Sabrina Velghe

Wave-front reconstruction from multidirectional phase derivatives generated by multilateral shearing interferometers.

To increase the accuracy of wave-front evaluation, we propose to exploit the natural capability of multiple lateral shearing interferometers to measure simultaneously more than two orthogonal phase derivatives. We also describe a method, based on Fourier-transform analysis, that uses this multiple information to reconstruct the wave-front under study.

Wollaston prism-like devices based on blazed dielectric subwavelength gratings

A Wollaston prism-like binary dielectric grating is presented and analyzed. It behaves like a transmission grating, differentially and symmetrically blazed for the two crossed polarization states, TE and TM. The phase profile is obtained by means of subwavelength structures etched in a high optical index isotropic dielectric medium (gallium arsenide, for instance). The performance of the device is illustrated by numerical examples and sketched in terms of spectral bandwidth and of extinction ratio. Some practical issues related to the fabrication are discussed.

Two-color multi-wave lateral shearing interferometry for segmented wave-front measurements

The possibility to measure segmented wave-front thanks to lateral shearing interferometry using diffraction grating is presented and analyzed. Aside from the response of such technique, the dynamic range is evaluated and shown to be limited. To greatly extend this one, a new method based on the use of two colors, not necessarily monochromatic, combined with an innovative Fourier treatment, is proposed. The two-color proposed in this paper is a high dynamic and low sensitivity technique; it can be completed by a one-color analysis, with low dynamics and high sensitivity, to reach high precision measurements. The ability of this method to measure Keck-like wave-front is demonstrated thanks to a computational analysis. Finally, a first experimental measurement of an etched substrate by using a quadri-wave lateral shearing interferometer is detailed.

In situ optical testing of infrared lenses for high-performance cameras.

We propose to evaluate infrared lenses with a dedicated analyzer having the same mechanical interface as the usual cameras. The proposed analysis is based on a wavefront measurement and allows a diagnostic of possible internal defects of the analyzed lens. The infrared lens analyzer described is constituted with a quadriwave lateral shearing interferometer and works with a blackbody light. We describe the response of this interferometer and an innovative method to obtain the wavefront under test. We finally present the experimental analysis of long-wavelength infrared lenses and the particular case of a modified lens that generates a large spherical aberration.

Visible and infrared wave-front metrology by Quadri-Wave Lateral Shearing Interferometry

Based on lateral shearing interferometry, a powerful technique, called the Quadri-Wave Lateral Shearing Interferometer (QWLSI), is used to evaluate the wave-front distortions with a high accuracy. Our device can be used for the characterization of complex and very aberrant optical devices, the testing of optical components, the control of adaptive optics and also for laser beam evaluation. The particular design of the QWLSI provides him interesting properties for wave-front metrology such as a high transverse resolution, a tunable sensitivity or also an evaluation of the measurement error. An innovative interferogram analysis allows also an increasing accuracy of the measurement. After dealing with these aspects, we will conclude by presenting an experimental wave-front analysis of a femtosecond laser pulse and an analysis in the far infrared spectral range [λ=8-14 μm].

Advanced wave-front sensing by quadri-wave lateral shearing interferometry

Based on multi-lateral shearing interferometry, a powerful technique, called the Quadri-Wave Lateral Shearing Interferometer (QWLSI) is used to evaluate the wavefront in an accurate and precise way. Our device can be used for the characterization of complex and very aberrant optical devices, the control of optical components and also for laser beam evaluation. This communication will detail the response of the QWLSI and its metrological performances, such as its high resolution, its adjustable sensitivity and dynamic. It will then be focused on two innovative applications of the QWLSI. The first application concerns the evaluation of infrared lenses dedicated to high-performance cameras. We will present experimental results recently completed by our prototype dedicated to the LWIR domain (λ=8-14μm). In a second part, we will study the possibility to analyze wave-fronts with discontinuities. Such wave-fronts can be produced by segmented mirrors, diffractive components or also bundle of single-mode fibers. We will finally present simulation results for this latter application.

Fast MTF and aberrations analysis of MWIR and LWIR imaging systems using quadri wave interferometry

We present the application of Quadri-Wave Lateral Shearing Interferometry (QWLSI), a wave front sensing technique, to characterize optical beams at infrared wavelengths from 2 to 16μm with a single instrument. We apply this technique to qualify optical systems dedicated to MWIR (λ within 3 and 5μm) and LWIR (λ within 8 and 14μm) wavelength ranges. The QWLSI offers the crucial advantage that it yields an analyzed wave front without the use of a reference arm and consequent time consuming alignment. The qualification of an optical system with QWLSI gives a complete diagnostic, from the aberration cartography to the PSF and MTF curves for every direction in one single measurement. In this paper, we first present the QWLSI technology and its main features, we also detail an experimental comparison between our MTF measurement and the results given by a classical MTF test bench. We finally show the experimental analysis of an infrared lens at two different wavelengths, one in the MWIR range (λ=3.39μm) and the other in the LWIR range (λ=10.6μm).

Segmented wave-front measurements by lateral shearing interferometry

The need for segmented wave front measurements has been rocketing for several years. The applications are various: thickness of metallic masks, diffracting elements, phasing of the primary segmented mirrors of telescopes, such as the Keck telescope, laser beam coherent recombination... Lateral shearing interferometers are common wave front sensors, used with success to test classical optical components. This technique does not require a reference wave, which is a major advantage. The lateral shearing interferometry has also proved successful to analyze segmented wave front; results of such a measurement by a diffraction-grating based interferometer are presented and analyzed. We dwell upon quadri-wave lateral shearing interferometers (QWLSI), which offer the possibility to characterize two-dimensionally the wave front, in a single measurement. This technique combines accuracy and qualities such as compactness and simplicity. Moreover, a chromatic regime of lateral shearing interferometers based on diffraction grating can be pointed out; this allows a two-color analysis to greatly extend the dynamic range. In the first parts we will present general considerations on QWLSI and segmented surface; then a technique to increase the dynamic range is investigated both theoretically and experimentally.

Accurate and highly resolving quadri-wave lateral shearing interferometer, from visible to IR

A powerful and achromatic interferometric technique based on recent advances in the technology of non-diffracting arrays is used to evaluate wave-front distortions with a high transverse resolution and an easily tunable dynamic range. The device presented here belongs to the family of Multiple Wave Lateral Shearing Interferometers (MWLSI) and has the natural capability to measure simultaneously four wave-front derivatives. In this communication, we propose to exploit all the information de facto included in the interferogram; in this perspective we suggest a new method of reconstruction of the wave-front knowing its derivatives. We will show that our device is already successful in the field of laser metrology in the near infrared domain and that it is promising for the far infrared domain with the presentation of a prototype dedicated to measurements at 10.6µm.

On-axis and off-axis characterization of MWIR and LWIR imaging systems using quadri-wave interferometry

The Quadri-Wave Lateral Shearing Interferometry (QWLSI) is an innovative wave front sensing technique that is commercially available for MWIR and LWIR applications. We present this technology and its application to the metrology, on and off-axis, of infrared imaging systems. The bench is only composed of a collimated reference beam that creates a source point at infinity, the objective to analyze and the sensor placed a few millimeters after the focal spot. Thanks to this direct measurement configuration, the alignment process is very simple and fast. A complete characterization (aberrations, MTF, field curvature) for several field points is possible within a few minutes.