Self-starting passively mode-locked all fiber laser based
on carbon nanotubes with radially polarized emission
Yong Zhou,
1
Jian Lin,
1
Xiaoqiang Zhang,
1
Lixin Xu,
1
Chun Gu,
1
Biao Sun,
1
Anting Wang,
1,
* and
Qiwen Zhan
2
1
Department of Optics and Optical Engineering, University of Science and Technology of China, Hefei 230026, China
2
Electro-Optics Program, University of Dayton, Dayton, Ohio 45469, USA
*Corresponding author: atwang@ustc.edu.cn
Received August 29, 2016; revised October 23, 2016; accepted October 27, 2016;
posted October 27, 2016 (Doc. ID 274777); published November 30, 2016
We demonstrate an all fiber passively mode-locked laser emitting a radially polarized beam by using a few-mode
fiber Bragg grating to achieve mode selection and spectrum filtering. An offset splicing of single-mode fiber with
four-mode fiber is utilized as a mode coupler in the laser cavity. Carbon nanotubes are introduced into the laser
cavity as the saturable absorber to achieve self-start mode locking. The laser operates at 1547.5 nm with a narrow
spectrum width of 0.3 nm at 30 dB. The emitted mode-locked pulses have a duration of 22.73 ps and repetition of
10.61 MHz. A radially polarized beam has been obtained with high mode purity by adjusting the polarization in
the laser cavity. © 2016 Chinese Laser Press
OCIS codes: (060.2410) Fibers, erbium; (060.3510) Lasers, fiber; (060.3735) Fiber Bragg gratings; (140.4050)
Mode-locked lasers; (160.4236) Nanomaterials.
http://dx.doi.org/10.1364/PRJ.4.000327
1. INTRODUCTION
Radially polarized beams (RPBs) have drawn considerable
attention recently due to their interesting properties of sym-
metrical polarization and amplitude emission [1]. These prop-
erties lead to applications in many areas under high numerical
aperture (NA) focusing, such as optical trapping [2,3],
surface plasmon excitation [4], electron acceleration [5], high
resolution metrology [6], and material processing [7,8].
Furthermore, a pulsed RPB is in more demand since most
of these applications need high peak power. A variety of spa-
tial polarization selective elements were used in a solid laser
cavity to generate continuous-wave (CW) RPBs [9–11]. There
have been a few recent papers demonstrating passively mode-
locked RPBs in solid lasers with the introduction of mode-
locking elements in laser cavities such as graphene absorber
[12] or semiconductor saturable absorber (SA) mirror [13].
In fiber lasers, some of our previous works had demon-
strated that a few-mode fiber Bragg grating (FM-FBG) is an
efficient transverse mode selector when the laser operates
within a narrow spectrum [14–16]. For those lasers utilizing
FM-FBG as mode selector with hybrid mode injection, high
reflection only occurs in a specific narrow wavelength range
for different modes [17]. As a result, the fiber laser should
operate within a narrow spectrum that matches the spectral
properties of the FM-FBG to guarantee high mode purity,
while the mostly passively mode-locked fiber laser is operated
within a broad spectral range. Several papers have already
demonstrated passively mode-locked RPBs based on artificial
SAs, but they face challenges with high sensitivity to
polarization and environmental changes. For example, [18]
demonstrated a passively mode-locked fiber laser based on
the nonlinear polarization rotation mechanism. However,
the spectrum width under the mode-locked state was so much
wider than the FBG reflection range that the mode purity of
the pulsed RPB was low. Reference [19] demonstrated a long
figure-8 cavity fiber laser generating a rectangular pulse with
radially polarized emission based on a nonlinear amplifying-
loop mirror; however, the laser structure was relatively com-
plicated. Our group also demonstrated actively mode-locked
RPB pulses to evade the sensitivity of polarization [20], but the
pulse duration was limited by the modulator, leading to low
peak power. Thus true SAs may be more suitable for passively
mode-locked RPB fiber lasers. Among variable true SAs, car-
bon nanotubes (CNTs) offer good stability, wide operating
wavelength, and insensitivity of polarization and ease of fab-
rication [21], and they are chosen as the SA in this work to
generate passively mode-locked RPB pulses.
In this paper, we demonstrated a ring cavity all fiber
passively mode-locked laser based on CNT that is capable of
producing high purity pulsed RPB output. A self-started mode-
locked pulse laser with narrow spectral width is successfully
demonstrated with a repetition rate of 10.61 MHz and pulse
duration of 22.73 ps under stable state. An RPB with high
mode purity of 98.03% is obtained through adjusting the
polarization in the laser cavity. Compared with the aforesaid
mode-locked RPB fiber lasers, this laser integrates all the
advantages of self-starting, high peak power, compact struc-
ture, high mode purity, and insensitivity to polarization.
2. EXPERIMENTAL SETUP
Figure 1 shows the schematic of the proposed fully integrated
fiber laser. A highly erbium-doped fiber (EDF, Liekki Er110-8/
125) with a length of 70 cm is pumped by a 974 nm laser
diode (LD) though a 974/1550 nm wavelength division multi-
plexer (WDM). A 9:1 optical coupler (OC) is placed behind
the EDF for temporal and spectral property detection. The
Zhou et al. Vol. 4, No. 6 / December 2016 / Photon. Res. 327
2327-9125/16/060327-04 © 2016 Chinese Laser Press