Holographic Fabry-Perot spectrometer.

We propose a spectrum analyzer based on the properties of a hologram recorded with the field transmitted by a Fabry-Perot etalon. The spectral response of this holographic Fabry-Perot spectrometer (HFPS) is analytically investigated in the paraxial approximation and compared with a conventional Fabry-Perot etalon of similar characteristics. We demonstrate that the resolving power is twice increased and the free spectral range (FSR) is reduced to one-half. The proposed spectrometer could improve the operational performance of the etalon because it can exhibit high efficiency and it would be insensible to environmental conditions such as temperature and vibrations. Our analysis also extends to another variant of the HFPS based on holographic multiplexing of the transmitted field of a Fabry-Perot etalon. This device increases the FSR, keeping the same HFPS performance.

Femtosecond spectral pulse shaping with holographic gratings recorded in photopolymerizable glasses.

The majority of the applications of ultrashort laser pulses require a control of its spectral bandwidth. In this paper we show the capability of volume phase holographic gratings recorded in photopolymerizable glasses for spectral pulse reshaping of ultrashort laser pulses originated in an Amplified Ti: Sapphire laser system and its second harmonic. Gratings with high laser induce damage threshold (LIDT) allowing wide spectral bandwidth operability satisfy these demands. We have performed LIDT testing in the photopolymerizable glass showing that the sample remains unaltered after more than 10 million pulses with 0,75 TW/cm2 at 1 KHz repetition rate. Furthermore, it has been developed a theoretical model, as an extension of the Kogelnik's theory, providing key gratings design for bandwidth operability. The main features of the diffracted beams are in agreement with the model, showing that non-linear effects are negligible in this material up to the fluence threshold for laser induced damage. The high versatility of the grating design along with the excellent LIDT indicates that this material is a promising candidate for ultrashort laser pulses manipulations.

Structure of the dielectric tensor in nematic liquid crystals with topological charge.

A q-plate is a stratified medium composed of a uniaxial nematic liquid crystal with an inhomogeneous orientation of the optical axis possessing a topological charge q, which looks promising as a switching device. This work reports an approach to diagonalizing the dielectric tensor in q-plates, giving a detailed study of nontrivial tensor coordinate transformations. The relationship between the diagonal dielectric tensor and the topological charge of the plate is established. From that, the dielectric tensor for a given q-plate can be explicitly calculated in a convenient frame, and hence the field propagation equations can be derived. These studies establish a theoretical basis that could further deepen the research on these particular structures for nonconventional manipulation of the light.

Generation of femtosecond paraxial beams with arbitrary spatial distribution.

We present an approach to generate paraxial laser beams with arbitrary spatial distribution in the femtosecond time regime. The proposed technique is based upon a pair of volume phase holographic gratings working in parallel arrangement. It exploits the spatial coherence properties of the incoming laser beam in a compact and robust setup that mitigates angular and spatial chirp. The gratings were recorded in a photopolymerizable glass with a high optical damage threshold and a large optical throughput. Setup performance is studied and experimentally demonstrated by generating Laguerre-Gaussian femtosecond pulses.

Fresnel diffraction effects in Fourier-transform arrayed waveguide grating spectrometer.

We present an analysis of Fourier-transform arrayed waveguide gratings in the Fresnel diffraction regime. We report a distinct spatial modulation of the interference pattern referred to as the Moiré-Talbot effect. The effect and its influence in a FT AWG device is explained by deriving an original analytical expression for the modulated field, and is also confirmed by numerical simulations using the angular spectrum method to solve the Fresnel diffraction integral. We illustrate the retrieval of spectral information in a waveguide Fourier-transform spectrometer in the presence of the Moiré-Talbot effect. The simulated device comprises two interleaved waveguide arrays each with 180 waveguides and the interference order of 40. It is designed with a Rayleigh spectral resolution of 0.1 nm and 8 nm bandwidth at wavelength lambda approximately 1.5 mum. We also demonstrate by numerical simulations that the spectrometer crosstalk is reduced from -20 dB to -40 dB by Gaussian apodization.

Fractionalization of the linear cyclic transforms.

In this study the general algorithm for the fractionalization of the linear cyclic integral transforms is established. It is shown that there are an infinite number of continuous fractional transforms related to a given cyclic integral transform. The main properties of the fractional transforms used in optics are considered. As an example, two different types of fractional Hartley transform are introduced, and the experimental setups for their optical implementation are proposed.

Effect of chronic poisoning with aluminum on the renal handling of phosphate in the rat.

The effects of aluminum on renal function and phosphate handling were studied using clearance techniques in chronically-intoxicated rats. Rats were given aluminum hydroxide (80 mg/kg b.w., i.p.), three times per week during 6 months. The phosphate tubular transport capacity was evaluated by determining the maximum tubular transport (TmRPi) and the fractional excretion of phosphate (FE% Pi) during the infusion of phosphate solutions with increasing concentrations (0, 9, 18, 33 mM). Parathyroid gland function was studied using indirect methods: calcemia recovery after EDTA administration and the nephrogenic excretion of cAMP as indicative of renal PTH actions, by RIA. The systemic acid base status was determined and food intake and rat growth were controlled in both groups. No changes were observed in the renal function. Pi reabsorption values per ml glomerular filtration rate (TRPi/GFR microg/ml) for different Pi plasmatic concentrations were distributed following a saturation curve compatible with a saturation kinetics. Aluminum increased TmRPi/GFR in treated animals (T) 76+/-4 as compared with control animals (C) 57+/-7 microg/ml, without a statistical modification in the apparent affinity. The FE% Pi and FE% Na were significantly lower in treated animals than in control animals. There were neither systemic variations in the acid-base balance nor in the Ca and Pi concentrations in plasma. The calcemia recovery following a hypocalcemic stimulus and the nephrogenic excretion of cAMP (T: 44+/-4; C: 91+/-7 pmol/min) were diminished. Considering all these facts, it can be postulated that the aluminum renal effect is associated from a decrease in PTH phosphaturic capacity. Nevertheless, other associated factors like minor phosphate intestinal absorption rate may not be disregarded, even though there were no significant intake variations.

[Growth and metabolism of calcium in rats chronically poisoned with aluminium hydroxide]. / Crecimiento y metabolismo del calcio en ratas sometidas a intoxicación crónica con hidróxido de aluminio.

The effects of aluminum on growth have been studied in rats chronically poisoned with aluminum hydroxide (80 mg/kg b.w.-i.p.-three times a week, during 6 months) and in control rats, between 3 and 26 weeks of age. The growth data was evaluated according to Parks 'theory of feeding an growth. At the end of the poisoning period, the calcium metabolism was studied through a balance of calcium and the determination of bone Ca++ accretion and resorption rates with the aid of 45Ca++. The parathyroid glands function was studied using an indirect method. Treated rats showed a significant decrease in asymptotic weights and in the initial efficiency of food conversion into biomass regarding controls. No differences were observed in food intake between both group. Aluminum affected neither the peak growth rate nor the time necessary to attain maturity. The calcium balance in treated rats was significantly less than in the control group. This was accompanied by a significant increase in the calcium excreted by faces, caused perhaps by a less intestinal absorption. An important amount of aluminum on the surface of the trabecular bone and a reduction in the skeletal Ca++ mass, was observed in all treated rats. Nevertheless there are no differences in the latter when expressed for 100 g of body weight. The rate of skeletal Ca++ accretion was found to be significantly decreased in treated group with respect to controls, without any changes in the bone Ca resorption rate. The reduction in bone turnover revealed by the decrease of Vo+/Vo- was accompanied by less recovery velocity of calcemia in the aluminum treated group, being indirectly related to the parathyroid gland response to calcium depletion. In the model that we studied the decreased bone turnover could have been caused by deposits of aluminum in bone; however there could exist associated factors such as dysfunction in the secretion of PTH, or less affinity between its receptors at the bone level.

Line spread function formulation proposed by W. H. Steel: a revision.

A revised formulation of the image light distribution of an incoherent line source proposed by Steel [Rev. Opt. 31, 334-340 (1952)] is presented. Analytical and numerical results based on this new representation are given. We explicitly show that a major error in Steel's final expression generates singularities, thereby preventing convenient numerical computation.

Incoherent spatial impulse response in variable-cross-section photoreceptors and frequency-domain analysis.

The spatial impulse response of a single variable-cross-section photoreceptor has been characterized assuming the incoming radiation to be an initial field propagating under confinement conditions inside the receptor. By application of Fourier analysis the total transfer function is determined. Both the outer and the inner segments are characterized as low-pass filters. The dependence of the transfer function on the modal parameter is analyzed.

Serum aluminum levels in Alzheimer's disease and other senile dementias.

The exact cause of Alzheimer's disease and the part played in it by aluminum is still speculative. We have studied serum aluminum in 356 healthy people, and we have observed that serum aluminum concentration is increased in aging people in relation to age. We suggest that this could be associated with an enhanced gastric permeability or by an increase in metal accumulation proportional to age. We have measured serum aluminum levels in patients with probable Alzheimer's disease, patients with other senile dementias, and age-matched group. Patients with probable Alzheimer's disease have statistically significant higher serum aluminum levels than patients with other types of senile dementias (alcoholic, vascular, multi-infart) and an age-matched control group. When we compare serum aluminum of patients with senile dementias from other causes with the age-matched control group, we do not find significant differences.

Single-image and double-image holographic coupler/optical fiber: a comparative study.

A mathematical model that describes a fiber-to-fiber holographic connector is presented. We consider both single- and double-image cases for the reconstruction of the second hologram. A comparative study of the two coupling conditions is presented. Fourier analysis of the coupling equation enables us to study the linearity of the system.

Holographic coupler-optical fiber system: mathematical model for the coupling optimization.

A mathematical model for describing a holographic coupler-optical fiber system is considered. The object wave to be recorded in the hologram is assumed to be a polarized wave which is scattered by the optical fiber. In the reconstruction the optimal condition for the diffracted wavefront to be coupled to a second fiber is obtained and numerically analyzed.

Initial field and energy flux in absorbing optical waveguides. II. Implications.

We present here numerical estimates of the density and flux of energy based on physical parameters associated with the absorption properties of certain types of absorbing optical waveguides. These results are based on a theoretical formalism established previously for the initial field incident upon the entrance pupil of an absorbing optical waveguide. Based on these results, the mechanisms of confined energy transmission and possible cross talk between neighboring waveguides are discussed. Some comments on the behavior of Bessel functions with complex arguments are also included in the discussion.

Initial field and energy flux in absorbing optical waveguides. I. Theoretical formalism.

An exact formulation of the electromagnetic field striking an optical waveguide is presented and compared with the decomposition relationship used in the literature. The expression for the critical angle is derived as a natural consequence of this formulation. Equations for the total fraction of power confined within a waveguide are derived and analyzed for the special case of absorbing waveguides. An explicit expression is derived for the fraction of energy confined within a waveguide supporting two sets of modes.

Linear behaviour in the aperture pupil of single photoreceptors: consequences related to the degree of inhomogeneity.

An exact treatment of the initial field falling on the entrance face of a photoreceptor, so that the scattered field far from the latter vanishes, is presented. Its physical interest and some consequences are discussed. By establishing certain similarities between the behaviour of the photoreceptor as light penetrates into and propagates along, and an optical linear system, it is possible to define a spatial transverse impulse response containing information on the diffraction capability of each individual receptor. The distribution of this function across a transverse plane depends upon the particular form of the refractive index in the photoreceptor. Thus, certain differences are seen to occur between the case of an homogeneous receptor and an inhomogeneous one, for particular values in the degree of inhomogeneity. This fact allows to interpret the presence of inhomogeneities in the photoreceptor structure.