Design and Synthesis of a 33-Layer Narrow Bandpass Filter with Graded Refractive Index Profiles
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ABSTRACT A novel bandp ass filter consisting of graded refractive index profiles and a three-cavity stack was designed. It exhibits good optical characteristics, i.e., high transmittance (>99%) at the designated wavelength of 1550±25 nm and high reflectance (>99%) in wavelength regions of 1400+ 50 nm and 1780± 100 nm. This designed 33-layer TiO 2-SiO
2
film with graded refractive
index profiles was prepared by helicon plasma sputtering. The measured transmittance spectrum exhibited good agreement with the designed spectrum. The microstructure of this multilay er film was investigated using electron microscopy. INTRODUCTION Fiber communication characterized by high speed and high capacity requires the development of dense wavelength division multiplexing(DWDM) technology. The high-quality optical filter is one of the key devices in such technology, so its design and fabrication have attracted considerable interest. Much research has been done on the development of high performance bandpass filter to date. 1-4 To achieve the desired optical characteristics, however, it is generally necessary to design
a normal cavity filter with more than one hundred of layers. For such a design, it is difficult to adjust the pass bandwidth and to obtain high transmittance at the pass band. Previously, we successfully designed and fabricated a new reflection filter using the concept of functionally graded materials (FGMs) that was initially proposed for the design of structural materials.' The reflection filter had a structure including graded refractive index profiles and effectively suppressed the sidelobes outside the stopband. 6.7 In the present study, we designed and synthesized a novel narrow bandpass filter combining a three-cavity filter and graded refractive index profiles. DESIGN OF OPTICAL MULTILAYER FILTER The general formula of the above-mentioned filter with a three-cavity structure and graded refractive index profiles is as follows: Substrate(BK7)ILABCDECFBG(xL)H(LH) m (yL)H(LH)m (zL)GAFBEDCBAL[Air
63 Mat. Res. Soc. Symp. Proc. Vol. 597 © 2000 Materials Research Society
(1)
where H and L are
Table I Data of TiO 2-SiO 2 composite films. No
Composition (mol:mol)
refractive deposition rate index (nm/min)
TiO 2 and SiO 2 films, respectively, and A, B, C, D, E, F and G are TiO 2-SiO 2 composite films, all with quarter-wave optical
thickness as shown in Table 1. "xH", "1'yH" and "zH" represent H layers
H
TiO 2
2.39
0.25
G
0.85TiO 2 -0.15SiO 2
2.28
0.30
F
0.75TiO 2 -0.25SiO 2
2.17
0.35
E
0.69TIO 2-0.31SiO 2
2.04
0.41
D
0.58TiO 2-0.42SiO 2
1.93
0.49
C
0.43TiO 2-0.57SiO 2
1.81
0.66
m times. The stacks of LABCDE and EDCBAL represent graded
B
0.32TiO 2-0.68SiO 2
1.71
0.58
refractive
A
0.19TiO 2-0.81SiO 2
1.59
0.46
L
SiO 2
1.47
with x, y and z times of optical thickness. "(LH)".. is a stack with alternating L and H layers repeated
index
multilayer
structures. For a given multilayer optical 0.36 film with N layers, the transmittance with incident wavelength at incident angle of
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