Calvin H. Gillespie

Calculation Of Surface Statistics From Light Scatter

The operation of a differential scatterometer developed at Montana State University is briefly described. The scatterometer takes and stores data under computer control. Analysis routines allow calculation of the surface power spectral density (PSD) function for the cases of one-dimensional surfaces [Z(x) - diamond-turned surfaces, for example] and isotropic two-dimensional surfaces [ Z(x,y) - polished surfaces, for example] . In addition, the zero and second moments of the PSD may be taken to provide bandwidth-limited values of the root mean square roughness (cr) and the root mean square slope (m). Results from several samples are used to check the vector perturbation theory [E. L. Church and J. M. Zavada, Appl. Opt. 14, 1788 (1975)] used by the computer to relate the scatter distribution function to the PSD. These experiments take advantage of the fact that the surface - and hence its PSD - remain a constant function during the measurements. Variations in the incident angle and polarization are introduced, and the resulting PSDs are calculated and compared. In another experiment, the min/max scatter angles (or, conversely, the min/max PSD spatial frequencies) are matched to those of a total integrated scatter (TIS) system. Integration over the light scatter data and the PSD allows direct comparison to the TIS and effective rms roughness obtained by the TIS system.

The Accurate Measurement Of Small Rotations By Modulating Polarization

An instrument developed to accurately measure small rotations (twist about an optical axis) uses a Faraday rotator to modulate the polarization of a HeNe laser beam. Twist sensors located in the beam path consist of a calcite polarizer (analyzer), a silicon detector and associated electronics. The sensors measure relative changes of the sensor analyzer and the null position of the modulated source beam. Reference measurements of the laser intensity and beam modulation are made to avoid accurately characterizing the modulation amplitude. This instrument has a sensitivity of 0.2 microradians at 5 Hz bandwidths over a 17.5 milliradians measuring range. This is accomplished by controlling the beam wander, reducing the laser divergence, and utilizing synchronous detection.

Design Review Of Three Reflectance Scatterometers

Design considerations for three scatterometers are presented. The first instrument, a total integrated scatter device, is currently used to make scatter measurements on flat reflectors in an optical finishing lab. The second scatterometer is a small research oriented system, nearly complete, which measures scattering intensity from relatively small diameter reflectors (a few inches) as a function of angle in the incident plane. The third system, which is still undergoing final design, also measures scattering intensity as a function of angle but will be able to do so in a nearly complete hemisphere in front of the sample. In addition, this third system will have variable angle of incidence capability and be able to measure scatter from any spot on flat reflectors up to 14 inches in diameter. Each of these systems is controlled by a small digital computer which, after initial system alignment, directs the measurement procedure, records data and analyzes results. This paper examines the problems associated with the mechanical motions of the systems, sample holding and orientation, system alignment, data compression and measurement of optical intensities that vary by as much as eight orders of magnitude. A short review of some applications for the data generated by these types of scatterometers is also given.

Comparison Of Roughness Measurements By Differential Scatter And Total Integrated Scatter

This paper discusses the problems involved in comparing the rms roughness values obtained from a plane of incidence differential scatterometer to those obtained from a TIS system. The theoretical justification for making the comparison and the importance of spatial frequency bandwidths are reviewed first. The two types of instruments are briefly described and referenced. Several practical problems relating to the types of samples that allow the comparison are presented. The comparison is independent of the surface height distribution (i.e., a Gaussian astribution is not required); however, because the data is taken in one plane only by the differential scatterometer only isotropic samples, one dimensional samples (grating like) or a combination of the two may be used to make the comparison. Then two less obvious mechanisms associated with TIS devices are discussed. These are the small angle scatter assumption and the angle of incidence variation at the detector. Both of these effects act to discriminate against high frequency roughness in the calculation of rms roughness from TIS data. Finally, some experimental comparisons of several samples are presented.

High-energy laser pulse multiplexing into a fused silica fiber array

A novel technique to multiplex high energy laser pulses from a Nd:YLF laser into an array of fibers at energies near the bulk damage limit of fused silica is presented with the object of delivering N equal high energy laser pulses with a minimal time dispersion. Characteristics of the multiple fiber system, diffractive grating splitter, and spatial mode structure of the laser to minimize fiber damage are presented along with preliminary results in scaling the system to larger fiber numbers (N approximately equals 200) with a high energy 10-Joule Nd:Glass laser system. Fiber array alignment techniques and morphologies of fiber damage will also be presented and discussed.

High-speed imaging of Raleigh-Taylor instabilities in laser-driven plates

We have previously reported our observations of the dynamic behavior of laser driven plates. Recent improvements and modification of the imaging techniques have identified and provided measurements of Raleigh-Taylor (R-T) instabilities that occur in these events. The microscope system in the LLNL Micro Detonics Facility, was converted to an epi- illuminated polarization configuration. A double pulse nanosecond illuminator and a second independently focusable frame camera were also added to the system. A laser driven plate, that is a dense solid driven by a laser heated, lower density plasma, is inherently R-T unstable. The characteristics and growth of the instability determine whether or not the plate remains intact. In earlier reports we correlated the surface patterning of thin plates with the fiber-optical transmission modes. In subsequent experiments we noted that the plasma burn through patterning in thin plates and the surface patterning of thicker plates did not correspond to the thin plate early time patterning. These observations led to the suspicion of R-T instability. A series of experiments correlating plate thickness and pattern spatial frequency has verified the instability. The plates are aluminum, deposited on the ends of optical fibers. They are launched by a YAG laser pulse traveling down the fiber. Plate velocities are several kilometers per second and characteristic dimensions of the instabilities are a few to tens of microns. Several techniques were used to examine the plates, the most successful being specularly reflecting polarization microscopy looking directly at the plate as it flies toward the camera. These images gave data on the spatial frequencies of the instabilities but could not give the amplitudes. To measure the amplitude of the instability a semi-transparent witness plate was placed a known distance from the plate. As above, the plate was observed using the polarization microscope but using the streak camera as the detector. Both the launch of the plate and its impact into the witness plate are observed on the streak record. Knowing the plate velocity function from earlier velocimetry measurements and observing the variations in the arrival time across the plate, the amplitude of the instability can be calculated.

Laser damage thresholds in fused silica structured light gratings

Computer-generated, ion-milled, holographic structures and gratings are being used for multiple splitting of high-power pulsed laser beams. The structures were manufactured by Teledyne Brown Engineering in Huntsville Alabama and both a linear grating and a 2D structure were tested. Damage thresholds were measured in single-shot exposure using a 300 mj, 1 micrometers wavelength laser with a nominal 15 ns gaussian pulse. Energy densities on the gratings were adjusted with a focusing lens and the energy distribution was mapped with an imaging profiler. The gratings were examined dynamically for sparking and breakdown. They were examined after exposure using phase contrast microscopy. Initial results indicate the grating surface is always damaged before the smooth surface independent of the beam direction. The initial indication is that the average energy density threshold for single-shot damage is in the excess of 12 j/cm2.

Wavelength Scaling Of BRDF Scatter Data

Light scattered by optical components is becoming an increasingly troublesome problem in many modern optical designs. Serious attention is now being paid to this issue by several government and industry labs who are using, or are planning to use, scatter measurements as acceptance criteria for optical components. One of the difficulties associated with scatter measurements is the large number of variables that can vary the scatter distribution. In addition to sample parameters, such as roughness, bulk defects and contamination, there are sample independent parameters such as polarization, angle of incidence and wavelength. Separating the various effects is not trivial; although, there can be strong economic motivations to do so. The usual approach has been to make scatter measurements under the conditions expected for actual use--that is, to use the polarization, incident angle and wavelength that are intended for eventual use to make the scatter measurements. Polarization and incident angle are relatively east to adjust in most instruments, but generating different wavelength capabilities is more expensive. In addition, the variable of incident angle has already been examined for the case of reflective optics. [1] This paper examines the possibility that scatter measurements made at one wavelength can be used through a process commonly referred to as wavelength scaling to predict scatter at other wavelengths.

The application of angular resolved scatter to the documentation of damage to smooth mirrors

Mirrors designed to survive exposure to damaging radiation are being irradiated and then measured to determine the mechanisms of failure and to improve the ability of analysis codes to predict an exposure damage threshold. The differences between survival and catastrophic failure are easily recognized and recorded by macro photography. However, the goal of this project is to quantify the onset of mirror degradation utilizing non contact methods that have good measurement sensitivity to small changes in reflectivity (material properties) and light scatter (roughness). A new angular resolved scatterometer is described that has an extended dynamic range and integrated analysis capable of displaying the surface power spectral density (PSD) over large bandwidths of surface spatial frequencies. Graphical displays of the scattered light power before and after exposure to the radiation are compared and integrated over equivalent spatial bandwidths of sensitivity for other instruments to compare calculated RMS roughness values.

Extreme Ultraviolet Explorer: Mirror Optical Tests And Results

The optical tests described here were performed after each step in the fabrication process of three Extreme Ultraviolet Explorer (EUVE) Type 1 telescope mirrors. The surface finish and irregularity was determined by image analysis and the geometrical figure was tested interferometrically. Images of a collimated point source were relayed to obtain magnifications and photographic exposures were adjusted to include all the scattered light while still resolving the smallest bright image core. By linearizing film sensitivity and taking digitized lineouts of the photographed image, less than 1 arc second full width half maximum (FWHM) intensities were measured. The integrated energy density was measured photometrically, and diffraction limited curves of 5 arc second half energy widths (HEW) were obtained. The test purpose, instruments employed, alignment methods, and test results are detailed.