Bernard Ségard

Observation of negative velocity pulse propagation

The propagation of suitably shaped millimetre wave pulses through a linear resonant molecular absorber is studied in order to give an experimental support to the concept of negative group velocity. Significant pulse advances with negligible distortion are actually observed with a true shape detection.

Slow light in saturable absorbers

In connection with the experiments recently achieved on doped crystals, biological samples, doped optical fibers and semiconductor heterostructures, we revisit the theory of the propagation of a pulse-modulated light in a saturable absorber. Explicit analytical expressions of the transmitted pulse are obtained, enabling us to determine the parameters optimizing the time-delay of the transmitted pulse with respect to the incident pulse. We finally compare the maximum fractional delay or figure of merit so attainable to those which have been actually demonstrated in the experiments.

Modes of destabilization of Brillouin fiber ring lasers

Abstract We theoretically analyze the stability of the Brillouin fiber ring lasers. This study not only allows us to draw the portrait of the unstable domains for a large range of the system parameters, but it also gives valuable information on the dynamical regimes encountered in these domains. The role of the cavity modes on this dynamics is pointed out, leading to the determination of frequencies characterizing the output power modulation. Incommensurable frequencies lead to quasiperiodic regimes, the complexity of which increases with the fiber length.

Noncritical slowing down in optical bistability

We have experimentally studied the dynamics of a bistable system prepared in an initial state, close to the unstable intermediate branch of the bistability curve but far from the turning points. Our observations are indicative of a divergence of the switching times in —logϵ, ϵ characterizing the departure of the initial state from the unstable branch. This result, contrasting with the ϵ−12 standard law in critical slowing down, is well interpreted by a simple model involving a single dynamical variable.

Microwave spectrometer for saturated absorption experiments.

A spectrometer has been built to perform Doppler-free saturated absorption experiments in the millimeter range (30-300 GHz); a plane-cylindrical resonator between Stark plates has been used. With that device, inverted Lamb-dips have been observed at 115 GHz with a width 25 times below the Doppler width. However, the essential feature of this apparatus is to allow the application of Stark field typically of 2500 V/cm, leading to such specific uses as the Stark tuned Lamb-dip, level-crossing, and mode-crossing experiments. Typical examples are given and other applications are proposed.

Switching delays in optical bistability: An experimental study

Abstract The switching process in optical bistability is experimentally investigated on a model system made of a passive resonator containing an absorbing gas. The (up or down) switching is achieved by superimposing rectangular pulses on a cw power holding the system near a limit point. The observations are in good agreement with the predictions derived from the asymptotic theory of Mandel and support in particular his pulse area scaling law.

Pulse normalization in slow-light media

We analytically study the linear propagation of arbitrarily shaped light pulses through an absorbing medium with a narrow transparency window or through a resonant amplifying medium. We point out that, under certain general conditions, the pulse acquires a nearly Gaussian shape, irrespective of its initial shape and of the spectral profile of the line. We explicitly derive in this case the pulse parameters, including its skewness, responsible for a deviation of the delay of the pulse maximum from the group delay. We illustrate our general results by analyzing the slow-light experiments having demonstrated the largest fractional pulse delays.

Optimal superluminal systems.

We demonstrate that significant effects in the superluminal propagation of light pulses cannot be observed without involving systems whose gain explodes outside the pulse spectrum. We explicitly determine the minimum norm of the gain to attain given superluminal effects and the transfer function of the corresponding optimal system. The gain norms, which would be required with the most efficient systems considered up to now (dispersive media, photonic barriers) to attain the same effects, are shown to exceed the minimum by several orders of magnitude. We finally estimate the largest superluminal advances which could be attained in a realistic experiment.

Generation of Electromagnetic Pulses by Stacking of Coherent Transients

Electromagnetic pulses, of amplitude significantly larger than that of the input field, have been generated in a linear resonant absorber, heavily absorbing in steady state. This counter-intuitive result has been obtained by using a burst-switching of the input field, timed in order that all the oscillatory transients, transmitted by the medium at each switching of the input field, present a maximum at the same moment.

Cooperative shortening and modulation of the resonance optical pulse stimulated by an off-resonance stepwise excitation

We present the first direct experimental evidence of the stimulated inelastic resonance fluorescence (SIRF) in a dilute, optically thick gas of 2-level systems. In particular, the shortening and modulation of the SIRF pulse and the rise time effects predicted by an analytical calculation are clearly pointed out. Previously computed shapes of the SIRF pulse are also interpreted.