LEOI-22 Precision Interferometer

Instruction Manual

Lambda Scientific Pty Ltd

6A Hender Avenue, Magill, South Australia 5072, Australia Phone: +61 8 8333 0382 Facsimile: +61 8 8333 0380

E-mail: sales@lambdasci.com Web: www.lambdasci.com

COPYRIGHT V2-0

COMPANY PROFILE

Lambda Scientific specializes in developing and manufacturing various

spectrophotometers, spectroscopic instruments, thin film measurement, physics education

kits, optical components, opto-mechanic parts, and light sources. Spectrophotometer

products include infrared spectrophotometer and ultraviolet-visible spectrophotometers.

Our spectroscopic instruments include laser Raman spectrometer, multifunctional grating

spectrographs, multi-channel optical analysers and monochromators, whose detectors are

PMT, single photon counter or CCD, etc. They are either manual or computer-controlled.

Physics experimental instruments and education kits cover wide range of experimental

projects in general physics, especially in geometrical, physical and modern optics, e.g. lens

measurements, interference phenomena, diffraction phenomena, analysis of polarization,

Fourier optics and holography etc. We also provide a variety of optical components,

optical mounts and optical breadboards. Light sources include xenon lamps, mercury

lamps, sodium lamps, bromine tungsten lamps and various lasers. Our products have been

sold around the world. Lambda Scientific is committed to providing high quality, cost

effective products and on-time delivery.

CONTENTS 1. Introduction ................................................................................................................................ 1

2. Specifications ............................................................................................................................. 1

3. Theory ........................................................................................................................................ 1

3.1 Interference ........................................................................................................................... 1

3.2 Michelson Interferometer...................................................................................................... 2

3.3 Fabry-Perot Interferometer ................................................................................................... 4

3.4 Twyman-Green Interferometer ............................................................................................. 4

4. Structure ..................................................................................................................................... 6

5. Operation of Experiment Examples ........................................................................................... 7

5.1 Michelson Interferometer...................................................................................................... 7

5.1.1 Interference Fringes Observation ................................................................................... 7

5.1.2 Equal Inclination Interference........................................................................................ 9

5.1.3 Equal Thickness Interference ....................................................................................... 10

5.1.4 White Light Interference Fringes ................................................................................. 10

5.1.5 Measurement of Wavelength of the Sodium D-lines ................................................... 12

5.1.6 Measurement of the Wavelength Separation of Sodium D-lines ................................. 13

5.1.7 The Refractive Index of Air ......................................................................................... 13

5.1.8 The Refractive Index of Transparency Slice ............................................................... 15

5.2 Fabry-Perot Interferometer ................................................................................................. 16

5.2.1 The Multi-beam Interference ....................................................................................... 16

5.2.2 Measurement of the Wavelength of He-Ne Laser ....................................................... 17

5.2.3 Observation of the Interference of Sodium D-lines ..................................................... 18

5.3 Twyman-Green Interferometer ........................................................................................... 19

5.3.1 Demonstrating the principle of a Twyman-Green interferometer ............................... 19

6. Maintenance ............................................................................................................................. 21

6.1 Optical components ............................................................................................................ 21

6.2 Driving mechanism ............................................................................................................. 21

7. Laser safety and lab requirements ............................................................................................ 21

8. Parts List ................................................................................................................................... 22

1

1. Introduction

The Michelson interferometer can be used for observing interference phenomena such as equal

inclination interference, equal thickness interference, white-light interference, precision

wavelength contrast, determine the zero-path difference and measuring refractive index of a

transparent medium slice or air. The Fabry-Perot interferometer is used for observing multiple-

beam interference and measuring the fine structure of spectra (e.g. wavelength disparity of

yellow sodium doublet lines). Twyman-Green interferometer is used to measure the defects of

optical components such as lenses, prisms and windows etc.

This equipment can be easily switched between the Michelson, Fabry-Perot and Twyman-

Green Interferometers. Its application is to optical experiments in physics teaching at

universities and other tertiary institutions.

2. Specifications

Flatness of Beam Splitter and Compensator Plate 0.1λ

Minimum travel reading 0.00025mm

Travel of moving mirror 0.625mm (travel of fine micrometer: 25mm)

Sodium-tungsten lamp Sodium: 10W; Tungsten:15W

He-Ne laser output 0.7 ~ 1mW@632.8nm

Overall dimension 350mm×350mm×245mm

Weight Approx. 15 Kg

3. Theory

3.1 Interference

Light has two vectors: an electric field vector and a magnetic field vector, and these two

vectors can be used to model light waves. When two or more beams of light meet in space,

these fields add according to the principle of superposition.

Generally, light beams which originate from separate sources have no fixed relationship. At

any instant there will be points in space where the fields add to produce the maximum field

strength. Because there is no fixed relationship between the oscillations, a point where there is

a maximum at one instant may have a minimum at the next. As the oscillations of visible light

are faster than the human eye can perceive, the human eye averages these results and perceives

a uniform intensity of light.

If the light beams come from the same source, there is generally some degree of correlation

between the frequency and phase of the oscillations. At one point in space the light from the

beams may always be in phase. So the combined field will always be a maximum and a bright

spot will be seen. At another point the light from the beams may be always out of phase and

minima, or dark spot, will be seen. This phenomenon is called interference.

2

In 1801, Thomas Young designed a method for producing such an interference pattern. He

allowed a single, narrow beam of light to fall on two narrow, closely spaced slits, and placed a

viewing screen opposite the slits. When the light from the two slits overlapped on the screen, a

regular pattern of dark and bright bands appeared. When first performed, Young’s experiment

offered important evidence to the wave nature of light.

Young’s slits can be used as a simple interferometer. If the spacing between the slits is known,

the spacing of the maxima and minima can be used to determine the wavelength of the light.

Conversely, if the wavelength of the light is known, the spacing of the slits could be

determined from the interference pattern.

3.2 Michelson Interferometer

In 1881, A. A. Michelson designed and built an interferometer using a similar principle.

Originally Michelson designed his interferometer as a means to test for the existence of the

ether, a hypothesized medium in which light was thought to propagate. Due in part to his

efforts, the ether is no longer considered a viable hypothesis. However, Michelson’s

interferometer has become a widely used instrument for measuring the wavelength of light, for

measuring extremely small distances by using a light source with the known wavelength, and

for investigating optical media.

E

BS CP M2

M1

M2'

S

The above figure shows a schematic diagram of a Michelson interferometer. The beam of light

from the light source S strikes the beam-splitter BS, which reflects 50% of the incident light

and transmits the other 50%. The incident beam is therefore split into two beams; one beam is

reflected toward the fixed mirror M1, the other is transmitted toward the movable mirror M2.

Both mirrors reflect the light directly back toward the beam-splitter BS. The light from M1 is

transmitted through the beam-splitter BS to the observer’s eye E, and the other light from M2 is

transmitted through the compensator plate CP and reflected from the beam-splitter BS to the

observer’s eye E.

Since the beams are from the same light source, their phases are highly correlated. When a

beam expander is placed between the light source and the beam-splitter, the light rays spread

out, and an interference pattern of dark and bright rings, or fringes, can be seen by the observer.

In above figure, M2’ is the virtual image of M2, and the light path of the Michelson

interferometer can be seen as the light path of the air plate between M1 and M2’. The

3

compensator plate CP parallel to the beam splitter BS has the same thickness and refractive

index with the BS. Because the light paths of the two beams are equal, and different light

waves have the same retardation, and it is easy for observing the white-light interference.

The forming process of the interference rings is shown in the following figure.

2d

d

E

L

PM1 M2'θL2L1

P''P'

P' P'' θ2d

2dcosθ

M2’ is the virtual image of M2, and parallel to M1. For simplicity, light source L is at the

observer’s position. L1 and L2 are the virtual images of L formed by M1 and M2’, and are

coherent. Let d be the distance between M1 and M2’, therefore the distance between L1 and L2

is 2d. So, if d = mλ/2 (m is an integer), the phases of the light beams from the normal direction

of L1 and L2 are the same, but the phases of the light beams from other direction are not always

the same. Light beams from the points P’ and P” to the observer have a path difference 2dcosθ.

If M1 is parallel to M2’, the two light beams have the same angle, θ, and are parallel to each

other. So, when 2dcosθ = nλ (n is an integer), the two light beams form a maximum field

strength. For definite n, λ and d, the value of θ is a constant, and the contour of the maximum

point becomes a ring. The centre of the ring is at the bottom in the line of viewing being

perpendicular to the mirror plane.

4

3.3 Fabry-Perot Interferometer

When one beam of light passes through a plane-parallel plate, it is reflected many times with

multiple-beam interference taking place, and the interference fringes are sharp and bright. That

is the basic principle of the Fabry-Perot interferometer.

G1 G2

Light Source

High reflectance coating

Observer θ

In above, two partial mirrors G1 and G2 are aligned parallel to one another, forming a reflective

cavity. When monochromatic light irradiates the reflective cavity with entrance angle θ, many

parallel rays that pass through the cavity form the transmitted light. The optical path difference

between two neighboring rays is:

θδ cos2nd= .

Thus, the transmitted light intensity is:

( ) λ

πδ2

2

0

sin1

41

1

R

RII

−+

=′ .

where, R is reflectivity, then, I’ varies with δ. When

λδ m= (m = 0, 1, 2…),

I’ becomes a maximum, and when

2/)12( λδ +′= m (m’ =0, 1, 2…),

I’ becomes a minimum.

3.4 Twyman-Green Interferometer

Twyman-Green interferometer is a variant of the Michelson interferometer and is mainly used

to measure the defects of optical components such as lenses, prisms, windows, laser rods, and

plane mirrors. The beam splitter and mirror arrangement in Twyman-Green interferometer

resembles Michelson interferometer. There is a slight difference between the Michelson and

Twyman-Green interferometers: The light source is usually an extended source (although it can

also be a laser) in a Michelson interferometer, but the light source in Twyman-Green

interferometer is always a point source such as a laser. An optical component, e.g. a lens, can

be tested by placing it into one beam path. Any irregularities of the lens can be observed from

5

the resulting interference pattern. In particular, spherical aberration, coma, and astigmatism

show up as specific variations in the fringe pattern.

M2

M1

W1

W2

Sample

B C

L2

L1

Screen

S A

BS

In above figure, if the sample has perfectly flat surfaces, the returning wave front is plane and

no fringes are observed. However, if the optical flat is not perfectly flat on either side, the wave

from M2 returning to the beam splitter is no longer plane. Thus, the phase difference between

the superimposed waves of M1 and M2 will vary across the field of view and a fringe pattern

appears. These fringes are a contour map of the distorted wave front, so that the imperfections

of the sample are displayed in terms of wave front aberrations.

6

4. Structure

This equipment combines a Michelson interferometer, a Fabry-Perot interferometer and a

Twyman-Green interferometer on one square base, which is made of thick steel plate and has a

stable rigid-frame.

10

0 105

0

45

40

5

1. Main stage 10. Fine micrometer

2. Side stage 11. Movable mirror

3. Light source (He-Ne laser or sodium-tungsten lamp) 12. Stage with mounting holes for 6 and 8

4. Beam expander 13. Compensator

5. Transparency slice clamp 14. Beam splitter

6. Ground glass screen 15. Extension arm

7. Rotational pointer and SOCKET3 16. Two in one screen

8. Fixed Mirror (It is also used as a F-P mirror) 17. SOCKET2 for beam expander

9. Presetting micrometer

The structure of the equipment is shown in Figure 4-1. On the side stage 2, there is one

extruded hole for installing light sources (He-Ne laser or sodium-tungsten lamp) and three

small holes for holding other items such as beam expander (4), transparency slice clamp (5)

and ground glass screen (6). Beam expander is installed in SOCKET2 (17) when in use. Fixed

mirror (8) is both the reference mirror of Michelson interferometer and used as the front mirror

of Fabry-Perot interferometer. Beam splitter (14) is plated semi permeable film on the inner

side. Compensator (13) has the same thickness as beam splitter (14) and is at 90° angle to (14).

The relative position of beam splitter and compensator has been adjusted before leaving the

factory and there should be no need for adjustment. Movable mirror (11) is controlled by fine

micrometer (10) which has a travel of 25mm. When the fine micrometer moves a distance of

0.01mm (resolution), movable mirror moves a distance of 0.00025mm with respect to main

stage. A ground glass screen (13) is used to receive the fringes of the Michelson interferometer,

and for protecting the eye from the laser light.

7

5. Operation of Experiment Examples

5.1 Michelson Interferometer

5.1.1 Interference Fringes Observation

He-Ne laser as the light source

Note:

a) Direct eye exposure to laser should be avoided.

b) DO NOT observe the laser interference fringes by using the reflecting mirror.

c) All experiments should be carried out under low-light conditions in order to

make viewing interference phenomena as easy as possible.

1) Place laser mount with a He-Ne laser in the mounting hole on the side stage and turn

on.

2) Place beam expander in SOCKET2. Adjust the height of the laser tube to let the beam

hit on the center of the beam expander. Remove the beam expander.

3) View the beam spot on the beam splitter; it should be approximately in the middle of

the beam splitter, and view also on the movable mirror. Adjust the laser tube until the

beam spots on both the beam splitter and movable mirror are at the same height.

Note: This may involve tilting the tube and so remember to re-adjust the height each

time tilting occurs. Place a piece of card (e.g. Business card) in front of the fixed mirror

to avoid multiple reflections.

4) Place a piece of card in front of the movable mirror.

5) Place the two in one screen in the extension arm in SOCKET1 and face the white

screen towards the beam splitter. A beam spot will be seen on the screen which comes

from the fixed mirror. There are also other spots on the screen with less brightness due

to multiple reflections. Align the centre of the white screen with the brightest beam

spot.

8

6) Remove the piece of card and view the white screen. Two bright spots should appear

(and less bright multiple reflections). Adjust the movable mirror until the two bright

spots coincide with each other at the centre of the white screen.

7) Position the beam expander into SOCKET2 with the lens lock facing the beam splitter.

If the expanded beam spot is not immediately incident on the movable mirror then

adjust the laser tube. The fringe pattern can be observed on the white screen.

Note: When adjusting the expanded beam spot, it may not be immediately obvious in

some cases. Hold a piece of paper behind the movable mirror to identify the location of

the beam spot. Adjust the two tilting screws on the laser holder to move the spot onto

the movable mirror.

If you achieve fringes but they are not circular and smaller as you may expect then make some

adjustments to the presetting micrometer to ‘zoom’ in or out to get a better view. If you did not

achieve fringes, closely follow the instructions from the beginning; otherwise contact

support@lambdasci.com for technical support.

Using sodium lamp as the source

1) Take away the He-Ne laser and beam expander. Place sodium lamp in the mounting

hole.

2) Flip the two in one screen to view the interference pattern on the mirror. Adjust the

height of the lamp, so that the sodium light strikes at the center of the mirror.

Commonly, Interference fringes can be observed from the reflected light.

Note: If you can not find any fringes, it means that the interference light path has

changed by the vibrations when you replaced the light source. To get back the fringes

we should do as the following processes.

3) Place a pinhole board (you can pierce a hole on a business card by a pin) on front of the

lamp and adjust the movable mirror until the two images of the pinhole coincide with

each other.

9

4) Remove the pinhole board and interference fringes will be observed by viewing the

mirror. Using the ground glass screen is optional in this experiment but if it is preferred

insert the ground glass in between the light source and the beam splitter (SOCKET2).

5.1.2 Equal Inclination Interference

Now let’s study the different kinds of fringes which are produced by Michelson interferometer.

As shown in the following picture,

Figure 5-3: Illustration of Equal Inclination Interference

M2’ is the virtual image of movable mirror M2. In the observer’s field of view, it looks like that

the two light beams are reflected from mirror M1 and M2’ and the interference pattern is just

like the thin air film between M1 and M2’.

He-Ne laser as the light source

1) Re-produce the interference image as per 5.1.1 which should be similar to (a).

2) Adjust the coarse micrometer such that images (a) to (e) are viewed in succession.

3) Set the fine micrometer to the middle of the scale (between 10mm to15mm).

4) Re-adjust the coarse micrometer as close as possible to image (c).

5) Use the fine micrometer to produce fringes of equal inclination.

10

5.1.3 Equal Thickness Interference

Figure 5-4: Fringes of equal thickness

Adjust the screws at the back of M2 and if M1 and M2’ has a very small angle with each other,

the fringes of equal thickness interference can be observed on the screen.

He-Ne laser as the light source

1) Install the He-Ne laser and remove the beam expander. Turn the fine micrometer to the

middle of the scale (between 10mm to15mm).

2) Adjust the laser and movable mirror to get interference pattern on the white screen.

3) Turn the coarse micrometer in the direction which interference rings disappeared at

their center, and the fringes will grow larger. Stop when there are only a few fringes on

the screen.

4) Turn the fine micrometer to move the movable mirror in the direction which

interference rings disappeared at their center, until there are only two or three rings left.

5) Adjust the movable mirror a little. If the image of movable mirror M2’ is tilted relative

to the fixed mirror M1, you will see the interference stripes.

6) Continue turn the fine micrometer to make the curved fringes move toward their center.

Some straight bands will appear in succession. That is the fringes of equal thickness.

5.1.4 White Light Interference Fringes

Because white light has a short coherence length, interference fringes can only be observed

when the optical path difference is nearly zero. Compared with the interference pattern created

by a laser or a Sodium lamp, obtaining white light interference is much more difficult. With the

help of a specially designed Sodium-Tungsten lamp, you may quickly find the point of zero

path difference.

11

1) After achieving equal inclination interference (5.1.2), replace the laser with a sodium-

tungsten lamp and remove the beam expander. Use the mirror of the two in one screen

as the observation screen.

2) Adjust the height of the light source until the yellow sodium

light and white tungsten light illuminate the upper and lower

half of the visual field respectively. Make sure that the visible

sodium fringes have good contrast and wide-spacing.

Interference fringes of sodium doublet may help you to locate

the point of zero path difference.

If you do not see any fringes, it means that the interference

light path has changed by the vibrations when you replaced

the light source. To get back the fringes we should do as the

following processes.

3) Place a pinhole board (you can pierce a hole on a business card by a pin) on front of the

lamp and adjust the movable mirror until the two images of the pinhole coincide with

each other, and then you will get interference fringes in the viewing mirror.

4) Search for white light fringes by turning the fine micrometer slowly and alter the path

length at such a rate that you can always see the yellow fringes traversing the field of

view. In this way you will be able to approach zero path difference at maximum speed

but will not miss the appearance of the white light fringes.

5) When color fringes gradually appear, a central dark interference fringe can be

identified. That is the interference at the position of zero light-path difference.

6) To observe the white light interference fringe clearly, you can turn off the Sodium lamp

and put the ground glass screen in SOCKET2.

12

5.1.5 Measurement of Wavelength of the Sodium D-lines

1) Place the sodium-tungsten lamp on the side-stage of the interferometer and warm it up

for about 5 minutes.

2) Adjust the interferometer to produce circular fringes in the field of view.

3) At the position with a clear equal inclination fringes, record the reading d0 of the fine

micrometer.

4) Count the number of fringes that appear (or disappear) in the center of the field of view

as the fine micrometer is turned slowly. After counting 50 fringes, record the

micrometer reading again

5) Continue above process through 250 fringes, recording the micrometer reading after

each set of 50 fringes has been counted. Calculate the ∆d. The actual mirror

movement, d∆ , is equal to λN∆ /2, where λ is the wavelength of the source.

The moving distance is:

2

λNd

∆=∆

And ∆N is the number rings counted. So,

N

d

∆

∆=

2λ

Notice:

a) Always turn the micrometer knob in one direction.

b) Set the micrometer screw somewhere near the middle of its total movement. In this

position, the relationship between the micrometer reading and the mirror movement is

most nearly linear.

c) Turn the micrometer screw a full turn before counting fringes to eliminate errors due to

backlash.

13

5.1.6 Measurement of the Wavelength Separation of Sodium D-lines

The Michelson interferometer can also be used for measurement of the wavelength separation

of Sodium D-lines. The yellow Sodium doublet includes two kinds of monochromatic lights

that have very tiny wavelength spacing with each other. Therefore, during the process of

moving the movable mirror, the interference fringes produced by two yellow lines will appear

periodically clear and blurry. The wavelength difference of the yellow sodium doublet lines is

given by

d∆=∆

2

2

λλ

Where λ is the averaged wavelength of two lines through the result of last experiment, ∆d is

the thickness of air membrane between the two mirrors M1 and M2’.

1) Adjust the interferometer to obtain a clear, wide-spaced interference pattern of sodium

doublet. Slowly turn the fine micrometer till all the fringes disappeared. Record the

reading d1 of the micrometer;

2) Continue to turn the micrometer in the same direction and new interference pattern

appears. Record the reading d2 where the interference pattern vanishes again;

3) Repeat this process in different places near zero path difference point to get an average

value of 21 ddd −=∆ .

5.1.7 The Refractive Index of Air

In Michelson interferometer mode, if we place an air chamber in the light path of M2 and then

change the density of the air (by deflating or pumping the air), the distance of the light path

will change byδ . And it will generate a certain number of interference fringes.

λδ Nnl =∆= 2 ,

Therefore,

lNn 2/λ=∆

Where, l is the length of the air chamber, λ is the wavelength of the light source, N is the

number of fringes you counted.

The refractive index of air is dependent upon temperature and pressure. It is near unity and the

quantity n-1 is directly proportional to the density ρ of the gas. For an ideal gas:

1

1

00 −

−=

n

n

ρ

ρ

T is the absolute temperature, P is the pressure. Therefore,

TP

PT

0

0

0

=ρ

ρ

So we get

1

1

00

0

−

−=

n

n

TP

PT,

14

When temperature is constant, then

PTP

Tnn ∆

−=∆

0

00 )1(

Because lNn 2/λ=∆ , we have

lNPTP

Tn2/

)1(

0

00 λ=∆−

So

P

P

l

Nn

∆×+=

21

λ

1) Align the equipment.

2) Adjust the movable mirror M2 to get clear fringes of equal inclination on the centre of

white screen using the He Ne Laser.

3) Put the air chamber with known length l in its holder (for accurate measurement, the

end plates of the air chamber must be perpendicular to the laser beam);

4) Pump in air to the chamber; record the reading of the gauge P∆ .

5) Release the valve and slowly deflate the air in the chamber till the pointer on the gauge

is back to 0. While you do this, count N. The refractive index of air in the experiment

environment is

P

P

l

Nn

∆×+=

21

λ

Note: This experiment should be carried out several times in order to get the average.

Notice: To protect the gauge, the reading of the gauge should not be over 40Kpa

15

5.1.8 The Refractive Index of Transparency Slice

When place a transparency slice in one optical arm of the Michelson interferometer, light path

of this arm will changes as the transparency slice is rotating. The difference of the light path

can be determined by counting the number of the fringes disappeared or appeared. And the

light path has a relation to the rotating angleθ , the thickness d and the refractive index n of

the slice.

If the entrance light is perpendicular to the transparency slice at first, and after rotating a

angleθ , the change of the number of fringes is N . The refractive index n is given by:

λθ

θ

Ndn

dnn

−−=

)cos1(2

sin

0

22

0

Where λ is the wavelength of the light source (the He-Ne laser), 0n is the refractive index of

air.

Experiment Procedures

1) Place the transparency slice clip in the mounting hole in SOCKET3.

2) Place the two in one screen on the extension arm and adjust the screws at the back of

the movable mirror to get a set of clear fringes on the white screen.

3) Mount the transparency slice on the clip. Adjust the clip and the rotational pointer and

make sure that the slice is approximately perpendicular to the optical path.

4) Rotate the clip using the pointer slowly and observe the fringes on the screen carefully

at the same time. The fringes in the center of the screen will disappear or appear. Until

at a position that the fringes do not disappear or appear, stop rotating. Now the slice

should now be perpendicular to the optical path.

5) Adjust the movable mirror to get a set of clear fringes on the screen. Slowly rotate the

slice by moving the lever arm. Count the number of fringe transitions that occur as you

rotate the slice from its current angle to its new angle (at least 10 degrees). If the

rotating angle isθ , and the number of fringes emission is N then we can get the

refractive index n by the equation:

16

λθ

θ

Ndn

dnn

−−=

)cos1(2

sin

0

22

0

Where 0n = the index of refraction of air (see Experiment above), λ = the wavelength

of He-Ne laser in vacuum, and N = the number of fringe transitions that you counted.

d = the thickness of the transparency slice (d = 0.1mm).

6) Repeat the last three processes 4, 5 and 6 three times and calculate the average of n.

5.2 Fabry-Perot Interferometer

5.2.1 The Multi-beam Interference

1) Turn the interferometer 90° to make the Fabry-Perot interferometer facing to the

observer who is at the position opposite to the movable mirror (see structure figure

in chapter 4).

2) Unscrew the F-P (fixed) mirror, and then mount it in the holes in front of the

movable mirror. Make sure that the surface of F-P mirror is faced towards the

movable mirror.

3) Adjust the three screws behind the movable mirror to make sure that the two mirrors

are parallel to each other approximately and the distance between them is about 2

mm.

4) Unscrew the screw on the top of the beam splitter and compensator and remove it.

Put it in a safe place (Fixing it in the place of fixed mirror is a good choice).

5) Set up the He-Ne laser at the light path of Fabry-Perot interferometer. Adjust the

laser to let the beam hit on the center of the F-P mirror. Adjust the top and right

screws behind the movable mirror to let the beam spots coincident. It means the two

mirrors are near parallel.

6) Place a beam expander and a ground glass screen into the light path to create area

light source, so that the observer can find a series of multi-beam interference rings.

To make it simple, as shown in above figure, you can also find the multi-beam fringes. If the

G1 and G2 are absolutely parallel to each other, the interference fringes on the ground glass

screen will look like a perfect circle (you should mount it in the extension arm, see dotted lines

in structure in chapter 4).

EyeG2G1Ground

Glass

Screen Beam

Expander He-Ne Laser

17

5.2.2 Measurement of the Wavelength of He-Ne Laser

The interference fringes of F-P interferometer are clearer and thinner than in Michelson’s. So if

we use the method of counting the rings to measure the wavelength of He-Ne laser, the result

will be more exact than Michelson interferometer.

1) Setup the F-P interferometer.

2) Adjust the interferometer carefully to produce clear circular fringes in the center of

ground glass screen.

3) At this time, record the reading d0 of the fine micrometer.

4) Count the number of fringes that appear (or disappear) in the center of the ground glass

screen as the micrometer is turned slowly. After counting 50 fringes, record the

micrometer reading again

6) Calculate the ∆d. The actual mirror movement d∆ is equal to λN∆ /2, where λ is the

wavelength of the source. ∆N is the number of rings we counted, here ∆N=50.

The moving distance is:

2

λNd

∆=∆

And ∆N is the number rings counted. So,

N

d

∆

∆=

2λ

7) To avoid any errors in counting the rings or recording the reading of the micrometer,

steps 1- 6 should be repeated at least 3 times.

Eye

G2G 1 Ground

Glass

Screen

Beam ExpanderHe-Ne Laser

18

5.2.3 Observation of the Interference of Sodium D-lines

The low pressure Sodium lamp is used as the light source. The light emitted by the Sodium

lamp actually has two different wavelengths which produces two different sets of concentric

interference fringes on the ground glass screen.

When rotating the fine micrometer, the observer can find that the two sets of interference

fringes will coincide at certain positions and separate at other positions.

1) Setup the instrument in F-P mode. Use the sodium lamp as the light source and turn

on the power.

2) Slowly move the movable mirror by adjusting the screws behind it till they are very

close to each other. The distance between them is about 1-2mm. (Notice: Do not let

them touch each other).

3) Place a pinhole plate in front of the lamp. Generally, the light beam through a hole in

front the lamp forms a series of light spots due to the reflections of the two mirrors

or it may look like a comets tail. Adjust the movable mirrors in order to make those

spots or tail coincide.

4) Move away the pinhole plate and adjust the movable mirror carefully till you get

clear interference fringes. For the convenience of observation, the ground glass

screen can be used in the mounting hole in front of F-P mirror (Fixed Mirror), as

shown in the above figure.

5) Slowly rotating the fine micrometer to observe the Separating- Coincidence-

Separating phenomena of the interference fringes.

Note: Be careful when adjusting the screws of the movable mirror to avoid the collision of the two mirrors.

S

Ground

Glass

Screen G1 G2

19

5.3 Twyman-Green Interferometer

5.3.1 Demonstrating the principle of a Twyman-Green interferometer

Twyman-Green interferometer is used to check optics by parallel light. Following figures are

the familiar checking modes of it.

a) Check flat mirror

M1

Test Mirror

L1

Screen

S

BS

M2

The figure shows the structure of checking flat mirror. If the test mirror has any defects, the

corresponding fringes can be observed on the screen.

b) Check transparent flat optic

M2

M1

Flat

L1

Screen

S

BS

The figure shows the structure of transparent flat optic. If the flat optic has any defects, the

corresponding fringes can be observed on the screen.

c) Check prism

M2

M1

Prism

L1

Screen

S

BS

20

The figure shows the sketch map of checking prism.

d) Check lens

M1

Lens

L1

Screen

S

BS

Perfect Spherical

Mirror

The figure shows the sketch map of checking lens.

Notice:

This instrument is design to demo the checking principle of Twyman-Green

interferometer and can not be used to check optics. Here we use mode b), and with the

help of expand light source (laser and beam expander) to demo the principle.

Experiment Procedures

1) Set up the instrument according to the Michelson interferometer. He-Ne laser is used

as the light source.

2) Adjust the movable mirror to get equal thickness fringes on the screen. Refer the

particular procedures in experiment 5.1.3.

3) Place the thin film (Sample 1 and sample 2 in turn) into the clamp and setup them in SOCKET3.

4) Observe the interference fringes on the screen. If the sample has perfectly flat

surfaces the fringes are perfect too. Otherwise the fringes will have the

corresponding deformation and tortuosity.

21

6. Maintenance

6.1 Optical components

Please do not wipe the optical components. If necessary, please whisk the dust by the clean soft

brush, and use absorbent cotton dipped by the mixture of ethanol and aether. The surface of the

optical components can not be touched by hand.

6.2 Driving mechanism

Careful adjustment required. Do not push the coarse micrometer and fine micrometer by force.

Please don’t disassemble the driving mechanism to avoid damage.

7. Laser safety and lab requirements

Follow the corresponding laser safety guidelines based on AS/NZS 2211.1:1997 and other lab

instructions about optical components etc.

This experimental laser is a laser which is designed only for

laboratory applications.

The output power of He-Ne laser is less than 1 mW at 632.8nm.

DANGER

LASER RADIATION

AVOID DIRECT EYE EXPOSURE

HELIUM-NEON LASER

1.5mW MAX OUTPUT at 632.8nm

CLASS IIIa LASER PRODUCT

22

8. Parts List

No. Description Specification Qty

1 Main interferometer 1

2 Ground Glass Screen Φ50mm 1

3 Extension arm 1

4 He-Ne Laser 0.7~1mW 1

5 Laser Holder 1

6 Sodium-Tungsten Lamp Sodium: 10W, Tungsten: 15W 1

7 Air Chamber and Air Pump with Gauge Range: 0~40KPa

Chamber length: 80mm 1

8 Two in one screen 1

9 Transparency slice clip 1

10 Beam expander 1

11 Samples Slice surface of different quality 2

12 Power cord 2