Laser Ultrasound for NDT: investigation of the generation beam shape

Usually laser ultrasonic systems use an infrared laser operating at 10 µm to generate the ultrasound in CFRP objects. However this system suffer from an important drawback: optical fiber cannot be used to transport the high energy beam from the laser output to the target which limits the flexibility of the system to investigated complex shaped objects. To overcome this issue, visible light can be used to generate ultrasound. In our case we use a fiber-coupled laser operating at 532 nm. The output end of the optical fiber is placed on an industrial robot arm. The investigated object remains stationary while the optical fiber mounted on the robot arm scan the object. This system offers large flexibility but laser generation at 532 nm is known to be less efficient than CO 2 system emitting at 10 µm. Increasing the visible pulse power is one of the options but optics and CRFF object will be damaged before the same echo level as CO2 generation can be obtained. An alternative solution consists in the investigation of the generation beam size and shape. We recently developed an optical design composed of an axicon lens able to generate annular beam. The diameter of the beam can be adapted. If the axicon lens is removed the optical system provides a disk whose diameter can also be adapted onto the investigated object. Ray-traycing simulations performed with ASAP and experimental beam profile investigated with a beam profiler show good agreement between theoretical and experimental optical design. An experimental comparison between ring and disk shapes will be presented. The comparisons take into account the beam diameter and the beam intensity but also the effect of the angle of incidence will be presented.TECCOM

Continuous Solar Simulator for Concentrator Photovoltaic Systems

A continuous solar simulator for measuring performance of concentrator photovoltaic (CPV) systems is presented. The illumination system is based on a Xenon lamp, a homogenizer rod, shaping optics and a 30cm diameter collimator. The design optimises the reproduction of the characteristics of direct solar illumination: 32’ divergence, high spatial homogeneity, sun-like spectral distribution, with a maximum intensity of 250W/m². It accommodates pass-band and attenuation filters to tune the beam output. It operates in continuous mode, allowing to investigate CPV thermal aspects as well. The present paper addresses the concept design of the solar simulator and associated performance results

Status of the SALTO demonstrator: project overview and first on-sky operations

SALT

Achromatization of nonimaging Fresnel lenses for photovoltaic solar concentration using refractive and diffractive patterns

In the field of concentrated photovoltaics, the main disadvantage of lenses compared to mirrors lies in their chromaticity: Snell's law is related to the refractive index which is wavelength dependent. Consequently, even for purely collimated beams under normal incidence, the maximum concentration achievable with typical lenses made of PMMA is limited to ~1000×. This maximum value becomes even lower when considering Sun's angular aperture. Since the law of reflection is not wavelength dependent, mirrors can theoretically achieve the thermodynamic limit of concentration which is about 46'000×. This thesis aims at the design and the manufacturing of an achromatic Fresnel lens suitable for photovoltaic solar concentration, i.e. combining high concentration, low production cost and tolerance to manufacturing errors. Firstly, we investigated a hybrid lens made of a refractive lens and a diffractive lens. The investigations showed that the concentration ratio could be multiplied by 4. A full chapter is dedicated to the optimisation of blazed diffraction gratings to finally achieve the design of the diffractive lens. Nevertheless, a bilayer diffractive lens is needed to obtain a high diffraction efficiency which makes the diffractive lens highly sensitive to manufacturing errors and consequently not suitable for photovoltaic solar concentration. Purely refractive achromatic Fresnel doublets were then investigated and several designs were compared. They allow for very high concentration ratios in the case of collimated beams under normal incidence, higher than 100'000×. Therefore, contrary to singlets, Fresnel doublets are much more affected by the angular size of the source than by the chromatic aberration. Moreover, it was shown that they are tolerant to manufacturing error, change of temperature and uncertainty on the refractive index. It emerges from this thesis that the concept of achromatic doublets is a tolerant and low-cost production solution to achieve a highly concentrated white flux. Although bilayer diffractive lenses are not suitable for concentrated photovoltaics, the combination of refractive with diffractive structures seems to be promised to a bright future for spectrum splitting applications, including spectrum splitting for concentrated photovoltaics

Achromatization of solar concentrator thanks to diffractive optics

Refractive solar concentrators suffer from important chromatic aberration. To enhance solar cell performance and increase the concentrating ratio, we propose an hybrid (diffractive/refractive) lens which allows a more uniform flux density and an achromatized (in the sense of achromatic doublet) illumination of the cell. To solve the problem of high diffraction efficiency, we turn to multilayer diffractive lenses to have broadband high diffraction efficiency optimized on solar spectrum. Rigorous coupled wave analysis simulations were performed in order to check the validity of scalar theory, they led to a new order of magnitude for the ratio period/wavelength which has been found to be around 100 in place of 10.WALI

Nonimaging achromatic shaped Fresnel lenses for ultrahigh solar concentration

The maximum concentration ratio achievable with a solar concentrator made of a single refractive primary optics is much more limited by the chromatic aberration than by any other aberration. Therefore achromatic doublets made with poly(methyl methacrylate) and polycarbonate are of great interest to enhance the concentration ratio and to achieve a spectrally uniform flux on the receiver. In this Letter, shaped achromatic Fresnel lenses are investigated. One lossless design is of high interest since it provides spectrally and spatially uniform flux without being affected by soiling problems. With this design an optical concentration ratio of about 8500× can be achieved

Achromatization of solar concentrator using diffractive optics

Photovoltaic energy suffers from payback time. High solar concentration (above 500 suns) is a very interesting way to reduce production cost. The use of lenses allows more flexibility to make light flux more uniform. In addition lenses are less prone to errors of manufacture and can be easily duplicated. Unfortunately, optical materials are very dependent on the wavelength which causes important chromatic aberrations. One can think about achromatic doublets but doublets are thick, heavy and require exotic glasses which make them expensive. During the conference, I would like to present an innovative way to achromatize solar concentrators using hybrid (refractive/diffractive) lenses. Indeed, an hybrid lens is as thin as a Fresnel lens and is made in only one glass. But diffractive lenses suffer from limited broadband diffraction efficiency due to spurious orders. To overcome the lake of efficiency we are investigating the use of multi-layer diffractive optical element. This technique allows to achieve an efficiency greater than 97% over the full visible spectrum using only three components (e.g. two optical elements + air)

Hybrid Lens for Solar Concentration: Optimization of the Multilayer Diffractive Lens

Manufacture process degrades the ideal shape of multilayer diffractive lens due to draft angle, half radius tool and slope error. We show some shape improvements using an extended scalar theory.Guide2Dy

Laser ultrasound flexible system for non-contact inspection of medium size and complex shaped composite structures made of carbon fiber reinforced polymer

We present the development of a contactless laser ultrasound system for nondestructive inspection of CFRP complex structures. Ultrasound are generated by a thermoelastic effect resulting from a green pulsed laser insulating a point of the inspected part. The resulting displacement of the surface point is probed by a two-wave mixing based interferometer working in the near infrared. The system is flexible and completely fiber-coupled. It is able to provide C-scans on complex shaped CFRP aeronautical structures.TECCOM