Optics and Photonics Journal, 2013, 3, 158-161
doi:10.4236/opj.2013.32B038 Published Online June 2013 (http://www.scirp.org/journal/opj)
Design of Thermo-Optic Variable Optical Attenu ator
Based on Quartz Substrate
Hongqing Dai1, Junming An1,2, Liangliang Wang1, Yue Wang1, Liyao Zhang1,
Jiashun Zhang1,2, Hongjie Wang1,2, Pan Pan1, Xiaogu ang Z hang1,
Ruidan Liu1, Jianguang Li1,2,Yuanda Wu1,2, Xiongwei Hu1
1Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China
2Henan Shijia Photons Technology Co., Ltd, Hebi, China
Received 2013
ABSTRACT
In this paper, we designed a thermo-optic variable optical attenuator (VOA) based on quartz substrate, which consists of
a Mach-Zehnder interferometer (MZI) and a thin film heater above the phase-modulation arm. The transmission proper-
ties of the waveguide and attenuation characteristics of the device have been simulated by beam propagation method
(BPM), and the simulated results illustrated that the designed VOA had good performance.
Keywords: VOA; MZI; Thermo-optic; Quartz Substrate
1. Introduction
Variable optical attenuator (VOA) is one of the most
basic optical passive devices in modern optical network,
with a wide range of applications in optical communica-
tion and its main function is to control and attenuate op-
tical signal power. With the application of dense wave-
length division multiplexing (DWDM) system, to realiz-
ing optical signal propagation in high speed correctly, we
need to monitor and balance the multi-channel optical
power, resulting in dynamic channel equalizer (DCE),
variable optical attenuator integrated multiplexer/demul-
tiplexer (VMUX), optical add-drop multiplexer and other
optical devices. VOA is the core component of these
devices and plays a very important role in the gain con-
trol of linear-repeaters for wavelength division multi-
plexed networks and channel power equalization in
wavelength division multiplexing (WDM) cross-connect
nodes. The study of PLC-VOA has just started in our
country, but the demand for VOAs is larger and larger
with the progress of optical communication. Only the
new optical attenuators of low cost, easy integration, and
good performance can meet the growing market demand
[1,2].
SiO2 material has a lot of advantages compared with
other materials that are used in the manufacture of optical
waveguide devices from early times. The coupling loss
of optical waveguide devices produced between SiO2
planar waveguide and single-mode fiber is quite low. The
transmission loss of the optical signal in SiO2 material is
only 0.2dB/km. The SiO2 material has certain thermo-
optic coefficient and thus can be used for the modulation
of refractive index. The devices based on SiO2 materials
can be directly grown on Si or quartz substrate, which
can be integrated with other Si based waveguide devices.
The SiO2 material also has great environment stability.
Internationally, most of the VOAs have been fabricated
were based on the Si substrate and there was an under-
cladding layer between the core and the substrate. In this
paper, we designed a Mach-Zehnder interferometer (MZI)
thermo-optic variable optical attenuator based on quartz
substrate. It eliminated the step of the fabrication of the
underclading layer, simplifying the manufacture process.
The core and the substrate could match better because
they were composed of the same material of SiO2. Re-
sults of the simulation of the waveguide transmission
properties and device attenuation properties illustrated
that the designed VOA had good performance.
2. The Principles of VOAs
The structure of the MZI thermo-optic VOA based on
quartz substrate is shown in Figure 1. It consists of in-
put/output waveguide, two Y-branches, two symmetrical
phase-modulation arms, and a metal film heater above
one of the arms. The input light signal is split to two
identical light signals in the first Y-branch region. Then
the two beams of light will pass through the two arms
separately and interfere in the second Y-branch region. If
the two arms are completely symmetrical, the output

H. Q. DAI ET AL. 159
light signal will emerge from the output waveguide, the
same with the input light signal in the case of no modula-
tion. In the device designed, one of the two phase-
modulation arms will be modulated to develop phase
difference between the two beams of light with same
intensity. After the transmission through the two arms,
the two beams of light will interfere. The intensity of the
output light varies from the maximum to the minimum
with the changes of the phase difference from 0 to π.
When one of phase-modulation arms is heated by the thin
film heater, its temperature rises and the refractive index
of the arm will be changed. The light going through the
arm will develop a corresponding phase shift
h
2πTL
λT
n 
 
h (1)
where is the wavelength of the light; n is the refrac-
tive index of the core of the arm waveguide;
λ
T

is the
temperature variation of the core after heating; nT


is the thermo-optic coefficient of the waveguide material;
Lh is the length of the heated waveguide. From (1) we
can get that when the thermal phase shift is π, the tem-
perature variation of the waveguide is that
1
h
λn
T2L T






 (2)
In the one-dimensional linear assumption, if the phase
shift is π, the power consumption P

could be derived
as [3]
10.88
2
wh w
h c
kw tn
PwT
t






 





(3)
where w is the thermal conductivity of the core and
the cladding layer. It is assumed that the thermal conduc-
tivity of the core and the cladding layer are the same be-
cause they are all based on SiO2 material and their re-
fractive index are very close; wh is the width of the heater
above the phase-modulation arm; tc is the location of the
core; tw is the total thickness of the core and the cladding
layer. As seen from (2), the device power consumption
and the core position tc is inversely proportional. How-
ever, large tc will affect the propagating velocity of heat
from the heater to the core, resulting in low modulation
k
Figure 1. Basic configuration structure of the VOA.
rate. Furthermore, to lower the device power consump-
tion, deep isolated grooves have been etched at the both
sides of two phase-modulation arms of the VOA, as
shown in Figure 2, better limiting the lateral diffusion of
heat in the modulation area.
3. Design of the Device
The schematic of the VOA with isolated grooves is
shown in Figure 2.
3.1. Design of the Waveguide
The material of the core and the overcladding layer is
SiO2. The cladding layer is directly grown on the core by
plasma enhanced chemical vapor deposition (PECVD)
technology based on the quartz substrate, as shown in
Figure 3.
In our experiment, the refractive index of the cladding
layer is 1.445 and its refractive index difference (

)
between the core and the cladding is 0.75%. The thick-
ness of the overcladding layer is 18μm and the cross-
section dimension of the core is . The simulation
results showed that under the condition of
2
66μm
λ1.5μm,

Figure 2. Schematic of the VOA with isolated grooves.
Figure 3. Cross-section diagram of the VOA.

H. Q. DAI ET AL.
160
0.75% , the waveguide could achieve good single-
mode propagation. We adopted the traditional Y-branch
MZI structure, as shown in Figure 4 [4,5]. It contains a
single input waveguide of length L1 and a reduced width
(10%) waveguide of length L2. The narrowed waveguide
has been induced to filter out the high order mode,
weaken the interference effect of the high order mode
with the fundamental mode and improve the uniformity
of the output light [6]. A waveguide with a 0.75%

has strong light confinement, allowing us to realize a
curvature radius as small as 5000 μm. Other parameters
in same situation, the radius of the bent waveguide is
larger, the propagation loss is lower. Considering the
device size, we adopted , ensuring that the
distance between two phase-modulation arms is 100 μm.
A segment junction occurs between the two bent wave-
guides with positive radius and negative radius respec-
tively. Through the simulation, we got that the optimal
value of the offset is 0.1 μm, as shown in Figure 5.
Where I0 is the input light intensity; I is the output light
intensity.
r80 m 00μ
3.2. Design of the Heaters and Grooves
Both ends of the heater are connected to the power sup-
ply via the metal leads. The heat generated, which is
proportional to the square of the current flowing, is con-
ducted to the core, achieving the purpose of thermo-optic
modulation.
Figure 4. Schematic of the Y-branc h (input section).
Figure 5. Results used to find optimal value of Offset.
The heater and leads are composed of titanium (Ti) of
high resistivity and aluminium (Al) of low resistivity
respectively to reduce the power consumption. The thick-
ness of the Ti is h
t0.3μm

, the length is h L 5000μm

,
the width is h w20μm

and the resistivity is
7Ω 
h
ρ4.2 10m

. The thickness of the Al is w t1μm

,
is w
L10000μm the length

, the width is w w10μm

stivity is w ρ2.65 10 and the resi8Ω
m


fundamental formula of resistance
. Using the
LL
Rρρ
Sw

t
(4)
The resistance of the heater can be calculated as
h
R350Ω

and the resistance of the leads as w R26.5Ω

,
so that the electric power is mainly converted into the
heat of the heater rather than the leads.
SiO2 has a certain thermal conductivity and the heat
generated by the heater could diffuse laterally and affect
the other arm, increasing the power consumption. In or-
der to reduce the power consumption, a groove has been
etched at the center of the two phase-modulation arms.
The other groove etched along the heated arm was to
prevent the heat from being lost. The third groove was
etched along the side of the unheated phase-modulation
arm for the high dynamic modulation range. The depth of
the grooves etched is 26 μm, slightly greater than the
total thickness of the cladding layer and core. Because
the air has a much lower thermal conductivity than the
silica, the heat will be better confined and reduce the
power consumption with the same attenuation.
4. Simulation of the Device Properties
4.1. Simulation of the Static Loss
Waveguide transmission loss associated with the wave-
guide structure and parameters is an important factor to
affect the device insertion loss. Figure 6 shows the re-
sults simulated of the device static loss. The length of the
input/output waveguide is L_(in(ou)) = 1500 μm, the
radius of the bent waveguide is r = 8000 μm. The amount
of the optical power is defined as
0
I
A10lg(
I
 )
(5)
From the Figure 6 and (5) we could calculate that the
static loss is about 0.46 dB after the transmission in the
MZI structure in the case of no modulation.
4.2. Simulation of Attenuation Characteristics
Connect the both ends of the heater with the power sup-
ply and the temperature of the waveguide will be
changed by the heat generated. As a result, the refractive
index of the waveguide will be changed and the phase
difference will be induced, achieving the purpose of the

H. Q. DAI ET AL.
161
Figure 6. Simulation result of the device static loss.
Figure 7. Relations between attenuation and power con-
sumption.
attenuation of the optical power. Equation (1) could be
written as another form
h
L
2πnλ    (6)
It can be calculated that when the phase difference is
that the corresponding variation of the refractive in-
dex is. From (2), as the phase difference
is , we could calculate that the corresponding tempera-
ture variation is
π
π
4
n1.5510

 
T13K

. Finally, we could get the re-
lation between attenuation and power consumption, as
shown in Figure 7. When the attenuation is 30 dB, the
power consumption is about 180 mW.
5. Conclusions
We designed a MZI thermo-optic VOA based on quartz
substrate and simulated its properties using the BPM. We
found that the static loss of the device is less than 0.5 dB
and the power consumption is only 180 mW with the
attenuation of 30 dB. Device fabrication and measure-
ment experiments are being carried on presently and the
results of the experiments will be published soon.
6. Acknowledgements
This work has been supported by the National High
Technology Research and Development Program of
China (863 Program) (Nos.2011AA010303 and 2013AA
031402), the National Natural Science Foundation of China
(Nos.61090390, 61274047, 61275029, and 61205044), the
Major Science & Technology Specific Project of He’nan
province of China, and the Independent Innovation
Foundation of He’nan Province of China.
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