2009 VCE Photonics WorkshopDraft program

To be organised by the Australian Institute of Physics (Victorian Branch) Education Committee, with the assistance of Craig Anderson from Leongatha Secondary College

Draft 2009 Program3:00pm Speaker: 4:00pm Practical activities: Three groups - three labs, forty minutes in each lab6:00pm Pizzas and soft drink 6:45pm Lecture on applications of Photonics: 7:30pm Finish

Speakers:

Practical ActivitiesLab 1

1. Bottle of Light (TIR and Critical angle)2. Optical Waveguide demonstrator set3. Making a simple ‘optical fibre’ and

exploring some of its properties4. Light makes its escape5. Modern optical fibre

6. Attenuation in optic fibre7. Estimating Semiconductor Band Gap using

LEDs8. Laser to Audio Receiver9. Optical Voice Link

Lab 2

10. Spectroscopic studies11. Measuring the speed of light using

microwaves12. Measuring the thickness of a hair using

a laser

13. Measuring the Wavelength of a Laser using a CD

14. Interference using a CD and a DVD

Lab 3 15. Ciderhouse Wavelength Division Multiplexing (WDM) kit activity16. Senko kit activity **

* Explanation provided by demonstrator** Notes will be with the equipment.Equipment for activities 1 - 9, 11 - 14 was supplied by Leongatha Secondary College.

IMPORTANT – LASER LIGHT CAN BE HAZARDOUSDo not bend down so that your head is level with the laser beam. Laser light is potentially hazardous to your eyes, so never look directly into the beam.

The prac groups are:A. Lab 1, then 2 & 3B: Lab 2, then 3 & 1C: Lab 3, then 1 & 2

1. Bottle of LightFibre optics – total internal reflection – critical angle

In 1870, before members of the prestigious British Royal Society, John Tyndall showed how a light beam could be guided in an arcing stream of water, Tyndall shined a bright light into a horizontal pipe leading out of tank of water. Then, when the water was allowed to flow out and downward in an arc, light rays traveled inside the water until they were broken up by the water striking a collection pan. With the help of a drink bottle and a laser pointer you will duplicate this experiment.

What it shows:A Beam of laser light can be trapped inside a stream of water by total internal reflection. This is the aquatics equivalent of a fibre optic cable.

How it works:A stream of water flows from a hole in the side of a drink bottle (figure 1). The critical angle of 490

is such that total internal reflection will occur in the stream even when it is reduced to almost a trickle. Imperfections in the stream (and scattering agents added) allow some of the light to escape, and the effect is seen as a sparkling waterfall.

The water reservoir is a 2L drink bottle, and the stream emanates from a hole just above the base section (about 8cm from the bottom). To ensure a smooth flow a hole larger than 5 mm is cut in the plastic and a piece if clear tape stuck across it – the hole being made using a regular hole punch; this avoids unclean edges that occur when the plastic is cu and then thin wall produces a more laminar flow.Figure 1 Total internal reflection within stream of water

Direct the stream of water onto a white tile in the sink, so you can see if the water optical fibre really works!Discussion:The index of refraction of water is 1.33, and thus the critical angle for a water/air interface is sin-1 (1/1.33) = 490. So long as the water stream does not bend at too sharp an angle, light traveling along the length of the stream strikes the water/air interface at an angle greater than 490 with respect to the normal to the interface and is this totally reflected.

2. Optical Waveguide: Demonstrator SetIntroductionThis kit, when used with a Laser pointer, dramatically and visually demonstrates the ability of a simple optical element to trap light inside the element with very little light loss. The optical elements that you will use to demonstrate this principle are optical wave guides. (The term optical wave guide is synonymous with the more common term -optical fibre.) You will better understand what these terms mean when you complete the activities in this kit. The result of shining a laser beam inside these two distinctly different wave guide shapes may surprise you. The kit contains two optical wave guides made from specially formulated polymer/acrylic material. One guide is a straight bar that will show you how light waves are trapped inside the bar by a process known as total internal reflection. You will learn that very little light escapes from the inside of the bar. The second bar of material 'is curved. How does curvature affect the passage (and possible loss) of light? You will learn that light can travel through optical light guides even when the light has to negotiate bends. This wave guide property of being able to bend light up and down is a very useful property of fibre optic communications systems, as used by long distance telephone companies.

Equipment Needed: straight optical light guide, 10H x 20W x 230L mm curved optical light guide, 10H x 20W x 230L mm Laser pointer that produces visible light.

Demonstrations:1. Using your thumb and forefinger of one hand, aim the laser pointer in a safe direction away

from people or .reflective surfaces.2. Turn the laser pointer on, then position the pointer and straight optical light guide as shown

below.

3. You should now see the laser beam reflecting off the top and bottom surfaces of the light guide in a zig~zag pattern.

4. Change the angle between the laser beam and the end face of the acrylic bar. Notice how the laser beam continues to bounce up and down inside the light guide.

5. Do you understand why these are called optical light guides?6. Still aiming the laser pointer into the end of the straight wave guide, note that the laser beam

bends slightly as it enters the guide. This bending of light is a known scientific phenomenon, and it can be explained by understanding what is called the refractive index of materials. See the section titled "Optical Fibre Theory" for explanation.

7. Now replace the straight bar with the curved light guide. Shine the laser beam into either end and observe the light beam travelling around the curve.

8. Return all the kit items to their proper storage containers so' you and/or others can repeat the activity as you wish.

SummaryIn the preceding steps you conducted a hands-on demonstration of the essentials of optical wave guide technology that are critical to operation of fibre optic cable and communications systems. The 'phenomenon of light reflecting off a fibre's internal surfaces is technically known as "total internal reflection", In the practical world, fibre optic light guides used in communications

applications are round in shape because there is potential for a small amount of light to be lost at the corners, if the guide is rectangular in shape. Early in this manual we described the nearly "loss-less" transmission of light. In fact, by intent, the optical light guides in this kit are not lossless. For demonstration purposes, we purposely mixed a specia1 material in the acrylic to scatter the light. This was to aid in yiewing the laser beam as it travels or bounces down the wave guide. This design is by no means ideal for transmitting the maximum amount of light the greatest distance, but it helps you see first-hand how light is reflected down the optical waveguide. Also keep in mind that when we measure or view any energy source such as light, voltage or electrical current, we must ‘tap’ a small amount of that energy. Therefore, there is no such thing as a loss-less energy measurement.

Optical Fibre TheoryFibre optics is the most rapidly growing portion of optics study in the world. It has grown so dramatically that some people may not think of it as even being part of the optics field. In terms of data communications they could be right, because fibre optic communications utilizes electronic and laser technology, in addition to optical fibre. The illustration, left, depicts the construction of basic

communication optical fibre, with concentric layers of materials called the core and cladding. These materials are bonded to each and are always composed of different materials because they must have different refractive indexes. With the wave guides contained in this kit the cladding layer is air. The acrylic bars are the cores. In optical fibres, the outer material always has a lower refractive index than the inner layer. Visualize a light ray - travelling at a specific angle - as it strikes the boundary between higher and lower refractive index materials.

As the light ray enters the lower refractive index material, it bends away at an increased angle, as shown. If the entry angle of the light ray increases, eventually the exiting ray will be parallel to the horizontal boundary between the materials.

Any further increases in the angle of the incident (entry) ray will actually cause the light rays to reflect back into the material with larger refractive index - the core. This reflection is commonly called "total internal reflection" and is the basis for the theory of how light travels through optical fibre. Because the cladding layer surrounds the core, the light is confined to two dimensions and travels lengthwise (also "rebounding" from side to side) from one end of the fibre to the other. Total internal reflection is essentially "loss-less" reflection. No light is lost at each successive reflection within the fibre, which makes this concept very suitable for transmitting light over long distances. Any minor loss of light in optical fibre is caused primarily by impurities in the core material. Commercial available optical fibre is usually made from glass or plastic.

Optical fibres can carry many times more information - faster and over longer distances - than conventional copper wire, and it is far less vulnerable to electromagnetic interference. Other advantages of fibre optics include ease of installation and the ability to transmit data with extremely low error rates. Optical fibre also does not "attract lightning strikes, since it is not electrically conductive.

Table 1. Refractive indices of some common materials

Material ValueAir 1.00029Water 1.33Glass 1.4 – 1.8Silicon 3.5Acrylic 1.49Diamond 2.0

3. Making a Simple "Optical Fibre" and exploring some of the properties of optical fibre

OverviewIn this activity you will construct a simple light guide using water and a length of vinyl tubing. The water and vinyl tubing will act as the core, while air will act as the cladding or boundary layer. The experiment will demonstrate how effective even a simple light guide is for coupling energy from a light source to a detector. You will also observe how the light guide can carry light "around "El corner" with relatively little loss compared to when light travels in a straight line

Materials required 20cm length of 12mm tubing A Tain data logger and

Phototransistor light probe A Laser pointer.

Water, a plastic transfer pipette. Small squares of plastic and a rubber

band

What to do!Make sure the Tain data logging equipment is on, use the program 'Optic fibre' and set it to 'run'. We are not going to use the equipment as a logger; we will simply observe the screen display.

Observing all safety requirements, have one team member hold the phototransistor light probe and the other shine the laser .pointer at the probe. Start very closely together and slowly move the pointer and probe apart. Note the readout on the computer.

Now, because we will be working with water, and the probe is not water tight, we will wrap the probe in plastic. To prove the plastic does not absorb or scatter the light repeat the experiment above.

Insert the probe in the end of the plastic tube. Hold the tube straight and shine the laser at the probe down the tube. Observe the screen display. How does the light level compare with just shining the laser at the probe?

___________________________________________________________________________Whilst shining the laser down the tube, slowly bend the tube and note what happens to the light being transmitted to the probe.

___________________________________________________________________________Carefully fill the tube with water using the pipette. Try not to get bubbles and fill the tube fully. Seal the end with plastic and a rubber band.

Holding the tube straight, shine the laser through the water and onto the probe. Compare the light levels to those when the tube was empty.

___________________________________________________________________________Whilst shining the laser down the tube of water, slowly bend the tube and note the level of light being transmitted to the probe.

___________________________________________________________________________Change the angle of the laser and note the light intensity at the probe.

___________________________________________________________________________

4. Light makes its EscapeThe light carrying portion of an optical fibre must be protected

In this activity you will observe how changing the optical density (refractive index) of the material surrounding the optic fibre will affect the fibre's light-transmitting ability.

Materials requiredLaser pointerPlasticineAcrylic fibreSmall piece of white cardPetri dishes of water and paraffin oil

MethodOBSERVE LASER SAFETY PRECAUTIONS WHILE DOING THIS PRAC

Insert the fibre into the end of the laser pointer. Observe the amount of light transmitting along the fibre by holding the card to the end of the fibre. Gently bend the fibre into a 'U' shape and observe the amount of light transmitted by the fibre.Slowly immerse the bent part of the fibre into the water in the petri dish. As you immerse the fibre in the water, observe the amount of light coming out the end of the fibre using the card.

Can you see light escaping from the fibre?Where does the light go? Repeat in the paraffin dish. Turn the laser off.

ResultsAs the bottom of the fibre is immersed, the amount of light coming out the far end of the fibre decreases. You should be able to see light escaping from the fibre by looking at the bottom of the dish.

DiscussionThe decrease in light from the fibre end is caused by the change in optical density outside the . fibre when it is dipped in water. The optical density of water is closer to that of the fibre than the optical density of air; therefore, it doesn't trap light as well. When the light in the fibre encounters the water, some of it escapes and travels to the bottom of the pan. The U-shape in the optic fibre increases the amount of light escaping when it is immersed in water.

The correct term for optical density as we have applied it really is "refractive index" or index of refraction. The refractive indices of the three materials that you worked with in this experiment are shown in the table below. You might now ask: What good are optical fibres if their ability to transmit light can be affected by conditions around them? If this were actually the case, they would not be very useful. Most fibre optics used for commercial applications are manufactured with a coating around the central light-carrying portion so that external conditions do not affect them. This coating is called "cladding" while the central "light-carrying" portion is called the "core". A fibre's cladding always has a lower refractive index than the core.

Water: 1.33 acrylic (plastic) 1.45 air: 1.00

5. Modern Optical fibreCommercial fibre is very pure and has a protective ‘cladding’

The acrylic fibre used in previous experiments carries light from one end to the other, but it doesn't really do a very good job. To transmit light long distances, commercial optical fibres must be composed of ultra pure transparent materials. For example, some commercial optical fibre material is so pure that the light lost when travelling through a one kilometre length is more than 90 percent of the light which 'entered the fibre. In the illustration is a basic optical fibre, with concentric layers of core and cladding. T he fibre you will use In this experiment contains a central "light carrying" core and a very thin (10 m) cladding layer to trap the light inside. (The cladding is also transparent. You probably won't be able to distinguish it from the core.)

Materials RequiredLaser pointerPlasticineOptic fibreSmall piece of white cardPetri dish full of water and paraffin oil

OBSERVE LASER SAFETY PRECAUTIONS WHILE DOING THIS PRACWe are going to repeat the last experiment, but using proper optic fibre, rather than just a length of plastic fibre. Insert the optic fibre into the end of the laser pointer. Observe the amount of light transmitting along the fibre by holding the card to the end of the fibre. Gently bend the fibre into a 'U' shape and observe the amount of light transmitted by the fibre.

Slowly immerse the bent part of the fibre into the water in the trough. As you immerse the fibre in the water, observe the amount of light coming out the end of the fibre using the card. Can you see light escaping from the fibre? Why not? Repeat in the petri dish full of paraffin. Turn the laser off.

ResultsAs the bottom of the optic fibre is immersed, the amount of light coming out the far end of the fibre stays the same. You should not be able to see light escaping from the fibre when looking at the bottom of the trough.

DiscussionLight is transmitted from one end of the fibre to the other because light is being 'guided by the central fibre core and trapped inside by the outer cladding layer. Light intensity doesn't change when you dip the fibre in the water because the refractive index (optical density) immediately surrounding the central core doesn't change as it did in previous experiment. The cladding layer remains constant and acts as an optical shield between the fibre core and the water.

The fibre you just finished experimenting with is made of plastic. It is one of the two most commonly used materials in commercial optical fibres. The other material is glass – commonly called ‘silica’ in the technical community.

6. Attenuation in Optic Fibre

Attenuation is the loss of optical signal as it travels along the optic fibre. Several factors can cause attenuation, but it is generally categorized as either intrinsic or extrinsic. Intrinsic attenuation is caused by substances inherently present in the fibre, whereas extrinsic attenuation is caused by external forces such as bending. The attenuation coefficient is expressed in decibels per kilometre and represents the loss in decibels per kilometre of fibre

Intrinsic AttenuationIntrinsic attenuation results from materials inherent to the fibre. It is caused by impurities in the glass during the manufacturing process. As precise as manufacturing is, there is no way to eliminate all impurities. When a light signal hits an impurity in the fibre, one of two things occurs: It scatters or it is absorbed. Intrinsic loss can be further characterized by two components: Material absorption and Rayleigh scattering.

Material AbsorptionMaterial absorption occurs as a result of the imperfection and impurities in the fibre. The most common impurity is the hydroxyl (OH-) ion, which remains as a residue despite stringent manufacturing techniques.

Rayleigh ScatteringAs light travels in the core, it interacts with the silica molecules in the core. Rayleigh scattering is the result of these elastic collisions between the light wave and the silica molecules in the fibre. Rayleigh scattering accounts for about 96 percent of attenuation in optical fibre. If the scattered light maintains an angle that supports forward travel within the core, no attenuation occurs. If the light is scattered at an angle that does not support continued forward travel, however, the light is diverted out of the core and attenuation occurs. Depending on the incident angle, some portion of the light propagates forward and the other part deviates out of the propagation path and escapes from the fibre core. Some scattered light is reflected back toward the light source. This is a property that is used in an optical time domain reflectometer (OTDR) to test fibres. The same principle applies to analysing loss associated with localized events in the fibre, such as splices.

Short wavelengths are scattered more than longer wavelengths. Any wavelength that is below 800 nm is unusable for optical communication because attenuation due to Rayleigh scattering is high. At the same time, propagation above 1700 nm is not possible due to high losses resulting from infrared absorption.

Looking at Rayleigh ScatteringRayleigh scattering (named after Lord Rayleigh, who first described it) is the scattering of light by particles much smaller than the wavelength of the light. It occurs when light travels in transparent solids and liquids, but is most prominently seen in gases. Rayleigh scattering of sunlight from particles in the atmosphere is the reason why the light from the sky is blue. We are going to explore the phenomenon of Rayleigh scattering. The materials we will use are A Hodson Light Box Some glue sticks to simulate optic fibre Black tape; clear tape A small piece of white card

Using the wide slit, modified with tape to the height of a glue stick shine the light into one end of a glue stick and hold the other end of the glue stick approximately one centimetre from the white card. Notice that the end of the glue stick closer to the light box is a different colour than the end nearer the white card. Notice the colour of the circle on the white card.

Place two glue sticks end to end, and attach them together with the clear tape. Repeat the investigation and notice any difference in the colours along the glue sticks and in the coloured circle on the white background. Continue to attach more glue sticks with the clear tape and to notice the changes in colour and Intensity along the glue sticks and in the colour circle. It is unlikely you will get transmission along more than two glue sticks. The change in colour along the glue sticks is caused by Rayleigh scattering. Which colour is closest to the light box

Place a blue filter in the light box. How far does the light travel along the glue stick? Change the filter to a red filter. How far does the light travel now?

Other related, but essentially immaterial informationThe light box emits white light. The glue stick scatters the blue light out of the light beam more than the yellow or the red light. Because the first colour, to be scattered is blue, the end of the glue stick nearest the light box appears blue and the other end is yellow to orange-red.

The glue stick scattering model offers a demonstration of why the sky is blue and sunsets are red. The sky is blue because blue light is most readily scattered from sunlight in the atmosphere, just as blue light was most readily scattered from white light in the glue sticks. If blue light was not scattered in the atmosphere, the sun would look a little less yellow and a little whiter and the rest of the sky would be black.

At sunset, the sun is low near the horizon, and light travels through a greater thickness of atmosphere before reaching your eyes than it does when the sun is higher in the sky. Just as the light travelling along the glue sticks got redder as the length of the glue stick path got longer, so the sunset is red when the atmospheric path through. which the sunlight travels gets longer. The scattering that produces the red sunset may be enhanced by pollution or other atmospheric conditions.

Extrinsic AttenuationExtrinsic attenuation can be caused by two external mechanisms: macrobending or microbending. Both cause a reduction of optical power. If a bend is imposed on an optical fibre, strain is placed on the fibre along the region that is bent. The bending strain affects the refractive index and the critical angle of the light ray in that specific area. As a result, light travelling in the core can refract out, and loss occurs.

A macrobend is a large-scale bend that is visible, and the loss is generally reversible after bends are corrected. To prevent macrobends, all optical fibre has a minimum bend radius specification that should not be exceeded. This is a restriction on how much bend a fibre can withstand before experiencing problems in optical performance or mechanical reliability. The second extrinsic cause of attenuation is a microbend. Microbending is caused by imperfections in the cylindrical geometry of fibre during the manufacturing process.

Microbending might be related to temperature, tensile stress, or crushing force. Like macrobending, microbendlng causes a reduction of optical power in the glass. Microbending is very localized, and the bend might not be clearly visible on inspection. With bare fibre, microbending can be reversible.

7. Estimating Semiconductor Band Gap using LEDs

Objective: Observe visible light-emitting diodes (LED's) in simple electrical circuits, and relate the composition of semiconductor materials with their behaviors. Estimate the band gap of a semiconductor material.

Review of Scientific Principles:Colored light can be produced in a number of ways. On the one hand, ordinary incandescent light bulbs may be used with filters that select out a portion of the complete color spectrum that is emitted from the glowing wire filament. On the other hand, the familiar orange-red glow of neon lights is generated by electrically ionizing very small amounts of gases inside sealed glass tubes. LED's contain neither a wire filament nor any gases. The light emitting portion of a solid state diode is quite small so you will need a magnifying glass to see it clearly. Even though the diode may be enclosed in a colored plastic lens, the lens is not the cause for the color of the light observed.

In LED's, electrical energy is converted into light energy. The voltage required to switch on the LED is proportional to the energy of the light emitted from that LED. Also by comparing the color of the light with a chart of the visible light spectrum, it is possible to assign a wavelength to the color of each LED. Using this wavelength, a simple calculation can be made to approximate the energy of the electron transition taking place at the junction in the diode. The colored light (made up of photons) is being produced by electrons that are relaxing across the energy gap in the semiconductor material. The reverse process may also be observed in which light shining on a diode can be converted into electrical energy.

Applications:LED's are very common and are frequently used as indicator lamps. When the light goes on, electricity is flowing. Whether it is a compact disc player, electric guitar amplifier, computer, monitor, or video game module, we always look for the little colored light to let us know it is working. As common as they are, however, most people have no idea how LED's produce their bright, colored light; they simply expect it to happen.

Time: One hour. (More time is required if students will be assembling the circuits themselves).Materials and Supplies:Each student group will need: variable power supply (at least 0-6 VDC) with leads one panel containing several different visible LED's (see note 1) digital multimeter (DMM) and test leads with small alligator clips magnifying lens 9V battery and snap connector, 4700 ohm resistor LED socket (see note 2) small, bright flashlight

General Safety Guidelines: Do not stare long at any of the brightly lit LED's. Some of the wires may have sharp edges. Do not grasp any bare wires or connections with your hands. Be sure the power supply is set for 0-6 V (DC). Turn it off when not in use.

Procedure:

1. Obtain a panel containing several different visible LED's.2. Connect the power supply 0-6V DC to the panel leads.3. Connect the DMM across the circuit, and set it for DC.4. With the power supply at its lowest setting, turn it on.5. Slowly dial up the voltage until and effect is noticed at one of the LED's.6. Continue to slowly dial up voltage until you have observed all the LED's (do not let the

voltage reading on your DMM exceed 2.5V).7. Now slowly dial the voltage back down, and observe the LED's.8. Repeat steps 5-7. Record the voltage at which you observe each LED to go on and off.9. With all LED's on and shining brightly, compare their colors with a chart of the visible light

spectrum. Or view the lighted LED's with a calibrated spectroscope. Record the wavelength in nanometers of the color that matches each LED.

10. Turn off the power supply, and disconnect only its positive lead from the circuit. Leave the DMM on and its leads in place across the LED circuit.

11. With the room darkened, shine a small, bright flashlight on each of the LED's in the panel. Record the maximum voltage that you read from the DMM for each LED.

12. Obtain a 9V battery and snap connector with appropriate resistor and socket. Insert a green light LED in the socket with the long leg on the red side. Shine it directly over and against each of the LED's in the panel. Record the maximum voltage for each that you read from the DMM.

13. Repeat step 12 replacing the green LED with a red LED. Record the voltage readings.I4. Test what happens when an LED is placed in the socket backwards.15. Using a magnifying glass, make two scaled drawings of an LED--one from the top and one

from the side. Indicate on your drawings where the light is produced.

Data and Analysis:LED

ColourTurn On Wavelength of

light emittedEnergy of

light emittedBand gap

of materialComposition of material

Questions:1. In what order do the LED's light when the voltage is increased slowly from zero?2. Place the LED's in order according to increasing wavelengths.3. What is the relationship between the lists in questions one and two?4. From your observations, what kind of mathematical relationship exists between the numerical

values for electrical energy (volts) and the wavelength of the colored light emitted?5. What effect does the white light source have on the LED's? Why?6. Which color of LED caused a voltage reading in every LED on the panel?7. Which color of LED caused a voltage reading in only one LED on the panel?8. Explain your answers to questions six and seven by discussing the relationship between

energy and color of light.9. In which LED is the diode composed of a material with the largest band gap?10. Calculate the bandgap for the material in each diode in units of electron volts (eV) by using

the equation E = 1240 / Where is the wavelength of light in nanometers (10-9 meters).11. Compare the values calculated with a list of semiconductors and their band gaps. Which

materials do you believe are present in the LED's that you used?

8. Laser to Audio Receiver

Equipment Needed: Modulated Laser Audio Source Laser-Audio Receiver

Procedure:1. Choose a flat, level surface about 60 - 120 cm in size.2. Collect all the items listed in the "EQUIPMENT NEEDED" section above. 3. Review the Rules for Laser Safety 4: Position the laser and the receiver as shown in Figure 1.

Figure 1. Side view of the laser and audio receiver5. Turn on the equipment.6. Dim the room lights to help you observe the laser beam. Position the receiver in line with the

laser so the beam strikes the audio receiver close to the photodetector access hole. Manually align the beam with the optical input (photo diode) of the audio receiver.

7. Turn on the sound source or microphone using the switch.8. Turn the ON/OFF volume knob on the audio receiver to the 12 o'clock position. The pilot

light (yellow LED) on the audio receiver should now be lit, showing that it is on.

Figure 2. Close up view of the alignment laser beam to receiver aperture

9. Listen for sound to come out of the audio receiver. If you do not hear sound on the first attempt, turn up the volume control clockwise on the audio receiver. If you reach maximum volume and still do not hear sound, realign the laser beam with the audio receiver.

10. When you have the audio receiver and thelaser properly aligned you should easily hear sound from the receiver.

11. A lens can be used to focus the diverged beam back down to increase the signal strength. Try this and observe the resulting increase in volume.

Questions:1. We want to make a laser transmitter that can reach the Moon (3.82 x 108 m from earth).

Assuming that this receiver needs 2 x 10-6 watts of light to reproduce the audio signal, what power laser would be required if a lens of diameter 1 m is used to focus the light onto the photo transistor? Assume beam divergence of 0.5°. Neglect atmospheric effects.

2. If we were to use our laser (1.6 milliwatts) how many photons would this detector on the Moon receive every second. Assume that the wavelength is 630 nm.

9. Optical Voice LinkA Hands-On Introduction to Fibre Optics CommunicationsPrefaceThe Optical Voice Link is a project-oriented introduction to optical fibre communications.

Experiments and Activities 1. Grasp the optical fibre near its tip with your thumb and forefinger. Point it toward a light

source and different coloured objects, while observing the other end of the fibre. Note the changes in brightness in that end as you move the other end around, or cover its tip with a finger. Do any colours seem to transmit better than others?__________________________________________________________________________

2. Holding the fibre about 0.5 mm from this page, move it left to right across the heading of this section. What changes do you observe in the brightness at the other end of the fibre?__________________________________________________________________________

3. Push the momentary switch on the transmitter board to energise this assembly. The red LED should light. If not check the battery.

4. Do you see a periodic signal on the oscilloscope display when humming or whistling? If-you change the pitch does the period of measured signal on the oscilloscope change?

__________________________________________________________________________

Insert the prepared fibre end through the cinch nut and into the connector until the core tip seats against the moulded lens inside the device package.

Screw the connector cinch nut down to a snug fit,. locking the fibre in p1ace.

Figure 1. Cross-section of fibre optic LED and cable.

5. Turn the receiver switch on and turn it clockwise to about the midway position. Press the momentary switch on the transmitter and speak into the microphone. You should hear your voice from the speaker at the receiver end. (If a high-pitched sound is produced by the receiver when pressing the momentary switch, reduce the volume at the receiver.) Describe the quality of your voice reproduction at the receiver.__________________________________________________________________________

6. Assuming that this receiver needs 2 x 10-6 watts of light to reproduce the audio signal, the transmitter launches 50 x 10-6 watts of power into the fibre and the fibre has 1 dB of attenuation per meter, determine the maximum length of cable that can be installed between transmitter and receiver and still function properly.

7. With the transmitter and receiver assemblies as far apart as possible, adjust the gain of the receiver to as high as possible without the receiver producing a high-pitched squeal when the transmitter momentary-switch is closed. Have somebody touch the transmitter microphone to a mechanical clock while holding down the momentary switch. Can you hear the gears inside the clock moving, through the receiver speaker? Repeat this experiment with an electric clock

__________________________________________________________________________

8. Disconnect the fibre from the transmitter, leaving the fibre connected to the receiver. Turn the volume on the receiver switch to maximum. Hold the transmitter end of the fibre up to a fluorescent light. What do you hear? The noise you hear is 50 Hz, the frequency of the 240 volt 50 Hz AC input. The fluorescent light is pulsing; it is not actually "on" all the time. Repeat this procedure, holding the fibre close to an incandescent light bulb. What do you hear now? Describe the difference.

__________________________________________________________________________

Circuit OperationMost fibre optic transmitters typically have an amplifier or buffer, driver, optical source, sometimes an optical connector or interface. The transmitter in this kit also has an acoustic microphone for converting sound waves to an electrical signal, and requires a nine-volt battery with holder to provide electrical power. Figure 2 shows the schematic of the circuits contained on the transmitter printed wiring board of this kit.

__________________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________

10. Spectroscopic studies

An important property of light is its frequency (or wavelength). The various methods used to generate light can create quite different ranges of frequencies (which your eyes detect as colours). A useful instrument to investigate light is one that can split light up into its constituent wavelengths – a spectroscope.

In this activity you will use spectroscopes to examine various light sources.

There are two types of spectroscope that you will use; one is a hand-held cardboard box, and the other is a fibre-coupled computerised device. Make sure that you spend some time using each of these.

Hand-held spectroscope (“cardboard box”)To examine a light source, look into the spectroscope through the circular hole at one end whilst positioning ensuring as much light as possible is entering the small slit at the other end. Glance across to the scale inside the box to see what colours/wavelengths are present in the light.

Fibre-coupled spectroscope (using the computer)To examine a light source, place it so that light is incident on the end of the fibre (the fibre end is held in a metal mount). The instrument is quite sensitive, so you do not need to put the light source too close to the fibre end. Use the mouse to control the spectrum that is graphed on the computer.

Use the spectroscopes to examine and describe the following light sources:

light from a fluorescent tube (i,e, the room lights)

________________________________________________________________________

________________________________________________________________________ light from a light bulb (“ordinary” bulb or a coloured bulb)

________________________________________________________________________

________________________________________________________________________ light from various light-emitting diodes (LEDs). In particular, what seems unusual about the

light from a “white” LED?

________________________________________________________________________

________________________________________________________________________ light from the mercury atomic vapour lamp. Is it similar to any of the other light sources that

you have examined?

________________________________________________________________________

________________________________________________________________________

11. Measuring the Speed of Light using Microwaves

Equipment: A microwave oven, A ruler Thermal paper (from an old fashioned fax roll) Some damp paper towel

MethodRemove the turntable from the microwave and replace the glass plate. Place a layer of damp paper towel on the plate, and lay a piece of fax paper over it. The water in the paper towel is what will heat up, causing the fax paper to discolour at the 'hot spots'. Heat until it just starts to discolour- about 20 seconds, depending on the power of the oven. The hot-spots will show up as blackish blobs - choose the approx centre of the blobs. The distance between the hot spots is half the wavelength of the microwaves; and the frequency of the microwaves will often be printed on the back of the oven. Our microwave has a frequency of 2450 megahertz. The speed of light is equal to the wavelength multiplied by the frequency of an electromagnetic wave (microwaves and visible light are both examples of electromagnetic waves

How it works:When you turn on your microwave oven, electrical circuits inside start generating microwaves - electromagnetic waves with frequencies (which we'll call ‘f’ later) around 2.45 gigahertz -. 2450000000 Hz. These waves bounce back and forth between the walls of the oven, the size of which is chosen so that the peaks and troughs of the reflected waves line up with the incoming waves and form a ‘standing wave’.

If you pluck a guitar string, you'll set it vibrating. Usually, you will excite the .first harmonic. - a standing wave that has the string stationary at the bridge and the fret, and vibrating back and forth in the centre. With effort, you might be able to excite the second harmonic (try plucking the string in opposite directions ¼ of the way in from either end), then you'll see the string vibrating back and forth, with the centre stationary. This pattern has three nodes or points with no displacement away from rest (there's a useful mnemonic - a NODe has NO Displacement): the ends and the centre, and two anti-nodes: ¼ and ¾ of the way along its length. There are infinitely many modes, one for each positive integer, with more and more nodes between the fret and the bridge.

A full wave is shaped like a ‘sine function’ going from zero to a maximum back through zero to a negative maximum and back to zero again - like the second harmonic in the figure on this page. So you can see that the distance between the maximum displacements of the wave is one half the wavelength.

The 1st, 2nd and 9th harmonics. The field is strongest where the wave has the highest amplitude

The electromagnetic field inside the microwave behaves in roughly the same way - except the vibrations are in ‘the electromagnetic field’. Where the vibrations are greatest (the anti nodes), you will see the greatest heating, but at the nodes, the paper will only change colour slowly as heat diffuses into those areas.

Thus, the distance between the melted regions (x) is equal to the distance between the antinodes, and equal to half the wavelength ()

So, the detailed calculation to find the speed of light (c) is c = * f = 2 * x * f.

12. Measuring the Thickness of a hair using a Laser

WHEN USING THE LASER BE CAREFUL NOT TO ALLOW THE LASER LIGHT TO REFLECT INTO PEOPLE'S EYES.

By taking measurements from the diffraction pattern produced when a laser hits a hair we can measure the thickness of the hair. The laser we are using has a wavelength, of 635 nanometres.

Position one of the three hairs that has been mounted on a wooden slide in the slide holder and place this at least 1 metre from the screen.

Measure the distance D; between the hair and the screen.

Point the laser at the hair and adjust the lasers position until a clear diffraction pattern is observed on the screen. This will be easier if the room is not well lit.

Measure the distance x, between two nodes. (If several nodes are visible you may measure the distance across several and then divide by the appropriate number giving greater accuracy)

Find the thickness of the hair d, using d = D/x.

Repeat for the other two hairs and one of your own.

Distance D Distance x Thickness dHair AHair BHair C ,Own Hair

You can also find the thickness by placing each slide into a projector and projecting an image of the hole and the hair onto1he screen. Measure the thickness of the image of the hair and the diameter of the image of the hole. The hole is 9.5 mm in diameter. Calculate the magnification achieved hence find the diameter of the hairs.

Diameter of Magnification Thickness of Thickness ofimage hair image . hair

Hair AHair BHair COwn Hair

13. Measuring the Wavelength of a Laser Using a CD

The spacing between tracks on a CD is 1.6 m. This can be used to determine the wavelength of a laser.

Position the CD about 15 cm from a wall and shine the laser onto the CD. Record the distance from the CD to the wall (D) and the distance between the two closet maxima on the wall/screen (x). BE CAREFUL NOT TO ALLOW THE LASER LIGHT TO REFLECT INTO PEOPLES' EYES

D = ______ mm x = ________ mm

Calculate the approximate wavelength of the laser light using:

= (xd)/(2D) .where d = the spacing on a CD (1.6 m).

LASER

CD

Wall/screenD

X

14. Interference using a CD and DVD

AIM: To measure the inter-track spacing on a CD and DVD

BACKGROUNDThe spacing of the tracks of a CD and a DVD is small enough so that they act as diffraction gratings. By measuring how laser light is scattered by these gratings, we can determine the inter-track spacing using the formula for Young's Double Slit experiment:

d sin = kwhered = spacing between slits = angle k = order of the minimum (0, 1, 2, ...) . = wavelength

EQUIPMENTSemiconductor laser for the light source CD and DVDRingstandsMeter stick

PROCEDURE Mount the laser and the CD on ringstands as shown, such that the reflected light from

the CD returns directly to the laser. With the light off you will se the interference maximum as shown.

Measure the distance to the wall, and the distances to each of the interference maxima using the centre laser spot as a reference.

Laser and CD mounted on Ringstands

Compute the angles (from the measurements) for each of the interference maxima. Graph the data using sin = k (/d) Do a linear regression on the line sin versus k . The slope of this line is d Compute d, the spacing between the tracks on the CD. Do the error analysis. Compare to the Iiterature value and compute a percentage error. Repeat the entire process for a DVD.

k = 0

k = 1

d1d2

d -1d -2

k = 2laser

CD/DVD

k = -1k = -2