In this paper, we report, to our knowledge, the first demonstration of the induction of long-period and Bragg gratings on surface-core optical fibers. Surface-core fibers described herein were fabricated from commercial silica tubes and germanium-doped silica rods by employing a very simple procedure. Being the core on the fiber surface, it can be sensitive to refractive index variations in the environment in which the fiber is immersed. Thus, results concerning the sensitivity of these gratings to environmental refractive index variations are presented. Besides, simulation data are presented for comparison to the experimental behavior and for projecting future steps in this research.
We report the use of the photo-emf effect in BTO photorefractive crystals to measure sub-micrometer-order amplitude
transverse vibrations. The method is based on the illumination of the surface under analysis by a direct laser beam of
wavelength λ = 532 nm and the collection of the back-scattered speckled patterns of light onto the photoconductor. A
pattern of space-charge electric field is built-up in the photoconductive material volume in a time-scale corresponding to the response time of the material that is essentially controlled by charge-transport phenomena. A pattern of free electrons
in the conduction band is simultaneously built-up with a much faster time-scale that depends on the excitation of
electrons from photoactive centers inside the material band gap into the conduction band. If the illuminated target surface
is static, the pattern of space-charge field and free electrons are in mutual equilibrium and no electric signal is detected.
However, if the target is laterally vibrating, the speckle pattern of light is simultaneously moving and the fast pattern of
free electrons follows. The pattern of space-charge field instead is comparatively much slower and is not able to follow it. In this way the free charge distribution and the pattern of electric field are mutually displaced proportionally to the
amplitude of the target vibration and are not any more in equilibrium. An alternating current is therefore produced that
can be detected to find out the size of the target vibration amplitude. We report experiments carried out with Bi<sub>12</sub>TiO<sub>20</sub>
We report the use of photo-electromotive-force effect in a speckled pattern of light onto a photorefractive Bi<sub>12</sub>TiO<sub>20</sub> crystal to measure the amplitude of transverse vibrations. Our theoretical model shows that, for vibrations much faster than the photorefractive response time, the photo-emf signal exhibits a maximum at a characteristic vibration amplitude value that may be used to calibrate the photo-emf sensor.
We report a mathematical formulation that successfully describes the holographic photocurrent produced in photo-emf experiments, with large oscillation amplitudes, in strongly absorbing photorefractive materials. The large amplitude produces a sensible enhancement of the photocurrent signal and in this way facilitates measurements. Accounting for bulk light absorption of the sample is essential in order to adequately describe the experiment. We measure pure and doped potorefractive Bi<sub>12</sub>TiO<sub>20</sub> (BTO) crystals and show that these data are in excellent agreement with theory. From data fitting we are able to determine some material's parameters.