Study on polarization spectrum and annealing properties
of 45°-tilted fiber gratings
Xi Guo (郭 曦)
1
, Zhikun Xing (邢志坤)
1
, Huabao Qin (覃华宝)
1
, Qizhen Sun (孙琪真)
1
,
Deming Liu (刘德明)
1
, Lin Zhang (张 琳)
2
, and Zhijun Yan (闫志君)
1,
*
1
School of Optical and Electronic Information, National Engineering Laboratory for Next Generation Internet
Access System, Huazhong University of Science and Technology, Wuhan 430074, China
2
Aston Institute of Photonic Technologies, Aston University, Birmingham B4 7ET, UK
*Corresponding author: yanzhijun@gmail.com
Received December 25, 2018; accepted January 25, 2019; posted online April 29, 2019
We have investigated the whole polarization-extinction-ratio (PER) spectrum and annealing properties of
45°-tilted fiber gratings (45°-TFGs). Experimental results show the PER spectrum of 45°-TFGs is a Gaussian-
like profile and covers a 540 nm bandwidth from 1260 to 1800 nm, in which the bandwidth with PER greater
than 10 dB is over 250 nm. The output polarization distribution of 45°-TFGs was analyzed by employing a bulk
linear polarizer, and the results show a perfect figure “8”, which indicates that the 45°-TFG is a type of linear
polarizer. Moreover, the annealing property of 45°-TFGs was measured up to 700°C, in which the PER of the
grating started to decrease at 300°C and reached the minimum at 700°C. Based on these results, the 45°-TFGs
can be used as an ultra-wide bandwidth in-fiber polarizing device.
OCIS codes: 060.2340, 230.5440, 230.1150, 050.2770.
doi: 10.3788/COL201917.050601.
The 45°-tilted fiber grating (45°-TFG), which can filter
out s polarization (TE) from the forward propagating core
mode, while the residual p-polarized light (TM) propa-
gates along the fiber core, has great potential as an
ideal in-fiber polarizer for applications in optical commu-
nication, sensor, and laser systems. Compared with the
traditional polarizing devices, the 45°-TFG contains a
number of prominent merits, including high polarization-
extinction-ratio (PER), low insertion loss, compact struc-
ture, good heat dissipation, simple fabrication method,
and nice compatibility with standard fiber devices, which
make the 45°-TFG more desirable for modern all-fiber sys-
tems. Since the first TFG structure was reported by Meltz
in 1990
[1]
, the TFGs have aroused researchers’ interests,
and many theoretical researches based on TFGs have been
undertaken in succession
[2–4]
. However, the fabrication
methods limited the development and application of
TFGs. Benefiting from the UV inscription technique,
which made the bulk production of TFGs more viable,
the TFGs were extensively investigated and found to have
good polarization properties when the tilted angle is ex-
actly 45° with respect to the normal of the fiber axis. Then,
various app lications based on 45°-TFGs were achieved,
such as an in-fiber polarizer
[5]
, in-fiber polarime ter
[6]
,
PER equalizer
[7]
, and interference filter
[8]
. Apart from
the applications in polarizing devices, the 45°-TFGs were
also implemented as in-fiber diffraction gratings in the
communication and imaging systems
[9–12]
. At present,
the main applications of 45°-TFGs are focused on the
all-fiber ultrafast mode-locked laser systems
[13–18]
. One of
the major achievements is a mode-locked pulse in a wide
wavelength range
[16]
, which has been realized due to the
considerably broad bandwidth of 45°-TFGs. However,
there is no report on comp rehensive analysis about the
whole PER spectrum of 45°-TFGs, which is essential
for further development towards real applications.
Nowadays, optical fiber devices are desirab le for many
harsh environmental applications
[19–21]
, especially in a
high-temperature environment, such as the oil and gas
industries
[22]
and the monitoring of engine turbines and
furnaces
[23]
. In view of these requirements, annealing
properties are of much significance in determining the
performance in high-temperature applications. However,
no systematic study concerning the high-temperature
characteristics of 45°-TFGs has been published yet. In this
Letter, we will experimentally investigate the whole PER
spectrum of the 45°-TFG, output polarization distribu-
tion, and its high-temperature annealing properties.
The principle of 45°-TFGs can be explained clearly with
Brewster’s law. As we know, the light incident at Brew-
ster’s angle on an optical interface will partially cease
its TE component. In a typical UV-inscribed fiber grating,
the Brewster’s angle can be given as
[24]
θ
B
¼ arctan
n
2
n
1
¼ arctan
n
core
þ Δn
n
core
; (1)
where n
core
and Δn are the refractive indices of the fiber
core and UV-induced refractive index modulation, respec-
tively. Because Δn (∼10
−4
) is far less than n
core
, the Brew-
ster’s angle for the fiber grating is calculated to be ∼45°.
Therefore, as depicted in Fig.
1, when unpolarized light
propagates through a 45°-TFG, the s-polarized light will
be coupled out from the side of the fiber, and p-polarized
light will transmit along the fiber core, which promotes the
45°-TFG as an ideal in-fiber polarizer.
COL 17(5), 050601(2019) CHINESE OPTICS LETTERS May 10, 2019
1671-7694/2019/050601(5) 050601-1 © 2019 Chinese Optics Letters