Abstract
The dynamic properties of a nonlinear directional coupler
made of Kerr materials are investigated using a normal-mode theory based on
even and odd supermodes. This analysis provides a good platform to study the
pulse propagation in nonlinear multimode waveguides, since the device is simple
and can be regarded as a bimodal waveguide. Although the nonlinear directional
coupler has already been well investigated using the standard coupled-mode
theory, this paper presents new knowledge concerning the device operation
from the standpoint of supermodes. Under the normal-mode model, power exchange
between the two supermodes is performed through non-phase-matched four-wave
mixing (FWM) and new frequency components are created on propagation, leading
to spectral broadening of optical pulses. It is found that the group-delay
mismatch between the two modes induces instability in collaboration to the
nonlinearity when the duration of input pulses is of the order of sub-picoseconds.
The dynamic properties of the nonlinear directional coupler are quite similar
to those of the nonlinear MMI coupler that were more recently investigated
by the present authors.
© 2013 IEEE
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