3rd elect. laser (2016)

8/16/2019 3rd Elect. Laser (2016)

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Laser

“Light Amplification by Stimulated Emission of Radiation”

Dr. Soheir Backar

8/16/2019 3rd Elect. Laser (2016)


!roperties of laser light

. "onochromatic #single spectral color and $a%elength&'

Laser light consists of one $a%elength. (he laser emits all photons withthe same wave length.

). *ollimation'*ollimation refers to the degree to $hich the beam remains parallel with

distance.Laser beams are %ery narro$ #thin beam& $ith %ery little di%ergence.

Dr. Soheir Backar )

8/16/2019 3rd Elect. Laser (2016)


!roperties of laser light #cont.&

+. *oherence #is a property of $a%es that

indicates the ability of $a%es to interfere $ith eachother&'Different parts of the laser beam are related to eachother in phase #similar in phase and direction&. (hese

phase relationships are maintained o%er long enoughtime, so that interference effects may be seen orrecorded photographically.

-. igh intensity' Laser beam is concentrated with diameter notmore than 1 mm. /ntensity %arying bet$een fe$ $attsto thousands of $atts.

Dr. Soheir Backar +

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Laser measurements

Diffractive measurements ofsmall dimensions

(ex. Wire diameter)

Profile and surface positionmeasurement

Measurement of productdimension

2- spot system

Triangulationdevice

traig!tness error

measurement

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Dr. Soheir Backar 0

Straightness error measurement'

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Dr. Soheir Backar 1

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R

y

h

2

d

2

R)

y

R3

R3

Position " # y δ

tan(2θ$2δ)%y&"'

!%d.sinδ

Dr. Soheir Backar 4

θ

θ 2

δ δ θ 22 +

θ δ θ +

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Diffracti%e measurements of smalldimension' 5ire diameter

"easurements can be achie%ed through obser%ations of the diffraction patternformed $hen a laser beam is diffracted around the edges of the ob6ect.

(he diffraction pattern produced by a $ire inserted in the path of a laser beam $ith diameter larger than the $ire is an array of spots e7tending in a lineperpendicular to the $ire.

(he diffracted spots lie in a line perpendicular to the $ire. The separationof the spots varies inversely with the wire diameter.

Dr. Soheir Backar 8

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Dr. Soheir Backar 9

5ire diameter

Angle of the nth spot from the direction of the laser beam

:o. of spot

Laser $a%elength=

=

=

=

λ

φ

n

D

n

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Dr. Soheir Backar 3

Example:

A diffracti%e measurement system is used to measure the diameter of a thin

$ire during $ire dra$ing. (he linear distance bet$een the un diffracted spot#n;3& and the )nd diffracted spot #n;)& of the diffraction pattern is 1.0 mm.(he distance bet$een the $ire and the un diffracted spot is )333 mm and thelaser beam $a%elength used in measurement is 3.1-- µm. *alculate the $irediameter.

By replacing the $ire $ith anotherone, it is found that the lineardistance bet$een the un diffractedspot #n;3& and the )nd diffracted

spot #n;)& of the diffraction patternis 3.33 mm. *alculate the $irediameter. 5hat can you obser%e<

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=igure sho$s ho$ the pattern changes as the $ire diameter decreases.

Advantages:

- :o contact $ith the $ire.

- "o%ement of the $ire along the direction of its length does not affect thediffraction pattern. (hus $ires can be measured $hile they are mo%ing, fore7ample, during e7trusion.

- >ne could obtain a continuous real?time measurement of changes in the $irediameter as it is e7truded or dra$n.

Dr. Soheir Backar

8/16/2019 3rd Elect. Laser (2016)


!rofile and surface position measurement'

@se of a light beam as an opticalprobe offers a number ofad%antages'

- no physical contact .

- no possibility of surfacedamage.

- Measurements can be made onsoft materials, like clay models.

- no wear on the measuring tool .

Triangulationdevices

To-spotsystem

Dr. Soheir Backar )

8/16/2019 3rd Elect. Laser (2016)


Triangulation devices:

A spot of light is pro6ectedfrom the measuring head

onto the surface. (he sourceis indicated as a laser diode.Light reflected from thesurface is imaged onto thedetector. /f the position of theob6ect surface changes by ∆,

the position of the imagedspot is shifted by ∆7. (hemagnitude of the shift in theposition of the imaged spot ismeasured by the detector,and the change in surfaceposition is determined from.

Dr. Soheir Backar +

/maging lens to %ie$ #focus& an image of anob6ect onto a camera sensor #remo%e paralla7 orpro%ide ad6ustable magnification&

(riangulation' a method of finding a position by measuring the lines andangles of a triangle on a map.

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Dr. Soheir Backar -

/ncident beam

λ

δ

f

f/maginglens

C

C

*

2

st

)nd

- Angle of incident beam .

- Angle of obser%ation .

- /nitial distance bet$eenimaging lens and ob6ect .

- Distance bet$een imaging lensand image plane.

- Spot displacement on image

plane .

λ

δ

f

f

C

It is required to find the relation between ! and .

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Dr. Soheir Backar 0

/ncident beam

λ

δ

f

f/maginglens

C

C

*

2

st

)nd

)]cos('[)]sin('[

cos'.

)cos(cos

)sin(.'.'

cos),cos(

coscos

)sin(.'.'

:)2(),1(

)2('.

'

'

'

)1()sin(cos

coscos

)sin(.)sin(

λ δ λ δ

λ

λ δ λ

λ δ

λ λ δ

λ λ

λ δ

λ δ λ

λ λ

λ δ λ δ

+−+

=∴

++

+=∴

=+=

+

+=

→

+

=∴

+

=

→+=∴

=⇒=

+=⇒=+

Y f

f Y Y

Y f

Y f Y

C

Y

C

X

X f

Y f Y

from X f

Z f Y

X f

Z

f

Y

Y Z

Y C

C

Y

C Z C

Z

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Dr. Soheir Backar 1

Example: A laser triangulation measurementsystem is used to e%aluate thedisplacement of a mo%ing ob6ect.E%aluate the ob6ect displacement basedon the follo$ing'? Angle of incident beam ; +3 degrees.? Angle of obser%ation ; -3 degrees.? /nitial distance bet$een imaging lensand ob6ect ; )03 mm.

? Distance bet$een imaging lens andimage plane ; -3 mm.? Spot displacement on image plane ;0mm.

/ncident beam

λ

δ

f

f/maginglens

C

C

*

2

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Two"spot system:

(his system uses t$o laser beams that strike the surface at a kno$n angle. (he t$o beams produce t$o spots of light on the surface. The separation of the spotsdepends on the distance from the sensor head to the surface. (he spotspacing S is gi%en by'

5here D is the displacement ofthe surface from a nominalreference position and θ is theangle bet$een the beam andthe plane of the surface. (het$o spots come together $henthe surface is at its referenceposition.

As the target surface changes position, the spots mo%e closer together or fartherapart, depending on $hether the surface position is mo%ing farther from ornearer to the sensor head.

Dr. Soheir Backar 4

θ tan

2 DS =

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#easurement of product dimension:

Dimensional comparison measurements in !ic! t!eposition of t!e surface of t!e product is determined

relative to t!e sensor !ead suc! as triangulation met!od.

Diffraction type measurements !ic! !ave *eendescri*ed earlier in relation to measurements of ire and

fi*er diameters.

Measurement of t!e o*scuration of laser *eam t!at passesacross t!e item.

Dr. Soheir Backar 8

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#easurement of the laser beam obscuration:

>bscuration techniFue in%ol%es interposing the ob6ect to be measured in thepath of a laser beam that is scanned across the ob6ect. (he beam is detected by aphoto detector #electronic component take light and transforms it to %olt&. (he %olt signal is amplified to be seen clearly on the screen. (hen, this signal isentered to a data acFuisition card $hich transform analog signal #changing $ithtime& to digital signal that can be recorded.

An ob6ect in the path of the beam interrupts the detector output for a time. /none arrangement, the interruption produces a timing signal.

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=or e7ample, the diameter ofdifferent cylindrical $ork piecescan be measured by mo%ing the beam along the length #profile&

of the ob6ect.

An alternati%e method uses astationary laser beam through $hich the ob6ect passes, as on acon%eyor belt or mo%able table.

(he table %elocity should be kno$n by makingcalibration #put block gauge $ith kno$n distance onthe table and according to tra%elling time $e cancalculate table %elocity&. By measuring time $e cancalculate the reFuired dimension.Dr. Soheir Backar )3

+

11 .t V D =

22 .t V D =

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Dr. Soheir Backar )

Sho$ ho$ to measure the taper angle of the sho$n $ork piece using opticalobscuration.- Dra$ the measuring set?up.- E7plain ho$ the %alue of the angle is e%aluated.

L

L

D,

D2

11 .t V D = 22 .t V D =

θ

2

12 D D −)2/)(

(tan 121

L

D D −=

−θ

3rd elect. laser (2016)

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