Digital Interferometry Research Articles

Digital Radial Shearing Interferometry: Testing Mirrors with a Central Hole

Abstract It has generally been assumed that a radial shearing interferometer cannot be used to test a mirror with a central hole. We show that this can be done by a proper choice of the shear ratio and the use of digital techniques of measurement and analysis.

Vibration-observation techniques for digital speckle-pattern interferometry

Vibration observation is a major application of digital speckle-pattern interferometry (DSPI), which is a variation on electronic speckle-pattern interferometry (ESPI). DSPI processes speckle patterns in a computer rather than with a frame grabber and analog electronics as in ESPI. A new method of observing vibration fringes is presented and compared with existing techniques as well as some variations on them. Fringe contrast and signal-to-noise ratio are used as a means of comparison since these quantities are dependent on the techniques used. This new technique involves continuously subtracting a reference frame containing only self-interference terms and no cross interference term from the time-averaged data frames of the vibrating object. This reference frame is created by vibrating a reference mirror at a high amplitude while the object is at rest. Comparisons of calculated fringe contrast with four other observation methods show that this method yields extremely good fringe contrast. Experimental results are shown for this new technique as well as for the most commonly used vibration-observation technique. These results show that the new technique is far superior to all the other methods for moderately unstable objects, which may slowly drift or deform in time.

Fringe scanning speckle-pattern interferometry

Digital speckle-pattern interferometry systems for automatic measurement of deformations of a diffuse object are presented, which are based on a fringe scanning method with phase-shifted speckle interferograms. A digital speckle pattern before deformation of an object is recorded in the mass storage device of a computer facility. After deformation, four digital speckle patterns are recorded as changing the phase of reference light such as 0, π/2, π, and 3π/2, respectively. Four speckle interferograms, whose phases are shifted by 0, π/2, π, and 3π/2, are generated by calculating the square of the differences between speckle patterns before and after deformation. These interferograms are low-pass filtered to reduce speckle noise. The calculation of the arctangent with four phase-shifted speckle interferograms gives the optical path difference which is proportional to the deformation. A correction of the discontinuity of the calculated phase gives the numerical data of the deformation in the whole object area. Some experimental results for the measurement of out-of-plane, in-plane, and 3-D deformations are presented.

High Precision Digital Interferometry: Its Application To The Production Of An Ultrathin Solid Fabry-Perot Etalon

Teflon polishing is capable of producing surfaces flat to 3 nm or better, but its wider application requires suitable testing methods to measure such small residual errors. Digital interferometry solves this problem and, because data can be stored and processed, makes it possible to carry out a variety of tests with high precision. The application of these techniques to the production and testing of a solid Fabry-Perot eta Ion with a diameter of 50 mm, a thickness of 0.352 mm, and a maximum deviation from parallelism of its faces of ±2 nm over the central 30 mm is described.

Optical isotope shifts and hyperfine structure in Cd

The spectral lines λ226.5, λ441.6, λ806.7 and λ853.3 nm of Cd II and λ508.6 nm of Cd I have been studied under high resolution by means of digital recording interferometry. The relative positions of all the stable isotopes of cadmium in all the lines have been deduced from the measurements. The main object of this study was to provide precise data for the simple p–s transitions λ226.5, λ806.7 and λ853.3 nm. Such lines are the most favourable for theoretical interpretation, and the new results are intended to provide a basis for the separation of mass and field effects in measured isotope shifts in cadmium. The hyperfine structure intervals, which are important in the interpretation of isotope shifts, have also been measured for all the levels involved in the five transitions.

Isotope shifts and hyperfine structure in the atomic spectrum of palladium

The lines λ340.5 nm, λ357.1 nm, and λ677.4 nm of the arc spectrum of palladium have been studied under high resolution by means of digital recording interferometry. The even-even shifts are claimed to be more accurate than previously published work; the relative shifts determined in λ357.1 nm are 102 Pd - 104 Pd, 1.03(2); 104 Pd - 106 Pd, 1.00; 106 Pd - 108 Pd, 1.02(1); 108 Pd - 110 Pd, 0.92(2); 104 Pd - 105 Pd, 0.23(1). An odd-even shift has been measured for the first time in this element, and shows appreciable staggering. The relative shift 102 Pd - 104 Pd has been found to be substantially greater than the values reported in the literature. Values of the change in the mean square radius of the nuclear charge distribution have been deduced from the measurements, all to an accuracy of Ŧ15% ; these are: 102 Pd - 104 Pd, 0.176 fm 2 ; 104 Pd - 106 Pd, 0.170 fm 2 ; 106 Pd - 108 Pd, 0.173 fm 2 ; 108 Pd - 110 Pd, 0.153 fm 2 .

Isotope shifts and hyperfine structure in the optical spectrum of platinum

The lines λ 444.2nm, λ 304.2nm and λ 306.5nm of the arc spectrum of platinum have been studied under high resolution by using digital recording interferometry. The work extends the range of nuclei for which isotope shifts have been measured to include 190 Pt, and has provided some improvement in accuracy for the shifts involving other isotopes. Data are now available for all the stable platinum nuclei; the relative shifts, which are closely proportional to the changes in mean square radius of the proton distribution, are as follows: 190, 192; 0.905 ± 0.028.192, 194; 0.969 ± 0.007. 194, 195; 0.456 ± 0.004. 194, 196; 1.0. 196, 198; 1.084 ± 0.004. The data are discussed in terms of recent measurements of nuclear deformation in platinum. The hyperfine splittings of some levels have also been deduced from the optical measurements.