Long Period Fiber Gratings Inscribed With an Improved Two-Dimensional Scanning Technique

Abstract

We demonstrated a promising $hbox{CO}_{2}$ laser irradiation system based on an improved 2-D scanning technique. Such a system could be used to inscribe high-quality long period fiber gratings (LPFGs) with good reproducibility of grating inscription, which attributes to the fact that our system includes a $hbox{CO}_{2}$ laser with an excellent power stability of less than $pm$2% and a 3-D ultraprecision motorized translation stages with an excellent bidirectional repeatability value of 80 nm. Moreover, a control program with an easy-to-use operation interface was developed in our system so that a high-quality LPFG could be achieved as soon as grating parameters, such as grating pitch and number of grating periods, are entered, which has a widespread commercial value and prospects for development. Additionally, near mode fields of the $hbox{CO}_{2}$-laser-induced LPFG were observed and simulated to investigate mode coupling in the gratings.

Full text

Long Period Fiber Gratings Inscribed
With an Improved Two-Dimensional
Scanning Technique
Volume 6, Number 4, August 2014
Xiaoyong Zhong
Yiping Wang, Senior Member, IEEE
Changrui Liao
Guolu Yin
Jiangtao Zhou
Guanjun Wang
Bing Sun
Jian Tang
DOI: 10.1109/JPHOT.2014.2337875
1943-0655 Ó2014 IEEE

Long Period Fiber Gratings Inscribed
With an Improved Two-Dimensional
Scanning Technique
Xiaoyong Zhong, Yiping Wang, Senior Member, IEEE, Changrui Liao,
Guolu Yin, Jiangtao Zhou, Guanjun Wang, Bing Sun, and Jian Tang
Key Laboratory of Optoelectronic Devices and Systems of the Ministry of Education and Guangdong
Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
DOI: 10.1109/JPHOT.2014.2337875
1943-0655 Ó2014 IEEE. Translations and content mining are permitted for academic research only.
Personal use is also permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
Manuscript received May 23, 2014; revised June 27, 2014; accepted July 1, 2014. Date of publica-
tion July 10, 2014; date of current version August 11, 2014. This work was supported in part by the
National Science Foundation of China under Grants 61308027, 61377090, and 11174064; by the Sci-
ence & Technology Innovation Commission of Shenzhen under Grants KQCX20120815161444632
and JCYJ20130329140017262; and by the Distinguished Professors Funding from Shenzhen Uni-
versity and Guangdong Province Pearl River Scholars. Corresponding author: Y. Wang (e-mail:
ypwang@szu.edu.cn).
Abstract: We demonstrated a promising CO2laser irradiation system based on an im-
proved 2-D scanning technique. Such a system could be used to inscribe high-quality
long period fiber gratings (LPFGs) with good reproducibility of grating inscription, which
attributes to the fact that our system includes a CO2laser with an excellent power
stability of less than 2% and a 3-D ultraprecision motorized translation stages with an
excellent bidirectional repeatability value of 80 nm. Moreover, a control program with an
easy-to-use operation interface was developed in our system so that a high-quality
LPFG could be achieved as soon as grating parameters, such as grating pitch and
number of grating periods, are entered, which has a widespread commercial value and
prospects for development. Additionally, near mode fields of the CO2-laser-induced
LPFG were observed and simulated to investigate mode coupling in the gratings.
Index Terms: Long period fiber gratings (LPFGs), optical fiber sensors, CO2laser
2-D scanning, fiber optics components.
1. Introduction
Long period fiber gratings (LPFGs) have been widely used in the field of optical fiber sensors,
communications, and lasers. A few inscription methods, such as UV laser exposure [1], CO2la-
ser irradiation [2]–[5], electric arc discharge [6], femtosecond laser exposure [7], [8], mechanical
microbends [9], etched corrugations [10], [11], and ion beam implantation [12], [13], have been
demonstrated to inscribe LPFGs in different types of optical fibers. Among these methods, the
CO2laser irradiation method is particularly flexible and low cost, as it could be applied to in-
scribe LPFG in almost all type of fibers without using a phase mask [14], [15]. Since Davis et al.
reported the first CO2-laser-induced LPFG in a conventional glass fiber in 1998 [16], various
CO2laser irradiation techniques have been demonstrated and/or improved to inscribe LPFGs in
different types of optical fibers such as SMFs [17], [18], PCFs [19], [20], and PBFs [21]. In
2003, Rao et al., reported a typical CO2laser inscribing system in which an industrial 2-D opti-
cal scanner with a poor bi-directional repeatability was employed so that the precision of grating
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IEEE Photonics Journal LPFGs Inscribed With 2-D Scanning Technique

pitch was not good [22]. In addition, in the CO2laser irradiation systems reported, an industrial
CO2laser with a maximum output power of 10 W usually was employed to inscribe LPFGs.
However, such a CO2laser has a poor power stability of 10% so that the reproducibility of
LPFGs is not good. In other words, the output power of the CO2laser employed has to be finely
adjusted to achieve a high-quality LPFG during each grating inscription.
In this letter, we demonstrated a promising CO2laser irradiation system based on an im-
proved 2-D scanning technique for inscribing high-quality LPFGs. Such a system employs a 3-D
ultra-precision motorized translation stages with an excellent bi-directional repeatability of
80 nm, a CO2laser with an excellent power stability of less than 2%, and a control program
with a easy-to-use operation interface to inscribe high-quality LPFGs. Moreover, near mode
fields of the achieved LPFGs was observed to investigate their mode coupling.
2. LPFG Inscription Setup
A promising LPFG inscribing system based on an improved 2-D scanning technique was dem-
onstrated by use of a focused CO2laser beam, as shown in Fig. 1(a). This system consisted of
an industrial CO2laser with a maximum power of 10 W (SYNRAD 48-1) and a power stability of
10%, an electric shutter for turning on/off the laser beam, an infrared ZNSE PO/CX lens with a
focused length of 63.5 mm, a four-times beam expander for decreasing the diameter of the
focused laser spot, and a 3-D ultra-precision motorized stage (Newport XMS50, VP-25X and
GTS30V) with a minimum incremental motion of 10 nm and a bi-directional repeatability of
80 nm. A closed loop control system was, for the first time, employed to improve the power sta-
bility of the CO2laser to 2%, which is a huge advantage of our LPFG inscribing system Our
experiment results showed that the power stability (2%) of the CO2laser improved effectively
the stability and reproducibility of grating inscription. For example, the success rate of grating in-
scription is almost 100% in our current experiments. In contrast, the success rate was about
Fig. 1. (a) Schematic diagram of the LPFG inscribing system based on a 2-D scanning technique
employingaCO
2laser. (b) Easy-to-use operation interface of the control program. Z-dimension
of the 3-D stage is used to focus the laser beam on the fiber, and X- and Y-dimensions are used
to realize the 2-D scanning of the laser beam.
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30% in our previous experiments with a CO2laser with a power stability of 10% [22], [23]. A
supercontinuum light source (NKT Photonics SuperK Compact) and an optical spectrum ana-
lyzer (YOKOGAWA AQ6370C) were employed to monitor the transmission spectrum of the
CO2-laser-inscribed LPFG during grating inscription.
A control program with a easy-to-use operation interface was developed by use of LabVIEW
software in order to control every devices in the system and to inscribe high-quality LPFGs. As
soon as the grating parameters, such as grating pitch, number of grating periods, number of
scanning cycles, are entered via the operation interface illustrated in Fig. 1(b) and the “Write”
button is clicked, a high-quality LPFG could be achieved. Of course, the grating inscribing pro-
cess could be paused or stopped at any time by means of clicking the “Pause”button or the
“Stop”button. Hence, such an improved LPFG inscription system could potentially be integrated
with a fiber drawing tower to inscribe continuously a large number of LPFGs during drawing a fi-
ber, which has the widespread commercial value and the prospects for development.
Our LPFG inscription could be described as follow. First of all, one end of a standard single
mode fiber (YOF Inc) is fixed on the 3-D motorized stage by use of a pair of fiber holders, and
another end of the fiber is attached by a small weight to provide a constant pre-strain in the
fiber, thus enhancing the efficiency of inscribing LPFGs [24], [25]. The CO2laser beam propa-
gates through the beam expander and the lens and then is focused on the fiber by means of ad-
justing Z-dimension of the 3-D stage. We achieved the diameter of the focused spot by means
of observing the CO2-laser-ablated zone on the surface of the fiber. As shown in Fig. 2(c), a
Fig. 2. (a) Transmission spectrum evolution of a CO2-laser-inscribed LPFG with 30 grating periods and
a grating pitch of 320 m while the number of scanning cycles (K) increases from 1 to 7. (b) Micro-
scope image of the CO2-laser-inscribed LPFG. (c) CO2-laser-ablated zone on the surface of the fiber.
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groove was carved on one side of the optical fiber by repeated scanning of a focused CO2laser
beam with a higher power of 5 W. The width of the groove was measured to be 30 m. So the
diameter of the focused laser spot is about 30 m. To the best of knowledge, this is the smallest
focused spot in the LPFG inscribing system employing a CO2laser so far [22]. Second, the
motorized stage is moved by 1 mm with a speed of 0.5 mm/s along the “Y”direction, i.e.,
the vertical orientation of the fiber axis, in order that the focused CO2laser beam scans/
irradiates cross the fiber. Therefore, the first period of LPFG is created. Thirdly, the motorized
stage is shifted by a grating pitch, e.g., 320 m, along the “X”direction, i.e., the fiber axis, and
then moved by 1 mm along the “-Y”direction in order that the focused CO2laser beam scans/
irradiates cross the fiber again. Therefore, the second period of LPFG is created. This scanning
and shifting processes are periodically carried out N times (N is the number of grating periods)
until the last grating period is created. The process above may be repeated for K cycles from
the first grating period to the last grating period until a desired LPFG is achieved.
3. Experiment Results
As shown in Fig. 2(a), with the increase of the number of scanning cycles, the resonant wave-
length of the LPFG shifts toward the shorter wavelength, the resonant attenuation is increased,
Fig. 3. Measured transmission spectrum of the CO2-laser-inscribed LPFGs with 30 grating periods
and different grating pitches of (a) 260 m, (b) 280 m, (c) 300 m, (d) 320 m, (e) 340 m,
(f) 360 m, (g) 380 m, and (h) 400 m.
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and the 3 dB bandwidth of the resonant dip is decreased. One high-quality LPFG with a large
dip attenuation of 35.66 dB at the resonant wavelength of 1547.8 nm and a low insertion loss
of less than 0.3 dB was achieved in a standard single mode fiber after only seven scanning cy-
cles were done. As shown in Fig. 2(b), no obvious physics deformation was observed on the
surface of the grating. This is due to the fact that, during the grating inscription, the CO2laser
power was decreased to 0.5 W in order to avoid to induce physics deformation (i.e. groves) on
the surface of the fiber. So residual stress relaxation and glass densification are the possible
mechanisms for refractive index modulation in our CO2-laser-induced LPFGs [2]. In contrast,
physical deformation is the dominant mechanism for refractive index modulation in the asym-
metric LPFGs with periodic grooves (i.e. physical deformation).
In our system, the CO2laser beam is immovable, and the employed fiber is periodically
moved/shifted along the “X”and “Y”directions via the 2-D ultra-precision motorized stage with
an excellent bi-directional repeatability of 80 nm and a minimum incremental motion of 10 nm.
In contrast, in the system reported in reference [22], the fiber is fixed, and the CO2laser beam
periodically scans the fiber via an industrial 2-D optical scanner with a poor bi-directional repeat-
ability. Compared with our 2-D scanning technique, providing a common point-to-point tech-
nique is used to inscribe a LPFG, the CO2laser beam has to be aligned with and focused on
the fiber core during each inscription of grating period, which is a very difficult work and is of dis-
advantage to the stability and repeatability of grating inscription.
To investigate the phase matching condition as function of a resonant wavelength, eight
LPFGs with the same number of grating periods ðN¼30Þand different pitches of 260, 280, ...,
and 400 m were inscribed in the standard SMF by use of the improved CO2laser system
above. As shown in Fig. 3, each LPFG has a large dip attenuation of more than 33 dB at the
resonant wavelength and a low insertion loss of less than 0.5 dB, as well as more than three
attenuation dips for each LPFG are observed from 1100 to 1700 nm, indicating that the funda-
mental mode is coupled to different cladding modes. As shown in Fig. 4, the CO2-laser-inscribed
LPFG with a longer grating pitch has a longer resonant wavelength corresponding to the same
order cladding mode, which is the same as the phase matching condition of the UV-laser-
inscribed LPFGs illustrated in Fig. 8 reported in reference [1]. Therefore, we can inscribe a high-
quality LPFG with a desired resonant wavelength by mean of determining a suitable grating
pitch from the curve illustrated in Fig. 4.
As shown in Fig. 5, another four LPFGs, i.e. LPFG1,LPFG
2,LPFG
3,LPFG
4, with different grat-
ing pitch of 420, 380, 320, and 280 m, respectively, were inscribed in a standard SMF in order
toinvestigatemodecouplingintheCO
2-laser-inscribed gratings. A single-wavelength light from
Fig. 4. The measured resonant wavelengths versus the grating pitches of the CO2-laser-inscribed
LPFGs illustrated in Fig. 3.
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a tunable laser with a wavelength range from 1510 to 1612 nm (EXFO FLS-2600B) was input
into one end of each LPFG. Another end of the LPFG was cleaved at the last grating period to
observed its near fields by use of an infrared camera (Model 7290A, Electro Physics Corp.) and
a microscope (Leica DM2500 M). As shown in Fig. 5(b), asymmetrical mode field profile was ob-
served at the resonant wavelength of each LPFG. That is, the fundamental mode of LPFG1at
the resonant wavelength of 1530.8 nm, LPFG2at the resonant wavelength of 1570.0 nm, LPFG3
at the resonant wavelength of 1548.2 nm, and LPFG4at the resonant wavelength of 1593.8 nm,
was coupled into the circularly asymmetric cladding mode of LP12,LP
13,LP
14, and LP15,
respectively.
Moreover, it is easy seen from Fig. 5(b) that the cladding mode energy on one side is obvi-
ously larger than that on another side, that is, the cladding mode in the CO2-laser-induced
LPFG is asymmetrical within the cross section of the fiber cladding. This is due to the fact that,
during the LPFG inscription, an circularly asymmetric refractive index modulation within the
cross section of fiber is induced by the asymmetric residual stress relaxation resulting from the
single side irradiation of CO2laser [26]–[28].
We simulated the cladding mode field in a LPFG written in a standard SMF by use of a mode
solver (COMSOL version 3.5) based on the Finite Element Method (FEM). It has been found
that, in case the CO2laser irradiation induces a low refractive index modulation in the LPFG, a
linear, quadratic or exponential refractive-index profile assumed in the numerical simulations
Fig. 5. (a) Transmission spectra, (b) experimental, and (c) simulated near field profiles of the CO2-
laser-inscribed LPFGs at the resonant wavelength, i.e., LPFG1at 1530.8 nm, LPFG2at 1570.0 nm,
LPFG3at 1548.2 nm, and LPFG4at 1593.8 nm.
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results in a small quantitative difference, rather than a qualitative change, in the simulation re-
sults [26]. Thus we assumed a linear refractive index profile within the cross-section of the grat-
ing to simply the simulation of near field profiles of the CO2-laser-inscribed LPFGs. Assuming
refractive index within the cross-section of the grating is linearly modulated with a relationship of
n¼n0þð1X=2RÞn(n0is the cladding refractive index before CO2laser irradiation; nis
the amplitude of refractive index modulation after CO2laser irradiation; Xis the distance of CO2
laser irradiation and R is the fiber radius). For n¼0:5106, the simulated near mode filed
profile of the four LPFGs are illustrated in Fig. 5(c), which is similar to the experimental results
shown in Fig. 5(b). Hence, the circularly asymmetric mode field profiles shown in Fig. 5 experi-
mentally and theoretically verify that asymmetry refractive index modulation are induced within
the cross section of the CO2-laser-induced LPFGs. However, nonuniform absorption of laser
energy results in an asymmetrical refractive index profile within the cross-section of the grating,
which is more complicated than a simple linear profile. As a result, the simulated near filed pro-
files are somehow different from the observed ones.
4. Conclusion
ApromisingCO
2laser irradiation system based on an improved 2-D scanning technique was
demonstrated to inscribe high-quality LPFGs. Compared with other CO2laser inscribing sys-
tems, in our system the laser beam was fixed and the employed fiber was periodically moved
along X-direction and shifted along Y-direction so that the focused laser beam periodically
scans/irradiates the fiber. About 5 minutes were required to inscribe a high-quality LPFG with a
large attenuation dip of 35.7 dB, a bandwidth of 87.8 nm, and 30 grating periods in a standard
single mode fiber by use of our current experimental system with an improved power stability of
less than 2% and the 2-D scanning technique. In contrast, more time, e.g., about 30 minutes,
have to be required to inscribe a LPFG with a small attenuation dip of about 25.1 dB, and a
bandwidth of 12.0 nm and 55 grating periods in the same type of optical fiber by use of our pre-
vious experimental system with a poor power stability of less than 10% [22], [23]. Circularly
asymmetric mode field profiles indicates asymmetry mode coupling in the CO2-laser-induced
LPFGs. Moreover, a control program with a easy-to-use operation interface was developed;
therefore, our system has the widespread commercial value and the prospects for development.
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