Supercontinuum Generation in Sub-Centimeter
Lengths of High-Nonlinearity Photonic Crystal Fibers
Fiorenzo G. Omenetto, Natalie A. Wolchover, Mackenzie R. Wehner, Matt Ross, Anatoly Efimov, Antoinette Taylor,
V. Ravi K. Kumar, Alan K. George, Jonathan C. Knight, Nicolas Y. Joly and Philip St. J. Russell
P
Optical power [dBm]
hotonic crystal fibers (PCFs) have
become one of the success stories of
modern photonics, re-energizing research
in nonlinear optical processes in waveguides.
Certainly, one of the most dramatic
manifestations of such effects in PCFs is
the nonlinear transformation of ultrafast
laser pulses into supercontinuum (SC)
radiation.1,2 Typically made of silica,
–20
–40
350
(a)
Pump wavelength
l = 1550 nm
–60
–80
600
1200
1800
2400
3000
Wavelength [nm]
Wavelength [nm]
1700
(b)
1300
1700
(c)
1300
10
20
30
40
50
60
70
Average power [mW]
(a) Ultrabroad supercontinuum trace
resulting from the propagation of 70 mW
average power (80 MHz), 1550 nm, 110
fs pulses in a 5.7 mm segment of high
nonlinearity SF6 PCF. (b, c) The spectral
behavior as a function of average power
for different PCF lengths (a) Z = 5.7 mm
and (b) Z = 70 cm. The data in (b) show
a smooth broadening of the pump pulse,
typical of a self-phase modulation dominated process. The data in (c) for the longer piece of fiber clearly show the spectral
signature of multiple soliton fission and
their subsequent red-shift (indicated by
the arrows in the figure).
PCFs that are used to generate SC provide
bandwidth in excess of one optical octave,
with limits imposed by the physical properties of the fiber itself (such as modes
supported, absorption, dispersion, etc.).
The cascaded nonlinearities make this SC
rich in spectral features. Their control and
stability is crucial in order for one to fully
take advantage of such a broad bandwidth
source.
We performed a series of experiments
in high-nonlinearity soft-glass (Schott
SF6) PCF by propagating 1 nJ, t = 110 fs,
l = 1550 nm pulses in several short
lengths of this fiber (down to a few millimeters).3 This glass has a nonlinear index
of refraction that is one order of magnitude higher than silica. The SC generated is smooth and dramatically broad,
spanning from 350 nm to beyond 3000
nm. This, in itself, is quite remarkable. It
offers a bright, low-coherence, broadband
fiber-based light source that extends to a
spectral region where few compact laser
sources are available.
The spectral quality of the SC radiation spectrum is dramatically altered by
selecting an appropriate (and rather
short) length of PCF. Different regimes
of nonlinear pulse transformation can be
clearly identified. When the fiber length
(Z = 5.7 mm in the experiment) is much
shorter than the dispersion length, soliton
propagation is not important and a symmetric SC spectrum arises from (almost)
pure self-phase modulation. Significant
spectral smoothing is observed in this
regime, where no soliton fission or other
high-order nonlinearity effect comes into
play. These effects reappear at longer fiber
lengths (Z > 10 cm), the SC is influenced
by the temporal breakup of multiple
Raman-shifting solitons (see figure).
These results underscore the importance of operating at appropriate lengths
in such highly nonlinear waveguides and
confirm that the defining physical interaction that underpins SC generation occurs
in the very first instances of propagation
in these fibers. Further advantages of
operating with such short fiber lengths
are the reduction of material absorption,
allowing for a broader spectrum to
be generated, and minimization of chromatic dispersion, thereby lessening the
temporal broadening that accompanies
SC generation and resulting in a short
SC pulse.
Working in a spectral broadening
regime largely dominated by self-phase
modulation will also offer the possibility for efficient pulse recompression to
generate controlled few-cycle femtosecond pulses across a wide bandwidth.
Finally, the suitability of these PCFs to
be pumped at infrared wavelengths (l >
1300) make them ideal for the realization
of broadband all-fiber sources bringing the versatility of nonlinear optics to
reduced and convenient dimensions.4 t
[ F.G. Omenetto ([email protected]) and N.A.
Wolchover are with the departments of biomedical engineering and physics at Tufts University. M.R. Wehner,
M. Ross, A. Efimov and A. Taylor are with the Los
Alamos National Laboratory, Los Alamos, N.M. V.R.K.
Kumar, A.K. George and J.C. Knight are with the Center
for Photonics and Photonics Materials, University of
Bath, Bath, United Kingdom. N.Y. Joly and P.St.J.
Russell are with the Max Planck Research Institute,
University of Erlangen-Nuremberg, Erlangen, Germany. ]
References
1. R. Alfano, The Supercontinuum Laser Source, Springer,
N.Y. (1989).
2. G.P. Agrawal, Nonlinear Fiber Optics, Academic Press, San
Diego, Calif. (2001).
3. F.G. Omenetto et al. Opt. Express 14(11), 4928-34.
4. F.G. Omenetto et al. “High-power all-fiber supercontinuum
generation in SF6 photonic crystal fiber,” CLEO, CThV4,
Long Beach, Calif. (2006).
OPN December 2006 | 35
F. Pozzi, M. Sorel, G.A. Siviloglou, S. Suntsov, R. El-Ganainy, R. Iwanow, G.I. Stegeman,
D.N. Christodoulides, D. Modotto, A. Locatelli, C. De Angelis and R. Morandotti
36 | OPN December 2006
up to 0.7 mm long. We also fabricated
tapers for efficient modal excitation and
output collection. To evaluate nonlinear
response, we studied the spectral evolution of 8 ps TM polarized pulses when
propagating in these nanoguides as a
function of the input power. Although
the nanowires were shorter than the
tapers, the nonlinear phase shift in the
nanowire was found to clearly dominate
the spectral evolution.
In spite of relatively high losses, these
prototype nanowires were orders of
magnitude more efficient in producing a
nonlinear phase shift and hence spectral
broadening than the more conventional
waveguides used previously for all-optical
devices (as in nonlinear directional couplers). More specifically, nonlinear phase
shifts of p were obtained for effective
nanowire lengths of 0.35 mm at 30 W.
By including linear as well as nonlinear absorption processes, we modeled the
nonlinear propagation in these sub-wavelength guiding structures and found them
to be in excellent agreement with the
experimental data. Such tightly confined
waveguides may be the key to realizing
all-optical switching devices that operate
at greatly reduced power levels and that
can be integrated at much higher packing
densities than previously possible. t
This research was sponsored in the U.S. by the NSF,
in Canada by NSERC, and in the U.K. by EPSRC.
The authors thank the staff of the James Watt Nanofabrication Centre at the University of Glasgow.
[ F. Pozzi and M. Sorel ([email protected])
are with the department of electrical and electronic
engineering at the University of Glasgow, Glasgow,
Scotland. G.A. Siviloglou, S. Suntsov, R. El-Ganainy,
R. Iwanow, G.I. Stegeman and D.N. Christodoulides
are with CREOL & FPCE, University of Central Florida,
Orlando, Fla. D. Modotto, A. Locatelli and C. De Angelis are with the Istituto Nazionale per la Fisica della
Materia, dipartimento di elettronica per l’automazione,
Università di Brescia, Brescia, Italy. R. Morandotti
([email protected]) is with the National Institute
of Scientific Research, University of Quebec, Quebec,
Canada. ]
References
1. P.N. Prasad. Nanophotonics, John Wiley & Sons, N.Y. (2004).
2. J.C. Knight. Nature 424, 847-51 (2003).
3. A.R. Cowan et al. Opt. Express 12, 1611-21 (2004).
4. E. Dulkeith et al. Opt. Express 14, 5524-34 (2006).
5. G.A. Siviloglou et al. Opt. Express 14, 9377-84 (2006).
2.5
2.0
1.5
Air
ne of the most fascinating challenges of the past few decades has been
to develop all-optical integrated circuits
for future telecommunications networks.
So far, the solution has remained elusive.
In principle, ultrafast all-optical switching can be achieved through nonlinear
materials with intensity-dependent
refractive indices. Over the years, there
has been considerable effort towards the
realization of photonic devices in which
all-optical routing can take place. However, in typical weakly guiding structures,
this can only be accomplished at prohibitively high power levels.
A simple alternative is to fabricate
structures with sub-micron transverse
dimensions (strongly guiding geometries)
in materials with high Kerr nonlinearities
and well-established fabrication technologies, such as IV-IV and III-V semiconductors.1 In general, optical nanowaveguide structures can provide superior
light confinement (because of their strong
index contrast) and are thus ideal for
nonlinear optics applications.2
For example, the fabrication of silicon
nanowires has today reached an extraordinary technological maturity, resulting
in nanoguides with linear losses on the
order of a few dB/cm. However, silicon’s
nonlinear optical response is limited by
multi-photon absorption.3,4
With this in mind, we have recently
explored the fabrication of high indexcontrast nanowires of AlGaAs, which is
well known for its excellent nonlinear
properties.5 The development of submicron cross-section AlGaAs nanowires
may eventually lead to ultra-compact,
low-power nonlinear devices operating at
watt power levels.5 Here, we report the
first observation of self-phase-modulation
enhancement due to strong confinement
in such nano-waveguides.
The prototype structures fabricated
were a few hundred nanometers wide and
x [mm]
O
1
0.5
L
3 mm
0
–0.5 0
0.5
850 mm–L/2
y [nm]
(a)
Power spectrum [a.u.]
NONLINEAR OPTICS
Enhanced Third-Order Nonlinear Effects
in Ultra-Compact AlGaAs Nanowires
W
(b)
1
L=0 mm
L=600 mm
0.8
0.6
0.4
0.2
1554
50 mm
1556
1558
Wavelength [nm]
(d)
1580
850 mm–L/2
50 mm
(c)
(a) Typical refractive index profile of the nanowaveguides used in the experiments reported here. Upper
and lower claddings are composed of Al0.7Ga0.3As,
while the core is formed from Al0.2Ga0.8As. (b) SEM picture of the nanowire rib waveguide on a GaAs substrate
(upper inset) and the complete structure as viewed
from above (in the plane of the substrate, lower inset).
(c) Typical TM mode field profile. (d) Spectral broadening measured for the L=0 and 600 mm samples for an
input power of 38 W (collected for a wire 550 nm wide).
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