Nghiên cứu tính toán và thiết kế mạch tích hợp quang băng rộng chuyển đổi và tách ghép mode (design and simulation of wideband photonic integrated circuits for multi mode (de)multiplexing and conversion) tt tiếng anh

Therefore, the objective of this dissertation is to proposed new Silicon on Insulator SOI rib/ridge waveguide mode demuxer designs based on some specific structures that are used as pass

MINISTRY OF EDUCATION AND TRAINING

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

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TRAN TUAN ANH

DESIGN AND SIMULATION OF PHOTONIC INTEGRATED CIRCUITS FOR MULTI-MODE (DE)MULTIPLEXING AND

This dissertation is completed at:

Hanoi University of science and technology

SUPERVISORS:

PROF DR TRAN DUC HAN

DR TRUONG CAO DUNG

Reviewer 1:

Reviewer 2:

Reviewer 3:

The dissertation will be defended before approval committee

at Hanoi University of Science and Technology: Time…, date… month….year…

The dissertation can be found at

1 Ta Quang Buu Library

2 Vietnam National Library

INTRODUCTION

Motivation

Main stream in optical communication research field is to improve

bandwidth for wideband applications There are some available

techniques, such as WDM multilevel modulation format, polarization

division multiplexing (PDM) On the other hand, (MDM) technology

has been being paid attention as an emerging technology to enhance

the capacity of the optical communication system besides mentioned

above technology Different from applications of WDM in

long-distance transmission, MDM is more suitable for transmission with

short distance, ultra large capacity, like intra-chip communication

Hence, MDM shows to be a promising technique applied in on-chip

optical communication, along with other (de)multiplexing schemes

Objectives

By reviewing existing papers and research works, there are some

designs with different structures and materials have been proposed

However, each of them have their own pros and cons and also are

suitable for different purposes Therefore, the objective of this

dissertation is to proposed new Silicon on Insulator (SOI) rib/ridge

waveguide mode (de)muxer designs based on some specific structures

that are used as passive devices working in C band and can overcome

existing disadvantages, thus resulting in better overall performances

regarding number of (de)multiplexed modes, loss and footprints

Research methodology

All the designs’ structures are build based on theoretical foundation

Then, each device optical properties are investigated and optimized by

numerical simulation methods, namely BPM and EIM Then, the

devices are evaluated based on performance criteria defined as follow:

I L 10log out

in

P

Where Pin is total power of input waveguides, Pout is the wanted output

power of the device and Punwantedis the total of unwanted powers to

wanted output port

Scientific contributions and practical applications

1) We proposed a new mode (de)muxer design based on ADC SOI

waveguide, which can (de)multiplex fundamental mode and 1st-order

3) We proposed a new mode sorting design based on branched bus SOI waveguides Our design can (de)multiplex up-to 4 modes so far One of the results has been published in Photonics and Nanostructures-Fundamentals and Applications

Our designs can operate within band C range with low insertion loss and crosstalk; have small footprint, thus can be promising candidates for high bitrate MDM on-chip photonics integrated circuits

Dissertation structure

The dissertation consists of 4 chapters Chapter 1 introduce SOI waveguide structures analysis and principles that used in this dissertation Chapter 2, 3 and 4 will demonstrate new specific SOI waveguide designs with MDM functionality, which are also the three

main scientific contributions of this dissertation

1.1 Shapes and functions of silicon-on-insulator waveguide

In this dissertation, rib/ridge waveguide structures are used for all designs Regarding material, core layer and substrate are made from silicon and cladding layer is made from silicon dioxide (silica) Those structure and material are also used in CMOS technology, that explains the reason why SOI waveguide is suitable for low cost manufacturing PIC

1.2 Optical waveguide analysis and simulation methods

1.2.1 Effective index method

Effective index method (EIM) is an appropriate analysis for calculating the propagation modes of the channel waveguides It applies the tools developed for planar waveguides to solve the problem

of two-dimensional structure It consists of solving the problem in one dimension; described by the x coordinate in such a way that the other coordinates (the y coordinate) acts as a parameter In this way, we obtain a y-dependent effective index profile; this generated index

profile is treated once again as a one-dimensional problem from which the effective index of the propagating mode is finally obtained

1.2.2 Finite difference method (FDM)

An arbitrary electromagnetic field propagating along a waveguide can

be decomposed into many elementary discrete guided modes, which can be also called eigenmodes With Finite Difference Method, the cross-section of the waveguide is made discretely with a rectangular grid of points which might be of identical or variable spacing Each grid of point is assigned to an arbitrary electric field value and adjacent electric field can be calculated correspondingly

Separation by Implanted Oxygen

(SIMOX)

Bond and Etch-back SOI (BESOI)

Wafer Splitting

Silicon Epitaxial Growth

Photolithography

- Oxygen ions density > >10 18 cm 2

- Temperature = 6000C during implantation

- Implantation energy of up to 200 keV

- Annealed at a temperature of 1300

- Two oxidized wafers are brought into

contact temperature room

- Wafer thinning via CMP

- Implant p-type ions into wafer, hence the

silicon lattice bonds are significantly

- Chemical vapor deposition (CVD)

- (SiH2Cl2) is often used as the source gas

- Temperature > 1000oC

Silicon Etching

(Using Plasmas Gas)

Using Deep Ultra Violet light, photoresist layer and mask to create waveguide structure

on upper layer

Dry etching using CF4 gas and AC power to achieve critical dimension requirements of specific waveguides

Fig 1.3.1 SOI waveguide fabrication process

1.2.3 Beam propagation method (BPM)

FDM can solve the waveguide eigenmode, but cannot be used to solve propagation characteristic in integrated optics or fiber optics The Beam Propagation Method (BPM) is a widely used to simulate the evolution of electromagnetic fields in arbitrary inhomogeneous medium Transparent Boundary Condition (TBC) is used in FD-BPM simulation to eliminate the back reflections or incoming fluxes into

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the analysis window, in which radiation disappear into the boundary without any reflection when reaching the edge of simulation window

1.3 Silicon-on-insulator waveguide fabrication

SOI waveguides fabrication technology is similar with CMOS ones The fabrication process is described in the diagram chart Fig 1.3.1

1.4 Silicon-on-insulator waveguide structure used for MDM functionality

As the core width W is quite large and the high-contrast waveguides,

the propagation constant is:

2 2

( 1)4

m eff

eff

m kn

eff o

(m=0) and mode m th can be expressed from Eq (1.4.8) and Eq (1.4.9)

1.4.2.2 Guide-mode propagation analysis (MPA)

An input field profile 𝛹(𝑥, 0) at the entrance of the multimode waveguide then can be decomposed into modal field distribution 𝜓𝑚(𝑥)



 (1.4.12)

exp  jz m m(  2) / 3 L   1 Those properties mentioned above

illustrate the self-imaging property of MMI Based on which and

where the field 𝛹(𝑥, 𝐿) is reproduced, there are general interference

(GI) and restricted interference (RI) mechanism This abstract will

introduce GI only as this mechanism is used in our design

1.4.2.3 General interference MMI

a, Single images can be fulfilled when z = p(3𝐿𝜋) with p = 0,1,2 …

b, Multiple images can be fulfilled when 𝐿 = 3𝑝𝐿𝜋/𝑁 𝑤𝑖𝑡ℎ 𝑝 =

1,3,5,7 …

The resulting in terms of phase and positions of reproduced imagines

are expressed as follow:

In the case of shortest devices when p=1, the optical phases of the

signals in a NxN MMI couplers are given by:

Where input ports i (i=1, 2…N) are numbered from bottom to top and

the output ports j (j=1, 2…N) are number from top to bottom in the

MMI coupler NxN and 𝜑0 = −𝛽0𝐿𝑀𝑀𝐼−𝜋

2 is the constant phase For shortest MMI length (p=1), 𝜑0 = 0

1.4.3 Asymmetric Y-junction waveguide

In case of two adjacent waveguides, if effective refractive index of arbitrary mode (mth) from the 1st waveguide is equal to one of arbitrary mode (nth) from the 2nd waveguide, then mode (mth) of field in 1st

waveguide can be coupled into mode (nth) of field of 2nd waveguide ( 1) ( 2)

1.5 Conclusion

This chapter has introduced waveguide structure, waveguide material that used in designing all our proposed devices Then, common numerical analysis methods are mentioned SOI waveguide fabrication is also briefly introduced Finally, three different waveguide structures and their corresponding working principles are introduced Each structures have its own pros and cons when applying

in MDM, which will be mentioned in the following chapters

CHAPTER 2 MODE DIVISION MULTIPLEXER BASED ON ASYMMETRIC DIRECTIONAL COUPLER (ADC)

ADC is one of the structures that attracts significant amount of research due to its simplicity, straight forward operating principles

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ADC structure has been used by several designs but there are still some drawbacks, e.g., high measured modal loss, modal dispersion while guiding into the waveguides and complicate structure Hence in this section, we propose a new design of two-mode DeMUXer based

on asymmetric directional SOI waveguide coupler with simple structure suggesting some ideas for further improvements

2.1 Two mode division (De)multiplexer based on an MZI asymmetric silicon waveguide

Fig 2.1.1 shows the configuration of the TM-(de)MUXer which is

based on submicron silicon strip waveguides

Fig 2.1.1 Schematic of the ADC based SOI waveguide

The device is designed for operating in electromagnetic transverse (TE) modes with the wavelength operation of 1550 nm comprising of

two asymmetric directional couplers, with the gap g is chosen as g=80

nm Silicon single mode waveguide is fabricated with the width and

the height to be chosen as w=500 nm and h2=500 nm, while two-mode waveguide has the width chosen as 2w=1000 nm and the height is set

to h1 The two-mode waveguide is designed to satisfy two conditions:

mode TE0 won’t be coupled partially to the single mode waveguide and mode TE1 will be coupled effectively to single mode waveguide

In this design, d is fixed as 1.2 µm, then optimal length of sinusoidal waveguides Ls is chosen by using BPM simulation as 105 µm As seen

on the Fig.2.1.2, the effective index of mode TE0 in the single mode waveguide is calculated by BPM method as 2.911 Therefore, the effective index of mode TE1 in the two-mode waveguide must also be

2.911, hence h1=550 nm Then, the coupling length of asymmetric

directional coupler must be satisfied the condition:

Lc  2 mLhb12  2 nLhb21 (2.1.1)

m and n are integer, L hb12 and Lhb21 are half beat-length of waveguides

corresponding to input port 1 and input port 2, respectively BPM

simulation found out that: 2Lhb12 37.9 µm and 2Lhb21 ≈ 36.9 µm as

plotted on the sub figure of Fig 2.1.3 Hence chosing Lc = 1440 µm

will be the least common multiple value

1 TE 0 2L

hb1 2Lhb2

Fig 2.1.3 Transmission characteristic of on dependence of the coupling

length of the asymmetric directional coupler

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By using BPM simulation, Fig 2.1.4 show field distributions of the mode (de)MUXer at the operating wavelength of 1550 nm when the fundamental mode (TE0) and first-order mode (TE1) of lights are launched spontaneously into input port 1

Fig 2.1.5 shows the wavelength dependence by the BPM simulation

of the transmission The mode conversion efficiency (from TE0-TE1

and vice versa) varies from 98.4% to 99.7% in the range ±1 nm around the center wavelength of 1550 nm while crosstalk is less than -25 dB Wavelength response also shows that the transmission is tunable in the range from 1545 nm to 1560 nm (around 5.5 nm wavelength spacing), hence being applicable in CDWM

Fig 2.1.6 plots crosstalk of the MUXer depending on the etched depth

tolerance The results show that for a tolerance of ΔH = ± 3 nm the

Cr.T is well below the value of -10 dB

Side wall roughness is determined by σ, which is the standard deviation of the roughness and Lcor Wavelength dependence of the

transmission loss from sidewall roughness of the modes are plotted in

Fig 2.1.7(a) and (b) for two cases of the couple (σ, Lcor) corresponding

to (2 nm, 50 µm) and (0.4 nm, 10 µm), respectively This loss reduces

to the maximum value of 0.016 dB/mm with σ = 0.4 nm and Lcor = 10

µm by using smoothing techniques to minimize the surface area, such

as silicon etching using KOH

-30 -25 -20 -15 -10 -5 0

Fig 2.1.6 Performance of MUXer and deMUXer devices as a function of

the etched depth tolerance: H=h1=500 nm

0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018

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of 1 nm and mode conversion efficiency up to 99.74% at the designed wavelength The device has low insertion loss, crosstalk and also low sidewall roughness scattering The proposed design shows some better aspects compared to similar previous works based on asymmetric directional couplers The whole size of the device can be integrated on

a footprint as 4µm×1600µm; therefore, it is potentially suitable to construct on-chip photonics integrated circuits

CHAPTER 3 MODE DIVISION MULTIPLEXER BASED ON MULTIMODE INTERFERENCE COUPLER

Compare to other structures, MMI proved to be a promising candidate for a compact mode (DE)MUXer with large bandwidth tolerance and good performance Therefore, this section will introduce new MDM structures based on symmetric Y-junctions and MMI waveguides

3.1 Three-mode division (De)multiplexer based on a trident coupler and two cascaded 3×3 MMI silicon waveguides

The designed structure of the three-mode division (de)muxer is presented schematically in Fig 3.1.1 The 1st MMI coupler, PS1 and PS2 are designed to split power of the fundamental mode and the first-order mode equally to two outermost output, while combines the second-order mode to its center output Then, the 2nd MMI coupler and

to the third PS is designed so that the fundamental mode and the order mode are switched to different output ports and the second-order mode propagating through the central output SOI rib waveguides

first-have the height H = 500 nm and the slab height ho = 100 nm The distance G is set as 1.1 µm and WMMI is set as 4.8 µm The stem width

of the trident coupler is set is W0=1.5 µm The width w of stem

waveguide is determined as 520 nm for single mode regime