Full text of DOI:10.1557/JMR.2000.0044

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Fabrication of a 33-layer optical reflection filter with stepwise
graded refractive index profiles
Xinrong Wang and Hiroshi Masumoto
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-Ku,
Sendai 980-8577, Japan
Yoshihiro Someno
Mechatronic Devices Division 1, ALPS Electric Co. Ltd., 6-1 Aza, Nishida Kakuda, Kakuda,
Miyagi Prefecture, 981-1505, Japan
Lidong Chen and Toshio Hirai
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-Ku,
Sendai 980-8577, Japan
(Received 3 May 1999; accepted 10 November 1999)
A novel optical multilayer filter characterized by symmetrically stepwise graded
refractive index profiles and high optical performance was designed. This 33-layer
TiO2–SiO2 filter was fabricated by helicon plasma sputtering. The prepared filter
exhibited a sharp cutoff reflection band around the central wavelength of 1340 nm and
wide pass regions with high transmittance. These results correspond well with
calculated estimations. Symmetrically stepwise graded refractive index profiles were
demonstrated to effectively contribute to suppression of the sidelobes and to be
potentially applicable in the design of other filters.
Dense wavelength division multiplexing (DWDM)
technology is essential in high-speed and large-capacity
optical fiber communication. Communication capability
can be enhanced by development of the DWDM system
in which many channels of different wavelengths of
light are combined in a single fiber. In general, the dif-
ferent wavelengths of light in the regions around 800,
1300, and 1500 nm are used for propagation of optical
signals in the short-, medium-, and long-distance areas,
respectively, because of the low loss in the windows of
glass located in these near-infrared regions.1 Therefore,
development of optical devices used in the above wave-
length regions has attracted considerable interest. An
optical filter is an essential component of the DWDM
system. To effectively separate channel waves, it is nec-
essary to develop filters that exhibit high selectivity in
the bandwidths in a certain wavelength area. To date,
there have been many reports on theoretical analysis and
experimental fabrication of optical filters.2–6 For the re-
flection filter, quarterwave stacks with alternating high-
and low-refractive index films are used as a filter with a
reflection band in specific wavelengths; an advantage of
such multilayer films is their ease of manufacture.7–9
However, the sidelobes attached to the simple quarter-
wave stacks detract from the transmission in the pass
regions. To solve this problem, an effective theoretical
design termed “rugate filter” has been developed in the
past decade. According to theoretical calculations, com-
plex rugate filters can achieve excellent performance in
reflection filters.10–12 Consequently, some experimental
approaches to fabrication of rugate filters have been
carried out by various processes such as electron-beam
co-evaporation,13,14 ion-beam-assisted deposition,14,15
the sol-gel method,16 and plasma-enhanced chemical va-
por deposition.17 However, it is difficult to fabricate ru-
gate filters because they require a continuous change of
refractive index in a sinusoidal or more complicated
manner with a large number of periods in the direction of
thickness.
To obtain reflection filters with high performance and
more reproducible films for practical applications, we
previously proposed a novel design of optical reflection
filters with stepwise graded refractive index profiles and
prepared a 31-layer TiO2–SiO2 multilayer filter with
such profiles, having a reflection band around 730 nm.18
However, there have been few reports on high-
performance optical multilayer filters with a central
wavelength in the regions around 1300 and 1500 nm.
Here we present a new design of optical reflection filter
with a central wavelength of 1340 nm based on our pre-
vious research. This is a 33-layer TiO2–SiO2 multilayer
film with symmetrically stepwise graded refractive index
profiles. We present experimental results of this film fab-
ricated by helicon plasma sputtering.
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We designed the filter with a configuration expressed
as LABCDECFBGAH(LH)5LGAFBEDCBAL, where L,
A, B, C, D, E, F, G, and H are the layers with an optical
thickness of a quarterwave and with the indices of 1.47,
1.58, 1.70, 1.82, 1.93, 2.04, 2.15, 2.27, and 2.39 (see
Table I), respectively. The thickness dependence of the
refractive index of the 33-layer TiO2–SiO2 multilayer
film is shown in Fig. 1(a). The total thickness of the
multilayer film was designed to be 6122 nm. In Fig. 1(a),
the part labeled W shows a central quarterwave stack
with simple high- and low-indices—i.e., a TiO2–SiO2
multilayer stack—the parts labeled S consist of the layers
with graded refractive indices, and the parts labeled M
are composed of the layers with an amplitude modulation
of refractive index difference. In the design, the central
quarterwave multilayer stack (W) performs high reflec-
tance of the reflection band. To suppress the sidelobes
that accompany the simple quarterwave stacks and de-
tract from the transmission in the pass regions, two parts
(M) and two symmetrical stack parts (S) with a multi-
layer stack (W) were introduced into the structure of the
filter. The calculated results of the transmittance spectra
for the designed filters indicated that only the M layers
placed on either side of a central quarterwave stack can
decrease the ripple near the reflection band, but they
cannot eliminate the sidelobes far from the stop band.
The symmetrical S parts placed on either side of a central
quarterwave stack can remarkably reduce the sidelobes
in the large wavelength range outside the reflection band.
It does not, however, markedly suppress the ripples near
the stop band. The symmetrical layers with graded re-
fractive indices, in combination with the layers that
modify the index difference on either side of a central
quarterwave stack, can play a significant role in elimi-
nating the sidelobes outside the reflection band. We in-
vestigated the effect of stepwise graded refractive index
profiles with linear and quadratic functions on the optical
filter characteristics, we analyzed the optical character-
istics of filters with some nonsymmetrical stepwise
graded refractive index profiles. The results showed that
FIG. 1. (a) Relationship between refractive index and thickness of a
designed 33-layer TiO2–SiO2 multilayer film with stepwise graded
refractive index profiles and a central wavelength of 1340 nm and (b)
transmittance spectra of a designed 33-layer multilayer film (solid
line) and a simple 33-layer H(LH)16 film (dotted line) with the same
central wavelength of 1340 nm. W, TiO2–SiO2 stack layers; M, layers
with an amplitude modulation of refractive index difference; S, layers
with graded refractive indices.
the above filter with the refractive index profiles of a
linear function is optimal. Detailed results will be re-
ported elsewhere.19
TABLE I. Data of TiO2, SiO2, and TiO2–SiO2 composite films.
Deposition
The transmittance spectrum of the designed filter un-
der an incident light at an incident angle of 0° was cal-
culated by means of the reflection equations of optical
multilayer films.20 The refractive indices of a BK7 sub-
strate (n ס 1.52) with a thickness of 1 mm and of the
reference films in the TiO2–SiO2 system (L, A, B, C, D, E,
F, G, and H, respectively) were used in the calculation.
The transmittance spectrum of the designed 33-layer
TiO2–SiO2 multilayer film with stepwise graded refrac-
tive index profiles and a central reflection wavelength of
1340 nm is shown by a solid line in Fig. 1(b). For com-
Composition
(mol:mol)
Refractive
index
rate
(nm/min)
No.
H
G
F
E
D
C
B
A
L
TiO2
2.39
2.27
2.15
2.04
1.93
1.82
1.70
1.58
1.47
0.27
0.31
0.35
0.41
0.49
0.63
0.55
0.44
0.34
0.88TiO2–0.12SiO2
0.78TiO2–0.22SiO2
0.69TiO2–0.31SiO2
0.58TiO2–0.42SiO2
0.45TiO2–0.55SiO2
0.31TiO2–0.69SiO2
0.18TiO2–0.82SiO2
SiO2
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parison, the transmittance spectrum of a simple alternat-
SiO2, and all TiO2–SiO2 composite films in the multi-
layer have an amorphous structure of a microstructure
containing nanoscale crystalline of TiO, in the amor-
phous TiO2 layers similar to other TiO2–SiO2 multilayer
films.21
Transmittance spectrum of the prepared 33-layer
TiO2–SiO2 multilayer film with stepwise graded refrac-
tive index profiles is shown as a solid line in Fig. 3. For
purposes of comparison, the transmittance spectrum of
the designed 33-layer TiO2–SiO2 multilayer film is also
shown as a dotted line in Fig. 3. The determination was
carried out with a Shimadzu UV-3101PC scanning spec-
trophotometer. In the measured transmittance spectrum,
the reflectance of the reflection band of the multilayer
ing 33-layer TiO2–SiO2 multilayer film [H(LH)16] with
the same central reflection wavelength of 1340 nm is also
shown by a dotted line in Fig. 1(b). For this film with
stepwise graded refractive index profiles, the reflectance
of the reflection band around 1340 nm is up to 99.89%,
being a full width at half-maximun (50% transmittance)
of about 460 nm from 1148 to 1610 nm. For the reflec-
tion band, the wavelength region with a reflectance of
more than 99.0% is from 1198 to 1520 nm. For the
transmission region, there is a high transmittance of
about 90% in the wavelength ranges from 800 to
1100 nm and from 1640 to 2200 nm. It is clear that the
designed 33-layer multilayer filter with stepwise graded
refractive index profiles can significantly eliminate the
sidelobes outside the reflection band in comparison with
the simple 33-layer alternating quarterwave stack filter.
The deposition of thin films was performed with a
helicon plasma sputtering system with a pair of helicon
cathodes. The chamber was evacuated below a baseline
vacuum level of 5 × 10−5 Pa before the deposition. In the
deposition, BK7 glass and Si (100) substrates were used
in rotation at room temperature. The ambient gas pres-
sure of the system was 1.8 × 10−1 Pa. Ar and O2 were
used as the sputtering gas and the reactive gas with par-
tial pressures of 1.2 × 10−1 Pa and 6.0 × 10−2 Pa, respec-
tively. First, TiO2–SiO2 composite films with various
refractive indices were synthesized by changing the radio
frequency powers of the TiO2 and SiO2 targets in the
respective cathodes. Their thickness and refractive indi-
ces were measured at 633 nm with a Gaertner L116-B
ellipsometer. The deposition rates, refractive indices, and
composition of TiO2–SiO2 system films are shown in
Table 1. Consequently, the designed 33-layer TiO2–SiO2
filter films were deposited simultaneously on BK7 and Si
(100) substrates under the same conditions as those of the
reference films.
FIG. 2. TEM micrograph of the cross-sectioned structure of a 33-layer
TiO2–SiO2 film with stepwise graded refractive index profiles and a
central wavelength of 1340 nm. W, TiO2–SiO2 stack layers; M, layers
with an amplitude modulation of refractive index difference; S, layers
with graded refractive indices.
Transmission electron microscopy (TEM) observation
of a cross-sectional specimen of the multilayer film was
performed with a JEOL-2000FX electron microscope.
Figure 2 shows a TEM micrograph of a cross-sectional
33-layer TiO2–SiO2 film on a Si (100) substrate. The
multilayer film has a total thickness of about 6120 nm. In
the TEM image, the darker bands are TiO2 layers or
TiO2-rich layers, and the brighter bands are SiO2 layers
or SiO2-rich layers. In Fig. 2, the W, M, and S parts can
be seen to correspond with those in Fig. 1(a). In the
microstructure of the S part with stepwise graded refrac-
tive index profiles, the brightness of the multilayer film
gradually decreases with the increase of the refractive
index of the layers—i.e., increasing TiO2 content in the
layers. The interfaces between the layers are well de-
fined. This indicates that smooth surfaces of TiO2, SiO2,
and TiO2–SiO2 composite films were formed during the
deposition. X-ray diffraction analysis revealed that TiO2,
FIG. 3. Calculated and experimental transmittance spectra of a 33-
layer TiO2–SiO2 film with stepwise graded refractive index profiles
and a central wavelength of 1340 nm on a BK7 substrate.
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film was more than 99.0% in the wavelength region from
ence, Sports and Culture, Government of Japan. Part of
this work was supported by a Grant-in Aid in Promotion
of Research of Advanced Materials (No. H1001p) from
the Laboratory for Developmental Research of Advanced
Materials, the Institute for Materials Research, Tohoku
University. We wish to thank Mr. E. Aoyagi, Mr.
U. Hayasaka, and Mr. S. Ito at the Institute for Materials
Research, Tohoku University, for the TEM observations.
1208 to 1518 nm, in which the reflectance is up to
99.92% at the central wavelength of 1340 nm. The full
width at half-maximum is about 480 nm from 1136 to
1614 nm, well consistent with the designed values.
Moreover, the experimental transmittance spectrum also
exhibited high transmittance in the transmission wave-
length region outside the reflection band because of sup-
pression of the sidelobes as in the case of the calculated
spectrum. There is one peak of small reflection near
1682 nm outside the reflection band in the experimen-
tal spectrum, which may be explained as being due to
random error of deposition rate. In general, however, it
is indicated that the measured transmittance spec-
trum corresponds well with the calculated results de-
scribed above.
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Moreover, it was demonstrated to be simpler and more
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ACKNOWLEDGMENTS
Part of this work was supported by a Grant-in-Aid
for Scientific Research (A) (No. 07405029) and a
Grant-in-Aid for Scientific Research on Priority Areas
(No. 08243102) from The Ministry of Education, Sci-
J. Mater. Res., Vol. 15, No. 2, Feb 2000
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