JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 22, NOVEMBER 15, 2014 3827
Amplification and Bandwidth Recovery of Chirped
Super-Gaussian Pulses by Use of Gain Shaping
in Ytterbium-Doped Fiber Amplifiers
Sijia Wang, Bowen Liu, Minglie Hu, and Chingyue Wang
Abstract—We demonstrate a simple passive method to gen-
erate broadband chirped super-Gaussian pulses from high-gain
ytterbium-doped fiber amplification system without obvious band-
width degradation. By use of the inherent gain shaping effect of
fiber amplifiers, pulses can recover the initial Super-Gaussian pro-
file and bandwidth with large gain and optical signal-to-noise ratio
(OSNR). In this method, fiber amplifier itself works as a spectral
filter with flexible transmission profiles and gain, which can be
optimized by pump power and fiber length. A spectrally resolved
numerical model with amplified spontaneous emission (ASE) is de-
veloped for broadband ytterbium-doped double-clad fiber pulse
amplifiers. In a proof-of-principle two-stage amplification system,
with a shorter fiber preamplifier working in the gain-narrowing
region as a pre-shaper, broadband Super-Gaussian pulses resem-
ble themselves after the second amplification stage with the largest
total gain and OSNR. Experimental results quantitatively confirm
the numerical predictions. The proposed gain shaping method also
provides a new way to compensate the pulse deformation and band-
width narrowing in high-gain fiber chirped pulse amplification
systems.
Index Terms—Fiber amplifiers, gain shaping, super-Gaussian
pulse, ytterbium.
I. INTRODUCTION
Y
TTERBIUM-DOPED fiber amplifiers are of great inter-
est for applications ranging from fundamental science to
industry, due to their outstanding performance in generating
high-average-power, high-repetition-rate ultrashort pulses with
excellent beam quality and stability [1]–[4]. In comparison with
bulk solid-state systems, the powerful ytterbium-doped fiber
lasers feature low-cost, robust, compact and easy-to-operate,
thus more suitable for real-world application. However, the spe-
cific fiber geometry also promotes an enhancement of nonlin-
ear effects, which set critical limitations on the pulse energy
and peak power scaling of fiber lasers. Hence to date, numer-
ous strategies for performance scaling of fiber ultrashort laser
Manuscript received May 22, 2014; revised July 27, 2014 and September 21,
2014; accepted September 21, 2014. Date of publication September 24, 2014;
date of current version October 15, 2014. This work was supported in part by
the National Basic Research Program of China under Grants 2011CB808101
and 2010CB327604, and by the National Natural Science Foundation of China
under Grants 61405245, 61322502, 61205131, and 11274239.
S. Wang is with the Qian Xuesen Laboratory of Space Technology, China
Academy of Space Technology, Beijing 100094, China (e-mail: sj0607@163.
com).
B. Liu, M. Hu, and C. Wang are with the School of Precision Instruments and
Optoelectronics Engineering, and Key Laboratory of Opto-Electronics Infor-
mation Technology, Tianjin University, Tianjin 300072, China (e-mail: bwliu@
tju.edu.cn; huminglie@tju.edu.cn; chywang@tju.edu.cn).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JLT.2014.2360206
systems have been developed. To scale up the pulse energy
or average power without nonlinear distortions, chirped pulse
amplification (CPA) technique is usually employed along with
large-mode-area gain fibers [5]. However, suffered from the
gain narrowing occurs with gains higher than 20 dB, this lin-
ear amplification technique could hardly generate broadband
pulses shorter than 100 fs. To obtain a high peak power, pulses
amplifications are always accompanied with active controlling
systems with feed-back loops [6]. Recently, the combinations of
ytterbium-doped fiber amplifiers and optical parametric ampli-
fication (OPA) technique have opened up a new way to generate
tunable ultrashort pulses with extremely high peak power, short
pulse duration, and high average power [7]–[9]. As pump of the
OPA process, flattop-like Super-Gaussian temporal profile can
significantly increase the conversion efficiency and gain band-
width. Furthermore, a broadband large-linear-chirped spectrum
can effectively lower the gain narrowing effect thus guarantee a
shorter pulse duration and a wider tuning range. Active pulse-
shaping techniques in fiber amplifiers aiming at compensation
of gain-narrowing limitation [6] or gain saturation induced de-
formations on nanosecond narrowband Super-Gaussian pulse
have been investigated [10]. For broadband large-linear-chirped
pulses, the small signal gain spectrum also affects the pulse tem-
poral shape due to the direct mapping between the time-domain
and spectral-domain [11]. On this condition, this gain shaping
effect makes an amplifier act like a kind of passive pulse-shaping
component, but with an additional gain and controllable trans-
mission profiles.
In this paper, we propose a promising fiber amplifier scheme
by taking full advantage of this spectral gain shaping mechanism
to directly output broadband Super-Gaussian pulses with high
gain and large OSNR in the absence of active control. Numer-
ical simulations and experiments of a proof-of-principle two-
stage amplification system show that broadband chirped Super-
Gaussian pulses resemble themselves with the largest gain and
OSNR, when the pre-amplifier provides gain-narrowed pulses
for the second amplification. System here can be easily extended
to multi-stage, and denote great potentials in the performance
scaling of the state-of-art fiber CPA pumped OPA systems. Fur-
thermore, studies here are also useful to fiber CPA systems to
overcome the gain-narrowing limitations on the output pulse
duration under a high gain.
The paper is organized as follows. In Section II, we present a
spectrally resolved numerical model for a backward cladding-
pumped ytterbium-doped fiber amplifier, based on the homo-
geneously broadened two-level rate and propagation equations.
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