Single-mode output by controlling the
spatiotemporal nonlinearities in mode-locked
femtosecond multimode fiber lasers
U ˘gur Te ˘gin ,
a,b,
* Babak Rahmani,
b
Eirini Kakkava ,
a
Demetri Psaltis,
a
and Christophe Moser
b
a
École Polytechnique Fédérale de Lausanne, Optics Laboratory, Lausanne, Switzerland
b
École Polytechnique Fédérale de Lausanne, Laboratory of Applied Photonics Devices, Lausanne, Switzerland
Abstract. The performance of fiber mode-locked lasers is limited due to the high nonlinearity induced by the
spatial confinement of the single-mode fiber core. To massively increase the pulse energy of the femtosecond
pulses, amplification is performed outside the oscillator. Recently, spatiotemporal mode-locking has been
proposed as a new path to fiber lasers. However, the beam quality was highly multim ode, and the calculated
threshold pulse energy (>100 nJ) for nonlinear beam self-cleaning was challenging to realize. We present an
approach to reach high energy per pulse directly in the mode-locked multimode fiber oscillator with a near
single-mode output beam. Our approach relies on spatial beam self-cleaning via the nonlinear Kerr effect, and
we demonstrate a multimode fiber oscillator with M
2
< 1.13 beam profile, up to 24 nJ energy, and sub-100 fs
compressed duration. Nonlinear beam self-cleaning is verified both numerically and experimentally for the first
time in a mode-locked multimode laser cavity. The reported approach is further power scalable with larger core
sized fibers up to a certain level of modal dispersion and could benefit applications that require high-power
ultrashort lasers with commercially available optical fibers.
Keywords: fiber lasers; spatiotemporally mode-locked lasers; multimode nonlinear fiber optics.
Received Jul. 17, 2020; revised manuscript received Sep. 1, 2020; accepted for publication Sep. 17, 2020; published online
Oct. 16, 2020.
© The Authors. Published by SPIE and CLP under a Creative Commons Attribution 4.0 Unported License. Distribution or
reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
[DOI: 10.1117/1.AP.2.5.056005]
1 Introduction
Fiber laser dynamics have been studied extensively in the past
decades to generate femtosecond pulses with high energies and
peak powers.
1
Numerous laser designs are developed to under-
stand nonlinear wave propagation under partial feedback condi-
tions. By tuning complex cavity dynamics in single-mode fiber
cavities, self-organization of longitudinal cavity modes with vari-
ous temporal profiles and central wavelengths has been realized,
such as soliton,
2
similariton,
3,4
and dissipative soliton.
5,6
Due to
the high spatial confinement in the small single-mode fiber core,
nonlinear effects appear at moderate peak power in solid cores,
and the accumulation of excessive nonlinear phase leads to pulse
breakup, which then limits the achievable pulse energies.
To overcome this limitation, custom-made sophisticated fi-
bers, having large single-mode areas in a photonic crystal fiber,
were proposed to reach the microjoule pulse energy level.
7,8
The
fiber used in these demonstrations with its mode field diameter
of 70 μm needed to be kept straight to avoid bending losses and
ensure stability. They share the same limitations as solid-state
lasers in the sense that they are rigid and cannot be spliced with
conventional techni ques. As a result, such lasers do not share
the features of fiber lasers that render them advantageous in
practice.
Recently, spatiotemporal mode-locking has been demon-
strat ed in commercially available multimode fi ber (MMF)
cavities w ith graded-index multim ode fibers (GRIN MMFs)
by harnessing their low modal dispersio n and inherent periodic
self-focusing to produce a coherent superp osition of transverse
and longitudinal modes in an all-normal dispersion regime.
9–11
These studies presented cavities with dissipative soliton pulse
operation with low-output beam quality despite utilizing gain
fibers with three modes. In a recent study, improvement in the
output beam profile of a spatiotemporal mode-locked fiber laser
*Address all correspondence to U˘gur Te˘gin, E-mail: ugur.tegin@epfl.ch
Research Article
Advanced Photonics 056005-1 Sep∕Oct 2020
•
Vol. 2(5)
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