Serge Habraken

Flat Fresnel doublets made of PMMA and PC: combining low cost production and very high concentration ratio for CPV

The linear chromatic aberration (LCA) of several combinations of polycarbonates (PCs) and poly (methyl methacrylates) (PMMAs) as singlet, hybrid (refractive/diffractive) lenses and doublets operating with wavelengths between 380 and 1600 nm - corresponding to a typical zone of interest of concentrated photovoltaics (CPV) - are compared. Those comparisons show that the maximum theoretical concentration factor for singlets is limited to about 1000 × at normal incidence and that hybrid lenses and refractive doublets present a smaller LCA increasing the concentration factor up to 5000 × and 2 × 10(6) respectively. A new achromatization equation more useful than the Abbé equation is also presented. Finally we determined the ideal position of the focal point as a function of the LCA and the geometric concentration which maximizes the flux on the solar cell.

Performance comparison of four kinds of flat nonimaging Fresnel lenses made of polycarbonates and polymethyl methacrylate for concentrated photovoltaics

Solar concentrators made of a single refractive primary optics are limited to a concentration ratio of about 1000× [Opt. Express 19, A280 (2011)], due only to longitudinal chromatic aberration, while mirrors are limited to ∼46,000× by the angular size of the Sun. To reduce the chromatic aberration while keeping cost-effective systems for concentrated photovoltaics, a study of four different kinds of flat Fresnel doublets made of polycarbonates and polymethyl methacrylate is presented. It reveals that Fresnel doublets may have fewer optical losses than non-Fresnel doublets, with a lower lateral chromatic split allowing for even higher concentration ratio.

Mosaiced and high line frequency VPH gratings for astronomy

To increase the size of the volume phase holographic gratings the Centre Spatial de Liege can produce, mosaic technic has been tested and characterized. This method consists of assembling VPH gratings recorded and processed independently into one larger grating. By this way, the final grating size becomes virtually unlimited and dispersive elements can accommodate the largest telescope beams. The second research line about VPH gratings was the high line frequency domain: ν > 3000 lp/mm. Actually, for these frequencies, diffraction according to TE and TM modes is maximum for different wavelengths. However, it is possible to tune the index modulation to three times what is usually required to use the first diffraction TE peak. In this case, the second TE maximum matches the first TM maximum and unpolarized light is so entirely diffracted. This article also summarizes our prospects in the field of very high index modulation gratings where Δn as high as 0.14 has been reached; cryogenic temperature operation for which we have demonstrated our VPH gratings stand -180°C without any Blaze modification; and wavefront correction by post-polishing to minimize diffracted beam aberrations. With this latter technique, λ/6 wavefront over 10 cm diameter has been obtained in the first trial.

Use of subwavelength gratings in TIR incidence as achromatic phase shifters

Nulling interferometry constitutes a very promising technique in observational astrophysics. This method consists in attenuating the signal of a bright astrophysical object in order to detect much fainter nearby features, e.g. exoplanets around their host star. An on-axis destructive interference is created by adjusting the phases of the beams coming from various telescopes. The huge flux ratio between the parent star and the planet (106 in the thermal infrared) requires unprecedented high performance broadband phase shifters. We present a new design for these key components called Achromatic Phase Shifters (APS). We propose to use subwavelength diffractive optical elements under total internal reflection (TIR) incidence. Our component can be seen as an evolution of the Fresnel Rhomb technology.

Large-scale DCG transmission holographic gratings for astronomy

The recent interest of the astronomer community for volume phase holographic gratings is directly related to the enhancement of spectrograph throughput since this kind of grating can rise higher diffraction efficiency. Indeed, dichromated gelatine technology has demonstrated capability for 70-90% efficiency. From the heritage of several diffractive and holographic projects and applications, the Centre Spatial de Liege has recently decided to invest in the large-scale DCG grating technology. This paper will present the new facility which is now fully operational, its capability and first results obtained.

A far infrared/terahertz micromechanical sensor based on surface plasmons resonance

This paper describes a new concept related to the bolometric micromechanical sensors for detecting far IR and THz radiation. We believe that this concept permits a low cost and ease of fabrication of large bi-dimensional array of sensors with an enhanced signal-to-noise ratio. The micromechanical sensor comprises a thermo-sensitive bi-material (multi-material) micro-cantilever beam with a selective absorber dedicated to far IR and THz radiation energy, and optical readout system based on surface plasmon resonance for detecting the bending of the micro-cantilever element. To increase the radiation detector sensitivity, the SPR phenomenon is used for cantilever deflection monitoring.

Performance of solmacs, a high PV solar concentrator with efficient optics

A new solar panel with high concentration photovoltaic technology (x700) has been designed, prototyped and tested in the SOLMACS project. The quality of concentrating optics is a key factor for high module efficiency. Therefore new dedicated PMMA Fresnel lenses were studied and produced by injection molding. Lens design, material and production process were optimized to achieve a high optical yield of 86%. Thorough lens performance assessment in optical laboratory was completed with lifetime UV aging tests. Another important aspect is the thermal control of the hot spot created under the solar cell that receives the concentrated flux of 700 Suns. A dedicated heat spreader was developed to achieve passive thermal control with minimum mass and cost. This was supported by thermal models and tests at both cell and module level. 35% triple junction cells were implemented in the module. Micro-assembly technologies were used for the cell packaging and electrical connections. In support to the research, a continu...

Infrared Achromatic Phase Shifters using Modulated Total Internal Reflection

We propose a new family of achromatic phase shifters that uses the modulated total internal reflection (TIR) phenomenon. These components can be seen as enhanced Fresnel rhombs for infrared applications like nulling interferometry and polarimetry. The TIR phenomenon comes with a differential phase shift between the polarization components of the incident light. Modulating the index transition at the TIR interface allows compensating for the intrinsic material dispersion in order to make the subsequent phase shift achromatic over broad bands. The modulation can be induced by a thin film of a well-chosen medium or a subwavelength grating whose parameters are specially optimized. We present results from theoretical simulations together with preliminary fabrication outcomes.

Holographic filter applied to LCD projector for improvement of laser pointer presentation

This paper presents the concept and feasibility demonstration of an improvement of the LCD projector optics. An innovative concept of integration of an holographic filter in the optical head allows for laser pointer tracking. The filter retrieves the positioning information of the pointer on a PSD (Position Sensitive Device). The detector is interfaced with the LCD matrix in order to project a pattern on the screen. Several software options can be implemented for remote control of the slide show. We present the holographic geometry optimization, the experimental recording, and results.