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Acousto-optic programmable dispersive filters (AOPDF) can compensate in real time for large amounts of group-delay dispersion. This feature can be used in chirped-pulse amplification femtosecond laser chains to compensate adaptively for dispersion. An analytical expression relating the group delay at the output of the AOPDF to the input acoustic signal is obtained with coupled-wave theory in the case of collinear and quasicollinear bulk acousto-optic interactions and also in the case of planar waveguides and optical fibers. With this relation, the acoustic signal that will induce an arbitrary group-delay variation with frequency can be easily obtained. Numerical simulations are shown to support the principle of arbitrary group-delay control with an AOPDF.

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With 31 FiguresThe light-induced changes of refractive indices in electro-optic crystals are based on the spatial modulation of photocurrents by nonuniform illumination. The generation of photocurrents at low light intensity depends on the presence of suitable donors, because most of the crystals of interest are intrinsically transparent in the visible. The electrons or holes, which are excited from the impurity centers by light of a suitable wavelength, are upon migration, retrapped at other locations, leaving behind positive or negative charges of ionized trap centers. The photoexcited charges will be reexcited and retrapped until they finally drift out of the illuminated region and are trapped. The resulting spacecharge field between the ionized donor centers and the trapped charges modulates the refractive indices via the electro-optic effect. Uniform illumination erases the space-charge fields and brings the crystal back to its original state (which process is known as optical erasure).

We describe the programmable spatial beam shaping of 100-kHz, 4-microJ amplified femtosecond pulses in a focal plane by wave-front modulation. Phase distributions are determined by a numerical iterative procedure. A nonpixelated optically addressed liquid-crystal light valve is used as a programmable wave-front tailoring device. Top-hat, doughnut, square, and triangle shapes of 20-microm size are obtained in a focal plane. Their suitability for femtosecond laser machining is demonstrated.

We experimentally demonstrate efficient optical carrier reduction of microwave signals with a single-mode 1.5-microm wavelength Brillouin all-fiber ring laser. Because of the tunable optical coupling, the lasing threshold of the short-length (20-m) fiber cavity is lower than 5 mW, and high conversion efficiencies (up to 60%) are obtained at any pump power up to approximately 200 mW. Using the single-mode Stokes beam as a seed for the stimulated Brillouin scattering process allows up to 40-dB optical carrier depletion with almost no added noise for an optically carried microwave signal at 6 GHz. In addition, using this resonator, we provide evidence of generation of high-spectral-purity beatnotes.

The experimental demonstration and the far-field pattern characterization of an optically controlled phased-array antenna are described. It operates between 2.5 and 3.5 GHz and is made of 16 radiating elements. The optical control uses a two-dimensional architecture based on free-space propagation and on polarization switching by N spatial light modulators of p × p pixels. It provides 2(N-1) time-delay values and an analog control of the 0 to 2π phase for each of the p × p signals feeding the antenna (N = 5, p = 4).