Ehud Shafir

Optical methods for distance and displacement measurements

This tutorial reviews various noncontact optical sensing techniques that can be used to measure distances to objects, and related parameters such as displacements, surface profiles, velocities and vibrations. The techniques that are discussed and compared include intensity-based sensing, triangulation, time-of-flight sensing, confocal sensing, Doppler sensing, and various kinds of interferometric sensing with both high- and low-coherence sources.

Fibre Bragg Grating sensor for shock wave diagnostics

We measured the response of short FBGs to a weak planar shock wave. The combined effect of the Photo-Elastic effect and the FBG strain was estimated theoretically depending on its orientation with respect to shock front (for 1550 nm FBG, parallel: 0.9 nm/kbar, perpendicular: -1.4 nm/kbar). The experimental results imply that the FBG/fibre survives for more than 1 μs at 5 kbar shock stress, and that our assumptions about the FBG behaviour under dynamic loading are valid, though more work is needed to fully quantify the effect.

Practical strain isolation in embedded fiber Bragg gratings

We describe a new method for isolating embedded fiber Bragg gratings from strain. The strain isolated grating may serve as a pure temperature sensor in environments where both strain and temperature variations occur. The grating need not be at the end of the fiber, thus enabling multiplexing of several strain-isolated gratings along a single fiber.

Multi-wavelength fiber-optic confocal position sensor with diffractive optics for enhanced measurement range

A multi-wavelength fiber-optic confocal position sensor, employing a diffractive optical element (DOE), is described. The DOE was designed with the aim of enhancing the chromatic dispersion of the optics, and thus improving the measurement range of the technique. A proof-of-principle experiment is presented, yielding a five-fold enhancement in the dispersion and thus in measurement range in excellent agreement with design simulation.

Comparison of FBG responses to static and dynamic pressures

FBGs respond to external pressures in ways that reflect both the strain-optic effect and the geometrical variations, both induced by the applied pressure. While the response to static isotropic pressure is quite straight forward and intuitive, the response to anisotropic shock waves is much more complex and depends also on the relative orientation between the fiber and the shock propagation direction. We describe and explain experimental results for both cases.

Size Effect in Fiber Optic Displacement Sensors

We report and explain anomalous results obtained when using small core optical fibers to probe the distance to some scattering surfaces, such as machined metal surfaces. Large intensity variations occur when different sections of the surface are probed at constant distance. This is explained using a model which includes ray tracing and reflection from a surface structure exhibiting a variable surface normal vector. When the collection fiber size is increased, a larger portion of the reflected light field is sampled, and the signal intensity variations weaken.

Multiple-Fiber and Multiple-Wavelength Confocal Sensing With Diffractive Optical Elements

Adding a diffractive element to the imaging optics of a fiber-based confocal sensor greatly enhances the chromatic dispersion of the optics. This can be exploited to enhance the sensing range and resolution of the sensor. Several examples are given using one and two fibers with simultaneous dual wavelength illumination to probe the displacement of a target object.

Multimode optical fiber photonic doppler velocimetery

A new optical fiber head for Photonic Doppler Velocimetry (PDV) made from a combination of fiber types [multimode (MM) and single-mode (SM)] and lenses is described. The input laser beam is delivered by a SM fiber and imaged onto the target by simple optics, including an imaging lens centered inside a larger lens, whose role is to collect and image the back reflected light into the MM collection fiber. The large core of the MM fiber enhances the collection efficiency and also reduces its dependence on the target angle. Transmission through the MM fiber reduces the heterodyne fringe visibility considerably, but the Fourier analysis still enables very accurate resolution of the fringe frequency (and hence the velocity). The new PDV head with 20 GHz bandwidth was tested in a dynamical shock wave experiment to measure velocities of ∼3 km/s (∼3.9 GHz), and the results agreed very well with measurements by a standard SM PDV.

Ultrafast pressure measurement in shock wave research using fiber Bragg grating sensors

Fiber Bragg Grating (FBG) sensors may probe ultrafast changes in pressure caused by shock waves propagating in solid and liquid media impacted by high velocity projectiles. The FBG spectra are measured using an optical system comprising economically priced electro-optical components offering 5 nsec temporal resolution and 0.8 – 1.6 nm spectral resolution. We present results showing evolution of 5 kBar shock wave pressure in approx. 100 nsec, as well as the dependence of the FBG response on the physical length of the sensor (1mm and 0.1mm), and on the relative orientation between the FBG axis and the shock wavefront.

Collimated Fibers with Bragg Gratings for Strain Sensing of Rapidly Rotating Mechanical Structures

Standard single mode fibers terminated with small diameter collimators are coupled to fiber Bragg gratings to demonstrate a generic approach for optical strain measurement in rapidly rotating structures which does not require optical ingress via the central rotation axis.