LG
11
-mode vortex Nd:YAG laser by applying
second-order circular Dammann grating for
annular pumping
Quan Zhang (张 权)
1,2
, Junjie Yu (余俊杰)
1
, Biran Shi (施必然)
1,2
,
Fanchun Tang (唐凡春)
1,2
, and Jianlang Li (李建郎)
1,
*
1
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
2
Center of Materials Science and Optoelectronics Engineering , University of Chinese Academy of Sciences, Beijing
100049, China
*Corresponding author: apuli@siom.ac.cn
Received December 16, 2018; accepted February 22, 2019; posted online May 13, 2019
The excitation of high-order Laguerre–Gaussian (LG) modes in a neodymium-doped yttrium aluminum garnet
(Nd:YAG) laser resonator was presented by applying the diffraction of a second-order circular Dammann gra-
ting (CDG) for annular pumping. In the study, the 808 nm pump light was shaped into a double-ring structure
by the CDG and matched spatially with that of an ideal LG
11
mode. As a result, the laser resonator generated an
LG
11
vortex mode, and the laser power reached 204 mW with 14.5% slope efficiency. Also, when the cavity mirror
was tilted, the laser output could switch to the LG
01
vortex mode. The results showed the convenience and
versatility of CDG in an annular-pumped vortex laser.
OCIS codes: 140.3580, 140.3480, 050.1950.
doi: 10.3788/COL201917.051402.
Laguerre–Gaussian (LG) modes, which are one set of
eigen solutions of the Helmholtz equation under paraxial
approximation in the cylindrical coordinate, exhibit
doughnut-like transversal intensity distribution and pos-
sess a helical phase wave-front
[1]
. Generally, LG modes
are described by LG
p;l
,wherep and l are the radial and
azimuthal indices
[2]
.LG
p;l
modes with l ≠ 0havep þ 1rings
in the transverse field, and a helical phase front with topo-
logical charge of l carries a z component of orbital angular
momentum (OAM) of l ℏ per photon
[3]
. That is why LG
modes are also called optical vortex beams
[4]
. Based on those
physical properties, LG beams have been widely used in
optical manipulation
[5]
, optical communication in free
space
[6]
, super resolution microscopy
[7]
, optical switching
[8]
,
and spinning object detection
[9,10]
.Comparedwithlower-
order LG modes, high-order modes show superior perfor-
mance in some fields
[11,12]
. For example, the high-order
LG
33
mode reduces the thermal noise of the mirrors, and
its presence in the application of gravitational-wave detec-
tion signifies high sensitivity and a high detection rate
[13–15]
.
The methods for generating LG modes include passive
and active ways. The former refers to the spatial conver-
sion of an incoming light beam after transmitting through
the optical elements, like computed holograms
[16]
, spiral
phase plate
[17]
, and spatial light modulator
[18]
. The latter
aims to the direct oscillation of the desired LG mode in
a laser resonator, either by deploying various mode selec-
tors inside it
[19]
or applying the annular pumping tech-
nique
[20–23]
. As the annular pumping profile, after some
shaping optics, intrinsically has good spatial overlapping
with target LG modes in the gain medium, this technique
favors the LG-mode excitation of one laser resonator with
high efficiency and high mod al purity.
As is known, a circular Dammann grating (CDG), a
binary-phase component with periodic and alternate
0 and π phases in the radial direction, can diffract an in-
coming light into a single ring or multiple concentric rings
with equal intensities at the far field
[24,25]
. We previously
reported the utilization of a first-order CDG to imple-
ment a single-ring-pumped LG
01
-mode neodymium-doped
yttrium aluminum garnet (Nd:YAG) laser. This scheme is
easy to realize by only inserting the CDG element into the
laser’s pump unit, and also, the fabrication of these ele-
ments is simple and with low cost.
In this study, to test the CDG’s ability in assisting the
excitation of high-order vortex mode, we firstly applied a
second-order CDG to shape the pump light into a double-
ring intensity profile at the far field; thereafter, we dem-
onstrated the LG
11
mode’s oscillation of a Nd:YAG laser.
Furthermore, the LG
11
mode’s oscillation was switched to
the LG
01
mode stably when we slightly adjusted the incli-
nation of the output coupler (OC) of the laser. The results
showed the versatility of CDG-based annular pumping for
a high-order LG-mode vortex solid-state laser.
Figure
1 shows the schematic diagram of the laser. An
808 nm fiber-coupled laser diode was used as the pump
source, and its tail fiber had a core diameter of 105 μm
and numerical aperture of 0.22. Behind the pigtail fiber,
two lenses L
1
and L
2
, with the focal lengths of 15 and
25.4 mm, respectively, were used to collimate the pump
radiation and then focus it into the gain medium. The gain
medium was a piece of Nd:YAG crystal that had 1 at. %
doping concentration of Nd ions, 10 mm diameter, and
1 mm thickness. The front surface of it was coated for high
transmission at 808 nm and high reflection at 1064 nm,
and its rear surface was anti-reflection coated at 1064 nm.
COL 17(5), 051402(2019) CHINESE OPTICS LETTERS May 10, 2019
1671-7694/2019/051402(5) 051402-1 © 2019 Chinese Optics Letters