Fabrice Devaux

Parametric amplification of a polychromatic image

The picosecond parametric amplification of a polychromatic image with a wavelength bandwidth of 140 nm and a gain of 15 dB has been obtained in a lithium triborate type-I crystal (LBO). Approximately 8 lines/mm in both lateral dimensions were resolved in the crystal plane. This resolution value is in good agreement with a numerical study of the phase-matching conditions near collinear degeneracy, where phase matching is noncritical for the signal beam in angle as well as in wavelength. The parametric amplification of a monochromatic image in LBO is also studied. Because their polarizations are identical, the signal and the idler can interfere, leading to phase-sensitive amplification.

Two-dimensional time-resolved direct imaging through thick biological tissues: a new step toward noninvasive medical imaging.

We report an original two-dimensional time-resolved direct imaging method for transillumination optical tomography that combines the time-gating and forward phase-conjugation properties of type II degenerate parametric amplification. An object with subcentimeter resolution embedded in 4-cm-thick chicken breast tissue was imaged with a signal-to-noise ratio of 2.

Image restoration through aberrant media by optical phase conjugation in a type II three-wave mixing interaction

Restoration of an image through an aberrant plate has been achieved by use of optical phase conjugation in a type II parametric interaction in which the amplified signal and idler waves are reflected back through the aberrant medium. The resolution of the restored phase-conjugate images is equal to 0.18 mm, and the amplification gain is ~4dB.

Computational temporal ghost imaging

Ghost imaging is a fascinating process, where light interacting with an object is recorded without resolution, but the shape of the object is nevertheless retrieved, thanks to quantum or classical correlations of this interacting light with either a computed or detected random signal. Recently, ghost imaging has been extended to a time object, by using several thousands copies of this periodic object. Here, we present a very simple device, inspired by computational ghost imaging, that allows the retrieval of a single non-reproducible, periodic or non-periodic, temporal signal. The reconstruction is performed by a single shot, spatially multiplexed, measurement of the spatial intensity correlations between computer-generated random images and the images, modulated by a temporal signal, recorded and summed on a chip CMOS camera used with no temporal resolution. Our device allows the reconstruction of either a single temporal signal with monochrome images or wavelength-multiplexed signals with color images.

Imaging through scattering media by parametric amplification of images: study of the resolution and the signal-to-noise ratio

An imaging scheme through scattering media in which parametric image amplification is used is presented. An image of a resolution chart through a solution of latex microspheres with an attenuation of 22 mean free paths is obtained with a resolution of 20 mum. The evolution of the signal-to-noise ratio with respect to the medium attenuation is studied and compared with a rough modeling of the imaging process.

Temporal ghost imaging with pseudo-thermal speckle light

We report ghost imaging of a single non-reproducible temporal signal with kHz resolution by using pseudo-thermal speckle light patterns and a single detector array with a million of pixels working without any temporal resolution. A set of speckle patterns is generated deterministically at a sampling rate of tens kHz, multiplied by the temporal signal and time integrated in a single shot by the camera. The temporal information is retrieved by computing the spatial intensity correlations between this time integrated image and each speckle pattern of the set.

Optimizing the signal-to-noise ratio in the measurement of photon pairs with detector arrays

To evidence multimode spatial entanglement of spontaneous down-conversion, detector arrays allow a full-field measurement, without any a priori selection of the paired photons. We show by comparing results of the recent literature that electron-multiplying CCD cameras allow, in the present state of technology, the detection of quantum correlations with the best signal-to-noise ratio (SNR), while intensified CCD cameras allow at best the identification of pairs. The SNR appears to be proportional to the square root of the number of coherence cells in each image, or Schmidt number. Then corrected estimates are derived for extended coherence cells and not-very-low and non-space-stationary photon fluxes. Finally, experimental measurements of the SNR confirm our model.

Integrated optofluidic index sensor based on self-trapped beams in LiNbO3

We show that self-trapped beams can form in structured monolithic lithium niobate chips. In particular, they are observed to be unaffected when crossing few hundred microns wide gaps. The technique is employed to fabricate an index sensor constituted of a buried circular optical waveguide crossing a fluidic channel in a lithium niobate substrate. Fluidic channels are realized by precision dicing while the optical waveguides are induced by photorefractive beam self-trapping controlled by the pyroelectric effect. The self-aligning property of this latter waveguides provides a simple fabrication technique of an integrated sensor that accurately measures the refractive index of transparent liquids.

The phase-mismatch vector and resolution in image parametric amplification

We derive up to second order the magnitude and direction of the phase-mismatch vector in parametric image amplification. This general analytical calculation is shown to be in agreement with our previous numerical studies of the image resolution at degeneracy.

Pump-power-dependent gain for small-signal parametric amplification in birefringent fibres

The nonlinear dependence of the parametric gain for small-signal amplification is experimentally studied in a normally dispersive highly birefringent single-mode fibre. As a result of the pump-induced nonlinear phase shift, the signal gain curve versus the pump power is bell shaped in the undepleted-pump regime with maximum gain attained away from perfect phase-matching. This dynamics is quite different from the quasi-exponential gain measured for spontaneous parametric emission growing from noise. Moreover, the experimental gain curves are more accurately predicted by modulation instability computations rather than four-wave mixing theory because of the influence of the non-phase-matched waves.