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.
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.
Bi-AGRIN cemented doublets, super corrected for zero axial color, spherical aberration and sphero-chromatism can show polychromatic performance in the range of 0.004 waves PV or better at the red and blue wavelengths for speeds up to F/2. These doublets are comprised of two elements of axial gradient index glass. The crown and flint elements are each designed with separate and distinct gradient glass lines, giving each element an axial gradient in refractive index and dispersion. This paper examines one design and its performance sensitivity to dispersion modeling via the Buchdahl and Sellmeier dispersion equations.
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.
LightPath has continued efforts to improve the range of uses for GRADIUMTM glass. New glass lines as well as new geometries have been demonstrated. LightPath continues to work with software vendors to improve ease-of-use. At the 1998 CLEO, a lab sample of a transverse gradient (a.k.a. `biaxial') was shown. New products are expected soon. Laboratory work has demonstrated the viability of other glass families. A GTi (GRADIUM Titania) family has been developed as well as a GSI (GRADIUM Spectrally Invariant) material, based on US Patent #5,689,374, also for use as an achromatic transverse gradient. The new GLaK glass line has one existing profile currently ready and will be publicly released this summer. Considerable efforts have also been made at improving the characterization of GRADIUM glasses, including more accurate profile measurements and investigations of appropriate techniques for modeling the dispersion. The improved accuracy is required for apochromatic systems. Finally, as a part of the ongoing process improvement effort, LightPath has formed a strategic partnership with Hikari glass. This relation ship will allow LightPath to have better control over the incoming glass supply and have a glass supply that is better suited for production of GRADIUM glasses. This change will alter the existing profiles; redefinitions will be available this summer.
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.
We demonstrate a four channel integrated wavelength division multiplexer (WDM) and demultiplexer (WDDM) based on volume holographic gratings and substrate-guided waves at near IR wavelengths. The four operating wavelengths are centered at 750, 780, 810 and 840 nm respectively. The WDM and WDDM are demonstrated using 50/125 multimode fibers. The channel-to- channel crosstalk level is measured to be less than -40 dB. The system insertion losses are -23 dB, -21 dB, -20 dB, -22 dB respectively for 750 nm, 780 nm, 810 nm and 840 nm.
We report a four channel integrated wavelength division multiplexer (WDM) and demultiplexer (WDDM) based on volume holographic gratings and substrate-guided waves at near IR wavelengths. The four operating wavelengths are centered at 750, 780, 810 and 840 nm respectively. The WDM and WDDm are demonstrated using 50/125 multimode fibers. The channel-to- channel crosstalk level is measured to be less than -40 dB. The system insertion losses are -23dB, -21dB, -22dB respectively for 750 nm, 780 nm, 810 nm and 840 nm.
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.
LightPath has been actively commercializing a family of axial-gradient glasses called the GSF glasses. These glasses have the same dispersion and refractive indices found in the conventional SF glasses. Therefore, in polychromatic design, it has only been possible to apply the extra degrees of freedom offered by GRADIUM® to negative elements by replacing a flint glass. This has shown that GRADIUM can significantly and positively affect polychromatic designs. For example, a seven element double Gauss lens was redesigned with GRADIUM, resulting in a six element design and a 27% improvement in rms spot size.1 Similarly, the Kodak Ektapro varifocal projection lens was redesigned reducing the element count from seven to five. Despite these impressive results, since most of the work done by any polychromatic system is done by positive elements— typically crowns, it is clear that the real potential of GRADIUM to polychromatic design could not be unleashed until GRADIUM crown materials were available. The GRADIUM GK glass family is one of several GRADIUM crown families under development. A number of examples will be presented to illustrate the advantages realized in optical design when GK glasses are used. Simple doublets as well as more complex multi-element systems will be discussed, some of which use both GK and GSF glasses and others which use only GK or GSF glasses.
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.
Precise optical design requires detailed knowledge of the refractive index. GRADIUM® glasses further complicate these calculations because the refractive index is a function of axial position within the lens blank. LightPath has used a 16-coefficient Buchdahl dispersion model to allow the computation of the appropriate profile as a function of wavelength. The initial coefficients published for the GSF glass family relied on published data for base glasses and were restricted to the visible regime.1 There are many opportunities for use of GRADIUM materials in the NIR. Because there was no experimental data available to confirm the dispersion of these materials in this regime, experiments were conducted to measure the refractive index profiles of various GRADIUM glasses in the visible through the NIR. With the availability of this information, GRADIUM glasses can be applied to NIR imaging, communications, or other applications requiring information about the refractive index profile at these wavelengths. From these experiments of refractive index versus axial position as a function of wavelength, a revised dispersion model is presented for the GSF glass family. In addition, a new crown GRADIUM glass, which is under development, were also tested.
One of the advantages offered by visible and NIR lasers over carbon-dioxide lasers is that they can be delivered through optical fibers. Fiber-optic beam delivery is ideal when the beam must be delivered along a complex path or processing requires complicated manipulation of the beam delivery optics. Harnessing the power of a high-power laser requires that knowledgeable and prudent choices be made when selecting the laser and its beam delivery system. The purpose of this paper is to discuss a variety of issues important when designing a beam delivery system; data obtained with high power Nd:YAG lasers will be used as illustrative examples. (1) Multimode optical fibers are used for high-power applications. The fiber imposes, to varying degrees, a structure on the beam that is different from the laser output. Fibers degrade the beam quality, although the degree of degradation is dependent on the fiber length, diameter and type. Smaller fibers tend to produce less degradation to beam quality, but the minimum usable fiber size is limited by the quality of the laser beam, focusing optic and the numerical aperture of the fiber. (2) The performance of the beam delivery system is ultimately determined by the quality of the optics. There for, well- corrected optics are required to realize the best possible performance. Tests with both homogeneous and GRADIUMTM lenses provide insights into evaluating the benefits offered by improvements in the output optics from gradient-index, aspheric and multi-element lens systems. Additionally, these tests illustrate the origins of variable focused spot size and position with increasing laser power. (3) The physical hardware used in the beam delivery system should have several characteristics which enhance its functionality and ease of use, in addition to facilitating the use of advanced diagnostics and monitoring techniques.
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