Fabry-Perot Cavity Optimization for Absolute Strain Sensing Using Finite Element Analysis.

The finite element method (FEM) was used to investigate the optical-mechanical behavior of a Fabry-Perot Interferometer (FPI) composed of a capillary segment spliced between two sections of standard optical fiber. The developed FEM model was validated by comparing it with theory and with previously published experimental data. The model was then used to show that the absolute strain on the host substrate is usually smaller than the strain measurement obtained with the sensor. Finally, the FEM model was used to propose a cavity geometry that can be produced with repeatability and that yields the correct absolute strain experienced by the host substrate, without requiring previous strain calibration.

Optical poling by means of electrical corona discharge.

Electrical corona discharge is employed in this work to deposit ions on the surface of an optical fiber, creating a strong electric field that is used for poling. Green laser light propagating in the core frees photocarriers that are displaced by the poling field. The technique presented can induce a higher optical nonlinearity than previously obtained in traditional optical poling with internal metal electrodes. To date, a maximum second order nonlinearity 0.13 pm/V has been achieved for a 15 kV corona discharge bias.

Electrooptic control of the modal distribution in a silicate fiber.

We demonstrate the use of the electrooptic effect to control the propagation constant of the guided modes in silicate few mode fibers with internal electrodes. The electrooptic effect induces a perturbation of the fiber's refractive index profile that controls intermodal interference. To increase the electrooptic effect the silicate fibers are poled. The response time is in the nanosecond range.

Linear electro-optical effect in silica fibers poled with ultraviolet lamp.

A second-order nonlinearity was induced in silica fibers poled by exposure to ultraviolet (UV) radiation and simultaneous high voltage applied to internal electrodes. The UV light source was a tubular lamp with spectral peak at 254 nm. The highest second-order nonlinear coefficient measured through the linear electro-optic effect was 0.062 pm/V. The erasure of the recorded voltage with UV excitation was studied, and the stability of the poled fiber at a temperature exceeding ~400 K was investigated. By eliminating the use of a focused laser beam as excitation source, the technique enables poling many pieces of fiber in parallel.

Characterizing Complex Mineral Structures in Thin Sections of Geological Samples with a Scanning Hall Effect Microscope.

We improved a magnetic scanning microscope for measuring the magnetic properties of minerals in thin sections of geological samples at submillimeter scales. The microscope is comprised of a 200 µm diameter Hall sensor that is located at a distance of 142 µm from the sample; an electromagnet capable of applying up to 500 mT DC magnetic fields to the sample over a 40 mm diameter region; a second Hall sensor arranged in a gradiometric configuration to cancel the background signal applied by the electromagnet and reduce the overall noise in the system; a custom-designed electronics system to bias the sensors and allow adjustments to the background signal cancelation; and a scanning XY stage with micrometer resolution. Our system achieves a spatial resolution of 200 µm with a noise at 6.0 Hz of 300 nTrms/(Hz)1/2 in an unshielded environment. The magnetic moment sensitivity is 1.3 × 10-11 Am². We successfully measured the representative magnetization of a geological sample using an alternative model that takes the sample geometry into account and identified different micrometric characteristics in the sample slice.

In-fiber Fabry-Perot interferometer for strain and magnetic field sensing.

In this paper we discuss the results obtained with an in-fiber Fabry-Perot interferometer (FPI) used in strain and magnetic field (or force) sensing. The intrinsic FPI was constructed by splicing a small section of a capillary optical fiber between two pieces of standard telecommunication fiber. The sensor was built by attaching the FPI to a magnetostrictive alloy in one configuration and also by attaching the FPI to a small magnet in another. Our sensors were found to be over 4 times more sensitive to magnetic fields and around 10 times less sensitive to temperature when compared to sensors constructed with Fiber Bragg Grating (FBG).

Optical creation and erasure of the linear electrooptical effect in silica fiber.

We study the creation and erasure of the linear electrooptical effect in silicate fibers by optical poling. Carriers are released by exposure to green light and displaced with simultaneous application of an internal dc field. The second order nonlinear coefficient induced grows with poling bias. The field recorded (~108 V/m) is comparable to that obtained through classical thermal poling of fibers. In the regime studied here, the second-order nonlinearity induced (~0.06 pm/V) is limited by the field applied during poling (1.2 × 108 V/m). Optical erasure with high-power green light alone is very efficient. The dynamics of the writing and erasing process is discussed, and the two dimensional (2D) field distribution across the fiber is simulated.