Watt-level Flat Supercontinuum Source Pumped by Noise-like Pulse
from an All-fiber Oscillator
He Chen, Shengping Chen, Zongfu Jiang and Jing Hou
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China
chenhhe@qq.com, chespn@163.com, jingzongfu7@163.com, houjing25@sina.com
Keywords: Supercontinuum Generation, Ultrafast Optics, Mode-locked Lasers, Fiber Lasers.
Abstract: We demonstrate Watt-level flat visible supercontinuum (SC) generation in photonic crystal fibers, which is
directly pumped by broadband noise-like pulses from an Yb-doped all-fiber oscillator. The novel SC
generator is featured with elegant all-fiber-integrated architecture, high spectral flatness and high efficiency.
Wide optical spectrum spanning from 500 nm to 2300 nm with 1.02 W optical power is obtained under the
pump power of 1.40 W. The flatness of the spectrum in the range of 700 nm~1600 nm is less than 5 dB
(including the pump residue). The exceptional simplicity, economical efficiency and the comparable
performances make the noise-like pulse oscillator a competitive candidate to the widely used cascade
amplified coherent pulse as the pump source of broadband SC. To the best of our knowledge, this is the first
demonstration of SC generation which is directly pumped by an all-fiber noise-like pulse oscillator.
1 INTRODUCTION
The phenomenon of supercontinuum (SC)
generation in nonlinear medium, especially in
optical fibers, is appreciated as one of the most
spectacular in nonlinear physics. Owing to the
unique combination of extremely broad spectral
bandwidths, high spectral power densities, and high
spatial coherence, fiber-based supercontinuum lasers
have found numerous applications in areas such as
spectroscopy and microscopy (Dudley et al., 2006).
There are two critical requisites for SC
generation, the first one is the nonlinear medium,
including various bulk materials and nonlinear
optical fibers, among which photonic crystal fiber
(PCF) had attracted the most intensive attentions
because of its unprecedented design freedom of the
dispersion properties (Russell, 2003, Knight et al.,
2000). Specifically, by design optimization of the
zero dispersion wavelength (ZDW), SC covering the
whole visible spectrum can be facilely generated
from fibers (Stone and Knight, 2008).
Beside the nonlinear medium, highly intensive
pump source plays another essential role in process
of the broadband SC generation. In terms of the
temporal durations of the pump pulses, there are
typically three commonly used pumping schemes,
which are pumping with fs pulse (Ranka et al.,
2000), ps-ns long pulse (Coen et al., 2001, Xiong et
al., 2006, Chen et al., 2011a, Chen et al., 2013) and
continuous wave (CW)/quasi-CW (Travers et al.,
2008, Cumberland et al., 2008), respectively. Each
kind of pump lasers evolve in partially different way
in the nonlinear medium, especially when they are
launched in the abnormal dispersion region.
Beside those three typical types of pump lasers,
there is another intriguing optical pulse form, called
noise-like pulse (NLP) or double-scale pulse
(Horowitz et al., 1997, Tang et al., 2005a, Zhao et
al., 2007, Kobtsev et al., 2009, Yu et al., 2014,
Churkin et al., 2015, Liu et al., 2015), which has
been recently widely investigated and was also
exploited as the pump source to achieve spectral
broadening and nonlinear frequency generation in
nonlinear medium (Zaytsev et al., 2013, Kobtsev et
al., 2014, Lin et al., 2014, Runge et al., 2014,
Smirnov et al., 2014, You et al., 2015). NLP is quite
distinct from those standard mode-locked pulses in
two ways. Temporally, NLP is regarded as
stochastic fs bunch localized in a ps-ns pulse packet.
Spectrally, it is featured with the ultra-broadband
spectrum, which is actually the superposition of
incoherent single pulses’ spectra (Runge et al.,
2013). Although the “unstable” nature usually makes
NLP regarded to own limited practical usability
because of its intractability, interestingly, pumping
nonlinear medium with NLP had shown several
advantages over similar single pulse sequence. For
example, it had been experimentally shown that
double-scale pulses manifest more efficient Raman
transformation comparing with single scale pulses,
leading to broader supercontinuum spectra (Kobtsev
et al., 2014). And similar conclusion was also drawn
in the scenarios of second-harmonic generation with
NLP (Smirnov et al., 2014).
Actually NLP-pumped SC generation has been
recently investigated in several kinds of nonlinear
fibers, such as standard SMF-28 (Zaytsev et al.,
2013), highly nonlinear fiber (Lin et al., 2014),
P2O5-doped fiber (Kobtsev et al., 2014), and widely
extended spectra have been obtained, however none
of them extended to visible spectral regions, which
is mainly restricted by the dispersion properties of
the used nonlinear fibers. Moreover, in all of the
related investigations, at least one stage of fiber
amplifier was included in the configuration to boost
the pump power.
In this manuscript, we report a novel elegant
scheme of Watt-level visible SC generation which
takes advantages both of the unique dispersion
properties of PCF and the special features of NLP.
Firstly, unlike fore-reported investigations on NLP-
pumped near-infrared SC generation, we use PCF to
expand the spectrum to both visible and infrared
directions. Secondly, no fiber amplifier is used in the
configuration, the SC is directly pumped by the NLP
from an ultra-simple all-fiber master oscillator, by
which the whole system is much more compact
comparing with its counterparts. Actually the work
is partially motivated by simplifying the architecture
of SC source. By significantly reducing the
complexity, and the cost of SC laser sources, Watt-
level pocket SC source will be possible and
applications within new areas can be enabled.
Besides, the differences in the mechanisms between
NLP-pumped SC generation and regular-pulse-
pumped one are discussed.
2 EXPERIMENTAL SETUP
The architecture of the all-fiber-integrated
supercontinuum generator is depicted in Fig.1,
which is constructed with an ultra-simple mode-
locked Yb-doped all-fiber oscillator and a section of
PCF. The Yb-doped fiber oscillator is formed with
two NOLMs and a cladding-pumped fiber amplifier.
The left NOLM was made by a 50:50 fused coupler
and a section of 35-meters-long twisted passive fiber
in the loop. The right NOLM was made by a 10:90
fused coupler. Both of the NOLMs act as the
reflective mirrors of the oscillator for laser
oscillating and artificial saturable absorber for
mode-locking simultaneously. Two fiber-squeezer-
based polarization controllers were applied on both
of the NOLMs to control their nonlinear reflection
characteristics, which are essential for manipulating
the mode-locking behaviors (Chen et al., In press).
The fiber amplifier is formed with a section of
double-cladding 10/125 Yb-doped fiber, a 976 nm
multimode laser diode as the pump source, and a
pump/signal combiner.
Figure 1: The schematic of the proposed supercontinuum
generator. PC: polarization controller; YDF: Yb-doped
fiber; PCF: photonic crystal fiber.
By proper polarization controlling, the fiber
oscillator before the PCF can be operated in the NLP
regime, where the pulse peak power can be
increased almost linearly with the pump power. The
repetition rate was measured to be 3.52 MHz by an
oscilloscope. Under the pump power of 5.8 W, the
highest power energy of 0.4 μJ and average peak
power of 7 kW were obtained. The corresponding
autocorrelator trace has a short 100-fs-wide peak and
a 57-ps-wide pedestal beneath it, which reveals the
noise-like nature of the output pulses: the pulse is
actually a pulse packet rather than an integrated
single pulse, which is formed with stochastic 100-fs
inner pulse. The output spectrum has a 3 dB spectral
width of 76 nm. The detailed characteristics of the
pump oscillator can be found in our previous
contribution (Chen et al., In press). Notably, it is
believed that stimulated Raman scattering (SRS)
plays an essential role in the formation of the NLP
and its ultra-broadband spectrum (Aguergaray et al.,
2013, Tang et al., 2005a).
Two sections of solid core PCFs were employed
in the experiment as the main nonlinear spectral
broadening medium. The first one has a core
diameter of 4.7 μm with 6-layers of hexagonal air
holes arranged in the cladding. The second one has
similar structure, but a larger core diameter of 7.5
μm. The cross profiles of the two PCFs are shown as
in Fig. 3. The calculated dispersion curves of the
PCFs are also illustrated in Fig. 3. As we can see,
the two PCFs have different zero-dispersion-
wavelengths (ZDWs) of 1035 nm and 1118 nm
respectively. We use "PCF 1" and "PCF 2" to refer
to the two PCF respectively. The 3 dB spectral
bandwidth of the pumping NLP ranges from 1070
nm to 1146 nm, which is totally in the abnormal
dispersion region of PCF 1, and partially normal/
partially abnormal dispersion region of PCF 2. The
PCF and the 10/125 output fiber were fusion spliced
with the technique of controlled hole collapse to
reduce the splicing loss (Chen et al., 2011b), which
is estimated to be around 0.5 dB.
Figure 2: The calculated dispersion curves and the cross
profiles of PCF 1 and PCF 2.
3 EXPERIMENTAL RESULTS
AND DISCUSSIONS
By propagating through a 3-meters-long PCF 1, the
spectrum of the output pulse was significantly
expanded. Figure 4 shows the evolution of the
generated SC the different pump power levels. The
spectra in the range of 400 nm-1200 nm and 1200
nm-2400 nm were separately measured by two
different optical spectrum analyzers with the same
spectral resolution of 0.5 nm, and were joined
together in Fig. 4. A 1400 nm cut-on long pass filter
was employed after the output port when measuring
the spectra of 1400 nm-2400 nm to avoid the second
order harmonics of 700 nm-1200 nm spectra. The
SC was delivered to the optical spectrum analyzers
through a short section of multimode fiber, and the
high order mode interference in the delivering fiber
caused the fine fluctuations on the spectral curves.
As the pump power increased from 0.5 W to 1.4
W, the spectrum expanded towards both of the long
and the short wavelength directions simultaneously,
as exhibited in Fig. 4. Under the highest pump
power of 1.4 W, a wide and flat SC spectrum with
total power of 1.02 W is obtained which spans over
1800 nm, ranging from ~500 nm to ~2300 nm. The
spectrum has excellent flatness in a large spectral
range. Unlike the traditional ps-pulse-pumped SC
with a high pump residue peak on the spectrum, in
this case, the broadband pump residue was merged
with the extended spectral components, forming an
integrated and super-flat SC. The 6 dB spectral
bandwidth spanned from 685 nm to 1620 nm, and 20
dB bandwidth spanned from 540 nm to 2040 nm
(including the pump residue).
The spectral flatness and smoothness as
exhibited in this NLP-pumped SC were regarded as
the typical feature and one of the great benefits of
ps-ns long pulse pumped SC, which indicated that
there is similar mechanism underlying the two
different pumping scheme. It is reasonable in a
qualitative perspective. For the case of SC
generation pumped with ps-ns long pulse in the
abnormal dispersion region, modulation instability
(MI) causes the temporal breakup of the long input
pulse and transform them into a bunch of numerous
stochastic fs-level solitons in the initial phase before
the dramatic spectral extending (Dudley et al.,
2009). The MI-induced incoherent soliton bunch, to
some extent, resembles the characteristics of the
incoherent NLP, which is also regarded as a bunch
of fs-level stochastic sub-pulses, and those sub-
pulses will be further broken down to narrower
solitons after the process of pulse compression and
soliton fission. Although the initial evolution process
of the two kinds of pumping pulses in PCF may be
different, the outcomes are similar, both of which
evolve to a great bunch of stochastic solitons, except
that the distributions of the solitons’ pulse duration
and peak power may differ from each other.
However, the exact differences between the MI-
induced soliton bunch and the NLP formed in the
mode-locking oscillator requires specific numerical
and experimental investigation.
After the process of solitons formation, the
spectra are dramatically extended driven by the
soliton dynamics, where the Raman induce soliton
self-frequency shift (SSFS) is mainly responsible for
the generation of long wavelength spectra, and the
spectral extension towards the short-wavelength-side
is driven by the corresponding dispersive wave
generation in the normal dispersion region. While
the soliton is being red-shifted due to SSFS, it traps
a packet of dispersive waves under the group-
velocity matched condition, which is called soliton
trapping of dispersive waves, so that the dispersive
waves are further blue-shifted resulting the group
velocity matching of the spectral edges (Gorbach
and Skryabin, 2007). The spectrum exhibits a
continuous broadening towards both sides of the
pump wavelength in this process. The spectrum
should be further expanded to both of the red and
blue sides by increasing the pump power.
Figure 3: The spectral evolution process of the generated
SC in PCF 1 under the different pump power levels.
Different from pumping PCF 1 in the abnormal
dispersion region, in this section, we exhibit the
experimental results of simultaneously pumping in
the normal and abnormal dispersion region of PCF
2, whose ZDW is at 1118 nm. As we can see in Fig.
3, the center wavelength of the broadband pumping
NLP is located very close to the ZDW of PCF 2,
nearly half of the pump lay in the normal dispersion
region, and the other half in the abnormal region.
The length of PCF 2 used in the experiment is 6 m,
as it has relatively large effective mode area and low
nonlinear parameter, it is elongated to sufficiently
enhance the fiber nonlinearities. Figure 5 shows the
evolution process of the generated SC under
different pump power levels. As we can see, the
bandwidth of the SC from PCF 2 at the same pump
power is much narrower than the one from PCF 1,
especially at the pump power of 0.5 W and 1.0 W.
However, under the pump power of 1.4 W, the
generated SC spans over more than 1750 nm,
ranging from 550 nm to 2300 nm, with the total
power of 1.05 W, which is similar with the case of
PCF 1, except that the short wavelength edge ceased
at 550 nm rather than 500 nm in PCF 1. The
difference in the short wavelength edges is mainly
determined by the different group velocity matching
curves of the two PCFs. Although nearly half of the
pump is located at the normal dispersion region of
the PCF, the generated SC shows similar
characteristics with the case of all abnormal
dispersion pumping.
The NLP-pumped SC generated in PCF 1 and
PCF 2 shown in the previous two sections have
manifested that NLP can be used to generating flat
and broadband SC even with a relatively low pump
power without any additional amplifiers. These
experimental results make essential contributions to
the subject of SC generation in two aspects.
The first one is that this NLP-pumped SC
generation scheme can be of great practical utility
Figure 4: The spectral evolution process of the generated
SC in PCF 2 under the different pump power levels.
because of its structural simplicity and the
comparable performances. Actually, it is a trait of
NLP that it can be generated directly from mode-
locked fiber oscillator with high pulse energy and
peak power. While, regular single pulse, like soliton
and dissipative soliton, which are directly produced
from all-fiber oscillator, always have limitations in
peak power and pulse energy scalability. Pulse
splitting or harmonic mode locking can be induced
by the peak power clamping effect under intensive
pump power (Tang et al., 2005b, Liu, 2010),
resulting a limitation in the output peak power,
which is critical for broadband SC generation.
Hence, traditional regular pulse pumped all-fiber SC
laser source always involves cascaded fiber
amplifiers. As an instance, the dual-NOLM-based
mode-locked fiber laser used in the experiment can
also produce single pulse sequence in the dissipative
soliton and dissipative soliton resonance regime by
polarization controlling (Chen et al., In press), but in
both regimes, the output peak powers are all limited
below 1 kW. The special facility of generating
pulses (packets) with high peak power directly from
fiber oscillator under intensive pump power was also
verified numerically (Knight et al., 2000). Hence, for
the applications where the coherence of SC is not a
requisition, the employment of intensively pumped
all-fiber NLP oscillator as the pump source of SC
generation is feasible and a much more
economically efficient and simplified scheme,
comparing with the traditional cascaded-amplifier-
based one. We believe, by further improving the
stability and robustness, this scheme may make
Watt-level pocket SC source possible, from which a
great many of applications could benefit. Even for
applications where highly powerful all-fiber SC
generation is required, the employment of high
power NLP fiber oscillator as the seed laser of the
cascaded fiber amplifiers will save one or two stages
of pre-amplifiers.
Beside the practical utility, there are also
scientific subjects involved in the physical process
of the NLP-pumped SC generation. As mentioned
before, the distinct temporal structure of incoherent
NLP makes the initial evolution process of NLP-
pumped SC generation different with the scenarios
of pumping with coherently mode-locked ps-ns long
pulses in abnormal dispersion region. Briefly
speaking, coherently mode-locked long pulses
evolve to incoherent soliton bunch under the
influence of MI, while NLP itself is incoherent
soliton-resembling bunch with stochastic sub-pulse
durations and peak powers. In other words, as a
qualitative description, the primary place where the
process of pulse breakup happens is different in the
two pumping schemes, one is in the mode-locking
oscillator under the main influence of SRS (Runge et
al., 2014, Aguergaray et al., 2013), while the other is
in the PCF under the influence of MI. As NLP often
owns diversified temporal and spectral structures,
which may involves complex processes like the
generation of optical rogue waves, there may be
some possibilities to manipulate the SC generation
by engineering the formation of NLP in the pumping
fiber oscillator. To answer those remaining questions
and verify the speculations, a theoretical framework
which incorporates both of the NLP dynamics and
the SC generation dynamics will be built in future
investigations.
It should be noted that the PCFs used in the
experiments are all commercial fibers without
specific dispersion engineering. We believe this
ultra-compact SC generation scheme with all-fiber
NLP oscillator as the pump can be further developed
towards higher output power and wider spectrum
extending to violet region by the employment of
specifically designed PCF and the optimization of
the pumping oscillator.
4 CONCLUSIONS
We demonstrate Watt-level flat visible SC
generation in PCFs, which is directly pumped by
broadband noise-like pulses from an Yb-doped all-
fiber oscillator. Two different PCFs were tested. In
PCF 1, wide optical spectrum spanning from 500 nm
to 2300 nm with 1.02 W optical power is obtained
under the pump power of 1.40 W. The flatness of the
spectrum in the range of 700 nm~1600 nm is less
than 5 dB (including the pump residue). The largely
simplified architecture, exceptional flatness, and
provide another feasible to the commonly used
scheme with single pulse sequence as the pump. We
believe the spectrum bandwidth and output power
can be further elevated by PCF optimizing the cavity
design of the pump oscillator and characteristics of
the NLP. With very limited amount of optical
components, it is probably the most economically
efficient watt-level visible SC source ever. By
reducing the complexity and the cost of Watt-level
visible SC laser sources, applications within new
areas can be enabled.
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