Kenneth E. Weiland

Evaluation of mechanical modal characteristics using optical techniques

In this paper the sensitivity of embedded fiber optic sensors to changes in modal characteristics of plates is discussed. In order to determine the feasibility of embedded fiber Bragg gratings for the detection of modal shapes and modal frequencies, a comparison of holographically imaged modes and the detected dynamic strain from embedded fiber optic Bragg gratings is made. Time averaged optical holography is used for the detection of mechanical defects, or damage, in various aerospace components. The damage is detected by measuring an alteration in structural dynamics, which is visually apparent when using time-averaged holography. These shifts in the mode shapes, both in frequency of the resonance and spatial location of vibration nodes, are caused by changes in parameters that affect the structures mechanical impedance, such as stiffness, mass and damping, resulting from cracks or holes. It is anticipated that embedded fiber optic sensor arrays may also be able to detect component damage by sensing these changes in modal characteristics. This work is designed to give an initial indication to the feasibility of damage detection through the monitoring of modal frequencies and mode shapes with fiber optic sensors.

Mode sorter and detector based on photon orbital angular momentum

Photons were generated possessing orbital angular momentum (OAM) in the form of Laguerre-Gaussian (LG) laser modes. Three Mach-Zehnder interferometer systems, with the use of Dove prisms, were constructed to sort and detect the LG modes based on their OAM state. Successful generating, sorting, and detecting of LG modes is the first step towards information encoding and decoding via lasers. It is possible to generate many LG modes, which can be orthogonally decoded. With many modes to encode information with it is possible to transfer more than one bit of information with a single photon using LG encoding. An experimental test of this concept was performed by demonstrating the generation and sorting of LG modes with attenuated intensities averaging below one photon passing through the system at a time. In summary, generating, sorting, and low-power detecting of specific LG modes were all demonstrated.

Single-photon nondiffracting Bessel beams

We present the results from a non-diffracting optical beam experiment that utilizes extremely low power levels (single-photon). The non-diffracting beam has a Bessel spatial distribution and demonstrates interesting single-photon self-interference effects such as spatial confinement. The single-photon Bessel beam is generated using two means: (1) an attenuated HeNe laser beam that statistically provides a single photon flux through the optical system, and (2) one photon from a pair of quantum entangled twin photons produced by spontaneous parametric down-conversion (SPDC) in a Beta Barium Borate (BBO) crystal pumped by a UV laser. The entangled nature of the single-photon Bessel beam using the SPDC source provides a high level of discrimination from ambient background noise photons that would otherwise severely limit the utility of such a technique to dark enclosures. The HeNe laser on the other hand, provides higher photon count rates and is more convenient to work with in contrast to the entangled photon source. We verify that a single-photon Bessel beam reforms itself beyond a circular obscuration by measuring the transmitted spatial distribution.

Sensitivity and calibration of nondestructive evaluation method that uses neural-net processing of characteristic fringe patterns

This paper answers some performance and calibration questions about a non-destructive-evaluation (NDE) procedure that uses artificial neural networks to detect structural damage or other changes from sub-sampled characteristic patterns. The method shows increasing sensitivity as the number of sub-samples increases from 108 to 6912. The sensitivity of this robust NDE method is not affected by noisy excitations of the first vibration mode. A calibration procedure is proposed and demonstrated where the output of a trained net can be correlated with the outputs of the point sensors usded for vibration testing. The calibration procedure is based on controlled changes of fastener torques. A heterodyne interferometer is used as a displacement sensor for a demonstration of the challenges to be handled in using standard point sensors for calibration.

Sources of error in heterodyne moire deflectometry

Alignment problems, and the accompanying errors, in heterodyne moire deflectometry are considered. A change in the x-directed offset between two states of a phase object causes a constant phase error, and it is noted that the relative x coordinate of the two photographic plates (the two states of the phase object) must be positioned typically within 1 micron to avoid a detectable offset. Possible solutions for assuring the relative alignment of the two deflectograms are considered, including a sandwich technique and a two-color double-exposure implementation.

Neural network for image-to-image control of optical tweezers

A method is discussed for using neural networks to control optical tweezers. Neural-net outputs are combined with scaling and tiling to generate 480X480-pixel control patterns for a spatial light modulator (SLM). The SLM can be combined in various ways with a microscope to create movable tweezers traps with controllable profiles. The neural nets are intended to respond to scattered light from carbon and silicon carbide nanotube sensors. The nanotube sensors are to be held by the traps for manipulation and calibration. Scaling and tiling allow the 100X100-pixel maximum resolution of the neural-net software to be applied in stages to exploit the full 480X480-pixel resolution of the SLM. One of these stages is intended to create sensitive null detectors for detecting variations in the scattered light from the nanotube sensors.