Serge Bielawski

Polarization dynamics of a fibre Fabry-Perot laser with feedback

Experimental results on the low-frequency polarization dynamics of a free-running Nd-doped optical fibre laser and a fibre laser with derivative feedback are presented. It is found that the power spectrum of each polarization mode reveals three relaxation peaks. Using the combined (negative and positive) derivative feedback it is possible to suppress noise in the region of the in-phase relaxation oscillations and to excite noise in the region of one of the two antiphase relaxation oscillations. The model developed of a class B laser with two orthogonal elliptically polarized modes is checked and explains the main experimental features of relaxation oscillation peaks.

Incompatibility between cavity resonances and wavevector matching: influence on threshold and beam structures of OPOs

We investigate theoretically and experimentally the interplay between cavity and double-refraction in continuous-wave optical parametric oscillators. We show that very basic geometrical effects can prevent transverse wavevector matching for the TEM/sub 00/ modes, and thus increase the threshold and change dramatically the beam structures when using a resonant (or double-pass) pump. We extend the work of Boyd and Kleinman to take these cavity-induced mismatches into account, and test experimentally the theoretical results using a cw type II KTP OPO. The transverse wavevector mismatch leads to the appearance of transverse modes with increasing order (up to TEM/sub 40-0/) and surprisingly induces structures that are different on signal and idler. The theory is able to predict quantitatively all selected transverse modes, and allows a simple interpretation in transverse Fourier space.

Control of laser chaos

Further modifications of the method proposed by Ott, Grebogi and Yorke to control chaos [Phys. Rev. Lett. 64, 1196 (1990)] have been achieved allowing us to stabilize and characterize unstable states (stationary or periodic) in their whole domain of existence. We demonstrate the possibility of stabilizing unstable periodic orbits in an experiment by applying a continuous feedback method. It has been checked experimentally on a CO2 laser with a modulated parameter. This kind of method is very attractive opening the way to the control of chaos in very fast systems.

Single shot observation of rogue waves in optical turbulence by using time microscopy

Fibers are considered as extraordinary tabletop laboratories to investigate optical turbulence and optical rogue waves (ORW). However ultrafast measurement of random optical power fluctuations is a challenging experimental task and the typical sub-picosecond timescale prevented-up to now-time-resolved observations of the awaited dynamics. We report the first single-shot recordings of ORW emerging from the propagation of random waves in optical fibers by using a specially designed ultrafast Time Microscope (TM) [1].

Phase and amplitude single-shot measurement by using time-lens and ultrafast time-holography

Despite recent advances in optical ultrafast measurement, simultaneous measurement of amplitude and phase of fast optical fields having complex evolutions over large time window remains a largely open problem. Here, we demonstrate two novel and complementary techniques that allow in particular the single-shot recordings of amplitude and phase of irregular waves with a high temporal resolution over a long time window.

Spatiotemporal chaos in a laser

This paper concentrates on spatiotemporally chaotic regimes. The investigation shows different types of spatiotemporal chaos (extensive and non-extensive) stemming from at least three different origins. The first mechanism is linked to spatial nonuniformities (here the Gaussian lineshape of the laser) that can induce low dimensional spatiotemporal chaos. A second scenario is the interaction between two patterns of different wavenumbers, the pattern with the smallest wavenumber playing the role of spatial nonuniformities for the other pattern. Numerical simulations show that it leads to extensive chaos (linear growth of the number of positive Lyapunov exponents with the size of the system). The third mechanism is a cascade of parametric instabilities: each of the two waves appearing at the primary instability act as a pump for the parametric excitation of two new traveling waves, leading eventually to spatiotemporal chaos.

Dynamics of class-B laser with two elliptically polarized modes

Experimental results containing the dynamical behavior of the Nd-doped optical fiber laser are presented. It is found that the state of pump polarization affects the intensity ratio of polarization modes. The intensity fluctuation spectrum of each polarization mode reveals three relaxation peaks. The model of a class B laser with two orthogonal elliptically-polarized modes is eligible for explanation of the main experimental features of relaxation oscillation peaks.

Multimode class-B lasers with combined derivative feedback

In these equations, x and y are proportional to the inversion of population and the intensity of the laser field, respectively y and 0 are proportional to the amplitude and the delay of the feedback, respectively. The quantity p is proportional to the ratio of the photon and population inversion lifetimes and is an 0(10-3) small quantity. The parameter w(>l) is the pump parameter nomalized by its threshold value. We next investigate the limit of small amplitude feedback and small damping