ABSTRACT
Large diameter, high-harmonic diffractive lenses could find applications in future space telescopes. Residual chromatic aberrations from these lenses can cause significant blurring. Solutions to reduce chromatic dispersion and other aberrations to diffraction-limited performance are discussed. A design example based on a 240-mm-diameter, 1-m focal length multi-order diffractive engineered lens operating over the astronomical R-Band (589-727 nm) is presented. The design example uses a relay subsystem with four times smaller diameter than the primary. This color corrector includes both refractive and diffractive optical elements and reduces the longitudinal chromatic aberrations by more than a factor of 30 compared to the primary lens alone, while maintaining the effective focal length and numerical aperture of the system.
ABSTRACT
Diffractive optical surfaces have attractive properties for use in optical systems, like reducing weight and correcting for chromatic aberrations, but fabrication of high-quality glass diffractive optics is challenging, preventing it from being widely adopted in commercial applications. In this Letter, we report on a fabrication method to address molding challenges for high-surface-quality diffractive glass optics at molding temperatures up to 550°C, including selection of mold material, mold fabrication, precision glass molding, durability, and stability of the mold. To enable optimal mold machining and easy mold release, nickel phosphorous (NiP) is chosen as the plating material for its cutting performance and anti-adhesion properties, and copper-nickel C71500 (CuNi) is selected as the mold substrate because its coefficient of thermal expansion (CTE) is close to NiP. By the proposed method, diffractive glass optics with 2 nm Sa surface roughness is demonstrated.
ABSTRACT
A multiple-order diffractive engineered surface (MODE) lens is introduced, in which focal position change with wavelength exhibits both refractive and diffractive characteristics. Engineering calculations are provided that indicate Strehl ratio and encircled energy performance over a large range of focal length and aperture diameter design space. A prototype lens is designed and constructed for the astronomical R-band (589 nm to 727 nm) wavelength range. Test results show that measured full-width-at-half-maximum focal spot diameter is 2.1 times larger than the ideal Airy spot diameter, and focal position versus wavelength is nearly identical to the design. The 48 mm aperture diameter, f/3.12 prototype telescope exhibits angularly resolved features in natural scenes at 0.006°, with subtense of the Airy spot diameter at 0.002°. Applications include eventual use in large aperture, ultralightweight space telescopes.
ABSTRACT
The concept of complex diversity is introduced that adequately accounts for special considerations in the design of the system and the reconstruction algorithm for single-shot phase retrieval techniques. Complex-number pupil filters containing both amplitude and phase values are extracted by numerical propagation from a computer-generated hologram design, which generates multiple images in a single acquisition. The reconstruction is performed by a Fourier iterative algorithm modified with an area restriction to avoid noise amplification. Numerical simulations show that the complex diversity technique estimates extrinsic Kolmogorov aberration better than conventional single-shot techniques for a distant point object. Experiments show that sensorless adaptive optics correction is achieved using the complex diversity technique.
ABSTRACT
Phase retrieval is an attractive approach for sensor-less adaptive optics (AO) because of its relatively simple implementation. Recently, random phase diversity has shown fast convergence for phase retrieval algorithms. In this study, design optimization using random phase diversity is discussed with respect to a sensor-less AO system using a liquid-crystal-on-silicon (LCoS) spatial light modulator. The extrinsic phase disturbances studied are due to Kolmogorov turbulence. Simulation analysis shows that the size of super-pixel segments of the random phase patterns on the LCoS and the cropped image area of the phasorgrams are determined by Fried's parameter for high-Strehl-ratio and low-iteration-number reconstruction. AO experiments with an LCoS spatial light modulator confirm the simulation results.
ABSTRACT
A new microscope is discussed, where the scanning illumination has a numerical aperture of 2.8 with λ = 1.56 µm femtosecond fiber laser. Samples are placed or grown on a silicon substrate. Multi-photon emission is imaged in transmission on a cooled CCD. Two-photon and three-photon effects are observed from the silicon/water interface and gold nanoparticles. Images of cells, reference spheres and gold nanoparticles illustrate imaging properties of the microscope. Spectral characteristics of individual particles are achieved with a blazed transmission grating. Emission properties of differently sized gold nanoparticles are studied in detail, which indicate that their emission is a two-photon effect due continuum generation. Interestingly, spectral shape and emission power are similar for 20nm, 40nm and 60nm diameter gold nanoparticles for the cases studied.
ABSTRACT
A scanning and rotating nanoslit is used to measure submicrometer features in focused spot distributions. Using a filtered backprojection technique, a highly accurate reconstruction is demonstrated. Experimental results are confirmed by simulating the scanning slit technique using a physical optics simulation program. Analysis of various error mechanisms is reported, and the reconstruction algorithm is determined to be very resilient. The slit is 125 nm wide and 50 µm long and is fabricated on a 120 nm thick layer of aluminum. The size of the image field is 15 µm, and simulations indicate that 200 nm Rayleigh resolution is possible with an infinitely narrow slit.
ABSTRACT
We introduce a microscope system using a solid immersion lens (SIL) to image Blu-ray disc samples without removing the protective cover layer. The aberration caused by the cover layer is minimized with a truncated SIL. A subsurface imaging simulation is achieved by using the rigorous coupled wave theory, partial coherence, vector diffraction, and the Babinet principle. Simulated results are compared with experimental images and atomic force microscopy measurements.
ABSTRACT
Optical designers are encouraged to adopt aspheric and free-form surfaces into an increasing number of design spaces because of their improved performance. However, residual tooling marks from advanced aspheric fabrication techniques are difficult to remove. These marks, typically in the mid-spatial frequency (MSF) regime, give rise to structured image artifacts. Using a theory developed in previous publications, this paper applies the fundamentals of MSF modeling to demonstrate how MSF errors are evaluated and toleranced in an optical system. Examples of as-built components with MSF errors are analyzed using commercial optical design software.
ABSTRACT
Off-axis polarized monopole illumination is applied to a hyper-numerical-aperture (NA) (NA>1) microscopic system. Illumination artifacts due to vector effects are observed, which are asymmetric and depend on illumination conditions. A model based on rigorous coupled wave theory is used to simulate image profiles for dielectric, semiconductor, and metal gratings with different monopole locations and polarization states. A solid immersion lens microscope is used to image different types of samples including MoSi photomask, patterned silicon wafer, and chrome photomask. The experimental images are in good agreement with simulation results.
ABSTRACT
Optical design and tolerancing of aspheric or free-form surfaces require attention to surface form, structured surface errors, and nonstructured errors. We describe structured surface error profiles and effects on the image point-spread function using harmonic (Fourier) decomposition. Surface errors over the beam footprint map onto the pupil, where multiple structured surface frequencies mix to create sum and difference diffraction orders in the image plane at each field point. Difference frequencies widen the central lobe of the point-spread function and summation frequencies create ghost images.
ABSTRACT
Aspheric and free-form surfaces are powerful surface forms that allow designers to achieve better performance with fewer lenses and smaller packages. Unlike spheres, these surfaces are fabricated with processes that leave a signature, or "structure," that is primarily in the mid-spatial-frequency region. These structured surface errors create ripples in the modulation transfer function (MTF) profile. Using Fourier techniques with generalized functions, the drop in MTF is derived and shown to exhibit a nonlinear relationship with the peak-to-valley height of the structured surface error.
ABSTRACT
A single scanning nano-slit is used to study aerial image characteristics. Finite-difference time-domain simulations reveal that, in the far field of such a slit, the detected image contrast is very high over a large spatial frequency range regardless of the polarization direction. In the near field, the TM polarization shows a decrease in contrast at larger spatial frequencies. Experiments verify this characteristic using a 125 nm wide slit on an aluminum mask at a wavelength of 658 nm. Unlike the light transmission characteristics of a nano-slit, which are greatly influenced by slit width and metal mask thickness, it is shown that image contrast measurement is almost insensitive to small changes in these parameters. It is found that defects on the metal mask play an important role in accurate analysis of the system.
ABSTRACT
Optical aberration effects on Gaussian speckle contrast are theoretically examined in an imaging system exhibiting partial spatial coherence. Analysis includes phase-perturbed random fields from a rough object illuminated by an extended source that generate speckle in the image plane. Results indicate that, unlike coherent illumination, speckle contrast in this partially coherent system depends on odd-functional aberrations, such as coma. In addition, calculations show that speckle contrast reduction as a function of coherence factor exhibits a stronger dependence on aberrations than for an aberration-free case.
ABSTRACT
Speckle image contrasts from a fractal rough-surface object are investigated in simulation, where the image surface is conjugate to the object surface. It is observed that the Hurst exponent H of fractal roughness affects speckle contrast and statistics dramatically. For example, a strong rough surface (sigma(h)>lambda) exhibits Rayleigh statistics over increasing ranges of point spread function widths as H decreases. Conceptual explanations are also presented for effects of H and the coherence factor on speckle in imaging systems.
ABSTRACT
Non-Gaussian speckle contrast from a phase-perturbed random object field in a spatially partially coherent system is simulated. A quasi-monochromatic extended incoherent source is modeled as a collection of independent point sources distributed on a regular grid. The source illuminates a phase screen object in a Kohler configuration. Speckle is calculated from the incoherent sum of irradiances in the image plane generated from the point sources. Simulated speckle contrasts are verified by an experiment with a fractallike rough surface distribution that is fabricated using a grayscale maskless lithography tool. Characteristics of the simulation method and physical quantities affecting speckle contrast are discussed.
ABSTRACT
Optical aberration effects up to the second moment of Gaussian laser speckle are theoretically investigated for both partially and fully developed speckle. In the development, a plane-wave illuminated diffuser generates a phase-perturbed random field in the object plane that creates speckle in the image plane. Theoretical derivations show that image field statistics are generally non-circular Gaussian due to aberrations. Speckle statistics are not affected by odd-functional aberrations, such as coma, and dependency of aberrations is asymptotically ignorable for very weak or strong diffusers. Furthermore, Gaussian speckle contrast as a functional of optical aberrations exhibits a stationary point for the aberration free condition, where apparently contrast does not achieve a local maximum. Calculations of speckle contrast for several aberration conditions are also presented.
ABSTRACT
We discuss how information available from ray-tracing techniques can be used to calculate optical forces and torques on particles. A general ray-trace computer code is augmented with the polarization and irradiance distributions of the illumination and Fresnel surface coefficients to give a reasonably accurate prediction of interaction with large particles out of the focal plane. Calculations of trapping location versus nonuniform illumination conditions are compared with an experiment. Other example calculations include trapping a hemispherical lens and a two-particle trap.
ABSTRACT
This paper gives a treatment for finding 3rd order aberrations in solid-immersion lenses (SIL) using spherical-aberration as the basis for a polynomial power expansion of the wavefront. Unlike previous work, the treatment is general for any incident and lens media, for any lens thickness, and any chief-ray specification. Using this treatment, a tolerance analysis is given with particular discussion on thickness tolerance and limitations on field of view. Major findings include very tight thickness tolerance for high-index hyperhemisphere SILs and a thickness tolerance window for hemispheres skewed to values less than the radius of curvature of the lens.
ABSTRACT
Image contrast enhancement, high lateral resolution, and height information are obtained with induced polarization evanescent imaging using a solid immersion lens. Experiments are conducted by imaging features on a patterned Si substrate. Imaging theory is used to predict optimum orientation of high-spatial-frequency samples, and a topographical image is derived from the induced polarization image through a calibration procedure. A numerical aperture of 1.5 is used in the experiment. Height accuracy of +/-2 nm is demonstrated with a known sample.
