Department of Electronic Engineering

Millimetre Wave and THz Research at QMUL

Professor Xiaodong ChenSchool of Electronic Engineering and

Computing ScienceQueen Mary University of London

Email: x.chen@elec.qmul.ac.uk

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Outline Where am I from?

History of QMUL Group

Some New Topics

Summary

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Queen Mary, University of London

Queen Mary and Westfield College was founded in 1889, one of four major Colleges of University of London, ranked in12/13 place last year.

The newly merged Medical Hospital of the College was founded in 1373, the first teaching hospital in London!

Sir Peter Mansfield (Co-winner of 2003 Nobel prize in medicine (MRI)) was a graduate in physics at Queen Mary, University of London.

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Department of Electronic Engineering Queen Mary, University of London

Antenna & Electromagnetics Group (since 1968)

Networks Group Centre for Digital Music Multimedia & Vision Group

22 full staff + 10 teaching staff

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The Antenna and Electromagnetic Group

Prof. Clive Parini (Director of Graduate School) Prof. Xiaodong Chen (Director of Graduate Studies) Prof. Yang Hao Dr Robert Donnan (Lecturer) Dr Akram Alomainy (Lecturer) Prof Peter Clarricoats, FRS (part time) Prof. Derek Martin (part time) Prof. Brian Collins (visiting professor) George Hockings (visiting professor) 10 Postdcotoral Research Assistants, 20 PhD Research Students

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Brief History: Major Milestones 1969-70 Analysis and Design of Corrugated

Horns. 1973 First UK Compact Antenna Range. 1974 First text on Geometric Theory of

Diffraction. 1976 First use of Optimisation in Reflector

Antenna Design 1977 First Design of Array Feeds with Mutual

Coupling for Satellite Antennas 1982 First Design Tools for Shaped-beam

Antennas for Spacecraft Applications 1983 Reflector Surface Metrology using

Ultrasound or Millimetrewaves. 1984 First text on Corrugated Horns.

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1985:Reflector Design of James Clerk Maxwell Radio Telescope.

With a diameter of 15m the James Clerk Maxwell Telescope (JCMT) is the largest astronomical telescope in the world designed specifically to operate in the submillimeter wavelength region of the spectrum. The JCMT is used to study our Solar System, interstellar dust and gas, and distant galaxies. It is situated close to the summit of Mauna Kea, Hawaii, at an altitude of 4092m.

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1991:200GHz clean room operation of single offset CATR

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5GHz to 200GHz single offset CATR

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1992: Successful Measurement of Advanced Microwave Sounding

Unit -B

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Mounted on NOAA weather satellite AMSU-B uses passive radiometry to determine upper

atmospheric water vapour content

Swath width approx 2000Km

15km

50Km

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Orbit covers the globe (except near poles)

28.8°Earth rotationPer orbitOrbit plane rotatesEastward 1° perday

530 miles

2 satellites cover the complete globe in 12 hours

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Passive radiometry around the water vapour absorption line (183.3GHz)

AMSU-B channels:-90GHz150GHz183GHz

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AMSU-B measured upper atmosphere water vapour

content

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AMSU-B QUASI-OPTICS

Mirrors and diochroic plates are used to select the various channels

Frequency 1

Frequency 2

Inputsignal

Diochroicplate

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New Tri-reflector CATR System (2005)Makes efficient use of main reflector

Absorber

Field Magnetude

QUIET ZONE

Field Magnetude

QUIET ZONE

TRI-REFLECTOR RANGE

SINGLE REFLECTOR

RANGE

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300GHz Tri-reflector CATR Demonstrator Currently under test

at QM*Spherical Main reflector diameter = 1M*Shaped subreflectors of order 350mm in diameter* rms error on all reflectors about 8 microns* Quiet zone size is 75% of main reflector diameter.* Spherical main reflector permits manufacture of large sizes with 1 micron rms for 1 THz operationusing optical mirror technology

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New Research Topics:

Antenna Technology Wireless Communications/GPS EM Healthcare Quasi-optics and Millimetre Wave

New analysis algorithm –DGBA Quasi-optical components/system

High Power THz Generation

Summary of the ProblemHigh-frequency methods of analysing reflectors

Analysis Objective: modular, efficient analysis tool

Physical Optics (PO) GTD

• Simple• Flexible• Non-singular fields

+• Ray-based method• Numerically efficient

• Inefficient for λ << Dλ: signal wavelength

D: reflector diameter

• Non-modular• caustics

06/23

Analysing Qausi-optical system

Component Structure Modular Analysis

GB ex

pans

ion

output plane

refle

ctor

to b

e ana

lyse

d

reflected beams

input plane

}

diffracted beams

}diffracted beams

GB

expansion

Prev

ious

re

flecto

r

Introducing: DGBA(Diffracted Gaussian Beam Analysis)

Gaussian Beam Expansion

expa

nsion

plane

f x y A

w x mL jn xw y L j y

mnnm

( , )

( ) exp( )( ) exp( )

09/23

Gaussian Beam Reflection

sr

qq

n

x si

i

i

h

hrrx

x

h

reflected beam

incident beam

incident beam:

1 12q R

jwi i

i

reflectedbeam:

1 12q R

jwr r

r

Gaussian beam optics: • w wr i

Rr by Geometrical Optics:1 1 1R R fr i “lens formula”:

10/23

Gaussian Beam Diffraction- canonical problem -

d

q

half screen

Gaussian beam

11/23

Gaussian Beam Diffraction - solution of the canonical problem -

U P Ui Pj jk d ys d yp

j jk d ys d yp

x xsx xs

( ) ( )erfc( ( ( ) ( ))

/)

erfc( ( ( ) ( ))/

);

112 0

1 1 2

12 0

1 1 2

U P( )

U Pi ( )

: Total diffracted field at the observation point

: Unperturbed (incident) field at the observation point

k d ys0 : Complex phase at the stationary point of the boundary-diffraction integrand

k d yp01 : Complex phase at the (first) pole

of the boundary-diffraction integrand

xs : Shadow boundary

12/23

Gaussian Beam Diffraction- normal incidence -

half screen

z

x

y az=-z

0

Q(x ,y ,z )0 0 0

P(x,y,z)s

s

w0

Gaussian beamamplitude (+1)

• Boundary diffraction theory gives asymptotic solution• GO incident beam is complemented by a diffracted field in terms of complementary error functions

• Solution is valid for normal incidence within the paraxial

region

13/23

DGBA test application:A Cassegrain-Gregorian Compact Antenna Range (CATR) – the spherical tri-reflector @ 90 GHz -

16/23

spherical

100100

shapedshaped

feed

300

DGBA - Numerical Results- spherical tri-reflector CATR test case -

-50

-40

-30

-20

-10

-1 -0.5 0 0.5 1

DGBA; E-planeDGBA; H-planePO; E-planePO; H-plane

field

in d

B

radius in m

co-polar

x-polar

field in the quiet zone (1200 from main reflector)

20/23

28

Dichroic Dichroics are well known for their frequency selective

characteristics at millimeter and sub-millimeter wave frequencies

There are two basic types of dichroic mirrors: Patch and Slot.

29

Two channel Quasi-Optical Network (QON)

Two channels: 54GHz (oxygen lines) and 89GHz (atmospheric windows)

High pass dichroic (transmits at 89GHz and reflects at 54GHz) is needed to achieve high pass QO system

M1-54

H-54

M1-89

H-89

M2

D

30

DS

High-pass dichroic Porosity value

High cut-off frequencyLow cut-

off frequency

The final design: D = 2.16mm, S = 2.46mm, Thickness = 2.5mm

31

Measurement Transmission measurement above 75GHz was

conducted by placing it in a quasi-optical measurement bench

H

32

Results analysis - 1

Integration of DGBA and PMMDual Channel Quasi-optical system

Integration of DGBA and PMMResults – 54GHz

M1-54

M2

Horn-54

-8.68dB Beamwidth Deg.

Simulation. ： H-21.51 E-20.92

Measured ： H-20.09 E-19.34

Integration of DGBA and PMMResults – 89GHz

Dichroic

Horn-89

M1-89

M2

-8.68dB Beamwidth Deg.

Simulation ： H-21.44 E-21.22

Measured ： H-19.15 E-19.69

36

One of the most difficult components to realise in sub-millimeter bands is the THz sources.

THz sources can be broadly divided into three categories: Solid state sources; Vacuum tube sources; Optical style sources.

Each of them has its strength and weakness.

THz sources

37

Overview: State of the art

  THz-emission power as a function of frequency Solid line: Conventional THz sources; Ovals: recent THz sources *1: M. Tonouchi, ‘Cutting-edge terahertz technology’, Nature photonics, Feb, 2007

BWO

Gyrotron

38

Solid state sources: are limited by reactive parasitics, or transit times (RC) rolloff, or heavy resistive losses;

Vacuum tube sources: suffer from physical scaling problem, metallic losses and need for extremely high fields;

Optical style sources: the photon energy level (~meV) too close to that of lattice phonons, needing cryogenic cooling.

Overview: Physical limitations

39

Micro-klystron

40

Micro-klystron beam source

PSD Experimental setup to test the scale down effect Experimental measured A-K voltage and current

41

Introduction – What is Pseudo-Spark Discharge?

electron beam

anode

insulator

hollowcathode

deff

• Occurs in special confining geometry• In various gases such as helium, nitrogen, argon, et al• Low pressure, 50-500mtorr, self-sustained, transient hollow cathode discharge, for a gap separation of

several mm• High quality electron beam and ion beam extraction before and during the conductive phase

(pd)min

pd [torr x cm]

VB [V]

0 2 4 6 8 10 12 14 16

200

400

600

800

1000

1200

1400vacuum breakdown

p seu d o sp ark reg io n

pseudosparkregion

Paschen curve and pseudospark regionSingle gap PSD geometry

42

PSD Process Phase 1: Townsend discharge

- low current pre-discharge- plasma formation

Phase 2: Hollow cathode discharge- hollow cathode effect- plasma expansion

Phase 3: Superdense glow discharge (conductive phase)- high-current phase (10 kA cm-2 )

43

Phase 1: Townsend discharge

Seed electrons Pre-discharge Plasma formation

44

Phase 2: Hollow cathode discharge

Hollow cathode effect Plasma expansion Secondary emission

45

Phase 3: Superdense glow discharge

Sheath contraction Primary emission Conductive phase

46

PSD Numerical Simulation

MAGIC: Particle-In-Cell and Monte-Carlo Collision (PIC-MCC)Ref: C.K. Birdsall et al, Computer Phy. Comm 87, 1995.

47

PSD - Gas Ionisation1. Electron-induced ionisation

2. Ion-induced ionisation

The cross section depends on:1. The energy of the impact electron;2. The gas type.

For different gases, the cross sections are different functions of impact electron energy. The functions can be achieved from experimental results.

48

PSD-2D Computational ModelMAGIC 2D Model:Constant A-K voltage 10kVAK gap d=6mmRadius = 25mmRoom temperature

Insulator: 6mm thick PerspexAnode aperture: 0.5mm radiusAnode thickness: 12mm

Cathode aperture: 1.5mm radiusTrigger radius: 1mm, cable outer radius: 6mm

Nitrogen 100mTorr

49

PSD-2D Phase 1&2Plasma formation at 30ns

Plasma expansion at 50ns

50

PSD-2D Phase 3

Plasma expansion and emission at 80ns

51

PSD Process

Detailed motion of all the particles in the system.

52

Simulation results

Observed voltage between the anode and the cathode. Observed current at the anode aperture.

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Summary MM/THz technology becomes

increasingly beneficial to our society. MM/THz technology has been advancing over one century – an old and young topic.

New applications have posed many technical challenges in MM/THz technology – needing fresh blood of microwave engineers.

Solutions lies in understanding and innovation in methodology and technology.

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Thank you!