Design the Structure of Vertical Multilayer Hybrid Silicon

Waveguide to Work in Anomalous Dispersion Region

Zengzhi Huang

1,2

, Shuai Yuan

and Jinsong Xia

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China

Keywords: Waveguide, Dispersion, Slot Waveguide, Silicon Photonics, Nonlinear Optical Device, Integration Optics.

Abstract: In order to decrease the dispersion of the silicon vertical slot waveguide, we propose a vertical multilayer

hybrid silicon waveguide. The optical mode distribution of the multilayer waveguide is simulated by a finite

element method. By a proper design of the waveguide parameters, the dispersion of waveguide can in the

range of ±300 ps/nm/km in 1510-1590 nm, with one zero-group-dispersion point in the C band. This

waveguide can be an alternative in on-chip nonlinear application, such as all optical signal processing.

1 INTRODUCTION

Silicon photonics has been an important branch in

modern optics. Due to its compact size and rich

nonlinear properties, silicon waveguides are

promising in on-chip signal transmission and all

kinds of signal processing. One main obstacle of the

application of silicon waveguides is its relatively

large waveguide dispersion, which induces optical

pulse broadening in signal transmission and phase-

mismatch in some nonlinear parametric process.

Conventionally, the process of efficient four wave

mixing occurs in a waveguide with anomalous

dispersion. Although the material dispersion of

crystalline silicon is large and normal, it can be

compensated by the structural dispersion through

optimization of waveguide structure parameters. For

silicon strip waveguide, broad-band anomalous

dispersion has been realized(Turner et al., 2006) and

optical parametric gain has been experimentally

demonstrated(Foster et al., 2006).

Silicon slot waveguide is proposed in

2004(Almeida et al., 2004) and broadly investigated

in the last decade(Zengzhi et al., 2015, Huang and

Xia, 2016). Combining high quality nonlinear

cladding material with silicon slot waveguide, the

hybrid slot waveguide is potential in on-chip all-

optical signal processing. A lot of researches relate

to the dispersion engineering of silicon slot

waveguides to achieve anomalous dispersion in

telecommunication wavelength to near infrared

wavelength. Most researches are based on a

horizontal slot waveguide structure, that is, the

direction of the slot is horizontal to the silica

substrate(Zhang et al., 2010b, Zhang et al., 2010a).

For vertical slot waveguides, it’s difficult to tailor

the dispersion to the anomalous region. Some

designs tailor the dispersion by shifting the slot

position off the waveguide centre of using dual-slot,

however, that leads to in some working wavelengths,

the optical field are not centred in the slot

region(Zhu et al., 2012).

In this article, we propose a novel hybrid

vertical slot waveguide structure intended to achieve

broad-band anomalous dispersion in

telecommunication wavelength and near infrared

wavelength. This novel vertical slot waveguide

structural is like a multilayer structure in horizontal

direction. By choosing appropriate low index

material, the hybrid silicon slot waveguide can have

a zero-group-dispersion wavelength in the

telecommunication region. What’s more, using this

novel waveguide, a large portion of the optical

energy contains in the low-index slot and cladding

region, which is beneficial in reducing the two photo

absorption and free carriers absorption in the silicon

layer.

2 WAVEGUIDE STRUCTURE

Before designing the waveguide structure, we first

take a look at the two familiar silicon waveguides,

channel waveguide and slot waveguide. The

294

Huang Z., Yuan S. and Xia J.

Design the Structure of Vertical Multilayer Hybrid Silicon Waveguide to Work in Anomalous Dispersion Region.

DOI: 10.5220/0006220802940297

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 294-297

ISBN: 978-989-758-223-3

dispersions of both waveguides have been

investigated by previous works. It’s known that

silicon has a large and normal dispersion near

1550nm. Fortunately, silicon waveguide has a strong

confinement of light, which induces large structural

dispersion. By optimizing waveguide structure

parameters, the structural dispersion can be

anomalous dispersion and compensate the material

dispersion, thus obtaining anomalous waveguide

dispersion.

We use Lumerical mode solution software to

simulate the waveguide dispersion. The material

dispersions of silicon and SiO

are taken into

consideration, in the form of Sellmeier Equation.

Fig.1 shows the dispersion of a channel waveguide.

The waveguide is on a 340nm SOI substrate. The

width of the waveguide is 500nm. We can see that

there is a zero group velocity dispersion point in

1567nm. For wavelength larger than 1567nm, the

channel waveguide shows anomalous dispersion.

Figure 1: Simulated dispersion curve of a silicon channel

waveguide, with a size of 500nm × 340nm (width ×

height).

However, it’s difficult to achieve anomalous

dispersion in vertical silicon slot waveguides. Fig. 2

is the dispersion curve for a slot waveguide. The

waveguide parameters are as following, h=340nm,

=200nm and W

slot

=100nm. The total width of the

slot waveguide is 500nm, same as the channel

waveguide calculated above. It can be seen that the

vertical slot waveguide has a large and normal

dispersion in the wavelength from 1610-1820 nm.

Compared Fig.2 with Fig. 1, we can conclude

that the insertion of the 100-nm slot in the middle of

the channel waveguide makes a big difference to the

waveguide dispersion. This is because the air slot

breaks the continuity of the electric field component

of the TE mode.

Figure 2: Simulated dispersion curve of a silicon vertical

slot waveguide. The top silicon layer thickness is 340nm.

The widths of the slot and silicon rail are 100nm and

200nm, respectively.

In order to decrease the normal dispersion of

the vertical slot waveguide, two ways are

demonstrated to be feasible. i). Design a waveguide

with larger cross section. That means increasing the

thickness of the top silicon and the width of the

waveguide. ii). Reduce the refractive index

difference between the silicon and slot region. Both

of these two ways are supposed to alleviate the

abrupt change of the optical mode induced by the

waveguide boundary.

3 MODAL PROFILE

Following these two designing principles, we

propose a vertical multilayer hybrid silicon

waveguide, shown like Fig. 3. It can be seen as a

silicon slot waveguide with a rectangular low-index

cladding. W and h denote the width and height of the

cladding, respectively. W

and W

slot

represent the

width of the silicon rail and the width of the central

low index region.

Figure 3: A schematic of the vertical multilayer hybrid

silicon waveguide (in cross view).

Design the Structure of Vertical Multilayer Hybrid Silicon Waveguide to Work in Anomalous Dispersion Region

295

An optical mode simulation is performed using

the COMSOL Multiphysics software. The

simulation region is 4μm×4μm. The simulation

boundary is set as scattering boundary condition.

The waveguide parameters are W=700 nm, h=500

nm, W

=100 nm and W

slot

=200 nm. The refractive

index of the low index material is set as 2.4, so the

electric field in the low index region is enhanced by

a factor of 1.45 on the Si-slot interface. The effective

index of the multilayer waveguide is 2.23 in 1550nm.

Fig. 4 is the electric field distribution of the

fundamental quasi-TE mode. We can observe that a

characteristics slot mode exists in the proposed

waveguide, in which the electric field in the low

index slot region is increased. The electric field

enhancement can also be reflected by a 2D cross line

view, as the yellow line in Fig. 4 shows.

Figure 4: Electric field distribution of the multilayer

waveguide.

Compared to the traditional silicon slot

waveguide, the multilayer vertical waveguide has

smaller W

and larger

slot

, due to two additional

low index regions. This feature is fascinating in

nonlinear application based on silicon slot

waveguide, because the two photon absorption effect

in silicon is suppressed and it is promising to

achieve a high figure of merit (FOM)(Zhang et al.,

2015).

4 WAVEGUIDE DISPERSION

We calculate the dispersion of the multilayer

waveguide, shown as Fig.5. For low index material

refractive index n=2.4, the waveguide dispersion is

less than ±300 ps/nm/km in 1510-1590 nm, with a

zero-group-dispersion point in 1542 nm. For

wavelength longer than 1542 nm, the multilayer

waveguide have anomalous dispersion.

Figure 5: Waveguide dispersion curves for n=2.3, 2.4 and

2.5, respectively. The other parameters are W=700 nm,

h=500 nm, W

=100 nm and W

slot

=200 nm.

Next we investigate how the waveguide

parameters influence the dispersion of the multilayer

waveguide. Varying n from 2.3 to 2.5, we can see in

Fig.5 that if n becomes larger, the dispersion curve

moves upwards and the zero-group-dispersion point

moves to shorter wavelength.

The waveguide height has small influence to the

waveguide dispersion. This is shown in Fig.6.

Figure 6: Waveguide dispersion curves for h=460nm,

480nm and 500nm, respectively. The other parameters

used for simulation are W=700 nm, W

=100 nm,

slot

=200 nm and n=2.4.

The dispersion curve for various W

is depicted

in Fig.7. The figure shows that W

is an important

factor that decides the waveguide dispersion. For

=90 nm, the multilayer waveguide remains in

anomalous dispersion region for the whole

calculation wavelength. However, W

=120 nm, the

waveguide works in normal dispersion.

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

296

Figure 7: Waveguide dispersion curves for W

=90nm,

100nm, 110nm and 120nm, respectively. The other

parameters used for simulation are W=700 nm, h=500 nm,

slot

=200 nm and n=2.4.

5 CONCLUSIONS

In this paper, we propose a vertical multilayer

silicon hybrid waveguide, to overcome the large

normal dispersion of the vertical silicon slot

waveguide. By proper design, the dispersion of

waveguide can in the range of ±300 ps/nm/km in

1510-1590 nm. This kind of waveguide can be as a

nonlinear optical waveguide, and find its

applications in on-chip all optical signal processing.

The multilayer waveguide provides additional

flexibility in tailoring the dispersion. A lower, flatter

dispersion can be obtained by further optimization.

ACKNOWLEDGEMENTS

This work is supported in part by the Major State

Basic Research Development Program of China

under Grant 2013CB632104 and Grant

2013CB933303, and in part by the National Natural

Science Foundation of China under Grant 61177049

and Grant 61335002.

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