1

Lasers from Islamabad

The LHC at CERN

and

Lahore

What Binds them Together

Shaukat Hameed Khan

13 May, 2010

Celebrating 50 Years of the Laser

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The Laser is 50 on 16th May 2010.

• Will discuss:• A small Pakistani contribution to the design of the

system which makes up the massive detectors in the Large Hadron Collidor at CERN in Geneva.

• Context of mankind’s greater quest over the centuries for precision, whether these relate to his position on earth or in space, or timekeeping.

• Role of Qaim Mohammad of Lahore

• Will present the principal features of the laser programme in Pakistan.

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3

Tribute to the Laser Pioneers

• The Laser should have been ‘discovered’ / built in the 1930s. All the theoretical constructs were in place.

– Einstein …. Spontaneous and Stimulated emissions– Quantisation of Blackbody Radiation– Gaseous discharges, crystallography, optics, etc

• Recognition? Professor vs Post-Doc

– Townes & Schawlow: Maser’s optical equivalent at Bell Labs. • Primary interest: Continuous laser for spectroscopic studies –

not pulsed - could not use ruby as the lasing medium.

– Maiman’s breakthrough, 1960 : Pulses of coherent light from a small ruby rod illuminated by a flash lamp.

– Initially a cinema projector lamp

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4

SOURCES OF LIGHT

NaturalLight

Cd M2

ManmadeLight

Lasers

Normal Vision

SunHor. Zen.Night

SkyDark Clear Sky

Sky

10-6 10 -2 10 +2 10+6 10+10

1Km 1 m 1 µm 1 AO 1 X-ray Unit

1014 1010 106 102 10-2 10-6 10-10

Wavelength: IR 7000 Ao 4000 Ao UV

(Microns)

The EM Spectrum

4

5

SNo

Source Power(Watts)

EmittingArea

(cm 2 )

EmittingSolidAngle

Ω

BW/cm2/

Ω

1 Sun 4*10 26 2.5*1023 4 Π ~ 130

2 MercuryArc Lamp

104 ~ 1.0 4 Π ~ 800

3 Laser Diode Pointer

10- 3 ~ 0.1 10 - 6 ~ 10 4

4 Nd : Yag( LRF )

1* 10 6 ~ 0.1 10 - 6 10 13

5 Nd : Glass( Fusion)

2*10 13-15 4*103 5*10 – 8 10 25

Brightness: Power / unit area / solid angle / unit λ

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6

Basically Three Types

DiagnostcTool

MetrologyAnalysisNDTFusionSensor/Probe

Change Agent Carrier

Material Working

Cut, Drill Weld

Surgery

Photochemistry

Communications

Controls/Guidance

Military Applications• Laser

Rangefinders• PGM’s• Weapon• Secure Comm

Laser Applications

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Some ApplicationsIndustry:o Welding of auto chassis, hermetic sealing of ICso Microdrilling, Cutting, Ablationo Photolithographyo Rapid Prototyping, Stereolithography (photo - polymers)

Medicineo Optical Coherence Tomography (laser ultrasound, under the tissue).o Photo dynamic therapy

Astronomy / Astrophysicso Giant Interferometers ( LIGO) - gravitational waves - precision < proton sizeo ‘Guide’ stars –Induced Fl., Na, 93Km - Adaptive optics of ground telescopes

Research: endless applicationso Optical tweezers – hold & manipulate μscopic particles – bacteria o Laser cooled atoms – Optical molasses – Atomic clockso Slow light o Quantum Computingo Fusion / Simulation of Nuclear Weapons

Communication, Entertainment ……….Defence……..

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Lasik, Moulding of the cornea

Guide star, 30 m telescope. 10x of Hubble

fsec photo-graphy

OCT of fingertip 1974 1982 1990 1998

2006

Argus

Shiva, 1X beam

Nova (single beam)

T3, Univ. of Rochester

10 TW Nova

LLNL, 100 TW, Nd laser

LLNL 1.25 Petawatt Nd laser

CEA Limeil

Rutherford Vulcan CPA

Long Pulse Technology

Early Lasers

Chirped pulse Amplifucation

NIF single beam

1,000

100

10

1

Peak

Las

er P

ower

, TW

NovaNIF

107

105

103

102

101

Joul

es

1970 1980 1990 2000 2010 Year

DPSSL

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Can we measure this slowing down ?

LASER RANGE FINDERS & LONG BASE INTERFEROMETRY ( lunar and satellite ranging); + GPS. ( < 0.01 msec )

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The Longest Day Last in the last 100 Years:: During 1912.The shortest day : August 2, 2001, > by 1 msec<

(Geophysicist Dr. Richard Gross.JPL)

LASER RANGING: Airborne lidar system

finds hidden fault lines

AA 3Laser Altimeter,

Accurate estimate of the position of the center of the moon:

Of paramount importance in mapping out the position and orbit and specially for studying the moon’s liquid core and testing ideas about gravity

April 22, Murphy et al UCSD/NASA ; laser pulses, the 3.5 m telescope at the Apache Point Observatory in New Mexico, found the Lunokhod 1 reflector and pinpointed its distance from earth to within 10 mm

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Profile of the Laser Programme in PakistanModest programme started : June 1969 at At. En. Centre, Lahore

Now > ~500 professionals at Nilore <Optics Labs , to NILOP in Islamabad

Design and Fabricate• Complete Laser Systems

– UV to IR ( Solid State, Metal Vapour, Liquid, Gas )– Pulsed (psec - nsec), Moderate Rep rates; Continuous– Fixed Frequency / tunable dye

• Infrastructure and Manpower for : Optical Components / Modules / Systems– Optical Coatings– Electronic systems– Precision Mechanics

• Applications– Spectroscopy, Photochemistry, LIS– PDT, Defence, Agriculture, Industry, Eu

• Exports of complete systems and sub-systems

Atomic Clocks / BECLaser Land Levelers

Generated Revenue:Rs 5 billion in 20 years

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Some Tributes / Some Names# Year Laser Group/ Optics Labs Others1 1969 Dr S.H. Khan, Dr. Aijaz karim

QAU1975 – 1980Dr. M. S. Razmi, Dr. G. Murtaza

1980-1990Dr. M.S. Zubairi, Dr. M. Aslam Baig

2008Dr. Khurshid Hasnain

2 1970 - 78

Dr. Nisar Ahmad; Dr Sh. ShahdinDr Qaisar Sultana;

Dr. Aslam Khan Dr. Ehsan Khawaja;

1975-85

Dr. Khalid RashidDr. Iqbal SiddiquiDr. Sarfaraz BhattiDr. Javed AkhtarDr. Badar SulemanDr. Masroor Ikram

Mr. Amjad ParvezMr. M. IqbalMr. Maqbool Ch.Mr. Afzal ChaudhryMr Shahid ButtMr Imtiaz Ahmad

3 1985-2010

Dr. M.S. FarooqiDr. M. NawazDr. A. FarooqDr. M ArshadDr. Ramiz ul IslamDr. M AtifDr. ShamraizFirdousDr M. Saleem

Dr. Ghazanfar HussainDr. M AslamDr Mushtaq AhmedDr. Zulfiqar AliDr. Abid AliDr. Jahanzeb ShahDr. Khalid MehmoodDr. Iqtidar Shakoor

Comsats IIT, Islamabad2006-

3 PhDs, led by Dr. Aslam Khan

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Down Memory Lane

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Nathiagali, 1976, with Prof Kastler

Islamabad, 2006; with C.H. Townes

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Some Current Laser Research

1. Atomic and Molecular Spectroscopy2. Building a Cesium Atomic Beam Clock at NILOP

A. Global Accuracy ~ 10-12; Short Term Stability ~ 10-10. B. MOTs (Total Electron Impact XSection on Cs)C. BEC … Free Expanding Cloud of Cold atoms as an Atomic

Standard: Ramsey Fringe Contrast . Initial stages3. PDT4. GaN. MBE with In-situ ellipsometeric Studies; RHEED (Reflection

High Energy Electron Diffraction)5. Laser Assisted Ablation/Deposition, Diamond like films6. LIDAR7. Nano Magnetic Films by Laser Ablation

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Epitaxial Growth Laboratory• MBE with In-situ ellipsometer; Reflection High Energy Electron Diffraction

Lithography Laboratory

• Spin Rinse dryer; Oxygen Plasma Stripper, Spin Coater, Mask Aligner

• Developer Station; Inspection Microscope; Surface Profiler

• Acid Wet Bench; ICP/RIE System

• E-beam System; Dicing Saw, Probe Station; Wire Bonder

Lapping and Polishing Laboratory

• Interferometer, Microscope, Diamond Cutting Machine

PVD Laboratory RF/DC Sputtering; Thermal/Electron beam evaporation; PLD

Characterization Laboratories Ellipsometer; Spectrophotometer; FTIR; Scan. Probe /Tunnel. μscope; Photo/Electroluminescence/Raman spectroscopy

Simulation and Designing Laboratories LaserMod software

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Comparative study of Group II-B elements

42930 42960 42990 43020 43050 43080

100

200

300

400

500

+

+

18d3 D

1

24s3 S

117

d3 D1

16d3 D

1

21s3 S

1

25s3 S

1

25d3 D

2

Ioni

zaio

n en

ergy

(arb

.uni

ts)

Laser energy cm-1

19d3 D

2

27s3 S

1

+173 S

1

72225 72250 72275 72300 72325 72350 72375

200

300

400

500

600

700

800

900

23d1 D 2

27d3D2

23d3D2

481nm

27s3 S 1

Ioni

zatio

n Si

gnal

(arb

.uni

ts)

Term Energy cm-1

23s3 S 1

480nm

21d3D221

d1 D 2

6s6p3P1

6s2 1S0

λ1= 253.7nm

84184.1 cm-1

λscan=247-223 nm

41580 41610 41640 41670 41700

200

300

400

500

600

700

800

900

23d1 D 2

21d1 D 2

23s3 S 1

27s3 S 1

21d3 D 2

23d3 D 2

Ioniza

tion S

ignal(

arb.un

its)

Laser Energy cm-1

27d3 D 2

Published in Eur. Phys. J. D 53, 147 (2009).

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PLD

Dye LaserYAG for CERN

TEA CO2 Laser

Some Infrastructure

Optical Fabrication

Tea N2 Laser

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1.

Laser Range Finder, AR 3

2.

Tank Range Finder; Models TR 2, TR 3

3.

Tank Driver’s Night Sights, DNS 3

AA 3Laser Altimeter,

4. 5. 6.

Hand-held Thermal Camera

8.7. 9.

Defence Products

ARS 134Airborne Radio

Tank Gunner Thermal camera

Replacement, LRF, T-80 UDTI Test Bench

EU 9.7 m contract

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LDR 3 Laser Designator

Day / Night Surveillance System

INU Calibration JigHUD Combiners

Conventional Systems for Defence

Laser Land Leveler

• 30% less water• Higher yields• Prevents

waterlogging & salinity

10. 12.

13.

11.

14.

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Optics Labs and CERN: A Chance Encounter , 1989

• A CERN team was visiting PINSTECH in Sep. 1989. • Looking for partners : design and fabricate parts and modules for

the massive CMS Detector (LHC) at CERN• Spare time, unscheduled visit to Optics Labs next door.....

Only dedicated / integrated Laser Lab in Pakistan:

Clear expertise : optics, lasers, electronics precision mechanics Excellent infrastructure:

Optics Lab and its precursor – the Laser Group at Pinstech involved in research , teaching, and production for over 36 years.

A natural partner for CERN.

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Includes 40 laser based Position Monitoring Modules

From Optics Labs, Pakistan.

2007: The Loaded Tracker Unit being Placed inside CMS.

Laser Research in Pakistan and CERN

2121

FOUR MAJOR CONTRIBUTIONS of PAEC* to CERN:

Transporter ~ 13 T

CMS Magnet Feet, ~ 28 T

1. Magnet Feet for CMS (Fabrication only)

2. Resistive Plate Chambers ( Assembe / Test)

3. Assembly and Test of Carbon Frames and RODS for TOBs

4. Laser Based Position Monitoring System for Tracker of CMS (design, fabrication, installation)

5. Part of Int. Data Processing - GRID

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CMS COLLABORATION HAD in 2000: 36 NATIONS 160 INSTITUTIONS 2008 SC. / ENG.

Total Weight : 12,500 TonsTotal Length : ~22 metresDiameter : ~15 metresMagnet : 4 Tesla

SC cable: 4.2oK, 20 kilo Amps 27000 A / mm^2

ASEPS: Asia-Europe Physics Summit 24-26 March 2010

2323

Main Purpose of CMS: p-p detector > Study physics underlying breakdown in the electroweak symmetry:Several possibilities: Higgs mechanism favoured in the context of Supersymmetry. BUT, Need to cleanly detect “signatures”

… photons, muons, electrons jets … over large energy range and large luminosities

CMS is Optimised for search of : Higgs Boson ; CP violations; Top Quark Studies; Onset of Quark Gluon Plasma Formation

Luminosities ≥ 1034 cm-2 s-1; Magnet 4 T, crystal e.m. calorimeter, powerful inner tracker

Leads to precisions of < 1% at 100 GeV

The CMS Challenge

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Depends as much on Intrinsic Detector Capability Stability of the Structure ( Design… Materials …

Stiffness / Stability )

Expected Movements Of The Assembled Structure ? Very Very Heavy /Large StructureWill Move and Distort Due to:

Gravity, Magnetic field, Temperature Gradients, Differential Expansions ( e.g, Si, Steel, Al , CF, quartz) ,

Even Affected by the Water Level In Lake Geneva

Shifts /distortions up to 20 mm !

Optics Labs Focussed on the CMS TrackerTracker Performance: Heart of The CMS

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• Dia x Length: 2m x 6m

• ~25,000 silicon strip detectors(A=210 m2 ), connected to 75,000APV chips;

• Readout channels: 9.6 million

• µbonds: 26 million

Tracker: Cylindrical FOUR mechanically independent components

The single most important feature of CMS is the configuration which is designed for high momentum resolution of the muons.

Places a very stringent demand upon the spatial resolutionand therefore the detector alignments.

TEC, Tracker End Cap

Beam Axis Pixel Det.

TIB, inner barrel

TOB, Tracker outer barrel

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X1,y1

Need to know where the detectors are w.r.t each other

X2,y2

Muon Momentum related to bending in transverse plane

Rad. of curvature ρ(m) = pt GeV/c / 0.3 BT. ρ ~d2/8s‘s’: muon trajectory sagita after travelling distance ‘d’

Error in s > error in muon measurement.

δs/s = δp/p α σs (mm) pt (TeV) / d2 (m2) B (T)26

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TIB,

TECTOB

Pixel Det.

Frame to Petal (Rod) Petal to Structure

Si Detector to CF Frame : 0.010 mm

CF Frame to Petal / Rod : 0.020 mm

Petal / rod to support structure : 0.020 mm

Quad. Sum: 0.030 mm

Detector to Frame

Beams run Parallel to Z-axis; Disc may rotate around Z (∆ Φ) or move perpendicular To Z (∆X, ∆Y)> Measure the Φ co-ordinates of the laser spot.

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TRACKER: Required Precision

X

Y

X

Y

Z

Vertical Position

R ( µm)

R - φ( µm)

Z ( µm)

200 mm 100 15 500

700 mm 300 15 500

1200 mm 600 50 2000

Max. distortion @ R=1200 mm: - 0.314 mm at top

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Key Features of the Position Monitoring SystemThe laser pulse produces photo-electrons in the silicon detector

Also transmitted to other detectors if correct λ.

The same electronic systemreads out the signal from the high energy particles as well as the laser beam.

Only a few tens of fCof charge to be produced to avoid saturation

λ=1064 nm

Response curve, Si

Transmission Curve of Si

T~30%~0.2A/W

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Laser passes through successive detectors:

Read the signal / laser position (c.o.g.) from each detector

One shot gives many relative positions of many detectors at the same instant

Repeat sequence

Read the Laser Centre of Gravity (COG)

Diffraction from Detector strips

Less coherent diode laser

Laser COG Profile

3131

250

200

150

100

50

0

269.6 269.8 270.0 270.2 270.4

Some Data

The C.O.G. Resolution :

0.2 µm in X ; 0.3 µm in Y

3232

Searching for Laser Beam Paths

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Ray 2

Ray 3Ray 4 /6/ 8

Ray 1

Si, 2-D Mini-disc

Periscope

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1

2

8

8

8

Lasers&

Trigger

Splitters

Optical Switch

Ray 2

Ray 3

Ray 4

Patch PanelConn.

Tracker Connections

Control Room

Collimator

Laser

Switch

Trigger

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10 Years of Collaboration with CERN Main Contribution of Optics Labs : 1. Position Monitoring System of Detectors in the Tracker, + work on

Link with End Caps / Muon Chambers:• Design, Fabrication,Testing, Assembly & Integration• Testing of Components for Radiation Damage• Fabricated / Tested Prototypes for Performance• Convergence between various proposals and sub-systems

from Ger., Hung., Spain, Port. and Pakistan• Have Supplied and Integrated 40 Modules

2. Fabrication of TOB Frames3. Assembly of the Tracker Outer Barrel RODs (TOB RODs) which are a

self-contained assembly• Design of Test Jigs /Processes for Individual Modules and

RODs

4. Installation, Validation, and Testing at CERN

5. Miscellaneous: Hard Gold Plating of Al Conductors35

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13 Diff. Glasses

3 Diff Optical cements

HR / AR / Metallic Coatings

Transmission recovery

Radiation Damage Studies::

n fluence > 4 x 1014 /cm 2

γ : 10 Mega Rad

Major App. : Space Optics

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AR coatings for Si > 10 layersIncreased From 29%-39%

37 37

o Transmission of Silicon Sensors : Max. number to be crossed Depends on : R and T of Silicon; and the signal-range of the readout electronics.

Designed Anti-reflection coatings on some discarded Si samples from CERN, to achieve max transmission

o Laser Wavelength: Optimised for Max Signal… 1064 nm

o Light Distribution System: Optical elements such as beam splitters, prisms and fibres …. Polarisation dependent

Proved its stability and linearity.

Built a full scale one-quarter mock-up of one laser channel

Quick Summary

Cut-out View

3838

Only Rad-Hard materials usable. n fluence: 4x1014 ; γ : 10 MegaRad

Used the 10 MeV Pinstech Reactor

13 Diff. Glasses; 3 Diff. Opt.Cements; Coatings: HR / AR /Metallic

[ Some glasses / coatings /cements had not been studied previously ]

Tests of adhesion and abrasion of coatings.

Recovery of glass Transmission?

Drew up the Specs. Of Optical Components Designed, Fabricated & Tested Prototypes Produced the Final Modules

Choice /Optimisation of Optical Components.

Stability Studies: Large-scale distribution system for laser light.

Stability Study of Carbon Fibre structures to possibly realize a stable reference structure/wheel, its ( temperature variations, radiation, change of humidity,etc.) >>> (Done in Portugal)

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Automatic Test Jigs and Carbon Fiber Frames for Tracker Outer and Inner Barrels ( TOB, TIB)

FERMILAB, USA; Add APV

CERN4 -5 persons from

Optics Labs. at Cern, since 2001

Assembly line for TOB Frames, 30 /day

Completely assembled Rod: 6 Detector Modules Under Test.

CF from Belgium

Assemble Detector Rods, CERN

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Collaborating Partners• S. H. Khan, B. Suleman, S.M.J. Akhtar, A. Parvez,

M. Ashraf, Farooq Ahmad, Liaquat Ali, Imtiaz Ahmad, Irshad Ahmad, Abid Ali -- (Opt. Labs - PK)

• R. Ribeiro, R. Alemany, H. Voss, K. Hopfner,S. Silva (Pedro Reis), C. Martinez, --- (CERN)

• B. Wittmer, A. Ostaptchouk, S. Schael --- (Aachen,- Ger)

• M. Vaz, J. D. Rodrigues, J. S. Gomes, T. Marques, A. Nicolau -- (FEUP & INEGI - PT)

• P. Rato, H. Gomes, P. Stallinga, M. Abreu -- (Univ. Faro & LIP PT)

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o Theorise o Observe o Measure

Man’s Greatest Enterprise:

Will Talk About: Precision of Measurement Distances, Positions and Time

plus some digressions

Always worried about AbsolutesWants answers to WHY, WHERE and WHEN

All the good things in life have to do with finding answers.. ….. some bad things also

… . and think again

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What is the scale of the Solar System? Universe?

What is the value of the Astronomical Unit, AU

We know that the Sun-Earth Distance ~ 150 million Km > 1 AU ; Immense figure in everyday scales

The Classical Issue: Our Place in the Universe

The Solar Parallax

and Pythagorus

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Angle between thecentres of the Moon andthe Sun when the Moon'sshadow is at the half waypoint: < 87° >.

Sun ~ 20 times furtheraway than the Moon

RELATIVE DISTANCES FOR THE MOON AND THE SUN

Aristarchus of Samos (c.310 - 230 BC)

More Measurements ::: Angle ; Sun more Distant, and Larger

Accepted to Brahe (1546 - 1601)

Kepler (1571- 1630) : < 1‘, observing Mars, instead of moon

Ratio ES / EM … ~ 229 (based on an angle of 89.75° ).

Modern Value ~ 390 (angle of 89.853o)

Precision Increase: 2.75o in 2000 y, 0.103o in next 300 y !43

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Apart from Theory, needed two more items;• Instrument: Galileo Telescope (1610) , Chronometer• Observors: • Method: Transit of Planets across the Sun

The Abs. Value of the A.U. ::: Planetary Transits

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Edmund Halley, 1656 – 1742 to carefully observe the transit of Venus in 1761 and 1769

International ‘frenzy’ : 1761 and 1769 to observe the transits

>> Mason & Dixon ( U.S.A.), Wales ( Canada), Chappe ( Siberia) Pingre ( Madagascar), de Gentil (India)

Capt. Cook, 1769

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Correct Measurements of the Solar Parallax

Kepler's laws : the relative distances of the planets from the Sun (in AU)

1769: absolute dimensions of the Solar System ( avg. of some dozens of observations !

Needed: A transit of Venus + two observers at different latitudes on Earth + telescopes + accurate timekeeping

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Solar parallax : between 8.55" and 8.88".

Modern accepted value = 8.794148”CERN June 8, 2004

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Astrolabe, Brass, 1634/5, Made by Qa’im Muhammad, Lahore, (Lewis Evans Collection, USA)

Depicts 50 stars + a gazetteer of longitudes &latitudes of 120 locations

Scales of Sines, Cosines & Cotangent (latter for determining prayer times … a scale for finding the direction to Mecca (the qibla)

The back includes a table of the planets, ...... and unusual solar calendar

Lahore School Of Astrolabe Makers Was Famous in the 16/17th Centuries

Moving About: Knowing Where You Are

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DEFINITION OF THE METRE

The metre (m): Length of the path traveled by light in vacuum during a time interval of 1 / 299 792 458 of a second.

1 m = c• t, with t = 1 / 299 792 458 s. Hence(?) c = 299 792 458 m/s.

The unit of length : depends < unit of time > .

PHYSICAL REALISATION OF THE METRE:

Uses a Laser of a known and highly stablefrequency.

Stabilised laser, λ = c/ν …. allows length to be measured by direct comparison.

The primary standard : a HeNe laser, whose optical frequency can be stabilised to absorption line of iodine gas. 47

48

Day One complete rotation time of the earth> is not a good measure of the unit of time.

a. "mean solar day" lengthened over the centuries; b. periodical (seasonal) and non-periodical variations.

Time Keeping

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World’s timekeeping system no longer has an astronomical basis

1967 -- The 13th General Conference on Weights and Measures defined the second on the basis of vibrations of the cesium atom;

The second : duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

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NIST- F1 Cesium Fountain Atomic Clock

Primary Time and Frequency Standard:

Developed at the NIST Labs (Boulder, Colorado).

Part of the international group of atomic clocks that define Coordinated Universal Time (UTC), the official world time.

Because NIST-F1 is among the most accurate clocks in the world, it makes UTC more accurate than ever before.

How Does it Work ?? 49

50

Laser Cooled Clocks

IMPROVEMENT IN CLOCK UNCERTAINTY

NIST-F1:; 1.7 in 1015, 1 sec in 20 X106 Years; (3 X better than NIST 7 )

NIST- F2 ~ 10 x more precise than NIST-F1

NIST-F2

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Atom Irradiated by a resonant photon (exact frequency)

Atom absorbs this photon; its momentum changed.

Atom is excited (typically few ns), then returns to the ground state, emitting a photon. Step repeated

Absorbed photons come from the same direction

Emission can take place in any direction in space with equal probability. On average, therefore, the momentum change over a number of emission events is equal to zero

v1, λ1 v2, λ2 …… λ = λ1+ λ2 + λ3 + λ4 ……Laser

Atomic beam

51

Laser Cooling (Nobel Prize for Physics in 1997).

52

Momentum change over a number of absorption events is not zero >> there is a net momentum transfer from light to the atom

For each absorption/emission cycle the speed of the Cs atom reduces by around 3.5 mm/s.

Apply to all three dimensions,

At the point of intersection of a number of laser beams, possible,, to cool a cloud of cesium atoms to a few µK.

v1, λ1 v2, λ2 …… λ = λ1+ λ2 + λ3 + λ4

LaserAtomic beam

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Fountain Clock: Uses a fountain-like movement of atoms to measure frequency and time interval.

Cesium atoms introduced into the clock's vacuum chamber.

Six IR laser beams directed at right angles to each other >>slow down the atoms, bunch them, and cool them to temperatures near absolute zero ( ~ µ K degrees )

Two vertical lasers gently toss the ‘ball’ upward (the "fountain" action), and then all of the lasers are turned off. This little push is just enough to lIft the ball about a meter high through a microwave-filled cavity. Under the influence of gravity, the ball then falls back down through the microwave cavity. 53

5454

Video Animation of Cs Fountain Atomic Clock

http://www.colorado.edu/physics/2000/bec/images/evap2.gif

NIST Web site

55

NEW CONCEPT : FEMTOSECOND LASERS

Modelocked Femtosecond Laser:

Large gain – bandwidth product :::: used to create a set of equidistant laser frequencies.

The mode separation is given by the free spectral range of the cavity or more precisely by the pulse repetition rate.

Optical frequency comb can be used like a ruler to measure large optical frequency differences. The Modes are Distributed in frequency space within 3 parts in 1017

Mode spacing equals the pulse repetition rate within 6 x10-16

Frequency comb

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A photonic crystal fiber creates additional modes via four wave mixing.The resulting frequency comb is wide enough to measure the frequency difference between a laser at f = 282 THz ( 1064 nm ) and its own second harmonic 2f ( 532 nm ).

We get the laser frequency itself: 2f - f = f.

Knowing f : know frequencies of all the modes in the comb

HENCE can be used for optical frequency measurements.

56