Russell J. Palum

Sub-Nyquist interferometry: implementation and measurement capability

Sub-Nyquist interferometry (SNI) provides a method for measuring wavefronts with large departures from a reference sphere, such as those encountered when testing steep aspheric surfaces. SNI allows wavefronts with several hundred waves of departure to be recorded and analyzed. The theory of SNI is reviewed, its experimental implementation described, and limitations in the hardware and potential improvements are discussed. The importance of calibrating the interferometer for non-null testing is demonstrated.

Wavelength control of a diode laser for distance measuring interferometry

A breadboard diode-laser-based distance-measuring interferometer which measures relative thsp1ement has been developed. A temperature-controlled Fabry-Perot etalon is used in a closed-loop system to lock the wavelength emitted by the diode laser source to one of its passbands. Experiments performed using a wavemeter and a commercial laser position transducer indicate that the locked wavelength is returned to the same absolute value to within 1 ppm (part per million) from turn-on to turn-on of the control system, and that the wavelength fluctuation is reduced to less than 0.1 ppm.

Sub-Nyquist Interferometry: Results And Implementation Issues

Phase-shifting interferometry (PSI) is the generally accepted technique for data acquisition and computer analysis of interferometric patterns. General purpose instruments that use this technique for the measurement of spheres and flats are available from a number of vendors. Recent improvements in the fabrication of aspheric surfaces have led to interest in a similar instrument for the measurement of aspheric surfaces. This instrument must be able to measure large departures from a reference surface, which is a limitation of PSI. Sub-Nyquist interferometry (SNI) extends the range of PSI by using a priori knowledge of the test surface. This paper will review SNI, discuss its hardware implementation, and give the results of preliminary wavefront analysis.

Resolving interferometric step-height measurement ambiguities using a priori information

One of the principal concepts of sub-Nyquist interferometry is the use of all available knowledge about a surface to extend the available measurement range. The use of a priori information in resolving the ambiguities found in interferometric step-height measurements is explored. Information about the height of a step that has been obtained by another measurement technique or through process parameters, such as etch rate, may be used for this purpose. The theory behind this type of measurement and experimental results are presented.

Resolving Interferometric Step Height Measurement Ambiguities Using A Priori Information

One of the principal concepts of sub-Nyquist interferometry is the use of all available knowledge about a surface to extend the available measurement range. This paper will explore the use of this a priori information in resolving the ambiguities found in interferometric step-height measurements. Information about the height of a step that has been obtained by another measurement technique or through process parameters, such as etch rate, may be used for this purpose. The theory behind this type of measurement and experimental results are presented.