JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

4-04
Polymer Optical Waveguides Fabricated by Using
Proton Beam Writing
K. Miura a) , Y. Machida a) , M. Uehara a) , O. Hanaizumi a) , Y. Ishii b) , T. Satoh b) , K. Takano b) ,
T. Ohkubo b) , A. Yamazaki b) , A. Inouye b) , M. Kouka b) , A. Yokoyama b) and T. Kamiya b)
a) Graduate School of Engineering, Gunma University,
b) Department of Advanced Radiation Technology, TARRI, JAEA
Proton beam writing (PBW) has recently attracted much
attention as a next-generation micro-fabrication technology.
It has many advantages compared with other technologies
such as electron beam (EB) writing. PBW facilitates
three-dimensional processes and has higher throughput than
1-3)
EB writing . Planar polymer optical waveguides are
becoming increasingly important recently in the field of
optical fiber communication. In this study, we proposed
and fabricated straight polymer waveguides (shown in
Fig. 1) by using PBW, and obtained single-mode
waveguides working at a wavelength of 1.55 μm.
An SiO2 film was deposited as an under-cladding on an
20 mm) by using radio-frequency
Si substrate (20 mm ×
sputtering. A PMMA (refractive index ~1.48) film was
spin-coated on the SiO 2 film at 1,350 rpm for 30 sec. The
sample was then baked at 120 C for 2 min. We repeated
these processes twice, and the PMMA film became 10 μm
thick. Straight waveguides were drawn by using the 3 MV
single-ended accelerator in TIARA. The proton irradiation
energy was 1.7 MeV, the proton beam current was 60 pA,
the dose was 100 nC/mm 2 , and the beam size was about
1 μm . We drew seven waveguides having widths of 4, 6,
8, 10, 12, 14, and 16 μm on the PMMA film. A 10-μm
thick PMMA film was deposited again on the sample as an
upper-cladding by spin-coating under the same condition as
the first PMMA layer. We cleaved both sides of the
sample to observe near field patterns (NFPs) of the
waveguides. We used a wavelength-tunable laser (Santec
Ltd., ECL-210) for our NFP measurements. The laser
wavelength was 1.55 μm, and the light was introduced
through a single-mode fiber (SMF) to edges of the
waveguides. We observed their NFPs by using a vidicon
camera (Hamamatsu Photonics Ltd., C2741-03).
Figure 2 presents the NFP image of the 8-μm width
waveguide. Its mode field diameter (MFD) was almost
10 μm, which is almost the same as the MFD of the SMF
(vertical size ~9.8 μm, horizontal size ~10.5 μm). It is
therefore considered that the coupling losses between
fabricated waveguides and SMFs are low. We regarded it
as a single-mode waveguide because no higher-order mode
was observed when the excitation condition was changed.
All waveguides were evaluated by the same method, and we
found that the waveguides having widths above 12 μm were
multimode waveguides. We assume that the proton beam
penetrated though the PMMA and SiO 2 films to the Si
substrate 4) , and all waveguides work by the difference of
refractive indices between the two proton-irradiated films.
JAEA-Review 2010-065
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We thus succeeded in fabricating straight polymer
waveguides by using PBW. We are planning to measure
the refractive indices of proton-irradiated PMMA and SiO 2
films, and to fabricate Y-junction waveguides and
thermo-optic polymer switches based on Mach-Zehnder
interferometer waveguides.
References
1) A. A. Bettiol et al., Nucl. Instrum. Meth. Phys. Res. B
231 (2005) 364.
2) F. Watt et al., Materials Today 10 (2007) 20.
3) N. Uchiya et al., Nucl. Instrum. Meth. Phys. Res. B 260
(2007) 405.
4) I. Rajta et al., Nucl. Instrum. Meth. Phys. Res. B 260
(2007) 400.
PMMA
PMMA
Core
(PBW)
SiO 2 ~15 m m
Substrate (Si)
10 m m
10 m m
Fig. 1 Schematic figure of the PMMA waveguide
utilizing the PBW technology.
10 m
Fig. 2 Observed NFP of the 8-μm width waveguide.