Boyd V. Hunter

Optical design with inhomogeneous glass: the future is here

For many years optical designers have been intuitively aware of the value and potential that an inhomogeneous refractive index distribution can bring to the manufacture of precision optical instruments. Even so, designers have been cautious when considering lens designs with inhomogeneous glass, partly because of design difficulties, but mostly because of the need for controlled and reliable materials. In this paper we demonstrate the feasibility of graded index lenses by addressing the index control requirements that are needed for a diffraction limited lens. We chose for our analysis a rather stressing case: an F/1.5 plano-convex singlet. A general analytic expression for the index of refraction is developed for a perfect axial gradient lens (single color, on axis). Index errors were then added to the perfect index and the lens evaluated for wavefront quality. We found that index errors on the order of 1.6x10-3 rms produced aberrations of 0.04 waves rms, which is within the bounds of a diffraction limited lens. LightPath now routinely fabricates glass with much less index variation, making feasible the fabrication of repeatable diffraction limited lenses with inhomogeneous glass.

How to design and tolerance with GRADIUM glass

Designing with axial-gradient materials can be a complicated task. The difficulties range from the speed of ray-tracing codes and the mechanics of specifying the material and appropriate variables to selecting the best gradient and orientation rom a set of fixed profiles. We propose a simple methodology for designing with axial-gradient glasses in modern ray-tracing codes. The first step is to determine locations where the gradient can be useful. This decision may be made by probing a design with aspheres or by analysis of the design to decide what needs to be corrected. The second step is to modify the design for appropriate base materials. GRADIUMTM lenses act as correctors in the optical system and the first-order optical properties still must be controlled in the normal manner. The third step is to design the optimal gradient for the applications. While the designer will only have the option of designing the gradient for actual use in a very limited set of cases, understanding the shape of the ideal gradient will allow the designer to select the profile and orientation that most closely matches the ideal. Then the designers can work on best implementing the design and fine-tuning the design. Tolerancing and preparation of the GRADIUM lens print require only a few additional steps and understanding of how the material is fabricated. For example, the maximum profile thickness is nominal and may not correspond to the physical dimensions of a blank, such as when a blank is pre- thinned.

Properties, specifications, and tolerances of GRADIUM glasses

Commercially available GRADIUM® glasses present lens designers with new freedoms to increase performance or reduce the lens count, weight, and cost of optical systems. These glasses possess an axial gradient through the entire glass thickness with large changes in refractive index (?n), dispersion (?v), or other properties. GRADIUM glass lenses containing large refractive index gradients are especially powerful for reducing aberrations in both monochromatic and chromatic lens systems. The purpose of this paper is to explain the general properties of GRADIUM glasses, how these glasses and lenses are manufactured, and the specifications and tolerances of the glasses and lenses. Using GRADIUM glass lenses is very straightforward; the lenses are fabricated with spherical surfaces and used like homogenous (single index) lenses. Comparisons between the theoretical design and actual lens performance for several commercial lenses are presented.

Differential excitation spectroscopy for detection of common explosives: ammonium nitrate and urea nitrate

Differential Excitation Spectroscopy (DES) is a new pump-probe detection technique (patent-pending) which characterizes molecules based on a multi-dimensional parameterization of the rovibrational excited state structure, pump and probe interrogation frequencies, as well as the lifetimes of the excited states. Under appropriate conditions, significant modulation of the ground state can result. DES results provide a unique, simple mechanism to probe various molecules. In addition, the DES multi-dimensional parameterization provides an identification signature that is highly unique and has demonstrated high levels of immunity from interferents, providing significant practical value for high-specificity material identification. Ammonium nitrate (AN) and urea nitrate (UN) are both components commonly used in IEDs; the ability to reliably detect these chemicals is key to finding, identifying and defeating IEDs. AN and UN are complicated materials, having a number of different phases and because they are molecular crystals, there are a number of different types of interactions between the constituent atoms which must be characterized in order to understand their DES behavior. Ab initio calculations were performed on both AN and UN for various rovibrational states up to J’ ≤ 3 and validated experimentally, demonstrating good agreement between theory and experiment and the very specific responses generated.

Differential excitation spectroscopy for detection of chemical threats: DMMP and thiodiglycol

Differential Excitation Spectroscopy (DES) is a new pump-probe detection technique (patent-pending) which characterizes molecules based on a multi-dimensional parameterization of the rovibrational excited state structure, pump and probe interrogation frequencies, as well as the lifetimes of the excited states. Under appropriate conditions, significant modulation of the ground state can result. DES results provide a unique, simple mechanism to probe various molecules. In addition, the DES multi-dimensional parameterization provides an identification signature that is highly unique and has demonstrated high levels of immunity from interferents, providing significant practical value for highspecificity material identification. Dimethyl methylphosphonate (DMMP) is used as a simulant for G series nerve agents and thiodiglycol as a simulant for sulfur mustard (HD). Ab initio calculations were performed on DMMP for various rovibrational states up to J’ ≤ 3 and validated experimentally, demonstrating good agreement between theory and experiment and the very specific responses generated. Thiodiglycol was investigated empirically. Optimal detection parameters were determined and mixtures of the two materials were used to demonstrate the immunity of the DES technique to interference from other materials, even those whose IR spectra show significant overlap.

Improved characterization of GRADIUM gradient-index glasses

Last year we presented the first experimental measurements of the dispersion in GRADIUMTM glasses. These measurements provided a glimpse into the materials properties that had previously been approximated. Unfortunately, although these valuable measurements provided broad spectrum information about the material, these measurements did not provide the precision required for many white-light applications. Furthermore, it was clear last year that the modeling must be improved because of the need to accurately model the material over the natural transmission window of the glass. Therefore, LightPath has undertaken a program to provide improved experimental characterization of current and new GRADIUM materials as well as improved modeling. Experimental data will be available soon. We will therefore evaluate the accuracy and limits of dispersion modeling using the modified Sellmeier approach. We will also present some empirical guidelines for the number of coefficients required for high-precision, wide spectral band modeling.