Claude Froehly

Propagation soliton et auto-confinement de faisceaux laser par non linearité optique de kerr

Abstract An experimental demonstration is presented of the stable monomode self-trapping of laser beams with adequate shape and power when they are propagating through homogeneous transparent dielectrics whose refractive index exhibits fluctuations proportional to the local intensity. The field evolution in such material is governed by a nonlinear scalar wave equation, that possesses stable solutions called solitons. Those solutions correspond to the exact balance of the natural diffraction divergence of the beam by the convergence of the self-induced refractive index gradient.

Picosecond steps and dark pulses through nonlinear single mode fibers

We experimentally observed transmission of “negative pulses” through a single mode optical fiber in conditions where self-phase modulation, due to self induced variations of silica refractive index, may be balanced by the positive group velocity dispersion (dVg/dδ#62;0). Soliton propagation could be expected at power levels where exact compensation of these two distorsions arises. The duration of the output dark pulses, determined by SHG correlation techniques, decreases when the launched power is increased, as predicted by computer simulation. For an input peak power of 0.2 W, corresponding to the theoretical soliton peak power, the emerging dark pulse looks like the input one.

White-light supercontinuum generation in normally dispersive optical fiber using original multi-wavelength pumping system

We report on the experimental demonstration of a white-light supercontinuum generation in normally dispersive singlemode air-silica microstructured fiber. We demonstrate that the simultaneous excitation of the microstuctured fiber in its normal and anomalous dispersion regimes using the fundamental and second harmonic signals of a passively Q-switched microchip laser leads to a homogeneous supercontinuum in the visible range. This pumping scheme allows the suppression of the cascaded Raman effect predominance in favor of an efficient spectrum broadening induced by parametric phenomena. A flat supercontinuum extended from 400 to 700 nm is achieved.

II Shaping and Analysis of Picosecond Light Pulses

Publisher Summary This chapter describes shaping and analysis of picosecond light pulses and the space-time coherence effects due to the new situation arising in optics such as the circumstances when large amounts of energy are carried by every one of the space-time modes of the radiation, a situation realized for example with picosecond laser pulses. The chapter presents basic pulse types frequently involved in operations of shaping or analysis—quasi-monochromatic pulses, having time frequency defined enough for them to obey simple scalar diffraction laws, and the corresponding notion of quasi single space frequency pulses, with well defined space frequency, obeying simple dispersion laws. The chapter explains deterministic pulse shaping by interaction with filters and modulators in terms of temporally coherent optics. In addition, the chapter reviews methods for measurements of pulse durations and devotes its main development to the observations of complex amplitude distributions at the picosecond or sub-picosecond time scale.

Spatial-soliton-induced guided waves in a homogeneous nonlinear Kerr medium

It has been predicted that an intense pump beam can induce the focusing of a weak probe beam at a different wavelength that is simultaneously propagating in a Kerr homogeneous material through cross-phase modulation. In Kerr-type nonlinear material, spatial solitons are the only stable waves able to propagate at high-intensity levels without leading to catastrophic breakdown. We propose the use of a soliton beam to induce the stable guiding of a probe beam. Experimental results obtained with picosecond pulses are reported that evidence spatial-solitoninduced guiding.

Spectro-temporal imaging of femtosecond events

We demonstrate a new technique for the direct, real-time, femtosecond scale temporal measurement based on the conversion of temporal information to the spectral domain. The potential of the method has been experimentally investigated with an optical fiber spectral compressor device, first stretching and up-chirping the pulses in a prism dispersive delay line, and afterwards compensating the induced chirp by means of cross-phase modulation in a single-mode fiber. Spectro-temporal imaging (STI) reduces the problem of high-resolution temporal measurements to standard spectrometry.

Nouvelle méthode de mesure de la réponse impulsionnelle des fibres optiques.

A new technique for measuring the temporal transfer function of optical fibers is described. The method consists of placing the fiber under test in one arm of a Mach-Zehnder interferometer excited by a broadband source. The temporal impulse response is obtained from a holographic reconstruction. The method requires only short lengths of single-mode or multimode fibers (less than 1 m). We have measured a dispersion of 0.3 nsec/km.nm at 0.59 microm with a single-mode fiber, in good agreement with theory. The arrival times of the various modes of multimode fibers are resolved.

Flattening of the spatial laser beam profile with low losses and minimal beam divergence

A new type of Nd:YAG laser is described that uses two spatial filters that are placed against the mirrors in the focal planes of a converging lens (Fourier planes). With the appropriate filters, we experimentally obtained a uniphase beam with a flat spatial profile and minimal beam divergence in the free-running regime.

Shaping of short laser pulses by passive optical Fourier techniques

Abstract It is shown that continuous pulse shaping may be achieved by spreading the coherent pulse (30 psec) emitted by a mode- locked laser with the help of a convenient diffracting pupil. Experimental results are given, showing that pulses have been obtained with controlled shapes over 700 psec duration.

Coherent measurement of short laser pulses based on spectral interferometry resolved in time.

We propose a new method for measuring the phase and the amplitude of a short laser pulse that is based on shearing interferometry in the spectral domain combined with time gating. The method has several features in common with spectral interferometry for direct electric-field reconstruction, in particular, fast and direct reconstruction of the phase. Accurate measurement of the phase added to an 80-fs pulse by a block of F4 glass demonstrates the technique.