IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. XX, NO. XX, XX 2013 1
Supercontinuum Generation with GHz Repetition
Rate Femtosecond-Pulse Fiber-Amplified VECSELs
C. Robin Head
1,∗
, Ho-Yin Chan
2,∗
, James S. Feehan
2
, David P. Shepherd
2
, Shaif-ul Alam
2
, Anne C. Tropper
1
,
Jonathan H. V. Price
2
and Keith G. Wilcox
1
Abstract—We report supercontinuum generation using a
mode-locked VECSEL emitting 400-fs pulses at a 3-GHz rep-
etition rate, amplified with a cascaded ytterbium-doped fiber
amplifier system up to 40 W of average power. The pulses
were then recompressed to their original duration via a high
throughput transmission grating compressor, and used to gen-
erate supercontinuum in two samples of photonic crystal fiber
(PCF); an all-normal dispersion PCF, and a PCF with a zero
dispersion wavelength of 1040 nm, creating 20 dB spectral
bandwidths of 200 nm and 280 nm respectively.
Index Terms—Fiber amplifiers, vertical-external-cavity
surface-emitting laser (VECSEL), supercontinuum generation,
GHz repetition rate
I. INTRODUCTION
T
HE generation of supercontinuum in photonic crystal
fiber (PCF) has found applications in a wide range of
research areas such as telecommunications, metrology and
spectroscopy. Typical laser sources used to generate super-
continuum are pulsed solid-state or fiber lasers that operate at
repetition rates up to approximately 100 MHz. While these
low-repetition-rate sources easily produce spectrally broad
supercontinuum due to their high peak power, higher repetition
rates are desirable for frequency combs, as the increased mode
spacing leads to more energy per frequency mode, and it is
possible to isolate individual modes using a simple diffraction
grating [1], [2]. The supercontinuum generated for frequency
combs should, however, be coherent, which typically requires
sub-100 fs pulse durations when using PCF pumped close
to its zero dispersion wavelength (ZDW) [3]. This limit can
be stretched as Stumpf et al. have demonstrated with 170-
fs pulses at a wavelength of 1.5 micron using state-of-the-
art dispersion-flattened, polarization-maintaining, highly non-
linear fiber [4]. Alternatively supercontinuum with a high
degree of coherence can be generated using all-normal dis-
persion PCF with pulse durations of several hundred fem-
toseconds [5], [6]. Mode-locked (ML) vertical-external-cavity
surface-emitting semiconductor lasers (VECSELs) typically
work at GHz repetition rates [7], emit near-transform-limited
pulses [8] with pulse durations down to 60 fs [9], and have
∗
robin.head@soton.ac.uk, hyc1g11@orc.soton.ac.uk
[1] School of Physics and Astronomy, University of Southampton,
Southampton SO17 1BJ, UK
[2] Optoelectronics Research Centre, University of Southampton,
Southampton SO17 1BJ, UK
Manuscript received October 12, 2012; revised XXXX XX, 2012. Copyright
(c) 2012 IEEE. Personal use of this material is permitted. However, permission
to use this material for any other purposes must be obtained from the IEEE
by sending a request to pubspermissions@ieee.org.
reached average powers up to 5.1 W with sub-picosecond
pulses [10]. Furthermore the mode-locking dynamics allows
continuous repetition rate tuning over a range of several GHz
by changing the length of the external cavity [11], [12]. Thus
VECSELs are potentially flexible sources for coherent GHz
supercontinuum generation. However the direct generation of
both sub-100 fs pulses and high average output power in
VECSELs has not been achieved yet. Sub-picosecond pulsed
fiber amplifier systems have reached output powers of 830 W
at a 78 MHz repetition rate with a 640 fs pulse duration
using a chirped pulse amplification architecture [13]. More
recently a fiber master oscillator power amplifier (MOPA)
system has achieved 110 W at a 1.3 GHz repetition rate
with a 890 fs pulse duration requiring only modest pulse
stretching [14]. As the repetition rate increases, the pulse
energy decreases and the linear amplification regime can be
used to correspondingly higher average powers before pulse
stretching becomes neccessary. Fiber MOPA systems provide
a complementary technology to ML-VECSELs to enable high
power GHz amplification and supercontinuum generation. In
previous work Dupriez et al. reported a femtosecond seeded
MOPA system where average powers of 53 W were reached
[15]. However only a few hundred milliwatts of the output
power could be used, to demonstrate that it was possible to
cleanly compress the pulses, due to the power handling of the
aluminum reflection grating compressor.
Recently Chamorovskiy et al. have generated a supercon-
tinuum with a VECSEL MOPA system where a 500 m long
GeO
2
-codoped silica fiber, driven by a 1.57 µm VECSEL
MOPA producing 15.5 ps pulses, was used. [16].
Here we report a VECSEL MOPA system producing 40 W
of average power. A 70% throughput grating compressor was
used giving 400 fs pulses. These pulses are then used to
generate supercontinuum in two samples of PCF; an all-normal
dispersion PCF, and a PCF with a zero dispersion wavelength
of 1040 nm.
II. EXPERIMENTAL SET-UP
The amplified pulse source, shown in Figure 1, consists
of distinct sections; The VECSEL, the cascaded fiber ampli-
fier system and the compressor. The compressed output was
then launched into the PCF to generate supercontinuum. We
describe the VECSEL in section II-A followed by the fiber
amplifier and pulse compression system in section II-B.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. XX, NO. XX, XX 2013 2
Optical
Pump
Output
Coupler
Gain
Sample
SESAM
Cascade of
Fiber Amplifiers
PCF
ML-VECSEL
Compressor
Pump 1
Pump 3 Pump 2
Pre-Amp 1
Pre-Amp 2
Final Stage
Amplifier
Taper Taper
Gratings
Roof-top
mirror
Pick off
mirror
Dichroic
Mirror
Dichroic
Mirror
Fig. 1. Schematic of the experimental set-up. The VECSEL was used
as a seed laser for a three-stage ytterbium-doped fiber amplifier system.
The amplified pulses were recompressed with a high throughput grating
compressor and free space launched into the PCF.
A. VECSEL
The ML-VECSEL in this experiment consists of the
gain structure, a Semiconductor Saturable Absorber Mirror
(SESAM) and a spherical output coupler. The gain structure
consists of six strain-balanced InGaAs quantum wells designed
for 1025 nm, grown on top of a 27.5 pair AlAs/GaAs
distributed Bragg reflector (DBR). The SESAM is used to
mode-lock the laser and has a single InGaAs quantum well
designed for 1025 nm, on top of a DBR designed at 1040 nm.
The design and optical properties of the gain structure and
SESAM have been described in [7]. The VECSEL is optically
pumped using an 830 nm fiber coupled diode laser, which
is focused to a 60 µm radius spot on the gain structure. A
V-shaped cavity is formed by a 0.3% output coupler with
a radius of curvature of 50 mm, the gain structure which
acts as a plane folding mirror, and the SESAM as the cavity
end mirror. The cavity is designed to give a fundamental
laser mode with a 60 µm radius waist on the gain and a
20 µm radius waist on the SESAM, ensuring that the SESAM
is saturated more strongly than the gain. The gain structure
is mounted on a water-cooled temperature-controlled copper
block. Neither the gain nor the SESAM structure has been
processed, limiting the maximum average output power [17].
All components are directly mounted on a single Invar plate
to reduce misalignment due to vibrations or thermal drift and
thus improve long-term mode-locked stability.
B. Amplifier System
The MOPA system used in this experiment is similar to the
set-up used by Chen et al. [18], [19] but with a VECSEL
as master oscillator. The system is based on polarization-
maintaining large mode area (LMA) fiber technology, ensuring
a robust and stable amplifier system. The first and second
pre-amplifiers are ytterbium-doped fibers with core diameters
of 5 µm and 25 µm and lengths of 2 m and 2.7 m respec-
tively. Both pre-amplification stages are reverse pumped using
975 nm fiberized diodes. The small core fiber in the first
amplifier was chosen in order to maintain a good signal-to-
noise ratio and to provide high gain with the low-power wave-
length division multiplexed coupled pump-diode. The second
preamplifier had a larger core to reduce nonlinear effects and
had sufficient output power to saturate the final amplifier.
We used fiber lengths long enough to efficiently absorb the
pump power but short enough to avoid gain narrowing [20].
Following each pre-amplification stage the signal is passed
through an isolator. After pre-amplification the signal is free-
space-coupled into the final-stage amplifier. The final-stage
amplifier fiber, similar to that used in the second pre-amplifier,
is a 3 m length of double-clad LMA ytterbium-doped fiber,
which is reverse pumped with a high power fiberized diode
laser source (Laserline LDM 200-200) at 976 nm. The pump
launch efficiency for the final two amplifiers was estimated to
be ∼ 70%. Single mode operation (M
2
<1.1) was achieved
from these few-mode LMA fibers by pre-tapering the input
to act as a mode filter as described by Chen et al. [18].
Following the final stage amplifier, the amplified signal passes
through an isolator and a high throughput compressor (Ibsen
FSTG-PCG-1250-1064 gratings) with a grating separation of
approximately 2 cm. The amplified and compressed VECSEL
signal is directly free-space coupled into PCF. To improve
thermal handling, and thus allow for a higher average power
to be launched, the end-caps of the PCF are collapsed and
polished. The two different PCFs used in this experiment are a
10 m long all-normal dispersion PCF (NL-1050-NEG-1) with
a dispersion minimum at 1050 nm; and a 1 m long PCF (SC-
5.0-1040) with a ZDW of 1040 nm, closely matched to the
wavelength of amplified signal.
III. EXPERIMENTAL RESULTS
A. Amplifier System
The target wavelength for the VECSEL seed is 1040 nm to
achieve optimal amplification in the ytterbium fiber amplifier
system. For this particular combination of gain and SESAM
wafers, a heat sink temperature of 15
◦
C and pump power of
1280 mW produced a laser spectrum centered at 1040 nm
with a Full Width Half Maximum (FWHM) of 2.85 nm,
corresponding to a train of near transform-limited 400 fs pulses
at a repetition rate of 3 GHz and an average output power of
20 mW. Figure 2 shows the optical spectrum and intensity
autocorrelation of the VECSEL output.
The average power of the signal after the first two pre-
amplification stages was 1.5 W with pulse durations of 1.7 ps.
Pump powers of 210 mW and 5.4 W were used for the
first and second stages respectively. The pulse duration after
the final stage power amplifier is 3 ps at an average power
of up to 40 W, limited by pump absorption. The pulse
elongation was predominantly due to the fiber dispersion in the
amplifier system. These average powers were still in the linear
amplification regime and so no significant spectral broadening
could be observed and no complex phase structure was added
by the amplifier chain. Thus it was possible to recompress the
pulses to their original duration of 400 fs, as shown in Figure
3. Scaling the average power to higher powers would enable
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. XX, NO. XX, XX 2013 3
1.0
0.8
0.6
0.4
0.2
0.0
SHG Intensity [a.u.]
6420-2
Delay Time [ps]
-30
-20
-10
0
SPD [dBm/nm]
106010401020
Wavelength [nm]
Fig. 2. Second harmonic generation (SHG) intensity autocorrelation of the
pulse (black) with a sech
2
fit (green). The inset shows the normalized spectral
power density (SPD) of the spectrum of the ML-VECSEL. The pulse duration
of the VECSEL is 400 fs and the center wavelength of the VECSEL is at
1040 nm with a full width at half maximum of 2.85 nm.
1.0
0.8
0.6
0.4
0.2
0.0
SHG Intensity [a.u.]
3020100-10
Delay Time [ps]
-40
-30
-20
-10
0
SPD [dBm/nm]
106010401020
Wavelength [nm]
Fig. 3. Autocorrelations of the uncompressed (black) and compressed (red)
pulse. The inset shows the spectrum after the final stage amplifier. The
uncompressed pulse duration is ∼3 ps: the recompressed pulses have a 400 fs
duration.
parabolic pulse amplification and allow for pulse compression
to sub-original durations.
B. Supercontinuum Generation
The recompressed 400 fs pulses are launched into the
two PCFs in turn, achieving in each case a transmission
efficiency of ∼ 50%. The maximum average powers mea-
sured after the PCFs are 2.5 W and 3.9 W for the PCF
pumped closely to its ZDW and all-normal dispersion PCF
respectively, limited by thermal management of the passively
cooled launch into the PCF. Figure 4 shows the spectra of the
supercontinuum generated with the VECSEL MOPA system.
The 20 dB bandwidth of the supercontinuum created with
the all-normal dispersion PCF is 200 nm. We expect this
supercontinuum to have a high degree of coherence [5], [6].
The pulse predominantly acquires a linear chirp and it should
be compressible with appropriate dispersion compensation. A
-50
-40
-30
-20
-10
0
Normalized Spectral
Power Density [dBm/nm]
140012001000800
Wavelength [nm]
Fig. 4. Measured input spectrum (red) and supercontinuum spectra produced
with the all-normal dispersion PCF (blue) and the PCF with a ZDW of
1040 nm (black). The 20 dB bandwidth of the all-normal dispersion PCF
supercontinuum is 200 nm and 280 nm for the PCF pumped closely to its
ZDW.
280 nm 20-dB-bandwidth supercontinuum is measured for
the PCF with a ZDW of 1040 nm. The supercontinuum had
spectral components between 750 nm and 1300 nm. The dip
at about 900 nm is typical for PCFs with a ZDW and is due to
the relation between the pump wavelength and the dispersive
properties of the PCF, however for a detailed explanation a
numerical model would be necessary [3]. In this case the
supercontinuum is thought to be temporally incoherent as the
pulse duration launched into the PCF is significantly above
100 fs [3].
IV. CONCLUSION
We have demonstrated that the combination of femtosecond
ML-VECSELs and fiber amplifiers can be used to gener-
ate supercontinuum at multi-GHz repetition rates at average
powers of several Watts in a robust and practical system.
Supercontinuum with a 20 dB bandwidth width of 280 nm
and an average output power of 2.5 W was achieved with a
PCF pumped close to its ZDW. For supercontinuum generated
with all-normal dispersion PCF a 20 dB spectral bandwidth of
200 nm was achieved with an average output power of 3.9 W.
The VECSEL-MOPA produced up to 40 W output power
at a 3 GHz repetition rate. The pulses were re-compressed
to 400 fs durations using a high throughput transmission
grating compressor. High power supercontinuum generation
has been reported by Chen et al. [19] and we aim to use
techniques developed in that work along with PCF with
optimized dispersion characteristics to generate higher average
power broader bandwidth supercontinuum in the future. A
VECSEL MOPA system employing a shorter seed pulse and
optimized amplifier design should enable parabolic regime
amplification to produce shorter pulses which could be used
to produce octave-spanning coherent supercontinuum.
ACKNOWLEDGMENT
C. Robin Head and Ho-Yin Chan have contributed equally
to this work. We thank R. Malik for advice and in-
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. XX, NO. XX, XX 2013 4
put. This work was undertaken with funding from EPSRC
(EP/G059268/1 and EP/I02798X/1). J. H.V. Price was sup-
ported by a RAEng/EPSRC Fellowship and K. G. Wilcox
holds an EPSRC Early Career Fellowship.
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