A simple method for optical alignment and positioning between two planes by means of a Circular Zone Plate (CZP), is proposed.The first plane is determinated by the zone plate and the other one is related to position of a reflecting surface situated in the forward space of the zone plate. Two points sources are generated by the same zone plate,when it is illuminated by a plane wave, and by several diffracted orders reflected by the reflecting surface.. Interference pattern produced by two spherical waves depend on the relative position of the generating sources. Tilting or axial misalignment between the planes determine the relative position of the two point sources and it yield fringe pattern variation.

Interferometric Alignment By A Circular Zone Plate

The usefullness of the zone plates (ZP) to the optical alignment and displacements measurement is discussed basing on examples of parabolic (PZP) and equilateral hyperbolic ZP (EHZP). Any displacement of the ZP from the position occupied during its recording creates the interference patterns between diffracted beam and its undiffracted counterpart. The shape of this pattern is explicitly determined by the kind and range of the misalignment. The PZP enables the accurate measurement of the linear motion in the direction parallel to the foci line with the control of undesired deviations in perpendicular directions. EHZP offers a possibility of strict point positioning for all possible degrees of movement freedom (including in-plane rotations), in contrary to the commonly used spherical ZP. Simple experiments supporting the analysis are presented.

Application of zone plates in interferometric positioning.

We propose a new metrologic application of zone plates, using interferometric techniques, in order to detect changes in the refraction index of a dielectric parallel plate under the influence of the environmental parameters. The key of this kind of interferometer is a couple of circular zone plates produced with the proper combination of initial phase shift, in order to suppress the constant phase factors introduced with the system in the equilibrium state. In this way we are in quadratic condition and then we can measure the variation parameters with great accuracy and with the advantage of knowing the sign of it produced under external actions. The influence of the several fabrications parameters are discussed.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

New optical sensors using circular zone plates with initial phase shift

This thesis is concerned with the use of zone plate interferometers for
the precise location of position. The thesis begins by discussing the
use of the zone plate as an optical component and examines the
wavefront aberrations that can occur in higher orders of diffraction.
Interferometers applicable to the measurement of position are then
reviewed, including interferometers that incorporate optical zone
plates as beam dividers and beam combiners.
A zone plate interferometer for defining and locating multiple
positions in three-dimensions is reported. Here, a three-dimensional
array of positions is defined by a two-dimensional array of reflective
zone plates. An optical probe containing two zone plates completes the
interferometer, and is used to locate the positions.
The displacement sensitivity of the probe can be controlled at the
optical design stage. The variation of sensitivity with wavefront
geometry is explored and expressions are derived for the transverse and
axial sensitivity in terms of the conjugates of the illuminating
wavefronts. Various wave front configurations were experimented with and
the resulting position location patterns recorded. The measured values
of sensitivity to displacement are compared to theory.
A version of this interferometer was designed and developed by the
author, for checking the performance of three-coordinate measuring
machines used in engineering metrology. The results of trials with the
interferometer, comparing the performance of measuring machines in
several government laboratories, are reported. The averaged
repeatability of setting, on one optically defined position, was found
to be 1 µm in the transverse directions and 6 µm in the axial
direction.
Resulting from this work, two simple devices for pointing are
described. Each uses a pair of zone plates to define an optical axis.
In one case, the axis is precisely located by positioning a coherent
light source and observing the interference bands generated by the
device. In the other, moire bands are observed in incoherent
illumination.

Richard F. Stevens

Papers

Zone plate interferometers for verifying three coordinate measuring machines