RESUMEN

The sea surface is a complex dynamic structure dependent on atmospheric conditions, and for which physical and chemical properties change from water to foam. Its roughness determines how the surface reflects, absorbs, and emits radiance, and depends on multiple parameters such as wind speed and direction, and foam and turbulence induced from natural waves or from object displacement. In this paper, a model description is given for laser reflection on the sea surface in open water driven by the wind. The model allows calculation of the reflected laser radiance from the sea surface toward a receiver as a function of the incoming laser radiance with a known beam intensity profile. Each subarea of the sea surface seen by one pixel of the receiver is considered as an ensemble of facets, where each facet is defined by its x and y directional slopes. The wind speed and orientation determine the probability density function of the sea surface facet slope occurrence. In this paper, we have analytically expressed the reflected radiance on the sea surface as a function of the wind speed, receiver range, receiver heading, laser position, laser output aperture, and laser incoming radiance. Using the tolerance ellipse, the reflected radiance expression was approximated, and both direct and approximated results were compared. The richness in behavior of the reflected radiance and its dependence on the geometry of the problem were studied showing the impact of the receiver position, the laser position, heading, and beam divergence.

RESUMEN

Recent works on plasma channels produced during the propagation of ultrashort and intense laser pulses in air demonstrated the guiding of electric discharges along the laser path. However, the short plasma lifetime limits the length of the laser-guided discharge. In this paper, the conductivity and lifetime of long plasma channels produced by ultrashort laser pulses is enhanced efficiently over many orders of magnitude by the electric field of a hybrid AC-DC high-voltage source. The AC electric pulse from a Tesla coil allowed to stimulate and maintain the highly conductive channel during few milliseconds in order to guide a subsequent 500 times more energetic discharge from a 30-kV DC source. This DC discharge was laser-guided over an air gap length of two metres, which is more than two orders of magnitude longer than the expected natural discharge length. Long plasma channel induced by laser pulses and stimulated by an external high-voltage source opens the way for wireless and efficient transportation of energetic current pulses over long air gaps and potentially for guiding lightning.

RESUMEN

We demonstrate for the first time the possibility to generate long plasma channels up to a distance of 1 km, using the terawatt femtosecond T&T laser facility. The plasma density was optimized by adjusting the chirp, the focusing and beam diameter. The interaction of filaments with transparent and opaque targets was studied.

RESUMEN

We report an efficient transfer of 800 nm energy into both the ultraviolet and the far infrared (IR) during the filamentation in air of an appropriately shaped laser pulse. The multiorder enhancement of the IR supercontinuum in the 3-5 µm atmospheric transmission windows was achieved thanks to spectral-step cascaded four-wave mixing occurring within the spectrum of the shaped femtosecond laser pulse. These results also point out the limit of the self-phase modulation model to explain the spectral broadening of a filamenting laser pulse.

RESUMEN

We have investigated the guiding and triggering of discharges from a Tesla coil type 280 kHz AC high voltage source using filaments created by a femtosecond Terawatt laser pulse. Without the laser the discharges were maximum 30 cm long. With the laser straight, guided discharges up to 110 cm length were detected. The discharge length was limited by the voltage amplitude of the Tesla coil.

RESUMEN

We report the impact of the spatial coherence distortion on the measured absorption spectra and the identification of materials analyzed by terahertz time-domain spectroscopy. It is shown that the deformation of the terahertz beam wave front can result into the overestimation of the electromagnetic absorption, the generation of artificial absorption peaks and even to the disappearance of characteristic absorption peaks. Obtaining clear absorption spectra without artifacts is crucial for applications based on terahertz imaging and spectroscopy.

Asunto(s)

Óptica y Fotónica , Imágen por Terahertz/métodos , Espectroscopía de Terahertz/métodos , Artefactos , Lactosa/química , Materiales Manufacturados , Modelos Estadísticos , Modelos Teóricos , Poliésteres/química , Politetrafluoroetileno , Polvos , Radiación

RESUMEN

We demonstrate that a femtosecond-laser filament in both molecular and atomic gases is birefringent for a copropagating probe pulse. Any input-probe polarization is decomposed into two orthogonal components, the optical axis being in the pump polarization direction. In molecular gases, the birefringence is mainly due to the delayed rotational molecular-wave packet; the probe pulse thus experiences several revivals in time. The two probe components end up spatially separated in the far field. In atomic gases such as argon, the effect is weaker and is attributed to the instantaneous electronic cross-phase modulation.

RESUMEN

Ultraviolet and infrared conical emissions were observed during the filamentation in air of powerful femtosecond laser pulses produced by a portable terawatt laser system. The broadband spectrum was measured from 200 nm up to 14 microm and covered the complete optical transmission window of the atmosphere. The angularly resolved spectrum showed some X-wave structure across the frequency range analyzed. However, we demonstrated that the strong conical emission observed in the mid- and far-infrared is mainly owing to the four-wave mixing between the pump pulse and its blueshifted conical emission.

RESUMEN

We demonstrate that a femtosecond laser pulse-induced filament can act as a "polarization separator" for a copropagating femtosecond probe pulse. The probe component with polarization parallel to the pump is guided along the propagation axis through the cross interactions with the pump pulse. And the probe component with polarization orthogonal to that of the pump is diffracted out into an outer ring by the plasma. The extinction ratio (I(probe)(min) / I(probe)(max)) along the propagation axis is better than 1:20.

RESUMEN

We introduce a new six-degrees-of-freedom alignment technique that enables the precise alignment of two optical device planes. The method combines linear diffraction gratings and cylindrical Fresnel lenses that allow the diagnosis of misalignment in each degree of freedom independently. The technique was used to align two 20 mm x 20 mm fused-silica substrates separated by 17 mm. The worst-case alignment precision was found to be better than +/- 5 microm laterally, +/- 20 microm longitudinally, +/- 0.036 degrees rotationally, and +/- 0.007degrees in tilt.

RESUMEN

A model for a clustered free-space optical interconnect is developed and is used to determine the maximum array density that can be achieved, together with the optimal cluster parameters that maximize this density. This model includes misalignment tolerance and the impact of multimode vertical-cavity surface-emitting laser beams. We find that for short interconnect distances, the maximum channel density is limited by the speed of the relay lenses, but as the interconnect distance increases, geometric aberrations become the limiting factor. We also determine the interconnect distance below which a micro-channel relay is more suitable and the distance above which a single-lens solution is adequate.

RESUMEN

A folded structured light generator is presented. This spot generator is to be used in a modulator-based free-space optical interconnect. Three cascaded diffractive optical elements produce 4 x 8 clusters on an 800 microm x 1600 microm pitch, in which each cluster is a 4 x 4 array of 13.1-microm-radius spots on a 90-microm pitch. The folded configuration is more compact than an existing linear spot-array generator and replaces 14 optical surfaces with eight surfaces. Details of the optical design, sensitivity analysis, alignment techniques, assembly, and test results are presented.

RESUMEN

A new compact in situ method of measuring the perpendicularity of a plane wave to a substrate is proposed. Off-axis cylindrical Fresnel lenses are used to focus a portion of the incident plane wave onto target lines. The displacement of the focal line from the targets is determined by the degree of angular misalignment. The proposed design has been incorporated into a 10-mm-thick fused-silica module, which enables us to obtain an alignment precision of better than 0.0083 degrees. This method is designed for use in optical assembly procedures that require an incident collimated beam that is normal to the alignment features. Experimental results are presented.

RESUMEN

A vertical-cavity surface-emitting laser based bidirectional free-space optical interconnect has been implemented to interconnect two printed circuit boards. A total of 512 clustered channels with a density of 2844 channels/cm2 are transmitted over a distance of 83 mm. The optical interconnect is a combination of refractive microlenses and diffractive minilens relays.