A side by side comparison reveals that SERDS technique delivers superior signal to noise ratio and better
detection limits in most situations, even when a longer excitation wavelength is employed for the purpose of elimination
of the fluorescence. We have applied the SERDS technique to the quantitative analysis of the presence of trace amounts
of methanol in red wines, which is an important task in quality control operations within wine industry and is currently
difficult to perform in the field. So far conventional Raman spectroscopy analysis of red wines has been impractical due
to the high degree of fluorescence.
Shifted Excitation Raman Difference Spectroscopy (SERDS) implemented with two wavelength-stabilized laser diodes
with fixed wavelength separation is discussed as an effective method for dealing with the effects of fluorescence in
Raman spectroscopic analysis. In this presentation we discuss the results of both qualitative and quantitative SERDS
analysis of a variety of strongly fluorescing samples, including binary liquid mixtures. This application is enabled by the
Volume Bragg Grating® (VBG®) technology, which allows manufacturing of compact low-cost high-power laser
sources, suitable for extending the SERDS methodology to portable Raman spectrometers.
Concentration of chloride ions was determined with a classical chemical titration method in three types of
samples: a cracked concrete core and an undamaged concrete core, both taken from a bridge in Iowa, and
also from concrete test samples prepared at Rutgers University. Chloride concentration profiles were
obtained. The same samples were the subjected to the near infrared spectrometric determinations of
chloride content by two manufacturers of spectrometric instruments. Very good correlation between the
chemical and spectrometric measurements was obtained [ R2> 0.96], thus opening the possibility of rapid
on-site chloride concentration determination in concrete structures.
We present a novel approach to achieving both wavelength stabilization and
wavelength agility in high-power two-dimensional stacks of high-power laser diodes. This
approach utilizes volume Bragg gratings® with Bragg period that varies as a function of
position within the clear aperture of the element according to a periodic function with period
equal to the spacing between the laser diode bars within the stack. The Bragg period varies
linearly within each period so that translation of the volume Bragg grating element results in
simultaneous tuning of the wavelength of all the bars in the stack. As a result, the wavelength
of the stack is adjustable, stable and the emission line is narrowed to < 0.5 nm. This kind of
laser diode stacks is particularly suitable for pumping of gaseous media with very narrow
absorption lines, e.g. atomic vapors of rubidium, cesium, potassium etc.
KEYWORDS: Semiconductor lasers, High power lasers, Reflectivity, Diodes, Chemical elements, Fiber Bragg gratings, Temperature metrology, Manufacturing, Laser resonators, Glasses
Volume Bragg grating (VBG) technology has been shown to improve spectral characteristics of the high-power laser diodes and arrays. We review the recent advances in VBG technology that lead to better spectral performance as well as improvements in spatial brightness of the high-power laser diode arrays. The VBG technology has been applied to manufacturing of high-power laser diode arrays suitable for spectral beam combining to achieve even higher spatial brightness. The approach involves fabrication of VBG elements with transverse chirp of the Bragg period. Wavelength tuning of the laser diode arrays and wavelength-shifted laser diode bar operation have been demonstrated, which will lead to manufacturing of spectrally combined high-brightness arrays.
Ann add/drop multiplexer/demultiplexer using volumetric holographic crystal Bragg gratings and without use of circulators has been demonstrated. Multiplexed gratings with angle multiplexed reflection filters provide wavelength- selective reflection of one or more channels into or out of the fiber without disturbing the through channels. Overall channel add/drop losses of less than 3 dB and through channel losses of less than 0.5 dB have been demonstrated. Fabrication of holographic filters with the desired passband characteristics has also been demonstrated.
The rapid improvement of the performance of photorefractive polymers over the past few years has generated a strong technological interest for these new materials. The areas of application include holographic storage, image processing, optical correlation, and phase conjugation. This paper reviews some of our recent advances in the field of organic photorefractive materials. We will first present several new polymeric composites that combine high efficiency and long shelf lifetimes. Then we will discuss the performance of a new class of organic photorefractive materials: photorefractive polymer dispersed liquid crystals. Finally, we will present two examples of applications based on photorefractive polymers: (i) optical correlators for security verification, and (ii) imaging through scattering media in the near infra-red.
Using anomalous large diffusion we report a recording of reflection type gratings in a PVK-based photorefractive polymer without any external electric field. Diffraction efficiency of the gratings was measured to be 7%. An efficient modulation of beams during two-beam coupling up to 12% was demonstrated by using phase mutilation technique. This practical scheme can be used in dynamic vibration measurements.
We report on novel high-performance photorefractive polymer composites based on poly(Nvinylcarbazole) using mixtures of two isomeric electro-optically active chromophores. We demonstrate that the shelf life time of devices using the eutectic mixture is considerably improvedcompared with devices containing just one isomer.
Keywords: photorefractive polymers, four-wave mixing, materials stability, aging
Photorefractive materials have many potential photonic applications, including dynamic holographic storage and image processing. Recently, the new class of photorefractive polymers has emerged, offering wide structural flexibility, easy processability, and low cost. Progress in this field has led to nearly 100% diffraction efficiency for the readout of a hologram stored in the material and to extremely large net gain coefficients of more than 200 cm-1. These excellent properties result from refractive index modulations as large as (Delta) n approximately equals 10-2 which could be generated in these materials with low- power lasers, such as HeNe laser or laser diodes.
The performance of photorefractive polymers has improved considerably over the last few years, leading to the recent observation of 86% diffraction efficiency and a net two-beam coupling gain of more than 200 cm-1 in a poly(N-vinylcarbazole)-based polymer doped with the azo dye 2,5-dimethyl-4-(p-nitrophenylazo)anisole. This performance by far surpasses the other organic PR materials reported to date. We report experimental results that evaluate the suitability of this polymer composite for dynamic holography and other applications.
Photorefractive (PR) polymers are a new class of PR materials in which large (approximately 10-3) photoinduced refractive index changes can be generated with very low light power density (approximately 100 mW/cm2 or less). They offer structural flexibility, ease of processing and lower cost compared with commonly used inorganic PR crystals. We have developed a PR polymer composite with significantly enhanced performance compared with the existing PR polymers: for the first time, complete diffraction of an incident beam into the first (Bragg) diffraction order was observed in a 105 micrometers thick layer of a PR polymer. Absorption and reflection losses limit the measured maximum diffraction efficiency to 86%. The material also exhibits a net two-beam coupling gain of more than 200 cm-1. These results show that this class of materials presents a good practical alternative to the inorganic PR crystals.
The studies of the photorefractive properties of the two polymer composites based on poly(N-vinylcarbazole) (PVK) are reported. The polymers were obtained by doping PVK with 2,4,7 trinitro-9- fluorenone as a sensitizer agent and two different azo dyes as nonlinear chromophores. Best results were obtained for the polymer doped with 2,5-dimethyl-4-p-nitrophenylazoanisole, which showed a maximum diffraction efficiency of 34% at 40 V/micrometers external electric field strength in 105 micrometers thick samples. The influence of different geometries of the hologram writing- retrieval on the diffraction efficiency is described. Net amplification of one of the writing beams in two-beam coupling within the material has been observed. Recording of the holograms of 2-dimensional objects in real time has been demonstrated with good resolution and image brightness. All the results were obtained with a low power visible laser diode emitting at 675 nm.
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