COL 12(8), 080602(2014) CHINESE OPTICS LETTERS August 10, 2014
Transmission characteristics of photonic crystal fiber gas
cell used in freque ncy stabilized laser
Chongde Huang (
ÂÂÂ
)
1,2
, Dijun Chen (
&&&
ddd
)
1∗
,
Haiwen Cai (
ééé
°°°
©©©
)
1∗∗
, Ronghui Qu (
YYY
JJJ
)
1
, and Weibiao Chen (
¥¥¥
III
)
1
1
Key Laboratory of Space Laser Communication and Detection Technology,
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
∗
Corresponding author: djchen@siom.ac.cn;
∗∗
corresponding author: hwcai@siom.ac.cn
Received March 3, 2014; accepted April 29, 2014; posted online July 18, 2014
We measure the transmission characteristics of hollow-core ph otonic crystal fiber (HC-PCF) gas cells with
ferrule- and fusion-spliced configurations, and near- and far-field images of the HC-PCF are observed.
Results show that the center of mass (COM) of the far-field image varies with the laser frequency and
temperature, and the moving COM relates to the oscillatory transmission. Using a model of the spatial
interference, we first demonstrate that mainly t he modes with asymmetric phase distributions affect t he
COM position. The frequency stabilization performances of the lasers are compared. The fusion-spliced
gas cell sh ows better performance than the ferrule-spliced one.
OCIS codes: 060.2270, 060.4005, 140.3425, 350.6090.
doi: 10.3788/COL201412.080602.
CO
2
is one of the most important greenhouse gases. Mea-
surements show that the CO
2
density in the atmosphere
is presently higher than that in the past 420000 years;
hence, understanding the evolution of CO
2
in the global
scale is important
[1]
. The integrated path differential ab-
sorption (IPDA) lidar possesses advantages, such as high
precision and low bias for measuring CO
2
density in the
atmosphere. A frequency stabilized laser with a r e ference
gas cell is a key in IPDA lidar technology
[2,3]
. Because
the laser frequency is locked at the weak CO
2
R18 ab-
sorption line near 1572.02 nm, a gas cell with lo ng o p-
tical path is necessary
[2−4]
. Hollow-core photonic crys-
tal fibers (HC-PCFs) have been used to produce com-
pact and robust gas ce lls with long absorption length
[5,6]
.
The HC-PCFs are recently commercially available
[7]
. To
build an all-fiber gas cell, coupling the HC-PCF with a
standard single mode fiber (SMF) or a multimode fiber
(MMF) is necessary. However, the gas cell o ften exhibits
an oscillatory background in its transmission spectrum,
which is considered due to the effect of HC-PCF’s sur-
face modes
[8−10]
. Our results show that the far -field of
the HC-PCF’s output changes with laser fre quency, and
leads to fluctuation of the power coupled to the subse-
quent fiber. Varia tions in background transmission re-
sult in spurious signals fo r the frequency reference a nd
degrade the frequency stability of the seed laser.
In this letter, we compare the transmission characteris-
tics of ferrule- and fusion-spliced HC-PCF gas cells. We
measure the fa r-field pattern and variations in center-of-
mass (COM) with the laser frequency and temperature.
By performing the method of sliding-window Fourier
transform on the transmission spectrum
[11,12]
, the co n-
tents of the HC-PCF are analyzed, and their effective
refractive indexes are deduced. A semi-empirical model
based on the spatial interference between the HC-PCF
modes is proposed to analyze the far-field variations.
Here we re veal that variations in COM are mainly at-
tributed to the coupling of the fundamental mode with
the non- axial symmetric phase distribution modes, such
as the LP
11
and LP
31
-like modes. The frequency stabi-
lization performance of the laser is presented, and the
fusion-spliced structure yields better results than the
ferrule-spliced one.
The HC-PCF (HC-1550 -02, NKT Photonics, USA) is
used to make a c ompact and robust CO
2
-filled gas cell
with long absorption length for the laser as the frequency
reference
[5,6]
. The core diameter of this HC-PCF is 10 µm
and its attenuation is about 30 dB/km
[7]
. Two configu-
rations of gas c e lls are made in our work, as shown in Fig.
1. The length of HC-PCF is 10 m in the ferrule-spliced
configuration and 15 m in the fusion-splice configuration
respectively. The input ports are both fusion-splicing
with standard SMFs. The losses of the input s plices are
about 2 dB and they are repeatable. Because the core
of the HC-PCF is larger than the SMF’s, we connect the
MMF of 50 µm core diameter to the HC-PCF’s output
end to reduce the insertion loss. For the ferrule-spliced
configuration, the HC-PCF is s e aled into a FC/PC ce-
ramic ferrule and connected to the MMF with cera mic
ferrule by a sleeve. There is a gap of about 20 µm be -
tween the two fibers, which is used to evacuate or fill the
gas. Afterward, the splice is sealed with epoxy to keep the
CO
2
gas with the re quired pressure. On the other hand,
for the fusion-spliced configura tion, the MMF is fusion
splicing to the HC-PCF after filling CO
2
in the HC-PCF
at the required pressure. The insertion loss of the ferrule-
spliced gas cell is 3.5 dB, and the fusion-spliced gas cell’s
is 2.8 dB.
Here the light from a distributed-feedback laser diode
(DFB-LD) is injected throug h the gas cells and a photo
detector (PD) is use d to record the transmission power.
The transmission spectrums of the two gas cells are ob-
tained by scanning the freq uency of this DFB-LD. Fig-
ure 2 s hows the measured curves in which fluctuations
are noticed obviously. The percentages of peak-to-peak
fluctuation to the average are 11.9% and 8.4%. Figure 3
1671-7694/2014/080602(5) 080602-1
c
2014 Chinese Optics Letters