Biomedical subjects
Publications and source records attributed to Eric G Johnson.
A coronagraph designed for high contrast imaging applications has been experimentally tested using coherent laser light and a vortex mask of topological charge m=2. Intensity contrast values of 95% were achieved in this first verification of the scheme. Improvements for approaching the theoretical value of 100% are suggested.
We summarize a novel integrated wavelength-stabilization scheme for broad stripe surface-emitting lasers. The method is based on two gratings fabricated on opposite sides of a device in which the first grating disperses light through the substrate to the opposite side, where the second surface has a feedback grating that operates under total internal reflection and in the Littrow condition to provide feedback into the gain medium. Experimental results have been obtained for both high power and a narrow linewidth, showing a CW slope of 0.85 W/A.
Fabrication of a thick analog profile with photoresist is a difficult task in photolithography. We demonstrate that a binary phase-grating photomask with an appropriate period and duty cycles is capable of manipulating the exposure illumination in an analog fashion and can be used for fabrication of the desired analog micro-optics profiles on the surface of a thick photoresist. By choosing the proper period and variation of duty cycle of the phase-grating mask, one can create the desired analog intensity of exposure illumination for an optical stepper. This allows the formation of a wide range of analog micro-optics profiles with an SPR 220-7 photoresist. The numerical convolution of the diffraction efficiency curve and resist exposure characteristics is used to predict the final resist profile and also to design the appropriate duty-cycle distribution for the binary phase grating. As a demonstration of this technology, we fabricated a variety of micro-optical elements, such as a positive lens, ring lens, prism, and vortex of approximately 100-200 microm diameter, by using a phase-grating mask fabricated in a poly(methyl methacrylate) electron-beam resist.
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We investigate a novel surface-emitting superluminescent diode configuration that uses two detuned grating outcouplers to suppress lasing. This device exhibits a shaped beam with a peak power of 1.5 W quasi-continuous wave with an 11 nm bandwidth centered on 970 nm.
The utilization of phase-mask technology for the fabrication of an analog micro-optics profile with a thick photoresist was investigated. A two-dimensional phase-grating mask with pi phase depth can produce a desired analog variation of exposure intensity, which allows one to vary the thickness of an analog photoresist after its exposure by a photolithographic stepper and development of the photoresist. A two-dimensional phase-grating mask of square pixels was simulated, designed, and fabricated. The fabrication of analog micro-optics in a thick SPR-220 photoresist by use of this phase mask was also demonstrated.
An innovative fabrication technique is introduced that is based on multiple-exposure techniques for micro-optics fabrication. This approach is compatible with conventional lithography systems used in integrated circuit manufacturing and can be applied to thick and thin photoresists and is based on additive lithographic techniques introduced elsewhere [Appl. Opt. 41, 6176 (2002)]. We chose a simple subdomain basis set to transform the two-dimensional basis patterns into a family of various three-dimensional micro-optic elements using exposure control to modulate the third dimension. We demonstrate the capability to sculpt the photoresist into a variety of three-dimensional micro-optic elements by performing multiple exposures using elements from the subdomain basis set, without resorting to multiple etching steps.
A comprehensive and fully three-dimensional model of holographic lithography is used to predict more rigorously the geometry and transmission spectra of photonic crystals formed in Epon SU-8 photoresist. It is the first effort known to the authors to incorporate physics of exposure, postexposure baking, and developing into three-dimensional models of photonic crystals. Optical absorption, reflections, standing waves, refraction, beam coherence, acid diffusion, resist shrinkage, and developing effects combine to distort lattices from their ideal geometry. These are completely neglected by intensity-threshold methods used throughout the literature to predict lattices. Numerical simulations compare remarkably well with experimental results for a face-centered-cube (FCC) photonic crystal. Absorption is shown to produce chirped lattices with broadened bandgaps. Reflections are shown to significantly alter lattice geometry and reduce image contrast. Through simulation, a diamond lattice is formed by multiple exposures, and a hybrid trigonal-FCC lattice is formed that exhibits properties of both component lattices.
A broad-area semiconductor laser diode with two monolithically integrated optical elements is presented. A 275-nm period detuned second-order diffraction grating on the p-doped side layer is combined with a refractive lens transferred into the GaAs substrate so that outcoupling and beam-shaping functions are decoupled. The high-power device produces a circular spot and demonstrates the potential of dual optics integration in broad-area semiconductor diodes.
In optical lithography the degradation of image quality due to aberrations present in the exposure tool is a serious problem. Therefore it is desirable to establish a reliable aberration measurement procedure based on the analysis of printed images in the photoresist. We present what is to our knowledge a new method for characterizing the aberrations of an exposure tool using a hybrid diffractive photomask. By utilizing each different impact on the aberrated image from each diffracted illumination, we were able to extract the aberration present in the stepper system. We experimentally verified this method with a G-line stepper and verified its spherical aberration astigmatism.
Fabrication of micro optics for fiber optics applications is a challenge due to their size and the issues associated with alignment of the optics to single-mode fibers. This study summarizes a method for fabricating diffractive optical elements on the ends of coreless fiber segments for passive alignment to single-mode fibers. Results are presented for passively aligned diffractive lens elements used for both collimation and beam shaping.
PURPOSE: The purpose of this pilot study was to compare disability self-assessment and upper quarter muscle balance female dental hygienists and non dental hygienist females. The upper quarter was operationally defined as the shoulder and neck region. Muscle balance was operationally defined as muscle flexibility and muscle performance. METHODS: A convenience sample of 41 working dental hygienists and 46 non dental hygienists participated in the study. Muscle flexibility of the upper quarter was measured by inclinometry or standard muscle length testing. Muscle performance was measured by timing the duration of four statically maintained positions. Subjects filled out the Northwick Park Neck Pain Questionnaire (NPNPQ), which is a disability self-assessment. Analysis of Covariance (ANCOVA) was used during data analysis to adjust for the mean age difference between the dental hygienist group (38.0 years) and the non-dental hygienist group (29.3 years). RESULTS: The results of this pilot study suggest that female dental hygienists are more likely than non dental hygienist females to develop tightness in the upper trapezius (p = 0.007) and the levator scapula (p = 0.01) of the non dominant upper quarter and lower fibers of the pectoralis major of the dominant upper quarter (p = 0.03) Muscle performance trends in the dental hygienist group supported muscle balance theory that short muscles remain strong while lengthened muscles become weak. The dental hygienist group had higher disability scores in all nine parts of the NPNPQ compared to the non-dental hygienist group, five of which were statistically significant (p < 0.05). CONCLUSION: The results of this pilot study suggest that muscle imbalances in the upper quarter are more common in female dental hygienists than in female non dental hygienists and may contribute to the numerous upper quarter pathologies associated with the practice of dental hygiene. Further research is needed to determine if upper quarter strengthening and flexibility exercises performed by dental hygienists can reduce disability self-assessment.
An innovative fabrication technique is introduced that is based on multiple-exposure techniques for micro-optics fabrication. This method utilizes various exposure times and combinations of binary and analog photo masks to sculpture complex photoresist profiles. It also demonstrates the fabrication of analog structures from the multilevel structures thus formed by using resist reflow.