This PDF file contains the front matter associated with SPIE Proceedings Volume 12530, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

This conference presentation was prepared for the Advanced Optics for Imaging Applications: UV through LWIR VIII conference at SPIE Defense + Commercial Sensing 2023

This paper discusses a passive vibration isolation system which has been specifically developed to protect the optical system of a MWIR Missile Warning Sensor deployed on a Fighter Aircraft from high level random vibration. The concept of the designed system is to protect especially those components from the excitation of the aircraft which are most susceptible to vibration. This is achieved by isolating an optical unit, consisting of a group of lenses and a MWIR detector, by means of custom-designed, vulcanized elastomeric isolators, creating a Single Degree of Freedom Oscillator (SDoF). The capabilities of this passive vibration isolation system inside a MWIR Missile Warning Sensor are analyzed in terms of load reduction onto the MWIR detector, the effects of displacement between optical elements and the impact of the latter on optical performance, especially on line-of-sight stability and distortion of the very wide field-of-view.

12530 This conference presentation was prepared for the Advanced Optics for Imaging Applications: UV through LWIR VIII conference at SPIE Defense + Commercial Sensing 2023

Terahertz (THz) near-field imaging promises new advances in medical diagnostics and material characterization. However, its spatial resolution is limited by the light diffraction on lenses/mirrors, which limits the optical resolution of a standard free-space imaging system to ~λ/2 (Abbe limit). Alternatively, super-resolution imaging can be achieved by employing a solid immersion lens (SIL) as the spot size of the focused THz beam is further reduced by a factor of 1/n, where n is the refractive index (RI) of the lens material. In this work, we present the design and fabrication of hemispherical THz SILs using powder mixes of titanium dioxide (TiO₂) and polypropylene (PP) having n_Tio2≈10 and n_pp≈1.51 at 1.0 THz. We present two different lens fabrication strategies that are simple and cost-effective. The first one uses pressing the TiO₂ powder with a PP powder at the Vicat temperature of PP while controlling the concentration of TiO₂ and the resultant lens porosity. The second one uses pressing the TiO₂ powder in a hollow hemisphere that is 3D printed using PP. The fabricated lenses are then characterized optically, and their RIs are compared to predictions of the Bruggeman model of the effective media. From the experimental characterization of the composite SILs, a resolution λ/5 was achieved at 0.09 THz (λ≈3.3 mm), which is one of the best resolutions for THz SILs reported in the literature. We believe that further improvements in material processing can reduce the resolution of the TiO₂-based THz SILs to their fundamental λ/20 limit .

Hot conditions force infrared imaging system designers to have thoughtful control of stray light. One must simultaneously optimize both the emission and reflection properties of an imaging system to effectively reject unwanted infrared radiation in a hot, compact environment. In aviation systems, infrared sensors may be placed behind severely angled windows to remain both aerodynamic and compact. We present a reflective baffle design that effectively rejects stray radiation and masks emission from hot structures, allowing an infrared imager to see outside of its harsh environment.

An afocal freeform telescope has been prototyped at MIT Lincoln Laboratory. The design leverages an upgraded version of the Laboratory’s FANO design code to produce designs in afocal-space rather than imaging-space. The FANO code is unique, as it optimizes the surfaces using a Non-Uniform Rational B-Spline (NURBS) description rather than a polynomial-based equation. This approach allows the mirrors to take the most optimal shape without the limit of number of polynomial terms often hard-coded into commercial design software. Design comparisons with polynomial-based approaches are discussed showing improved performance when designing with NURBS shapes. Test results from the 150-mm diameter, 6x afocal magnification, 4° field-of-view prototype unit operating in the near-infrared band are shown.

The conventional on-axis reflective systems suffer from a diffraction effect on the Point Spread Function (PSF) due to the secondary mirror obscuration. Meanwhile, the unobscured off-axis reflective systems’ imaging performance may be impacted by linear astigmatism aberration. The Linear Astigmatism Free-Three Mirror System (LAF-TMS) is a confocal off-axis reflective system that eliminates linear astigmatism and enables a wide Field of View (FoV). We present an enhanced design of LAF-TMS, called ”wide-wide”, which has an aperture of D=40mm, an effective focal length of f=75mm, and a wide FoV of 8.25°(Horizontal) × 6.21°(V ertical) combined with a wide spectral bandwidth capability suitable for Unmanned Aerial Vehicle (UAV) applications. To evaluate the performance of this compact and fast optical system design, we use the Photon Simulator (PhoSim) to model physically accurate PSF under different conditions of the mirror surface, mechanical environment, and atmosphere. As a benchmark, we compare and analyze the PhoSim PSF results with other ray tracing software such as Zemax and CodeV. Additionally, PhoSim is capable of simulating infrared spectral imaging cases with a user-defined Spectral Energy Distribution (SED), intensity, and emissivity of each pixel. The comprehensive simulation results demonstrate the high performance of the LAF-TMS with a wide-wide FoV and multispectral capabilities.

The adoption of neural networks for optical component design has increased rapidly in recent years. In this design framework, the numerical simulation of optical wave propagation and material wave modulation are encoded directly as layers of a neural network. This direct encoding enables the optimization of physical quantities (e.g., the transmissivity values of the diffractive optical elements) by gradient descent and the backpropagation algorithm. For the body of work which uses these networks for simulation and optimization, there is a tendency to treat the training process as identical to traditional deep neural networks. However, to the best of our knowledge, there is yet an explicit evaluation of training parameters to support this intuition. This work aims to help fill this gap by providing an exploration and evaluation of data variety to help accelerate those in the community who wish to use this emerging design framework.The application of neural networks in optical component design has witnessed rapid growth in recent years. This design framework encodes the numerical simulation of optical wave propagation and material wave modulation directly within neural network layers, enabling the optimization of physical quantities, such as transmissivity values of diffractive optical elements, through gradient descent and backpropagation algorithms. Physics-informed neural networks have been employed in designing diffractive deep neural networks, optimizing holograms for near-eye displays and creating multi-objective traditional optics. However, there remains a lack of evaluation for training parameters, and discrete sampling considerations are often overlooked. To address these gaps, this study examines the impact of dataset variety on physics-informed neural networks in optimizing lenses that either satisfy or violate the Nyquist sampling criteria. Results show that increased data variety enhances optimized lens performance across all cases. Optimized lenses demonstrate improved imaging performance by reducing diffraction orders present in aliased analytical lenses. Moreover, we reveal that low data variety leads to overfit lenses that function as selective imagers, providing valuable insights for future lens design and optimization.

Metallic mirror coatings are a key part of intelligence gathering, surveillance, and reconnaissance systems (ISR). As collecting accurate and clear information is critical in these applications, coatings working in these applications must withstand extreme environments, including high humidity and corrosion for extended periods. Gold mirror coatings are a common choice, being chemically inert to be relatively unaffected by environmental conditions and benefiting from high reflectivity from 600nm to into the mid-IR. Aluminum has a lower overall reflectivity than gold but extends its high reflectivity band further into the UV than gold. Silver performs better than gold in the full visible range but suffers from weak environmental survivability performance. As such, silver requires a durable protective coating to withstand exposure to ambient and corrosive environments. Hence, these silver-based coatings must be protected by inert coatings without reduction of their spectral performance in the spectral region of interest. Besides spectral performances, these coatings must also meet other durability requirements according to military specification documents like MIL-F 48616, MIL-810G amongst a few. Amongst the harshest requirements are exposure to Salt Fog spray and extended heat and humidity. Low stress coatings are also desired to meet surface figure requirements for some of these optics. In this article, we provide an overview of performance of our protected silver coating, with reflectance > 95% from 0.45-20 um that can withstand exposure to salt fog spray for over 120 h. These coatings were also exposed for over 24 h of high humidity, temperature, and thermal cycling without any significant deterioration in performance.

In recent years, applied optics has been pushed into two opposite directions, deep ultraviolet (DUV)/extreme ultraviolet (EUV) for semiconductors, and infrared (IR) for infrared optics. To increase resolution, coatings are critical for IR optics on various IR transparent substrates. Outer window coatings pose a huge challenge in meeting severe environmental conditions including corrosion resistance, mechanical durability, or even biological threats. In this research, AR antireflective (AR) coatings for short wavelength to mid-wavelength IR (SWIR-MWIR) applications on IR transparent substrates were developed through physical vapor deposition (PVD) processes. By careful materials selection and processes tunning, Corning has developed a coating that can pass durability tests in severe conditions including fluid, ozone, bacteria, and enhanced humidity exposure. The developed coating has successfully passed corrosion resistance of SO₂ salt-fog test for 168 hrs (7days) and H₂SO₄+salt-fog solution test for 672 hrs (28days) without degradation optically and mechanically. Atomic force microscope (AFM) data show that the RMS roughness is greatly reduced after corrosion testing and the surface is smoother with less scatter centers, which is consistence with transmission improvement. Time-of- Flight Secondary Ion Mass Spectrometry (ToF-SIMS) data confirms no degradation after SO₂ salt fog corrosive testing for 168Hrs. The success of this super-durable coating development was dependent upon dense, low defect, stress balanced, and chemically inert selection of the material system of SiO₂/Si.

Transparent magnesium aluminate spinel (MgAl2O4) has been developed as an optical ceramic for a variety of applications, including as windows. As a broadband, ultraviolet (UV) thru midwave infrared (MIR) material, it has been developed for windows and has many desirable properties compared with alternative infrared glasses and other transparent ceramics. Current efforts to advance high strength spinel manufacturing processes have demonstrated progress toward large format windows. Although low-absorption spinel, specifically in the near-infrared (NIR) has been demonstrated previously, additional processing is required for new, large-scale spinel manufacturing processes to decrease the effects of impurities near one-micron wavelengths. In this work we present recent results that show measured absorption near 1 μm is reduced by annealing, which reduces effects of trace impurities. Experimental results from photo-thermal common-path interferometer measurements are reported.

Spaceborn optics are exposed to extreme environmental conditions that may have short- and long-term effects on their performance. The periodic day/night transition over the orbit can induce strong temperature transient that may affect the optical figure due to e.g. high thermo-optic coefficients. Besides radiative heating, the ionizing radiation (high energetic e-, H+ and photons) can also damage the optics via solarization and/or compaction. Due to their low thermo-optical coefficients combined with low density, SCHOTT IRG InfraRed Glasses are a material of choice for astrospace applications. In this study we also show that these glasses are resilient to ionizing radiation. IRG 22, 24, 25, 26 and 27 chalcogenide glasses from the SCHOTT portfolio were irradiated by means of ⁶⁰Co gamma irradiation at the ESA ESTEC irradiation facility to an estimated ionizing dose of 10 kGy. The comparison of the transmittance and refractive index prior and after irradiation showed only marginal changes.

NRL is developing new materials that transmit across wide wavelength ranges. MILTRAN is a new rugged optical ceramic that transmits visible through LWIR and is 3.5 times harder than ZnS. With a negative dn/dT, it is well suited as an internal lens element. NRL-series moldable glasses transmit SWIR through LWIR and may be bonded to each other in an adhesive-free thermal process. NRL-200-series glasses transmit visible through MWIR and expand the glass map for multispectral lens designs. These new materials enable greater flexibility for designers of lenses for advanced defense applications and potentially reduce the size, weight and cost of next-generation optics. This paper will discuss new optical materials, their properties and the advantages of using NRL materials in optics designs.

We present a high performance nanohole metasurface lens design operating at 1550nm. We use a simple basis with features that can be made using DUV lithography so that the lens could be manufactured scalably and economically. Our basis consists of cylindrical nanoholes etched into a silicon wafer where the hole radius is the only degree of freedom. We design an anti-reflection (AR) coating for the exposed meta-atom surface to enable high transmission across the basis; we evaluated three candidate materials and selected ZnS because it yielded the best optical performance. Lens layout was performed using a parametric meta-atom library generated with finite-element analysis in CST studio under the local uniformity approximation. We modelled performance of the lens, including both transmission and diffraction losses, using a parametric blazed-grating library to represent small sections of the aperture and then sum across the aperture. We compare the lens performance against a bulk singlet.

Wide field-of-view optics are crucial optical elements with extensive applications in imaging, display, and sensing. Conventional wide FOV optics rely on cascading multiple refractive optical elements to assemble ‘fisheye lenses’ that correct third-order Seidel aberrations. Here we experimentally demonstrate a flat lens design that achieves 140◦ FOV in the long-wave infrared band. The metalens was fabricated on a monolithic float zone Si wafer leveraging photolithography and deep reactive ion etching. Large metalenses with diameter exceeding 4 cm have been realized using this approach. Thermal imaging at ambient temperature was validated by coupling the metalens with a LWIR focal plane array.

Optical metasurfaces enable devices to interact with light in unique ways by modulating phase, polarization, or intensity. A metasurface, composed of individual subwavelength scatterers known as meta-atoms, can be designed to provide unparalleled control of wavefronts for a variety of optical applications, yet the design of such devices is often unintuitive and challenging due to computationally expensive forward simulations and the number of free parameters. To overcome this, there is interest in developing inverse design methods as an alternative to conventional forward design. Inverse design leverages machine learning algorithms to effectively search a problem space, starting from application and resulting in solution parameters. In this work, we adopt an inverse design approach that involves targeted forward simulations of arbitrary meta-atoms. To ensure that the dataset captures all possible shapes and rotations of near field responses with second order accuracy, it is constructed using meta-atoms with varying geometries and corresponding phase shifts, including the effect of nearest neighbors. A custom deep learning system is developed to extract meaningful features from this near field response. The proposed framework provides flexibility to produce an inverse design paradigm for generalized metasurface applications without the need for repeated forward simulations. Additionally, the machine learning model is highly effective in reconstructing electric fields, irrespective of the loss function used.

We combine metasurface optics and refractive optics to form hybrid lenses, where the refractive elements provide the optical power but metasurfaces are used to correct aberrations. We introduce an algorithm to optimize layout of metasurfaces (MSs) in hybrid lens designs where the MSs can be located anywhere in the optical train. This algorithm uses a ray-based, scalar field method to propagate through refractive optics with speed comparable to Fourier methods, which are limited to propagation between planar surfaces. This method supports propagation of real optical fields and derived adjoint fields, both forward and backward, which enables inverse design with adjoint gradient methods to optimize the MS nanostructured layout. In contrast to previous image-space optimizations which neatly partition the problem into separate ray-optics and wave-optics domains, this algorithm provides the freedom to arbitrarily interleave metasurface optics with refractive optics during hybrid lens design. A hybrid lens design example in mid-wave infrared is presented to demonstrate this framework.