With new detectors that are capable of imaging across multiple wavelength bands, new methods need to be developed to reduce the lens count and improve performance across these multiple bands while minimizing the SWAP-c (Size, Weight, power and cost) of the system. One method that was proposed was using an update to the classical γν-ν diagram. This method which, uses instantaneous Abbe number and minimum dispersion wavelength to select materials that minimize the chromatic and thermal focal shift over the desired spectral region. A MWIR/LWIR lens was designed using this method to minimize the lens count. The lens has a continuous 3x zoom range. The lens was manufactured to determine the validity of the method that was used and to evaluate the new materials that are being developed. A comparison of the nominal design to the manufactured design is discussed. This includes a comparison of MTF performance.
Chalcogenide glasses have been steadily advancing infrared imaging capabilities and systems since the mid 1900s. Rochester Precision Optics has recently invested in bolstering their infrared glass manufacturing capabilities. While vertically integrating to reduce costs and to support the current and expanding demand for their precision glass molding, diamond turning, and assemblies that use the classic chalcogenide glasses; the optical design team has been able to capitalize on the new infrared materials further expanding the infrared optical glass map for S.W.A.P. enhancement in their designs.
The focus of this work is to highlight some of the capabilities and recent innovations in chalcogenide glass manufacturing leading to low cost methods of producing optical materials, elements, unique or previously difficult geometries.
Due to changes in the fictive temperature as a result of the precision glass molding process there is an induced change in the index of refraction. This can be on the order of 0.001 in oxide glasses and as high as 0.02 in the chalcogenide glasses. It is important to accurately define the expected index of refraction and the tolerance of it after molding as there may be an impact on the optical design tolerances and system performance. We report on the measured change in index of refraction in common chalcogenide glasses due to the Rochester Precision Optics (RPO) precision glass molding process. We will compare the change in index of refraction between as advertised, as measured, as molded, and we will look at post mold annealing recovery. Utilizing an upgraded M3 refractometer we will be able to measure the index from the visible to the LWIR.
One of the difficulties in designing infrared optical systems is the comparative lack of glasses from which to design lenses. In visible optical systems, the designer has a palette of hundreds of glass options with varying dispersions and mechanical properties. In contrast, the designer of infrared optical systems has perhaps a dozen materials options from which to choose.
Instead, what if the infrared transparent materials were designed specifically for various applications? Using a material with a targeted index dispersion profile, the designer can complete a system using fewer lens surfaces and in many cases with increased functionality such as athermalization.
Next comes the question of how to obtain such a material. One approach is somewhat scattershot: to melt series of glasses, measure each of their properties, and settle on one composition for scale-up to production volumes. This approach is both time- and resource-consuming, as the measurements for many properties require specialized equipment and sample preparation.
In contrast to this scattershot method, the principle of intelligent material design allows glass scientists to design glasses with intentionally chosen mechanical and optical properties, and greatly reduces the number of test melts required to obtain a final production solution. Intelligent material design consists of leveraging the existing literature data to make informed decisions about which glass compositions are likely to exhibit the desired properties. By describing the variation of the properties over the glass family with mathematical functions, the material design problem is reduced to the simultaneous solution of a set of equations.
We show successful printing of chalcogenide glass using two different techniques. Additive manufacturing is still a fairly new field, but is increasing rapidly. We compare some of the first tests of selective laser melting and direct laser processing techniques to chalcogenide glass.
Single aperture multispectral systems are becoming prevalent thanks to advances in multispectral detectors, new optical materials, and new methods for selecting materials that minimize chromatic and thermal focal shift. This design study focuses on design of a three field-of-view, multispectral lens operating across the MWIR and LWIR spectral regions. The lens in question will have an f-number of f/3 with a 3X zoom ratio. The narrow full field-ofview of the lens is 3.33° with a wide full field-of view of 9.99°, the length of the system is 163 mm. The performance goal for the lens is diffraction limited over the thermal region. The study will provide an overview of material selection using an updated γv-v diagram, to provide achromatic and athermal characteristics. The study will then step through first order layout, optimization with key constraints, and tolerancing for manufacturability. Finally, the study will provide detailed analysis of system performance including as-built MTF over temperature, aberration analysis, and NETD contributions from narcissus.
With the move to more and more lightweight and cost-effective design, a move to multiband or multi-spectral optics is required. These systems are becoming more prevalent in the market as new detector technologies have been developed. However, the lens designs are only starting to be considered with the addition of new materials in the MWIR and the LWIR. For the VIS/SWIR region the designs have been possible, but a lack of detector technology has resulted in few designs being considered for actual manufacturing. These designs are also difficult due to changes in the Abbe number in the different wavebands. Where the glass map is robust in the visible region, there exists a lack of crown glasses in the SWIR, and one is left with mostly flint glasses. This proves challenging from a chromatic perspective. The challenge becomes even more difficult if one wants to incorporate athermalization.
Recently, optical materials have been developed by Schott and NRL to improve material selection in the SWIR, MWIR, and LWIR wavelength regions. In addition, new multiband detectors are reaching maturity, leading to a natural push for common aperture lens systems. Detectors that can span the SWIR/MWIR, MWIR/LWIR or SWIR/MWIR/LWIR wavelengths regions will require complex optical systems to effectively utilize their full potential. Designing common aperture wide-band systems that are both achromatized and passively athermal, especially while maintaining SWAP-c (size, weight, power and cost), poses significant challenges. Through use of the updated γν-ν diagram, which provides guidance on material combinations that both achromatize and athermalize, part of that challenge is reduced. This updated γν-ν diagram uses instantaneous Abbe number and peak wavelength. The instantaneous Abbe number is a function of wavelength and is the scaled reciprocal of the instantaneous dispersion. The instantaneous Abbe number is defined at the peak wavelength, which occurs when the second derivative of the index of refraction goes to zero. Three examples will be presented using this updated athermal/achromatic glass map to demonstrate its effectiveness. These design examples will include a SWIR/MWIR design, a MWIR/LWIR design and, a SWIR/MWIR/LWIR design.
Over the past few years, new detector technologies have enabled multiband detection through a single aperture. This creates significant SWAP advantages (size, weight and power) and has spurred significant interest in multiband optics (for instance SWIR/MWIR, MWIR/LWIR, etc.). However, due to the small number of materials available in the infrared regions, passive optical athermalization and achromatization can be challenging even over single waveband. This becomes even more challenging in the case of multiband optics. One method for determining appropriate material combinations for athermalization and achromatization is use of a y ∗ v vs. v diagram. We examine an updated form of the y ∗ v vs. v diagram using instantaneous Abbe number. While Abbe number is an effective metric for dispersion within single bands, it becomes less reliable when extended to wider wavelength ranges. Instantaneous Abbe number allows for a wider waveband to be defined, without a loss of generality; and this allows for an updated definition of the y ∗ v vs. v diagram for the development of multiband optics. We present an example of a multiband lens as well as compare the typical definition of Abbe number with instantaneous Abbe number to determine the validity of the updated model.
With the move to smaller pixel sizes in the longwave IR region there has been a push for shorter focal length lenses that
are smaller, cheaper and lighter and that resolve lower spatial frequencies. As a result lenses must have better correction
for both chromatic and monochromatic aberrations. This leads to the increased use of aspheres and diffractive optical
elements (kinoforms). With recent developments in the molding of chalcogenide materials these aspheres and kinoforms
are more cost effective to manufacture. Without kinoforms the axial color can be on the order of 15 μm which degrades
the performance of the lens at the Nyquist frequency. The kinoforms are now on smaller elements and are correcting
chromatic aberration which is on the order of the design wavelength. This leads to kinoform structures that do not
require large phase changes and therefore have 1.5 to just over 2 zones. The question becomes how many zones are
required to correct small amounts of chromatic aberration in the system and are they functioning as predicted by the lens
design software? We investigate both the design performance and the as-built performance of two designs that
incorporate kinoforms for the correction of axial chromatic aberration.
A method for performing optical beam shaping in the near-field region using diffractive optical elements generated by
Fresnel based Phased Optimised General Error Diffusion algorithm (POGED) was developed and investigated by means
of numerical simulations. POGED was found to deliver significantly higher signal to noise ratio than iterative
Gerchberg-Saxton type algorithm.
The transposition of signals in space is fundamental to the optical interconnection of electronic systems. Previous free-space implementations employed a scheme based on imaging systems that are prone to aberrations. The system proposed here is useful for implementation of a three-stage Clos network, an optical transpose sector switch, a reconfigurable optical transpose system, and an optical cross connect switch. The purpose of this paper is to propose a non-imaging system for an optical transpose interconnect system, where a macrolens is inserted between the two mesolenses arrays, at the centre. The proposed macrolens is a Fourier transform lens system designed to be virtually aberration free.