Resonator-free sub-MHz spectral dip in a twisted gain medium

Ultra-narrow spectral features are desirable for a broad range of applications, from precision spectroscopy to atomic clocks to slow-light and microwave photonics, and are conventionally realized using either ultrahigh-Q resonant structures or atomic resonances. Ultrahigh-Q structure often involves microfabrication, and suffers from loss mechanisms and manufacturing variations that cannot be easily compensated, whereas atomic resonances suffer from signal attenuation and tunability is a challenge. Here, we propose an entirely new way to achieve a sub-MHz and tunable spectral feature in a resonator-free gain medium, exploiting polarization pulling in a medium with frequency dependent polarization eigenmodes. To demonstrate a specific realization, we use Brillouin gain in a commercial spun fiber and experimentally achieve a 0.72 MHz spectral dip, which is to our knowledge, the narrowest spectral Brillouin feature ever reported. Furthermore, the simulation shows that the dip linewidth can be reduced to <0.1MHz, equivalent to a Q of almost 2 billion, by optimizing the birefringence and spun rate of the fiber. We also show that the linewidth, depth, and spectral location of this dip are all tunable on demand by controlling the pump frequency, pump power, and the input polarization of the signal. Its simplicity in implementation and broad applicability, its ultra-narrow linewidth, its tunability makes this approach extremely attractive for applications such as high precision metrology and microwave photonics.

Fabrication of Active Polymer Optical Fibers by Solution Doping and Their Characterization

This paper employs the solution-doping technique for the fabrication of active polymer optical fibers (POFs), in which the dopant molecules are directly incorporated into the core of non-doped uncladded fibers. Firstly, we characterize the insertion of a solution of rhodamine B and methanol into the core of the fiber samples at different temperatures, and we show that better optical characteristics, especially in the attenuation coefficient, are achieved at lower temperatures. Moreover, we also analyze the dependence of the emission features of doped fibers on both the propagation distance and the excitation time. Some of these features and the corresponding ones reported in the literature for typical active POFs doped with the same dopant are quantitatively similar among them. This applies to the spectral location of the absorption and the emission bands, the spectral displacement with propagation distance, and the linear attenuation coefficient. The samples prepared in the way described in this work present higher photostability than typical samples reported in the literature, which are prepared in different ways.

CARACTERIZAÇÃO ESPECTRAL NA REFLECTÂNCIA DE Eucalyptus grandis

O objetivo do trabalho foi comparar níveis de reflectância das folhas de Eucalyptus grandis Hill ex Maiden através de medidas espectrais a partir da localização das mesmas em relação à árvore e aos pontos cardeais. O experimento foi conduzido na área experimental pertencente à FEPAGRO, localizada no município de Santa Maria-RS. O trabalho compreendeu a instalação de delineamento blocos ao acaso. Com a utilização de um espectrorradiômetro FieldSpec®3, foram obtidas as medidas de radiância refletida da face superior das folhas e posteriormente trabalhados no software R. Na Banda 1 (400 a 700 nm), notou-se um pico que está relacionado ao total de clorofila na folha dentro da faixa do visível. Observou-se que folhas na posição leste foram as que apresentaram maior reflectância, e as folhas que estavam ao sul e ao oeste apresentaram as menores. Para a Banda 2 (700 a 1000 nm), observou-se diferença estatística significativa apenas entre os valores médios das reflectâncias no par de posições leste-oeste, contrastando com a Banda 1, que obteve diferença significativa entre as reflectâncias médias de todos os pares. A resposta espectral das folhas revelou-se estatisticamente diferente, confirmando, assim, que há diferença espectral relacionada à localização das árvores e posições de coletas das folhas.Palavras-chave: Espectrorradiometria; sensoriamento remoto; curva de reflectância. AbstractSpectral reflectance characterization in Eucalyptus grandis. The objective was to compare levels of reflectance of leaves of Eucalyptus grandis Hill ex Maiden using spectral measurements from their location in relation to the tree and the cardinal points. The experiment was carried out in the experimental area belonging to FEPAGRO, located in Santa Maria-RS. The work included the installation of a randomized block design. With the use of a spectroradiometer FieldSpec ® 3, it was measured the radiance reflected from the upper surface of leaves for further work with the software R. In the first band (400 to 700 nm), it was realized that a peak is related to total chlorophyll within the visible range. It was observed that the leaves heading east presented the highest reflectance and the leaves at south and west had the lowest one. For the second band (700 to 1000 nm), we observed a statistically significant difference only between the average values of reflectance in the pair of east-west positions, contrasting with the band that got a significant difference between the average reflectance of all pairs. The spectral response of leaves proved to be statistically different; it confirms that there are differences related to the spectral location of trees and locations of collected leaves.Keywords: Spectroradiometry; remote sensing; reflectance curve.

Large-Scale Rossby Normal Modes during Some Recent Northern Hemisphere Winters

Large-scale Rossby normal modes are studied for the Northern Hemisphere winters of 2005, 2006, 2008, and 2009 using global observational meteorological analyses spanning the 0–92-km altitude range. Spectral analysis of geopotential height fields shows pronounced peaks at westward-propagating zonal wavenumber 1 near the theoretical locations of the free Rossby waves at 25, 16, 10, and 5 days that, in some cases, have amplitudes significantly larger than the estimated background spectrum. Evidence is also found for a wavenumber-2 free mode near 4 days. A coherence analysis is used to extract the amplitude and phase of the waves, and to isolate those regions of the latitude/altitude plane where the signals are statistically significant. Although the spectral location, temporal evolution, and vertical structure of several of these waves are suggestive of the presence of Rossby normal modes, this study shows that in the real atmosphere the waves only occasionally have the global properties of classical normal modes. Moreover, no evidence is found that the amplitudes of these modes are enhanced during stratospheric sudden warmings.

Human visual pigments: microspectrophotometric results from the eyes of seven persons

The material for this work was obtained from seven eyes removed because of malignant growths. Foveal and parafoveal samples of the retinas were taken and transverse measurements were made of the absorbance spectra of the outer segments of the rods and cones, using a Liebman microspectrophotometer. Four kinds of spectra were obtained with absorbance peaks at the following wavelengths: rods, 496.3 ± 2.3 nm ( n = 39); red cones, 558.4 ± 5.2 nm ( n = 58); green cones, 530.8 ± 3.5 nm ( n = 45); blue cones, 419.0 ± 3.6 nm ( n = 5). The distribution of the peaks was unimodal for the rods. For the red and green cones, however, there was evidence for bimodal distributions, with sub-population maxima at 563.2 ± 3.1 nm ( n = 27) and 554.2 ± 2.3 nm ( n = 31) for the reds and at 533.7 ± 2.1 nm ( n = 23) and 527.8 ± 1.8 nm ( n = 22) for the greens. A substantial difference in mean spectral location of the red cones was observed between patient 1 (561 nm) and patient 4 (553 nm). Both patients were classified as normal trichromats by all clinical tests of colour vision but there was a clear difference in their relative sensitivities to long-wave fields. In both direction and magnitude, this difference proved to be that required by the microspectrophotometric results.