Mid-infrared ZBLAN fiber supercontinuum source using picosecond diode-pumping at 2 μm

Article (PDF Available)inOptics Express 21(20):24281-24287 · October 2013with42 Reads
DOI: 10.1364/OE.21.024281 · Source: PubMed
Abstract
We present the first demonstration of mid-IR supercontinuum generation directly pumped with picosecond pulses from a Thulium fiber-amplified gain-switched laser diode at 2 µm. We achieve more than two octaves of bandwidth from 750 - 4000 nm in step-index ZBLAN fiber with Watt-level average power and spectral flatness of less than 1.5 dB over a 1300 nm range in the mid-IR from 2450 - 3750 nm. The system offers high stability, power-scaling capability to the 10 W regime, and demonstrates an attractive route towards relatively inexpensive, versatile and practical sources of high power broadband mid-IR radiation.
Mid-infrared ZBLAN fiber supercontinuum
source using picosecond diode-pumping at 2 µm
A. M. Heidt,
*
J. H. V. Price, C. Baskiotis, J. S. Feehan, Z. Li, S. U. Alam,
and D. J. Richardson
Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, United Kingdom
*
A.Heidt@soton.ac.uk
Abstract: We present the first demonstration of mid-IR supercontinuum
generation directly pumped with picosecond pulses from a Thulium fiber-
amplified gain-switched laser diode at 2 µm. We achieve more than two
octaves of bandwidth from 750 – 4000 nm in step-index ZBLAN fiber with
Watt-level average power and spectral flatness of less than 1.5 dB over a
1300 nm range in the mid-IR from 2450 - 3750 nm. The system offers high
stability, power-scaling capability to the 10 W regime, and demonstrates an
attractive route towards relatively inexpensive, versatile and practical
sources of high power broadband mid-IR radiation.
©2013 Optical Society of America
OCIS codes: (190.4370) Nonlinear optics, fibers; (060.2390) Fiber optics, infrared.
References and links
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Received 24 Jul 2013; revised 22 Sep 2013; accepted 25 Sep 2013; published 3 Oct 2013
(C) 2013 OSA
7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.024281 | OPTICS EXPRESS 24281
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1. Introduction
Supercontinuum generation (SCG) in non-silica fibers is a promising approach to meet the
growing demands for broadband, high brightness mid-IR radiation in various research areas
such as molecular fingerprinting and chemical sensing [1,2]. Heavy metal fluoride (ZBLAN)
fibers are particularly attractive due to their technological maturity and transparency in the
mid-IR, and SCG in this fiber type has been demonstrated with numerous pumping schemes,
predominantly using either nanosecond or femtosecond pumping [1–7].
For SCG in the near-IR and visible wavelength regions, picosecond pumping has been
successful in both research and commercial products as it provides many of the proven
advantages associated with femtosecond systems, but with great benefits in terms of reduced
system cost and complexity, as well as higher spectral power densities [8–10]. When seeded
by electrically gain-switched semiconductor diode lasers, picosecond pump systems offer a
high level of control, compactness, and reliability with intriguing prospects for controlling the
properties of the continuum that is generated [11]. The pumping of ZBLAN fibers with diode-
seeded picosecond fiber amplifier systems therefore represents a particularly attractive route
towards practical mid-IR SC generation.
Current implementations of diode-pumped mid-IR SCG in ZBLAN fibers are based on
amplified nanosecond - rather than picosecond – pulsed laser diodes emitting around 1550 nm
and therefore rely on the decay of the pump pulses into noisy femtosecond sub-pulses prior to
the coupling into the nonlinear fiber in order to reach the required peak powers for the SCG
process [3]. While this technique leverages the maturity of telecommunication components, it
also relinquishes much of the potential control offered by diode pumping. As ZBLAN fibers
typically have zero-dispersion wavelengths (ZDWs) between 1.65 – 1.9 µm, pumping at
longer wavelengths in the anomalous dispersion region is more favourable for efficient SCG.
Pumping close to 2 µm results in both an extension of bandwidth and an increase of
conversion efficiency towards mid-IR wavelengths [12]. Seed diodes around 1550 nm
therefore require the conversion of the pump pulses to longer wavelengths. Several methods
including soliton self-frequency shift, a two-stage SCG process with intermediate
amplification in Thulium-doped fiber amplifiers (TDFAs) or the gain-switching of a TDF
laser cavity (pumped using a pulsed 1550 nm laser diode) have been applied for this purpose
[2,13], neither of which are ideal from a system simplicity and reliability point of view.
In this paper we present what we believe to be the first report of a mid-IR SC source
pumped directly with high peak power picosecond pulses from a fiber-amplified gain-
switched laser diode at 2 µm. We achieve a spectral bandwidth of more than two octaves
spanning from 750 - 4000 nm, limited only by the transparency window of the ZBLAN fiber.
The SC source delivers Watt-level average power, high stability, and spectral flatness of
better than 1.5 dB in the range 2450 - 3750 nm, which is unprecedented in the mid-IR
wavelength region and enables, for example, spectroscopic measurements with uniform
spectral sensitivity and high signal-to-noise ratios. The system should be power-scalable to
the 10 W regime and represents a significant improvement in system simplicity compared to
existing diode-pumped mid-IR sources.
#194526 - $15.00 USD
Received 24 Jul 2013; revised 22 Sep 2013; accepted 25 Sep 2013; published 3 Oct 2013
(C) 2013 OSA
7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.024281 | OPTICS EXPRESS 24282
2. Experimental setup
Fig. 1. (a) Schematic experimental setup of the diode-seeded Thulium-doped fiber amplifier
(TDFA) pump system and the SC generation stage. LD: laser diode; FBG: fiber Bragg grating;
LMA-TDF: large mode area Thulium-doped fiber; DM: dichroic mirror; HWP: half-wave
plate; ISO: isolator. (b) Calculated dispersion profiles of the fundamental LP
01
mode and the
first three higher order modes of the ZBLAN fiber.
The experimental setup of the pump system and the SCG stage is shown in Fig. 1(a). The
picosecond pump pulses are directly generated by electrical gain-switching of a discrete-
mode laser diode (Eblana Photonics) operating at 2008 nm and are subsequently amplified in
a multistage Thulium-doped fiber amplifier (TDFA) chain. The system consists of two pre-
amplifiers, both core-pumped by an in-house built 1565 nm Er/Yb fiber laser, and a power
amplifier stage using large-mode area TDF with a core/cladding diameter of 25/250 µm
(Nufern), free space cladding-pumped in a counter-propagating configuration by a 800 nm
fiber-coupled laser diode with up to 75 W of pump power. A fiber Bragg grating (FBG) based
spectral filter removes excess amplified spontaneous emission (ASE) after the first pre-
amplifier. The system delivers 33 ps pulses with up to 3.5 µJ pulse energy and 100 kW peak
power at variable repetition rates in the range 1 MHz – 1 GHz with near-diffraction limited
beam quality, as reported in [14,15]. For the SC generation, the high power picosecond pulses
are extracted from the pump system with a dichroic mirror and coupled into 7 m of ZBLAN
fiber (9 µm core diameter, 0.25 NA, IR Photonics) with 60% coupling efficiency using an
aspheric lens. We performed experiments with a hand-cleaved fiber input facet as well as a
polished and connectorized fiber, but observed no significant difference in coupling
efficiency or power handling capability. A polarizing isolator and a pair of half-wave plates
protect the pump system from back-reflections, allow variable attenuation of the pump beam
and the control of its polarization state. The generated continuum was characterized using two
optical spectrum analyzers (OSAs), a Yokogawa AQ6370 for the wavelengths up to 1600 nm
and an AQ6375 in the range 1600 – 2200 nm, as well as a monochromator (Bentham
TMc300V) with liquid nitrogen cooled PbS detector for wavelengths above 2200 nm. The
measurement resolution was set to 2 nm for the OSAs and 10 nm for the monochromator. All
instruments were corrected for their respective wavelength responses, and the relative power
levels were calibrated by recording data with ~100 nm measurement overlap between the
instruments.
Figure 1(b) shows the dispersion profiles of the fundamental LP
01
mode and the first three
higher order modes of the ZBLAN fiber, calculated with a fully vectorial finite element
method using the material dispersion supplied by the manufacturer and assuming a constant
NA for all wavelengths. At the pump wavelength of 2008 nm, the fiber is multimoded and
supports both LP
01
and LP
11
modes. The ZDW of the fundamental mode is estimated at 1650
nm, i.e. the pump is located deep in the anomalous dispersion region with D
13 ps/(nm·km)
at 2008 nm. The LP
11
mode has low normal dispersion (D
-3 ps/(nm·km)) at the pump
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Received 24 Jul 2013; revised 22 Sep 2013; accepted 25 Sep 2013; published 3 Oct 2013
(C) 2013 OSA
7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.024281 | OPTICS EXPRESS 24283
wavelength and cuts off above ~2800 nm, i.e. the fiber is single-moded at mid-IR
wavelengths. All other higher order modes have cut-off wavelengths much shorter than the
pump.
3. Results and discussion
3.1 Spectral characteristics and SC generation dynamics
Fig. 2. Supercontinuum spectra generated with 185 nJ (black), 300 nJ (red), 600 nJ (green),
and 1100 nJ (blue) input pulse energy (offset for clarity). Also shown are the calculated group
indices of the fiber modes with identical colour code as in Fig. 1(b). The wavelength axis of
the calculated group indices is shifted by about 100 nm with respect to the measurement, as
indicated by the lower x-axis (blue). The insets show a magnification of the mid-IR part of the
broadest spectrum (top right) and the far-field mode profile of the ZBLAN fiber output at
visible wavelengths (top left).
The generated supercontinuum spectra are shown in Fig. 2 as a function of pulse energy
measured at the output of the ZBLAN fiber. The maximum spectral bandwidth of more than
two octaves spanning from 750 – 4000 nm is reached for 1.1 µJ pump pulses. A further
increase in pump power did not lead to a significantly enhanced spectral broadening,
indicating that the bandwidth of the SC at mid-IR wavelengths is likely limited by the
transparency window of the ZBLAN fiber. The influence of fiber attenuation is evident from
the steepening of the spectral slope on the long wavelength edge of the continuum at
wavelengths above 3800 nm. This coincides with the general attenuation curve provided by
the manufacturer indicating exponentially increasing losses for wavelengths above 3750 nm
[16]. We have therefore demonstrated a spectral coverage close to the achievable maximum
for this particular fiber and pumping scheme, because the short- and long-wavelength extrema
of the SC are linked via group-velocity-index matching, as will be discussed below.
The shape of the SC is typical for picosecond pumping in the anomalous dispersion region
of a fiber, consisting of a residual peak at the pump wavelength and the continuum forming
approximately 20 dB below the pump peak. The spectra exhibit a remarkable flatness in the
mid-IR with a power variation as low as 1.5 dB over a 1300 nm wide spectral range from
2450 – 3750 nm for 1.1 µJ pump pulses, as shown in the inset of Fig. 2. For this measurement
the monochromator was purged with nitrogen in order to minimize absorption by water
vapour and CO
2
during the free-space propagation through the instrument. Note that the
narrower spectra shown in Fig. 2 were taken without nitrogen purging and exhibit
(unresolved) water absorption peaks in the range 2600 – 2800 nm. The high degree of spectral
flatness of the generated SC is unprecedented in this wavelength region and combined with
the high average output power enables broadband mid-IR spectroscopic measurements with
uniform spectral sensitivity and high signal-to-noise ratios (results in preparation).
#194526 - $15.00 USD
Received 24 Jul 2013; revised 22 Sep 2013; accepted 25 Sep 2013; published 3 Oct 2013
(C) 2013 OSA
7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.024281 | OPTICS EXPRESS 24284
The observed SCG dynamics are determined by the dispersion profile of the fundamental
mode, leading to modulation instability (MI) initiated break-up of the injected pulse into
fundamental solitons and subsequent self-frequency shifting to longer wavelengths [17]. The
contribution of the LP
11
mode to the spectral broadening process is limited by its
predominantly normal dispersion and the low cut-off wavelength. Since the soliton number of
the input pulse is estimated to be of the order N = 100, a large sea of solitons with mutually
overlapping spectra is expected, which in superposition form the flat SC spectrum observed
experimentally. The calculated group indices of the fiber modes, shown in Fig. 2, reveal that
the red-shifting solitons are group-index matched to an ensemble of modes at short
wavelengths, i.e. pulses in these modes propagate with the same velocity through the fiber.
The co-propagation enables nonlinear interaction and energy transfer from the solitons to
these modes in the form of a dispersive wave [18]. A recent study demonstrated that the
dispersive waves can be generated directly in higher order modes, although the solitons
propagate exclusively in the fundamental mode for wavelengths above 2800 nm [19]. The
ZBLAN fiber therefore emits the short wavelengths of the SC as a superposition of higher
order modes, as is shown in the top left corner inset in Fig. 2, which displays the far-field
collimated beam profile of the visible part of the continuum.
An intuitive insight that summarizes the main components of these rather complex SCG
dynamics is given by the visual overlap of the experimental results with the calculated group-
velocity indices of the ZBLAN fiber modes seen in Fig. 2, which shows good agreement of
the experimental long- and short-wavelength extrema of the continuum with the phase-
matching predicted from the group-index matched positions. Due to uncertainties in our
computational assumptions, in which we had to rely on the fiber geometry and material
dispersion reported by the manufacturer, we shifted the wavelength axis of the calculated
group indices by about 100 nm towards longer wavelengths, as was done similarly by others
in previous publications [6].
3.2 Power and stability
Fig. 3. Power conversion efficiency (black) to selected mid-IR wavelengths regions and
average power levels (red) in these regions at 1 MHz pump repetition rate as a function of
pump pulse energy.
Figure 3 shows the power conversion efficiency to selected mid-IR wavelength regions
(defined as percentage of total SC output power) as well as the average power levels in these
regions at a pump repetition rate of 1 MHz as a function of coupled pump pulse energy. We
achieve up to 1.1 W of total average output power, with more than 21% (235 mW) at
wavelengths > 2500 nm and more than 12% (130 mW) at wavelengths > 3000 nm. The pump
depletion is in the order of 60% for all spectra shown in Fig. 2. Higher efficiencies have been
reported recently (e.g. 27% for wavelengths > 3800 nm [12]), but in contrast to these reports
we observe significant bi-directional spectral broadening both to near- and mid-IR regions,
and apply direct picosecond pumping, which inherently leads to lower pump depletion than
femtosecond pumping [17], but also results in higher system simplicity and reliability.
#194526 - $15.00 USD
Received 24 Jul 2013; revised 22 Sep 2013; accepted 25 Sep 2013; published 3 Oct 2013
(C) 2013 OSA
7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.024281 | OPTICS EXPRESS 24285
Note that the average power levels in Fig. 3 refer to the lowest investigated pump
repetition rate, as the system was reliably operated over several weeks for many hours per day
without observing any degradation. However, the average power levels can easily be scaled
by increasing the pump repetition rate, which can simply be done electronically via the pulse
generator used to gain-switch the seed diode. This highlights the simplicity and high potential
for automated electronic control of our approach. In its current configuration the pump system
can deliver the necessary pulse energy for maximum spectral broadening (1.1 µJ) up to
repetition rates of more than 15 MHz [14], enabling a potential SC power in the order of 10
W
Fig. 4. Source stability over a 30-minute interval at a test wavelength of 3350 nm.
and more than 1 W at wavelengths > 3000 nm. When we increased the total SC power level
to 2 W in our experiments, fiber damage occurred at about 80 cm distance from the input end.
The fiber end facet was unaffected. The damage mechanism is still unclear, but we suspect
coating inhomogeneities to be responsible for the failure, as inspection with a visible light
source revealed several leakage points along the fiber length. Earlier demonstrations have
shown that ZBLAN fibers are capable of handling more than 10 W of average power [20],
and we consequently consider the fiber damage a technical rather than a fundamental
limitation.
Since the SCG dynamics are initiated by noise-seeded MI, large shot-to-shot fluctuations
in the spectral and temporal domain are expected, i.e. the continuum is temporally incoherent
[17]. However, when using slow detectors such as power meters or spectrometers averaging
over thousands of shots, the spectrum appears extremely stable. Figure 4 shows a long-term
power stability measurement at a test wavelength of 3350 nm, which was recorded with an
extended-wavelength OSA (Yokogawa, measurement range up to 3400 nm) set to 2 nm
resolution. Although the test wavelength is separated from the pump by over half an octave,
we record less than 0.5 dB power variation over a 30-minute interval. No special effort was
taken to minimize thermal effects on the ZBLAN fiber or to stabilize the polarization state of
the pump system, which might improve this value even further. The excellent stability of the
continuum is enabled by the low amplitude and timing jitters of the pulse generation process
in the gain-switched discrete-mode seed diode and their preservation in the TDFA chain [14],
highlighting the high practicality of our pumping approach.
4. Conclusion
To the best of our knowledge, we present the first supercontinuum source pumped directly by
fiber-amplified picosecond pulses from a gain-switched 2 µm laser diode, achieving
broadband spectral coverage from 750 to 4000 nm, Watt-level average power, exceptional
spectral flatness in the mid-IR, and high time-averaged stability. By seeding and amplifying
high peak power picosecond pulses directly at 2 µm for efficient pumping of ZBLAN fiber,
we achieve a significant improvement in system simplicity and stability compared to earlier
diode-pumped SC sources. The system should be scalable to average powers of more than 10
W using fibers with better power handling capabilities, and a fully fiberized implementation
#194526 - $15.00 USD
Received 24 Jul 2013; revised 22 Sep 2013; accepted 25 Sep 2013; published 3 Oct 2013
(C) 2013 OSA
7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.024281 | OPTICS EXPRESS 24286
of the source is also easy to envisage. Maturing fabrication technology for glass materials
with larger mid-IR transparency window offers future potential for a further bandwidth
increase with this pumping scheme, which represents an attractive route towards relatively
inexpensive, versatile and practical sources of high power broadband mid-IR radiation.
Acknowledgments
We acknowledge OFS Denmark, Nufern, Eblana Photonics and Yokogawa for providing core
and cladding pumped thulium doped fiber, laser diodes and the extended wavelength OSA,
respectively, as well as M. Becker and M. Rothhardt of the IPHT Jena for supplying the 2 µm
FBG. A. M. Heidt acknowledges funding from the EU People Programme (Marie Curie
Actions) under grant agreement 300859 (ADMIRATION). This work was supported by the
EU 7th Framework Program under grant agreement 258033 (MODE-GAP) and by the UK
EPSRC through grant EP/I01196X/1 (HYPERHIGHWAY).
#194526 - $15.00 USD
Received 24 Jul 2013; revised 22 Sep 2013; accepted 25 Sep 2013; published 3 Oct 2013
(C) 2013 OSA
7 October 2013 | Vol. 21, No. 20 | DOI:10.1364/OE.21.024281 | OPTICS EXPRESS 24287
    • "Pumping near a zero dispersion wavelength (ZDW) with higher nonlinearity not only reduces its power requirement but also smoothes the generated supercontinuum power spectra. Tremendous progress has been made4567891011121415161718192021 during the last fifteen years in the generation of SC employing photonic crystal fibers. The success of this progress is mainly due to large effective fiber nonlinearity and dispersive profile which can be tailored at will using dispersion engineering. "
    [Show abstract] [Hide abstract] ABSTRACT: This paper presents broadband supercontinuum generation in photonic crystal fibers employing cosh-Gaussian pulses. The supercontinuum spectrum of cosh-Gaussian pulses consists of a large number of internal oscillations which increases with the increase in the value of cosh factor. Unlike the supercontinuum spectra created by Gaussian pulses, which consists of two distinct minima, the spectra created by cosh-Gaussian pulses of equal peak power and pulse duration characterized by virtually flat and broadband supercontinuum. Improvement in the flatness of the generated supercontinuum increases with the increase in the value of cosh factor.
    Full-text · Article · Nov 2015
    • "In fact, some fiber-based supercontinuum (SC) sources covering the visible and near-infrared band have already emerged as commercial products [3]. However, SCG in the mid-infrared regime or beyond has been the key interest of researchers in recent years, and, as a result, innumerable designs of advanced fiber-based SC sources have been proposed456. In most of the theoretical investigations, researchers intended to maximize the bandwidth by pumping in the anomalous dispersion regime of the host fiber. "
    [Show abstract] [Hide abstract] ABSTRACT: A flat-top, coherent supercontinuum generation (SCG) spanning ∼1540 nm from the near-infrared to shortwave infrared (NIR–SWIR) band in a host lead–silicate-based binary multi-clad microstructure fiber (BMMF) is analyzed and reported. This ultra wide band (903–2443 nm) SCG with flatness <5 dB is theoretically achieved with a combination of a low input pump power source (peak power ˆ 25 kW, pulse width ˆ 75 fs) and a short fiber length (∼8 cm) by controlling the nonlinear dynamics of propagating ultrashort pulses accurately through multi-order dispersion engineering. Simulations reveal that by appropriately controlling the fourth-order dispersion coefficient, a great enhancement in the spectral flatness can be achieved when the device is operated close to the maximum dispersion wavelength in the all-normal dispersion regime.
    Full-text · Article · Jun 2015
    • "Recently, successful realizations of mid-IR supercontinuum generation in PCFs and conventional fibers have been reported by several groups in lead silicate [2], ZBLAN [3] and tellurite [4] glasses. However, in general, the spectra demonstrated in these experiments were rather irregular with reported differences in output power across the spectra of 20 dB over 2700 nm range [3], 40 dB over 2500 nm range [2] and 24 dB over 4000 nm [4]. Practical use of supercontinuum with such large variation of intensity in the spectrum is limited, which prompts further research for enhanced spectral flatness. "
    [Show abstract] [Hide abstract] ABSTRACT: In this paper we report a two octave spanning supercontinuum generation in a bandwidth of 700-3000 nm in a single-mode photonic crystal fiber made of lead-bismuth-gallate glass. To our knowledge this is the broadest supercontinuum reported in heavy metal oxide glass based fibers. The fiber was fabricated using an in-house synthesized glass with optimized nonlinear, rheological and transmission properties in the range of 500-4800 nm. The photonic cladding consists of 8 rings of air holes. The fiber has a zero dispersion wavelength (ZDW) at 1460 nm. Its dispersion is determined mainly by the first ring of holes in the cladding with a relative hole size of 0.73. Relative hole size of the remaining seven rings is 0.54, which allows single mode performance of the fiber in the infrared range and reduces attenuation of the fundamental mode. The fiber is pumped into anomalous dispersion with 150 fs pulses at 1540 nm. Observed spectrum of 700-3000 nm was generated in 2 cm of fiber with pulse energy below 4 nJ. A flatness of 5 dB was observed in 950-2500 nm range.
    Full-text · Article · Jun 2014
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