Garry Berkovic

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.

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.

Fiber optic sensing in rapidly rotating mechanical structures.

A novel, generic approach for fiber-optical sensing in rapidly rotating structures is presented. This approach does not require optical ingress via the central rotation axis. In this work, strain sensing at rotation speeds of up to 5000 rpm is demonstrated, and higher speeds should be possible. We demonstrate measurement of the rotation-induced strain in a rotating body at 500 rpm intervals up to 5000 rpm, and results agree very well with predictions.

Compact and rotationally invariant design of angle-polished optical fiber assembly emitting along the mechanical axis

This paper describes a miniature assembly for optical fiber tips that ensures both ultralow backreflection and an on-axis light beam exiting the assembly, resulting in a rigid, high-performance, and compact component combined with ease of assembly.

Temperature effects in chromatic confocal distance sensors

We show, both theoretically and experimentally, the effect of changes in temperature on measurements of distance and displacement by a typical commercial chromatic confocal sensor. A temperature change of even 1°C can lead to an error which is larger than the specified absolute accuracy of the sensor. The dominant factor in this effect is the change of the refractive index of the optics with temperature. A correction procedure is described which yields high accuracy (better than 5 μm) over a temperature range from -5 to 55°C.