Download Novel sol-gel material for fabrication of a long period waveguide

SCIENCE CHINA
Physics, Mechanics & Astronomy
• Research Paper •
November 2011 Vol.54 No.11: 1967–1971
doi: 10.1007/s11433-011-4463-1
Novel sol-gel material for fabrication of a long period waveguide
grating filter as a precise thermometer
WANG Xin1, LI XiaoChan2, ZHANG Tao2, HU CanDong1, ZHU QiuXiang1, CHEN SiHai3,
LI Yun1, XU Jia1, HE Miao1*, NIU QiaoLi1, ZHAO LingZhi1, LI ShuTi1 & ZHANG Yong1
1
Key Laboratory of Electroluminescent Devices of Guangdong Provincial Education Department, Institute of Optoelectronic Materials &
Technology, South China Normal University, Guangzhou 510631, China;
2
College of Information Science & Engineering, Wuhan University of Science & Technology, Wuhan 430081, China;
3
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science & Technology, Wuhan 430074, China
Received December 31, 2010; accepted January 25, 2011; published online August 15, 2011
A new kind of organic-inorganic hybrid HfO2/SiO2 sol-gel material with a large thermo-optic coefficient and a wide linear
tunable temperature range has been developed for fabrication of a long period waveguide grating (LPWG) filter, whose parameters were optimized and designed by using finite difference time domain (FDTD) simulations. The LPWG filter, a periodic rectangle-corrugated grating structure, was easily fabricated with soft-lithography technique. At a temperature range from
19°C to 70°C, the fabricated LPWG filter element demonstrated a high temperature sensitivity of about 6.5 nm/°C and a wide
linear tunable temperature range of 51°C, so that it can be used as a precise thermometer. Our results are useful for the designs
of LPWG filters for the implementation of a wide range of thermo-optic functions.
LPWG filter, soft-lithography, sol-gel material, temperature sensitivity
PACS: 42.79.Gn, 81.20.Fw, 42.79.Dj, 42.82.Cr
1
Introduction
Long-period grating (LPG) devices are useful in both optical communication and sensor fields. Much research efforts
have been devoted to the study of long-period fiber gratings
(LPFGs), which can transfer light energy from the guide
mode to the selected cladding modes of the fiber at specific
resonance wavelength. One of the most important applications of LPGs is temperature sensors [1]. But it is worth
nothing that the resonance wavelength of an ordinary LPFG,
written in a conventional communication fiber, shifts by
only 3–10 nm for a temperature change of 100°C , which is
not sufficient for many applications. Because of the geometry and material constraints of optical fibers, it is difficult to
improve the tunability of LPFGs [2,3]. To relax the con*Corresponding author (email: [email protected])
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
straints of fibers, LPGs formed in planar waveguides, referred to as long period waveguide gratings (LPWGs), have
been proposed [4,5]. Compared with the LPFGs, LPWGs
fabricated in planar waveguides have more advantages in
the structure fabrication and material choice, and the design
of the transmission spectrum is more flexible. There have
been a number of experimental demonstrations on fabrications of the LPWGs in different materials, such as glass [6],
lithium niobate [7], semiconductor [8], and polymer [4,9].
In addition, we have noticed that the sol-gel materials became more and more popular in waveguide material preparation, due to their low cost and simple processing steps.
Especially, the thermo-optic coefficients of the sol-gel materials are in general much larger than that of the other optical materials, so LPWGs in sol-gel materials may show
higher temperature sensitivity and a larger temperature tunable range. At present, the fabrication of LPWGs has been
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Wang X, et al.
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mostly limited to the costly semiconductor fabrication techniques such as reactive ion etching (RIE) and ultraviolet
(UV) writing before an alternative fabrication technique
utilizing the UV-curable sol-gel material, called soft-stamp
replica-molding lithography and shortened as soft-lithography, appeared [10]. In the soft-lithography technique, a
mechanically flexible and elastomeric Poly (di-methylsiloxane) (PDMS) mold stamp was used to transfer the desired pattern because its relatively low surface energy results in weak adhesion to other materials [11]. Because of
its convenience, effectiveness and low cost for the formation and manufacture of micro- and nano-structures, the
soft-lithography technique gradually plays an important role
in surface chemistry, materials science, optics, Micro-Electro-Mechanical Systems (MEMS) and microelectronics. In fact, soft-lithography techniques have been used
to fabricate optical devices, such as distributed feedback
structures [12], waveguides [13], micro lens arrays [14], and
optical resonators [15]. In this paper, we have reported on
the development of a new kind of organic-inorganic hybrid
HfO2/SiO2 sol-gel material with a large thermo-optic coefficient and a wide linear tunable temperature range, with
which a long-period grating has been fabricated on the surface of a planar waveguide core by using soft-lithography.
2
Analysis and design
In order to realize a resonance-wavelength-tunable long
period waveguide grating (LPWG) filter which can offer
higher tuning temperature sensitivity and much wider tunable temperature range, we have designed a multi-layer
structure as shown in Figure 1. Figure 1 shows the
cross-section of a designed LPWG filter, which is in fact a
periodic rectangle-corrugated planar waveguide. The grating structure of the filter is fabricated on the interface between the waveguide core and the upper cladding of the
planar waveguide, and the waveguide core is made from a
kind of organic-inorganic hybrid HfO2/SiO2 sol-gel material
with a large thermo-optic coefficient. When a light beam is
incident into and propagating along the z-direction in the
core, the propagation of the light wave will include core
modes and cladding modes according to the coupling mode
theory. The LPWG can couple the core modes and the cladding modes, and the couplings among the cladding modes
are so weak that they can be ignored. We can adjust the
geometry, sizes and refractive indexes of the core and the
claddings to allow only the fundamental core modes to exist
in the core so that only the coupling between the fundamental core modes and the first cladding modes needs to be
taken into account. Owing to the LPWG, the light energy
coupled from the fundamental core modes to the first cladding modes can be greatly intensified and reach maximum
when the phase-matching condition  co  1cl  2 /  is
November (2011) Vol. 54 No. 11
Figure 1
The cross-section of a LPWG filter.
fulfilled [16], where  co and 1cl are respectively the
propagation constant of the fundamental core modes and the
first cladding modes, and  is the grating period. On the
phase matching condition, the transmission spectrum of a
LPWG filter exhibits several resonance loss peaks, corresponding to different couplings. Thus the phase matching
condition can be also called as resonance condition and
more simply expressed as 0  ( N co  N1cl ), where 0 is
the response wavelength, N co and N1cl are respectively
effective refractive indexes of the fundamental core modes
and the first cladding modes, which vary with the temperature. Provided that the cladding is thick enough and the
small thermal expansion of the grating is ignored [17], the
temperament sensitivity of the resonance wavelength
d 0 / dT can be derived and expressed as：
d0
   C co  C cl 0co  1cl  ,
dT

 


1
 dN co dN1cl
1 

 d
d




,
(1)
where T is the temperature, C co and C cl are respectively
the thermal-optic coefficients of the core and the cladding,
and 0co and 1cl are the fraction powers of the guided
mode and the first cladding mode in the core. According to
eq. (1), the temperature sensitivity of the LPWG filter depends on the grating geometry, the modal dispersion parameter , and the thermal-optic coefficients of the core and
cladding materials.
In order to realize minimum transmission of a light wave
propagating through a LPWG filter, the parameters of the
waveguide and the LPWG need to be optimized and designed by using the FDTD simulations. The dependence of
resonance wavelength on the grating period can be calculated and illustrated in Figure 2, where the five lines represent five cladding modes, and obviously the resonance
wavelength is linear with the grating period. In addition, the
parameters should be chosen so that the core mode was
coupled to only the first cladding mode. With the FDTD
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Figure 2 The resonance wavelength vs. the grating period.
simulations, the optimized parameters were as follows. The
thicknesses of the core, the upper cladding and the lower
cladding are 1 m, 2 m and 2 m, respectively. The refractive indexes of the core, the upper cladding and the
lower cladding were 1.502, 1.496 and 1.496, respectively.
The depth, period, length and duty cycle of the LPWG were
respectively 86 nm, 450 m, 20 mm and 0.5.
3 Fabrication
The hybrid HfO2/SiO2 sol-gel material developed in this
study was prepared by uniform hydrolysis and subsequent
copolymerization of [3-(methacryloxy) propyl] trimethoxysilane (MAPTMS) and Hafnium (IV) in nitrogen atmosphere. Firstly, MAPTMS was hydrolyzed in isopropanol
and deionized water in a molar ratio of 1:2:2 with 0.03 M
hydrochloric acid as the catalyst, and stirred at 70°C for 30
min. Next, Hafnium (IV) (70 wt% solution in n-butanol)
was hydrolyzed in methacrylic acid in a molar ratio of 1:2
with 0.02 M acetic acid as the catalyst, and stirred at 40°C
for 40 min. Finally, these two solutions were mixed with a
Si/Hf molar ratio of 3:2, and the polycondensation of the
mixture was allowed to occur at room temperature with
vigorous stirring for 12 h. At last, 4 wt% of photo initiator,
a mixture of bis (2,4,6-trimethylbenzoyl)-phenylphosphine
oxide and 1-hydroxycyclohexyl phenylketone in a molar
ratio of 3:1, was added into the mixed sol solution to make
the final HfO2/SiO2 sol UV-sensitive. Refractive index of
the prepared sol-gel material can be adjusted by changing
the Si/Hf molar ratio.
The LPWG filter was fabricated by using soft-lithography technique including micro-contact printing and subsequent pattern transfer. Firstly, master structure with the desired grating pattern was patterned by conventional photolithography, and then the polydimethylsiloxane (PDMS)
liquid was poured onto the top of the master to cover it and
cured for 3 h at 50°C. After the thermal curing process, the
PDMS material, transformed into a silicone elastomer, was
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easily peeled off the master as a negative mold against the
master. Afterwards, a mechanically-flexible PDMS stamp
was formed and peeled from the master substrate, and the
elastomeric stamp having grating structure on its surface
was then used to transfer the pattern onto the UV-curable
hybrid HfO2/SiO2 sol-gel layer to form waveguide core, and
the replication process is illustrated in Figure 3.
Initially, a film in the prepared solgel material I was spincoated on the surface of a 1.5-m-thick SiO2 buffer layer of
a silicon substrate to be the lower cladding of the waveguide.
To improve the adhesion of the sol-gel film to silicon substrate, the sample was soft-baked on a hotplate at 80°C for 5
min to remove the excess solvent in the film, and then subjected to UV exposure for 5 min and heated on a hotplate at
150°C for 60 min. With a prism coupler (Metricon Corporation), the thickness and refractive index of the prepared solgel lower cladding layer were measured as 2 m and 1.496
at a wavelength of 1320 nm at room temperature. In a subsequent replication process, a film in the prepared sol-gel
material II was spin-coated onto the lower cladding to be
the waveguide core, and the thickness and refractive index
of the prepared core layer were measured as 2 m and 1.502
at 1320 nm at room temperature. Then the PDMS stamp
was placed and pressed on the sol-gel film so that the stamp
was kept in contact with the surface of the core layer film.
After the core layer was exposed upon UV radiation and
solidified, the PDMS stamp was mechanically removed off
so that the corrugated grating structure was printed on the
surface of the core layer. Afterwards, the sample was developed in acetone for 15 s, and the unilluminated zones
were washed away. Finally, a 2-m-thick film in the prepared sol-gel material I was spin-coated onto the grating
surface as the upper cladding. After the sample was subjected UV radiation and baked at 170°C for 3 h, the desired
LPWG filter was fabricated successfully.
4
Measurement results
Figure 4 shows the profile image of the cross-section of the
Figure 3
structure.
Schematic diagram of the replication process of the LPWG
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Figure 4 100× optical image of the cross-section of the fabricated rectangle-corrugated LPWG.
fabricated rectangle-corrugated LPWG taken by a chargecoupled device (CCD) microscopic camera (DP25, Olympus). It is noted that the fabricated element profile has strict
periodicity and steep wall, and the period is about 450 m,
in agreement with the design.
In order to measure the transmission characteristics of
the LPWG filter, a broadband infrared source was fed
through a polarizer and input into the LPWG filter. The
output light was collected through a 40× microscope objective, and the transmission spectra of LPWG filter for TE
polarization at different temperatures were measured and
recorded by using an optical spectrum analyzer (OSA). As a
result, a series of transmission loss peaks with a wavelength
range of 1260–1630 nm and a temperature range from 19°C
to 70°C were observed from the transmission spectra. The
temperature dependence of the resonance wavelength of
LPWG filter for the TE polarization is shown in Figure 5.
As shown in Figure 5, the resonance wavelength of LPWG
filter shifted toward longer wavelength as the temperature
increased. Because of the negative thermo-optic coefficient
of the material of the core and cladding, the effective refractive index of the core and cladding decreased while the
temperature increased, resulting in the shifts towards longer
wavelengths as per resonance condition. We can conclude
from the data in Figure 5 that the fabricated LPWG filter
had a temperature sensitivity of 6.5 nm/°C, much larger
than 0.8 nm/°C of tunable filters and 0.89 nm/°C of polymer
LPWG [18]. In addition, the fabricated LPWG filter had a
linear tunable temperature range of 51°C, wider than that of
the past work of others. Thus, the fabricated LPWG filter
can be used as a precise thermometer.
In summary, we have successfully demonstrated the fabrication of a LPWG filter based on a new kind of UV-tunable
sol-gel material using soft-lithography technique, and it had
a high temperature sensitivity and wide linear tunable temperature range so that it can be used as a precise thermometer. In addition, the sol-gel waveguide elements can be easily integrated with other optical components on the same
substrate to form new functional planar light wave circuits.
It should be mentioned that the fabrication procedure of the
LPWG filter is very simple and cost-effective so that it is
suitable for manufacture. Furthermore, the novel hybrid
organic-inorganic sol-gel materials have great applicability
for mass-production of other micro-optical elements.
This work was supported by the Natural Science Foundation of Guangdong
Province, China (Grant Nos. 8251063101000007, 10151063101000009
and 9451063101002082), the Scientific & Technological Plan of Guangdong Province (Grant Nos. 2008B010200004, 2010B010600030 and
2009B011100003), the National Natural Science Foundation of China
(Grant Nos. 61078046 and 10904042), the Key Project of Chinese Ministry
of Education (Grant No. 210157), the Scientific & Technological Project of
Education Department of Hubei Province (Grant No. D20101104), the
Fundamental Research Funds for the Central Universities (Grant No.
HUST 2010MS069), and Program for New Century Excellent Talents in
University, China (Grant No. 07-0319).
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Figure 5 Dependence of the wavelengths of LPWG transmission loss
peaks on temperatures.
Conclusions
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