Nonlinear Self-Switching and Multiple
Gap
Soliton
Formation in a Fibre Bragg Grating
D. Taverner, N.G.R. Broderick, D.J. Richardson, M. Ibsen.
Optoelectronics Research Centre, Southampton University, Southampton, SO1 7 1 BJ, UK.
The interplay of the Kerr-induced nonlinear refractive index changes and dispersion in nonlinear Fiber
Bragg Gratings (FBGs) leads to a plethora of nonlinear phenomena, the most
&ii&g
of which is
perhaps the formation and propagation of gap solitons [ 11. Whilst a considerable amount of theoretical
work has been performed in this area [ 1,2,3,4] experimental observations of nonlinear
grating
behaviour
are limited, principally by the difficulty in getting sufficiently high power densities within the core of
a FBG in a suitable spectral and temporal range. In order to reduce the nonlinear threshold for
gap
soliton formation one can use the somewhat weaker dispersive properties of FBGs outside ofthe
band
gap and indeed recent experiments have yielded the first strong evidence of Bragg
grating gap soliton
formation by this means [5,6]. However, the strongest and most manifestly nonlinear effects are
predicted to occur at wavelengths within the band gap, close to the Bragg wavelength of the
grating
structure and it is therefore essential to make measurements within this regime. In this paper we report
what we believe to be the first clear experimental observation of nonlinearity within the band gap of an
FBG, namely nonlinear self-switching and, at higher intensities, multiple gap soliton formation.
Our experiments are made within the quasi-cw regime using nanosecond pulses of a physical
length several times that of the FBG. Earlier theoretical work (see RefIl] and reference therein) show
that Bragg gratings under CW excitation should exhibit optical bistability within their band-gap. Stability
analysis indicates that under certain operating conditions robust, bistable operation is obtained allowing
for optical switching. In other operating regimes instability occurs resulting in the formation of periodic
trains of gap solitons, that once formed propagate stably through the grating. The instability has been
equated to some form of modulation instability [ 1,6].
The experimental set-up is shown in Fig. 1. High power nanosecond pulses were coupled into a
FBG through a polarising beam-splitter and quarter wave-plate arrangement that decoupled the incident
and reflected signal pulses, allowing simultaneous measurements of the incident, transmitted and
reflected beams. The placement of a X/2 waveplate before this PBS allowed control of the power incident
on the grating. The transmitted, reflected and incident signals were detected with a single-mode fibre
coupled PIN photodiode and sampling oscilloscope. Our temporal resolution was -50~s. The fibre
grating was written into a germanosilicate fibre with a mode-area of 30pm* (N.A.=0.25,&=125Onm)
using a moving fibre/phase-mask scanning beam technique [7]. The grating was 8cm long, unchirped,
with a 0-n sinusoidal apodisation profile along its length. The grating had 98% reflectivity
at its peak
wavelength of 1536nm and was measured to have a transform-limited, 3dB bandwidth of 3 .~GI-Iz The
grating was mounted in a glass capillary and angle polished front and back to remove unwanted
reflections from these surfaces. The high power pulses were obtained from a large mode area, erbium
doped fibre amplifier chain seeded with ns pulses from a directly-modulated, wavelength-tunable,
semiconductor DFB laser [8]. The source was capable of producing nanosecond pulses with energies
~1 OOpJ and peak powers >l OOkW at kHz repetition rates. For the purposes of this experiment the source
was operated at 4kHz repetition rate giving 25pJ pulses with -2ns duration (see
insert Fig. 1). Note that
the pulse is highly asymmetric due to amplifier saturation effects and has a sharp -30~s feature on the
leading edge due to chirp on the diode seed pulse. This feature becomes more prominent when examining
the pulse transmission through the FBG with the pulse spectrum tuned to lie within the band-gap (see
Fig 2, trace a). The chirped spike lies outside the band gap and is transmitted whereas the
main body Of
the pulse is reflected (see Fig.3, trace a). Although aesthetically undesirable this feature actually Proved
a valuable calibration aid, giving a direct measure in the transmitted pulse time domain of the
inPut
Pulse
power. Note that the corresponding physical pulse lengths were significantly longer than the grating*
we were therefore well within the quasi-cw regime and anticipated clean switching and the potential ‘@
generate a significant number of gap soliton pulses. The pulse spectrum at the FBG input was measured
to have a 3dB spectral bandwidth of 1.2 GHz, considerably less than the FBG bandwidth. The central
wavelength of the source could be continuously and accurately temperature-tuned to wavelengths in and
around the FBG band gap.
We set the source wavelength close to the short wavelength side of the center of the bandgap and
examined the grating pulse transmission and reflection characteristics as a function of increasing peak
power. The transmission results are summarised in Fig. 2. At low powers (Fig.2 trace a) the pulse is
seen to be almost completely reflected from the grating other than our chirped rising edge marker.
However as the pulse peak power is increased strong pulse reshaping becomes apparent. Fig.2 traces
b-d show various stages in the growth of the nonlinearly transmitted pulses. Initially one gap soliton is
formed (around 3kW peak power), however as the intensity is increased more solitons are generated.
Each subsequent pulse then narrows to -100~s and moves forward allowing additional pulses to form
at the rear of the bunch. We observed the generation of up to 5 gap solitons in our experiments (Fig.2,
trace d). Note that the absence of Raman scattering or other nonlinear spectral distortion was confirmed
by direct spectral measurements. The corresponding effects of the pube formation are readily observed
in the reflected domain, see Fig.3 traces a-d where progressively larger ‘chunks’ of energy are seen to
be switched from the front of the pulse.
We evaluated the percentage energy transmittance of the FBG as a function of peak power both
by integration of the transmitted pulse forms and through direct measurements of the incident,
transmitted and reflected average powers. Good agreement was obtained between the two approaches.
The results are shown in Fig.4 where it is seen that the transmission switches from about 3% in the linear
regime to a saturated level of about 40% for peak powers of -4kW and above, representing -1ldB
switching contrast. Note that the data presented in Fig.4 has been processed to eliminate the contribution
of the chirped leading spike to the calculated total transmission.
In conclusion, we report the first observation of optical switching and multiple gap soliton
generation within the bandgap of a FBG. Switching from 3% to 40% of the pulse energy is obtained
for nanosecond pulses at internal field strengths of order lo- IS GW/cm* . At higher intensities multiple
gap solitons were obtained. We have experimentally observed the formation of up to 5 gap soliton with
durations in the range lOO-500~s. We believe these results to represent a significant step in the study of
nonlinear FBG effects and indicate that the combination of high power fiber based 1550 nm sources,
coupled to recent improvements in FBG fabrication techniques should allow for further advancements
in such studies.
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PI
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