Measurement of wave-front aberration in soft contact lenses by use of a Shack-Hartmann wave-front sensor

Lower- and higher-order wave-front aberrations of soft contact lenses were accurately measured with a Shack-Hartmann wave-front sensor. The soft contact lenses were placed in a wet cell filled with lens solution to prevent surface deformation and desiccation during measurements. Aberration measurements of conventional toric and multifocal soft contact lenses and a customized soft contact lens have proved that this method is reliable. A Shack-Hartmann wave-front sensor can be used to assess optical quality of both conventional and customized soft contact lenses and to assist in enhancing lens quality control.     

Spectral properties of multiorder diffractive lenses

Diffractive lenses have been traditionally designed with the first diffracted order. The spectral characteristics of diffractive lenses operating in higher diffracted orders differ significantly from the first-order case. Multiorder diffractive lenses offer a new degree of freedom in the design of broadband and multispectral optical systems that include diffractive optical elements. It is shown that blazing the surface-relief diffractive lens for higher diffraction orders enables the design of achromatic and apochromatic singlets. The wavelength-dependent optical transfer function and the associated Strehl ratio are derived for multiorder diffractive lenses. Experiments that illustrate lens performance in two spectral bands are described, and the results show excellent agreement with the theoretical predictions.     

Achromatic fourier processor with holographic optical lenses

An optical Fourier processor that allows the use of broadband light sources and colored inputs is designed, fabricated, and tested. We develop a design technique based on phase manipulation in the Fourier plane to construct an image processor that provides a chromatically corrected image making use of the good aberrations behavior of symmetrical optical systems. Only a small number of diffractive lenses and one achromatic refractive lens are required to obtain a real image. We verify our design experimentally using holographic lenses, which are presented, owing to their versatility, as a good alternative to expensive blazed diffractive elements.     

Refractive index variation in compression molding of precision glass optical components

Compression molding of glass optical components is a high volume near net-shape precision fabrication method. In a compression molding process, a variation of the refractive index occurs along the radial direction of the glass component due to thermal treatment. The variation of refractive index is an important parameter that can affect the performance of optical lenses, especially lenses used for high precision optical systems. Refractive index variations in molded glass lenses under different cooling conditions were investigated using both an experimental approach and a numerical simulation. Specifically, refractive index variations inside molded glass lenses were evaluated by measuring optical wavefront variations with a Shack-Hartmann sensor system. The measured refractive index variations of the molded glass lenses were compared with the numerical simulation as a validation of the modeling approach.     

Anamorphic white light Fourier processor with holographic lenses

Two anamorphic and achromatic Fourier processors were designed and constructed using diffractive and refractive cylindrical lenses. The diffractive lenses are holographic lenses recorded on silver halide material. In both processors the achromatic one-dimensional Fourier transform plane was obtained with two holographic lenses and one refractive cylindrical lens. The image with the same magnification in both directions at the output plane was formed with two different combinations of lenses. The differences between the two processors are analyzed, and in both cases the chromatic aberration in the Fourier plane and in the output plane is evaluated. Even though single cylindrical refractive lenses were used to image in one direction, good results were obtained.     

Lens design using group indices of refraction

Abstract

An approach to lens design is described in which the ratio of the group velocity to the speed of light (the group index) in glass is used, in conjunction with the more familiar phase index of refraction, to control certain chromatic properties of a system of thin lenses in contact. It is shown that, at the wavelength of a maximum or minimum (where the phase power of a lens is locally independent of wavelength), the group power is equal to the phase power. It is further shown that, in a lens consisting of three or more elements, the phase and group powers can be constrained to be both equal and independent of wavelength (achromatic) at one or more wavelengths. In the neighbourhood of such wavelengths, both the first and the second derivatives of phase power with respect to wavelength are zero, giving this type of lens (in principle) an exceptionally high degree of achromatism not previously described, herein called group achromatism. The first-order design of thin-lens systems is illustrated by examples with the help of a computer program incorporating the methods described.     

Effects of primary spherical aberration, coma, astigmatism and field curvature on the focusing of ultrashort pulses: homogenous illumination

We analyze the spatiotemporal intensity of pulses with durations of 20 fs and shorter and a carrier wavelength of 810 nm at the paraxial focal plane of an achromatic doublet lens. The incident pulse is well-collimated, and we use the Seidel aberration theory for thin lenses to evaluate the phase change due to the aberrations of the lens. In a set of cemented thin lenses with the stop at the lens, there is only spherical aberration, coma, astigmatism and field curvature, whereas the distortion aberration in the phase front is zero. We analyze the effect of these aberrations in the focusing of ultrashort pulses for homogenous illumination. We will show that the temporal spreading introduced by these aberrations in pulses shorter than 20 fs at 810 nm is very small but the spatial spreading is not, which reduces the intensity of the pulse considerably.     

Effects of primary spherical aberration, coma, astigmatism, and field curvature on the focusing of ultrashort pulses: Gaussian illumination and experiment

We analyze the spatiotemporal intensity of Gaussian temporal envelope pulses with initial durations of 200 fs and a carrier wavelength of 810 nm at the paraxial focal plane of an achromatic doublet lens for a well-collimated incoming pulse beam by using the Seidel aberration theory for thin lenses with the stop at the lens. We analyze the effect of these aberrations in the focusing of ultrashort pulses for Gaussian illumination and present experimental results for 200 fs pulses focused by a near-IR achromatic doublet.     

Active optics null test system based on a liquid crystal programmable spatial light modulator

We present an active null test system adapted to test lenses and wavefronts with complex shapes and strong local deformations. This system provides greater flexibility than conventional static null tests that match only a precisely positioned, individual wavefront. The system is based on a cylindrical Shack-Hartmann wavefront sensor, a commercial liquid crystal programmable phase modulator (PPM), which acts as the active null corrector, enabling the compensation of large strokes with high fidelity in a single iteration, and a spatial filter to remove unmodulated light when steep phase changes are compensated. We have evaluated the PPM's phase response at 635 nm and checked its performance by measuring its capability to generate different amounts of defocus aberration, finding root mean squared errors below λ/18 for spherical wavefronts with peak-to-valley heights of up to 78.7λ, which stands as the limit from which diffractive artifacts created by the PPM have been found to be critical under no spatial filtering. Results of a null test for a complex lens (an ophthalmic customized progressive addition lens) are presented and discussed.     

Achromatic hybrid refractive-diffractive lens with extended depth of focus

A method for designing achromatic hybrid refractive-diffractive elements that can produce beams with long focal depths while they preserve the entire aperture for capture of light and high transverse resolution is presented. Its working principle is based on the combination of a diffractive optical element that generates a long range of pseudonondiffractive rays and a refractive lens of opposite dispersion to form an achromatic hybrid lens. A hybrid lens with a fast f-number (f/1) that works in the entire visible wave band (400-700 nm) was designed and fabricated. Simulation results demonstrate a factor-of-10 improvement in depth of focus compared with that of a conventional f/1 lens, with matching 1-microm lateral resolution. Experimental results confirm the effectiveness of the proposed method through demonstration of an achromatic hybrid lens with better than a factor-of-7 improvement in depth of focus and 1-microm transverse resolution.     

Finite conjugate embedded relay lens hyperspectral imaging system (ERL-HIS)

We present a novel embedded relay lens hyperspectral imaging system (ERL-HIS) with high spectral resolution (nominal spectral resolution of 2.8 nm) and spatial resolution (30 μm×8 μm) that transfers the scanning plane to an additional imaging plane through the internal relay lens so as to alleviate all outside moving parts for the scanning mechanism used in the traditional HIS, where image scanning is achieved by the relative movement between the object and hyperspectrometer. The ERL-HIS also enables high-speed scanning and can attach to a variety of optical modules for versatile applications. Here, we also demonstrate an application of the proposed ERL-HIS attached to a microscopic system for observing autofluorescent images of sliced cancer tissue samples.     

Replicated arrays of hybrid elements for application in a low-cost micro-spectrometer array

Abstract

We report on the fabrication and characterization of replicated hybrid elements for low-cost micro-spectrometer array applications. An array of hybrid elements, where one surface combines the fairly large dimensions of refractive microlenses with the submicron features of a diffraction grating, was successfully replicated by hot embossing. The parameters are: lens diameter=990 μm, height=60 μm, grating period=1 μm, linewidth=300 nm and grating depth=2 μm. These replicated spectrometer elements showed a maximum resolution of 2.25 nm and a stray-light suppression better than 30 dB.     

Third-order dispersion effects generated by non-ideal achromatic doublets on sub-20 femtosecond pulses

Gaussian temporal envelope pulses with initial durations of 10?fs, 15?fs and 20?fs and a carrier wavelength of 810?nm were analyzed at the paraxial focal plane of non-ideal achromatic doublet lenses for well-collimated incoming pulses parallel to the optical axis. The wave vector is expanded up to third order, to investigate the effect of third-order group velocity dispersion on the pulse and the results are compared to those obtained when the wave number is expanded up to second order. The propagation time difference and the primary spherical aberration were included in the calculations using the thin lens approximation theory. Results are presented for a homogenous illumination beam.     

Compensation of chromatic errors in high na molded objective lenses

High NA molded glass objective lenses are commercially available for use in the 680 nm to 850 nm range. We show that a simple positive lens can be used with an objective lens to compensate for wavelength-induced aberration in the 400 nm to 700 nm range. There is a direct relationship between the optimum power of the corrector lens and the wavelength of interest. We also show that a single positive lens and a holographic optical element can be used with an objective lens to compensate for wavelength-induced aberration over a wide range of wavelengths.     

Design of Achromatic Anamorphotic Telescopes

Thin lens equations are given for the correction of the longitudinal and transverse chromatic aberrations of an anamorphotic telescope consisting of four thin lenses, and it is shown how a thin lens solution for such a system having a given Petzval sum and a given angular magnification in the first principal section, may be obtained. From this, two conditions, which the optical constants of the glasses of the lenses have to satisfy for simultaneous correction of the two chromatic aberrations and the two aberrations of astigmatism and distortion, resulting from the anamorphotic effect, are then derived. Finally a method for the selection of glasses which makes achromatic correction possible, is given.     

Probability of Failure of Sylgard-Mounted Optical Lenses in Head-Up Display During Thermal Cycle Testing

Lenses made from optical glass do not have a single characteristic strength. The strength of any optical glass is highly dependent on the area under stress and its surface finish. The nominal design strength of manufactured glass is typically in the 6.9–70 MPa range. This study presents an analysis of measured strain data and an approach to quantify the failure probability of optical lenses that are used in head-up display systems for aircraft applications when these lenses are subjected to accelerated thermal cycling. The thermal cycling produces thermal stress within the elastomerically mounted optical lens of the head-up display. We used a combination of experimental methods and finite element modeling to determine the thermal strain in the optical lens. The thermal strain values that were obtained using these methods were validated and were found to be in a cumulative distribution. This cumulative strain provides a quantitative estimate of the effects of thermal stress in the optical lens. We then determined the failure probability using Weibull parameters.     

Focal plane position detection with a diffractive optic for shack-hartmann wave-front sensor fabrication

During construction of a Shack-Hartmann wave-front sensor it is critical that the spacing between the lens array and the detector array be equal to the lens array focal length to obtain accurate and precise measurements of a wave front. This separation is often difficult to determine with large f/# lenses, because their focal spot diameter does not change substantially for small displacements on either side of the focal plane. We describe a method with an array of off-axis lens segments for determining the location of the focal plane. Because the lenses are off axis, changes in the distance from the optic to the detector array result in transverse focal spot position variations as a function of their separation from the lenses. By analyzing the focal spot pattern on a CCD, we achieved 12-mum rms error in the axial position measurement while moving a 4-mm-focal-length optic over 1 mm.     

Eliminating chromatic aberration in Gauss-type lens design using a novel genetic algorithm

Two different types of Gauss lens design, which effectively eliminate primary chromatic aberration, are presented using an efficient genetic algorithm (GA). The current GA has to deal with too many targets in optical global optimization so that the performance is not much improved. Generally speaking, achromatic aberrations have a great relationship with variable glass sets for all elements. For optics whose design is roughly convergent, glass sets for optics will play a significant role in axial and lateral color aberration. Therefore better results might be derived from the optimal process of eliminating achromatic aberration, which could be carried out by finding feasible glass sets in advance. As an alternative, we propose a new optimization process by using a GA and involving theories of geometrical optics in order to select the best optical glass combination. Two Gauss-type lens designs are employed in this research. First, a telephoto lens design is sensitive to axial aberration because of its long focal length, and second, a wide-angle Gauss design is complicated by lateral color aberration at the extreme corners because Gauss design is well known not to deal well with wide-angle problems. Without numbers of higher chief rays passing the element, it is difficult to correct lateral color aberration altogether for the Gauss design. The results and conclusions show that the attempts to eliminate primary chromatic aberrations were successful.     

Static and dynamic Fresnel zone lenses for optical interconnections

Abstract

Static and dynamic Fresnel zone lenses were fabricated in quartz glass by means of microstructuring techniques. Two types of on-axis and offaxis lenses with different focal lengths and of different apertures were designed to operate at wavelengths of 1·52 μm and 633 nm. The blazed profile of the onaxis and off-axis lenses was approximated by up to 16 and up to four discrete levels respectively. Dynamic, that is electrically switchable, lenses have been realized by filling the structured surface with liquid crystal. The optical properties of the lenses, such as the focal spot sizes and the diffraction efficiencies, were investigated. Further the switching behaviour of the dynamic lenses was studied. The design and fabrication of the static and dynamic, on-and off-axis Fresnel zone lenses as well as their optical and switching properties will be presented.     

Birefringent properties of diametrically loaded gradient-index lenses

Gradient-index (GRIN) lenses have been widely used as collimators in various fiber-optic sensors and as optical coupling devices in components designed for optical communication systems. However, relatively little attention has been paid to the birefringent properties of GRIN lenses and the potential for using them as photoelastic sensing elements in optical transducers. Analytical and experimental results are described that were obtained for the intensity distribution produced by studying a GRIN lens by using a polariscope. The residual birefringence inherent in an unloaded lens is initially studied. The lens is then assumed to be diametrically loaded and the superposition is studied by the method of ray tracing. When the results obtained from the simulation for a Selfoc, 0.25-pitch lens are compared with experimental data, an excellent agreement is obtained. Intensity increases monotonically with load, confirming that the lens would be a good choice for the sensing element of an optical transducer designed as part of a strain or acceleration measurement system. The numerical simulation is then used to study the influence of residual stress on sensitivity.