Design and simulation of wideband photonic intergrated circuits for multimode (DE) multiplexing and conversion746

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 passive

MINISTRY OF EDUCATION AND TRAINING

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

-0 - 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: Assoc Prof PhD Le Nhat Thang

Reviewer 2: Assoc Prof PhD Pham Ngoc Nam

Reviewer 3: PhD Le Van Hai

The dissertation will be defended before approval committee

at Hanoi University of Science and Technology:

Time 14h, date 21 month 12 year 2020

The dissertation can be found at

1 Ta Quang Buu Library

2 Vietnam National Library

1

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 passive devices working in C band and can overcome as

existing disadvantages, thus resulting better overall performances in

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

P Cr T 10 out

unwanted

P log

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

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

1.2 Optical waveguide analysis and simulation methods

1.2.1 Effective index method

obtain a y-dependent effective index profile; this generated index

3

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 discrete with a rectangular lygrid 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 = 600 0 C 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)

- (SiH2 2 is often used as the source gas Cl )

- Temperature > 1000 o C

Silicon Etching (Using Plasmas Gas)

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

on upper layer Dry etching using CF gas and AC power to 4 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

4

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

1.4.1 Directional coupler

Coupling can be regarded as a scattering effect The field of

waveguide 1 (which has propagation constant = and amplitude a1)

is scattered from waveguide 2 (which has propagation constant =

and amplitude a2), creating a source of light that changes the amplitude

of the field in waveguide 2 The field of waveguide 2 has a similar

effect on waveguide 1 Coupled equations are as below:

1 0 2 2( , ) *

2

ij n n x y E E dxdy i j

Differential equations system (1.4.1) and (1.4.2) above has the roots

that represented as follows:

n W (1.4.8) corresponds to m=0 and corresponds to m=1 By defining as the half-beat length of the two lowest-order modes:

2

1

43

eff o

n W

L (1.4.9)

The different between the propagation constants of fundamental mode (m=0) and mode mth can be expressed from Eq (1.4.8) and Eq (1.4.9 ) ( 2)

3

m m

L (1.4.10) 1.4.2.2 Guide-mode propagation analysis (MPA)

An input field profile at the entrance of the multimode waveguide then can be decomposed into modal field distribution

1

0

( , ) ( )exp

3

M

m m m

m m

L (1.4.13) Input field will be reproduced if the phase term satisfies the condition:

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 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(3 ) with p = 0,1,2 …

b, Multiple images can be fulfilled when

The resulting in terms of phase and positions of reproduced imagines

are expressed as follow:

1

0

1

N

j

in q q

C (1.4.19) with

q q

W

N q

p N q

N

(1.4 ) 20

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

signals in a NxN MMI couplers are given by:

7

MMI coupler NxN and is the constant phase For shortest MMI length (p=1),

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

neff waveguidem( 1) neff waveguiden( 2) (1.4.22)

Another factor that needs to be considered is Mode Conversion Factor (MCF) The equation of MCF is expressed as below:

in MDM, which will be mentioned in the following chapters

ASYMMETRIC DIRECTIONAL COUPLER (ADC)

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

8

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

Fundamental mode TE 0

First-order mode TE 1

Output port 2 Output port 1

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 is chosen as =80 g g

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 2 =1000 nm and the height is set w

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, is fixed as 1.2 µm, then optimal length of sinusoidal dwaveguides 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

0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 1.5

2 2.5 3 3.5

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Length of asymmetric directional couplers L c ( m)

1 , Input1-Output1 TE

0 , Input2-Output2 TE

0 , Input2-Output1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

TE

0 TE

1

TE1 TE02Lhb1

2Lhb2

Fig 2.1.3 Transmission characteristic of on dependence of the coupling

length of the asymmetric directional coupler

10

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 2.1.4 Electric field patterns for the mode (de)MUXer

-35 -30 -25 -20 -15 -10 -5 0

1 , Output1 TE

0 , Output1 TE

1 , Output2

11

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 , which is the standard σdeviation 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 = 0.4 nm and σ Lcor = 10

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

as silicon etching using KOH

x 10-3-30

-25 -20 -15 -10 -5 0

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

0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018

Fig 2.1.7 Sidewall roughness loss calculation for two modes and two

2.2 Conclusion

The results show that the device can work as a tunable CWDM device (with four wavelengths and 5.5 nm spacing) with the spectrum width

12

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

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

i i 2 0

i 2

1e 4

2 2 1

2

3 1e 4

,

i 2 i 1

i 2

e 1

2 e

,

i 2

2 2

i 2

e 1

3 e

Length of the sinusoidal waveguides, L s ( m)

Fig 3.1.2 BPM simulation for transmittance properties of the trident

coupler as a function of the length of the sinusoidal

The first MMI coupler was designed with the chosen length: LMMI2=Lπ The second MMI coupler was designed with the chosen length:

LMMI2=3Lπ/2 As the 2nd MMI only divides 0th and 1st order mode,

hence it can be considered as a 2x2 MMI The relation of the amplitudes and phases between input and output of the two MMI couplers can be obtained as follows:

M

2

(3.1.2)

14

By mathematical calculation, three PS value must be ΔΦ1= ΔΦ2=2п/3

ΔΦ3= п/2 so that each order mode can be (de)multiplexed to different output Phase shifter dimension is initially set up as Lps = 20

µm and Wps = 0.52 µm We choose Wps1=Wps2= 410 nm and Wps3= 590

nm to satisfy the corresponding phase shift value, as in Fig 3.1.3 Fig 3.1.4 shows field distributions of the mode (de)MUXer Fig 3.1.6(a) shows that I.L more than -1 dB and Cr.T smaller than -15 dB are achieved for 3 modes as the branching angle varies within 6 degrees

to 13 degrees (Ls=15 µm equivalent to 8.4 degree) Fig 3.1.7(a) shows that variation of LMMI2 within ± 1 µm, the I.L is more than -0.72 dB and Cr.T are smaller than -25 dB Fig 3.1.7(b) shows that variation of

W0 within ± 0.2 µm, the I.L more than -0.6 dB and Cr.T is smaller isthan -20 dB Fig 3.1.8 shows that the variation of etching depth within

± 50 nm I.L more than -2 dB and Cr.T is smaller than -18 dB is

0 1.5708 3.1416

4 4.712

2 6.283

Central width of the phase shi er, Wps ( m)

Fig 3.1.3 BPM simulation for the phase angle Φ is a function of the

central width of the phase shifter

Fig 3.1.4 Electric field patterns of the proposed three-mode (de)MUXer