Design and Optimisation of Photonics Devices: Supporting a Key Enabling

Technology

Prof. B. M. A. Rahman,

City, University of London, UK

B.M.A.Rahman@city.ac.uk

EngiTek 2020 Congress16th June, 2020, Irbid, Jordan

Evolution of Engineering to ElectronicsEmergence of PhotonicsBecoming a Key Enabling TechnologyOverview of ModellingBriefly some our resultsConclusions

Outline of my talk

Michael Faraday

FRS

in London

1791-1867: 1831 relation between time varying magnetic field and current

Origin of Electrical Engineering

Origin of Electronics

• 1904: Sir John Ambrose Fleming; Vacuum Tube – Diode• Head of EE, University College London

• 1906: Lee De Forest: Triodes

Start of electronics

Sir John Ambrose Fleming

1849-1945: invented electronic valve in 1904

1947: Invention of transistor in Bell Labs –started the revolution of electronics

Rapid Development of Electronics• Electronic valves• Transistors• IC > LSI > VLSI

• This rapid development had profound affect on all aspects of our life in the 20th Century

Moore’s Law

Let’s follow

• Emergence and progress of the Information Technology

James Clerk Maxwell, FRS

1864: Maxwell’s Equations: predicted electromagnetic waves

1887: Henrich Hertz demonstrated experimentally

King’s College, London; Cambridge

• Rapidly followed by the invention of

• Radio in 1897• TV 1927• Phone 1876• Mobile phone 1973• Internet 1969

The

Supported growth of Internet Dec 08: 1.5 billion users

Growth 1000% during last 20 years

In 2019 4.5 Billion Internet users out of 7.7 B populations

Global Internet traffic

2G 3G 4G 5G

DATA VOLUME1000x mobile data

CONNECTED DEVICES10x - 100x

~5x LOWER LATENCY

HIGH END-USER DATA RATES10x – 100x

10x POWER SAVINGFor low powered devicesSource: METIS

We talked about revolution of electronics and continuous progress of communication technologies

What role can the photonics play here?

What is Photonics?

Electronics manipulates Electrons

but Photonics manipulates photons or light

Emergence of Photonics in 1960s• Semiconductor lasers• Optical Fibres

Nobel prize 1964

• Townes, Basov and Prokhorov (from the Soviet Union) won the Nobel Prize in 1964 for their work on both microwave and optical lasers

• Schawlow won the Nobel prize in 1979 for work on laser spectroscopy

Nobel prize in 2009

Optical fibre development

• Concept of modern clad optical fibre developed by Kao and Hockham in the UK at STL in 1966

• Presented at IEE in January 1966• Identified 1000 dB/km loss due to impurities

• By 1970 Corning reported loss reduced to 20 dB/km

Photonics

• Most of the major inventions were from the need of telecommunication sectors

• Most of the market is related to consumer products

New communication systems

FLAG Pacific -1: 22000 km, 8 pairs of fibres

Each fibre WDM 64 λ @ 10 Gb/s = 5 Tb/sec

= 60 million simultaneous telephone channels

Optical communication is a part of Photonics

Impact of Photonics• Telecommunications• Appliances: CD/DVD Players, Display, scanner,

Laser printers, illuminations • Industrial uses: material processing• Medical applications, corneal sculpting• Sensing: physical, chemical, biological

But now big market is consumer products such asToday: Photonics for storage & retrieval

Storage 25 GB Blue Ray DVD

Today: Photonics for Display

New Flat Screen TV, New Flat Screen TV, HD or UHD (4k = 3840x2160 pixels)

also screen for mobile phones

Photonics for printing

Today’s colour laser printer

Laser in healthcare

LASIK: Vision correction

Ultraviolet Excimer laser for eye surgery

Photonic for illuminations

More efficient than incandescent or fluorescent lights

Optical sensors: a massive market

Optical pressure sensor

There can be 200 sensors in a car

Laser in material processing

The University for businessand the professions

Car and plane’s body parts are processing by high power lasers

Stents are fabricated by high power lasers

Photonics

• Horizon 2020 EU Research Programme

• Photonics – A Key Enabling Technology (KET)

Photonics technology: The pillars

• Materials• Devices• Systems

• Exploitations

Photonic Devices Work at City University

Various types of Optical Waveguides

Optical Modulators, 3dB couplers, MxNsplitters, filters, Bragg gratings, Spot-size converters, Compact bends, Nonlinear Devices, VCSELs, Polarization Splitters, Polarization Rotators, Polarization Controllers, etc.

For my contribution over last 41 years is the developing finite element based numerical modelling tools for photonic devices

• Fellow of IEEE • Fellow of OSA• Fellow of SPIE

Types of Photonic Devices

• Uniform Optical Waveguides: Modal Solutions

• Nonuniform Guided-wave structures• Butt-coupled uniform sections• Junction Analysis• Arbitrarily nonuniform structures• Beam Propagation Methods

• Time-domain approaches

Uniform in z-direction: n(x,y)Find modal field E(x,y) or H(x,y) and γ = α + j β

Optical waveguides are key components

Modal solutions for optical waveguides

• Semi-analytical approach• Effective Index Method • Numerical approaches• Fourier-based method : Spectral Index Method• Finite difference method• Finite element method• Transfer Matrix method• Beam Propagation Method•

Finite Element Method (FEM)

• Structural problems• Fluid dynamics• Thermodynamics• Electromagnetics

• Electrical machine designs• Radio frequency, microwave• Optical waveguides & Devices

DiscretizationIn the FEM the structural cross-section is

subdivided into a finite number of elements.

Discretisation

In the FEM the waveguide cross-section (x,y) is subdivided into a finite number of elements.

Any structure can be represented

element

The Variational Formulation

( ) ( ) ( ) ( )

∫

∫∫Ω⋅⋅

Ω⋅⋅∇⋅∇

+Ω×∇⋅×∇

=

−

dHH

dHHdHH

µεαε

ωˆ

)ˆ

*

*

0

1*

2

IEEE JLT p.682, 1984

Full Vectorial

Naturally satisfies boundary conditions

Exact-in-the-limit

Valid for general anisotropic refractive index

Citations: 1400+

Discontinuity AnalysisMisalignment

Directional Coupler

MMI

Butt-coupled uniform sectionsTo obtain modal coefficients: needs junction analysis

Least Squares Boundary Residual method

The energy functional J is given by

J E E Z H H dtI

tII

tI

tII= − + ⋅ −∫

202

2α

Ω

Ω

Continuity of Et and Ht is enforced

IEEE JLT p.52, 1988

Cited 120+ times

Beam Propagation Method

essential for z-variantn(x,y,z) type of structures

• Fourier, FDM or FEM-based• Scalar, Semivectorial, or Vectorial formulation• ABC, TBC, PML Boundary condition

• H-field based: JLT 2000 paper cited 140+ times

Time-domain approach

• FDTD• Valid for general electromagnetic problems• Particularly useful for pulse propagation, strong

discontinuities (photonic crystals, nanoparticles) and strong nonlinearity

• Computationally very versatile but computer intensive as being 4-dimesnional (x,y,z,t)• Often approximation is used to reduce 1D• Poor in representing very fine features• Poor in representing curved/slanted surfaces

Needs and emerging areas of research in photonics

• Higher data rate for communications

• Silicon photonics

• Plasmonics

• Nonlinear Photonics

• Bio-Photonics

• Metamaterials

The University for businessand the professionsWaveguide: after optical fibre > Photonic Crystal Fibre

First reported by Prof. Philip Russel, Univ Bath, England

Single material

Low loss

Adjustable spot size

Endlessly single mode (nearly)

Adjustable GVD

A 2-D, Hx contour field, for the Hx11 mode

-4 -3 -2 -1 0 1 2 3 4-4

-3

-2

-1

0

1

2

3

4

Air Holes

Defect Region

Transmission Capacity in Optical Fibers

Fig. The evolution of transmission capacity in optical fibers due to technological breakthroughs.*

*White Paper (2013) “Space Division Multiplexing: A new milestone in the evolution of fiber optic communication,” Nokia Siemens Networks, http://modegap.eu/?p=767. 52

Data demand increasing rapidly and continuously

• WDM

• Polarisation division multiplexing

• Space division multiplexing

• Vortex modes?

8-Core MCF InterconnectΛ = core to core pitch

2Λ= distance between the two rows

r1 = core radius

r2 = distance between the center of core and the inner edge of trench

r3 = distance between the center ofcore and the outer edge of trench

Δ1 = relative refractive-index differencebetween core and cladding

Δ2 = relative refractive-index differencebetween trench and cladding

W = width of the trench layer Fig. Schematics of (a) 8-core MCF and (b) trench-assisted index profile.

4

Results and Discussion Contd…

Fig. Variations of mode coupling coefficientand coupling length with the core to corepitch for an 8-core TA-MCF, for differentΔ2, when r2/r1=2.0, r3/r1=3.0, and W/r1=1.0

Fig. Variations of crosstalk with core tocore pitch for an 8-core TA-MCF, fordifferent Δ2 values.

5

IEEE PJ 2017

Multicore Fiber (MCF)

Crosstalk is a potential disadvantage of MCF

Fig. Schematic of MCFs (a)homogeneous MCF* (b) heterogeneousMCF# (c) trench-assisted MCF* and (d)hole-assisted MCF$.*

56

Results and Discussion Contd…

Fig. HY field of the fundamental mode for 8-core step index and TA-MCF, when r2/r1= 2.0, r3/r1= 3.0, and W/r1= 1.0.

Fig. Variation of coupling length and crosstalk with the r2/r1 for 8-core TA-

MCF OI, when r1 = 4.45 μm, Δ1 = 0.35%, Λ = 45 μm, and W/r1= 1.0.

7

Opt Lett. 2015, IEEE Pj 2016, Opt Comm 2016

Mode splitter for mode division multiplexing

• Important device for multimode transmission systems

H𝑦𝑦11

H𝑦𝑦11

H𝑦𝑦11

H𝑦𝑦11, H𝑦𝑦

21 H𝑦𝑦11, H𝑦𝑦

21, H𝑦𝑦31

SiO2

Air Si Si

S

WMWS

H

Asymmetric Directional Coupler

JLT May 2016

Also in OSA Continuum 2019

SHG, supercontinuum sources

• SHG in LiNbO3 in 1997

• SHG in GaAs in GaAs 2000

• SHG in ZnO in 2013

• Four wave mixing in PCF Opt Lett May 2015, • THz generation by FWM: IEEE STQE Inv paper Apr 2016,

and IEEE PTL Aug 2016 issue

Opt Exp Dec 14 and May 15Cover page IEEE QE April2017

Pump wavelength 2.0 µm

Black-solid line curve represents the SCspectrum for the waveguide containingGe11.5As24S64.5 glass for its lower claddingand red-dashed line curve represents thespectrum for the structure employingMgF2 as its lower cladding.

Spectral evolution with a peak power of 500 Wfor (a) air-clad all-chalcogenide waveguide; (b)air-clad chalcogenide core employing MgF2for its lower cladding.

JOSA B Nov 2015, Feb 2018, JAP 2018, PTL Nov 2017, A0 June 2020Cited more than

210+ time

Electronics vs Photonics

• The electronics revolution has been possible with the invention of transistor in 1947 and followed by their integration to IC → LSI → VLSI

• But for photonics, this integration is still in the early stage

• But why?

But so far integration of photonics components have been modest

So far Best in Photonics

• Best waveguides: lowest loss – silica fibres 0.2 dB/km• Best lasers: InP based• Best modulators: Lithium Niobate – but too long• Detectors: Ge• Isolator?? YIG? Not yet integrated!

• As there is not a single material which is good or reasonably good for all the functions, integration of the functions have been poor.

• What about silicon?

Silicon

• Best material for electronics• Heavy investments from semiconductor industries• Well developed CMOS foundry• Low cost, super precisions• Wonder material for electronics

• But is it good for photonics?

Silicon Photonic Waveguides

• Use of well developed Si CMOS Processing Technologies for electronics

• Compact waveguides and devices• Compact bends (< 5 µm) and systems

• Which will allow more components in a chip• Yield is much better• More functionality• More reliable, lower cost • Also can put electronics and photonics together

Silicon Strip Nanowire

Silicon n = 3.45

SiO2 n = 1.45

Silicon n = 3.45

Si Substrate

SiO2 Buffer Layer

3 μm

1.5 μm

260 nm

Width

Air / SiO2

Variations of Hy along X-axis and Y-axis for the Hy11 mode

Variations of the Ex field along the X and Y-axes for the Hy

11 mode

Optics Express 2010

Poly-Si Layers L

SiO2

Si

Poly-Si

h

sp

sH

w1

w2

Cross-section of the SSC

Schematic diagram of the multilayersbased spot-size converter

Schematic diagram of the SSC

Schematic of polarization-independent SSC based on the multi-layer. (a) Schematicdiagram for coupling process; (b) Cross-section of the multi-layer structure.

Scientific Reports, 2020

Example: Silicon Slot Waveguide

A coupled structure where individual guide cannot support a mode but together they can support only one supermode

Contour plot of Ey field for Hx11 mode

Horizontal slot as bio-sensor

Optimization of sensor designs

JLT May 2015

Sensing arm

Reference arm

Gas chamberInlet Outlet

LASER Optical detection

MZI

Wav

egui

de cr

oss-

sect

ion

𝑯𝑯𝒙𝒙− 𝒇𝒇𝒇𝒇𝒇𝒇𝒇𝒇𝒇𝒇

𝑬𝑬𝒚𝒚− 𝒇𝒇𝒇𝒇𝒇𝒇𝒇𝒇𝒇𝒇

Fast data transfer in computer• Rack-to-rack optical interconnection• Chip-to-chip• On-chip optical signal processing

• Use optical fibre for rack-to-rack• Use silicon nanowires for intra-chip

• Consider electronics and photonics on the same Si chip

GST – fabricated by Shanghai Jiao Tong University, China

GST – IEEE Photonics J Feb 2018

Global energy consumption

Illuminations

Also helps in fighting Global warming

Global electricity consumptions

Google data centre

Energy consumption of a data centre

• Google’s Joe Kava & Heather Dooley Data Centre• Energy consumption in 2015 was 5.7 TWhr

• Where power is consumed in a data centre?• 50% power consumed in transferring data from CPU to

memory through fine gold wire connectors

Power consumptions

• Global data centres consumes 416,000 GW• 3% of whole world’s electricity consumption

• 50-60% consumes in the gold wires transferring data in the processors

• By using optical waveguides, rack-to-rack, chip-to-chip or intra-chip data transfer this can be reduced

Global electricity generation

Pioneer in Renewable energy: solar cells

• Sun is source of all energy sources

• Including fossil fuel, hydro, or wind

• But, we can convert solar energy directly

• Silicon solar cells is the dominant technology now

• Research continuing to increase the efficiency and lower cost

A simple low-index contrast silica waveguide

Displacement vector profiles (a) UX, (b) UY and (c) UZ components of UX11

(a)

(b)

(c)

Ux1,1

Uy2,2

Uz2,1

JOSA May 2016

And effect of higher frequencyIEEE QE 2015

Photonics

• Support rapid data demanded by 5G, IoT,

• Support increased data rate using SDM, MDM

• Rack-to-rack to chip-to-chip data transfer by photonics

• Support lower power consumptions by data centre

• Combine photonics and electronics on one chip

• Reduce global warming through use of better solar cells

• Its uses in healthcare, industrial, consumer products

Conclusions

• At City, University of London, we have one of the strongest research groups in the world on Photonics Modelling.

• Today, I have briefly discussed the emergence, development of Photonics and particularly its ability to shape the future associated technology.

• Thanks to the organiser for arranging my talk.

• But missed to opportunity to visit Jordan – may be next time.