Design of 50 G nonpolarizing dense wavelength
division multiplexer angle-tuning bandpass filter
Xianming Chen,
1
Junxian Ma,
1,
* and Yatao Yang
2
1
Advanced Technology Research Center, College of Information Engineering,
Shenzhen University, 3688 Nanhai Avenue, Shenzhen, Guangdong, 518060, China
2
JDS Uniphase (Shenzhen) Corporation, 3 Hongmian Road, Futian Free Trade Zone,
Shenzhen, Guangdong, 518060, China
*Corresponding author: majx@szu.edu.cn
Received 28 July 2010; accepted 16 August 2010;
posted 18 August 2010 (Doc. ID 132108); published 22 September 2010
Transmission characteristic differences of a narrow bandpass filter between p- and s-polarized light,
especially the central wavelength separation, will corrupt the performance of the filter when the inci-
dence is oblique. In this paper, by adding high-index materials asymmetrically to both sides of a
low-index spacer, which tunes the equivalent index of the spacer, the central wavelengths of the two
polarizations coincide perfectly when in 20° incidence; with different reflected layers on the two sides
of the spacers and replacing some reflected layers with equivalent layers, the 0:5 dB normalized
passband width is kept at 0:2 nm, which meets the requirement of the 50 G dense wavelength division
multiplexer filter. © 2010 Optical Society of America
OCIS codes: 310.0310, 310.6860, 060.4510, 230.7408.
1. Introduction
Recently, polarization-related problems have become
the crucial factors that restrict the development of
the angle-tuning narrow bandpass filter, which se-
lects the pass wavelength with incidence angle tun-
ing. Those problems are mainly manifested in the
fact that different polarizations, say p and s polariza-
tion, have different effective refractive indices, and,
as a result, the central wavelengths and bandwidths
of different polarizations will be different. Polariza-
tion-related problems are very important for optical
narrowband filters, which are one of the fundamen-
tal devices in the optics application area, especially
in optical communication. With the recent establish-
ment and commercial application of the dense wave-
length division multiplexer (DWDM) standard, a
100 GHz filter or even 50 GHz is required to be re-
searched and applied. According to the International
Telecommunication Union standard, 100 G and 50 G
DWDM filters require that 0:5 dB normalized
passband widths should be 0:4 nm and 0:2 nm,
respectively.
Among those methods of designing a narrowband
filter, multilayer thin film is regarded to have a sim-
ple structure, excellent reliability, and low cost. As a
result, its polarization-related problem has attracted
widespread concern [1,2]. Moreover, some methods of
improving performance have been brought forward.
To that end, the use of median-refractive-index ma-
terial as the spacer was proposed by Cushing [3]; the
maximum allowable incidence angle of the filter, in
which the central wavelength and bandwidths of
p-polarized refracted light coincide with that of s-
polarized light, is 20°. However, considering the
well-known fact that it is difficult to control a coating
machine with more than two materials, the above
proposal of using median-index material will face rig-
orous technique difficulties. Moreover, those median-
index materials actually rarely exist. Gu designed
one type of spacer structure that adds high-index
materials symmetrically to both sides of the low-
index spacer, so as to tune the equivalent index of
0003-6935/10/285271-05$15.00/0
© 2010 Optical Society of America
1 October 2010 / Vol. 49, No. 28 / APPLIED OPTICS 5271