Modulation instabilities in nonlinear optics: from losses to gain…

Modulation instability is an old but evergreen research topic in nonlinear science in general and in nonlinear optics in particular. It consists in the destruction (destabilization) of a wave leading to the exponential amplification of small perturbations (small waves having different frequencies). It is an ubiquitous phenomenon occurring in various natural and engineering systems from oceans to lasers and optical fibres. In the latter system when a powerful wave of light propagates through a fiber (under specific conditions) it can loose energy being eventually destroyed at expenses of amplification of waves with other frequencies.

Instead of viewing instabilities as purely detrimental phenomena to be tamed, suppressed and controlled we should try to engineer them in order to produce some effects which may result useful in technology. An example of such approach is the fibre optic parametric amplifier, where modulation instability is used for amplification of broadband signals in order to increase the information capacity of current fibre optical communication systems.

I’m very interested in the role played by dissipation, in particular by optical losses, in the modulation instability dynamics. It is common sense that when waves oscillating at a certain frequency are damped by some absorption process their amplitude decreases up to eventually vanishing. Surprisingly enough this is not always the case, in nonlinear systems strong losses acting selectively on certain frequencies can unexpectedly lead to the amplification of the damped waves themselves!

Such gain-through-loss process can be used to achieve selective and tuneable amplification of optical waves at desired frequencies in resonators or waveguides where standard techniques are not available (more technically when phase-matching is difficult to achieve) and could be used to initiate optical frequency comb formation in normal dispersion resonators or, when the dissipation is acting periodically and alternating to various frequencies, to design Mamyshev oscillators which operate in the harmonic mode-locking regime (dissipative Faraday instability).

My works in this field: