  Ability to selectively enhance the amplitude and maintain high coherence of
the supercontinuum signal with long pulses is gaining significance. In this
work an extra degree of freedom afforded by varying the dispersion profile of a
waveguide is utilized to selectively enhance supercontinuum. As much as 16 dB
signal enhancement in the telecom window and 100 nm of wavelength extension is
achieved with a cascaded waveguide, compared to a fixed dispersion waveguide.
Waveguide tapering, in particular with increasing width, is determined to have
a flatter and more coherent supercontinuum than a fixed dispersion waveguide
when longer input pulses are used. Furthermore, due to the strong birefringence
of an asymmetric silicon waveguide the supercontinuum signal is broadened by
pumping simultaneously with both quasitransverse electric (TE) and
quasi-transverse magnetic (TM) mode in the anomalous dispersion regime. Thus,
by controlling the dispersion for the two modes selective signal generation is
obtained. Such waveguides offer several advantages over optical fiber as the
variation in dispersion can be controlled with greater flexibility in an
integrated platform. This work paves the way forward for various applications
in fields ranging from medicine to telecom where specific wavelength windows
need to be targeted.
