Auro Michele Perego

Pattern Generation by Dissipative Parametric Instability

Nonlinear instabilities are responsible for spontaneous pattern formation in a vast number of natural and engineered systems, ranging from biology to galaxy buildup. We propose a new instability mechanism leading to pattern formation in spatially extended nonlinear systems, which is based on a periodic antiphase modulation of spectrally dependent losses arranged in a zigzag way: an effective filtering is imposed at symmetrically located wave numbers k and -k in alternating order. The properties of the dissipative parametric instability differ from the features of both key classical concepts of modulation instabilities, i.e., the Benjamin-Feir instability and the Faraday instabiltyity. We demonstrate how the dissipative parametric instability can lead to the formation of stable patterns in one- and two-dimensional systems. The proposed instability mechanism is generic and can naturally occur or can be implemented in various physical systems.

Mode-locking via dissipative Faraday instability

Emergence of coherent structures and patterns at the nonlinear stage of modulation instability of a uniform state is an inherent feature of many biological, physical and engineering systems. There are several well-studied classical modulation instabilities, such as Benjamin–Feir, Turing and Faraday instability, which play a critical role in the self-organization of energy and matter in non-equilibrium physical, chemical and biological systems. Here we experimentally demonstrate the dissipative Faraday instability induced by spatially periodic zig-zag modulation of a dissipative parameter of the system—spectrally dependent losses—achieving generation of temporal patterns and high-harmonic mode-locking in a fibre laser. We demonstrate features of this instability that distinguish it from both the Benjamin–Feir and the purely dispersive Faraday instability. Our results open the possibilities for new designs of mode-locked lasers and can be extended to other fields of physics and engineering.

Self-Induced Faraday Instability Laser

We predict the onset of self-induced parametric or Faraday instabilities in a laser, spontaneously caused by the presence of pump depletion, which leads to a periodic gain landscape for light propagating in the cavity. As a result of the instability, continuous wave oscillation becomes unstable even in the normal dispersion regime of the cavity, and a periodic train of pulses with ultrahigh repetition rate is generated. Application to the case of Raman fiber lasers is described, in good quantitative agreement between our conceptual analysis and numerical modeling.

Gain through losses in nonlinear optics

Instabilities of uniform states are ubiquitous processes occurring in a variety of spatially extended nonlinear systems. These instabilities are at the heart of symmetry breaking, condensate dynamics, self-organisation, pattern formation, and noise amplification across diverse disciplines, including physics, chemistry, engineering, and biology. In nonlinear optics, modulation instabilities are generally linked to the so-called parametric amplification process, which occurs when certain phase-matching or quasi-phase-matching conditions are satisfied.In the present review article, we summarise the principle results on modulation instabilities and parametric amplification in nonlinear optics, with special emphasis on optical fibres. We then review state-of-the-art research about a peculiar class of modulation instabilities (MIs) and signal amplification processes induced by dissipation in nonlinear optical systems. Losses applied to certain parts of the spectrum counterintuitively lead to the exponential growth of the damped mode themselves, causing gain through losses. We discuss the concept of imaging of losses into gain, showing how to map a given spectral loss profile into a gain spectrum. We demonstrate with concrete examples that dissipation-induced MI, apart from being of fundamental theoretical interest, may pave the way towards the design of a new class of tuneable fibre-based optical amplifiers, optical parametric oscillators, frequency comb sources, and pulsed lasers.Optics: Exploiting dissipation-induced instabilities opens door to new nonlinear optics technologiesBy reviewing the state of the art in nonlinear optics, scientists have explored how nonlinear effects reinforce waves generated from noise by instabilities in optical systems, paving the way for a range of new optical technologies. Pattern forming instabilities occur in a variety of physical, chemical, and biological systems, and can be viewed as a detrimental factor, limiting or damaging performances of engineering systems. These instabilities, however, have recently been exploited as a source of useful behaviour in nonlinear optical systems. This led Auro M. Perego from Aston University, Birmingham in the United Kingdom, and colleagues, to research a type of modulation instability and amplification process produced by dissipation in nonlinear optical systems. The work could lead to new tuneable fibre-based optical amplifiers, optical parametric oscillators (OPOs), and pulsed laser sources.

Coherent effects in mode-locked lasers: new theory and experiments

We present the predictions of a new theory for mode-locking in lasers valid also for fast gain media (e.g. semiconductor lasers). Substantial deviations from Haus theory are found, which are validated by experimental results.

High repetition rate multi-similariton laser

We present the numerical demonstration of an harmonically mode-locked multi-similariton laser supporting a low jitter, stable train of self similar high repetition rate pulses exploiting, as mode-locking mechanism, the principle of dissipative Faraday instability (DFI) induced by zigzag modulation of spectral losses [1, 2]. At variance with the theoretical and experimental studies on the DFI [1, 2], where the amplification was distributed along the fiber, we propose here a lumped amplification scheme suitable for a more flexible design of mode-locked lasers pumped by rare-earth gain medium (Erbium, Ytterbium). We have considered an unidirectional all normal dispersion ring resonator with two lumped amplifying sections separated by two passive nonlinear dispersive fibers. Just before each amplifying section is located a spectral filter. The two filters differ by having the transmittance profile respectively blue- and red-detuned relatively to the amplifiers central frequency. The detuned spectral filters provide the necessary periodic zigzag modulation of the spectral losses needed to trigger the DFI. The CW operation of the laser is unstable and the growth of spectral sidebands results in a temporal modulation of the field temporal profile leading to the formation of a pulse train with repetition rate corresponding to the instability frequency around 0.1 THz and pulse duration of about 3 ps. Propagation in the fibers has been modeled using the generalized nonlinear Schrödinger equation and the lumped amplification by a saturable gain term with spectral bandwidth typical of rare-earth amplifiers.

Raman polarizer based on a fiber with a random birefringence

Raman polarizers are devices able to amplify and simultaneously repolarize optical signals, exploiting the polarization attraction phenomenon induced by the Raman gain anisotropy [1, 2]. To characterize the degree of polarization (DOP) of the signal as a function of the Raman gain (G) in the case of the co-propagating pump and signal pulses, the following formula for ideal Raman polarizer has been recently derived [1]: DOP = 1 – G−1.

Dissipative Faraday instability mode-locking in a Raman fiber laser

There is a new type of parametric instability that was recently theoretically predicted - dissipative Faraday instability. In this work we experimentally demonstrate this new type of instability by successfully achieving mode-locking in a simple configuration. Applying a combination of passive spatial modulation of spectrally dependent loss and group velocity dispersion, we achieve high harmonic mode-locking in a Raman fiber laser. The results not only open the possibilities for novel designs of mode-locked lasers, but extend beyond the field of laser physics.

Self-pulsating nonlinear systems via dissipative parametric instability

The recently discovered dissipative parametric instability is presented in the framework of the universal complex Ginzburg-Landau equation. The pattern formation associated with the instability is discussed in connection to the relevant applications in nonlinear photonics especially as a new tool for pulsed lasers design.

Dissipative parametric instability: A new tool for pattern formation engineering in nonlinear optical systems

We present the novel dissipative parametric instability which leads to pattern formation and pulse train generation in nonlinear optical systems, under zig-zag modulation of the dissipation applied on symmetrically located spectral regions.