10 research outputs found
Beam self-cleaning in multimode optical fibers and hydrodynamic 2D turbulence
We experimentally demonstrate the conservation of the average mode number in the process of Kerr beam self-cleaning in a graded-index multimode optical fiber, in analogy with wave condensation in hydrodynamic 2D turbulence
Mode decomposition of multimode optical fiber beams by phase-only spatial light modulator
Multimode optical fibers (MMF) recently attracted a renewed attention, because of their
potential for spatial division multiplexing, medical imaging and high-power fiber lasers, thanks
to the discovery of new nonlinear optical effects, such as Kerr beam self-cleaning,
spatiotemporal mode-locking, and geometric parametric instability, to name a few. The main
feature of these effects is that many transverse modes are involved in nonlinear interactions. To
advance our understanding, it is necessary to analyse the modal content of beams at the output
of MMFs. In this work, based on a computer digital holography method using a phase-only
spatial light modulator (SLM) as a correlation filter, we experimentally demonstrate a method of
mode decomposition involving a large (≃80) number of fiber modes. To obtain this, we carried
out a SLM calibration, and numerically investigated the most critical parameters which affect
the fidelity of the decomposition, by comparing experimental and reconstructed beam patterns
in both the linear (speckled structures) and in the nonlinear (self-cleaned beams) propagation
regime
Mode decomposition method for investigating the nonlinear dynamics of a multimode beam
We overview our recent experimental studies on the nonlinear spatial reshaping of multimode
beams at the output of multimode optical fibers. We use a holographic mode decomposition
technique, which permits to reveal the variation of the spatial mode composition at the fiber
output, as determined by either conservative (the Kerr effect) or dissipative (Raman scattering)
nonlinear processes. For the first case, we consider the effect of spatial beam self-cleaning, and
we compare experimental mode decompositions with predictions based on the thermodynamic
theory, including the case of beams carrying nozero orbital angular momentum. For the second
case, we analyze the beam mode content at the output of a Raman laser based on a graded
index multimode fiber
Statistical mechanics of beam self-cleaning in GRIN multimode optical fibers
Since its first demonstration in graded-index multimode fibers, spatial beam
self-cleaning has attracted a growing research interest. It allows for the
propagation of beams with a bell-shaped spatial profile, thus enabling the use
of multimode fibers for several applications, from biomedical imaging to
high-power beam delivery. So far, beam self-cleaning has been experimentally
studied under several different experimental conditions. Whereas it has been
theoretically described as the irreversible energy transfer from high-order
modes towards the fundamental mode, in analogy with a beam condensation
mechanism. Here, we provide a definitive theoretical description of beam
self-cleaning, by means of a semi-classical statistical mechanics model of wave
thermalization. This approach is confirmed by an extensive experimental
characterization, based on a holographic mode decomposition technique,
employing laser pulses with temporal durations ranging from femtoseconds up to
nanoseconds. An excellent agreement between theory and experiments is found,
which demonstrates that beam self-cleaning can be fully described in terms of
the basic conservation laws of statistical mechanics
Mode decomposition method for investigating the nonlinear dynamics of a multimode beam
We overview our recent experimental studies on the nonlinear spatial reshaping of multimode
beams at the output of multimode optical fibers. We use a holographic mode decomposition
technique, which permits to reveal the variation of the spatial mode composition at the fiber output,
as determined by either conservative (the Kerr effect) or dissipative (Raman scattering) nonlinear
processes. For the first case, we consider the effect of spatial beam self-cleaning, and we compare
experimental mode decompositions with predictions based on the thermodynamic theory, including
the case of beams carrying nozero orbital angular momentum. For the second case, we analyze the
beam mode content at the output of a Raman laser based on a graded index multimode fiber
Thermalization of Orbital Angular Momentum Beams in Multimode Optical Fibers
We report on the thermalization of light carrying orbital angular momentum in multimode optical fibers, induced by nonlinear intermodal interactions. A generalized Rayleigh-Jeans distribution of asymptotic mode composition is obtained, based on the conservation of the angular momentum. We confirm our predictions by numerical simulations and experiments based on holographic mode decomposition of multimode beams. Our work establishes new constraints for the achievement of spatial beam self-cleaning, giving previously unforeseen insights into the underlying physical mechanisms
Thermalization of orbital angular momentum beams in optical fibers
We experimentally and theoretically study the thermalization of orbital angular momentum beams in nonlinear multimode graded-index optical fibers. Our results shed new light on the physical mechanism of the beam self-cleaning effect
Thermalization of Orbital Angular Momentum Beams in Multimode Optical Fibers
We report on the thermalization of light carrying orbital angular momentum in multimode optical fibers,
induced by nonlinear intermodal interactions. A generalized Rayleigh-Jeans distribution of asymptotic
mode composition is obtained, based on the conservation of the angular momentum. We confirm our
predictions by numerical simulations and experiments based on holographic mode decomposition of
multimode beams. Our work establishes new constraints for the achievement of spatial beam self-cleaning,
giving previously unforeseen insights into the underlying physical mechanisms