This PDF file contains the front matter associated with SPIE Proceedings Volume 10181, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

There is a strong desire to reduce size and weight of single and multiband IR imaging systems in ISR operations on hand-held, helmet mounted or airborne platforms. NRL is working on developing new IR glasses that transmit from 0.9 to > 12 µm in wavelength, with refractive index ranging from 2.38 to 3.17, to expand the glass map and provide compact solutions to multispectral imaging systems. These glasses were specifically designed to have comparable glass molding temperatures and thermal properties so that they can be laminated and co-molded into optics with reduced number of air-glass interfaces (lower Fresnel reflection losses). These new NRL glasses also have negative or very low dn/dT, making it easier to athermalize the optical system. Our multispectral optics designs using these new materials demonstrate reduced size, complexity and improved performance. This presentation will cover discussions on the new optical materials, multispectral designs, as well fabrication and characterization of new optics.

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.

Drag force effect is an important aspect of range performance in missile applications especially for long flight time. However, old fashioned gimbal approaches force to increase missile diameter. This increase has negative aspect of rising in both drag force and radar cross sectional area. A new gimbal approach was proposed recently. It uses a beam steering optical arrangement. Therefore, it needs less volume envelope for same field of regard and same optomechanical assembly than the old fashioned gimbal approaches. In addition to longer range performance achieved with same fuel in the new gimbal approach, this method provides smaller cross sectional area which can be more invisible in enemies’ radar. In this paper, the two gimbal approaches - the old fashioned one and the new one- are compared in order to decrease drag force and radar cross sectional area in missile application. In this study; missile parameters are assumed to generate gimbal and optical design parameters. Optical design is performed according to these missile criteria. Two gimbal configurations are designed with respect to modeled missile parameters. Also analyzes are performed to show decreased drag force and radar cross sectional area in the new approach for comparison.

A low-loss, high-speed optical phased array (OPA) has been designed and fabricated. Two different platforms have been utilized in combination to leverage electro-optic (EO) tuning. A lithium niobate (LiNbO₃) optical phased array was fabricated and used in conjunction with a silicon nitride (Si₃N₄) 8x8 waveguide array that condenses the output pitch and utilizes the Triplex^TM waveguide technology. This OPA allows for the non-mechanical beam steering (NMBS) of 1550 nm light on an edge coupled optic platform and takes advantage of the high electro-optic coefficient and high speed capability of LiNbO₃ for electro-optic phase tuning. This coupled OPA has an overall insertion loss of ~3.5 dB which is advantageous to silicon-on-insulator OPAs that have shown overall insertion losses of ~14 dB. To characterize and tune this device, a 3 lens imaging system was employed to produce both near- and far- field intensity patterns of the output of the OPA on a static image plane. At the image plane, a high resolution infrared camera was used to observe the resulting intensity pattern. The control software for tuning the OPA reads the intensity incident at a specified position on the detector array, and has a PWM interface to drive the electro-optic phase controls. Beam steering was accomplished using an iterative tuning algorithm.

Previous work developed a first-order theory for picking optimal pairs of materials for gradient index (GRIN) achromatic singlets. This work extends that concept to include the addition of a third material to a GRIN blend, to improve performance further. Several ternary-based GRIN lens designs are compared to binary versions. Implications for material development in gradient index optics are discussed.

Military infrared systems generally must exhibit stable optical performance over a wide operating temperature range. We present a model for the first-order optical design of radial gradient-index systems, based on a form of the thermo-optic glass coefficient adapted to inhomogeneous material combinations. We find that GRIN components can significantly reduce the optical power balance of athermal, achromatic systems, which introduces the scope for a new class of broadband infrared imaging solutions. This novel first-order modelling technique is used to generate a starting point for optimisation of a SWIR/LWIR multispectral optical design.

Starting with a literature-based design for a rifle scope, we demonstrate the potential to replace large, heavy glass lenses with polymer-based GRIN lenses with little optical penalty, while saving 50% of the lens weight in the system. Several properties of the primary design are chosen to favor manufacturability, such as leaving glass for the external surfaces and choosing polymers and GRIN geometries based on proven fabrication techniques. Compared to the reference design which weighed 154 g, the GRIN-substituted design weighed 78 g while maintaining visually constant performance from 0-40°C.

Infrared (IR) transmitting gradient index (GRIN) materials have been developed for broad-band IR imaging. This material is derived from the diffusion of homogeneous chalcogenide glasses has good transmission for all IR wavebands. The optical properties of the IR-GRIN materials are presented and the fabrication and design methodologies are discussed. Modeling and optimization of the diffusion process is exploited to minimize the deviation of the index profile from the design profile. Fully diffused IR-GRIN blanks with Δn of ~0.2 are demonstrated with deviation errors of ±0.01 refractive index units.

Previous work identified a lens design in which a polymer GRIN element was able to simultaneously correct for firstorder color and temperature variations of a glass singlet. This work presents a materials-based theory, rooted in paraxial optics, that explains this correction and identifies relationships among the GRIN and glass materials which must hold for the correction to be effective. This result provides a path for using polymer optics, with their relatively large thermophysical and thermo-optic coefficients, in passively corrected optical systems over significant temperature ranges.

Changes in the position of best focus over temperature are a major source of contrast degradation in the long-wave infrared. The prime sources of this focus shift are the difference between thermal expansion coefficients of lens material and housing material, and the change in refractive index over temperature ∂n/∂T. These parameters, combined with the limited depth of focus when using lenses for uncooled detectors, can rapidly degrade performance with changing temperature. Firstorder paraxial calculations to model these changes are discussed, with a demonstration of its application to single-element imaging systems. The model is then expanded to include two-element systems where both elements are made of the same optical material, or the more general case where different materials are combined. It is shown how a chalcogenide glasses are well suited for athermalization, and how a combination of material choice and optical prescription can lead to an improved passive optical athermalization scheme, i.e. stable performance over temperature with no moving components. The limits of the used model are discussed and examples given for various focal lengths.

Compact multispectral imagers often require lightweight, inexpensive, flat beamsplitters working in convergent light. Unfortunately, the use of tilted plane parallel plates (PPPs) as an optomechanical design technique is severely restricted due to the aberrations that PPPs generate in non-collimated spaces. We propose a digital correction method of the image blur introduced by tilted flat beamsplitters working in convergent light. The deconvolution-based results are compared against some known non-symmetric aberration correctors advocated by classic, optical hardware based solutions. This method offers a promising solution for reduction of size, weight and cost of high-performance compact multispectral imaging systems.

Lenses for staring-array point-source detection sensors must maintain good signal-to-noise ratio (SNR) over fields of view often exceeding 100 degrees. Such lenses typically have f-θ distortion to provide constant solid angle sampling in object space. While the relative illumination calculation is often used to describe flux transfer from a Lambertian extended object for imaging applications, maximizing SNR for point-source detection depends primarily on maximizing collected irradiance at the entrance pupil, the shape of which can vary dramatically over field. We illustrate this field-dependent SNR calculation with an example lens and outline the calculations needed to derive a simple aberration-based expression for the field dependence of point-source SNR.

We describe a case study in which a telescope system, originally designed for a large format, visible camera, needed MWIR imaging capabilities while maintaining its original setup. The dedicated telescope system was adapted to share its existing optics with a new imaging module via a skew path concept. The challenges of non-rotationally symmetric design are explored along with an explanation of the methodology used to analyze and address the unique configuration.

Infrared-transmitting optics used in imaging systems have high refractive indices (n=1.4 to n > 3) that require antireflective (AR) coatings. These coatings have limitations in that they can delaminate in operational environments, which is a problem particularly for broadband coatings that consist of multiple layers of dissimilar materials. In addition, residual reflections within an imaging system can cause ghost reflections, degrading performance. Recently, new methods have been developed for fabrication of anti-reflective surface structures (ARSS) on optics that significantly reduce reflection losses at the surface. The ARSS approach provides a more robust solution by using surface structures built directly into the actual surface of the optics, without the need for a coating with extraneous materials. We present recent results that demonstrate superior ARSS performance on a variety of optics for use in the infrared spectral region. These materials have been successfully patterned with ARSS using reactive ion etching (RIE) or using photolithography and etching. We report on reflection losses as low as 0.02% for fused silica at 1.06 microns, and have also demonstrated low reflection losses for ARSS on germanium, spinel ceramic, and sapphire, all of which are important for mid- to long-wave infrared imaging applications.

In this paper we show how to systematically design anti-reflective metasurfaces for the mid-infrared wavelength range. To do so, we have utilized a multilayer arrangement involving a judiciously nano-perforated surface, having air holes, arranged in a hexagonal fashion. By exploiting an effective medium approach, we optimized the dimensions of the surface features in our design. Here, we report a broadband reflectivity 3.5 − 5.5 μm that is below 10% over a broad range of incident angles 0⁰ ≤ θ_𝑖 ≤ 70⁰ , irrespective of the incident polarization (TE, TM). Our experimental results are in excellent agreement with full-wave finite element simulations. This systematic approach can be used to design a wide variety of patterned metasurfaces, capable of controlling the phase of the incident optical field.

Pulse magnetron sputtering is very well suited for the deposition of optical coatings. Due to energetic activation during film growth, sputtered films are dense, smooth and show an excellent environmental stability. Films of materials like SiO₂, Al₂O₃, Nb₂O₅ or Ta₂O₅ can be produced with very little absorption and scattering losses and are well suited for precision optics. FEP's coating plant PreSensLine, a deposition machine dedicated for the development and deposition of precision optical layer systems will be presented. The coating machine (VON ARDENNE) is equipped with dual magnetron systems (type RM by FEP). Concepts regarding machine design, process technology and process control as well as in situ monitoring are presented to realize the high demands on uniformity, accuracy and reproducibility. Results of gradient and multilayer type precision optical coatings are presented. Application examples are edge filters and special antireflective coatings for the backlight of 3D displays with substrate size up to 300 x 400mm. The machine allows deposition of rugate type gradient layers by rotating a rotary table with substrates between two sources of the dual magnetron system. By combination of the precision drive (by LSA) for the substrate movement and a special pulse parameter variation during the deposition process (available with the pulse unit UBS-C2 of FEP), it is possible to adjust the deposition rate as a function of the substrate position exactly. The aim of a current development is a technology for the uniform coating of 3D-substrates and freeform components as well as laterally graded layers.

We are currently developing linear variable filters (LVF) with very high wavelength gradients. In the visible, these filters have a wavelength gradient of 50 to 100 nm/mm. In the infrared, the wavelength gradient covers the range of 500 to 900 microns/mm. Filter designs include band pass, long pass and ulta-high performance anti-reflection coatings. The active area of the filters is on the order of 5 to 30 mm along the wavelength gradient and up to 30 mm in the orthogonal, constant wavelength direction. Variation in performance along the constant direction is less than 1%. Repeatable performance from filter to filter, absolute placement of the filter relative to a substrate fiducial and, high in-band transmission across the full spectral band is demonstrated. Applications include order sorting filters, direct replacement of the spectrometer and hyper-spectral imaging. Off-band rejection with an optical density of greater than 3 allows use of the filter as an order sorting filter. The linear variable order sorting filters replaces other filter types such as block filters. The disadvantage of block filters is the loss of pixels due to the transition between filter blocks. The LVF is a continuous gradient without a discrete transition between filter wavelength regions. If the LVF is designed as a narrow band pass filter, it can be used in place of a spectrometer thus reducing overall sensor weight and cost while improving the robustness of the sensor. By controlling the orthogonal performance (smile) the LVF can be sized to the dimensions of the detector. When imaging on to a 2 dimensional array and operating the sensor in a push broom configuration, the LVF spectrometer performs as a hyper-spectral imager. This paper presents performance of LVF fabricated in the far infrared on substrates sized to available detectors. The impact of spot size, F-number and filter characterization are presented. Results are also compared to extended visible LVF filters.

The bandwidth of an optical interference filter when used in an optical system is limited by the angular range of its illumination. The limitation has two main aspects: The central wavelength of the filter shifts to shorter wavelength and the shape of the transmission band degrades. The first effect results in filters with increased transmission width and decrease average transmission while the second results in reduction in the average transmission of the filter. For compact systems with high numerical aperture these effects can be pronounced. By minimizing the propagation angles in the filters these effects can be substantially reduced in the near infra red (NIR) to levels similar to IR filters. We have recently developed materials and design techniques that permit us to obtain low angle shift coating that maintain very high transmission in the spectral range from 800 to 1100 nm. In this paper we will demonstrate the performance, design and fabrication, of fully blocked (200 to 1200 nm) narrow band pass filters and their measured performance over a range of angles (0-30°). Filters with bandwidths from 15 to 100 nm will be shown to illustrate the versatility of these techniques.

It has been verified that a broadband high-reflection (HR) film could restrain electric-field intensity (EFI) enhancement effect in the nodular defects at normal incidence. However, it’s impossible to design an omnidirectional HR coatings to avoid the light penetration from all incident angles at oblique incidence. In this paper, the EFI enhancement is simulated by using a three-dimensional finite-difference time-domain (FDTD) code. Two types of polarizers that prevent light penetration at low and high incident angular range (IAR) are proposed to explore the influence of transmission band at different angles in the case of oblique incidence. The damage morphologies of nodules initiating from different diameter silica microspheres in polarizers reproduce the simulated EFI distributions very well. These results indicate that light penetration at high IAR rather than at low IAR contributes mostly to EFI enhancement. Then, the conclusion is proved further by the films with low and high IAR at normal incidence. Controlling the angle position of transmission band at small angle can reduce the EFI enhancement in the usual case and increase the laser-induced damage threshold (LIDT) of films.

Indium Tin Oxide, ITO, is the industry standard for transparent conductive coatings. As such, the common metrics for characterizing ITO performance are its transmission and conductivity/resistivity (or sheet resistance). In spite of its recurrent use in a broad range of technological applications, the performance of ITO itself is highly variable, depending on the method of deposition and chamber conditions, and a single well defined set of properties does not exist. This poses particular challenges for the incorporation of ITO in complex optical multilayer stacks while trying to maintain electronic performance. Complicating matters further, ITO suffers increased absorption losses in the NIR – making the ability to incorporate ITO into anti-reflective stacks crucial to optimizing overall optical performance when ITO is used in real world applications. In this work, we discuss the use of ITO in multilayer thin film stacks for applications from the visible to the NIR. In the NIR, we discuss methods to analyze and fine tune the film properties to account for, and minimize, losses due to absorption and to optimize the overall transmission of the multilayer stacks. The ability to obtain high transmission while maintaining good electrical properties, specifically low resistivity, is demonstrated. Trade-offs between transmission and conductivity with variation of process parameters are discussed in light of optimizing the performance of the final optical stack and not just with consideration to the ITO film itself.

Thick HfO₂ single layers derived from a reactive plasma ion assisted deposition were investigated with a designed film thickness of 800 nm. The film structure was modeled by fitting the corresponding variable angle spectroscopic ellipsometric data and correlated to the ratio of plasma ion momentum transfer during the film deposition. Scatter loss was calculated according to a multilayer model as well as a single surface model. Water absorption in the MWIR was used to confirm the revealed film structure. The results indicate that the scatter loss of the HfO₂ based high reflective optics can be estimated by using a single surface model in a first-order approximation from the DUV to the MWIR. A linear relationship between the refractive index inhomogeneity and the amount of plasma ion momentum transfer during the deposition was established. The total loss at 2.95 μm is dominated by the absorptance loss, whereas both the absorptance and the scatter losses are reduced as the ratio of plasma ion momentum transfer increases. Appropriately optimizing and selecting deposition parameters enable low loss and environmentally stable HfO₂ coatings, leading to numerous defense applications from the DUV to the MWIR.

The market for thermal imaging sensors and cameras has been increasingly focused on higher volumes and lower costs. Precision glass molding (PGM) is a high volume, low cost method which has been utilized for decades to produce lenses from oxide glasses. Due to the recent development of high quality precision-molded chalcogenide glasses, which are transparent at critical thermal imaging wavelengths, PGM has emerged as the enabling technology for low cost infrared optics. Since the price of germanium is high and volatile, it plays a large role in the high price of chalcogenide glasses that contain it. As40Se60 has previously been investigated as a lower-cost alternative to germanium-containing chalcogenide glasses and was found suitable for the PGM process. This paper investigates the composition-dependence of PGM-relevant properties for As38Se62 and standard As40Se60 and presents a comparison of molding behavior and lens performance.

In this work, the effect of adding Se, Te, In, Cs, Y to gallium lanthanum sulphide glass was studied. Structural modifications to the glassy network were achieved by substitution of sulphur, gallium or lanthanum using a melt-quench method in an inert atmosphere. Optical, thermal and mechanical characterisation of the samples revealed tailorable features according to the nature and the amount of glass modifier. In particular, the addition of selenium and tellurium resulted in an extended transmission in the infrared up to 12 μm. Furthermore, for small amounts of selenium, the position of the bandgap did not change significantly, maintaining visible transmission. The addition of indium led to the formation of glasses with longer transmission in the infrared and a cut-off edge around 600nm in the UV-visible range. Over-all, the addition of these modifiers resulted in stronger materials with improved thermal stability and similar mechanical properties to original Ga-La-S glass. The outcome of this work aims to bring a new family of chalcogenide glasses for applications in the infrared and visible range.

Additive manufacturing is proving its relevancy across a wide spectrum of development, prototyping and manufacturing in the US. However, there is a desire to move the capability beyond modeling and structural components. The use of additive manufacturing techniques to fabricate low-cost optics and optical systems is highly desirable in a number of markets. But processes and techniques for successfully printing an optic are currently very new. This paper discusses early advances in printing optics suitable for commercial and military applications. Data from and analysis of early prototype lenses fabricated using one possible technique will be included and discussed. The potential for additive manufacturing of optics to open the design space for complex optics and reduce development time, lowering cost and speeding up time to market, will also be discussed.

Near net shape parts can be produced using some very old processes (investment casting) and the relatively new direct metal laser sintering (DMLS) process. These processes have significant advantages for complex blank lightweighting and costs but are not inherently suited for producing high performance mirrors. The DMLS process can provide extremely complex lightweight structures but the high residual stresses left in the material results in unstable mirror figure retention. Although not to the extreme intricacy of DMLS, investment casting can also provide complex lightweight structures at considerably lower costs than DMLS and even conventional wrought mirror blanks but the less than 100% density for casting (and also DMLS) limits finishing quality. This paper will cover the progress that has been made to make both the DMLS and investment casting processes into viable near net shape blank options for high performance aluminum mirrors. Finish and figure results will be presented to show performance commensurate with existing conventional processes.

High-precision chalcogenide molded micro-lenses were produced to collimate mid-infrared Quantum Cascade Lasers (QCLs). Molded cylindrical micro-lens prototypes with aspheric contour (acylindrical), high numerical aperture (NA~0.8) and small focal length (f<2 mm) were fabricated to collimate the QCL fast-axis beam. Another innovative freeform micro-lens has an input acylindrical surface to collimate the fast axis and an orthogonal output acylindrical surface to collimate the slow axis. The thickness of the freeform lens is such that the output fast- and slow-axis beams are circular. This paper presents results on the chalcogenide molded freeform micro-lens designed to collimate and circularize QCL at 4.6 microns.

Innovative mid-infrared imaging fiber bundle has been developed that is flexible, rugged and high fiber-count for use with infrared cameras for thermal imaging applications. High-quality chalcogenide fibers are used to produce coherent fiber bundle that is 2 meters in length, 4000 fibers in a 3mm diameter bundle, minimum bend radius of 10cm, and low attenuation over the spectral range of 1.5-6.5 microns. Individual fiber pixel size is 34 microns and the NA is 0.3. This paper presents the fabrication process and the optical characterization of the mid-infrared imaging fiber bundle.

Calibration of the emitted radiation from UV sources peaking at 365 nm, is necessary to perform the ASTM required 1 mW/cm² minimum irradiance in certain military material (ships, airplanes etc) tests. These UV “black lights” are applied for crack-recognition using fluorescent liquid penetrant inspection. At present, these nondestructive tests are performed using Hg-lamps. Lack of a proper standard and the different spectral responsivities of the available UV meters cause significant measurement errors even if the same UV-365 source is measured. A pyroelectric radiometer standard with spectrally flat (constant) response in the UV-VIS range has been developed to solve the problem. The response curve of this standard determined from spectral reflectance measurement, is converted into spectral irradiance responsivity with <0.5% (k=2) uncertainty as a result of using an absolute tie point from a Si-trap detector traceable to the primary standard cryogenic radiometer. The flat pyroelectric radiometer standard can be used to perform uniform integrated irradiance measurements from all kinds of UV sources (with different peaks and distributions) without using any source standard. Using this broadband calibration method, yearly spectral calibrations for the reference UV (LED) sources and irradiance meters is not needed. Field UV sources and meters can be calibrated against the pyroelectric radiometer standard for broadband (integrated) irradiance and integrated responsivity. Using the broadband measurement procedure, the UV measurements give uniform results with significantly decreased uncertainties.

The mid-wave infrared (MWIR) portion of the electromagnetic spectrum is critically important for a variety of applications such as LIDAR and chemical sensing. Concerning the latter, the MWIR is often referred to as the “molecular fingerprint” region owing to the fact that many molecules display distinctive vibrational absorptions in this region, making it useful for gas detection. To date, steering MWIR radiation typically required the use of mechanical devices such as gimbals, which are bulky, slow, power-hungry, and subject to mechanical failure. We present the first non-mechanical beam steerer capable of continuous angular tuning in the MWIR. These devices, based on refractive, electro-optic waveguides, provide angular steering in two dimensions without relying on moving parts. Previous work has demonstrated non-mechanical beam steering (NMBS) in the short-wave infrared (SWIR) and near infrared (NIR) using a waveguide in which a portion of the propagating light is evanescently coupled to a liquid crystal (LC) layer in which the refractive index is voltage-tuned. We have extended this NMBS technology into the MWIR by employing chalcogenide glass waveguides and LC materials that exhibit high MWIR transparency. As a result, we have observed continuous, 2D MWIR steering for the first time with a magnitude of 2.74° in-plane and 0.3° out-of-plane.