All-fiber passively mode-locked laser using
nonlinear multimode interference
of step-index multimode fiber
TAO CHEN,
1,
*QIAOLI ZHANG,
1
YAPING ZHANG,
1
XIN LI,
1
HAIKUN ZHANG,
1
AND WEI XIA
1,2
1
School of Physics and Technology, University of Jinan, Jinan 250022, China
2
e-mail: sps_xiaw@ujn.edu.cn
*Corresponding author: taochen426@hust.edu.cn
Received 31 July 2018; revised 2 September 2018; accepted 11 September 2018; posted 13 September 2018 (Doc. ID 340879);
published 16 October 2018
We experimentally demonstrate for the first time, to the best of our knowledge, an all-fiber passively mode-locked
laser operation based on the nonlinear multimode interference of step-index multimode fiber. Such a structure
couples the light in and out of the multimode fiber via single-mode fibers, and its physical mechanisms for satu-
rable absorption have been analyzed theoretically based on the third-order nonlinear Kerr effect of multimode
fiber. Using the nonlinear multimode interference structure with 48.8 mm length step-index multimode fiber, the
modulation depth has been measured to be ∼5%. The passively mode-locked laser output pulses have a central
wavelength of 1596.66 nm, bandwidth of 2.18 nm, pulsewidth of ∼625 fs, and fundamental repetition rate of
8.726 MHz. Furthermore, the influence of total cavity dispersion on the optical spectrum, pulse width, and
output power is investigated systematically by adding different lengths of single-mode fiber and dispersion
compensation fiber in the laser cavity.
© 2018 Chinese Laser Press
https://doi.org/10.1364/PRJ.6.001033
1. INTRODUCTION
Passively mode-locked fiber lasers exhibit the advantages of
compact and simple all-fiber structure, low-cost, and good
compatibility with optical fiber systems. Various saturable
absorption materials have been developed to actuate mode-
locking based on a semiconductor saturable absorption mirror
[1], single-walled carbon nanotube [2–4], and the 2D nanoma-
terials such as graphene [5–8], transition metal chalcogenides
[9–11], topological insulators [12,13], and black phosphorus
[14,15]. These materials often suffer from optical-power-
induced thermal damage and oxidation, thus restricting the
damage threshold and long-term stability of the fiber laser
[16,17]. The alternative techniques to address the issues are
the use of nonlinear polarization evolution [18,19], nonlinear
amplifying loop mirror [20,21], and nonlinear mode-coupling
[22]. Different from the precise control of the polarization state
in nonlinear polarization evolution and the accurate selection
of the coupler split ratio in a nonlinear amplifying loop mirror,
the nonlinear mode-coupling can simply achieve stable and
robust passive mode-locking through four structures. The first
structure implementing nonlinear mode-coupling is the wave-
guide array coupling [22,23]. An AlGaAs waveguide array was
used as a saturable absorber to experimentally achieve passive
mode-locking in an Er-doped fiber laser (EDFL). Using the
multicore fiber coupling is the second structure to mode-lock
a fiber laser based on nonlinear mode-coupling [24–27]. The
dual-core fiber couplers for a passively mode-locked fiber laser
were numerically simulated, and a tapered seven-core telecom-
munication fiber for saturable abs orption was experimentally
measured. The third nonlinear mode-coupling structure is the
long-period fiber gra ting coupling [28]. Although the long-
period fiber grating is theoretically capable of supporting
mode-locked fiber laser operation, the length required to
achieve complete coupling is on the order of meters and limits
its usefulness for mode-locked experiments. Nonlinear multi-
mode interference (MMI) is the fourth structure to realize non-
linear mode-coupling [29–37]. The MMI structures usually
consist of a multimode fiber (MMF) used as an intermediate
coupler between two other fibers [e.g., single-mode fiber (SMF)
or photonic crystal fiber]. They have been adopted for all-fiber
bandpass filters with tens of centimeters of MMF and are also
widely applied in the mode-field transformation between differ-
ent types of fiber.
MMF is the key component of an MMI structure. Recently,
graded-index multimode fibers (GIMFs) have attracted exten-
sive investigation of nonlinear optical interactions [38,39],
as the group velocities of all modes of the GIMF are nearly
Research Article
Vol. 6, No. 11 / Novembe r 2018 / Photonics Research 1033
2327-9125/18/111033-07 Journal © 2018 Chinese Laser Press