Optics and Photonics Journal, 2013, 3, 69-72
doi:10.4236/opj.2013.32B017 Published Online June 2013 (http://www.scirp.org/journal/opj)
Copyright © 2013 S ciRes. OPJ
High Peak Power, Single-polarized, Sub-nanoseconds
Pulses Generation of a Yb-doped Rod-type Photonic
Crystal Fiber Amplifier*
Ziwei Wang1,2, Songtao Du1, Zhaokun Wang1,2, Jing He1,2, Yunrong Wei1, Qihong Lou1, Jun Zhou1
1Shanghai Key Laboratory of All Solid-St ate Lasers and Applied Techniques, Shang ha i Insti tute of Optic s and Fi ne Me c ha ni c s ,
Chinese Academy of Scien ces , Shanghai, China
2Universi ty of Chinese Academy of Scien ces , Beijing, China
We report a high-peak-power, single-polarized master oscillator power amplification system employing polarization-
maintaining Yb-doped rod-type photonic crystal fiber. The MOPA system comprises of a Q-switched microchip laser
generating ~630ps pulses at 8.6 kHz repetition-rate and two amplification stages employing double cladding fiber and
rod-type PCF respectively. The MOPA system obtains narrow spectral bandwidth, single-polarized pulses of 9W max-
imum output average power, corresponding to peak power of 1.7MW.
Keywords: Lasers; MOPA; Rod-type PCF; Single Poliriza tion; High-peak-power
High-peak-power, single-polarized laser sources operat-
ing in sub-nanosecond regime are widely used in a range
of applications including frequency conversion , ma-
terial processing  and remote sensing. Rare-earth-
doped fiber lasers and amplifiers with single-polarization,
narrow spectral linewidth, high signal-to-noise ratio and
excellent beam quality offer manifest advantages in the
applications above. In the past few years, large mode
area (LMA) double cladding fibers (DCFs) are widely
used in short-pulsed fiber amplifiers. In 2005, Fabio Di
Teodoro et al.  reported a three-stage amplifier gene-
rating pulses whose peak power in excess of 4.5MW
while pulse duration was 450ps and repetition rateswas
13.4 kHz. In 2010, Shaif-ul Alam et al.  introduced a
single-polarized master oscillator power amplification
(MOPA) system with output peak power of 39 kW at
20ps pulse duration 113.8MHz repetition rate. Although
LMA DCF can support high power for pulsed lasers, it
would become multiple transverse modes with beam
quality reduction ; moreover, there isn’t polariza-
tion-maintaining (PM) LMA DCF whose core diameter
is larger than 30 μm in the market. In 2007, a ~ 85ps
pulse duration, 105 kHz repetition-rate, 3MW peak pow-
er Yb-doped fiber amplifier exhibiting narrow spectrum
and diffraction-limited beam quality was reported by O.
Schmidt et al. . T he significant component in the am-
plifier was a very-large-core, intrinsically single-mode,
rod-type photonic crystal fiber (PCF)  in power ampli-
fication stage. The rod-type fiber combines advantages of
fiber lasers and bulk solid lasers by particular micro-
structure design. Through enlarging core diameter and
reducing length, the peak power in core is lower. Fur-
thermore, it is designed as an extraordinary small refrac-
tive-index difference between core and cladding to be
intrinsically single mode. Although the rod- type fiber
cannot be coiled as ordinary fibers, it presents a range of
eminent features including single-mode operation, pump
acceptance angle and efficient pump absorption. The
paper discussed above reported a narrow pulse,
high-peak-power results, however, the damage of main
amplifier was observed at maximum power and pulse
distortion was also occurred when peak power was ~2MW.
The papers about rod-type PCF amplifier are mainly fo-
cused on non-polarize d amplificatio n so far, neverthe less
high-peak-power PM amplifier is rarely reported.
In this paper, we present a single-polarized dual-stage
master oscillator power amplification (MOPA) system
for ge nera ting hi gh-peak-po wer and si ngle -polarized pulses.
We employ a low-repetition-rate passively Q-switched
microchip laser as seed, and introduce Yb-doped, PM,
LMA double cladding fiber and 80cm-long, 100μm-core,
PM, rod-type PCF as gain medium in pre-amplification
and power amplification stage respectively. From this
system, we obtained ~600ps, 8.6kHz repetition-rate, center
*The work has been sponsored by the national foundation of
Z. W. WANG ET AL.
Copyright © 2013 S ciRes. OPJ
wavelength 1064nm, and spectrum linewidth 0.46nm,
polarization extinction ratio (PER) 13dB laser output of
average power 9W, corresponding to a peak power ex-
ceeding 1.7MW and pulse energy exceeding 1mJ.
2. Experimental Setup
The experimental architecture is shown in Figure 1. A
fiber-pigtailed Nd:YAG/Cr4+:YAG, single-polarized, pas-
sively Q-switched microchip laser is used as the ma ster
oscillator( M O) coupling into amplifier. Due to the low
gain of short Yb-doped fibers in 1064nm，the MO o utput
is amplified in a dual - stage, PM fiber amplifier c hai n t ha t
affords effective suppression of nonlinearities such as
amplified spontaneous emission(ASE) and simulated
Raman scattering(SRS). Polarization independent optical
isolators are used between am plification stages to p revent
backward laser leakage. Cladding power stripper is also
introduced to protect MO against backward pump laser.
For the first amplification stage, we use a 3m-long PM
Yb-doped double cladding fiber(core/cladding diameter
25/250 μm, core/cladding NA 0.06/0.46 and pump ab-
sorption ~5.10dB/m @975nm) as gain medium. The fi-
ber is backward pumped by two fiber-pigtailed diode
lasers at 975nm through a (2+1) ×1 combiner whose end
is spliced with endcap and angle (6°) polished to prevent
facet damage and backward-reflected power from coupl-
ing into fiber core. We employ two half-wave plates to
control the polarization of pre-amplified output pulses.
To remove all remaining ASE and preventing backward
pump light of the second stage, an inter-stage band-pass
filter is specialized designed, emitting only a spectral
band of 10nm centered at 1064nm. The filtered pre-am-
plified pulses are coupled into a 80cm-long, PM Yb-
doped rod-type PCF (core/cladding diameter 100/285μm,
core/cladding NA 0.02/0.6, and pump absorption ~30dB/
m@975nm) which is free-space backward pumped by a
LD at 975nm through a set of lens and dichroic mirrors
(1064nm HT&975nm HR). The rod-t ype P CF is endcap-
ping with large AR-coating silica columns(length 8mm
and diameter 8.2mm) in both ends in order to provide
appropriate beam expansion，thus reduce end facet in-
tensity and avoid damage of end facet for a 0° angle facet.
The amplified beam output s th rough a collimating len s.
The output characterization of the fiber amplifier in-
cludes average power, spectrum and temporal pulse pro-
file. We measure average power by Thorlabs S370C
power meter. Spectrum and temporal profile are detected
by Yokogawa AQ6370 optical spectrum analyzer and
LeCroy Wavesurfer 104MXs 1GHz oscilloscope con-
nected to a high speed InGaAs photodiode respectively.
3. Results and Discussion
The spectrum (a), pulse temporal profile (b) of MO are
presented in Figure 2. The average output power of MO
is ~170 mW with center wavelength at 1064.8 nm, 3dB
spectral bandwidth of 0.29 nm, pulse duration of 628 ps
and 8.6 kHz repetition-rate. And the PER of the MO is
Figure 3 provides the average output power of the
MOPA system as the function of launched pump power.
After pre-ampli fication, there is average power ~500mW
filtered pulses coupled into rod-t ype P C F . The maximu m
output average power is 9W corresponding to peak pow-
er exceeding 1.7MW, when pump power is 31W. The
slope efficiency extracted form a linear fit is ~44%. It is
emerged that signal power increases along with pump
power increase; the pump threshold of rod-type fiber is
~5W. Moreover, the shift in wavelength of pump
Figure 1. experimental setup. MO: master-oscillator; ISO:
optical is olat or ; CPS ：claddi ng power stripper ; PM-YDCF:
polarization-maintaining Yb-doped double cladding fiber;
LD: laser diode; HWP: half-wave plate; BPF: band-pass
filter; ROD-PM-PCF: rod-type polarization-maintaining
photonic c rystal fi ber; DF: dichroic mirror.
Figure 2. Seed’s features of (a) spectrum and (b) temporal
Z. W. WANG ET AL.
Copyright © 2013 S ciRes. OPJ
would move towards the Yb-absorption peak at 975nm
with pump power increasing. Owing to the endcaps, the
maximum in-core peak intensity of 68 GW/cm2 is re-
duced to ~10MW/cm2, which is far less than the meas-
ured surface damage threshold for sub-nanosecond pulses
in silica . We suggest that signi ficant higher amplified
signal power scaling would be possible through increas-
ing pump power as power saturation or deduction isn’t
occurred. T hrough i ncreasi ng the p ower of p re-amplified
pulses, both the output power and optical-to-optical effi-
ciency would increase notably . Furthermore, we meas-
ure the PER to be ~13dB at maximum output operation,
that means the amplifier maintaining the polarization of
In Figure 4, the typical spectrum of seed pulses (a) is
presented comparing with output pulses of average power
at 9W (b). The output center wavelength is 1064.9nm,
and 3dB spectral bandwidth is 0.46nm. The amplified
spectrum is relatively broader than seed’s, it is due to
self-phase modulation (SPM)  in high peak power.
Thanks to the effective management of continuous wave
ASE, the signal to peak(~1040nm) ASE ratio exceeds
Figure 3. Aver ag e o utp ut power of si gn al lase r v ersus p ump
Figure 4. (a) seed spectrum and (b) amplified spectrum at
output power of 9W.
Figure 5. Amplified pulse (a) temporal profile and (b) se-
quence at output powe r of 9W.
40dB; SRS and four-wav e-mixing (FWM) are not en-
gendered in the measurement. Furthermore, because of
backward pump, residual pump is hardly appeared. In
general, the spectrum represents an excellent signal-to-
noise ratio o f t he syste m, and it d e monstrates the bene fits
of rod-type PCFs in minimizing nonline ar effects.
Figure 5 shows the temporal profile (a) and pulse se-
quence (b) of output pulses at maximum average power.
The output pulse duration is 632 ps while repetitio n ra te is
8.6 kHz. Comparing to the seed pulse profile, an irregu-
lar temporal pulse distortion is exhibited owing to gain
saturation and SPM corresponding to the spectrum broa-
dening we discussed above. SPM would become signifi-
cant constraint in further power amplification scaling.
In this paper, we report on a single-polarized, dual-stage
MOPA system combining Q-switched microchip laser
with Yb-doped very-large-mode-area rod-type PCF in
sub-nanosecond regime. The amplifier generates high
peak power in excess of 1.7MW (corresponding to aver-
age power 9W, pulse duration 638ps and repetition rate
8.6 kHz) pulses in single-polarized, spectrally narrow,
high sig nal-to-noise ratio beam, which haven’t been gen-
erated in Yb-doped double cladding fiber amplifiers.
This work represents the highest peak power obtained
from single-polarized rod-type PCF amplifiers. With the
excellent properties including high-peak-power and sin-
gle-polarization, the MOPA system is a promising can-
didate to be used in applications including frequency
conversion, material processing and LIDAR. In the fu-
ture work, we would donate attempts to increase pulse
Z. W. WANG ET AL.
Copyright © 2013 S ciRes. OPJ
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