Design of high-performance supersphere solid immersion lenses

Two types of novel solid immersion lens are designed and investigated theoretically using the vector diffraction theory. The advantages of these so-called high-performance supersphere solid immersion lenses (HPSILs) are that they can improve the Strehl ratio of the focused spot and increase the focal depth of near-field optical systems. Both the spot size and the sidelobe intensity are not increased, however, compared with those of the standard Weierstrass solid immersion lens. These HPSILs will be useful for near-field optical data storage and photolithography.     

Optical intensity distribution of a plano-convex solid immersion mirror

Ignoring the effect of the small aperture, we deduce the optical field distribution of the so-called plano-convex solid immersion mirror with a small aperture on the apex (PC-SIM) by using the vector diffraction theory. The simulation results show that a PC-SIM, like a solid immersion lens (SIL), can achieve high resolution. Unlike the SIL, the PC-SIM can effectively reduce the spreading of the spot size with increasing distance from the interface. The size and intensity of the spot are related not only to the refractive index of the solid immersion medium but also to the structure parameter of the PC-SIM. The size of a spot smaller than a quarter wavelength can be obtained simply by optimizing the structure parameter of a PC-SIM but not by decreasing the size of the small aperture.     

Comparison of simulated quenching algorithms for design of diffractive optical elements

We compare the performance of very fast simulated quenching; generalized simulated quenching, which unifies classical Boltzmann simulated quenching and Cauchy fast simulated quenching; and variable step size simulated quenching. The comparison is carried out by applying these algorithms to the design of diffractive optical elements for beam shaping of monochromatic, spatially incoherent light to a tightly focused image spot, whose central lobe should be smaller than the geometrical-optics limit. For generalized simulated quenching we choose values of visiting and acceptance shape parameters recommended by other investigators and use both a one-dimensional and a multidimensional Tsallis random number generator. We find that, under our test conditions, variable step size simulated quenching, which generates each parameter's new states based on the acceptance ratio instead of a certain theoretical probability distribution, produces the best results. Finally, we demonstrate experimentally a tightly focused image spot, with a central lobe 0.22-0.68 times the geometrical-optics limit and a relative sidelobe intensity 55%-60% that of the central maximum intensity.     

Spot size, depth-of-focus, and diffraction ring intensity formulas for truncated Gaussian beams

Simple polynomial formulas to calculate the FWHM and full width at 1/e2 intensity diffraction spot size and the depth of focus at a Strehl ratio of 0.8 and 0.5 as a function of a Gaussian beam truncation ratio and a system f-number are presented. Formulas are obtained by use of the numerical integration of a Huygens-Fresnel diffraction integral and can be used to calculate the number of resolvable spots, the modulation transfer function, and the defocus tolerance of optical systems that employ laser beams. I also derived analytical formulas for the diffraction ring intensity as a function of the Gaussian beam truncation ratio and the system f-number. Such formulas can be used to estimate the diffraction-limited contrast of display and imaging systems.     

Influence of thermal deformations of the output windows of high-power laser systems on beam characteristics

By using the well-known Green's function methods, we study the three-dimensional temperature distributions and thermal deformations of the output windows of unstable optical resonators induced by an incident annular laser beam. Some expressions and theoretical profiles of the temperature distributions and thermal deformations as functions of the radius and of the thickness of optical windows are obtained. Moreover, the influence of the thermal deformations of sapphire, silica, and silicon windows within unstable optical resonators on the Strehl ratio and on the far-field laser intensity distribution is also discussed. Under conditions of 50-kW intense laser irradiation during 5 s, the maximum thermal deformation in sapphire, silica, and silicon substrates is 1.993, 0.393, and 6.251 microm, respectively. Under the same conditions the Strehl ratio of sapphire is higher than that of silica.     

Optical data storage system with a planoellipsoidal solid immersion mirror illuminated directly by a point light source

A new solid immersion mirror called the planoellipsoidal (PE) solid immersion mirror (SIM) for the near-field optical storage is proposed and developed. The PE SIM has a small aperture on the apex of the ellipsoidal surface. The intensity distribution of the transmitted field is calculated by using the vector diffraction theory. Compared with a conventional solid immersion lens (SIL), the proposed PE SIM has the following features. A PE SIM replaces three optical elements of the collimator, objective, and SIL in a conventional SIL optical storage system, so that the optical system equipped with the PE SIM is not only simple in its assembly but is also effective in making an optical head unit. The PE SIM obtains light from a point light source and focuses it directly on the recording layer, which may be useful for a compact optical data storage system. The convex ellipsoidal surface of the PE SIM can reduce the risk of the SIM touching the surface of the recording medium. In addition, the spreading of the spot size with the increase of distance is very small in the PE SIM.     

Optical tunneling effect calculation of a solid immersion lens for use in optical disk memory

A solid immersion lens (SIL) has the advantage of easily decreasing the spot size for high data density in optical recording. To accurately obtain the optical tunneling effect for a high-N.A. SIL, we calculated the optical tunneling beam characteristics, using electromagnetic theory. Tunneling beam spot-size dependence on polarization direction and energy-transfer efficiency are also clearly shown.     

Superresolution along extended depth of focus with binary-phase filters for the Gaussian beam

In the paraxial Debye regime, simple and power-efficient pupil filters are designed to break the diffraction limit along a large depth of focus (DOF) for the Gaussian beam. Dependences of the superresolution factor, DOF gain, Strehl ratio, sidelobe strength, and axial intensity nonuniformity on the Gaussian profile in the pupil plane are characterized using the numerical method. Optimal filter designs are proposed for either high-resolution or ultra-large-DOF applications followed by experimental verifications.     

Microwave nondestructive detection and evaluation of voids in layered dielectric slabs

A microwave nondestructive testing technique is discussed for detection and evaluation of voids in layered dielectric media backed by a conducting plate. This technique utilizes the phase properties of the effective reflection coefficient of the medium as a microwave signal penetrates inside the dielectric layers and is reflected by the conducting plate. Properties of the difference between this phase in the absence and presence of an air gap is investigated as a function of the void thickness, frequency, and dielectric properties of the layers. Utilizing a simple experimental apparatus measurements were also conducted, the results of which were compared with the theoretical predictions.     

Tunable beam focusing based on optical bistability with a composite Kretschmann configuration involving a Kerr medium

A beam focuser with a composite Kretschmann configuration involving a Kerr medium is investigated theoretically. The structure employs a silver film with four slits filled with the Kerr medium. Optical bistability and beam focusing are demonstrated, and changes to the incident intensity result in optical bistability characteristics in the reflection, focused intensity, spot size, and depth of focus. The proposed structure has the potential to be applied for optical switching and nano-illumination.     

Simulation of Contaminates Around the Solid Immersion Lens in a Near-Field Optical Recording System

Accurate predictions of contamination in next-generation optical storage drives are paramount when active gap control is employed. In near-field recording devices, the read/write interface can be on the order of 20-30 nm, which means that the gap could be quite susceptible to contamination. Predictive modeling approaches for studying the behavior of contaminates in nanoscale hydrodynamic interfaces are needed. Here, we present such a model. The interface consists of a flat disk surface translating under a solid immersion lens (SIL) of hemispherical geometry. We present the computational modeling simulation results for nano-scale contaminates around the near-field SIL. The simulation shows that the discrete contaminates actually circumnavigate the SIL/disk interface during operation. We identify and discuss the external influences on the discrete contaminate particle behavior     

Optimization of sensing parameters for a confocal signal-based wavefront corrector in microscopy

The accuracy of a confocal signal-based wavefront corrector depends on several parameters such as spatial variation of optical properties within the specimen, aberration magnitude and composition, time required for the correction, etc. Here, a numerical analysis has been performed with the aim to improve system performance. The goal of the search algorithm in a confocal signal-based wavefront corrector is to estimate the Zernike coefficients of the aberrations. High-magnitude aberrations show low Strehl ratios. Repeating the correction process results in higher Strehl ratios, but at the cost of increased time. An in-focus on-axis specimen results in higher Strehl ratio compared to an out-of-focus and off-optical-axis specimen. For all cases, the wavefront correction accuracy is better, when the diameter of the pinhole is chosen to be equal to that of the Airy disk. The lower limit on the pinhole size for detecting small magnitude aberrations is set by noise.     

Tunable three-dimensional intensity distribution by a pure phase-shifting apodizer

The three-dimensional distribution of light intensity that is modulated by a pure phase-shifting apodizer is studied. Results show that the Strehl ratio can be altered by the proposed apodizer and by the waist width of incident Gaussian beams. By changing geometrical parameters of the proposed apodizer, we can increase the focal depth to several times that of the original system. The proposed apodizer can also be used to realize focal splitting and local minimum of intensity, which may be advantageous for constructing an optical trap. Furthermore, the local minimum of intensity number is tunable by changing the parameters of the apodizer.     

Far field intensity of a partially locked optical phased array

The effects of imperfect frequency locking on the performance of an optical phased array were investigated. An analytic expression was obtained for the far field intensity pattern in terms of the degree of mutual coherence between lasers. The results of the analytical study were applied to determine the Strehl ratio of an injection coupled optical phased array for different degrees of slave resonator length control, ratios of injected to slave laser intensities, and pulse durations.     

Image-quality metrics for characterizing adaptive optics system performance

Adaptive optics system (AOS) performance is a function of the system design, seeing conditions, and light level of the wave-front beacon. It is desirable to optimize the controllable parameters in an AOS to maximize some measure of performance. For this optimization to be useful, it is necessary that a set of image-quality metrics be developed that vary monotonically with the AOS performance under a wide variety of imaging environments. Accordingly, as conditions change, one can be confident that the computed metrics dictate appropriate system settings that will optimize performance. Three such candidate metrics are presented. The first is the Strehl ratio; the second is a novel metric that modifies the Strehl ratio by integration of the modulus of the average system optical transfer function to a noise-effective cutoff frequency at which some specified image spectrum signal-to-noise ratio level is attained; and the third is simply the cutoff frequency just mentioned. It is shown that all three metrics are correlated with the rms error (RMSE) between the measured image and the associated diffraction-limited image. Of these, the Strehl ratio and the modified Strehl ratio exhibit consistently high correlations with the RMSE across a broad range of conditions and system settings. Furthermore, under conditions that yield a constant average system optical transfer function, the modified Strehl ratio can still be used to delineate image quality, whereas the Strehl ratio cannot.     

Psychometrically appropriate assessment of afocal optics by measurement of the Strehl intensity ratio

Several different performance criteria have been proposed for assessing the quality of visual afocal sights. Earlier research by one of the authors (Haig) has shown that a high degree of correlation exists between a subjective assessment of performance and the Strehl intensity ratio of the optical system. We discuss some of the problems in choosing an objective performance criterion for visual sights and describe equipment that has been developed for measuring the line Strehl ratio of binoculars, both on and off axes. The equipment can be modified for testing other types of visual sight. It can also be used for measuring several additional performance parameters such as the modulation transfer function, transmission, and field curvature.     

Ellipsometer measurements using focused and masked beams

While optical spectroscopic measurements using ellipsometry may be made in air and are non-destructive, the relatively large (> 2 mm) spot size has limited their use to surface regions greater than 2 mm in lateral extent. Recent developments in focusing instruments have made spot sizes on the order of 20 to 25 μm possible. The work to be presented explores the use of the 25 μm spot size to probe non-uniform nanostructured thin films. Measurements were performed on a highly non-uniform film (0 to 2 μm in thickness across 4 mm in lateral dimension) using such a 25 μm spot. Further reduction of the spot size is possible using mechanical masking with a slit. Measurements have been made to the range of a few microns in width. The practical resolution limits of beam masking may be decreased by increasing incident light intensity, improving slit alignment, and improving detection methods.     

Optical properties and size distribution of aerosols derived from simultaneous measurements with lidar, a sunphotometer, and an aureolemeter

A new method is proposed to derive the optical properties and size distribution of aerosol in an air column from simultaneous measurements of the backscattering coefficient, the optical thickness, and the solar aureole intensity with lidar, a sunphotometer, and an aureolemeter. Inasmuch as the backscattering properties and the optical thickness depend on both the complex refractive index and the size distribution, whereas the forward-scattering properties depend mainly on the size distribution, real and imaginary indices of refraction and size distributions of aerosol are retrieved from these measurements. The real and the imaginary parts of the complex refractive index of an aerosol at a wavelength of 500 nm during the period from November 1991 to March 1992 obtained in Tsukuba, Japan, were estimated to be 1.46-1.48 and 0.005-0.014, respectively. It is inferred from the size distribution and an optical thickness fraction of stratospheric aerosols in the total columnar aerosols that these results reflect the influences of stratospheric aerosols that originated from the Mt. Pinatubo eruption.     

Optical field study of near-field optical recording with a solid immersion lens

The use of a solid immersion lens (SIL) is an important technique for increasing areal density in optical recording. Here an approximate method is presented for analyzing the optical fields in a SIL above a half-space and a SIL above a multilayer recording medium. Both propagating and evanescent components are included in the distribution of fields below the SIL. An approximate closed-form expression is given for the decay of the intensity away from the SIL surface above a half-space. In the case of a SIL above a recording medium the model describes the strong oscillations that are observed in the reflected Kerr rotation and ellipticity as the medium spacing is varied. These oscillations are attributed to standing waves in the propagating field component.     

Continuous radial superresolution filters with large focal depth

A new set of continuous superresolution filters is proposed which exhibits a radial superresolution performance with an extended depth of focus in an optical system by properly choosing the design parameters. Numerical simulation results of the performance parameters of the superresolution gain, the radial central core size, the Strehl ratio, the side-lobe factor and the depth of focus with different design parameters for the optimized patterns are displayed. We also give a design example for this kind of filter characterized by a birefringent element inserted between two parallel polarizers. This kind of filter would be useful in fields such as optical data storage systems.