Evaluation of narcissus for multilayer diffractive optical elements in IR systems

The influence of narcissus effect for multilayer diffractive optical elements (MLDOEs) is evaluated from the viewpoint of diffraction efficiency and the narcissus intensity. A modified paraxial evaluation criterion for the reflected narcissus radiation of MLDOEs has been deduced. A practical 8-12 μm IR optical system designed with one two-layer diffractive element has been given to illustrate the distribution of incident narcissus energy among various diffraction orders in the waveband. The narcissus intensities of the two diffractive surfaces have been calculated for those diffraction orders that have the maximum diffraction efficiency. This method can be used in the process of evaluation and control of the narcissus influence in IR optical systems with MLDOEs.     

High-efficiency diffractive micromachined chopper for infrared wavelength and its application to a pyroelectric infrared sensor

We have developed a diffractive micromachined chopper (DMC) for an IR wavelength of ~10 mum. This device operates mechanically by movable reflection grating beams. It modulates the diffraction efficiency by controlling the displacement of grating beams by an electrostatic force. For a CO(2) laser beam, a high modulation efficiency of 84% with an -0.8-dB small insertion loss was obtained by detecting 0th-order diffracted light. A novel pyroelectric IR microsensor with a DMC and a diffractive multilevel Si microlens was proposed and it demonstrated the detection of human existence.     

Two-element diffractive systems for generation of Bessel fields

We demonstrate experimentally a combination of two apodizing diffractive elements, which generate nearly propagation-invariant Bessel fields efficiently with almost uniform axial intensity distributions over the specified region recently introduced by two of the present authors. Strongly reduced intensity fluctuations near both ends of the propagation-invariant range are verified. Various design and alignment issues of the two-element systrem are addressed numerically.     

Use of infrared thermography for quantitative non-destructive evaluation in FRP strengthened bridge systems

The increasing use of fiber reinforced polymer (FRP) composites as externally bonded reinforcement for strengthening of concrete structures has created a need for the development and implementation of rapid methods of field level non-destructive evaluation (NDE) both for quality assurance during installation of the material and also for long-term monitoring of in-field performance. These methods must provide inspectors with the ability not only to inspect FRP strengthened structures for defects, but also should provide the means to quantify the shape, size and severity of the defect for continued monitoring and comparison. Infrared (IR) thermography, which uses data from temperature differentials to detect and characterize defects and anomalies, presents a potentially efficient technique for non-contact, real-time inspection and quantitative data interpretation. In this research a three-girder two bay reinforced concrete bridge deck segment was loaded under field representative loading conditions and the girders and slabs were sequentially strengthened with FRP composites to study the effect of FRP strengthening at systems level. The two slabs were strengthened with two different composite systems, prefabricated strips and field impregnated fabric laminates. The appearance and progression of damage in the FRP systems and at the FRP–concrete interface with an increase in the level of loading was quantitatively monitored using IR thermography. Based on the thermal intensity, the damage could be classified into types such as interlaminar debonding inside the composite or composite–concrete interface debonding. Also based on the magnitude of the thermal intensity, the severity of the damage could be quantitatively monitored. The results were also correlated to the visual observations of crack patterns and failure mechanisms as well as to strain and displacement data measured during the test.     

Modeling parameters for the spectral behavior of infrared frequency-selective surfaces

Comparisons of experiment and theory are presented for transmission spectra over the range 2-15 mum of a set of frequency-selective surfaces consisting of arrays of simple dipole patches of aluminum on or in silicon. The arrays are fabricated by direct-write electron-beam lithography. Important parameters controlling the spectral shape are identified, such as dipole length, spacing, resistance, and dielectric surroundings. The separate influence of these variables is exhibited. Encouraging agreement between simple model calculations and the measurements is found.     

Speckle reduction in laser projection systems by diffractive optical elements

In laser projection systems the observer in the far field of the image points on the screen will recognize serious speckle noise. There are many methods to reduce or eliminate speckles in the near field by reducing or eliminating temporal or spatial coherence of the laser. But for the far field it is hardly possible to change the coherence properties of laser sources so that speckles will disappear. We propose a new method for eliminating speckles in the far field by using a diffractive optical element. The intensity modulation depth in the far-field speckle pattern can be reduced to a few percent while good beam quality is preserved.     

Diffracted-ray approach to the fast evaluation of diffractive lenses

We employ a combination of asymptotic methods to speed up the computation of fields in the focal region of a diffractive lens (DL). The DL is treated locally as a linear grating with a slowly varying period and groove orientation. We employ rigorous electromagnetic diffraction theory locally to obtain the field just behind the DL. A simple diffracted-ray formula is derived for the field in the focal region of the DL at observation points that are not in the immediate vicinity of the optical axis. A careful study of the range of validity of this formula is made. For observation points that are not in the immediate vicinity of the optical axis the new algorithm is 3 x 10(5) times faster than the application of numerical integration to the double integrals involved and approximately 1000-1200 times faster than a recently published algorithm based on using asymptotic theory to replace the double integral with a single integral.     

Magnetooptic sensor for remote evaluation of surfaces

A new magnetooptic (MO) detection method utilizing changes in the optical path through a transparent MO thin film has been developed and studied for evaluation of surface deformation created by subsurface or internal defects in materials. Investigation of defects cannot be performed on nonconducting and nonmagnetic materials using conventional electromagnetic techniques such as eddy-current or magnetic flux leakage. The new method utilizes the controlled periodic displacement of a domain wall in the MO thin film and can be used to measure remotely mechanical deformation of a surface of any type of material by measuring the changes of width of the shoulder in the intensity versus time waveform.     

Production and evaluation of silicon immersion gratings for infrared astronomy

Immersion gratings, diffraction gratings where the incident radiation strikes the grooves while immersed in a dielectric medium, offer significant compactness and performance advantages over front-surface gratings. These advantages become particularly large for high-resolution spectroscopy in the near-IR. The production and evaluation of immersion gratings produced by fabricating grooves in silicon substrates using photolithographic patterning and anisotropic etching is described. The gratings produced under this program accommodate beams up to 25 mm in diameter (grating areas to 55 mm x 75 mm). Several devices are complete with appropriate reflective and antireflection coatings. All gratings were tested as front-surface devices as well as immersed gratings. The results of the testing show that the echelles behave according to the predictions of the scalar efficiency model and that tests done on front surfaces are in good agreement with tests done in immersion. The relative efficiencies range from 59% to 75% at 632.8 nm. Tests of fully completed devices in immersion show that the gratings have reached the level where they compete with and, in some cases, exceed the performance of commercially available conventional diffraction gratings (relative efficiencies up to 71%). Several diffraction gratings on silicon substrates up to 75 mm in diameter having been produced, the current state of the silicon grating technology is evaluated.     

Calculation of piezoelectric modulators of infrared radiation

The methods and results of numerical calculations of the stress—strain state and flexural strains of a cantilevered, monolithic, piezoceramic plate under the influence of nonuniform poling along its thickness are discussed. Optical modulators with prescribed amplitude—frequency responses can be constructed by varying the thickness distribution of the polarization through the diffusion of impurities or variation of the external electric field. Translated from Izmeritel'naya Tekhnika, No. 7, pp. 58–60, July, 1995.     

Wavelength-multiplexing diffractive phase elements: design, fabrication, and performance evaluation

We report on the wavelength-multiplexing diffractive phase element (WMDPE) capable of generating independent spot patterns for different wavelengths. The iterative method proposed by Bengtsson [Appl. Opt. 37, 1998] for designing a kinoform that produces different patterns for two wavelengths is extended to the WMDPE for multiple wavelengths (more than two wavelengths). Effectiveness of the design algorithm is verified by design and computer simulations on the WMDPE's for four and nine wavelengths. The WMDPE for three wavelengths (441.6, 543.5, and 633 nm) is designed with five phase levels and is fabricated by electron-beam lithography. We observed that the individual spot patterns are reconstructed for the design wavelengths correctly. Performance of the WMDPE is evaluated by computer simulations on the uniformity error, the light efficiency, and the contrast. On the basis of the results, the characteristics of the WMDPE's are discussed in terms of various conditions of fabrication and usage.     

Dedicated spectrometers based on diffractive optics: Design,modelling and evaluation

Abstract

The described design of diffractive optical elements for low cost IR-spectrometers gives a built-in wavelength reference and allows ‘spectral arithmetic’ to be implemented in the optical performance of the DOE. The diffractive element combines the function of the lenses and the grating and eliminates the need for alignment of those components in the standard scanned grating spectrometer design. The element gives out a set of foci, each with one spectral component, which are scanned across a detector, thus relaxing the demands for scan angle control. It can thus be regarded as an alternative solution to a beam splitter and band pass filter instrument. Software tools have been designed to ease the adaptation of the design to different applications. To model the performance of the spectrometers we have implemented a scalar Rayleigh-Sommerfeldt diffraction model. The gold-coated elements are produced by injection moulding using a compact disc (CD) moulding technique and mould inlays mastered by e-beam lithography. The optimized selection of wavelength bands and the classification of the measured signal use a combination of principal component analysis and robust statistical methods. Typical applications will be material characterization of recycled plastics and gas monitoring. Spectrometers for two different applications have been built and tested. Comparisons between the design goals and the measured performance have been made and show good agreements.     

Polarization of the reflectivity of paints and other rough surfaces in the infrared

In this study the IR reflectivity of painted and rough surfaces was investigated, and an attempt was made to represent the surfaces by a complex refractive index. A CO(2) laser was used as a collimated source in the thermal IR region, and the polarization properties of reflected radiation were measured. The samples chosen were flat surfaces of sandblasted aluminum, concrete, painted metal, and asphalt. Values of the bidirectional reflectance function were obtained in the two orthogonal states of polarization, based on sulfur as the Lambertian standard. Many samples, such as painted metals, showed specular behavior and could be characterized by Fresnel equations. For some of these surfaces optical constants were calculated from the reflectivity measurements. Good agreement was obtained between the calculated and measured values of the percent of polarization for these surfaces.     

Tactile measuring systems for the recognition of unknown surfaces

A novel smart tactile sensor that recognizes the nature of the surfaces is presented. The approach is based on the idea of analyzing the signal produced when the sensor touches and stimulates the surface. An "intelligent probing" system for material recognition has been developed. It is based on the use of bimorph piezo-ceramic actuators and sensors that allow the unknown surface to be stimulated and the response signal sensed. Two different experimental prototypes of the tactile sensing system have been realized and their performance has been characterized. Several interesting applications have been considered with particular emphasis on the problems of "humanitarian demining" and automatic waste material recycling. Experimental results are given to show the efficiency of the smart measuring system     

Design, fabrication, and evaluation of a multichannel diffractive optic rotary joint

A multichannel diffractive optic rotary joint was designed, fabricated by electron-beam lithography, and evaluated with regard to cross talk and to output signal power variations. High cross-talk margin (>25 dB) and low output signal power variations (<2 dB) were achieved. The sensitivity to input-light-beam wavelength uncertainty was investigated. Two design examples are presented. The first design eliminates cross talk due to unwanted diffraction orders and shows that for a ten-channel joint the wavelength uncertainty of an 850-nm emitting laser must be less than 8 nm. In the second design cross talk due to the second diffraction order is permitted, which results in a tolerance level that is three times better for wavelength uncertainty.     

Simplified design and evaluation of cam surfaces

A method is presented to determine the location and magnitude of cam profile errors. The impact of this method centers around the simplification of the design and evaluation of cam surfaces. The proposed technique can evaluate errors in newly machined or worn cams. Inputs are cam rotational speed and a set of theoretical data points, which lists cam radial displacement and acceleration versus angular position. A recursion formula for the cam profile errors is derived using a finite-difference scheme. Application of the method is discussed. A. H. Soni. Professor of Mechanical & Aerospace Engineering,David L. Smith, Research Associate at the Oklahoma State University, Still-water, Oklahoma.     

Angular variation of thermal infrared emissivity for some natural surfaces from experimental measurements

Multiangle algorithms for estimating sea and land surface temperature with Along-Track Scanning Radiometer data require a precise knowledge of the angular variation of surface emissivity in the thermal infrared. Currently, few measurements of this variation exist. Here an experimental investigation of the angular variation of the infrared emissivity in the thermal infrared (8-14-mum) band of some representative samples was made at angles of 0 degrees -65 degrees (at 5 degrees increments) to the surface normal. The results show a decrease of the emissivity with increasing viewing angle, with water showing the highest angular dependence (~7% from 0 degrees to 65 degrees views). Clay, sand, slime, and gravel show variations of approximately 1-3% for the same range of views, whereas a homogeneous grass cover does not show angular dependence. Finally, we include an evaluation of the impact that these data can produce on the algorithms for determining land and sea surface temperature from double-angle views.     

Focusing diffractive cylindrical mirrors: rigorous evaluation of various design methods.

The performance characteristics of focusing diffractive mirrors designed with various methods are evaluated by using the rigorous boundary element method. Quantitative results are presented for (1) conventional-zero-thickness mirror designs, (2) alternative-zero-thickness designs that incorporate an off-axis correction factor and (3) finite-thickness designs. For TM polarization, the mirrors designed by using the alternative-zero-thickness method perform considerably worse than those designed by using the conventional-zero-thickness method, which contradicts predictions made in an earlier paper.