Population inversion in sodium D
2
transition based on
sodium-ethane excimer pairs
Shu Hu (胡 墅)
1
, Baodong Gai (盖宝栋)
1,2
, Jingwei Guo (郭敬为)
1,
*,
Pengyuan Wang (王鹏远)
1
, Xueyang Li (李学杨)
1,2
, Hui Li (李 慧)
1
, Jinbo Liu (刘金波)
1
,
Shan He (何 山)
1,2
, Xianglong Cai ( 蔡向龙)
1
, Dong Liu (刘 栋)
1
, Ying Chen (陈 莹)
1
,
Fengting Sang (桑凤亭)
1
, and Yuqi Jin (金玉奇)
1
1
Key Laboratory of Chemical Lasers, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian 116023, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*Corresponding author: jingweiguo@dicp.ac.cn
Received July 10, 2017; accepted July 28, 2017; posted online August 21, 2017
Sodium-ethane excimer pairs are studied and proved to be a great choice of excimer pumped sodium laser (XPNaL)
gain media. The lifetime of the sodium D
2
line is studied in a sodium-ethane excimer system excited by a 553 nm
laser, and the observed phenomenon of lifetime lengthening is discussed. Amplified spontaneous emission (ASE) of
the sodium D
2
line is successfully obtained, and its time-resolved and spectroscopic characteristics are studied
experimentally. According to the intensity of the ASE signal under different sodium vapor atom densities, the
sodium D
2
line gain feature of sodium-ethane excimer pairs excited by the 553 nm laser is concluded.
OCIS codes: 140.1340, 140.2180, 020.0020.
doi: 10.3788/COL201715.111401.
In recent years, the diode pum ped alkali laser (DPAL)
has attracted extensive attention and study in the field
of high power laser due to its advantages, such as high gain
coefficient, small medium volume, high quantum effi-
ciency, good beam quality, and excellent flowing and heat
dissipation properties
[1,2]
. DPALs based on potassium
[3]
,
rubidium
[4]
, and cesium
[5]
vapors have achieved great
progress recently. The excimer pumped alkali laser
(XPAL)
[6,7]
is a new class of optically pumped laser, which
is developed from the DPAL. Thanks to the excimer
pairs, XPAL can realize broad absorption; this feature
avoids the problem of linewidth matching between the
absorption of alkali metal vapor and the emission of
diode laser. XPAL based on Cs vapor has been reported
of lasing on the D
1
line (894 nm)
[8]
and D
2
line (852 nm)
[9]
.
The D
2
line (780 nm) of Rb vapor laser has also been
demonstrated experimentally
[10]
. Furthermore, the blue
light can be realized in Rb vapor based on frequency
upconversion
[11]
.
For the splitting between energy levels of 3
2
P
3∕2
and
3
2
P
1∕2
of just 17.2 cm
−1
[1]
, it will be difficult to realize a
DPAL operating as a three-level system based on sodium
vapor. Fortunately, XPAL is a four-level or five-level sys-
tem, which can avoid the energy level splitting problem
mentioned above, and can make lasing of sodium vapor
possible. The sodium D
1
∕D
2
line laser from the excimer
pumped sodium laser (XPNaL) will be suitable for the
application of the sodium beacon laser, which is the
light source of the sodium guide star. The sodium guide
star can improve the image resolution of the adaptive
optics (AO) syste m
[12,13]
at high altitudes and is essential
for applications such as universe observation, space
target recognition, and laser telecommunications
[14,15]
. The
prevalent sodium beacon lasers are mainly based on meth-
ods of sum-frequency generation (SFG)
[16]
of 1064 and
1319 nm
[17]
, stimulated Raman scattering (SRS)
[18]
, and
an optical parametric amplifier (OPA)
[19]
. If XPNaL is uti-
lized as a sodium beacon laser, a wavelength correction
system is unnecessary. Sodium-ethane excimer pairs will
be formed after the collision happens between sodium
atoms of the ground state (3
2
S
1∕2
) and ethane molecules,
and a “blue satellite” pumping laser will excite excimer
pairs from the X
2
Σ
þ
1∕2
state to the B
2
Σ
þ
1∕2
state. Since
the B
2
Σ
þ
1∕2
is a repulsive state, excimer pairs dissociate
quickly to produce sodium atoms of an excited state
(3
2
P
3∕2
), then the sodium D
2
line laser can be observed
from the stimulated emission of 3
2
P
3∕2
→ 3
2
S
1∕2
and ac-
curately match the resonance line of the sodium atom,
so XPNaL provides a novel technical approach for the
sodium beacon laser.
In 2012, Hewitt and Eden realized XPNaL based on
Na–Xe excimer pairs lasing at sodium D
1
and D
2
lines
[20]
, however, suffering from the low pumping efficiency,
the laser power was too low. So, the key issue of developing
XPNaL is exploring excellent excimer pairs. In our previous
work, we expanded the sodium-rare gas pairs to sodium-
alkane gas pairs, and studied the absorption coefficients
of different excimer pairs, including Na-C
2
H
6
excimer pairs,
both experimentally and theoretically
[21]
.
In this work, the fluorescence lifetime of the sodium D
2
line based on Na-C
2
H
6
excimer pairs is measured, and the
kinetic process of fluorescence lifetime lengthening is
given. The time-resolved and spectroscopic characteristics
of the sodium D
2
line amplified spontaneous emission
(ASE), also the sodium D
2
line gain feature of Na-C
2
H
6
excimer pairs, is investigated through the ASE intensity
COL 15(11), 111401(2017) CHINESE OPTICS LETTERS November 10, 2017
1671-7694/2017/111401(5) 111401-1 © 2017 Chinese Optics Letters
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