Mane-Si Laure Lee

Wide-field-angle behavior of blazed-binary gratings in the resonance domain.

Blazed-binary gratings for which a blazed effect with binary etches is achieved under normal incidence offer first-order diffraction efficiencies larger than those of blazed-échelette gratings in the resonance domain [Opt. Lett. 23 1081 (1998)]. We provide further insight into the behavior of blazed-binary gratings and show that they operate efficiently under symmetrical mounting and over a wide field-angle interval. These properties are illustrated with theoretical and experimental results obtained for an approximately 1000-line/mm grating at 633 nm.

Broadband blazing with artificial dielectrics

The efficiency of conventional diffractive optical elements with échelette-type profiles drops rapidly as the illumination wavelength departs from the blaze wavelength. We use high dispersion of artificial materials to synthesize diffractive optical elements that are blazed over a broad spectral range (approximately 1 octave) or for two different wavelengths.

Blazed binary diffractive gratings with antireflection coating for improved operation at 10.6 μm

We report on the design fabrication and characterization a 3l-period grating composed of subwavelength ridges of progressively varying widths for operation at 10.6 mm. The grating is blazed into the first transmitted order (an efficiency of 80% is measured) under TM po- larization and over a broad range of angles of incidence. The fabrication involves contact photolithography, reactive-ion etching, and an evapora- tion 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-mm IR band).

Imaging with blazed-binary diffractive elements

Blazed diffractive elements are currently fabricated with continuous profiles or with staircase approximations. Blazed-binary gratings are more recent diffractive elements which implement continuous phase delays through a gradient-index artificial material with subwavelength binary etches. The performance of these diffractive elements in the resonance domain opens up new perspectives for manufacturing fast lenses. The objective of this paper is to review the main and unique properties of blazed-binary diffractive elements and to consider the consequences for monochromatic imaging systems.

Infrared hybrid optics with high broadband efficiency

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.

Blazed-binary diffractive elements with periods much larger than the wavelength

Blazed-binary optical elements with only binary ridges or pillars are diffractive components that mimic standard blazed-echelette diffractive elements. We report on the behavior of one-dimensional blazed-binary optical elements with local periods much larger than the wavelength. For this purpose, an approximate model based on both scalar and electromagnetic theory is proposed. The model is tested against electromagnetic-theory computational results obtained for one-dimensional blazed-binary gratings with large periods. An excellent agreement is obtained, showing that the model is able to predict quantitatively the wavelength and the incidence-angle dependences of the diffraction efficiency of blazed-binary structures.

Q-Band Millimeter-Wave Antennas: An Enabling Technology for MultiGigabit Wireless Backhaul

The bandwidth demands in mobile communication systems are growing exponentially day by day as the number of users has increased drastically over the last five years. This mobile data explosion, together with the fixed service limitations, requires a new approach to support this increase in bandwidth demand. Solutions based on lower-frequency microwave wireless systems may be able to meet the bandwidth demand in a short term. However, with the small-cell mass deployment requiring total capacities of 1 Gb/s/km2, scalable, multigigabit backhaul systems are required. Millimeter-wave technology fits nicely into these new backhaul scenarios as it provides extended bandwidth for high-capacity links and adaptive throughput rate, which allows efficient and flexible deployment. Besides these advantages, millimeter-wave solutions become even more attractive when the cost of backhaul solutions and the cost of spectrum licenses are factored in. Compared to the cost of laying fiber to a cell base station, which is the only other scalable solution, the millimeter-wave solution becomes the most appropriate approach.

Real-time increase in depth of field of an uncooled thermal camera using several phase-mask technologies.

Imaging systems that combine a phase mask in the pupil and digital postprocessing may have better performance than conventional ones. We have built such a system to enhance the depth of field of an uncooled thermal camera. The phase masks are binary, their structures are optimized thanks to an image quality criterion, and they have been realized with three different technologies that give equivalent results. The deconvolution postprocessing is performed in real time with a graphics processing unit. A significant increase of the depth of field of a factor 3 has been obtained.

Transmission blazed-binary gratings for visible light operation: performances and interferometric characterization

We present experimental results obtained for blazed-binary gratings with periods between 1.6? and 114.7? blazed at ? = 633?nm. These gratings are composed of subwavelength pillars with pulse-width modulation within the periods. For small periods up to 3?, the measured efficiencies agree well with the theoretical predictions. But for large periods, the discrepancy between the theoretical and experimental efficiencies is significant, reaching almost 20% for the grating with the largest period. In order to identify the origin of this discrepancy, a Mach?Zehnder interferometer dedicated to the characterization of small-size gratings (?400 ? 400??m2) is set up. The interferometric characterization combined with scanning-electron-microscope observations reveal that the 20% discrepancy is due to a deficiency of the fabrication process for the smallest pillars and to an overall fabrication of pillars with actual widths smaller than their nominal values. These two fabrication errors can reasonably be expected to be eliminated in future work.

Blazed-binary diffractive gratings with antireflection coating for improved operation at 10.6 μm

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).