Investigation of ellipticity and pump power in a passively
mode-locked fiber laser using the nonlinear polarization
rotation technique
H. Ahmad
1,
*, S. I. Ooi
1
, M. Z. A. Razak
2
, S. R. Azzuhri
1
, A. A. Jasim
1
,
K. Thambiratnam
1
, M. F. Ismail
1
, and M. A. Ismail
1
1
Photonics Research Centre, University of Malaya, Kuala Lumpur, Malaysia
2
Institute of Microengineering and Nanoelectronics, Research Complex, University Kebangsaan Malaysia, 43600 Bangi,
Selangor, Malaysia
*Corresponding author: harith@um.edu.my
Received November 30, 2016; accepted February 10, 2017; posted online March 3, 2017
An elliptical initial polarization state is essential for generating mode-locked pulses using the nonlinear polari-
zation rotation technique. In this work, the relationship between the ellipticity ranges capable of maintaining
mode-locked operation against different pump power levels is investigated. An increasing pump power, in con-
junction with minor adjustments to the polarization controller’s quarter waveplate, results in a wider ellipticity
range that can accommodate mode-locked operation. Other parameters such as noise, pulsewidth, and average
output power are also observed to vary as the ellipticity changes.
OCIS codes: 140.3510, 140.4050, 060.2410, 060.4370.
doi: 10.3788/COL201715.051402.
Over the last few years, fiber lasers have become the key
focus in the development of new laser applications due to
their significant advantages over their solid state counter-
parts. The key advantages of fiber lasers include their low
weight and compact size, as well as relatively lower losses
and ease of alignment. These characteristics make fiber
lasers a crucial component of today’s optical communica-
tion, carrying terabits of information across continents.
While fiber lasers can be configured to suit multiple ap-
plications, pulsed fiber lasers in particular have recently
garnered substantial attention in generating high energy
pulses. These pulses serve a variety of applications, includ-
ing communications, sensing, manufacturing, and medi-
cine. Pulsing in fiber lasers can be achieved by multiple
active and passive mean s, with the most common being
Q-switching and mode-locking. Of the two, mode-locking
is preferred for applications requiring high frequency
pulses but at a lower power.
Fiber lasers can be mode locked in three ways; either
actively, passively, or as a hybrid of both. Active
modulation is reliable and offers tunability of various
parameters, but generates a broad pulsewidth
[1]
, and also
requires bulky and expensive components. Passive modu-
lation, on the other hand, modulates the lo ss and gain of
the laser using saturable absorbers (SAs)
[2–5]
or by inducing
SA action, such as nonlinear amplifying loop mirrors
(NALMs)
[6]
and nonlinear polarization rotation (NPR)
[7,8]
.
Pulses generated in this method have pulsewidths shorter
than those generated through active means, typically in the
sub-picosecond region. Another approach would be hybrid
methods, which combine active and passive methods to
generate self-starting mode-locked laser outputs and a
shorter pulsewidth
[9]
.
Generating mode-locked pulses using the NPR
technique requires two key components; namely a polar-
izer and an analyzer
[10]
. The simplicity and versatility of
the NPR technique can support a large variety of
applications and devices, such as the generation of super-
continuum spectra
[11,12]
, vector solitons
[13–15]
, all-optical
flip-flop memories
[16]
, and also in the development and
fabrication of semiconductor optical amplifiers
[17]
.Fur-
thermore, the NP R allows for fast saturable absorption.
Recently, Smirnov et al.
[18]
has demonstrated an all-
normal-dispersion mode-locked fiber laser using the
NPR technique that can support a large variety of out-
puts by adjusting the settings of the polarization control-
ler (PC). Different PC settings yield different spectrum
energies, durations, shapes, and widths. Oliver et al.
[19–21]
demonstrated th is b y measuring the output polarization
state, analyzing the inter-mode beat spectrum and by de-
termining the allowed operating regimes. In this work,
the relationship between pump power levels and the ellip-
ticity of a mode-locked fiber laser of the NPR technique is
successfully demonstrated. The work shows that the
higher the pump power, the wider the degree of ellipticity
that can accommodate a mode-locked operation.
Figure
1 shows the schematic of the experimental setup.
The laser is configured in a ring cavity. A 980 nm laser
diode with a maximum power of 600 mW is used as a pump
source for the laser and is connected to the 980 nm port
of a 980/1550 nm wavelength division multiplexer
(WDM). The common output of the WDM is connected
to a commercially available 65 cm long erbium-doped fiber
(EDF) with a cut-off wavelength between 870–970 nm, a
numerical aperture of 0.21–0.23, and an absorption coeffi-
cient of 20 dB/m at 1531 nm. The output from the EDF is
COL 15(5), 051402(2017) CHINESE OPTICS LETTERS May 10, 2017
1671-7694/2017/051402(5) 051402-1 © 2017 Chinese Optics Letters