NONLINEAR FIBER OPTICS

Tremendous technological advances during the past four decades have allowed information imprinted on a beam of light to be transmitted over thousands of kilometers. These developments have enabled mankind to enter the modern information age of today, in which instantaneous communication across the world is orthodox. The enabling breakthrough in light-based telecommunications has been the development of low-loss optical fibers; micrometer-size glass cylinders in which light can remain trapped over extended distances.

Light and matter interact linearly when the brightness of the light source is weak. This is the case with everyday light sources such as Sunlight. In fiber-telecommunication systems strong laser light is tightly trapped inside the micrometer-size fiber, and the interaction is no longer linear. Because nonlinear fiber optics is linked to a variety of physical systems through mathematically similar theoretical frameworks, the study of nonlinear wave phenomena in a controlled fiber-optic setting can provide insights across the boundaries of physics disciplines.

My research aims to explore and exploit the nonlinear dynamics of waves in fibers and related systems by adopting a broad perspective that ranges from multidisciplinary fundamental physics to device development.

MODULATION INSTABILITY AND SOLITONS

Modulation instability is a central nonlinear process that is associated with the exchange of energy between a periodic perturbation and a plane wave background. In an optical fiber context the instability manifests as the break-up of a continuous wave field into a train of pulses. We have investigated the dynamics of modulation instability using analytical solutions of the underlying nonlinear Schrodinger equation. This approach has led us to experimentally identify a form of higher-order instability, where instability sidebands are sequentially amplified in a harmonic series.

Soliton is the archetype of a nonlinear wave. Where ordinary wave packets spread through diffraction or dispersion, the soliton retains its shape with the spreading being balanced by a counteracting nonlinearity. We have studied the interactions of solitons in systems where the nonlinear Schrodinger equation is perturbed. In this case colliding solitons can emit radiation or exchange energy.

SUPERCONTINUUM GENERATION AND ROGUE WAVES

Supercontinuum generation refers to the process in which narrowband (single color) light is transformed into a broadband spectral continuum (rainbow). Specially tailored, highly nonlinear photonic crystal fibers offer an ideal platform for spectral broadening because light can interact with the host fiber over extended distances. The nonlinear dynamics in fiber supercontinuum generation are immensely rich with both modulation instability and soliton effects playing a crucial role. Our investigations have led us to discover new types of soliton interactions that can spontaneously take place in the context of supercontinua and lead to the generation of new spectral components.

Understanding the nature in which noise couples to the supercontinuum field is paramount for applications such as frequency metrology. Scientists have found that rare events of extreme qualities can manifest in regimes where the dynamics exhibit chaotic sensitivity to initial conditions. These sporadic events bear similarities with rogue waves observed across the oceans of the world which has aroused immense interest in a variety of physical systems. We have conducted extensive investigations of rogue wave -like events in fiber supercontinua, producing insights into their formation dynamics and characteristics.

PASSIVE RESONATORS AND CAVITY SOLITONS

When the two ends of an optical fiber are joint together the device becomes a resonator. This means that the system allows only for certain optical frequencies to undergo large-amplitude oscillations. By continuously exciting the resonator with laser light the nature of the system is dramatically changed from that of the single-pass fiber configuration. Remarkably, solitons can exist even in a driven resonator. The dispersion of such cavity solitons is balanced by the material nonlinearity, but in addition their dissipation is balanced by the coherent driving of the cavity. This allows cavity solitons to circulate indefinitely inside the resonator.

The cavity length of conventional fiber-based resonators is typically measured in tens or even hundreds of meters. Recent years have witnessed the surge of micrometer-scale resonators. These microring devices can be utilized to perform low-footprint broadband frequency conversion with high efficiency. We have recently established that despite the physical down-scaling of multiple orders-of-magnitude the physics behind microring resonators remain largely the same as those governing macroscopic fiber resonators. This has allowed us to develop computationally efficient models to describe the nonlinear dynamics inside microscopic cavities.

MODE-LOCKED FIBER LASERS

Passive resonators contain two key ingredients required to qualify as lasers: mechanism of feedback and a source of driving energy. Yet, passive resonators are not lasers for they lack an active medium that can amplify the optical signal via stimulated emission. By replacing a section of the cavity with an active optical fiber doped with rare-earth elements such as Erbium or Ytterbium a resonator can be converted into a fiber laser.

In its simplest form a fiber laser emits a temporally continuous output. By inserting an element that promotes the formation of high peak power pulses inside the cavity the laser can be made to deliver trains of ultrashort pulses. Such mode-locked fiber lasers possess numerous benefits compared to their solid-state counterparts such as compactness, low-manufactuing cost and lack of sensitivity against misalignment. We have developed new laser architectures that combine environmental stability with high-energy outputs.

© 2017 Miro Erkintalo. All rights reserved.