The rupture of periodicity caused by one defect (defect layer) in a one-dimensional photonic crystal (1DPhC) results in a narrow transmission spectral line in the photonic band-gap, and the field distribution shows a strong confinement in the proximity of the defect layer. In this work, we present a theoretical model to calculate the frequency of defect modes caused by defect layers induced by localized mechanical stress. Two periodical arrangements were studied: one with layers of poly(methyl-methacrylate) (PMMA) and polystyrene (PS), PMMA-PS; the other with layers of PMMA and fused silica (SiO2), PMMA-SiO2. The defect layers were induced by localized compression (tension). The frequencies of the defect modes were calculated using elasto-optical theory and plane wave expansion and perturbation methods. Numerical results show that the frequency of the defect mode increases (decreases) when the compression (tension) increases. Based on the theoretical model developed, we show that compression of n layers of a 1DPhC induces n defect modes whose frequencies depend on the compression magnitude in the case of normal incidence of electromagnetic waves, in accordance with the results reported for other types of defect layers. The methodology shows the feasibility of the plane wave expansion and perturbation methods to study the frequency of the defect modes. Both periodical arrangements are suitable for designing mechanically tunable (1DPhC)-based narrow pass band filters and narrow reflectors in the (60, 65) THz range.

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Photonic crystal fibers are characterized by their periodic structure with dimensions in the nanometer to micrometer range, which gives them the potential to be applied in various technical areas. In this work, we study the microstructure of a hexagonal photonic crystal fiber through a macroscopic localized compression test and measurements of relative intensity changes of a transmitted signal in the photonic crystal fiber. Our experimental study was carried out by controlling the orientation of the localized compression respective to the cross-section microstructure of the photonic crystal fiber. To complete the study, we developed a theoretical model based on the elasto-optic effect, and the numerical solution obtained with the model was compared with the experimental results. With both experimental and theoretical results, we obtained a causal correlation between the loss of relative intensity of the signal traveling through the hexagonal photonic crystal fiber and the orientation (respective to the fiber plane) of a localized compression on photonic crystal fiber. In this way, we can explore the cross-section microstructure of a photonic crystal fiber and its orientation in a device with a macroscopic compression test.

A computer controlled high-gain amplifier has been designed for sensitive detection of optical signals. Based on the use of digital-to-analog converters DACs, the gain and offset of a transimpedance amplifier are adjusted through a computer trying to match ideal amplifier parameters for optical detection with different experimental conditions. The amplifying modules have been developed for optical-rheometry techniques that provide information about the microstructural properties of fluids by measuring optical anisotropies induced by transient flows. Characterization of these programmable gain amplifiers shows that they provide gains and bandwidths more than adequate for experiments involving signals that evolve rapidly and with a large dynamic range. Hence, the use of DACs allows for the possibility of computer controlling both gain and offset in real time, with a significant reduction of the spurious contributions from the amplifying stages during data acquisition. In spite of being designed for optical rheometry, the amplifiers could be useful for any other laboratory arrangements requiring an amplifying stage with real-time gain and offset adjustment capabilities.

Heterodyne detection of a monochromatic laser beam scattered by molecules in a transparent gas gives a signal that is proportional to the spatial Fourier transform of density fluctuations for a wave vector determined by the optics. In a turbulent air jet, density fluctuations are due to either acoustic waves or to entropy fluctuations. This technique captures both. The method can then be used as a nonintrusive microphone to study the propagation of acoustic waves inside the jet. At each point in the flow the direction of propagation of the acoustic wave is determined, and the acoustic field can be described. This will eventually help to localize the sources of the waves in the flow and hopefully determine the hydrodynamic event that generates these waves.