Evolution of light trapped by a soliton in
a microstructured fiber
S. Hill, C. E. Kuklewicz, U. Leonhardt, F. K¨ onig
School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife,
KY16 9SS, UK.
Abstract: We observe the dynamics of pulse trapping in a microstructured
fiber. Few-cycle pulses create a system of two pulses: a Raman shifting
soliton traps a pulse in the normal dispersion regime. When the soliton
approaches a wavelength of zero group velocity dispersion the Raman
shifting abruptly terminates and the trapped pulse is released. In particular,
the trap is less than 4ps long and contains a 1ps pulse. After being released,
this pulse asymmetrically expands to more than 10ps. Additionally, there is
no disturbance of the trapping dynamics at high input pulse energies as the
supercontinuum develops further.
© 2009 Optical Society of America
OCIS codes: (060.7140) Ultrafast processes in fibers; (190.5530) Pulse propagation and tem-
poral solitons; (190.4370) Nonlinear optics, fibers
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When a short and intense pulse of light is launched into a microstructured fiber (MF), non-
linear effects can significantly broaden the spectrum to much more than an octave . These
supercontinua are used in a variety of applications such as ultrafast optical switching, spec-
troscopy, optical coherent tomography, optical clocks, etc. Routinely, around 100-fs pulses
are used at a wavelength close to a zero-group velocity dispersion wavelength (ZDW) of the
fiber. Using nearly octave–spanning input pulses [2, 3, 4], the phenomenon of ‘pulse trapping’
[5, 6, 7, 8, 9, 10] can be observed. In this effect a non-dispersing pulse can form at the short
wavelength end of the broad spectrum. This is surprising because the pulse exists in a region
of normal dispersion in the fiber, where the dispersion-induced chirp cannot be cancelled by
self-phase modulation. The non-dispersing pulse is trapped behind a fundamental soliton in the
anomalous dispersion regime that was generated by soliton fission. Soliton fission can create
multiple fundamental solitons, which shift to longer wavelengths via the soliton self-frequency
shift (SSFS) .
Nishizawa and Goto  were the first to demonstrate that a soliton undergoing the SSFS can
trap light behind it. The trapped light adjusts to a wavelength that is group velocity matched
to the soliton. Because it is forced to travel with the soliton and lies in the normal dispersion
regime, it has to shift to shorter wavelengths in order to keep the same group-velocity as the
soliton. Gorbach and Skryabin provided an alternative view of pulse trapping : the light is
trapped because the soliton accelerates. In simulations they turned off the (negative) accelera-
tion induced by the SSFS and the trapping ceased. The acceleration of the soliton provides a
‘gravity-like’ potential to the trapped pulse.
In this paper we study experimentally the phenomenon of pulse trapping in a fiber with two
ZDWs. We use intense few-cycle pulses to generate a soliton and a trapped pulse and focus in
particular on the trap dynamics as the soliton reaches the longer zero-dispersion wavelength and
30 150 15 30
Fig. 11. SHG-FROG measurement of the trapped pulse for a pulse energy of 295pJ corre-
sponding to figure 10. The trace is similar to previous FROG measurements of the trapped
light after the trapping has ended (see figure 8).
(compare to figure 8). The length of the tail is determined by the dispersion over the full fiber
length. The wavelength of the pulse peak is arrested at ∼590nm. This stabilizes the trapped
pulse and makes it independent of coupled input energy in this energy range.
The spectrum continuesto reach shorter wavelengths, considerably belowthe group velocity-
matched point (figure 10). We attribute the additional peak of light appearing below 550nm to
nonlinearly phase matched resonant wave mixing at the very input of the fiber [25, 17, 26]. This
is consistent with the observation that the peak shifts to the blue with increasing energy. Similar
behaviour has been seen in [19, 4]. In our case there is a particularly pronounced spectral gap at
550nm, independent of input energy and of constant shape. In the infrared we see that multiple
solitons, created from the input pulse by soliton fission, have shifted towards the long ZDW.
The soliton that is arrested just short of the long ZDW is at a minimum of group velocity. The
initially trapped light still trails the soliton and thus is of a wavelength shorter than the group
velocity-matched wavelength. Light in the trap that had been propagating ∆v faster than the
soliton before release has undergone a spectral blueshift at the soliton to a group velocity ∆v
slower than the soliton .
By measurements in both the temporal and spectral domains, we show how the dynamics of a
soliton determine both the wavelength and pulse-like nature of the short wavelength end of the
supercontinuum. Light in the normal dispersion regime forms a trapped pulse governed by a
potential consisting of a nonlinear barrier and a ‘gravity-like’ linear potential. The linear part
is due to the SSFS which causes a negative acceleration of the soliton. As long as the soliton
is accelerating, the trapping confines the light in time. When the SSFS terminates, the trapped
light falls behind the soliton, moving to a wavelength associated with a group–velocity slower
than the soliton. This effect dominates the shape of the supercontinuum at short wavelengths
until resonant wave mixing with the input pulse further broadens the spectrum.
This setup allows for the creation and release of wavelength tunable picosecond pulses in the
normal dispersion regime at any location along a microstructured fiber. The wavelength for the
trapped pulse is obtained by choosing an appropriate dispersion profile of the fiber. The most
important parameter is the longer zero dispersion wavelength. If the fiber has a long ZDW that
is longer than 1160nm, then the SSFS would take the soliton further into the infrared before
the acceleration terminates. The trapped pulse would shift to even shorter wavelengths and
still be confined in time. Once the soliton is arrested, the trapped pulse has reached the final
wavelength which is largely independent of pulse energy. Hence, the trapped pulse wavelength
could be increased from this by use of lower input pulse energies or chirped input pulses.
We are indebted to Dmitry Skryabin, Andrey Gorbach, Scott Robertson, Franz K¨ artner, Peter
Staudt, Klaus Metzger and Wilson Sibbett for discussions and technical support. This work is
supported by the EPSRC, the Royal Society, and the Leonhardt Group Aue.