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Media featuring both optical gain and disorder, such as random lasers,represent formidable challenges as subjects of research due to the highcomplexity of the light propagation within them; however, dramatic advances inthis nascent field have been furnished by the paradigm of applying wavefrontshaping techniques to the beam pumping the system. We present here atheoretical and experimental study employing this approach in a gain mediumwhere the disorder arises not from random scattering but rather from themultimodality of a waveguide: an amplifying multimode fiber. The shaping of thecore-guided pump prior to its injection affects the complex, speckle-likepatterns of excitation within the fiber volume. Thus we are offered theintriguing prospect of manipulating the spatially heterogeneous gain in asystem where the disorder, albeit highly complex, may be fully understood interms of the discrete eigenmodes of a well-known waveguide. We study our mediumin two different configurations: as an optical amplifier, and as a fiber laser.In the first configuration we show that dependence upon the pump configuration,of the amplifier transmission function, surprisingly survives several physicalmechanisms which at first sight would appear to severely limit it. This insightis then carried on to the lasing cavity configuration, where we find strikingparallels between the behavior of the system and that of random lasers; moreimportantly, the pump shaping shows a strong ability to control the complexemission, in particular - to selectively favor individual spectral lines andstabilize single-mode operation.

Gain As Configurable Disorder-Adaptive Pumping for Control of Multimode Fiber Amplifiers and Lasers