Optics and Photonics Journal, 2011, 1, 106-109
doi:10.4236/opj.2011.13018 Published Online September 2011 (http://www.SciRP.org/journal/opj)
Copyright © 2011 SciRes. OPJ
Optical Properties of the Coupling Interface for Planar
Optical Waveguides
Yu Zheng1,2, Ji-An Duan1,2
1College of Mechanical & Electrical Engineering, Centr al S outh university, Changsha, China
2StateKey laboratory of High Performance and Complex Manufacturing, Changsha, China
E-mail: zhengyu@csu.edu.cn
Received July 15, 2011; revised August 18, 2011; accepted August 29, 2011
Abstract
Planar optical waveguides are the key elements in a modern, high-speed optical network. An important pro-
blem facing the optical fiber communication system, specifically planar optical waveguides, is coupling. The
current study presents a coupling model for planar optical waveguides and optical fibers. The various effects
of the optical properties of the coupling interface were analyzed by the scalar finite difference beam propa-
gation method, including the thickness, with or without the matching refractive index of the interface adhe-
sive. The findings can serve as a guide for planar optical waveguide packaging.
Keywords: Planar Optical Waveguide, Coupling Interface, Beam Propagation, Optical Property
1. Introduction
Planar optical waveguides refer to the fabrication and
integration of several optical components on a common
planar substrate, such as beam splitters, optical switchers,
variable attenuators, interleave filters, and wavelength
multiplexers [1,2]. Planar optical waveguides have the
advantages of low transmission loss, low connection loss
to optical fibers, compact size, high reliability, and high
reproducibility. They stand at the frontier of develop-
ment and can provide important support to the next gen-
eration of optical information technology.
Planar optical waveguide packaging involves bonding
dozens or even hundreds of optical channels, with cross-
sections of 4 × 4 μm to 8 × 8 μm, to single mode optical
fibers, with a core diameter of 8 - 9 μm, as shown in Fig-
ure 1. Different bonding methods to connect the planar
optical waveguides to the optical fibers, such as adhe-
sives, welding, soldering, metal oxide bonding, and an-
odic bonding, have been tested in planar optical wave-
guide packaging [1,3-5]. Among the bonding methods,
ultraviolet (UV)-cured adhesives offer several advan-
tages, such as low cost, strong bonding, and excellent
reliability.
UV-cured adhesives have two functions: to bond pla-
nar optical waveguides and optical fibers together, and to
act as a light propagation medium in optical paths [4]. As
a bonder, adhesives should provide strong and stable
adhesion, and maintain their mechanical properties at a
certain level while avoiding failure under all the condi-
tions in an optical fiber communication system. As a
light propagation medium, adhesives should be trans-
parent at the wavelength used for optical fiber commu-
nication and be able to combine a wide range of wave-
lengths and optical power with long-term stability. Un-
fortunately, the optical properties of the coupling inter-
face between a planar optical waveguide chip and the
optical fibers are sensitive to adhesive properties (e.g.,
adhesive thickness, refractive index, and shrinkage of
adhesive). Therefore, in this paper, the various effects of
the optical properties of coupling interface are analyzed
by the scalar finite difference beam propagation method,
including the thickness of the interface adhesive, with or
without the matching refractive index of the adhesive.
2. Scalar Beam Propagation Analysis
The beam propagation method is a particular approach
Input fiber array Output fiber array
Waveguide Chip
Adhesive
Figure 1. Schematic illustration of a planar optical wave-
guide packaging.
Y. ZHENG ET AL.107
for approximating the exact wave equation for mono-
chromatic waves and solving the resulting equation nu-
merically. For the monochromatic type of waves, such as

,, exp
x
yzi t
, in a three-dimensional waveguide,
the beam propagation method satisfies the scalar equa-
tion as follows [6,7]:

222 2
2
2222 ,, 0nxyz
xyzc


 
 (1)
where is the propagation direction of waves,
z
is
the angular frequency, is the refractive index
of the incident medium, 0 is in-
troduced for the spatially-dependent wave number, and
is the wave number in free space. If
,,nxyz
,,kxy

exp n

,,z knxyz
0
k

,,

,,
x
yz uxyzikz
, then is the con-
stant number to be chosen to make , where
is slowly varying.
n
k

,,uxyz z
22
dudz can then be neglected with
respect to ddku z. With this assumption and after a
slight rearrangement, the above equation is reduced to
the following:

22
22
22
2n
n
uiu u
kku
zkxy



(2)
The three propagation media in the coupling interface
between the planar optical waveguide chip and optical
fibers are the core of the optical fiber, adhesive, and core
of the planar optical waveguide chip. To facilitate calcu-
lation, only the electric field E is considered. The mag-
netic field H can be calculated in the same way as the
electric field E. The propagation field may be discon-
tinuous in the coupling interface. The boundary can be
given by:
12
12
EnEn
Dn Dn
 
 (3)
where is the normal direction of input light, 1 and
2 are the electric field of the first and second media,
respectively, and 1 and 2 are the electric flux den-
sity of the first and the second media, respectively.
nE
ED D
3. Results and Discussion
The light coupling between a planar optical waveguide
chip and an optical fiber can be calculated by the overlap
integral of the optical field distributions in a reference
plane. As the goal of the present study is to understand
the properties of the coupling interface for planar optical
waveguide packaging, a sample model, as shown in Fig-
ure 2, is used to investigate them. The proposed model
presents the majority case in planar optical waveguide
packaging. It consists of a single mode fiber, a regular
square-type planar optical waveguide cross-section, and
an adhesive with the thickness of d in between. The re-
Fiber Core AdhesiveIntegrated Photonic Chip Core
d
Figure 2. Model for the optical fiber-planar optical wave-
guide coupling simulation.
fractive index of the fiber core and cladding is 1.4494
and 1.445, respectively, at a wavelength of 1.31 μm (Δ =
0.3%). The refractive index of the planar optical wave-
guide core and cladding is 1.4486 and 1.445, respectively,
(Δ = 0.25%) [2,8]. The diameter of the fiber core is 8.2
μm, and the planar optical waveguide core dimension is
8 × 8 μm. The adhesive has excellent transmittance at a
wavelength of 1.26 - 1.65 μm, which is the optical com-
munication wavelength.
The coupling loss at the coupling interface is calcu-
lated using the scalar BPM algorithm. The coupling loss
in optical power through a connection is defined simi-
larly to that of signal attenuation through a waveguide.
Coupling loss is also a log relationship. The coupling
loss (C.L.) in optical power through a connection is de-
fined as:
10
.. 10i
o
P
CLLog P
 (4)
Po is the power emitted from the source optical
waveguide in a connection. Pi is the power accepted by
the connected optical waveguide. In any fiber optic con-
nection, Po and Pi are the optical power levels measured
before and after the joint, respectively.
3.1. Refractive Index and Thickness of Adhesive
Figure 3 shows the coupling loss of an optical fiber and
a planar optical waveguide chip with respect to the re-
fraction index of the adhesive for different adhesive
thicknesses. The present study assumes that no lateral
misalignment and angular misalignment exist between
the optical fiber and the planar optical waveguide chip.
Moreover, the propagation wavelength is considered
1.31 and 1.55 μm (optical communication wavelength).
In Figure 3(a), at a wavelength of 1.31 μm, the coupling
loss is less than 0.12 dB for the adhesive thickness of
less than 9 μm. When the adhesive thickness is greater
than 9 μm, the slope of the variation is large. In Figure
3(b), at a wavelength of 1.55 μm, the coupling loss is
less than 0.01 dB for various adhesive thicknesses, and
the slope of the variation is small.
As shown in Figure 3(a) and (b), a large adhesive
Copyright © 2011 SciRes. OPJ
Y. ZHENG ET AL.
108
1.0 1.2 1.4 1.6
0.04
0.08
0.12
0.16
0.20
Coupling Loss /dB
Refractive Index of Adhesive
d=0m
d=1.5m
d=3.0m
d=4.5m
d=6.0m
d=7.5m
d=9.0m
d=10.5m
d=12.0m
d=13.5m
d=15.0m
nm
(a)
1.0 1.2 1.4 1.6
0.04
0.08
0.12
0.16
Coupling Loss /dB
Refractive Index of Adhesive
d=0.0m
d=1.5m
d=3m
d=4.5m
d=6m
d=7.5m
d=9m
d=10.5m
d=12m
d=13.5m
d=15.0m
n
m
(b)
Figure 3. Calculated coupling loss vs. index and thickness of
adhesive. (a) λ = 1.31 μm; (b) λ = 1.55 μm.
thickness leads to a high coupling loss; upon increasing
the adhesive thickness from 0 to 15 μm, the coupling loss
increases about 60%. Figure 3 shows that coupling loss
is not equal to 0 when the adhesive thickness is 0. This
finding is due to the non-matching mode fields of the
optical fiber and the planar optical waveguide chip; the
optical fiber core is round, whereas the planar optical
waveguide chip is square. The coupling loss of an optical
fiber and a planar optical waveguide chip is no more than
0.1 dB per coupling point. Thus, controlling the adhesive
thickness at less than 10 μm is recommended. Further-
more, coupling loss is not sensitive to the refractive in-
dex of the adhesive if the adhesive thickness is less than
10 μm.
3.2. Propagation Wavelength
Figure 4 shows the relationship between propagation
wavelength and coupling loss with different refractive
indexes of the adhesive when the adhesive thickness is 5
and 9 μm. As the adhesive thickness increases, so does
the coupling loss. Wavelength-dependent loss (WDL) is
linked to the uniformity of coupling loss in the consid-
ered spectral bandwidth. It is defined as the difference in
dB between the maximum and minimum values of cou-
1.2 1.3 1.4 1.5 1.6 1.7
0.05
0.06
0.07
0.08
0.09
Coupling Loss /dB
Propagation Wavelength /m
RI=1.000
RI=1.075
RI=1.150
RI=1.225
RI=1.300
RI=1.375
RI=1.450
RI=1.525
RI=1.600
d=5m
(a)
1.2 1.3 1.41.5 1.6 1.7
0.07
0.08
0.09
0.10
0.11
0.12
Coupling Loss /dB
Propagation Wavelength /m
RI=1.000
RI=1.075
RI=1.150
RI=1.225
RI=1.300
RI=1.375
RI=1.450
RI=1.525
RI=1.600
d=9m
(b)
Figure 4. Calculated coupling loss vs. propagation wave-
length. (a) Adhesive thickness d = 5 μm; (b) Adhesive thick-
ness d = 9 μm.
pling loss due to the variation of the propagation wave-
length. WDL is calculated as follows:
WDL (dB)= CLmax (dB) CLmin (dB) (5)
Figure 4(a) shows that for the adhesive thickness of 5
μm, the WDL is no more than 0.09 dB, whereas the re-
fractive index of the adhesive changes from 1.00 to 1.60.
When the adhesive thickness is 9 μm, as shown in Figure
4 (b), the WDL is no more than 0.12 dB, whereas the
refractive index of the adhesive changes from 1.00 to
1.60. According to GR-1221-CORE, the coupling loss
changes no more than 0.2 dB over a full optical spectrum
from 1.260 - 1.625 μm [9]. This finding indicates that the
refractive index of the adhesive has little effect on the
WDL of the planar optical waveguides.
4. Conclusions
The various effects of the optical properties of the cou-
pling interface between a planar optical waveguide and
optical fibers are analyzed by the scalar finite difference
beam propagation method, including the thickness of the
interface adhesive, with or without the matching refrac-
tive index of the adhesive. The following can be con-
Copyright © 2011 SciRes. OPJ
Y. ZHENG ET AL.
Copyright © 2011 SciRes. OPJ
109
cluded:
A large adhesive thickness leads to high coupling loss.
Upon increasing the adhesive thickness from 0 to 15
μm, the coupling loss increases about 60%.
The coupling loss of an optical fiber and a planar op-
tical waveguide chip is no more than 0.1 dB per cou-
pling point. Thus, controlling the adhesive thickness
at less than 10 μm is recommended.
The refractive index of the adhesive has little effect
on the wavelength-dependent loss of planar optical
waveguides when the refractive index of the adhesive
changes from 1.00 to 1.60.
5. Acknowledgements
This work was supported by the National Natural Sci-
ence Foundation of China (Grant No. 50735007 and No.
51075402), and the National High-Tech Research and
Development Program of China (Contract No. 2007
AA04Z344).
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