Highly-Efficient, Octave Spanning Soliton Self-Frequency
Shift Using a Photonic Crystal Fiber with Low OH Loss
Stephen A. Dekker(1), Ravi Pant(1), Alexander C. Judge(1), C. Martijn de Sterke(1) Benjamin J. Eggleton(1),
Itandehui Gris-Sánchez(2), Jonathan C. Knight(2)
(1)Centre for Ultrahigh-Bandwidth Devices for Optical Systems, Institute for Photonics and Optical Science, School of Physics, The University of
Sydney, NSW 2006, Australia.
(2)Centre for Photonics and Photonic Materials, Dept. of Physics, University of Bath, Bath BA2 7AY, UK.
Author email address: email@example.com
Abstract: We report the first demonstration of octave spanning soliton self-frequency shift in a
OH absorption reduced fiber with widely-spaced zero-dispersion wavelengths. To our knowledge,
this is the largest reported frequency tuning for a fiber-based source.
OCIS codes: (190.4370) Nonlinear optics, fibers; (320.7110) Ultrafast nonlinear optics; (320.7120) U ltrafast phenomena
1. Introduction and Background
Pulsed, wavelength tunable sources are desirable for numerous applications in areas such as communications,
analog-to-digital conversion and spectroscopy [1, 2]. The Soliton Self-Frequency Shift  (SSFS), which is a result
of the red-shift induced by intra-pulse Raman scattering, is ideal for realizing such sources. A wavelength tunable
source employing the SSFS is attractive because the red-shift can be continuously tuned by varying the power of the
input pulse, it can provide a very large tuning range, and it leads to sources with short output pulses (~ 10’s fs
There have been several demonstrations of the SSFS
in different types of silica fiber [4, 5]. The maximum
shift achievable in these experiments is limited on
the long wavelength side by the intrinsic absorption
of silica at wavelengths above λ ≈ 2 μm . Since
the SSFS requires anomalous dispersion , the
separation of the first and second zero-dispersion
wavelengths, λZD1 and λZD2, also limits the achievable
red-shift. The λZD1 can be lowered by using Photonic
Crystal Fibers (PCFs) with small cores, but this tends
to increase the OH loss peak around a wavelength λ
≈ 1400 nm , thus, in effect, creating an upper limit
on the SSFS when pumping at shorter wavelengths.
Here we present the first experimental demonstration
of octave spanning soliton self-frequency shift in
which pulses from a mode-locked Ti:Sapphire laser
at a wavelength λ = 783 nm are shifted to λ = 1650
nm in a PCF. The efficiency of this process is 54%;
this fraction of the output energy is contained in the
most red-shifted soliton. The concept of the octave
spanning SSFS is shown in Fig. 1: widely spaced
wavelength interval with anomalous dispersion,
which, in principle, is available for the SSFS.
However, in commercially available fibers with these
parameters the high loss associated with the OH peak
around a wavelength of 1400 nm prevents this full
interval from being exploited. In contrast, we used a
specially designed PCF which combines a reduced OH loss peak with widely spaced zero dispersion wavelengths,
allowing almost the entire interval with anomalous dispersion to be exploited.
ensure a large
Our PCF had a length of 22 m, a core diameter of 1.5 μm, and a peak OH-associated attenuation of 0.09 dB/m; an
SEM of the fiber’s core area is shown in the inset of Fig. 1. Using the SEM of the fiber cross-section and a
Fig. 1. Schematic of the soliton self-frequency shift showing an input
soliton shifting from near the first zero-dispersion wavelength to the second
zero-dispersion wavelength while crossing the traditional region of OH
absorption. Inset shows an SEM of the fiber core region.
Fig. 2. Schematic of the experimental set-up: pulses from a mode-locked
Ti:Sappire laser are coupled into 22 m of PCF, via an attenuator and a
polarization controller. The output is analyzed on an OSA.
commercially available finite-element software package, the anomalous dispersion region was calculated to range
from λZD1 = 700 nm to λZD2 = 1870 nm (see Fig. 1). This window, wherein the nonlinear coefficient varies
monotonically from γ = 0.11 (Wm)−1 at λZD1 to γ = 0.03 (Wm)−1 at λZD2, permits an SSFS over an octave for an input
wavelength of 783 nm.
2. Fiber fabrication
The fiber was fabricated using the stack-and-draw process but with additional steps to reduce spectral attenuation.
Previously published data on attenuation in such small-core PCF’s shows a strong increase in the spectral
attenuation for core diameters below about 2 μm, due to extrinsic OH contamination during stacking, and structural
damage to the silica matrix during the drawing. These together cause the increased attenuation both at the OH
overtones and at other wavelengths within the transparency window of silica. Previous efforts to reduce these effects
using halogen-based dehydration were only partially successful . In our fibers we greatly reduced these effects by
annealing the preform in a dry environment immediately prior to fiber drawing , which allowed us to fabricate
low attenuation, small-core fibers with zero dispersion wavelengths suitable for a large SSFS in the near-IR.
3. Experiment and Results
The experimental setup is shown schematically in Fig. 2.
Pulses from a mode-locked Ti:Sapphire laser with a
repetition rate of 83 MHz and centre wavelength of 783 nm
were launched into the PCF using a 40× microscope
objective. An optical isolator was used at the laser output to
avoid feedback. The pulse width, measured using an
autocorrelator, had a full-width at half-maximum of 260 fs,
assuming a Gaussian pulse. Simultaneous measurements of
the pulse spectrum show that the pulses had a linear chirp
with a chirp parameter C ≈ 2.4. Due to the birefringence of
our PCF, a quarter-wave plate was used to optimize the
input polarization such that the red-shift was maximized at
the highest input power. Subsequently, the input power was
tuned using a variable attenuator and the output spectrum
recorded using an optical spectrum analyzer.
Figure 3 shows output spectra for different input powers.
The wavelength of the strongest soliton increases
continuously as the input power varies from 4 mW to 155
mW. At the highest power it has a wavelength of λ = 1650
nm, a red-shift of more than an octave relative to the pump
wavelength of 783 nm. To the best of our knowledge, this is
a record frequency shift obtained in an SSFS experiment,
and the first time an octave shift has been achieved.
We have demonstrated a record SSFS over more than an octave, from 783 nm to 1650 nm using a PCF with reduced
OH loss. For the largest SSFS observed, the fraction of the output energy in the most red-shifted soliton is 54%. This
is thus a high efficiency, pulsed source with wavelength tunable over more than an octave. It is likely that by further
optimizing the input pulse, which in our experiments was linearly chirped, even larger shifts can be observed.
 J. H. Lee et al., “Soliton Self-Frequency Shift: Experimental Demonstrations and Applications,” IEEE J. Sel. Top. Quantum Electron. 14,
 S. Oda and A. Maruta, “SSFS: Experimental Demonstrations and Applications,” Opt. Express 14, 7895 (2006).
 F. M. Mitschke and L. F. Mollenauer, “Discovery of the soliton self-frequency shift,” Opt. Lett. 11, 3330 (1986).
 X. Liu et al., “Soliton self-frequency shift in a short tapered air–silica microstructure fiber,” Opt. Lett. 26, 358 (2001).
 M.-C. Chan et al., “1.2- to 2.2-μm Tunable Raman Soliton Source Based on a Cr : Forsterite Laser and a Photonic-Crystal Fiber,” IEEE
Photonics Technol. Lett. 20, 900 (2008).
 J. P. Gordon, “Theory of the soliton self-frequency shift,” Opt. Lett. 11, 662 (1986).
 A. Montville et al., “Low Loss, Low OH, Highly Non-linear Holey Fiber for Raman Amplification,” Proc. CLEO/QELS’06 CMC1 (2006).
 I. Gris-Sánchez et al., “Reducing spectral attenuation in solid-core photonic crystal fibers,” Proc. OFC’10 OWK1 (2010).
Fig. 3. PCF output spectra for a range of average input powers.
The pump wavelength and the longest wavelength obtained are
indicated with vertical dashed lines.