Design of Thermo-Optic Variable Optical Attenuator Based on Quartz Substrate

doi:10.4236/opj.2013.32B038

Paper Menu >>

Journal Menu >>

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

Email: [email protected]

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

2πTL

λT



 

h

(1)

where is the wavelength of the light; n is the refrac-

tive index of the core of the arm waveguide;



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

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

wh w

kw tn

PwT

























(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

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

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



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

Rρρ



(4)

The resistance of the heater can be calculated as

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

A10lg(

 )

(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

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

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.

REFERENCES

[1] X. Z. Jin and X. Wang, “Summary for Technology De-

velopment of Variable Optical Attenuator,” Optical Com-

munication Technology, Vol. 27, 2003, pp. 29-32.

[2] C. J. Sun, K. M. Schmidt and W. H. Lin, “Silica

Waveguide Devices and Their Applications,” Proceed-

ings of SPIE, Vol. 5729, 2005, pp. 9-17.

doi:10.1117 /12.593540

[3] X. L. Zhao, “Silica Mach-Zehnder Interferometer Thermo

-Optic Switch,” Harbin Institute of Technology, 2004, pp.

91-94.

[4] M. G. Lee, L. G. Alexei and D. S. Zhou, Optical Beam

Splitter, United States, US20100046890A1, Feb 2010.

[5] R. Yamamoto and N. B. Miyadera, “Optical Waveguide

Structure Including at least First, Second and Third

Waveguides and Coupling Waveguide,” United States,

US007664353B2, Feb 2010.

[6] A. Klekamp, P. Kersten and W. Rehm, “An Improved

Single-mode Y-branch Design for Cascaded 1:2 Split-

ters,” Journal of Lightwave Technology, Vol. 14, 1996.