All-in-fiber amplification and compression of
coherent frequency-shifted solitons tunable
in the 1800–2000 nm range
GRZEGORZ SOBO
´
N,
1,
*TADEUSZ MARTYNKIEN,
2
DOROTA TOMASZEWSKA,
1
KAROL TARNOWSKI,
2
PAWEŁ MERGO,
3
AND JAROSŁAW SOTOR
1
1
Laser & Fiber Electronics Group, Faculty of Electronics, Wroclaw University of Science and Technology,
Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
2
Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology,
Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
3
Laboratory of Optical Fiber Technology, Maria Curie-Sklodowska University, pl. M. Curie-Sklodowskiej 3, Lublin, Poland
*Corresponding author: grzegorz.sobon@pwr.edu.pl
Received 17 January 2018; revised 19 February 2018; accepted 19 February 2018; posted 21 February 2018 (Doc. ID 320061);
published 18 April 2018
We report an all-fiber, all-polarization maintaining (PM) source of widely tunable (1800–2000 nm) ultrashort
pulses based on the amplification of coherent self-frequency-shifted solitons generated in a highly nonlinear fiber
pumped with an Er-doped fiber laser. The system delivers sub-100 fs pulses with energies up to 8.6 nJ and is built
entirely from PM optical fibers, without any free-space optics. The all-fiber alignment-free design significantly
increases the suitability of such a source for field deployments.
© 2018 Chinese Laser Press
OCIS codes: (190.4370) Nonlinear optics, fibers; (140.4050) Mode-locked lasers; (060.4370) Nonlinear optics, fibers.
https://doi.org/10.1364/PRJ.6.000368
1. INTRODUCTION
Widely tunable and ultrashort-pulsed laser sources operating
in the 1.8–2.0-μm spectral band are in demand for many ap-
plications in various fields of industry and science. Compact
and efficient lasers in this spectral region can be used, e.g., for
supercontinuum generation in nonlinear fibers [1], as pumps
for mid-infrared parametric oscillators [2], or as sources in
surgery or dermatology [3]. Most commonly, ultrashort pulses
in this spectral region are generated from mode-locked fiber
lasers based on Tm- or Ho-doped gain fibers, utilizing vari-
ous mode-locking techniques, e.g., nonlinear polarization
evolution (NPE) [4,5], semiconductor saturable absorber mir-
rors (SESAM) [6], graphene [7], and carbon nanotube satu-
rable absorbers [8].
Alternatively, broadband and ultrashort pulses in the 1.8–
2.0 μm region can be obtained via nonlinear frequency conver-
sion in highly nonlinear fibers (HNLFs). In principle, a
near-infrared pump (e.g., from a mode-locked erbium-doped
fiber laser, EDFL) launched to a dispersion-engineered
HNLF might be converted toward longer wavelengths due
to Raman-induced soliton self-frequency shift (SSFS). The
concept of using compact 1.56-μm EDFLs and shifting their
output radiation via SSFS toward the 2-μm band utilizing long
sections of conventional single-mode fibers was presented by
Nishizawa and Goto [9]. Nowadays, usually photonic crystal
fibers (PCFs) [10,11], suspended-core fibers [12], or
GeO
2
-doped silica fibers [13] are used as nonlinear media
for SSFS. The usage of fluoride fibers allows for generation
of solitons shifted up to 4.3 μm[14]. Recently, we have re-
ported generation of sub-100-fs pulses tunable from 1700 to
2100 nm via SFSS in a microstructured HNLF pumped by
a compact EDFL [15]. The main advantage of this approach
over classical Tm- or Ho-doped fiber lasers is that the output
radiation is easily tunable over a very broad spectral range, un-
reachable for fiber lasers based on rare-earth-doped gain fibers.
Untypical wavelengths like 1650–1800 nm, interesting for deep
bio-imaging applications [16], can be easily obtained just by ad-
justing the pump power launched into the nonlinear fiber [15],
without the necessity of building complex short-wavelength
Tm-doped lasers [17]. The pulses obtained via SSFS are usually
closely transform-limited [18,19] and might maintain their co-
herence [20], and they also have a broad and smooth spectral
envelope [15,19,20]. The SSFS output power can be further
scaled by external amplification using the chirped pulse ampli-
fication (CPA) technique. The usage of an SSFS-based seed
source for the CPA instead of a fixed-wavelength Tm- or
Ho-doped laser makes the entire source much more flexible,
thanks to the broad tuning capabilities. However, the most
368
Vol. 6, No. 5 / May 2018 / Photonics Research
Research Article
2327-9125/18/050368-05 Journal © 2018 Chinese Laser Press