COL 11(12), 121406(2013) CHINESE OPTICS LETTERS December 10, 2013
Transverse mode transition and LG
01
-mode generation in
an end-pumped Nd:YVO
4
laser
Yao Yao (
), Kegui Xia (
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), Minqiang Kang (
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),
Zhiqiang Fang (
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), and Jianlang Li (
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)
∗
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
∗
Corresponding author: apuli@siom.ac.cn
Received October 8, 2013; accepted November 19, 2013; posted online December 9, 2013
A laser-diode end-pumped Nd:YVO
4
crystal laser is demonstrated to emit the first-order Laguerre-
Gaussian (LG
01
) mode with 502-mW laser power and 22% slope efficiency. The LG
01
-mode is lased
only when the pumping area locates in the central part of the laser crystal’s front surface, and thereafter
the symmetrical LG
01
-HG
01
-TEM
00
mode transition happens when laser crystal is moved laterally inside
several-tens-micron area. The possible mechanism responsible for the phenomenon of symmetrical mode
transition is also discussed.
OCIS codes: 140.6810, 140.3530, 140.3480, 140.3580.
doi: 10.3788/COL201311.121406.
In the past years, the first-order Laguerre-Gaussian mode
(LG
01
), which is known as one solution of the paraxial
wave equation and has doughnut-shaped cross section
with spiral phase fronts
[1,2]
, has attracted much atten-
tion due to its important applications in many fields like
optical manipulation
[3,4]
, super-resolution microscopy
[5]
,
and gravitational waves detection
[6]
, etc.
Techniques to produce such beams are usually catego-
rized into passive and active methods. The former mainly
mentions the beam transforming of fundamental Gaus-
sian mode or Hermit-Gaussian mode by using diffractive
optical elements
[7,8]
, spatial light modulators
[3,9,10]
or
cylindrical lenses pairs converter
[11]
, etc. The latter
is to force a laser resonator to oscillate in the desired
mode by adopting the annular pumping
[12−15]
, spot-
defect or grating mirror of laser cavity
[16−18]
or thermal-
twisted cavity
[18]
, etc. As we know, these ever-reported
methods mentioned the utilization of purpose-designed
device
[16−19]
, high pumping power
[20]
or reshaped pump
profile
[12−15]
.
In this letter, we demonstrated a laser-diode (LD) end-
pumped Nd-doped yttrium vanadate (Nd:YVO
4
) crystal
laser which emitted LG
01
mode easily and efficiently.
This laser had plano-plano cavity and consisted of only
two components, laser crystal and plane-mirror-based
output coupler. Although our laser cavity seemed sim-
ilar to that in Refs. [12,13], in our scheme, the pump
light reached to laser crystal had circular and solid cross
section instead of annular cross section. Further, our ex-
perimental results indicated that the laser’s beam pattern
was sensitive to the lateral position of laser crystal, and
LG
01
-mode emission was realized only when the pump-
ing area was located in the center of laser crystal’ front
surface. When the pumping area deviated laterally from
the center of laser crystal’s front surface within a range
of several-tens microns, the lasing mode experienced a
rapid and symmetrical transition from LG
01
mode to the
first-order Hermite-Gaussian (HG
01
) mode and then to
TEM
00
mode. This result revealed the observation of an
interesting phenomenon in LD end-pumped Nd:YVO
4
laser. The details of our investigation were given in the
following sections.
Figure 1 plotted our experimental setup schematically.
The pump source was an 808-nm LD coupled by a fiber
with a 105-µm core diameter and 0.22 numimerical aper-
ture (NA). The radiation from this LD was collimated
by lens L1 with 8-mm focal length and then focused into
the front surface of the gain medium by lens L2 with
40-mm focal length. The gain medium was a piece of
1.5 at.-% Nd-doped YVO
4
crystal with a dimension of
5×5×5 (mm) and its c axis was oriented parallel to cavity
axis. Its front surface was dichromatic coated for high
transmission at 808 nm and high reflection at 1064 nm,
while its rear surface was anti-reflectively coated at 1064
nm. The four lateral faces of this crystal were wrapped
by indium film and mounted on an aluminum holder,
and no active cooling system was used. A plane mirror
with 90% reflectivity at 1064 nm was used as the output
coupler (OC). As shown in Fig. 1, this laser resonator
had simple plano-plano cavity, formed between the front
face of laser crystal and the OC. In the experiment, the
laser cavity was kept at a length of 15 mm. And the
laser power and beam profile were monitored by a power
meter and charge-coupled deviced (CCD) camera, re-
spectively.
The use of Nd:YVO
4
crystal as gain medium was based
on the following considerations. Firstly, its high absorp-
tion cross section at pumping wavelength and high emis-
sion cross section make itself beneficial for low-threshold
oscillation of laser resonator. Furthermore, its c-cut
Fig. 1. Schematic diagram of laser-diode end-pumped
Nd:YVO
4
laser.
1671-7694/2013/121406(4) 121406-1
c
2013 Chinese Optics Letters