Bruno Toulon

Collective phase measurement of an array of fiber lasers by quadriwave lateral shearing interferometry for coherent beam combining

We present a new configuration of quadriwave lateral shearing interferometer dedicated to phase detection for beam-combining purposes. Assuming that the fibers are disposed in a matrix arrangement, our scheme gives direct access to the phase step between adjacent fibers in two dimensions. Experimentally recorded interferograms are made only of two-wave interference fringes that scroll as the phase evolves in the fibers. This simplicity allows fast treatment by the spatial demodulation process, and the phase map from the fibers can be estimated in real time. No external reference is required, and the technique is fully compatible with a high number of fibers.

Design of a fiber-collimated array for beam combining

In this paper, we present the design of a very precise collimated fiber array that meets requirements for beam combining. Calculations permit to determine the tolerances toward key parameters and specify the components to manufacture. Thus, the collimated fiber array is composed of a high quality commercial microlens array and an especially dedicated fiber holder that we design and realize experimentally. Manufacture techniques for both the microlens and the holder are chosen to be collective and then compatible with a high number of fibers. With the collimated fiber array hence obtained, the individual beam quality was measured to be λ/10 and the pointing accuracy is under 0.6 mrad.

Holistic characterization of complex transmittances generated by infrared sub-wavelength gratings.

We present a characterization technique of wide-area subwavelength structures. The optical bench is based on lateral shearing interferometry, which allows an accurate complex transmittance (phase and amplitude) measurement. The experimental validation is made in the long-wavelength infrared domain; more precisely we work in the integrated 8-9 microm spectral range. Measurements of the transmitted amplitude and phase shift reveal a good agreement with respectively experimental results based on Fourier Transform infrared spectrometry, and theoretical simulations.

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.

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.

Optical wavefront sensor based on sub-wavelength metallic structures

Lateral shearing interferometers (LSIs) are efficient tools for optical analysis. They allow classical optical wave-front aberrations measurements as well as the precise evaluation of abrupt steps. The basic element of an LSI is the transmittance grating, which diffracts a number of orders (two in the case of a mono-dimensional LSI, ideally three or four non coplanar orders in the case of bi-dimensional LSI). This brings the need for specifically designed transmittance gratings. For instance, a mono-dimensional LSI needs a sinusoidal-shaped transmittance, since its Fourier transform carries exactly 2 orders. Such transmittances are however either impossible or at least extremely costly to design using classical macroscopic techniques, mainly because the usual thin film deposition techniques require several technological steps, in order to get the desired light filtering effect. Given these constraints, we made use of sub-wavelength structures in order to build a new class of LSI. They are made of sub-wavelength lamellar metallic gratings specifically designed for the mid-infrared, and allow the precise coding of the desired transmission shape all over the LSI grating.

Phase and amplitude interferometric characterization of infrared nanostructured gratings

Sub-wavelength gratings allow to code complex transmittance functions that introduce both amplitude and phase variations in the propagation of a given wavefront. These micro-structures are a promising technique to miniaturize optical functions such as light polarizing, light confinement, spectral filtering... Realizations in the visible and the infrared domain have been fulfilled: for example micro-lenses, anti-reflection coatings or sinusoidal-transmittance can easily be coded. This technique is all the more advantageous in the mid-wavelength infrared (MWIR) or long-wavelength infrared (LWIR) spectral range since there are only a few materials available in this spectral range. However the characterization of these structures is problematical, since it involves phase and amplitude measurements. It is even more complicated in the far infrared domain (8 - 14 μm), as will be detailed. Besides, the finite size of the gratings introduces phase steps, which is well-known to be a problematic issue. We describe here a dedicated bench to characterize sub-wavelength gratings in the LWIR spectral range. The core of the bench is a quadri-wave lateral shearing interferometer based on a diffraction grating, which allows a complete two-dimensional characterization of both phase and amplitude in a single measurement. We present here theoretical and experimental results of a characterization of such a sub-wavelength grating.