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.
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.
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.
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.
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