Laser Beam Cleaning

7/24/2019 Laser Beam Cleaning

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THE AMERICAN COLLEGE, MADURAIPOST-GRADUATE PHYSICS DEPARTMENT

TITLE: OPTICAL SPATIAL FILTERING (Laser beam clea ! "#

INTRODUCTION: This is the first experiment in coherent optics. A laser beamFrom HeNe laser) is used in this experiment. We need to remember all precautionswhile working with laser. If laser light is di erged with a magnif!ing lens and isincident on a screen" one can see the patch of light non#uniform with $pits andalle!s%. &ee Fig.'. This needs to be cleaned for uniform illumination for opticaldata processing(shear interferometer etc. This cleaning up process is also calledspatial filtering of laser light.Two important functions of a con erging lens A con erging lens performs twoimportant functions '. imaging and *. Fourier transform of the ob+ect transparenc!.The first one is more ob ious than the second. When a laser light ,parallel beam) isincident on a microscope ob+ecti e" er! close to the lens the laser light con erges to

a small area at the focal plane which is about - / mm for -0x ob+ecti e. Theillumination on the small area is the Fourier transform of laser beam. &ee Fig. *. The$pits and alle!s% are mapped as sidebands of higher spatial fre1uencies which areunwanted and ha e to be filtered out.

THE PHYSICAL SITUATION: An optic bench of a meter and a half with six up#rights for mounting optical elements" He#Ne laser ,* - mW)" a microscope ob+ecti eof -0x" pin#hole ,diameter approx '/ micron) mounted at the center of a plate. Themicroscope and pin#hole mount are in one unit and fixed on to an up#right of annoptic bench. A camera is needed to take the patterns.

PROCEDURE: 2eam alignment. Align the laser beam along the axis of the optic bench from one end to the other. 3ake +udgment on the positions of arious opticalelements !ou will mount. 4onsider another pin#hole ,of si5e ' * mm) at the center of a cardboard mounted on one up#right and mo e it along the optic bench from one endto other while the laser light alwa!s passes through it. All other up#rights areremo ed in this case. Ad+ust the base screws and uni ersal ad+ustments of the laser

platform to achie e this. This alignment is er! important. This is the prelude tomake a $-f% s!stem with two lenses of same focal length.

The spatial filter assembl! is mounted on another up#right so that laser beam passesthrough the microscope ob+ecti e and on the pinhole ,'/ micron). 2! carefull!ad+usting the distance the Air! pattern ,circular rings) can be seen on a screen keptclose to the pin#hole. What is important is the central maximum which is almost ofuniform illumination. When the pin#hole is exactl! at the correct position" The centralmaximum co ers the field and uniform illumination results. &ee Fig. 6. Now thehigher spatial fre1uencies are cut#off.


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The si5e of the pin#hole for filtering is gi en b!

d = 1.22

0f

With = 632.8 nm, 0 = 2 mm f = 4 mm,d isdetermined asabout 15 20 micron .

SOME REFERENCES'. 7 8oodman" Introduction to Fourier optics" ch.9.*. :ipson and :ipson" ;ptical ph!sics" pp '69#*6-" pp *90#*9


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encouraged to add some experiment. He(she first demonstrates the experiment in aseminar and then it is added in the lab session suitabl!.

; er the !ears the instruction sheet gets modified and some experiments are remo edand new experiments are added.

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Fig. '. ?nfiltered laser light from a magnif!ing lens. $Bits and alle!s% ,non#uniformillumination) can be seen. This is due to the presence of dust particles on the mirrors of the


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Fig. *. The Fourier transform of non#uniform laser light is at the back focal plane ofmicroscope ob+ecti e. The pin#hole of si5e */ micron cuts off the side#band ,higher spatialfre1uencies) and allows the central region of almost uniform illumination. The microscopeob+ecti e and the pin#hole are in one block. C#D# E motion control is needed for the pin#hole mount.

Fig. 6. The same laser light after remo ing the $pits and alle!s%. 4omparing withthe Fig. '" we can be satisfied with the filtering action of the pin#hole. This beamis now read! for other further acti ities on the optic bench.


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Fig. -. The filtered beam emerges as a di erging light from B and the lens : has itsfocal plane at B" so the beam is rendered parallel. ;n another mount a shear plate ismounted and the arieties of shear interferograms are obser ed &ee Fig. /. When the

point B is exactl! exact ad+ustment is the focal plane" either a broad dark or a broad bright fringe is seen as shear interferogram. Fig. 6 is such a pattern. When the shear plate is remo ed and the beam is intercepted on a screen" it diameter remains thesame for a considerable length of propagation" so exact collimation is achie ed.


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&ome arieties of shear interferograms with aberrations of a test lens.

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