Steven Richard Kitchen

Holographic common-path interferometer for angular displacement measurements with spatial phase stepping and extended measurement range.

A novel technique for extending the unambiguous measurement range for differential measurements of angular deflections is presented. The technique utilizes a common-path interferometer that simultaneously probes the out-of-plane displacement of three points on the object surface. The system is based on a single laser diode, and all the optical functions of the system are implemented in a dedicated holographic optical element (HOE). The HOE automatically provides spatially phase-stepped interference signals for real-time phase measurement. It is therefore not necessary to employ any polarizing optics or active elements to introduce the phase stepping. The common-path scheme combined with the HOE provides a system that is inherently stable, since the HOE operates as both transmitter and receiver in the system. The system is compact, is robust, and has the potential for being mass-produced at a low cost and is thus well suited for industrial use, such as in commercial vibrometers. The technique is demonstrated in a system for measuring angular deflections of a plane mirror. The technique, however, is not restricted to this use alone and can easily be configured to probe other types of surface displacements, e.g., the deflection of a diaphragm. In the present configuration, the system can measure angular deflections with a sensitivity of 2.5 x 10(-7) rad over a measurement range that is approximately 3.5 x 10(-3) rad, i.e., a dynamic range of approximately 1:14,000. Furthermore, the system can easily be reconfigured for a desired angular sensitivity and measurement range.

Quasi-achromatic laser Doppler anemometry systems based on a diffractive beam splitter

We propose a new beam-splitter system that makes it possible to use nonstabilized laser diodes for laser Doppler anemometry (LDA) systems by making the system wavelength independent. The beam splitter consists of two linear diffraction gratings that produce two parallel beams with a beam spacing that is wavelength dependent. This ensures passive wavelength compensation for the fringe spacing in the measurement volume. One can choose the distance between the two parallel beams by changing the distance between the two gratings, whereas the distance to the measurement volume can be designed by choice of a condensing lens with the proper focal length. This means that the system can be designed to have a desired fringe spacing in the measurement volume. The gratings are implemented as surface-relief holograms in photoresist, which makes it possible to mass produce the beam-splitter system at low cost through replication of the structure. The method for passive wavelength compensation for the fringe spacing is demonstrated both theoretically and experimentally.

Holographic common-path interferometer for measuring tilt or vibrations utilizing a spatial heterodyne principle

ABSTRACT A novel technique for extending the unambiguous range for differential measurements of linear surface deflectionsis presented. The principle utilises a new type of spatial heterodyne system, where a common-path interferometerprobes the axial displacement of three points on the object surface simultaneously. The system is based on a singlelaser diode with all the optical functions of the system implemented in a single holographic optical element (HOE). The common-path scheme combined with the HOE provides a system that is inherently stable, since the HOEoperates both as transmitter and receiver in the system. The HOE at the same time provides a simple and all passive quadrature phase measurement without the need for any active elements or polarising optics. The system iscompact, robust, and has the potential for being mass-produced at a low cost and is thus well suited for industrialuse. The demonstrated system can measure vibrations or tilt with an unambiguous dynamic range of approximately1:14,000. Furthermore, the system can easily be reconfigured for a desired sensitivity and dynamic range.Keywords: Common-path interferometry, holographic optical elements ,

Compact optically based systems for linear and angular displacement sensing

Two new, compact sensor systems for measuring linear and angular displacements are presented. Both systems have been designed with the aim of being compact, reliable, low-priced, yet performs with reasonable accuracy. Common for both systems is the use of Vertical Surface Emitting Lasers (VCSELs) as the coherent light source. The first system is a device for measuring one component of a linear translation, being it of a solid surface or scattering particles in a flow. A VCSEL array constitutes the illuminating source creating a fringe-like pattern in the measuring volume by imaging. Scattered light from the object is detected with a single detector and a spectral analysis of the signal reveals the traverse velocity, the fringe distance being known. For measuring rotation of a semi-reflective steel ball, a compact system has been developed, again based on a single, lensless VCSEL as the source. This system is further reduced in size, as compared to the previous system and provides adequate accuracy for its intended purpose.

VCSEL array for compact time-of-flight sensor

A novel and low-cost time-of-flight system is presented. The system, which is based on a vertical cavity surface emitting laser (VCSEL) array as the light source, images the near field of all the VCSEL beams into the measurement volume. Each VCSEL provides a light sheet or a light spot in the measurement volume, and thus adds to the total optical power present in the measurement volume. The multiple-beam time-of-flight system provides operation at a lower signal-to-noise ratio and determinant of target-motion direction with a single photodetector. The concept provides a simple and compact sensor for surface velocity and fluid flow measurments, where the calibration of the system is almost temperature independent. This paper will demonstrate the concept and quantify the system tolerance with respect to temperature changes.

Impulse response function for common-path interferometers

The concept of an impulse response function (IRF) for interferometers - and especially for common-path mterferometers - will be introduced as a tool for depicting fringe appearance in ESPI systems based on the shearing effect-1.The impulse response function relates the measured phase change at an arbitrary position in the detector plane with a displacement in the object plane. Thus the TRY depicts the function of a specific filter placed in the Fourier plane ofthe common-path interferometer. A suite of filters will be introduced to show the specific way in which the filter will control the fringe interpretatio