Brigitte Loiseaux

Programmable focal spot shaping of amplified femtosecond laser pulses

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.

Signal-beam amplification by two-wave mixing in a liquid-crystal light valve

A new two-wave-mixing interaction with gain through a Bi(12)SiO(20) liquid-crystal light valve is presented. We show that the diffraction of a pump beam in the direction of a weak signal leads to a net amplification of the signal beam, with no need for a phase shift between the interference pattern and the induced index grating. A two-wave-mixing gain of 10 and a 150-ms response time are obtained with a 8.8-microm -thick liquid-crystal layer, a total intensity of the interacting beams of only 200 muW/cm(2), and an ac external voltage of +/-6 V . Image amplification is also demonstrated.

NEARLY DIFFRACTION-LIMITED LASER FOCAL SPOT OBTAINED BY USE OF AN OPTICALLY ADDRESSED LIGHT VALVE IN AN ADAPTIVE-OPTICS LOOP

We demonstrate correction of laser wave-front distortions by use of an adaptive-optical technique based on a light valve. The setup consists of an achromatic and adjustable-sensitivity wave-front sensor and a wave-front corrector relying on an optically addressed liquid-crystal spatial light modulator. Experimental results with strongly aberrated beams focused close to the diffraction limit are presented for the cw regime. Additional experiments with pulses and measurement of damage thresholds show that this approach is relevant for spatial phase correction of ultraintense laser pulses.

Increase in depth of field taking into account deconvolution by optimization of pupil mask

We consider optimization of hybrid imaging systems including a phase mask for enhancing the depth of field and a digital deconvolution step. We propose an image quality criterion that takes into account the variability of the systems point-spread function along the expected defocus range and the noise enhancement induced by deconvolution. Considering the classical cubic phase mask as an example, we show that the optimization of this criterion may lead to filter parameters that are significantly different from those usually proposed to ensure the strict invariance of the PSF.

Superresolution along extended depth of focus with binary-phase filters for the Gaussian beam

In the paraxial Debye regime, simple and power-efficient pupil filters are designed to break the diffraction limit along a large depth of focus (DOF) for the Gaussian beam. Dependences of the superresolution factor, DOF gain, Strehl ratio, sidelobe strength, and axial intensity nonuniformity on the Gaussian profile in the pupil plane are characterized using the numerical method. Optimal filter designs are proposed for either high-resolution or ultra-large-DOF applications followed by experimental verifications.

Spatial mode control of a diode-pumped Nd:YAG laser by an intracavity liquid-crystal light valve

We present a new beam-shaping technique with an intracavity optically addressed liquid-crystal spatial light modulator. The Nd:YAG resonator is able to deliver beams with various spatial profiles such as flat-topped super-Gaussian and square-shaped beams.

Dispersive holographic microlens matrix for single-LCD projection

Single-LCD projection is the simplest and most compact architecture for LCD projection. It is simply made of a lamp, a LCD with color filters and a projection lens. However it suffers from poor luminous efficiency due to color filter losses, since each filter transmits only about one third of the flux emitted by the lamp. We report a new bright single-LCD architecture based on innovative use of an holographic microlens matrix, which provides the spatio- chromatic illumination of the LCD. This specific component illuminates each LCD pixel with the primary color corresponding to the color video signal driving the pixel. Correct RGB spatial repartition is obtained by taking advantage of the holographic lens spectral dispersion. When illuminated by white light, the focus is spectrally spread out. This effect has been utilized to illuminate three adjacent LCD pixels within red, green and blue light, the LCD being located at the focal plane. The use of holographic microlens arrays allow the suppression of color filters and a compensation of LCD aperture ratio. Thus, this new compact single-LCD projection architecture ensures a flux gain factor above three compared to classical ones, as well as saturated primary colors compatible with high quality LCD video image projection.

Phase volume holographic optical components for high-brightness single-LCD projectors.

Projection systems based on liquid-crystal displays (LCDs) offer new opportunities to display high-definition and large-size TV images. There are two types of LCD projector architectures: the 3-LCD architecture uses one LCD for each primary color, red, green, and blue, whereas a single-LCD configuration employs only one LCD paved with color filters. The single-LCD projector is simple and compact but suffers from a poor luminous efficiency because of losses in the color filters: each filter transmits only ~1/3 of the flux emitted by the lamp. To increase this optical efficiency, we propose to introduce volume holographic elements in the architecture of a single-LCD projector. Innovative systems are presented in which volume holographic elements realize the spatiochromatic illumination of the LCD. This illumination consists of selectively directing all the light that corresponds to a primary color, red, green, or blue, in the pixel addressed with the corresponding video composite signal and exploits the spectral selectivity and dispersion properties of volume holographic gratings and lenses. The two main advantages of such illumination are the suppression of the color filters and the recovery of the light lost in a classical architecture by absorption of the color filters. A complete luminous efficiency analysis of spatiochromatic illumination with volume holographic elements is presented. The achieved performances are compared with classical single-LCD projectors.

Comparison of depth-of-focus-enhancing pupil masks based on a signal-to-noise-ratio criterion after deconvolution

We consider optimization of hybrid imaging systems including a pupil mask for enhancing the depth of field and a digital deconvolution step. In a previous paper [Opt. Lett. 34, 2970 (2009)] we proposed an optimization criterion based on the signal-to-noise ratio of the restored image. We use this criterion in order to optimize different families of phase or amplitude masks and to compare them, on an objective basis, for different desired defocus ranges. We show that increasing the number of parameters of the masks allows one to obtain better performance.

Analysis of blazed diffractive optical elements formed with artificial dielectrics

A new hybrid method for the analysis of diffractive optical elements, which combines fully vectorial and scalar theories, is presented. It is suitable for use with elements of arbitrary large zone, even when the local feature size is of the order of the wavelength. To assess its applicability, we have performed cross-checking tests. The model is shown to accurately predict many optical properties of diffractive optical elements based on two-dimensional artificial dielectrics, like the useful energy diffracted into the order of interest or the deterministic loss into high diffraction orders for an illumination with a wavelength different from the design wavelength or for highly oblique incidence.