Advances in high power, short pulse, fiber laser systems and
technology
J.H.V. Price, A. Malinowski, A. Piper, F. He, W. Belardi, T.M. Monro, M. Ibsen, B.C. Thomsen, Y. Jeong,
C. Codemard, M.A.F. Roelens, P. Dupriez, J.K. Sahu, J. Nilsson* and
D.J. Richardson*
Optoelectronics Research Centre (ORC), University of Southampton, Southampton, SO17 1BJ, United
Kingdom.
*also with Southampton Photonics Incorporated, Chilworth Science Park, Southampton SO16 4NS, United Kingdom.
Tel: +44 2380 594524, Fax: +44 2380 593142, Email: djr@orc.soton.ac.uk
Abstract: We review recent advances in Yb fiber lasers and amplifiers for high power short pulse
systems. We go on to describe associated recent developments in fiber components for use in
such systems. Examples include microstructured optical fibers for pulse compression and
supercontinuum generation, and advanced fiber grating technology for chirped-pulse amplifier
systems.
1. Introduction
There is an increasing demand for high-power ultrashort-pulse laser systems for industrial and scientific
applications. Fiber laser technology is emerging as an attractive option for pulsed systems – not least due to the fact
that during the past few years, the favourable heat-dissipation geometry of fibers has enabled tremendous increases
in the average continuous wave output powers that can be reached. Powers of up to 1.4kW in a single transverse
mode have now been achieved [1], and multi-mode systems are now approaching the 10kW regime. Furthermore,
the broad gain bandwidths of Er and Yb doped fibers can support ultrashort ~100 fs pulses. However, due to
nonlinear effects, the creation of practical high power pulsed fiber systems still requires new amplification
techniques and the development of new fiber components.
Nonlinear effects in the fiber core, the most important being self-phase modulation, stimulated Raman and
Brillouin scattering, quickly become significant with ultrashort pulses due to the high peak powers. In order to
overcome the nonlinear limits, and hence increase the pulse energies and powers that can be achieved, it is necessary
to employ more complex amplification schemes, which enable reducing the nonlinearity of the gain medium, and
controlled temporal stretching of the pulses during amplification. The technique of chirped pulse amplification
(CPA) [2] is commonly used, in which ultrashort pulses from an oscillator are stretched to several hundred
picoseconds duration before amplification, and recompressed at the output of the amplifier by a matching
compressor. This reduces the peak powers in the amplifier below the threshold for nonlinear pulse distortion,
allowing for amplification to much higher pulse energies. However CPA systems incorporate many separate
components in order to stretch and recompress the pulses, and due to the large temporal stretching ratios, the
compressor gratings have ~1 m separation, and do allow for a truly compact system. As a consequence, the
application of such systems has so far been limited, but recently demonstrated fiber technology should lead to
practical future systems.
This paper is structured as follows. To highlight the improvements in large mode area fibers, we first consider a
high average power parabolic pulse system that we demonstrated recently [3]. We also show our compact gain-
switched laser diode source. We then report the development of a CFBG pulse stretcher with both 2
nd
and 3
rd
order
dispersion to match the bulk grating compressor demonstrated in an Yb fiber CPA system. Finally, the unique
dispersion and nonlinear properties of microstructured optical fibers (MOF) are reviewed, and illustrations given of
new applications, such as supercontinuum generation, and how MOF can replace bulk gratings in mode-locked
oscillators and CPA compressors. We conclude with a summary, and consider the future challenges.
2. High power pulsed systems based on large core fibers
Since the main performance limitation of rare-earth-doped fibers is nonlinearity in the fiber core, by applying fibers
with large-core-area and short absorption lengths, higher pulse energies can be achieved. Another aspect is that the
energy storage capability of a doped fiber scales as well with the core area. The first system considered is a
femtosecond source delivering high average power. The fiber used for the final amplifier had a 40 micron core, with
low NA to enable higher power and pulse energy than had previously been achieved from such a system.
Furthermore, by careful launching of the seed, single mode output was obtained from this multi-mode amplifier.
When moderate energy pulses are required, a parabolic amplifier scheme can be used, which omits the stretcher
typically found in a CPA system, and thereby enables a much more compact compressor to be used. It has been
shown that high power pulses in a fiber amplifier with normal dispersion evolve towards parabolic pulses with a
linear chirp, which enables recompression to short durations despite significant self-phase modulation in the fiber
[4]. Parabolic pulses from Yb fiber amplifiers have previously been demonstrated with average powers up to 17 W
(pulse energy 230 nJ) using a bulk glass seed laser [5], and up to 13 W (pulse energy 260 nJ) using a fiber based
seed laser [6]. Here we review our demonstration showing high average power of >25 W, and high pulse energies
of 410 nJ from an all Yb fiber system. A conventional bulk-grating compressor was used to remove the linear chirp,
resulting in 100 fs pulses [3]. The setup is shown in Fig. 1.
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Fig. 1. Schematic of parabolic amplification system.
Our femtosecond mode-locked Yb fiber oscillator (developed in-house) [7] produces ~30pJ pulses with a
50 MHz repetition rate. The pulses are centered at 1055 nm with a spectral FWHM of 18.6nm with an
autocorrelation FWHM of ~110fs.
The output of the amplifier system is shown in Fig. 2. It can be seen that the spectrum of the pulses broadens at
higher energies. The spectral bandwidth of the high energy pulses was ~20 nm. The measured autocorrelations
correspond to pulse durations of ~160 fs at low energy and ~110 fs at 410 nJ. The autocorrelations demonstrate that
the pulses did not have large pedestals and that the pulse quality was good (
τ
ν
∆
∆
~0.6) for pulses with energies up
to 410 nJ. The system also produced pulses at higher average powers of up to 40 W without the onset of significant
Raman scattering, but for these higher energy pulses the nonlinear spectral broadening exceeded the amplifier
bandwidth, and the quality of the recompressed pulses was reduced.
wavelength (nm)
1000 1020 1040 1060 1080 1100 1120 1140
signal (dB)
-70
-60
-50
-40
-30
-20
5 nJ
410 nJ
(a)
delay (fs)
-1500 -1000 -500 0 500 1000 1500
signal
0.0
0.2
0.4
0.6
0.8
1.0
5 nJ
410 nJ
(b)
Fig. 2. (a) spectra and (b) autocorrelations of low (5 nJ) and high (410 nJ) energy pulses from parabolic amplifier system.
The second system considered again uses a large mode amplifier, operating at 1550nm using Er/Yb co-doped fiber.
The system incorporates a novel pulse source based on a low power pulsed semiconductor laser, and which shows
the compatibility of high power fiber technology with compact, low cost sources. Using this setup, we demonstrated
a 60 W average power, 4.5 ps pulses, at a repetition rate of 10 GHz [8]. The 1.5 µm region is attractive for such
applications as free space, space-based communications, or LIDAR because it is relatively eye-safe, compatible with
existing telecommunications hardware, and suitable for detecting aerosols, clouds and pollution.
The short pulse amplification scheme is shown in Fig. 3(a). A 1555 nm DFB laser in a high-speed fiber
pigtailed package is gain-switched by driving with a 10 GHz sinusoid at 30 dBm RF power and a DC bias current of
54 mA. The linear chirp of the pulses was compensated in 120 m of dispersion compensating fiber (DCF) to
produce 4.5 ps duration pulses with an average power of 3 dBm. The pulses are launched into a preamplifier,
consisting of a commercial Erbium doped fiber amplifier to amplify the average power up to 33 dBm. This signal is
then launched into the power amplifier. The power amplifier is pumped by a diode stack at 975 nm that is free space
coupled into the double-clad gain fiber pumped in a counter-propagating configuration [9]. The gain fiber has a 30-
µm diameter Er/Yb co-doped phosphosilicate core with a numerical aperture (NA) of 0.20. The D-shaped inner
cladding has a 400/360 µm diameter for the longer/shorter axis and is coated with a low-refractive-index polymer
outer cladding which provides a nominal inner-cladding NA of 0.48. The fiber length is ~3 m which leads to ~10 dB
pump absorption. The signal is launched into the gain fiber using a tapered splice that provides efficient mode
matching between the single mode fiber (SMF-28) and the core of the gain fiber. The power amplifier efficiency is
shown in Fig. 3(b). The output power increases linearly with launched pump power and the overall slope efficiency
is 22%.
10 GHz
DC
Bias
Seed
Laser
MOPA
120m
DCF
EDFA
Pre-Amp
MOD
10 Gbit/s
PRBS
Output
975nm Pump
Diode Stack
Dichroic Mirror
HR@1555nm
HT@975nm
Er/Yb co-doped
double-clad fiber
Tapered
Splice
0 50 100 150 200 250 300
0
10
20
30
40
50
60
70
Average signal power [W]
Launched pump power [W]
Measured
Linear fit
Slope efficiency: 22%
Fig. 3. (a) Experimental setup. (b) Power amplifier efficiency.
-50
-40
-30
-20
-10
0
Normalised Intensity (dB)
157015601550
Wavelength (nm)
(a)
1.0
0.8
0.6
0.4
0.2
0.0
Intensity (au)
3020100
-10-20-30
Delay (ps)
(b)
Fig. 4. (a) Optical spectrum after the high power amplifier at an average power of 60 W and a pulse repetition rate of 10 GHz.
(b) Autocorrelation traces of the 10 GHz pulses before (dashed line) and after high power amplification (solid line).
The optical spectrum at the output of the MOPA is shown in Fig. 4(a). At 60 W average output power the
signal peak is 25 dB above the peak of the background ASE. There was no spectral broadening observed
confirming that the nonlinear effects are minimized in the large core amplifier fiber of the MOPA. The pulse quality
after amplification was verified using autocorrelation measurements shown in fig. 4(b). The input pulse duration of
4.5 ps, determined from the input autocorrelation trace, is maintained after amplification to 60 W. The low level
pedestal observed on the output autocorrelation trace is from a small coupling of the first-order mode into higher-
order modes within the amplifier fiber. We have also used the modulator to reduce the repetition rate of the pulses
from 10 GHz down to 10 MHz to investigate the peak power scaling effects in this amplifier system. Peak powers
of 1.3 MW have been obtained, however further work is required to optimize the preamplifiers to improve the
OSNR for lower repetition rates.
3. CFBG with both 2
nd
and 3
rd
order dispersion compensation
In a typical femto-second CPA system, pulse stretching is done with bulk gratings, but in the interests of moving as
close as possible to an all-fiber system, we use a chirped fiber Bragg grating (CFBG). Due to the high pulse energies
bulk gratings must still be used for the compressor. The bulk grating compressor inevitably has a large 3
rd
order
phase term in addition to the 2
nd
order dispersion. In order to achieve femtosecond pulse durations both these terms
must be matched in the pulse stretcher.
Here, we present for the first time, to our knowledge, an Yb fiber CPA system using a CFBG with both 2
nd
and
3
rd
order dispersion. The CFBGs presented here have acceptance bandwidths of up to 18nm and were produced
using a scanning technique previously developed to produce precision gratings for telecommunication applications
[10, 11]. A CFBG with 2
nd
and 3
rd
order dispersion matched to a bulk grating has previously been demonstrated in a
CPA system at 1550nm [12], but the maximum energy and minimum pulse duration will be limited using Er
compared to Yb amplifiers. A CFBG that matched the diffraction grating compressor dispersion was presented
recently operating at a wavelength of 1.05µm [13]. However, in that study no amplification was implemented and
the recompressed pulses had poor contrast.
Fig 5. Schematic of CPA system. (AOM,EOM=Accousto/Electro-optic modulator, LD=laser diode, LMA=large mode area
fiber.)
Fig. 5 shows the schematic of our CPA system. Pulses from our femtosecond mode-locked Yb fiber oscillator [7]
were stretched with the CFBG and then passed through two single-mode (5µm core) core-pumped Yb doped fiber
amplifiers. Electro-optic and acousto-optic modulators were used to reduce the repetition rate and filter ASE
between amplifier stages. Power amplification took place in a 9m length, cladding pumped Large Mode Area, fiber
with a core diameter 16.5µm, doped with 7000 ppm Yb
3+
ions, and a cladding diameter of 200µm. The fiber, which
was effectively single mode in operation, was pumped from opposite ends with 915nm and 975nm pump diodes.
Fig. 6a) shows the power output of the final stage amplifier as a function of pump power - the slope efficiency was
~70%. The maximum average power achieved before compression was 17 W. Fiberised and free-space polarization
controllers were distributed through the system as necessary.
The group delay response of the gratings reported in this paper was measured by an RF phase-delay
measurement technique. Fig. 6b) shows the reflection spectrum and delay as a function of wavelength for a CFBG
centered at 1053nm with an acceptance bandwidth of 18nm. The solid curve through the delay data is a fit to 2
nd
and
3
rd
order dispersion, giving values of D x length = 44.65±0.4 ps/nm (
(
)
=∂∂=
22
2
ωφβ
−26.4
ps
2
/rad)
,
and dD/d
λ
x
length = 1.48
±
0.2 ps/nm
2
(
(
)
=∂∂=
33
3
ωφβ
0.545
ps
3
/rad
2
) closely matching the values for our 1500 lines/mm
reflection grating compressor.
Fig 6. a) Power performance of CPA system LMA amplifier. b) Reflection spectrum and measured delay as a function of
wavelength for an 18nm CFBG. The solid curve through delay data is a fit to 2
nd
and 3
rd
order terms.
The bandwidth of the amplifiers was considerably less than the spectral bandwidth of the seed laser, and in the
CPA system we stretched the pulses with a relatively narrow band grating (7 nm) in order to avoid distortions due to
gain narrowing effects. The 2
nd
and 3
rd
order dispersion of the 7 nm CFBG were approximately the same as for the
18 nm CFBG described above.Fig. 7a) shows the autocorrelation of the recompressed output pulses.
Fig 7. a) Autocorrelation after recompression (demonstrates importance of higher order dispersion compensation), and b) the
spectrum of the pulses from CPA system.
The output pulse width is estimated at ~500 fs. Fig. 7a) also includes an autocorrelation of a pulse through the
same system which was stretched with a 30 nm bandwidth grating which had only 2
nd
order dispersion. It can be
seen that uncompensated 3
rd
order dispersion has resulted in a substantially broader autocorrelation (~3 ps) with a
broad pedestal. Fig. 7b) shows the spectral output of the final amplifier with the 7nm grating. The bandwidth of the
output pulse is ~5nm (
τ
ν
∆
∆
~0.7). Pulse energies from the system were limited to <10
µ
J because the fiber
amplifiers were not optimized for maximum gain and minimum nonlinearities, but clearly with a more optimal
choice of fiber this system has the potential to reach similar pulse energies to those previously achieved using fiber
amplifiers and bulk stretchers. The minimum duration of our pulses was limited by the bandwidth of our amplifier.
4. Microstructured optical fibers
Microstructured optical fibers (MOF) are of great practical interest because the incorporation of air holes to define
the cladding region allows for an increased range of fiber parameters compared to conventional fiber [14]. These
fibers are now being researched to replace bulk components in high power pulsed fiber laser systems. For MOF
with a solid core, guidance is due to the average index contrast between the core and microstructured surround.
Photonic bandgap (PBG) fibers [15, 16] use diffraction, rather than total internal reflection for light guidance, and
thus enable fibers in which the light field is confined to an air, or even a vacuum, core.
For average index guiding fibers, the large index difference between air and glass leads to a range of unique
dispersion and nonlinear properties. As an example, the dispersion has been shown to be particularly sensitive to the
hole arrangement, and a wide range of dispersion properties have been demonstrated, including anomalous
dispersion down to visible wavelengths, broadband flattened dispersion [17] or large normal dispersion [18]. The
effective mode area in a MOF can also be tailored by up to three orders of magnitude by altering the scale of the
transverse refractive index profile [17] opening up possibilities for fibers with either high or low optical
nonlinearities as required for a wide variety of applications.
Average index guiding MOF fabricated to have anomalous dispersion at wavelengths below 1.3 µm must have a
small core (typically <2.5 µm diameter) and a high air fill fraction in the cladding. This results in a strong
(anomalous) waveguide contribution to the dispersion to compensate for the (normal) material dispersion of silica at
these wavelengths. For fiber based femtosecond mode-locked lasers, the pulse formation is determined by the
balance of dispersion and nonlinearity [19]. The grating pairs typically used to provide anomalous in Yb-fiber
mode-locked lasers require precise alignment, and an Yb fiber laser in which anomalous GVD was provided by a
solid core MOF has recently been demonstrated [20, 21]
