Freeform optics bring new degrees of freedom to optical systems and require the abilities both to describe any surface (continuous or not) and to optimize their shape together with the geometry of the entire system. This increases the number of variables, and therefore the complexity of the fitness function to be minimized in order to obtain highest optical performance. Most proprietary algorithms from commercial solutions cannot handle more than tens of variables and/or noisy function landscape limiting the implementation of such free-form in optical systems. Here, CMA-ES algorithm is coupled to parallel computation of ray tracing simulations able to cover the high computational demand. The benefits of such state-of-the-art evolutionary optimization algorithms is a one-step convergence by exploring the entire landscape of solutions without the need of any starting optical architecture.
We report on nanoimprint and plasma etching technologies development for high-density sub-wavelength surface structuration at a scale from 1.5μm to 200nm or below, to get multifunctional windows (of size ~2”-3”) offering both outstanding optical and fluidic properties. Such windows are of interest for outdoor surveillance systems, which need to operate whatever the environmental conditions. We demonstrate the realization of multifunctional surfaces enabling antireflection and water repellency properties on different optical materials, i.e. glass/silica, silicon and germanium, for applications from visible to longwave infrared domains. Illustration of such multifunctional window advantages for imaging is provided thanks to its integration in front of a MWIR camera and image analysis in presence of water droplet.
We exploit micro-nano structuration to achieve multifunctional windows offering outstanding optical and fluidic properties to enhance the operation of surveillance or detection devices under rainy conditions. These windows are based on synthesis of an artificial index gradient for antireflection properties and improvement of their water repellency property thanks to their structuration at a subwavelength scale with controlled conical geometries. We demonstrate the realization of multifunctional germanium windows for LWIR camera, using two approaches: nanoimprint lithography, well-known for its very high resolution enabling applications from visible to thermal infrared domain, followed by etching techniques, and 3D direct laser writing based on Two-Photon Polymerization (TPP), which is of interest thanks to its ability to manufacture complex 3D structuration directly. Optical characterization shows the ability of such windows to improve optical transmission within 8-14μm spectral range, as compared to non-structured window. In terms of water repellency, the structured windows enable an increase of the contact angle up to 160° with a very low hysteresis. To evaluate the advantage of the multifunctional windows for imaging devices, the windows are integrated in front of a thermal infrared camera and images analysis shows that the camera sensitivity is increased for the nanoimprint window thanks to the multifunctional window and high water repellency in presence of water.
We report on the design and fabrication of a reflection grating for hyperspectral applications operating in the range from 340 nm to 1040 nm wavelength. The blazed grating is based on an effective medium approach, where the desired functionality is realized using a binary surface relief structure. For each period, a gradient in size of the local grating features mimics an interface which adds a linear phase profile to the illuminating beam – thus introducing diffraction. The surface relief structure is composed of 2D structures - pillars with diameters from 200 nm to 350 nm to voids with diameters from 300nm to 120 nm. Overall, an entire number of ~50 such features are arranged to establish an overall unit cell of the grating over a length of 30 μm. By purposeful design of size, shape and arrangement of the sub-wavelength features such gratings offer novel opportunities in tailoring the spectral response, i.e. particular broadband efficiency or the enhancement of the efficiency in specific sub-domains of the spectrum. We will present measured performance results of a grating covering a circular area of 80mm in diameter manufactured on a 4inch-wafer. Finally, we will give an outlook on how such structures can be applied to curved surfaces and even ultra-broadband operation.
Today, optical instruments with FOV of ±30 degrees or more can be achieved by combining several optical modules thus increasing the complexity at instrument level in the design, integration, validation and calibration phases. For instance the MERIS instrument on the ENVIronment SATellite (ENVISAT) has a 68.5 degree FOV shared between 5 identical modules. Each one of those modules needs to be calibrated independently and the modules require to be co-registrated, which leads to stringent constraints at system level in terms of stability. Reducing the number of modules to achieve large FOVs will have potentially beneficial impact on the mass and volume of the instrument and will ease the calibration and co-registration process.
Increasing the capture volume of visible cameras while maintaining high image resolutions, low power consumption and
standard video-frame rate operation is of utmost importance for hand-free night vision goggles or embedded surveillance
systems. Since such imaging systems require to operate at high aperture, their optical design has become more complex
and critical. Therefore new design alternatives have to be considered. Among them, wavefront coding changes and
desensitizes the modulation transfer function (MTF) of the lens by inserting a phase mask in the vicinity of the aperture
stop. This smart filter is combined with an efficient image processing that ensures optimal image quality over a larger
depth of field. In this paper recent advances are discussed concerning design and integration of a compact imaging system
based on wavefront coding. We address the design, the integration and the characterization of a High Definition (HD)
camera of large aperture (F/1.2) operating in the visible and near infrared spectral ranges, endowed with wavefront coding.
Two types of phase masks (pyramidal and polynomial) have been jointly optimized with their deconvolution algorithm in
order to meet the best performance along an increased range of focus distances and manufactured. Real time deconvolution
processing is implemented on a Field Programmable Gate Array. It is shown that despite the high data throughput of an
HD imaging chain, the level of power consumption is far below the initial specifications. We have characterized the
performances with and without wavefront coding through MTF measurements and image quality assessments. A depth-of-
field increase up to x2.5 has been demonstrated in accordance with the theoretical predictions.
The military uncooled infrared market is driven by the continued cost reduction of the focal plane arrays whilst maintaining high standards of sensitivity and steering towards smaller pixel sizes. As a consequence, new optical solutions are called for. Two approaches can come into play: the bottom up option consists in allocating improvements to each contributor and the top down process rather relies on an overall optimization of the complete image channel. The University of Rennes I with Thales Angénieux alongside has been working over the past decade through French MOD funding’s, on low cost alternatives of infrared materials based upon chalcogenide glasses. A special care has been laid on the enhancement of their mechanical properties and their ability to be moulded according to complex shapes. New manufacturing means developments capable of better yields for the raw materials will be addressed, too. Beyond the mere lenses budget cuts, a wave front coding process can ease a global optimization. This technic gives a way of relaxing optical constraints or upgrading thermal device performances through an increase of the focus depths and desensitization against temperature drifts: it combines image processing and the use of smart optical components. Thales achievements in such topics will be enlightened and the trade-off between image quality correction levels and low consumption/ real time processing, as might be required in hand-free night vision devices, will be emphasized. It is worth mentioning that both approaches are deeply leaning on each other.
An external cavity with a binary phase grating has been developed to achieve the coherent beam addition of five
quantum-cascade lasers emitting at 4.65 μm. The combining of these five emitters is achieved by a binary phase grating
or Dammann grating able to separate an incident beam into five beams of equal intensities with a 75% efficiency. A CW
output power of ~ 0.65 W corresponding to a combining efficiency of 70% with a good beam quality is obtained. More
results concerning output power, combining, efficiency stability and beam quality and spectrum are exposed.
An external cavity with a binary phase grating has been developed to achieve the coherent beam addition of five
quantum-cascade lasers emitting at 4.65 μm. The combining of these five emitters is achieved by a binary phase grating
or Dammann grating able to separate an incident beam into five beams of equal intensities with a 75% efficiency. A CW
output power of ~ 0.5 W corresponding to a combining efficiency of 66% with a good beam quality is obtained. More
results concerning output power, combining, efficiency stability and beam quality and spectrum are exposed.
Diffractive optical elements in the form of surface-relief 'blaze' (echelette-type) structures diamond-turned onto the surface of conventional refractive lens elements are well-established and widely used. However, they suffer from limited broadband diffraction efficiency, which prevents the full benefits of hybrid optics from being realised. A family of diffractive optics, the blazed-binary optical element, is investigated to improve the broadband efficiency.
Blazed-binary optical elements are diffractive components, composed of subwavelength (ie. with size smaller than the wavelength) ridges, pillars or other simple geometries carefully etched in a dielectric film, that mimic standard blazed-echelette diffractive elements. Their operation exploits effective-medium theory. We show that by exploiting the high dispersion of artificial material, diffractive optical elements which are blazed over a broad spectral range can be synthesized. A blazed-binary grating is designed to validate the broadband behaviour and practical aspects are investigated through the manufacture of sub-wavelength structures in a Gallium Arsenide substrate.
Hybrid refractive-diffractive optics are widely used in infrared systems, but their performance is limited by reduced diffraction efficiency away from the design wavelength. Two techniques are currently being investigated to improve broadband efficiency; dual-layer blaze structures and blazed-binary optics. This paper discusses the design of dual-layer blaze structures in detail, and presents some athermalised lenses which benefit from this approach. A brief summary of using blazed-binary structures to improve efficiency is presented.
We report on the design fabrication and characterization a 3-period grating composed of subwavelength ridges of progressively varying widths for operation at 10.6 µm. The grating is blazed into the first transmitted order (an efficiency of 80% is measured) under TM polarization and over a broad range of angles of incidence. The fabrication involves contact photolithography, reactive-ion etching, and an evaporation deposition over the etched structure. The result validates the use of photolithography, a low-cost technology, for the manufacture of efficient blazed binary diffractive elements for thermal imaging (the 8- to 12-µm IR band).
Blazed-binary optical elements are diffractive components, composed of subwavelength ridges, pillars or other simple geometries carefully etched in a dielectric film, that mimic standard blazed-echelette diffractive elements. Recent experimental results in the visible showed that, blazed-binary optical elements offer high diffraction efficiencies and unique properties that cannot be achieved by standard echelette diffractive elements. Meanwhile, the manufacture of these optical elements for operation in the visible represents a challenge for today’s technologies since they involve both sub-micron sizes and high aspect ratios. In this paper, we extend the study to the thermal infrared, where the fabrication constraints are compatible with simple manufacture process such as photolithography. A 3λ-period blazed-binary grating etched into a silicon substrate, implementing an antireflection function (zinc sulphide deposition over the etched structure), was designed for operation under TM polarization at 10.6 μm. Its fabrication involved contact photolithography, reactive ion etching and an evaporation deposition over the etched structure. A first-order transmitted diffraction efficiency of 80 % was measured under TM polarization at 10.6 μm. This result validates the use photolithography, a low-cost technology, and an antireflection deposition, for the manufacture of efficient blazed-binary diffractive elements operating for thermal imaging (8-12μm infrared band).
With today’s organic photopolymer materials allowing high resolution, it is possible to record subwavelength volume holographic index gratings, i.e. index gratings with a period much smaller than the wavelength operating in the form birefringence regime. Such components are of interest for the direct view liquid crystal displays that suffer from a narrow viewing angle. In fact, by providing high birefringence, they compensate the intrinsic birefringence of the liquid crystal and thus, improve the viewing angle properties of the liquid crystal displays. Recently, we introduced a hybrid holographic compensator combining an “in plane” commercially available birefringent film and a slanted optical-axis subwavelength volume index grating recorded in a DuPont photopolymer. In this paper, we present the hybrid holographic compensation and its interesting features for TN-LCD viewing angle improvement. More particularly, a hybrid holographic compensator is designed for avionics application. It is based on a 120nm-period volume index gratings achieving an index modulation of 0.045 with a 33°-slant angle of the optical axis. The hybrid holographic compensator was manufactured and implemented on an avionics equipment from Thales Avionics. Visual inspection as well as contrast measurements showed a good improvement of the viewing angle characteristics.
Recent experimental results clearly evidence that blazed-binary diffractive elements, a family of diffractive components composed of subwavelength binary features, offers unusual and attractive properties in the resonance domain. In this paper, we provide further insight into the field-angle behavior of these diffractive elements. We show that blazed-binary gratings operate efficiently under symmetrical mount and over a wide field-angle interval. This interesting feature makes blazed-binary diffractive elements attractive candidates for imaging systems.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.