5 research outputs found
Theory of Transverse Mode Instability in Fiber Amplifiers with Multimode Excitations
Transverse Mode Instability (TMI) which results from dynamic nonlinear
thermo-optical scattering is the primary limitation to power scaling in
high-power fiber lasers and amplifiers. It has been proposed that TMI can be
suppressed by exciting multiple modes in a highly multimode fiber. We derive a
semi-analytic frequency-domain theory of the threshold for the onset of TMI
under arbitrary multimode input excitation for general fiber geometries. We
show that TMI results from exponential growth of noise in all the modes at
downshifted frequencies due to the thermo-optical coupling. The noise growth
rate in each mode is given by the sum of signal powers in various modes
weighted by pairwise thermo-optical coupling coefficients. We calculate
thermo-optical coupling coefficients for all pairs of modes in a
standard circular multimode fiber and show that modes with large transverse
spatial frequency mismatch are weakly coupled resulting in a banded coupling
matrix. This short-range behavior is due to the diffusive nature of the heat
propagation which mediates the coupling and leads to a lower noise growth rate
upon multimode excitation compared to single mode, resulting in significant TMI
suppression. We find that the TMI threshold increases linearly with the number
of modes that are excited, leading to more than an order of magnitude increase
in the TMI threshold in a 82-mode fiber amplifier. Using our theory, we also
calculate TMI threshold in fibers with non-circular geometries upon multimode
excitation and show the linear scaling of TMI threshold to be a universal
property of different fibers
Theory of Stimulated Brillouin Scattering in Fibers for Highly Multimode Excitations
Stimulated Brillouin scattering (SBS) is an important nonlinear optical
effect which can both enable and impede optical processes in guided wave
systems. Highly multi-mode excitation of fibers has been proposed as a novel
route towards efficient suppression of SBS in both active and passive fibers.
To study the effects of multimode excitation generally, we develop a theory of
SBS for arbitrary input excitations, fiber cross section geometries and
refractive index profiles. We derive appropriate nonlinear coupled mode
equations for the signal and Stokes modal amplitudes starting from vector
optical and tensor acoustic equations. Using applicable approximations, we find
an analytical formula for the SBS (Stokes) gain susceptibility, which takes
into account the vector nature of both optical and acoustic modes exactly. We
show that upon multimode excitation, the SBS power in each Stokes mode grows
exponentially with a growth rate that depends parametrically on the
distribution of power in the signal modes. Specializing to isotropic fibers we
are able to define and calculate an effective SBS gain spectrum for any choice
of multimode excitation. The peak value of this gain spectrum determines the
SBS threshold, the maximum SBS-limited power that can be sent through the
fiber. We show theoretically that peak SBS gain is greatly reduced by highly
multimode excitation due to gain broadening and relatively weaker intermodal
SBS gain. We demonstrate that equal excitation of the 160 modes of a
commercially available, highly multimode circular step index fiber raises the
SBS threshold by a factor of 6.5, and find comparable suppression of SBS in
similar fibers with a D-shaped cross-section
Exploiting spacetime symmetry in dissipative nonlinear multimode amplifiers for output control
Time-reversal symmetry enables shaping input waves to control output waves in many linear and nonlinear systems; however energy dissipation violates such symmetry. We consider a saturated multimode fiber amplifier in which light generates heat flow and suffers nonlinear thermo-optical scattering, breaking time-reversal symmetry. We identify a spacetime symmetry which maps the target output back to an input field. This mapping employs phase conjugation, gain and absorption substitution but not time reversal, and holds in steady-state and for slowly varying inputs. Our results open the possibility of output control of a saturated multimode fiber amplifier
Dataset for "Mitigating stimulated Brillouin scattering in multimode fibers with focused output via wavefront shaping"
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