Hybrid femtosecond laser system based on a Yb:KGW
regenerative amplifier for N
P
polarization
Dongyu Yan (闫东钰), Bowen Liu (刘博文), Yuxi Chu (储玉喜)*, Huanyu Song (宋寰宇),
Lu Chai (柴 路), Minglie Hu (胡明列), and Chingyue Wang (王清月)
Ultrafast Laser Laboratory, Key Laboratory of Opto-electronic Information Science and Technology of Ministry of
Education, School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China
*Corresponding author: chuyuxi@tju.edu.cn
Received November 23, 2018; accepted January 25, 2019; posted online April 8, 2019
We report a hybrid femtosecond laser system based on a femtosecond Yb-doped fiber laser and a Yb-doped
potassium gadolinium tungstate (Yb:KGW) regenerative amplifier. To match the central wavelength of the
seed source, a Yb:KGW crystal is used in the regenerative amplifier for N
p
polarization. We study and optimize
the dynamics of nonlinear amplification to alleviate the gain narrowing effect. With optimization, the system can
output 270 fs pulses with 21 μJ pulse energy at a 60 kHz repetition rate.
OCIS codes: 140.3280, 140.3615, 140.7090.
doi: 10.3788/COL201917.041404.
In recent years, femtosecond lasers with high pulse energy
have gained significant attention in many research and
industrial applications. Ultrafast fiber and solid-state laser
amplification systems are both potentially important
techniques for producing high-power, high-energy femto-
second laser sources
[1–3]
. The fiber scheme offers major ad-
vantages in terms of compactness, lack of misalignment,
and environmental insensitivity
[4–7]
compared with mode-
locked solid-state laser
[8]
, while amplifiers with bulk gain
media extract intrinsically high output energies
[9,10]
. Thus,
a hybrid system with a fiber seed source and a solid-state
amplifier can inherit the advantages of both. The regen-
erative amplification scheme, which offers a gain of several
orders, can be used to amplify a nanojoule (nJ)-level fiber
laser source to tens of microjoules (μJ) at multi-kilohertz
repetition rates
[11,12]
. The regenerative amplifier combined
with the fiber seed source is an alternative that offers high-
energy outp ut with a stable and compact structure.
For hybrid femtosecond laser amplification systems,
spectral matching between fiber laser sources and bulk
media in amplifiers is an essential issue. Among various
materials, Yb-doped rare-earth potassium tungstates
[Yb-doped potassium gadolinium tungstate (Yb:KGW),
Yb-doped potassium yttrium tungstate (Yb:KYW)] are
promising for high-emission cross sections, broad gain
bandwidth, and good thermal conductivity
[13]
.But,optical
properties of Yb:tungstates exhibit anisotropy for different
polarizations, owing to their crystalline structure
[14]
.
Most previous works with Yb:KGW/Yb:KYW crystals
have concentrated upon laser amplification for
N
m
-polarized modes, which show larger gain coefficients
than N
p
-polarized modes
[15–17]
. Unfortunately, the gain
spectrum of Yb:KGW crystals with N
m
polarization does
not match the spectrum of a 1040 nm femtosecond fiber
laser, which is usually used as a seed source. In order to
overcome this problem, a regenerative amplifier with an
electric field parallel to the N
p
axis is applied in this Letter.
The gain bandwidth of Yb:KGW is ∼18 nm, which can
support sub-200-fs pulses. However, pulses output from
Yb:KGW regenerative amplifiers are normally com-
pressed to 300– 500 fs due to gain narrowing in ampli-
fiers
[18]
. To obtain shorter pulses, several methods have
been developed to extend the output spectrum. Using dua l
Yb:KGW crystals with two different optical axes (N
m
and
N
p
), the output spectra are broadened to ∼20 nm
[19,20]
.
However, this makes the systems rather complex and
bulky. Another strategy is nonlinear amplification, which
broadens spectra by self-phase modulation (SPM) to alle-
viate the gain narrowin g effect
[21,22]
.
In this Letter, we demonstrate a hybrid amplification
system based on a femtosecond Yb-doped fiber laser
and a Yb:KGW regenerative amplifier. For central-
wavelength matching, Yb:KGW is utilized for N
p
polarization. To obtain short pulses, nonlinear spectral
broadening is induced by SPM in the amplification. Mean-
while, the nonlinear regenerative amplification dynamics
of different round trips is investigated at the optimal gate
time. By optimizing the nonlinear process in the amplifier,
the system can output 270 fs pulses with 21 μJ pulse
energies after compression. To the best of our knowledge,
this is the shortest pulse that has been reported for
N
p
-polarized-laser output of a Yb:KGW crystal.
The experimental setup is depicted in Fig.
1. The hybrid
femtosecond amplifier consists of an oscillator, a pre-chirper,
a pre-amplifier, an acousto-optic modulator (AOM), a Yb:
KGW regenerative amplifier, and a grating compressor. The
home-made oscillator is a mode-locked Yb
3þ
-doped fiber
laser operating in the all-normal-dispersion (ANDi) regime.
The oscillator delivers 4.5 ps pulses with a 138 mW average
power at a 60 MHz repetition rate, with excellent power
stability of less than 0.2% root mean square (RMS). The
output spectrum is centered at 1038 nm with a 16 nm band-
width, as shown in Fig.
2(a). With a 1200 line/mm grating
pair, pulses from the fiber laser oscillator can be dechirped to
COL 17(4), 041404(2019) CHINESE OPTICS LETTERS April 10, 2019
1671-7694/2019/041404(4) 041404-1 © 2019 Chinese Optics Letters