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EOP4056 Optical Metrology and Testing: Experiment OM1
EOP4056 Optical Metrology and Testing
Experiment OM1: Setting Up of a Michelson Interferometer
1.0 Objectives
To set up a Michelson interferometer from discrete optical components
To observe the Michelson interferometer behaviors and characteristics
2.0 Apparatus (number in the brackets is the number of sets)
Optical breadboard (1)
1.5mW HeNe laser with mounting assembly (1)
Beam steering mirror with mounting assembly (4)
Bi-concave lens, f = -25.4 mm, with mounting assembly (1)
Bi-convex lens, f = +200 mm, with mounting assembly (1)
Broad band beam splitter, R/T  50/50 @  = 480 – 700 nm, AOI = 45o, with
mounting assembly (1)
Beam stop (2) and viewing screen (1)
2 mm plastic aperture (1) and 2 mm paper apertures (2)
Plastic ruler (1) and cardboard marked with 90o vertical line (1)
3.0 Introduction
The Michelson interferometer is an important example of interferometers based on division
of amplitude. A partially reflecting mirror is used to divide a wave (beam) into two resulting
waves (beams) which wave fronts (beam sizes) maintain the original width but have reduced
amplitudes. These two beams are sent in quite different directions against plane mirrors, from
where they are brought together again to form interference fringes.
M1
S
L
BS
C
M2
Observe
Figure 1: The Michelson interferometer. S is a light source. L is a diffusing ground-grass
plate or a lens to extend the small light source S. BS is a partially reflecting mirror or
beam splitter. M1 and M2 are highly polished plane mirrors. C is a compensating plate
which is identical to BS except the partially reflecting coating.
The Michelson interferometer had been invented before the first ruby laser was built on 1960.
An extended light source is required to extend the field of view of an observer. For small
light source, an extender L is required. The light coming from the source S is divided into (1)
a reflected and (2) a transmitted beam of equal intensity by the beam splitter. These two
beams are reflected by mirrors M1 and M2 and return to the beam splitter. Part of the beam
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EOP4056 Optical Metrology and Testing: Experiment OM1
coming from M1 passes through the beam splitter and part of the beam coming from M2 is
reflected by the beam splitter. Thus, the two beams are brought together to form interference
fringes.
Since one beam passes through BS 3 times, a compensating plate C is required for another
beam so that both beams pass through equal thickness of glass. The inclusion of a
compensating plate negates the effect of dispersion (optical path varying with wavelength)
due to the glass medium of the beam splitter. Hence, a very board bandwidth source can
generate fringes. However, C is not required if laser source is used. To obtain fringes, the
mirrors M1 and M2 are made exactly or closely perpendicular to each other. Depending on the
bandwidth (coherency) of the source, the optical distance of M1 and M2 to the partially
reflecting coating must be closely equal. However, if a laser source is used, optical path
difference up to 10 cm can still produce fringes.
3.1 Circular fringes
To understand the origin of the fringes formed, Figure 1 is redrawn with all the elements in a
straight line.
d
l2
l1
P
2d
l2
l1

P1’
P2’
P1’ 2d
P2’

2dCos
O
Ob
De
M1 M2 ’
L
L1’
L2’
Figure 2: A conceptual rearrangement of the Michelson interferometer. Ob is the
observer viewing at BS where all the elements can be seen in BS. M2’ is the image of M2
formed by reflection in BS. Assume the distance of M2 to BS is larger than that of M1 to
BS and M1 and M2’ are parallel. L is swung over about BS so that it is in line with M1 and
BS. L1’ and L2’ are the images of L in M1 and M2’, respectively. A large collecting lens
can be put at De position to form fringes on a screen located at the focal plane of the lens.
From the figure, the two virtual sources L1’ and L2’ are coherent in that the phases of
corresponding points in the two virtual sources are exactly the same at all instants. If d is the
separation of M1 and M2, the separation of L1’ and L2’ is 2d. When d is exactly an integral
number of half wavelengths, all rays of light reflected normal to the mirrors will be in phase.
However, rays of light reflected at an angle will not in phase in general.
Consider a single point source P on L emitting light in all directions (the same from P 1’ and
P2’). Let a ray from P reflected at an angle  by M1 and M2’ to form two rays (actually
formed by BS). Since M1 and M2’ are parallel, the two rays are also parallel. The path
difference between the two rays coming to the observer from corresponding points P 1’ and
P2’ with an angle  to the optical axis is 2dCos.
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EOP4056 Optical Metrology and Testing: Experiment OM1
Hence, it can be generalized that split parallel rays will reinforce each other to produce
maximum intensity (constructive interference) fringes for those angles  satisfying the
relation
2dCos = m ------ Eq. 1
where m is an integer and  is the wavelength. For a given d, m and ,  is constant,
constructive interference will lie in the form of circles with their centers on the optical axis,
which each  is corresponding to an m value.
The intensity distribution across the rings is given by
I  A2 = 4a2cos2(/2) ----- Eq. 2
where a is the amplitude of the split waves, A is that of their resultant and  is the phase
difference given by
 = (2/)*2dCos ------ Eq. 3
Fringes of this kind, where parallel beams are brought to interference with phase difference
determined by the angle of inclination , are often referred to as fringes of equal inclination.
A particular ring corresponds to a fixed order m. As M2’ is moved toward M1, d decreases,
from Eq. 1, Cos increases and therefore  decreases. Hence, the rings shrink toward the
center, with the highest-order one (Cos = 1) disappearing whenever d decreases by /2.
Each remaining ring broadens as more and more fringes vanish at the center, until only a few
fringes fill the whole field of view. By the time d is equal to zero, the center fringe will have
spread out, filling the entire field of view. When M2’ is further moved, fringes reappear at the
center and move outward.
3.2 Fringes of equal thickness
If the mirror M2’ and M1 are not exactly parallel, fringes will still be seen with
monochromatic light. In this case the space between the mirrors is wedge-shaped.
P2’
P
P’
O
P1’
d
L
Ob
M1 M 2 ’
L2’
L1’
Figure 3: The formation of fringes with inclined mirrors in the Michelson
interferometer
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EOP4056 Optical Metrology and Testing: Experiment OM1
The two rays reaching the observer Ob from a point P on the source are not no longer
parallel, but appear to diverge from a point P’ near the mirrors (note the schematic is drawn
not in scale). For various positions of P on the extended source, it can be showed that the path
difference between the two rays remains constant but that the distance of P’ from the mirrors
changes. If the angle between the mirrors is not too small, the distance of P’ from the mirrors
is not great. If the distance between the mirrors, d = 0, the fringes are straight because the
variation of the path difference across the field of view is due primarily to the variation of the
thickness of the air-film between the mirrors. With a wedge-shaped film, the locus of points
of equal thickness is a straight line parallel to the edge of the wedge. If d has an appreciable
value, the fringes are not exactly straight because there is also some variation of the path
difference with angle as mentioned in section 3.1. They are in general curved and are always
convex toward the thin edge of the wedge. If d is decreased (M2’ moves towards M1 without
changing the inclination of M2’), the fringes will move to ‘curve-in’ side, a new fringe
crossing the center each time d changes by /2. When d approaches 0, the fringes become
straighter until when d = 0, M2’ intersects M1, the fringes are perfectly straight. When M2’
moves further, the fringes curve in the opposite direction. For large path differences, large d,
the fringes cannot be seen. Because the principal variation of path difference results from a
change of the thickness d, these fringes have been termed fringes of equal thickness.
4.0 Warnings and precautions
Students are responsible to be careful the below warnings and precautions.
Students are responsible to own and other personal safety.
4.1 Laser safety
The helium-neon (HeNe) laser used is a class IIIa laser which can cause permanent damage
to your vision (retina). Never look at a direct laser beam or a direct reflection of a laser
beam from a specular (mirror, glass, metal, etc.) surface. Never put your eyes at the plane
where a laser beam is guided to traverse by optical components. Do not wear rings, watches
or other shiny jewelry when working with lasers. (All these objects could send laser beams
towards your eyes or those other persons nearby). Never insert an optical component directly
into a laser beam (to avoid any possible beam reflections from the component, e.g. from the
chamfers of the component). Never simply flip an optical component in a laser beam (to
avoid any possible beam reflections from other specular objects located within the same
workspace). Use only diffuse reflectors (e.g. rough surface white papers) for viewing or
tracing HeNe laser beam. Always block laser beam close to the laser when the experiment is
left unattended.
4.2 Partial and diffuse reflections of laser beam
In a darkened room, our pupils will be expanded and will let in 60 times more light than in a
lighted room. This experiment has many partial reflections (from lens, transparent apertures,
anti-reflection surface of a beam splitter) and diffuse reflections (from various objects:
viewing screen, holders, mounts, posts, etc.) Hence, this experiment will be performed in a
lighted room. Furthermore, the light intensity of the fringes on the viewing screen is
sufficiently high to be viewed in lighted room.
4.3 Tracing laser beam
An experiment normally involves more than one optical component and mechanical part
which can give total or partial reflections of laser beam. It is always required to know a
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EOP4056 Optical Metrology and Testing: Experiment OM1
laser beam direction and position. Tracing technique is always used. To do this tracing, put
a beam stop (a rough surface white paper for HeNe laser) at a position where a laser beam
direction and position are known and move the beam stop away in the laser beam direction
until to the desired distance or location.
4.4 Handling optical components
The optical components used are expensive. Never touch the optical surfaces of lenses,
mirrors, beam splitters, etc with your skin (finger, nose, etc.) or any objects (except lens
tissues). The coatings on the surfaces can be degraded by the fatty acids of human grease or
scratched by the objects. It is the same of the air blown out from human mouth which
contains acidic moisture. In this experiment, all the optical components have been mounted
on their holders with mounting posts, always carry the optical components at the
mounting posts. Never remove the optical components from their holders.
4.5 Adjustment knobs of adjustable mirror mounts
Never turn an adjustment knob of a mirror mount more than a few turns. It should never be
far from its medium position. The spring of the mirror mount could be damaged if it is over
stretched.
4.6 Clamping screws
There are clamping screws on the post holder and the laser mounting assembly. Do not over
tighten these screws. This may damage the screw thread or break the mechanical clamping
parts. Instead, tighten the screws until the holders are sufficient to hold the required parts
without moving. E.g. tighten the clamping screw of a post holder until it is just sufficient to
hold its mounting post without sliding down. Note that the required strength for tightening a
clamping screw depends on the load to be held without moving.
4.7 No rush work
You are advised not to carry out this experiment in rush to avoid any mistakes which could
cause the damages as mentioned previously, especially your eyes. As an example, a cutting
of a mounting post across a laser beam may send a reflected laser beam towards your or your
co-worker’s eyes. Although the laser beam sweeps across your eyes in a short instant, it may
temporarily cause a ‘dark line’ existing in your vision.
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EOP4056 Optical Metrology and Testing: Experiment OM1
5.0 Experiment
3”
3”
4”
12”
BSB
M1
A2
HeNe laser
A1
L1
Horizontal
steering (X)
Spring
Screen
Vertical
steering (Y)
A3
7”
Adjustable mirror mount
Mark to indicate
AR & wedge side
L2
4”
Fulcrum
(steel
A4
M2
S1
A5
5”
90o
M4
S2
BS
A6
S1
(50/50)
S2
(AR)
M3
12”
5”
Beam splitter (BS)
Figure 4: Schematic view of Michelson Interferometer. The mounting assemblies of the
components are not shown in the schematic diagram. M1, M2, M3 and M4 are the beam
steering mirrors. L1 is the bi-concave lens and L2 is the bi-convex lens. BS is the beam
splitter, where S1 is the beam splitting surface and S2 is the anti-reflection surface. BSB
is the beam stop for blocking laser beam close to the laser. A1, A2, A3, A4, A5 and A6
are the aperture or beam-stop locations along the setup of the Michelson interferometer.
The numbers at the outside of the optical breadboard indicate the distances in inches. The
distance between adjacent screw holes is one inch. The adjustable mirror mount has
two fine thread screws for adjustment, a steel ball and two pulling springs.
The experimental procedures below only include the important steps (including the safety
steps) for carrying out this experiment. They do not contain all the details on the adjustment
and alignment of the laser beam. You need to think and feel on them, e.g. how much and
how light to turn an adjustment knob of a mirror mount for a small beam movement in the
required direction. The below are mechanical parts for optical alignment.
i. Laser mounting assembly:
a. Laser tube height: slide post clamp up/down along its mounting post
b. Laser tube horizontal tilting: rotate post clamp about its mounting post
Loose the post clamping screw a little bit (don’t loose too much) for movement. Do not
over-tighten the clamping screw. Laser tube vertical tilting is not allowed.
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EOP4056 Optical Metrology and Testing: Experiment OM1
ii. Adjustable mirror mount:
a. Laser beam horizontal fine steering (X): turn the horizontal fine adjustment screw
b. Laser beam vertical fine steering (Y): turn the vertical fine adjustment screw
iii. Mounting post and its post holder
a. Laser beam horizontal coarse steering (X): rotate the mounting post in its post holder
b. Optical component height: slide the mounting post up/down in its post holder
Loose the post holder clamping screw for movement. Do not over-tighten the clamping
screw. Laser beam vertical coarse steering (Y) is not allowed. The optical component
here can be mirror, lens, beam splitter, etc.
iv. 2 mm aperture:
It is always useful for optical alignment to mark laser beam center or the optical axes of
optical components and to act as a screen to monitor the reflections from optical
components that are inserted in the beam after the aperture.
Student must read and understand sections 4.0 to 4.7 and 5.0 before performing the
experiment below.
Note that optical alignment needs patience and time. You must make sure not to knock
down any optical component along the optical alignment.
This experiment is carried out in a lighted room. Never switch off the room lights. If
necessary, you may block the lights from the room lamps to reach to the screen.
5.1 Procedures for setting up a Michelson Interferometer
5.1.1 U-shaped beam alignment
i. The laser beam will traverse in a U-shaped path as shown in Figure 4 and in a plane
which is parallel with the surface of the optical breadboard. Hence, you should never
put your eyes at this laser traversing plane level.
ii. Make sure the laser is not turned on. Remove L1, L2, BS and all the apertures from
the laser beam path.
iii. Laser is still off. Check the directions of the laser output, M1 and M2 so that when the
laser is turned on, the laser beam will traverse approximately in a U-shaped path.
Using a plastic ruler, check whether the center-heights of the laser output, M1 and M2
are about 12 cm. If necessary, correspondingly adjust the mounting assemblies of the
laser (do not loose the clamping screw too much, else the clamp will slide down),
M1 and M2. (Note: normally, they are in the correct positions unless someone has
moved their positions.)
iv. Laser is still off. Put a beam stop (BST or a rough surface white paper) close to the
laser. Turn on the laser. Trace the laser beam until a position close to M1. (Note that
if you are sure the approximate position of the laser beam close to M1, you may
put BST directly close to M1.) With the laser beam falling on BST, make sure M1
can capture the laser beam if BST is removed. Adjust the laser mounting assembly if
necessary. Put another beam stop (BSB) close to the laser (to block the laser beam).
Move BST to A2 position. Remove BSB (to unblock the laser beam). Check
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EOP4056 Optical Metrology and Testing: Experiment OM1
whether the laser beam intersects M1 at its center. Adjust the laser mounting assembly
if necessary.
v. Note that from this onwards, procedures will be mentioned in more simple
forms, including precaution steps. It is your responsibility to make sure you can
understand the meanings of the simple forms. With BST at A2, trace the beam
until M2. Make sure M2 can capture the beam. Adjust M1 mounting post if necessary
(do not accidentally knock down BST). Move BST to A5 (block the beam first).
Using a plastic ruler, record the center-height h of the beam just after M1. (Caution:
Never touch the mirror surface! The value h may not be the actual value. Make sure
you use the same end of the ruler touching the breadboard surface for other centerheight measurements). Check whether the beam in the M1-M2 arm intersects M2 at
its center and has the same center-height h just before M2. If necessary,
correspondingly adjust the adjustment knobs and mounting post of M1 and the
mounting post of M2 (don’t knock down BST). (For easy alignments for the rest
of experiments, make the M1-M2-arm beam in parallel with the right edge of the
breadboard. You may use the cardboard marked with 90o vertical line and a line
of screw holes on the surface of the breadboard for this parallel alignment. Note
that the beam is not on top of a line of screw holes but offset about 3 mm from
the screw hole center.)
vi. With BST at A5, trace the beam until A6 position. Using the cardboard marked with
90o vertical line, check whether the beam in M2-M4 arm is parallel to the front edge
of the breadboard (or a line of screw holes on the surface of the breadboard, offset
about 3mm from screw hole center). The beam at A6 must have the same centerheight h. (This parallel arrangement is important for easy alignment for the rest
of experiments.) If necessary, correspondingly adjust the adjustment knobs and
mounting post of M2 (don’t let the beam shoot outside BST). Now, the beam has
been aligned to traverse in a U-shaped path and in a plane parallel to the breadboard
surface.
5.1.2 Laser beam expander alignment
i. An inverted telescope is used to expand a laser beam. In this experiment, Galilean
telescope is used.
ii. Lens alignment: It is important to know how a lens is aligned properly. There are
two partial reflections from a lens, one from each surface, which form two spots on an
aperture screen. In this experiment, the alignment sequences are: (1) Slide up/down
the lens mounting post until the spots centers are at the same height of the aperture.
(2) Slide left/right the post holder until the two spots are overlapping. (3) Rotate the
post holder until the two spots are centered about the aperture. (Note that the lens
mounting assembly does not allow vertical tilting. Hence, the spots may not be
coincident at the top or bottom of the aperture.) Note on the movements of the
spots with respect to each of the adjustments. Repeating up/down, left/right, rotate
movements may be required to align the lens properly. This alignment consumes time,
depending on individual alignment skill. Consider geometrical optics of ray reflection
and refraction.
iii. Put the 2 mm plastic aperture at A1 position and align it so that the beam passes its
center. (For easy viewing the beam passing the aperture center, put a rough
surface white paper close to the aperture at the exit side, the back-scattered laser
light will help you to view the aperture opening area.) Put a 2 mm paper aperture
at A4 position and align its center at the beam center. Put another 2 mm paper
aperture at A2 position and align its center at the beam center.
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EOP4056 Optical Metrology and Testing: Experiment OM1
iv. Block the beam. (Caution: Never turn the lens chuck screws! The lens can easily
drop out from the lens chuck.) Put in L1 (Bi-concave lens) at about 4 screw-holes
distance from M1 and align roughly its center at the beam path. (Use the centers of
A2 and A4 apertures or 5.1.1 v as a guide and use a ruler for L1 center-height.)
Unblock the beam. Use the lens alignment guides, align L1 so that the transmitted
beam is centered on A4 aperture and the two reflected beams are centered about A2
aperture. (They may be at the top or bottom of the aperture. Remove the aperture
at A2, you can see the two large-diameter, low-intensity spots at A1. One of the
spot may have concentric circular bright and dark rings).
v. Put the paper aperture (initially at A2) at A3 position and align its center at the beam
center. (Note that the diffraction of the 2 mm aperture may affect the beam
direction after the aperture. Hence, the aperture must be properly center at the
beam center. The aperture at A4 is a useful reference for this aperture
centering.) Block the beam. Put in L2 (Bi-convex lens) at about 7 screw-holes
distance from L1 and align roughly its center at the beam path (including its centerheight). Unblock the beam. Use the lens alignment guides, align L2 so that the
transmitted beam is centered on A4 aperture and the two reflected beams are centered
about A3 aperture. With the information in 5.1.1 vi, the beam center at A6 should be
the same position (height = h, offset about 3mm from screw hole center).
vi. Make sure M4 can reflect back the beam to M2. Remove BST at A6. Finely turn the
M4 adjustment knobs until the reflected (returning) beam passes through A4 aperture
and then A3 aperture, and lastly its center falls at about 1 mm from the edge of either
side of A1 aperture. Remove A3 and A4 aperture. Carefully move L2 back and forth
along the beam until the returning beam size at A1 is the same size as the incoming
beam passing through the aperture. (Note that the returning beam must be located
about 1 mm from the edge as that before moving the lens. You can use a white
paper to determine the incoming beam size that passes the aperture.) The beam
after L2 is well collimated. Measure and record the beam sizes at A4, dA4 = ____ mm
and A6, dA6 = _____ mm.
5.1.3 Michelson interferometer alignment
i. The beam splitting surface of the beam splitter has reflectance, R  50% and
transmittance, T  50% at wavelengths,  = 480 – 700 nm (broad band) and angle of
incidence, AOI = 45o. The anti-reflection surface is not totally 0% reflectance. Hence,
it is tilted about 30 arc minutes to the beam splitting surface to avoid interference due
to multiple internal reflections.
ii. Put a paper aperture at A5 position and align its center at the beam center. Use the
information in 5.1.1 vi to confirm the aperture centering (diffraction effect).
iii. Block the beam. Put in BS with the surface marked with ‘>’ facing to M4. Roughly
align S1 surface center (Use the information in 5.1.1 vi. Make sure the centerheight of BS is equal to h) and its orientation (45o between S1 surface and the
beam path) so that when the beam is unblocked, part of the incoming beam will be
reflected to M3 by S1 surface. The angle between the reflected beam to M3 and the
incoming beam from M2 is roughly 90o.
iv. Unblock the beam. Perform repeating adjustment of moving the BS base plate back
and forth along the incoming beam from M2 and rotate the base plate until the angle
between the incoming beam from M2 and the reflected beam to M3 is close or equal
to 90o. At the same time, the reflected beam falls on about the center of M3. (Use the
cardboard marked with 90o vertical line. The holes on the breadboard surface
can help to determine 90o angle. This alignment consumes time!) After the 90o has
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EOP4056 Optical Metrology and Testing: Experiment OM1
been achieved, note on the position and orientation of the base plate relative to the
screw holes in the incoming beam direction. (This noting is important for following
adjustment.) Adjust M3 height if necessary. (Note that the center-height of the
beam on M3 may not be exactly equal to h and the transmitted beam to M4 is
not centered. All these are fine.)
v. Rotate the mounting post of M3 if necessary. Turn the adjustment screws of M3 so
that the beam reflected from M3 passes through the aperture at A5, then the aperture
at A2 and lastly falls on the same position (about 1 mm from the same edge). (Note
that the beam initially may not fall on the paper of the aperture at A4. Hence,
you need to trace the beam with a rough surface white paper to know its position
outside the aperture paper. The alignment consumes time and requires very fine
adjustment to get the final position.) Remove the aperture at A5. The coincident
position of the two spots on A1 aperture should not be changed. If they are changed,
adjust accordingly to get the same position (about 1 mm from the same edge). At
this coincidence of the two spots, a series of bright and dark fringes appears on the
viewing screen.
vi. The S1 surface may not be at the center of the beam at M3-BS direction after A5
aperture is removed. Put BST (or a paper aperture) at the front of M4, move the base
plate along M3-BS direction until the spot on BST is the best without blocking by the
lens chucks. (Note the position of the base plate in M2-BS direction must be the
same as noted in 5.1.3 iv). (The BS has been aligned in all X, Y and Z directions).
The interferometer alignment has been completed.
Note that aligning the two beams on A1 aperture is good for a quick rough alignment to
get fringes appearing on the screen. However, this is only applied for a collimated or
closely collimated beam.
5.2 Procedures for observation of interferometer behaviors and characteristics
i. After the interferometer alignment is completed, one of the interferometer mirrors will
not be moved anymore. Let M3 is fixed all the time. This section will observe how the
orientation and separation of the fringes changed with respect to the orientation
and tilting angle of the M4 mirror movements. Finely turn (repeat to turn a small
step and release) the adjustment screws of M4 so that approximately five bright
fringes appear across the beam on the screen. Perform the following experiment in
sequence. You may sketch the fringes change after every movement of M4 for
easy discussion.
ii. Use a rough surface white paper. View the fringes on the white paper from the screen
position to a position close to BS. Record this observation in words.
iii. Finely turn the horizontal adjustment screw of M4 clockwise and observe the
movement of the fringes on the screen until the number of bright fringes is equal to 2
or 10 (depending the number decreasing or increasing). Record this observation in
words (for fringes changed with respect to mirror movements).
iv. Finely turn the horizontal adjustment screw of M4 counter-clockwise and observe
the movement of the fringes on the screen until the number of bright fringes is equal
to 2 or 10. Record this observation in words.
v. The observations are the same for turning the vertical adjustment screw of M4. Now,
finely turn these two screws until approximately five horizontal bright fringes.
Draw the fringes on a graph paper in details (relative separation, thickness and
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EOP4056 Optical Metrology and Testing: Experiment OM1
intensity of the bright and dark fringes). Ask your instructor to verify your drawn
fringes and the fringes appearing on the screen.
vi. By selecting the correct adjustment screw so that only the number of horizontal bright
fringes changes without changing the orientation of the fringes. Finely turn this
adjustment screw clockwise and counter-clockwise and observe the number of the
bright fringes. Make sure you turn the screw for the whole range until the number of
fringes becomes very large or equals to 1 (or 0) and then increases again. Record this
observation in words. (Let 0o tilting angle is assigned for 1 or 0 number of fringes,
clockwise produces positive tilting angles and counter-clockwise produces
negative tilting angles. Record only comparative, no quantitative.)
vii. Turn the correct adjustment screw so that the number of fringes becomes 1 (or 0) and
then turn another adjustment screw to get approximately 5 vertical bright fringes.
Record this observation with respect to the mirror tilting angle and direction in this
sequence of screw adjustments.
viii. Turn the correct adjustment screw so that the number of fringes becomes 1 (or 0).
At this position, let assign the horizontal adjustment screw angle as x = 0o and the
vertical adjustment screw angle as y = 0o. Turn the horizontal adjustment screws and
observe x until there are about 10 bright fringes on the screen, let assign x = x1.
Turn the vertical adjustment screw with the same turning angle, i.e. y = x1. Record
what you observe on the fringes changed. Then, turn the vertical adjustment screw so
that y = 0o and then further turn it until y = -x1. Record what you observe on the
fringes changed.
ix. Turn the correct adjustment screw to get approximately 5 vertical bright fringes.
Move L2 closer to L1 by 4 screw-holes distance. Align L2 as mentioned in 5.1.2.
Record the fringes changed in words. Turn the correct adjustment screw to bring to
one or two bright fringes. Record the fringes changed in words. Further turn the
adjustment screw and also another adjustment screw until a single spot (either bright
or dark) with side ring. Draw the fringes.
x. Move L2 closest to L1 but the base plates of L2 and L1 are not touching each other.
Put BST or a paper aperture at A6 to align L2. Draw the fringes. Turn one of the
adjustment screws and record the fringes changed. Turn back the adjustment screw to
the previous fringes before go next step.
xi. Remove L2 from the laser beam path (block the beam first). Draw the fringes. Turn
one of the adjustment screws and record the fringes changed. Turn back the
adjustment screw to the previous fringes before go next step.
xii. Remove L1 from the laser beam path (block the beam first). Draw the fringes. Turn
one of the adjustment screws and record the fringes changed.
Now, you should be able to control the number of fringes and their orientation, etc.
(Cognitive – Evaluating, Level 5)
[5 marks]
6.0 Other information
Relevant studies
Types of interference fringes: real or virtual; Localization of interference fringes: localized
and non-localized; Theory on the telescope and laser beam expander; Fringes produced by
plane waves (collimated laser beam);
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EOP4056 Optical Metrology and Testing: Experiment OM1
Discussions
1. Discuss on the matters along with the experiments
(Cognitive – Understanding, Level 2)
[10 marks]
2. Analyse the setup of the interferometer
(Cognitive – Analysing, Level 4)
[10 marks]
3. Justify how a compensator can be used to improve the result of your experiment.
(Cognitive – Evaluating, Level 5)
[10 marks]
4. Compare on the experiment observation related to theory
(Cognitive – Evaluating, Level 5)
[10 marks]
Conclusion
Conclude based on the discussed matters.
(Cognitive – Evaluating, Level 5)
[3 marks]
MARKING SCHEME
1. Experiment objectives – 2%
2. Procedures, results, answers and discussions for all questions and assignments – 45%
3. Conclusion – 3%
LABORATORY REPORT
Date of submission: within 14 days after performing the experiment
Place of submission: submit to the laboratory where you conducted the experiment
Length of report:
Your definition. Write the necessary things.
Report contents:
Report must include the following:
i.
Experiment observations
ii.
Discussion
iii.
Conclusion
)
End of Lab Sheet
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